Applied Gate Voltage Research Articles

Positive and negative chemical pressure effects investigated in electron-doped FeSe films with an electric-double-layer structure

We investigated chemical pressure effects in electron-doped (e-doped) FeSe by fabricating an electric-double-layer structure with single crystalline FeSe films on LaAlO$_3$ with Se substituted by isovalent Te and S. Our method enables transport measurements of e-doped FeSe. Electron doping by applying gate voltage of 5 V increases $T_{\mathrm c}$ of the FeSe$_{1-x}$Te$_x$ and FeSe$_{1-y}$S$_y$ films with $0 \leq x \leq 0.4$ and $0 \leq y \leq 0.25$, while the e-doped $x=0.5$ film showed lower $T_{\mathrm c}$ than that of the undoped one. Both positive and chemical pressure suppress $T_{\mathrm c}$ of the e-doped FeSe. The obtained superconducting phase diagram in the e-doped samples is rather different from that in undoped samples. This might suggest that the superconductivity mechanism is different between undoped and e-doped systems. Alternatively, this is possibly explained by the absence of the nematic order in e-doped samples.

Polarization Field on Edge States of Single-layered MoS2

Operating quantum materials through the application of strong electric field holds great promise for the development of new-generation electronic and photonic technologies with currently inaccessible functionalities. The conventional way of applying gate voltage to produce sufficiently high electric field faces challenge in micro-nano level. Here, we explore polarization field of two-dimensional piezoelectric semiconductors under externally applied strain. The results suggest that the intensity of polarization field can exceed 10 MV/cm near the boundaries of single-layered MoS2 ribbon. Such polarization field can trigger the metallicity-to-semiconductor (M-S) phase transition of one-dimensional edge states, and even lead to metallic bulk states. Similar phase transition can also be driven by a pure external electric field but the required intensity is in the order of MV/cm. Electric field driving the semiconducting phase of edge states presents the sensitive dependence of ribbon width but relatively robust for polarization field. This study opens a new avenue to manipulate quantum materials by high polarization field.

Improved Thermoelectric Power Factor of InGaZnO/SiO₂ Thin Film Transistor via Gate-Tunable Energy Filtering

Progress on improving the thermoelectric power factor (PF) of InGaZnO thin films by various surface treatment techniques has been seriously hindered by the innate coupled but opposing relationship between its electrical conductivity ( σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ) and Seebeck coefficient (S). Therefore, proposing unique solutions to decouple these properties is crucial in advancing its thermoelectric performance. This study demonstrates that the S - σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> coupled relationship can be suppressed by integrating InGaZnO in an InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film transistor, leading to a significantly enhanced PF compared to pristine InGaZnO thin films. An exponential increase in σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> was observed without the typical severe decline in S. An extremely narrow charge accumulation layer near the InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface could have formed under a positive applied gate voltage ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> ), causing a dramatic reduction of the thermoelectrically active region to an almost two-dimensional surface. By utilizing the Kamiya-Nomura percolation modeling, it was observed that energy filtering occurs at the InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface. This likely causes a change in the shape of the potential barriers, which leads to the suppression of the S - σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> coupling relationship.

Zoology of spin and orbital fluctuations in ultrathin oxide films

Many metallic transition-metal oxides turn insulating when grown as films that are only a few unit-cells thick. The microscopic origins of these thickness induced metal-to-insulator transitions however remain under dispute. Here, we simulate the extreme case of a monolayer of an inconspicuous correlated metal -- the strontium vanadate SrVO$_3$ -- deposited on a SrTiO$_3$ substrate. Crucially, our system can have a termination to vacuum consisting of either a SrO or a VO$_2$ top layer. While we find that both lead to Mott insulating behavior at nominal stoichiometry, the phase diagram emerging upon doping -- chemically or through an applied gate voltage -- is qualitatively different. Indeed, our many-body calculations reveal a cornucopia of nonlocal fluctuations associated with (in)commensurate antiferromagnetic, ferromagnetic, as well as stripe and checkerboard orbital ordering instabilities. Identifying that the two geometries yield crystal-field splittings of opposite signs, we elucidate the ensuing phases through the lens of the orbital degrees of freedom. Quite generally, our work highlights that interface and surface reconstruction and the deformation or severing of coordination polyhedra in ultra-thin films drive rich properties that are radically different from the material's bulk physics.

