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III-nitride materials, such as GaN and its alloys, are essential for modern microelectronics and optoelectronics due to their unique properties. Focused ion beam (FIB) techniques play a crucial role in their prototyping and characterization at the micro- and nanoscale. However, conventional FIB milling with Ga ions presents challenges, including surface amorphization and point defect introduction, prompting the exploration of alternative ion sources. Xenon-based inductively coupled plasma or plasma FIB has emerged as a promising alternative, offering reduced damage and better sample property preservation. Despite extensive research on FIB-induced damage in GaN, systematic comparisons between Ga and Xe ion milling on the luminescence characteristics of GaN remain limited. This study aims to fill this gap by evaluating and comparing the extent of FIB-induced damage caused by Ga and Xe ions in wurtzite and zincblende GaN through cathodoluminescence measurements. Our findings indicate that Xe ion milling yields higher integrated intensities compared to Ga ion milling, attributed to shallower implantation depths and reduced lattice disorder. We also observe a decrease in integrated intensity with increasing ion beam acceleration voltage for both wurtzite and zincblende GaN layers. This study provides valuable insights into optimizing FIB-based sample preparation techniques for III-nitride materials, with implications for enhancing device performance and reliability. [ABSTRACT FROM AUTHOR]

Distributed Bragg reflectors (DBRs) based on alternating layers of porous and non-porous GaN have previously been fabricated at the wafer-scale in heteroepitaxial GaN layers grown on sapphire substrates. Porosification is achieved via the electrochemical etching of highly Si-doped layers, and the etchant accesses the n+-GaN layers through nanoscale channels arising at threading dislocations that are ubiquitous in the heteroepitaxial growth process. Here, we show that the same process applies to GaN multilayer structures grown on silicon substrates. The reflectance of the resulting DBRs depends on the voltage at which the porosification process is carried out. Etching at higher voltages yields higher porosities. However, while an increase in porosity is theoretically expected to lead to peak reflectance, in practice, the highest reflectance is achieved at a moderate etching voltage because etching at higher voltages leads to pore formation in the nominally non-porous layers, pore coarsening in the porous layers, and in the worst cases layer collapse. We also find that at the high threading dislocation densities present in these samples, not all dislocations participate in the etching process at low and moderate etching voltages. However, the number of dislocations involved in the process increases with etching voltage. [ABSTRACT FROM AUTHOR]

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

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

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

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

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

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

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

The full width at half maximum (FWHM) of the luminescence of visible InGaN quantum well (QW) based emitters increases with wavelength. This broadening of the luminescence decreases the color saturation from 100% to about 70% up a wavelength shorter than 515 nm. For emission wavelengths longer than 515 nm, the saturation surprisingly increases again and reaches 95% beyond 560 nm. More important, in the yellow-red range, the perceived hue is strongly blue-shifted by a broad emission. This phenomenon is known as Abney effect, and it originates from the spectral sensitivities of the cone cells of the human eyes. As a consequence, for red InGaN QW based LEDs, the peak wavelength must be even further in the red. Based on a large set of QWs grown on different crystal orientations, we correlated the wavelength and the luminescence FWHM of InGaN/GaN QWs to build a model which predicts the perceived chromaticity, i.e., the apparent hue and saturation of yellow-red nitride LEDs. We also applied this model to (11-22), a-plane, and m-plane QWs and compared our data to the state-of-the-art of the literature. We concluded that the FWHM of the luminescence is a critical parameter to design and a further challenge for red InGaN-based light-emitting diodes. [ABSTRACT FROM AUTHOR]

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices. [ABSTRACT FROM AUTHOR]

