Gate dielectric and contact effects in hydrogenated amorphous silicon-silicon nitride thin-film transistors

The characteristics of glow-discharge hydrogenated amorphous silicon-silicon nitride (a-Si/a-SiNx) thin-film transistors (TFTs) are reported for various deposition conditions. TFTs incorporating a N-rich nitride gate dielectric, a-SiN1.6, are superior to a-Si TFTs with a Si-rich gate nitride, a-SiN1.2. In particular, the N-rich gate nitride TFTs show considerably less interface or near-interface charging during operation, improved stability, and a higher field-effect mobility. The average field-effect mobility μFE is found to be 0.27 and 0.41 cm2/V s for the Si- and N-rich gate nitride TFTs, respectively. A further improvement in mobility, μFE =0.61 cm 2/V s, is achieved by increasing the N-rich gate nitride deposition temperature from 250 to 450 °C. These results suggest that N-rich a-SiN x, deposited at elevated temperatures, yields a more abrupt or "cleaner" a-SiNx/a-Si interface. We also show, for the first time, that using n+ μc-Si source-drain contacts in place of n+ a-Si improves device performance, yielding an average field-effect mobility of 0.84 cm2/V s, with an activation energy of 0.069 eV.