SOFT LASER SPUTTERING OF GAAS SEMICONDUCTOR (100) SURFACE

We have studied the soft laser sputtering of (100)GaAs with 337 nm pho tons, starting from the threshold for particle emission (a few tens of mJ/cm(2)) to some 300 mJ/cm(2) fluences. Atoms and molecules sputtere d from the irradiated surface are detected, their relative number meas ured, and their time of flight determined using laser resonant ionizat ion mass spectrometry. The surface after laser irradiation is examined by scanning electron microscopy and electron microprobe analysis. One observes a significant preferential emission of arsenic in the form o f As-2. This leads to the formation of perturbed Ga-rich surface struc ture which appears even at low fluence and after a few tens of laser s hots on the same spot. This initial transformation seems to determine the further evolution of the irradiated surface. First, Ga atoms aggre gate to form Ga islands on the surface; after a sufficient number of s hots, micrometric structures are produced which finally behave as pure Ga metal. This evolution of the surface state after multipulse irradi ation appears practically the same for low and medium laser fluences, the only difference being in the number of shots required to obtain th e same microscopic structure. The velocity distribution of Ga atoms an d As-2 molecules is well fitted by half-space Maxwellian distributions . The kinetic temperatures are in broad agreement with the results obt ained from a model of laser heating of the surface. The gross features of the experimental results can be interpreted from the particular th ermodynamics properties of GaAs which exhibits very large As-2 pressur e above the solid as soon as the temperature exceeds 950 K. After a fe w laser shots, corresponding to particle emission from defect sites, t he thermodynamics of GaAs appears to govern the further evolution of t he laser-sputtered surface. Two sputtering regimes are evidenced: In t he low-fluence regime (from threshold to 90 mJ/cm(2)) sputtering appea rs to be dominated by surface defect emission, whereas for higher flue nces emission is more characteristic of thermal process accompanied by preferential sputtering of arsenide. According to these experimental results, a simple analytical model was developed which relates the qua ntitative surface to the quantitative sputtered cloud compositions. (C ) 1995 American Institute of Physics.