Excitonic fine structure in the charge-transfer spectra of GaP:Fe

Calorimetric-absorption spectroscopy at ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ temperatures reveals fine structure in the absorption spectra of Fe-doped GaP. A series of sharp lines is observed near 0.83 eV at the onset of the ${\mathrm{Fe}}^{3+}$→${\mathrm{Fe}}^{2+}$ charge-transfer transition resembling a fine structure previously observed in the spectra of Fe-doped InP samples. In both cases, absorption lines are related to the formation of bound excitonic states in the ${\mathrm{Fe}}^{3+}$→${\mathrm{Fe}}^{2+}$ charge-transfer transition. The present data indicate that the formation of such bound electron-hole states in the charge-transfer spectra of deep-level impurities is a common phenomenon in III-V semiconductors.