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Cambridge Centre for Gallium Nitride


June 2024:

Sub-surface Imaging of Porous GaN Distributed Bragg Reflectors via Backscattered Electrons

M. Sarkar, F. Adams, S. A. Dar, J. Penn, Y. Ji, A. Gundimeda, T. Zhu, C. Liu, H. Hirshy, F. C. P. Massabuau, T. O’Hanlon, M. J. Kappers, S. Ghosh, G. Kusch, and R. A. Oliver

Microscopy and Microanalysis 30, 208-225 (2024)

In this article, porous GaN distributed Bragg reflectors (DBRs) were fabricated by epitaxy of undoped/doped multilayers followed by electrochemical etching. We present backscattered electron scanning electron microscopy (BSE-SEM) for sub-surface plan-view imaging, enabling efficient, non-destructive pore morphology characterization. In mesoporous GaN DBRs, BSE-SEM images the same branching pores and Voronoi-like domains as scanning transmission electron microscopy. In microporous GaN DBRs, micrographs were dominated by first porous layer features (45 nm to 108 nm sub-surface) with diffuse second layer (153 nm to 216 nm sub-surface) contributions. The optimum primary electron landing energy (LE) for image contrast and spatial resolution in a Zeiss GeminiSEM 300 was approximately 20 keV. BSE-SEM detects porosity ca. 295 nm sub-surface in an overgrown porous GaN DBR, yielding low contrast that is still first porous layer dominated. Imaging through a ca. 190 nm GaN cap improves contrast. We derived image contrast, spatial resolution, and information depth expectations from semi-empirical expressions. These theoretical studies echo our experiments as image contrast and spatial resolution can improve with higher LE, plateauing towards 30 keV. BSE-SEM is predicted to be dominated by the uppermost porous layer’s uppermost region, congruent with experimental analysis. Most pertinently, information depth increases with LE, as observed.

DOI: /10.1093/mam/ozae028


May 2024:

Monitoring of the Initial Stages of Diamond Growth on Aluminum Nitride Using In Situ Spectroscopic Ellipsometry

W. Leigh, S. Mandal, J. A. Cuenca, D. Wallis, A. M. Hinz, R. A. Oliver, E. L. H. Thomas, and O. Williams

ACS Omega 8(33), 30442-30449 (2023)

The high thermal conductivity of polycrystalline diamond makes it ideally suited for thermal management solutions for gallium nitride (GaN) devices, with a diamond layer grown on an aluminum nitride (AlN) interlayer atop the GaN stack. However, this application is limited by the thermal barrier at the interface between diamond and substrate, which has been associated with the transition region formed in the initial phases of growth. In this work, in situ spectroscopic ellipsometry (SE) is employed to monitor early-stage microwave plasma-enhanced chemical vapor deposition diamond growth on AlN. An optical model was developed from ex situ spectra and applied to spectra taken in situ during growth. Coalescence of separate islands into a single film was marked by a reduction in bulk void fraction prior to a spike in sp² fraction due to grain boundary formation. Parameters determined by the SE model were corroborated using Raman spectroscopy and atomic force microscopy.

DOI: /10.1021/acsomega.3c03609


April 2024:

Disentangling the Impact of Point Defect Density and Carrier Localization-Enhanced Auger Recombination on Efficiency Droop in (In,Ga)N/GaN Quantum Wells

R. M. Barrett, J. M. McMahon, R. Ahumada-Lazo, J. A. Alanis, P. Parkinson, S. Schulz, M. J. Kappers, R. A. Oliver, and D. Binks

ACS Photonics 10, 8, 2632-2640 (2023)

The internal quantum efficiency of (In,Ga)N/GaN quantum wells can surpass 90% for blue-emitting structures at moderate drive current densities but decreases significantly for longer emission wavelengths and at higher excitation rates. This latter effect is known as efficiency “droop” and limits the brightness of light-emitting diodes (LEDs) based on such quantum wells. Several mechanisms have been proposed to explain efficiency droop including Auger recombination, both intrinsic and defect-assisted, carrier escape, and the saturation of localized states. However, it remains unclear which of these mechanisms is most important because it has proven difficult to reconcile theoretical calculations of droop with measurements. Here, we first present experimental photoluminescence measurements extending over three orders of magnitude of excitation for three samples grown at different temperatures that indicate that droop behavior is not dependent on the point defect density in the quantum wells studied. Second, we use an atomistic tight-binding electronic structure model to calculate localization-enhanced radiative and Auger rates and show that both the corresponding carrier density-dependent internal quantum efficiency and the carrier density decay dynamics are in excellent agreement with our experimental measurements. Moreover, we show that point defect density, Auger recombination, and the effect of the polarization field on recombination rates only limit the peak internal quantum efficiency to about 70% in the resonantly excited green-emitting quantum wells studied. This suggests that factors external to the quantum wells, such as carrier injection efficiency and homogeneity, contribute appreciably to the significantly lower peak external quantum efficiency of green LEDs.

DOI: /10.1021/acsphotonics.3c00355


March 2024:

Strategy for reliable growth of thin GaN Caps on AlGaN HEMT structures

A. M. Hinz, S. Ghosh, S. M. Fairclough, J. T. Griffiths, M. J. Kappers, R. A. Oliver, and D. J. Wallis

J. Cryst. Growth 624, 127420 (2023)

This paper presents the growth of thin GaN capping layers on standard AlGaN HEMT structures. It has been found that the reliable growth of thin (≤ 5 nm) GaN capping layers by organometallic vapour phase epitaxy is challenging as GaN is unstable at high growth temperatures even in atmospheres with high ammonia partial pressure. To overcome this challenge a growth strategy based on the controlled desorption of GaN has been adopted. By intentionally growing thicker than desired capping layers and controlling the desorption during the cool down after growth it is feasible to reliably grow high quality GaN capping layers with a specific target thickness. The development of the controlled desorption process has been simplified by predicting the desorption based on the computer controlled cooling ramp and the temperature dependent GaN desorption rate. The latter was obtained by analysing in-situ reflectance traces for relevant growth conditions. Moreover, examples on how to identify exposed AlGaN barriers, i.e. without intact GaN caps, by TEM and AFM are presented.

DOI: /10.1016/j.jcrysgro.2023.127420


February 2024:

Porous pseudo-substrates for InGaN quantum well growth: morphology, structure and strain relaxation

Y. Ji, M. Frentrup, X. Zhang, J. Pongrácz, S. M. Fairclough, Y. Liu, T. Zhu, and R. A. Oliver

J. Appl. Phys. 134, 145102 (2023)

Strain-related piezoelectric polarization is detrimental to the radiative recombination efficiency for InGaN-based long wavelength micro-LEDs. In this paper, partial strain relaxation of InGaN multiple quantum wells (MQW) on the wafer scale has been demonstrated by adopting a partially relaxed InGaN superlattice (SL) as the pseudo-substrate. Such a pseudo-substrate was obtained through an electro-chemical etching method, in which a sub-surface InGaN/InGaN superlattice was etched via threading dislocations acting as etching channels. The degree of strain relaxation in the MQWs was studied by x-ray reciprocal space mapping, which shows an increase of the in-plane lattice constant with the increase of etching voltage used in fabricating the pseudo-substrate. The reduced strain in the InGaN SL pseudo-substrate was demonstrated to be transferable to the InGaN MQWs grown on top of it, and the engineering of the degree of strain relaxation via porosification was achieved. The highest relaxation degree of 44.7% was achieved in the sample with the porous InGaN SL template etched under the highest etching voltage. Morphological and structural properties of partially relaxed InGaN MQWs samples were investigated with the combination of atomic force and transmission electron microscopy. The increased porosity of the InGaN SL template and the newly formed small V-pits during QW growth are suggested as possible origins for the increased strain relaxation of InGaN MQWs.

DOI: /10.1063/5.0165066


January 2024:

Additive GaN Solid Immersion Lenses for Enhanced Photon Extraction Efficiency from Diamond Color Centers

X. Cheng, N. K. Wessling, S. Ghosh, A. R. Kirkpatrick, M. J. Kappers, Y. N. D. Lekhai, G. W. Morley, R. A. Oliver, J. M. Smith, M. D. Dawson, P. S. Salter, and M. J. Strain

ACS Photonics 10 (9), 3374–3383 (2023)

Effective light extraction from optically active solid-state spin centers inside high-index semiconductor host crystals is an important factor in integrating these pseudo-atomic centers in wider quantum systems. Here, we report increased fluorescent light collection efficiency from laser-written nitrogen-vacancy (NV) centers in bulk diamond facilitated by micro-transfer printed GaN solid immersion lenses. Both laser-writing of NV centers and transfer printing of micro-lens structures are compatible with high spatial resolution, enabling deterministic fabrication routes toward future scalable systems development. The micro-lenses are integrated in a noninvasive manner, as they are added on top of the unstructured diamond surface and bonded by van der Waals forces. For emitters at 5 μm depth, we find approximately 2× improvement of fluorescent light collection using an air objective with a numerical aperture of NA = 0.95 in good agreement with simulations. Similarly, the solid immersion lenses strongly enhance light collection when using an objective with NA = 0.5, significantly improving the signal-to-noise ratio of the NV center emission while maintaining the NV’s quantum properties after integration.

DOI: /10.1021/acsphotonics.3c00854


December 2023:

Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide

C. Chen, S. Ghosh, F. Adams, M. J. Kappers, D. J. Wallis, and R. A. Oliver

Ultramicroscopy 254, 113833 (2023)

The scanning capacitance microscope (SCM) is a powerful tool to characterise local electrical properties in GaN-based high electron mobility transistor (HEMT) structures with nanoscale resolution. We investigated the experimental setup and the imaging conditions to optimise the SCM contrast. As to the experimental setup, we show that the desired tip should be sharp (e.g., with the tip radius of ≤25nm) and its coating should be made of conductive doped diamond. Most importantly, its spring constant should be large to achieve stable tip-sample contact. The selected tip should be positioned close to both the edge and Ohmic contact of the sample. Regarding the imaging conditions, we also show that a dc bias should be applied in addition to an ac bias because the latter alone is not sufficient to deplete the two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure. The approximate range of the effective dc bias values was found by measuring the local dC/dV-V curves, yielding, after further optimisation, two optimised dc bias values which provide strong, but opposite, SCM contrast. In comparison, the selected ac bias value has no significant impact on the SCM contrast. The described methodology could potentially also be applied to other types of HEMT structures, and highly-doped samples.

DOI: /10.1016/j.ultramic.2023.113833


November 2023:

Quantitative analysis of carbon impurity concentrations in GaN epilayers by cathodoluminescence

K. Loeto, G. Kusch, S. Ghosh, M.J. Kappers, and R.A. Oliver

Micron 172, 103489 (2023 )

In this work, a technique for quantifying carbon doping concentrations in GaN:C/AlGaN buffer structures using cathodoluminescence (CL) is presented. The method stems from the knowledge that the blue and yellow luminescence intensity in CL spectra of GaN varies with the carbon doping concentration. By calculating the blue and yellow luminescence peak intensities normalised to the peak GaN near-band-edge intensity for GaN layers of known carbon concentrations, calibration curves that show the change in normalised blue and yellow luminescence intensity with carbon concentration in the 1016 − 1019 cm-3 range were derived at both room temperature and 10 K. The utility of such calibration curves was then examined by testing against an unknown sample containing multiple carbon-doped GaN layers. The results obtained from CL using the normalised blue luminescence calibration curves are in close agreement with those from secondary-ion mass spectroscopy (SIMS). However,the method fails when applying calibration curves obtained from the normalised yellow luminescence likely due to the influence of native VGa defects acting in this luminescence region. Although this work shows that indeed CL can be used as a quantitative tool to measure carbon doping concentrations in GaN:C, it is noted that the intrinsic broadening effects innate to CL can make it difficult to differentiate between the intensity variations in thin ( < 500 nm) multilayered GaN:C structures such as the ones studied in this work.

DOI: /10.1016/j.micron.2023.103489


October 2023:

Advanced Transfer Printing With In-Situ Optical Monitoring for the Integration of Micron-Scale Devices

B. Guilhabert, S. P. Bommer, N. K. Wessling, D. Jevtics, J. A. Smith, Z. Xia, S. Ghosh, M. J. Kappers, I. M. Watson, R.A. Oliver, M. D. Dawson, and M. J. Strain

IEEE Journal of Selected Topics in Quantum Electronics 29, 7900111 (2023 )

Transfer printing integration of planar membrane devices on photonic and electronic circuits is becoming a well established technology. Typical systems incorporate a single planar layer printed into full contact with the host substrate. In this work we present an advanced transfer print system that enables printing of optical devices in non-planar geometries and allows in-situ optical monitoring of devices. We show micro-resonators with air-clad whispering gallery modes coupled to on-chip waveguides, inverted device printing and three dimensionally assembled micro-cavities incorporating semiconductor micro-lenses and nanowire lasers. We demonstrate printing onto non-standard substrates including optical chip facets and single-mode fibre ends. The optical fibre printing was carried out with alignment assistance from in-situ optical coupling through the transfer printing system in real-time allowing active alignment of the system.

DOI: /10.1109/JSTQE.2022.3227340


September 2023:

MOVPE studies of zincblende GaN on 3C-SiC/Si (001)

T.J. Wade, A. Gundimeda, M.J. Kappers, M. Frentrup, S.M. Fairclough, D.J. Wallis, and R. A. Oliver

Journal of Crystal Growth 611, 127182 (2023 )

Cubic zincblende GaN films were grown by metalorganic vapour-phase epitaxy on 3C-SiC/Si (001) templates and characterized using Nomarski optical microscopy, atomic force microscopy, X-ray diffraction, and transmission-electron microscopy. In particular, structural properties were investigated of films where the growth temperature of a GaN epilayer varied in the range of 830°C to 910°C and the gas-phase V/III-ratio varied from 15 to 1200 at a constant reactor pressure of 300 Torr. It was observed that with increasing epi temperature at a constant V/III-ratio of 76, the film surface consisted of micrometer-sized elongated features aligned along [1-10] up to a temperature of 880°C. The zincblende phase purity of such samples was generally high with a wurtzite fraction of less than 1%. When grown above 880°C the GaN surface morphology degraded and the zincblende phase purity reduced as a result of inclusions with the wurtzite phase. A progressive narrowing of the 002 reflection with increasing epi growth temperature suggested an improvement of the film mosaicity. With increasing V/III-ratio at a constant growth temperature of 880°C, the film surface formed elongated features aligned along [1-10] at V/III values between 38 and 300 but the morphology became granular at both lower and higher V/III values. The zincblende phase purity is high at V/III values below 300. A slight broadening of the 002 X-ray diffraction reflection with increasing V/III-ratio indicated a small degradation of mosaicity. Scanning electron diffraction analyses of cross-sectional transmission-electron micrographs taken of a selection of samples illustrated the spatial distribution, quantity and structure of wurtzite inclusions within the zincblende GaN matrix. Within the limits of this study, the optimum epilayer growth conditions at a constant pressure of 300 Torr were identified to be at a temperature around 860°C to 880°C and a V/III-ratio in the range of 23 to 76, resulting in relatively smooth, zincblende GaN films without significant wurtzite contamination.

DOI: /10.1016/j.jcrysgro.2023.127182


August 2023:

Complications in silane-assisted GaN nanowire growth

N. Jiang, S. Ghosh, M. Frentrup, S. M. Fairclough, K. Loeto, G. Kusch, R. A. Oliver, and H.J. Joyce

Nanoscale Adv. 5, 2610-2620 (2023 )

Understanding the growth mechanisms of III-nitride nanowires is of great importance to realise their full potential. We present a systematic study of silane-assisted GaN nanowire growth on c-sapphire substrates by investigating the surface evolution of the sapphire substrates during the high temperature annealing, nitridation and nucleation steps, and the growth of GaN nanowires. The nucleation step – which transforms the AlN layer formed during the nitridation step to AlGaN – is critical for subsequent silane-assisted GaN nanowire growth. Both Ga-polar and N-polar GaN nanowires were grown with N-polar nanowires growing much faster than the Ga-polar nanowires. On the top surface of the N-polar GaN nanowires protuberance structures were found, which relates to the presence of Ga-polar domains within the nanowires. Detailed morphology studies revealed ring-like features concentric with the protuberance structures, indicating energetically favourable nucleation sites at inversion domain boundaries. Cathodoluminescence studies showed quenching of emission intensity at the protuberance structures, but the impact is limited to the protuberance structure area only and does not extend to the surrounding areas. Hence it should minimally affect the performance of devices whose functions are based on radial heterostructures, suggesting that radial heterostructures remain a promising device structure.

