Kinetics of hydrogen desorption from germanium-covered Si(100)

Two models from the recent literature, proposed to describe the apparent effect of Ge coverage on silicon monohydride desorption kinetics, are evaluated based on their ability to fit temperature-programmed desorption data, an evaluation of the physical consistency of the estimated kinetic constants, and a comparison with the effects of other atomic impurities (B, P, C) on hydrogen desorption from Si(100). The first model attributes the decrease in the peak temperature for silicon monohydride (beta(1)) desorption with increasing Ge coverage to a long-range electronic effect that reduces the activation barrier uniformly over the entire surface. It is shown that this model fails to fit the high Ge coverage data unless the preexponential factor also decreases by approximately nine orders of magnitude, which is physically implausible. The second model considers the possibility of an alternate pathway to depopulate the silicon monohydride phase, in which hydrogen diffuses from the silicon to the germanium surface phase, and desorbs rapidly from a short-lived GeH intermediate. The estimated activation barrier for the surface migration step of 25+/-1 kcal mol(-1) is thermodynamically consistent with the energetics of hydrogen desorption from Si and Ge, and its magnitude is intermediate between the estimated activation barriers for hydrogen surface diffusion on Si and Ge. Although Ge may modify the energetics of the interaction of hydrogen and silicon, it is concluded that such an effect alone is insufficient to describe the shift and broadening of the beta(1) feature, while the GeH intermediate model succeeds, even in the absence of any such effect.