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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.

We report on the characterization of the intrinsic, anisotropic, dielectric properties of wurtzite gallium nitride in the spectral range of 0.5–11 THz, using terahertz time-domain spectroscopy. The ordinary ( e ˜ ⊥ ) and extraordinary ( e ˜ ∥ ) components of the complex dielectric function were determined experimentally for a semi-insulating, m-plane gallium nitride single crystal, providing measurements of the refractive indices ( n ⊥ , ∥ ) and absorption coefficients ( α ⊥ , ∥ ) . These material parameters were successfully modeled by considering the contribution of the optical phonon modes, measured using Raman spectroscopy, to the dielectric function, giving values for the relative static dielectric constants of e 0 ⊥ = 9.22 ± 0.02 and e 0 ∥ = 10.32 ± 0.03 for wurtzite gallium nitride.

The authors image the spatial dependent luminescent properties of InGaN quantum wells (QWs) in light-emitting diodes (LEDs) using fluorescence microscopy with selective excitation of the QWs through the transparent sapphire substrate. The authors measure strong carrier escape with the associated photovoltaic effect in the device under open and short circuit conditions. The addition of electrical contacts allows comparison of the images under both optical and electrical excitation. A lateral distribution of the junction potential is measured in LEDs with structured metal contacts. An ohmic contact creates an equipotential surface and influences the collective emission. The technique offers useful insights into the spatial properties of the recombination processes in InGaN materials and LED fabrication processes at low forward bias.

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) InxGa1−xN 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.

The mechanisms governing efficiency droop in an In0.18Ga0.82N/GaN multiple quantum well structure were investigated using a combination of ultrafast time-resolved terahertz and photoluminescence spectroscopy. From excitation fluence dependent studies, a reduction in the room temperature photoluminescence efficiency to 3% of its maximum value was observed for an excitation fluence of 0.96 mJcm-2. A correlation was found between the onset of efficiency droop and the emergence of a peak on the high-energy side of the quantum well emission with a 1/e decay time of 19.6 ps. These characteristics were attributed to the saturation of localised states and the population of higher energy delocalised states. Time-resolved studies revealed different scaling behaviours between the terahertz and photoluminescence decay dynamics, suggesting that the saturation of localised hole states may be playing a part in the onset of efficiency droop. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross-sectional scanning capacitance microscopy measurements of unintentionally doped model structures for InGaN quantum wells, GaN barriers and AlInN electron-blocking layers showed n -type conductivity for the In-containing layers. Secondary ion mass spectrometry indicated that oxygen impurities are the likely source of the electron density in the model layers. The n -type conductivity as well as the oxygen impurity level increases to ∼1018 cm-3 for AlInN lattice-matched to GaN. These results suggest that the background electron concentration due to oxygen impurities in indium-containing layers needs to be considered in the design and theoretical modelling of device structures. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 −1 0] 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.

The impact of the InGaN growth temperature and of the trimethylindium flux on trench defects has been investigated. We show that the density of defects is affected by both conditions but also their morphology and their emission properties. Therefore, the impact of such defect on the performance of quantum well structures can be controlled by adjusting these two growth conditions. Under usual conditions for making blue light emitting diodes, we observe that the enclosed region of the defect emits light at a longer wavelength. Nevertheless, our data also demonstrate that emission at a shorter wavelength is possible under certain growth conditions.