DOI: /10.1039/D2NA00939K


July 2023:

Three-photon excitation of InGaN quantum dots

V. Villafañe, B. Scaparra, M. Rieger, S. Appel, R. Trivedi, T. Zhu, J. Jarman, R. A. Oliver, R. A. Taylor, J. J. Finley, and K. Müller

Phys. Rev. Lett. 130, 083602 (2023 )

We demonstrate that semiconductor quantum dots can be excited efficiently in a resonant three-photon process, while resonant two-photon excitation is highly suppressed. Time-dependent Floquet theory is used to quantify the strength of the multiphoton processes and model the experimental results. The efficiency of these transitions can be drawn directly from parity considerations in the electron and hole wave functions in semiconductor quantum dots. Finally, we exploit this technique to probe intrinsic properties of InGaN quantum dots. In contrast to nonresonant excitation, slow relaxation of charge carriers is avoided, which allows us to measure directly the radiative lifetime of the lowest energy exciton states. Since the emission energy is detuned far from the resonant driving laser field, polarization filtering is not required and emission with a greater degree of linear polarization is observed compared to nonresonant excitation.

DOI: 10.1103/PhysRevLett.130.083602


June 2023:

Design of step-graded AlGaN buffers for GaN-on-Si heterostructures grown by MOCVD

S. Ghosh, A. M. Hinz, M. Frentrup, S. Alam, D. J. Wallis, and R. A. Oliver

Semicond. Sci. Technol. 38, 044001 (2023 )

For the growth of low-defect crack-free GaN heterostructures on large-area silicon substrates, compositional grading of AlGaN is a widely adapted buffer technique to restrict the propagation of lattice-mismatch induced defects and balance the thermal expansion mismatch-induced tensile stress. So far, a consolidation of the design strategy of such step-graded buffers has been impaired by the incomplete understanding of the effect of individual buffer design parameters on the mechanical and microstructural properties of the epilayers. Herein, we have analyzed a series of metal-organic chemical vapor deposition grown GaN/graded-AlGaN/AlN/Si heterostructures through in situ curvature measurements and post-growth x-ray diffraction (XRD). Our results reveal that in such epi structures, the GaN layer itself induces more compressive stress than the AlGaN buffer, but the underlying AlGaN layers dictate the magnitude of this stress. Furthermore, for a fixed AlGaN buffer thickness, the mean-stress accumulated during the GaN growth is found to be correlated with its structural properties. Specifically, one µm thick GaN layers that acquire 1.50 GPa or higher compressive mean-stress are seen to possess 20-21 XRD ω-FWHM values less than 650 arc-sec. Also, the evolution of instantaneous stresses during the growth of the AlGaN layers is found to be a valuable indicator for buffer optimization, and composition difference between successive layers is established as a crucial criterion. The results also show that increasing the total buffer thickness (for a fixed number of steps) or increasing the number of steps (for a fixed total buffer thickness) may not always be beneficial. Irrespective of the buffer thickness, optimized high electron mobility transistor structures show similarly low sheet-resistance (∼350 Ω □)^−1 and high mobility (∼2000 cm2 V^−1 s^−1) at room temperature.

DOI: 10.1088/1361-6641/acb9b6


May 2023:

Compositional Mapping of the AlGaN Alloy Composition in Graded Buffer Structures Using Cathodoluminescence

K. Loeto, G. Kusch, S. Ghosh, M. Frentrup, A. Hinz, and R. Oliver

Phys. Status Solidi A 2023, 2200830 (2023)

Herein, the use of cathodoluminescence (CL) hyperspectral mapping in the quantification of the AlGaN alloy composition in graded buffer structures is explored. The quantification takes advantage of the known parabolic dependence of the AlGaN bandgap on the alloy composition allowing the AlGaN near-band-edge (NBE) emission energy recorded from CL to be converted to a composition. The proposed quantification method is first applied to cleaved cross-sections of two nominally step-graded AlGaN buffer structures each containing five AlGaN layers with different compositions. By comparing the compositions obtained from CL to those calculated using X-ray diffraction, a close agreement between values from both techniques is observed. However, due to a change in the bowing parameter, some deviation is observed for layers with compositions near 75%. The method is then applied to cleaved cross-sections of an AlGaN buffer whose group III precursor flow molar ratio is varied linearly throughout the growth. Herein, the hyperspectral nature of the CL datasets is exploited such as to produce composition maps by converting the relevant AlGaN-NBE emission energy at each pixel of the CL data to a composition.

DOI: 10.1002/pssa.202200830


April 2023:

Polarity determination of crystal defects in zincblende GaN by aberration-corrected electron microscopy

H. Xiu, S. M. Fairclough, A. Gundimeda, M. J. Kappers, D. J. Wallis, R. A. Oliver, and M. Frentrup

Journal of Applied Physics 133, 105302 (2023)

Aberration-corrected scanning transmission electron microscopy techniques are used to study the bonding configuration between gallium cations and nitrogen anions at defects in metalorganic vapor-phase epitaxy-grown cubic zincblende GaN on vicinal (001) 3C-SiC/Si. By combining high-angle annular dark-field and annular bright-field imaging, the orientation and bond polarity of planar defects, such as stacking faults and wurtzite inclusions, were identified. It is found that the substrate miscut direction toward one of the 3C-SiC ⟨110⟩ in-plane directions is correlated with the crystallographic [1–10] in-plane direction and that the {111} planes with a zone axis parallel to the miscut have a Ga-polar character, whereas the {111} planes in the zone perpendicular to the miscut direction have N-polarity. The polarity of {111}-type stacking faults is maintained in the former case by rotating the coordination of Ga atoms by 180° around the ⟨111⟩ polar axes and in the latter case by a similar rotation of the coordination of the N atoms. The presence of small amounts of the hexagonal wurtzite phase on Ga-polar {111} planes and their total absence on N-polar {111} planes is tentatively explained by the preferential growth of wurtzite GaN in the [0001] Ga-polar direction under non-optimized growth conditions.

DOI: 10.1063/5.0138478


March 2023:

Core–Shell Nanorods as Ultraviolet Light-Emitting Diodes

D. Cameron, P.-M. Coulon, S. Fairclough, G. Kusch, P. R. Edwards, N. Susilo, T. Wernicke, M. Kneissl, R. A. Oliver, P. A. Shields, and R. W. Martin

Nano Letters 23 (4), pp. 1451–1458 (2023)

Existing barriers to efficient deep ultraviolet (UV) light-emitting diodes (LEDs) may be reduced or overcome by moving away from conventional planar growth and toward three-dimensional nanostructuring. Nanorods have the potential for enhanced doping, reduced dislocation densities, improved light extraction efficiency, and quantum wells free from the quantum-confined Stark effect. Here, we demonstrate a hybrid top-down/bottom-up approach to creating highly uniform AlGaN core–shell nanorods on sapphire repeatable on wafer scales. Our GaN-free design avoids self-absorption of the quantum well emission while preserving electrical functionality. The effective junctions formed by doping of both the n-type cores and p-type caps were studied using nanoprobing experiments, where we find low turn-on voltages, strongly rectifying behaviors and significant electron-beam-induced currents. Time-resolved cathodoluminescence measurements find short carrier liftetimes consistent with reduced polarization fields. Our results show nanostructuring to be a promising route to deep-UV-emitting LEDs, achievable using commercially compatible methods.

DOI: 10.1021/acs.nanolett.2c04826


February 2023:

Fabrication and transfer printing based integration of free-standing GaN membrane micro-lenses onto semiconductor chips

N. K. Wessling, S. Ghosh, B. Guilhabert, M. J. Kappers, A. M. Hinz, M. Toon, R. A. Oliver, M. D. Dawson, and M. J. Strain

Optical Materials Express 12 (12), pp. 4606-4618 (2022)

We demonstrate the back-end integration of optically broadband, high-NA GaN micro-lenses by micro-assembly onto non-native semiconductor substrates. We developed a highly parallel process flow to fabricate and suspend micron scale plano-convex lens platelets from 6" Si growth wafers and show their subsequent transfer-printing integration. A growth process targeted at producing unbowed epitaxial wafers was combined with optimisation of the etching volume in order to produce flat devices for printing. Lens structures were fabricated with 6 − 11 µm diameter, 2 µm height and root-mean-squared surface roughness below 2 nm. The lenses were printed in a vertically coupled geometry on a single crystalline diamond substrate and with µm-precise placement on a horizontally coupled photonic integrated circuit waveguide facet. Optical performance analysis shows that these lenses could be used to couple to diamond nitrogen vacancy centres at micron scale depths and demonstrates their potential for visible to infrared light-coupling applications.

DOI: 10.1364/OME.472999


January 2023:

Cubic GaN and InGaN/GaN quantum wells

D. J. Binks, P. Dawson, R. A. Oliver, and D. J. Wallis

Applied Physics Reviews 9, 041309 (2022)

LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.

DOI: 10.1063/5.0097558


December 2022:

Radiation effects in ultra-thin GaAs solar cells

A. Barthel, L. Sayre, G. Kusch, R. A. Oliver, and L. C. Hirst

Journal of Applied Physics 132, 184501 (2022)

Ultra-thin solar cells are of significant interest for use in space due to their intrinsic radiation tolerance, which may allow them to be used in particularly harsh radiation environments, where thicker cells would degrade rapidly and enable reduction in cover glass thickness to reduce launch mass. In this study, devices with an 80 nm GaAs absorber layer were irradiated with 3 MeV protons. It is shown that integrated light management in these ultra-thin devices offers enhanced efficiency, in addition to extended lifetime through radiation resilience. Time-resolved cathodoluminescence is employed to map the introduction of radiation-induced defects with increasing proton fluence and characterize a decrease in carrier lifetime from (198 ± 5) ps pre-radiation to (6.2 ± 0.6) ps, after irradiation to 2×10^14 cm^-2 fluence. Despite the substantial reduction in carrier lifetime, short-circuit current does not degrade up to a proton fluence of 1×10^15 cm^-2, beyond which a collapse in short-circuit current is observed. This exposure correlates with the point at which the carrier lifetime, extrapolated from cathodoluminescence, becomes comparable to the transit time for carriers to cross the ultra-thin device. Variation in current–voltage behavior with carrier lifetime and fluence shows that the recombination statistics are similar to those of a Shockley–Read–Hall single deep-level trap model, but that bimolecular recombination does not fully describe the observed behavior. An implication of these highly radiation tolerant cells for space power systems is shown to offer significant savings in cover glass mass, compared with a thicker cell.

DOI: 10.1063/5.0103381


November 2022:

Dismantling barriers faced by women in STEM

J. M. Jebsen, K. Nicoll Baines, R. A. Oliver, and I. Jayasinghe

Nature Chemistry. 14, pages 1203-1206 (2022)

Governments worldwide are committing more funding for scientific research in the face of the ongoing pandemic and climate crises. However, the funding process must be restructured to remove the barriers arising from conscious and unconscious biases experienced by minoritized groups, including women, and particularly women of colour.

DOI: 10.1038/s41557-022-01072-2


October 2022:

Characterization of buried interfaces using Ga Kα hard X-ray photoelectron spectroscopy (HAXPES)

B. F. Spencer, S. A. Church, P. Thompson, D. J. H. Cant, S. Maniyarasu, A. Theodosiou, A. N. Jones, M. J. Kappers, D. J. Binks, R. A. Oliver, J. Higgins, A. G. Thomas, T. Thomson, A. G. Shard, and W. R. Flavell

Faraday Discuss. 236, 311 (2022)

The extension of X-ray photoelectron spectroscopy (XPS) to measure layers and interfaces below the uppermost surface requires higher X-ray energies and electron energy analysers capable of measuring higher electron kinetic energies. This has been enabled at synchrotron radiation facilities and by using lab-based instruments which are now available with sufficient sensitivity for measurements to be performed on reasonable timescales. Here, we detail measurements on buried interfaces using a Ga Kα (9.25 keV) metal jet X-ray source and an EW4000 energy analyser (ScientaOmicron GmbH) in the Henry Royce Institute at the University of Manchester. Development of the technique has required the calculation of relative sensitivity factors (RSFs) to enable quantification analogous to Al Kα XPS, and here we provide further substantiation of the Ga Kα RSF library. Examples of buried interfaces include layers of memory and energy materials below top electrode layers, semiconductor heterostructures, ions implanted in graphite, oxide layers at metallic surfaces, and core–shell nanoparticles. The use of an angle-resolved mode enables depth profiling from the surface into the bulk, and is complemented with surface-sensitive XPS. Inelastic background modelling allows the extraction of information about buried layers at depths up to 20 times the photoelectron inelastic mean free path.

DOI: 10.1039/D2FD00021K


September 2022:

The influence of threading dislocations propagating through an AlGaN UVC LED

Douglas Cameron, Paul R. Edwards, Frank Mehnke, Gunnar Kusch, Luca Sulmoni, Marcel Schilling, Tim Wernicke, Michael Kneissl, and Robert W. Martin

Appl. Phys. Lett. 120, 162101 (2022)

During the epitaxy of AlGaN on sapphire for deep UV emitters, significant lattice mismatch leads to highly strained heterojunctions and the formation of threading dislocations. Combining cathodoluminescence, electron beam induced current and x-ray microanalysis reveal that dislocations with a screw component permeate through a state-of-the-art UVC LED heterostructure into the active region and perturb their local environment in each layer as growth progresses. In addition to acting as non-radiative recombination centers, these dislocations encourage high point defect densities and three-dimensional growth within their vicinity. We find that these point defects can add parasitic recombination pathways and compensate intentional dopants.

DOI: 10.1063/5.0086034


August 2022:

Photocurrent detection of radially polarized optical vortex with hot electrons in Au/GaN

Y. Hou, M. Kappers, C. Jin, and R. A. Oliver

Appl. Phys. Lett. 120, 202101 (2022)

We report a GaN based metal–semiconductor–metal (MSM) infrared photodetector enabled with azimuthally distributed sub-wavelength gratings fabricated on one of the working electrodes. Under illumination, hot electron transfer is introduced by the plasmonic resonance in the infrared waveband formed at the interface of Au/GaN. Without the help of using any external optical polarizers, the device is able to detect radial polarization vortices in the form of photocurrents with a prescribed response spectrum. The detector exhibits a 10%–90% rise and fall time of 0.9 ms under modulated light, much faster than that of conventional ultraviolet GaN MSM photodetectors based on the band edge absorption. This work provides a viable way to measure spatially variant polarization beams with a compact plasmonic photodetectors fabricated from wide bandgap semiconductors.

DOI: 10.1063/5.0094454


July 2022:

Investigationof wurtzite formation in MOVPE-grown zincblende GaN epilayers on AlGaNnucleation layers

A. Gundimeda, M.Frentrup, S.M Fairclough, M. J. Kappers, D.J. Wallis, and R.A. Oliver

Journal of Applied Physics 131, 115703 (2022)

The influence of AlGaN nucleation layers on zincblende GaN epilayers was studied to investigate the formation of wurtzite phase inclusions in the epilayer. GaN epilayers grown on AlGaN nucleation layers with varying aluminum contents suffer from the increasing presence of wurtzite inclusions as the aluminum content of the nucleation layer increases. High-resolution transmission electron microscopy along with four-dimensional scanning transmission electron microscopy is used to investigate the origin of the wurtzite inclusions in the nucleation layer and at the GaN/AlGaN interface. It was observed that a GaN nucleation layer and an AlGaN (x = 0.95) nucleation layer grew in the zincblende and wurtzite phase, respectively. These phases were then adopted by the overgrown GaN epilayers. For a GaN epilayer on an AlGaN (x = 0.29) nucleation layer, wurtzite inclusions tend to form at the GaN/ AlGaN (x = 0.29) interface due to strong {111}-type faceting observed in the zincblende nucleation layer. This strong faceting is correlated with an enrichment of aluminum in the upper part of the nucleation layer, as observed in energy dispersive x-ray spectroscopy, which may influence the kinetics or thermodynamics controlling the surface morphology.

