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Monte Carlo simulation of exciton hopping reveals the features of the potential profile in AlGaN (with Al content of ∼26%) responsible for S-shaped and W-shaped temperature dependences of photoluminescence band peak position and bandwidth, respectively. One of the samples was grown using the conventional metal-organic chemical vapor deposition (MOCVD), while the second one was grown with the insertion of a buffer layer grown by migration enhanced MOCVD (MEMOCVDTM) technique. The potential profile is shown to be double-scaled in both Al0.26Ga0.74N epilayers, and the exciton hopping occurs through random potential fluctuations within isolated low-potential regions with the average energy of the localized states dispersed on the second scale. Both energy fluctuation scales were found to be in the range of 18-20 meV and were independent of the growth technique used. Meanwhile, the carrier lifetime was 30 ps in the sample grown by conventional MOCVD and 190 ps in the sample grown using the MEMOCVDTM technique. We conclude that the difference in the lifetimes is caused by the density of nonradiative recombination centers rather than by the potential fluctuations affecting the carrier motion. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

We report on using Migration Enhanced Metal Organic Chemical Vapor Deposition (MEMOCVDTM) buffers for increasing lifetime of non-equilibrium carriers in GaN and AlGaN players grown on sapphire and SiC substrates. Photoluminescence (PL) and the light-induced grating technique (LITG) were used for the comparative study of the GaN and AlGaN epilayers with the MEMOCVDTM multilayered buffers and with conventional buffers. Measurements on GaN layers grown on sapphire and SiC show carrier lifetime more than 300 ps respectively with MEMOCVD buffer layers which is more than 4 times higher compared to the layers on conventional MOCVD buffers. In the AlGaN layers, PL intensity increases for MEMOCVD buffer layer compared to conventional MOCVD layers possibly due to reduction in defect density. The LITG results also show higher carrier lifetime for Al0.23Ga0.77N (more than 190ps) with MEMOCVD AlN buffers compared to layers with conventional buffer layers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

AlGaN epilayers with different aluminum molar fractions have been comparatively studied by photoluminescence (PL) and light-induced transient grating (LITG) techniques. The carrier diffusion length determined by LITG is shown to be close to the average distance between two first-neighbor dislocations, which was roughly estimated by using the dislocation density obtained by etch pit technique. For epilayers containing the same amount of Al, the carrier lifetime and PL intensity are inversely proportional to the density of the dislocations, which limit the carrier lifetime by serving as nonradiative recombination centers. For AlGaN epilayers with similar lifetimes but with different Al contents, the PL intensity decreases with increasing Al fraction, which is explained by the Al content dependence of the radiative recombination rate. It is demonstrated that introduction of AlN/AlGaN superlattices on sapphire substrates by using Migration Enhanced Metal Organic Chemical Vapor Deposition (MEMOCVDTM) enables growth of AlGaN with an increased nonequilibrium carrier lifetime and enhanced photoluminescence intensity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) more...

The structural, electrical and optical properties of Mg-doped GaN epitaxial layers grown by metalorganic chemical vapor deposition were investigated in order to optimize the p-type conduction. A series of samples exhibiting different Mg concentrations were involved with this study, and post-growth thermal annealing at temperatures ranging from 650°C to 900°C were performed under flowing N2. Under our growth conditions, these two factors, i.e. the doping level and annealing temperature, were found to strongly control the physical characteristics of the samples. The highest hole concentration, of about 7×10 17 cm −3 , was recorded for the specimen prepared with a Mg source flow rate of 0.3 μmol min−1 and subsequently annealed at 900°C without damaging the surface morphology. The corresponding as-grown wafer also exhibited the minimum X-ray rocking curve full width at half maximum in both the symmetric (0 0 2) and asymmetric (1 0 2) diffraction planes. more...

Surface morphology studies of GaN and AlGaN grown by metalorganic chemical vapor deposition (MOCVD) have been carried out using atomic force microscopy. The open core dislocation and steps connecting two threading dislocations of opposite direction are commonly observed in undoped and doped GaN. Structural studies on AlGaN epitaxial layers grown on undoped GaN revealed the formation of open-core dislocation with width upto 300 nm. The nanopipes originate from the threading dislocation formed due to large lattice mismatch between sapphire and GaN. The mismatch also leads to high strain in the epilayers resulting in cracking effect at the edges of the hexagonal V-type defect. The self organized quantum dots features on the smooth surface of AlGaN epitaxial layer exhibit the Stranski–Krastanov(SK) mode of island growth. more...

