Optical absorption related to Fe impurities in TlGaSe2

Absorption measurements of TlGaSe2 crystals intentionally doped with Fe show growing optical transitions below the band gap energies as impurity concentration increase. The comprehensive theoretical analysis is performed for elucidation of subband absorption spectra by using valence band-to-Fe centre optical transitions. We identify three deep donor levels at 1.33, 1.475, and 1.77 eV, respectively, with rather similar doping and temperature dependences. Energy position of levels follows temperature shrinkage of the lowest direct band gap rather than of the indirect band gap. Electron–phonon and static structural broadening is evaluated and the corresponding configurational coordinate diagram of Fe centres is constructed. Subtraction of calculated band-to-Fe centre absorption from the total absorption spectrum shows that the fundamental absorption edge structure of the TlGaSe2 crystal remains almost unchanged at least to the doping level of Fe = 0.5%. However, Fe impurities cancel low-temperature photoluminescence.