The photoluminescence spectra of a zincblende GaN epilayer grown via metal-organic chemical vapour deposition upon 3C-SiC/Si (001) substrates were investigated. Of particular interest was a broad emission band centered at 3.4 eV, with a FWHM of 200 meV, which extends above the bandgap of both zincblende and wurtzite GaN. Photoluminescence excitation measurements show that this band is associated with an absorption edge centered at 3.6 eV. Photoluminescence time decays for the band are monoexponential, with lifetimes that reduce from 0.67 ns to 0.15 ns as the recombination energy increases. TEM measurements show no evidence of wurtzite GaN inclusions which are typically used to explain emission in this energy range. However, dense stacking fault bunches are present in the epilayers. A model for the band alignment at the stacking faults was developed to explain this emission band, showing how both electrons and holes can be confined adjacent to stacking faults. Different stacking fault separations can change the carrier confinement energies sufficiently to explain the width of the emission band, and change the carrier wavefunction overlap to account for the variation in decay time. [ABSTRACT FROM AUTHOR]

The efficiency of light emitting diodes (LEDs) remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper, the measured variations in the external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells (QWs) are shown to compare closely with the predictions of a revised ABC model, in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well (MQW) region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region. [ABSTRACT FROM AUTHOR]

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 InGaNbased 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. [ABSTRACT FROM AUTHOR]

In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves. [ABSTRACT FROM AUTHOR]

In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple quantum well structure containing the InGaN prelayer, originating from the first quantum well in the stack. Band structure calculations revealed that a reduction in the resultant electric field occurred in the quantum well immediately adjacent to the InGaN prelayer, therefore leading to a reduction in the strength of the quantum confined Stark effect in this quantum well. The partial suppression of the quantum confined Stark effect in this quantum well led to a modified (higher) emission energy and increased radiative recombination rate. Therefore, we ascribed the origin of the high energy emission band to recombination from the 1st quantum well in the structure. Study of the temperature dependent recombination dynamics of both samples showed that the decay time measured across the spectrum was strongly influenced by the 1st quantum well in the stack (in the sample containing the prelayer) leading to a shorter average room temperature lifetime in this sample. The room temperature internal quantum efficiency of the prelayer containing sample was found to be higher than the reference sample (36% compared to 25%) which was thus attributed to the faster radiative recombination rate of the 1st quantum well providing a recombination pathway that is more competitive with non-radiative recombination processes. [ABSTRACT FROM AUTHOR]

In this paper, we present a combined cathodoluminescence (CL) and electron beam induced current (EBIC) study of the optical and electrical properties of InGaN light emitting diodes grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2 T), whereas in the other device, each barrier was grown in a two step process with the first few nanometers at a lower temperature (Q2T). It was found that in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2 T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal. [ABSTRACT FROM AUTHOR]

Micron-scale mapping has been employed to study a contacted InGaN/GaN LED using combined electroluminescence (EL), cathodoluminescence (CL), and electron beam induced current (EBIC). Correlations between parameters, such as the EBIC and CL intensity, were studied as a function of applied bias. The CL and EBIC maps reveal small areas, 2-10 μm in size, which have increased nonradiative recombination rate and/or a lower conductivity. The CL emission from these spots is blue shifted, by 30-40 meV. Increasing the reverse bias causes the size of the spots to decrease, due to competition between in-plane diffusion and drift in the growth direction. EL mapping shows large bright areas (~100 μm) which also have increased EBIC, indicating domains of increased conductivity in the p and/or n-GaN. [ABSTRACT FROM AUTHOR]

We have studied the carrier recombination dynamics in an InGaN/GaN multiple quantum well structure as a function of emission energy and excitation density between temperatures of 10K and 100K. Under relatively low levels of excitation, the photoluminescence (PL) intensity and decay time of emission on the high energy side of the luminescence spectrum decrease strongly between 10K and 50 K. In contrast, for emission detected on the low energy side of the spectrum, the PL intensity and decay time increase over the same temperature range. These results are consistent with a thermally activated carrier redistribution process in which the (temperature dependent) average timescale for carrier transfer into or out of a localised state depends on the energy of the given state. Thus, the transfer time out of shallow, weakly localised states is considerably shorter than the arrival time into more deeply localised states. This picture is consistent with carriers hopping between localisation sites in an uncorrelated disorder potential where the density of localised states decreases with increasing localisation depth, e.g., a exponential or Gaussian distribution resulting from random alloy disorder. Under significantly higher levels of excitation, the increased occupation fraction of the localised states results in a greater average separation distance between unoccupied localised states, causing a suppression of the spectral and dynamic signatures of the hopping transfer of carriers. [ABSTRACT FROM AUTHOR]