In this paper we describe the implementation and the characterisation of five different in-situ defect reduction techniques for non-polar (a -plane) GaN growth on r -plane sapphire. Sample 1 (3D/2D) employs a methodology frequently applied on the c -plane, involving a low temperature nucleation layer (LTNL) followed by 3D GaN island formation and lateral coalescence. For Sample 2 (d3D) GaN islands are grown directly onto the sapphire with no LTNL, followed by lateral growth. Sample 3 (d3D Si) follows a similar procedure, but with high silicon doping in order to adjust the 3D GaN island shape. Sample 4 (SiNx) utilises a silicon nitride interlayer between a LTNL and subsequent growth of a GaN layer. Sample 5 is grown by epitaxial lateral overgrowth (ELOG) coupled with a SiNx interlayer. X-ray diffraction, scanning electron microscopy and cathodoluminescence are used to identify defects, and determined the threading defect density to vary from 1 x 1010–1 x 109 cm–2 and basal-plane stacking fault (BSF) density to vary from 5 x 105 – 5 x 103 cm-1. The improvement in crystal quality is reflected in the photoluminescence spectra by a comparison of the ratio of the GaN near band edge (NBE) emission to the BSF associated emission. It was determined that the ELOG method was most successful in blocking BSFs, with a density reduction of 2 orders of magnitude resulting in a fifteen-fold increase in the NBE:BSF emission ratio increase. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the low temperature time-integrated and time-resolved photoluminescence properties of an (0001) InGaN/GaN multiple quantum well structure. From excitation power dependent studies, we observe a clear correlation between the onset of efficiency droop and the broadening of the time-integrated luminescence spectra. Time-resolved spectroscopy reveals that this spectral broadening is associated with a large and rapid red-shift of the spectrum with increasing time after the excitation pulse. This dynamic shift of the spectrum also modifies the form of the luminescence transients, yet the spectrally integrated luminescence dynamics have the same non-exponential shape as those obtained under much lower excitation levels. This observation is consistent with the emission originating from the recombination of independently localised carriers, even under very high excitation levels. Nevertheless, at these high excitation levels we propose that there also is a population of weakly localised carriers, probably electrons. We suggest that these more mobile carriers are effective in screening the built-in electric field across the quantum well as well as being more susceptible to non-radiative loss. The collapse in this population, due to radiative and non-radiative recombination, restores the built-in polarisation induced field and results in the observed time dependent redshift of the luminescence spectrum. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN quantum dot (QD) structures have been grown by metal organic vapour phase epitaxy (MOVPE) on non-polar (11-20) GaN surfaces by employing an anneal step in nitrogen immediately after the growth of the InGaN. Here, we compare the growth of such structures on pseudo-substrates grown using an epitaxial lateral overgrowth (ELOG) technique and on pseudo-substrates grown using a simpler in situ SiNx interlayer. The less complex defect reduction approach results in a significantly higher defect density, but does not detrimentally effect the QD formation. For both types of pseudo-substrate, sharp peaks with resolution limited widths are observed in both cathodoluminescence at 9 K and micro-photoluminescence at 4.2 K. The QDs demonstrate significantly reduced exciton lifetimes compared to structures grown on c-plane, which has advantages for possible applications in single photon sources. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O'Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O'Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290–i03–06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.

The effects of the threading dislocation (TD) density on the optical properties of a series of comparable InxGa1–xN/ GaN multiple QW structures were studied. The TD density ranged from 2 × 107 cm–2, for a structure grown on a free-standing GaN substrate, to 5 × 109 cm–2 grown on a sapphire substrate. Room temperature internal quantum efficiencies (IQEs) were determined by temperature dependent photoluminescence (PL); no systematic dependence of the IQE on the TD density was found. The excitation power density dependence of the efficiency was investigated, which also showed no systematic dependence on TD density. PL excitation spectroscopy was used to verify that equivalent carrier densities were generated within the QWs of each structure. The lack of systematic dependence of the optical properties on TD density is attributed to the strong carrier localisation in InGaN/GaN QWs. At the highest density of TDs studied, it is estimated that the average defect separation greatly exceeds the in-plane diffusion lengths of electrons and holes; consequently the majority of carriers in the system are isolated from the TDs. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have studied the carrier dynamics in an InGaN/GaN multiple quantum well as a function of recombination energy between temperatures of 10 K and 100 K. For emission detected on the high energy side of the luminescence spectrum, the luminescence intensity and its decay time decrease strongly between 10 K and 50 K. In contrast, for emission detected on the low energy side of the spectrum, both of these quantities increase over the same temperature range. We attribute this behaviour to the thermally activated redistribution of carriers via energy-loss hopping between localised states. On the basis of the respective changes in decay timescales for shallow and deep states, the average energy-loss hopping times are inferred to increase with increasing energy depth of localisation and are thus consistent with spatially uncorrelated localisation within a Gaussian or exponential density of localised states. Under higher excitation densities this behaviour is suppressed due to the onset of saturation of the localised states. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we report on the effects of including of an InGaN prelayer in an InGaN/GaN multiple quantum well (MQW) structure. Two equivalent 10 period In0.16Ga0.84N MQW structures were studied, one with a 24 nm thick In0.05Ga0.95N prelayer, one without. X-ray diffraction measurements showed no evidence of strain relaxation in the structure containing the prelayer, while AFM revealed a 15% increase in average diameter of “V”-defects. Temperature dependent photoluminescence (PL) showed the MQW structure, with the prelayer, to exhibit a higher room temperature internal quantum efficiency than the reference structure. PL excitation spectroscopy revealed an absorption and carrier transfer process associated with the InGaN prelayer, which created an additional recombination pathway resulting in a feature on the high energy side of the PL spectrum. Time-resolved PL studies showed that this recombination had a weaker temperature sensitivity to that of the primary luminescence feature, and thus may be responsible for the enhanced PL intensity measured at 300 K. Monochromatic cathodoluminescence (CL) images showed the high energy feature originated from the “V”-defects. It is thus suggested that the InGaN prelayer may provide a transfer mechanism for carriers in the prelayer into the semi-polar QWs on the “V”-defect facets. Alternatively the high energy emission may arise from QWs at the base of the stack that have parameters that are significantly different from the rest of the QWs. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of the miscut of a (0001) c -plane substrate on the structural and optical properties of InGaN/GaN quantum wells grown by metal-organic vapour phase epitaxy using a two-temperature method has been investigated. The two-temperature growth method involves exposure of the uncapped InGaN quantum well to a temperature ramp in an ammonia atmosphere before growth of the GaN barrier at a higher temperature. The resulting quantum well, consists of interlinking InGaN strips containing gaps which may impede carrier diffusion to dislocations. By increasing the substrate misorientation from 0° to 0.5° we show that the density of InGaN strips increases while the strip width reduces. Our data show that the PL efficiency increases with miscut and that the peak efficiency occurs at a lower excitation power density.

