INVESTIGATION OF THE HYDROSTATIC-PRESSURE DEPENDENCE OF THE E(0) GAP, THE EXCITONIC BINDING-ENERGY AND THE REFRACTIVE-INDEX OF MOCVD-GROWN ZNTE LAYERS

We have determined the pressure dependence of the excitonic transition energy in thin MOVPE ZnTe layers by measuring the absorption coefficient and the refractive index. Pressures up to 12 GPa were applied in a diamond anvil cell. A special gasket technique was used to suppress the scattered light, The absorption coefficient was measured in 1-mu-m thick films at T = 115 K and at T = 300 K up to 50 000 cm-1. The spectra are dominated by a sharp excitonic structure. Its pressure dependence is evaluated using a model which allowed us to determine the rate of pressure-induced shift of the binding energy dE(x)/dP and the rate of change of the Rydberg energy dR(x)*/dP. The refractive index n was measured on 2.4-mu-m-thick films at T = 300 K and at T = 115K by observing the pressure shift of the.interference extrema in the transparency region. The resulting refractive index was extrapolated to the frequency of the longitudinal optical phonon to obtain the constant epsilon(infinity). The static dielectric constant epsilon(st) was obtained using the Lyddane-Sachs-Teller relation. Using this value and the determined effective masses at the applied pressures, we calculated the exciton binding energy. At the pressure P = 9.4 GPa, a phase transition was observed. The obtained spectra for this phase were compared with a model describing indirect transitions in semiconductors. A further phase transition was observed at P = 10.9 GPa.