Structure and Electronic Properties of H-passivated silicon nanowires

1. Structure and Electronic Properties of H-passivated silicon nanowires N. Lu1, Li Huang1,2, T. L. Chan1, C. V. Ciobanu3, F.-C. Chuang1, J. A. Yan2, M. Y. Chou2, C. Z. Wang1, K. M. Ho1 1 Ames Laboratory-USDOE and Department of Physics, Iowa State University, Ames, Iowa, 50011 USA 2School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 3 Division of Engineering, Colorado School of Mines, Golden, Colorado 80401, USA We have investigated the structure of thin H-passivated Si nanowires oriented along the [110] and [112] directions using an efficient optimization procedure based on a genetic algorithm (GA), followed by structural refinements at the density functional theory (DFT) level [1,2]. We found that in the presence of hydrogen, the silicon atoms of the nanowire can maintain their bulk-like bonding environment down to sub-nanometer wire dimensions. Furthermore, our calculations reveal that, as the number of atoms per length is increased, there emerge three distinct types of wire configurations with low formation energies (magic wires) for the [110] wires (Fig. 1). Two of these structures have a plate-like aspect in cross section, which have not been observed so far. The third one has a hexagonal section, which is consistent with recent experiments for Si and Ge wires [1]. For the [112] wires, we find that at certain values of the hydrogen chemical potential the nanowires can take relatively stable (magic) structures with rectangular cross sections bounded by monohydride {110} and {111} facets with dihydride wire edges [2]. We also investigated the electronic properties of H-passivated Si nanowires (SiNWs) oriented along the [112] direction [4], with the atomic geometries retrieved via global genetic search. We show that [112] SiNWs remain an indirect band gap even in the ultrathin diameter regime, whereas the energy difference between the direct fundamental band gap and the indirect one progressively decreases as the wire size increases, indicating that larger [112] SiNWs could have a quasi-direct band gap. We further show that this quasi-direct gap feature can be enhanced when applying uniaxial compressive strain along the wire axis. Moreover, our calculated results also reveal that the electronic band structure is sensitive to the change of the aspect-ratio of the cross sections. [1] T. L. Chan, C. V. Ciobanu, F.-C. Chuang, N. Lu, C. Z. Wang, and K. M. Ho, Nanolett. 6(2), 277-281 (2006). [2] N. Lu, C. V. Ciobanu, T. L. Chan, F.-C. Chuang, C. Z. Wang, K. M. Ho, J. Phys. Chem. C111, 7933 (2007). [3] D. D. D. Ma, C. S. Lee, F. C. K. Au, S. Y. Tong, S. T. Lee, Science 299, 1874 (2003). [4] Li Huang, N. Lu, Jia-An Yan, M. Y. Chou, C. Z. Wang, K. M. Ho, submitted (2008).

2. Fig. 1 Magic nanowires (perspective view) found as minima of the formation energy per atom. The chain (a) and double-chain (b) are characterized by the number of complete six-atom rings R and double-rings D, respectively. The configurations with hexagonal cross-section have a number L of full concentric layers L=2 in panel (c) above) of six-rings, and are consistent with recent observations of H-passivated SiNWs. The facet orientations of magic wires are shown on the right. Fig. 2 Perspective view along the axis of [112] H-passivated Si nanowires with monohydride (a) and trihydride (b) (111) facets. The (110)-type facets are covered with monohydrides in both cases. Fig. 3 Calculated energy band gaps with LDA (circle) and GW (square) versus the diameter for [112] SiNWs. For comparison the measured band gaps of [112] wires by Ma et al (diamond) are plotted. The solid horizontal line indicates the LDA gap of bulk Si. The dashed lines are fitted to the data points.