Localized Vibrational Modes of Boron-Lithium Pairs in Si-Rich Ge–Si Alloys

Abstract

Localized vibrational modes of boron‐lithium pairs have previously been observed in silicon; the ¹⁰B(¹¹B) localized mode is split into an axial mode at 584(564) cm⁻¹ and a doubly degenerate transverse mode at 681(655) cm⁻¹. In this work the infrared absorption bands of these modes were observed in boron‐doped lithium‐compensated Ge_xSi_1−x for 0≤x≤0.12. The bands were slightly lowered in frequency and asymmetrically broadened by the addition of the germanium. The asymmetric shape results from the superposition of the absorption from boron atoms with different second‐neighbor configurations; the line shape is fit well by a simple model for interactions between the boron and Ge second neighbors and the assumption of random distribution of Ge throughout the crystal. The broadening is attributed primarily to lattice strain, the effect of the mass change being calculated to be far too small, and to modification of the Li–B interaction. It is found that the Li–B pairing distance must be smaller then the previously reported values of 2.4–2.9 Å. New bands at 553 and 651 cm⁻¹, attributed to boron with a Ge first neighbor, are split off from the previously observed boron localized modes in the sample with 12% Ge. These are weaker than would be predicted by a random distribution of Ge, indicating that boron and Ge tend to avoid first‐neighbor pairs, or that the oscillator strength of the boron mode is lessened by a Ge first neighbor.