Faceting and lateral overgrowth have been investigated for Ge selectively grown on Si(001) substrates in trench regions bound by SiO2 sidewalls. In wet-etched large trenches with sloped sidewalls, Ge faceting behavior was similar to Si and SixGe1-x faceting: slow-growing {113} facets dominate, with {111} facets expanding as the layer became thicker. However, the {111} facet length for Ge was much smaller than that of Si; this can be explained in terms of mass transport and accumulation, as well as energy minimization and the higher surface diffusivity of Ge. In dry-etched small trenches with vertical sidewalls, minimization of the high-energy interface area between Ge and SiO2 appears to be most critical in determining faceting morphology. Overgrowth of Ge led to void formation at the oxide interface, presumably to avoid the high-energy Ge/SiO2 interface. Upon coalescence of lateral-growth regions, fast-growing (001) forms and dominates subsequent growth. Thus, the total thickness of the overgrown Ge layer was closely related to the width of the SiO2 region between trenches. (c) 2008 American Vacuum Society.