DOI: 10.1063/5.0077186


June 2022:

Decreased Fast Time Scale Spectral Diffusion of a Nonpolar InGaN Quantum Dot

C. Kocher, J. C. Jarman, T. Zhu, G. Kusch, R. A. Oliver, and R. A. Taylor

ACS Photonics 9, 275-281 (2022)

Spectral diffusion can lead to considerable broadening of the line width of nitride quantum dots. Here, InGaN quantum dots grown on a nonpolar plane were shown to exhibit a decreased spectral diffusion rate compared to polar nitride dots. A robust intensity correlation method was used to measure the spectral diffusion rate of six quantum dots. A maximum spectral diffusion time of 1170 ± 50 ns was found. An increase of the rate with increasing power was observed. The decreased internal field leads to a lifetime for the nonpolar dots that is shorter than that for polar dots; the important ratio of spectral diffusion time to lifetime is more favorable for nonpolar quantum dots, thereby increasing the chances of generating indistinguishable photons.

DOI: 10.1021/acsphotonics.1c01613


May 2022:

Defect characterization of {10-13} GaN by electron microscopy

G. Kusch, M. Frentrup, N. Hu, H. Amano, R. A. Oliver, and M. Pristovsek

Journal of Applied Physics 131, 035705 (2022)

Advances in obtaining untwinned (10-13)-oriented semi-polar GaN enable a new crystal orientation for the growth of green and red LED structures. We present a scanning electron microscopy study that combines the structural characterization of electron channeling contrast imaging with the optical characterization of cathodoluminescence hyperspectral imaging on a (10-13) GaN layer. An extensive defect analysis revealed that the dominant defects consist of basal plane stacking faults (BSFs), prismatic stacking faults, partial dislocations, and threading dislocations. With a defect density of about an order of magnitude lower than in comparable. The optical properties of the defects have been characterized from 10 K to 320 K, showing BSF luminescence at room temperature indicating a reduced density of non-radiative recombination centers in the as-grown samples compared to established semi- and non-polar orientations. Our findings suggest that growth along (10-13) has the potential for higher radiative efficiency than established semi-polar orientations.

DOI: 10.1063/5.0077084


April 2022:

Carrier dynamics at trench defects in InGaN/GaN quantum wells revealed by time-resolved cathodoluminescence

G. Kusch, E. J. Comish, K. Loeto, S. Hammersley, M. J. Kappers, P. Dawson, R. A. Oliver, and F. C.-P. Massabuau

Nanoscale 14, 402-409 (2022)

Time-resolved cathodoluminescence offers new possibilities for the study of semiconductor nanostructures – including defects. The versatile combination of time, spatial, and spectral resolution of the technique can provide new insights into the physics of carrier recombination at the nanoscale. Here, we used power-dependent cathodoluminescence and temperature-dependent time-resolved cathodoluminescence to study the carrier dynamics at trench defects in InGaN quantum wells – a defect commonly found in III-nitride structures. The measurements show that the emission properties of trench defects closely relate to the depth of the related basal plane stacking fault within the quantum well stack. The study of the variation of carrier decay time with detection energy across the emission spectrum provides strong evidence supporting the hypothesis that strain relaxation of the quantum wells enclosed within the trench promotes efficient radiative recombination even in the presence of an increased indium content. This result shines light on previously reported peculiar emission properties of the defect, and illustrates the use of cathodoluminescence as a powerful adaptable tool for the study of defects in semiconductors.

DOI: 10.1039/D1NR06088K


March 2022:

Influence of AlGaN nucleation layers on MOVPE-grown zincblende GaN epilayers on 3C-SiC/Si(001)

A.Gundimeda, M. Rostami, M. Frentrup, A. Hinz, M. J. Kappers, D. J.Wallis, and R. A.Oliver

Journal of Physics D: Applied Physics 55, 175110 (2022)

The suitability of AlGaN nucleation layers (NLs) with varying Al fraction x for the metal organic vapour phase epitaxy of zincblende GaN on (001) 3C-SiC was investigated, using x-ray photoelectron spectroscopy, atomic force microscopy, and x-ray diffraction. The as-grown NLs exhibited elongated island structures on their surface, which reduce laterally into smaller, more equiaxed islands with increasing AlN composition. During high-temperature annealing in a mixture of NH3 and H2 the nucleation islands with low Al fraction ripened and increased in size, whereas this effect was less pronounced in samples with higher Al fraction. The compressive biaxial in-plane strain of the NLs increases with increasing AlN composition up to x = 0.29. GaN epilayers grown over NLs that have low Al fraction have high cubic zincblende phase purity and are slightly compressively strained relative to 3C-SiC. However, those samples with a measured Al fraction in the NL higher than 0.29 were predominantly of the hexagonal wurtzite phase, due to formation of wurtzite inclusions on various {111} facets of zb-GaN, thus indicating the optimal Al composition for phase-pure zb-GaN epilayer growth.

DOI: 10.1088/1361-6463/ac4c58


February 2022:

Understanding the Role of Grain Boundaries on Charge‐Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

Z. Li, S. P. Senanayak, L. Dai, G. Kusch, R. Shivanna, Y. Zhang, D. Pradhan, J. Ye, Y.‐T. Huang, H. Sirringhaus, R. A Oliver, N. C. Greenham, R. H. Friend, and R. L. Z. Hoye

Advanced Functional Materials 31, 2104981 (2021)

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air, and recent demonstrations of long charge-carrier lifetimes that can exceed 1 µs. In particular, Cs2AgBiBr6 is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs2AgBiBr6 thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field-effect transistor and temperature-dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 µm found in Cs2AgBiBr6 single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single-crystal properties.

DOI: 10.1002/adfm.202104981


January 2022:

Method for inferring the mechanical strain of GaN-on-Si epitaxial layers using optical profilometry and finite element analysis

B. F. Spiridon, M. Toon, A. Hinz, S. Ghosh, S. M. Fairclough, B. J. E. Guilhabert, M. J. Strain, I. M. Watson, M. D. Dawson, D. J. Wallis, and R. A. Oliver

Optical Materials Express 11, 1643-1655 (2021)

GaN-on-Si has become a useful fabrication route for many GaN devices and applications, but the mechanical stress incorporated throughout the material stack can impact the viability of this approach. The transfer printing of GaN membrane devices, a promising emerging technology, is most effective with flat membranes, but in practice many GaN structures released from their Si substrate are highly bowed due to the strain in the epitaxial nitride stack. Our approach uses the optical profiles of epitaxial wafers and membranes as inputs for inferring the mechanical strain state of the material by multi-variable numerical model fitting using COMSOL Multiphysics. This versatile, adaptable and scalable method was tested on samples from two GaN-on-Si wafers, revealing the relationship between built-in strain and material bow in principal-component fashion, returning 3–4×10−4 strain estimates for the AlGaN (compressive) and GaN (tensile) layers, and suggesting the occurrence of plastic deformation during transfer printing.

DOI: 10.1364/OME.418728


December 2021:

Multimicroscopy of cross-section zincblende GaN LED heterostructure

B. Ding, M. Frentrup, S. M. Fairclough, G. Kusch, M. J. Kappers, D. J. Wallis, and R. A. Oliver

Journal of Applied Physics 130, 115705 (2021)

Zincblende GaN has the potential to bridge the “green gap” due to the absence of internal electric fields with respect to wurtzite GaN. However, at present, the quality of zincblende GaN light emitting diodes (LEDs) is not yet sufficient for useful efficient green devices. One of the major challenges is the poor spectral purity of the emitted light. A multimicroscopy approach, combining scanning electron microscopy-cathodoluminescence (SEM-CL), scanning transmission electron microscopy (STEM), and scanning electron diffraction (SED), is applied on a single feature to enable cross correlation between techniques and to investigate the possible causes for the broad optical emission of a zincblende GaN LED structure. This investigation demonstrates that SEM-CL on a site-specific TEM cross section prepared by focused ion beam (FIB) microscope can provide access to nanoscale light emission variations that can be directly related to structural differences seen in STEM. We demonstrate that the general large quantum well (QW) emission peak width relates to quantum well thickness and In content fluctuations. Multiple low-energy QW emission peaks are found to be linked with stacking fault bunches that intersect the QWs. Splitting of the QW emission peak is also found to be caused by the formation of wurtzite-phase inclusions associated with twins formed within the zincblende matrix. Our characterization also illustrates the quantum well structure within such wurtzite inclusions and their impact on the optical emission.

DOI: 10.1063/5.0058429


November 2021:

Dislocations at coalescence boundaries in heteroepitaxial GaN/sapphire studied after the epitaxial layer has completely coalesced

T.J.O'Hanlon, T.Zhu, F.C.-P.Massabuau, and R.A.Oliver

Ultramicroscopy 231, 113258 (2021)

We have performed cross-sectional scanning capacitance microscopy (SCM), cathodoluminescence (CL) microscopy in the scanning electron microscope (SEM) and transmission electron microscopy (TEM) all on the same few-micron region of a GaN/sapphire sample. To achieve this, it was necessary to develop a process flow which allowed the same features viewed in a cleaved cross-section to be traced from one microscope to the next and to adapt the focused ion beam preparation of the TEM lamella to allow preparation of a site-specific sample on a pre-cleaved cross-section. Growth of our GaN/sapphire samples involved coalescence of three-dimensional islands to form a continuous film. Highly doped marker layers were included in the sample so that coalescence boundaries formed late in the film growth process could be identified in SCM and CL. Using TEM, we then identified one or more dislocations associated with each of several such late-coalescing boundaries. In contrast, previous studies have addressed coalescence boundaries formed earlier in the growth process and have shown that early-stage island coalescence does not lead to dislocation formation.

DOI: 10.1016/j.ultramic.2021.113258


October 2021:

The effect of thermal annealing on the optical properties of Mg-doped zincblende GaN epilayers

D. Dyer, S. A. Church, M. Jain, M. J. Kappers, M. Frentrup, D. J. Wallis, R. A. Oliver, and D. J. Binks

Journal of Applied Physics 130, 085705 (2021)

The effects of thermal annealing on the optical properties of Mg-doped cubic zincblende GaN epilayers grown by metalorganic chemical vapor deposition on 3C-SiC/Si (001) substrates are investigated. The photoluminescence spectra show near band edge features and a blue luminescence band that depend on Mg concentration, temperature, and excitation power density. Annealing the sample in a N2 atmosphere causes the intensity of the blue band to increase by a factor of 5. Power dependent photoluminescence measurements show a reduction in the laser excitation density required for saturation of the blue band after annealing, indicating an increase in the recombination lifetime. Time decay measurements confirm this increase, which is attributed to a reduction in the concentration of non-radiative defects after annealing. The results presented here are compared to those reported previously for Mg-doped hexagonal wurtzite GaN.

DOI: 10.1063/5.0057824


September 2021:

Photoluminescence efficiency of zincblende InGaN/GaN quantum wells

S. A. Church, M. Quinn, K. Cooley-Greene, B. Ding, A. Gundimeda, M. J. Kappers, M. Frentrup, D. J. Wallis, R. A. Oliver, and D. J. Binks

Journal of Applied Physics 129, 175702 (2021)

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite, crystal phase has the potential to improve efficiency. However, optimization of the emission efficiency of these heterostructures is still required to compete with more conventional alternatives. Photoluminescence time decays were used to assess how the quantum well width and number of quantum wells affect the recombination rates, and temperature dependent photoluminescence was used to determine the factors affecting recombination efficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increase weakly with well width, consistent with a change in the exciton binding energy. The relative efficiency at room temperature was found to increase by a factor of five when the number of wells was increased from one to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2.5 to 7.5 nm. These results indicate that thermionic emission is the most important process reducing efficiency at temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombination on well width means that increasing well thickness is an effective way of suppressing thermionic emission and thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in the wurtzite phase.

DOI: 10.1063/5.0046649


August 2021:

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

S. Shukla, M. Sood, D. Adeleye, S. Peedle, G. Kusch, D. Dahliah, M. Melchiorre, G.-M. Rignanese, G. Hautier, R. Oliver, and S. Siebentritt

Joule 5, 1816–1831 (2021)

Cu(In,Ga)S2 is a high-potential material for its usage in tandem solar cells; however, its power conversion efficiency has remained limited so far. High bulk recombination losses and interface losses both account for the performance limitation. In this work, we adopt a holistic approach to address both bulk and interface recombination losses. We show that bulk recombination losses can be substantially suppressed by controlling the Cu deficiency in the material. From theoretical calculations, we argue that Cu deficiency reduces the antisite defects that are probably the most detrimental defects. Additionally, we effectively passivate the interface through the usage of Zn(O,S) buffer layer, thereby minimizing the losses at the interface. This leads to a solar cell device performance of over 15% from 1.6-eV-band-gap Cu(In,Ga)S2 from a completely non-toxic process. The path to further performance improvement is discussed to increase the viability of Cu(In,Ga)S2 toward tandem application.

DOI: 10.1016/j.joule.2021.05.004


July 2021:

Defect structures in (001) zincblende GaN/3C-SiC nucleation layers

P. Vacek, M. Frentrup. L. Y. Lee, F. C.-P. Massabuau, M. J. Kappers, D. J. Wallis, R. Gröger, and R. A. Oliver

J. Appl. Phys. 129, 155306 (2021)

The defect structure of zincblende GaN nucleation layers grown by metalorganic vapor-phase epitaxy on 3C-SiC/Si (001) was investigated by high-resolution scanning transmission electron microscopy. Perfect dislocations, partial dislocations, and stacking faults are present in the layers. Perfect dislocations are identified as 60° mixed-type and act as misfit dislocations to relieve the compressive lattice mismatch strain in GaN. Stacking faults are mainly bounded by 30° Shockley partial dislocations and rarely by Lomer-Cottrell partial dislocations, both of which are able to relieve the compressive lattice mismatch strain in the layer. We propose that the stacking faults and their partial dislocations originate from the dissociation of perfect dislocations present in the zincblende GaN layer, and by direct nucleation of partial dislocations loops from the surface. These are the two main mechanisms, which lead to the final defect structure of the zincblende GaN nucleation layers.

DOI: 10.1063/5.0036366


June 2021:

Origin(s) of Anomalous Substrate Conduction in MOVPE-Grown GaN HEMTs on Highly Resistive Silicon

S. Ghosh, A. Hinz, S. M. Fairclough, B. F. Spiridon, A. Eblabla, M. A. Casbon, M. J. Kappers, K. Elgaid, S. Alam, R. A. Oliver, and D. J. Wallis

ACS Appl. Electron. Mater. 3 (2), 813–824 (2021)

The performance of transistors designed specifically for high-frequency applications is critically reliant upon the semi-insulating electrical properties of the substrate. The suspected formation of a conductive path for radio frequency (RF) signals in the highly resistive (HR) silicon substrate itself has been long held responsible for the suboptimal efficiency of as-grown GaN high electron mobility transistors (HEMTs) at higher operating frequencies. Here, we reveal that not one but two discrete channels distinguishable by their carrier type, spatial extent, and origin within the metal-organic vapor phase epitaxy (MOVPE) growth process participate in such parasitic substrate conduction. An n-type layer that forms first is uniformly distributed in the substrate, and it has a purely thermal origin. Alongside this, a p-type layer is localized on the substrate side of the AlN/Si interface and is induced by diffusion of group-III element of the metal-organic precursor. Fortunately, maintaining the sheet resistance of this p-type layer to high values (∼2000 Ω/□) seems feasible with particular durations of either organometallic precursor or ammonia gas predose of the Si surface, i.e., the intentional introduction of one chemical precursor just before nucleation. It is proposed that the mechanism behind the control actually relies on the formation of disordered AlSiN between the crystalline AlN nucleation layer and the crystalline silicon substrate.

