The electrical conductivity of magnesium oxide at high fields

Abstract

Measurements reported earlier on the electrical conductivity of magnesium oxide single crystals in the temperature range 400-750°C and at low fields have been extended to fields exceeding 2 MV cm−1. A limited number of electric breakdown measurements are also reported at fields as high as 9 MV cm−1. These studies show that two conduction processes operate. One has a zero-field activation energy of 2·8 eV which then decreases as the field is raised so that it is dominant at high fields. The other which predominates at low fields and low temperatures has an activation energy of 1·5 eV.The results are best described in terms of a model involving the hopping of oxygen ions between appropriate sites in the crystal. It is suggested that the activation energy of 1·5 eV is that for migration of carriers (ion vacancies or possibly trapped electrons) through the lattice and that the higher energy of 2·8 eV is required for a two-stage activation process involving firstly vacancy formation and secondly vacancy migration through the lattice.When the field and temperature are high the conduction may become increasingly electronic. A current instability which sets in under these conditions and can lead to breakdown may correspond to the onset of trap-filled, space-charge controlled conduction.