Electrically induced change in HfO2/1-monolayer TiO2/SiO2 metal-oxide-semiconductor stacks: capacitance–voltage and hard X-ray photoelectron spectroscopy studies

Metal-oxide-semiconductor capacitors with HfO2/1-monolayer TiO2/SiO2 stacks were examined to explore the origin of the interface dipole modulation. The capacitance–voltage (C–V) measurements exhibited that the polarity of the interface dipole layer changes depending on the gate bias. The hard X-ray photoelectron spectroscopy measurements demonstrated that an applied gate voltage induces small changes in the Ti–O chemical bonding and potential profile around the HfO2/SiO2 interface.

Circuit Quantum Electrodynamics with Carbon-Nanotube-Based Superconducting Quantum Circuits

Hybrid circuit quantum electrodynamics (QED) involves the study of coherent quantum physics in solid state systems via their interactions with superconducting microwave circuits. Here we present an implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bi-layer, and implement voltage tunability of the qubit frequency using a local electrostatic gate. We demonstrate strong dispersive coupling to a coplanar waveguide resonator via observation of a resonator frequency shift dependent on applied gate voltage. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of $10-200~\rm{ns}$.

Quantum dynamics of a single fluxon in Josephson-junction parallel arrays with large kinetic inductances

We present a theoretical study of coherent quantum dynamics of a single magnetic fluxon (MF) trapped in Josephson junction parallel arrays (JJPAs) with large kinetic inductances. The MF is the topological excitation carrying one quantum of magnetic flux, $\Phi_0$. The MF is quantitatively described as the $2\pi$-kink in the distribution of Josephson phases, and for JJPAs with high kinetic inductances the characteristic length of such distribution ("the size" of MF) is drastically reduced. Characterizing such MFs by the Josephson phases of three consecutive Josephson junctions we analyse the various coherent macroscopic quantum effects in the MF quantum dynamics. In particular, we obtain the MF energy band originating from the coherent quantum tunnelling of a single MF between adjacent cells of JJPAs. The dependencies of the band width $\Delta$ on the Josephson coupling energy $E_J$, charging energy $E_C$ and the inductive energy of a cell $E_L$, are studied in detail. In long linear JJPAs the coherent quantum dynamics of MF demonstrates decaying quantum oscillations with characteristic frequency $f_{qb}=\Delta/h$. In short annular JJPAs the coherent quantum dynamics of MF displays complex oscillations controlled by the Aharonov-Casher phase $\chi \propto V_g$, where $V_g$ is an externally applied gate voltage. In the presence of externally applied dc bias, $I$, a weakly incoherent dynamics of quantum MF is realized in the form of macroscopic Bloch oscillations leading to a typical "nose" current-voltage characteristics of JJPAs. As ac current with frequency $f$ is applied the current-voltage characteristics displays a set of equidistant current steps at $I_n=2en f$.

Traps at the hBN/WSe2 interface and their impact on polarity transition in WSe2

Semiconducting two-dimensional (2D) materials-based devices usually exhibit inferior electrical performance compared to their theoretical predictions, which is mainly attributed to the presence of high density of interfacial defect induced trap states within the bandgap of 2D materials. It is pertinent to control the density of interface traps (D it) and identify their respective energy levels inside the band gap of the 2D materials to understand the tailored device performance. Here, we report the large modulation of D it by electrical gating and varying the channel thickness of tungsten diselenide (WSe2) placed on ultra-clean hexagonal boron nitride (hBN) gate insulator in a metal–insulator–semiconductor structure, which is revealed by performing multi-frequency capacitance and conductance measurements. Analysis of the 2D hBN/WSe2 interface reveals that with the increase of WSe2 thickness, D it at the midgap of WSe2 is reduced to 6 × 109 cm−2 eV−1, which is less than D it reported for SiO2/Si interface (∼1010 cm−2 eV−1). Furthermore, by increasing thickness and applying gate voltage, D it distribution is systematically modulated inside the WSe2 band gap from valence band edge to mid-gap to conduction band edge, thereby changing the Fermi level of WSe2, and inducing versatile device polarity. Our results show that D it and its spatial energy distribution within the thickness tailored WSe2 band gap primarily control polarity modulation in WSe2.

Electron-Vibration Entanglement of Resonating Dimers in Quantum Transport.

