Electrical and structural properties of a-Ge:H alloys [solar cells]

As part of an effort to find optimal deposition conditions for a-Si_1-xGe_x:H alloys, the authors undertook an investigation of the deposition conditions required for making high-quality a-Ge:H films. The goal of this study was to find regions of parameter space which might prove to be fruitful for depositing improved alloy material. The a-Ge:H films were deposited using DC plasma-enhanced CVD (chemical vapor deposition) of germane as a function of temperature, pressure, power level, and either H₂ or Ar dilution. The film properties that were measured included the slope of the Urbach edge, the activation energy for dark conductivity, the ratio of photoconductivity to dark conductivity, the μτ product, and the infrared absorption. Generally, the best films were made with H₂ dilution (H₂/GeH₄⩾5:1) and at a relatively high temperature (i.e. with the heater temperature set at ~50°C higher than that used for depositing high-quality a-Si:H)