Self-Interstitial Injection Effects on Carbon Diffusion in Silicon At High Temperatures
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SELF-INTERSTITIAL INJECTION EFFECTS ON CARBON DIFFUSION IN SILICON AT HIGH TEMPERATURES L.A. Ladd and J.P. Kalejs Mobil Solar Energy Corporation, 16 Hickory Drive, Waltham, Massachusetts 02254, USA ABSTRACT Carbon diffusivity is reported for different ambient conditions imposed during annealing of silicon in the temperature range from 800 to 11000 C, which produce varying levels of silicon self-interstitial supersaturation. The diffusivities are deduced from SIMS analysis of carbon out-diffusion profiles. Carbon diffusivity is increased by up to a factor of 70 in annealing with phosphorus in-diffusion, and by a factor of as much as seven in an oxidizing ambient, when compared to anneals in a nitrogen ambient. The enhancements tend to decrease above 1100 0C. This behavior can be explained by attributing the increase in carbon diffusivity to self-interstitial supersaturation which increases the concentration of highly mobile carbon selfinterstitial pairs. Significant time dependent effects were also observed for 800 and 900 0 C phosphorus in-diffusion conditions. INTRODUCTION We have previously reported [1] that carbon diffusivity in silicon during annealing at 900 0C can vary with the ambient conditions imposed at the surface of the sample. It was concluded from these results that the diffusivity enhancement with respect to a nitrogen ambient anneal, produced by an oxidizing ambient or with phosphorus in-diffusion, is due to increased availability of highly mobil carbon-silicon self-interstitial pairs arising from self-interstitial supersaturation. In this paper, we have extended our measurements of the carbon diffusivity to cover the temperature range from 800 0C to 11000 C. Comparison is made of the carbon diffusivities obtained to the values deduced by extrapolating to lower temperatures the data of Newman and Wakefield [2] taken above 1100 0C. Time dependent effects observed for anneals for different times are also discussed. EXPERIMENTAL PROCEDURES The carbon diffusivity was measured in single crystal wafers of high carbon concentration (9 x lO /cm3 ) CZ silicon grown by General Diode Corporation. The wafer orientation was and the dislocation count was about 1 x lO/cm'. The interstitial oxygen content was 10 x lO17 /cm 3 . The wafers were boron doped to 1 Q-cm. The procedures used for sample preparation, annealing and carbon out-diffusion profile measurement using secondary ion mass spectroscopy (SIMS) were the same as described previously [1]. The SIMS measurements were carried out at Charles Evans and Associates. The phosphorus diffusions in all cases were carried out using a high surface concentration spin-on dopant0 in a nitrogen ambient. Anneals were carried out at 800, 900, 1000 and 1100 C for each ambient condition. The anneal times were chosen to be long enough at each temperature to give an out-diffusion profile of at least 1 pm depth so that it could be accurately measured. For the case of the oxygen ambient, the time was kept within the range where the [calculated] interstitial generation rate would
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