Magnetic resonance studies of ZnO

Abstract

We have used EPR and ODMR to study state-of-the-art bulk ZnO single crystals. Most of the samples are n-type; however, under certain conditions (e-irradiated or annealed), we observe a center due to residual nitrogen ( g ∥=1.9953, g ⊥=1.9633 and A iso=1.225 mT, A aniso=0.864 mT). The N center is a nitrogen p-orbital along the c-axis, very similar to P or As in ZnSe, which have a Jahn-Teller distorted, deep state, seen in EPR. The intensity of the N center is strongly anisotropic, as would be expected for shallow heavy-hole acceptors with m J=± 3 2 in which the magnetic dipole transition is forbidden for B ∥ c and becomes allowed away from that orientation due to mixing with m J=± 1 2 states. The EPR spectra for the n-type samples are dominated by delocalized donors ( g ∥=1.9570 and g ⊥=1.9551). In one sample, the donor appears to be localized. These crystals also exhibit a broad PL band at 2.5 eV. ODMR on the 2.5 eV “green” PL band reveals an S=1 triplet center (not seen in EPR) with spin-Hamiltonian parameters of D=0.763 GHz, g ∥=1.9710, and g ⊥=2.0224. ODMR of shallow donors is also observed as a luminescence-decreasing feature on the 2.5 eV green emission band.