Development of High Temperature Optical Interference Filters
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Development of High Temperature Optical Interference Filters Thomas C. Parker1 and John D. Demaree1 U.S. Army Research Lab, 4600 Deer Creek Loop, Aberdeen Proving Grounds, MD 21005, U.S.A. 1
ABSTRACT Oblique angle deposition (OAD) is a self-organizing physical vapor deposition (PVD) technique that has been used to grow sculpted 3D nanostructures including helices, slanted rods, and zigzag structures, and other shapes. OAD structures can be fabricated from virtually any material that can be deposited using PVD including: polymers, metals, semiconductors, oxides, and nitrides. The control over the nano-scale structural anisotropy of these materials allows one to tailor their electrical, magnetic, mechanical, crystalline, and optical properties. Through the careful design of the OAD structure and material selection this technique can be used to create photonic materials (1D, 2D, and 3D) with unique properties. We will discuss ongoing work using OAD to develop oxide thin film interference filters that can withstand extreme temperatures (800-1000° C) at mTorr vacuum levels, which are being developed for thermal photovoltaic applications. INTRODUCTION The development of efficient, portable thermal photovoltaic (TPV) power systems depends on the ability to maximize the power transfer from a hot radiating emitter to a photovoltaic element. Thin film optical filters may be used to achieve this goal by rejecting long wavelength light which will not be efficiently captured by the particular photovoltaic material, effectively “recycling” these photons to increase the efficiency of the system. In terrestrial-based light weight TPV systems (350 nm), therefore, are entirely due to the deposited SiO2/Y2O3 filter. There are slight changes in the optical properties of the filters after annealing, primarily a reduction in overall reflectance on all samples, likely due to increased surface roughness and hence increased diffuse scattering. Most spectral features remain unchanged in wavelength, with very small shifts either toward longer wavelengths (samples on silicon) or shorter wavelengths (samples on sapphire). The red shift in Figure 2a is likely due to the growth of the 42 nm thermal oxide under the filter, as discussed in the RBS section above. In effect the original low index porous SiO2 layer is no longer sandwiched between the Y2O3 and the Si substrate. The addition of the thermally grown SiO2 to the film stack effectively increases the height of the low index layer and hence the interference fringes are red shifted. The blue shift noted in the annealed filters on sapphire (Figure 2b) must be due to some compaction of the porous SiO2 during the annealing, but we do not yet have other data to support this supposition. Similarly, the unexpectedly small red shift in the Si sample due to the thermal oxide, as will be discussed in the simulation section below, gives evidence of compaction of the porous SiO2 layer. Optical Simulation The reflectance of the thin films stacks were simulated using the Essential Macleod software. In mod
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