Abstract: A method for simultaneously forming an epitaxial silicon layer on a surface of a silicon substrate, and a polysilicon layer on a silicon dioxide (SiO.sub.2) layer which is formed on the silicon substrate using a low pressure silicon vapor deposition method, employing silicon hydride gas, particularly disilane (Si.sub.2 O.sub.6), as a silicon source gas. A crystal growing temperature ranging from 780.degree. C. to 950.degree. C. and a reaction gas pressure ranging from 20 Torr to 300 Torr are desirable. An extended silicon epitaxial region is achieved under a higher temperature and a higher gas pressure, and with a substrate of a (100) orientation. A polysilicon layer having an even surface and joining smoothly to an epitaxial silicon layer which is simultaneously formed, is obtained under a lower temperature and a lower gas pressure, and with a substrate of a (111) orientation.