Two-electron photo-ionization from deep states in semiconductors

Abstract

Photo-ionization (P-I) from deep states in semiconductors can be used to explore the band structure of the crystal as well as to give detailed information about the symmetry and other properties of the bound states themselves. When 2 electrons are bound to a defect the P-I spectrum at high photon energy reflects the initial and final state properties of both electrons. Only at low energies, where one electron is transferred into its 1-electron ground state does the P-I process of a 2-electron state resemble that of a 1-electron state. This fact offers a resolution of the two different interpretations of P-I from the 0- state in GaP as reported by Henry et al. from photo-capacitance studies and by Grimmeiss et al. from photo-conductivity. In the latter the 4 lowest enercry thresholds (Ei = 0.65 to 1.19 eV) correspond to “1-electron” P-I with the 0 center left in its neutral ground state. Only above about 1.4 eV do the “2-electron” processes appear. In photo-capacitance the “2-electron” processes dominate (the “1-electron” thresholds being too weak or too diffuse to be identified), and the spectrum is strongly modified by relaxation of the lattice around the 0+ ion.