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A p-n junction on a silicon substrate doped with boron ions (d- dopant) is made in following manner. A shallow silicon surface layer including a n-type dopant is first obtained by ion implantation of the substrate with arsenic atoms. The arsenic-doped silicon layer at the surface has a relatively low initial reflectivity. Then, radiation from a pulsed carbon dioxide laser is directed onto the doped surface. A portion of the pulsed radiation causes melting of the thin arsenic-doped layer at the solid surface, giving the shallow melted surface a reflectivity greater than the initial reflectivity of the solid surface. The increased reflectivity of the melted surface prevents an additional portion of the pulsed radiation from causing further melting, thus controlling the depth of melting. The melted surface is then allowed to cool and solidify to form a p-n junction at a thin (less than 200 angstrom) junction depth. 6 figs.