The current research demonstrates the effectiveness ofboth silicon and germanium as transmissive materials for use within the far infrared wavelength range of2O to 160 microns. This study involves samples with a wide range ofresistivities and temperatures including: n-type Si of4000, 2000, 160, 65, 12, and 2.6 ohm-cm and p-type Si of 500 and 60 ohm-cm within a temperature range of -100°C to 250°C and n-type Ge of39, 25, 14.5, 5.0, 2.5, and 0.5 ohm-cm within a temperature range of-100°C to 100°C. Far infrared absorption mechanisms are briefly discussed. The experimental absorption data are used to discuss the interaction between absorption by lattice resonance and free carrier absorption. Highly resistive germanium and silicon are both found to be excellent transmissive materials in the far infrared. These studies may be used to develop the feasibility of silicon and germanium as optical windows or lenses within an extraterrestrial environment.

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The current report demonstrates the temperature vs. transmission vs. resistivity relationship for the less explored infrared wavelength range of 6 to 22im for silicon (Si) and 10 to 22im for germanium (Ge) over the temperature range of -1 00°C to 25°C. These studies involve a wide range of resistivities. Material samples include n-type Si of 4000, 160, and 12 ohm-cm, and n-type Ge of 35, 2.5, and 0.5 ohm-cm. Silicon has useable transmission bands (above 20%) only between 1.2 and 8.5im. between 14 and 15 .6jnn, and greater than 2OFim with best transmission occurring between 1.2 and 6.5tm. Germanium has a useable transmission band between 2 and l7tm with best transmission between 2 and I 1 .5gm. The temperature dependence of infrared transmission becomes more pronounced with increasing wavelength (6 or 10 to 22gm): 1.5% to I 1.5% and 3% to 9.5% for silicon and germanium respectively over the temperature range of -100°C to 25°C. The 4000 ohm-cm Si sample (Float Zone) exhibits significantly greater transmission at wavelengths of both 9.0 and 19.5 microns. The temperature dependence of lattice absorption is observed in germanium. This study builds a bridge between previously determined absorption mechanisms of the near and far infrared ranges and may be used to develop the feasibility of silicon and germanium as optical windows or lenses within an extraterrestrial environment.

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Both silicon and germanium are widely used as transmissive elements in the infrared region ofthe spectrum. Both materials are typically used in applications where significant temperature ranges exist. In this work we report transmission in the wavelength range of 1.39 to 22 un and in the temperature range ofroom temperature (25°C) down to -100°C for silicon and germanium samples of various resistivities. The data presented indicate an orderly change in transmission with decreasing temperature for the various sample resistivities. Absorption coefficients are calculated from the transmission data.