ELECTRONIC PROCESSES IN LASER-INDUCED GA(0) EMISSION AND LASER-ABLATION OF THE GAP(110) AND GAAS(110) SURFACES

We have carried out sub-monolayer sensitivity measurements of laser-in duced Ga0 emission from the GaP(110) and GaAs(110) surfaces using a re sonant ionization technique. The laser ablation threshold fluence is d efined as the critical fluence above which an increase of the Ga0 emis sion yield is observed by repeated irradiation with laser pulses on th e same spot but below which a decrease of the yield is observed. The p hoton-energy dependences of the laser ablation threshold fluence and t he sub-ablation emission yield have been investigated for photons belo w, near and above the bulk band-gap energies in both GaP and GaAs. For photons below the band-gap energies, the sub-ablation emission and la ser ablation are observed and ascribed to photon absorption by defects on the surfaces. The sub-ablation emission is found to be reduced sub stantially at a photon energy slightly below the band-gap energies for both GaP and GaAs; the reduction is of resonance type for GaAs and st epwise for GaP. Only a small amount of emission is observed for GaP fo r photons above the band-gap energy. The ablation threshold fluence fo r GaAs shows a resonant-type increase, corresponding to the resonance- type reduction of the sub-ablation emission yield, while the ablation threshold fluence for GaP does not change on crossing the band-gap ene rgy. For photons above the band-gap energies, the ablation threshold f luence decreases with increasing photon energy in a similar manner for both GaP and GaAs. The decrease in the ablation threshold fluence for the GaAs(110) surface appears to be correlated to the increase in the sub-ablation emission yield as observed previously. These results of the photon energy dependence of the sub-ablation emission yield and ab lation threshold fluence are explained in terms of the electronic exci tation of defects on surfaces and of surface occupied states.