The present invention relates to the discovery that pyrolysis of certain fluorohydridodisilanes or mixtures of fluorohydridodisilanes can result in the formation of a highly stable, highly reflective, highly abrasion resistant, photoconductive, semiconductor film on a substrate.
Amorphous polymeric materials of silicon and hydrogen (hereafter referred to as a-SiH) have emerged as a new class of semiconductors in recent years. The materials are generated as thin films from the decomposition of silane (SiH.sub.4) in electrical discharges or, less frequently, from the thermal decomposition of silane or higher hydrogen-containing silanes (e.g., Si.sub.2 H.sub.6, Si.sub.3 H.sub.8, etc.) as described in U.S. Pat. No. 4,459,163, issued July 10, 1984 to MacDiarmid et al.
A disadvantage of the use of silane, disilanes or polysilanes (Si.sub.n H.sub.2n+2) as a starting material for generation of a-SiH relates to the significant explosion hazard of silane/air mixtures. An operational consequence of this explosion hazard is the frequent use of inert carrier gases as diluents in these systems. Both chemical vapor deposition (CVD) and discharge (plasma CVD) routes to a-SiH from silane also have disadvantageous features. Discharge systems require relatively sophisticated and expensive equipment. Silane-based CVD systems, while simple, suffer from the tendency to undergo vapor nucleation unless the pressure is very low. This behavior restricts the range of permissible operating conditions for such thermal decompositions.
When it is desirable to include additional elemental constituents in the amorphous films, co-reactants such as phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) are commonly added to the starting materials. When fluorine is to be incorporated into an amorphous film, tetrafluorosilane (SiF.sub.4) is most commonly added to the reactant mixture. This is described for example in U.S. Pat. No. 4,217,374 granted to Ovshinsky and Izu on Aug. 12, 1980.
U.S. Pat. No. 4,374,182, issued Feb. 15, 1983 to Gaul et al., discloses decomposing halogenated polysilanes at an elevated temperature. Gaul et al., however is limited to the pyrolysis of polychloropolysilanes and does not teach the use of fluorohydridodisilanes. Further, Gaul et al. specifies the formation of elemental silicon (i.e., with no halogen) in a solid state transformation rather than from a vapor phase deposition. Gaul et al., is therefore distinguished from the instant invention directed to the production of reflective, amorphous films produced from flurohydridodisilanes.
U.S. Pat. Nos. 2,606,811, issued on Aug. 12, 1952 to Wagner and 4,079,071, issued on Mar. 14, 1978 to Neale, addressed the decomposition at elevated temperatures of halogenated disilanes. However, these patents are distinguished from the instant invention because both are directed toward the hydrogenation of di- and polysilanes for the formation of silanes, and more specifically, the formation of monosilanes. Wagner teaches the process of making a compound with a single silicon atom by heating in the presence of hydrogen a compound containing a halogen and a Si--Si bond. Neale teaches the metal catalyzed process of preparing compounds of the formula H.sub.a (CH.sub.3).sub.x SiCl.sub.4 - (a+x) by contacting a polysilane with hydrogen gas under pressure and heat. Neither Wagner nor Neale teach the vapor phase deposition of amorphous silicon films from the thermal decomposition of fluorohydridodisilanes as taught in the instant invention. In fact, Wagner and Neal are specifically directed toward the hydrogenation of chlorosilanes, not fluorosilanes.
United Kingdom Pat. No. 2,148,328, issued to M. Hirooka, et al., on May 30, 1985, teaches the decomposition of various silanes, including monomeric halosilanes (SiX.sub.4), cyclic polymeric halosilanes (SiX.sub.2).sub.n, where n is greater than or equal to 3, di- and polysilanes such as Si.sub.n HX.sub.2n+1 and Si.sub.n H.sub.2 X.sub.2n. These materials are decomposed via electric discharge, or photolysis, or high temperature or catalytically and, unlike the instant invention, are mixed with a requisite second stream consisting of a vapor phase material selected from the group consisting of H.sub.2, SiH.sub.4, SiH.sub.3 Br, or SiH.sub.3 I wherein the second stream has also been decomposed. The obvious disadvantage of such prior art, one which clearly distinguishes it from the instant invention, is the necessity of having two materials to decompose. The United Kingdom patent requires the second stream as the source of hydrogen to facilitate the reduction of the silane to the amorphous silicon. The instant invention, however, has sufficient hydrogen and silicon in the single stream of fluorohydridodisilane to produce the desired amorphous silicon film. In addition, the hydrogen-containing films produced by the instant invention possess the low defect density and high dopability characteristic of a-SiH films. The films of the instant invention, retaining fluorine bonded to silicon, exhibit the high thermal stability characteristic of fluorosilicon materials. Yet the instant invention does not require a second source of either fluorine or hydrogen as a decomposable starting material.
United Kingdom Pat. No. 2,156,385, published Oct. 9, 1985 and issued to Tanaka et al., teaches the use of fluorinated monosilanes and one partially fluorinated disilane, Si.sub.2 H.sub.4 F.sub.2, as precursor materials for the formation of silicon films on a substrate. However, United Kingdom Pat. No. 2,156,385 is directed solely to plasma discharge deposition.
U.S. Pat. No. 4,485,121, issued Nov. 27, 1984 to Matsumura teaches the use of electric discharge in the presence of a mixed gas of silicon difluoride or silicon monofluoride gas with hydrogen gas and depositing the decomposed gas on a substrate. Matsumura does not teach the use of flurohydridodisilanes.
Thus the instant invention is distinguished from prior art utilizing monosilanes, disilane (Si.sub.2 H.sub.6), polysilanes, and systems requiring dual streams of starting materials one of which is a reducing agent, e.g., hydrogen gas, and systems dependent on plasma, glow or other electric discharges. Furthermore, the instant invention, utilizing selected fluorohydridodisilanes with fixed and known ratios of constituents on the silicon atom, provides greater control and uniformity in the distribution of fluorine and hydrogen in the resulting amorphous silicon-containing film than does the relatively crude alternative of coreacting materials such as tetrafluorosilane and hydrogen.