January 2026:
Effect of buffer layer thickness on recombination in zincblende InGaN/GaN quantum wells
X Xu, D Dyer, M Frentrup, W R Fieldhouse-Allen, M J Kappers, G Kusch, D J Wallis, R A Oliver and D J Binks
Journal of Physics D: Applied Physics 58, 475101 (2025)
InGaN/GaN quantum wells grown in the zincblende phase along the [001] direction are free of the internal electric fields that reduce the radiative recombination rate in conventional quantum wells grown along the c-axis in the wurtzite phase. However, heteroepitaxial growth and reduced thermodynamic stability compared to the wurtzite phase typically results in a significant density of stacking faults (SFs) in zincblende GaN, which impacts emission efficiency when they intersect quantum wells. Here it is shown that increasing the buffer layer thickness that lies between the substrate and the active region significantly reduces the density of SFs reaching the quantum well, and thereby increases the emission efficiency.
December 2025:
Porous GaN: Anion-Specific Electrochemical Etching Mechanisms and Morphological Control
T. R. Harris-Lee, B. Thornley, J. Zhang, M. J. Kappers, and R. A. Oliver
ACS Applied Materials & Interfaces 17, 64931-64941 (2025)
Porous GaN has emerged as a promising material for enhancing the performance of optoelectronic devices and broadening the range of possible GaN applications. However, the electrochemical etching (ECE) process used to create porosity remains poorly understood, particularly regarding the impact of the chemical environment on pore morphology. Here, the controlled ECE of n-type GaN is systematically investigated across a range of etchant chemicals and pH values. It is shown that the identity, speciation, and relative concentrations of anionic species play dominant roles in dictating porous morphology. Through deliberate manipulation of anion compositions within an etchant solution, for example, by adjusting initial polyprotic acid concentration and/or addition of a conjugate salt, porous morphology and surface structure can be controlled and tuned effectively. Further, ECE-generated current oscillations, previously interpreted as evidence for an oxidation–dissolution ECE mechanism, are shown to correlate with the presence of dynamic anion equilibria, providing an additional mechanistic interpretation of n-type GaN ECE. This furthered understanding enables more tailored and application-specific control over porous structure, offering opportunities for optimized, bespoke GaN-based porous architectures.
November 2025:
Understanding Local Crystallography in Solar Cell Absorbers with Scanning Electron Diffraction
A. Griesi, Y. P. Ivanov, S. M. Fairclough, A. V. Oli, G.Kusch, R. A. Oliver, P. De Padova, C. Ottaviani, U. Wijesinghe, S. Siebentritt, A. Di Carlo, O. S. Hutter, G. Longo, and G. Divitini
Small Methods xx, xxx (2025)
In thin film photovoltaic devices, the control of grain structure and local crystallography are fundamental for high power conversion efficiency and reliable long-term operation. Structural defects, grain boundaries, and unwanted phases can stem from compositional inhomogeneities or from specific synthesis parameters, and they need to be thoroughly understood and carefully engineered. However, comprehensive studies of the crystallographic properties of complex systems, including different phases and/or a large number of grains, are often prohibitively challenging. Here, the use of 4D Scanning Transmission Electron Microscopy (4D-STEM) is demonstrated on cross-sections to unravel the nanoscale properties of three different materials for photovoltaics: Cu(In,Ga)S2, halide perovskite, and Sb2Se3. These materials are chosen because of the variety of challenges they present: the presence of multiple phases and complex stoichiometry, electron beam sensitivity, and very high density of grains. 4D-STEM provides comprehensive insights into crystallinity and microstructure, but navigating its large datasets and extracting actionable, statistically sound information requires advanced algorithms. How unsupervised machine learning, including dimensionality reduction and hierarchical clustering, can extract key information from 4D-STEM datasets is demonstrated. The analytical framework follows FAIR principles, employing open-source software and enabling data sharing.
