Lattice dynamics of mixed semiconductors (Be,Zn)Se from first-principles calculations

TL;DR

This study uses first-principles calculations to analyze the vibrational properties of mixed Zn(1-x)Be(x)Se semiconductors, revealing how Be-Se chain formation influences phonon modes and matches experimental Raman spectra.

Contribution

It provides a microscopic understanding of anomalous phonon modes in mixed semiconductors by explicitly modeling Be-Se chain formation using first-principles methods.

Findings

Identification of phonon mode softening and splitting due to Be-Se chains.

Correlation of force constants with interatomic distances.

Agreement with experimental Raman spectra observations.

Abstract

Vibration properties of Zn(1-x)Be(x)Se, a mixed II-VI semiconductor haracterized by a high contrast in elastic properties of its pure constituents, ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation and frozen phonons. The calculations within the local density approximation has been done with the Siesta method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure endsystems were moreover done also by all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes early detected in Raman spectra in the intermediate region (20 to 80%) of ZnBe concentration. This was early discussed on the basis of a percolation phenomenon, i.e., the result of the formation of wall-to-wall --Be--Se-- chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of Be--Se bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence on the short-range order.