Surface charges and subsurface space-charge distribution in magnesium oxides containing dissolved traces of water

MgO crystals which contain dissolved traces of H₂O eventually acquire an excess oxygen content, MgO_1+δ, because OH^- pairs, associated with cation vacancies, convert in part into molecular H₂ and peroxy anions, O₂^2-. While H₂ molecules are lost from the crystal, the vacancy-bound O₂^2- remain as spin-paired, self-trapped positive holes. Probably at this stage the MgO surface is negatively charged. Around 800 K the O₂^2- decay, releasing unbound positive holes, i.e., highly mobile O^- states. They lead to an oxygen release from the crystal surface and a positive surface charge. With the assumption that the vacancy-bound holes are essentially immobile the space-charge distribution in the subsurface region and the associated electric field and potential were calculated for different temperatures and as a function of charge carrier concentration. The width of the space-charge region is of the order of 10-50 nm, decreasing with increasing concentration, while the field at the surface increases. At a concentration as small as 10-20 ppm the field at the surface is of the order of 3.8 ×{} 10⁷ V m^-1. At higher temperatures when cation vacancies are mobilized, the negatively charged bound holes will diffuse and drift toward the surface modifying the surface charge and subsurface space-charge layer. When all excess oxygen and excess cation vacancies are annealed out the MgO surface will probably again become negatively charged.