October 2025:
Point Defect Induced Potential Wells across the m-Plane of Core/Shell GaN Nanowires
S. Rezaie, G. Kusch, L. Samuelson, J. B. Wagner, and S. Yazdi
physica status solidi (RRL)–Rapid Research Letters 19, 2500145 (2025)
Nanowires are promising structures for next-generation photonic devices due to their superior structural, optical, and electronic properties compared to thin films. In this study, unexpected electrostatic potential wells across the non-polar m-plane and at the core/shell interface in n-type GaN core/shell nanowires, grown via metal-organic vapor phase epitaxy, are reported. Using advanced electron microscopy, including off-axis electron holography, electrostatic potential distributions are mapped and shallow quantum wells are identified at the core/shell interface and core center. High-resolution transmission electron microscopy ruled out planar and line defects, implicating point defects as their source. Valence electron energy loss spectroscopy revealed localized bandgap narrowing due to strain from concentrated point defects. Hyperspectral cathodoluminescence linked lower potential in the core to CN defects, while the absence of related luminescence at the core/shell interface suggests VGaON defect complexes as plausible causes. These findings highlight the critical role of point defects in GaN nanowires, with significant implications for device performance.
September 2025:
Point defect luminescence associated with stacking faults in magnesium doped zincblende GaN
X. Xu, M. Frentrup, G. Kusch, R. Shu, C. Hofer, P. A. J. Bagot, M. P. Moody, M. J. Kappers, D. J. Wallis, and R. A. Oliver
Journal of Applied Physics 137, 235301 (2025)
The luminescence characteristics and the relation between the distribution of impurities and stacking faults (SFs) in Mg-doped zincblende gallium nitride (zb-GaN:Mg) have been investigated by cathodoluminescence (CL) and atom probe tomography (APT). Four peaks have been identified in the CL emission spectrum, and the possible related recombination mechanisms have been proposed. The main peak at 3.23 eV is associated with excitonic transitions, while the other three, having lower energies at about 3.15, 3.02, and 2.92 eV, respectively, are related to donor-to-acceptor (DAP) transitions involving different acceptor energy levels. These DAP peaks were significantly more intense on or close to SFs compared to the surrounding defect-free material, indicating an enrichment of point defects near SFs. This finding was supported by APT measurements, where Mg showed a tendency to segregate toward SFs in zb-GaN.
August 2025:
Microwave plasma modelling for thick diamond deposition on III-nitrides
J. A. Cuenca, A. Al-Moathin, M. J. Kappers, S. Mandal, M. Kuball, R. A. Oliver, C. Li, O. and A. Williams
Carbon 241, 120349 (2025)
Microwave plasma chemical vapour deposition (MP-CVD) of thick polycrystalline diamond (PCD) (t > 100 µm) is demonstrated on flipped III-nitrides (III-N)/gallium nitride (GaN) on Si using a sample holder designed using iterative microwave plasma modelling. The damage of flipped III-N/GaN in H plasma is due to superheating, caused by expansion of voids in the bonding layer from the flipping process and etching of the III-N/GaN film at an onset of above 720 °C. This study demonstrates that holders with a tapered base allow rapid sample cooling (T ~ 669 °C) to mitigate damage in a reactive hydrogen plasma at high-power and pressure. This holder enables, high quality thick PCD deposition and demonstrates the importance of microwave plasma modelling for cost-effective iteration of sample holder/susceptor design for temperature regulation.
DOI: 10.1016/j.carbon.2025.120349
July 2025:
All-site alloyed perovskite for efficient and bright blue light-emitting diodes
Y. Chen, R. Wang, G. Kusch, B. Xu, C. Hao, C. Xue, L. Cheng, L. Zhu, J. Wang, H. Li, R. A. Oliver, N. Wang, W. Huang, and J. Wang
Nature Communications 16, 3254 (2025)
Perovskite light-emitting diodes have drawn great attention in the fields of displays and lighting, especially for applications requiring high efficiency and high brightness. While three-dimensional perovskite light-emitting diodes hold promise for achieving higher brightness compared to low-dimensional counterparts, efficient blue three-dimensional perovskite light-emitting diodes have remained a challenge due to defect formation during the disordered crystallization of multiple A-cation perovskite. Here we demonstrate an all-site alloy method that enables sequential A-site doping growth of formamidinium and cesium hybrid perovskite. This approach significantly reduces the trap density of the perovskite film by approximately one order of magnitude. Consequently, we achieve efficient and bright blue perovskite light-emitting diode with an external quantum efficiency of 23.3%, a luminous efficacy of 33.4 lm W−1, and a luminance of approximately 5700 cd m−2 for the emission with a peak at 487 nm. This work provides a strategy for growing high-quality multicomponent perovskite for optoelectronics.
