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 (G SMBE) from GeH4, has been investigated in situ, by elevated-temperatur e scanning tunnelling microscopy. While Si grows epitaxially in the Fr ank-van der Merwe mode, Ge grows heteroepitaxially in the Stranski-Kra stanow mode. Nevertheless, at low coverages and/or low temperatures, c omparison with Si growth from Si2H6 yields similar behaviour. At cover ages less than one monolayer, the epitaxial strain is low enough to al low for such an Si-like growth, and at temperatures below 600 K neithe r Ge nor Si can grow properly due to hydrogen blockage of surface diff usion. 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 import ant growth parameter since, together with the temperature, it determin es the diffusion length. At coverages higher than one monolayer, Ge gr owth 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 a rrow(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 di fference can be attributed to the surfactant role of hydrogen present on the surface in the GSMBE process. A detailed Ge/Si(001) growth diag ram, covering the temperatures mentioned above, is presented. (C) 1997 Elsevier Science B.V.