Growth optimization and doping with Si and Be of high quality GaN on Si(111) by molecular beam epitaxy

GaN layers have been grown by plasma-assisted molecular beam epitaxy on AIN-buffered Si(111) substrates. An initial Al coverage of the Si substrate of approximately 3 nm lead to the best AIN layers in terms of x-ray diffraction data, with values of full-width at half-maximum down to 10 arcmin. A (2 x 2) surface reconstruction of the AIN layer can be observed when growing under stoichiometry conditions and for substrate temperatures up to 850 degrees C. Atomic force microscopy reveals that an optimal roughness of 4.6 nm is obtained for AIN layers grown at 850 degrees C. Optimization in the subsequent growth of the GaN determined that a reduced growth rate at the beginning of the growth favors the coalescence of the grains on the surface and improves the optical quality of the film. Following this procedure, an optimum x-ray full-width at half-maximum value of 8.5 arcmin for the GaN layer was obtained. Si-doped GaN layers were grown with doping concentrations up to 1.7 x 10(19) cm(-3) and mobilities approximately 100 cm(2)/V s. Secondary ion mass spectroscopy measurements of Be-doped GaN films indicate that Be is incorporated in the film covering more than two orders of magnitude by increasing the Be-cell temperature. Optical activation energy of Be accepters between 90 and 100 meV was derived from photoluminescence experiments.