A method for depositing an epitaxial Ge--Sn layer on a substrate in a CVD reaction chamber includes introducing into the chamber a gaseous precursor comprising SnD4 under conditions whereby the epitaxial Ge--Sn layer is formed on the substrate. the gaseous precursor comprises SnD4 and high purity H2 of about 15-20% by volume. The gaseous precursor is introduced at a temperature in a range of about 250.degree. C. to about 350.degree. C. Using the process device-quality Sn--Ge materials with tunable bandgaps can be grown directly on Si substrates.