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 (SiH/sub 4/) diluted in hydrogen (H/sub 2/) gas and 100% ammonia (NH/sub 3/) 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 /spl Aring//min, respectively, when the SiH/sub 4/ 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/spl times/10/sup -11/ Scm/sup -1/ were obtained at NH/sub 3/ gas fraction of 60% at SiH/sub 4/ flow rate of 7 sccm. It is also shown that a wider optical bandgap for a-SiN:H can be obtained at a SiH/sub 4/ gas flow rate of 5 sccm, where its value reaches 3.97 eV at NH/sub 3/ gas fraction of 25%, whilst its dark conductivity reaches a lower value of 1.05/spl times/10/sup -12/ Scm/sup -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.