DOI: 10.1038/s41467-025-58470-6
June 2025:
Beyond transmission electron microscopy imaging: Atom probe tomography reveals chemical inhomogeneity at stacking fault interfaces in InGaN/GaN light-emitting diodes
R. Shu, R. A. Oliver, M. Frentrup, M. J. Kappers, H. Xiu, G. Kusch, D. J. Wallis, C. Hofer, P. A. J. Bagot, and M. P. Moody
Materialia 40, 102417 (2025)
In this study, we present an atom probe tomography investigation of zincblende InGaN-based multi-quantum well light-emitting diode (LED) structures with a specific focus on the influence of stacking faults within the system. We demonstrate that the visualisation of stacking faults in atom probe reconstructions is possible due to previously documented sensitivities of measured composition in III-V materials to local variations in electric field during the experiment. Meanwhile, we quantify the composition of indium (In) in the InGaN quantum wells and establish that elongated regions exist, parallel to ridges on the sample surface, in which the indium content is increased. We discuss this observation in the context of previous scanning transmission electron microscopy (STEM) data which suggested that such In rich regions are associated with stacking faults. Our experiments not only showcase the feasibility of stacking fault characterization in InGaN-based multi-quantum well LEDs through atom probe tomography (APT) but also offer a practical pathway towards three-dimensional imaging and compositional analysis of stacking faults at the atomic scale.
DOI: 10.1016/j.mtla.2025.102417
May 2025:
TUNA-EBSD-CL Correlative Multi-microscopy Study on the Example of Cu (In, Ga) S2 Solar Cell Absorber
Y. Hu, G. Kusch, D. Adeleye, S. Siebentritt, and R. A. Oliver
Journal of Microscopy 298 (1), pp 106-117 (2025)
Multi-microscopy offers significant benefits to the understanding of complex materials behaviour by providing complementary information from different properties. However, some characterisations may strongly influence other measurements in the same workflow. To acquire reliable and valid datasets, optimising multi-microscopy procedure is necessary. In present work, we studied the influence of the measurement order on the quality of multi-microscopy datasets. Multi-microscopy incorporating tunnelling current AFM (TUNA), electron backscatter diffraction (EBSD), and cathodoluminescence (CL) on a polycrystalline solar cell absorber, Cu(In,Ga)S2 (CIGS), is used as an example. The investigation revealed potential characterisation-induced contaminations, such as surface oxidation and hydrocarbon layer coating, of the sample surface. Their subsequent influence on the measurement results of following correlation techniques was examined. To optimise the dataset quality, multi-microscopy should be carried out in TUNA-EBSD-CL order, from the most to the least surface sensitive techniques. With the optimised multi-microscopy measurement order on a CIGS absorber, we directly correlated the local changes in electrical and opto-electronic properties with the microstructure of grain boundaries (GBs). The described methodology may also provide insightful concepts for applying other AFM-SEM-based multi-microscopy on different semiconductor materials.
