Analysis of Raman Spectra of GeSi Ultrathin Superlattices and Epilayers

Abstract

Molecular beam epitaxy (MBE) techniques^1,2 are now capable of fabricating artificial layered structures with virtually “one-atomic-layer” accuracy. Thus one may construct a structure by depositing a few atomic layers of Ge on a silicon substrate, or build a superlattice (GemSin)p where m atomic layers of Ge are followed by n atomic layers of Si to form a super-cell, which is repeated p times along the growth direction. A versatile method for investigating the structural properties of these epilayers (ELs) and superlattices (SLs) is Raman spectroscopy. In earlier papers^{3, 4} we studied the low-lying acoustic phonons (Δω > 100 cm^-1) in such structures and found them to be sensitive to the overaL.L. periodicities, boundary conditions, and average properties of the lattices. In a more recent paper⁵ we have studied the optical phonons, which yield information regarding local structure, interface smudging, and lattice-strain effects. There we calculated the Raman spectra of ideal unstrained (GemSin)p structures, then successively included strain and interface smudging in the theoretical model. In this paper we wiL.L. review and extend the results of Ref. 5, covering epilayers as well. Since we are concerned with backscattering-like Raman spectra for longitudinal vibrations along the [001] direction, we use a linear chain model⁴ to calculate the phonons, while a bond-polarizability approach^5–8 is used to evaluate the Raman scattering intensity.