E. Franke

Infrared switching electrochromic devices based on tungsten oxide

Different types of electrochromic devices for thermal emittance modulation were developed in the spectral region from mid- to far-infrared (2–40 μm). In all devices polycrystalline and amorphous tungsten oxide have been used as electrochromic and ion storage layer, respectively. Two types of all-solid-state devices were designed, one with a metal grid for the top and bottom electrode deposited on a highly emissive glass substrate, and another with a top metal grid electrode and a highly reflecting bottom metal electrode layer. Tantalum oxide is used as an ion conductor in both device types. The third device type consists of a polymeric ion conductor. All solid-state constituent layers were grown by either reactive or nonreactive dc or rf magnetron sputtering in a high vacuum environment. Modulation of the emittance is accomplished by reversible insertion of Li ions into polycrystalline WO3 by applying and switching a small voltage across the structure. Spectrally dependent measured reflectance modulation ...

Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry

Amorphous tantalum oxide thin films were deposited by reactive rf magnetron sputtering onto [001] silicon substrates. Growth temperature, oxygen partial pressure, and total gas pressure have been varied to obtain thin films with different densities. The thin films were analyzed by glancing angle-of-incidence x-ray diffraction, atomic force microscopy, and variable angle-of-incidence spectroscopic ellipsometry in the near infrared to vacuum ultraviolet spectral region for photon energies from E=1 to 8.5 eV, and in the infrared region from E=0.03 to 1 eV. We present the dielectric function of amorphous tantalum oxide obtained by line shape analysis of the experimental ellipsometric data over the range from E=0.03 to 8.5 eV (40 μm–145 nm). In the infrared spectral region the ellipsometric data were analyzed using Lorentzian line shapes for each absorption mode observed in the spectra. Amorphous tantalum oxide optical properties in the near infrared to vacuum ultraviolet spectral region were extracted by usin...

All-solid-state electrochromic reflectance device for emittance modulation in the far-infrared spectral region

All-solid-state electrochromic reflectance devices for thermal emittance modulation were designed for operation in the spectral region from mid- to far-infrared wavelengths (2–40 μm). All device constituent layers were grown by magnetron sputtering. The electrochromic (polycrystalline WO3), ion conductor (Ta2O5), and Li+ ion-storage layer (amorphous WO3), optimized for their infrared (IR) optical thicknesses, are sandwiched between a highly IR reflecting Al mirror, and a 90% IR transmissive Al grid top electrode, thereby meeting the requirements for a reversible Li+ ion insertion electrochromic device to operate within the 300 K blackbody emission range. Multicycle optical switching and emittance modulation is demonstrated. The measured change in emissivity of the device is to 20%.

ANISOTROPY OF BORON NITRIDE THIN-FILM REFLECTIVITY SPECTRA BY GENERALIZED ELLIPSOMETRY

Generalized variable angle spectroscopic ellipsometry (gVASE) over the photon energy range from 1.5 to 3.5 eV has been used to study and distinguish the hexagonal and cubic phases of boron nitride in thin films (50–500 nm) deposited by magnetron sputtering onto (100) silicon. Furthermore, gVASE is used to characterize the anisotropic reflectivity of the hexagonal phase films with different microstructures.

Optical properties of amorphous and polycrystalline tantalum oxide thin films measured by spectroscopic ellipsometry from 0.03 to 8.5 eV

Amorphous tantalum oxide thin films were deposited by reactive r.f. magnetron sputtering onto silicon[001] substrates. Growth temperature, oxygen partial pressure and total gas pressure have been varied for optimizing thin film density. The as-deposited films were annealed in atmosphere at 700°C and transformed into a polycrystalline state. The thin films were analyzed by glancing-angle-of-incidence X-ray diffraction, and variable angle-of-incidence spectroscopic ellipsometry in the near infrared (NIR) to vacuum-ultra-violet (VUV) spectral region for photon energies from 1 to 8.5 eV, and in the infrared (IR) region from 0.03 to 1 eV. We present the IR dielectric functions of amorphous and polycrystalline tantalum oxide, which were obtained by analyzing the experimental ellipsometric data with Lorentzian lineshapes for each phonon resonance absorption. We observe a shift of the characteristic phonon absorption in tantalum oxide to lower frequencies upon sample annealing. The optical properties in the NIR to VUV were analyzed by using a parametric model approach. The dielectric functions obtained thereby were further locally fitted by Lorentzian lineshapes in order to analyze critical point structures due to electronic band-to-band transitions.

