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Paper of the Month

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.

DOI

 

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.

DOI

 

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.

DOI

 

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.

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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.

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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.

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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'HanlonA. 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.

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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.

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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.

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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.

DOI

 

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).

DOI

 

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.

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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.

DOI

 

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

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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)

pssb

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.

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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

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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.

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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.

Download open access paper

DOI

 

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.

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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.

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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.

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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.

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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.

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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)

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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)

 

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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)

 

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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)

 

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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)

 

DOI

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)

 

DOI

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)

DOI

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

 

DOI

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)

DOI

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)

DOI

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)

DOI

 

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