The potential of 2D materials in future CMOS technology is hindered by the lack of high-performance p-type field effect transistors (p-FETs). While utilization of the top-gate (TG) structure with a p-doped spacer area offers a solution to this challenge, the design and device processing to form gate stacks pose serious challenges in realization of ideal p-FETs and PMOS inverters. This study presents a novel approach to address these challenges by fabricating lateral p+-p-p+ junction WSe2 FETs with self-aligned TG stacks in which desired junction is formed by van der Waals (vdW) integration and selective oxygen plasma-doping into spacer regions. The exceptional electrostatic controllability with a high on/off current ratio and small subthreshold swing (SS) of plasma doped p-FETs is achieved with the self-aligned metal/hBN gate stacks. To demonstrate the effectiveness of our approach, we construct a PMOS inverter using this device architecture, which exhibits a remarkably low power consumption of approximately 4.5 nW.

A great challenge is presented when metals have contact with a 2D semiconducting material because the contact resistances (Rc) induced at the metal‐graphene interfaces hinder the performances of 2D devices, and therefore low resistance Ohmic contacts need to be developed to achieve unique and high performance of the 2D devices. This study demonstrates that edge‐contacted graphene devices of multiple stacked 2D hetero‐structures with hexagonal boron nitride (hBN) exhibit superior performances in carrier transport across channel and contact regions, compared to surface‐contacted devices. In surface‐contacted graphene devices, Rc and contact resistivity (ρc) are calculated by applying the modified transfer length (LT*) obtained from the contact‐end‐resistance method, while Rc and ρc in edge‐contacted graphene devices are estimated by replacing the LT* with the thickness of graphene. The edge‐contacted device is fabricated via a controlled plasma etching that allows each layer of graphene and hBN consisting hetero‐structures to be removed evenly at a uniform speed. Four‐point probe measurements are conducted in addition to transmission line method and confirms that ρc is lower for edge contact than surface contact. ρc of a graphene edge‐contacted device (≈10 Ω µm2) is much lower than that of a surface‐contacted device (≈230 Ω µm2).

It becomes clear that, in two-dimensional (2D) materials-based devices, sheet resistances underneath electrodes change due to a metallic contact, leading to substantial errors in determining a transfer length. Thus, the extraction of transfer length and corresponding contact resistivity must be revisited to assess the performance of 2D devices. In this study, we present the three different approaches of determining the contact resistivity in 2D WSe 2 field effect transistors for the first time by theoretical analysis using the resistive network model as well as electrical measurements using the contact-end resistance and transfer length methods, based on the followings: (a) contact resistance multiplied by transfer length ( R cf W ⋅ L Tk ), (b) integrated contact resistance ( ∫ 0 L R ( x ) ⋅ W d x ), and (c) contact resistance raised to a constant power R cf W α ). These different extraction methods give rise to almost identical transfer length and contact resistivity, validating our model and its accuracy from the results obtained by using various contact metals and plasma doping conditions. This work serves as a foundation for future research on the determination of physical parameters responsible for the carrier transport at the metallic contact interface in 2D semiconductor devices.

Effective control of 2D transistors polarity is a critical challenge in the process for integrating 2D materials into semiconductor devices. Herein, a doping‐free approach for developing tungsten diselenide (WSe 2 ) logic devices by utilizing the van der Waals (vdWs) bottom electrical contact with platinum and indium as the high and low work function metal respectively is reported. The device structure is free from chemical disorder and crystal defects arising from metal deposition, which enables a near ideal Fermi‐level de‐pinning. With effective controllability of device polarity through metal work function change, a complementary metal‐oxide‐semiconductor field effect transistor inverter with a gain of 198 at a bias voltage of 4.5 V is achieved. This study demonstrates an ultrahigh performance 2D inverter realized by controlling the device polarity from using Fermi‐level pinning‐free vdWs bottom contacts.

We investigate the development of gate-modulated tungsten diselenide (WSe2)-based lateral pn-homojunctions for visible and near-infrared photodetector applications via an effective oxygen (O2) plasma treatment. O2 plasma acts to induce the p-type WSe2 for the otherwise n-type WSe2 by forming a tungsten oxide (WOx) layer upon O2 plasma treatment. The WSe2 lateral pn-homojunctions displayed an enhanced photoresponse and resulted in open-circuit voltage (VOC) and short-circuit current (ISC) originating from the pn-junction formed after O2 plasma treatment. We further notice that the amplitude of the photocurrent can be modulated by different gate biases. The fabricated WSe2 pn-homojunctions exhibit greater photoresponse with photoresponsivities (ratio of the photocurrent and incident laser power) of 250 and 2000 mA/W, high external quantum efficiency values (%, total number of charge carriers generated for the number of incident photons on photodetectors) of 97 and 420%, and superior detectivity values (magnitude of detector sensitivity) of 7.7 × 109 and 7.2 × 1010 Jones upon illumination with visible (520 nm) and near-infrared lasers (852 nm), respectively, at low bias (Vg = 0 V and Vd = 1 V) at room temperature, demonstrating very high-performance in the IR region superior to the contending two-dimensional material-based photonic devices. These superior optoelectronic properties are attributed to the junctions induced by O2 plasma doping, which facilitate the effective carrier generation and separation of photocarriers with applied external drain bias upon strong light absorption.

In this study, a simple and controllable chlorine doping method of MoS2 using a remote inductively coupled plasma (ICP) was studied and the effect of doping on the properties of MoS2 was investigated by adjusting the work function of MoS2.

