Multilayer MoS2 Field-effect Transistors Research Articles

Effects of Channel Length Scaling on the Electrical Characteristics of Multilayer MoS2 Field Effect Transistor

With the rapid miniaturization of integrated chips in recent decades, aggressive geometric scaling of transistor dimensions to nanometric scales has become imperative. Recent works have reported the usefulness of 2D transition metal dichalcogenides (TMDs) like MoS2 in MOSFET fabrication due to their enhanced active surface area, thin body, and non-zero bandgap. However, a systematic study on the effects of geometric scaling down to sub-10-nm nodes on the performance of MoS2 MOSFETs is lacking. Here, the authors present an extensive study on the performance of MoS2 FETs when geometrically scaled down to the sub-10 nm range. Transport properties are modelled using drift-diffusion equations in the classical regime and self-consistent Schrödinger-Poisson solution using NEGF formulation in the quantum regime. By employing the device modeling tool COMSOL for the classical regime, drain current vs. gate voltage (ID vs. VGS) plots were simulated. On the other hand, NEGF formulation for quantum regions is performed using MATLAB, and transfer characteristics are obtained. The effects of scaling device dimensions, such as channel length and contact length, are evaluated based on transfer characteristics by computing performance metrics like drain-induced barrier lowering (DIBL), on-off currents, subthreshold swing, and threshold voltage.

Dramatically enhanced ambient effects in a multi-layer MoS2 transistor with channel thickness near maximum depletion width

It is known that semiconducting two-dimensional transition metal dichalcogenides such as MoS2 and WSe2, widely attracted as advanced field-effect transistors (FETs) due to good surface roughness in nano-scale and outstanding gate control with the desired bandgap, are significantly affected by oxygen and water molecules in air. Here, the channel thickness-dependent ambient effects on operation of multi-layer MoS2 FETs are investigated for the first time. In particular, a multi-layer MoS2 FET with channel thickness similar to the maximum depletion width (Dmax) exhibited dramatic changes in the on-current to off-current (Ion/Ioff) ratio under ambient conditions. The results were verified using numerical simulations. Our work is important in terms of the development and optimization of highly sensitive chemical or gas sensors, and it furthers our understanding of how multi-layer MoS2 FETs operate.

Multilayer MoS2 Back‐Gate Transistors with ZrO2 Dielectric Layer Optimization for Low‐Power Electronics

Herein, high‐performance back‐gate molybdenum disulfide (MoS2) field‐effect transistors (FETs) with high‐quality sub‐20 nm high‐k dielectric layers are developed for high‐performance and lower‐power consumption applications. The 20 nm ultrathin ZrO2 dielectric layers are deposited by thermal atomic layer deposition (ALD) method, where the growth temperature is varied and it shows a significant impact on the electrical characteristics of the deposited ZrO2 materials. A polydimethylsiloxane (PDMS) transfer process is used to transfer multilayer MoS2 flakes onto a 20 nm ZrO2/p–Si substrate with an optimized dielectric growth temperature without any subsequent processing, resulting in back‐gate MoS2 FET device architecture. These transistors demonstrate excellent electrical characteristics with on–off current ratio up to 1.8 × 107, subthreshold swing as low as 70 mV decade−1 and field‐effect mobility as high as 3.9 cm2 V−1 s−1. Furthermore, an enhancement‐mode device operation and a high complementary metal–oxide–semiconductor (CMOS) ION/IOFF ratio of 107 are achieved. The excellent electrical performance is attributed to the low interface state traps and high‐quality ZrO2 dielectric layer, indicating the great potential of our multilayer MoS2 FETs technology for low‐power applications.

