Percolation-type multi-phonon pattern of Zn(Se,S): Backward/forward Raman scattering and ab initio calculations

Abstract

We perform a combined backward/near-forward Raman study of the multi-oscillator Zn(Se,S) zincblende alloy with particular attention to the transverse optical modes. Besides the three [1×(Zn–Se), 2×(Zn–S)] known modes apparent in backscattering, one novel Zn–Se mode is revealed in near-forward scattering, suggesting a four-mode [2×(Zn–Se), 2×(Zn–S)] percolation behavior (1-bond→2-mode) for Zn(Se,S). The latter four-mode pattern appears to violate most admitted rules behind the current version of the percolation scheme for a zincblende- or diamond-type alloy, in reference to Ga(As,P) and (Ge,Si), respectively, concerning (i) the number of Raman modes, and the composition dependence of their (ii) frequencies and (iii) intensities. It is explained within a refined version of the percolation scheme, now suggested, in which the 1-bond→2-mode behavior is echoed among all constituent bonds of an alloy – referring to (i), the phonon dispersion is taken into account besides the local strain – referring to (ii), and both the Zn–Se and Zn–S bonds exhibit an unusual sensitivity to their local environment at the second-neighbor scale – referring to (iii). The discussion is supported by ab initio phonon calculations using prototype (S,Se)-impurity motifs, and by a contour modeling of the multi-phonon-polariton Zn(Se,S) near-forward Raman spectra within the formalism of the linear dielectric response, based on ellipsometry measurement of the Zn(Se,S) refractive index. Particular attention is awarded to the coupling of neighboring transverse optical modes via their developing transverse electric field when entering the phonon-polariton regime.