(Ga,Mn)As has become a model diluted ferromagnetic semiconductor, in which ferromagnetic ordering between substitutional Mn²⁺ ions occurs owing to interaction with spin-polarized holes, as it was successfully explained within the frames of the p-d Zener model just over a decade ago [1]. However, the theoretical understanding of the band structure and the underlying ferromagnetic interactions are still under debate. An alternative model assuming an anticrossing-induced formation of a Mn-related impurity band in the band gap and the Fermi-level location in that band was proposed [2]. The band anticrossing model was developed previously to account for the observed strong red shift of the interband optical transition and a giant spin-orbit splitting in Ga(Bi,As) alloy [3].

In the present study, we have investigated thin 50-100nm (Ga,Mn)As and Ga(Bi,As) layers (Mn: 4% - 7%, Bi: 1 - 2%). These films were grown on semi-insulating (001) GaAs substrates with LT-GaAs and InGaAs (with the In content from 3.4% to 30%) buffers in order to introduce compressive or tensile strain in the (Ga,Mn)As or Ga(Bi,As) epitaxial layers. Some of the Ga(Bi,As) epi-layers were grown pseudomorphically on the LT-GaAs and InGaAs (with 10% of In content) buffers or partially and fully relaxed on InGaAs buffers with 20% and 30% of In content. The most of (Ga,Mn)As epitaxial films were fully strained, only few of them, grown on the InGaAs with high In composition were partially relaxed. The alloys compositions and the nature and magnitudes of the strain in the investigated epitaxial films were analysed by the high resolution X-ray diffractometry results. These investigations were supported by Raman spectroscopy measurements. The analysis of the fundamental properties of the epi-layers were performed using the modulation PR spectroscopy measurements, which enabled the determination of the band gap (E₀) and spin-orbit split-off (E_SO) band-to-conduction-band optical transitions. The full-line-shape analysis of experimental PR spectra allowed us to determine the energies of interband transitions and the electro-optic energies for light and heavy holes in all the investigated layers. Photoreflectance results for Ga(Bi,As) have shown significant red shift of E₀ transition energy with increasing  Bi content. This shift is strongly depended on the strain parameters. On the other hand, small changes of fundamental gap (blue or red shift) have been observed in (Ga,Mn)As epitaxial layers subjected to different value of Mn content and type and magnitude of the strain.

 Our experimental results are consistent with the valence-band origin of mobile holes mediating ferromagnetic ordering in the (Ga,Mn)As and in agreement with the anti-crossing model in case of the Ga(Bi,As).

[1] T. Dietl, H. Ohno and F. Matsukura, Phys. Rev. B 63, 195205 (2001)

[2] K. Alberi, K.M. Yu, P.R. Stone, O.D. Dubon, W. Walukiewicz, T. Wojtowicz, X. Liu and       J.K. Furdyna, Phys. Rev. B 78, 075201 (2008)

[3] K. Alberi, J. Wu, W. Walukievicz, K. M. Yu, O.D. Dubon, S.P. Watkins, C.X. Wang,       X. Liu, Y.-J. Cho and J. Furdyna, Phys. Rev. B 75, 045203 (2007)

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