  Hexagonal boron nitride (hBN) is gaining considerable attention as a
solid-state host of quantum emitters from the ultraviolet to the near infrared
spectral ranges. However, atomic structures of most of the emitters are
speculative or unknown, and emitter fabrication methods typically suffer from
poor reproducibility, spatial accuracy, or spectral specificity. Here, we
present a robust, deterministic electron beam technique for site-specific
fabrication of blue quantum emitters with a zero-phonon line at 436 nm (2.8
eV). We show that the emission intensity is proportional to electron dose and
that the efficacy of the fabrication method correlates with a defect emission
at 305 nm (4.1 eV). We attribute blue emitter generation to fragmentation of
carbon clusters by electron impact and show that the robustness and
universality of the emitter fabrication technique are enhanced by a
pre-irradiation annealing treatment. Our results provide important insights
into photophysical properties and structure of defects in hBN and a framework
for deterministic fabrication of quantum emitters in hBN.
