Si(001) surface variation with annealing in ambientH2

Monohydride termination and surface variation of $\mathrm{Si}\mathrm{}\left(001\right)-2\mathrm{}\times 1$ by annealing in ${\mathrm{H}}_2$ ambient has been investigated with scanning-tunneling-microscopy (STM). A clean $\mathrm{Si}\mathrm{}\left(001\right)-2\mathrm{}\times 1$ surface was annealed and cooled to room temperature in ${\mathrm{H}}_2$ ambient with various partial pressure of $P\left({\mathrm{H}}_2\right).\mathrm{}$ Hydrogen termination of the surface is observed when $P\left({\mathrm{H}}_2\right)$ exceeds ${10}^{-3}\mathrm{torr},$ and complete monohydride termination is obtained with $P{(\mathrm{H}}_2)\mathrm{\sim }0.1\mathrm{}\mathrm{torr}.$ The results are reproduced well by the calculation using the temperature-dependent sticking coefficient of molecular ${\mathrm{H}}_2$ on Si(001). [Kolasinski et al., J. Chem. Phys. 101, 7082 (1994)]. The surface obtained by ${\mathrm{H}}_2$ annealing shows characteristic $S_B$-step shape, which has a long kink-free portion connected by a long kink. It is shown that the etching of Si surface by ${\mathrm{H}}_2$ is not responsible for the determination of the shape of $S_B$ steps. Instead, the presence of passivating hydrogen at the temperature above the frozen temperature of step motion on Si(001) is the origin of the characteristic step shape.