Yaoqiang Zhou1,Lei Tong1,Yue Pang1,Jianbin Xu1
The Chinese University of Hong Kong1
Yaoqiang Zhou1,Lei Tong1,Yue Pang1,Jianbin Xu1
The Chinese University of Hong Kong1
Two-dimensional (2D) materials are envisaged to extend Moore’s Law and ultimate the down-scaling transistor due to their ultra-thin bodies and dangling-bong-free surfaces. Compared to conventional silicon device, the 2D transistor also show a unique doping-free polarity control (p-type or n-type) which is achieved by contact engineering. These 2D transistors are regarded as the Schottky-barrier-field-effect transistor (SBFET) in which the selective carrier injection of holes or electrons is determined by the Schottky barrier formed at the metal/semiconductor (MS) interfaces. However, the SBFETs usually show intrinsic ambipolar behaviors which limit their applications in low-power consumption electronics due to the relatively low on/off ratio and high leakage current in the off state.<br/>The optimization of contact materials of 2D SBFETs is the efficient approach to achieving the ambipolar to unipolar polarity conversion and suppressing off-state leakage current, but it is hindered by the Fermi level pinning (FLP) effect. Emerging 2D layered semimetals such as graphene and WTe<sub>2</sub> provide a state-of-the-art approach to building the van der Waals MS interface and optimizing the performance of 2D SBFET due to the immunity to FLP effect.<br/>Here, we report on a unipolarity-tunable WSe<sub>2</sub> SBFET with asymmetric semimetal WTe<sub>2</sub> and graphene contacts, in which the WTe<sub>2</sub> was synthesized by the one-step MoTe<sub>2</sub> epitaxial growth. By using the graphene/WTe<sub>2</sub> asymmetric contacts, the WSe<sub>2</sub> SBFET shows the arbitrary carrier polarity determined by the drain bias. The leakage currents of SBFET were effectively suppressed to below 10<sup>-13 </sup>A and showed the controllable <i>I</i><sub>ds </sub>on/off ratios with a maximum of 10<sup>6</sup>. Benefiting from the immunity to the FLP effect, the n-type and p-type contact formed through applying positive and negative source-drain voltage <i>V</i><sub>ds</sub>: An electron Schottky barrier height Φ<sub>B-n</sub>= 90 meV and a hole Schottky barrier height Φ<sub>B-p</sub>= 140 meV was achieved when <i>V</i><sub>ds</sub> = 1V and <i>V</i><sub>ds</sub> = -1V, respectively. The SBFET can work as the gate-modulated Schottky rectifier showing the near-unity ideality factors of ~1.0, an ultralow reverse tunneling current of 10<sup>-15</sup> A, and a high rectifying ratio of 3×10<sup>6</sup>. Furthermore, with electrically gate-tuned band engineering, the asymmetric SBFET showed a reconfigurable photovoltaic performance in which the sign of the responsivity and external quantum efficiency be substantially tuned. This WTe<sub>2</sub>/graphene contact strategy is generally applicable to other 2D materials such as WTe<sub>2</sub>/MoS<sub>2</sub> gate-tunable n-type Schottky diode. This general modulating approach of carrier injection in 2D materials provides an alternative method to reduce the logic-circuit complexity and enhance the performance of the reconfigurable photodetector.<br/><br/>The work is in part supported by Research Grants Council of Hong Kong, particularly, via Grant Nos. AoE/P-701/20, 14206721, N_CUHK438/18, RGC Postdoctoral Fellowship, and CUHK Group Research Scheme.