摘要

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe2 p-type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near-ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe2 channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.

前言

Schottky junctions, which are formed at a metal–semiconductor interface, are characterized by a current rectifying energy barrier. Ideally, the barrier is determined by only the metal work function and semiconductor electron affinity, in a case known as the Schottky–Mott limit. Typically, however, defect states at the metal–semiconductor interface induce Fermi-level pinning in the metal and dictate the energy barrier height.[1] Experiment has approached the Schottky–Mott limit in 2D semiconductors contacted with 3D metal contacts.[2,3] However, despite theoretical predictions,[4,5] experimental observation of the Schottky–Mott limit using 2D metals has been illusive.[6–20] Here, we report measurements on a boron-nitride-passivated graphene–tungsten diselenide (WSe2) Schottky junction which exhibits near-ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe2, enabling experimental verification of the Schottky–Mott limit in a single device. Utilizing this exceptional gate control, we demonstrate a dynamically tunable diode ideality factor which is enabled by the lack of Fermi-level pinning in our device. Our results provide a pathway for defect-free electrical contact to 2D semiconductors and open up possibilities for circuits with efficient switching characteristics and higher efficiency optoelectronic devices.

Van der Waals (vdW) heterostructures,[21] especially when passivated with hexagonal boron nitride ($h$-BN),[22,23] present an excellent platform for studying the Schottky–Mott limit. Graphene,[24,25] a semimetal with a gate-tunable work function,[26] is a promising alternative to traditional bulk metal electrical contacts to 2D semiconductors.[16] In lieu of using different metals, we propose a modified Schottky–Mott rule for gate-tunable Schottky junctions in which the gate voltage ($V_{\rm{G}}$) directly modulates the barrier height ($\Phi_{\rm{B}}$), $\left | \frac{\mathrm{d} \Phi_{\rm{B}}}{\mathrm{d} V_{\rm{G}}} \right | = S_{\rm{G}}$. When $S_{\rm{G}} = 1$, the system is operating at the Schottky–Mott limit. Here, we present measurements on a gated graphene–WSe2 Schottky junction for which $S_{\rm{G}} \approx 1$.

结论

Our results demonstrate verification of the Schottky–Mott rule in a single device. We provide an avenue for studying the Schottky–Mott limit in gated Schottky junctions, circumventing the requirement for fabricating many separate devices. The ability to create unpinned graphene–2D semiconductor junctions within the existing vdW heterostructure framework will enable researchers to probe exotic physics requiring high-quality electrical contact. Furthermore, our gated Schottky diodes result in a tunable effective ideality factor. Tuning $n_{\rm{Eff}} \lt 1$ will enable new circuits with efficient switching characteristics. Finally, tuning $n_{\rm{Eff}} \gt 1$ while minimizing the reverse bias leakage current is promising for creating higher-efficiency PV devices.