MoTe2 Field-effect Transistors Research Articles

Controlling Polarity of MoTe2 Transistors for Monolithic Complementary Logic via Schottky Contact Engineering.

Two-dimensional (2D) layered molybdenum ditelluride (MoTe2) crystals, featuring a low energy barrier in the crystalline phase transition and a sizable band gap close to that of silicon, are rapidly emerging with substantial potential and promise for future nanoelectronics. It has been challenging, however, to realize n-type MoTe2 field-effect transistors (FETs), thus complementary logic, because MoTe2 FETs mainly exhibit p-type behavior. Here, we report a dopant-free method for controlling polarity of MoTe2 FETs by modifying Schottky barriers at their MoTe2-metal contacts via thermal annealing. Upon annealing, MoTe2 FETs encapsulated by hexagonal boron nitride (h-BN) are consistently changed from hole to electron conduction, displaying an on/off current ratio of 105 or higher. When the MoTe2 channel is sandwiched between top and bottom h-BN thin layers (h-BN/MoTe2/h-BN FETs), higher field-effect mobility is attained, up to 48.1 cm2 V-1 s-1 (hole) and 52.4 cm2 V-1 s-1 (electron) before and after thermal annealing, respectively. The thermally controlled FET polarity change further enables high-performance MoTe2 monolithic complementary inverters with gain as high as 36, suggesting this simple and effectual approach may lead to compelling possibilities of rationally controlling transport polarity, on demand, in atomically thin transistors with metal contacts and their 2D integrated circuits.

Oxygen-Sensitive Layered MoTe2 Channels for Environmental Detection.

The oxygen (O2)-dependent resistance change of multilayered molybdenum ditelluride (MoTe2) channels was characterized. A variation of the channel resistance could reproducibly determine relative O2 content (denoted as the O2 index). We found that Joule heating in a layered MoTe2 field-effect transistor caused the O2 index to decrease drastically from 100 to 12.1% in back gate modulation. Furthermore, Joule heating caused effective O2 desorption from the MoTe2 surface and repeatable O2 detection by multilayered MoTe2 channels was realized. This work not only explored the influence of O2 on the electrical properties of multilayered MoTe2 channels but also revealed that MoTe2 channels are promising for sensing O2 in an environmental condition.

Advanced Multifunctional Field Effect Devices Using Common Gate for Both 2D Transition‐Metal Dichalcogenide and InGaZnO Channels

AbstractVarious functions are introduced from a unique field‐effect device structure which combines or merges transition metal dichalcogenide (TMD) and InGaZnO (IGZO) channels together on one common gate: multivalue field effect transistors (FETs), photodetecting devices, and signal frequency doublers. Judging by the individual transfer characteristics of two FET devices, the n‐IGZO FET always shows higher drain current and more positive‐side turn‐on voltage than those of n‐TMD channel FETs. As a result, a combined transfer characteristic presents two‐step drain current levels, so that their load‐resistance inverter might demonstrate three value output voltage signals. Those ternary value inverter devices with n‐IGZO/n‐ReSe2 or n‐IGZO/n‐WSe2 combination also operate as a photodetector, responding to visible/near infrared (IR) photons with a fast photodynamics of 30 ms rising/falling time. The most interesting is the ambipolar device achieved from n‐IGZO FET/p‐MoTe2 FET combination circuit. The ambipolar device circuit operates as an alternating current (AC) signal frequency doubler and demonstrates twofold organic light emitting diode blinking in one AC period. This unprecedented technique for the multifunctional field effect TMD devices incorporating n‐IGZO FETs will open a new path toward future electronics.