We investigated the properties of a GaN epilayer grown by metalorganic vapour phase epitaxy on a c-plane bulk GaN substrate obtained by ammonothermal growth. X-ray diffraction measurements showed that the epilayer and substrate were fully relaxed, had a miscut angle of 0.3±0.05° towards m and had omega rocking curve width values of 20–30 arcsec, limited by the instrumental broadening. Scanning capacitance microscopy data of the sample in cross-section indicated that the substrate had n-type conductivity with a carrier concentration of at least 1019 cm−3. Combined optical Nomarski microscopy, atomic-force microscopy and scanning electron microscope-cathodoluminescence studies showed the presence of large hexagonal pyramids on the surface, each associated with one or two dislocations with a screw-component threading from the substrate. This observation leads us to calculate a lower limit of the threading dislocation density of 3×102 cm−2. We predict that the formation of such hexagonal hillocks during epitaxy can be avoided with a slightly larger miscut angle of 0.4° or 0.5°. Another type of defect observed were ridge-like surface structures with narrow arrays of edge-type threading defects with a local density of 109 cm−2. However, the absence of threading defects below the regrowth interface at a ridge suggested that this type of structure is linked to (polishing) damage to the substrate surface and is therefore rated as an avoidable problem.

Two solar cells based on an InGaN/GaN p–i–n hetero-junction, but having different dislocation densities, were fabricated and characterized. The structures were grown on c-plane (0001) GaN-on-sapphire templates with different threading dislocation (TD) densities of 5×108 and 5×109 cm−2. Structural characterization revealed the presence of V-defects in the InGaN epilayer. Since each V-defect was associated with a TD, the structural as well as the optical properties worsened with a higher TD density in the GaN/sapphire template. It was also found that additional dislocations were generated in the p-GaN layer over the V-defects in the InGaN layer. Because of its superior structural quality, the peak external quantum efficiency (EQE) of the low TD density sample was three times higher than that of the high TD density sample.

The structural and optical properties of trench defects, which are poorly understood yet commonly occurring defects observed on the surfaces of InGaN multiple quantum wells (MQW), are reported. These defects comprise near-circular trenches which enclose areas of MQW which give rise to a red shift in peak photoluminescence emission and a change in cathodoluminescence intensity with respect to the surrounding material. Atomic force microscopy shows that the height of trench-enclosed areas differs from that of the surrounding quantum well structure, and that trenches are unrelated to the commonly observed V-defects in InGaN films, despite being occasionally intersected by them. Cross-sectional electron microscopy analysis of trenches with raised centres suggests that the red shift in the observed cathodoluminescence peak emission may be due to the quantum wells being thicker in the trench-enclosed regions than in the surrounding quantum well area. The mechanism of trench formation and its implication for the control of the emission properties of light-emitting diodes is discussed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mg-doped GaN films have been grown on (0 0 0 1) sapphire using metal organic vapour phase epitaxy. Use of different buffer layer strategies caused the threading dislocation density (TDD) in the GaN to be either approximately 2×109 cm−2 or 1×1010 cm−2. Frequency-dependent capacitance and conductance measurements at temperatures up to 450 K have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Admittance spectroscopy of the films finds a single impurity-related acceptor level with an activation energy of 160±10 meV for [Mg] of about 1×1019 cm−3, and 120±10 eV as the Mg precursor flux decreased. This level is thought to be associated with the Mg acceptor state. The TDD has no discernible effect on the trap detected by admittance spectroscopy. We compare these results with cathodoluminescence measurements reported in the literature, which reveal that most threading dislocations are non-radiative recombination centres, and discuss possible reasons why our admittance spectroscopy have not detected electrically active defects associated with threading dislocations.

