THEORETICAL AND EXPERIMENTAL-STUDY OF THE QUASI-STATIC CAPACITANCE OFMETAL-INSULATOR HYDROGENATED AMORPHOUS-SILICON STRUCTURES - STRONG EVIDENCE FOR THE DEFECT-POOL MODEL

The density of localized states in hydrogenated amorphous silicon (a-S i:H) is studied by means of the quasistatic capacitance technique appl ied to metal-insulator a-Si:H structures. Calculations in the framewor k of the defect-pool model show that the changes in the quasistatic ca pacitance 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 compa rison of theoretical qs-CV curves with experimental curves obtained in a wide range of bias-anneal voltages V-ba on several kinds of structu res (top gate oxide, top gate nitride, and the most commonly used bott om gate nitride structures) strongly support the defect-pool model, an d values for the model parameters are deduced, it is shown that for al l structures the dominant phenomenon for bias annealing at positive V- ba (i.e., under electron accumulation) is the creation of defects in t he lower part of the gap in the a-Si:H. Bias annealing under hole accu mulation 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 de pends 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].