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 (R). 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 MoS and MoTe 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 MoS and MoTe, 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 MoS. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that R 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 (MoS) 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-MoS 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.

A systematic modulation of the carrier type in molybdenum ditelluride (MoTe ) field-effect transistors (FETs) is described, through rapid thermal annealing (RTA) under a controlled O environment (p-type modulation) and benzyl viologen (BV) doping (n-type modulation). Al O capping is then introduced to improve the carrier mobilities and device stability. MoTe is found to be ultrasensitive to O at elevated temperatures (250 °C). Charge carriers of MoTe 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 MoTe -transistor performance are confirmed to originate from the physical and chemical absorption and dissociation of O , especially at tellurium vacancy sites. The electron branch is modulated by varying the BV dopant concentrations and annealing conditions. Unipolar n-type MoTe FETs with a high on-off ratio exceeding 10 are achieved under optimized doping conditions. By introducing Al O capping, carrier field effect mobilities (41 for holes and 80 cm V s for electrons) and device stability are improved due to the reduced trap densities and isolation from ambient air. Lateral MoTe 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.

The accessibility of both n-type and p-type MoS2 FET is necessary for complementary device applications involving MoS2. However, MoS2 PFET is rarely achieved due to pinning effect resulting high Rc at metal-MoS2 interface and the inherently strong n-type property of the MoS2 material. In this study, we realized a high-performance multi-layer MoS2 PFET via controllable chemical doping, which has an excellent on/off ratio of 10 7and a maximum hole mobility of 72 cm 2 /Vs at room temperature, and these values are further exceeding to 10 9 and 132 cm 2 /Vs at 133K. In addition, we revealed that large Rc hindered the polar transition of MoS2 FET from n-type to p-type, meanwhile channel Rs