Tailoring the electrical properties of MoTe2 field effect transistor via chemical doping

Abstract Regulating electrical properties is basic to accomplish a dynamic exhibition of electronic devices such as field effect transistors (FETs). Chemical doping is advantageous in this regard being cost effective, efficient and over-simplistic collated to other methods used in semiconductor electronics for effectual tailoring of electrical properties. Here we report on controlling electrical characteristics of multilayer MoTe2 field effect transistor via chemical doping with ammonia (NH3). Shifting of threshold voltage from positive to negative back-gate voltage (Vbg) indicates n-type doping which is further complied with the results of Raman spectroscopy wherein A1g characteristic peak shifted towards lower wave number from 173.5 cm−1 to 169.4 cm−1. Further, the full-width at half maximum (FWHM) for A1g peak is observed to be 3.2–6.0 cm−1 that is broader relative to E12g characteristic peak. After doping, the current on/off ratio is improved from 105 to 107. The electron mobility and charge carrier density was increased from 43 cm2V−1s−1 to 180 cm2V−1s−1 and from 4.2 × 1012 cm−2 to 6.9 × 1012 cm−2, respectively. Our study reveals exceptional results by illustrating improved performance of the MoTe2 FETs, which could be utilized for potential applications in electronic devices.

Electric Double-Layer Gating of Two-Dimensional Field-Effect Transistors Using a Single-Ion Conductor.

Electric double-layer (EDL) gating using a custom-synthesized polyester single-ion conductor (PE400-Li) is demonstrated on two-dimensional (2D) crystals for the first time. The electronic properties of graphene and MoTe2 field-effect transistors (FETs) gated with the single-ion conductor are directly compared to a poly(ethylene oxide) dual-ion conductor (PEO:CsClO4). The anions in the single-ion conductor are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the negative electrode and a corresponding cationic depletion layer at the positive electrode. Because the cations are mobile in both the single- and dual-ion conductors, a similar enhancement of the n-branch is observed in both graphene and MoTe2. Specifically, the single-ion conductor decreases the subthreshold swing in the n-branch of the bare MoTe2 FET from 5000 to 250 mV/dec and increases the current density and on/off ratio by two orders of magnitude. However, the single-ion conductor suppressed the p-branch in both the graphene and the MoTe2 FETs, and finite element modeling of ion transport shows that this result is unique to single-ion conductor gating in combination with an asymmetric gate/channel geometry. Both the experiments and modeling suggest that single-ion conductor-gated FETs can achieve sheet densities up to 1014 cm-2, which corresponds to a charge density that would theoretically be sufficient to induce several percent strain in monolayer 2D crystals and potentially induce a semiconductor-to-metal phase transition in MoTe2.

MoTe2 Lateral Homojunction Field-Effect Transistors Fabricated using Flux-Controlled Phase Engineering.

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe2) phase, metallic monoclinic (1T') MoTe2 phase, and their lateral homojunctions can be selectively synthesized in situ by chemical vapor deposition due to the small free energy difference between the two phases. Here, we have investigated, in detail, the structural and electrical properties of in situ-grown lateral 2H/1T' MoTe2 homojunctions grown using flux-controlled phase engineering. Using atomic-resolution plan-view and cross-sectional transmission electron microscopy analyses, we show that the round regions of near-single-crystalline 2H-MoTe2 grow out of a polycrystalline 1T'-MoTe2 matrix. We further demonstrate the operation of MoTe2 FETs made on these in situ-grown lateral homojunctions with 1T' contacts. The use of a 1T' phase as electrodes in MoTe2 FETs effectively improves the device performance by substantially decreasing the contact resistance. The contact resistance of 1T' electrodes extracted from transfer length method measurements is 470 ± 30 Ω·μm. Temperature- and gate-voltage-dependent transport characteristics reveal a flat-band barrier height of ∼30 ± 10 meV at the lateral 2H/1T' interface that is several times smaller and shows a stronger gate modulation, compared to the metal/2H Schottky barrier height. The information learned from this analysis will be critical to understanding the properties of MoTe2 homojunction FETs for use in memory and logic circuity applications.

