Ab initio DFT study of physical properties of silicon nanostructures

 

S. Mahtout*

 

Laboratoire de Physique Théorique, Université A. Mira, Béjaïa, Algérie

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In this work, we have used the first-principles simulated annealing generalized gradient approximation density functional calculations based on norm-conserving pseudopotentials to investigate the geometric and electronic structure of low energy silicon nanostructures (clusters) in the range size 10 to 20 atoms (Sin, n = 21-30). Our interest is motivated by the fact that the atomic clusters constitute a bridge between molecular and condensed matter physics. Thus, by studying the properties of clusters as a function of clusters size, one hopes to learn how they evolve towards bulk properties and to find unique properties for specific cluster sizes that differ largely from their bulk counterparts. These facts mark the enormous potential of clusters studies for a wide range of applications. For theses reasons, the study of atomic clusters has become one of the most active areas of research in the last two decades. Silicon is one of the most important semiconductors with widespread applications which have been extensively studied. It is a semiconductor with diamond lattice structure preferring sp3 hybridization. Due to its unique electrical properties it is the most important technological material in the electronic industry.

To perform our calculation, we have used a density functional electronic structure calculation implemented in the SIESTA simulation package [1-2]. The generalized gradient approximation (GGA) of the density functional has been used with the exchange-correlation energy functional parameterized by Perdew and Zinger [3] and Perdew, Burke, and Ernserhof (PBE) [4]. Self-consistent field calculations are carried out with convergence criterion of 10-4 a.u. on the total energy and electron density. During simulation, the volume of the system was kept constant and to avoid interaction between the clusters a big supercell of 38 Å was used. The k=0 points was used for Brillouin zone sampling.

We find that new structures are obtained for each cluster size comparatively for those reported in the literature. In all cases of this size range, the bond length of Sin  clusters is longer than the one in bulk silicon. The average number of coordination is larger than those in the bulk crystal index indicating that the bulk-like behaviour is still far away. The second differences of cluster energies show that the lowest energy isomers of Si22, Si23, Si25, Si27 and Si29 are more stable than neighbouring clusters. The binding energies generally increase while the HOMO-LUMO gap generally decreases with the increase of clusters size.

 

Keywords: nanostructures, silicon, DFT simulation, structural properties, electronic properties