Session A36: Carbon Nanotubes: Electrical Transport and Noise

The Device Physics of Experimentally Validated Analytical Theory of Transport in Ballistic Carbon Nanotube Transistors

We have developed a fully analytical ballistic theory of carbon nanotube field effect transistors (CNFETs) enabled by the development of an analytical surface potential capturing the temperature dependence and gate and quantum capacitance electrostatics. The analytical ballistic theory is compared to the experimental results of a ballistic transistor with good agreement. The validated analytical theory enables qualitative and quantitative intuitive insight into transport in CNFETs, provides techniques for extracting device parameters such as the bandgap and the surface potential from experimental current-voltage characteristics, and elucidates on the relatively new device physics of drain optical phonon scattering and its role in reducing the linear conductance and intrinsic gain of the transistor. These results apply to all ballistic CNFETs with a channel length that is less than the acoustic phonon mean free path.   

Unusual behaviors of heat-treated nanotube devices with platinum contact metal

Modest heat treatments typically lead to performance improvements in devices made from single-walled carbon nanotubes (SWCNTs) [1]. We report highly unusual behavior, however, in the response SWCNTs contacted with Pt electrodes. Instead of monotonically improving, the contact resistance can either increase or decrease by one order of magnitude depending on the processing temperature. Furthermore, we observe anomalous changes in the device transconductance, such that SWCNTs previously identified as metallic acquire gate-dependent characteristics. The results appear intrinsic to the Pt-SWCNT interface and are not due to contamination or environmental effects, because measurements are performed \textit{in situ} during heating in ultra-high vacuum. Complimentary electrochemical and spectroscopic testing reveals the influences of Pt-SWCNT interface chemistry. These results have particular importance for high power applications requiring refractory metal contact electrodes. [1] A. Kane et al. \textit{App. Phys. Lett}. \textbf{92} 038506 (2008).   

Length dependent transport measurements in long channel semiconducting carbon nanotubes

In order to understand the intrinsic electronic properties of CNTs, it is important to eliminate the contact effects from the measurements. We accomplish this by using a conductive-tip atomic force microscope cantilever as a movable electrode to obtain length dependent transport measurements. We report on the resistance versus length$ R(L)$ for several long channel ($L$ up to 130 $\mu $m) semiconducting CNTs at room temperature. In the on state,$ R(L)$ of semiconducting CNTs is linear. In the depleted state, $R(L)$ is linear for long channel lengths ($>$ 10 $\mu $m), but non-linear for short channel lengths due to the long depletion lengths in one-dimensional semiconductors. Transport remains diffusive under all depletion conditions, due to both low disorder and high temperature.   

Simulation of Electronic Transport in Carbon Nanotube Field Effect Transistors

In recent years, Carbon nanotube (CN) field effect transistors (CNFETs) with a sub-threshold slope of 40mV/dec have been demonstrated, which is less than the thermal limit of 60mV/dec\footnote{Appenzeller, J., \emph{et al.}, \emph{IEEE Trans. Elec. Dev.}, \textbf{52}, 2568, (2005)}. By exploiting inter-band tunneling, the transmission ratio comes to depend on the density of states rather than the thermal distribution of carriers in the contacts. Using tight-binding approximations to the Hamiltonian in the Keldysh non-equilibrium Green's function (NEGF) formalism, we study the transport properties of CNFETs under a tunneling mode bias. Phonon coupling effects are included through the self-consistent Born approximation (SCBA). The mode-space approach to decoupling the Hamiltonian\footnote{Venugopal, R., \emph{et al.}, \emph{J. Appl. Phys., }\textbf{92}, 3730, (2002)} is extended to include chiral nanotubes, such that a more realistic class of CNs may be treated with computational efficiency. Further, a comparison is made between the $\pi$-orbital and $\pi+\sigma$-orbital tight-binding models. Here, we find that transport is minimally affected. The geometry and electrostatic contact doping are examined to optimize device performance.   

Effect of gate electrode contact on transport properties of carbon nanotubes

Recently, considerable effort has been devoted to developing carbon nanotube devices. One of the important issues in the developments of carbon nanotube devices is the control of contact effects of the electrodes. To detect electric signals through nanotubes, electrodes must be connected to the nanotubes. The contact with the electrodes sensitively influences their electronic structures and transport properties. Therefore, it is important to discuss the transport properties on the basis of the detailed electronic state calculations that include the effect of the contact with the electrodes. We have investigated quantum transport in carbon nanotubes bridged between metallic electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We have analyzed transport properties of the finite size of carbon nanotubes bridged between metallic electrodes on a gate electrode, and discuss the contact effect of the electrodes on the transport properties.   