Electron transport in a single molecule resulting from the superposition of its vibronic states depends on the coupling strength with the metallic leads. However, dynamical coherence and Fermionic correlation in molecule-molecule and molecule-lead coupling necessitates a critical approach to treat the current and its noise level, especially in the presence of a variable external bias for temperature-dependent conduction. Primarily, this work is a generalization of the theoretical approach of the atomic dimers to incorporate the effect of vibrational modes in current and conductance characteristics. The variation of current and differential conductance due to the external bias reveals a vibrational Coulomb blockade structure corresponding to the functioning vibrational mode in the system. The numerical demonstration for a diverse class of molecules generically shows that electron-vibration interaction can quantitatively predict the nature of coherent electron transport and current noise. Secondly, an attempt has been made to illustrate the effect of magnitude of coherence-induced noise suppression of current as a signature of electron-vibration entanglement. Finally, temperature-dependent conductance of the molecular junction in dimer structure has been estimated along with the peak shifts due to the applied gate voltage.

Improved Contact Resistance by a Single Atomic Layer Tunneling Effect in WS2 /MoTe2 Heterostructures.

Manipulation of Ohmic contacts in 2D transition metal dichalcogenides for enhancing the transport properties and enabling its application as a practical device has been a long‐sought goal. In this study, n‐type tungsten disulfide (WS2) single atomic layer to improve the Ohmic contacts of the p‐type molybdenum ditelluride (MoTe2) material is covered. The Ohmic properties, based on the lowering of Schottky barrier height (SBH) owing to the tunneling barrier effect of the WS2 monolayer, are found to be unexpectedly excellent at room temperature and even at 100 K. The improved SBH and contact resistances are 3 meV and 1 MΩ µm, respectively. The reduction in SBH and contact resistance is confirmed with temperature‐dependent transport measurements. This study further demonstrates the selective carrier transport across the MoTe2 and WS2 layers by modulating the applied gate voltage. This WS2/MoTe2 heterostructure exhibits excellent gate control over the currents of both channels (n‐type and p‐type). The on/off ratios for both the electron and hole channels are calculated as 107 and 106, respectively, indicating good carrier type modulation by the electric field of the gate electrode. The Ohmic contact resistance using the tunneling of the atomic layer can be applied to heterojunction combinations of various materials.

Hysteresis Behavior of the Donor-Acceptor-Type Ambipolar Semiconductor for Non-Volatile Memory Applications.

Donor–acceptor-type organic semiconductor molecules are of great interest for potential organic field-effect transistor applications with ambipolar characteristics and non-volatile memory applications. Here, we synthesized an organic semiconductor, PDPPT-TT, and directly utilized it in both field-effect transistor and non-volatile memory applications. As-synthesized PDPPT-TT was simply spin-coated on a substrate for the device fabrications. The PDPPT-TT based field-effect transistor showed ambipolar electrical transfer characteristics. Furthermore, a gold nanoparticle-embedded dielectric layer was used as a charge trapping layer for the non-volatile memory device applications. The non-volatile memory device showed clear memory window formation as applied gate voltage increases, and electrical stability was evaluated by performing retention and cycling tests. In summary, we demonstrate that a donor–acceptor-type organic semiconductor molecule shows great potential for ambipolar field-effect transistors and non-volatile memory device applications as an important class of materials.

Enhanced Superconductivity in Monolayer Td-MoTe2.

Crystalline two-dimensional (2D) superconductors (SCs) with low carrier density are an exciting new class of materials in which electrostatic gating can tune superconductivity, electronic interactions play a prominent role, and electrical transport properties may directly reflect the topology of the Fermi surface. Here, we report the dramatic enhancement of superconductivity with decreasing thickness in semimetallic Td-MoTe2, with critical temperature (Tc) increasing up to 7.6 K for monolayers, a 60-fold increase with respect to the bulk Tc. We show that monolayers possess a similar electronic structure and density of states (DOS) as the bulk, implying that electronic interactions play a strong role in the enhanced superconductivity. Reflecting the low carrier density, the critical temperature, magnetic field, and current density are all tunable by an applied gate voltage. The response to high in-plane magnetic fields is distinct from that of other 2D SCs and reflects the canted spin texture of the electron pockets.

Electrical transport and rectification in a typical heterostructure based on fullerene–pentacene suspended to copper leads

In this work, we study quantum electrical transport through a heterojunction molecular bridge based on a fullerene and a pentacene as an electron-poor and electron-rich, respectively, coupled to two copper semi-infinite electrodes. Our calculations are based on the Green’s function approach within the nearest-neighbor tight-binding approximation. For this reason, a theoretical generalized derivation is introduced to calculate the self-energy of a centered-cubic lattice electrode using a unitary transformation. Based on this model, the effects of different geometrical ways of contacts and also various values of the applied gate voltage are examined, comprehensively. The estimations obviously clarify that the value of the transmission coefficient of carriers for single contact is significantly different from multiple contacts, particularly near to the Fermi level. The model predicts that tunable rectification can be achieved via gate voltage and also different ways of coupling of the C $$_{60}$$ to the electrode. The reification ratio value of the device with six contacts of the C $$_{60}$$ molecule to the left electrode approaches over than 1400, when the gate voltage of 2.0 V is applied. The results can be applied in electronic devices and logic circuits based on molecular diodes.

