NOVELTY - The silicon-based precursor used, is silicon tetrachloride. High purity silicon tetrachloride is vaporized with one or more other precursors from a series of chlorides or hydrides. A mixture is formed with a carrier gas. The mixture is introduced into a reaction chamber with the preheated substrate to be coated. The substrate is preheated to 900 - 1390degreesC. On the substrate a thin, doped layer is deposited. Volatile reaction byproducts are removed from the chamber.    USE - To form a silicon layer on a substrate surface by gas- (or vapor-) phase deposition. To manufacture thin layer solar cells or crystalline silicon thin layer solar cells (claimed).    ADVANTAGE - A new method is offered, for preparing thin silicon layers on a substrate surface, especially for manufacturing solar cells. Use of silicon tetrachloride is found to be simple and efficient. Disadvantages of monosilane, dichlorosilane or trichlorosilane precursors are avoided. Costs of precursor transport, storage and disposal are markedly reduced. A large comparative cost reduction is realized when producing thicker layers for solar cells. Quality is comparable to that achieved using trichlorosilane.    DETAILED DESCRIPTION - Gas phase deposition is carried out by thermal decomposition of highly-pure silicon tetrachloride at 0.8 - 1.2 bar absolute. The gas mixture containing carrier gas and precursors, is in the chamber for a mean residence time of 0.05 - 5 s. The process employs a multi-crystalline silicon substrate surface. Substrate heating in the chamber is thermal, electrical or by irradiation. Exposure to reaction conditions lasts 2-30 minutes. An epitaxial silicon layer is deposited at a rate of 2000 - 6000 nm per minute. As precursor, the silicon tetrachloride is mixed with chlorine or hydrogen compounds carried in the gas phase. These compounds contain elements of the third, fourth or fifth main groups of the periodic system. The coated substrate is processed to form a solar cell. It is cleaned or textured in known manner. Then diffusion takes place at 800-1000degreesC from the gas phase or another doping source. The glass layer formed by diffusion is removed. On the electronically-active silicon layer, a thin anti-reflection coating is deposited. Subsequently, screen printing applies metal contacts onto the front and rear of the substrate. These contacts are thermally-inlaid. The silicon tetrachloride is used to produce a silicon-based layer by gas phase deposition, on a substrate comprising SiC, SiNx or SiOx, where x is 0.1 - 2,.