Interference enhanced Raman spectroscopy of ultra thin crystalline Ge films

Abstract

We report Raman study of ultra thin Ge films using interference enhanced Raman scattering which uses a trilayer structure of Al, CeO₂ and crystalline Ge films. The use of CeO₂ allows the growth of crystalline Ge films at relatively low substrate temperatures (300°C). With a decrease of Ge film thickness, the Raman line exhibits an increased red shift of the peak position and line broadening. The latter can be quantitatively explained on the basis of phonon confinement in the growth direction. Raman spectra of the 2 nm and 4 nm thick Ge films show shoulder at similar to -280 cm^-1 which could be attributed to surface phonons. The changes in the Raman shift as a function of thickness showed that the films were compressively strained up to a thickness of similar to -7 nm beyond which the strain is released.