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

Influences of silicon nitride (SiNx) films on the electrical performances of hydrogenated amorphous silicon thin film transistors (a-Si : H TFTs) are studied. Relatively low temperature (200 degrees C) SiNx films are prepared by plasma enhanced chemical vapour deposition at different radio-frequency powers. Results indicate that the SiNx films at a radio-frequency power of 340 W (Power density = 1.96 x 10(-1) W cm(-2)) are near-stoichiometric and have better interface quality. Therefore, a-Si : H TFTs with this SiNx 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 3.18V. 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 SiNx gate dielectric and a-Si : H active layers. However, when the surface roughness of the SiNx film is poor, the degradation from the a-Si : H/SiNx interface is predominated. Therefore, if the SiNx film is used as a gate dielectric layer to fabricate a-Si : H TFTs, the surface roughness and chemical composition of the SiNx film should be considered simultaneously.