A Lifetime Study of Oxygen Agglomeration Induced Defects in CZ Silicon Crystal by Surface Photovoltage (SPV)
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A LIFETIME STUDY OF OXYGEN AGGLOMERATION INDUCED DEFECTS IN CZ SILICON CRYSTAL BY SURFACE PHOTOVOLTAGE (SPV) KAMAL MISHRA*, W. HUBER* AND JACEK LAGOWSKI** * MEMC Electronic Materials, Inc., St. Peters, MO *
*
Semiconductor Diagnostics, Inc., Tampa, FL
ABSTRACT In this study the effect of oxygen agglomeration on minority carrier diffusion length in as-grown p-CZ silicon has been studied in detail. Oxygen-related defects were found acting as minority carrier traps in p-CZ silicon. These defects are not found in either oxygen-free FZ silicon or in n-type CZ silicon samples. The traps have profound effect on low excitation level diffusion length values leading to an apparent lifetime decrease by as much as an order of magnitude. This effect can be eliminated by a steady state "bias light" superimposed on the chopped excitation light. The traps can be annihilated and re-generated by thermal treatments. Our study has also revealed oxygen-induced recombination centers. Significant improvement in lifetime is realized in p-type CZ silicon after heat treatment between 550 0 C and 800 0C. INTRODUCTION Oxygen is one of the most important impurities incorporated in silicon and the behavior of oxygen in silicon has been the subject of extensive study[1-5]. Generally, oxygen is present in silicon at a concentration level that exceeds the solubility limit during IC processing or during the crystal cool-down period. This non-equilibrium concentration of oxygen necessitates the formation of oxygen agglomeration-induced defects, i.e., thermal donors and oxide precipitates, during any post-growth thermal processing as soon as the kinetic limitations are overcome. These oxygen agglomeration process-induced defects play an important role in VLSI technology. It is commonly accepted that intentionally grown oxide precipitates in CZ silicon act as recombination centers[6-8]. However, the effect of oxygen-related ingrown defects on lifetime has received very little attention[9]. It has been reported[9] that lifetime changes dramatically, going from the center to the edge of a crystal, due to oxygen-induced defects present in an "as grown" CZ silicon. The "as grown" precipitates were believed to act, directly or indirectly, as recombination centers. The objective of this work was to gain an understanding of the effect of oxygen-related "as grown" defects on minority carrier lifetime using surface photovoltage (SPV) minority carrier diffusion length, LD, measurements. EXPERIMENTAL The majority of p-type silicon samples used in this study came from two groups of p-type (B-doped) CZ silicon: (a) Resistivity 4-6 flcm, [0] = 10-13 ppm; and (b) Resistivity 26-30 Qcm, [01= 15-16 ppm. In addition, limited runs were conducted with n-type (P-doped) CZ, p- and n-type FZ, and MCZ silicon (4-6 0cm, [0] = 5 ppm) samples. Wafers 2 to 3 mm thick were utilized which permitted Mat. Res. Soc. Symp. Proc. Vol. 262. @1992 Materials Research Society
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measurement of the diffusion length up to 700-800jpm without limitations provided by the wafer thickness.
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