Comparison Studies of Hydrogenated Amorphous Silicon Films Prepared from Silane-Hydrogen and Silane-Helium Mixtures
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ETED ANOIRP!US SILICON FILMS PREPARED COWPARISON STUDIES OF HYD I AND SILMNE-HELIU MIXTURES* SILANE-HYDR R.I. PATEL, D.J. OLSEN, J.R. SHIRCK, AND N.T. TRAN Electronic and
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Laboratories,
3M Company,
St. Paul,
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ABSTRACT Hydrogenated amorphous silicon films were produced from silane/hydrogen and silane/helium gas mixtures by RF glow discharge. We examined the optical and electrical properties of films produced with these gas mixtures, at various RF power levels and silane fractions. Film quality was analyzed by measuring the dark and photoconductivity, optical band gap, and activation energy. Optical emission spectroscopy was also used as a diagnostic tool for studying the plasma during glow discharge depositions. Experimental results indicate that amorphous silicon films made from silane/helium mixtures exhibit improved optoelectronic properties, higher deposition rates, and higher emission intensity ratios (I, /I ) as compared to films produced from silane/hydrogen mixtures. In preparing films from silane/helium mixtures, the onset of dust/powder formation occurs at considerably higher RF powers as compared to silane/hydrogen, thus making this approach an attractive commercial option for depositing films at high rates. INTRgxxxTIcOq
Hydrogenated amorphous silicon (a-Si:H) films prepared from the plasma decomposition of silane have become an increasingly attractive for a variety of applications including large scale terrestrial solar cells, field effect transistors for displays, xerographic photoreceptors, and electronic imaging devices [1i. In conventional RF glow discharge depositions high quality a-Si:H films have been deposited at rates of 1 to 3 g/second using monosilane (SiH ) as the process gas. The deposition rate can be increased by increasing tae power density in the glow discharge. However, energetic ions/neutrals produced at high RF powers adversely affect film properties, and, also lead to increased nucleation and growth resulting in SiH powder formation. To avoid such reactions, silane is commonly dilufed with hydrogen gas. Most of the work reported in the literature for a-Si:H depositions has been performed with 10% silane/hydrogen mixtures. The effect of inert gas dilutions (He, Ne, Ar, and Kr) of silane on the material quality of plasma deposited a-Si:H films is that substantial differences in the electrical, optical, and structural properties are observed [2]. The authors reported that helium or neon dilutions produced fewer deleterious effects on the material quality as compared to argon and krypton. They also obtained higher deposition rates with He and Ne which was attributed to the high levels of hydrogen incorporation in the form of silicon monohydride (SiH) in the deposited films. In this report, we present comparison studies of a-Si:H films prepared from various silane/hydrogen and silane/helium mixtures. The optical and electrical properties have been measured for films prepared at various RF powers and silane fractions. Optical emission spectroscopy was used
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