Experimental studies on n-type conduction in Bi Doped Ge-Se glasses : role of phase-separation and defects

Abstract

In summary, the earlier evidence for phase separation in Ge??Se????Bi? glassy systems was discussed. Phase?separation evidence at both microscopic and macroscopic levels was also found in another Bi?doped glass system, Ge??Se????Te??Bi?. In this latter system, a three?fold coordination for Bi atoms can better explain the changes in configurational heat capacity and the activation energies for glass transition and crystallization. To support the phase?separation evidence, the ??Bi?Se? phase (having three?fold coordinated Bi atoms) has been considered instead of ??Bi?Se? (having six?fold coordinated Bi atoms). Carrier?type determination in ??Bi?Se? revealed that it is p?type. Based on the evidence of phase separation and the p?type characteristics of ??Bi?Se?, the role of phase separation in bringing about p?type conduction in Bi?doped glasses is explained. This explanation is consistent with the evidence for three?fold coordinated Bi atoms. A discussion on the interdependence of inherent defects, gap states, and photoluminescence was provided. Detailed photoluminescence studies were carried out in a?Se, Bi?doped Ge??Se??, and Ge??Se??Te?? glasses. For all glasses that showed photoluminescence, the spectra contained fine structure revealing PL transitions at three distinct energies. These spectra were deconvoluted into a combination of three Gaussians. The individual deconvoluted spectra were identified with transitions involving gap states associated with VAPs (random D? and D? defects) and IVAPs (non?random D? and D? defects). With this identification, it was found that Bi addition in both Ge??Se?? and Ge??Se??Te?? glass systems brings about a relative reduction in D? defects, which subsequently leads to p?type conduction. Thus, the role played by defects in producing p?type conduction in Bi?doped glasses has been clearly established. In summary, the glassy system Ge??Se??????In?Bi? provides scope for studying the effects of incorporating additional elements in n?type Bi?doped samples. The studies show that indium (In) addition induces a reversal of structural and electrical changes produced by bismuth (Bi) addition. When the contribution of extra electrons accompanies structural inhomogeneities, the current in the corresponding glassy matrix increases significantly, as in Ge??Se??In?Bi??. The present studies also reveal that improved p?type conduction with Bi addition is associated with a significant shift in the Fermi level toward the valence band. This is augmented by microscopic?level inhomogeneities and a substantial reduction in the relative concentration of D? defects.