Multilayered interference coatings based on titanium- and zirconium nitride and designed for solar control have been prepared using reactive d c magnetron sputtering. Preparation effects and degradation mechanisms were investigated. It was shown that the quality of the nitride strongly depends on the degree of crystallinity in the underlying oxide. It has been shown that the nitride layer partly oxidizes as the top oxide layer is deposited. The degradation is enhanced with temperature. A thin sacrificial layer of aluminium deposited between successive depositions of nitride and oxide is shown to improve the optical performance of the coating as preparedm as well as after accelerated ageing tests.
The optical properties of opaque and semitransparent films of zirconium nitride have been studied. A thorough investigation of the influence of composition, deposition rate, substrate temperature and film thickness on the optical response of the film was performed. Both photometric and ellipsometric methods were used to determine thicknesses and the optical constants at wavelengths ranging from 0.23 to 25 μm. The resulting values of n and k, in the wavelength intervals where these independent methods are applicable, have been shown to agree extremely well. The results so far indicate an even larger potential for zirconium nitride based solar control coatings as compared to the titanium nitride based.
Access to optical constants derived from films of zirconium nitride of variable quality made multilayer modelling a powerful tool in the design and analysis of solar control coatings.
Triple-layer structures of TiO2TiN/TiO2 and quadruple layer structures of TiO2Al/TiN/TiO2 have been sputtered on glass substrates at temperatures ranging from room temperature to 300°C. The reflectance and transmittance were measured in the visible and the near-IR wavelength regions. The thin layer of aluminium, in the quadruple layer, oxidizes and forms a dense diffusion barrier. The multilayers exhibit improved optical selectivity which also improves with substrate temperature up to 300°C.
Opaque and semitransparent dc magnetron-sputtered ZrN films on glass and silicon have been optically characterized with spectral reflectance measurements and ellipsometry. High rate sputtered ZrN has good optical selectivity, i.e., higher than 90% infrared reflectance and a pronounced reflectance step in the visible to a reflectance minimum of less than 10% at 350 nm. The results are comparable with those obtained for single crystalline samples and those prepared by chemical vapor deposition. The complex optical constant (N = n v ik) for opaque films has been determined in the 0.23-25-µm wavelength range with Kramers-Kronig integration of bulk reflectance combined with oblique incidence reflectance for p-polarized light. A variable angle of incidence spectroscopic ellipsometer has been used for determination of the optical constants in the 0.28-1.0-µm wavelength region. The results of the two methods show excellent agreement. The results indicate that ZrN is free electronlike and the Drude model can be applied. The best opaque films had Drude plasma energies (ħω(p) between 6.6 and 7.5 eV and relaxation energies (ħ/τ) between 0.29 and 0.36 eV. Ellipsometer data for the semitransparent films show that the refractive index (n) in the visible increases with decreasing film thickness whereas the extinction coefficient (k) is essentially unchanged. The optical properties are improved by deposition upon a heated substrate.
Thin semi-transparent ZrN films have been prepared using reactive dc magnetron sputtering. The films had thickness from 11 to 43 nm and were grown on heated and room temperature glass substrates. The optical constants, N=n+ik, of the thin films have been determined with an RT inversion method in the wavelength interval 0.40 to 2.0 μm. The thickness of the films was determined from the photometric measurements. The optical properties of the thin films on glass were compared to opaque and thin ZrN films grown on single crystalline Si. The Drude parameters were calculated from the measured optical constants in the relaxation region of the thin films. The relaxation time, τ, of the thin films was found to increase with film thickness, substrate temperature and substrate crystallinity. The relaxation time is the mean free time for the electrons between collisions and a long relaxation time corresponds to a film with high optical quality. The observed decrease of τ with decreasing film thickness can be explained by the higher statistical probability of the electrons in a thin film to collide with the two surfaces of the film. Another explanation to the decrease of τ with film thickness is scattering from grain boundaries and lattice impurities. The higher optical quality of films grown on heated substrates is probably due to an increased grain size. The measured optical constants were compared with calculated optical constants, using the Drude model, and the optical behaviour of thin ZrN films was found to be well described by the screened free-electron model.
Transparent heat mirror coatings based on thin zirconium nitride films have been prepared using reactive magnetron sputtering. The zirconium nitride films have been sandwiched between layers of zirconium oxide. It is shown that the multilayer configuration ZrO2/ZrN/ZrO2 can be used as solar control coatings on window glazings. A visible transmittance of around 60% and a thermal emittance lower than 0.2 can be obtained, and the ratio between visible transmittance and total solar transmittance can be as high as 1.7. The influence of substrate temperature on the optical quality of the films is evaluated and it is shown that the crystal structure of the first oxide layer is of importance for the optical quality of the nitride. The influence of preparation conditions and accelerated ageing has been modelled using the optical constants of thin films prepared under identical conditions as the films in the multilayer coatings.