Reaction dynamics of H2 and D2 on Si(100) andSi(111)

Abstract

Experimental and theoretical results on the dynamics of dissociativeadsorption and recombinative desorption of hydrogen molecules onsilicon are reviewed.The temperature dependence of the desorption rates for H2 and D2on Si(100) and Si(111) corresponds to a desorption barrier of2.3-2.4 eV. Adsorption is phonon assisted: the stickingcoefficient increases strongly with surface temperature,corresponding to an activation energy of 0.65 eV at a gastemperature of 300 K (with little isotope effect). It alsoincreases with incident gas energy. The adsorption barrier isstrongly reduced by the presence of defects and steps on the surfaceand by preadsorbed H atoms at inter-dimer positions. The barrierat steps, for instance, is only of the order of 0.1 eV.The state-resolved energetics of the desorbing particles shows an excessenergy (including translational energy) above thermal which is smallcompared to the activation barrier to adsorption. The angulardependence of sticking and desorption is strongly forward peaked(∝(cos θ)10 to 11 or ∝(cos θ)3 to 4 depending on azimuth).Molecular vibrations show vibrational heating in desorption(with a strong isotope effect) and vibrationally assisted stickingat higher temperatures. On the other hand, molecular rotations showcooling in desorption.Ab initio generalized gradient approximationslab calculations for the H2 interaction withthe dimers of Si(100)2×1 and with the Si `adatoms' onSi(111)7×7 all indicate the existence of strong latticerelaxations near the adsorption sites and the transition stategeometries in qualitative agreement with the observedphonon-assisted sticking. The energetically lowest transition stateon Si(100) is the inter-dimer state which also has the highestelastic relaxation energy (about 0.33 eV). The barrier of theasymmetric intra-dimer state is slightly higher but the relaxationenergy is only half as big. There is also a symmetric intra-dimertransition state with an even higher barrier. It may, nevertheless,contribute to sticking and desorption due to a largephase space.The slab calculations yield good values for the desorption barrierheights of 2.3 to 2.5 eV. They also lead to a good semiquantitativeunderstanding of the existence of highly reactive sites near stepsand preadsorbed H atoms at inter-dimer states. The absolute valuesof adsorption barriers, however, come out consistently too low byabout 0.2 to 0.4 eV. Since in our review we are not so muchinterested in pursuing the consequences of ab initiocalculations in detail but more in a model describing theexperimental data quantitatively, we allow ourselves to readjust theab initio results slightly to obtain good fits to the data.Because of the uncertainties of the ab initio results, therelation of model parameters to the various transition states isuncertain as well.The strong phonon assistance of sticking can be modelledfairly well by taking only a single lattice displacementcoordinate at the adsorption site into account. Together with thesix degrees of freedom of the molecule, one obviously needs a modelwith at least seven degrees of freedom. We will present aseven-dimensional Hamiltonian which we treat in a seven-dimensionaltime-independent coupled-channel calculation. The results of thiscalculation agree well with experimental data. Most of the resultsare more or less independent of the particular transition statemediating the reaction. An exception, perhaps, is the angulardependence of sticking coefficients and desorption fluxes. Wediscuss several alternatives for explaining the observed angulardistributions in terms of possible transition states.