We have performed a detailed study of the impact of basal plane stacking faults (BSFs) on the optical properties of both a-plane InGaN/GaN quantum wells (QWs) and GaN template samples grown on r-sapphire. In particular, we have used polarised photoluminescence excitation spectroscopy (P-PLE) to investigate the nature of the low temperature recombination as well as extracting information on the valence band (VB) polarisation anisotropy. Our low temperature P-PLE results revealed not only excitons associated with intersubband quantum well transitions and the GaN barrier material but also a transition associated with creation of excitons in BSFs. The strength of this BSF transition varied with detection energy across the quantum well emission suggesting that there is a significant contribution to the emission line width from changes in the local electronic environment of the QWs due to interactions with BSFs. Furthermore, we observed a corresponding progressive increase in the VB splitting of the QWs as the detection energy was varied across the quantum well emission spectrum. [ABSTRACT FROM AUTHOR]

Semi-polar group III nitrides and their alloys (AlGaN, InGaN) show great promise for future opto-electronic devices. For these orientations, specific X-ray diffraction (XRD) methods have been developed to measure the alloy content. The XRD methods proposed in the literature all use approximations at different levels. Here, we introduce a novel exact model, against which we compare each simplifying assumption previously used. The relevant approximations are then assembled to produce an accurate linearized model, which shares the same mathematical form as that of the standard polar or non-polar nitride analysis. This linearized model can be further simplified to provide an analytic expression for correction of the alloy content when a tilt (partial relaxation) is found between the alloy epilayer and the semi-polar GaN template. For a given model-alloy content and tilt angle- we can compute the expected XRD data. Vice-versa, the alloy content can be obtained from the experimental measurements-tilt angle and difference in d-spacing. This work focuses on the typical semi-polar planes studied in the literature: inclined a-planes ([formula]), e.g., ([formula]), and inclined m-planes ([formula]), e.g., ([formula]) or ([formula]). [ABSTRACT FROM AUTHOR]

There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum well light emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localization induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localization. Over the range of excitation power density where the depth of the S shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively. [ABSTRACT FROM AUTHOR]

The growth of high density vertically aligned carbon nanotube forests on conductive CoSi2 substrate layers is characterized by in situ x-ray photoemission spectroscopy and x-ray diffraction. We use in situ silicidation to transform as loaded, low conductivity CoSi supports to highly conductive CoSi2 during nanotube growth. These cobalt silicide films are found to be stable against oxidation and carbide formation during growth and act as an excellent metallic support for growth of aligned nanotubes, resembling the growth on the insulating Fe/Al2O3 benchmark system. The good catalytic activity is attributed to interfacial reactions of the Fe catalyst particles with the underlying CoSi2 support. We obtain ohmic conduction from the support layer to the carbon nanotube forest. [ABSTRACT FROM AUTHOR]

Time-integrated and time-resolved microphotoluminescence studies have been performed on Inx Ga1-xN quantum disks at the tips of GaN nanocolumns. The results are analyzed in the context of current theories regarding an inhomogeneous strain distribution in the disk which is theorized to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. It is concluded that no lateral separation of carriers occurs in the quantum disks under investigation. Internal field screening by an increased carrier density in the QDisks at higher excitation densities is observed via a blue-shift of the emission and a dynamically changing decay time. Other possible explanations for these effects are discussed and discounted. Cathodoluminescence studies have also been carried out on the nanocolumns to provide insight into the physical origin of the luminescence. [ABSTRACT FROM AUTHOR]

