  Single photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered
significant attention over the last few years due to their superior optical
properties. However, despite the vast range of experimental results and
theoretical calculations, the defect structure responsible for the observed
emission has remained elusive. Here, by controlling the incorporation of
impurities into hBN and by comparing various synthesis methods, we provide
direct evidence that the visible SPEs are carbon related. Room temperature
optically detected magnetic resonance (ODMR) is demonstrated on ensembles of
these defects. We also perform ion implantation experiments and confirm that
only carbon implantation creates SPEs in the visible spectral range.
Computational analysis of hundreds of potential carbon-based defect transitions
suggest that the emission results from the negatively charged VBCN- defect,
which experiences long-range out-of-plane deformations and is environmentally
sensitive. Our results resolve a long-standing debate about the origin of
single emitters at the visible range in hBN and will be key to deterministic
engineering of these defects for quantum photonic devices.
