Two-wavelength micro-Raman spectroscopy and atomistic modeling of n-InSb:Te single crystals: orientation dependence of phonon scattering and the influence of carrier concentration

We present a systematic study of Raman scattering in tellurium-doped InSb single crystals by two-wavelength micro-Raman spectroscopy at excitation wavelengths of 532 and 785 nm. Samples of three crystallographic orientations — (100), (111) and (211) — with carrier concentrations from 5.84·10¹⁷ to 1.6·10¹⁸ cm⁻³ were examined. All eight spectra were analysed in a single global decomposition into four components, in which the position of the coupled L⁻ mode is fixed at the value predicted by phonon–plasmon coupling theory for the independently measured carrier concentration. The decomposition yields one transverse-optical frequency, ωTO = 180.94 ± 0.19 cm⁻¹, common to all samples and both excitations, and resolves the coupled L⁻ mode at 175–178 cm⁻¹ in six of the eight spectra. The LO-phonon fraction of the spectrum is determined by the crystallographic face. At 532 nm it increases in the sequence (111) < (211) < (100): 9.9 and 29.1 % for the (111) and (211) faces, and 49.8 and 72.9 % for the unpolished and polished (100) samples. At 785 nm it does not exceed 1 % on the (111) and (211) faces and amounts to 13.3 and 25.6 % on the (100) face, in agreement with the selection rules for the zinc-blende structure as governed by the surface-field contribution, whose weight is set by the probing depth. Under 785 nm laser excitation a 2TA overtone of zone-boundary acoustic phonons (~82 cm⁻¹) is observed; in the 100–125 cm⁻¹ interval the relative weight of two-phonon scattering at 532 nm is lower by at least an order of magnitude. The mode assignments were tested against atomistic modelling of the Raman spectra using molecular dynamics with the machine-learned MACE-MH-1 potential.