Effect of substrate and temperature on the electronic properties of monolayer molybdenum disulfide field-effect transistors

The use of two-dimensional nanostructured molybdenum disulfide (MoS₂) films in field-effect transistors (FETs) in place of graphene was investigated. Monolayer MoS₂ films were fabricated by chemical vapor deposition. The output and transfer curves of supported and suspended MoS₂ FETs were measured. The mobility of the suspended device reached 364.2 cm²?V^?1?s^?1 at 150?°C. The hysteresis of the supported device in transfer curves was much larger than that of the suspended device, and it increased at higher temperatures. These results indicate that the device mobility was limited by Coulomb scattering at ambient temperature, and surface/interface phonon scattering at 150?°C, and the injection of electrons, via quantum tunneling through the Schottky barrier at the contact, was enhanced at higher temperatures and led to the increase of the hysteresis. The suspended MoS₂ films show potential for application as a channel material in electronic devices, and further understanding the causes of hysteresis in a material is important for its use in technologies, such as memory devices and sensing cells.     

Variability of electrical contact properties in multilayer MoS2 thin-film transistors

We report the variability of electrical properties of Ti contacts in back-gated multilayer MoS₂ thin-film transistors based on mechanically exfoliated flakes. By measuring current–voltage characteristics from room temperature to 240 °C, we demonstrate the formation of both ohmic and Schottky contacts at the Ti–MoS₂ junctions of MoS₂ transistors fabricated using identical electrode materials under the same conditions. While MoS₂ transistors with ohmic contacts exhibit a typical signature of band transport, those with Schottky contacts indicate thermally activated transport behavior for the given temperature range. These results provide the experimental evidence of the variability of Ti metal contacts on MoS₂, highlighting the importance of understanding the variability of electronic properties of naturally occurring MoS₂ for further investigation.     

Interface contact and modulated electronic properties by external vertical strains and electric fields in graphene/MoS2 heterostructure

Based to the first-principles calculations, we study the electronic properties of graphene/MoS₂ heterostructure by modulating the vertical strains and applying external electric field. Graphene/MoS₂ heterostructure is a van der Waals heterostructure (vdWH) with the interlayer spacing is 3.2 Å for the equilibrium state, and the contact property of the interface is n-type Schottky contact. The Schottky barrier height (SBH) changes with vertical strains which induces a change of charge transfer between graphene and MoS₂ layer. In addition, with strain or without strain, the applied positive electric field can effectively promote the charge transfer from graphene to MoS₂, while the negative electric field has the opposite effect. These findings support for the design of field effect transistors based on graphene vdWHs.     

Direct observation of the hysteretic Fermi level modulation in monolayer MoS2 field effect transistors

The hysteresis in the transfer curve of MoS₂ has significant impact on the device performance. However, the hysteresis mechanism is still not clear. Here, we investigate the hysteresis of the monolayer MoS₂ by probing the local Fermi level variations as a function of the back gate voltage in different atmosphere using the Kelvin probe microscopy. While the Fermi level of the MoS₂ in air is much lower than that in vacuum, both the MoS₂ devices in vacuum and air show large Fermi level hysteresis. The Fermi level hysteresis direction is clock-wise, identical to that observed in the transfer curves. Both the hysteresis in Fermi level and transfer curve can be explained consistently by taking into account the charge trapping. Our findings confirm that carrier density modulation in MoS₂ plays a vital role in the hysteresis, and provide insight into the hysteresis mechanism for the optimization of the device performance.     

