Infrared Absorption of Lattice Modes and the Silicon Local Mode in Ge<i>x</i>Si1−<i>x</i> Alloys

Abstract

Measurements have been made of the infrared absorption of GexSi1−x alloys for x≤0.12 and x≥0.88. The measurements were made in the range 100–700 cm−1 at temperatures from 10 to 300°K. In Si-rich alloys (x≤0.12) absorption bands are seen near 485, 400, and 125 cm−1 which are essentially temperature independent, indicating a single-phonon process. The bands do not correspond well with the predictions of simple models for resonance mode absorption, but are more nearly characteristic of the single-phonon density of states for Si. In the Ge-rich alloys (x≥0.88) the Si localized mode is observed as an absorption band at 389 cm−1 for x=0.99, increasing to 394 cm−1 at x=0.88. The absorption cross section αpeak/[Si] is only 4×10−21 cm2, smaller by 102−103 than observations for other local modes in semiconductors. Two-phonon absorption bands at about 200 and 290 cm−1 in Ge are enhanced by the addition of Si. The over-all temperature dependence of the absorption strength is large enough that it is not obvious whether the observed enhancement is temperature dependent or is a temperature-independent absorption, caused by the Si, superposed on a temperature-dependent background.