We demonstrate selective growth of high-quality GaN by gas-source molecular
beam epitaxy on Si(111) wafers patterned with SiO2. GaN was grown on wafer
s having two different buffer layers. The first buffer layer contains two A
lGaN/GaN superlattices, separated by GaN spacer, grown on AlN, with a total
thickness of 400 nm. The second is a thin AlN (1.5 nm) buffer layer. X-ray
diffraction confirms (0001) growth orientation, smooth interfaces, and coh
erence lengths comparable to the layer thickness in both samples. In the ca
se of the thin AlN buffer layer, the tensile stress measured by the E-2 Ram
an line shift is attributed to the mismatch in the thermal expansion coeffi
cients of GaN and Si. However, when the AlGaN/GaN superlattice buffer layer
is grown first, a reduced stress is measured. High carrier concentrations
(approximate to 10(18) cm(-3)) are seen in the GaN grown on the thin AlN bu
ffer layer, which we attribute to the incorporation of silicon from the sub
strate during the growth process. The superlattice buffer layer is seen to
inhibit this diffusion, resulting in a carrier concentration of < 10(17) cm
(-3) in the GaN. (C) 2000 American Institute of Physics. [S0003-6951(00)012
14-6].