High-performance Te-doped p-type MoS2 transistor with high-K insulators

Molybdenum disulfide (MoS2) films possess intrinsic n-type conductivity, and thus development of p-type MoS2 films for realizing practical next-generation complementary metal oxide semiconductor devices is extremely challenging. The use of dopants is a well-known conventional approach for engineering intrinsic conductivity with improved device performance. Herein, we demonstrate the n-type conductivity suppressing property of the tellurium (Te)-doped multilayer MoS2, grown by metal-organic chemical vapor deposition. The back-gated as-grown Te-doped multilayer MoS2 field-effect transistor (FET) is observed to exhibit p-type behavior with a maximum mobility of 0.036 cm2/V·s, ON/OFF current ratio of 7.8×103, and a Schottky barrier height of 32 meV. A quantitative Schottky barrier height of a p-type multilayer MoS2-based FET with Au electrode has been obtained by analyzing its low-temperature transport characteristics. Enhanced device performance has been achieved by doping the p-type MoS2 transistors in a back-gate structure and high-K dielectric gate insulators with Te. pFET devices containing Al2O3 dielectric insulators exhibit extremely high performance, including a maximum mobility of ∼182 cm2/V·s, maximum ON/OFF current ratio of ∼105, and a low subthreshold swing of ∼215 mV/dec. These improvements have been attributed to the charge screening effect associated with the high-K dielectrics and a low Schottky barrier height.

Experimental and modeling study of 1/f noise in multilayer MoS2 and MoSe2 field-effect transistors

In field-effect transistors (FETs) with two-dimensional (2D) transition metal dichalcogenide channels, the dependence of field-effect mobility on atomic layer thickness has been studied and interpreted in terms of interface scattering and interlayer coupling resistance (Rint). However, a model for 1/f noise, such as in MoS2 and in MoSe2 FETs, for various contact metals and layer number thicknesses has not been reported. In this work, we have experimentally studied current–voltage and 1/f noise on MoS2 and MoSe2 FETs with source and drain contacts of high and low work function metals to understand both the mobility and the noise behavior. We have developed a noise model incorporating layer number dependent Hooge parameters and Rint. The noise and mobility models utilize screening lengths for charge, mobility, and Hooge parameter to describe the variation of these quantities with a layer number. Using our single model topology with appropriate fitting parameters for each material and each contact metal, the model captures the experimentally observed layer thickness dependence of the Hooge parameter. Our noise analysis is fully comprehensive and, hence, could be applied to any 2D layered systems.

Efficient and Versatile Modeling of Mono- and Multi-Layer MoS2 Field Effect Transistor

Two-dimensional (2D) materials with intrinsic atomic-level thicknesses are strong candidates for the development of deeply scaled field-effect transistors (FETs) and novel device architectures. In particular, transition-metal dichalcogenides (TMDCs), of which molybdenum disulfide (MoS2) is the most widely studied, are especially attractive because of their non-zero bandgap, mechanical flexibility, and optical transparency. In this contribution, we present an efficient full-wave model of MoS2-FETs that is based on (1) defining the constitutive relations of the MoS2 active channel, and (2) simulating the 3D geometry. The former is achieved by using atomistic simulations of the material crystal structure, the latter is obtained by using the solver COMSOL Multiphysics. We show examples of FET simulations and compare, when possible, the theoretical results to the experimental from the literature. The comparison highlights a very good agreement.

Tuning the on/off current ratio in ionic-liquid gated multi-layer MoS2 field-effect transistors

Ionic-liquid gated multi-layer MoS2 field-effect transistors were fabricated and their electrical properties related to maximum depletion depth, bulk neutral and surface accumulation conduction were investigated in detail. A negative back-gate bias resulted in a depletion of the bulk neutral channel at the bottom interface, which significantly enhanced transistor performance in terms of the on-current to off-current (Ion/Ioff) ratio and the off-state current level. The experimental results were verified by numerical simulations and analytical equations. These findings are critical for the optimization of the Ion/Ioff ratio of multi-layer MoS2 transistors for energy efficient electronics and low power applications.

