Optical Absorption Due to Excitation of Electrons Bound to Si and S in GaP

Infrared transmission measurements of GaP with S or Si donor impurities at liquid-helium temperature reveal absorption lines due to excitation of the donor electron. Strong absorption peaks corresponding to the 1s→2p_± transition are observed, and their spectral positions are used to determine optical ionization energies of 104.2 and 82.5 meV (±{}0.3 meV), respectively, for S and Si donors in GaP. A fitting of Faulkner's effective-mass calculation for donor states to the observed levels yields the effective masses of the conduction band: m_⊥=(0.191±0.005)m₀ and mII=(1.7±0.2)m₀. The p-like final states observed in these measurements are found to differ significantly in binding energy from the excited donor-electron states observed in the "two-electron" recombination of the exciton bound to neutral sulfur in GaP.