The anomalous variation of band gap with alloy composition: cation vs anion substitution in ZnTe

The energy of the excitonic signature, E-g(x), in the wavelength modulated reflectivity spectrum of Zn1-xMgxTe shows a monotonic increase with x in co ntrast to a large downward bowing of E-g(x) in ZnSxTe1-x and ZnSexTe1-x. Fr ee and bound excitonic signatures in the photoluminescence spectra of the s ame samples provide additional confirmation for these trends. Raman spectra of these ternaries display a remarkable resonance enhancement of the zone center longitudinal optical (LO) phonon and its overtones when the scattere d photon energy approaches the free excitonic transition energy: for a fixe d exciting photon energy, the intensities of Raman lines with shifts of 1LO , 2LO, 3LO,... are selectively enhanced in sequence as a function of x, but in the reverse order in Zn1-xMgxTe as compared to that in ZnSexTe1-x, unde rscoring the monotonic increase of E-g (x) in the former and a significant downward bowing in the latter. (C) 1999 Elsevier Science Ltd. All rights re served.