External Gate Electrode Research Articles

Spin transport in proximity-induced ferromagnetic phosphorene nanoribbons

Abstract We investigate the spin-dependent transport properties in normal/ferromagnetic/normal phosphorene nanoribbon (PNR) junctions where an external gate electrode is attached to the ferromagnetic region. The exchange splitting in ferromagnetic phosphorene is induced by ferromagnetic proximity effect. We found that the current through this junction is spin polarized due to the exchange field induced in phosphorene. The spin polarization oscillates with the external gate voltage, and the spin polarization can be reversed from 100% to −100% by a slight change of the gate voltage. The controllable spin transport in phosphorene nanoribbons can have useful applications in the development of spintronic devices.

Analytical study of nano-scale logical operations

A complete analytical prescription is given to perform three basic (OR, AND, NOT) and two universal (NAND, NOR) logic gates at nano-scale level using simple tailor made geometries. Two different geometries, ring-like and chain-like, are taken into account where in each case the bridging conductor is coupled to a local atomic site through a dangling bond whose site energy can be controlled by means of external gate electrode. The main idea is that when injecting electron energy matches with site energy of local atomic site transmission probability drops exactly to zero, whereas the junction exhibits finite transmission for other energies. Utilizing this prescription we perform logical operations, and, we strongly believe that the proposed results can be verified in laboratory. Finally, we numerically compute two-terminal transmission probability considering general models and the numerical results match exactly well with our analytical findings.

Work Function Engineering for Performance Improvement in Leaky Negative Capacitance FETs

We analyze the effects of ferroelectric leakage on the performance of a negative capacitance field-effect transistor (NCFET), which has an intermediatemetallic layer between the ferroelectric and the high-K dielectric. We show that, when designed without taking the dielectric leakage into account, the NCFET performance can actually degrade significantlywith respect to that of the baseline FET. To overcome these detrimental effects of leakage, we propose the concept of work-function engineering, where metals of dissimilar work-functions are used for the external gate electrode and the intermediate metallic layer. Using this approach, the ferroelectric charge–voltage characteristic is shifted along the voltage axis, which results in superior performance of the NCFET.

Gate-tunable electron interaction in high-\u03ba dielectric films

The two-dimensional (2D) logarithmic character of Coulomb interaction between charges and the resulting logarithmic confinement is a remarkable inherent property of high dielectric constant (high-κ) thin films with far reaching implications. Most and foremost, this is the charge Berezinskii-Kosterlitz-Thouless transition with the notable manifestation, low-temperature superinsulating topological phase. Here we show that the range of the confinement can be tuned by the external gate electrode and unravel a variety of electrostatic interactions in high-k films. We find that by reducing the distance from the gate to the film, we decrease the spatial range of the 2D long-range logarithmic interaction, changing it to predominantly dipolar or even to exponential one at lateral distances exceeding the dimension of the film-gate separation. Our findings offer a unique laboratory for the in-depth study of topological phase transitions and related phenomena that range from criticality of quantum metal- and superconductor-insulator transitions to the effects of charge-trapping and Coulomb scalability in memory nanodevices.

Graphene field-effect transistor array with integrated electrolytic gates scaled to 200 mm

Ten years have passed since the beginning of graphene research. In this period we have witnessed breakthroughs both in fundamental and applied research. However, the development of graphene devices for mass production has not yet reached the same level of progress. The architecture of graphene field-effect transistors (FET) has not significantly changed, and the integration of devices at the wafer scale has generally not been sought. Currently, whenever an electrolyte-gated FET (EGFET) is used, an external, cumbersome, out-of-plane gate electrode is required. Here, an alternative architecture for graphene EGFET is presented. In this architecture, source, drain, and gate are in the same plane, eliminating the need for an external gate electrode and the use of an additional reservoir to confine the electrolyte inside the transistor active zone. This planar structure with an integrated gate allows for wafer-scale fabrication of high-performance graphene EGFETs, with carrier mobility up to 1800 cm2 V−1 s−1. As a proof-of principle, a chemical sensor was achieved. It is shown that the sensor can discriminate between saline solutions of different concentrations. The proposed architecture will facilitate the mass production of graphene sensors, materializing the potential of previous achievements in fundamental and applied graphene research.

