Presentation on theme: "CNT devices Since their first discovery and fabrication in 1991, CNTs have received considerable attention because of the prospect of new fundamental science."— Presentation transcript:
CNT devices Since their first discovery and fabrication in 1991, CNTs have received considerable attention because of the prospect of new fundamental science and many potential applications.
Stretching And confined deformation Strain of less than 1% results in the CNT changing from metal to semiconductor.
Twisting and bending
The encapsulated fullerenes can rotate freely in the space of a (10, 10) tube at room temperature, and the rotation of fullerenes will affect 10) peapod electronic properties significantly; generally, orientational disorderwill remove the sharp features of the average density of states (DOS). However, the rotation of fullerenes cannot induce a metal–insulator transition. Unlike the multicarrier metallic 10) peapod, the 0) peapod is a semiconductor, and the effects of the encapsulated fullerenes on tube valence bands and conduction bands are asymmetrical. The distances between the centres of the fullerenes are and nm for the 10) peapod and 0) peapod, respectively. Peapods J. Chen, and J. Dong, J. Phys. Condens. Matter, 16, 1401 (2004)
It is shown that, by appropriate work function engineering of the source, drain and gate contacts to the device, the following desirable properties should be realizable: a sub-threshold slope close to the thermionic limit; a conductance close to the interfacial limit; an ON/OFF ratio of around 1000; ON current and transconductance close to the low-quantum-capacitance limit. Semiconducting behavior in nanotubes was first reported by Tans et al. in Fig. 5 shows a measurement of the conductance of a semiconducting SWNT as the gate voltage applied to the conducting substrate is varied. The tube conducts at negative Vg and turns off with a positive Vg. The resistance change between the on and off state is many orders of magnitude. This device behavior is analogous to a p-type metal–oxide– semiconductor field-effect transistor (MOSFET), with the nanotube replacing Si as the semiconductor. At large positive gate voltages, n-type conductance is sometimes observed, especially in larger-diameter tubes. McEuen et al., IEEE Trans. Nanotechn., 1, 78 (2002)
Semiconducting nanotubes are typically p-type at Vg=0 because of the contacts and also because chemical species, particularly oxygen, adsorb on the tube and act as weak p-type dopants. Experiments have shown that changing a tubes chemical environment can change this doping levelshifting the voltage at which the device turns on by a significant amount. This has spurred interest in nanotubes as chemical sensors. Adsorbate doping can be a problem for reproducible device behavior, however. Controlled chemical doping of tubes, both p- and n-type, has been accomplished in a number of ways. N-type doping was first done using alkali metals that donate electrons to the tube. This has been used to create n-type transistors, p-n junctions, and p-n-p devices. Alkali metals are not air-stable, however, so other techniques are under development, such as using polymers for charge-transfer doping Scattering sites in nanotubes: I–V characteristics at different Vgs for a p- type SWNT FET utilizing an electrolyte gate in order to improve gate efficiency. McEuen et al., IEEE Trans. Nanotechn., 1, 78 (2002) Implying a mean-free path of approx. 700 nm.Maximum transconductance dI/dVg=20uA/V at Vg=-0.9V. Normalizing this to the device width of ~2nm: 10mS/um.
Bottom - gated CNT FET
Calculated conductance vs gate voltage at room temperature, varying (a) the work function of the metal electrode, and (b) doping of the NT. In (a) the work function of the metal electrode is changed by -0.2 eV (red dashed), -0.1 eV (orange dashed), 0 eV (green), +0.1 eV (light blue), and +0.2 eV (blue), from left to right, respectively. In (b) the doping atomic fraction is n-type (red), (orange), and (green), and p-type (blue dashed), from left to right, respectively. Thus the gate field induces switching by modulating the contact resistance (the junction barriers). Oxygen adsorption at the junctions modifies the barriers (i.e. the local band-bending of the CNT) and affects the injection of carriers (holes or electrons).
The inverse subthreshold slope, which is a measure of the efficiency of the gate field in turning on the device, decreases with a decrease in gate oxide thickness. This behavior cannot be explained by conventional field-effect transistor models, and has in fact been shown to be a result of the presence of Schottky barriers at the metal/nanotube interface at the source and drain.
