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

Silicon carbonitride (SiCXNY) thin films have been deposited by magnetically confined 13.56 MHz PECVD, using SiH4, CH4, and NH3 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. SiCXNY materials produced with low SiH4 concentration (20%) presenting resistivities greater than 1014 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 SiNX dielectric and also with SiCX 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 cm2 V−1 s−1 for the sample with the composition SiC0.3N1.3:H0.93, corresponding to gas percentage of SiH4=20%, CH4=20% and NH3=60%. Small amounts of carbon in the SiNX network can improve the mobility in more than 13%. Carbon-rich SiCXNY 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.