Photoemission from activated gallium arsenide. I. Very-high-resolution energy distribution curves

Abstract

The energy distribution curves (EDC's) of the photoelectrons emitted from the (100) face of a p-type doped (\ensuremath{\sim}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) GaAs crystal, activated to negative electron affinity in ultrahigh-vacuum conditions, is investigated. The study is performed at 300 and 120 K under well-focused ${\mathrm{Kr}}^{+}$-laser excitation and with a very-high-energy resolution (20 meV). The analysis of the EDC's as a function of the photon energy, mainly at low temperature, is shown to provide a very direct picture of the GaAs band structure away from the Brillouin-zone center. The experimental results are well fitted by a spherical, nonparabolic k\ensuremath{\rightarrow}\ensuremath{\cdot}p\ensuremath{\rightarrow} perturbation calculation of the coupled conduction and valence bands, for electron kinetic energies up to 1 eV in the central \ensuremath{\Gamma} valley. The essential role played by the subsidiary L and X minima in the energy relaxation and photoemission processes is evidenced. The main contribution to the total emitted current is due to electrons which were thermalized in the bulk \ensuremath{\Gamma} minimum and have lost an average energy \ensuremath{\simeq}130 meV in the band-bending region prior to emission into vacuum. The band-bending value is shown to be \ensuremath{\ge}0.5 eV. The yield and time evolution of GaAs photocathodes are discussed. This detailed study leads to a reexamination of the pioneer work of L. W. James and J. L. Moll [Phys. Rev. 183, 740 (1969)] and to a good understanding of the photoemission properties of activated GaAs.