Vibronic spectrum of c-BN measured with cathodoluminescence

An extended vibronic spectrum (up to six phonon replicas) has been measured by low temperature cathodoluminescence in pure c-BN microcrystalline samples obtained by high temperature high pressure method. The zero phonon line, found at 3.573 eV, should be connected to a center identified previously as PF1, which is intrinsic in c-BN. As a consequence, the vibronic spectrum is not associated with the BN1 center at 3.293 eV, which is created by irradiation, as reported in literature. The spectrum is in very good agreement with the Franck-Condon model, formerly developed for color centers in alkali halides and from which in this case a value of 5.26 for the Huang-Rhys factor can be obtained. From the spectrum it has been possible to determine very precisely the value of phonon energy (141±3 meV), which is in good agreement both with the results of ab initio calculations of LO phonon energy at the X point of the Brillouin zone of c-BN and with the value measured by second-order Raman spectroscopy. The connection of this center with the indirect gap and the large energy threshold for excitation could explain why this center with its vibronic band was hardly seen before by photoluminescence. Since the measured displacement of the equilibrium normal coordinate of the excited level was found to be 0.17 nm, this level could be attributed, according to calculations from first principles, to a boron antisite and not to a Frenkel pair (a nitrogen vacancy coupled to a boron interstitial) as previously indicated.