Integrated advantages from perovskite photovoltaic cell and 2D MoTe2 transistor towards self-power energy harvesting and photosensing

Perovskite photovoltaic (PV) cell researches have been extensively conducted due to its unprecedentedly excellent power conversion efficiencies (PCEs), which are reported to be over 23.7% in maximum. Despite the high PCEs, there are several unresolved issues on optimal interface states in the cell and device stabilities. Moreover, beyond the simple PV device issues, extended device or circuit applications have rarely been reported although it seems equally important to fully reveal the self-power energy harvesting functions of the perovskite cells to modern ubiquitous environment. Here, we integrate a realistic self-power circuit utilizing a few nm-thin molybdenum ditelluride (MoTe2) field effect transistors (FETs) as well as perovskite PV cells. The PV cells intrinsically show more than 0.9 V of open circuit voltage (VOC) under artificial sun (AM 1.5) or visible photon illumination, so we thus used a red-light emitting diode (LED) to simply generate ~1 V of photovoltage. Two modes for practical operation were consequently observed from the circuit, depending on the light intensity: self-power current/voltage source mode (high intensity) and photosensor mode (low intensity). We now conclude that our self-power circuit approaches integrating perovskite cells and 2D MoTe2 FET could attract much attention for future ubiquitous electronics and energy harvest applications.

Tuning the Polarity of MoTe2 FETs by Varying the Channel Thickness for Gas-Sensing Applications.

In this study, electrical characteristics of MoTe2 field-effect transistors (FETs) are investigated as a function of channel thickness. The conductivity type in FETs, fabricated from exfoliated MoTe2 crystals, switched from p-type to ambipolar to n-type conduction with increasing MoTe2 channel thickness from 10.6 nm to 56.7 nm. This change in flake-thickness-dependent conducting behavior of MoTe2 FETs can be attributed to modulation of the Schottky barrier height and related bandgap alignment. Change in polarity as a function of channel thickness variation is also used for ammonia (NH3) sensing, which confirms the p- and n-type behavior of MoTe2 devices.

Low-Frequency Noise Characteristics in Multilayer MoTe2 FETs With Hydrophobic Amorphous Fluoropolymers

This letter investigates the low-frequency noise (LFN) properties of the multilayer MoTe2 field-effect transistors (FETs) before and after hydrophobic amorphous polymer (CYTOP) encapsulation in the subthreshold and linear regimes. The noise spectrum density of drain current (SID) shows that the LFN in the multilayer MoTe2 FETs nicely fits to a 1/ $f^{\gamma }$ power law with $\gamma \sim \textsf {1}$ in the frequency range of 10–200 Hz. From the dependence of SID on the drain current, carrier number fluctuation ( $\Delta n$ ) is considered as a dominant LFN mechanism from all operation regimes in the multilayer MoTe2 FETs. Extracted trap density (Nt) based on the McWhorter model in this letter was reduced at least more than one order level compared with the multilayer MoTe2 FETs without CYTOP passivation.

Accurate change of carrier types within ultrathin MoTe2 field-effect transistors with the time exposed to ambient air

Two-dimensional MoTe2 attracts much attention due to its novel properties; however, the stability limits its application. Here, the electrical stability of ultrathin MoTe2 was systematically researched and the law of how carrier types within MoTe2 change when exposing to air with a long time period of 180 days was obtained. MoTe2 field-effect transistors (FETs) were prepared using mechanical exfoliation and dry transfer technology; then they were exposed to the air. Their electrical behaviors confirm that due to the air adsorption the carrier type within ultrathin MoTe2 gradually changes from initial ambipolar (electron and hole conduction) to the unipolar hole conduction (P type) after 100 days of the air exposure. The mobility of ambipolar MoTe2 FETs shows a non-monotonic law with the time. The electron mobility decreases first, then almost keeps unchanged between 24 and 47 days, since then it quickly drops to near zero. The hole mobility increases with the time and reaches a maximum value at the 47th day; then it reaches a stable value after passing a negligible reduction. This polarity shift can be attributed to the oxygen molecules adsorption, which was further proved by the X-ray photoelectron spectroscopy and the photocurrent response of FETs under ultraviolet illumination. Furthermore, at the 180th day, the device shows unique P-type conductivity and excellent output characteristics, which provides a very convenient method to make unipolar P-type MoTe2 FET from ambipolar MoTe2.

