Various electrode materials for single-walled carbon nanotube (SWNT) transistors were investigated. Pd electrodes have been used for Ohmic contacts (Javey et al., 2003). Ti electrodes have been used for Schottky contacts for hole conduction (Heinze et al., 2002; Martel et al., 2001; Kamimura & Matsumoto, 2005), and Mg and Ca electrodes for electron conduction (Nosho et al., 2006). Moreover, in the case of sub-μm order channel length at low temperature, SWNT transistors with Ohmic contacts have shown resonant tunneling transistor (RTT) characteristics (Liang et al., 2001), which are also called as Fabry-Perot characteristics, and SWNT transistors with Schottky contacts have shown single-hole transistor (SHT) characteristics (Suzuki et al., 2001), in which the Schottky barriers act as tunneling barriers. Therefore, electrodes materials should be chosen to obtain the desired characteristics.
In this study, we succeeded in fabricating a multifunctional quantum transistor using the particle nature and wave nature of holes in SWNT. This transistor can operate as an RTT and also as an SHT. An RTT is a device that uses the wave nature of hole and an SHT uses the particle nature of hole in the SWNT. Both devices need tunneling barriers at both sides of the quantum island. The RTT needs strong coupling while the SHT needs weak coupling between the quantum island and the electrodes. Usually, these tunneling barriers are made from thin oxide layers, etc. Therefore, the thickness of the tunneling barriers and the coupling strength cannot normally be controlled in a given device. In the present device, however, the Schottky barriers act as the tunneling barriers between the SWNT quantum island and electrodes. Therefore, the thickness of the tunneling barriers and the coupling strength between the SWNT and electrodes can be controlled by the applied gate voltage
Moreover, a SWNT is a cylindrical material with a diameter of several nanometers. The small diameter makes it possible to detect an electrical field from even a single-charge. Moreover, by observing the relative energy difference between the conducting carrier and the single-charges to be measured, it is possible to define the potential energy of the single-charges to be measured. However, as described in many reports (Kim et al., 2003; Radosavljevic et al., 2002), SWNT electron devices show hysteresis characteristics in gate voltage-drain current characteristics. The hysteresis characteristics are caused by gate-voltage-dependent charge fluctuation, e.g., adsorption of water molecules around a SWNT (Kim et al., 2003), charging into insulator layer around a SWNT (Radosavljevic et al., 2002), and charging into amorphous carbon around a SWNT (Martel et al., 2001) By eliminating these origins of the hysteresis characteristics, the number of fluctuating charges becomes small and a single-charge can be detected by a SWNT multi-functional quantum transistor.
2. Materials and methods
We have eliminated the three origins of the hysteresis characteristics of a SWNT field effect transistor mainly pointed out in current reports (Martel et al., 2001; Kim et al., 2003; Radosavljevic et al. 2002; Kamimura & Matsumoto, 2004). To burn out amorphous carbon, we annealed a SWNT at low temperature in oxidizable atmosphere (Kamimura & Matsumoto, et al., 2004). To reduce the number of adsorbed atmosphere molecules, we covered the channel with a silicon dioxide layer. To reduce the number of trap sites in the insulator, we reduced channel length to 73 nm. The SWNT multi-functional quantum transistor fabricated by the process mentioned above shows almost no hysteresis characteristics in the gate voltage rage from -40 to 40 V. Moreover, an abrupt discrete switching of the source-drain current is observed in the electrical measurements of the SWNT multi-functional quantum transistor at 7.3 K. These random telegraph signals (RTS) are attributed to charge fluctuating charge traps near the SWNT multi-functional quantum transistor conduction channel. The current-switching behavior associated with the occupation of individual electron traps is demonstrated and analyzed statistically.
