Control of Carrier Polarity and Schottky Barriers in Layered PdSe2 Field-Effect Transistors

Palladium diselenide (PdSe2) has been discovered as an intriguing two-dimensional (2D) semiconductor for its unique pentagonal crystalline structure, electrical and optical anisotropy, thickness-modulated band gap, robust air stability, and high carrier mobility, demonstrating great potential in field-effect transistors (FETs), photodetectors, and thermoelectric devices. Controlling the carrier polarity and electrical contact Schottky barriers is of great significance for realizing high-performance PdSe2 optoelectronic devices. Here, by combining work-function measurement and electrical transport, we observe a thickness-modulated carrier polarity transition in layered PdSe2 FETs from n-type unipolar to intrinsic ambipolar and p-type unipolar behavior by simply varying the number of layers of PdSe2. Due to the weak Fermi-level pinning in few-layer PdSe2, n-type and ambipolar FETs are achieved by contacting PdSe2 to Sc, Ti, and Pd electrodes, respectively. The Schottky barrier heights are determined by temperature-dependent transport, showing a 15 meV barrier for the Sc contact at the electron side and 62 and 93 meV barriers for Ti and Pd contacts, both at the hole side, respectively. We further demonstrate ozone-treatment-induced hole doping in PdSe2, which effectively converts the PdSe2 FETs from n-type to p-type. The doping is robust in the ambient environment over 3 months. In-plane homojunction on a PdSe2 flake is realized by selective ozone doping, demonstrating typical diode rectifying behavior. The ability to control the carrier polarity and Schottky barriers in layered PdSe2 makes this 2D semiconductor highly feasible for complementary metal-oxide semiconductor device integration and high-performance photodetectors based on atomically thin p–n junctions.