Two-dimensional MoS2 has emerged as promising material for nanoelectronics and spintronics due to its exotic properties. However, high contact resistance at metal semiconductor MoS2 interface still remains an open issue. Here, we report electronic properties of field effect transistor devices using monolayer MoS2 channels and permalloy (Py) as ferromagnetic (FM) metal contacts. Monolayer MoS2 channels were directly grown on SiO2/Si substrate via chemical vapor deposition technique. The increase in current with back gate voltage shows the tunability of FET characteristics. The Schottky barrier height (SBH) estimated for Py/MoS2 contacts is found to be +28.8 meV (zero-bias), which is the smallest value reported so-far for any direct metal (magnetic or non-magnetic)/monolayer MoS2 contact. With the application of gate voltage (+10 V), SBH shows a drastic reduction down to a value of -6.8 meV. The negative SBH reveals ohmic behavior of Py/MoS2 contacts. Low SBH with controlled ohmic nature of FM contacts is a primary requirement for MoS2 based spintronics and therefore using directly grown MoS2 channels in the present study can pave a path towards high performance devices for large scale applications.
This work deals with the electron beam irradiation of the Schottky metal contacts in monolayer molybdenum disulfide (MoS$_2$) field-effect transistors (FETs). We show that the exposure of the Ti/Au source/drain leads to an electron beam improves the transistor conductance. We simulate the path of the electrons in the device and show that most of the beam energy is absorbed in the metal contacts. Hence, we propose that the transistor current enhancement is due to thermally induced interfacial reactions that lower the contact Schottky barriers. We also show that the electron beam conditioning of…
Two-dimensional material-based field effect transistors (2DM-FETs) are playing a revolutionary role in electronic devices. However, after years of development, no device model can match the Pao-Sah model for standard silicon-based transistors in terms of physical accuracy and computational efficiency to support large-scale integrated circuit design. One remaining critical obstacle is the contacts of 2DM-FETs. In order to self-consistently include the contact effect in the current model, it is necessary to perform self-consistent calculations, which is a fatal flaw for applications that priorit…
To explore the potential of field-effect transistors (FETs) based on monolayers of the two-dimensional semiconducting channel(SC) for spintronics, the two most important issues are to ensure the formation of variable low resistive tunnel ferromagnetic contacts(FC), and to preserve intrinsic properties of the SC during fabrication. Large Schottky barriers lead to the formation of high resistive contacts and methods adopted to control the barriers often alter the intrinsic properties of the SC. This work aims at addressing both issues in fully encapsulated monolayer WSe$_2$ FETs by using bi-laye…
When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance ($C_\textrm{G}$) becomes smaller than the oxide capacitance ($C_\textrm{ox}$) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS$_\textrm{2}$ as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS$_\textrm{2}$ capacitance is smaller than its quantum capacitance, limiting the single-gated $C_\text…
Two-dimensional (2D) semiconductors offer a promising prospect for high-performance and energy-efficient devices especially in the sub-10 nm regime. Inspired by the successful fabrication of 2D $β$-TeO$_2$ and the high on/off ratio and high air-stability of fabricated field effect transistors (FETs) [Nat. Electron. 2021, 4, 277], we provide a comprehensive investigation of the electronic structure of monolayer $β$-TeO$_2$ and the device performance of sub-10 nm metal oxide semiconductors FETs (MOSFETs) based on this material. The anisotropic electronic structure of monolayer $β$-TeO$_2$ plays …
Layered semiconductors, such as MoS2, have attracted interest as channel materials for post-silicon and beyond-CMOS electronics. Much attention has been devoted to the monolayer limit, but the monolayer channel is not necessarily advantageous in terms of the performance of field-effect transistors (FETs). Therefore, it is important to investigate the characteristics of FETs that have multilayer channels. Here, we report the staircase-like transfer characteristics of FETs with exfoliated multilayer MoS2 flakes. Atomic force microscope characterizations reveal that the presence of thinner terrac…
Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 $μA/μm$ at 80 K and >200 $μA/μm$ at 300 K) and relatively low contact resistance (1.2 to 2 $kΩ\cdotμm$ from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other con…
Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low temperature (< 200$^\circ$ C) sub-stoichiometric AlO$_x$ provides a stable n-doping layer for monolayer MoS$_2$, compatible with circuit integration. This approach achieves carrier densities > 2x10$^{13}$ 1/cm$^2$, sheet resistance as low as ~7 kOhm/sq, an…
We report here gate-tunable memristors based on monolayer MoS2 grown by chemical vapor deposition (CVD). These memristors are fabricated in a field-effect geometry with the channel consisting of polycrystalline MoS2 films with grain sizes of 3-5 um. The device characteristics show switching ratios up to 500, with the resistance in individual states being continuously gate-tunable by over three orders of magnitude. The resistive switching results from dynamically varying threshold voltage and Schottky barrier heights, whose underlying physical mechanism appears to be vacancy migration and/or ch…
The contact resistance limits the down-scaling and operating range of OFETs. With the monolayer (1L) organic crystals and non-destructive metal/semiconductor interfaces, intrinsic mobility of 12.5 cm2V-1s-1 and Ohmic contact resistance of 40 ohm-cm were achieved. The on/off ratio maintained at 10^3 even at a small VDS of -0.1 mV. High current density of 4.2 uA/um was achieved with the 1L-crystal as the active layer. At such high current density, the velocity saturation and channel self-heating effects are observed in OFETs for the first time. In addition to the low contact resistance and high-…
We report, for CVD-grown monolayer MoS2, the very first results on temporal degradation of material and device performance under electrical stress. Both low and high field regimes of operation are explored at different temperatures, gate bias and stress cycles. During low field operation, current is found to saturate after hundreds of seconds of operation with the current decay time constant being a function of temperature and stress cycle. Current saturation after several seconds during low field operation occurs when a thermal equilibrium is established. However, high field operation, especi…
Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOT), excellent gate control, and low leakage currents. Here, we report liquid metal printed ultrathin and scalable Ga2O3 dielectric for 2D electronics and electro-optical devices. We directly visualize the atomically smooth Ga2O3/WS2 interfaces enabled by the c…
Strain engineering is an important method for tuning the properties of semiconductors and has been used to improve the mobility of silicon transistors for several decades. Recently, theoretical studies have predicted that strain can also improve the mobility of two-dimensional (2D) semiconductors, e.g. by reducing intervalley scattering or lowering effective masses. Here, we experimentally show strain-enhanced electron mobility in monolayer MoS2 transistors with uniaxial tensile strain, on flexible substrates. The on-state current and mobility are nearly doubled with tensile strain up to 0.7%,…
The chemical activity of ionized hydrogen describes the potency of protons in solution and is used to define the pH with wide-ranging applications in biological and materials sciences. Measuring pH with graphene field-effect transistors (FETs) has been described in the past but has yet to be characterized with a monolayer hexagonal boron nitride (hBN) capping layer. hBN capping is hypothesized to lower the chemical affinity of protons to oxidized defects in the graphene crystal and lower the standard deviation of pH measurement via screening charge density. First, the electronic properties of …
We have fabricated 4-element Graphene/Silicon on Insulator (SOI) based Schottky barrier photodiode array ()PDA and investigated its optoelectronic device performance. In our device design, monolayer graphene is utilized as common electrode on lithographically defined linear array of n-type Si channels on SOI substrate. As revealed by wavelength resolved photocurrent spectroscopy measurements, each element in the PDA structure exhibited a maximum spectral responsivity comparable to that of commercially available photodiodes operating under self-powered operational mode. Time-dependent photocurr…
We fabricate monolayer MoS2 field effect transistors and study their electric characteristics from 10^-6 Torr to atmospheric air pressure. We show that the threshold voltage of the transistor increases with the growing pressure. Hence, we propose the device as an air pressure sensor, showing that it is particularly suitable as a low power consumption vacuum gauge. The device functions on pressure-dependent O2, N2 and H2O molecule adsorption that affect the n-doping of the MoS2 channel.
