Charge-transfer transitions of Fe ions in InP

We study, in detail, optical transitions of Fe ions in InP. Spectra recorded at different sample temperatures allow reliable assignments of absorption peaks to 3d internal transitions of Fe²⁺. Charge-transfer processes from Fe³⁺ to Fe²⁺ plus a bound hole give rise to multiple absorption bands in the energy range from 6100 to 10 000 cm^-1. For both the Fe²⁺ ion and the bound hole, a series of excited states are observed. In the ground state of the bound hole, a strong interaction with the d-shell electrons of the Fe²⁺ ion diminishes the spin-orbit coupling to 72% of its original value. By comparison with shallow effective-mass-like acceptor series, the hole binding energy of the Fe²⁺ ion is derived. This allows an exact location of the Fe²⁺ levels in the band gap to be determined.