  A great deal of interest is directed nowadays towards the development of
innovative technologies in the field of quantum information and quantum
computing, with emphasis on obtaining reliable qubits as building blocks. The
realization of highly stable, controllable and accessible hole spin qubits is
strongly dependent on the quality of the materials hosting them. Ultra-clean
germanium/silicon-germanium heterostructures have been predicted and proven to
be promising candidates and due to their large scalability potential, they are
opening the door towards the development of realistic and reliable solid state,
all-electric, silicon-based quantum computers. In order to obtain ultra-clean
germanium/silicon-germanium heterostructures in a reverse grading approach, the
understanding and control over the growth of Ge virtual substrates and thin
films is key. Here, we present a detailed study on the growth kinetics,
morphology, and crystal quality of Ge thin films grown via chemical vapor
deposition by investigating the effects of growth temperature, partial pressure
of the precursor gas and the use of Ar or H2 atmosphere. The presence of
carrier gases catalyzes the deposition rate and induces a smoothening on the
surfaces of films grown at low temperatures. We investigated the surface
roughness and threading dislocation density as a function of deposition
temperature, partial pressure and gas mixture. Ge thin films deposited by
diluting GeH4 in Ar or H2 were employed as virtual substrates for the growth of
full Ge/SiGe QW heterostructures. Their defect density was analyzed and their
electric transport properties were characterized via Hall measurements. Similar
results were obtained for both carrier gases used.
