SURFACE AND BULK ABSORPTION CHARACTERISTICS OF CHEMICALLY VAPOR-DEPOSITED ZINC SELENIDE IN THE INFRARED

Chemically vapor-deposited zinc selenide exhibits outstanding properties in the infrared and has been established as a prime material for transmissive optics applications. Here we Present and discuss data relating to the surface and the bulk absorption forward-looking infrared- (FLIR-) grade chemically vapor-deposited ZnSe, at wavelengths (2-20 mum) and temperatures (100-500 K) of current interest. This investigation is based on both spectral emittance measurements and infrared transmission spectroscopy performed in the context of a systems development program. Surface effects can be detected at wavelengths of up to 14 mum and usually predominate at wavelengths of less than 8 mum. Fractional surface absorptions are temperature independent from approximately 200 to 400 K and cam be fitted to a Fourier series, at wavelengths ranging from 3.5 to 13.5 mum. The bulk absorption coefficient (beta(V)) is strongly dependent on temperature as well as wavelength, but it can be approximated by a bivariate polynomial expression that yields recommended values. At wavelengths lambda less than or similar to 10 mum, beta(V) decreases with increasing temperature; it is shown that a wavelength-independent Debye-Waller factor provides a correct description of the temperature dependence, thus pointing to infrared-active localized modes. At wavelengths lambda greater than or similar to 14 mum, beta(V) increases with temperature and exhibits temperature dependencies (T1.7, T2.6) that reflect three- and four-phonon summation processes. Finally, an analysis of the temperature dependence of beta(V) at 10.6 mum demonstrates that the intrinsic lattice dynamical contribution to bulk absorption at this wavelength should be close to 4 x 10(-4) CM-1, in accord with the results of earlier laser calorimetry tests performed on exceptionally pure laser-grade chemically vapor-deposited ZnSe.