The Effect of Hydrogen Incorporation on the Performance of Nanocrystalline Silicon Thin Film Transistors Fabricated by M
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The Effect of Hydrogen Incorporation on the Performance of Nanocrystalline Silicon Thin Film Transistors Fabricated by Microwave ECR Plasma CVD Lihong Teng and Wayne A. Anderson Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14226. ABSTRACT Nanocrystalline silicon (nc-Si:H) TFT’s with the active layers deposited by microwave ECR-CVD were fabricated and characterized using hydrogen dilution as a variable during the Si film deposition. The bottom-gated TFT’s were fabricated on SiO2, glass and polyimide substrates at a temperature of 200-400°C with the introduction of 4-12 mTorr H2. The correlation between the H2 dilution and the TFT characteristics was studied and the experimental results were correlated with some theoretical considerations. The TFT on a glass substrate deposited at 400°C with 10 mTorr H2 showed a field effect mobility of 13.3 cm2/V-s and an ON/OFF current ratio of 4.9x106 with the OFF-state leakage current of 3.7x10-11 A. We found that, for the same Si film deposition conditions, the TFT’s fabricated on polyimide foil have comparable characteristics to the TFT’s on glass substrates. INTRODUCTION Thin film transistors made on nanocrystalline silicon are important because of their lowtemperature fabrication process, eliminated need of post-deposition anneal and compatibility with the low-cost, flexible substrates, which are important in the applications of active matrix liquid crystal displays [1]. Compared with a-Si:H, nc-Si:H has a higher electrical conductivity, a low optical absorption coefficient and a better stability. As a result, the p-i-n solar cell incorporating nc-Si:H as the absorbing layer produces a high photocurrent and is stable against light soaking [2]. Thin film transistors with nc-Si channel layers have higher ON-current and higher mobility than a-Si TFT’s [3]. The dilution of silane in hydrogen has been recognized as a critical parameter for the transition from the amorphous to the nanocrystalline phase. Atomic H on the growing surface gives termination of dangling bonds and an extraction of SiH3 radicals. The supply balance between the atomic H and SiH3 radicals is a key factor in determining the film structure [4]. The presence of excess hydrogen in the gas mixture passivates the dangling bonds on the growing surface and etches the growing surface. Etching eliminates part of the disordered structure and enhances the crystalline phase because the crystalline structure is the lowest energy configuration, which is the most stable [5]. Research has been oriented towards depositing a more stable Si film with larger grain size and with an acceptable deposition rate. Various methods for the growth of nc-Si:H have been proposed, including RF PECVD [6], VHF PECVD [3], ECR-CVD [7] and hot-wire CVD [8]. However, there are fewer reports on the TFT’s fabricated on the as-deposited nc-Si:H. In this work, we fabricate nc-Si TFT’s on glass and polyimide substrates with the Si films deposited by microwave ECR-CVD. Microwave powe
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