Study On Preparation And Properties Of In-Doped ZnO Thin Films

ZnO is an important II-VI semiconductor oxide material with a wide direct band gap of 3.3eV. Due to its good conductivity and high transmittance, ZnO based TCO films are widely used in light-emitting devices, liquid crystal display, solar cell, gas sensor, and transparent electrode, etc. Compared with the commonly used transparent conductive films In₂O₃:Sn and SnO₂:F, ZnO is inexpensive, with higher thermal and chemical stability exposured to active hydrogen and hydrogen plasma. In doping can improve the opto-electrical properties of ZnO thin films effectively. In-doped ZnO thin films have high sensibility to NO₂, also it has good thermoelectricity properties at room temperature. So it is interasting to prepare high quality In-doped ZnO thin films and study the properties.In this paper, high transparent conductive ZnO:In thin films were prepared on glasses substrates by RF magnetron sputtering. The influence of In content, substrate temperature, sputtering pressure, sputtering power, annealing temperature, etc. on the film properties have been studied systematically. The films were characterized and analyzed by XRD, AFM, SEM and ultraviolet-visible-infrared light spectrophotometer, etc. The structure, opto-electrical properties, NO₂ sensitivity and thermoelectrical properties of the films were also studied. The main results obtained are as follows:1. All the ZnO:In films were polycrystalline hexagonal wurtzite structure with (002) orientation perpendicular to the subastrate. In and Zn ions are in +3 and +2 valence state, and no orther valence was found.2. The surface of the films was affected by the substrate temperature, In concentration, sputtering power and sputtering pressure. The improvement of substrate temperature and a lower In concentration are propitious to a denser and smoother surface.3. The Hall measurements indicate that all the films are n-type, In-doping concentration, substrate temperature, sputtering pressure, sputtering power and annealing have great influence on the resistivity of ZnO:In films. The conductivity of the films shows increase and then decrease with increasing In doping, substrate temperature and sputtering power. The carrier concentration decreases and the resistivity increases with sputtering pressure and annealing. The optimized parameters of In doping concentration, sputtering power, sputtering pressure and substrate temperature for high conductive ZnO:In thin films are 5at.%, 120W, 2Pa and 250oC respectively. The lowest resistivity of the films prepared at room temperature under the optimized perameter without annealing is 4.3×10^-4Ω·cm.4. All the fimls shows high transmittance of above 80% (substrate included) in the visible region. And the transmittance of the films does not shift with the doping concentration, substrate temperature, sputtering power and annealing temperature significantly. The transmittance increases slightly with sputtering pressure. The optical band gap of the films shows red shift after annealing in air and it decreades with the increade of the anneaing temperature. A linear relationship between optical band gap and the strain in the ZnO:In films was found after annealing.5. There are four PL emissions: 396nm (3.13eV, violet), 446nm (2.78eV, blue), 482nm (2.57eV, green) and 527nm (2.35eV, green) for all the films, the PL intensity of emission peaks increase after annealing.6. The measurement of NO₂ sensor shows that ZnO:In thin films has intense sensitivity to NO₂, and the working temperature is at 275oC. The 1at.% In-doped ZnO thin films shows highest sensitivity, and the sensitivity increases with the decrease of the film thickness. The films of 90nm with 1at.% In content has a high sensitivity of 16 to NO₂ of 20ppm.7. ZnO:In thin film has high Seebeck coefficient of negative at room temperature, implying n-type semiconductor. The Power factor of the films with 1at.% doped ZnO thin film is as high as 10^-4 W/K²Â·m, which is higher than the value of the widely studied p-type thermoelectricitive material CuAlO₂, showing potential usage in the field.