Fundamental Optical Absorption Edge in MnIn2Te4 Single Crystals

Abstract

The optical properties are studied in the region of the fundamental absorption edge of MnIn2Te4 single crystals (142m, defect stannite) oriented along (100) and (001). The width of the energy gap is measured to be Eg = 1.55, 1.425, and 1.41 eV, for the temperatures 77, 300, and 370 K, respectively. The α(ℏω) spectra are established to follow Urbach's rule throughout the temperature range studied, the steepness of the absorption edge remaining constant. Crystal annealing followed by fast cooling shifts the absorption edge longward by 50 meV, the exponential character of α(ℏω) persisting within the whole range T = 77 to 370 K. An analysis shows the optical absorption in the region of the fundamental edge to be governed by the sum of the effects of the density-of-states tails associated with a high concentration of lattice vacancies and of the electron-phonon interaction. The optical linear dichroism of the absorption edge of MnIn2Te4 crystals with defect stannite structure is discovered and studied. It is found that the crystals belonging to the tetragonal syngony, such as MnIn2Te4 (142m) and CuInSe2, CuInTe2 (142d), and exhibiting a weak tetragonal lattice deformation τ ≈ 0, reveal, in contrast to most chalcopyrites with τ > 0, a positive linear dichroism of transmittance close to Eg. Minimal direct transitions in MnIn2Te4 are allowed in the E ⊥ c polarization, the crystal splitting of the valence band at the Γ point being positive. [Russian Text Ignored].