La Doping Effect on Temperature-Dependent Carrier Transport Behavior of Zn_1–x La _x O Nanocrystalline Films

Zn_1–x La _x O nanocrystalline films have been fabricated to study the La doping effect on the temperature dependence carrier transport behavior. Zn_1–x La _x O (x = 0, 0.01, 0.02, and 0.04) nanocrystalline films were fabricated separately on the glass substrates by sol–gel spin-coating method. X-ray diffraction patterns of the Zn_1–x La _x O films show the same wurtzite hexagonal structure and (002) preferential orientation. SEM images illustrate that grain size decrease with increasing La doping concentration. From UV/Vis/IR transmittance spectra, the Zn_1–x La _x O films reveal a decrease of transmission and optical band gaps after the doping of La into the ZnO film. Photoluminescence spectra of La-doped ZnO films show a slightly red-shifted decrease of UV emission and an enhancement of visible emission originated from doping induced defects. Temperaturedependent conductivity reveals a semiconductor transport behavior for all Zn_1–x La _x O nanocrystalline films and decreases with increasing La doping. The resulting conductivity can be expressed by the combination of thermal activation conduction and Mott variable range hopping (VRH) conduction. The temperature-dependent conductivity fits the relationship, σ(T) = σ _h0 exp[–(T ₀/T)^1/4, indicating the behavior of Mott VRH in low temperature range. By contrast, in the high temperature range, the temperature-dependent conductivity can be described by the Arrhenius equation, σ(T) = σ ₀ exp[–(E_a/kT)], which shows the behavior of thermal activation conduction. The results reveal that the crystallization and the corresponding carrier transport behavior of the Zn_1–x La _x O nanocrystalline films are obviously affected by La doping.