Band-to-band optical absorption in narrow-gap Hg_1-xCd _xTe semiconductors

The infrared absorption spectra have been measured at temperatures from 4.2 to 300 K for thin bulk samples of Hg_1-xCd_xTe with compositions, x, from 0.170 to 0.443 with thicknesses smaller than 10 μm. The spectral range of the investigations includes the exponential absorption edges and intrinsic absorption spectra determined by electron transitions from the valence band to the conduction band. From the measured absorption spectra, the energy-band gap was determined by the photon energy at the absorption transition point between the Urbach exponential region and the Kane region. The data were fitted by a calculation of the intrinsic optical transition using Kane's theory. The dependence of the energy-band gap on composition and temperature was determined in the alloy composition range from x=0 to 0.443 and for x=1, for temperatures from T=4.2 to 300 K. The absorption coefficient, α_g, at the band-gap energy versus composition and temperature, and the exponential rule satisfied by the absorption edge are also obtained.