Bias Stress Instability in Multilayered MoS2 Field-Effect Transistors Under Pulse-Mode Operation

In this article, the bias stress instability (BSI) of multilayered MoS2 field-effect transistors (m-MoS2 FETs) encapsulated in hydrophobic polymers [cyclized transparent optical polymer (CYTOP)] was systematically investigated under the pulse-mode operation. Compared with the dc mode, the threshold voltage shift ( $\Delta {V}_{\text {th}}$ ) under positive-bias stress was inversely larger than that for negative-bias stress. BSI in the m-MoS2 FETs with CYTOP was negligibly affected by the external oxygen/moisture. Shallow hole traps and electron detrapping effects were strongly identified as the main causes of bias-polarity dependence on $\Delta {V} _{{\text {th}}}$ in the pulse mode. This can potentially be attributed to the sulfur vacancies in the m-MoS2, which have a short trap lifetime of hundreds of microseconds. During the pulse-mode operation, various parameters, such as duty cycle, timeframe, bias polarity, and electric field strength, were systematically adjusted and experimentally applied to elucidate the origins of the asymmetry of the BSI in pulse mode. Atomic- and energy-band models, as well as recovery time constant extraction, are suggested to support the key mechanism of bias-polarity effects on BSI, which are, nicely, consistent with previous reports.

Staircase-like transfer characteristics in multilayer MoS2 field-effect transistors

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 terraces at the edges of the flakes accompanies the staircase-like characteristics. The anomalous staircase-like characteristics are ascribable to a difference in threshold-voltage shift by charge transfer from surface adsorbates between the channel center and the thinner terrace at the edge. This study reveals the importance of the uniformity of channel thickness.

Transient characteristics of back-gated multilayer MoS2 and WSe2 channel n-type metal oxide semiconductor field effect transistors: A comparative study

The surface and interface effects of back-gated transition metal dichalcogenide channel MOSFETs are of great importance to device applications. This paper reports the transient current-time (I-t) characteristics of back-gated multilayer MoS2 and WSe2 channel n-type MOSFETs due to the charge trapping into the surface and interface traps of the devices. By investigating the current-voltage and I-t results measured from the devices with a similar structure and bias conditions under ambient and vacuum conditions, we find that the WSe2 devices show more significant surface and interface effects as compared to the MoS2 devices. The comparison of the experimental results with the technology computer aided design simulation shows that a single type trap model can account for the main characteristics of the transient process observed from the MoS2 and WSe2 devices. As compared to the trap on the MoS2 device, the surface trap on the WSe2 device has higher density, lower energy, and smaller trapping time. A further experimental comparison with WS2 devices suggests that the difference is microscopically originated from the chalcogen of Se and S rather than the metal element of Mo and W.

Thickness-Dependent Electrical Properties of MoS₂ Field-Effect Transistors Fabricated on Sol-Gel Prepared AlOX Layer.

Solution-processed high-k oxide layer, which is typically deposited using atomic layer deposition (ALD), has been proposed and recently demonstrated on molybdenum disulfide (MoS2) field-effect transistors (FETs). In this report, we statistically investigate electrical performance of multilayer MoS2 FETs fabricated on sol-gel AlOX gate-dielectric. More than 10 sample devices with different MoS2 thickness are characterized and compared. For electrical parameters extraction, Y -function method is adopted in order to minimize S/D electrode contact-induced variations. In spite of the relatively rougher surface of the sol-gel AlOX film, no significant difference of electrical performance is observed. The sol-gel prepared AlOX can be considered as a promising high-k gate dielectric for high-performance large-area transistion metal dichalcogenides (TMDs) devices fabrication.

Enhanced photoresponse characteristics of transistors using CVD-grown MoS2/WS2 heterostructures

Semiconductor heterostructures based on transition metal dichalcogenides provide a broad platform to research two-dimensional nanomaterials and design atomically thin devices for fundamental and applied interests. The MoS2/WS2 heterostructure was prepared on SiO2/Si substrate by chemical vapor deposition (CVD) in our research. And the optical properties of the heterostructure was characterized by Raman and photoluminescence (PL) spectroscopy. The similar 2 orders of magnitude decrease of PL intensity in MoS2/WS2 heterostructures was tested, which is attribute to the electrical and optical modulation effects are connected with the interfacial charge transfer between MoS2 and WS2 films. Using MoS2/WS2 heterostructure as channel material of the phototransistor, we demonstrated over 50 folds enhanced photoresponsivity of multilayer MoS2 field-effect transistor. The results indicate that the MoS2/WS2 films can be a promising heterostructure material to enhance the photoresponse characteristics of MoS2-based phototransistors.

