Unusual electrical properties of hydrothermally grown ZnO

Bulk ZnO of high structural and optical quality can be grown by the hydrothermal method. An X-ray rocking-curve linewidth of 18 arcsec has been measured for the (002) reflection, and a photoluminescence linewidth of 0.3 meV has been found for one of the donor–bound exciton lines. However, the electrical properties are unusual in that shallow donors are not dominant, as is the case for vapor-phase-grown and melt-grown bulk crystals. This situation can be greatly modified by annealing in forming gas (5% H₂ in N₂) at , with bulk shallow donors then becoming completely dominant for , and near-surface donors at lower temperatures. As T_A is varied from 100–650 C, both the mobility and carrier concentration vary in nonmonotonic fashion, due to changes in the relative strengths of the bulk and surface components of conduction.