Halide Vapor Phase Epitaxy of Gallium Nitride Films on Sapphire and Silicon Substrates
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Pasadena, CA 'University of Wisconsin, Dept. of Chemical Engineering, Madison, WI ABSTRACT A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN substrates rests in the application of halide vapor phase epitaxy (HVPE) to produce GaN films at high growth rates. In this paper, we describe the growth of thick GaN films via the HVPE technique on (0001) sapphire and (111) Si substrates. At a temperature of 1030'C, films are grown at rates between 70 and 90 rtm/hr, yielding total thicknesses exceeding 200 jim on sapphire. DCXRD measurements of GaN/sapphire indicate FWHM values less than 220 arcsec on 180 jim thick films. Room temperature PL measurements of GaN/sapphire indicate strong emission at 3.41 eV, with a FWHM value of 65 meV. Moreover, no detectable deep level emission was found in room temperature PL measurement. Under optimized conditions, films are morphologically smooth and optically clear. The GaN morphology appears to be a strong function of the initial nucleation conditions, which in turn are strongly affected by the partial pressure of GaC1. HVPE growth on (11) Si substrates is accomplished using an AIN MOVPE buffer layer. INTRODUCTION The nitride family of wide bandgap semiconductors, including GaN and its associated alloys with AIN and InN, have recently gained prominence as proven materials for blue-green light emitting diode devices [1,2,3,4] and as promising candidates for blue-green laser devices. Despite rapid technological progress in the field, a number of basic scientific issues must be resolved to understand the broader applicability of the nitride family of semiconductors in visible light emitting devices. Many of the current materials efforts revolve around problems arising from the use of heteroepitaxial substrates in the growth of GaN-based devices. The most widely used substrate for the growth of nitride-based devices has been (0001) sapphire. However, sapphire suffers from a poor lattice match and substantial coefficient of thermal expansion difference with GaN, leading to the formation of threading defects in the epitaxial layer and fracture of thick layers during cooldown. Existing nitride device structures possess dislocation densities on the order of 10 10cm 2, which is an extremely high value by conventional arsenide and phosphide device standards [5]. The relationship between dislocation density and nonradiative recombination in the nitride semiconductors is not fully understood at present, but does not impede the efficent performance of some devices [5]. The GaN/sapphire system also suffers from nucleation problems, which can be ameliorated by the application of a thin low temperature buffer layer of GaN or AIN [6,7]. Alternatively, thick GaN layers produced via the HVPE technique on heteroepitaxial substrates may be used as GaN homoepitaxial substrates [8]. Studies by other researchers suggest that the application of thick
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