Temperature-dependent Hall effect studies of ZnO thin films grown by metalorganic chemical vapour deposition

The electrical properties of zinc oxide (ZnO) thin films of various thicknesses (0.3-4.4 mu m) grown by metalorganic chemical vapour deposition on glass substrates have been studied by using temperature-dependent Hall-effect (TDH) measurements in the 18-300 K range. The high quality of the layers has been confirmed with x-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence techniques. TDH measurements indicate the presence of a degenerate layer which significantly influences the low-temperature data. It is found that the measured mobility generally increases with increasing layer thickness, reaching a value of 120 cm(2) V-1 s(-1) at room temperature for the 4.4 mu m thick sample. The lateral grain size of the layers is also found to increase with thickness indicating a clear correlation between the size of the surface grains and the electrical properties of corresponding films. Theoretical fits to the Hall data suggest that the bulk conduction of the layers is dominated by a weakly compensated donor with activation energy in the 33-41 meV range and concentration of the order of 10(17) cm(-3), as well as a total acceptor concentration of mid-10(15) cm(-3). Grain boundary scattering is found to be an important limiting factor of the mobility throughout the temperature range considered.