Investigation Of Thermal Stability Of Low-emissivity Films

The high-temperature thermal stability of low-emissivity （Low-E） films hasimportant theoretical significance and practical worth, but has not been investigated andcharacterized systematically. F-doped SnO₂（FTO） films deposited by chemical vapordeposition （CVD） and Ag-based multilayer films deposited by magnetron sputteringmethod were studied and post-treated at high temperatures in the muffle furnace andtoughening furnace, respectively. The temperature and time effects on the structures, filmmorphologies and photoelectrical properties were investigated by X-ray diffraction （XRD）,field emission scanning electron microscope （FESEM）, atomic force microscope （AFM）,UV-Vis spectrophotometer and physical property measurement system （PPMS）. Theroom-temperature photoluminescence （PL） mechanism of FTO films were studied withthe micro-Raman Spectroscopy. The element distribution in the films has been researchedby X-ray photoelectron spectroscopy （XPS） and Auger electron spectroscopy （AES） andthe conductive mechanism has been discussed. The specific research contents are asfollows:FTO films with SnO₂:F/SiO_xCystructure possess good high-temperature thermalstability, the temperature resistance can be improved to600℃and the longest time is15min. When the temperaute and time are higher or longer than this condition, the opticaland electrical properties of FTO films become deteriorative. The main component in thefilm interior is non-stoichiometric SnO_2-x:F （x≈0.8⁰.9） with polycrystalline cassiteritetetragonal crystal structure and the most obvious preferred orientation of （200） plane, Fatoms occupy the substitutional positions in SnO₂lattice. The room temperaturephotoluminescence spectra of FTO films can be divided into three parts in the region of325⁶50nm. The ultraviolet emission is attributed to a recombination of photogeneratedelectrons with the holes. The violet emission at385.20nm is related with the fluorinesubstitutions, while the violet emissions at409.33nm and441.69nm belong to oxygenvacancies with a positive chargeVO and with two positive chargesVO, respectively.The bluegreen emission is related with the surface states and structural defects. The [O]/[Sn] ratio in the film interior has been less effected by tempering temperature andtime and has little contribution to the deterioration of conductivity. With the increasingtemperature and time, the relative content of oxygen vacancies at the surface layerdecreases, the diffusion and lattice distortion between FTO layer and SiO_xCylayerbecome more severe, which is in good agreement with the increase of film resistivity.Compared with the Ag-based films with glass/TiO₂/ZnO/Ag/NiCrO_x/SnO₂/Si₃N₄multilayer structure, when pre-deposited1.1nm NiCrO_xand12nm Si₃N₄under Ag layer,the modified Ag-based films with glass/Si₃N₄/TiO₂/NiCrO_x/ZnO/Ag/NiCrO_x/SnO₂/Si₃N₄multilayer structure possess better thermal stability and the highest temperature resistancecan be improved to600℃. When the modified Ag-based films are treated at60±1℃withthe humidity of90±2%, the water resistence has been improved to17days. The multilayerfilms contain a crystalline Ag layer with3C structure and amorphous layers. Atransformation of the preferred orientation from （111） plane to （220） plane occurs whenthe temperature is higher than600°C. When the modified Ag-based films were temperedat675±25℃for6min in the toughening furnace, some amount of Ag2O exists in Ag layer.The ratio of the relative content of CrIbonded with oxygen and CrIIin amorphous states inthe under NiCrO_xlayer increases from0.62to0.74after tempered for6min, whichindicates the oxidization of Ag layer may be resulted from the diffusion from the underlayers. The improved compactness and crystallinity of the functional Ag layer aftertempering play a positive role in decreasing the film resistivity.