The effect of post-metal annealing on the electrical performance and stability of two-step-annealed solution-processed In2O3 thin film transistors

In this work, solution-processed indium oxide (In₂O₃) thin film transistors (TFTs) were fabricated by a two-step annealing method. The influence of post-metal annealing (PMA) temperatures on the electrical performance and stability is studied. With the increase of PMA temperatures, the on-state current and off-state current (I_on/I_off) ratio is improved and the sub-threshold swing (SS) decreased. Moreover, the stability of In₂O₃ TFTs is also improved. In all, In₂O₃ TFT with post-metal annealing temperature of 350°С exhibits the best performance (a threshold voltage of 4.75 V, a mobility of 13.8 cm²/V, an I_on/I_off ratio of 1.8 × 10⁶, and a SS of 0.76 V/decade). Meanwhile, the stability under temperature stress (TBS) and positive bias stress (PBS) also show a good improvement. It shows that the PMA treatment can effectively suppress the interface trap and bulk trap and result in an obviously improvement of the In₂O₃ TFTs performance.     

Large-scale chemical vapor deposition growth of highly crystalline MoS2 thin films on various substrates and their optoelectronic properties

Large-scale growth of mostly monolayer molybdenum disulfide (MoS₂) on quartz, sapphire, SiO₂/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS₂ on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS₂ is monolayer with high quality and uniformity. The MoS₂ field effect transistors based on the as-grown MoS₂ exhibit carrier mobility of 1–2 cm²V^?1s^?1 and On/Off ratio of ~10⁴ while showing large photoresponse. Our results provide a simple approach to realize MoS₂ on various substrates for electronics and optoelectronics applications.     

Photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors and the effect of its phase transition

We investigate the origin of photo-induced electrical instability in hydrogenated amorphous silicon based thin-film transistors (a-Si:H TFTs). Photo instability alone was accompanied by a positive shift in the threshold voltage (V_TH) caused by photo irradiation, and even larger positive or negative shift further exacerbated the instability caused by photo-induced electrical bias stress. Such phenomena can occur as a result of extended charge trapping and/or the creation of defect-states at the semiconductor/dielectric interface or in the gate dielectric. The mechanism for such is difficult to describe through chemical interactions of electron-donating and -withdrawing molecules that exhibit a shift in V_TH in only one direction. We also prove that a transition from an amorphous to a protocrystalline phase improves the photo-induced electrical stability. Such results originate from a reduction in the density of the localized states in protocrystalline-Si:H films relative to that of a-Si:H. We believe that this study provides significant information on the device physics of optoelectronics, which commonly exhibit photo-induced instability and charge transport, as a result of prolonged exposure to photo irradiation.     

Ultrasonication-assisted liquid-phase exfoliation enhances photoelectrochemical performance in α-Fe2O3/MoS2 photoanode

This study successfully manufactured a p-n heterojunction hematite (α-Fe₂O₃) structure with molybdenum disulfide (MoS₂) to address the electron–hole transfer problems of conventional hematite to enhance photoelectrochemical (PEC) performance. The two-dimensional MoS₂ nanosheets were prepared through ultrasonication-assisted liquid-phase exfoliation, after which the concentration, number of layers, and thickness parameters of the MoS₂ nanosheets were respectively estimated by UV–vis, HRTEM and AFM analysis to be 0.37 mg/ml, 10–12 layers and around 6 nm. The effect of heterojunction α-Fe₂O₃/MoS₂ and the role of the ultrasonication process were investigated by the optimized concentration of MoS₂ in the forms of bulk and nanosheet on the surface of the α-Fe₂O₃ electrode while measuring the PEC performance. The best photocurrent density of the α-Fe₂O₃/MoS₂ photoanode was obtained at 1.52 and 0.86 mA.cm⁻² with good stability at 0.6 V vs. Ag/AgCl under 100 mW/cm² (AM 1.5) illumination from the back- and front-sides of α-Fe₂O₃/MoS₂; these values are 13.82 and 7.85-times higher than those of pure α-Fe₂O₃, respectively. The results of electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis showed increased donor concentration (2.6-fold) and decreased flat band potential (by 20%). Moreover, the results of IPCE, ABPE, and OCP analyses also supported the enhanced PEC performance of α-Fe₂O₃/MoS₂ through the formation of a p–n heterojunction, leading to a facile electron–hole transfer.     

