Laser-induced electronic processes on GaP (110) surfaces: Particle emission and ablation initiated by defects.

Abstract

We have carried out high-sensitivity measurements of ${\mathrm{Ga}}^{0}$ emission from the GaP (110) surface by laser pulses of photon energies above the indirect-band-gap energy ${\mathit{E}}_{\mathit{G}}$ (range I), below ${\mathit{E}}_{\mathit{G}}$ but above the energy gap ${\mathit{E}}_{\mathit{V}\mathit{S}}$ between the valence band and the unoccupied surface band (range II), and below ${\mathit{E}}_{\mathit{V}\mathit{S}}$ (range III). Below the ablation threshold, we find that laser pulses of energy ranges II and III induce particle emission, the yield of which is reduced as the laser shots are repeated on the same spot. The dependence of the yield on the number of shots shows rapidly (A) and slowly (S) decreasing components. The A component is found to be enhanced by ${\mathrm{Ar}}^{+}$ bombardment and reduced by subsequent annealing. Measurements of the dependence of the pulse width show that the yield is scaled by the fluence for the A component. The yields for both A and S components are superlinear functions of fluence, exhibiting apparent threshold laser fluences. The threshold laser fluence for the A component is found to be smaller than that for the S component. The yield-fluence relations can also be fitted to power functions: The power index for the A component is 2--3 and, for the S component, 4--6. These components are not induced by photons in energy range I. Above the ablation threshold the yield on the same spot is found to increase by repeating laser pulses. Nearly the same ablation threshold fluence was observed for photons in energy ranges I and II, while the ablation threshold is scattered for photons in energy range III. The results are interpreted in terms of a breaking of the bonds of loosely bound atoms near defects on surfaces by multiple electron-hole localization. Three types of defects are differentiated: adatom type, kink type on steps, and vacancy type. The A and S components are ascribed to be initiated by the adatom- and kink-type defects, respectively, and the laser ablation is ascribed to be initiated by vacancies. A model of localization is suggested.