Carbon Precipitation in Cz and Efg Silicon
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CARBON PRECIPITATION IN CZ AND EFG SILICON S.Q. Feng , J.P. Kalejs and D.G. Ast+ + Materials Science and Engineering, Cornell University, Ithaca, NY14853, U.S.A. * Physics Department of Peking University, PRC ** Mobil Solar Energy Corporation, 16 Hickory Drive, Waltham, MA02154, U.S.A. ABSTRACT The defects formed in carbon rich CZ and EFG silicon after a 900'C, 160 or 560 min long, phosphorous diffusion were studied with TEM. Besides misfit dislocations, numerous precipitates were observed in the junction region in both materials. However, no precipitates were found in the bulk. The precipitates in CZ had a larger strain field than those in EFG material. The difference in the extent of the strain field of precipitates in CZ and EFG silicon can be ascribed to the higher oxygen content in CZ silicon. It appears that oxygen is involved in the Si-self interstitial promoted precipitation of carbon. INTRODUCTION EFG silicon is grown in contact with graphite and contains carbon at During junction diffusion, the carbon is in supersaturation and will, if the kinetics is sufficiently fast, precipitate out. Since precipitates will degrade the junction quality, a study of carbon during diffusion is of technological interest. The defect structure of carbon doped (>1-3"101 7 cm- 3 ) diffusion annealed FZ silicon crystals was investigated by Graff et al.[1]. They reported that precipitates were formed in carbon-rich FZ silicon crystals as the result of diffusion, but that these precipitates did not appear to contain carbon. Ladd et al.[2] pointed out that phosphorous in-diffusion can enhance the carbon diffusion coefficient at 900 0C by a factor 40 and attributed a small peak in the carbon SIMS profile at a depth of about 0.3-0.4 pm (i.e., in the junction region) to carbon precipitation in this region. Gleichmann et al.[3] studied the influence of a 1 hr. pre-anneal at 1200'C on the EBIC image of phosphorous-diffused EFG at 900 0 C for 30 min. They concluded that gettering centers were formed throughout the bulk as a result of the anneal. Since the TEM failed to detect these centers, they assumed that carbon precipitated with the aid of silicon self-interstitials into (nearly) strain free complexes with contrast too low to be visible. In order to clarify the interaction between carbon, oxygen, and interstitials, we studied with TEM the influence of a 900 0 C phosphorous diffusion, which is known to generate silicon self-interstitials[4,5], on the precipitation in carbon-rich CZ and EFG silicon.
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EXPERIMENTAL Processed carbon-rich single crystal CZ silicon wafers with 8.5x1O17 cm- 3 carbon and 1Ox1O07 cm-3 oxygen and processed carbon-rich EFG silicon ribbon with 9x101 7 cm- 3 carbon and
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