Phillip V. Smith

Extension of the Brenner empirical interatomic potential to CSiH systems

The Brenner hydrocarbon potential has been parameterized to include interactions with silicon. This extended Brenner (XB) potential gives a good representation of the low-index planes of silicon, as well as small hydrocarbon and silane molecules. Classical molecular dynamics calculations have been performed simulating the chemisorption of hydrocarbon molecules on Si(001).

Structures of small hydrocarbons adsorbed on Si(001) and Si terminated β-SiC(001)

Abstract In this paper a theoretical examination of the adsorption of hydrocarbons atop Si(001) and silicon terminated β-SiC(001) is presented. In each case the addition of a hydrocarbon species on top of each silicon dimer of the originally clean (2 × 1) silicon terminated substrate is examined. Both C 2 H 2 and C 2 H 4 species are considered. The total energies of the various potential candidate structures are determined to find the preferred structure. Each structure is obtained by varying the atomic positions in the first four layers of the appropriate unit cell to obtain the minimum total energy configuration for each proposed structure. The resulting structures are then compared to see whether one particular structure is strongly preferred or whether more than one structure may coexist. The results for the two substrates are compared and show that a c(2 × 2) arrangement of carbon dimers is strongly preferred for the SiC(001) substrate. However this structure is not determined for the Si(001) substrate, where the bridging of adjacent silicon atoms by carbon pairs is found to be more energetically favourable.

The stable configurations for oxygen chemisorption on the diamond (100) and (111) surfaces

Abstract The structures resulting from oxygen chemisorption onto the diamond (100) and (111) ideal and reconstructed surfaces have been extensively studied using the SLAB-MINDO molecular orbital method. For the C(100) surface, we find that the lowest energy configuration for monolayer chemisorption is a 1 × 1 on-top structure whilst that for half-monolayer chemisorption is a 2 × 1 dimer bridge site topology. For the C(111) surface, both the single-bond cleavage and triple-bond cleavage surfaces have been studied. Bridge site molecular oxygen chemisorption parallel to the surface is found to be responsible for monolayer oxygen coverage on the single dangling bond C(111) surface. For half-monolayer coverage, chemisorption of atomic oxygen onto the bridge sites of the 2 × 1 π-bonded chain model reconstructed surface is the preferred mechanism. Molecular oxygen is found to be directly dissociative on the triple dangling bond C(111) surface. The lowest energy monolayer configuration on the triple-bond cleavage surface is found to be a tilted on-top model of the single chain reconstructed surface whilst a half-monolayer exposure prefers the bridge sites of the surface single chains. Both the monolayer and half-monolayer minimum energy configurations are found to retain the 2 × 1 topology of the clean C(111) surface. These results are compared with previous studies of oxygen chemisorption onto the Si(100) and (111) surfaces.

The surface structure of β-SiC(100): The clean and monohydride 2 × 1 phases

Abstract The SLAB-MINDO method has been employed to determine the topologies of the 2 × 1 clean and monohydride structures of the β-SiC(100) surfaces. The positions of the carbon and silicon atoms within the first four layers of each structure have been calculated. These topologies are obtained from the minimisation of the total energy for a β-SiC(100) film of 14 layers. The co-ordinates of the hydrogen atoms with respect to the nearest neighbours in the surface layer have also been derived for the monohydride structures. The optimum geometries are found to be the symmetric dimer for the clean carbon terminated surface and a buckled dimer for the clean silicon terminated surface. Symmetric dimer structures are obtained for both monohydride surfaces. Whilst the situation regarding the clean carbon terminated surface is unclear, our prediction of a buckled dimer structure for the clean Si-terminated surface is in agreement with previously published experimental work.

The structure of the Si(100)2 × 1: H surface

Abstract The SLAB-MINDO method has been employed to determine the structure of the Si(100)2 × 1: H surface. The positions of the adsorbed hydrogen atoms with respect to the nearest neighbour silicon atoms have been calculated, along with the displacements of all the silicon atoms within the first four silicon layers from their ideal bulk positions. The structure is obtained from the minimisation of the total energy for a film of 16 layers. The optimum geometry is found to be that of a symmetric dimer with a Si-Si dimer bond length close to the bulk nearest neighbour distance. This is in contrast to the significantly shorter dimer bond length which has been found to characterise the clean surface. The significant atomic displacements which occur within the deeper layers of the Si(100)2 × 1 topology are found to be only slightly modified by the formation of the monohydride.

A molecular dynamics study of the chemisorption of C2H2 and CH3 on the SI(001)-(2 × 1) surface

Abstract Chemisorption of C 2 H 2 and CH 3 molecules onto the dimerized (001) surface of silicon has been simulated using the extended Brenner potential. For reference, chemisorption energies and minimum-energy geometries have also been obtained from Hartree-Fock and Becke3LYP DFT calculations performed with the Gaussian-94 suite. Various chemisorption sites have been identified. Optimal C 2 H 2 chemisorption was found to occur in a cross-dimer configuration, parallel to the dimer rows. Optimal CH 3 chemisorption occurred with the CH 3 bonding directly to the surface dangling bonds. A second-layer chemisorption site for CH 3 has also been identified, which may be important in the formation of diamond films on a silicon substrate.

