Infrared Absorption of Lattice Modes and the Silicon Local Mode in GexSi1−x Alloys

Abstract

Measurements have been made of the infrared absorption of Ge_xSi_1−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 α_peak/[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.