Impurity Distribution Profiles and Surface Disorder after Laser Induced Diffusion
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IMPURITY DISTRIBUTION PROFILES AND SURFACE DISORDER AFTER LASER INDUCED DIFFUSION. E. FOGARASSY, R. STUCKP. SIFFERT Centre de Recherches Nucl6aires Laboratoire PHASE , 67037 STRASBOURG CEDEX France F. BROUTETJ. C. DESOYER Facult6 des Sciences POITIERS France ABSTRACT A model for laser induced diffusion is proposed, assuming the melting of Si surface under pulsed laser irradiation and the diffusion ,in liquid phase, of a thin filmof impurity deposited according to the "limited source" conditions. Depending on the thickness of the film, it results in a dopant distribution profile with a surface disordered layer induced either by segregation effects or precipitation of the dopant in excess of the solubility limit achieved by laser annealing. Experimental results, obtained for a ruby laser irradiation of thin films of different impurities of group III and V, like antimony, bismuth, gallium and indium deposited on silicon substrates are in good agreement with the model. Their effective segregation coefficients have been deduced by fitting the experimental amount of dopant precipitated in the disordered surface layer with the numerical calculations. A cellular structure is seen on surface. INTRODUCTION Laser induced diffusion which consists in a thin film deposition on silicon followed by pulsed laser treatment has been demonstrated to be an interesting technique for junction formation especial ly for solar cell fabrication [ 1,21*Like ion implantation combined with laser anneal i ng it allows to tailor the doping profile and to reach d ping levels in excess of the thermal equilibrium solubility IimitL 3 1 • Whereas several models allowing to calculate the doping concentration profiles have been presented for ion implantation with subsequent laser anneal ing [ 4, 5 ], no model exists so far, for laser induced diffusion. Such a model is necessary to determine the concentration of dopant which is not int egrated in substitutionnal lattice sites, either because it is accumulated at the surface due to segregation effect, or precipitated because it exceeds the solubility limit achieved by laser treatment. It allows thus to analyze the role of the thickness of the deposited film on the formation of the disordered surface layer. The model presented here is a strictly thermal model assuming diffusion of the dopant in the liquid phase occuring from a limited source located at the surface. A good agreement is obtained between the profiles calculated by this model and those measured by Rutherford Backscattering SPectrometry (R. B. S.). The effecti ve segregation coefficients of Sb, Ga, Bi and In have been deduced from the experimental amount of dopant precipitated in the disordered surface layer, which presents a cellular structure, as demonstrated by transmission electron microscopy.
Mat.
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13 (1983)D Elsevier Science Publishing Co.,
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312
THE MODEL Basically the calculations have been performed for irradiations with a Q-switched ruby laser ( k = 0. 69 Pm ) single pulses of 20 ns duration. We
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