Low-temperature growth morphology of singular and vicinal Ge(001)

Scanning tunneling microscopy is used to study the nonequilibrium surface morphology of singular and vicinal Ge(001) grown by molecular-beam epitaxy. Growth on substrates with ≈0.1° miscut produces patterns of nearly symmetrical growth mounds over a wide range of growth temperature, 60<T<230°C and film thickness, 0.5<h<1000 nm. The characteristic slope or aspect ratio of the growth mounds increases with film thickness. Analysis of the onset of mound formation gives an estimate of the Ehrlich-Schwoebel length; l_ES is approximately equal to the surface lattice constant and independent of temperature. This small value for l_ES implies either a weak repulsive barrier (ΔE_d∼k_BT) at descending steps or a step-adatom attraction (ΔE_a >k_BT) at ascending steps. Buffer layers grown at T=365°C on vicinal substrates (9° miscut towards [110]) show (115) facets. Low-temperature growth on vicinal surfaces (6° and 9° miscuts at T=155 and 230°C) produces highly anisotropic growth ridges oriented along the miscut direction with larger roughness amplitude and smaller in-plane length scales than mounds produced by the same growth conditions on singular substrates. At 230°C, the slopes of the growth ridges are stabilized by the (105) surface.