Role of Single Defects in Electronic Transport through Carbon Nanotube Field-Effect Transistors

The influence of defects on electron transport in single-wall carbon nanotube field-effect transistors (CNFETs) is probed by combined scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). SGM images are used to quantify the depletion surface potential, and from this the Fermi level, at individual defects along the CNFET length. SIM is used to measure the voltage distribution along the CNFET. When the CNFET is in the conducting state, SIM reveals a uniform potential drop along its length, consistent with diffusive transport. In contrast, when the CNFET is “off,” potential steps develop at the position of depleted defects. High-resolution imaging of a second set of weak defects is achieved in a new “tip-gated” SIM mode.

Figure 1

(a) Setup for scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). In SGM, transport current is measured as a function of tip-gate position. In SIM, the local voltage oscillation is measured. (b) AFM topography image of the CNFET. (c) Schematic of valence band energy modulation due to defects. Vertical black bar is the local Fermi energy. The tip depletes the shaded part of the tube below it.

Figure 2

(a)– (e) SGM images with tip voltage of 1, 2, 4, 6, and 8 V, respectively. Lateral bias is 0.3 $V_{pp}$ for (a)– (c), 0.1 $V_{pp}$ for (d), (e). Back-gate voltage is $-1\mathrm{V}$. (f)– (j) Simultaneously acquired SIM images. Weak defects are clearly resolved at high tip voltage (see text). Defects 1–4 and the edges of the electrical contacts are shown in (c). The diagonal line in (c) is used for the SIM line scans of Fig. 4.

Figure 3

SGM imaged defect diameter increases linearly with tip voltage. The slope of the line gives the depletion surface potential of each defect.

Figure 4

(a) When the CNFET is in the “off” state, SIM shows voltage steps at the position of strong defects. The bias, backgate, and tip voltages are $V_{\mathrm{a}\mathrm{c}}=0.1$ $V_{pp}$, $V_g=0.7\mathrm{V}$, and $V_{\mathrm{t}\mathrm{i}\mathrm{p}}=-1\mathrm{V}$. The lateral bias is $V_{\mathrm{a}\mathrm{c}}=0.1$ $V_{pp}$. At the voltage of $V_{\mathrm{t}\mathrm{i}\mathrm{p}}=-1\mathrm{V}$ the tip is a noninvasive probe. (b) SIM images a uniformly increasing potential along the CNFET in the “on” state ($\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}\mathrm{g}\mathrm{a}\mathrm{t}\mathrm{e}=0\mathrm{V}$). The line for both scans is indicated in Fig. 2c.