Sb-mediated homoepitaxial growth on Ge(0 0 1)

Antimony-mediated Ge growth on Ge(0 0 1) was studied using scanning tunneling microscopy and other surface characterization techniques. A critical temperature (470 K) was detected below which Sb did not segregate to the surface. In the absence of Sb–Ge exchange at lower temperatures, a high density of small, three-dimensional Ge clusters were observed. The high cluster density was attributed to nucleation at defects such as anti-phase domain boundaries in the Sb-terminated substrate. At higher temperatures where Ge–Sb exchange was active, the island densities were lower but still far greater than those seen on bare Ge(0 0 1), thus Sb enhanced nucleation both above and below the segregation temperature. In this regime Sb also caused the island shapes to change from severely elongated along the island dimer row direction to nearly isotropic. Even at the higher temperatures, however, growth was not layer-by-layer; rather the surface rapidly reached a regime where its morphology did not change with coverage and the roughness was bounded. In contrast, growth on bare Ge(0 0 1) led to continuous roughening. Above 8 ML, growth on the Sb-terminated substrate resulted in smoother surfaces. Thus above 470 K Sb is an effective surfactant for Ge(0 0 1) homoepitaxy: it continuously floats to the surface during growth and it limits surface roughening. It is suggested that Sb limits the surface roughness by decreasing the island anisotropy which increases the attempt frequency to descend steps at the end of island dimer rows. Because diffusion is much faster along the dimer rows, this enhances the downward flux.