Low-orbit-environment protective coating for all-solid-state electrochromic surface heat radiation control devices

We suggest use of low-orbit space ambient protective surface coatings for our recently reported all-solid-state electrochromic surface-heat emissivity modulation device in order to (i) to improve the infrared emissivity modulation upon use of the protective layers antireflection effect; and (ii) protect the layer stack within hazardous space environments. We study the optical properties of ZnSe as potential coating material before and after electron cyclotron resonance oxygen plasma treatment, which is a simulation of the low earth orbital environment. The influence on the device performance is evaluated.

Phase and microstructure investigations of boron nitride thin films by spectroscopic ellipsometry in the visible and infrared spectral range

Spectroscopic ellipsometry over the spectral range from 700 to 3000 cm−1 and from 1.5 to 3.5 eV is used to simultaneously determine phase and microstructure of polycrystalline hexagonal and cubic boron nitride thin films deposited by magnetron sputtering on (100) silicon. The results are obtained from a single microstructure-dependent model for both infrared and visible-light thin-film anisotropic dielectric functions. The optical behavior of high c-BN content thin films is described by an effective medium approximation. We obtain the amount of h-BN within high c-BN content thin films. A thin oriented nucleation layer between the silicon substrate and the high c-BN content layer is demonstrated. The preferential arrangement of the grain c axes within the h-BN thin films are found to be dependent on the growth parameters. The results from the infrared and visible spectral range ellipsometry model are compared to each other and found to be highly consistent.

In situ ellipsometry growth characterization of dual ion beam deposited boron nitride thin films

Pure hexagonal h, as well as mixed-phase cubic/hexagonal c/h boron nitride (BN) thin films were deposited onto [001] silicon substrates using the dual ion beam deposition technique. The BN thin films were grown under UHV conditions at different substrate temperatures and ion beam bombarding parameters. Thin-film growth was monitored using in situ spectroscopic ellipsometry at 44 wavelengths between 420 and 761 nm. The in situ ellipsometric Ψ and Δ data were compared with two-layer growth model calculations for the mixed-phase c/h BN, and with one-layer growth model calculations for pure h-BN growth. In situ data provide information on the optical properties of deposited h-BN and c/h-BN material, film thickness, and BN growth rates. A virtual interface approach is employed for the optical properties of the silicon substrate. The growth and nucleation of c-BN observed here confirms the cylindrical thermal spike model. The results for composition and thickness of the BN films were compared to those obtained ...

Infrared ellipsometry on hexagonal and cubic boron nitride thin films

Infrared spectroscopic ellipsometry (IRSE) over the wavelength range from 700 to 3000 cm−1 has been used to study and distinguish the microstructure of polycrystalline hexagonal and cubic boron nitride thin films deposited by magnetron sputtering onto (100) silicon. The IRSE data are sensitive to the thin-film layer structure, phase composition, and average grain c-axes orientations of the hexagonal phase. We determine the amount of cubic material in high cubic boron nitride content thin films from the infrared optical dielectric function using an effective medium approach.

Oxygen plasma effects on optical properties of ZnSe films

Zinc selenide is an infrared transparent semiconductor material being considered for use in space as an infrared optical coating. In this work, zinc selenide thin films of different thicknesses were exposed to an electron cyclotron resonance generated oxygen plasma, often used to “simulate” the low earth orbital environment. The maximum fluence used in our experiments was equivalent to ∼16 years in the low earth orbital environment. ZnSe thin film optical constants (both before and after oxygen plasma exposure) were determined using variable angle spectroscopic ellipsometry from the vacuum ultraviolet at 146 nm through the middle infrared to 40 μm. A parametric dispersion model (Herzinger–Johs) was successfully used to fit the optical data over the entire range from ultraviolet to infrared. Comparing the pre- and post-oxygen plasma exposure data, few changes were observed in the middle infrared region, while drastic changes were seen in the vacuum ultraviolet through visible to near infrared (0.73–8.5 eV). This suggests that chemical changes upon plasma exposure, including oxidation, are found mainly in a thin layer near the surface. As the proposed application is for infrared coatings, and few infrared changes were seen under conditions roughly equivalent to 16 years in low earth orbit, ZnSe may indeed be useful for space infrared applications. Performance simulations of ZnSe coated infrared-operating electrochromic thermal-control surfaces confirm this conclusion.