Deep-UV reflectivity in hexagonal boron nitride: from monolayer to bulk samples

Abstract

Hexagonal boron nitride (hBN) is an ultrawide bandgap semiconductor with a large range of basic applications relying on its low dielectric constant, high thermal conductivity, and chemical inertness. The growth of high-quality crystals in 2004 has revealed that hBN is also a promising material for light-emitting devices in the deep ultraviolet domain, as illustrated by the demonstration of lasing at 215 nm by accelerated electron excitation [1], and also the operation of field emitter display-type devices in the deep ultraviolet [2]. With a honeycomb structure similar to graphene, bulk hBN has gained tremendous attention as an exceptional substrate for graphene with an atomically smooth surface, and more generally, as a fundamental building block of Van der Waals heterostructures [3]. I will discuss here our recent measurements by reflectivity spectroscopy shining a new light on the efficient light-matter interaction in hBN. I will first present experiments in monolayer hBN epitaxially grown on graphite. Compared to the reflectivity spectrum of the bare graphite substrate, a huge contrast is observed in the reflectivity of atomically-thin hBN deposited on graphite, demonstrating a radiative efficiency close to unity in monolayer hBN. I will then address the optoelectronic properties of bulk hBN where high-resolution measurements in high-quality samples allow to resolve the respective contributions of indirect and direct optical transitions and quantify the strength of the light-matter interaction in bulk hBN.