Modelling of a Multi-Valent Impurity, Such as GaAs:Cr
Chromium may participate as a deep lying impurity in GaAs in several ways: substitutional as CrGa or CrAs, or as a member of a complex. The usual Cr impurity studied in most work is believed to be CrGa. Unusual conditions are pointed out that might create forms such as CrAs. The remainder of the paper is specific to CrGa, for which the lattice-neutral condition requires three electrons lost to bonding, CrIII. The acceptor condition CrII is well-known, and there has been considerable evidence for the doubly-ionized acceptor CrI. Less evidence exists for CrGa as an amphoteric impurity with a charge state CrIV (one trapped hole). However, that state is also taken into account in writing a set of coupled equations for transition processes among the charge conditions CrI through CrIV. Complicated splittings of the bound states are discussed, as arise from the anisotropic crystal field, spin-orbit coupling and Jahn-Teller distortions. Energy changes of the system (impurity + host) when an electron or hole is emitted or captured are contrasted with enthalpies of activation. An example from CrIII⇋CrII capture/emission data shows that internal consistency is possible in describing communication of Cr with the conduction and valence bands.
KeywordsCharge State Phonon Transport Capture Coefficient Hole Emission GaAs Lattice
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