Comparative study of interfacial strain dependent phonon localization in the beryllium-zinc chalcogenide superlattices

Abstract

A comprehensive study is reported for the novel (BeX)m/(ZnX)n (X = S, Se and Te) superlattices (SLs) to assess their acoustic and optical phonon characteristics. The use of an elastic continuum model has offered implicit equations for assessing folded acoustic modes (FAMs) and their splitting at the center and at the edge of mini-Brillouin zone. In (BeTe)m/(ZnSe)n SLs grown with Zn–Te interfaces, the Raman spectroscopy measurements revealed well resolved first order doublet and frequency shifts which compared favorably well with the simulated results. For the graded (BeX)10-Δ/(Be0.5Zn0.5X)Δ/(ZnX)10-Δ/(Be0.5Zn0.5X)Δ SLs, we have adopted a modified linear-chain model by meticulously integrating the effects of alloying and interfacial layer thickness Δ to study phonon dispersions (ωj(q→SL)) and Raman intensity profiles. By changing Δ from 1 to 3 monolayers, our study has distinctly revealed (a) a dramatic increase in the Raman scattering intensity of ZnX quasi-confined optical modes (Q-COMs) with slight upward shifts in frequencies, (b) an insignificant change in Raman intensity lines of BeX confined optical modes (COMs) with appreciable downward shifts (up to ∼ - 113 cm−1) in frequencies, and (c) trivial modifications in the FAMs. These atypical effects noticed in the optical modes are attributed to the localization of atomic displacements at the BeX-ZnX interfacial region. The variations of phonon mode frequencies and enhancement of Raman lines for the ZnX Q-COMs can be used as vital tools for establishing interfacial structures in graded SLs of technological importance.