April 2025:
Buffer-less Gallium Nitride High Electron Mobility Heterostructures on Silicon
S. Ghosh, M. Frentrup, A. M. Hinz, J. W. Pomeroy, D. Field, D. J. Wallis, M. Kuball, and R. A. Oliver
Advanced Materials 37, 2413127 (2025)
Thick metamorphic buffers are considered indispensable for III-V semiconductor heteroepitaxy on large lattice and thermal-expansion mismatched silicon substrates. However, III-nitride buffers in conventional GaN-on-Si high electron mobility transistors (HEMT) impose a substantial thermal resistance, deteriorating device efficiency and lifetime by throttling heat extraction. To circumvent this, a systematic methodology for the direct growth of GaN after the AlN nucleation layer on six-inch silicon substrates is demonstrated using metal-organic vapor phase epitaxy (MOVPE). Crucial growth-stress modulation to prevent epilayer cracking is achieved even without buffers, and threading dislocation densities comparable to those in buffered structures are realized. The buffer-less design yields a GaN-to-substrate thermal resistance of (11 ± 4) m2 K GW−1, an order of magnitude reduction over conventional GaN-on-Si and one of the lowest on any non-native substrate. As-grown AlGaN/AlN/GaN heterojunctions on this template show a high-quality 2D electron gas (2DEG) whose room-temperature Hall-effect mobility exceeds 2000 cm2 V−1 s−1, rivaling the best-reported values. As further validation, the low-temperature magnetoresistance of this 2DEG shows clear Shubnikov-de-Haas oscillations, a quantum lifetime > 0.180 ps, and tell-tale signatures of spin-splitting. These results could establish a new platform for III-nitrides, potentially enhancing the energy efficiency of power transistors and enabling fundamental investigations into electron dynamics in quasi-2D wide-bandgap systems.
March 2025:
Behaviour of Ti/Al/Ti/Au contacts to AlGaN/GaN heterostructures at low temperature
F. Adams, S. Ghosh, Z. Liang, C. Chen, N. Suphannarat, M. J Kappers, D. J. Wallis, and R.A. Oliver
J. Phys. D: Appl. Phys. 58, 135117 (2025)
Ohmic contacts to wide bandgap nitrides have been realised, but little is known about their behaviour at low temperatures. To address this, an established Ti/Al/Ti/Au contact stack on AlGaN/GaN heterostructures has been characterised from 320 K to 80 K. Two structures were investigated, with very similar ambient 2D electron gas transport characteristics despite their difference in AlGaN barrier thickness and composition. This allowed for direct comparison of contact behaviour across different heterostructures. Upon annealing at < 800 °C for samples with 29 nm AlGaN barriers, contacts which had Ohmic characteristics at room temperature exhibited a gradual onset of Schottky behaviour as the measurement temperature was lowered. When non-Ohmic behaviour was observed, a combination of direct tunnelling, Fowler–Nordheim tunnelling and a thermally assisted Fowler–Nordheim mechanism is suggested to describe the carrier transport. In this case, annealing at 800 °C for 30 s proved sufficient to ensure Ohmic behaviour when tested from 320 K to 80 K. For a heterostructure with 8 nm AlGaN, the required annealing temperature to maintain consistent Ohmic behaviour across the temperature range was reduced to 750 °C. From these observations, the determining factor for Ohmic behaviour is suggested to be the thickness of the AlGaN barrier–either as-grown, or the effective thickness following the formation of TiN protrusions into the AlGaN barrier during annealing. The understanding provided here allows tailoring of either the processing conditions or the heterostructure, and may aid with design of novel devices for low temperature operation.
February 2025:
Investigating the exciton dynamics in InGaN/GaN core-shell nanorods using time-resolved cathodoluminescence
K. Loeto, G. Kusch, O. Brandt, P.-M. Coulon, S. Hammersley, J. Lähnemann, I. Girgel, S. Fairclough, M. Sarkar, P. A Shields, and R.A. Oliver
Nanotechnology 36, 025703 (2024/2025)
This study examines the exciton dynamics in InGaN/GaN core–shell nanorods using time-resolved cathodoluminescence (TRCL), which provides nanometer-scale lateral spatial and tens of picoseconds temporal resolutions. The focus is on thick (>20 nm) InGaN layers on the non-polar, semi-polar and polar InGaN facets, which are accessible for study due to the unique nanorod geometry. Spectrally integrated TRCL decay transients reveal distinct recombination behaviours across these facets, indicating varied exciton lifetimes. By extracting fast and slow lifetime components and observing their temperature trends along with those of the integrated and peak intensity, the differences in behaviour were linked to variations in point defect density and the degree and density of localisation centres in the different regions. Further analysis shows that the non-polar and polar regions demonstrate increasing lifetimes with decreasing emission energy, attributed to an increase in the depth of localisation. This investigation provides insights into the intricate exciton dynamics in InGaN/GaN nanorods, offering valuable information for the design and development of optoelectronic devices.