2.2. Sample preparations
A schematic of the sample structure is shown in Fig. 1. SWNT was prepared as follows. An n+-Si wafer with a thermally grown 300 nm thick oxide was used as a substrate. Layered Fe/Mo/Si (2 nm/20 nm/40 nm) catalysts were evaporated using an electron-beam evaporator under a vacuum of 10-6 Pa. These layered catalysts were patterned on the substrate using the conventional photo-lithography process. SWNT was grown by thermal chemical vapor deposition (CVD) using the mixed gases of hydrogen and argon-bubbled ethanol. After the growth of the SWNT, it was purified by burning out the amorphous carbon around the SWNT in an air atmosphere at a temperature of several hundred degrees Celsius (Kamimura and Matsumoto, 2004). Ti (30 nm) electrodes were deposited on the patterned catalysts as the source and drain, and on the back side of the n+-Si substrate for the gate, using the electron-beam evaporator under a vacuum of 10-6 Pa. The distance (
3. Results and discussions
3.1. Particle-Wave duality
Fig. 2(a) shows the differential conductance d
An oscillation characteristic with two oscillation periods is also observed in Fig. 2(a). A large oscillation period of Δ
as shown in Fig. 2(c) and (d), respectively, where
The shape of the Coulomb oscillation peaks must be Gaussian, which is attributed to thermal broadening, while the resonant tunneling current peaks must be Lorentzian, which is attributed to energy uncertainty (Radosavljevic et al., 2002). Therefore, the d
Fig. 3 shows the d
and shows a constant value independent of
The energy separation
Fig. 4 shows a contour plot of the d
Fig. 5(a) shows the Coulomb charging energy
The dependence of the full-width at half maximum (FWHM) of the resonant tunneling current peak characteristic on
The FWHM is estimated from an
In the particle nature mode of
where α is the ratio of the modulated energy to applied
The logarithmic dependence of the drain current
3.2. Hysteresis elimination
Fig. 8 shows the temperature dependence of differential conductance as a function of the gate voltage.
The quantum levels peaks became blurred with increasing temperature. However, even at
The static characteristic measurement of the devise with silicon dioxide layer on the SWNT channel shown in Fig. 9 was carried out using an Agilent B 1500.
In the measurement, the data were integrated for a few seconds to eliminate the effect of noise, which was set by the equipment automatically. After that, the data were recorded. In the drain current
From the small period of drain current oscillation of
In Fig. 9(d), the gray (black) line shows drain current vs. applied gate voltage from -40 (40) to 40 (-40)V in a forward (backward) sweep of gate voltage. The SWNT multi-functional quantum transistor showed almost no hysteresis characteristics. The SWNT multi-functional quantum transistor was covered with a silicon dioxide layer, which prevents the SWNT from absorbing and releasing molecules in ambient, because fluctuation of molecules on the SWNT usually induces the hysteresis characteristics in the drain current-gate voltage characteristics. Moreover, the SWNT multi-functional quantum transistor has a small channel length of 73 nm, which reduced the number of trap sites near the SWNT and, at the same time, the number of trapped carriers causing hysteresis characteristics. We believe that the purification was effective to reduce the hysteresis characteristics. Thus, the hysteresis characteristic in the drain current–gate voltage characteristic was eliminated.
3.2. Single charge sensitivity of SWNT multi-functional quantum transistor
Figure 10(a) shows the time dependence of the conductance of the SWNT multi-functional quantum transistor at 7.3 K with a gate bias of
The applied gate voltage was under the Fabry-Perot interference region. The SWNT multi-functional quantum transistor shows RTS, as shown in Fig. 10(a), The RTS showed three levels, n, n+1 and n+2, of the conductance shown in Fig. 10(a). At a lower applied gate voltage, current levels higher than n+2 such as n+3 and n+4 appeared. The multiple levels of RTS are attributed to charge fluctuating charge storages near the conduction channels of the SWNT multi-functional quantum transistor. Moreover, because there was a single-charge storage including multiple energy levels or were some charge storages being at almost the same distances from the conductance channel of the SWNT multi-functional quantum transistor, the RTS appeared. Figures 10(b) and 3(c) show histograms of the conductance levels of RTS at
The gate voltage dependences of the natural log of the ratio between the
The charge transition is modeled, as shown in Fig. 12(a), in which the energy barrier is between the SWNT and the charge storage. Figures 11(b)-11(e) are enlargement plots of each
Equation (2) is the transformed Arrhenius equation, in which VG is the parameter.
In summary, we succeeded in fabricating and demonstrating a multi-functional quantum transistor using the particle nature and wave nature of holes in SWNT. This transistor can operate in the wave nature mode as an RTT and in the particle nature mode as an SHT. We were able to reveal that the principle of the characteristic transition from an SHT to an RTT is the modulation of the coupling strength between the SWNT quantum island and the electrodes by the applied VG.