Surface Potential, Field-Effect Mobility, and Surface Conductivity of ZnO Crystals

Measurements of surface properties of ZnO crystals were made at 300°K, both in dry ${\mathrm{O}}_2$ and in high vacuum. The dark conductivity was changed by illumination with ultraviolet. Surface potentials of crystals with diameters down to 0.002 cm were measured by the Kelvin method with a sensitivity of 0.002 v. Comparison of the measured and calculated dependence of dark conductivity on dark surface potential showed that the latter could be changed from about 0.1 v below to 0.5 v above the neutral point. Application of a transverse electric field produces a fast change in conductivity in less than 50 μsec and a slow change in which part of the fast change decays with a time constant ranging from minutes to hours depending on ambient and surface potential. The field effect mobility increases with increasing surface potential from a value which is sometimes smaller than one to a plateau value between 70 and 145 ${\mathrm{cm}}^2$ ${\mathrm{v}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ and decreases again for the largest value of surface potential. Evidence is given that the low mobility values are caused by surface states. Combined measurements of surface potential, field-effect mobility, and surface conductivity together with quantum efficiency measurements of the surface conductivity by Collins and Thomas yield the quantum efficiency of the hole-trapping process at the surface which is approximately 1 for a neutral surface. A quantitative treatment of the hole-trapping process is in good agreement with the experimental results and shows that the bulk diffusion length for holes is >1000 A and that the ratio of hole surface trapping velocity and diffusion constant equals 1.7×${10}^5$ ${\mathrm{cm}}^{-1\mathrm{}}$.