Raman study of Cd1-xZnxTe phonons and phonon-polaritons-Experiment and ab initio calculations

摘要

Backward/near-forward Raman scattering and ab initio Raman/phonon calculations are combined, together with x-ray diffraction and ellipsometry measurements to further inform the debate on the compact phonon behavior of the II-VI Cd1-xZnxTe alloy. The compacity favors the coupling of polar optic modes in both the transverse and longitudinal symmetries via the related (E-L,E-T ) long-wave electric fields. The E-L-coupling achieves maximum in the Zn-dilute limit, which enhances the (upper) ZnTe-like (impurity) mode at the expense of the (lower) CdTe-like (matrix-like) one, leaving the impression of a unique {Cd-Te,Zn-Te}-mixed longitudinal optic (LO) phonon across most of the composition domain. However, the purely mechanical (non-polar) transverse optic (PM-TO) phonons, that hardly couple, reveal an underlying three-mode {1 x (Cd-Te),2 x (Zn-Te)} fine structure that distinguishes between Zn-Te vibrations in Zn- and Cd-like environments up to second neighbors. Further refinement arises by exploring the phonon-polariton (i.e., polar-TO) regime at large Zn content. On reducing the scattering angle, the E-T-coupling develops into a sequential softening of phonon-polaritons from ZnTe- down to CdTe-like ones, which transiently unveils a bimodal pattern behind the Cd-Te signal. Altogether, this results in a (rare) canonical four-mode {2 x (Cd-Te),2 x (Zn-Te)} percolation pattern for Cd1-xZnxTe, i.e., a close II-VI replica of the twin III-V In1-xGaxAs one-yet differing by two apparent LO modes and a sensitivity of bond vibrations limited to first-neighbors. Retrospectively, the difference in sensitivity of bond vibrations to the local environment between In1-xGaxAs (limited to first neighbors) and Cd1-xZnxTe (extending up to second neighbors) emerges as a rule throughout common (covalent) III-V and (ionic) II-VI semiconductor alloys.