Electrical Properties of Cu-As-Te-Se Amorphous Semiconductors

Abstract

Cu-As-Te-Se bulk glasses are characterized utilizing a suite of experimental techniques to understand their electrical properties with the goal of providing new strategies for the production of a novel class of advanced amorphous thermoelectric materials. Currently, the electrical properties of Cu-As-Te-Se glasses are two orders of magnitude too low to yield competitive thermoelectrics; hence, a deeper understanding of their electrical transport properties may provide the needed insight to enhance their figure of merit, ZT. To that avail, the structural, electrical, optical and defect characteristic of bulk Cu-As-Te-Se glasses were investigated. The atomic structure of Cu-As-Te ternary glasses was investigated with 64Cu, 125Te NMR, and Raman spectroscopy to determine the local environment of the copper atoms. It was shown that copper exists in a four-fold coordination environment and generates CuTe4 tetrahedra in all CuAs-Te glasses. The structure of these glasses was then compared to that of Ge-As-Te glasses where Ge is also tetra-coordinated. It was found that Ge-As-Te glasses contain GeTe4 tetrahedra only at low Ge concentration and Te remains two-coordinated, unlike in the case of Cu which increases Te’s coordination to three through dative bonding at all concentrations. The contrast between the two glass structures is further exemplified by the inherent difference copper plays in modifying the structure as observed in the increase in density of Cu-As-Te with Cu content while the Ge-As-Te glasses show an opposite trend with Ge content. The electrical properties of Cu-As-Te-Se glasses over large concentrations of Cu, up to 25%, were then characterized to determine the effect of copper on the conduction properties. Electrical measurements revealed three conduction mechanisms: extended state conduction at high temperature, localized state conduction at medium temperature, and variable range hopping (VRH) at low temperature. Introduction of Cu had the effect of decreasing the activation energy of localized and extended state conduction and greatly increased the extent of VRH and the density of mid-gap states. This effect was measured as a function of concentration. In order to determine whether the decrease in conduction activation energy was the result of a shift in the Fermi level or a reduction in band gap energy, optical properties of the glasses were characterized as a function of Cu content. A comparison of the optical band gap energy with the electrically measured band-gap Ec- EF revealed that the fermi-level does not shift with Cu addition despite the large increase in mid-gap states responsible for VRH. This pinning of the Fermi level indicates that the population of defects in Cu-As-Te-Se glasses is not sufficient to achieve doping. In order to gain a deeper understanding of the gap states and their contribution to the electrical properties, glassy samples of Cu-AsTeSe and Cu-AsTe were characterized by EPR spectroscopy. A strong EPR signal is observed in these glasses indicating that large populations of unpaired electrons associated with dangling bonds are present as copper is added to the matrix. The spin density found by EPR is found to be equivalent to the gap state density derived from conductivity measurement. The observation of high spin densities in these glasses is contrary to expectation from conventional chalcogenide glasses where unpaired electrons normally relax into Valence Alternation Pairs (VAPs). It is also found that the population of unpaired spins is temperature dependent. Overall, the sum of the data obtained from NMR, Raman, density, optical, electrical and EPR experiments permitted to derive a comprehensive picture of the band structure in Cu-As-Te-Se glasses. It is found that the gap states lie below the Fermi level, but above the negatively charged valence alternation pair states. This also indicates that the density of state is negligible at the Fermi level. While the exact nature of the unpaired electron defects was not resolved by EPR, it is strongly correlated to the Cu atom at all concentrations and likely originates from a defective Cu-Te bond resulting from structural disorder. The weaker Cu-Te bond and the structural rigidity of the fourfold Cu in the glass network may favor the formation of these stable unpaired defects.