Raman study of the random ZnTe–BeTe mixed crystal: Percolation model plus multimode decomposition

The Verleur and Barker-like multiphonon description proposed for the refined understanding of the two-mode transverse optical (TO) and longitudinal optical (LO) Raman responses from the stiff∕short Be–Se bond in the percolation range (0.19⩽Be⩽0.81) of the random BeSe–ZnSe mixed crystal [O. Pagès et al., Phys. Rev. B 70, 155319 (2004)] is extended to the similar Be–Te bond in the percolation range of the random BeTe–ZnTe mixed crystal. Moreover this description is shown to remain valid just outside the percolation range, in spite of appearances. We deduce that it is not activated by the close intermixing of the stiff and soft continua at the mesoscopic scale but relies directly on the contrast in the bond properties at the microscopic scale. The whole study is supported by extensive ZnBeTe Raman data, and full contour modeling of the TO and LO Raman line shapes by using our phenomenological 1-bond→2-mode percolation model. Care is taken that our ZnBe(Se,Te) epitaxial layers undergo large substrate-induced residual biaxial strains at the two edges of the percolation range. This generates significant phonon shifts, which have to be taken into account.