Laser implantation of impurity atoms into silicon and gallium-arsenide single crystals

Abstract

Sn and Fe have been incorporated into the bulk of silicon single crystals by means of an irradiation of thin metallic surface layers of these elements with light pulses from a Nd-glass laser. Analogously, Sn has been implanted into gallium-arsenide single crystals and furthermore, a successful implantation of Sn into GaAs has been achieved by an irradiation from the backside of the sample. Depth profiles of the implants have been determined by Rutherford backscattering analysis. The state of the impurity atoms has been studied by conversion-electron Mössbauer spectroscopy. The surface topography of Sn-implanted silicon was analyzed by scanning electron microscopy. The dependence of the number of implanted impurity atoms and their depth profiles on the laser power density and the thickness of the deposited surface layer has been investigated. The fraction of atoms implanted from the surface layer decreases with increasing layer thickness. For a given layer thickness, the implanted fraction reaches a maximum for a certain laser power density. The impurity atoms can be implanted to a depth of several thousand angstroms with nearly homogeneous depth profiles for suitably chosen conditions. The concentrations exceed the solid solubilities by orders of magnitude. These findings are consistent with a qualitative model for the implantation process assuming a detailed balance between an evaporation of the metallic surface layers and the incorporation of the impurity atoms into the bulk by diffusion in a molten substrate layer. After the recrystallization, as inferred from the Mössbauer spectra, Sn is found on substitutional lattice sites in silicon and on substitutional Ga sites in GaAs, whereas Fe in silicon precipitates in agglomerates. Upon thermal annealing, the depth profiles relax towards more homogeneous distributions. No segregation of Sn from the supersaturated solution is indicated; however, drastic changes in the Fe Mössbauer spectra after annealing indicate that a fraction of these atoms may occupy well-defined lattice sites, whereas the majority remains in agglomerates.