Antibonding configurations of hydrogen in silicon-germanium alloys

Abstract

Defects with a hydrogen atom at an antibonding site are identified in germanium-rich SiGe alloys $({\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x})$ implanted with protons at low temperatures. Infrared absorption spectra of proton-implanted ${\mathrm{Ge}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$ reveal hydrogen-related absorption lines, which are similar to those observed previously in pure germanium, together with three unreported absorption lines. For crystals with a silicon content of 1.2 at. % these three lines are observed at 815.8, 1430.2, and $1630.7\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The three lines are ascribed to the wag mode, the stretch mode, and to the first overtone of the wag mode, respectively, of the negatively charged state of a hydrogen atom located at an antibonding site and attached to a substitutional silicon atom. The assignment is based on the comparison between the observed frequencies and those known for similar structures, the variation of line intensities with the alloy composition, the absorption frequency shifts resulting when hydrogen is replaced by deuterium, and the thermal stability of the absorption lines, as well as comparison with the results of ab initio calculations. A conversion process takes place at temperatures in the range 20-125 K between negatively charged hydrogen at a germanium antibonding site to a silicon antibonding site. This provides direct evidence that silicon acts as an effective trap for negatively charged hydrogen in SiGe alloys. Based on the data obtained for samples with different silicon contents we give an estimate of the low temperature diffusion length of hydrogen in Ge-rich SiGe before it becomes trapped at an antibonding site attached to Ge and discuss why local vibrational modes of negatively charged hydrogen remain unobserved for silicon and silicon-rich SiGe alloys.