There is a clear difference in the inverse subthreshold slope for the case of sweeping all gate segments together (S = 400 mV/dec) versus sweeping only the inner segments (S = 180 mV/dec). We attribute the observed change in S to a change from Schottky barrier modulation to bulk switching. (b) shows linear plots of the subthreshold portion ( where the current is dominated by carrier density ) of the transfer characteristics when the inner gate segments are swept together or separately. The current nearly identical, despite the fact that the effective gate lengths differ by a factor of 1.6. This is in contrast to the expected behavior of diffusive transport, where the current varies inversely with the gate length.
Calculated output characteristics of the symmetric ( dashed lines) and the asymmetric ( solid lines) CNFET.
We have introduced nanotemplate to control selective growth, length and diameter of CNT. Ohmic contact of the CNT/metal interface was formed by rapid thermal annealing (RTA). Diameter control and surface modification of CNT open the possibility to energy band gap modulation.
Diode-like rectifying behavior was observed in a CNx /C multiwalled nanotube due to its being one half doped with nitrogen. FETs based on an individual CNx /C nanotube were fabricated by focused ion-beam technology. The nanotube transistors exhibited n-type semiconductor characteristics, and the conductance of nanotube FETs can be modulated more than four orders of magnitude at room temperature. The electron mobility of a CNx /C NT FET estimated from its transconductance was as high as cm2/Vs. The n-type gate modulation could be explained as due the effect of bending of the valence band in the Schottky-barrier junction.
CNTs doped with fullerenes inside nanotubes (so-called peapods) are interesting materials for novel CNT FET channels. Transport properties of various peapods such as C60-, and have been studied by measuring FET I-V characteristics. Metallofulleren peapod FETs exhibited ambipolar behavior both p- and n-type characteristics by changing the gate voltage, whereas C60-peapod FETs showed unipolar p-type characteristics similar to the FETs of intact single-walled nanotubes. This difference can be explained in terms of a bandgap narrowing of the single-walled nanotube due to the incorporation of metallofullerenes. The bandgap narrowing was large in the peapods of metallofullerene, where more electrons are transferred from encapsulated metal atoms to the fullerene cages. The entrapped fullerene molecules are capable of modifying the electronic structure of the host tube. It is, therefore, anticipated that the encapsulation of fullerene molecules can play a role in band gap engineering in nanotubes and hence that peapods may generate conceptually novel molecular devices.
Schematic illustration of elastic strain distributed around the site of metallofullerenes in a small-diameter nanotube peapod and the corresponding changes in conduction and valence band edges. Charge transport in a partially filled peapod FET in metal-on-top setup. (a) Transfer characteristics at various temperatures. Data were taken at Vds = 0.3 V.
CNT junction Current vs. voltage characteristics of an all-carbon transistor with semiconducting nanotube as channel, with different voltages at the carbon gate. The back gate is kept at 0 V. The measurements were carried out at 4 K.
The band profile of the SB CNTFET at the minimal leakage bias (VG=0V) for VD=0.6V. The band profile of the MOS CNTFET when the source-drain current is low. (VD=0.6V and VG=-0.3V). The channel is a (13,0) nanotube. Ambipolar conduction leads to a large leakage current that exponentially increases with the power supply voltage, especially when the tube diameter is large. An asymmetric gate oxide SB CNTFET has been proposed as a means of suppressing ambipolar conduction. SB CNTFETs of any type, however, will likely suffer from the need to place the gate electrode close to the source (which increases parasitic capacitance) and metal-induced gap states, which increase source to drain tunneling and limit the minimum channel length.
Id vs. Vd characteristics at VG = 0.4V for the MOS CNTFET (the solid line) and the SB CNTFETs (the dashed lines). The off-current of all transistors (defined at Vd=0.4V and Vg=0) was set at 0.01µA by adjusting the flat band voltage for each transistor. For the SB CNTFETs, three barrier heights we simulated. The channel is a (13,0) nanotube, which results in a diameter of d 1 nm, and a bandgap of Eg 0.83 eV. Id vs. Vg characteristics at Vd = 0.4V for the zero barrier SBFET and the MOS CNTFET. The gated channel of both transistors is a 5nm-long, intrinsic (13, 0) CNT. By eliminating the Schottky barrier between the source and channel, the transistor will be capable of delivering more on-current. The leakage current of such devices will be controlled by the full bandgap of CNTs (instead of half of the bandgap for SB CNTFETs) and band-to-band tunneling. These factors will depend on the diameter of nanotubes and the power supply voltage.