Precisely controllable n-type doping in MoTe2 field effect transistors by hydrazine treatment

An accurate and controllable n-type doping method for MoTe2 field effect transistors is developed by hydrazine treatment. It is shown that hydrazine molecules are absorbed physically without a new substance being formed, as determined by X-ray photoelectron spectroscopy measurements. The conduction type, electron concentrations, and minimum conductivity points in the transfer characteristics could be modulated precisely for a wide range by varying the concentration of hydrazine solution. Compared to pristine MoTe2 control devices, the electron concentrations are changed from 9.67 × 1011 cm−2 to 3.46 × 1012 cm−2 in the n-type regime, while the electron current on/off ratio is increased from 104 to 106 after 5 wt. % of hydrazine treatment. It is also interesting to observe that the ambipolar window increases almost linearly as the device temperature is reduced from 260 K to 5 K. The effective Schottky barrier heights for electrons in hydrazine treated MoTe2 field-effect transistors are extracted and found to be lower than 0.024 eV, indicating that the thermionic emission is not dominated. Furthermore, a lateral MoTe2 p–n junction with a rectification ratio higher than 103 and an ideality factor of around 1.66 is demonstrated through selective doping. This developed unique method of n-type doping and p-n junction creates an opportunity to fabricate high performance functional devices based on 2D layered materials.

Specific and Highly Sensitive Detection of Ketone Compounds Based on p-Type MoTe2 under Ultraviolet Illumination.

The selective and sensitive detection of chemical agents is demanded by a wide range of practical applications. In particular, sensing of volatile organic compounds (VOCs) at parts-per-billion level is critical for environmental monitoring, process control, and early diagnosis of human diseases. In this report, we demonstrate a specific and highly sensitive detection of ketone compounds using two-dimensional (2D) molybdenum ditelluride (MoTe2). We investigated the effects of UV activation on the sensing performance to a variety of VOCs. It is found that the MoTe2 field-effect transistor (FET) exhibits an opposite sensing response to ketone compounds before and after UV light activation, whereas the responses to other types of VOCs remain in the same direction regardless of the illumination. This unique behavior enables the discriminative detection of ketone molecules including acetone and pentanone from other VOCs in a gas mixture. The activation of UV light also results in a very high sensitivity and low detection limit toward acetone (∼0.2 ppm). Moreover, the MoTe2 FET shows a stable sensing performance in a high humidity environment. The results demonstrate the potential of MoTe2 as a promising candidate for high-performance acetone sensors in important applications such as human breath analysis. The scheme of light-tunable sensing can be applied to a broad range of sensing platforms based on 2D materials.

Control of polarity in multilayer MoTe2 field-effect transistors by channel thickness.

In this study, electronic properties of field-effect transistors (FETs) fabricated from exfoliated MoTe2 single crystals are investigated as a function of channel thickness. The conductivity type in FETs gradually changes from n-type for thick MoTe2 layers (above ≈ 65 nm) to ambipolar behavior for intermediate MoTe2 thickness (between ≈ 60 and 15 nm) to p- type for thin layers (below ≈ 10 nm). The n-type behavior in quasi-bulk MoTe2 is attributed to doping with chlorine atoms from the TeCl4 transport agent used for the chemical vapor transport (CVT) growth of MoTe2. The change in polarity sign with decreasing channel thickness may be associated with increasing role of surface states in ultra-thin layers, which in turn influence carrier concentration and dynamics in the channel due to modulation of Schottky barrier height and band-bending at the metal/semiconductor interface.

