Amorphous Silicon Thin Film Transistors With High Electron Field Effect Mobility

Abstract

ABSTRACTNormal staggered hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT) were prepared by rf plasma deposition through a three-step process. The TFTs were constituted by an a-SiN/a-Si:H structure grown on NiCr source-drain electrodes evaporated on glass substrates. The intrinsic a-Si:H active layer (Fermi level at EC-EF = 0.7 eV) was deposited from pure SiH4 rf plasma, and the insulator layer of a-SiN was grown using a high rf power plasma (200 mW/cm2) of SiH4-N2 mixture with a SiH4 fraction of 0.5 %. Ellipsometric measurements showed that a very transparent a-SiN film was grown with an abrupt interface insulator/a-Si:H. TFTs with 0.2 μm thick a-Si:H layer and 10 μm channel length have on-off current ratios of 5 104, electron field effect mobility of 1.5 cm2/V-s (dielectric constant εri ≈ 7.9), and threshold voltage around 5 V. The results are discussed in terms of low hydrogen content and low porosity of these a-SiN films prepared from silane-nitrogen.