Theoretical Study of Multiple-Trap Correlations in Random Telegraph Signals of a Carbon Nanotube Field-Effect Transistor

We develop a theoretical model to explain the observation of high amplitude, multiple-trap random telegraph signatures (RTS) in the electronic transport of a one-dimensional field effect transistor (FET) with a carbon nanotube channel. A unique RTS pattern is observed, with an initial strong blockade of the current that continues over a well-defined bias window, and subsequent reversal of the blockade through a separate RTS series. We ascribe our observations to correlated electrostatic effects between multiple charge traps along the channel, whereby one trap 'passivates' the other purely electrostatically and without any direct chemical bond. We present a robust quantum transport model that provides quantitative validation of this hypothesis. We assert that this effect, which has not been reported in bulk silicon devices, arises from the logarithmic electrostatic potential profile of the 1-D channel that allows the trap levels to slip past each other under the action of a remote gate, ultimately reversing their energy hierarchy and annihilating each other. Our results suggest that multiple-trap behavior in low-dimensional field-effect devices may be adaptable for several new transistor and sensor technologies.   

Reversible current blockade of carbon nanotube through well resolved multitrap-interactions

We report the observation of a new kind of multiple-trap random telegraph signal (RTS) in the electronic transport of a carbon nanotube field effect transistor at room temperature. RTS from one charge trap precedes a transition to strong current blockade continues over a well-defined bias window, and subsequently reverses to unblock through a separate RTS series arising from a second, adjacent trap thru pure electrostatic interaction. Our results suggest that multiple-trap behavior in low-dimensional field-effect devices may be adaptable for several new transistor and sensor technologies.   

Scanning Gate Microscopy of Single-Walled Carbon Nanotubes

The one dimensional nature of single-walled carbon nanotubes (SWCNTs) causes their low energy properties to be described by Luttinger liquid theory. Using low-temperature scanning probe microscopy and electrical transport measurements, we have investigated the electronic properties of SWCNTs. Individual SWCNTs were contacted with Pd electrodes and located using atomic force microscopy. The AFM operating at 300 mK was used to probe the SWCNTs using scanning gate microscopy. Differential conductance as a function of source-drain bias voltage and gate voltage showed Coulomb diamond patterns. Using a voltage on the AFM tip, we are able to probe the spatial dependence of the conductance. The ability to perform simultaneous electrical transport and scanning probe microscopy measurements allows us to test theoretical predictions about Luttinger liquids including spin-charge separation.   

Electron Transport in Fluorinated Singe Wall Nanotubes

Taking advantage of helical symmetry, we present results for the electronic structure and electron transport properties of fluorinated chiral single-wall carbon nanotubes, within an all-electron, local density functional approach. In this talk, we discuss the effect of pairs of fluorine substituents on metallic nanotubes. Our results show that this fluorination results in resonance features in the vicinity of the Fermi level. The resonance behavior comes out as a result of the interaction of the C-C atoms those are close to the F-attached C atoms. We also discuss the change of atomic structure caused by fluorination. To our knowledge this is the first electron transport calculation that uses the helical symmetry within a first-principles approach. We find that the use of helical symmetry has important advantages in electron transport calculations of systems with local defect sites. We will also briefly discuss about the application of this technique on Si nanowire systems with defects.   

Electron-phonon scattering effects on transport properties of carbon nanotubes - From diffusive to ballistic regimes -

Recently, nanotechnology has fabricated various nanoscale electronic devices. In these systems, the mean free path is comparable to the system size. Therefore, it is important to understand how the transport property changes from the ballistic to the diffusive regimes by various scattering effects. In this work, we study the transport properties of carbon nanotubes using the time-dependent wave-packet approach [1]. Combining with the molecular dynamics simulations, we can treat the electron transport from diffusive to ballistic regimes from atomistic point of views [2]. We investigated the transport properties of metallic carbon nanotubes and the channel-length dependence of resistance from diffusive to ballistic regimes. The obtained mean free path and relaxation time are consistent with experimental observations. Furthermore, we investigate the mobility of semiconducting nanotubes. In the presentation, we will discuss the detail analysis of the origin of resistance. [1] H.Ishii \textit{et al.}, Phys.Rev.B 76(2007) 205432 [2] H.Ishii \textit{et al.}, to be published in Applied Physics Express.   

