Local Gate Voltage Research Articles

High-performance local back gate thin-film field-effect transistors using sorted carbon nanotubes on an amino-silane treated hafnium oxide surface

Wafer-scale fabrication and characterization of local back gate semiconducting nanotube thin-film transistors (SN-TFTs) are reported in this paper. The local back gate voltage of the corresponding SN-TFT controls the individual transistor switching. In order to achieve high performance, a high-k dielectric material is employed as a gate oxide and this helped to achieve low-voltage operations, much steeper sub-threshold voltage swings and higher transconductance values. A simple procedure to deposit a high-density single-walled carbon nanotube thin film on an amino-silane-treated hafnium oxide (HfOX) surface is suggested such that a good density of nanotubes is realized without degrading the device on–off current ratio and mobility values. The density of the nanotubes achieved on the silanized HfOX surface is about 40–45 nanotubes µm−2. SN-TFTs exhibit an excellent p-type output characteristic with distinct linear and saturation regions. Local back gate SN-TFTs exhibit an on–off current ratio exceeding 104 and a steep sub-threshold slope of 400 mV/decade. SN-TFTs achieve a maximum current density of 13 μA µm−1, an average threshold voltage of −0.5 V, a maximum normalized transconductance of 18.5 μS µm−1 and exhibit a maximum carrier mobility of 60.6 cm2(Vs)−1.

Anomalous magnetic transport in ferromagnetic graphene junctions

We investigate magnetotransport in a ferromagnetic-normal-ferromagnetic graphene junction where a gate electrode is attached to the normal segment. It is shown that the charge conductance can be maximal at an antiparallel configuration of the magnetizations. Moreover, we demonstrate that both the magnitude and the sign of the spin-transfer torque can be controlled by means of the gate voltage in the normal segment. In this way, the present system constitutes a spin-transfer torque transistor. These anomalous phenomena are attributed to the combined effect of the exchange field and the Dirac dispersion of graphene. Our prediction opens up the possibility of moving domain walls parallel or antiparallel to the current in a controllable fashion by means of a local gate voltage.

Mirror-Image Photoswitching in a Single Organic Thin-Film Transistor

In this study, we detailed our design for making a photoactive semiconductor/dielectric interface in order to study charge dynamics at the interface in combination with photoexcitation, using organic thin-film transistors (OTFTs) as local probes. The photoactive semiconductor/dielectric interface is formed from a hybrid gate dielectric (polymethyl methacrylate (PMMA) and titanium oxide (TiO2)). Because the photogenerated electrons from TiO2 nanoparticles under UV illumination can serve as the trapping centers for quenching hole carriers, p-type semiconductors showed the decrease of the hole mobility. Interestingly, n-type semiconductors displayed symmetric, opposing photoswitching effects at the same condition, showing that the photogenerated holes can function as a local positive gate voltage for improving the electron mobility. On the basis of this understanding, a remarkable example of mirror-image photoswitching effects in a single ambipolar OTFT was realized. These results may offer grounds for the creation of the future practical/multifunctional organic optoelectronic devices.

Conformation-Induced Electrostatic Gating of the Conduction of Spiropyran-Coated Organic Thin-Film Transistors

A facile method to make optoelectronic devices formed from organic thin-film transistors that are functionalized by photochromic spiropyran dyes in a nondestructive manner has been developed. When PDMS stamps are employed, alkane-containing spiropyran molecules can be coated simply onto the surface of organic thin films in a noninvasive manner. Upon UV irradiation, the molecules undergo isomerization from the neutral closed form to the charge-separated merocyanine form, producing the local electrostatic environment. This photoinduced electrostatic environment can function as a local negative gate voltage, thus increasing the electrical conductivity in p-type devices and decreasing the electrical conductivity in n-type devices. Further irradiation with visible light or keeping the devices in the dark can switch the device conductance back to their initial value. This method is reversible and reproducible on different devices with different thickness over a long period of time.

Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes

Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.

Role of a spin-flip scatterer in a magnetized Luttinger liquid

We study the spin-dependent scattering of charge carriers in a magnetized one dimensional Luttinger liquid from a localized non-homogeneous magnetic field, which might be brought about by the stray field of magnetic tip near a uniform liquid, or by a transverse domain wall (DW) between two oppositely magnetized liquids. From a renormalization group treatment of the electron interactions we deduce scaling equations for the transmission and reflection amplitudes as the bandwidth is progressively reduced to an energy scale set by the temperature. The repulsive interactions dictate two possible zero temperature insulator fixed points: one in which electrons are reflected in the same spin channel and another where the electron spin is reversed upon reflection. In the latter case, a finite spin current emerges in the absence of a charge current at zero temperature and the Friedel oscillations form a transverse spiraling spin density. Adding a purely potential scattering term has no effect on the fixed points of a uniformly magnetized liquid. For a DW we find that the introduction of potential scattering stabilizes the spin-flip insulator phase even if the single-particle spin-flip scattering produced by the DW is arbitrarily weak. The potential can be induced externally, e.g. by a local gate voltage or a constriction, providing a means for controlling the transport properties of the wire.

Electronic transport in locally gated graphene nanoconstrictions

We have developed the combination of an etching and deposition techniques that enables the fabrication of locally gated graphene nanostructures of arbitrary design. Employing this method, we have fabricated graphene nanoconstrictions with local tunable transmission and characterized their electronic properties. An order of magnitude enhanced gate efficiency is achieved adopting the local gate geometry with thin dielectric gate oxide. A complete turn off of the device is demonstrated as a function of the local gate voltage. Such strong suppression of device conductance was found to be due to both quantum confinement and Coulomb blockade effects in the constricted graphene nanostructures.