Fluid-FET: An Ionic Controller for Lab-on-a-Chip

A highly sensitive ions flow controller, for lab-on-a-chip devices, operating through the nanochannels and low external potentials is demonstrated. The ion concentration can be modulated upto 50% while working in sub-micro molar range. The device is a fluidic-field-effect transistor (Fluid-FET) which has nanodimensional fluidic channel, connecting two fluid reservoirs and is capped with high quality thermal SiO2 (i.e., low surface charges compared with plasma enhanced chemical vapor deposition SiO2), which is used as gate oxide; analogous to a channel between source–drain of a conventional FET. The ionic conductance modulation in the range of 40%–50% is demonstrated at low voltages (≤2 V) through an external gate potential, providing a simpler way to modulate the surface charges of the channel. The conductance of nanochannel is modeled as a function of gate voltage and gate electrode position (symmetrical and asymmetrical with respect to nanochannels). The precise movement of the ions in one preferred direction (diodic behavior) is achieved by asymmetrically positioned external gate electrode. Fluid-FETs are realized using standard silicon process techniques with polysilicon as sacrificial material, allowing thermally grown silicon oxide as capping layer with low surface charges. The devices are analyzed using 3-D optical imaging, scanning electron microscopy, and electrical characterization in the presence of various ionic solutions.

Whispering-gallery mode quantum resonators in graphene

Recently, a research group from the Center for Nanoscale Science and Technology at the National Institute of Standards and Technology (NIST), and the Massachusetts Institute of Technology in the United States has demonstrated a new type of quantum electro-optic phenomenon, whispering-gallery mode resonators.1 The resonators are generated by a scanning tunneling microscope (STM) in proximity to graphene devices (Figure 1). On the basis of the quantum effect of electron tunneling, STM is a powerful technique to investigate the local electronic properties of both metallic and semiconducting systems with atomic resolution. Graphene, the most acclaimed material of the last decade, has enabled new horizons for STM research. The graphene surface can be directly probed by the scanning tip, whereas remaining chemically stable and clean even exposure to ambient air for days. Charged carriers in graphene can be readily tuned from holes to electrons using an external gate electrode. Furthermore, the charge carriers in graphene, often called Dirac particles, behave like electromagnetic waves, setting the stage for graphene to realize quantum electro-optic phenomena such as Veselago lensing2 and Klein tunneling.3

Laser tests of the DEPFET gated operation

DEPFET is an active pixel particle detector, in which a MOSFET is integrated in each pixel, providing first amplification stage of readout electronics. Excellent signal over noise performance is provided this way. The DEPFET sensor is planned to be used as an inner pixel detector in the BELLE II experiment at electron-positron SuperKEKB collider in Japan. Gated operation of the DEPFET sensor is a unique function which allows making sensor insensitive for incoming radiation for defined time interval. Charge previously integrated in the DEPFET's internal gate is saved and integration can continue afterwards. The insensitive mode of the DEPFET is achieved by a suppressed clear mechanism, which clears newly generated charge in the detector bulk, but keeps charge previously stored in the internal gate by a capacitive coupling to the external gate electrode. The properties of the gated operation were evaluated with the laser beam. It was proven, that DEPFET can operate this way. The average charge selection in the insensitive mode is lower than 0.4% and the suppressed clear mechanism does not cause charge loss higher than 200 electrons. Such fast mechanism which can define a time window, where detector stops integration of new charge, can be used for example to select out noisy bunches injected in an accelerator.

Chemical control of the charge state of nitrogen-vacancy centers in diamond

We investigate the effect of surface termination on the charge state of nitrogen vacancy centers, which have been ion-implanted few nanometers below the surface of diamond. We find that, when changing the surface termination from oxygen to hydrogen, previously stable NV- centers convert into NV0 and, subsequently, into an unknown non-fluorescent state. This effect is found to depend strongly on the implantation dose. Simulations of the electronic band structure confirm the dissappearance of NV- in the vicinity of the hydrogen-terminated surface. The band bending, which induces a p-type surface conductive layer leads to a depletion of electrons in the nitrogen vacancies close to the surface. Therefore, hydrogen surface termination provides a chemical way for the control of the charge state of nitrogen-vacancy centers in diamond. Furthermore, it opens the way to an electrostatic control of the charge state with the use of an external gate electrode.

