Optical and Electrical Properties of Selenium-Tellurium Alloys

The optical and electrical properties of selenium-tellurium alloys have been studied. These properties depend upon the degree of order of the alloys. The bandgap of disordered evaporated layers of the alloys Se3Te and SeTe have been estimated to be 1.87 and 1.58 eV, respectively, from the break from the approximately exponential absorption edge. This is consistent with a linear variation of bandgap with composition. From the dispersion of the refractive index, another characteristic frequency larger than the bandgap is found. The disordered layers convert to a crystalline form when heated above room temperature. The conversion process has been followed by observing the change of resistivity of the layers with time. During much of the conversion, this change is approximately exponential. The time constant in this region has an exponential temperature dependence, being characterized by activation energies of 1.4 and 0.8 eV for Se3Te and SeTe, respectively. The rate of conversion of the alloys increases with increasing tellurium content. As with selenium, the ordered alloys are more conducting than the disordered alloys and the absorption in the low photon energy region is greater. An absorption band has been found in the ordered alloys which is associated with transitions between tellurium levels. The strength of the band is directly proportional to the tellurium concentration; the energy of separation of the levels, which is a reflection of the local crystal field, varies linearly with the composition of the alloys.