Anomalous temperature dependence of absorption edge in narrow-gap HgCdTe semiconductors

Abnormal temperature dependence of absorption edge is reported for narrow-gap Hg_1-xCd_xTe semiconductors at low temperature. Infrared absorption spectra are taken for bulk and molecular-beam epitaxial Hg_1-xCd_xTe, respectively, in the temperature range of 11-300 K. The results indicate an abnormal shift of the absorption edge around the temperature range of 30-70 K. Analysis suggests that (i) the phenomenon is introduced by Hg vacancies in the samples, of which the energy level locates about 9-12 meV above the Hg_1-xCd_xTe valence band, and (ii) the conventional criterion for the determination of band gap energy, E _g, from absorption spectra is not accurate enough as soon as Hg vacancies exist, especially at a temperature above 77 K. It hence provides an explanation why there should exist difference between the cutoff wavelength of the detector and the absorption-edge (E_g) wavelength of the material the detector was made of.