The deep level optical spectroscopy by the so called DLOS technique gives the spectral shape of both σon(hv) and σ op(hv), the optical cross sections for the transitions between a deep level and the conduction and valence bands, in a very sensitive and selective way. We have applied this technique to the optical study of the HL1 trap related to chromium in GaAs, in very pure LPE layers of n and p type doped with chromium. We can therefore determine what kind of charge states of chromium we are dealing with by çomparing σo n and σop with the optical absorption spectra in bulk materials. Moreover, comparisons between the excitation spectra of luminescence and the photoionization cross section σo n and σop have given unambiguous interpretation of the origin of the two luminescence bands at 0.57 and 0.62 eV found by YAG laser excitation in bulk Cr doped GaAs depending on the Fermi level position. They are interpreted as the radiative transitions between the (Cr3+-Cr2+ ) centre and the bands. In this way, we give a coherent description of the thermal and optical properties of the single deep acceptor level corresponding to the Cr 3+ or Cr2+ states according to its electronic filling

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In order to explain the band tails experimentally observed in undoped semiconductors especially in GaAs, a model for band tails associated with inhomogeneous strain fields has been developed. An analytical law for the optical absorption edge is deduced and compared with experimental results obtained in bulk or epitaxial GaAs samples. Two cases are considered: spherical and cylindrical strain fields. The origin of such strain fields, dislocations or precipitates, is discussed.