Band gap measurements of monolayer h-BN and insights into carbon-related point defects

TL;DR

This study provides direct measurements of the electronic band gap and exciton binding energy of monolayer h-BN, revealing the influence of carbon-related point defects on its optical and electronic properties.

Contribution

It offers new insights into the electronic structure and defect states of monolayer h-BN using STM, STS, and optical spectroscopy, which were previously elusive.

Findings

Band gap of defect-free monolayer h-BN is 6.8 eV.

Exciton binding energy is approximately 0.7 eV.

Carbon-related defects introduce intragap states around 2.0 eV.

Abstract

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the ultraviolet and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer. Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is $(6.8\pm0.2)$ eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of $(0.7\pm0.2)$ eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.