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Growth mechanisms and material quality of the laterally overgrown cubic-phase gallium nitride (c-GaN) and hexagonal-phase gallium nitride (h-GaN) on stripe-patterned GaAs (001) substrates were investigated using transmission electron microscopy (TEM). Investigational results show that h-GaN is only laterally overgrown along the (111)B facets of the c-GaN stripes with the growth direction of (0001)h-GaN//(111)B c-GaN for all the mask stripe orientations. Dislocation density in the laterally overgrown h-GaN regions for the [110]-stripe pattern is reduced to be lower than 104 cm–2, which is six orders of magnitude smaller than that in the conventionally grown h-GaN films. On the other hand, the laterally overgrown c-GaN with lower planar defect (stacking faults and twins) density presents in the region just above the stripe windows for the [10]-stripe pattern. In addition, a large reduction of planar defect density was found in the laterally overgrown c-GaN regions for the [100] stripe direction. Also, a model is used to describe the cubic-to-hexagonal structural transition in lateral-overgrown GaN on patterned GaAs (001) substrates for the purpose of lower dislocation and lower planar defect densities in the laterally overgrown h-GaN and c-GaN, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the structural and optical properties of InxGa1−xAs1−yNy (x = 0.116, ∼0.3 and y = 0–0.031) alloy films grown by low-pressure metalorganic vapour epitaxy (LP-MOVPE), performing high-resolution transmission electron microscopy (HRTEM) and photoluminescece (PL) measurements. The effects of rapid thermal annealing (RTA) at 700 °C under N2 ambient were also examined. The TEM images showed that with increasing In content, not only misfit dislocations near the InGaAsN/GaAs interface due to the large lattice mismatch with the GaAs subatrate, but also many defects in the InGaAsN layer due to the relaxation of local strain increased. From energy dispersive X-ray (EDX) analysis of the high In-content layers (x = 0.3–0.355), we found a significant In fluctuation in the InGaAsN layer. The PL peaks of In0.116Ga0.884As1−yNy (y = 0–0.031) measured at 7 K shifted to the high-energy side after RTA, while those of InxGa1−xAs1−yNy (x = ∼0.3, y = 0–0.015) shifted unusually to the low-energy side. This unsual behavior of the high In-content layer after RTA may be attributed to the enlargement of quantum-dot-like regions, which are formed by the compositional fluctuation in microscopic scale. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrically-biased photoreflectance (PR) spectrum measurements were carried out for the Au/cubic-AlGaN/GaN/n-GaAs(001) Schottky diode structure. The built-in electric-field strength at the GaAs side of the GaN/GaAs interface and its dependence on the external-bias voltage, evaluated by the Fast Fourier Transform analysis of the Franz-Keldysh oscillation in the PR spectrum, suggest the formation of the hole channel in the vicinity of the GaN/GaAs heterointerface even under the zero-biased condition. The band-lineup and band-bending at this heterointerface were discussed in relation to the parallel conduction phenomena observed in the Hall effect and photoconductivity measurements. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have proposed the use of cubic-GaN (c-GaN) as an intermediate layer for the metalorganic vapor phase epitaxy (MOVPE) growth of hexagonal-GaN (h-GaN) on GaAs (111)B substrates. Insertion of the c-GaN layer at the h-GaN (0001)/GaAs (111) interface significantly improves the crystallinity of the h-GaN layer. Although, we have used [111]-oriented c-GaN layer so far, the lattice-mismatch between h-GaN (0001) and c-GaN (111) is expected to be less than 0.1%, which is much smaller than that for the other commonly used substrate materials. Furthermore, the c-GaN layer was grown at a relatively low growth temperature (Tg = 600°C) to prevent the GaAs substrate from thermal decomposition and to provide a strain relief template layer. This technique enables us to succeed in obtaining nearly strain free h-GaN layers on GaAs (111)B substrates. In this report, the relationship between the nature of the c-GaN intermediate layer and the cubic-to-hexagonal structural transition machanisms are discussed.

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.

Photoconductivity (PC) and electroreflectance (ER) spectrum measurements were carried out for the cubic GaN/GaAs and cubic AlGaN/GaN/GaAs heterostructures grown by LP-MOVPE. Both spectra contained significantly overlapped signals from the c-(Al)GaN epitaxial layer and the underlying GaAs layer, but the latter was successfully suppressed and distinguished from the former by the use of an additional steady-state illumination of Ar + laser as an optical bias. The temperature dependence of the optically-biased PC spectrum showed that there remained structural disorder in the c-GaN layer indicated by the temperature-independent Urbach energy. From the temperature dependence of the ER spectrum, the E 0 critical point of c-GaN observed likely corresponds not to excitonic transition but to free-carrier transition as it shifted by ∼30 meV to higher energies than the reported value, probably due to the dissociation of exciton by the relatively large modulation field applied to the c-(Al)GaN layer.

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.

Selective-area growth (SAG) of cubic GaN (c-GaN) was performed on patterned GaAs (1 0 0) 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 [0 1 1]-stripe pattern, by decreasing the growth temperature from 960°C to 900°C, the top surfaces changed from inclined (8 1 1)B facets to planar (1 0 0) 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 [0 0 1]-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 [0 1 1]-stripe pattern, while it increases for SAG c-GaN on the [0 0 1]-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 31 min for SAG c-GaN on [0 1 1]-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.

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.

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.

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.

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.

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.

