Abrupt metal-insulator transition observed in VO2 thin films induced by a switching voltage pulse

An abrupt metal-insulator transition (MIT) was observed in VO2 thin films during the application of a switching voltage pulse to two-terminal devices. Any switching pulse over a threshold voltage for the MIT of 7.1 V enabled the device material to transform efficiently from an insulator to a metal. The characteristics of the transformation were analyzed by considering both the delay time and rise time of the measured current response. The extrapolated switching time of the MIT decreased down to 9 ns as the external load resistance decreased to zero. Observation of the intrinsic switching time of the MIT in the correlated oxide films is impossible because of the inhomogeneity of the material; both the metallic state and an insulating state co-exist in the measurement volume. This indicates that the intrinsic switching time is in the order of less than a nanosecond. The high switching speed might arise from a strong correlation effect (Coulomb repulsion) between the electrons in the material.Comment: 5 pages, 5 figure

Observation of First-Order Metal-Insulator Transition without Structural Phase Transition in VO_2

An abrupt first-order metal-insulator transition (MIT) without structural phase transition is first observed by current-voltage measurements and micro-Raman scattering experiments, when a DC electric field is applied to a Mott insulator VO_2 based two-terminal device. An abrupt current jump is measured at a critical electric field. The Raman-shift frequency and the bandwidth of the most predominant Raman-active A_g mode, excited by the electric field, do not change through the abrupt MIT, while, they, excited by temperature, pronouncedly soften and damp (structural MIT), respectively. This structural MIT is found to occur secondarily.Comment: 4 pages, 4 figure

Observation of Mott Transition in VO_2 Based Transistors

An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.Comment: 4 pages, 4 figure

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS₂ Device

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS₂) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on–off ratio drops by 3 orders of magnitude due to the increase in “off” current for both single and few layer MoS₂ FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 10³ and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS₂ interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices

Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS₂/WSe₂ van der Waals Heterostructure

Two-dimensional (2D) material-based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto-electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS₂ and few layered WSe₂ has been demonstrated in which an atomically thin MoS₂ layer has been employed as a tunneling layer to the underlying WSe₂ layer. In this way, simultaneous contacts to both MoS₂ and WSe₂ 2D layers have been established by forming a direct metal–semiconductor to MoS₂ and a tunneling-based metal–insulator–semiconductor contacts to WSe₂, respectively. The use of MoS₂ as a dielectric tunneling layer results in an improved contact resistance (80 kΩ μm) for WSe₂ contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature-dependent measurement. Furthermore, this unique contact engineering and type-II band alignment between MoS₂ and WSe₂ enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 10⁴ for both electron (MoS₂) and hole (WSe₂) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS₂ and the heterostructure device, which agrees with the carrier recombination-induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high-performance opto-electrical devices and applications

Gate Tunable Self-Biased Diode Based on Few Layered MoS₂ and WSe₂

The operation of a self-biased diode (SBD) based on MoS₂ has been demonstrated by using an asymmetric top gate comprising metal-hexagonal boron nitride (h-BN)-MoS₂ structure. The rectification is achieved by asymmetric modulation of effective Schottky barrier and carrier density in the channel during forward and reverse bias, and a rectification factor of 1.3 × 10⁵ is achieved in <i>I–V</i> characteristics. The modulation of effective Schottky barrier is verified by temperature dependent measurement in a range of 173 to 373 K, and a difference of 300 meV is observed in effective Schottky barrier height for forward and reverse bias. The electrical characteristics of SBD exhibit close resemblance with an ideal thermionic emission model with an ideality factor of 1.3. SBD also exhibits strong photoelectrical response with a specific detectivity of 150 A/W and responsivity of 2.1 × 10¹⁰ Jones under 450 nm laser light illumination. In the end, to demonstrate the diversity of the proposed scheme, SBD based on WSe₂ has also been fabricated and the results have been discussed. These results suggest a new route toward the SBD based numerous electronics and optoelectronics applications and can in principle be implemented using other two-dimensional materials as well