Mechanism Of Ito Thin Film Infrared Low Emissivity

In this paper, ITO thin film samples were deposited on polyethylene terephthalate (PET) substrate by DC magnetron sputtering with different deposition conditions. Their structural, optical and electrical properties were measured by X-ray diffraction (XRD), stylus profiler, UV-Vis-NIR spectrophotometer, FT-IR spectrometer and Hall Effect measurement, respectively. The effects of several parameters on the properties of ITO films were studied. In addition, the author analyzed the relationships between structural and other functional properties of ITO thin films and tried to explain the reasons for their low infrared emissivity. Several important results were obtained:Firstly, the influence of different sputtering conditions on the properties of ITO thin films was discussed. (1) With the sputtering time increasing, a linear growth of the film thickness was observed and the films are amorphous. The resisitivity of the samples also decreased with increasing deposition time, reaching a minimum of 4.79×10-3Ωcm in this research. The increased carrier concentration might be the main reason. The transmission of the samples went down in the visible and near infrared region and some characteristic absorption peaks appeared in the near infrared region, so in the spectra, the absorption peak grew stronger with longer sputtering time and moved to the shorter wavelength. In the middle infrared region, the transmission tended to stay even with different film thickness. (2) With increasing sputtering power, the (400) oriented XRD peak was observed. The resisitivity decreased to a minimum of 1.2×10-3Ωcm, because the mobility became much bigger with higher power. The transmission decreased in visible region and the reflection increased to over 0.8 in middle infrared region. (3) With increasing sputtering pressure, the thickness and resistivity first increased and then decreased, reaching their minimums at the pressure of 1.1Pa. Comparatively, the influence of this parameter on the optical properties was not so significant. (4) What's more important, there was a very valuable relationship between the resistivity and infrared emissivity, according to both the experimental and theoretical analysis. As a result, detailed study about the electrical properties of ITO thin films could become a key point for explaining their infrared emissivity.Secondly, the relations of crystalline properties and the surface and electrical properties were discussed. For amorphous ITO films, the resistivity generally increased with growing surface roughness. Classical Kronig-Penney model and Neugebauer-Webb model were introduced to explain this phenomenon. However, for crystal ones, the variation would be different, because of the grain growth and their different effect on the electrical properties.Thirdly, the correlations of band structure and optical and electrical properties of ITO thin films were discussed. Both direct and indirect transitions could be observed in ITO films, which might be related with the surface and interface conditions. At the same time, some calculations about the Fermi level and mobility edge could effectively explain some fine experimental results.