In this work we will discuss the epitaxial growth of both germanium (Ge) and germanium tin (Ge_1-xSn_x) by thermal chemical vapor deposition. The focus of the discussion is on the challenges we encountered in the development of a low-temperature process to form a Ge layer with a smooth surface and a reasonably low defect level, a Ge_1-xSn_x layer in a metastable state, or a Ge/Ge_1-xSn_x stack structure. Examples of surface roughening, relaxation, intermixing, and segregation are given. Device fabrication from individual Ge/Ge_1-xSn_x layers and stacks will be mentioned.

For Ge growth on Si, we further discuss the importance of process conditions both to minimize the in-film defectivity and to enhance the surface smoothness. The growth selectivity to various dielectric materials shows some interesting dependence on in situ doping and other growth parameters. The dependence of film properties, such as crystallinity and resistivity, on substrate type, such as silicon, silicon germanium, and germanium, is studied. Growth on patterned Si substrates will also be discussed.

With an optimized low-temperature growth process, Sn can be uniformly distributed in Ge to form a homogeneous phase of Ge_1-xSn_x. The dependence of growth rate and Sn concentration on the Ge and Sn sources is discussed. Sn concentration up 16% can be achieved with decent film quality on a relaxed Ge layer. Fully strained Ge_1-xSn_x alloys and Ge/Ge_1-xSn_x stacks have both been used in the fabrication of electronic and photonic devices. The techniques used in stack formation are discussed together with in situ boron and phosphorus doping of Ge/Ge_1-xSn_x.