Effect of Nh 3 /SiH 4 Gas Ratios of Top Nitride Layer on Stability and Leakage in a-Si:H Thin Film Transistors
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ABSTRACT In 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 (SiH 4) 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 = NH 3/SiH 4) 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.
INTRODUCTION The a-Si:H TFT is widely used as a switching element in large area electronics such as liquid crystal displays [1], and more recently, in optical and x-ray image sensor arrays [2]. However, since its development a few years ago, there still remains several issues. Among them are the stability in threshold voltage (VT) and leakage current, which subsequently limit the widespread use of the TFT in large area electronics. The instability in a-Si:H TFTs, i.e. the variation in threshold voltage (AVT) after prolonged gate bias, should be low to enable fast and reliable readout of the signal charge in imaging arrays. The leakage current should be small enough for retention of the signal charge on the sensor during the OFF-state of the TFT. The instability can be attributed to two mechanisms: carrier injection into the insulating layers [3] and the dangling bond formation associated with weak Si-Si bonds in the a-Si:H region [4]. The latter occurs when the TFT is biased into strong accumulation. The leakage current has been identified to stem mainly from carrier injection in the reverse biased parasitic p-i-n diode at the drain region and from ohmic conduction associated with diffusion of phosphorous atoms into the a-Si:H layer from the highly doped microcrystalline silicon (n+.ic-Si:H) source and drain contact layers. In inverted staggered TFT structures, since the top nitride layer is in direct contact with the active a-Si:H region, it must influence the stability of the VT as well as the leakage current. While there have been few reports relating the composition of the top nitride on stability, its effect on the leakage current has hardly been broached (see [5]). Nakamura et al. have attributed the instability to electron injection from the a-Si:H into the nitride. However, the nitride layer in their samples did not vary in nitrogen content [6]. In contrast, Choi et al. have associated the instability in their TFTs with hole trapping at the top a-Si:H/a-SiNx:H interface, and have proposed an experimental procedure to separate the effects of the top and gate nitride on the 73
Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Soci
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