Ellipsometric study of the electronic structure of Ga_1−xMn_xAs and low-temperature GaAs

We have measured the optical constants of Ga_1−xMn_xAs from 0.62 to 6 eV, using spectroscopic ellipsometry. The second derivatives of the dielectric function are examined through a critical point analysis, allowing us to inspect interband transitions from different points in k⃗ space. The evolution of the band structure over a broad doping range is determined. Specifically, the E₁ critical point shifts to higher energies with increased doping of Mn, while all other critical points appear unaffected. The evolution of the critical points results from the interplay between band-gap renormalization due to ionized impurities and sp-d hybridization of the Mn induced impurity band with GaAs valence and conductions bands.