Percolation effect in the vibrational spectra of mixed crystals with highly contrasted bond stiffness

Abstract

We present a detailed investigation by Raman spectroscopy of the zone-center longitudinal (LO) and transverse (TO) optical phonons from Zn 1− x Be x Te epilayers with special emphasis on samples with low-medium Be content. On top of the expected ZnTe- and BeTe-like modes, which obey the standard modified-random-element-isodisplacement (MREI) model, we observe an additional BeTe-like mode at intermediate frequency. At medium Be-content (0.2< x<0.61), this extra mode is a real (TO, LO) doublet. We show that its frequency dependence versus x obeys a ‘restricted’ MREI description. On the strength side, it is remarkable that the extra mode grows at the cost of the other BeTe-like mode when x increases; the balance is equilibrated at x∼0.5. At low Be content ( x≤0.2), the Raman response from the extra mode changes in nature; it turns from a restricted MREI-like (TO, LO) doublet into a TO–LO degenerated mode with fixed frequency. One key point is that the latter stays below the local mode of Be in ZnTe. The present results enrich our ZnBeSe-based percolative picture for multimode description in Be–chalcogenide alloys that open the attractive class of mixed crystals with a sharp contrast in the bond stiffness.