Growth study of wide bandgap a-Si:H and a-SiN:H by PECVD method for application in thin film transistor

The growth of hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN:H) films was studied by plasma enhanced chemical vapor deposition (PECVD) method. 10% silane (SiH₄) diluted in hydrogen (H₂) gas and 100% ammonia (NH₃) gas were used as gas sources. The optical band-gap and deposition rate of a-Si:H film were found to vary from 1.70 to 1.95 eV and 51 to 84 Å/min, respectively, when the SiH₄ gas flow rate varied from 5 to 11 sccm. The widest optical bandgap of a-SiN:H films of 3.69 eV and lowest dark conductivity of 1.07×10^-11 Scm^-1 were obtained at NH₃ gas fraction of 60% at SiH₄ flow rate of 7 sccm. It is also shown that a wider optical bandgap for a-SiN:H can be obtained at a SiH₄ gas flow rate of 5 sccm, where its value reaches 3.97 eV at NH₃ gas fraction of 25%, whilst its dark conductivity reaches a lower value of 1.05×10^-12 Scm^-1. The application of the films as a gate insulator in the thin film transistor (TFT) device was also studied. The lowest dark conductivity of the a-SiN:H film resulted in a better device threshold voltage