We have investigated the optical properties of a series of non-polar InGaN/GaN single quantum wells (QWs) grown on r-plane sapphire with QW widths ranging from 7 to 45 A. The emission is attributed to recombination in regions of the QW intersected by basal plane stacking faults. For QW widths

The purpose of this study is to use the two main techniques in analytical transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDXS) and energy-loss spectroscopy (EELS), to investigate the elemental distribution in different aluminium gallium nitride epitaxial layers grown by different methods. It is shown that an accurate EDX quantification is obtained when beam broadening and fluorescence of stray X-rays by the corundum substrate, Ga contamination due to focused ion beam (FIB) sample preparation and thickness-dependent k -factors are taken into account (T. Walther, Proc. EMAG 2009, Sheffield, J. Phys. Conf. Ser. 241, 012016 (2010) [1]). The EDXS quantification results are then compared to EELS quantification results. Both, analysis of Al and Ga K-lines by EDXS and analysis of Al K-line and Ga L-line in EELS can yield the Al content with a standard error of Δx ≈ 0.01–0.02 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron holography was used to measure the electrostatic potential distribution in an LED chip at high spatial resolution, from which the internal polarization field can be derived. A cross-sectional TEM lamella on a workable LED die containing five InGaN/GaN quantum wells (QWs) connected to electrodes that fit into an in-situ biasing TEM holder was prepared using a focused ion beam (FIB). Off-axis electron holography under Lorentz mode was carried out to study the p -MQW-n region and phase profiles were derived at different reverse biasing conditions. p -n contrast has been shown in the calibrated electrostatic potential mapping. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Mg-doped GaN films have been grown on sapphire by metalorganic vapour phase epitaxy. Two different buffer layer schemes between the film and the sapphire substrate were used, giving rise to differing threading dislocation densities (TDDs) in the GaN. Frequency-dependent capacitance and conductance measurements have been used to study the electronic states associated with the Mg doping, and to determine how these are affected by the TDD. Current–voltage and admittance spectroscopy measurements were carried out using Al/Au Schottky contacts and Ni/Au Ohmic contacts fabricated in a dot-and-ring pattern. Frequency-dependent measurements on these devices were corrected to remove the series resistance effects arising from the well known high resistivity of Mg-doped GaN using formulae derived from the equivalent parallel circuit model. Thermal admittance spectroscopy, in which the conductance is monitored as a function of temperature, verified the existence of a single impurity-related acceptor level in samples with different TDDs at 165 ± 10 meV. For all of the samples the extrapolated capture cross-section values were very small, in the range of ~ 10− 20 cm− 2, suggesting that the acceptor level could be very repulsive. Since admittance spectroscopy detects only majority carrier traps, we believe these defect signature values are most probably associated with the Mg acceptor state as they are very close to the results previously reported by other research groups using different techniques.

The growth of nonpolar a -plane GaN on r -plane sapphire by metalorganic vapor-phase epitaxy has been investigated using a low-temperature nucleation layer (LT-NL) and using direct growth without a LT-NL. The dislocation densities in a -plane GaN films can be reduced significantly from (6.2±0.3)×10 10 cm −2 to (2.1±0.3)×10 9 cm −2 using direct growth. The dislocation densities can also be reduced from (6.2±0.3)×10 10 cm −2 to (2.1±0.3)×10 9 cm −2 by introducing a SiN x interlayer into the film grown with a LT-NL. However, introducing a similar SiN x interlayer into the film grown without a LT-NL produces no further dislocation reduction. Both the SiN x interlayers and the growth without a LT-NL reduce the dislocation density through the formation of islands, which promote dislocation bending and annihilation.

Introducing a small flow of hydrogen during the metal organic vapour phase epitaxy of InAlN will moderately reduce the indium incorporation into an epilayer while significantly reducing the volume of metallic indium on the surface as droplets, without significantly changing the nanoscale surface structure. Comparison with previous results suggests that this is a better way of removing surface indium while minimising the reduction of incorporated indium than increasing the temperature, and is similarly effective to change the TMI:TMA ratio. These results are consistent with the presence of an indium adlayer on the InAlN surface during growth facilitating indium incorporation.

