Cryogenic transmittance measurements of large high-resistivity CdZnTe for immersion grating applications in the GREX-PLUS mid-infrared high-resolution spectrometer

We are developing a high-resolution mid-infrared spectrograph (HRS) with a cadmium zinc telluride (CdZnTe) immersion grating (IG) for the GREX-PLUS mission. The HRS aims to achieve a resolving power of R∼30,000 over 10–18 µm, requiring a high-index material with sufficiently low absorption at cryogenic temperatures. In our previous work, we demonstrated that high-resistivity CdZnTe (∼10^10 Ωcm) satisfies this requirement at ≤20 K. Building on this result, large (∼6 inch) CdZnTe ingots have recently been fabricated, enabling extraction of IG-sized blanks. We report new transmission measurements of 1 and 10 mm-thick samples cut near the extraction region of a large ingot with the same crystallographic orientation as the IG optical axis. The samples show a resistivity of ∼10^9 Ωcm. We also improved thermal stabilization of the measurement system and investigated the rapid increase in transmittance below ∼10 K, possibly due to freeze out of residual electrons.