Mass transport mechanisms during excimer laser nitriding of aluminum

Surface layers of aluminum nitride were formed by irradiating pure aluminum substrates in nitrogen atmosphere with a pulsed excimer laser. The beam was focused on the sample placed inside a chamber filled with nitrogen gas. The irradiation was carried out at various laser fluences, nitrogen gas pressures, and numbers of pulses in order to investigate the influence of each parameter on the nitrogen incorporation and the mass transport mechanisms. X-ray diffraction showed the formation of polycrystalline AlN phase with the wurtzite structure, and the analysis of the nitrogen depth profiles by means of resonant nuclear reaction Analysis revealed a monotonic increase of the nitrogen concentration with the ambient gas pressure and the number of laser shots. It has been found that the laser fluence directly determines the temperature of the substrate and strongly changes the transport mechanism. The thermal simulations and the experimental evidence show that for fluences higher than 3 ${\mathrm{J}/\mathrm{c}\mathrm{m}}^2$ the temperature of the substrate exceeds 2900 K. This value is higher than the dissociation temperature $\left(\sim 2400\hspace{0.5em}\mathrm{K}\right)$ and close to the melting point $\left(\sim 3070\hspace{0.5em}\mathrm{K}\right)$ of AlN, which can therefore dissociate or melt. The atomic nitrogen can rapidly diffuse to greater depths in the liquid Al matrix or it can degas (outgas) through the surface of the sample, leading to the formation of rather homogeneous concentration profiles. For fluences lower than 3 ${\mathrm{J}/\mathrm{c}\mathrm{m}}^2$ the temperature of the substrate is not sufficient to destroy the nitride phase and the AlN grains can move inside the molten Al. In this case, the material transport can be attributed to Brownian motion and thermophoretic drift, which in turn are correlated with the chemical and thermal gradient, respectively.