Structural and dynamical properties of Bridgman-grown CdSexTe1−x(0<x≤0.35) ternary alloys

Abstract

Measurements of the Raman scattering and extended x-ray-absorption fine-structure (EXAFS) spectroscopy are reported on a series of Bridgman-grown zinc-blende CdTe${}_{1\ensuremath{-}x}$Se${}_{x}$ (0.35 \ensuremath{\ge} $x$ > 0.05) ternary alloys to empathize their lattice dynamical and structural properties. Low-temperature Raman spectra have revealed the classic CdTe-like (TO${}_{1}$, LO${}_{1}$) and CdSe-like (TO${}_{2}$, LO${}_{2}$) pairs of optical phonons. The composition-dependent peak positions of the LO${}_{2}$ modes exhibited shifts towards the higher-energy side, while those of the LO${}_{1}$ phonon frequencies have unveiled the slight redshifts. Detailed analyses of EXAFS data by using the first-principles bond orbital model have enabled us to estimate both the lattice relaxations and nearest-neighbor radial force constants around the Se/Te atoms in the CdTe/CdSe matrix. These results are methodically integrated in the ``average $t$-matrix'' formalism within the Green's-function theory for defining the impurity perturbations to comprehend the composition-dependent optical phonons in CdTe${}_{1\ensuremath{-}x}$Se${}_{x}$ alloys. Based on our comprehensive calculations of impurity modes in the low-composition regime $x\ensuremath{\rightarrow}$ 0, we have assigned the weak phonon feature observed near \ensuremath{\sim}175 cm${}^{\ensuremath{-}1}$ in the low-temperature infrared reflectivity spectroscopy study to a Se${}_{\mathrm{Te}}$ localized vibrational mode.