Infrared absorption in HgTe

A detailed theoretical study on the infrared-absorption behavior of HgTe is presented. An 8×{}8 second-order k{·}p approach is employed to evaluate the absorption coefficient from first principles. This method is suitable for treating direct interband transitions remote from the Brillouin-zone center. Results of our full second-order calculation are in good agreement with experimental data up to at least {}{ω}{}500 meV or k{}0.08×{}2{π}/a₀. Even in the case of HgTe with a Γ₆-Γ₈ energy gap as narrow as -120 meV at T=300 K, our method is still an improvement over the first-order k{·}p approach by about 17% in the neighborhood of {λ}=10 {μ}m ({}{ω}=124 meV) and even more at shorter wavelengths. A substantial enhancement is expected for a wider-energy-gap ({}Eg{}{}1 eV) zinc-blende-structure material. Dominant interband transitions in the infrared region are identified for different compositions of Hg_1-x{Cd}ₓ$Te. This difference between the major types of transitions involved is one of the reasons for a stronger absorption in a semimetallic sample than in a semiconducting sample. The dependence of the absorption coefficient (HgTe) on temperature and dopant concentration is then investigated and explained in terms of free carriers, the joint density of states, and the location of the Fermi level. Direct interband absorption in HgTe is dominant for wavelengths around, and shorter than, {λ}=10 {μ}m at T{}380 K with NA,ND{}5×{}10¹⁷ cm^-3. Effects of interactions with higher-energy bands on the joint density of states and the momentum-matrix elements are also examined through a comparison with results obtained by the first-order k{·}p approach.