Effect of quantum confinement and influence of extra charge on the electric field gradient in ZnO

By means of electron-nuclear double resonance (ENDOR), it is shown that the Al impurity, which acts as a shallow donor in ZnO, leads to a significant reduction of the electric field gradient in ZnO single crystals. In ZnO quantum dots, however, the gradient on the Al sites remains virtually unchanged. When the Zn$^{2+}$ ion is substituted by Mn$^{2+}$ in a ZnO single crystal, the electric field gradient slightly increases (by about $20\%$). Therefore, the Mn$^{2+}$ ions can be used as probes to monitor the electric field gradients in ZnO crystals.