Properties of a-Si:H TFTs using silicon carbonitride as dielectric

Silicon carbonitride (SiC _XN _Y ) thin films have been deposited by magnetically confined 13.56 MHz PECVD, using SiH ₄, CH ₄, and NH ₃ as gas sources. The ternary system (Si, C, N) was exploited in order to determine the production conditions which lead to a material suitable for gate dielectric application in TFTs. RBS and ERD measurements were performed to determine the elemental composition of the films. SiC _XN _Y materials produced with low SiH ₄ concentration (20%) presenting resistivities greater than 10 ¹⁴ Ω cm were found to be promising gate dielectrics and were applied to a-Si:H TFT devices. The performance of these TFTs was compared to those made with standard SiN _X dielectric and also with SiC _X dielectric. Results show that threshold voltage and leakage current decrease with increasing carbon and hydrogen concentrations, while the mobility presents a maximum of 0.43 cm ² V ⁻¹ s ⁻¹ for the sample with the composition SiC _0.3N _1.3:H _0.93, corresponding to gas percentage of SiH ₄=20%, CH ₄=20% and NH ₃=60%. Small amounts of carbon in the SiN _X network can improve the mobility in more than 13%. Carbon-rich SiC _XN _Y presents good off-state characteristics, like low threshold voltage and leakage current, but lower on-state performance due to its lower dielectric constant and bulk resistivity.