Use of hydrogenation in structural and electronic studies of gap states in amorphous germanium

A method of hydrogenating sputtered amorphous Ge has been developed for systematically studying the variation of the electronic properties with the number of dangling bonds. The role of incorporated hydrogen was established by the correlation of x-ray diffraction, electron paramagnetic resonance, and optical spectroscopy. A model for the logistics of incorporation of hydrogen was also developed and compared with experiment. The origin of the states in the gap of partially coordinated amorphous Ge was traced to the dangling bonds on void surfaces. These states span a large fraction of the energy gap of partially coordinated material, to the extent that transitions between them completely determine the optical absorption below photon energies of about 0.6 eV. (Optical absorption, across the band gap itself, in fully coordinated material, does not occur until about 1 eV.) However, the electron spin density is much smaller than the dangling-bond density, indicating that electrons most often pair to form weak molecular bonds or bipolarons on the void surfaces. Finally, the temperature dependence of the EPR linewidth appears to be correlated with the temperature dependence of the nonactivated conductivity.