  Focused MeV ion microbeams are suitable tools for the direct writing of
conductive graphitic channels buried in an insulating diamond bulk, as
demonstrated in previous works with the fabrication of multi-electrode ionizing
radiation detectors and cellular biosensors. In this work we investigate the
suitability of the fabrication method for the electrical excitation of colour
centres in diamond. Differently from photoluminescence, electroluminescence
requires an electrical current flowing through the diamond sub-gap states for
the excitation of the colour centres. With this purpose, buried graphitic
electrodes with a spacing of 10 micrometers were fabricated in the bulk of a
detector-grade CVD single-crystal diamond sample using a scanning 1.8 MeV He
micro-beam. The current flowing in the gap region between the electrodes upon
the application of a 250 V bias voltage was exploited as the excitation pump
for the electroluminescence of different types of colour centres localized in
the above-mentioned gap. The bright light emission was spatially mapped using a
confocal optical microscopy setup. The spectral analysis of electroluminescence
revealed the emission from neutrally-charged nitrogen-vacancy centres ($NV^0$,
$\lambda_{ZPL}$ = 575 nm), as well as from cluster crystal dislocations
(A-band, {\lambda} = 400-500 nm). Moreover, an electroluminescence signal with
appealing spectral features (sharp emission at room temperature, low phonon
sidebands) from He-related defects was detected ($\lambda_{ZPL}$ = 536.3 nm,
$\lambda_{ZPL}$ = 560.5 nm); a low and broad peak around {\lambda} = 740 nm was
also observed and tentatively ascribed to Si-V or GR1 centres. These results
pose interesting future perspectives for the fabrication of
electrically-stimulated single-photon emitters in diamond for applications in
quantum optics and quantum cryptography.