Controlled Formation of TiO2/MoS2 Core–Shell Heterostructures with Enhanced Visible‐Light Photocatalytic Activities

Most recently, much attention has been devoted to photocatalytic materials that may help to solve the global energy crisis and may provide environmental protection. Herein, novel cocatalysts based on few layered MoS₂ and TiO₂ nanomaterials have been designed by growing MoS₂ nanosheets on the surface of TiO₂ nanospheres through a facile hydrothermal method. The method allows the formation of TiO₂/MoS₂ core–shell heterostructures of uniform morphologies and stable structure and provides a good control over shell thickness. The mechanism that forms these heterostructures is discussed in detail. In addition, as cocatalyst, MoS₂ nanosheets can enlarge the light harvesting window to include visible light and improve the photocatalytic ability of TiO₂. Using Rhodamine B as the model, the resultant heterostructure is demonstrated to possess excellent and stable photocatalytic activity in the degradation of organic pollutants under visible light illumination. The TiO₂/MoS₂ heterostructures possess this catalytic activity due to their large surface area and their excellent interface for separating holes and electrons. Therefore, this novel heterostructure nanomaterials possess potential applications in water treatment, degradation of dye pollutants, and environmental cleaning.     

Effect of Intercalated LiF on Interface Contact Characteristics of MoS2 Field Effect Transistor: A First-Principles Study

Based on the first-principles calculation, the effect of intercalated LiF on the contact characteristics of the interface between Au electrode and MoS₂ layer is studied. It is found that adding LiF film can change the contact type between metal electrode Au and MoS₂ layer from Schottky contact to ohmic contact, which is accompanied by interfacial charge transfer from LiF layer to MoS₂ layer and the downward movement of d (d_xy and d_z2) orbital of Mo atom and p (p_x and p_y) orbital of S atom to Fermi level. And the interlayer spacing between LiF layer and Au electrode has a great impact on the interface contact characteristics. The electric field effect and stress effect of interface contact of Au, LiF and MoS₂ (Au/LiF/MoS₂)is more obvious than that of interface contact of Au and MoS₂ (Au/MoS₂). Au/LiF/MoS₂ shows ohmic contact with the interlayer spacing between Au layer and LiF layer less than 3.05 Å and with the electric field less than 0.15 VÅ⁻¹, respectively, while Au/MoS₂ still shows N-type Schottky contact. These findings are helpful to control the contact resistance and have guiding significance for high performance MoS₂ field effect transistor and other electronic components.     

Decreased n-type behavior of monolayer MoS2 crystals annealed in sulfur atmosphere

Herein, we report decreased n-type behavior of mechanically exfoliated monolayer MoS₂ crystals via annealing in sulfur atmosphere. The Raman, photoluminescence, and X-ray photoelectron spectroscopy (XPS) measurements consistently suggested decreased n-type behavior of the monolayer MoS₂ crystals after an hour of thermal annealing at 200 °C in sulfur atmosphere. Such decreased n-type behavior could be attributed to the reduced concentration of sulfur vacancies after the annealing, suggested by the analysis of XPS spectra. Furthermore, after the annealing in sulfur atmosphere, the monolayer MoS₂ transistors exhibited positively shifted threshold voltages and reduced on-currents, confirming decreased n-type conduction. These results demonstrate that the reduction of the concentration of sulfur vacancies decreases n-type behavior in monolayer MoS₂, providing valuable information on understanding the effect of sulfur vacancies on the performance of monolayer MoS₂ devices.     

