Influences of low temperature silicon nitride films on the electrical performances of hydrogenated amorphous silicon thin film transistors

Influences of silicon nitride (SiN x ) films on the electrical performances of hydrogenated amorphous silicon thin film transistors (a-Si : H TFTs) are studied. Relatively low temperature (200 °C) SiN x films are prepared by plasma enhanced chemical vapour deposition at different radio-frequency powers. Results indicate that the SiN x films at a radio-frequency power of 340 W (Power density = 1.96 × 10 −1 W cm −2 ) are near-stoichiometric and have better interface quality. Therefore, a-Si : H TFTs with this SiN x gate dielectric layer have a high field effect mobility and sustain the bias stress. The field effect mobility is 0.59 cm 2 V −1 s −1 and the threshold voltage shift after a constant voltage stress (CVS) for 2.8 h is 3.18 V. The electrical degradation mechanism of a-Si : H TFTs is studied from the capacitance–voltage measurement. The degradation of the a-Si : H TFT after CVS is due to the defect generation in the SiN x gate dielectric and a-Si : H active layers. However, when the surface roughness of the SiN x film is poor, the degradation from the a-Si : H/SiN x interface is predominated. Therefore, if the SiN x film is used as a gate dielectric layer to fabricate a-Si : H TFTs, the surface roughness and chemical composition of the SiN x film should be considered simultaneously.