Abstract
This study mainly focuses on the fabrication and electrical properties of Li-doped ZnO thin film, which includes four parts: I. Feasibility study on application of Vienna abinitio simulation package to the doping effect in p-type ZnO. II. Synthesis of Li-doped ZnO powder by soft chemical routes and the target fabrication. III. Thin film deposition of Li-doped ZnO by DC pulsed sputtering and study on the fabrication parameters. IV. Study of the electrical and optical properties of Li-doped ZnO thin film by impedance measurement. We first evaluated the feasibility of using the first principle method, namely Vienna abinitio simulation package, to study the doping effect in p-type ZnO by discussing the relationships between dopant and electrical properties or p/n type behavior of the doped ZnO material. With respect to the synthesis of Li-doped ZnO powder and target fabrication, we used solution-based soft chemical routes to obtain high reproducible, homogeneous particles in the size range between 50~100nm. This kind of solution-based synthetic method may generate dynamic-stabilized crystal phase while the solid state reaction route can only synthesize thermodynamic-stabilized crystal structures. It also provides more accurate control over the homogeneity of dopant and the composition design of crystal phase as well as covalent bonding. The Li-doped ZnO powder with 0.05 to 0.8 mole Li doping level was formed and made as a target for further use. For fabrication of stable Li-doped ZnO thin film and to understand the effect and control mechanism of fabrication parameters on the electrical and optical properties, we utilized DC pulsed sputtering to deposit Li-doped ZnO thin film, secondary ion mass spectrometry to determine Li element content, and Hall effect instrument to measure the electrical properties and confirm the p/n type behavior. By using 0.2 mole% Li doped ZnO powder, we successfully obtained transparent p-type ZnO thin film at room temperature. The characteristics of this thin film are: film thickness, 573 nm; resistivity, 1.302 (Ω-cm); mobility, 0.94 (cm2/V•s); carrier concentration, 5.08 x 1018 (cm-3); transmittance, 95%. We found the major factors in fabrication process are temperature of substrate, flow rate of Ar carrier gas and sputtering power. As the temperature of substrate or the flow rate of Ar carrier gas increased, the doping concentration of Li decreased and led to a poorer p-type property. However, the higher the sputtering power, the richer the Li doping concentration, which enhances the p-type property. We further applied the electrical impedance measurement to the study of Li doping effect. By simulating Li atoms existing in the grain or on the grain boundary with equivalent circuit, the influence of Li dopant on the electrical and optical properties of Li-doped ZnO thin film could be analyzed. Accordingly, the optimal Li dopant content and fabrication parameters for stable Li-doped ZnO thin films were found. In the final part of this study, we demonstrated Li-doped ZnO with Li doping level of 0.1 to 0.3 mole could form stable p-type thin film by characterization with current-voltage I-V curve for p-n junction measurement.