二维辉钼材料及器件研究进展

经过几十年的发展, 集成电路的特征尺寸将在10–15年内达到其物理极限, 替代材料的研究迫在眉睫. 石墨烯曾被寄予厚望, 但由于其缺乏带隙限制了在数字电路领域的应用. 近年来, 单层及多层辉钼材料由于具有优异的半导体性能, 有可能超过石墨烯成为硅的替代者而引起了微纳电子领域的广泛关注. 本文对近二年国际上辉钼半导体器件研制、辉钼半导体材料的性能 表征及制备方法研究等方面的进展进行了综述, 并对大面积单层材料的研制提出了值得关注的方向. 关键词： 2')" href="#">MoS₂ 辉钼材料 纳米材料 集成电路     

Novel molybdenum disulfide nanosheets–decorated polyaniline: Preparation,characterization and enhanced electrocatalytic activity for hydrogen evolution reaction

Novel molybdenum disulfide nanosheets–decorated polyaniline (MoS₂/PANI) was synthesized and investigated as an efficient catalyst for hydrogen evolution reaction (HER). Compared with MoS₂, MoS₂/PANI nanocomposites exhibited higher catalytic activity and lower Tafel slope for HER in H₂SO₄ solution. The amount of 19 wt% PANI for coupling with MoS₂ resulted in a high current density of 80 mA cm⁻² at 400 mV (vs. RHE). In addition, the optimal MoS₂/PANI nanocomposite showed impressive long-term stability even after 500 cycles. The enhanced catalytic activity of MoS₂/PANI nanocomposites was primarily ascribed to the effective electron transport channels of PANI and the increase of electrochemically accessible surface area in composite materials, which was advantageous to facilitate the charge transfer at catalyst/electrolyte interface.     

The effect of carrier gas flow rate on the growth of MoS<Subscript>2</Subscript> nanoflakes prepared by thermal chemical vapor deposition

Molybdenum disulfide nanoflakes (MoS₂) are superior material for their semiconducting properties. For bulk and monolayer MoS₂ the band gap changes from indirect-to-direct, respectively. So, it exhibits promising prospects in the applications of optoelectronics and valleytronics, such as solar cells, transistors, photodetectors, etc. In this research, the influence of different Ar flow rates as the carrier gas, is investigated for growing MoS₂ nanoflakes on silicon substrates using one-step thermal chemical vapor deposition by simultaneously evaporating of solid sources like sulfur and molybdenum trioxide powders. The structural and optical properties of the obtained nanoflakes are assessed by using X-ray diffraction pattern, scanning electron microscopy, UV–visible absorption, photoluminescence and Raman spectroscopy. It is shown that, Ar gas flow rate is strongly affects on the final products as few-layer MoS₂ structures. Moreover, the abundance of MoS₂ in comparison to MoO₂ and MoO₃ structures, in the obtained nanoflakes, is influenced by the Ar flow rate.     

Three‐Dimensional Multilayer Assemblies of MoS2/Reduced Graphene Oxide for High‐Performance Lithium Ion Batteries

Three‐dimensional (3D) multilayer molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) nanocomposites are prepared by a solution‐processed self‐assembly based on the interaction using different sizes of MoS₂ and GO nanosheets followed by in situ chemical reduction. 3D multilayer assemblies with MoS₂ wrapped by large RGO nanosheets and good interface are observed by transmission electron microscopy. The interaction of Na⁺ ions with oxygen‐containing groups of GO is also investigated. The measurement of lithium ion batteries (LIBs) shows that MoS₂/RGO anode nanocomposite with a weight ratio of MoS₂ to GO of 3:1 exhibits an excellent rate performance of 750 mAh g^?1 at 3 A g^?1 outperforming many previous studies and a high reversible capacity up to ≈1180 mAh g^?1 after 80 cycles at 100 mA g^?1. Good rate performance and high capacity of MoS₂/RGO with 3D unique layered‐structures are attributed to the combined effects of continuous conductive networks of RGO, good interface facilitating charge transfer, and strong RGO sheets preventing the volume expansion. Results indicate that 3D multilayer MoS₂/RGO prepared by a facile solution‐processed assembly can be developed to be an excellent nanoarchitecture for high‐performance LIBs.     

