Hydrogen Dilution effects on Amorphous Hydrogenated Silicon (a-Si:H) Prepared by Glow Discharge Near Room Temperature
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HYDROGEN DILUTION EFFECTS ON AMORPHOUS HYDROGENATED SILICON (a-Si:H) PREPARED BY GLOW DISCHARGE NEAR ROOM TEMPERATURE
MAN KEN CHEUNG AND MARK A. PETRICH Department of Chemical Engineering, The Robert R. McCormick School of Engineering and
Applied Science, Northwestern University, Evanston, IL 60208-3120, USA
ABSTRACT We report here the first phase of our attempt to deposit "device-quality" a-Si:H films by glow discharge near room temperature (50 *C). We have found that, at a low rf power density (40 mW/cm 2 ) and high hydrogen/silane ratios (L 2), film quality is not uniquely determined by the substrate temperature, T,. The microstructure of our low hydrogen content films as revealed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies is similar to that of "device-quality" films deposited at standard T.. The next phase of our work is to ascertain whether or not similar IR and NMR characteristics between our low Ts films and the standard Ts films imply also similar opto-electronic properties.
INTRODUCTION "Device-quality" a-Si:H films are commonly deposited by glow discharge under standard process conditions wizh substrate temperatures, T., between 200 0 C and 350 *C. These films typically have a total concentration of bonded hydrogen, CH, ranging from 10 to 15 at. %. Films deposited at lower T, usually have a CH exceeding 20 at. %. Increasing in CH with the lowering of T, leads to formation of microstructural features, and to degradation of opto-electronic properties [1-3]. In the surface reaction model of film growth, diffusion of adsorbed precursors on the hydrogen-saturated growing surface is important to the formation of dense and smooth "devicequality" materials [4]. An increase in Ts will increase the surface diffusion coefficient but there is an upper limit on Ts because too high of a T. will drive the hydrogen off of the surface causing hinderance to the surface diffusion of precursors. There are published experimental data, however, which suggest that hydrogen-related microstructure is not uniquely determined by T,, nor any other single process variable, but is determined by CH [5]. If the conclusion of that work is generally applicable, the implication is that manufacturing of "device-quality" films at room temperature is feasible. The possibility of depositing "device-quality" films at room temperature is intriguing because it offers tremendous
Mat. Res. Soc. Symp. Proc. Vol. 219. ©1991 Materials Research Society
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potential for research and development of complex integrated circuits on a wide range of substrates, even on novel conductive plastics.
EXPERIMENTAL The a-Si:H samples for the present study were prepared in a home-built stainless steel parallel-electrode glow discharge reactor. The electrodes were 17.8 cm in diameter, and were 3.5 cm apart. The deposition conditions are listed in Table I. Hydrogen contents were determined by a Perkin-Elmer 240 elemental analyzer. Film thicknesses were measured by a Tencor Instruments Alpha-step 200 stylus. IR absorption spectra were
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