Electrical observation of one dimensional sub-band structure of carbon nanotube in schottky barrier transistor

We have succeeded in observing the one dimensional sub-band structure of single walled carbon nanotube (SWNT) in the electrical measurement through the quantum capacitance effect, which could be observed even at room temperature. The sample structure was shown in Fig. 1. The SWNT was grown by chemical vapor deposition on a p-type silicon wafer with a thennally grown oxide. Pt/Au electrodes were deposited on the both side of the SW-NT for source and drain, and the back side of Si substrate for the gate. The channel length was 4 1tm. Thus, the back gate type SWNT-FET was fabricated. The left axis of Fig. 2 shows the gate voltage VG dependence of the drain Schottky barrier height of the SWNT-FET after the elimination of the adsorbed oxygen molecules by applying the Electrical Heating Process (EHP) I"2]. The SWNT-FET after EHP operates as a Schottky barrier transistor (SWNT-SBT). The barrier height qP was as large as qpB=1OO meV at VG-40 V. (The inset of Fig. 2 shows the band diagram at VG-40V near the drain contact.) The right axis of Fig. 2 shows the drain current IDVG characteristic of the SWNT-SBT with the constant drain voltage VD of I V, which indicates the ambipolar characteristics. Further increase of the drain voltage up to VD=10 V at 8.6 K, current step characteristics became appeared in the semi-logarithmic ID-VGcharacteristics as shown in Fig. 3, which are attributed to the oscillation characteristics of the quantum capacitance CQ [31. The gate capacitance consists of two components, one is the insulator capacitance Ci, which is originated from the geometry of the SWNT-SBT. And the other is the quantum capacitance CQ which is originated from the one dimensional sub-band structure of SWNT. In the present device, the CQ limits the operation of the device. The current step characteristics are resulted in the fact that the drain Schottky barrier is modulated stepwise by the effect of the quantum capacitance CQ. The CQ has the oscillation property corresponded to the saw teeth structure of the one dimensional sub-band structure of SWNT as shown in Fig. 4. Therefore, the current step characteristics in Fig. 3 indicate directly the sub-band structure in SWNT. The ratio of the gradients of the drain current IDin Fig. 3 at the step region and the slope region was estimated to be 3, which is almost in agreement with the ratio of the maximum and minimum of the CQ. The gate modulation coefficient a (which implies the ratio of the applied V0 and modulated potential energy in the SWNT-SBT) was estimated to be 0.071 from the other experimental results with the same sample structure device (not shown). Moreover, step width of the current step characteristics is about MaV6=0.5 eV, which is also in good agreement with the typical sub-band energy separation of 0.5 eV ofSWNT. Although a larger noise was superimposed, similar current step characteristics were also observed even at room temperature, owing to the large sub-band energy separation compared to the thermal energy of25 meV. This measurement technology includes a large potential to identify the chirality ofthe individual SWNT even after the fabrication ofthe electrical device.