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. C 111, 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 L2 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.