Electron-spin resonance of hole centers in x-irradiated Mg- and Zn-doped tetragonal GeO2crystals

Abstract

Ionizing radiation at 77 K generates trapped-hole centers in Mg- or Zn-doped single crystals of tetragonal Ge${\mathrm{O}}_{2}$. The electron-spin resonance spectrum exhibits a spin-$\frac{1}{2}$ defect corresponding to a hole localized in a $2{p}_{z}$ orbital on one of four (out of six) oxygen anions surrounding the substitutional impurity cation. There are four magnetically inequivalent anion sites for each of two inequivalent cation sites. Each ${\mathrm{O}}^{\ensuremath{-}}$ resonance line is doubled by weak proton hyperfine interaction (confirmed by electron-nuclear double resonance) from a charge-compensating O${\mathrm{H}}^{\ensuremath{-}}$ ion at a neighboring anion site displaced one lattice constant from the defect along a [001] direction. From fitted spin-Hamiltonian parameters one can deduce that the hydroxyl proton projects toward an interstitial lattice site. Pulse-annealing studies show that Mg and Zn trapped-hole centers are more stable than hole centers associated with trivalent impurity cations. The Mg hole center begins to anneal at approximately 210 K; the Zn hole center anneals at about 225 K. Subsequent to annealing, a similar hole center of low concentration and without hyperfine splitting is detected; this anneals at about room temperature. It is attributed to the same type of center but without local charge compensation.