The hexagonal domain suppression-effects in cubic-GaNAs grown by metalorganic chemical-vapor deposition (MOCVD) is reported. A thin buffer layer (20 nm) was first grown on a substrate at 853 K using trimethylgallium and dimethylhydrazine (DMHy), and GaNAs samples were grown at different AsH3 flow rates (0 ∼ 450 μmol/min) at 1193 K. As a result, three types of surface morphologies were obtained: the first was a smooth surface (AsH3 = 0 μmol/min); the second was a mirrorlike surface having small and isotropic grains (AsH3 : 45 ∼ 225 μmol/min ); and the third involved three-dimensional surface morphologies (above 450 μmol/min of AsH3 flow rate). Furthermore, it was confirmed using X-ray diffraction that the mixing ratio of hexagonal GaNAs in cubic GaNAs decreased with an increase of the AsH3 flow rate. We could obtain GaNAs having a cubic component of above 85% at AsH3 flow rates above 20 μmol/min. Therefore, the MOCVD growth method using AsH3 and DMHy was mostly effective for suppressing hexagonal GaNAs. It was observed that the photoluminescence intensity of GaNAs was decreased with increase of arsine flow rate.

A systematic study for evaluating the effect of introducing short‐period superlattice waveguide layers on the threshold current of single quantum‐well diode lasers is reported. The waveguide layers which consist of short‐period GaAs/AlAs superlattices, in place of AlGaAs alloys, resulted in a decrease by a factor of 4–6 in the lasing threshold current density of GaAs/AlGaAs single quantum‐well diode lasers of separate confinement heterostructure. A time‐resolved photoluminescence study of those laser structures revealed that the threshold current reduction was the result of an improvement of overall nonradiative carrier recombination lifetime of single quantum wells by more than a factor of 10.

The photoluminescence decay of GaAs/AlGaAs quantum wells was successfully measured at room temperature by using 660‐nm picosecond light pulses from an AlGaInP diode laser driven by short pulse current. Our experimental results reveal that picosecond light pulses from visible diode lasers may be widely applicable, in place of mode‐locked gas, solid‐state, or dye lasers, for studying the fast carrier recombination properties of many kinds of materials including GaAs and AlGaAs in the wavelength region of deep red to infrared.

We report on the compositional and optical investigation of InGaAs(N) alloy films grown on GaAs (001) substrates by low-pressure (60 Torr) metalorganic vapor phase epitaxy (MOVPE). The alloy films with the room-temperature photoreflactance (PR) signal (Eo transition) wavelength range of 0.98–1.36 m have been grown. The variation in PL characteristics of the InGaAs(N) alloy films has been investigated as a function of alloy composition, excitation power and temperature. At low temperatures (TxGa1-xAs1-yNy (x = 10.5% and 17.0% and y < 2%) are excellent with a single near-band-edge emission peak corresponding to their own Eo transition. The evolution of PL spectra with excitation power and temperature led to an insight into the nature of the near-band-edge states. The temperature dependence of integrated PL intensity indicates the presence of a large density of non-radiative recombination centers, showing a behavior characterized by two activation energies. Our results suggest that the origin of localization in InGaAsN alloy films is the alloy inhomogeneities of both In and N, which may results in the characteristic carrier dynamics.

Anisotropic X-Ray diffraction (XRD) and transport properties of cubic GaN grown on GaAs substrates correspond to the features of low-temperature grown GaN (LT-GaN) buffer layer. When the LT-GaN layer is grown on the surface tilted from (001) to [1-10] with annealing in arsenic ambient, the macroscopic step edges along [1-10] direction are modified by either the ambient of thermal annealing, or substrate misorientation. A parallel conduction in GaN, GaAs, and GaN/GaAs hetero-interface was observed by photoconductivity measurements. Transmission electron microscope (TEM) observation shows that self-annihilations for (-111) B stacking faults are preferentially occurred near GaAs interface when GaN film grown on the surface tilted from (001) toward [1-10] (As step edge) is annealed in arsenic ambient. TEM observation also shows that stacking faults and dislocations are preferentially generated near GaN/GaAs interface. It is suggested that anisotropic transport properties correspond to the well-like potential generated by band bending at GaN/GaAs interface. The nearly isotropic mobility of 3,000 cm2/Vsec at 77K is obtained by improving interface property.

ZnS epilayers were grown by pyrolytic metalorganic vapour phase epitaxy using diethyldisulphide (Et2S2) as sulphur alkyl precursor in conjunction with dimethylzinc-triethylammine. It is demonstrated that Et2S2 lowers the temperatures of the process by about 150°C with respect to Et2S, leading to growth rates around 0.5 mu m/hr just above 400°C. Mass spectrometry measurements allowed to determine the Et2S2 relative bond strengths by looking at the fragmentation path of Et2S2+. It turned out that the S-S bond is stronger than the S-C bonds in the Et2S2 molecule, possibly leading to a destabilization of the latters, which finally results into lower pyrolysis temperatures for the alkyl. 10 K cathodoluminescence (CL) spectra of ZnS show both free-to-bound and intense self-activated bands peaked at 3.683 eV and 2.58 eV, respectively. These emissions are attributed to unintentional Na and halogen (most probably Cl) doping of ZnS epilayers. Monochromatic CL images of ZnS samples evidenced the interplay between point-defects induced radiative emissions and non-radiative recombination associated to the ZnS extended defects.