Formation of buried Sb dopant profiles in silicon by pulsed laser epitaxy
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W. C. Sinke FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098SJ, Amsterdam, The Netherlands
R. Pretorius Ion-Solid Interaction Division, Van de Graaff Group, National Accelerator Centre, P.O. Box 72, Faure, 7131, South Africa (Received 28 December 1990; accepted 14 December 1992)
In this investigation buried Sb dopant profiles in single crystal silicon have been formed from evaporated layers using laser annealing. For irradiations carried out in air, severe oxidation of the surface layers inhibited epitaxy. Oxygen concentrations as high as 5 X 1017 atoms/cm 2 (equivalent to about 105 nm SiO2) were measured. It was found that both the thin (less than 3 nm) Sb layer and the free volume in the a-Si, deposited by evaporation onto a cold substrate, need to be present for this degree of oxidation to take place. However, when silicon was evaporated onto a substrate heated to 350 °C, dense packing of the silicon atoms was obtained and even for irradiations in air good epitaxy (minimum yield of 7%) and no oxidation occurred. To form buried Sb profiles, laser energies only slightly higher than the threshold for epitaxy were used to prevent excessive spreading due to an increase in liquid state diffusion obtained at higher energies. Under these conditions the width of the buried Sb profile was typically about 120 nm, and up to 90% of the Sb atoms were found to occupy lattice sites.
I. INTRODUCTION Dopant profiles buried in single crystal silicon are important in bipolar devices1 and are usually formed by ion-implantation into the silicon lattice followed by an annealing step to restore crystal integrity and to electrically activate the dopants. The depth and the concentration of doping are tailored by the energy and dose of the implantation beam, respectively. Hitherto, furnace annealing served the electronic industry well, but the shrinking real estate on the silicon chip requires that materials scientists investigate other processing techniques. One of the drawbacks of furnace annealing is that very abrupt junctions are not readily obtainable. Furthermore, the disadvantages associated with bulk heating become a problem as the industry moves toward ultralarge-scale integration. Pulsed laser beams can produce intense localized heating and temperatures exceeding the melting temperature of semiconductor materials, in the near surface regions for very short (ns-ms) time periods. For these very short interaction times, dopant redistribution is restricted to the molten zone, making abrupt junctions possible.2 Doping of silicon with Sb by ion-implantation, followed by laser annealing, has been successfully carried out.3 Lasers have also been used to dope silicon from evaporated Sb layers4 or SbCl3 solutions.5 In this inJ. Mater. Res., Vol. 8, No. 4, Apr 1993 http://journals.cambridge.org
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vestigation we have studied the formation of buried Sb dopant profiles by successive evaporation of Sb and an amorphous silicon layer, followed by irradiation with a pulsed, 2 thickness of 105 nm) were
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