The present invention relates to a method of forming a layer of material with the desired thickness and more particularly a method of forming a layer of high quality SiC (silicon carbide) on a single crystal substrate by using the chemical vapor deposition technique. Because of its excellent physical and electrical properties, the compound silicon carbide is widely applied as electrical insulating material, abrasive material, cutting tool and material for heat engine such as gas turbine, etc. The method most widely adopted to deposit a silicon carbide layer on a single crystal substrate comprises chemical vapor deposition technique wherein gaseous sources of silicon and carbon such as ethyl silane and propyl trimethyl silane are introduced in a reactor. But since these sources have the undesirable tendency to form graphite inclusions, the resulting layer has a poor quality. This is especially true when single crystal SiC is deposited on a single crystal Si substrate, since there is a great difference in their lattice constants. The conventional methods for forming high quality SiC on a single crystal Si substrate are: (1) The method which comprises applying a buffer layer such as SiO2, Al2O3, BN, etc. or chemical vapor deposition (CVD) on a Si substrate and then depositing a silicon carbide layer by using gaseous sources of silicon and carbon; or PA1 (2) The method which comprises pre-treating a substrate at high temperature (about 1100.degree. C. to 1200.degree. C.) and then depositing SiC. In the case of the above-mentioned method (1), the quality of the SiC layer is improved to some extent but the presence of buffer layer will reduce the heat transfer coefficient and the thermal conduction of the SiC layer thus making the use of SiC layer impractical for structural applications requiring good heat conduction. In the case of the method (2), the quality of the SiC layer is improved remarkably but the cost of raw material is higher than other conventional methods and the processing time is longer than method (1). To eliminate the above-mentioned drawbacks, one of the inventors has proposed an improved method, as disclosed in the Japanese Patent Application No. 59-124986 filed on July 24, 1984 and No. 61-140589 filed on July 9, 1986, comprising an improved steps wherein gaseous sources of silicon and carbon are reacted at a high temperature above 1200.degree. C. in a reactor without forming the graphite inclusions. This method, however, is not yet satisfactory for practical application because the cost of raw material is still higher than other conventional methods. The present inventors have continued the search for a method of forming high quality SiC by using chemical vapor deposition technique which could satisfy all the requirements of the industry. Accordingly it is the primary object of the present invention to provide a method of forming a SiC layer which is of excellent quality and can be mass-produced at a low cost. It is another object of the present invention to provide a method of forming a SiC layer with good heat conduction. It is a further object of the present invention to provide a method of forming a SiC layer of better quality in shorter processing time. The further objects and advantageous features of the present invention will be made more clear in the following description taken in conjunction with the accompanying drawing showing a preferred embodiment of the present invention.