HYDROGEN EFFECTS IN CVD - SELECTIVE ETCHING AND METASTABLE PHASES

We present a scanning tunneling microscopy (STM) study of the mechanis m of chemical vapor deposition (CVD) of Ge on Si(111)-7 x 7 from GeH4 and Ge2H6, and compare with results obtained by molecular beam epitaxy (MBE). We show that, depending on the growth conditions, the presence of hydrogen in CVD can produce two important effects: (a) the hydroge n can act as a site-selective ''etchant'', rendering substrate atoms l abile, and (b) it can lead to the generation of new, metastable struct ures not produced by MBE deposition. Explanations for the above effect s are proposed.