Raman scattering as a probe of the tensile strain distribution in GaAs grown on Si(111) by molecular beam epitaxy

Abstract

Epitaxial layers of GaAs grown on Si substrates, where the layer thickness greatly exceeds any critical thickness based on mismatch in lattice constant alone, have been shown to be under tensile strain for temperature at or below 300 K. This "thermal" strain arises from the difference in thermal expansion coefficients between GaAs and Si. We have performed Raman experiments on GaAs layers grown on both Si (001) and Si (111) substrates. We have observed a shift in the optical modes towards lower frequencies which is indicative of tensile strain in the GaAs layers, this is greater in the (111) growth direction than in the (001) one. In order to investigate the strain distribution as a function of distance from the GaAs/Si interface we have measured Raman spectra after successive removing of the epitaxial layer by chemical etching. We have found out that the strain decreases with increasing distance from the interface. We have developed the theory of Cerdeira et al. (1) to determine quantitatively the strain present in the heteroepitaxial layers. We have used, for the first time, polarization selection rules to separate the various components of the optical phonon modes. According to the theory we have observed that the doubly degenerate TO phonon line exhibits both a splitting and shift with strain, while only a shift is observed for the LO phonon line. In conformity with Cerdeira we have remarked that the strain dependence of the LO phonon is equal to that of the TO phonon mode observed in crossed polarization configuration.