EFFECTS OF PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITION SUBSTRATE HEATING ON THE ELECTRICAL-PROPERTIES OF ALPHA-SI-H THIN-FILM TRANSISTORS

Hydrogenated amorphous silicon thin film transistors (alpha-Si:H TFTs) have been developed by plasma enhanced chemical vapor deposition (PECVD) at low fabrication temperatures from 150 to 330-degrees-C. Silicon nitride, silicon oxide and alpha-Si:H films deposited at different substrate heating temperatures were studied. Electrical properties of silicon nitride were tested by time-zero-dielectric-breakdown (TZDB) measurements. Breakdown field of silicon nitride about 8-10 MV/cm can be achieved at deposition temperature of 330-degrees-C. Two different reaction mechanisms were observed for the deposition of alpha-Si:H from 150 to 330-degrees-C. In the lower temperature regime, the deposition is gas limited by reaction in the gas phase, whereas surface reaction dominates the higher temperature regime. In both regimes, refractive index of alpha-Si:H increases with the substrate heating temperature. The transition temperature is 250-degrees-C. As a result, a drop of refractive index of alpha-Si:H film deposited at 250-degrees-C is observed. From the IR spectra, the amorphous silicon films deposited at the substrate heating temperatures from, 150 to 330-degrees-C have the absorption in the ''stretching'' mode region centered at about 2020 cm-1 and ''wagging'' mode region at 630 cm-1. These peaks are the characteristic absorptions of predominant SiH bonding. Furthermore, the influences of silicon nitride and hydrogenated amorphous silicon films deposited at different substrate heating temperatures from 150 to 330-degrees-C on the device characteristics, i.e., on/off current ratio, threshold voltage, and field effect mobility were investigated. The experimental results indicate that the electric properties of TFTs are influenced by the substrate heating temperatures, as is the refractive index of alpha-Si:H. A drop of electric performance was observed for the TFT deposited at 250-degrees-C owing to a transition of reaction mechanism at 250-degrees-C. The electrical performances of TFT increase with substrate heating temperature in both reaction regimes due to less defects in the deposited films at higher temperature.