Analysis of channel mobility in GaN-based metal-oxide-semiconductor field-effect transistors

The factors limiting channel mobility in AlSiO/p-type GaN-based metal-oxide-semiconductor field-effect transistors (MOSFETs) were systematically investigated. MOSFETs with various thin interfacial layers (ILs) between Al0.78Si0.22Oy films and Mg-doped GaN layers were prepared and found to exhibit different channel mobilities. The maximum effective mobility showed a significant correlation with the threshold voltage (Vt) and the hysteresis (ΔVt) in the transfer characteristics of these devices, such that the mobility decreased with increasing Vt and ΔVt. This effect can be explained by electron capture in and emission from border traps situated near the conduction band minimum for GaN. The insertion of a 3-nm-thick SiO2 IL drastically enhanced the effective mobility and simultaneously reduced the ΔVt value. Hall effect measurements with an applied gate voltage were used to determine the mobility of free electrons while excluding electrons captured in the border traps. The Hall effect mobility was much higher than the effective mobility, indicating that mobility was in fact reduced by the capture of electrons by the border traps. The ratio of electrons captured by border traps to the overall electrons induced by a gate bias was greatly lowered in a MOSFET incorporating a SiO2 IL. When a high vertical electric field of approximately 1 MV/cm was present in the device channel, the Hall effect mobility was slightly increased following the insertion of an IL. These results suggest that the IL reduced the interfacial roughness and/or affected the screening out of scattering due to potential fluctuations of the AlSiO.

Role of strain and composition on the piezoelectric and dielectric response of AlxGa1−xN: Implications for power electronics device reliability

Gallium nitride (GaN) and AlxGa1−xN, its solid solution with Al, play a vital role in a variety of high-power applications owing to their high breakdown voltage, drift velocity, and sheet charge density. Their piezoelectric nature is critical for both the operation and reliability of GaN-based devices, and this is compounded by the lack of lattice-matched substrates and the lattice mismatch between GaN and AlxGa1−xN, which invariably results in strained films. Thus, accurate models of performance and reliability require knowledge of how strain affects dielectric and piezoelectric response. We used density functional theory to calculate the piezoelectric and dielectric constants for different compositions of AlxGa1−xN as a function of biaxial strain and use Gaussian process regression to develop models, including uncertainties, from the ab initio results. We find that the dielectric constants decrease with compressive biaxial strain and increasing Al content due to an increase in phonon frequencies. Meanwhile, the piezoelectric constants increase with compressive biaxial strain and with Al doping. Our results show that the presence of strain can explain discrepancies in experimental measurements of dielectric constants but not piezoelectric ones. Interestingly, the piezoelectric constants e33 and e31 (which control the elastic energy induced by the application of gate voltage in GaN high electron mobility transistors, which have been related to their degradation) vary by almost 100% within a biaxial strain range of ∼3%. These results indicate that incorporating strain-dependent and composition-dependent piezoelectric response into current degradation models based on inverse piezoelectricity is crucial for accurate reliability predictions in GaN-based transistors.

HfO2 as gate insulator on N-polar GaN–AlGaN heterostructures

To further increase the operation frequency of GaN high electron mobility transistors (HEMTs) with high power gain in the mm-wave band (30–300 GHz), channel thickness must be scaled. High-k dielectrics such as HfO2 can be inserted as gate insulator to reduce the leakage, while maintaining good gate control. In this work, we investigated HfO2 on N-polar HEMT structure by performing frequency dependent capacitance–voltage measurements on metal-oxide-semiconductor capacitors (MOSCAPs). The impact of annealing on the quality of HfO2 was studied. Moreover, we compared the CV characteristics of MOSCAPs with HfO2 deposited on the GaN channel with that deposited on the AlGaN cap layer. We showed that HfO2 deposition on AlGaN cap leads to Fermi level pinning, which prevents the channel being pinched off by applying gate voltage. On the other hand, well-behaved CV profiles were achieved by depositing HfO2 directly on GaN or SiN dielectric.

