Properties of gallium-doped hydrogenated amorphous germanium

The effects of adding small quantities of gallium atoms to hydrogenated amorphous germanium (a-Ge:H) on its dark-conductivity, band-gap, electronic density of states and the hydrogen bonding, were studied in detail by dark-conductivity, optical and infrared-transmission, and photothermal-deflection-spectroscopy measurements. Films of a-Ge:H having relative Ga atomic concentrations ranging between 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ and 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$ were deposited by the cosputtering of solid Ge and Ga targets in a rf-plasma sputtering chamber. The Ga incorporation was found to produce significant changes in the conductivity in the whole studied concentration range. The sign of the thermopower signal reveals a change from n- to p-type conduction for Ga concentrations reaching a value of about (1--5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. No variations in the magnitude of the band gap were detected for Ga atomic concentrations lower than 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. For higher Ga contents a gradual narrowing of the band gap is observed, which is not correlated by modifications in the hydrogen content nor in the H bonding structure as deduced from the infrared transmission spectra. The present results show that Ga acts as a p-type dopant in a-Ge:H, with a doping efficiency comparable to that of boron in a-Ge:H.The position of the Fermi level with respect to the conduction path levels was determined from the conductivity results, assuming a common constant prefactor for the observed thermally activated dark conductivity for both n- and p-type samples. The total energy region swept by the Fermi level in the studied Ga concentration range is approximately 0.3 eV, which is about the same as that reported in the literature for B doping of a-Ge:H. A semiquantitative electronic density of states distribution for a-Ge:H and its evolution with Ga doping was deduced systematically by fitting an analytical model of the optical absorption data. A significant increase in the defect states density was found to develop for Ga contents higher than about 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$. This effect seems to be correlated with the Fermi level approaching the valence band tail region. Conductivity prefactors for both electrons and holes in the 140--260 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ range were deduced from the analysis of the present conductivity and absorption data, which are in fair agreement with values reported in the literature for hydrogenated amorphous silicon and theoretical estimates. Significant tendency for Ga segregation was observed for a-Ge:H films having Ga atomic concentrations approaching 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}2}$.