Epitaxial Growth and Properties of Al x Ga1 − x  N  by MOVPE

Abstract

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...

Electronic structure of GaInN semiconductors investigated by x-ray absorption spectroscopy

“…The lattice constant for the c-axis obtained from the ͑0002͒ diffraction peak of the InGaN films decreases linearly with the increase in the Ga composition, indicating that Vegard's law is valid in the GaInN alloy, which is in good agreement with the reported results. 15,16 The magnitude of the full-width at half maximum ͑FWHM͒ of the ͑0002͒ diffraction peaks is obtained to be about 60 arc min. We note that no diffraction peak was observed for the GaInN XRD results.…”

Donor spectroscopy at large hydrostatic pressures and transport studies in compound semiconductors

“…Finally, in figures 2-8a through 2-8d, the calculated mobilities are plotted together with experimentally obtained data points (Yoshida 1982, Khan 1983, Koide 1986, Zhang 1995. In two out of the four sets of data (Yoshida 1982, Koide 1986), the mobility can be seen to remain roughly constant except for a slight drop near A1 alloying fractions of 0.2.…”

“…As the energy of the conduction band approaches that of the defect level, it becomes energetically less favorable for defects to be ionized and a "freeze out" of carriers originating from these defects is observed (Yoshida 1982, Khan 1983, Koide 1986, Zhang 1995. This is the same reduction in carrier concentration observed in GaN under large hydrostatic pressures (perlin 1995, Wetzel 1996).…”

“…In two out of the four sets of data (Yoshida 1982, Koide 1986), the mobility can be seen to remain roughly constant except for a slight drop near A1 alloying fractions of 0.2. Comparing this data with theory, it can be seen that were it not for resonant scattering, the mobilities would remain very nearly constant across the entire measured range of alloying concentrations.…”

“…With a few simple modifications, it is easy to extend our mobility calculations to model A1Ga.N alloys as well. Experiments have shown that the electron mobilities in AlGaN alloys remain relatively constant upon varying of the alloy concentration (Yoshida 1982, Koide 1986. In a few of the data sets, a temporary decrease in the mobility is also observed for a certain range of Al fraction.…”