Non-cubic Er centres in ZnSe studied by electron paramagnetic resonance and optical analysis

Abstract

EPR and optical (4f-4f photoluminescence and photoluminescence excitation) spectra due to Er centres are studied on bulk ZnSe crystals, which were grown by the high-pressure Bridgman technique and doped with ErF3 (and partly, in addition, with Li2CO3) during crystal growth. Besides the well known, almost isotropic signal with g=5.94, which has been assigned to isolated Er3+ on Zn lattice sites, we observe three strongly anisotropic EPR spectra (A, B and B') which are due to transitions in non-cubic Er3+ (spectrum A) and Er3+ (spectra B and B') centres. The symmetry axes of these centres have directions close to (111) (A) and parallel to (110) (B and B'). The angular dependences of the signals are influenced by twinning effects. In the crystals doped with ErF3 alone, only a single type of centre, which is obviously identical with the type A EPR centre, manifests itself in photoluminescence. A crystal-field analysis of the corresponding EPR and optical spectra shows that this centre has a Gamma 6-type ground level and is characterized by a crystal-field parameter ratio As<r6>/A4<r4>=-0.22. We think that this centre is a complex consisting of Er3+ on a Zn site and F on a nearest-neighbour interstitial site. The g-factors found for the EPR signals B and B' can be explained on the basis of non-Kramers doublet ground levels of Er2+ 4f12 which result from the splitting of cubic Gamma 5 triplets due to non-cubic crystal-field components. These two signals are ascribed to Er3+ on the two zincblende-type interstitial sites, respectively, each forming a complex with some other kind of atom on its next-nearest-neighbour interstitial site. A discussion is given of the result that the EPR spectra (unlike the optical spectra) were detected only in the case of Li2CO3 codoping.