Epitaxy of NiTe on WS for the p-Type Schottky Contact and Increased Photoresponse.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have great potential applications in the electronic and optoelectronic devices. Nevertheless, due to the difficulty in the efficient doping of atomic-thickness TMDCs or Fermi level pinning (FLP) effects at the metal/semiconductor interface, most TMDC devices exhibit the n-type conduction polarity, which significantly limits their functional applications based on the p-n junction. Here, 2D semi-metal NiTe nanosheets were epitaxially grown on the WS monolayer by a two-step chemical vapor deposition route. The microstructure and optical characterizations confirm that the vertically stacked NiTe/WS heterostructures are formed by van der Waals epitaxy. Interestingly, p-type WS field-effect transistors can be obtained with the hole mobility of ∼4.22 cm/V·s, when the epitaxial NiTe sheets act as the source/drain electrodes. This is attributed to the decreased FLP effect and hence the low potential barrier for holes at the van der Waals contacts. Furthermore, the photodetectors based on the heterostructures show a 2 orders of magnitude increase in the switch ratio, responsivity, and detectivity and a 1 order of magnitude increase in the rise and decay speeds relative to those based on pristine WS. This work paves the way to realize the p-type contact for monolayer WS with significantly enhanced optoelectronic performance.