Hexagonal MoTe2 with Amorphous BN Passivation Layer for Improved Oxidation Resistance and Endurance of 2D Field Effect Transistors

Environmental and thermal stability of two-dimensional (2D) transition metal dichalcogenides (TMDs) remains a fundamental challenge towards enabling robust electronic devices. Few-layer 2H-MoTe2 with an amorphous boron nitride (a-BN) covering layer was synthesized as a channel for back-gated field effect transistors (FET) and compared to uncovered MoTe2. A systematic approach was taken to understand the effects of heat treatment in air on the performance of FET devices. Atmospheric oxygen was shown to negatively affect uncoated MoTe2 devices while BN-covered FETs showed considerably enhanced chemical and electronic characteristic stability. Uncapped MoTe2 FET devices, which were heated in air for one minute, showed a polarity switch from n- to p-type at 150 °C, while BN-MoTe2 devices switched only after 200 °C of heat treatment. Time-dependent experiments at 100 °C showed that uncapped MoTe2 samples exhibited the polarity switch after 15 min of heat treatment while the BN-capped device maintained its n-type conductivity for the maximum 60 min duration of the experiment. X-ray photoelectron spectroscopy (XPS) analysis suggests that oxygen incorporation into MoTe2 was the primary doping mechanism for the polarity switch. This work demonstrates the effectiveness of an a-BN capping layer in preserving few-layer MoTe2 material quality and controlling its conductivity type at elevated temperatures in an atmospheric environment.

Measurement of Two Dimensional Van Der Waals Materials' Bandgap Using Ambipolar Field Effect Transistor with Graphene Contact and hBN Passivation

Two dimensional (2D) semiconductor materials haven been extensively studied for the last decade, regarded as one kind of the most important breakthrough materials toward future device technologies. Band gap of monolayer and few layer in 2D semiconductors has been reported, measured by optical probing such as photoluminescence (PL). However, if their exfoliated thickness is as large as multilayer over 10 layers (10L), PL measurements become less effective and inaccurate because the optical transition of 2D semiconductor is changed from direct to indirect mode. At the moment, the energy band gap also becomes smaller with the semiconductir thickness. In fact, for thick multilayer 2D semiconductors, practical band gap measurement is very difficult due to lack of measurementment technique, so that density function theory (DFT)-based calculations might be only way to conjecture the bandgap. Here, we introduce another way to measure such bandgap of multilayer 2D semiconductors; that is using field effect transistor (FET) as a platform. We have thus fabricated FETs with multilayer 2D channel using graphene contact and top passivation for this study. Because graphene contact would secure ambipolar behavior and Schottky contact barrier tuning of FETs with the assitance of top passivation, bandgap measurement was successfully carried out. Our FETs with WSe2, MoTe2, BP channel, show ambipolar behavior and their maximum Schottky barrier height are measured by temperature-dependent transfer curve characteristics. Bandgap values from WSe2 FET1, WSe2 FET2, MoTe2 FET, and BP FET were 1.16,1.44, 0.92, 0.42 eV, respectively obtained by twice the maximum Schottky barrier height. The values from WSe2 FETs were compared with those of PL measurement, to be confirmed for their compatibilities and according to literature, our values seems reasonable.

Carrier-Type Modulation and Mobility Improvement of Thin MoTe2.

A systematic modulation of the carrier type in molybdenum ditelluride (MoTe2 ) field-effect transistors (FETs) is described, through rapid thermal annealing (RTA) under a controlled O2 environment (p-type modulation) and benzyl viologen (BV) doping (n-type modulation). Al2 O3 capping is then introduced to improve the carrier mobilities and device stability. MoTe2 is found to be ultrasensitive to O2 at elevated temperatures (250 °C). Charge carriers of MoTe2 flakes annealed via RTA at various vacuum levels are tuned between predominantly pristine n-type ambipolar, symmetric ambipolar, unipolar p-type, and degenerate-like p-type. Changes in the MoTe2 -transistor performance are confirmed to originate from the physical and chemical absorption and dissociation of O2 , especially at tellurium vacancy sites. The electron branch is modulated by varying the BV dopant concentrations and annealing conditions. Unipolar n-type MoTe2 FETs with a high on-off ratio exceeding 106 are achieved under optimized doping conditions. By introducing Al2 O3 capping, carrier field effect mobilities (41 for holes and 80 cm2 V-1 s-1 for electrons) and device stability are improved due to the reduced trap densities and isolation from ambient air. Lateral MoTe2 p-n diodes with an ideality factor of 1.2 are fabricated using the p- and n-type doping technique to test the superb potential of the doping method in functional electronic device applications.

