  Doping of substrates at desired locations is a key technology for spin-based
quantum memory devices. Focused ion beam implantation is well-suited for this
task due to its high spacial resolution. In this work, we investigate ion-beam
implanted erbium ensembles in Yttrium Orthosilicate crystals by means of
confocal photoluminescence spectroscopy. The sample temperature and the
post-implantation annealing step strongly reverberate in the properties of the
implanted ions. We find that hot implantation leads to a higher activation rate
of the ions. At high enough fluences, the relation between the fluence and
final concentration of ions becomes non-linear. Two models are developed
explaining the observed behaviour.
