Indirect absorption edge of TlGaSe2 crystals

Absorption spectra of high-quality layered TlGaSe2 single crystals were measured in a wide temperature range. Results obtained at low temperatures show that the direct excitonic absorption initiating at EGdir = 2.127 eV is perturbed by several series of step-like features inherent for the indirect type transitions. Spectral modelling of these features revealed that: (i) the ground state of indirect exciton, n = 1, has a large Rydberg energy of about 100 meV, and (ii) the enhanced absorption to excited indirect excitonic states, n = 2, 3, exceeds the classical prediction for scaling factors K1ind/n3. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)