Low temperature cathodo- and photoluminescence has been performed on nonpolar a-plane GaN films grown using epitaxial lateral overgrowth. In films overgrown at a low V–III ratio, the emission spectrum is dominated by “yellow” and “blue” luminescence bands, attributed to recombination at point defects or impurities. The intensity of this emission is observed to decrease steadily across the window region along the -c direction, possibly due to asymmetric diffusion of a point defect/impurity species. When overgrown at a higher V–III ratio, the near band edge and basal-plane stacking fault emission intensity increases by orders of magnitude and a donor–acceptor pair band is observed. Using monochromatic cathodoluminescence imaging, the various emission features are correlated with the microstructure of the film. In particular, the peak energy of the basal-plane stacking fault emission is seen to be blueshifted by ∼15 meV in the wing relative to the window region, which may be related to the different strain states in the respective regions. [ABSTRACT FROM AUTHOR]

Four samples were grown by epitaxial lateral overgrowth (ELOG) using different magnesium precursor fluxes during the coalescence stage. These samples were studied in cross section using scanning capacitance microscopy and scanning electron microscopy. The resulting images revealed the existence of several differently doped regions in the ELOG structures including n-type doping in the GaN initially grown through the ELOG mask’s windows, and undoped GaN after coalescence had been completed. In addition, samples for which a magnesium precursor flux had been present during the coalescence of the GaN stripes also exhibited p-type doping. From an analysis of the spatial distribution of the unintentional doping in the ELOG material, it has been possible to propose that the incorporation of the n-type dopant was slower on the (0001) facet than the [formula] facets. This facet dependent difference in incorporation rates also helps to explain the nonuniformity in thickness of n-type conductive layers seen at the GaN/sapphire interface when a three-dimensional growth mode is employed in the early stages of growth. [ABSTRACT FROM AUTHOR]

The three-dimensional atom probe has been used to characterize green- and blue-emitting InxGa1-xN/GaN multiple quantum well structures with subnanometer resolution over a 100 nm field of view. The distribution of indium in InxGa1-xN samples with different compositions is analyzed. No evidence is found wherein the indium distribution deviates from that of a random alloy, which appears to preclude indium clustering as the cause of the reported carrier localization in these structures. The upper interface of each quantum well layer is shown to be rougher and more diffuse than the lower interface, and the existence of monolayer steps in the upper interfaces is revealed. These steps could effectively localize carriers at room temperature. Indium is shown to be present in the GaN barrier layers despite the absence of indium precursor flux during barrier layer growth. A strong evidence is produced to support a mechanism for the presence of indium in these layers, namely, that a layer of indium forms on the surface of the growing InxGa1-xN quantum well, and this layer then acts as a source of indium during GaN barrier layer growth. [ABSTRACT FROM AUTHOR]

Gross well-width fluctuations are observed in InGaN quantum wells (QWs), and have been linked to an overall three-dimensional morphology consisting of a network of interlinking InGaN strips. While these networks may be clearly observed in atomic force microscopy of appropriate uncapped epilayers, the surface morphology of capped (QW) samples reveals little or nothing of the subsurface structure of the InGaN. Hence transmission electron microscopy (TEM) must be used to determine the variations in QW thickness and composition. The network structure may be observed in either bright-field TEM or scanning transmission electron microscope high angle annular dark field images in plan-view studies of single QWs. It is suggested that multi-QWs could be imaged in cross section and the anisotropy of the network structure used as a potential signature of the three-dimensional morphology. The anisotropic nature of the network structure correlates well with the alignment of step edges in the GaN pseudosubstrates, and it is suggested that the network structure forms via the decomposition of In-rich regions arising at or near step edges. [ABSTRACT FROM AUTHOR]

Medium energy ion scattering (MEIS) has been used to measure at the monolayer scale the strain profile of self-organized GaN quantum dots grown on (11–20) or a-plane AlN by molecular-beam epitaxy. By confronting the MEIS results with a structural analysis carried out by atomic force microscopy, it is established that the strain profile is anisotropic, i.e., fully elastic along [1–100] and a combination of plastic and elastic along [0001]. High resolution transmission electron microscopy measurements reveal the presence of misfit dislocations with 1/2 [0001] Burgers vector, consistent with MEIS data. [ABSTRACT FROM AUTHOR]