Soft laser sputtering of the GaAlAs (100) surface

We have studied the soft laser sputtering of (100)GaAlAs with 337 nm photons, starting from the threshold for particle emission (a few tens mJ/cm2) up to about 300 mJ/cm2. Atoms and molecules sputtered from the irradiated surface are detected, their relative number measured and their time-of-flight determined using laser resonant ionisation mass spectrometry (RIMS). After laser irradiation the surface is examined by scanning electron microscopy (SEM) and electron microprobe analysis (EMA). From the shot number and the fluence dependencies of the sputtering yield, it is shown that two sputtering regimes exist. For low fluence (< 150 mJ/cm2), the sputtering results mainly from the absorption and excitation of defect sites. At higher fluences the process is similar to thermal evaporation. One observes preferential emission of As in the form of As2 molecules and the correlated Ga and Al-enrichment of the surface with formation of GaAl micron-sized structures. However, As preferential laser sputtering is rapidly counterbalanced by excess surface Ga and Al atoms leading to a stationary sputtering regime after a few thousand laser shots.