The present application relates to methods for depositing a smooth, germanium rich epitaxial film by introducing silylgermane as a source gas into a reactor at low temperatures. The epitaxial film can be strained and serve as an active layer, or relaxed and serve as a buffer layer. In addition to the silylgermane gas, a diluent is provided to modulate the percentage of germanium in a deposited germanium-containing film by varying the ratio of the silylgermane gas and the diluent. The ratios can be controlled by way of dilution levels in silylgermane storage containers and/or separate flow, and are selected to result in germanium concentration greater than 55 atomic % in deposited epitaxial silicon germanium films. The diluent can include a reducing gas such as hydrogen gas or an inert gas such as nitrogen gas. Reaction chambers are configured to introduce silylgermane and the diluent to deposit the silicon germanium epitaxial films.