Photoacoustic spectroscopic study of energy gap, optical absorption, and thermal diffusivity of polycrystalline ${\mathrm{ZnSe}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{-}\mathit{x}}$ (0≤x≤1) alloys

Using the photoacoustic spectroscopic (PAS) technique, we report on the results of composition-dependent optical band gap (${\mathit{E}}_0$), absorption coefficient (α), and thermal diffusivity (σ) of the ${\mathrm{ZnSe}}_{\mathit{x}}$ ${\mathrm{Te}}_{1\mathrm{-}\mathit{x}}$- (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 ${\mathit{A}}^{\mathrm{I}}$ ${\mathit{B}}^{\mathrm{III}}$ ${\mathit{C}}_2^{\mathrm{VI}}$ chalcopyrite-structure alloys [Phys. Rev. B 44, 163 (1991)], the nonlinear variation of ${\mathit{E}}_0$ with x for the Zn-Se-Te system cannot be explained by the electronegativity difference between the substituting atoms. However, the gap parameter ${\mathit{E}}_0$(x) can be expressed as ${\mathit{E}}_0$(x)=a+bx+${\mathit{cx}}^2$ (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 (${\mathit{m}}_{\mathit{e}}^{\mathrm{*}}$) 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.