Processing Effects on the Electrical and Optical Properties of Sulfur-Related Defect Centers in Silicon and Similarities

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PROCESSING EFFECTS ON THE ELECTRICAL AND OPTICAL PROPERTIES OF SULFUR-RELATED DEFECT CENTERS IN SILICON AND SIMILARITIES TO THE OXYGEN DONOR*

RICHARD A. FORMAN, ROBERT D. LARRABEE, DAVID R. MYERS, WILLIE E. PHILLIPS AND W. ROBERT THURBER Electron Devices Division, National Bureau of Standards, Washington D.C. 20234

ABSTRACT The properties of sulfur-related defects in silicon are shown to differ dramatically from those that would have been expected on the basis of effective mass theory for a simple substitutional double donor. The ratio of the densities of the sulfur states as measured by capacitance-voltage techniques has been observed to vary in specimens fabricated from the same starting resistivity. Optical absorption studies have shown that the deepest sulfur level has a manifold of ground states which anneal at unequal rates at 550*C. Deep-level measurements show that the thermal emission rate at a given temperature and the variety of effects produced depends on annealing history and total sulfur density. The variability of properties of samples of sulfur-doped silicon is similar to those found for the oxygen donors in silicon, thus suggesting a chemical trend for the column VI impurities in silicon.

INTRODUCTION Impurities with energy levels that are much greater than thermal energy the deep-level impurity centers) are recogfrom the nearest band edge (i.e., nized to be important in the performance of a variety of silicon devices [1-3]; however, methods for the unambiguous characterization of these centers Specimens of silicon doped with sulhave not yet been adequately developed. fur have provided an excellent test for both the experimental and conceptual aspects of the problems associated with the development of an adequate methodPrevious studies of the sulology for deep-level impurity characterization. fur system have shown that the apparent electrical activation energy of the deeper center varies from sample to sample [4], that the two dominant electrically active centers do not arise from two charge states of the same impurity center [5], and that the optical absorption spectrum near 2 M is not assignable to a single center [6]. These results are inconsistent with the prevailing theoretical model that identifies the two dominant electrically active centers as arising from an isolated substitutional sulfur atom [7] and instead suggests that these levels may arise from different sulfur-related complexes. As defect complexes introduced in silicon by radiation damage are known to react and evolve under thermal treatment [8], the changes in characteristics of the sulfur-related centers with thermal annealing were examined in order to investigate the nature of these defects and also to explore the *The work was conducted as part of the Semiconductor Technology Program at NBS. Portions of this work were supported by the Division of Electric Energy Systems, Department of Energy (Task Order A021-EES). Not subject to copyright.

80 causes of specimen-to-specimen variations in present study includes isotherma

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