Gas-source growth of group IV semiconductors: III. Nucleation and growth of Ge/Si(001)

The growth of Ge on Si(001) using gas-source molecular beam epitaxy (GSMBE) from GeH4, has been investigated in situ, by elevated-temperature scanning tunnelling microscopy. While Si grows epitaxially in the Frank-van der Merwe mode, Ge grows heteroepitaxially in the Stranski-Krastanow mode. Nevertheless, at low coverages and/or low temperatures, comparison with Si growth from Si2H6 yields similar behaviour. At coverages less than one monolayer, the epitaxial strain is low enough to allow for such an Si-like growth, and at temperatures below 600 K neither Ge nor Si can grow properly due to hydrogen blockage of surface diffusion. The growth can be roughly divided into three different regimes, T<600 K (H-induced roughening), 600<T<700 K (island-nucleation mode), and T>700 K (step-flow mode), where the gas flux is the second important growth parameter since, together with the temperature, it determines the diffusion length. At coverages higher than one monolayer, Ge growth no longer resembles that of Si, and a series of complex Ge/Si(001) phase transitions takes place. At 600<T<700 K the sequence of phase transitions is as Follows: (2x 1)double right arrow(2xN)double right arrow(MxN)double right arrow "hut" clusters. Growth at T>700 degrees K changes the sequence to (2 x 1)double right arrow(2 x N)double right arrow(Mx N)double right arrow "hut" pits double right arrow "hut" clusters. In this regime the wetting layer (8-9 ML) is significantly thicker than in the analogous solid-state MBE regime (3-4 ML). This difference can be attributed to the surfactant role of hydrogen present on the surface in the GSMBE process. A detailed Ge/Si(001) growth diagram, covering the temperatures mentioned above, is presented. (C) 1997 Elsevier Science B.V.