The electronic characteristics of semiconductor-based devices are greatly affected by the local dopant atom distribution. In Mg-doped GaN, the clustering of dopants at structural defects has been widely reported, and can significantly affect p-type conductivity. We have studied a Mg-doped AlGaN/GaN superlattice using transmission electron microscopy (TEM) and atom probe tomography (APT). Pyramidal inversion domains were observed in the TEM and the compositional variations of the dopant atoms associated with those defects have been studied using APT. Rarely has APT been used to assess the compositional variations present due to structural defects in semiconductors. Here, TEM and APT are used in a complementary fashion, and the strengths and weaknesses of the two techniques are compared.

InAlN is a relatively new addition to the palette of nitride semiconductor alloys, with potential applications in distributed Bragg reflectors and high electron mobility transistors. However relatively little is known about the effects of different growth conditions on InAlN’s structure and properties and more importantly what these effects can tell us about the surface processes of growth. Here we have investigated the effects of varying various metal fluxes. First, we varied the total fluxes of all the precursors while maintaining their ratios. This led to an increase in growth rate, of itself very desirable, but at the considerable cost of significantly roughened surfaces. Analysis of these surfaces using power spectral density functions suggests that they were all produced by a combination of stochastic roughening and smoothing by surface diffusion, suggesting that at a given temperature increasing the growth rate will always lead to roughening. In addition, we examined the effect of varying just the trimethylindium flux (and therefore varying the indium to gallium ratio). As this flux was increased the indium incorporation initially increased but then levelled off, and for further increases the amount of indium on the surface as droplets increases significantly, suggesting that there is a limit to the indium incorporation that than be achieved at a given temperature and pressure. This suggests that there are practical limits to simultaneously achieving high growth rates, high indium contents and low surface roughnesses.

In this paper we demonstrate a strain-driven GaN interlayer method to reduce dislocation densities in GaN grown on (111) oriented silicon by metal organic vapour phase epitaxy (MOVPE). In order to achieve crack-free GaN layers of reasonable thicknesses and dislocation densities it is crucial to integrate both dislocation reduction and strain management layers. In contrast to techniques like FACELO or nanoELO we show the in situ formation of GaN islands directly on the AlN nucleation layer without the need to deposit a SiO 2 or SiN x mask. A graded AlGaN layer for strain management can be grown on top of this dislocation reducing 3D GaN inter-layer in order to achieve crack-free GaN layers grown on top of the AlGaN strain management layer. Furthermore, an additional SiN x layer for subsequent dislocation reduction can also be incorporated into the structure and is shown to efficiently reduce the dislocation density down to the low 10 9 cm -2 . The structural properties of the 3D GaN island buffer layer and overgrown samples are studied by means of SEM, cross-sectional, and plan view TEM. Cathodoluminiscence in an SEM is employed to correlate the dislocation microstructure as observed by plan view TEM with luminescent properties.

The convergent beam electron diffraction (CBED) methodology was developed to investigate the lattice distortions in wurtzite gallium nitride (GaN) from a single zone-axis pattern. The methodology enabled quantitative measurements of lattice distortions (alpha, beta, gamma and c) in transmission electron microscope (TEM) specimens of a GaN film grown on (0,0,0,1) sapphire by metal-organic vapour-phase epitaxy. The CBED patterns were obtained at different distances from the GaN/sapphire interface. The results show that GaN is triclinic above the interface with an increased lattice parameter c. At 0.85 microm from the interface, alpha=90 degrees , beta=8905 degrees and gamma=11966 degrees . The GaN lattice relaxes steadily back to hexagonal further away from the sapphire substrate. The GaN distortions are mainly confined to the initial stages of growth involving the growth and the coalescence of 3D GaN islands.

An interesting recent development in the Group III nitrides is the growth of InAlN lattice matched to GaN, with applications in distributed Bragg reflectors (DBRs), high electron mobility transistors (HEMTs) and as etch-layers. This work presents a systematic study of the effects of changing the key growth conditions of ammonia flux and growth temperature in InAlN growth by metal-organic vapour phase epitaxy (MOPVE) and describes our current optimised parameter set. We also particularly concentrate on the details of surface morphology assessed by atomic force microscopy (AFM). The nanoscale surfaces are characterised by low hillocks and dislocation pits, while at a larger scale microscopic indium droplets are also present. However, these droplets are eliminated when the layers are capped with GaN. Other trends observed are that increasing the growth temperature will lower the indium incorporation approximately linearly at a rate of approximately 0.25% per °C, and that increasing the ammonia flux from 44.6 to 178.6 mmol min−1 increased the indium incorporation, but further increases to 446 mmol min−1 did not result in any further increase.

