Structural and tunneling properties of Si nanowires

We investigate the electronic structure and electron transport properties of Si nanowires attached to Au electrodes from first principles using density functional theory and the nonequilibrium Green's function method. We systematically study the dependence of the transport properties on the diameter of the nanowires, on the growth direction, and on the length. At the equilibrium Au-nanowire distance we find strong electronic coupling between the electrodes and nanowires, which results in a low contact resistance. With increasing nanowire length we study the transition from metallic to tunneling conductance for small applied bias. For the tunneling regime we investigate the decay of the conductance with the nanowire length and rationalize the results using the complex band structure of the pristine nanowires. The conductance is found to depend strongly on the growth direction, with nanowires grown along the $\langle 110\rangle$ direction showing the smallest decay with length and the largest conductance and current.

Figure 1

(a) Cross sections of $\langle 100\rangle$, $\langle 110\rangle$, and $\langle 111\rangle$ Si NWs. Blue and red spheres represent Si and H atoms, respectively. The diameter is $1.5$ nm. (b) Configuration of the two-probe system comprising two electrodes and the central scattering region. For the leads we use both Si NWs (left) and Au $\langle 111\rangle$ metallic electrodes (right).

Figure 2

Projected DOS for the Au-NW-Au system ($D=1.0$ nm, $l=30.72$ Å) for different Au-NW distances $d$. The Si${}_1$ atom belongs to the plane in contact with Au, while Si${}_2$ is located approximately in the middle of the Si NW between the Au planes.

Figure 3

Transmission coefficient for different diameters of the Si NW: $0.6$, $1.0$, $1.5$, and $2.0$ nm ($l=30.72$ Å and $d=2.3$ Å). The growth direction is $\langle 110\rangle$.

Figure 4

Transmission coefficient for the Au-NW-Au system ($l=30.72$ Å and $d=2.3$ Å) for different diameters. Transport gap around $E_F$ as a function of the diameter: The dashed line is the value for bulk Si.

Figure 5

Transmission coefficient for the Au-NW-Au system for lengths $7.68$, $15.36$, $23.04$, and 30.72 Å ($D=1.0$ nm and $d=2.3$ Å) in the direction $\langle 110\rangle$. There is a smooth transition from metallic to semiconducting behavior as the NW length increases.

Figure 6

Complex (left) and real (right) band structure for a Si NW with $\langle 110\rangle$ growth direction ($D=1.0$ and $1.5$ nm).

Figure 7

Transmission coefficient (top), complex band structure (bottom, color), and damping coefficient (bottom, black) for NWs with diameters $D=1.0$ nm (left) and $D=1.5$ nm (right), and $\langle 110\rangle$ growth direction.

Figure 8

Isosurface wave functions for the (left) HOMO and (right) LUMO. The NW growth direction is (top) $\langle 110\rangle$, (middle) $\langle 100\rangle$, and (bottom) $\langle 111\rangle$, and the diameter is $D=1.0$ nm.

Figure 9

Isosurface transmission wave functions for the (a) HOMO, (b) gap region, and (c) LUMO, for the Au-Si NW-Au system with Si NW growth direction $\langle 110\rangle$.

Figure 10

Complex and real band structures for Si NWs with $D=1.0$ nm and growth directions (top) $\langle 100\rangle$ and (bottom) $\langle 111\rangle$.

Figure 11

Transmission coefficient (top), complex band structure (bottom, color), and damping coefficient (bottom, black) for NWs with growth directions $\langle 100\rangle$ (left) and $\langle 111\rangle$ (right), and diameter $D=1.0$ nm.

Figure 12

$I$-$V$ characteristics for Au-NW-Au systems: (a) $\langle 110\rangle$ Si NWs with diameters $D=1.0$ and 1.5 nm. (b) Si NWs with $D=1.0$ nm, grown in the three principal directions.