High-performance p-type field-effect transistors using substitutional doping and thickness control of two-dimensional materials

In silicon field-effect transistors (FETs), degenerate doping of the channel beneath the source and drain regions is used to create high-performance n- and p-type devices by reducing the contact resistance. Two-dimensional semiconductors have, in contrast, relied on metal-work-function engineering. This approach has led to the development of effective n-type 2D FETs due to the Fermi-level pinning occurring near the conduction band, but it is challenging with p-type FETs. Here we show that the degenerate p-type doping of molybdenum diselenide and tungsten diselenide—achieved through substitutional doping with vanadium, niobium and tantalum—can reduce the contact resistance to as low as 95 Ω µm in multilayers. This, though, comes at the cost of poor electrostatic control, and we find that the doping effectiveness—and its impact on electrostatic control—is reduced in thinner layers due to strong quantum confinement effects. We, therefore, develop a high-performance p-type 2D molybdenum diselenide FET using a layer-by-layer thinning method to create a device with thin layers at the channel and thick doped layers at the contact regions.