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

TL;DR

This study uses microwave microscopy to map the spatial conductivity changes of black phosphorus over time, revealing degradation mechanisms and how encapsulation with hafnium oxide or boron nitride affects stability.

Contribution

It introduces a microwave impedance microscopy method to spatially monitor black phosphorus degradation and compares effects of different encapsulation layers on stability.

Findings

Conductivity of bare black phosphorus degrades within a day.

Hafnium oxide encapsulation significantly slows degradation, stable over a week.

Boron nitride encapsulation alters degradation to a edge-dominated, diffusive process.

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.