A photoacoustic spectroscopic investigation of the optical energy gap in Zn–Mn–Se type dilute magnetic semiconductors

Using photoacoustic spectroscopy (PAS) we have measured the composition dependent optical absorption coefficient and band gap of the Znl−xMnxSe (0≤x≤0.5) diluted magnetic semiconductors at room temperature. The band gap E0 estimated from the PAS spectra varies nonlinearly, showing a downwards bowing with the Mn-concentration. The exchange interaction of electrons in conduction and valence bands with the d-electrons of Mn ion interactions, effects of polytypes, micro-structures and mixed crystallization (zinc-blende and wurtzite structures) are considered for the analysis of the data. The observed exponential edge (Urbachʹs edge) can be considered as an internal Franz–Keldish effect arising from the charged impurity-generated and “frozen-in” optical phonon-generated fields. This can be described in the framework of the Tauc and Dow–Redfield model. The phonon-assisted indirect transition at the band tail regions for some samples are also observed from the present studies.