Metastability under high-intensity light of device-quality He-diluted, H₂-diluted and standard a-Si:H films deposited between 50°C and 350°C

We report the results of a study of the metastability under illumination by high intensity red light of device quality a-Si:H thin films deposited using a wide range of deposition conditions. The process variables included substrate temperature, pressure, rf power, and dilution of silane by He or H₂. In-situ monitoring of the sample conductivity and defect density during light-soaking provides the kinetics of the degradation of the electronic properties of the films. We observe equilibration of the photoconductivity and of the defect density. The characteristic time of equilibration τ_se of the defect density varies by more than an order of magnitude, dividing the samples into two groups: one group with a τ_se on the order of 103 seconds, the other with a τ_seon the order of 10⁴ seconds. Low steady state defect densities combined with high n|rt products are observed for "standard" a-Si:H deposited between 100°C and 250°C and He-diluted films deposited above 250°C.