Lateral epitaxial overgrowth (LEO) of GaN by sublimation has been carried out to understand the dependence of growth conditions on the morphology and quality of the layers. Growth conditions have been optimised to have flat and smooth lateral overgrown surfaces with (1 1 2 0) stripe window opening. The changing feature from triangular stripe to rectangular stripe growth with change in growth temperature have been observed using scanning electron microscopy (SEM). Atomic force microscopy (AFM) result shows the dominance of step growth mechanism leading to two-dimensional growth in the lateral overgrown GaN layers. more...

Nano-scale InGaN/GaN multi-structures have been grown on amorphous Si substrates using metal-organic chemical vapor deposition (MOCVD). The density, lateral size and height of the grown nano-scale InGaN/GaN multi-structures were estimated to be 10 10 cm −2 , 120 nm and 50 nm, respectively, by atomic-force microscopy (AFM). The optical properties of these nano-structures have been studied by photoluminescence (PL) and Raman scattering. Compared to the band gap emission from a reference InGaN/GaN multi-quantum wells sample, an evident blue-shift of the emission peak from the InGaN/GaN nano-structures could be observed from PL spectra obtained at 20 K. This result suggests that the carriers in the InGaN/GaN nano-structures are three-dimensionally confined. more...

Bulk single crystals and selectively grown gallium nitride (GaN) have been obtained using the sublimation technique. Crystals of size about 2–3 mm in length and 0.8–1.0 mm in width have been grown successfully. Atomic force microscopy has been employed to analyze the surface morphology of the as-grown samples to understand the growth mechanism. In free standing bulk GaN single crystals, two-dimensional growth is dominated by step growth mechanism. However, in the selective growth of GaN by sublimation, spiral growth originating from screw dislocation dominates. more...

We report on the structural properties and optical and electrical characteristics of InN epitaxial layers grown on highly resistive GaN templates using migration enhanced metalorganic chemical vapor deposition (MEMOCVD). The material quality of InN improved significantly for the layer thickness larger than 150 nm. The highest extracted value of the room temperature electron mobility was close to 850 cm2/V s for samples with electron carrier concentration of ∼4×1018 cm−3. The measured dependence of the electron mobility on electron concentration is interpreted using the model accounting for ionized impurity scattering, polar optical scattering, and compensation. The MEMOCVD-grown material exhibited stronger photoluminescence (PL) compared to InN deposited using conventional metalorganic chemical vapor deposition. Room temperature PL spectra were similar to InN grown using molecular beam epitaxy (MBE) with peak emission at 0.8 eV. The obtained results demonstrate the potential of the MEMOCVD technique for d... more...

We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters. more...

Concentration profiles of As in front of a GaAs crystal interface growing in a Ga rich solution have been determined during the electroepitaxial growth of GaAs layers using a computer simulation technique. The effect due to Peltier heating or cooling and electromigration during growth have been incorporated to simulate the concentration profiles. The growth velocity in the absence and presence of convection, due to Peltier effect and electromigration, are calculated under different conditions. It is observed that there is a transition in the movement of arsenic atoms towards the GaAs crystal interface from smooth and orderly to turbulent and wavy as the intensity of electric field increases during the electroepitaxial growth of GaAs. more...

A “pulsed metalorganic chemical vapor deposition” technique has been developed for lateral overgrowth of GaN thin films on SiC with conducting buffer layers for vertically conducting devices. Growth was carried out at temperatures as low as 950 °C keeping a constant gallium flux while pulsing NH3. We demonstrate that, by varying the NH3 pulse duration, growth morphology can be gradually changed from triangular to rectangular for the lateral overgrowth. Even at a V/III ratio as low as 550, high quality smooth layers with (1100) vertical facets were successfully grown with a lateral to vertical growth rate ratio as high as 4:1. Atomic force microscopic measurements show the root-mean-square roughness of the laterally overgrown layers to be 7.0 A. Scanning thermal microscopy was used to measure a thermal conductivity of 1.7 and 1.5 W/cm K, respectively, for the laterally overgrown film and the GaN deposition in the window region. more...

InGaN quantum dots (QDs) have been formed by metalorganic chemical vapor deposition selective growth on Si-patterned GaN/sapphire substrates. The QDs were fabricated on GaN plinths grown within nanoscale circular windows opened in the Si mask by both focused ion-beam irradiation and photoassisted wet chemical etching. Using this process, the epitaxial growth of GaN plinths could be achieved on an underlying GaN layer spared from process-induced damage. The optical properties of the InGaN QDs have been investigated by low-temperature cathodoluminescence measurements. Compared to the band-gap emission from a reference sample of InGaN/GaN multiple quantum wells, the evident blueshift of the emission peak from the InGaN QDs was demonstrated. This result suggests that the carriers in the InGaN QDs were three-dimensionally confined. more...

