How Important Is the Metal-Semiconductor Contact for Schottky Barrier Transistors: A Case Study on Few-Layer Black Phosphorus?

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Black phosphorus (BP) is a recently rediscovered layered two-dimensional (2D) semiconductor with a direct band gap (0.35–2 eV), high hole mobility (300–5000 cm2/Vs), and transport anisotropy. In this paper, we systematically investigated the effects of metal–semiconductor interface/contacts on the performance of BP Schottky barrier transistors. First, a “clean” metal–BP contact is formed with boron nitride (BN) passivation. It is found that the contact resistance of the clean metal–BP contact is seven times less than the previously reported values. As a result, high-performance top-gate BP transistors show a record high ON-state drain current (Ion) of 940 μA/μm. Second, BN tunneling barriers are formed at the source/drain contacts to help understand the abnormally high OFF-state drain current (Ioff) in devices with clean metal–BP contacts. This high Ioff is attributed to the electron tunneling current from the drain to the channel. Finally, the Ion/Ioff of BP field-effect transistors can be significantly improved by using an asymmetric contact structure. By inserting a thin BN tunneling barrier at the drain side, Ioff is reduced by a factor of ∼120 with a cost of 20% reduction in Ion. This case study of contacts on BP reveals the importance of understanding the metal–semiconductor contacts for 2D Schottky barrier transistors in general.