Locally Doped Transferred Contacts for WSe2 Transistors

Abstract

While two-dimensional (2D) materials have shown great promise for scaling technology nodes beyond the limits of silicon devices, key challenges remain for realizing high-quality and practical 2D field-effect transistors (FETs), including lowering contact resistance, demonstrating device structures with high electrical stability, reducing interface charge trapping, and integrating n- and p-FETs for beyond-complementary metal oxide semiconductor devices. High contact resistance often stems from Schottky contacts and Fermi level pinning and can be reduced by local doping or transferred contacts, respectively. However, these approaches to date have been mutually incompatible. Here, we combine both into a single structure and demonstrate a locally doped, transfer-contact stack containing access regions adjacent to the metal via contacts embedded in hexagonal boron nitride. Doping is applied by oxygen plasma treatment of access regions, while the fully encapsulated WSe₂ channel remains pristine, creating a lateral p⁺–i–p⁺ junction. We demonstrate a reduction in contact resistance by up to >30,000 times with the contact strategy, with a lowest individual contact resistance of ∼3.6 kΩ · μm, limited by the doping density at the contacts. Our results highlight increasing doping in the contact region as being crucial for achieving improved contact resistance in p-type WSe₂ devices. For our FET devices, the geometry of gates, doped access regions, and the channel are all defined by an electron beam lithography giving full and precise control over size and position. The p-FET behavior is strongly enhanced with a high on/off ratio up to 10⁷, but ambipolar characteristics from the intrinsic channel are still retained. Negligible, temperature-independent hysteresis is achieved from T = 10 to 300 K, with only back gate carrier control. High electrical stability is evident in the excellent reproducibility of transfer characteristics between multiple contact sets on a single device and different devices. The doping reduces contact resistance by reducing the Schottky barrier height and width, achieving Ohmic IV characteristics. The doping appears very stable, with negligible degradation of performance, keeping the device for 50 days in atmosphere. This reasonably simple device structure incorporates two important strategies to enhance contact quality, improving p-FET performance and retaining intrinsic channel quality.