We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride (h-BN) film grown on Ni by molecular beam epitaxy. After transfer to a SiO 2 /Si substrate, sharp lines are observed in photo-and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multi-layer islands that form at the nucleation centers of the h-BN film. The comparison of their energy, polarization and phonon replica emission with previous theoretical predictions suggest that the N B V N anti-site could be one of the light emitters present in our sample. Moreover, we have also observed evidences of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources. * alberto.h.minguez@pdi-berlin.de 1 I. INTRODUCTION The controlled generation and manipulation of single photons is a key factor for the development of quantum information technologies [1]. One of the most promising platforms for the realization of single photon sources (SPSs) are atom-like emitters based on solid-state systems, since they combine the optical properties of atoms with the scalability of solid-state nanostructures [2]. In addition to the well-known quantum dots [3] and color centers in solids [4], the family of solid-state SPSs includes new low-dimensional systems like carbon nanotubes [5] and two-dimensional materials like WSe 2 [6, 7]. One of the most recent additions to the last group has been hexagonal boron nitride (h-BN), where luminescent defects acting as SPSs have been reported both in the visible [8] and in the ultraviolet (UV) spectral ranges [9]. In the last years, several groups have intensively studied the optical properties of the SPSs emitting in the visible spectrum both in bulk h-BN [10-12], as well as in flakes [13-21].Although the structural nature of such centers is still under discussion [8,[22][23][24][25], their good polarization properties [8,13], and the fact that they maintain their quantum character even at elevated temperatures [19], makes them strong candidates as optoelectronic components in integrated quantum devices. Future applications of such quantum centers, however, will depend on (i) the possibility of growing h-BN over large areas with high structural and morphological quality, and (ii) the control over the kind of luminescent defects, as well as their spatial distribution within the h-BN film.
Among the various approaches to synthesize h-BN films, molecular beam epitaxy (MBE)is very promising as it allows the formation of h-BN on different kinds of substrates with precise control over the growth conditions. Recent results have demonstrated the feasibility of MBE for the large-area growth of h-BN films on Ni [26], as well as on pyrolytic graphite [27] and on graphene on SiC [28]. Moreover, the MBE-growth of vertical heterostructure films combining...