Electrical Characteristics of the Concentric-Shape Carbon Nanotube Network Device in pH Buffer Solution

A carbon nanotube network device having concentric-shaped electrodes (source and drain) is analyzed to examine the self-gating effect of various pH buffer solutions on its electrical properties. Using the 2-D homogeneous percolation theory, current-voltage characteristics of the devices are described as the classical MOSFET formula, in which the device is modeled as the p-type transistor with positive threshold voltage and the gate is tied with the drain (at positive bias) or source (at negative bias). To determine the apparent threshold voltage change due to the corresponding pH value, the ∂VD/∂ID (VD:drain voltage; ID:drain current) curve is extracted from the measured current-voltage characteristics to find VD_SAT(= -VTH, VQS = 0). A threshold voltage shift according to the pH value is observed without an external gate electrode, showing the possibility of relaxing the requirement of the external gate electrode. By grafting protonation/deprotonation which occurs in the carboxylated single-walled carbon nanotubes, the decaying current as pH increases is explained. Better sensitivity according to the operation regime is examined by the device modeling.

Low-power bacteriorhodopsin-silicon n-channel metal-oxide field-effect transistor photoreceiver

A bacteriorhodopsin (bR)-silicon n-channel metal-oxide field-effect transistor (NMOSFET) monolithically integrated photoreceiver is demonstrated. The bR film is selectively formed on an external gate electrode of the transistor by electrophoretic deposition. A modified biasing circuit is incorporated, which helps to match the resistance of the bR film to the input impedance of the NMOSFET and to shift the operating point of the transistor to coincide with the maximum gain. The photoreceiver exhibits a responsivity of 4.7 mA/W.

Single-electron transport and magnetic properties ofFe−SiO2nanocomposites prepared by ion implantation

The electric transport, magnetic, and magnetotransport properties of $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ nanocomposites prepared by Fe-ion implantation into silica were investigated. The structural studies revealed bcc Fe nanoparticles of an average size of $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ dispersed in a $100\text{\ensuremath{-}}\mathrm{nm}$-thick nanocomposite layer formed within the silica substrate. Using special thin-film electrodes that were only $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ apart, in-plane electrical measurements were performed in a temperature range of $4--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Though no external gate electrode was used, single-electron transport phenomena (Coulomb blockade and Coulomb staircase) were observed at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The presence of Coulomb steps in $I\text{\ensuremath{-}}V$ curves implies that the electric transport was realized by the tunneling of electrons via a random quasi-one-dimensional chain of a few isolated iron nanoparticles. The magnetic properties of the nanoparticles were determined by surface effects and by the superparamagnetic behavior. The nanoparticles exhibited enhanced anisotropy and were dipolarly interacting. However, the tunneling current was found to be independent of external magnetic field; i.e., no tunneling magnetoresistivity (TMR) was measured at $77\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At this temperature the nanoparticles were superparamagnetic. Presumably, a low volumetric concentration of Fe nanoparticles $(<14%)$ and a spin-flip process due to residual single Fe atoms present in the silica barriers were responsible for the absence of the TMR effect.

Interference and interactions in mesoscopic rings

Abstract We consider a mesoscopic ring connected to external reservoirs by tunnel junctions. The ring is capacitively coupled to an external gate electrode and may be pierced by a magnetic field. Due to strong electron–electron interactions within the ring the conductance shows Coulomb blockade oscillations as a function of the gate voltage, while Aharonov–Bohm interference effects lead to a dependence on the magnetic flux. The Hamiltonian of the ring is given by a Luttinger model that allows for an exact treatment of both interaction and interference effects. We conclude that the positions of conductance maxima as a function the external parameters can be used to determine the interaction parameter g , and the shapes of conductance peaks are strongly affected by electron correlations within the ring.