January 2025:
Enhanced Excitonic Nature of MAPbBr3 Nanocrystals in Nanoporous GaN
X. Bai, S.M. Fairclough, L. Dai, M. Sarkar, P.H. Griffin, A. Gundimeda, Y. Sun, N.C. Greenham, M.I. Dar, R.A. Oliver, and R.H. Friend
Adv. Optical Mater. 12, 2400221 (2024)
Blue gallium nitride (GaN) light-emitting diodes (LEDs), combined with red/green fluorescent converters, have broad potential for display applications. Metal halide perovskites now show excellent luminescence properties and may be suitable as light converters. Here a simple solution-processed method is reported to prepare a methylammonium lead bromide (MAPbBr3) nanoporous GaN composite. Fast (within 2 ps) energy transfer is demonstrated from photoexcited nanoporous GaN to encapsulate MAPbBr3 nanocrystals, as observed by transient absorption spectroscopy. The spatial confinement of the perovskite within the nanoporous GaN is shown to increase the perovskite radiative recombination rate. These results offer guidelines for developing high-performance perovskite/nanoporous GaN optoelectronics.
December 2024:
Impact of stacking faults on the luminescence of a zincblende InGaN/GaN single quantum well
A. Gundimeda, G. Kusch, M. Frentrup, H. Xiu, R. Shu, C. Hofer, P. A. J. Bagot, M. P. Moody, M. J. Kappers, D. J. Wallis, and R. A. Oliver
J. Phys. D: Appl. Phys. 58, 025112 (2024)
In this paper, we investigate the optical properties of a zincblende InGaN single quantum well (SQW) structure containing stacking faults (SFs). Cathodoluminescence studies revealed the presence of sharp emission features adjacent to SFs, identified as quantum wires (Qwire) via their spatial anisotropy. Scanning transmission electron microscopy provided evidence of indium rich regions adjacent to SFs which intersect the QW along the [110] and [1–10] directions, whilst atom probe tomography revealed that the indium rich regions have an elongated structure, creating a Qwire. This work sheds light on the intricate relationship between SFs and Qwires in zincblende InGaN SQW structures, offering insights into the underlying mechanisms governing their optical behavior.
November 2024:
Halide homogenization for low energy loss in 2-eV-bandgap perovskites and increased efficiency in all-perovskite triple-junction solar cells
J. Wang, L. Zeng, D. Zhang, A. Maxwell, H. Chen, K. Datta, A. Caiazzo, W. HM Remmerswaal, N. RM Schipper, Z. Chen, K. Ho, A. Dasgupta, G. Kusch, R. Ollearo, L. Bellini, S. Hu, Z. Wang, C. Li, S. Teale, L. Grater, B. Chen, M. M. Wienk, R. A. Oliver, H. J. Snaith, R. A. J. Janssen, and E. H. Sargent
Nat. Energy. 9, 70-80 (2024)
Monolithic all-perovskite triple-junction solar cells have the potential to deliver power conversion efficiencies beyond those of state-of-art double-junction tandems and well beyond the detailed-balance limit for single junctions. Today, however, their performance is limited by large deficits in open-circuit voltage and unfulfilled potential in both short-circuit current density and fill factor in the wide-bandgap perovskite sub cell. Here we find that halide heterogeneity — present even immediately following materials synthesis — plays a key role in interfacial non-radiative recombination and collection efficiency losses under prolonged illumination for Br-rich perovskites. We find that a diammonium halide salt, propane-1,3-diammonium iodide, introduced during film fabrication, improves halide homogenization in Br-rich perovskites, leading to enhanced operating stability and a record open-circuit voltage of 1.44 V in an inverted (p–i–n) device; ≈86% of the detailed-balance limit for a bandgap of 1.97 eV. The efficient wide-bandgap sub cell enables the fabrication of monolithic all-perovskite triple-junction solar cells with an open-circuit voltage of 3.33 V and a champion PCE of 25.1% (23.87% certified quasi-steady-state efficiency).
