Optimization of n(+) nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H-2 dilutions. High H-2 dilution resulted in n(+) nanocrystalline silicon films (n(+) nc-Si:H) with the lower resistivity (rho similar to 0.7 Omega cm) compared to that of doped amorphous silicon films (similar to 900 0 cm) grown at low H-2 dilution. The change of the lateral rho of n(+) nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The rho values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n(+) nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (similar to 92 Omega cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiNx:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH3/SiH4 gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiNx:H film. Having lowest leakage currents, two a-SiNx:H films with NH3/SiH4 ratios of similar to 19 and similar to 28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH3/SiH4 similar to 19 ratio showed higher device performance than the TFT containing a-SiNx:H with the NH3/SiH4 28 ratio. This was correlated with the N-H/Si-H bond concentration ratio optimized for the TFT application