Structural and electronic properties of glow-discharge-deposited a-Ge:H films as a function of the substrate potential

Abstract

Amorphous hydrogenated germanium films (a-Ge:H) were deposited in a glow discharge reactor starting with hydrogen diluted GeH 4. The deposition conditions were related to the film properties by means of an extended plasma characterization. The substrate bias voltage relative to the plasma was measured by a Langmuir probe for a wide range of applied RF power values and for electrode configurations of different area ratios. Electronic transport, defect structure and structural properties of a-Ge:H films deposited on both electrodes were determined by photoconductivity and dark conductivity, photothermal deflection spectroscopy and infrared absorption measurements. The main experimental results are: (a) The normalized photoconductivity (ημτ product) shows a monotonic rise with increasing negative bias voltage independent of the reactor configuration up to a value of 4 × 10 −7 cm 2/V at a substrate bias of −300 V. (b) At a threshold value of −100 V for the substrate bias, a step-like increase in the deposition rate is observed, the homogeneity of the films improves and the defect density and Urbach tail width decrease to previously unachieved minimum values of 6 × 10 16 cm −3 and 50 meV, respectively. Some general prerequisites for high quality a-Ge:H films are supported by these observations. They relate to either surface-related phenomena being influenced by the amount of ion bombardment on the growing film surface or the gas phase chemistry changing with the electron energy in the plasma. Possible candidates for film precursors in the low and high power regime are discussed.