DOI: 10.1021/acsaelm.0c00966


May 2021:

Point Defects in InGaN/GaN Core–Shell Nanorods: Role of the Regrowth Interface

K. Loeto, G. Kusch, P.-M. Coulon, S. M. Fairclough, E. Le Boulbar, I. Girgel, P. A. Shields, and R. A. Oliver

Nano Ex. 2, 014005 (2021)

Core-shell nanorod based light-emitting diodes (LEDs) with their exposed non-polar surfaces have the potential to overcome the limitations of planar LEDs by circumventing the quantum confined stark effect. In this experiment, InGaN/GaN core-shell nanorods were fabricated by a combination of top-down etching and bottom-up regrowth using metal-organic vapour phase epitaxy. When viewing the nanorods along their long axis, monochromatic cathodoluminescence maps taken at the GaN near-band-edge emission energy (3.39 eV) reveal a ring-like region of lower emission intensity. The diameter of this ring is found to be 530 (±20)nm corresponding to the ~510 nm diameter nickel etch masks used to produce the initial GaN nanopillars. Thus, the dark ring corresponds to the regrowth interface. To understand the origin of the ring, scanning transmission electron microscopy (STEM) and cathodoluminescence (CL) hyperspectral mapping at 10K were performed. STEM imaging reveals the absence of extended defects in the nanorods and indeed near the regrowth interface. Monochromatic CL maps recorded at 10K show that the ring remains dark for monochromatic maps taken at the GaN near-band-edge emission energy (3.47 eV) but is bright when considering the donor-acceptor pair emission energy (3.27 eV). This peculiar anticorrelation indicates that the dark ring originates from an agglomeration of point defects associated with donor-acceptor pair emission. The point defects are incorporated and buried at the GaN regrowth interface from the chemical and/or physical damage induced by etching and lower the radiative recombination rate; limiting the radiative efficiency close to the regrowth interface.

DOI: 10.1088/2632-959X/abe990


April 2021:

Combined SEM-CL and STEM investigation of green InGaN quantum wells

B. Ding, J. Jarman, M. J. Kappers, and R. A. Oliver

J. Phys. D: Appl. Phys. 54, 165107 (2021)

The microstructure of green-emitting InGaN/GaN quantum well (QW) samples grown at different temperatures was studied using cross-section scanning transmission electron microscopy (STEM) and plan-view cathodoluminescence (CL). The sample with the lowest InGaN growth temperature exhibits microscale variations in the CL intensity across the sample surface. Using STEM analysis of such areas, the observed darker patches do not correspond to any observable extended defect. Instead, they are related to changes in the extent of gross-well width fluctuations in the QWs, with more brightly emitting regions exhibiting a high density of such fluctuations, whilst dimmer regions were seen to have InGaN QWs with a more uniform thickness.

DOI: 10.1088/1361-6463/abddf8


March 2021:

Ti Alloyed α-Ga2O3: Route towards Wide Band Gap Engineering

A. Barthel, J. Roberts, M. Napari, M. Frentrup, T. Huq, A. Kovács, R. Oliver, P. Chalker, T. Sajavaara, and F. Massabuau

Micromachines 11(12), 1128 (2020)

The suitability of Ti as a band gap modifier for α-Ga2O3 was investigated, taking advantage of the isostructural α phases and high band gap difference between Ti2O3 and Ga2O3. Films of (Ti,Ga)2O3 were synthesized by atomic layer deposition on sapphire substrates, and characterized to determine how crystallinity and band gap vary with composition for this alloy. We report the deposition of high quality α-(Ti,Ga)2O3 films with x = 3.7%. For greater compositions the crystalline quality of the films degrades rapidly, where the corundum phase is maintained in films up to x = 5.3%, and films containing greater Ti fractions being amorphous. Over the range of achieved corundum phase films, that is 0% ≤ x ≤ 5.3%, the band gap energy varies by ∼270 meV. The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a modification in band gap, shows promising prospects for band gap engineering and the development of wavelength specific solar-blind photodetectors based on α-Ga2O3.

DOI: 10.3390/mi11121128


February 2021:

Efficient light-emitting diodes from mixed-dimensional perovskites on a fluoride interface

B. Zhao, Y. Lian, L. Cui, G. Divitini, G. Kusch, E. Ruggeri, F. Auras, W. Li, D. Yang, B. Zhu, R. A. Oliver, J. L. MacManus-Driscoll, S. D. Stranks, D. Di, and R. H. Friend

Nature Electronics 3, 704-710 (2020)

Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20%. However, the performance of visible perovskite light-emitting diodes has been hindered by non-radiative recombination losses and limited options for charge-transport materials that are compatible with perovskite deposition. Here, we report efficient, green electroluminescence from mixed-dimensional perovskites deposited on a thin (~1 nm) lithium fluoride layer on an organic semiconductor hole-transport layer. The highly polar dielectric interface acts as an effective template for forming high-quality bromide perovskites on otherwise incompatible hydrophobic charge-transport layers. The control of crystallinity and dimensionality of the perovskite layer is achieved by using tetraphenylphosphonium chloride as an additive, leading to external photoluminescence quantum efficiencies of around 65%. With this approach, we obtain light-emitting diodes with external quantum efficiencies of up to 19.1% at high brightness (>1,500 cd m^−2).

DOI: 10.1038/s41928-020-00487-4


January 2021:

Enhanced piezoelectricity and electromechanical efficiency in semiconducting GaN due to nanoscale porosity

Y. Calahorra, B. Spiridon, A. Wineman, T. Busolo, P. Griffin, P. K. Szewczyk, T. Zhu, Q. Jing, R. Oliver, and S. Kar-Narayan

Applied Materials Today 21, 100858 (2020)

Electrical polarization phenomena in GaN are important as they have significant impact on the operation of modern day energy efficient lighting and are fundamental to GaN-based high power and high frequency electronics. Controlling polarization is beneficial for the optimization of these applications. GaN is also piezoelectric, and therefore mechanical stress and strain are possible handles to control its polarization. Nonetheless, polar semiconductors in general, and GaN in particular, are weak piezoelectric materials when compared to ceramics, and are therefore not considered for characteristic electromechanical applications such as sensing, actuation and mechanical energy harvesting. Here, we examine the effect of nanoscale porosity on the piezoelectricity of initially conductive GaN. We find that for 40% porosity, the previously conductive GaN layer becomes depleted, and exhibits enhanced piezoelectricity as measured using piezoresponse force microscopy, as well as by using a mechanical energy harvesting setup. The effective piezoelectric charge coefficient of the porous GaN, d33,eff, is found to be about 8 pm/V which is 2-3 times larger than bulk GaN. A macroscale device comprising a porous GaN layer delivered 100 nW/cm² across a resistive load under a 150 kPa mechanical excitation. We performed finite element simulations to analyze the evolution of the piezoelectric properties with porosity. The simulations suggest that increased mechanical compliance due to porosity gives rise to the observed enhanced piezoelectricity in GaN. Furthermore, the simulations show that for stress-based excitations, the porous GaN electromechanical figure of merit is increased by an order of magnitude and becomes comparable to that of barium titanate piezoceramics. In addition, considering the central role played by GaN in modern electronics and optoelectronics, our study validates a very promising research direction when considering stress-based electromechanical applications which combine GaN's semiconducting and piezoelectric properties.

DOI: 10.1016/j.apmt.2020.100858


December 2020:

Alloy segregation at stacking faults in zincblende GaN heterostructures

B. Ding, M. Frentrup, S. M. Fairclough, M. J. Kappers, M. Jain, A. Kovács, D. J. Wallis, and R. A. Oliver

Journal of Applied Physics 128, 145703 (2020)

Current cubic zincblende III-Nitride epilayers grown on 3C-SiC/Si(001) substrates by metal-organic vapor-phase epitaxy contain a high density of stacking faults lying on the {111} planes. A combination of high-resolution scanning transmission electron microscopy and energy dispersive x-ray spectrometry is used to investigate the effects of alloy segregation around stacking faults in a zincblende III-nitride light-emitting structure, incorporating InGaN quantum wells and an AlGaN electron blocking layer. It is found that in the vicinity of the stacking faults, the indium and aluminum contents were a factor of 2.3 ± 1.3 and 1.9 ± 0.5 higher, respectively, than that in the surrounding material. Indium and aluminum are also observed to segregate differently in relation to stacking faults with indium segregating adjacent to the stacking fault while aluminum segregates directly on the stacking fault.

DOI: 10.1063/5.0015157


November 2020:

Sequential plan-view imaging of sub-surface structures in the transmission electron microscope

F. C-P. Massabuau, H. P. Springbett, G. Divitini, P. H. Griffin, T. Zhu, and R. A. Oliver

Materialia 12, 100798 (2020)

Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section.

DOI: 10.1016/j.mtla.2020.100798


October 2020:

Mixed-size diamond seeding for low-thermal-barrier growth of CVD diamond onto GaN and AlN

E. J. W. Smitha, H. Piracha, D. Field, J. W. Pomeroy, G. R. Mackenzie, Z. Abdallah, F. C.-P. Massabuau, A. M. Hinz, D. J. Wallis, R. A. Oliver, M. Kuball, and P. W. May

Carbon 167, 620 (2020)

We report a method of growing a diamond layer via chemical vapour deposition (CVD) utilizing a mixture of microdiamond and nanodiamond seeding to give a low effective thermal boundary resistance (TBReff) for heat-spreading applications in high-frequency, high-power electronic devices. CVD diamond was deposited onto thin layers of both GaN and AlN on Si substrates, comparing conventional nanodiamond seeding with a two-step process involving sequential seeding with microdiamond then nanodiamond. Thermal properties were determined using transient thermoreflectance (TTR), and the samples were also analysed with SEM and X-ray tomography. While diamond growth directly onto GaN proved to be unsuccessful due to poor adhesion, films grown on AlN were adherent and robust. The two-step mixed-seeding method gave TBReff values < 6 m2 K GW−1 that were 30 times smaller than for films grown under identical conditions but using nanodiamond seeding alone. Such remarkably low thermal barriers obtained with the mixed-seeding process offer a promising route for fabrication of high-power GaN HEMTs using diamond as a heat spreader with an AlN interlayer.

DOI: 10.1016/j.carbon.2020.05.050


September 2020:

Stacking fault-associated polarized surface-emitted photoluminescence from zincblende InGaN/GaN quantum wells

S. A. Church, B. Ding, P. W. Mitchell, M. J. Kappers, M. Frentrup, G. Kusch, S. M. Fairclough, D. J. Wallis, R. A. Oliver, and D. J. Binks

Appl. Phys. Lett. 117, 032103 (2020)

Zincblende InGaN/GaN quantum wells offer a potential improvement to the efficiency of green light emission by removing the strong electric fields present in similar structures. However, a high density of stacking faults may have an impact on the recombination in these systems. In this work, scanning transmission electron microscopy and energy-dispersive x-ray measurements demonstrate that one-dimensional nanostructures form due to indium segregation adjacent to stacking faults. In photoluminescence experiments, these structures emit visible light, which is optically polarized up to 86% at 10 K and up to 75% at room temperature. The emission redshifts and broadens as the well width increases from 2 nm to 8 nm. Photoluminescence excitation measurements indicate that carriers are captured by these structures from the rest of the quantum wells and recombine to emit light polarized along the length of these nanostructures.

DOI: 10.1063/5.0012131


August 2020:

The relationship between the three-dimensional structure of porous GaN distributed Bragg reflectors and their birefringence

P. H. Griffin, K. M. Patel, T. Zhu, R. M. Langford, V. S. Kamboj, D. A. Ritchie, and R. A. Oliver

J. Appl. Phys. 127, 193101 (2020)

Porous GaN distributed Bragg reflectors offer an opportunity to provide the high reflectance, lattice-matched components required for efficient GaN vertical cavity surface emitting lasers. The birefringence of these structures is, therefore, of key interest as it could be used to control the polarization of the emitted light. Here, we present a detailed analysis of the optical birefringence for both laterally etched, patterned structures and self-assembled radial porous structures. We correlate this with the 3D structure of the pores, which we measure through the use of FIB milling and serial block-face SEM imaging. This is a powerful method for imaging the internal nanostructure of the sample and allows the internal pore morphology to be viewed in a reconstruction of any 3D plane. We measure the birefringence of our porous GaN layers as Δn = 0.14 with a lower refractive index parallel to the pores (∥) than perpendicular to them (⟂). Using finite element modeling, we accurately reproduce the experimentally observed birefringence trends and find that this can be done by modeling GaN as a perfect dielectric. This indicates that the birefringence arises from the limited width across the pores. This also shows that standard modeling approaches can be used to design porous GaN birefringent devices effectively.

DOI: 10.1063/5.0005770


July 2020:

Polar (In,Ga)N/GaN Quantum Wells: Revisiting the Impact of Carrier Localization on the “Green Gap” Problem

D. S. P. Tanner, P. Dawson, M. J. Kappers, R. A. Oliver, and S. Schulz

Phys. Rev. Applied 13, 044068 (2020)

We present a detailed theoretical analysis of the electronic and optical properties of c-plane InGaN/GaN quantum-well structures with In contents ranging from 5% to 25%. Special attention is paid to the relevance of alloy-induced carrier-localization effects to the “green gap” problem. Studying the localization length and electron-hole overlaps at low and elevated temperatures, we find alloy-induced localization effects are crucial for the accurate description of (In,Ga)N quantum wells across the range of In content studied. However, our calculations show very little change in the localization effects when moving from the blue to the green spectral regime; that is, when the internal quantum efficiency and wall-plug efficiencies reduce sharply, for instance, the in-plane carrier separation due to alloy-induced localization effects changes weakly. We conclude that other effects, such as increased defect densities, are more likely to be the main reason for the green-gap problem. This conclusion is further supported by our finding that the electron localization length is large, when compared with that of holes, and changes little in the In composition range of interest for the green-gap problem. Thus, electrons may become increasingly susceptible to an increased (point) defect density in green emitters and as a consequence the nonradiative-recombination rate may increase.

DOI: 10.1103/PhysRevApplied.13.044068


June 2020:

​Dislocations as channels for the fabrication of sub-surface porous GaN by electrochemical etching

F. C.-P. Massabuau, P. H. Griffin, H. P. Springbett, Y. Liu, R. Vasant Kumar, T. Zhu, and R. A. Oliver

APL Materials 8, 031115 (2020)

Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers while leaving undoped layers undamaged, allowing the realization of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in-distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching the whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometer-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer, respectively. Results on AlGaN and non-polar GaN multilayers indicate that the same mechanism is operating, suggesting that this approach may be applicable in a range of materials.

DOI: 10.1063/1.5142491


May 2020:

GaN-on-diamond technology platform: Bonding-free membrane manufacturing process

M. D. Smith, J. A. Cuenca, D. E. Field, Y. Fu, C. Yuan, F. Massabuau, S. Mandal, J. W. Pomeroy, R. A. Oliver, M. J. Uren, K. Elgaid, O. A. Williams, I. Thayne and M. Kuball

AIP Advances 10, 035306 (2020)

GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm × 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. Atomic force microscopy and transmission electron microscopy were used to confirm the formation of high quality, void-free AlN/diamond interfaces. The bond between the III-nitride layers and the diamond was validated by strain measurements of the GaN buffer layer. Demonstration of this technology platform is an important step forward for the creation of next generation high power electronic devices.

DOI: 10.1063/1.5129229


April 2020:

Cross-shaped markers for the preparation of site-specific transmission electron microscopy lamellae using focused ion beam techniques

T. J. O'Hanlon, A. Bao, F. C.-P. Massabuau, M. J. Kappers and R. A. Oliver

Ultramicroscopy 212, 112970 (2020)

We describe the use of a cross-shaped platinum marker deposited using electron-beam-induced deposition (EBID) in a focused ion beam – scanning electron microscope (FIB-SEM) system to facilitate site-specific preparation of a TEM foil containing a trench defect in an InGaN/GaN multiple quantum well structure. The defect feature is less than 100 nm wide at the surface. The marker is deposited prior to the deposition of a protective platinum strap (also by EBID) with the centre of the cross indicating the location of the feature of interest, while the arms of the square cross make an acute angle of 45° with the strap's long axis. During the ion-beam thinning process, the marker may be viewed in cross-section from both sides of the sample alternately, and the coming together of the features relating to the arms of the cross indicates increasing proximity to the feature of interest. Although this approach does allow increased precision in locating the region of interest during thinning, it also increases the time required to complete the sample preparation. Hence, this method is particularly well suited to directly correlated multi-microscopy investigations in previously characterised material where high yield and the precise location are more important than preparation time. In addition to TEM lamella preparation, this method could equally be useful for preparing site-specific atom probe tomography (APT) samples.

