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The Infrared Absorption of GaP Single Crystals with Silicon and Oxygen Impurities

Infrared transmission measurements of GaP with Si and O donor impurities at liquid helium and room temperatures reveal absorption lines due to excitation of the donor electrons and excitations of localized phonons. Observed spectra for Si donor are used to determine the optical ionization energy of 82.5±0.05 meV for Si donors substituting for the Ga site in the GaP crystal. A fitting of Kasami's effective-mass calculation for donor states to the observed silicon donor levels yields the effective masses of the conduction band: m⊥=(0.198±0.002)m0 and m//=(1.65±0.02)m0. The absorption line at 464 cm-1 is due to the excitation of localized oxygen phonons substituting for the P site, and the absorption line at 495 cm-1 is due to the excitation of localized silicon phonons substituting for the Ga site, taking account of Lifshitz's theory of lattice vibrations around an impurity of different mass.