Nanocrystalline Silicon by Microwave CVD for Thin Film Transistors and Solar Cells
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NANOCRYSTALLINE SILICON BY MICROWAVE CVD FOR THIN FILM TRANSISTORS AND SOLAR CELLS
Young J. Song, Hak-Gyu Lee, Lihong Teng and Wayne A. Anderson, SUNY at Buffalo, Dept. of Electrical Engineering, 208 Bonner Hall, Buffalo, NY 14260
Abstract Microwave chemical vapor deposition (MCVD) is utilized to deposit nanocrystalline silicon (nc-si) thin films onto a variety of substrates for application to thin film transistors (TFT’s) and solar cells. It is especially important to gain reproducible control of the processing. Thus, an in-situ mass spectrometer (MS) records the plasma conditions with variation of process conditions such as gas selection, pressures, partial pressures, and substrate temperature. These data are correlated with electrical and optical properties of the films. Raman spectra show a FWHM of 11/cm with position at 522/cm as desired for crystalline Si. Typical film thickness is 100nm with grain size of 20-30 nm, using standard deposition, and 50-80 nm when the substrate is intensely optically illuminated during deposition, called photon assist (PA). Hydrogen dilution serves to increase the crystallinity of the films. The ratio of photo-to dark conductivity exceeds 10+5 with dark conductivity as low as 1.5 x 10-10 S/cm. Thin film transistors have been fabricated with Ion/Ioff of 10+7. Hetrojunction solar cells were fabricated using amorphous Si/ nc-Si/ crystlline Si giving a conversion efficiency of above 10.5%, without an antireflection coating. The use of MS in device design will be emphasized.
Introduction We are studying deposition of amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si) by microwave-generated plasma electron cyclotron resonance (ECR) with application to thin film transistors and solar cells [1,2]. Microwave-generated plasma processing has several advantages over dc- or rf-generated plasma processing, such as high fraction of ionization and dissociation, high electron kinetic temperature, and no electrode inside [3]. The in-situ mass spectrometer (MS) is widely used in the plasma-assisted chemical vapor deposition (CVD) to investigate plasma process conditions. The MS profile shows relative and absolute intensities of atomic mass population in deposition which are crucial to reproduce the same plasma conditions and to find the best condition. The early research has focused on the importance of deposition pressure and microwave power [4], and showed that those have been essential in deciding electro-optical properties of devices. In this paper, we emphasize on reproducibility of plasma conditions, such as gas selection, partial pressure of gases, and inlet gas pressure which are variables in the microwavegenerated plasma ECR, to show how these parameters influence plasma conditions and electrooptical properties, especially values of photo and dark conductivity.
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