Charged dangling bonds in undoped amorphous silicon

We report a basic disproportionality between light-induced changes in the defect density of undoped hydrogenated amorphous silicon (a-Si:H) determined from optical absorption and determined from spin density measurements. The results yield a lower limit for the ratio of charged to neutral dangling bonds in annealed a-Si:H of around 2–2.5, and in degraded material this ratio is reduced. The derived minimum ratio holds if charged dangling bonds are caused by unintentional impurity doping, a ratio approximately twice as large is obtained if the charged dangling bonds are formed according to thermodynamical equilibration models. The observed Fermi level shifts and optical absorption spectra during light soaking and annealing favor the thermodynamical equilibration model.