Improving the Stability of High-Performance Multilayer MoS2 Field-Effect Transistors.

In this study, we propose a method for improving the stability of multilayer MoS2 field-effect transistors (FETs) by O2 plasma treatment and Al2O3 passivation while sustaining the high performance of bulk MoS2 FET. The MoS2 FETs were exposed to O2 plasma for 30 s before Al2O3 encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoOx layer formed during the plasma treatment was found between MoS2 and the top passivation layer. The MoOx interlayer prevents the generation of excess electron carriers in the channel, owing to Al2O3 passivation, thereby minimizing the shift in the threshold voltage (Vth) and increase of the off-current leakage. However, prolonged exposure of the MoS2 surface to O2 plasma (90 and 120 s) was found to introduce excess oxygen into the MoOx interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS2 FETs were also subjected to gate-bias stress tests under different conditions. The MoS2 transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in Vth were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS2 transistors are expected to open up new opportunities for the development of sophisticated electronic applications.

Ambipolar MoS2 Transistors by Nanoscale Tailoring of Schottky Barrier Using Oxygen Plasma Functionalization.

One of the main challenges to exploit molybdenum disulfide (MoS2) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS2 FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS2 surface can be tailored at nanoscale using soft O2 plasma treatments. The morphological, chemical, and electrical modifications of MoS2 surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS2 to a broader distribution (from 0.2 to 0.8 eV) after 600 s O2 plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS2 multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS2 FETs, fabricated by self-aligned deposition of source/drain contacts in the O2 plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio Ion/Ioff ≈ 103 and field-effect mobilities of 11.5 and 7.2 cm2 V-1 s-1 for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O2 plasma functionalized MoS2 surface.

Mechanisms of Hysteresis Generation in Multi-Layered MoS2 Field Effect Transistor

Recently, two-dimensional transition metal dichalcogenides (2D TMDs) with layered structure have attracted much attention due to their unique electrical and optical properties. In particular, they exhibit a great potential for realizing the field effect transistors (FETs) which can apply for low power applications. Among them, molybdenum based 2D TMD (i.e., MoS2) is one of the most studied compounds. However, FETs with multi-layered MoS2 employed as a semiconductor layer must tackle many practical problems. For example, it has been speculated that either adsorbed molecules (e.g., water molecule) on the top of exposed MoS2 channel layer or traps at the MoS2-insulator interface causes the device instability such as the hysteresis generation in MoS2 FETs. Until now, research on the hysteresis phenomenon of FETs with multi-layered MoS2 has been focused on finding the origin of the hysteresis. However, the underlying mechanism(s) for this phenomenon has not yet been clearly clarified. Although recent several studies have suggested that the charge trapping and de-trapping processes at the MoS2-insulator interface cause the device instabilities including hysteresis, it is not sufficient to explain the origin of traps giving rise to the hysteresis phenomenon of the MoS2 FETs. In this work, we analyzed the hysteresis characteristics of the MoS2 capacitors, using high frequency capacitance-voltage (C-V) measurements. Based on this analysis, we will demonstrate the roles of the interface traps originated from sulfur vacancy defects on the hysteresis generation in MoS2 FETs. Based on these experimental results, we will propose a model to explain the hysteresis phenomenon observed in multi-layered MoS2 FETs.

The doping mechanism and electrical performance of polyethylenimine‐doped MoS­2 transistor

We present a systematic investigation of polyethylenimine (PEI) doping mechanism and its effects on the multilayer MoS2 field effect transistors (FETs). The threshold voltages of MoS2 FETs before (i.e., pristine) and PEI doping are observed at 3.7 and 0.72 V, respectively. This negative threshold voltage shift clearly reveals that the PEI molecules effectively act as n‐type dopants. The electrical properties are improved by absorption of PEI molecules onto MoS2 channel because the width of Schottky barrier (SB) is narrowed by the induced interfacial dipole between PEI molecules and MoS2 layers. Through the density function theory (DFT) calculation and X‐ray photoelectron spectroscopy (XPS) analysis, we confirm that formation of MoN bond generates new energy state into the bandgap. Consequently, the hole carriers can easily tunnel through the barrier under negative gate voltage. Furthermore, PEI doping improve photoresponsivity and time‐resolved photo‐switching characteristics because of the new energy state. Our studies demonstrate the PEI doping method has a great potential for improving electrical and optical properties of MoS2‐based devices.

