Interfacial strain-dependent localization of phonons in (CdS)m/(CdTe)n superlattices

Abstract

Comprehensive results of the acoustic and optical phonon traits are reported for the strained (CdS)m/(CdTe)n superlattices (SLs) using microscopic and macroscopic methods. Simulations by realistic rigid-ion-model provided atypical vibrational characteristics in the growth [001] as well as in-plane ([100], [110]) directions of SLs exhibiting anisotropy of zone-center optical-, acoustic-anti-crossing phonons, mini-gap formation, confinement, and gap modes. In (CdTe)n/(CdS)m and (CdS)n/(ZnS)m, the use of elastic continuum techniques offered accurate values of folded acoustic modes with proper shifts of the longitudinal-optical (ΔωLO) and transverse-optical (ΔωTO) phonons. Changes in ΔωLO have instigated strong motives for calculating the Raman intensity profiles in the graded (CdTe)10-Δ/(CdTe0·5S0.5)Δ/(CdS)10-Δ/(CdTe0·5S0.5)Δ SLs by meticulously including the interfacial layer thickness Δ (≡ 1 to 3 monolayers (MLs)). With Δ (≡ 3 ML), the Raman intensity features have exhibited downwards (upwards) shifts of CdS (CdTe)-like ωLO modes by ∼ - 20 cm−1 (∼+18 cm−1) in good agreement with the strain-induced results. Appraisals of Raman intensity shifts in the confined optical mode regions (see: Supplementary material) are ascribed to the localization/delocalization of atomic amplitudes at the CdS–CdTe interface. We strongly believe that the reported phonon aspects in CdTe/CdS SLs will encourage Raman spectroscopists to validate our theoretical conjectures and engineers to evaluate potentials of the CdTe-based SLs to improve/design electronic devices of many contemporary applications.