Atomic and Electronic Properties of Small Hydrogenated Silicon Clusters: Si
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ABSTRACT The atomic and electronic structures of Si6H2m, and SilH+i clusters have been investigated in the framework of density-functional theory. For both neutral and ionized clusters we found the structure to belong to one of four distinct structural families. A molecular-orbital picture of hydrogenation is presented. From the calculated formation energies of these clusters, we infer the relative stability of the different structural families discussed. INTRODUCTION Due to their peculiar geometry, small silicon clusters (Sin) with n -' 10 have been subject of considerable interest in the past[l]. It has been well established that the Si, clusters have compact structures which are quite different from the bulk crystal structure and that their electronic structure also dramatically deviates from that given by sp3 hybridization in the crystal. However, because of the rapidly increasing complexity of the structures of larger clusters, a complete answer to fundamental questions such as the structure "evolution" of the clusters as a function of n has not been obtained yet [2]. The structure of hydrogenated silicon clusters (Si,,H,) are even more complicated. The reason is that the stability of the SiH 2, results from a delicate balance between two competing factors: the global stability of a "host" Si,, cluster and the local stabilization induced by attachment of H atoms and creation of s9-like configuration. We have recently performed ab initio optimization of neutral SisH 2m clusters as a typical example[3]. We found that the geometries of those clusters can be classified in terms of the arrangement of Si atoms in distinct structural families and demonstrated that the attachment of H atoms to the cluster proceeds in a systematic way. In the present work we extend the ab initio study of the structure and energetics to the positively charged clusters, namely SiH+,,+i. We discuss the evolution of the structures of the Si6 cluster due to hydrogenation in both neutral and ionized forms from an electronic-state viewpoint. We also discuss the relative stability of these clusters. CALCULATION METHOD The energetics and structural optimization are based on the density functional theory[4] augmented with the generalized gradient approximation[5] to the exchange-correlation energy. We used the DMol program[6], which uses the wavefunction expansion in the localized orbitals with double-numerical-plus-polarization (DNP) basis. For Si, the DNP
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Mat. Res. Soc. Symp. Proc. Vol. 452 o1997 Materials Research Society
)=:# I 4 01 * WAW (a) PBN
(b) TBN-I
(C) TBN-I1
(d) Bulk
Figure 1: Calculated structures of Si 6 H 2m clusters, classified into four geometry families in terms of the positions of Si atoms. Large, dark circles represent Si atoms, and small, bright ones are H atoms, respectively. The same convention applies also to Figs.2-4.
basis set contains all of the occupied orbitals of a neutral atom and the 3s, 3p and 3d orbitals of an ion with a +2 charge. Occupancies of the ls and 2s orbitals are frozen. For H, the basis i
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