Interpretation of luminescence in GaAs : Cr : 0.839 eV and 0.574 eV lines

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Abstract
The 0.839 eV luminescence line in GaAs : Cr is generally interpreted as an internal transition in Cr2+ ion (5E-5T 2) on gallium site. This interpretation does not agree with E.P.R. and far-infrared absorption results. White has proposed an interpretation as an excitonic recombination on an isoelectronic deep centre. We propose an alternative explanation in which the 0.839 eV line would be an internal transition in a 5D ion (d4 or d6) in C 3V symmetry. The 0.839 eV line would correspond to a 5E*- 5E transition and the 0.75 eV hump to the 5E*-5A 1 transition. The excited state 5E* is satisfactory described by a crystal field treatment whereas the 5E ground state needs a vibronic description E ⊗ ε in trigonal symmetry. For that state we get a ratio EJT /ħω = 2.2. The fine structure levels I and J respectively at 1.50 and 2.56 meV are interpreted as the first two excited vibronic states (A1 and A2) by including a weak non linear coupling. The fine structure of the ground state 5E is described using an operator Lz submitted to a reduction factor p = 0.03. One électron considerations lead to the interpretation of the 0.839 eV line as a transition related either to a Cr2+ ion (d4) on gallium site coupled to a donor (sixth column) on arsenic site or to a Cr0 ion (d6) interstitial coupled to an acceptor on arsenic site. Moreover, the luminescence line at 0.574 eV is interpreted as the transition corresponding to the capture of a hole on the same centre