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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.