Electrical properties of individual tin oxide nanowires contacted to platinum electrodes

A simple and useful experimental alternative to field-effect transistors for measuring electrical properties (free electron concentration $n_d$, electrical mobility $\mu$, and conductivity $\sigma$) in individual nanowires has been developed. A combined model involving thermionic emission and tunneling through interface states is proposed to describe the electrical conduction through the platinum-nanowire contacts, fabricated by focused ion beam techniques. Current-voltage $\left(I\text{−}V\right)$ plots of single nanowires measured in both two- and four-probe configurations revealed high contact resistances and rectifying characteristics. The observed electrical behavior was modeled using an equivalent circuit constituted by a resistance placed between two back-to-back Schottky barriers, arising from the metal-semiconductor-metal $\left(M\text{−}S\text{−}M\right)$ junctions. Temperature-dependent $I\text{−}V$ measurements revealed effective Schottky barrier heights up to ${\Phi }_{BE}=0.4\mathrm{eV}$.

Figure 1

$\mathrm{Sn}{\mathrm{O}}_2$ nanowire of radius $r=27\pm 3\mathrm{nm}$ and length $L=11\mu \mathrm{m}$ electrically contacted using dual-beam FIB nanolithography techniques.

Figure 2

$I\text{−}V$ curve of an individual NW (Fig. 1) at $T=21°\mathrm{C}$ exhibiting a slightly rectifying response.

Figure 3

Band diagram of the back-to-back Schottky configurations existing in FIB contacted NWs. Voltage drops are indicated and related to the resistances used in the equivalent circuit.

Figure 4

Two-probe resistance of the NW (Fig. 1) as a function of the applied voltage $V$ at $T=21°\mathrm{C}$. Reduction of $R$ occurs when electrons overcome the reverse-biased junction more easily due to a decreased barrier height caused by increasing voltage.

Figure 5

Electrical conductivity $\sigma$ of the NW shown in Fig. 1 as a function of increasing temperature $T$. The observed negative temperature coefficient is due to the semiconducting properties of $\mathrm{Sn}{\mathrm{O}}_2$.

Figure 6

$\log \left(I\right)$ vs $\left(V_{SI}\right)$ plot of the NW shown in Fig. 1 at $T=21°\mathrm{C}$. The linear behavior demonstrated that the experimental data fit with the proposed conduction model (line). NW free carrier concentration $(n_d\sim {10}^{18}{\mathrm{cm}}^{-3})$ was calculated from the observed slope.

Figure 7

Reverse-biased Schottky resistance $R_{SI}$ of NW (Fig. 1) as a function of the temperature. Based on the experimental data, the effective Schottky barrier height was found to be ${\Phi }_{BE}=0.28\pm 0.02\mathrm{eV}$.

Figure 8

SEM image of $\mathrm{Sn}{\mathrm{O}}_2$ NW (shown in Fig. 1) after a few hours under working conditions. The piece of NW placed between the inner contacts is not affected by self-heating effects; however, the contact area (inset) is destroyed. Melting temperature of $\mathrm{Sn}{\mathrm{O}}_2$ is close to $T=1100°\mathrm{C}$, demonstrating that local temperature close to the Schottky barriers can be much higher.