A method of preparing a clean substrate surface for blanket or selective epitaxial deposition of silicon-containing and/or germanium-containing films. In addition, a method of growing the silicon-containing and/or germanium-containing films, where both the substrate cleaning method and the film growth method are carried out at a temperature below 750° C., and typically at a temperature from about 700° C. to about 500° C. The cleaning method and the film growth method employ the use of radiation having a wavelength ranging from about 310 nm to about 120 nm in the processing volume in which the silicon-containing film is grown. Use of this radiation in combination with particular partial pressure ranges for the reactive cleaning or film-forming component species enable the substrate cleaning and epitaxial film growth at temperatures below those previously known in the industry.