Properties of gallium-doped hydrogenated amorphous germanium

D. COMEDI; F. FAJARDO; I. CHAMBOULEYRON

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

Año: 1995 vol. 52 p. 4974 - 4985

0163-1829

The effects gallium impurities on the dark-conductivity, band-gap, electronic density of states and hydrogen bonding in a-Ge:H were studied by dark-conductivity, optical and Infrared-transmission, and photothermal-deflection-spectroscopy measurements. The relative Ga concentrations in a-Ge:H films was varied between 3x10(-5) and 1x10(-2) by the cosputtering of solid Ge and Ga targets in a rf-plasma sputtering chamber. Significant changes in the conductivity in the whole studied concentration range were found. The sign of the thermopower reveals a change from n- to p-type conduction for Ga concentrations reaching a value of about (1-5) x 10(-4). The band gap remained unchanged for Ga concentrations lower than 1x10(-3). For higher Ga contents, a gradual narrowing of the band gap is observed. 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 was determined from the conductivity results. The total energy region swept by the Fermi level in the studied Ga concentration range is approximately 0.3 eV, similar to that reported in the Literature for B doping. A semiquantitative electronic density of states for a-Ge:H and its evolution with Ga doping was deduced by fitting an analytical model to the optical absorption data. A significant increase in the defect states density was found to develop for Ga contents higher than about 5x10(-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 Omega(-1) cm(-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 ca-Ge:H films having Ga atomic concentrations approaching 1x10(-2).