This work assesses the relative effectiveness of four techniques to reduce the defect density in heteroepitaxial nonpolar a -plane GaN films grown on r -plane sapphire by metalorganic vapour phase epitaxy (MOVPE). The defect reduction techniques studied were: 3D–2D growth, SiN x interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Plan-view transmission electron microscopy (TEM) showed that the GaN layer grown in a 2D fashion had a dislocation and basal-plane stacking fault (BSF) density of (1.9±0.2)×10 11 cm −2 and (1.1±0.9)×10 6 cm −1 , respectively. The dislocation and BSF densities were reduced by all methods compared to this 2D-grown layer (used as a seed layer for the interlayer and ELOG methods). The greatest reduction was achieved in the (0 0 0 1) wing of the ELOG sample, where the dislocation density was 6 cm −2 and BSF density was (2.0±0.7)×10 4 cm −1 . Of the in-situ techniques, SiN x interlayers were most effective: the interlayer with the highest surface coverage that was studied reduced the BSF density to (4.0±0.2)×10 5 cm −1 and the dislocation density was lowered by over two orders of magnitude to (3.5±0.2)×10 8 cm −2 .

Unintentional n-type doping is commonly observed to occur during the growth of nominally undoped GaN on sapphire. Scanning capacitance microscopy reveals that a layer of GaN adjacent to the GaN/sapphire interface is n-type, whilst the remainder of the epilayer appears insulating. If two-dimensional GaN growth is commenced immediately after the growth of the nucleation layer, then the unintentionally doped layer is not present and the material is highly resistive. However, such samples have a high threading dislocation density (ca. 5×109 cm−2). To reduce the threading dislocation density, the GaN nucleation layer is annealed and growth is then continued at a low V:III ratio to promote three-dimensional island formation. These islands are eventually coalesced using an increased V:III ratio and temperature. Longer coalescence times lead to lower threading dislocation densities. The width of the unintentionally n-doped layer is found to increase as the coalescence time increases, whilst the carrier concentration in the layer is not observed to change. The dopant in the unintentionally conductive layer is shown, using secondary ion mass spectrometry, to be oxygen.

The structural and optical properties of a series of GaN/Al0.18Ga0.82N multiple quantum well (MQW) structures, in which the well thickness was varied between 2 and 8 nm, grown on a-plane (1 1 0) GaN on r-plane (1 0 2) sapphire substrates have been investigated. High-resolution X-ray diffraction and low-angle X-ray reflectivity measurements were used to determine the well and barrier thicknesses and the barrier composition after matrix transformation of the (binary) elastic constants for the appropriate coordinates, and assuming a pseudo-morphically strained system. The microstructure of the (1 1 0) samples is dominated by I1-type basal-plane stacking faults (BSF) terminated by partial dislocations or prismatic stacking faults, as determined by conventional and high-resolution transmission electron microscopy. The low temperature photoluminescence (PL) spectra of the QW structures show two emission bands which are assigned (partly based on photoluminescence excitation (PLE) spectroscopy) to excitons that are confined solely by the quantum wells and, at lower energy, those carriers that recombine in the region where the wells are intersected by BSFs. Both bands shift to higher energy with decreasing quantum well thickness. The optical data indicate that the non-polar (1 1 0) GaN/AlGaN system is free of polarization-induced electric fields, since the QW exciton emission energy is not below the band-edge emission energy of the GaN template.

We have observed oscillations in the low temperature (T < 50 K) photoluminescence excitation spectra of an InGaN/GaN single quantum well at photon energies above the GaN band-gap. We attribute the features in the spectra to excitation of electrons at multiples of the LO phonon energy above the GaN conduction band edge. The rapid cooling of these electrons; to the GaN conduction band edge and their subsequent capture leads to a, shift, in the photolumineseence, spectrum and hence-the oscillations in photoluminescence. We interpret the shift in the spectrum as being due to a modification of the occupation of the distribution of localization centres by electrons. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In our earlier studies, we showed that the InxGa1–xN quantum wells in some green-emitting commercial light emitting diodes are highly non-uniform, containing gross width fluctuations on a 50–100 nm lateral scale. We reproduced such fluctuations in a research laboratory context by either annealing the InxGa1–xN epilayer, or subjecting it to a temperature ramp prior to growth of the GaN cap. We showed that the width fluctuations relate to a network of interlinking InxGa1–xN strips, and that threading dislocations tend to intersect this network between the strips. Here we investigate whether such networks may also be formed under a range of other growth conditions. By examining very thin InxGa1–xN epilayers which are expected to be fully strained, and showing they form networks upon thermal treatment, we rule out any role of strain relaxation in network formation. By examining samples grown at temperatures up to 790 °C, with low indium contents, and observing the formation of a network structure, we eliminate the possibility that spinodal decomposition is involved in network formation. The growth conditions examined are relevant to blue and ultra-violet light emitters, and we thus suggest that network structures may have broader implications than previously realised. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a study of the effect of externally applied vertical electric field on the optical properties of single InGaN/GaN quantum dots via microphotoluminescence spectroscopy. This is achieved by incorporating the quantum dot layer in the intrinsic region of a p-i-n diode structure. We observe a large blue energy shift of similar to 60 meV, which is explained by the partial compensation of the internal piezoelectric field. The energy shift dependence on the applied field allows the determination of the vertical component of the permanent dipole and the polarizability. We also present theoretical modelling of our results based on atomistic semi-empirical tight-binding simulations. A good quantitative agreement between the experiment and the theory is found. (C) 2007 Elsevier Ltd. All rights reserved.

