Determination of electric-field gradients in semiconductors with high precision and high sensitivity

Wurtzite-type semiconductors such as GaN and ZnO exhibit intrinsic electric-field gradients at the nuclear sites due to the hexagonal lattice symmetry. We have determined these field gradients VZZ for all the nuclear sites by a magnetic double-resonance scheme. Shallow donors, which are present in these semiconductors either unintentionally or by doping, are used to enhance the nuclear magnetic resonance (NMR) sensitivity through the double-resonance technique. The value VZZ for ⁶⁷ Zn in ZnO is in excellent agreement with the value obtained by M\"ossbauer spectroscopy. For the Ga position in GaN and for the O position in ZnO we improved the precision by an order of magnitude compared to previous NMR results. The VZZ value for the N position in GaN was determined. The field gradients are directly related to the bond ionicities in these III-V and II-VI semiconductors.