Probing the Electronic States in Black Phosphorus Vertical Heterostructures

TL;DR

This paper uses vertical black phosphorus heterostructures and capacitance spectroscopy to explore electronic states, revealing disorder effects, temperature-dependent phenomena, and defect-related charge traps impacting device performance.

Contribution

It introduces a method to probe electron states in monolayer and few-layer phosphorene at low temperatures, highlighting disorder effects and defect states with combined experimental and theoretical analysis.

Findings

Electronic states are accessible over wide temperature and frequency ranges.

Disorders cause exponential band tails in phosphorene.

Charge trap and defect states explain temperature-dependent electron populations.

Abstract

Atomically thin black phosphorus (BP) is a promising two-dimensional material for fabricating electronic and optoelectronic nano-devices with high mobility and tunable bandgap structures. However, the charge-carrier mobility in few-layer phosphorene (monolayer BP) is mainly limited by the presence of impurity and disorders. In this study, we demonstrate that vertical BP heterostructure devices offer great advantages in probing the electron states of monolayer and few-layer phosphorene at temperatures down to 2 K through capacitance spectroscopy. Electronic states in the conduction and valence bands of phosphorene are accessible over a wide range of temperature and frequency. Exponential band tails have been determined to be related to disorders. Unusual phenomena such as the large temperature-dependence of the electron state population in few-layer phosphorene have been observed and systematically studied. By combining the first-principles calculation, we identified that the thermal excitation of charge trap states and oxidation-induced defect states were the main reasons for this large temperature dependence of the electron state population and degradation of the on-off ratio in phosphorene field-effect transistors.