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

Abstract

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.

Tunable Schottky barriers in ultrathin black phosphorus field effect transistors via polymer capping

“…The unchanged Raman spectra suggest that the PMMA capping can protect the BP flake from degradation effectively. In addition, it is known that the degradation can reduce the field effect mobilities by an order of magnitude [19,55,56]. Hence, a standard BP MOSFET was fabricated to investigate the protective effect of the PMMA layer.…”

Thermal sublimation: a scalable and controllable thinning method for the fabrication of few-layer black phosphorus

“…Though the oxidation of BP caused by trace impurities such as water and oxygen in nitrogen gas during heating might take place, we attributed the self-separation of BP was the main reason for the sublimation instead of oxidationsublimation. The oxidation conditions have been widely studied before: Favron et al [41] first indicated that light, oxygen and water were three factors cause the oxidation of BP; Luo et al [18] reported that with 5% oxygen/Ar or 2.3% H 2 O/Ar, the oxidation rate is <5 Å for 5 h from XPS results; Wang et al [42] reported BP could remain stable in pure water without the presence of oxygen molecules from nuclear magnetic resonance spectroscopy results; Li et al [43] reported that heating could remove the metastable oxygen adsorbed on the surface of BP. Therefore, according to those studies on oxidation processes and mechanisms of BP, it could be concluded that with the protection of high purity (99.995%) nitrogen gas, the sublimation of BP in this work could be described as a physical process of layer separation.…”

“…here we eliminated the oxidation of BP as the main factor facilitating sublimation. Favron 39 first indicated that light, oxygen and water were the three factors cause the oxidation of BP; Luo 17 reported that with 5% oxygen/Ar or 2.3% H 2 O/Ar, the oxidation rate is < 5Å for 5h from XPS results; Wang 40 reported BP could retain stable in pure water without the presence of oxygen molecules from nuclear magnetic resonance (NMR) spectroscopy results; Li 41 reported that annealing could remove the metastable oxygen adsorbed on the surface of BP. Hence according those studies on oxidation mechanism of BP, as well as considering protection of high purity (99.995%) nitrogen gas in this work, we attributed the self-separation of BP was the main reason for the sublimation.…”

Recovery of the Pristine Surface of Black Phosphorus by Water Rinsing and Its Device Application

“…Despite all of the advantageous properties, the lack of stability under ambient conditions limits further study of BP and its implementation. The previous research on degradation of BP proved that BP starts to degrade immediately once exposed to oxygen, water, and light, resulting in the formation of “bubbles” on the surface of BP. − Especially, oxygen plays a critical role in the degradation process and changing the properties of BP; theoretically, oxygen molecules change the BP surface hydrophilic so that water can easily interact with the oxidized surface . The degradation process has been observed as follows.…”