Characteristic features of the laser radiation–target interactions during reactive pulsed laser ablation of Si targets in ammonia

Abstract

We performed electron microscopy studies of targets subjected to the multipulse laser irradiation in the regime characteristic of reactive pulsed laser deposition from bulk Si in low-pressure ammonia. Experimental evidence is provided concerning the expulsion of liquid droplets from the crater that forms on the target surface. In our opinion, the main mechanisms responsible for droplets spraying directly from the crater are in this case liquid phase expulsion, under the action of the recoil pressure of the ablated substance and hydrodynamic instabilities at the target surface. Cracks of various sizes and orientations have been observed beneath the crater surface. These cracks might cause an increase in the ablation and deposition rates with time. A qualitative theoretical model was developed to account for crack formation based on the peculiar behavior of Si, which has a smaller density in bulk than in liquid phase.