SLAB-MINDO calculations on the Si(100) surface

Abstract Previous work has indicated that the SLAB-MINDO molecular orbital method may well provide a computationally viable and reliable surface analytical technique. In this paper we extend these earlier studies by using this semi-empirical approach to perform a complete optimization of the topology of the Si(100)2 ×1 surface. The lowest energy configuration is found to be that of a symmetrical dimer with a bondlength which is considerably shorter than the bulk nearest-neighbour distance. Significant reconstruction and charge transfer is shown to occur down to the sixth layer. The resulting bandstructure is also found to be semiconducting, in agreement with experiment. The occurrence of 2×2 domains of alternating buckled dimers on the Si(100) surface is also examined via the SLAB-MINDO method. It is concluded that while such domains cannot form stable structures on the Si(100) surface in their own right, they have energies very close to the corresponding 2×1 symmetrical dimer domains and thus should be readily stabilized by defects.

Chemisorption of HCl, Cl2 and F2 on the Si(100) surface

Abstract In an earlier publication we have presented the results of some preliminary calculations on the chemisorption of HF and F 2 onto the Si(100)2 × 1 surface using the SLAB-MINDO molecular orbital method. In this paper we extend this work to the case of HCl and Cl 2 , and report on some new results for F 2 . All of the species are assumed to dissociate and adsorb onto the surface silicon atoms, and the optimum configuration appropriate to a uniform monolayer coverage determined by minimising the total energy with respect to variations of the atomic coordinates within the six topmost surface layers. The behaviour of HCl on the Si(100) surface is found to be very similar to that obtained previously for HF. The H and Cl atoms chemisorb onto the dangling bonds of the surface silicon atoms at close to the expected tetrahedral angles whilst retaining a 2 × 1 dimer topology for the silicon substrate as suggested by experiment. The adsorption of Cl 2 is also found to yield a 2 × 1 symmetric dimer topology with each Si-Cl bond being tilted away from the surface normal at an angle of 14.8°. The lowest energy configuration for Cl 2 on the Si(100) surface, however, is determined to be a 2 × 1 bridge site model in which the two chlorine atoms are sited 0.708 A and 1.132 A above the surface plane. An analogous bridge site model is also found for F 2 chemisorption with the fluorine atoms lying 0.390 A and 0.015 A above the silicon surface. While this topology is in complete contrast to the molecular type topology reported earlier for a monolayer coverage of F 2 on the Si(100)2 × 1 surface, both configurations are found to have equivalent energies and might thus be expected to coexist during the early stages of fluorine chemisorption.

The chemisorption behaviour of oxygen on the Si(100) surface

Abstract Starting from the 2 × 1 reconstruction the chemisorption of oxygen on the Si(100) surface has been studied extensively by the ASED-MO method. While molecular oxygen adsorption on the reconstructed 2 × 1 surface prefers the dimer-bridge site, dissociation follows immediately with the Si(100)2 × 1 structure reverting back to the ideal unreconstructed surface so that the dissociated oxygen atoms occupy the dimer-bridge sites of a 1 × 1 oxygen superlattice. Further molecular oxygen chemisorption is found to favor the non-dimer-bridge sites as a peroxy bridge model. At elevated temperatures the oxygen molecules are able to overcome the corresponding energy barrier to chemisorb on the short-dimer-bridge sites (between the first layer and second layer silicon atoms). Molecular chemisorption is found to be a precursor to the chemisorption of atomic oxygen onto the short-bridge sites and coexists with atomic chemisorption under certain conditions. Chemisorption at the short-bridge sites can only occur, however, after occupation of the dimer-bridge sites by oxygens.

Empirical potential study of the chemisorption of C2H2 and CH3 on the β-SiC(001) surface

We have used an extended form of the Brenner empirical hydrocarbon potential to study the reconstructions of the clean β-SiC(001) surface, and the chemisorption of C2H2 and CH3 molecules onto the silicon terminated (2 × 1) surface. For reference, chemisorption energies and minimum-energy geometries have also been obtained from Becke3LYP HF/DFT cluster calculations. Optimal C2H2 chemisorption at low coverage was found to occur with the adsorbate molecule situated above the cave site, with the carbon-carbon bond oriented in the [110] direction parallel to the dimer rows. At 1 monolayer coverage this was also the preferred structure, with the adsorbate molecules positioned in a staggered c(2 × 2) arrangement. Optimal CH3 chemisorption was found to occur with the CH3 radicals bonding directly to the surface dangling bonds.