Interaction of hydrogen atoms with Si(111) surfaces: Adsorption, abstraction, and etching

The interaction of H atoms with Si(111) surfaces with respect to adsorption , abstraction, and etching was investigated using thermal desorption and pr oduct detection techniques. The study covers a wide range of coverages and the temperature range 100-1000 K. After H admission to Si(111) at 100 K in H-2 desorption spectra decomposition of trihydride (t), dihydride (d), and monohydride (m) was observed around 455, 700, and 820 K, respectively. Adso rption of H at 380 K leads to desorption from d and m, and after admission of H at 680 K desorption from m was observed. The kinetics of m, d, and t d esorption is according to first-order kinetics, only the m peak exhibits at small coverages second-order phenomenology. H exposure above 400 K leads t o desorption of subsurface alpha -hydrogen at 920 K in thermal desorption s pectra. Nonstationary etching via silane formation was monitored around 630 K. The nonstationary silane etch peak occurs through a quasi-first-order p rocess in the admission temperature range 100-500 K and assumes a second-or der phenomenology at admission temperatures between 500 and 600 K. This sil ane is formed through the recombination of surface silyl (t) and H in silyl ene (d) groups. Its yield decreases with the temperature at which H was adm itted and is negligible after admission above 620 K since silyl groups are no longer available on the surface. Stationary etching during subjecting th e surface with a continuous H flux occurs via a direct reaction step betwee n the incoming H and surface silyl groups. The stationary etch yield decrea ses from 200 to 600 K due to depletion of surface silyl groups. In parallel to stationary etching, H abstraction proceeds with much higher probability . The kinetics of D abstraction by H from the monodeuteride phase at 680 K, measured through the HD product rate, as well as the formation of homonucl ear D-2 products contradict the operation of an Eley-Rideal (ER) mechanism, but are in excellent agreement with the solutions of a hot-atom (HA) react ion kinetic model which was recently successfully applied to abstraction on metal surfaces. This model is based solely on hot-atom processes and inclu des competition of reaction and sticking of hot atoms. Four parameters are needed to reproduce the measured HD rate data. At 680 K the abstraction cro ss section is 3.2 Angstrom (2) and about 5% of the adsorbed D occurs in D-2 products. Subsurface alpha -D is abstracted at 680 K or higher temperature s with a cross section of 1.2 Angstrom (2). Abstraction at lower temperatur es, either from monodeuteride surfaces or from surfaces saturated with di- and trideuteride proceeds with a smaller cross section and a reduced D-2 pr oduct yield. At 100 K the HD cross section is only 2.2 Angstrom (2) (monode uteride) or 1.4 Angstrom (2) (saturated surface), the HD kinetics is phenom enologically like that required by the ER mechanism, and a negligible quant ity of D-2 is formed. The HA reaction model allows one to reproduce these f eatures by adjusting the model parameters accordingly. (C) 2001 American In stitute of Physics.