Tunable Mobility in Double-Gated MoTe2 Field-Effect Transistor: Effect of Coulomb Screening and Trap Sites.

There is a general consensus that the carrier mobility in a field-effect transistor (FET) made of semiconducting transition-metal dichalcogenides (s-TMDs) is severely degraded by the trapping/detrapping and Coulomb scattering of carriers by ionic charges in the gate oxides. Using a double-gated (DG) MoTe2 FET, we modulated and enhanced the carrier mobility by adjusting the top- and bottom-gate biases. The relevant mechanism for mobility tuning in this device was explored using static DC and low-frequency (LF) noise characterizations. In the investigations, LF-noise analysis revealed that for a strong back-gate bias the Coulomb scattering of carriers by ionized traps in the gate dielectrics is strongly screened by accumulation charges. This significantly reduces the electrostatic scattering of channel carriers by the interface trap sites, resulting in increased mobility. The reduction of the number of effective trap sites also depends on the gate bias, implying that owing to the gate bias, the carriers are shifted inside the channel. Thus, the number of active trap sites decreases as the carriers are repelled from the interface by the gate bias. The gate-controlled Coulomb-scattering parameter and the trap-site density provide new handles for improving the carrier mobility in TMDs, in a fundamentally different way from dielectric screening observed in previous studies.

Reconfigurable Complementary Monolayer MoTe2 Field-Effect Transistors for Integrated Circuits.

Transition metal dichalcogenides are of interest for next generation switches, but the lack of low resistance electron and hole contacts in the same material has hindered the development of complementary field-effect transistors and circuits. We demonstrate an air-stable, reconfigurable, complementary monolayer MoTe2 field-effect transistor encapsulated in hexagonal boron nitride, using electrostatically doped contacts. The introduction of a multigate design with prepatterned bottom contacts allows us to independently achieve low contact resistance and threshold voltage tuning, while also decoupling the Schottky contacts and channel gating. We illustrate a complementary inverter and a p-i-n diode as potential applications.

Role of Charge Traps in the Performance of Atomically Thin Transistors.

Transient currents in atomically thin MoTe2 field-effect transistors (FETs) are measured during cycles of pulses through the gate electrode. The curves of the transient currents are analyzed in light of a newly proposed model for charge-trapping dynamics that renders a time-dependent change in the threshold voltage as the dominant effect on the channel hysteretic behavior over emission currents from the charge traps. The proposed model is expected to be instrumental in understanding the fundamental physics that governs the performance of atomically thin FETs and is applicable to the entire class of atomically thin-based devices. Hence, the model is vital to the intelligent design of fast and highly efficient optoelectronic devices.

Uniform Benchmarking of Low-Voltage van der Waals FETs

Monolayer MoS2, MoSe2, MoTe2, WS2, WSe2, and black phosphorous field effect transistors (FETs) operating in the low-voltage (LV) regime (0.3V) with geometries from the 2019 and 2028 nodes of the 2013 International Technology Roadmap for Semiconductors (ITRS) are benchmarked along with an ultra-thin-body Si FET. Current can increase or decrease with scaling, and the trend is strongly correlated with the effective mass. For LV operation at the 2028 node, an effective mass of ~0.4 m0, corresponding to that of WSe2, gives the maximum drive current. The short 6 nm gate length combined with LV operation is forgiving in its requirements for material quality and contact resistances. In this LV regime, device and circuit performance are competitive using currently measured values for mobilities and contact resistances for the monolayer two-dimensional materials.