Optical properties of epitaxial SrHfO3 thin films grown on Si

The perovskite SrHfO3 can be a potential candidate among the high-permittivity materials for gate oxide replacement in future metal-oxide semiconductor field-effect transistor technology. Thin films of SrHfO3 were grown by molecular beam epitaxy and compared with SrTiO3 films. Their optical properties were investigated using spectroscopic ellipsometry and analyzed with respect to their structural properties characterized by x-ray diffractometry, atomic force microscopy, and transmission electron microscopy. A band gap of E-g=6.1 +/- 0.1 eV is measured optically, which renders this material better suited for gate dielectric applications than SrTiO3 with E-g similar to 3.4 eV. At similar equivalent oxide thickness, SrHfO3 also exhibits lower gate leakage current than SrTiO3 does. (c) 2007 American Institute of Physics.

Nanostructured Coatings on Glazing for Active Solar Facades

Estelle De Chambrier, Dipanwita Dutta, Maria Cristina Munari Probst, Christian Roecker, Jean-Louis Scartezzini, Josef Andreas Schuler

Thin films on glazing play an important role in the design of solar energy conversion systems as thermal collectors or photovoltaic panels. Antireflective coatings based on porous silicon dioxide are well established in the field of solar collector glazing. Regarding architectural integration of thermal collectors, colored glass panes based on multilayer interference stacks of dielectric thin films offer a smart alternative to the coloration of the absorber sheets. Materials with tunable refractive index are highly desirable in such coatings in order to give access to optimised designs. Thin films of silicon titanium mixed oxides TixSi1-xO2 have been deposited in a particle free atmosphere by a sol-gel dip-coating process. Optical properties of thin films have been characterized by spectrophotometry and spectral ellipsometry. A nanocomposite structure has been evidenced by Transmission Electron Microscopy (TEM). As low tunable refractive index material, novel nanocomposite thin films based on the elements Mg, F, Si and O have been deposited. Transparent and homogenous thin layers were achieved with variable Mg:Si molar ratios. Surprisingly low refractive index values have been found, reaching desired values for anti- reflective applications. Broad spectral transmittance maxima are observed with values up to 99.8 %. The nanostructure of quaternary Mg-F-Si-O thin films has been investigated by TEM, showing evidence of embedded crystalline nanoparticles. In comparison to existing anti- reflective materials, the novel quaternary thin films might be highly interesting regarding improvement of hardness and improved aging stability with respect to pores filling by hydrocarbons. Intermediate size A4 colored glazing based on TixSi1-xO2 films have been produced exhibiting 14.0 and 12.6 % visible reflectance and only small energy losses (1.4 and 3.4 % with respect to the uncoated substrate) showing the high potential of such materials for colored solar facades. A real-sized prototype of colored thermal solar collector has been realized, providing a convincing demonstration device.

EPFL2007

Sol-gel deposition and characterization of low refractive index nanocomposite Mg:F:Si:O thin films for solar and protective coatings

Estelle De Chambrier, Dipanwita Dutta, Christian Roecker, Jean-Louis Scartezzini, Josef Andreas Schuler

Coatings for thermal solar energy conversion have to be abrasion-resistant, stable at elevated temperatures, and often have to protect the substrate against corrosion. Due to the conditions of refractive index matching between the substrate and the ambient air, low refractive index materials are desired regarding a large variety of optical applications. Novel nanocomposite thin films based on the elements Mg, F, Si and O have been deposited by a sol-gel dip-coating process starting from trifluoroacetic acid, magnesium acetate and alcoxide precursors. Optical properties of this new material have been characterised by spectrophotometry and spectral ellipsometry, and show surprisingly low refractive index values. The experimental data obtained for varying chemical composition have been compared to the optical properties as predicted by Bruggeman and Ping-Sheng effective medium theories. Strong deviations from both theories are reported, suggesting modifications of the involved crystalline phases and/or formation of nanopores. Evidence for embedded crystalline nanoparticles has been found by Transmission Electron Microscopy (TEM). Compared to well known existing low refractive index materials such as porous SiO2 and MgF2, the quaternary Mg:F:Si:O thin films can be expected to show higher hardness and improved aging stability with respect to pore filling by hydrocarbons. The novel thin film material opens promising perspectives for new solar and protective coatings.

2007

Nanocrystalline Low-Refractive Magnesium Fluoride Films Deposited by Reactive Magnetron Sputtering: Optical and Structural Properties

Stefan Mertin, Paul Muralt, Silviu Cosmin Sandu, Jean-Louis Scartezzini

In this work, we study MgF2 thin-film synthesis by reactive pulsed DC magnetron sputtering from a metallic magnesium target in a gas mixture of argon, oxygen, and carbon tetrafluoride (CF4). Nanocrystalline films on silicon and glass substrates with excellent properties for optical application are achieved. The plasma discharge is analyzed with a differentially pumped mass spectrometer before and during the deposition process. Without breaking the vacuum, monochromatic photoelectron spectroscopy (XPS) is performed for in situ determination of the atomic C and O concentration. Film microstructure, topography, and thickness are investigated by electron microscopy (SEM and TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD). The optical constants n and k are determined by spectroscopic ellipsometry and spectrophotometry: a consistent parametric fit of the ellipsometric angles and spectral transmittance and reflectance based on three Lorentz oscillators to determine n and k is achieved for a wide spectral range (300–2 300 nm). At 550 nm, a refractive index of 1.382 and near-zero absorption is obtained, which is in excellent agreement with n = 1.383 of polycrystalline MgF2. The measured light reflection at 760 nm is reduced by 3% for a quarter-wave nanocrystalline MgF2 coating on glass compared to the uncoated glass substrate.

Wiley-Blackwell2015

Nanostructured Coatings on Glazing for Active Solar Facades

Estelle De Chambrier, Dipanwita Dutta, Maria Cristina Munari Probst, Christian Roecker, Jean-Louis Scartezzini, Josef Andreas Schuler

EPFL2007