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 approximate to 0.1 degrees miscut produces patterns of nearly symmetrical growth mounds over a wide range of growth temperature, 60<T<230 degrees 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 (Delta E-d similar to k(B)T) at descending steps or a step-adatom attraction (Delta E-a >k(B)T) at ascending steps. Buffer layers grown at T=365 degrees C on vicinal substrates (9 degrees miscut towards [110]) show (115) facets. Low-temperature growth on vicinal surfaces (6 degrees and 9 degrees miscuts at T=155 and 230 degrees 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 degrees C, the slopes of the growth ridges are stabilized by the (105) surface.