Evidence for volume boiling during laser ablation of single crystalline targets

The surface morphology of monocrystalline Ge and Si samples irradiated by the fundamental, second and forth harmonic of a pulsed Nd:YAG laser at fluences typically used for laser ablation has been investigated by scanning electron microscopy. Whenever the samples were irradiated by the 1064 nm radiation, which is poorly absorbed by the solid targets, round-shaped micrometer-sized cavities were found on the irradiated area even after one single laser pulse. Irradiation by the 532 nm or 266 nm radiation resulted in a smooth, featureless surface. It is shown here that the only possible cause for the cavities formation during irradiation by the 1064 nm radiation is the subsurface, explosive, volume boiling. Numerical estimations of the temperature depth-profiles evidenced the formation in this case of a thick and superheated liquid layer, a condition which favours the occurrence of volume boiling. q1999 Elsevier Science B.V. All rights reserved.