Properties of Evaporated Films of Lead Telluride and their Application to Infra-Red Multilayer Filters
C S Evans
PhD Thesis - The University of Reading, Department of Cybernetics (1970)
Precise narrow band filters are required for satellite radiometry in the range 10 to 20µm. In addition short wave blocking filters are needed throughout the range 4 to 60µm for order sorting in grating spectrometers and to supplement narrow band filters. The rejection problem is severe at wavelengths beyond the Plank peak, and adequate multilayer filtering requires both materials with outstandingly high refractive index, such as PbTe, and also low index materials.
Results for the transparency of a series of heavy-halide and covalent low-index films are presented. Preliminary attempts to realise a Fabry-Perot multilayer filter for use at 15µm showed that the absorption in the PbTe layers seriously diminished the peak transmission. Baking in air was found to reduce the absorption, and a systematic investigation of this effect was undertaken, using single films of PbTe. The absorption was found to be due to free carriers, and the heat-treatment is predominantly a reaction with Oxygen. Computations show that, over a useful range, spectral transmission and thickness measurements alone yield the refractive index, to sufficient accuracy for determining concentrations, and also the absorption index. Films of PbTe have up to 8x1018 carriers per cm3 in the absence of Oxygen, falling to less than 5x1017 per cm3 after Oxygen reaction.
Single film measurements showed that the absorption index of the PbTe is significantly affected by the type of vapour source, substrate material, substrate temperature and its subsequent cooling rate, and the chamber pressure of Oxygen. It appeared that the most useful disposable parameters for control of film properties are substrate temperature and Oxygen pressure. Optimum values for these parameters were determined, holding the other parameters of deposition constant.
The effects of substrate temperature and Oxygen pressure are interdependent as a result of variation in PbTe stoichiometry and in Oxygen reaction rate, both with temperature. Oxygen reaction during deposition yields higher transparency than post-deposition treatment at atmospheric pressure, particularly in the region of 15µm for use at 20°C. The choice of deposition temperature is limited by a reduction in refractive index at low deposition temperatures which is not related to free carriers.
PbTe was combined with ZnS and CsI in a series of new multilayer filters designed to give low ripple pass-band performances as well as adequate blocking, for order sorting, up to 40µm. The use of CsI layers, and of PbTe layers for their absorption, are novel features of these designs. A critical narrow band filter at 15µm possessed properties which, as a result of using the determined optimum conditions for PbTe, had carrier concentrations near the intrinsic level of 2x1016 per cm3.