Transmission and reflection measurements in the frequency range (700–2000 cm−1) have been carried out on high-quality bulk GaN grown by the sublimation technique. The transmission spectra are polarization dependent and show combination bands arising from anharmonic forces. The combination absorption is much stronger for E∥c polarization than that for E⊥c polarization. A high value of 350 cm−1 for the absorption coefficient in the combination band can be explained by the strong anharmonic forces in GaN. more...

The joining of defect-free AlN stripes is observed to trigger the generation of a large density of threading dislocations in the vicinity of the coalescence point. The AlN structure was grown by pulsed lateral epitaxy on shallow-grooved sapphire substrates. In the precoalescence stage, the dislocation density in the lateral epitaxial region ( ) are generated at the coalescence point as a result of relaxation of compressive stress that develops due to temperature gradients during growth. They bend toward the surface during the postcoalescence growth stage, leading to a high density of pure-edge threading dislocations in the lateral growth regions. Some threading dislocations form loops on prismatic planes in the crystal and the basal segments are observed to glide in the [0001] direction under the electron beam in the microscope. more...

The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150°C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films. more...

Time-resolved photoluminescence and light-induced transient grating measurements of GaN epilayers show that the photoluminescence decay can be described by two coupled exponential terms and that carrier mobility and lifetime in GaN epilayers are correlated within the model which accounts for nonradiative carrier recombination predominantly at dislocations. The obtained results demonstrate that migration-enhanced metalorganic chemical vapor deposition (MEMOCVD™) allows for growth of high-quality GaN epilayers on sapphire substrates with the dislocation density close to 108cm−2, carrier lifetime as long as 2 ns, and ambipolar diffusion coefficient of 2.1cm2s−1 corresponding to the hole mobility of approximately 40cm2V−1s−1. more...

Monte Carlo simulation of phonon-assisted localized exciton hopping has been employed to describe the photoluminescence linewidth variation with temperature and to reveal band potential profile of ternary AlGaN epilayers with different carrier lifetimes. The lifetimes of 30 and 190 ps were experimentally determined in the layers with AlN buffers grown by conventional metal-organic chemical vapor deposition (MOCVD) and by migration-enhanced MOCVD (MEMOCVD™), respectively. The potential profile in AlGaN is shown to consist of double-scaled fluctuations. Exciton hopping in Al0.26Ga0.74N occurs within the random potential fluctuations (on the scale σ≈19meV) in isolated low-potential regions with the average localization energy dispersed on the scale Γ≈19meV. Such a pattern of band potential profile was found to be independent on the growth technique used for the deposition of their AlN buffer layers. This implies that the large difference in carrier lifetimes estimated in the AlGaN epilayers with the same Al ... more...

We demonstrate the use of high-quality thin InGaN films as the reactive ion etching (RIE) stop layer for fabrication of recessed gate high-microwave-power AlGaN∕InGaN∕AlGaN∕GaN heterostructure field-effect transistors. We used migration-enhanced-metalorganic-chemical-vapor-deposition grown InGaN layer sandwiched in AlGaN barrier yields better than 10 times RIE selectivity of AlGaN and InGaN compared to our conventional standard AlGaN∕GaN high electron mobility transistors. The fabricated devices exhibited a 50% increase in the breakdown voltage, which is attributed primarily due to the improved electric field distribution at the gate edge and to the increased distance between the AlGaN surface and the device channel. The continuous wave microwave power was measured at the drain bias as high as 60V. The maximum output power and power added efficiency were 8.9W∕mm and 40%, respectively. The obtained results demonstrate a potential of this technique for development of the next-generation high-power transistors. more...

The selective overgrowth method has been used to grow bulk-like GaN layers by sublimation method. Si and SiO2, which have a different evaporation rate, thermal conductivity, and thermal expansion coefficient, have been chosen as mask materials. The effect due to the reduction of dislocation density with different mask materials has been discussed. The lateral growth rates strongly depend on the direction of the mask stripe. For the stripe windows aligned in GaN〈1100〉 direction, the lateral growth rate is approximately four times higher than with stripe direction in GaN〈1120〉. The microstructure of selectively regrown GaN has been investigated by transmission electron microscopy, scanning electron microscopy, and cathodoluminescence to understand the lateral growth mechanisms in sublimation. The threading dislocations in the region of laterally regrown GaN are extended in two different ways. First, the threading dislocations are perpendicularly propagated into the top surface in the window region. In thi... more...