Direct evidence of tensile strain in wurtzite structuren−GaNlayers grown onn−Si(111)using AlN buffer layers

We present results on precise frequency measurements of first-order Raman-scattering spectra from a thin $n-\mathrm{GaN}$ layer grown by molecular-beam epitaxy on $n-\mathrm{Si}\left(111\right)\mathrm{}$ substrate using an optimized AlN buffer. It is found that the optical-phonon line at 565.43 ${\mathrm{cm}}^{\mathrm{-}1}$, which we attributed to the high-frequency $E_2\left(\Gamma \right)$ phonon mode indicates that the grown GaN layer possesses wurtzite structure. No evidence of cubic GaN zinc-blende structure is observed. At the same time, the frequency of the $E_2\left(\Gamma \right)$ phonon mode deviates significantly from that of the bulk GaN and exhibits negative shift, while the triply degenerate Si optical phonon mode exhibits small positive shift in comparison to that of free standing Si substrate. The obtained experimental data demonstrate the complex nature of the strain distribution at the GaN/AlN/Si(111) interface. We show that the epitaxial growth of GaN induces changes on both sides of $n-\mathrm{G}\mathrm{a}\mathrm{N}/\mathrm{A}\mathrm{l}\mathrm{N}/n-\mathrm{Si}\left(111\right)\mathrm{}$ interface: the GaN layer itself exhibits biaxial tensile strain while the Si surface at interface is under biaxial compressive strain. This behavior appears to be a common problem for heterostructures independent of the growth technique employed.