Strain effect in silicon-on-insulator materials: Investigation with optical phonons - art. no. 035309

We report a detailed experimental and theoretical investigation of the effe ct of residual strain and strain relaxation, which manifests itself at the Si/SiO2 interfaces in commercial silicon-on-insulator (SOI) wafers. SOI mat erial is made of a single-crystal silicon overlayer (SOL) on top of an insu lator (buried SiO2 layer) sitting on a handle silicon wafer. Infrared refle ctivity spectra show that the buried SiO2 layer relaxes continuously when t hinning the SOL. At the same time, the SOL surface roughness and the linewi dth of optical phonons in Si near the Si/SiO2 interface (probed by micro-Ra man spectroscopy) increase. In the as-delivered wafers, this comes from a s light expansion of Si on both sides of the buried SiO2 layer, which, conver sely, is compressed. Thinning the SOL modifies these initial equilibrium co nditions. To get quantitative results, we have modeled all our Raman spectr a using a theory of inhomogeneous shift and broadening for optical phonons, which takes into account the phonon interaction with the static strain flu ctuations. From the variation of linewidth versus interface distance, we ha ve found that the mean-squared strain continues to relax in the bulk of the wafer through a depth on the order of several mum. We also show that the S OL surface roughness is related to strain fluctuations near the Si/SiO2 int erfaces.