Alloying of a thin Al layer with p-Si under laser irradiation

Abstract

Aluminium is not quite suitable for doping Si by thermic diffusion in a furnace because of the small solubility of Al atoms in Si and the formation of eutectic between them at C. Laser bombardment, as well as ion implantation, enhances the solubility of Al atoms in Si and a high concentration of impurity atoms is reached. Under these conditions during solidification a gradient of the concentration of the impurity atoms arises in front of the moving L-S boundary, which could lead to instability of the L-S boundary and to the formation of inhomogeneities in the near-surface region of Si. These can deteriorate the electrical characteristics of the semiconductor elements. Inhomogeneities in the near-surface region of Si, occurring upon irradiation of a thin aluminium layer deposited in vacuum on p-Si by an Nd:YAG laser, have been tested. The cross-sections of the irradiated samples were photographed, electron microscope studies performed and x-ray diffraction patterns taken. It was found out that when laser energy density is higher than intensive mixing of the melted Al layer and the melted near-surface region of the Si substrate arises during laser treatment. Inclusions, enriched with impurity atoms, as well as a cellular structure, have been observed. The results have been explained by overcooling, developed during solidification in the areas which are just in front of the moving L-S boundary.