Potential of magnetron sputtered magnesium fluoride containing thin films for the multilayer design of coloured coatings for solar collector glazing

In this work, the potential of magnetron sputtered magnesium fluoride (MgF2) and MgF2 containing composite coatings for coloured solar collector glazing is investigated. Coloured thin-film interference coatings on the reverse side of the collector cover glass give solar collectors an aesthetic appearance, which facilitates their integration into the building’s envelope. Hereby, integration means that the solar collector is no longer recognisable as technical device. Moreover, its appearance is the one of an architectural design element. Four years ago we showed that it is possible to match the colours of solar glazing with those of commercial sun protection glasses by means of thin-film optical filters. These filters are based on alternating high- and low-refractive index materials. Adding MgF2 or Mg–F–Si–O as a second low-refractive-index material enables even more flexibility and freedom to design coatings with a specific reflection colour, especially for bright colours. A novel concept of coloured filters involving MgF2 has been developed. They exhibit – independent of their colour hue – a uniform high solar transmittance 85–85.7% combined with a bright reflection. First results are very promising and confirm the future potential of MgF2-containing multilayers for coloured solar collectors

High-fidelity patterning of AlN and ScAlN thin films with wet chemicaletching

We report on the anisotropic wet etching of sputtered AlN and Sc0.2Al0.8 N thin films. With tetramethyl ammonium hydroxide at 80 degrees C, the etch rates along the c-axis were 330 and 30 nm/s for AlN and Sc0.2Al0.8N, respectively. Although the etching was anisotropic, significant lateral etching below the mask occurred, perpendicular to the c-axis. With a 1 mu m Sc0.2Al0.8 N film, it could be up to 1800 nm. We studied the lateral etching with molybdenum, SiO2, SiNx and TiO2 masks, and found the leading cause for the lateral etching to be modification of the AlN or Sc0.2Al0.8 N surface caused by ion bombardment and surface oxidation by ambient air. The lateral etching was reduced by optimizing the mask deposition and with thermal annealing. With Sc0.2Al0.8 N, the lateral etching was reduced down to 35-220 nm depending on the mask, while with AlN, it was reduced to negligible. These results can be used for developing optimised mask deposition processes for better etch characteristics and for microfabrication of AlN and ScxAl1-xN thin-film structures.Peer reviewe

Air cooling powered by façade integrated coloured opaque solar thermal panels

For building integration of solar-powered energy systems, aesthetic aspects play an importantrole. Covering a standard solar collector with a coloured glazing, opaque to the human eye but highly transparent to solar energy, permits a perfect architectural integration of solar thermal panels into glazed building façades. The thermal energy produced can be used for both solar heating and cooling, as well as for domestic hot water. The principle of the coloured appearance is based on interference in the thin-film coating on the reverse side of the cover glass. Different interference filters based on nano-composite materials deposited by the solgel method were presented at CISBAT 2007 [1]. Currently, we are developing new plasma-deposition processes, which are more suitable for industrial large-scale production. A new state-of-the-art ultra-high vacuum (UHV) system for magnetron sputtering deposition of novel nano-composite solar coatings has recently been designed, constructed, and installed at the Solar Energy and Building Physics Laboratory (LESO-PB). Up to five different magnetron sources can be used simultaneously, in reactive and non-reactive mode. The geometric configuration of the chamber has been optimised for best film homogeneity and allows the deposition on substrates up to 100 mm in diameter. The optical and electronic properties of thin films are closely interrelated and highly relevant for solar coatings. Photoelectron spectroscopy provides information on the coating structure, the deposited material and its chemical state inside the coating, as well as the nature of the interface between different layers. A system for ESCA analysis (Electron Spectroscopy for Chemical Analysis) has recently been installed and put into operation at LESO-PB. By ellipsometry and spectrophotometry, we can determine exactly the different optical properties of the coating, such as layer thickness, refractive index, or absorption coefficient. This provides best conditions for highly efficient research and development on new materials for further optimisation of the coloured interference filters.First results have been obtained with our new experimental infrastructure and will be presented in this contribution

Coloured coatings for glazing of active solar thermal façades by reactive magnetron sputtering

For building integration of solar-powered energy systems, aesthetic aspects play an important role. Covering a standard solar collector with a coloured glazing, opaque to the human eye but highly transparent to solar energy, permits a perfect architectural integration of solar thermal panels into glazed building façades. The thermal energy produced can be used for both solar heating and cooling, as well as for domestic hot water. The principle of the coloured appearance is based on interference in the thin-film coating on the reverse side of the cover glass. Different interference filters based on nano-composite materials deposited by the solgel method were presented at CISBAT 2007 [1]. Currently, we are developing new plasma-deposition processes, which are more suitable for industrial large-scale production. A new state-of-the-art ultra-high vacuum (UHV) system for magnetron sputtering deposition of novel nano-composite solar coatings has recently been designed, constructed, and installed at the Solar Energy and Building Physics Laboratory (LESO-PB). Up to five different magnetron sources can be used simultaneously, in reactive and non-reactive mode. The geometric configuration of the chamber has been optimised for best film homogeneity and allows the deposition on substrates up to 100 mm in diameter. The optical and electronic properties of thin films are closely interrelated and highly relevant for solar coatings. Photoelectron spectroscopy provides information on the coating structure, the deposited material and its chemical state inside the coating, as well as the nature of the interface between different layers. A system for ESCA analysis (Electron Spectroscopy for Chemical Analysis) has recently been installed and put into operation at LESO-PB. By ellipsometry and spectrophotometry, we can determine exactly the different optical properties of the coating, such as layer thickness, refractive index, or absorption coefficient. This provides best conditions for highly efficient research and development on new materials for further optimisation of the coloured interference filters. First results have been obtained with our new experimental infrastructure and will be presented in this contribution

Growth, Properties, and Applications of Al1-xScxN Thin Films

The discovery of enhanced piezoelectricity in solid solutions of AlN and ScN is certainly one of the most important events in piezoelectric MEMS. As compared to pure AlN, it brought a crucial factor 2 to 3 improvement in a number of figures of merit governing the performance of MEMS devices. The aim of this contribution is to give a short overview on actual topics in processing, properties, and applications in RF filters and sensors

Ex-situ AlN seed layer for (0001)-textured Al0.84Sc0.16N thin films grown on SiO2 substrates

It is more difficult to nucleate AlN-ScN alloy thin films (AlScN) in pure (0001)-texture than it is with pure AlN thin films. AlN thus can serve as seed layer for AlScN. Equipment limitations may lead to the problem of a vacuum break between AlN and AlScN deposition, as it leads to oxidation of the AlN surface. This issue was studied with high resolution TEM and electron diffraction. The formed oxide layer disturbed a lot the epitaxial growth, leading to additional grain orientations. A mild RF etching step introduced before AlScN deposition was able to remove the oxide layer, and allowed for growing Al0.84Sc0.16N in local epitaxy on AlN, as shown by Hyper map EDX images. The resulting AlScN films show a pure (0001) texture. Double beam laser interferometry and finite element modeling were used to determine d(33),(f) of both layers together as 6.6 pm/V, and of 6.85 pm/V for AlScN alone when using the standard value of 3.9 pm/V for pure AlN. At the same time, the relative dielectric constant of Al0.84Sc0.16N was determined as 14.1