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

Abstract

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 $\ensuremath{\approx}0.1\ifmmode^\circ\else\textdegree\fi{}$ miscut produces patterns of nearly symmetrical growth mounds over a wide range of growth temperature, $60lTl230\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and film thickness, $0.5lhl1000\mathrm{nm}.$ 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; ${l}_{\mathrm{ES}}$ is approximately equal to the surface lattice constant and independent of temperature. This small value for ${l}_{\mathrm{ES}}$ implies either a weak repulsive barrier $(\ensuremath{\Delta}{E}_{d}\ensuremath{\sim}{k}_{B}T)$ at descending steps or a step-adatom attraction $(\ensuremath{\Delta}{E}_{a}g{k}_{B}T)$ at ascending steps. Buffer layers grown at $T=365\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ on vicinal substrates (9\ifmmode^\circ\else\textdegree\fi{} miscut towards [110]) show (115) facets. Low-temperature growth on vicinal surfaces (6\ifmmode^\circ\else\textdegree\fi{} and 9\ifmmode^\circ\else\textdegree\fi{} miscuts at $T=155$ and 230 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}C, the slopes of the growth ridges are stabilized by the (105) surface.