Band tails and deep-defect density of states in hydrogenated amorphous germanium

A correlation between the midgap-defect-state density and the Urbach energy is experimentally observed in undoped hydrogenated amorphous germanium (a-Ge:H) thin films prepared by the glow-discharge technique under different conditions. Since these samples show thermally induced metastable effects, the data are analyzed with a thermal-equilibration model recently proposed for a-Si:H. This yields a neutral Ge dangling-bond (D/sup 0/) energy at 0.50-0.55 eV above the valence-band edge