LASER-INDUCED REACTIVITY OF NH3 ON GAAS SURFACE

We have studied the reactivity of NH3 on GaAs (100) surface irradiated by 280 nm laser beam at low fluence (< 50 mJ cm(-2) per pulse) in an experimental set-up which allows a very sensitive characterization of the surface state using mass spectrometry. Thanks to this analysis tec hnique we could follow desorbed NHx (x = 2,3) species during the laser treatment; this phase achieved, GaN+ could be identified as an eviden ce of N fixation using laser desorption mass spectrometry of the treat ed surface. This characterization mode demonstrates the fluence and la ser shot number dependencies on laser treatment efficiency. At 280 nm the N fixation is maximum for laser fluence of about 25 mJ cm(-2) (per pulse). Still even in the best nitridation conditions, the process ap pears to be of little efficiency, since it leads to the formation of l ess than one monolayer of GaN. These results combined with a numerical model of GaAs laser heating give evidence for the laser-induced nitri dation to be thermally assisted.