DOI: 10.1016/j.ultramic.2020.112970


March 2020:

Low temperature growth and optical properties of α-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition

J. W. Roberts, P. R. Chalker, B. Ding, R. A. Oliver, J. T. Gibbon, L. A. H. Jones, V. R. Dhanak, L. J. Phillips, J. D. Major and F. C-P. Massabuau

Journal of Crystal Growth 528, 125254 (2019)

Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200 °C amorphous Ga2O3 films were deposited. Between 250 °C and 350 °C the films became predominantly α-Ga2O3. Above 350 °C the deposited films showed a mixture of α-Ga2O3 and ε-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the α-Ga2O3 phase deposited at 250 °C.

DOI: 10.1016/j.jcrysgro.2019.125254


February 2020:

Impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN quantum​

A. A. Roble, S. K. Patra, F. Massabuau, M. Frentrup, M. A. Leontiadou, P. Dawson, M. J. Kappers, R. A. Oliver, D. M. Graham and S. Schulz

Scientific Reports 9, 18862 (2019)

We report on a combined theoretical and experimental study of the impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN multi-quantum well systems. The presence of carrier localization effects in this system was demonstrated by experimental observations, such as the “S-shape” temperature dependence of the photoluminescence (PL) peak energy, and non-exponential PL decay curves that varied across the PL spectra at 10 K. A three-dimensional modified continuum model, coupled with a self-consistent Hartree scheme, was employed to gain insight into the electronic and optical properties of the experimentally studied c-plane GaN/AlGaN quantum wells. This model confirmed the existence of strong hole localization arising from the combined effects of the built-in polarization field along the growth direction and the alloy fluctuations at the quantum well/barrier interface. However, for electrons these localization effects are less pronounced in comparison to the holes. Furthermore, our calculations show that the attractive Coulomb interaction between electron and hole results in exciton localization. This behavior is in contrast to the picture of independently localized electrons and holes, often used to explain the radiative recombination process in  c -plane InGaN/GaN quantum well.

DOI: 10.1038/s41598-019-53693-2


January 2020:

Thick, Adherent Diamond Films on AlN with Low Thermal Barrier Resistance​

S. Mandal, C. Yuan, F. Massabuau, J. W. Pomeroy, J. Cuenca, H. Bland, E. Thomas, D. Wallis, T. Batten, D. Morgan, R. Oliver, M. Kuball and O. A. Williams

ACS applied materials & interfaces 11(43), 40826 (2019)

The growth of >100-μm-thick diamond layers adherent on aluminum nitride with low thermal boundary resistance between diamond and AlN is presented in this work. The thermal barrier resistance was found to be in the range of 16 m2·K/GW, which is a large improvement on the current state-of-the-art. While thick films failed to adhere on untreated AlN films, AlN films treated with hydrogen/nitrogen plasma retained the thick diamond layers. Clear differences in ζ-potential measurement confirm surface modification due to hydrogen/nitrogen plasma treatment. An increase in non-diamond carbon in the initial layers of diamond grown on pretreated AlN is seen by Raman spectroscopy. The presence of non-diamond carbon has minimal effect on the thermal barrier resistance. The surfaces studied with X-ray photoelectron spectroscopy revealed a clear distinction between pretreated and untreated samples. The surface aluminum goes from a nitrogen-rich environment to an oxygen-rich environment after pretreatment. A clean interface between diamond and AlN is seen by cross-sectional transmission electron microscopy.

DOI: 10.1021/acsami.9b13869


December 2019:

Structural characterization of porous GaN distributed Bragg reflectors using x-ray diffraction​

P. H. Griffin, M. Frentrup,  T. Zhu, M. E. Vickers, and  R. A. Oliver

Journal of Applied Physics 126, 213109 (2019)

Porous GaN distributed Bragg reflectors (DBRs) provide strain-free, high-reflectivity structures with a wide range of applications across nitride optoelectronics. Structural characterization of porous DBRs is currently predominantly achieved by cross-sectional scanning electron microscopy (SEM), which is a destructive process that produces local data and has accuracy limited to around 3% by instrument calibration uncertainty. Here, we show that high-resolution x-ray diffraction (XRD) offers an alternative, non-destructive method for characterizing porous nitride structures. XRD scans of porous GaN DBRs show that despite the constant lattice parameter across the DBR layers, characteristic satellite peaks still arise, which are due to the interference between x-rays reflected from the porous and nonporous layers. By comparing the intensities and positions of the satellite peaks through diffraction patterns simulated from a kinematic model, the structural properties of the porous GaN DBRs can be analyzed. Using our method, we have measured a series of DBRs with stop bands from the blue wavelength region to the IR and compared their structural values with those from SEM data. Our results show that the XRD method offers improvements in the accuracy of determining layer thickness, although uncertainty for the value of porosity remains high. To verify the results gained from the XRD and SEM analysis, we modeled the optical reflectivity using the structural values of both methods. We found that the XRD method offered a better fit to the optical data. XRD, therefore, offers accurate, nondestructive characterization of porous DBR structures based on macroscale measurements and is suitable for full wafer analysis.

DOI: 10.1063/1.5134143


November 2019:

On-Chip Thermal Insulation Using Porous GaN​

B. F. Spiridon, P. H. Griffin, J. C. Jarman, Y. Liu, T. Zhu, A. De Luca, R. A. Oliver, and F. Udrea

Proceedings 2(13), 776 (2018)

This study focuses on the thermal characterization of porous gallium nitride (GaN) using an extended 3ω method. Porous semiconductor materials provide a solution to the need for on-chip thermal insulation, a fundamental requirement for low-power, high-speed and high-accuracy thermal sensors. Thermal insulation is especially important in GaN devices, due to the intrinsically high thermal conductivity of the material. The results show one order of magnitude reduction in thermal conductivity, from 130 W/mK to 10 W/mK, in line with theoretical predictions for porous materials. This achievement is encouraging in the quest for integrating sensors with opto-, power- and RF-electronics on a single GaN chip.

DOI: 10.3390/proceedings2130776


October 2019:

Effects of microstructure and growth conditions on quantum emitters in gallium nitride

M. Nguyen, T. Zhu, M. Kianinia, F. Massabuau, I. Aharonovich, M. Toth, R. Oliver, and C. Bradac

APL Materials 7, 081106 (2019)

Single-photon emitters in gallium nitride (GaN) are gaining interest as attractive quantum systems due to the well-established techniques for growth and nanofabrication of the host material, as well as its remarkable chemical stability and optoelectronic properties. We investigate the nature of such single-photon emitters in GaN with a systematic analysis of various samples produced under different growth conditions. We explore the effect that intrinsic structural defects (dislocations and stacking faults), doping, and crystal orientation in GaN have on the formation of quantum emitters. We investigate the relationship between the position of the emitters determined via spectroscopy and photoluminescence measurements—and the location of threading dislocations characterized both via atomic force microscopy and cathodoluminescence. We find that quantum emitters do not correlate with stacking faults or dislocations; instead, they are more likely to originate from point defects or impurities whose density is modulated by the local extended defect density.

DOI: 10.1063/1.5098794


September 2019:

Investigation of MOVPE-grown zincblende GaN nucleation layers on 3C-SiC/Si substrates

L. Y. Lee, M. Frentrup, P. Vacek, F. C.-P. Massabuau, M. J. Kappers, D. J. Wallis, and R. A. Oliver

Journal of Crystal Growth 524, 125167 (2019)

Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1-10] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H2/NH3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth.

DOI: 10.1016/j.jcrysgro.2019.125167


August 2019:

Insight into the impact of atomic- and nano-scale indium distributions on the optical properties of InGaN/GaN quantum well structures grown on m-plane freestanding GaN substrates

F. Tang, T. Zhu, W.-Y. Fu, F. Oehler, S. Zhang, J. T. Griffiths, C. Humphreys, T. L. Martin, P. A. J. Bagot, M. P. Moody, S. Kanta Patra, S. Schulz, P. Dawson, S. Church, J. Jacobs, and R. A. Oliver

Journal of Applied Physics 125, 225704 (2019)

We investigate the atomic scale structure of m-plane InGaN quantum wells grown on bulk m-plane GaN templates and reveal that as the indium content increases there is an increased tendency for nonrandom clustering of indium atoms to occur. Based on the atom probe tomography data used to reveal this clustering, we develop a k · p model that takes these features into account and links the observed nanostructure to the optical properties of the quantum wells. The calculations show that electrons and holes tend to colocalize at indium clusters. The transition energies between the electron and hole states are strongly affected by the shape and size of the clusters. Hence, clustering contributes to the very large line widths observed in the experimental low temperature photoluminescence spectra. Also, the emission from m-plane InGaN quantum wells is strongly linearly polarized. Clustering does not alter the theoretically predicted polarization properties, even when the shape of the cluster is strongly asymmetric. Overall, however, we show that the presence of clustering does impact the optical properties, illustrating the importance of careful characterization of the nanoscale structure of m-plane InGaN quantum wells and that atom probe tomography is a useful and important tool to address this problem.

DOI: 10.1063/1.5097411


July 2019:

Light-output enhancement of InGaN light emitting diodes regrown on nanoporous distributed Bragg reflector substrates

J.C. Jarman, T. Zhu, P.H. Griffin, and R.A. Oliver

Jpn. J. Appl. Phys. 58, SCCC14 (2019)

Utilising our novel wafer-scale electrochemical porosification approach which proceeds through the top surface by means of nanoscale vertical etching pathways, we have prepared full 2 inch wafers containing alternating solid GaN and nanoporous GaN (NP-GaN) layers that form distributed Bragg reflectors (DBRs), and have regrown InGaN-based light emitting diode (LED) heterostructures on these wafers. The NP-GaN DBR wafer is epi-ready and exhibits a peak reflectance of 95% at 420 nm prior to growth of the LED heterostructure. We observe a 1.8× enhancement in peak intensity of LED electroluminescence from processed devices, and delayed onset of efficiency droop with increased injection current.

DOI: 10.7567/1347-4065/ab0cfd


June 2019:

Optical and structural properties of dislocations in InGaN

F.C.-P. Massabuau, M.K. Horton, E. Pearce, S. Hammersley, P. Chen, M.S. Zielinski, T.F.K. Weatherley, G. Divitini, P.R. Edwards, M.J. Kappers, C. McAleese, M.A. Moram, C.J. Humphreys, P. Dawson, and R.A. Oliver

J. Appl. Phys. 125, 165701 (2019)

Threading dislocations in thick layers of InxGa1−xN (5% < x < 15%) have been investigated by means of cathodoluminescence, time-resolved cathodoluminescence, and molecular dynamics. We show that indium atoms segregate near dislocations in all the samples. This promotes the formation of In-N-In chains and atomic condensates, which localize carriers and hinder nonradiative recombination at dislocations. We note, however, that the dark halo surrounding the dislocations in the cathodoluminescence image becomes increasingly pronounced as the indium fraction of the sample increases. Using transmission electron microscopy, we attribute the dark halo to a region of lower indium content formed below the facet of the V-shaped pit that terminates the dislocation in low composition samples (x < 12%). For x > 12%, the facets of the V-defect featured dislocation bundles instead of the low indium fraction region. In this sample, the origin of the dark halo may relate to a compound effect of the dislocation bundles, of a variation of surface potential, and perhaps, of an increase in carrier diffusion length.

DOI: 10.1063/1.5084330


May 2019:

Spectral diffusion time scales in InGaN/GaN quantum dots

K. Gao, H. Springbett, T. Zhu, R. A. Oliver, Y. Arakawa and M. J. Holmes

Appl. Phys. Lett. 114, 112109 (2019)

A detailed temporal analysis of the spectral diffusion phenomenon in single photon emitting InGaN/GaN quantum dots (QDs) is performed via measurements of both time-varying emission spectra and single photon emission intensity autocorrelation times. Excitation dependent phenomena are investigated via the optical excitation of carriers into the GaN barrier material and also directly into InGaN. Excitation into InGaN reveals that the fastest environmental fluctuations occur on timescales as long as a few hundreds of nanoseconds: an order of magnitude longer than previously measured in GaN QDs. Such long time scales may in future allow for the generation of indistinguishable photons in spite of the fact that the experimentally measured linewidths are broad.

DOI: 10.1063/1.5088205


April 2019:

Investigation of stacking faults in MOVPE-grown zincblende GaN by XRD and TEM

L. Y. Lee, M. Frentrup, P. Vacek, M. J. Kappers, D. J. Wallis and R. A. Oliver

J. Appl. Phys. 125, 105303 (2019)

X-ray diffraction and bright-field transmission electron microscopy are used to investigate the distribution and density of {111}-type stacking faults (SFs) present in a heteroepitaxial zincblende GaN epilayer with high phase purity, grown on a 3C-SiC/Si (001) substrate by metalorganic vapour-phase epitaxy. It is found that the 4° miscut towards the [110] direction of the substrate, which prevents the formation of undesirable antiphase domains, has a profound effect on the relative densities of SFs occurring on the different {111} planes. The two orientations of SFs in the [−110] zone, where the SF inclination angle with the GaN/SiC interface is altered by the 4° miscut, show a significant difference in density, with the steeper (111) SFs being more numerous than the shallower (−1−11) SFs by a factor of ∼5 at 380nm from the GaN/SiC interface. In contrast, the two orientations of SFs in the [110] zone, which is unaffected by the miscut, have densities comparable with the (−1−11) SFs in the [−110] zone. A simple model, simulating the propagation and annihilation of SFs in zincblende GaN epilayers, reproduces the presence of local SF bunches observed in TEM data. The model also verifies that a difference in the starting density at the GaN/SiC interface of the two orientations of intersecting {111} SFs in the same zone reduces the efficiency of SF annihilation. Hence, (111) SFs have a higher density compared with SFs on the other three {111} planes, due to their preferential formation at the GaN/SiC interface caused by the miscut.

DOI: 10.1063/1.5082846


March 2019:

Encapsulation of methylammonium lead bromide perovskite in nanoporous GaN

KTP Lim, C Deakin, B Ding, X Bai, P Griffin, T Zhu, R. A. Oliver and D Credgington

APL. Mat. 7, 021107 (2019)

Halide perovskites hold exceptional promise as cheap, low temperature solution-processed optoelectronic materials. Yet they are hindered by poor structural and chemical stability, rapidly degrading when exposed to moisture or air. We demonstrate a solution-phase method for infiltrating methylammonium lead bromide perovskite (CH3NH3PbBr3, or MAPbBr3) into nanoporous GaN which preserved the green photoluminescence of the perovskite after up to 1 year of storage under ambient conditions. Besides a protective effect, confinement within the porous GaN matrix also resulted in a blueshift of the perovskite emission with decreasing pore size, suggesting an additional templating effect of the pores on the size of the perovskite crystals within. We anticipate that our method may be generalised to related perovskite materials, offering a route to producing composites of interest for use in optoelectronic devices for various applications.

DOI: 10.1063/1.5083037


February 2019:

Improvement of single photon emission from InGaN QDs embedded in porous micropillars

H. P. Springbett, K. Gao, J. Jarman, T. Zhu, M. Holmes, Y. Arakawa and R. A. Oliver

Appl. Phys. Lett. 113, 101107 (2018)

In many InGaN/GaN single photon emitting structures, significant contamination of the single photon stream by background emission is observed. Here, utilizing InGaN/GaN quantum dots incorporated in mesoporous distributed Bragg reflectors (DBRs) within micropillars, we demonstrate methods for the reduction of this contamination. Using the resulting devices, autocorrelation measurements were performed using a Hanbury Brown and Twiss set-up, and thus, we report a working quantum dot device in the III-nitride system utilizing mesoporous DBRs. Uncorrected g(2)(0) autocorrelation values are shown to be significantly improved when excited with a laser at longer wavelengths and lower powers. Through this optimization, we report a g(2)(0) value from a blue-emitting InGaN/GaN quantum dot of 0.126 ± 0.003 without any form of background correction.

DOI: 10.1063/1.5045843


January 2019:

Characterisation of InGaN by Photoconductive Atomic Force Microscopy

T. F. K. Weatherley, F. C.-P. Massabuau, M. J. Kappers and R. A. Oliver

Materials. 11, 1794 (2018)

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) In x Ga 1−x N films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with x≤12 %. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.

