SCANNING-TUNNELING-MICROSCOPY (STM) STUDIES OF THE CHEMICAL-VAPOR-DEPOSITION OF GE ON SI(111) FROM GE HYDRIDES AND A COMPARISON WITH MOLECULAR-BEAM EPITAXY

Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and tip-induced desorption are used to study the mechanism of the chemical vapor deposition (CVD) of Ge on Si(111)-7 X 7 from GeH4 and Ge2H6. The chemical and structural changes that follow the dissociation of the hydride molecules are investigated as a function of the substrate temperature, and the structure and growth mode of the resulting films are compared to those of films generated by molecular beam epitaxy (MBE). At room temperature, only Ge2H6 reacts appreciably with the Si(111) surface. The reaction proceeds via a mobile precursor state, but does not exhibit a site selectivity with respect to surface sites of the 7 X 7 unit cell. For temperatures in the neighborhood of 400-degrees-C, continuous film growth is observed for both GeH4 and Ge2H6, despite the fact that hydrogen is present at the surface. The film growth mode undergoes a qualitative change in a relatively narrow temperature range approximately 400-degrees-C. At 370-degrees-C, the film grows in the form of monolayers with a hydrogen-stabilized 1 X 1 structure. This is in contrast to growth by MBE which involves 7 X 7 and 5 X 5 reconstructed bilayers. At the same time, a highly site-selective substrate etching process favoring the center-adatom sites is observed and characterized. This etching process leads to a chemically induced Si-Ge intermixing at the growing interface. At 430-degrees-C, the CVD film growth mode involves bilayers with 7 X 7 and 5 X 5 reconstructions, but in addition, areas with a new square-root 3 x square-root 3R30-degrees reconstruction which is not observed in MBE are formed. Short-lived GeH species acting similarly to column III elements are probably the building blocks of this structure. We show that all of the above observations can be understood by considering the effect of hydrogen on the film growth process. The ability of the hydrogen to influence the growth process can, in turn, be tuned by varying its surface concentration and residence time by the substrate temperature.