Variable-Range Hopping and Thermal Activation Conduction of Y-Doped ZnO Nanocrystalline Films

ZnO and Y-doped ZnO nanocrystalline films were separately fabricated on the glass substrates by sol-gel spin-coating method. X-ray diffraction patterns of the films show the same wurtzite hexagonal structure and (0 0 2) preferential orientation. Scanning electron microscope images show that grain size and thickness of the nanocrystalline films decrease with increasing doping concentration. The decrease of optical bandgap with the increase of Y doping is deduced from the transmittance spectra. Temperature-dependent resistivity reveals a semiconductor transport behavior for all ZnO and Y-doped ZnO nanocrystalline films. The resulting conductivity originates from the combination of thermal activation conduction and Mott variable-range hopping (VRH) conduction. In the high-temperature range, the temperature-dependent resistivity can be described by the Arrhenius equation, σ (T) = σ₀ exp[ -(E_a/kT)], which shows the thermal activation conduction. The activation energy Ea increases from 0.47 meV for ZnO film to 0.83 meV for Zn_0.98Y_0.02O film. On the contrary, in the low-temperature range, the temperature-dependent resistivity can be fitted well by the relationship, σ(T) = σ_{h 0}exp[-(T₀/T)^1/4], which indicates the behavior of Mott VRH. The results demonstrate that the crystallization and the corresponding carrier transport behavior of the ZnO and Y-doped ZnO nanocrystalline films are affected by Y doping.