High hole mobility and non-localized states in amorphous germanium

Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to occur through the hopping mechanism via these localized states. The carrier mobility of these materials is usually very low, in the order of ∼10⁻³-10⁻² cm²/Vs at room temperature. In this study, we show that pure high-density amorphous Ge has exceptionally high carrier mobility, in the order of ∼100 cm²/Vs, and a high hole concentration of ∼10¹⁸ cm⁻³. The temperature-dependent conductivity of the material is also very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results provide direct evidence for a largely preserved band structure and non-localized states within the valence band in high-density amorphous Ge, as previously suggested by Tauc et al. from optical characterization alone.