Summary Description

Cadmium telluride is a medium refractive index material (2.67 @ 10 microns) with excellent long wavelength transmission in the infrared. It is non-hygroscopic with  very good chemical resistance. Since it is a multiple  crystalline material, some lots of material can exhibit  large Mie scattering at the shorter wavelengths. It has  been marketed in the past as IRTRAN-6.

Windows, including those used for cooled infrared  detectors, must be thicker than those made from other materials because of the low tensile strength.

Advantages

1. Non-hygroscopic
2. Good chemical resistance except for the very strong acids, for example, HNO₃
3. Excellent spectral range

Disadvantages

1. Expensive
2. Brittle
3. Low tensile strength
4. Mie scattering can reduce the transmission of CdTe in the shorter wavelengths
5. Multi-crystalline structure can vary on a lot basis. This can produce a variability in both the short wavelength transmission and strength of the material
6. CdTe is toxic and expensive

Far-IR Transmission – 1 mm thick CdTe Window

Far-IR Transmission – 4 mm thick CdTe Window

CdTe also transmits at wavelengths longer than 80 microns at very low temperatures. However the transmission is temperature dependent and Si is generally a better choice for a window for this spectral region.

Physical Data¹

Melting Point: 1092 °C
Density: 6.2 g/cm³
Solubility in H₂O: Insoluble
Hardness: 56 kg/mm₂
Appearance: Black, cubic crystalline material, either polished or satin finish

Refractive Index²

Specific index listed. Generic: 2.67 @ 10 microns
**Sopra S. A Material Data Base

Spectral Range

Short Wavelength Limit: 17,000 cm^-1 (1 mm)
Long Wavelength Limit: 313 cm^-1 (1 mm), 340 cm^-1 (4 mm)

Coatings

CdTe can be coated with a BBAR (Broad Band Antireflection) coating. However, this coating will not improve transmission at wavelengths longer than about 16 microns.

Typical Uses

• IR detector windows-CdTe absorbs HeNe laser at 6328. This makes it an attractive window for MCT detectors used in FT-IR systems.
• Liquid cell windows

Notes

Short and Long Wavelength Ranges for which transmissivity is greater than 50% of stated crystal thickness.

1. F. Buch and C. N. Ahlqvist, The yield strength of polycrystalline CdTe as a function of grain size, Material Science and Engineering 13, 194–196 (1974).
2. A.G. DeBell et al. Cryogenic refractive indices and temperature coefficients of cadmium telluride from 6 μm to 22 μm, Appl. Opt. 18, 3114 –3115 (1979).