Energy saving and emission reduction has become a hot topic which human concerned, due to the decrease of global oil resources and environmental deterioration. Significantly, building energy consumption accounts for about 30% of the total energy consumption. More significantly, heat loss through glass occupies a large proportion in building energy consumption. Therefore, it is very necessary to improve the thermal insulation performance of glass. The The simplest and most effective method is plating low-emissivity coatings on the surface of glass^[1]. That glass is called as low-emissivity (Low-E) glass. There are two main categories of low-emissivity coatings: metal films and transparent conductive oxide (TCO) films^{[2, 3]}. The emissivity of metal films is low, but its transmittance is also low^[4]. Compared with metal films, the transmittance of TCO films is higher. Among all kinds of TCO films, Al doped ZnO (AZO) films become the research focus due to nontoxicity, rich raw materials and low cost^{[5, 6]}. In this study, AZO films were deposited by DC magnetron sputtering. Effects of temperature on the emissivity were researched in detail.

The AZO films were deposited on glass substrates by DC magnetron sputtering. AZO target (2 Wt% Al₂O₃) was used. The thickness was kept around 500 nm. After deposition, the AZO films were put in muffle furnace for heat treatment. Films were kept at 100~400 ℃ respectively for 1 h and then cooled to room temperature in the furnace.

The film thickness was examined using a surface profiler (Ambios, XP2). The phase was examined by X-ray powder diffraction (Philips X’ Pert diffractometer). The surface morphology was observed with a scanning electron microscope (SEM) (JSM-6360, Japan). The resistivity was obtained by four-point probe method. The transmission was measured by UV-Vis spectrophotomtery (Shimadzu uv3150, Japan). The mean emissivity in 8~14 μ m wavebands was measured with an infrared emissivity measurement instrument (ISTP IR-2, China) by adopting the reflection method^[7].

Fig.1 shows X-ray diffraction (XRD) spectra of AZO films. All AZO films show hexagonal wurtzite structure and obvious (002) preferred orientation, which is a typical texture for AZO film^{[8, 9]}. With the annealing temperature rising to 300 ℃, the intensity of (002) diffraction peak increases, and the full width of half maximum (FWHM) narrows down. Whereas the intensity of (002) peak greatly decreases and the intensity of (102) diffraction peak obviously increases with the temperature further rising to 400 ℃. An appropriate increase of annealing temperature is beneficial to improve the crystalline quality. But if the annealing temperature is too high, the random thermal motion of atoms enhances greatly, which is not beneficial to the preferred orientation of the films.

Fig.1

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	Fig.1 XRD results of AZO films before and after annealed

Fig.2 shows the surface morphologies of the AZO films. After annealed, the grain size becomes larger. During the process of annealing, film atoms will get more energy at higher temperature to migrate, leading to the growth of grains.

Fig.2

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	Fig.2 SEM micrographs of the AZO films after annealing (a) as-deposited, (b) 200 ℃, (c) 300 ℃

Fig.3 shows the resistivity of AZO film. The resistivity firstly decreases and then increases with the increasing temperature, which is consistent with previous report^{[10, 11]}. The resistivity of the AZO film annealed at 200 ℃ is the lowest as 0.9× 10^-3 Ω · cm. The decrease of resistivity is attributed to the growth of grains. In addition, the film annealed in air will absorb oxygen, resulting in the reduction of oxygen vacancies and carrier concentration^[12]. Therefore, the resistivity increases, when the annealing temperature is further increased.

Fig.3

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	Fig.3 Resistivity of the AZO film before and after annealed

Fig.4 shows the mean emissivity of AZO film. The infrared emissivity decreases and then increases with the increasing temperature. It possesses the minimum value of 0.48 at 200 ℃. According to the results of XRD and SEM, the grains grow up after air annealing, which leads to the decrease of the scatter of infrared photons. Therefore, the interaction between the film and the infrared photon decreases, and the infrared emissivity decreases. When the annealing temperature increases further, the free electron concentration decreases. Reflection of electrons to infrared light decreases and the infrared emissivity increases.

Fig.4

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	Fig.4 Mean infrared emissivity of the film before and after annealed

Fig.5 shows the emissivity of AZO film during the process of heating and cooling from room temperature to 350 ℃. The emissivity increases with the increasing measuring temperature. For the same film, its emissivity increases, after the whole process, of heating and cooling. With the increase of measuring temperature, relative vibration between different ions becomes stronger. It will cause a stronger

production and absorption in the films for infrared photons. Therefore, the infrared emissivity increases during the heating process. The whole process of heating and cooling is equal to the annealing process. So the emissivity of the film after the whole process increases.

Fig.5

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	Fig.5 Infrared emissivity of AZO film as a function of measuring temperature (a): As-deposited; (b): Annealed at 200 ℃

Fig.6 shows the transmittance spectra of AZO films and average transmittance in 400~800 nm wavebands. The average transmittance firstly decreases and then increases, with the lowest as 82% at 200 ℃. The decrease of transmittance of the films after annealed at 100 and 200 ℃ is conducive to more Al element substituting Zn-site. The increase of carrier concentration leads to the increase of photons scattering, resulting in the reduction of transmission. The scattering mechanism in AZO film is mainly ionic hybrid scattering. When the ion hybrid scattering is dominant, the increase of the carrier concentration will lead to the decrease of the transmission of light. When the annealing temperature is more than 200 ℃, the crystallinity of the film is greatly improved, the scattering of the photon is reduced, so the transmission of the film increases.

Fig.6

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	Fig.6 Transmittance spectra of the AZO films (a) and the average transmittance (b)

(1) The AZO films deposited at room temperature and annealed in air show hexagonal wurtzite structure and (002) preferred orientation. The air annealing can promote grain growth and improve the crystallinity of the films.

(2) With the increase of annealing temperature, the resistivity and infrared emissivity of the AZO film decrease firstly and then increase, which is mainly due to the growth of grains and the changes of carrier concentration. The AZO film annealed at 200 ℃ shows the lowest resistivity (0.9× 10^-3 Ω · cm) and the lowest infrared emissivity (0.48).

(3) The infrared emissivity increases with the increase of measuring temperature from room temperature to 350 ℃, which is due to relative vibration between different ions in the AZO films.