Spatial conductivity mapping of unprotected and capped black phosphorus using microwave microscopy

Abstract

Thin layers of black phosphorus present an ideal combination of a 2D material with a tunable direct bandgap and high carrier mobility. However the material suffers from degradation in ambient conditions due to an oxidation reaction which involves water, oxygen and light. We have measured the spatial profile of the conductivity on flakes of black phosphorus as a function of time using scanning microwave impedance microscopy. A microwave excitation (3 GHz) allows to image a conducting sample even when covered with a dielectric layer. We observe that on bare black phosphorus, the conductivity changes drastically over the whole surface within a day. We demonstrate that the degradation process is slowed down considerably by covering the material with a 10 nm layer of hafnium oxide. It is stable for more than a week, opening up a route towards stable black phosphorus devices in which the high dielectric constant of hafnium oxide can be exploited. Covering black phosphorus with a 15 nm boron nitride flake changes the degradation process qualitatively, it is dominated by the edges of the flake indicating a diffusive process and happens on the scale of days.

Stabilizing ultra-thin black phosphorus with in-situ-grown 1 nm-Al2O3 barrier

“…Following this idea, capping by 2D material layers, such as hexagonal boron nitride and graphene, has been tested using a "flake to flake" transfer technique. 4,8,[13][14][15][16][17][18][19] However, particular care must be taken to ensure both a clean and dry interface and a complete coating by the BP flake to avoid edgeand grain boundary-dominated degradation. Furthermore, this technique is hardly transferrable to a large scale manufacturing technology up to now.…”

“…4,7,20 Aiming at the same result, thick oxide layers grown on BP have been probed [e.g., 100 nm of SiO 2 grown by inductively coupled plasma chemical vapor deposition 21 or 10 nm of HfO 2 grown by atomic layer deposition (ALD) 19 ] and found to secure the environmental stability of BP at a week scale. Also, numerous attempts have focused on the passivation via a relatively thick (10 nm or above) Al 2 O 3 layer grown on top of the BP flakes by ALD.…”

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

“…Previous reports have proved that the BP degradation process can be slowed down considerably by covering the material with appropriate passivation layers, and it is stable for more than a week [17, 18]. Here we passivated the device with h -BN and then fabricated a top-gate.…”

Black Phosphorus: Critical Review and Potential for Water Splitting Photocatalyst

“…The combined trilayer builds the type-II heterojunction alignment and PCE is predicted to be around 18%. In addition to managing recombination rate, heterostructures of the black phosphorus with particular materials such as HfO 2 can slow down the degradation of black phosphorus [70]. …”