Tungsten diselenide (WSe2) is attracting attention because of its superior electronic and optoelectronic properties. In recent years, the number of research works related to the WSe2-based field-effect transistors (FETs) has increased dramatically. Nonetheless, the performance of 2D WSe(2)is influenced sensitively by metal-semiconductor (MS) interface states, where Fermi-level pinning is substantial. This research explores Fermi-level depinning by doping with an n-type polymer. In this work, spin-coated polyvinyl alcohol (PVA) is used as an n-type dopant for achieving low-contact-resistance WSe2FETs in cases of both high-work-function (Pd) and low-work-function (In) metals. Interestingly, the increase in the Schottky barrier height resulting from the application of PVA gives rise to Fowler-Nordheim tunneling for a doped Pd-WSe(2)contact. By contrast, only direct tunneling is observed for an In-WSe(2)contact irrespective of whether the dopant is used. The barrier-height modification after doping reveals that the improvement of the contact resistance is correlated to the enhancement of tunneling current after doping, which is consistent with the measurement results. This work suggests a practical direction for contact engineering of future WSe2-based electronic devices and expands the current understanding of charge transport at the MS contact when a polymeric n-type dopant is applied.

2D molybdenum ditelluride (MoTe 2 ) has recently received significant attention due to its unique phase transition and ambipolar behavior as well as thickness‐dependent bandgap. The phase transition and electrical breakdown of various thickness MoTe 2 field‐effect transistors observed under high electric fields are addressed. Interestingly, the MoTe 2 exhibits phase transition from a semiconducting 2H phase to a metallic 1T′ almost simultaneously with electrical breakdown, and this is confirmed by a Raman peak of 1T′‐MoTe 2 at 125 cm −1 . Using Raman mapping results of MoTe 2 FETs obtained after the breakdown, it is revealed that the phase transition is initiated from the metal contacting electrode regions of source and drain. All the Raman peaks of MoTe 2 shifted to low frequency with increasing drain voltage. Based on the Raman peak shifts, the temperature change in the MoTe 2 FETs while device operation is in progress is estimated. The maximum temperature and dissipated power of a tri‐layer MoTe 2 device are found to reach 495 K and 5.85 mW, respectively, at an electric field of 6.5 V µm −1 . This research provides guidelines for circuit design toward the application of 2D semiconductor devices, related to the energy dissipation and electrical breakdown unique to 2D phase transitional materials.

Electrical metal contacts formed with 2D materials strongly affect device performance. Here, we used scanning transmission electron microscopy (STEM) and energy-dispersive spectroscopy (EDS) to characterize the interfacial structure formed and physical damage induced between MoS2 and the most commonly used metals, Ti, Cr, Au, and Pd. We further correlated the electrical performance with physical defects observed at the 2D interfacial structure. The contact resistances were higher in the order of Ti, Au, Pd, and Cr contacts, but all 4-point probe mobilities measured with metals in contact with identical quadrilayer MoS2 were ∼65 cm2 V-1 s-1, confirming the reliability of the devices. According to the STEM and EDS analyses, the Ti contact gave rise to a van der Waals gap between the clean quadrilayer MoS2 and the Ti contact. By contrast, Cr migrated into MoS2 while Mo and S counter-migrated into the SiO2 substrate. Au and Pd formed glassy layers that resulted in the migration of Mo and S into the Au and Pd electrodes. These interfacial structures between MoS2 and contact metals strongly correlated with the electrical performance of 2D MoS2 FETs, providing practical guidelines to form van der Waals contacts.

Tungsten diselenide (WSe2) has received significant attention because it shows the pristine ambipolar property arising from the Fermi level located near the midgap and can be converted to uni-polar form. In this study, we observe the formation of tungsten oxide (WOx) on the WSe2 surface after oxygen plasma treatment and show that the p-type WOx dopes WSe2. In our devices that underwent plasma treatment, it was interesting to find a strong correlation between the changes in the work function of WSe2 and a gold electrode, and the channel and contact resistances. The channel resistance changes very sensitively at a rate of 64 meV per dec with the increase in the WSe2 channel work function, which is close to the thermal limit; this indicates the defect-free oxidized WSe2 channel. The carrier transport in the oxidized WSe2 FET is shown to change to a high performance p-type device with greatly reduced channel and contact resistances with the increase in the plasma oxidation time.

Two‐dimensional (2D) black phosphorus (BP) has attracted increasing interest for next‐generation solid‐state device applications due to its unique blend of versatile properties. The ultrathin physique and low thermal conductivity (40–20 Wm −1 K −1 ) of BP make it susceptible to premature Joule breakdown under moderate electric field induced by inefficient and nonhomogeneous energy dissipation. Here, it is reported that the back‐gate BP device suffers Joule breakdown merely under 4 MV m −1 electric field value with the centrally localized fracture. The spatial micro‐Raman spectroscopy confirms uneven thermal spreading in BP channel with the center being 20% hotter than the lateral ends. Furthermore, to mitigate the early breakdown and uneven spreading, vertical van der Waals structure is assembled. The results show that the vertical BP device exhibits 230 times higher field strength and one order enhancement in power sustainability than those of lateral devices due to the integration of thermally favorable constituent materials and formation of the optimal path for self‐heat removal.

Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

Diverse diode characteristics were observed in two-dimensional (2D) black phosphorus (BP) and molybdenum disulfide (MoS2) heterojunctions. The characteristics of a backward rectifying diode, a Zener diode, and a forward rectifying diode were obtained from the heterojunction through thickness modulation of the BP flake or back gate modulation. Moreover, a tunnel diode with a precursor to negative differential resistance can be realized by applying dual gating with a solid polymer electrolyte layer as a top gate dielectric material. Interestingly, a steep subthreshold swing of 55 mV/dec was achieved in a top-gated 2D BP-MoS2 junction. Our simple device architecture and chemical doping-free processing guaranteed the device quality. This work helps us understand the fundamentals of tunneling in 2D semiconductor heterostructures and shows great potential in future applications in integrated low-power circuits.