Effect of Nh3/SiH4 Gas Ratios of Top Nitride Layer on Stability and Leakage in a-Si:H Thin Film Transistors

Abstract

ABSTRACTIn this paper, we present measurement results of stability and leakage current characteristics in inverted staggered hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) for different compositions of the top (passivation) nitride (a-SiNx:H). Here, we varied the deposition parameters, i.e., the ammonia (NH3) to silane (SiH4) gas ratio, of the passivation material for fixed composition of the (nitrogen-rich) gate nitride. When stressed with a prolonged gate bias, the observed shift in both threshold voltage (VT) and leakage current was largest in samples where the gas ratio (R = NH3/SiH4) was small. In the samples considered, R varied from 5 to 25. The shift in VT can be attributed to injection of energetic carriers from the a-Si:H and their subsequent trapping in the top a-SiNx:H layer. The trapping is reduced when the layer is nitrogen-rich.