DOI: 10.3390/ma11101794


December 2018:

Effect of growth temperature and V/III-ratio on the surface morphology of MOVPEgrown cubic zincblende GaN

L. Y. Lee, M. Frentrup, M. J. Kappers, R. A. Oliver, C. J. Humphrey and D. J. Wallis

J. Appl. Phys. 124, 105302 (2018)

The influence of growth temperature and V/III-ratio on the surface morphology of (001) cubic zincblende GaN epilayers during metal organic vapour phase epitaxy growth has been investigated using atomic force microscopy and transmission electron microscopy. The zincblende phase purity as determined by X-ray diffraction was found to be above 98% for most GaN epilayers studied. As the growth temperature was increased from 850 C to 910 C and as the V/III-ratio was separately increased from 38 to 300, surface features were found to be elongated in the [1-10] direction, and the ratio of the length to width of such surface features was found to increase. Faceting was observed at V/III-ratios below 38 and above 300, which in the latter case was accompanied by a reduction of the zincblende phase purity. An explanation for these morphological trends is proposed based on effects such as the reduced symmetry of the top monolayer of the (001)-oriented zincblende GaN lattice, diffusion of Ga and N adatoms on such a surface, and the relative energies of the crystal facets.

DOI: 10.1063/1.5046801


November 2018:

Resonant excitation of quantum emitters in gallium nitride

M. Kianinia, C. Bradac, M. Nguyen, T. Zhu, M. Toth, R. A. Oliver, and Igor Aharonovich

Optica 5, 932 (2018)

We demonstrate resonant excitation of quantum emitters in gallium nitride (GaN). The emitters are stable under non-resonant excitation and exhibit nearly Fourier-transform-limited lines of ∼250 MHz under coherent excitation, the narrowest reported to date for GaN.

DOI: 10.1364/OPTICA.5.000932


October 2018:

Porous AlGaN-Based Ultraviolet Distributed Bragg Reflectors

P. Griffin, T. Zhu and R. A. Oliver

Materials 11, 1487 (2018)

Utilising dislocation-related vertical etching channels in gallium nitride, we have previously demonstrated a simple electrochemical etching (ECE) process that can create layered porous GaN structures to form distributed Bragg reflectors for visible light at wafer scale. Here, we apply the same ECE process to realise AlGaN-based ultraviolet distributed Bragg reflectors (DBRs). These are of interest because they could provide a pathway to non-absorbing UV reflectors to enhance the performance of UV LEDs, which currently have extremely low efficiency. We have demonstrated porous AlGaN-based UV DBRs with a peak reflectance of 89% at 324 nm. The uniformity of these devices is currently low, as the as-grown material has a high density of V-pits and these alter the etching process. However, our results indicate that if the material growth is optimised, the ECE process will be useful for the fabrication of UV reflectors.

DOI: 10.3390/ma11091487


September 2018:

Ultra-low-threshold InGaN/GaN quantum dot micro-ring lasers

D. Wang, T. Zhu, R. A. Oliver and E. L. Hu

Optics Letters 43, 799 (2018)

In this work, we demonstrate ultra-low-threshold, optically pumped, room-temperature lasing in GaN microdisk and micro-ring cavities containing InGaN quantum dots and fragmented quantum wells, with the lowest measured threshold at a record low of 6.2 μJ∕cm2. When pump volume decreases, we observe a systematic decrease in the lasing threshold of micro-rings. The photon loss rate, γ, increases with increasing inner ring diameter, leading to a systematic decrease in the post-threshold slope efficiency, while the quality factor of the lasing mode remains largely unchanged. A careful analysis using finite-difference time-domain simulations attributes the increased γ to the loss of photons from lower-quality higher-order modes during amplified spontaneous emission.

DOI: 10.1364/OL.43.000799


August 2018:

Structure and magnetic properties of an epitaxial Fe(110)/MgO(111)/GaN(0001) heterostructure

N. Khalid, J.-Y. Kim, A. Ionescu, T. Hussain, F. Oehler, T. Zhu, R. A. Oliver, I. Farrer, R. Ahmad, and C. H. W. Barnes

J. Appl. Phys. 123, 103901 (2018)

We present the structural and magnetic properties of fully epitaxial Fe(110)/MgO(111)/GaN(0001) tunnel barrier structures grown by molecular beam epitaxy. In-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction measurements indicate epitaxial Fe(110) films on top of an epitaxial 2 nm MgO(111) tunnel barrier on GaN(0001). X-ray reflectivity measurements confirm a roughness of approximately 0.3 nm and 0.7 nm for the MgO/GaN and the Fe/MgO interfaces, respectively. Results of in-situ magneto-optical Kerr effect measurements indicate that 1 nm thick Fe film shows signs of in-plane ferromagnetism at room temperature. Vibrating sample magnetometer measurements determine the saturation magnetisation of the 5 nm thick film to be 1660 ± 100 emu/cm3 and show that this system has a predominant uniaxial anisotropy contribution despite the presence of cyclic twinned crystals. We estimate the values of effective uniaxial (KeffU) and cubic (Keff1) anisotropy constants to be 11700 ± 170 erg cm−3 and −3300 ± 700 erg cm−3 by fitting the angular dependence of the magnetising energy.

DOI: 10.1063/1.5022433


July 2018:

Effect of stacking faults on the photoluminescence spectrum of zincblende GaN

S. A. Church, S. Hammersley, P. W. Mitchell, M. J. Kappers, L. Y. Lee, F. C.-P. Massabuau, S. L. Sahonta, M. Frentrup, L. J. Shaw, D. J. Wallis, C. J. Humphreys, R. A. Oliver, D. J. Binks, and P. Dawson

J. Appl. Phys. 123, 185705 (2018)

The photoluminescence spectra of a zincblende GaN epilayer grown via MOCVD upon 3C-SiC/Si (001) substrates were investigated. Of particular interest was a broad emission band centered at 3.4 eV, which extends above the bandgap of both zincblende and wurtzite GaN. Photoluminescence excitation measurements show that this band is associated with an absorption edge centered at 3.6 eV. Photoluminescence time decays for the band are monoexponential, with lifetimes that reduce from 0.67 ns to 0.15 ns as the recombination energy increases. TEM measurements show no evidence of wurtzite GaN inclusions, which are typically used to explain emission in this energy range. However, dense stacking fault bunches are present in the epilayers. A model for the band alignment at the stacking faults was developed to explain this emission band, showing how both electrons and holes can be confined adjacent to stacking faults. Different stacking fault separations can change the carrier confinement energies sufficiently to explain the width of the emission band, and change the carrier wavefunction overlap to account for the variation in decay time.

DOI: 10.1063/1.5026267


June 2018:

The 2018 GaN power electronics roadmap

H. Amano, Y. Baines, E. Beam, Matteo Borga, Stefan Zeltner, Yuhao Zhang et al.

J. Phys. D: Appl. Phys. 51, 163001 (2018)

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

DOI: 10.1088/1361-6463/aaaf9d


May 2018:

Resonant photoluminescence studies of carrier localisation in c-plane InGaN/GaN quantum well structures

W. E. Blenkhorn, S. Schulz, D. S. P. Tanner, R. A. Oliver, M. J. Kappers, C. J. Humphreys and P. Dawson.

J. Phys.: Condens. Matter 30, 175303 (2018)

In this paper we report on changes in the form of the low temperature (12K) photoluminescence spectra of an InGaN/GaN quantum well structure as a function of excitation photon energy. As the photon energy is progressively reduced we observe at a critical energy a change in the form of the spectra from one which is determined by the occupation of the complete distribution of hole localisation centres to one which is determined by the resonant excitation of specific localisation sites. This change is governed by an effective mobility edge whereby the photo-excited holes remain localised at their initial energy and are prevented from scattering to other localisation sites. This assignment is confirmed by the results of atomistic tight binding calculations which show that the wave function overlap of the lowest lying localised holes with other hole states is low compared with the overlap of higher lying hole states with other higher lying hole states.

DOI: 10.1088/1361-648X/aab818


April 2018:

α-Ga2O3 grown by low temperature atomic layer deposition on sapphire

J. W. Roberts, J. C. Jarman, D. N. Johnstone, P. A. Midgley, P. R. Chalker, R. A. Oliver, and F. C-P. Massabuau

J. Cryst. Growth. 487, 23-27 (2018)

α-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with α-In2O3 and α-Al2O3. α-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature (~500 °C). In this study, we report the growth of α-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 Å/cycle, and predominantly consists of α -Ga2O3 in the form of (0001)-oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the α phase columns and most likely comprising ε-Ga2O3. The remainder of the Ga2O3 film – i.e. nearer the surface and between the α-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that α-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

DOI: 10.1016/j.jcrysgro.2018.02.014


March 2018:

Highly polarized electrically driven single-photon emission from a non-polar InGaN quantum dot

C. C. Kocher, T. J. Puchtler, J. C. Jarman, T. Zhu, T. Wang, L. Nuttall, R. A. Oliver, and R. A. Taylor

Appl. Phys. Lett. 111, 251108 (2017)

Nitride quantum dots are well suited for the deterministic generation of single photons at high temperatures. However, this material system faces the challenge of large in-built fields, decreasing the oscillator strength and possible emission rates considerably. One solution is to grow quantum dots on a non-polar plane; this gives the additional advantage of strongly polarized emission along one crystal direction. This is highly desirable for future device applications, as is electrical excitation. Here, we report on electroluminescence from non-polar InGaN quantum dots. The emission from one of these quantum dots is studied in detail and found to be highly polarized with a degree of polarization of 0.94. Single-photon emission is achieved under excitation with a constant current giving a g(2)(0) correlation value of 0.18. The quantum dot electroluminescence persists up to temperatures as high as 130 K.

DOI: 10.1063/1.5008720


February 2018:

Nanoscale structural and chemical analysis of F-implanted enhancement-mode InAlN/GaN heterostructure field effect transistors

Fengzai Tang, Kean B. Lee, Ivor Guiney, Martin Frentrup, Jonathan S. Barnard, Giorgio Divitini, Zaffar H. Zaidi, Tomas L. Martin, Paul A. Bagot, Michael P. Moody, Colin J. Humphreys, Peter A. Houston, Rachel A. Oliver, and David J. Wallis

Journal of Applied Physics 123, 024902 (2018)

We investigate the impact of a fluorine plasma treatment used to obtain enhancement-mode operation on the structure and chemistry at the nanometer and atomic scales of an InAlN/GaN field effect transistor. The fluorine plasma treatment is successful in that enhancement mode operation is achieved with a +2.8 V threshold voltage. However, the InAlN barrier layers are observed to have been damaged by the fluorine treatment with their thickness being reduced by up to 50%. The treatment also led to oxygen incorporation within the InAlN barrier layers. Furthermore, even in the as-grown structure, Ga was unintentionally incorporated during the growth of the InAlN barrier. The impact of both the reduced barrier thickness and the incorporated Ga within the barrier on the transistor properties has been evaluated theoretically and compared to the experimentally determined two-dimensional electron gas density and threshold voltage of the transistor. For devices without fluorine treatment, the two-dimensional electron gas density is better predicted if the quaternary nature of the barrier is taken into account. For the fluorine treated device, not only the changes to the barrier layer thickness and composition, but also the fluorine doping needs to be considered to predict device performance. These studies reveal the factors influencing the performance of these specific transistor structures and highlight the strengths of the applied nanoscale characterisation techniques in revealing information relevant to device performance.

DOI: 10.1063/1.5006255


January 2018:

InGaN µLEDs integrated onto colloidal quantum dot functionalized ultra-thin glass

K. Rae, C. Foucher, B. Guilhabert, M. S. Islim, L. Yin, D. Zhu, R. A. Oliver, D. J. Wallis, H. Haas, N. Laurand, and M. D. Dawson

Optics Express Vol. 25, Issue 16, pp. 19179-19184 (2017)

Red-, orange-, and green-emitting integrated optoelectronic sources are demonstrated by transfer printing blue InGaN µLEDs onto ultra-thin glass platforms functionally enhanced with II-VI colloidal quantum dots (CQDs). The forward optical power conversion efficiency of these heterogeneously integrated devices is, respectively, 9%, 15%, and 14% for a blue light absorption over 95%. The sources are demonstrated in an orthogonal frequency division multiplexed (OFDM) visible light communication link reaching respective data transmission rates of 46 Mbps, 44 Mbps and 61 Mbps.

DOI: 10.1364/OE.25.019179


December 2017:

Temperature and Bias Dependent Trap Capture Cross Section in AlGaN/GaN HEMT on 6-in Silicon With Carbon-Doped Buffer

Sandeep Kumar, Priti Gupta, Ivor Guiney, Colin J. Humphreys, Srinivasan Raghavan, R. Muralidharan, and Digbijoy N. Nath

IEEE Transactions on Electron Devices (Volume: 64, Issue: 12, Dec. 2017)

We report on the estimation of trap capture cross section in AlGaN/GaN HEMTs as a function of bias and temperature. Conductance dispersion technique was employed to study the AlGaN/GaN interface of the devices with a carbon-doped GaN buffer grown on 6-in silicon. While a negligible shift in the threshold voltage (VTH) was observed in temperature-dependent IDSVGS sweeps, we observed a spread in the capacitance-voltage ( CV ) measurements, indicating a contribution of interface traps. When biased near depletion, G/ ω versus frequency plot for AlGaN/GaN interface exhibits two peaks which correspond to a pair of trap density ( Dit ) and trap time constant ( Tit ) values. This was explained using a circuit model in conjunction with energy band diagram. The Dit and Tit values for one peak were in the range from ~ 0.3– 7×1012 /eV cm2 and 0.6– 10 μs while for the other peak, DitTit were in the range of ~0.1– 35×1012 /eV cm2 and ~0.06– 0.3 μs at 25 °C. From the Tit values, electron capture cross section ( σ) for both the traps was extracted and was found to be decreasing with increasing temperature in the range of1.1×10201×1019 cm 2 and 4.5×10201×1017 cm2 for slow traps and fast traps, respectively. A multiphonon emission effect was invoked to explain the temperature dependence of capture cross section.

DOI: 10.1109/TED.2017.2757516


November 2017:

X-ray diffraction analysis of cubic zincblende III-nitrides

Martin Frentrup, Lok Yi Lee, Suman-Lata Sahonta, Menno J Kappers, Fabien Massabuau, Priti Gupta, Rachel A Oliver, Colin J Humphreys and David J Wallis

Journal of Physics D: Applied Physics. 50 433002 (2017)

Solving the green gap problem is a key challenge for the development of future LED-based light systems. A promising approach to achieve higher LED efficiencies in the green spectral region is the growth of III-nitrides in the cubic zincblende phase. However, the metastability of zincblende GaN along with the crystal growth process often lead to a phase mixture with the wurtzite phase, high mosaicity, high densities of extended defects and point defects, and strain, which can all impair the performance of light emitting devices. X-ray diffraction (XRD) is the main characterization technique to analyze these device-relevant structural properties, as it is very cheap in comparison to other techniques and enables fast feedback times. In this review, we will describe and apply various XRD techniques to identify the phase purity in predominantly zincblende GaN thin films, to analyze their mosaicity, strain state, and wafer curvature. The different techniques will be illustrated on samples grown by metalorganic vapor phase epitaxy on pieces of 4'' SiC/Si wafers. We will discuss possible issues, which may arise during experimentation, and provide a critical view on the common theories.

DOI: 10.1088/1361-6463/aa865e


October 2017:

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures

Tong Wang, Tim J. Puchtler, Saroj K. Patra, Tongtong Zhu, John C. Jarman, Rachel A. Oliver, Stefan Schulz & Robert A. Taylor

Scientific Reports 7, 12067 (2017)

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions.

DOI: 10.1038/s41598-017-12233-6


September 2017:

Automatized convergence of optoelectronic simulations using active machine learning

B. Rouet-Leduc, C. Hulbert, K. Barros, T. Lookman, and C. J. Humphreys

Appl. Phys. Lett. 111, 043506 (2017)

A fundamental problem of optoelectronic simulations is to achieve convergence. We use statistical analysis and machine learning to effectively guide the selection of the next device to be examined based upon the expected convergence of the simulation. This active learning strategy rapidly constructs a model that predicts Poisson-Schr€odinger simulations of devices and that simultaneously produces fully converged simulations.

