Electronic Properties and Transport in Silicon Nanowires

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In an integrated silicon nanotechnology environment, it may be desirable to replace metallic inter-connects like Al, with Silicon metallic wires. This motivated us to investigate the electronic and transport properties of quasi one-dimensional (1D) Silicon nanowires. We have employed the pseudopotential density functional method to obtain the stable structural arrangements of such nanowires. For an infinite 1D wire, the computed nearest neighbor distance (d = 2.2 ) and the cohesive energy (3.4 eV) are both well below the corresponding bulk values. The electronic structure, as expected, shows a half-occupied p-band that crosses the Fermi-level at the middle of the zone-edge. This is a classical case demanding a rearrangement of atoms to create a Peierls distorted 1D chain. The calculations indeed confirm that, the dimerized structure has a gap at the new zone-edge but the cohesive energy change from uniform 1D chain is rather insignificant. Thus this dimerized 1D structure is neither metallic nor likely to be easily fabricated (because of low cohesive energy). We then allowed the nanowires to distort in the xy-plain to find other stable structures. Figure 1 is the calculated E vs. S curve. Here E is the total energy/atom (in eV) and S is the horizontal distance (in ) between the two atoms in the primitive unit cell comprising the chain. This total energy is measured relative to the energy of the free Si atom. The negative value indicates that the structure is stable, the cohesive energy being the absolute value. The plot reveals two stable zigzag quasi 1D structures as deduced from local energy minima in Figure 1. One structure has a shallow minimum, a wide-angle zigzag structure (labeled as W) with s = 1.9 and d = 2.2 . The cohesive energy of W structure is 3.79 eV, far from the bulk value. The charge density plot shown for this structure in Figure 2 shows directional bonding among atoms, highly reminiscent of the bulk Si. The electronic structure reveals that the system is semi conducting and hence not suited as metallic nanowires. The second zigzag structure, labeled as T in Figure 1 is much more promising. It consists of nearly equilateral triangles. The cohesive energy is high (4.4 eV/atom) and the inter-atomic distance (2.4 ) is close to the bulk value. The charge density plot of this structure (see Figure 3) shows significant charge delocalization as for free electron metals. The electronic structure of this nanowire displays metallic behavior. From the computed electronic structure we obtain an approximate value of conductance. For this we use the maximum channel capacity arguments, which we motivate using the Heisenbergs uncertainty principle. This quantum mechanical principle must play an important role in the nanodomain. If such metallic Silicon nanowires were to be fabricated, then one could achieve a major integration in nanotechnology.

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Journal: TechConnect Briefs
Volume: 2, Technical Proceedings of the 2003 Nanotechnology Conference and Trade Show, Volume 2
Published: February 23, 2003
Pages: 206 - 209
Industry sectors: Advanced Materials & Manufacturing | Sensors, MEMS, Electronics
Topic: Modeling & Simulation of Microsystems
ISBN: 0-9728422-1-7