K. Onabe

MOVPE Growth and Luminescence Properties of GaAsN Alloys with Higher Nitrogen Concentrations

Highly luminescent GaAsN alloy films have been successfully obtained with N concentrations up to 3.1% by metalorganic vapor phase epitaxy (MOVPE). The band-gap bowing parameter estimated from the photoluminescence (PL) peak energy is not constant but dependent on the N concentration; 24.0 eV for N 1% at 20 K, and 22.4 eV for N 1% at room temperature. From the characteristic features of the luminescence properties, the band-edge states are found to be much localized below the conduction band edge, and the emission process at low temperatures is associated with these localized states as in GaPN alloys.

Photoluminescence Study on Temperature Dependence of Band Gap Energy of GaAsN Alloys

We have studied the temperature dependence of photoluminescence (PL) spectra of GaAsN alloys. The PL peak energy shift due to the temperature change decreases with increasing N concentration of GaAsN alloys. The localized state emission partly contributes to the decrease in the PL peak energy shift. In addition, the small PL peak energy shift at high temperatures is due to the reduction in the temperature dependence of the band gap energy. From the analysis using the Bose-Einstein statistical expression, the average phonon energy is much larger than that expected from the linear interpolation between GaAs and GaN, indicating that the interaction between electrons and phonons localized at N atoms plays an important role in the reduction of the temperature dependence of the band gap energy of GaAsN alloys.

MBE growth and photoreflectance study of GaAsN alloy films grown on GaAs (001)

The GaAsN alloy has attracted much attention due to the scientific interest for the huge bandgap bowing as well as the technological importance for optoelectronic devices. The study of the bandgap energy for the higher N contents, however, is scarcely reported because of the extreme immiscibility. In this work, the bandgap energy and its temperature dependence of GaAsN (N=0-4.5%) alloy films have been investigated by photoreflectance (PR) spectroscopy.

RF-MBE growth of InAsN layers on GaAs (001) substrates using a thick InAs buffer layer

InAs_l-xN_x epilayers even with x=0.0353 were successhlly grown on GaAs (001) substrates by using the ~0.8μm thick InAs buffer layer. The N concentration was well controlled by altering T_g between 420°C and 510°C. With decreasing T_g the N concentration of the InAsN layer increased. The cross-sectional SEM and AFM images indicate that the fairly flat surface and interface of InAsN epilayer were obtained.

Reduction of Planar Defect Density in Laterally Overgrown Cubic‐GaN on Patterned GaAs(001) Substrates by MOVPE

The metalorganic vapor phase epitaxy (MOVPE) growth of cubic-GaN (c-GaN) layers has been performed on the [110]-stripe patterned GaAs (001) substrates. The surface morphology of the laterally overgrown layer was much improved with the formation of the flat (311)A surfaces as the growth proceeded. It is shown that the c-GaN layer thicker than 20 μm was grown on GaAs(001) substrates with the stripe direction in [110] for 90 min. Microstructure and extended defect distribution in the laterally overgrown c-GaN films have been analysed by transmission electron microscopy (TEM). It is found that the planar defect (stacking faults and twins) density drastically decreases at the region away from the substrate toward the top of the c-GaN stripe. On the other hand, the dislocations become dominant. The density of the dislocations was found to be lower than 10 8 cm -2 . These results suggest that the lateral overgrowth on [110]-stripe-patterned GaAs (001) substrates via MOVPE is an efficient method for obtaining a low planar defect density c-GaN layer.

Effects of growth temperature in selective-area growth of cubic GaN on GaAs (100) by MOVPE

