Optical phonons via oblique-incidence infrared spectroscopy and their deformation potentials in In1−xGaxAs

Abstract

The concentration dependence of optical phonons in In1−xGaxAs epilayers grown by chemical beam epitaxy and lattice matched to InP(100), where the epilayer strain is zero for x=0.46809, has been characterized for 0.25<x<0.8 with polarized far infrared reflectivity measurements at an oblique angle of incidence. In this method, the reflectance spectra contain sharp Berreman peaks exactly at the optic phonon frequencies. For radiation polarized in the plane of incidence (p polarized), Berreman peaks for both transverse optic (TO) and longitudinal optic (LO) modes are observed, whereas for s polarization only TO modes are seen. The derivatives of the measured spectra were fitted to Lorentzian line shapes to evaluate the various phonon modes, which included GaAs-like TO and LO phonons, disorder induced TO and LO modes, and InAs-like TO and LO phonons. The concentration dependences of the phonon frequencies were used to estimate the phonon deformation potentials of the bulk alloy. Indirect evidence is obtained for LO mode repulsion at low x values from the concentration dependences of the GaAs-like modes in both strained and bulk In1−xGaxAs and from an unphysical variation with x of the GaAs-like LO phonon deformation potential.