DOI: 10.1063/1.4996233


August 2017:

Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots

T. Wang, T. J. Puchtler, T. Zhu, J. C. Jarman, L. P. Nuttall, R. A. Oliver and R. A. Taylor

Nanoscale 2017; 9; 9421-9427

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g(2)(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems.



July 2017:

Effects of Wavelength and Defect Density on the Efficiency of (In,Ga)N-Based Light-Emitting Diodes

Markus Pristovsek, An Bao, Rachel A. Oliver, Tom Badcock, Muhammad Ali, and Andrew Shields

Physical Review Applied 2017; 7; 064007

We measure the electroluminescence of light-emitting diodes (LEDs) on substrates with low dislocation densities (LDD) at 106 cm−2 and low 108 cm−2, and compare them to LEDs on substrates with high dislocation densities (HDD) closer to 1010 cm−2. The external quantum efficiencies (EQEs) are fitted using the A B C model with and without localization. The nonradiative-recombination (NR) coefficient A is constant for HDD LEDs, indicating that the NR is dominated by dislocations at all wavelengths. However, A strongly increases for LDD LEDs by a factor of 20 when increasing the emission wavelength from 440 to 540 nm. We attribute this to an increased density of point defects due to the lower growth temperatures used for longer wavelengths. The radiative recombination coefficient B follows the squared wave-function overlap for all samples. Using the observed coefficients, we calculate the peak efficiency as a function of the wavelength. For HDD LEDs the change of wave-function overlap (i.e., B ) is sufficient to reduce the EQE as observed, while for LDD LEDs also the NR coefficient A must increase to explain the observed EQEs. Thus, reducing NR is important to improving the EQEs of green LEDs, but this cannot be achieved solely by reducing the dislocation density: point defects must also be addressed.



June 2017:

Structural impact on the nanoscale optical properties of InGaN core-shell nanorods

J. T. Griffiths, C. X. Ren, P.M. Coulon, E. D. Le Boulbar, C. G. Bryce, I. Girgel, A. Howkins, I. Boyd, R. W. Martin, D. W. E. Allsopp, P. A. Shields, C. J. Humphreys, and R. A. Oliver

Applied Physics Letters 2017; 110; 172105

III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1–100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1–100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.



May 2017:

Wafer-scale Fabrication of Non-Polar Mesoporous GaN Distributed Bragg Reflectors via Electrochemical Porosification

T. Zhu, Y. Liu, T. Ding, W.Y. Fu, J. Jarman, C.X. Ren, R.V. Kumar and R.A. Oliver

Scientific Reports 2017; 7; 45344

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. For many device applications, it is highly desirable to achieve not only high reflectivity and low absorption, but also good conductivity to allow effective electrical injection of charges. Here, we demonstrate the wafer-scale fabrication of highly reflective and conductive non-polar gallium nitride (GaN) DBRs, consisting of perfectly lattice-matched non-polar (11–20) GaN and mesoporous GaN layers that are obtained by a facile one-step electrochemical etching method without any extra processing steps. The GaN/mesoporous GaN DBRs exhibit high peak reflectivities (>96%) across the entire visible spectrum and wide spectral stop-band widths (full-width at half-maximum >80 nm), while preserving the material quality and showing good electrical conductivity. Such mesoporous GaN DBRs thus provide a promising and scalable platform for high performance GaN-based optoelectronic, photonic, and quantum photonic devices.



April 2017:

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

H. Qian, K.B. Lee, S. Hosseini Vajargah, S.V. Novikov, I. Guiney, Z.H. Zaidi, S. Jiang, D.J. Wallis, C.T. Foxon, C.J. Humphreys, P.A. Houston

Journal of Crystal Growth 2017; 459; pp. 185-188

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.



March 2017:

Defects in III-nitride microdisk cavities

C X Ren, T J Puchtler, T Zhu, J T Griffiths and R A Oliver

Semiconductor Science and Technology 2017; 32 (3)

Nitride microcavities offer an exceptional platform for the investigation of light–matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Microdisk geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we review the effect of defects on the properties of nitride microdisk cavities fabricated using photoelectrochemical etching of an InGaN sacrificial superlattice (SSL). Threading dislocations originating from either the original GaN pseudosubstrate are shown to hinder the undercutting of microdisk cavities during the photoelectric chemical etching process resulting in whiskers of unetched material on the underside of microdisks. The unetched whiskers provide a pathway for light to escape, reducing microdisk Q-factor if located in the region occupied by the WGMs. Additionally, dislocations can affect the spectral stability of quantum dot emitters, thus hindering their effective integration in microdisk cavities. Though dislocations are clearly undesirable, the limiting factor on nitride microdisk Q-factor is expected to be internal absorption, indicating that the further optimisation of nitride microdisk cavities must incorporate both the elimination of dislocations and careful tailoring of the active region emission wavelength and background doping levels.



February 2017:

Carrier localization in the vicinity of dislocations in InGaN

F. C-P. Massabuau, P. Chen, M. K. Horton, S. L. Rhode, C. X. Ren, T. J. O'Hanlon, A. Kovács, M. J. Kappers, C. J. Humphreys, R. E. Dunin-Borkowski, and R. A. Oliver

Journal of Applied Physics 2017 121:1

We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaN-based devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.



January 2017:

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires

Tim J. Puchtler, Tong Wang, Christopher X. Ren, Fengzai Tang, Rachel A. Oliver, Robert A. Taylor, and Tongtong Zhu

Nano Lett., 2016, 16 (12), pp 7779–7785

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g(2)(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts.



December 2016:

Nano-cathodoluminescence reveals the effect of electron damage on the optical properties of nitride optoelectronics and the damage threshold

James T. Griffiths, Siyuan Zhang, Jeremy Lhuillier, Dandan Zhu, Wai Yuen Fu, Ashley Howkins, Ian Boyd, David Stowe, David J. Wallis, Colin J. Humphreys and Rachel A. Oliver

J. Appl. Phys. 120, 165704 (2016)

Nano-cathodoluminescence (Nano-CL) reveals optical emission from individual InGaN quantum wells for applications in optoelectronic devices. We show the luminescent intensity decays over time with exposure to the electron beam for energies between 80 and 200 keV. Measurements of the CL intensity over time show an exponential decline in intensity, which we propose is due to the formation of nitrogen Frenkel defects. The measured CL damage decreases with reductions in the electron acceleratingvoltage and we suggest that the electron induced structural damage may be suppressed below the proposed damage threshold. The electron beam induced damage leads to a non-radiative region that extends over the measured minority carrier diffusion length. Nano-CL may thus serve as a powerful technique to study III-nitride optoelectronics.



November 2016:

Structural and optical properties of (112-2) InGaN quantum wells compared to (0001) and (112-0)

T. J. Badcock, M. Ali, T. Zhu, M. Pristovsek, R. A. Oliver and A. J. Shields

Applied Physics Letters, 109, 151110 (2016)

We study the photoluminescence internal quantum efficiency (IQE) and recombination dynamics in a pair of polar and non-polar InGaN/GaN quantum well (QW) light-emitting diode (LED) structures as a function of excess carrier density and temperature. In the polar LED at 293 K, the variation of radiative and non-radiative lifetimes is well described by a modified ABC type model which accounts for the background carrier concentration in the QWs due to unintentional doping. As the temperature is reduced, the sensitivity of the radiative lifetime to excess carrier density becomes progressively weaker. We attribute this behaviour to the reduced mobility of the localised electrons and holes at low temperatures, resulting in a more monomolecular like radiative process. Thus we propose that in polar QWs, the degree of carrier localisation determines the sensitivity of the radiative lifetime to the excess carrier density. In the non-polar LED, the radiative lifetime is independent of excitation density at room temperature, consistent with a wholly excitonic recombination mechanism. These findings have significance for the interpretation of LED efficiency data within the context of the ABC recombination model.



October 2016:

Structural and optical properties of (112-2) InGaN quantum wells compared to (0001) and (112-0)

Markus Pristovsek, Yisong Han, Tongtong Zhu, Fabrice Oehler, Fengzai Tang, Rachel A Oliver, Colin J Humphreys, Darius Tytko, Pyuck-Pa Choi, Dierk Raabe, Frank Brunner and Markus Weyers

Semiconductor Science and Technology 31(8):085007 (2016)

We benchmarked growth, microstructure and photo luminescence (PL) of (11-22) InGaN quantum wells (QWs) against (0001) and (11-20). In incorporation, growth rate and the critical thickness of (11-22) QWs are slightly lower than (0001) QWs, while the In incorporation on (11-20) is reduced by a factor of three. A small step-bunching causes slight fluctuations of the emission wavelength. Transmission electron microscopy as well as atom probe tomography (APT) found very flat interfaces with little In segregation even for 20 % In content. APT frequency distribution analysis revealed some deviation from a random InGaN alloy, but not as severe as for (11-20). The slight deviation of (11-22) QWs from an ideal random alloy did not broaden the 300 K PL, the line widths were similar for (11-22) and (0001) while (11-20) QWs were broader. Despite the high structural quality and narrow PL, the integrated PL signal at 300 K was about 4 times lower on (11-22) and more than 10 times lower on (11-20).



September 2016:

n-Type conductivity bound by the growth temperature: the case of Al0.72Ga0.28N highly doped by silicon


A. Kakanakova-Georgieva,S.-L. Sahonta, D. Nilsson, X. T. Trinh, N. T. Son, E. Janze and C. J. Humphreys

J. Mater. Chem. C (2016) 4, 8291

High-Al-content AlxGa1-xN layers, for x ~ 0.72, have been grown by metal organic chemical vapour deposition (MOCVD) at a temperature ranging from 1000 to 1100 &#176C, together with high flow rate of the dopant precursor silane (SiH4) in order to obtain highly Si-doped Al0.72Ga0.28N layers, ~ 1 x 1019 cm-3 as measured by secondary ion mass spectrometry (SIMS). Further characterization of the layers by capacitance–voltage (C–V), electron paramagnetic resonance (EPR), and transmission electron microscopy (TEM) measurements reveals the complex role of growth temperature for the n-type conductivity of high-Al-content AlGaN. While increasing temperature is essential for reducing the incorporation of carbon and oxygen impurities in the layers, it also reduces the amount of silicon incorporated as a donor.




August 2016:

The microstructure of non-polar a-plane (11-20) InGaN quantum wells


James Griffiths, Fabrice Oehler, Fengzai Tang, Siyuan Zhang, Wai Yuen Fu, Tongtong Zhu, Scott Findlay, Changlin Zheng, Joanne Etheridge, Tomas Martin, Paul Bagot, Michael Moody, Danny Sutherland, Philip Dawson, Menno Kappers, Colin Humphreys and Rachel Oliver

J. Appl. Phys. 119, 175703 (2016)

Atom probe tomography and quantitative scanning transmission electron microscopy are used to assess the composition of non-polar a-plane (11-20) InGaN quantum wells for applications in optoelectronics. The average quantum well composition measured by atom probe tomography and quantitative scanning transmission electron microscopy quantitatively agrees with measurements by X-ray diffraction. Atom probe tomography is further applied to study the distribution of indium atoms in non-polar a-plane (11-20) InGaN quantum wells. An inhomogeneous indium distribution is observed by frequency distribution analysis of the atom probe tomography measurements. The optical properties of non-polar (11-20) InGaN quantum wells with indium compositions varying from 7.9 % to 20.6 % are studied. In contrast to non-polar m-plane (1-100) InGaN quantum wells, the non-polar a-plane (11-20) InGaN quantum wells emit at longer emission wavelengths at the equivalent indium composition. The non-polar a-plane (11-20) quantum wells also show broader spectral linewidths. The longer emission wavelengths and broader spectral linewidths may be related to the observed inhomogeneous indium distribution.



July 2016:

Gallium nitride-based visible light-emitting diodes


Rachel Oliver

Materials Science and Technology, Volume 32, Issue 8 (2016)


Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency "droop" (the reduction in efficiency of nitride LEDs with increasing drive current density), the "green gap" (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.

open source pdf



June 2016:

Structure and composition of non-polar (11-20) InGaN nanorings grown by modified droplet epitaxy


Helen Springbett, James Griffiths, Christopher Ren, Tom O'Hanlon, Jonathan Barnard, Suman-Lata Sahonta, Tongtong Zhu and Rachel Oliver

Physica Status Solidi B vol.5 p793 (2016)


Nitride-based quantum dots (QDs) show promise as sources for single photon emission, enabling comparably high temperature emission and access to the blue and green spectral region. Some droplets forming during modified droplet epitaxy on non-polar (11-20) surfaces of InGaN epilayers on GaN are associated with underlying ring-like structures. The work by Springbett et al. (pp. 840–844) discusses droplet etching as a possible mechanism for ring formation, whereby In and Ga atoms are incorporated into the droplet and diffuse to the edges. Thereafter, recrystallization occurs, leading to the formation of a ring. It is hypothesised that the droplet then creeps in a direction determined by the crystallographic anisotropy and the surface energetics. Once this movement halts, etching continues. The resulting structure consists of a recession enclosed by a double ring. Transmission electron microscopy (TEM) analysis shows the droplets move along the ⟨0001⟩ c-axis, and energy dispersive X-ray spectroscopy (EDXS) indicates that they have a very high In content. These studies may help reveal the underlying QD formation mechanism during modified droplet epitaxy.



June 2016:

Optimisation of GaN LEDs and the reduction of efficiency droop using active machine learning


Bertrand Rouet-Leduc, Kipton Barros, Turab Lookman and Colin J. Humphreys

Scientific Reports 6, 24862 (2016)


A fundamental challenge in the design of LEDs is to maximise electro-luminescence efficiency at high current densities. We simulate GaN-based LED structures that delay the onset of efficiency droop by spreading carrier concentrations evenly across the active region. Statistical analysis and machine learning effectively guide the selection of the next LED structure to be examined based upon its expected efficiency as well as model uncertainty. This active learning strategy rapidly constructs a model that predicts Poisson-Schrödinger simulations of devices, and that simultaneously produces structures with higher simulated efficiencies



May 2016:

Comparative study of (0001) and (11-22) InGaN based light emitting diodes

Markus Pristovsek, Colin J. Humphreys, Sebastian Bauer, Manuel Knab, Klaus Thonke, Grzegorz Kozlowski, Donagh O’Mahony, Pleun Maaskant and Brian Corbett

Japanese Journal of Applied Physics 55, 05FJ10 (2016)

We have systematically investigated the doping of (11-22) GaN with Si and Mg by metal–organic vapour phase epitaxy for light emitting diodes (LEDs). By Si doping of GaN we reached electron concentrations close to 1020 cm-3, but the topography degrades above mid 1019 cm-3. By Mg doping we reached hole concentrations close to 5 x 1017 cm-3, using Mg partial pressures about 3 times higher than those for (0001). Exceeding the maximum Mg partial pressure led to a quick degradation of the sample. Low resistivities as well as high hole concentrations required a growth temperature of 900 °C or higher. At optimised conditions the electrical properties as well as the photoluminescence of (11-22) p-GaN were similar to (0001) p-GaN. The best ohmic p-contacts were achieved by NiAg metallisation. A single quantum well LED emitting at 465 nm was realised on (0001) and (11-22). Droop (sub-linear increase of the light output power) occurred at much higher current densities on (11-22). However, the light output of the (0001) LED was higher than that of (11-22) until deep into the droop regime. Our LEDs, as well as those in the literature, indicate a reduction in efficiency from (0001) over semi-polar to non-polar orientations. We propose that reduced fields open a loss channel for carriers.



April 2016:

Analysis of Defect-Related Inhomogeneous Electroluminescence in InGaN/GaN QW LEDs


C. X. Ren, B. Rouet-Leduc, J.T. Griffiths, E. Bohacek, M. J. Wallace , P. R. Edwards, M.A. Hopkins, D. W. E. Allsopp, M. J. Kappers, R. W. Martin, R.A. Oliver

In press, Superlattices and Microstructures (April 2016)


The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.

