Metastable supersaturated solutions of nitrogen in rapidly-solidified silicon
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I. INTRODUCTION Nitrogen is playing an increasingly important technological role in crystalline Si and at Si/SiO 2 interfaces. Although group V elements such as P or As are readily incorporated at high concentrations into Si as n-type dopants, N is not. From a technological viewpoint, intentional introduction of low concentrations of N has been shown to increase the strength of low oxygen content wafers,1'2 and beneficial effects on electrical characteristics have been reported for incorporation of N at Si/SiO 2 interfaces.3 In other applications, ion implantation has been used to introduce N into Si for local oxidation control4 and for buried dielectric layer formation.5 To optimize these beneficial N-related effects it is important to understand the atomic bonding configurations and electrical characteristics of N in Si. Several systems exhibit impurity incorporation in excess of equilibrium solubility during rapid crystallization following a pulsedlaser-induced melt.6 Studies of impurity-impurity and impurity-defect interactions in such materials complement studies near equilibrium. Incorporation of highly nonequilibrium impurity concentrations is readily accomplished for impurities with small segregation coefficients and low equilibrium solid-solubility. Nitrogen in crystalline Si with an equilibrium solid solubility of —4.5 x 1015 cm"3 near the melting point and a segregation coefficient7 of ~10~ 3 is an example of such an impurity, and we have found that laser-induced melting is a powerful technique for the study of this technologically important system. We report here on N incorporation above equilibrium solubility in rapidly solidified Si layers as measured by infrared (ir) absorption, secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and 616
electrical conductivity. Nitrogen was implanted into crystalline Si to concentrations four orders of magnitude above maximum equilibrium solid solubility. The effects of pulsed laser-induced melt and rapid crystallization upon the implanted N profiles, crystalline quality, chemical bonding configurations, and electrical activity were measured. II. EXPERIMENTAL DETAILS A four-inch diameter (111) high resistivity float zone Si wafer was polished and implanted on both sides at approximately 50 °C with 1015 cm"2 ions of 14N at each of the energies, 60, 100, and 180 keV, where the calculated8 projected ranges are 165, 275, and 480 nm and standard deviations are 60, 80, and 115 nm. Following implantation the wafer was diced into 0.63 X 0.63 cm2 samples for ir absorption, SIMS, and TEM measurements after selected furnace annealing and laser-induced melting. In order to have an insulating substrate under the N-implanted Si layers for electrical measurements, N was implanted at 50, 85, and 150 keV at doses of 0.5, 0.7, and 1.0 x 1015 cm" 2 , respectively, into 0.5 mm thick heteroepitaxial (100) Si on sapphire (SOS) substrates. Calculated ranges8 for these energies are 135, 230, and 410 nm with standard deviations of 50, 75, and 100 nm. The doses in
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