UV Light-Induced Changes to the Surface Conduction in Hydrothermal ZnO

High quality, bulk ZnO crystals grown by Tokyo Denpa using the hydrothermal process typically exhibit a room temperature carrier concentration in the 1013–1014 cm−3 range and a low mobility, conductive surface layer, observed at low temperature, with a sheet concentration on the order of 1012–1013 cm−2. In the sample discussed here, bulk conduction is controlled by two donor levels at 50 and 400 meV with concentrations of 1.2×1016 and 1.5×1016 cm−3, respectively. Temperature-dependent photo-Hall-effect measurements, using blue/UV light, in vacuum show an increase in the surface sheet carrier density to more than 1×1013 cm−2 at low to intermediate temperatures while the two bulk donors continue to dominate the high temperature behavior, up to 400 K. Long-lived persistent photoconductivity (PPC) is observed when the sample is returned to the dark. When the PPC is allowed to fully relax and the sample is exposed to air, there is surprisingly no longer any surface conduction at low temperature, while the two bulk donors remain unaffected. In this state, the 50 meV bulk donor level is observed to control the conduction over five orders of magnitude, down to a carrier concentration of 3.0×108 cm−3. This corresponds to an upper limit for the surface sheet carrier density of 1.6×107 cm−2. This is the lowest surface concentration we have ever observed in any ZnO sample and demonstrates that blue/UV light irradiation, in vacuum, at moderate temperatures is very effective at cleaning the surface. A subsequent 30 min anneal at 600 °C in forming gas (5% H2 in N2) increases the carrier concentration by almost two orders of magnitude. The forming gas anneal produces no changes in the concentrations of the 50 and 400 meV bulk donor levels and no new bulk donors are observed. However, the bulk acceptor concentration decreases from 2×1016 to 1.2×1016 cm−3, most likely as a result of passivation by hydrogen