Theoretical and experimental study of the quasistatic capacitance of metal-insulator hydrogenated amorphous silicon structures: Strong evidence for the defect-pool model

The density of localized states in hydrogenated amorphous silicon (a-Si:H) is studied by means of the quasistatic capacitance technique applied to metal-insulator a-Si:H structures. Calculations in the framework of the defect-pool model show that the changes in the quasistatic capacitance versus gate bias curves (qs-CV curves) after bias annealing reveal the changes in the density of dangling-bond states predicted by the model, and are sensitive to the defect-pool parameters. The comparison of theoretical qs-CV curves with experimental curves obtained in a wide range of bias-anneal voltages V-ba on several kinds of structures (top gate oxide, top gate nitride, and the most commonly used bottom gate nitride structures) strongly support the defect-pool model, and values for the model parameters are deduced, it is shown that for all structures the dominant phenomenon for bias annealing at positive V-ba (i.e., under electron accumulation) is the creation of defects in the lower part of the gap in the a-Si:H. Bias annealing under hole accumulation reveals the creation of defects in the upper part of the gap of a-Si:H, but the precise dependence of the qs-CV curves upon V-ba depends on the nature of the insulator-a-Si:H interface. In particular, it is affected by a higher density of interface trap levels in the top gate nitride structures, and by hole injection and trapping from the a-Si:H into the nitride layer in the bottom gate nitride structures. [S0163-1829(98)03040-9].