Strain-induced Ge segregation on Si at high temperatures

Surface morphology transformation under high-temperature annealing of the Ge layers grown on Si(1 1 1) at about 450 °C is studied with scanning tunneling and scanning reflection microscopies. The surface of the grown Ge layers, covered with relatively low-sloped facets, becomes composed of islands and continuous ridges with steep facets on their sidewalls under annealing at 700-850 °C. Such Ge segregation on Si, taking place at temperatures significantly lower than the Ge melting temperature, is initiated by the Ge-Si lattice strain. The strain relaxation occurs through Ge amount reduction at the Ge/Si interface. This leads to an increase in the ridges height-to-width aspect ratio up to about 0.41 and, hence, to the increase in surface energy. The role of surface energy minimization consists in the formation of energetically preferable facets on sidewalls of ridges and in the determination of their configuration. The observed surface morphology is suggested to be similar to that of Ge thin-layer solidification on Si.