Effects of Temperature on the Infrared Emission Performance of AZO Films

With the decrease of global resources and environmental deterioration, energy saving and emission reduction have become a hot topic and Low-E glass with thermal insulation performance is becoming the research focus. To improve the thermal insulation performance of glass, the simplest and most effective method is plating low-emissivity coatings on the surface of them. The Al doped ZnO (AZO) film is the most potential low-emissivity layer for Low-E glass, due to rich raw materials, high conductivity and high transparency and so on. Effects of temperature on the infrared emission performance of AZO films were researched and mechanisms of changes were analyzed in this study. The change of infrared emissivity of AZO film kept at a certain temperature for some time was studied firstly, and then the change of infrared emissivity in variable temperature environment was studied. 500 nm-thick AZO films were deposited on glass substrates at room temperature by direct current magnetron sputtering and then were put in muffle furnace for heat treatment. Films were kept at 100 similar to 400 degrees C for 1 h in air and then cooled to room temperature in the furnace. The phase of AZO films was analyzed by X-ray diffraction and the surface morphology was observed by scanning electron microscope. The resistivity was measured by four probe method and the infrared emissivity was measured using infrared emissivity measurement instrument. Visible spectrum was measured by visible spectrophotometer. The results showed that AZO film before and after annealed all show hexagonal wurtzite structure and (002) preferred orientation. With the annealing temperature rising to 300 degrees C, the intensity of (002) diffraction peak increases, the full width of half maximum (FWHM) narrows down and grain size increases. With the increase of annealing temperature, the resistivity decreases firstly and then increase. The film annealed at 200 degrees C shows the lowest resistivity of 0.9X10(-3) Omega . cm. The decrease of resistivity is attributed to the growth of grains. The film annealed at higher temperature in air will absorb oxygen, resulting in the decrease of resistivity. The change of the infrared emissivity with annealing temperature agrees to that of resistivity. The film annealed at 200 degrees C shows the lowest emissivity of 0.48. Infrared photons are strongly reflected by free electrons. When the resistivity is low and the concentration of free electrons is high, more infrared photons are reflected, the infrared radiation weakens, and the infrared emissivity decreases. The transmittance decreases firstly and then increases. It is the lowest at 200 degrees C but still up to 82%. This change is caused by the change of free electron concentration. Free electron strongly reflects visible light. The infrared emissivities of the films as-deposited and annealed at 200 degrees C were measured during the process of heating and cooling between room temperature and 350 degrees C. The sample was fixed on the heated stage and its emissivity was recorded every 25 degrees C. It was found that the infrared emissivity increases with the increase of temperature during the heating process, and decreases during the cooling process. After the whole process, the infrared emissivity of the AZO film increases.