Erroneous p-type assignment by Hall effect measurements in annealed ZnO films grown on InP

We report on incorrect carrier type identification achieved by Hall effect measurements performed on ZnO films grown by pulsed laser deposition on InP substrates and subsequently annealed for 1 h at 600 C in air. While Hall measurements, after post-growth annealing, reveal a change in the electrical properties of the films, from n-type to p-type, both photocurrent-based and standard C V measurements performed on the same samples show no change in the native n-type doping of the ZnO films. A possible interpretation of the two results is reported. In particular, p-type conductivity observed by Hall effect may be ascribed to a highly conductive thin layer formed during the annealing process at the ZnO/InP interface, which dominates the Hall effect measurements and does not influence the photo-electrochemical behavior of ZnO as well as the measured differential capacitance. The conflicting results here reported show that for this kind of samples, Hall effect measurement can be misleading with respect to the real nature of the analyzed material, instead both C V and photocurrent-based characterization techniques are more reliable and therefore could be alternatively used when particularly ambiguous results are expected by Hall effect measurements.