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March 2016:

Difference in linear polarization of biaxially strained InGaN alloys on nonpolar a-plane and m-plane GaN


Siyuan Zhang, Ying Cui, James T. Griffiths, Wai Y. Fu, Christoph Freysoldt, Jorg Neugebauer, Colin J. Humphreys and Rachel A. Oliver

PHYSICAL REVIEW B 92, 245202 (2015)


InGaN structures epitaxially grown on a-plane or m-plane GaN exhibit in plane optical polarization. Linear elasticity theory treats the two planes equivalently and is hence unable to explain the experimentally observed higher degree of linear polarization for m-plane than a-plane InGaN. Using density functional theory, we study the response of InGaN random alloys to finite biaxial strains on both nonpolar planes. The calculated m-plane InGaN valence band splitting is larger than that of the a plane, due to a greater degree of structural relaxation in a-plane InGaN. We provide a parametrization of the valence band splitting of InGaN strained to a-plane and m-plane GaN for In compositions between 0 and 0.5, which agrees with experimental measurements and qualitatively explains the experimentally observed difference between a-plane and m-plane polarization.



February 2016:

Self-assembled Multilayers of Silica Nanospheres for Defect Reduction in Non- and Semi-polar Gallium Nitride Epitaxial Layers


Tongtong Zhu,Tao Ding, Fengzai Tang, Yisong Han, Muhammad Ali, Tom Badcock, Menno J. Kappers, Andrew J. Shields, Stoyan K. Smoukov and Rachel A. Oliver

Crystal Growth and Design (2016) 16, 1010−1016


Non- and semi-polar GaN have great potential to improve the efficiency of light emitting devices due to much reduced internal electric fields. However, heteroepitaxial GaN growth in these crystal orientations suffers from very high dislocation and stacking faults densities. Here, we report a facile method to obtain low defect density non- and semi-polar heteroepitaxial GaN <i>via</i> selective area epitaxy using self-assembled multilayers of silica nanospheres (MSN). Non-polar (11−20) and semi-polar (11−22) GaN layers with high crystal quality have been achieved by epitaxial integration of the MSN and a simple one-step overgrowth process, by which both dislocation and basal plane stacking fault densities can be significantly reduced. The underlying defect reduction mechanisms include epitaxial growth through the MSN-covered template, island nucleation <i>via</i> nanogaps in the MSN, and lateral overgrowth and coalescence above the MSN. InGaN/GaN multiple quantum well structures grown on a non-polar GaN/MSN template show a more than 30-fold increase in luminescence intensity compared to a control sample without the MSN. This self-assembled MSN technique provides a new platform for epitaxial growth of nitride semiconductors and offers unique opportunities for improving the material quality of GaN grown on other orientations and foreign substrates or heteroepitaxial growth of other lattice-mismatched materials.



January 2016:

Dislocation core structures in Si-doped GaN

S. K. Rhode, M. K. Horton, W. Y. Fu, S.-L. Sahonta, M. J. Kappers, T. J. Pennycook, C. J. Humphreys, R. O. Dusane, and M. A. Moram

Applied Physics Letters 107, 243104 (2015)


Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in plan-view geometry of GaN films with a range of Si-doping levels and dislocation densities. All a-type (edge) dislocation core structures in all samples formed 5/7-atom ring core structures, whereas all (a+c)-type (mixed) dislocations formed either double 5/6-atom, dissociated 7/4/8/4/9-atom, or dissociated 7/4/8/4/8/4/9-atom core structures. This shows that Si-doping does not affect threading dislocation core structures in GaN. However, electron beam damage at 300 keV produces 4-atom ring structures for (a+c)-type cores in Si-doped GaN.



December 2015:

Nanocathodoluminescence Reveals Mitigation of the Stark Shift in InGaN Quantum Wells by Si Doping

James T. Griffiths, Siyuan Zhang, Bertrand Rouet-Leduc, Wai Yuen Fu, An Bao, Dandan Zhu, David J. Wallis, Ashley Howkins, Ian Boyd, David Stowe, Menno J. Kappers, Colin J. Humphreys and Rachel A. Oliver

Nano Lett. 2015, 15, 7639−7643


Nanocathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations, which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nanocathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.



November 2015:

Effects of quantum well growth temperature on the recombination efficiency of InGaN/GaN multiple quantum wells that emit in the green and blue spectral regions

S. Hammersley, M.J. Kappers, F.C.-P. Massabuau, S.-L. Sahonta, P. Dawson, R.A. Oliver and C.J. Humphreys

Appl. Phys. Lett. 107, 132106 (2015)

InGaN-based light emitting diodes and multiple quantum wells designed to emit in the green region exhibit, in general, lower internal quantum efficiencies than their blue-emitting counter parts, a phenomenon referred to as the green gap. One of the main differences between green-emitting and blue-emitting samples is that the quantum well growth temperature is lower for structures designed to emit at longer wavelengths, in order to reduce the effects of In desorption. In this paper, we report on the impact of the quantum well growth temperature on the optical properties of InGaN/GaN multiple quantum wells designed to emit at 460 nm and 530 nm. It was found that for both sets of samples increasing the temperature at which the InGaN quantum well was grown, while maintaining the same indium composition, led to an increase in the internal quantum efficiency measured at 300 K. These increases in internal quantum efficiency are shown to be due to reductions in the non-radiative recombination rate which we attribute to reductions in incorporation.



October 2015:

High Performance GaN High Electron Mobility Transistors on Low Resistivity Silicon for x-Band Applications

A. Eblabla, X. Li, I. Thayne, D. J. Wallis, I. Guiney, K. Elgaid

Physica Status Solidi B, Volume 252, 866-872 (2015)


This letter reports the RF performance of a 0.3-μm gate length AlGaN/AlN/GaN HEMT realized on a 150-mm diameter low-resistivity (LR) (σ < 10 Ωcm) silicon substrate. Short circuit current gain (fT) and maximum frequency of oscillation (fMAX) of 55 and 121 GHz, respectively, were obtained. To our knowledge, these are the highest fT/fMAX values reported to date for GaN HEMTs on LR silicon substrates.

July 2015:

A study of the inclusion of prelayers in InGaN/GaN single- and multiple-quantum-well structures

Matthew Davies, Philip Dawson, Fabien Massabuau, Adrian Le Fol, Rachel Oliver, Menno Kappers and Colin Humphreys

Physica Status Solidi B, Volume 252, 866-872 (2015)


We report on the effects on the optical properties of blue-light emitting InGaN/GaN single- and multiple-quantum-well structures including a variety of prelayers. For each single-quantum-well structure containing a Si-doped prelayer, we measured a large blue shift of the photoluminescence peak energy and a significant increase in radiative recombination rate at 10 K. Calculations of the conduction and valence band energies show a strong reduction in the built-in electric field across the quantum well (QW) occurs when including Si-doped prelayers, due to enhancement of the surface polarization field which opposes the built-in field. The reduction in built-in field across the QW results in an increase in the electron-hole wavefunction overlap, increasing the radiative recombination rate, and a reduction in the strength of the quantum confined Stark effect, leading to the observed blue shift of the emission peak. The largest reduction of the built-in field occurred for an InGaN: Si prelayer, in which the additional InGaN/GaN interface of the prelayer, in close proximity to the QW, was shown to further reduce the built-in field. Study of multiple QW structures with and without an InGaN:Si prelayer showed the same mechanisms identified in the equivalent single-quantum-well structure.

June 2015:

Ultra-low threshold gallium nitride photonic crystal nanobeam laser

Nan Niu, Alexander Woolf, Danqing Wang, Tongtong Zhu, Qimin Quan, Rachel A. Oliver and Evelyn L. Hu, Appl. Phys. Lett., 106, 231104 (2015)


The Cambridge GaN Centre and our collaborators at Harvard have developed nanobeam lasers which exhibit record-breaking lower threshold powers for optically-pumped room temeprature lasing. The key discovery which has enabled this is the use of a discontinuous InGaN quantum well structure as the active, light-emitting layer. The distcontinuities prevent carrier diffusion to the surfaces of the nanobeam where non-radiative recombination could take place, reducing the loss mechanisms and hence improving performance.

May 2015:

Practical Issues for Atom Probe Tomography Analysis of III-Nitride Semiconductor Materials

Fengzai Tang, Michael Moody, Tomas Martin, Paul Bagot, Menno Kappers, and Rachel Oliver, Microscopy and Microanalysis Firstview Article, pp. 1-13 (2015)


Various practical issues affecting atom probe tomography (APT) analysis of III-nitride semiconductors have been studied as part of an investigation using a c-plane InAlN/GaN heterostructure. Specimen preparation was undertaken using a focused ion beam microscope with a mono-isotopic Ga source. This enabled the unambiguous observation of implantation damage induced by sample preparation. In the reconstructed InAlN layer Ga implantation was demonstrated for the standard “clean-up” voltage (5 kV), but this was significantly reduced by using a lower voltage (e.g., 1 kV). The characteristics of APT data from the desorption maps to the mass spectra and measured chemical compositions were examined within the GaN buffer layer underlying the InAlN layer in both pulsed laser and pulsed voltage modes. The measured Ga content increased monotonically with increasing laser pulse energy and voltage pulse fraction within the examined ranges. The best results were obtained at very low laser energy, with the Ga content close to the expected stoichiometric value for GaN and the associated desorption map showing a clear crystallographic pole structure.

April 2015:

Segregation of In to Dislocations in InGaN

Matt Horton, Sneha Rhode, Lata Sahonta, Menno Kappers, Sarah Haigh, Tim Pennycook, Colin Humphreys, Rajiv Dusane and Shelly Moram, Nano Letters, 15, 2, 923-930 (2015)


Dislocations are one-dimensional topological defects that occur frequently in functional thin film materials and that are known to degrade the performance of InxGa1-xN-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure are expected to occur in and around dislocation cores in InxGa1-xN alloy thin films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films.

March 2015:

Indium clustering in a-plane InGaN quantum wells as evidenced by atom probe tomography

Fengzai Tang, Tongtong Zhu, Fabrice Oehler, Wai Yuen Fu, James T. Griffiths, Fabien C.-P. Massabuau, Menno J. Kappers, Tomas L. Martin, Paul A. J. Bagot, Michael P. Moody and Rachel A. Oliver, Appl. Phys. Lett. 106, 072104 (2015)


Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN, and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.

February 2015:

Low defect large area semi-polar(11¯22) GaN grown on patterned(113) silicon

Markus Pristovsek, Yisong Han, Tongtong Zhu, Martin Frentrup, Menno J. Kappers,Colin J. Humphreys, Grzegorz Kozlowski, Pleun Maaskant, and Brian Corbett, Phys. Status Solidi B, 1–5 (2014)


We report on the growth of semi-polar GaN (11¯22) templates on patterned Si (113) substrates. Trenches were etched in Si(113) using KOH to expose Si {111} sidewalls. Subsequently an AlN layer to prevent meltback etching, an AlGaN layer for stress management, and finally two GaN layers were deposited. Total thicknesses up to 5 microns were realised without cracks in the layer. Transmission electron microscopy showed that most dislocations propagate along [0001] and hence can be covered by overgrowth from the next trench. Dislocation densities were below 108cm−2 and stacking fault densities were less than 100 cm−1. These numbers are similar to reports on GaN on patterned r-plane sapphire. Typical X-ray full width at half maxima (FHWM) were 500″ for the asymmetric (00.6) and 450″ for the (11.2) reflection. These FHWMs were 50 % broader than reported for patterned r-plane sapphire, which is attributed to different defect structures and total thicknesses. The surface roughness shows strong variation on templates. For the final surface roughness, the roughness of the sidewalls of the GaN ridges at the time of coalescence are critical.

January 2015:

Effect of Threading Dislocations on the Quality Factor of InGaN/GaN Microdisk Cavities

Tim J. Puchtler, Alexander Woolf, Tongtong Zhu, David Gachet, Evelyn L. Hu and Rachel A. Oliver. ACS Photonics 2015, 2, 137−143


In spite of the theoretical advantages associated with nitride microcavities, the quality factors of devices with embedded indium gallium nitride (InGaN) or gallium nitride (GaN) optical emitters still remain low. In this work we identify threading dislocations (TDs) as a major limitation to the fabrication of high quality factor devices in the nitrides. We report on the use of cathodoluminescence (CL) to identify individual TD positions within microdisk lasers containing either InGaN quantum wells or quantum dots. Using CL to accurately count the number, and map the position, of dislocations within several individual cavities, we have found a clear correlation between the density of defects in the high-field region of a microdisk and its corresponding quality factor (Q). We discuss possible mechanisms associated with defects, photon scattering, and absorption, which could be responsible for degraded device performance.

December 2014:

Growth of non-polar (11-20) InGaN quantum dots by metal organic vapour phase epitaxy using a two temperature method

J. T. Griffiths, T. Zhu, F. Oehler, R. M. Emery, W. Y. Fu, B. P. L. Reid, R. A. Taylor, M. J. Kappers, C. J. Humphreys and R. A. Oliver. APL Mat. 2, 126101 (2014)


Non-polar (11-20) InGaN quantum dots (QDs) were grown by metal organic vapour phase epitaxy. An InGaN epilayer was grown and subjected to a temperature ramp in a nitrogen and ammonia environment before the growth of the GaN capping layer. Uncapped structures with and without the temperature ramp were grown for reference and imaged by atomic force microscopy. Micro-photoluminescence studies reveal the presence of resolution limited peaks with a linewidth of less than ∼500 μeV at 4.2 K. This linewidth is significantly narrower than that of non-polar InGaN quantum dots grown by alternate methods and may be indicative of reduced spectral diffusion. Time resolved photoluminescence studies reveal a mono-exponential exciton decay with a lifetime of 533 ps at 2.70 eV. The excitonic lifetime is more than an order of magnitude shorter than that for previously studied polar quantum dots and suggests the suppression of the internal electric field. Cathodoluminescence studies show the spatial distribution of the quantum dots and resolution limited spectral peaks at 18 K

November 2014:

Sulfuric acid and hydrogen peroxide surface passivation effects on AlGaN/GaN high electron mobility transistors

Z. H. Zaidi, K. B. Lee, I. Guiney, H. Qian, S. Jiang, D. J. Wallis, C. J. Humphreys and P. A. Houston. J. Appl. Phys. 116, 244501 (2014)


In this work, we have compared silicon nitride passivation, hydrogen peroxide, and sulfuric acid treatment on AlGaN/GaN HEMTs surface after full device fabrication on Si substrate. Both the chemical treatments resulted in the suppression of device pinch-off gate leakage current below 1 μA/mm, which is much lower than that for SiNx passivation. The greatest suppression over the range of devices is observed with the sulfuric acid treatment. The device on/off current ratio is improved (from 104–105 to 107) and a reduction in the device sub-threshold (S.S.) slope (from ∼215 to 90 mV/decade) is achieved. The sulfuric acid is believed to work by oxidizing the surface which has a strong passivating effect on the gate leakage current. The interface trap charge density (Dit ) is reduced (from 4.86 to 0.90 × 1012 cm−2 eV−1), calculated from the change in the device S.S. The gate surface leakage current mechanism is explained by combined Mott hopping conduction and Poole Frenkel models for both untreated and sulfuric acid treated devices. Combining the sulfuric acid treatment underneath the gate with the SiNx passivation after full device fabrication results in the reduction of Dit and improves the surface related current collapse.

October 2014:

Structure and strain relaxation effects of defects in InxGa1−xN epilayers

S. L. Rhode*, W. Y. Fu*, M. A. Moram, F. C.-P. Massabuau, M. J. Kappers, C. McAleese, F. Oehler, C. J. Humphreys, R. O. Dusane and S.–L. Sahonta. (*co-first author). Appl. Phys. 116 , 103513 (2014)


In this paper, we investigated different types of defects in InGaN epilayers, which are used in LEDs, laser diodes and solar cells. We have discovered that these defects (including trench defects, basal plane stacking faults, V-defects and dislocations) do not seem to relieve compressive strain in the film as observed by X-Ray diffraction (XRD). The formation of these defects is discussed, and some are likely to originate from a combination of the small in-plane misorientation of the sapphire substrate, and the thermal mismatch strain between the GaN and InGaN layers grown at different temperatures.

Figure:The TEM image shows threading dislocations terminating at V-defects, and horizontal dislocations in the underlying GaN which we propose form as a result of thermal mismatch and substrate miscut.

September 2014:

“Distinctive signature of indium gallium nitride quantum dot lasing in microdisk cavities”

Alexander Woolf, Tim Puchtler, Igor Aharonovich, Tongtong Zhu, Nan Niu, Danqing Wang, Rachel Oliver, Evelyn L. Hua. PNAS 2014. , doi: 10.1073/pnas.1415464111