Study On Carrier Transport Properties In Undoped ZnO Thin Films

ZnO is a representative of the third generation of wide bandgap semiconductors with a band gap of 3.37 eV and a high exciton binding energy of 60 meV at room temperature,which has great application prospects in the optoelectronic device field.After nearly 20 years of research,the ZnO-based materials has achieved certain results in the thin film growth,impurity doping,and the device applications.However,some problems,such as the difficultly of preparing high quality ZnO single crystals,low mobility,and unstable p-type conduction,etc.,have hindered the practical application of ZnO-based optoelectronic materials and devices,which are closely related to the defects and impurities in Zn O thin films.In this paper,we are devoted to study on carrier transport properties in undoped ZnO thin films,offering an important way to understand these defects and impurities.The main contents are as follows:（1）A series of undoped ZnO thin film was prepared on c-Al₂O₃ substrate by PLD technology with ZnO ceramic targets as source materials.The influences of the target-substrate distance,laser pulse frequency,and laser pressure on the Zn O thin films were studied.By characterizing the crystal structure,morphology,photoluminescence and electrical properties of the prepared samples,the optimum ZnO film growth process parameters were obtained.（2）The conduction mechanism of the prepared ZnO thin films was systematically studied,and the results show that ZnO thin films have different conduction mechanisms at different temperatures.In the temperature range of 125³00 K,the conductance of ZnO thin films is participated by electron conduction in conduction band and nearest-neighbor-hopping（NNH）conduction.At lower the temperatures of 30¹00 K,the main mechanism is Mott variable-range-hopping（Mott-VRH）conduction.At extremely low temperatures of 2²0 K,the conduction mechanism changes from Mott-VRH conduction to Efros-Shklovskii variable-range-hopping（ES-VRH）conduction.（3）By measuring temperature-dependence of carrier concentration and mobility,the electrical transport properties of carriers in undoped ZnO thin film were studied.It was found that the deviation of the electrode preparation had a significant influence on the electrical measurement.A correction formula was introduced to effectively correct the influence of the electrode.It was considered that,there is a high-conductivity layer with high carrier concentration in the interface of the ZnO thin film and c-Al₂O₃substrate due to the band offset,which leads to abnormal variations of carrier concentration and mobility with the temperature.A two-layer conduction model is proposed to,exclude the influence of the high-conductivity layer,and the obtained electrical parameters from epitaxial layer have been significantly improved.By calculating the conductivity of sample after subtracting the high-conductivity layer,it was found that this sample has only band conduction at higher temperatures.（4）The relation between the magnetoresistance and magnetic field intensity in ZnO thin films with various carrier concentrations was studied at different temperatures.It was found that the lower the carrier concentration is,the higher the localization degree is and the larger the magnetoresistance value is.The positive magnetoresistance was observed in the sample with the two-layer conduction model,which was attributed to the high-conductivity layer at extremely low temperature.The emergence of this phenomenon was due to two kinds of carriers with different concentration and mobility are involved in the transport process.