We have investigated iron-doped semi-insulating and conducting InP using a variety of characterization techniques. The occupation of the iron acceptor level was determined from measurements of the Fe2+ intracenter absorption and the Fe3+ electron paramagnetic resonance signal. These iron concentrations were then related to the total iron content, the free carrier concentration, and the net donor concentration as determined from an impurity analysis. When comparing the sum [Fe2+]+[Fe3+] with the total iron content of the samples measured by glow discharge mass spectroscopy we find that below about 1×1017 cm−3 no significant formation of precipitates occurs. Analysis of the free carrier concentration in semi-insulating crystals allows us to determine the temperature shift of the iron acceptor level with respect to the conduction band. We obtain a value of 4.7×10−4 eV/K somewhat larger than the band-gap shift of InP (3.8×10−4 eV/K). Due to this temperature shift the thermodynamic position of the iron acceptor level at room temperature is about 0.49 eV below the conduction band. This value is significantly smaller than the low temperature value of 0.63 eV. We believe that the temperature shift of deep electronic levels provides insight into the local electron-phonon coupling. Finally we find that the net donor concentration determined by an impurity analysis appears to be too low to account for the observed [Fe2+] concentrations. We present evidence that the missing donor is related to the well-known hydrogen related local vibrational mode at 2315 cm−1. These results indicate that both hydrogen passivation and native defects are necessary for a quantitative understanding of the compensation mechanism in semi-insulating InP.

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