Effect of Plasma Damage on Interface State Density Between a-Si:H and Insulating Films

ABSTRACTThe interface state density between a-Si:H and insulating film is very important in characteristics of a-Si:H thin film transistors. In this study, the interface state density was measured by photothermal deflection spectroscopy (PDS). Layered structures of a-SiN, 1.7:H on a-Si:H and a-SiO2.0 on a-Si:H were deposited by P-CVD method. The a-SiN1.7:H layer was grown from a gas mixture of SiH4 and NH3 and the a-SiO2.0 layer was grown from a gas mixture of SiH4 and N2O. While the interface state density of a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density of a-Si:H, that of a-SiO2.0 on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens is discussed in terms of plasma damage of a-Si:H surface by the source gases during deposition of insulating layer. Surface of the a-Si:H was treated by plasma of NH3 or N2o gas which is dominant constituent of source gases of insulating layer. Although the surface state density of the N2o plasma-treated sample increases, that of NH3 plasma treated sample does not increase. The shape and intensity of the spectra of N2o plasma-treated sample is similar to that of the a-Sio2.0 on a-Si:H structure. These results indicate that the interface defect between a-Sio2.0and a-Si:H layer was induced by plasma damage of the a-Si:H surface.