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

Abstract

Silicon carbonitride (SiC X N 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 X N 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 X N 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.