Infrared and Raman studies on a-Ge1-xSnx:H thin films.

Abstract

Infrared and Raman measurements were carried out on a-${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$:H thin films grown on Si substrates. The hydrogen concentration is found to decrease with increasing tin content. Preferential formation of monohydride groups (GeH) is observed with increasing Sn concentration. The tin dependence of the GeH and ${\mathrm{GeH}}_{2}$ stretching is explained as being due to the lower electronegativity of Sn in comparison to that of Ge. The study reveals that the stability-ratio electronegativity of germanium should be smaller than 3.59 for the a-Ge:H system. Also, the GeH wagging and ${\mathrm{GeH}}_{2}$ roll modes show that there is a large bond-angle variation with increasing Sn concentration. Additionally, the changes in the frequency, width, and intensity of the Ge-Ge (TO)-like phonon, clearly indicate that the structural disorder increases with increasing tin content. Auger-electron microprobe and Raman studies with different excitation laser lines show differences near the surface in comparison to the bulk, which are produced by the partial segregation of tin atoms, formation of Sn clusters in the surface, and reduction of the germanium concentration near the surface, as well as preferential attachment of hydrogen to Ge rather than to Sn.