The Role of Air Adsorption in Inverted Ultrathin Black Phosphorus Field-Effect Transistors

Few-layer black phosphorus (BP) attracts much attention owing to its high mobility and thickness-tunable band gap; however, compared with the commonly studied transition metal dichalcogenides (TMDCs), BP has the unfavorable property of degrading in ambient conditions. Here, we propose an inverted dual gates structure of ultrathin BP FET to research the air adsorption on BP. In fabrication process of back-gate BP FET, BP was transferred directly onto a wafer covered with electrodes. Thus, we can exclude the BP degradation during the process of electrodes fabrication, such as electron beam lithography (EBL) and thermal evaporation process. Furthermore, without any electrode covering BP, BP could be in full contact with the air; then the accurate effect of the air adsorption on BP can be researched in detail. The results clearly show that annealing can remove the p-doping resulted from the metastable oxygen adsorbed on the surface of BP, but the adsorption can be restored in a few hours exposure. In addition, both back and top gate inverted BP FETs exhibit a favorable performance. Therefore, this inverted structure is also an optional structure to reduce the influence of the instability of BP devices.