Optical Absorption of Cr4+ in GaAs:Cr

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Abstract
Optical absorption experiments are reported on p‐type GaAs:Cr:Zn from which the spectral shape of the absorption due to stable Cr4+ is derived in the energy region from the onset at about 0.4 eV up to the GaAs absorption edge. This absorption is interpreted as due to the photoionization transition Cr4+ + hv → Cr3+ + holevb superimposed by the simultaneously occurring transition Cr3+ + hv → Cr2+ + holevb. Its spectral shape at T = 78 K can well be fitted by a calculated one taking into account the uppermost three valence bands, the 4T1(F) ground state of Cr3+ and the 5T2 ground state of Cr2+ located within the gap, and the electron‐phonon coupling of these localized states. The fit yields the optical threshold energy E3 = 0.68 eV and the broadening parameter Γ3 = 0.24 eV of the Cr4+ → Cr3+ transition indicating a strong coupling of the 4T1(F) state of Cr3+ with TA phonons.