Unipolar n-Type Conduction in Black Phosphorus Induced by Atomic Layer Deposited MgO

Two-dimensional black phosphorus (BP) holds great promise for future nanoscale field-effect transistors owing to its finite bandgap and expected high carrier mobility. Nevertheless, while most studies have reported unipolar p-type or ambipolar BP transistors with a hole conduction dominance, excellent n-type transport in BP has been a critical challenge. Here, we report unipolar n-type BP transistors realized by a MgO capping layer via atomic layer deposition. By coverage with 20-nm-thick MgO, remarkable electron conduction is obtained in transistors fabricated on BP flakes of thicknesses from 2.6 to 7.6 nm, accompanied by fully suppressed hole transport. Furthermore, the unipolar electron transport is found to retain (or even improve) after a period of six months, with the highest extrinsic electron mobility reaching 135.9 cm2/Vs. The effective approach to realizing unipolar n-FETs of BP demonstrated in this letter paves the way to the implementation of BP-based CMOS digital logic circuits.