DOI: 10.1038/s41560-023-01406-5
October 2024:
Microstructure and reflectance of porous GaN distributed Bragg reflectors on silicon substrates
S. Ghosh, M. Sarkar, M. Frentrup, M. J. Kappers, and R. A. Oliver
J. Appl. Phys. 136, 043105 (2024)
Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage.
September 2024:
Microscopy studies of InGaN MQWs overgrown on porosified InGaN superlattice pseudo-substrate
Y. Ji, M. Frentrup, S. M. Fairclough, Y. Liu, T. Zhu, and R. A. Oliver
Semicond. Sci. Technol. 39, 085001 (2024)
In this study, possible origins of small V-pits observed in multiple quantum wells (MQWs) overgrown on as-grown and porosified InGaN superlattice (SL) pseudo-substrates have been investigated. Various cross-sectional transmission microscopy techniques revealed that some of the small V-pits arise from the intersection of threading defects with the sample surface, either as part of dislocation loops or trench defects. Some small V-pits without threading defects are also observed. Energy dispersive x-ray study indicates that the Indium content in the MQWs increases with the averaged porosity of the underlying template, which may either be attributed to a reduced compositional pulling effect or the low thermal conductivity of the porous layer. Furthermore, the porous structure inhibits the glide or extension of the misfit dislocations (MD) within the InGaN SL. The extra strain induced by the higher Indium content and the hindered movement of the MDs combined may explain the observed additional small V-pits present on the MQWs overgrown on the more relaxed templates.
DOI: /10.1088/1361-6641/ad575b
August 2024:
Threshold voltage mapping at the nanoscale of GaN-based high electron mobility transistor structures using hyperspectral scanning capacitance microscopy
C. Chen, S. Ghosh, P. De Wolf, Z. Liang, F. Adams, M. J. Kappers, D. J. Wallis, and R. A. Oliver
Appl. Phys. Lett. 124 232107 (2024)
Hyperspectral scanning capacitance microscopy (SCM) measures (dC/dV - V) spectra at every XY location of a semiconductor sample surface area. We report its application to GaN-based high electron mobility transistor (HEMT) structures to map threshold voltage (Vth) at the nanoscale. The consistency between the conventional SCM data and the hyperspectral SCM data set of the same area on the HEMT surface provides evidence for the reliability of hyperspectral SCM. We developed a method to extract a map of Vth distribution across the surface of the HEMT structure at the nanoscale from the hyperspectral SCM data set. The map reveals that most of the fissures (i.e., enlarged pits formed at threading dislocation surface endings) on the nitride sample surface reduce local Vth. Other variations in Vth in regions free of the fissures could be a result of thickness and/or composition inhomogeneities in the AlGaN barrier layer. Conventional SCM and other techniques cannot provide these detailed insights obtained through hyperspectral SCM.
July 2024:
Characterisation of the interplay between microstructure and opto-electronic properties of Cu(In,Ga)S2 solar cells by using correlative CL-EBSD measurements
Y. Hu, G. Kusch, D. Adeleye, S. Siebentritt, and R. A. Oliver
Nanotechnology 35 (29), 295702 (2024)
Cathodoluminescence and electron backscatter diffraction have been applied to exactly the same grain boundaries (GBs) in a Cu(In,Ga)S2 solar absorber in order to investigate the influence of microstructure on the radiative recombination behaviour at the GBs. Two different types of GB with different microstructure were analysed in detail: random high angle grain boundaries (RHAGBs) and Σ3 GBs. We found that the radiative recombination at all RHAGBs was inhibited to some extent, whereas at Σ3 GBs three different observations were made: unchanged, hindered, or promoted radiative recombination. These distinct behaviours may be linked to atomic-scale grain boundary structural differences. The majority of GBs also exhibited a small spectral shift of about ±10 meV relative to the local grain interior (GI) and a few of them showed spectral shifts of up to ±40 meV. Red and blue shifts were observed with roughly equal frequency.
DOI: /10.1088/1361-6528/ad3bbd
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.
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.
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.
