Cubic BN optical gap and intragap optically active defects

TL;DR

This study comprehensively characterizes the optical properties of cubic boron nitride (c-BN), revealing its optical gap exceeds 10 eV, and investigates defect-related emissions and potential single-photon sources using advanced microscopy and spectroscopy techniques.

Contribution

The paper provides the first detailed experimental and theoretical analysis of c-BN's optical gap and defect emissions, including nanoscale localization and single-photon emission potential.

Findings

Optical gap of c-BN exceeds 10 eV.

Nanoscale defect emissions with tens of nanometers localization.

Evidence of single-photon emission behavior.

Abstract

We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV. Further theoretical analysis in the framework of the Bethe-Salpeter equation of many-body perturbation theory supports this result. The spatial localization of defect-related emissions has been investigated using nanometric resolved cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a c-BN powder, we have promoted phase transitions in nanometric domains which have been detected by the appearance of specific hexagonal-phase signatures in both EELS and CL spectra. A high number of intragap optically active centers are known in c-BN, but the literature is rather scattered and hence has been summarized here. For several emission lines we have obtained nano-CL maps which show emission spot sizes as small as few tens of nanometers. Finally, by coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have addressed individual spots in order to identify the possible presence of single-photon sources. The observed CL bunching effect is compatible with a limited set of single-photon emitters and it permits obtaining emission lifetimes of the order of the nanosecond.