Effects of Al2O3 Capping and Post-Annealing on the Conduction Behavior in Few-Layer Black Phosphorus Field-Effect Transistors

Ambient instability has been proven challenging in black phosphorus field-effect transistors (BP FETs) and a capping layer is thus needed for their practical applications. In this paper, we have examined the effects of Al₂O₃ capping and O₂ post-annealing on the conduction characteristic of BP FETs. With the Al₂O₃ capping, it forms p-type into ambipolar transport and the electron mobility dramatically increases to 20-110 cm²/V·s in our case. Interestingly, with the O₂ post-annealing, the transport can be tuned from ambipolar back to p-type as the annealing time extends. It is attributed that the Al₂O₃ capping introduces an n-type doping in BP channel while the O₂ post-annealing dopes BP back into p-type. Moreover, after the O₂ post-annealing the interfacial POx might be formed, resulting in the degradation of subthreshold swing and on/off current ratio.