Impurity Interactions with Dislocations in Silicon
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IMPURITY INTERACTIONS WITH DISLOCATIONS IN SILICON KOJI SUMINO The Research Institute for Iron, Steel and Other Metals, 2-1-1 Katahira, Sendai 980, Japan
Tohoku University,
ABSTRACT A review is presented of some aspects on the interactions of light element impurities with dislocations in silicon crystals on the basis of the works done by the author's group. Topics taken up are gettering of impurities by dislocations, electrical and optical activities of impurity atoms accumulated at the dislocation core, immobilization of dislocations due to impurity gettering, and the mechanism of strengthening of silicon wafers due to doping of impurities such as oxygen and nitrogen. INTRODUCTION Interactions between impurities and dislocations offer many interesting subjects to be studied from both fundamental and practical view points. The dislocation core has a peculiar atomic arrangement that is never realized in a bulk crystal. Impurity atoms absorbed in the dislocation core may be in the state different from that in the matrix crystal. The electrical and optical activities of impurity atoms in the dislocation core may be different from those in the matrix. There may also be the possibility that some special chemical reaction takes place along the dislocation core. Practically, interaction of impurities with dislocations is now widely utilized in electronic industry to remove harmful impurities from electroor opto-active regions of device elements. This is the gettering of impurities with the use of dislocations. Further, slip and warpage of silicon wafers due to thermal cycling in device fabrication process are now effectively avoided by the use of silicon that involves oxygen or nitrogen at a relatively high concentration. This utilizes the strengthening of silicon originating from impurity-dislocation interaction. This paper gives a brief review on some fundamental aspects of impurity -dislocation interactions in silicon related to the above subjects on the basis of the investigations conducted by the author's group. GETTERING OF IMPURITIES BY DEFORMATION-INDUCED DEFECTS During high temperature deformation of a silicon crystal involving supersaturated impurities, the concentration of the impurity atoms dissolved in the crystal decreases at a very high rate that is never realized in static annealing of a dislocation-free crystal. The decrease in the concentration of dissolved oxygen during deformation proceeds so rapidly that almost all the supersaturated oxygen atoms are removed within 1 h at 900 'C while about 100 h is needed in the dislocation-free crystal to accomplish such a reduction of supersaturated oxygen atoms by static annealing at the same temperature [l]. Similar enhanced reduction of dissolved impurities by deformation is also observed for nitrogen atoms dissolved in a silicon crystal [2]. Plastic deformation at elevated temperature induces various kinds of defects into a silicon crystal according to transmission electron microscopic observations as shown in Fig. l(a) [3]. They consist of usual dislocations
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