Transition-metal and hydrogen in ZnO: a source of shallow donors

The structural and electrical properties of Fe, Co, Ni and Cu in ZnO, and their complexes with hydrogen have been studied using local-density-functional methods. Out of the many defect structures examined several centres are of particular relevance to the issue of n-type conductivity in as-grown material. Cu_Zn-H₂ is bound by 0.9 eV and exhibits a shallow donor level. Fe_Zn, Co_i and Fe_i also exhibit donor levels which may be shallow. Such centres are much more thermally stable than isolated hydrogen which is thought to be mobile at low temperatures. Combinations of transitional metal and hydrogen migth explain at least part of the n-type conductivity seen at room temperature in as-grown ZnO.