Cadmium Telluride (CdTe)
Of the long wavelength (>18µm) transparent II-IV materials available, cadmium telluride has proven to provide good optical performance across a wide range of temperatures and has provided adequate mechanical robustness to be used as a substrate material. Compared to the limited selection of alternative materials capable of transmitting in these long wavelengths (viz. KRS-5, KRS-6, CsI, CsBr, Diamond) CdTe has a high resistance to moisture sensitivity, is available at a reasonable price and can operate at elevated filter deposition temperatures without disassociating. It is however also the softest of the II-VI materials and is most easily scratched or prone to cleaving. The external transmittance spectrum has an electronic absorption edge at approximately 0.83µm which corresponds to an energy band gap of 1.49eV at 293K, and a far-infrared multi-phonon absorption edge commencing at approximately 27µm.
Marple et al originally measured the refractive index at 300K from a prism of melt-grown material with polished surfaces in 1962 from which the following modified Sellmeier equation was derived;
where A = 5.68, B= 1.53, and C2 = 0.366
For CVD material, in 1966 Ladd was able to fit a modified Sellmeier equation of the following form and established a temperature coefficient of 1.0x10-4 /K
n2-1 = A1λ2/(λ2-λ21) + A2λ2/(λ2-λ22)
with A1 = 6.1977889, A2 = 3.2243821, λ21 = 0.1005326µm2, and λ22 = 5279.518µm2.
Subsequent measurements of CVD CdTe by Harvey and Wolfe across a range of reduced temperatures (80-300K) provided adequate data for Barnes et al to fit a temperature-dependant Sellmeier dispersion equation of the form;
where ;A = -2.973x10-4 T + 3.8466, B = 8.057x10-4 T + 3.2215
C = -1.10x10-4 T + 0.1866, D = -2.160x10-2 T + 12.718
E = -3.160x101 T + 18753
The temperature-dependence of this model were derived in both linear form and using a quadratic polynomial. The temperature deviation from a straight line fit to the quadratic however resulted in a greater deviation in the quadratic than the experimental evidence suggested.