Raman scattering investigation of aGe/SiO2/Sinanocrystal system under hydrostatic pressure

Abstract

We have studied the hydrostatic pressure dependence of Ge nanocrystals embedded in a ${\mathrm{SiO}}_{2}$ matrix on a Si substrate by Raman scattering at room temperature. During the first cycle of increasing pressure, we observed a step change in the Ge Raman mode from 310.4 to $313.8{\mathrm{cm}}^{\ensuremath{-}1}$ at $\ensuremath{\sim}23\mathrm{kbar}.$ The linear pressure coefficients \ensuremath{\alpha} obtained before and after the step change at $\ensuremath{\sim}23\mathrm{kbar}$ are 0.42 and $0.64{\mathrm{cm}}^{\ensuremath{-}1}{\mathrm{kbar}}^{\ensuremath{-}1},$ respectively. Upon decreasing pressure, the Ge mode follows a single slope of pressure coefficient $\ensuremath{\alpha}=0.64{\mathrm{cm}}^{\ensuremath{-}1}{\mathrm{kbar}}^{\ensuremath{-}1}.$ A finite-element analysis was carried out to investigate the elastic-field distribution in the ${\mathrm{G}\mathrm{e}/\mathrm{S}\mathrm{i}\mathrm{O}}_{2}/\mathrm{Si}$ nanocrystal system, where the discontinuity of the specific geometric configuration with different elastic constants causes local areas of stress concentration around the interface. The step change of the Raman shifts with pressure at $\ensuremath{\sim}23\mathrm{kbar}$ was attributed to complete delamination between the ${\mathrm{SiO}}_{2}$ film and the Si substrate.