The doping mechanism and electrical performance of polyethylenimine‐doped MoS­2 transistor

Seongin Hong et al. (article no. 1600262) have investigated the polyethylenimine (PEI) doping mechanism and its effect on the electrical and optical properties of multilayer MoS2 field effect transistors (FETs). Density functional theory (DFT) calculation and X-ray photoelectron spectroscopy (XPS) measurement confirm that the PEI molecules were successfully doped and formed Mo–N bonds on the MoS2 channel, generating new energy states near the valence band. The strong n-doping changed the threshold voltage as well as the Schottky barrier width attributed to the induced interfacial dipoles. Therefore, the ON-current of the doped MoS2 FETs was improved in comparison with the pristine FETs. Furthermore, the PEI doping also enhanced the photoresponsivity of the MoS2 FETs from 0.14 A/W to 4.41 A/W. This study suggests that PEI molecular doping could be widely applicable to two-dimensional materials in order to improve the electrical and optical properties of respective devices.

Effects of plasma treatment on the electrical reliability of multilayer MoS2 field-effect transistors

Electric stability and recovery properties of multilayer molybdenum disulfide (MoS2) field-effect transistors (FETs) were investigated by plasma treatment via inductively coupled plasma and polymethyl methacrylate (PMMA) passivation. Plasma treatment is proposed as an efficient method for cleaning the surface of MoS2. The results indicate that performance degradation of MoS2 field-effect transistors owing to oxygen absorption is largely restored by the plasma treatment. It is also shown that PMMA passivation (7% chlorobenzene) degrades the performance of the MoS2 FET. The performance degradation of the device can be restored by the plasma treatment. It is suggested that high-density plasma treatment with minimized damage to the surface can be an efficient method of surface cleaning. These findings are especially important for the nanolithography and nanodevice fabrication of two-dimensional devices made of MoS2, graphene, etc., using PMMA resistors.

Enhanced photoresponsivity of multilayer MoS2 transistors using high work function MoOx overlayer

Using thin sub-stoichiometric molybdenum trioxide (MoOx, x < 3) overlayer, we demonstrate over 20-folds enhanced photoresponsivity of multilayer MoS2 field-effect transistor. The fabricated device exhibits field-effect mobility (μFE) of up to 41.4 cm2/V s and threshold voltage (VTH) of −9.3 V, which is also modulated by the MoOx overlayer. The MoOx layer (∼25 nm), commonly known for a high work function (∼6.8 eV) material with a band gap of ∼3 eV, is evaporated on top of the MoS2 channel and confirmed by the transmission electron microscope analysis. The electrical and optical modulation effects are associated with interfacial charge transfer and thus an induced built-in electric field at the MoS2/MoOx interface. The results show that high work function MoOx can be a promising heterostructure material in order to enhance the photoresponse characteristics of MoS2-based devices.

Hopping conduction and random telegraph signal in an exfoliated multilayer MoS2 field-effect transistor

We investigate the hopping conduction and random telegraph signal caused by various species of interface charge scatterers in a MoS2 multilayer field-effect transistor. The temperature dependence of the channel resistivity shows that at low temperatures and low carrier densities the carrier transport is via Mott variable range hopping with a hopping length changing from 41 to 80 nm. The hopping conduction was due to electron tunneling through localized band tail states formed by the scatterers located in the vicinity of the MoS2 layer. In the temperature range of 40–70 K, we observed random telegraph signal (RTS) that is caused by the capture and emission of a carrier by the interface traps that are located away from the layer. These traps form strong potential that interact with the layer and change the potential profile of the electron system. The characteristics of RTS depend strongly on gate bias and temperature, as well as the application of a magnetic field.