Studies of gas-surface reactivity and film growth selectivity in group-IV semiconductor heteroepitaxial systems via supersonic molecular beam techniques

Supersonic molecular beam techniques have been employed to examine the dissociative adsorption of GeH4 and Ge2H6 on the Si(100) and Si(111) surfaces. At sufficiently high incident kinetic energies (≥1 eV), both species on both surfaces react via a direct dissociation mechanism. At sufficiently low incident kinetic energies (≤0.6 eV) and substrate temperatures, however, dissociation of Ge 2H6 on the Si(100) surface proceeds primarily through a trapping, precursor-mediated dissociation channel. This "mixed-crystal" system (GenH2n+2 on Si is an example) allows comparisons to two related systems: the reactions of these same gasphase species (GeH4 and Ge2H6) on Ge surfaces, and the reactions of silanes (SiH4 and Si2H6) on these same Si surfaces. We find that the Si surfaces are much more reactive than their Ge counterparts; whereas, under similar reaction conditions, and on the same Si surface, the Ge hydrides are moderately more reactive than the Si hydrides.; The reactivities of ultrathin (∼2 monolayers) Ge(a) epitaxial layers and strained Si1-xGex epitaxial overlayers have also been examined. For deposition on Si(100) substrates, we find that these ultrathin strained overlayers, both pure component [i.e., Ge/Si(100)] and alloy [i.e., Si1--xGex/Si(100)], exhibit enhanced chemical reactivity when compared to their bulk (relaxed) counterparts, illustrating the role that strain can play in gassurface reactivity, heretofore an almost unexamined subject.; Two complementary in situ surface analytical techniques---low energy ion scattering spectrometry (LEISS) (first monolayer sensitive) and x-ray photoelectron spectroscopy (XPS) (several monolayers sensitive) have been employed to quantify the Ge concentration in the near-surface strained Si1--xGex layers as a function of substrate temperature and gasphase Ge concentration. We find that Ge surface segregation is significant and Ge enrichment occurs not only in the surface layer, but also in several of the sub-surface layers. The extent of Ge segregation indicated by each technique follows the trend: LEISS-Ge% " XPS-Ge% " bulk-Ge%. Interestingly, the composition of the bulk is nearly independent of substrate temperature at a fixed gas composition for the reaction conditions examined.; Concerning selective-area epitaxy on patterned Si-SiO2 substrates, the combination of a supersonic molecular beam of the thin film precursor and an independently controlled flux of atomic hydrogen was employed to examine the effects of substrate temperature (Ts), incident precursor kinetic energy (Ei), flux of atomic hydrogen, and beam composition on thin film nucleation and the kinetics of steady-state growth. The incubation time for films to nucleate on SiO2 surfaces increases with decreasing Ts and decreasing Ei. Under the conditions of low Ts and high Ei, the presence of Ge dramatically enhances selectivity when compared to a pure Si system. Adding atomic H further prolongs the incubation periods, where the effect is more pronounced at lower Ts and in pure Si systems. However, the additional atomic hydrogen also suppresses film growth. These results indicate that only limited selectivity can be achieved by this approach with continuous exposure of atomic H and thin film precursor fluxes.