Adsorption studies of Pd on MoS2

Adsorption of Pd on the basal plane of MoS₂ layer compound has been studied in an UHV system with AES, LEED and WF measurements. For substrate temperatures at or below 500 K the deposited Pd formed initially 2D islands. Upon further Pd deposition a number of the 2D islands, depending on temperature, changed to 3D particles. At 700 K Pd grew to 3D particles from the early stages of its deposition. Heating of Pd-covered MoS₂ at RT caused a change of the 2D islands to 3D particles which coalesce to larger and fewer. The Pd adatoms did not interact with the surface S atoms as Fe does. The Pd particles thus remain clean on MoS₂ which is promising in heterogeneous catalysis. At 1200 K a small amount of Pd atoms remained about uniformly distributed and submerged under the S atoms of the outmost layer of MoS₂.     

Effect of temperature–bias annealing on the hysteresis and subthreshold behavior of multilayer MoS2 transistors

The transfer characteristics (I_D–V_G) of multilayers MoS₂ transistors with a SiO₂/Si backgate and Ni source/drain contacts have been measured on as‐prepared devices and after annealing at different temperatures (T_ann from 150 °C to 200 °C) under a positive bias ramp (V_G from 0 V to +20 V). Larger T_ann resulted in a reduced hysteresis of the I_D–V_G curves (from ～11 V in the as‐prepared sample to ～2.5 V after T_ann at 200 °C). The field effect mobility (～30 cm² V^–1 s^–1) remained almost unchanged after the annealing. On the contrary, the subthreshold characteristics changed from the common n‐type behaviour in the as‐prepared device to the appearance of a low current hole inversion branch after annealing. This latter effect indicates a modification of the Ni/MoS₂ contact that can be explained by the formation of a low density of regions with reduced Schottky barrier height (SBH) for holes embedded in a background with low SBH for electrons. Furthermore, a temperature dependent analysis of the subthreshold characteristics revealed a reduction of the interface traps density from ～9 × 10¹¹ eV^–1cm^–2in the as‐prepared device to ～2 × 10¹¹ eV^–1cm^–2after the 200 °C temperature–bias annealing, which is consistent with the observed hysteresis reduction.

Schematic representation of a back‐gated multilayer MoS₂ field effect transistor (left) and transfer characteristics (right) measured at 25 °C on an as‐prepared device and after the temperature–bias annealing at 200 °C under a positive gate bias ramp from 0 V to +20 V.     

Hall effect measurements using low ac magnetic fields and lock-in technique on field effect transistors with molybdenum disulfide channels

Hall effect measurements conventionally rely on the use of dc magnetic fields. For electronic devices made of ultrathin semiconducting materials, such as molybdenum disulfide (MoS₂), the dc Hall effect measurements have practical difficulties. Here, we report the results of the Hall effect measurements using ac magnetic fields and a lock-in detection of the Hall voltage for field effect transistors with ultrathin MoS₂ channels. The ac Hall effect measurements have some advantages over the dc measurements. The carrier concentration and the Hall mobility were estimated as a function of gate voltage from the results of the ac Hall effect measurements. They used a magnetic field strength that was lower by two orders of magnitude than those used in prior studies on MoS₂ devices, which relied on dc magnetic fields.     

NiSi金属栅电学特性的热稳定性研究

研究了NiSi金属栅的各种电学特性及其热稳定性，提出一个物理模型用于解释当形成温度大于500 ℃时NiSi功函数随退火温度升高而增大的现象.测量了不同退火温度形成的NiSi材料的方块电阻，当退火温度大于400 ℃时，方块电阻达到最小，并在400—600 ℃范围内稳定.比较各种温度下形成的NiSi材料X射线衍射谱的变化，说明温度在400—600 ℃范围内NiSi相为最主要的成分.制备了以NiSi为金属栅的金属氧化物半导体电容.通过等效氧化层电荷密度及击穿电场E_bd的分布研 关键词： 金属栅 NiSi 炉退火 快速热退火     

Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity σ of single crystals of lithium heptagermanate Li₂Ge₇O₁₅ is studied in an electric field in the frequency range 0.5–100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity σ with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li₂Ge₇O₁₅ compound.