The current analysis of a single electron transistor based on double graphene nanoscroll island

The single electron transistor (SET) is nanoscale device which operates fast based on tunneling of single electrons in potential barriers. It has low energy consumption, small size and simplified equivalent circuit. This transistor contains source, gate, drain electrodes and an island which is surrounded with three electrodes. The material of SET island affects on operation of this device therefore graphene as a famous carbon based material is selected for the island. Two islands comprised of graphene nanoribbon (GNR) and graphene nanoscroll (GNS) are chosen for SET. The GNS-SET has lower coulomb blockade region and coulomb diamond area than GNR-SET. Moreover the double GNR-SET and double GNS-SET are simulated and their charge stability diagrams are compared together. The result shows that double GNS-SET has lower zero current region and better operation than double GNR-SET. Therefore double GNS is selected for SET island and impact of important parameters on its current are comprehensively investigated. The increasing applied gate voltage, temperature, GNS length and GNS spiral length lead to current increase in double GNS-SET. Moreover, decreasing the number of turns for GNS increases the current in double GNS-SET. These parameters can be utilized for controlling and tunable current in double GNS SET.

Quantum Sensor for Nanoscale Defect Characterization

The optical and electronic properties of semiconductors are strongly affected by structural and stoichiometric defects. The precise incorporation of dopants and the control of impurities are essentially what makes semiconductors useful materials for a broad range of devices. The standard defect and impurity characterization methods are sensitive only on a macroscopic scale, like the most widely used method of deep-level transient spectroscopy (DLTS). We perform time-resolved measurements of the resonance fluorescence of a single self-assembled $(\mathrm{In},\mathrm{Ga})$$\mathrm{As}$ quantum dot (QD) at low temperatures ($4.2\phantom{\rule{0.2em}{0ex}}\mathrm{K}$). By pulsing the applied gate voltage, we are able to selectively occupy and unoccupy individual defects in the vicinity of the dot. We address the exciton transition of the QD with a tunable diode laser. Our time-resolved measurements exhibit a shift of the resonance energy of the optical transition. We attribute this to a change of the electric field in the dot's vicinity, caused by electrons tunneling from a reservoir to the defect sites. Furthermore, we are able to characterize the defects concerning their position and activation energy by modeling our experimental data. Our results thus demonstrate how a quantum dot can be used as a quantum sensor to characterize the position and activation energy of individual shallow defects on the nanoscale.

Modeling and Simulation of PGSPAD-Based Silicon Photomultipliers

This article presents and develops a novel comprehensive simulation model for perimeter gated single-photon avalanche diode-based silicon photomultipliers (PGSPAD SiPMs). A PGSPAD is a modification of single-photon avalanche diodes (SPADs) that contains a third terminal to mitigate perimeter edge breakdown. It does this without excessively enlarging the device area. The applied gate voltage modulates the dynamic range, breakdown voltage, and dark current characteristics. The proposed PGSPAD SiPM model correctly mimics the current–voltage transfer function and the avalanche-quench process of the SiPM detector and is verified by the experimental measurements of a PGSPAD SiPM fabricated in a commercial 0.5- $\mu \text{m}$ CMOS process. The model includes the dark current of the PGSPAD device. This was not included in previous SiPM models. The effect of the number of triggered microcells versus the total number of microcells and the impact of different parameters on the PGSPAD SiPM performance with added gate terminal are evaluated. The photon–electron spectrum with the effect of an extra gate terminal has been studied. Simulation results show very good agreement with experimental measurements.

Room-Temperature Exciton-Based Optoelectronic Switch.

Excitons, bound pairs of electrons and holes, could act as an intermediary between electronic signal processing and optical transmission, thus speeding up the interconnection of photoelectric communication. However, up to date, exciton-based logic devices such as switches that work at room temperature are still lacking. This work presents a prototype of a room-temperature optoelectronic switch based on excitons in WSe2 monolayer. The emission intensity of WSe2 stacked on Au and SiO2 substrates exhibits completely opposite behaviors upon applying gate voltages. Such observation can be ascribed to different doping behaviors of WSe2 caused by charge-transfer and chemical-doping effect at WSe2 /Au and WSe2 /SiO2 interfaces, respectively, together with the charge-drift effect. These interesting features can be utilized for optoelectronic switching, confirmed by the cyclic PL switching test for a long time exceeding 4000 s. This study offers a universal and reliable approach for the fabrication of exciton-based optoelectronic switches, which would be essential in integrated nanophotonics.