27Aland69Gaimpurity nuclear magnetic resonance in ZnO:Al and ZnO:Ga

Abstract

We report ${}^{27}\mathrm{Al}$ and ${}^{69}\mathrm{Ga}$ impurity and ${}^{67}\mathrm{Zn}$ host species NMR experiments on ZnO powders containing Al or Ga dopants in the range 0.03--3 at. %. Impurity resonance shift and nuclear spin-lattice relaxation data are analyzed to yield values of the impurity nucleus chemical shift, Knight shift, and hyperfine coupling. The electric quadrupole coupling of dilute ${}^{69}\mathrm{Ga}$ in ZnO is determined to be ${(e}^{2}{qQ/h)}_{69}=3.9\ifmmode\pm\else\textpm\fi{}0.3$ MHz. The concentration dependence of the Knight shifts and relaxation rates indicate rapid development of an impurity band as the dopant concentration falls below about 0.5 at. %. At all concentrations investigated, Knight shifts and relaxation rates satisfy the Korringa relation; electron correlation and localization effects are essentially absent. The shapes of ${}^{69}\mathrm{Ga}$ spin echoes reveal that a second, more distorted gallium site is present with a relative concentration that increases with the ratio of oxygen to total metal content. Magnetic hyperfine coupling of carriers to the distorted sites is very weak and the apparent carrier density at substitutional sites decreases with the fraction of observed substitutional sites. These results imply that the distorted sites are part of a diamagnetic carrier trap involving gallium complexed with a defect. We suggest that the complex consists of two gallium ions and an oxygen interstitial, i.e. ${\mathrm{Ga}}_{2}^{3+}{\mathrm{O}}_{i}^{2\ensuremath{-}}.$