Abstract Selective-area growth (SAG) of cubic GaN (c-GaN) was performed on patterned GaAs (1xa00xa00) substrates by metalorganic vapor phase epitaxy. It was found that the growth feature and the crystal structure are sensitive to the growth temperature. For the SAG c-GaN on [0xa01xa01]-stripe pattern, by decreasing the growth temperature from 960°C to 900°C, the top surfaces changed from inclined (8xa01xa01)B facets to planar (1xa00xa00) facets. While, no effect of the growth temperature on the side walls of SAG c-GaN are observed. In contrast, for the SAG c-GaN on [0xa00xa01]-stripe pattern, the growth features are independent of the growth temperature. A rough surface and no distinct facets are formed. The X-ray diffraction results show that by decreasing the growth temperature, the FWHM of ω -scan decreases for the SAG c-GaN on the [0xa01xa01]-stripe pattern, while it increases for SAG c-GaN on the [0xa00xa01]-stripe pattern. The decrease in the FWHM of ω -scan indicates that the distribution of the crystal orientation becomes narrower. The narrowest FWHM was around 31xa0min for SAG c-GaN on [0xa01xa01]-stripe pattern at ∼900°C with a high cubic phase purity (>85%). Strong emission peaks of c-GaN were observed in the photoluminescence spectra without an emission peak of h-GaN for this sample.

Time-Resolved Photoluminescence of Cubic GaN Grown by Metalorganic Vapor Phase Epitaxy

Time-resolved photoluminescence of high quality cubic GaN was measured to clarify the origin of the emissions observed in cubic GaN. The 3.27 eV emission decay was fast and the intensity decreased with a lifetime of 260 ps at early decay and with a lifetime of 900 ps afterwards. The decay of the 3.18 eV emission was much slower than that of the 3.27 eV emission and the lifetime was 3.8 ns. In time-resolved spectra, the 3.18 eV emission had a broadening on the high-energy side at early times and the peak moved to lower energies with increasing time. These results can be explained in terms of the model for a donor±acceptor pair transition.

Cubic GaN Films on GaAs (001) Substrates without Deep-Level Luminescence Grown by Metalorganic Vapor Phase Epitaxy

Cubic GaN films on GaAs (001) substrates which show a low-temperature photoluminescence (PL) without the deep-level orange (2.0 to 2.1 eV) band were grown by metalorganic vapor phase epitaxy (MOVPE). The orange luminescence recovers its relative intensity at room temperature, though the dominance of band-edge free exciton is still significant. The surface morphology was much smoother and the inclusion of hexagonal phase was much reduced for the samples with the suppressed orange band. It is suggested that the role of the GaN buffer layer is very essential in the optimized growth for high optical quality films, providing a flat cubic structure template as well as avoiding thermal damage of GaAs surface at high temperatures.

Laterally Overgrown GaN on Patterned GaAs (001) Substrates by MOVPE

The characteristics of single-crystal GaN regions obtained by selective-area and subsequent lateral overgrowth on stripe-patterned GaAs (001) substrates by MOVPE were studied. Under certain growth conditions, the surface kinetics of the MOVPE process result in lateral-growth of both hexagonal-GaN (h-GaN) and cubic-GaN (c-GaN) stripes with the appropriate mask stripe orientation, namely [110] and [110], respectively. The facet structure comprises the c-GaN stripes surrounded with (111) B facets and mixture facets of (311) A, (111) A, and an inversed (111) B for the stripe window opening along the [110] and [110] directions, respectively. The cross-sectional TEM micrographs showed that the h-GaN is laterally overgrown along the (111) B facets of the c-GaN stripes. For the [110]-stripes, the laterally overgrown h-GaN regions contained a very low density of dislocations (< 10 8 cm -2 ). On the other hand, for the [110]-stripes, a large reduction of stacking fault density was found in the overgrown c-GaN regions. We demonstrated that both high quality h-GaN and c-GaN films were successfully grown on the stripe-patterned GaAs (001) substrates by MOVPE.

Spectroscopic ellipsometry study on the electronic structure near the absorption edge of GaAsN alloys

Spectroscopic ellipsometry has been used to investigate the electronic structure near the fundamental absorption edge of GaAsN alloys grown by metalorganic vapor phase epitaxy. The fundamental absorption edge is clearly observed in the imaginary part of the dielectric function and shifts to lower energies with increasing N concentration. In addition, the absorption structure is observed near the E 0 gap energy of GaAs even in GaAsN alloys. This unequivocally shows that the fundamental absorption edge of GaAsN is not shifted from the E 0 gap of GaAs but newly formed by the N incorporation. Thus, the formation of the narrowest band gap of GaAsN alloys is found to be completely different from that of conventional compound semiconductor alloys, such as AlGaAs and GaAsP.