Infrared photoconductivity of shallow impurities in GaP

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Abstract
The extrinsic photoconductivity of GaP doped with the donors S, Se, Te, or Si and with the acceptors C or Zn has been measured at liquid‐helium temperatures in the 400–3000‐cm−1 region of the infrared. The ionization energy of these impurities was determined to be 103.5, 102, 90, and 78 meV, respectively, for the donors and 49 and 71 meV, respectively, for the acceptors. Dips in the spectral response of the donors are interpreted as primarily due to electron capture from the conduction band to excited states of the donor atoms together with emission of LA(X) and LO(Γ) phonons. Excited states are observed in the photoconductive spectrum to which transitions from the ground state are forbidden in optical absorption. The spectral response of the acceptors show a prominent oscillatory response due to interaction with LO(Γ) phonons and to capture into the acceptor ground state. Smaller dips due to hole capture from the valence band to excited states are also observed in the p‐type crystals, but the excited states responsible have not all been identified in the acceptor spectra.