  The recent discovery of single-photon emitting defects hosted by the
two-dimensional wide band gap semiconductor hexagonal boron nitride (hBN) has
inspired a great number of experiments. Key characteristics of these quantum
emitters are their capability to operate at room temperature with a high
luminosity. In spite of large theoretical and experimental research efforts,
the exact nature of the emission remains unresolved. In this work we utilize
layer-by-layer etching of multilayer hBN to localize the quantum emitters with
atomic precision. Our results suggest the position of the emitters correlates
with the fabrication method: emitters formed under plasma treatment are always
in close proximity to the crystal surface, while emitters created under
electron irradiation are distributed randomly throughout the entire crystal.
This disparity could be traced back to the lower kinetic energy of the ions in
the plasma compared to the kinetic energy of the electrons in the particle
accelerator. The emitter distance to the surface also correlates with the
excited state lifetime: near-surface emitters have a shorter compared to
emitters deep within the crystal. Finite-difference time-domain and density
functional theory simulations show that optical and electronic effects are not
responsible for this difference, indicating effects such as coupling to surface
defects or phonons might cause the reduced lifetime. Our results pave a way
toward identification of the defect, as well as engineering the emitter
properties.
