PHOTOACOUSTIC SPECTROSCOPIC STUDY OF ENERGY-GAP, OPTICAL-ABSORPTION, AND THERMAL-DIFFUSIVITY OF POLYCRYSTALLINE ZNSEXTE1-X (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) ALLOYS

Using the photoacoustic spectroscopic (PAS) technique, we report on the results of composition-dependent optical band gap (E0), absorption coefficient (α), and thermal diffusivity (σ) of the ZnSexTe1-x- (0≤x≤1) type alloys. The band gap is found to vary nonlinearly with composition, showing downward bowing with a minimum around x=0.32. Unlike AIBIIIC2VI chalcopyrite-structure alloys [Phys. Rev. B 44, 163 (1991)], the nonlinear variation of E0 with x for the Zn-Se-Te system cannot be explained by the electronegativity difference between the substituting atoms. However, the gap parameter E0(x) can be expressed as E0(x)=a+bx+cx2 (where a, b, and c are real parameters calculated for these alloys). Similar downward bowing is also observed from the concentration-dependent band-edge effective mass (me*) of the carriers (also obtained from the present experimental data) indicating that the bowing is an intrinsic property of the Zn-Se-Te-type materials. The exponential edge (Urbach edge) is observed for these samples. The absorption process can be considered as an internal Franz-Keldysh effect arising due to the phonon-generated electric field. It could be described in the framework of the Dow-Redfield model. The determination of the thermal diffusivity and the phonon-assisted indirect transition at the band tail of these samples are also results for these materials obtained from the present PAS studies. © 1995 The American Physical Society.