Scanning gate microscopy of electronic inhomogeneities in single-walled carbon nanotube (SWCNT) devices

The electronic properties of SWCNT devices are primarily determined by the contact metal and the SWCNT bandstructure. However, inhomogeneities such as substrate imperfections, sidewall defects, and mobile contaminants also contribute. In extreme cases, metallic SWCNTs have transistor-like behaviors due to these inhomogeneities. We investigate methods of identifying and distinguishing these different scattering mechanisms using scanning gate microscopy. For example, we can readily identify a sidewall defect in the presence of substrate charge traps, because the two types of disorder respond differently to gate electric fields. We present methods of optimizing the imaging conditions to make such distinctions. This research has been partly supported by the NSF (DMR-0801271 and ECCS-0802077).   

Tuning the electronic states of carbon nanotube based devices under magnetic field

Carbon nanotubes have already demonstrated their wide potential in nanoelectronics and optoelectronics. In our study, we demonstrate that an applied magnetic field, along with a control of the electrostatic doping, drastically modifies the electronic band structure of a carbon nanotube based transistor. Several examples will be addressed in this presentation. In a parallel configuration (B parallel to the tube axis), a quantum flux threading the tube induces a giant Aharonov-Bohm conductance modulation mediated by Schottky barriers which profile is magnetic field dependent. In the perpendicular configuration, the applied magnetic field breaks the revolution symmetry along the circumference and non conventional Landau states are expected in the high field regime. By playing with a carbon nanotube based electronic Fabry-Perot resonator, we bring evidence that the electronic transmission of the device can be modified by a transverse magnetic field. The field dependence of the resonant states of the cavity reveals the onset of the first landau state at zero energy. These experiments also enlighten the outstanding efficiency of magneto-conductance experiments to probe the electronic properties of carbon based nano-materials.   

Transport and Magnetism in Template Synthesized Hydrogenated Multiwalled Carbon Nanotubes

In this work, we synthesize highly disordered carbon nanotubes by CVD in porous alumina templates. We show that, due to the disorder in the nanotubes, they can easily be made to uptake hydrogen by annealing. We show that this induces ferromagnetism in the nanotubes, and we perform a magnetic study. We also measure the transport properties of the nanotubes. First, we find a rate dependent hysteretic magnetoresistance. We explain the rate dependence through strong magneto-viscosity effects, and we attribute the hysteresis to anisotropic magnetoresistance. We also discover a magnetic field-driven temperature dependent transition from positive to negative magnetoresistance in the ferromagnetic nanotubes that is not observed in similarly disordered un-hydrogenated carbon nanotubes. We attempt to explain this behavior by considering it an order-disorder transition described by the Bright model due to several scattering pathways, that are present in the ferromagnetic nanotubes that are not present in the non-ferromagnetic tubes.   

Magnetotransport of hybrid nanoparticle-nanowire systems

Semiconductor nanowires decorated with metal nanoparticles have a number of interesting electronic and photonic properties. For example, top-gated field effect transistors based on these hybrid systems have shown charge storage when operated in a floating-gate architecture. In addition, recent measurements have demonstrated that spin relaxation and phase coherence lengths can be extracted from magnetoconductance patterns in the gating response of bare nanowires. Together, these results suggest the possibility of \textit{in situ} tuning of the spin relaxation length in hybrid systems via modulation of the floating-gate potential. Initial efforts along these lines will be presented, including gating response and low temperature magnetotransport in 50 nm diameter InP nanowires decorated with Au nanoparticles from 20 -- 250 nm in diameter. The potential utility of these systems as testbeds for the exploration of spin scattering and transport will be discussed.   

Exploration of conductance peak splitting in carbon nanotube field effect transistors at critical field strengths

Carbon nanotube field effect transistors were produced by chemical vapor deposition growth of nanotubes on oxidized silicon substrate. Samples were back gated on doped silicon and contacted with gold/chrome contacts. Conductance measurements were performed at low temperature and high magnetic field using a dilution refrigerator and a superconducting magnet. Data was taken at 0.5 Tesla increments from 0-11Tesla. The differential conductance (dI/dV) shows an interesting asymmetry with bias voltage as well as a near zero bias conductance peak. The near zero bias conductance peak demonstrates splitting at two critical magnetic field strengths on the 0.5T scale. These two critical regimes are further explored on a finer magnetic field scale.