Germanium-doped Metallic Ohmic Contacts in Black Phosphorus Field-Effect Transistors with Ultra-low Contact Resistance

In this work, we demonstrate for the first time an ultra-low contact resistance few-layered black phosphorus (BP) transistor with metallic PGe(x) contacts formed by rapid thermal annealing (RTA). The on-state current of the transistor can be significantly improved and the I(ON)/I(OFF) ratio increases by almost 2 order. The hole mobility is enhanced by 25 times to 227 cm(2)V(−1)s(−1). The contact resistance extracted by the transfer length method is 0.365 kΩ∙μm, which is the lowest value in black phosphorus transistors without degradation of I(ON)/I(OFF) ratio. In addition, the I-V curve of the transistor with PGe(x) contact is linear compared to that with Ti contact at 80 K, indicating that a metallic ohmic contact is successfully formed. Finally, X-ray photoelectron spectroscopy is used to characterize the PGe(x) compound. A signal of P-Ge bond is first observed, further verifying the doping of Ge into BP and the formation of the PGe(x) alloy.