Abstract: Heteroepitaxial methods are described herein for the growth of germanium-tin alloy layers directly on silicon substrates. A method of heteroeptiaxial growth of a germanium-tin alloy layer comprises placing a silicon substrate in a cold wall ultra-high vacuum chemcial vapor deposition chamber and depositing the germanium-tin alloy layer directly on the silicon substrate from a gaseous mixture in the deposition chamber, the gaseous mixture comprising a germanium source and a tin source.