Raman spectra of n-type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high-frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line-shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation-potential and electro-optic mechanisms.

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Epitaxial layers of AI~Ga~_.~N alloys are grown on sapphire (0001) and silicon (111) substrates by MOVPE in an ambient H2 gas at atmospheric pressure. By optimizing the reactor design and the growth conditions, parasitic reactions of metalorganic compounds with gaseous NH3 are remarkably reduced and the composition of AI~Ga~_~N layers can be controlled fairly well for the first time. The vapor-solid distribution coefficient for A1 is found to be approximately unity. At respective substrate temperatures of 1020~ for sapphire and 1050~ for silicon, single-crystal layers of AI~Ga~_~N with x up to 0.40 are obtained. The lattice constant is proportional to the molar fraction of A1N following Vegard's law. The electrical resistivity, carrier concentration, and Hall mobility of the layers are studied as a function of the composition.GaN and A1N have attracted much interest as the materials for optical devices in the short wavelength region, because they have direct energy bandgap of 3.4 (1) and 6.2 eV (2) at room temperature, respectively. For such device applications, single crystals with desired electrical properties are required. However, it has been fairly difficult to control their electrical Properties such as the type of conduction and the conductivity. As-grown GaN is usually an n-type material with relatively low resistivity, while A1N is always insulating. It is interesting to study, therefore, the growth of their alloy At~Ga~_xN single crystals and the properties of this alloy, because both GaN and AIN have a wurtzite structure. Although few reports of the growth of this alloy have appeared during the last several years, all of these reports have shown that the control of composition of the alloy is yet to be achieved for single crystals with good electrical properties.The growth of AI~GaI_~N layers by hydride vaporphase epitaxy (HVPE) was reported by Hagen et al. AI~.Gal_~N layers grown by MBE under high vacuum conditions are inferior to those grown by HVPE at atmospheric pressure (4) and metalorganic vapor-phase epitaxy (MOVPE) (7). This is probably due to the high equilibrium pressure of nitrogen over the alloy at typical growth temperatures resulting in high density of nitrogen vacancies, which may behave as electron donors. Furthermore, the lattice constant was not proportional to the molar fraction of A1N, suggesting the presence of some kinds of lattice defect. MOVPE has proven its suitability for the epitaxial growth of GaN of good quality (8), but very few works have been reported on this AlxGa~_~N alloy. Recently, Khan et al. (7) reported:the electrical and optical properties of AI~Gal_xN singlecrystal layers over the whole range of the composition prepared by using the low pressure MOVPE. However, no investigations of the controllability of the composition and the composition dependence of lattice constant were reported. In this paper, we report the epitaxial growth of AI~Gal_xN layers on sapphire and silicon substrates by MOVPE at atmospheric pressure and the achievement of good control o...