ESR and ENDOR studies of holes trapped at cation vacancies in BeO

Abstract

The $V^{-}$ center (cation vacancy with a trapped hole) has been identified by ESR and electron-nuclear double resonance (ENDOR) in neutron- and electron-irradiated BeO crystals. At liquid-helium temperature, the defect is observed in two distinct configurations labeled "axial" and "nonaxial" where the hole is localized on an anion situated either along the $\overrightarrow{\mathrm{c}}$ axis from the vacancy or on one of the three other concerns of the tetrahedron, respectively. This model of an ${\mathrm{O}}^{-}$ ion next to a vacancy is supported by an analysis of the $\tilde{g}$ tensor. At room temperature, the hole jumps from one anion site to another around the vacancy, giving an isotropic averaged ESR spectrum. For the axial $V^{-}$ center, ENDOR measurements were made at 1.5°K of the hyperfine interactions with the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$ nuclei which are nearest neighbors of the ${\mathrm{O}}^{-}$ ion and with two shells of next-nearest-neighbor nuclei. An analysis of the dipolar magnetic interaction with these nuclei gives detailed information about the electronic structure of the ground state of the defect and the lattice distortions in its vicinity. A comparison between the ENDOR spectra of the nonaxial $V^{-}$ center and the $V^0$ center (cation vacancy with two trapped holes) confirms the close relation between these two defects. $V^{-}$ centers associated with a trivalent impurity ${\mathrm{B}}^{3+}$ have been observed in $\gamma$-irradiated BeO crystals.