Two methods for GaN growth on sapphire by metal-organic vapour phase epitaxy are discussed. The first involves only two-dimensional (2D) growth, and results in a high dislocation density, but also a high electrical resistivity. The second involves initial growth of three-dimensional (3D) islands employing a low V:III ratio, followed by island coalescence at a high V:III ratio. It is often assumed that threading dislocations (TDs) form via the coalescence of 3D islands, but detailed atomic force microscopy studies on partially coalesced samples find no evidence of an increased TD density at coalescence boundaries, suggesting that other possible origins for TDs should be considered. The 3D–2D growth method allows TD densities as low as 1.1 × 108 cm−2 to be achieved, but unlike the 2D growth samples these layers are not highly resistive. Scanning capacitance microscopy is used to demonstrate the presence of an unintentionally doped layer close to the GaN/sapphire interface. To simultaneously achieve a reduced TD density compared to 2D growth samples and a high resistivity, a high temperature AlN buffer layer may be employed.

Gallium nitride was deposited using metal-organic vapour-phase epitaxy (MOVPE) on Si(1 1 1) substrates, using buffer layers of scandium nitride grown by gas-source molecular beam epitaxy (GS-MBE). A series of ScN buffer layers with varying roughness, thickness and crystallinity were used. The buffer layer crystallinity was shown to affect the morphology, residual strain and crystallinity of the subsequently grown GaN as well as the degree of wetting of ScN by GaN. The orientation of the GaN with respect to the ScN and the Si substrate was (0 0 0 1) GaN ∥(1 1 1) ScN ∥(1 1 1) Si , [0 1 1¯ 0] GaN ∥[1 1 2¯] ScN ∥[2 1¯ 1¯] Si . GaN grown directly onto ScN buffer layers at 1020 °C displayed island growth with limited wetting of the ScN. The reduced ScN–GaN interfacial area resulted in the growth of dislocation-free GaN islands of several microns in diameter. Predeposition of small amounts of GaN on the ScN at either 750 or 540 °C promoted wetting of the ScN by GaN and hence increased coalescence, allowing a continuous film to form; however, the increased ScN–GaN interfacial area resulted in the presence of threading dislocations. These were predominantly screw and mixed-type dislocations.

We demonstrate how growth processes affect on ZnO film properties, which are to be essential guides to prevent defect formation in order to synthesize reproducible high quality ZnO films. First, we reveal that deposition at a low temperature is indispensable to transfer underlying GaN atomic terraces to ZnO surface. As the film thickness is increased, however, the terraces disappear to develop island morphology. It is found that the thick film surface is smoothed to the extent that atomic terraces can be seen after an appropriate thermal treatment. Adverse effects associated with high annealing temperatures are then demonstrated as evidenced by cracks formation, increased yellow cathode-luminescence and intermixing at the interface.

Scanning capacitance microscopy (SCM), scanning spreading resistance microscopy (SSRM) and secondary electron emission within a scanning electron microscope (SEM) were used to image two magnesium-doped GaN samples, grown under identical conditions. In only one of these samples were the dopants activated by annealing. Under normal imaging conditions for SCM and SSRM, no discernable change in contrast was observed in the unactivated sample, whilst for SEM imaging contrast increased over ca. 40 s of imaging time, indicating some activation of the dopants by the electron beam. In addition, various sample preparation techniques were attempted in order to minimise the roughness of the GaN cross-section used for SCM imaging. Some techniques which reduce the surface roughness lead to a poor SCM signal-to-noise ratio. However, a cleaving technique is described which produces suffi-ciently large and flat areas within multi-layered GaN specimens for the extraction of useful SCM data. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A model based on the overall energy balance is used to calculate the critical thickness for an InGaN epitaxial layer on a GaN substrate. The critical thickness values as a function of the indium content are found to be lower than values predicted by models proposed by Fischer or People and Bean. We also used the energy balance model to estimate the effect of the hexagonal symmetry of wurtzite materials on the critical thickness; this results in reduction of the critical thickness by as much as 20% of its corresponding isotropic value. From the small amount of experimental data available we conclude that the energy balance model is more appropriate for describing the critical thickness of the InGaN/GaN material system than the models developed by Fischer or People and Bean.

