Self-Aligned Top-Gate Structure in High-Performance 2D p-FETs via van der Waals Integration and Contact Spacer Doping.

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.

Gate-Modulated Ultrasensitive Visible and Near-Infrared Photodetection of Oxygen Plasma-Treated WSe2 Lateral pn-Homojunctions.

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.

Metallic contact induced van der Waals gap in a MoS2 FET.

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.

The device level modulation of carrier transport in a 2D WSe2 field effect transistor via a plasma treatment.

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.

Colossal grain growth yields single-crystal metal foils by contact-free annealing.

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.

Modulation of Quantum Tunneling via a Vertical Two-Dimensional Black Phosphorus and Molybdenum Disulfide p-n Junction.

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.

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides.

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.