Direct evidence of tensile strain in wurtzite structure n-GaN layers grownon n-Si(111) using AlN buffer layers

We present results on precise frequency measurements of first-order Raman-s cattering spectra from a thin n-GaN layer grown by molecular-beam epitaxy o n n-Si(111) substrate using an optimized AlN buffer. It is found that the o ptical-phonon line at 565.43 cm(-1), which we attributed to the high-freque ncy E-2(T) phonon mode indicates that the grown GaN layer possesses wurtzit e structure. No evidence of cubic GaN zinc-blende structure is observed. At the same time, the frequency of the E-2(Gamma) phonon mode deviates signif icantly from that of the bulk GaN and exhibits negative shift, while the tr iply degenerate Si optical phonon mode exhibits small positive shift in com parison to that of free standing Si substrate. The obtained experimental da ta demonstrate the complex nature of the strain distribution at the GaN/AlN /Si(111) interface. We show that the epitaxial growth of GaN induces change s on both sides of n-GaN/AlN/n-Si(111) interface: the GaN layer itself exhi bits biaxial tensile strain while the Si surface at interface is under biax ial compressive strain. This behavior appears to be a common problem for he terostructures independent of the growth technique employed. [S0163-1829(99 )07447-0].