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 $1s\to 2p_{\pm }$ 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: $m_{\perp }=(0.191\mathrm{}\pm 0.005)m_0$ and $m_{\mathrm{II}}=(1.7\mathrm{}\pm 0.2)m_0$. 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.