Vertical Diodes

Precise doping-profile engineering in van der Waals (vdW) heterostructures is a key element to promote optimal device performance in various electrical and optical applications with two-dimensional vdW materials. Here, we report tungsten diselenide-(WSe₂) based pure vertical diodes with atomically defined p-, i- and n-channel regions. Externally modulated p- and n-doped layers are respectively formed on the bottom and the top facets of WSe₂ single crystals by direct evaporations of high and low work-function metals platinum and gadolinium. Since metal-induced dopings are restricted to the first few layers of WSe₂, atomically sharp p-i-n heterojunctions are naturally formed in the homogeneous WSe₂ layers. As the number of layers increases, charge transport through the vertical WSe₂ p-i-n heterojunctions is characterized by a series of quantum tunneling events, namely direct tunneling, Fowler–Nordheim (FN), and Schottky emission (SE). With optimally selected WSe₂ thickness, where FN and SE tunneling events prevail, our vertical heterojunctions show superb diode characteristics of an unprecedentedly high current density (≥ 2 × 10⁵ A/cm²) and low turn-on voltages while maintaining good current rectification.