Builiding on earlier studies of single InGaN quantum dots (QDs), we are considering their potential for use in blue- and green-emitting single photon sources. Envisaging a device based on a resonant cavity light emitting diode, we have studied the effect of growing QDs on an underlying AlN/GaN distributed Bragg reflector, and have shown that enhanced single QD emission may be obtained. Additionally, we have studied the effect of the growth and activation of a p-type cap on an underlying QD layer and have shown that the QDs survive the anneal process.

The recent development of a commercial band-gap thermometry system for wide band-gap materials such as GaN (the k-Space Associates’ “ BandiT TM ”) has allowed a systematic study of the relationship between pyrometric or thermocouple temperature-monitoring methods and the directly determined layer temperature. The wide band gap of GaN gives a weak and sample-dependent coupling to radiant heat sources, and it is common in molecular beam epitaxy (MBE) to use a sputtered Mo coating on the rear of the sapphire substrate to improve the efficiency and consistency of heat transfer. We have investigated the role of this backing layer and the use of a PBN diffuser in an MBE chamber, but similar results are expected to be obtained from band-gap thermometry in a metal-organic vapour phase epitaxy (MOVPE) system. The wide range spectrometer used for the band-edge determination can also be employed in a pyrometric mode, at wavelengths both within the band gap of GaN and above it. The latter is insensitive to thickness oscillations, and is less affected by the presence or absence of a Mo backing layer. Results are presented comparing all these measurement techniques, and analysed to show that the Mo backing does not significantly increase the power coupling into the epitaxial layer, although it does improve the accuracy of pyrometric measurements.

Semi-polar (1122) GaN films have been grown by organometallic vapour-phase epitaxy (OMVPE) on m-plane (1100) sapphire. The in-plane orientation of the GaN with respect to the sapphire substrate was determined to be [1 100] GaN ∥[1 120] sapphire and the projections of [0001] GaN and [1120] GaN ∥[0001] sapphire . The smooth planar films displayed a distinct in-plane anisotropic mosaicity with the symmetric (1122) reflection along the GaN m-direction (XRD ω FWHM of 1080arcsec) broader than that along the projected c-direction (665 arcsec). TEM analysis indicated the presence of line defects threading through the layer but few stacking faults. Two semi-polar (1122) InGaN/GaN 10-period quantum well structures with wells thicknesses (alloy compositions) of 3.4nm (12%) and 3.9nm (19%) showed strong photoluminescence peaks at 430 and 500 nm, respectively.

Films of Sc and Ti (5 and 20 nm thick) were deposited on 500 nm GaN/Al 2 O 3 templates, then annealed in mixed NH 3 and H 2 at 1070°C in an MOCVD reactor to produce ScN and TiN films. This resulted in a dense array of holes in the TiN layers, but had little effect on ScN layers. The dislocation densities in GaN grown on the masked templates were reduced significantly from 5.0±0.5 x 10 9 cm -2 on the templates to 1.5±0.2 x 10 8 cm -2 on the thin TiN layers and were reduced almost a 100-fold to 6.4±0.7 x 10 7 cm -2 on the thin ScN layers, despite the absence of holes in the latter. TiN layers produced GaN films under compressive residual stress and ScN layers gave GaN films under tensile residual stress; the levels of stress varied with mask thickness. The masking layers therefore provide a convenient method to control both dislocation densities and wafer curvature, both of which are necessary for successful device fabrication.

We have studied the optical properties of a series of InGaN/AlInGaN 10-period multiple quantum wells (MQW) with differing well thickness grown by metal-organic vapor-phase epitaxy that emit at around 380 nm. The aim of this investigation was to optimise the room temperature internal quantum efficiency, thus the quantum well (QW) thicknesses were accordingly chosen so that the overlap of the electron/hole wave function was maximised. At low temperature, we observed a reduction of the photoluminescence decay time with decreasing well width in line with the theoretical predictions. For a structure with well thicknesses of 1.5 nm, we measured a photoluminescence internal quantum efficiency of 67% at room temperature with a peak emission wavelength of 382 nm.