Influence of Rapid Thermal Annealing on Recombination Properties of Polycrystalline Silicon
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INFLUENCE OF RAPID THERMAL ANNEALING ON RECOMBINATION PROPERTIES OF POLYCRYSTALLINE SILICON M. KITrLER AND W. SEIFERT Institute of Semiconductor Physics, W.-Korsing-Str. 2, POB 409 D-O 1200 Frankfurt (Oder), Germany
ABSTRACT Rapid thermal annealing was shown by EBIC to increase the minoritycarrier diffusion length in cast polycrystalline silicon.The beneficial effect is due to a deactivation of intragrain defects (mainly dislocations) and is stable against post-RTA annealing up to at least 600 °C/ 10 min, INTRODUCTION Cast polycrystalline silicon is a low-cost material widely used in solar cell production. The disadvantage of this material type is the existence of a large number of extended crystal defects, mainly grain boundaries and dislocations. By interaction with impurities, e. g. oxygen, carbon, transition metals, these defects may act as recombination sites and reduce the minority-carrier diffusion length (DL), with inhomogeneous defect distribution leading to a corresponding spatial variation of DL. The final result is a degradation of the solar cell efficiency. Therefore, techniques as passivation, i. e. deactivation of defect-related recombination centres, and gettering, i. e. removal of impurities causing recombination centres (mainly TM), are required in solar cell production, see e. g. [11. So the improvement of our knowledge about electrical activity of defects, in particular about behaviour during thermal processing and interaction with impurities, is of continuing interest. Slow cooling is reported in the literature 121 to be necessary for satisfactory DL.The aim of the present paper is a study of the influence of Rapid Thermal Annealing (RTA), including an Inherent quenching step, on the recombination properties. EXPERIMENTAL The material investigated was p-type (- 10 fcm) cast polycrystalline silicon of two different suppliers. The columnar grain structure allowed to obtain parallel wafers of nearly identical defect structure, very convenient for comparing different treatments. Samples 10 mm x 10 mm in size were cut from the wafers, lapped and finally polished chemico-mechanically with After careful cleaning a part of the samples so prepared was subjected to a rapid thermal annealing (RTA) in 02, N2 or Ar + H2 at 900 °C for 60 s. The cooling rate of the samples after RTA was about 70K/s. In some cases, conventional furnace annealing (900 °C/10 min in N 2 or 1000 °C/15 min in evacuated quartz ampoules) was carried out for comparison. Post-RTA annealing up to 230 °C/2 h in air and up to 600 °C/ 10 min in N2 was used to prove the stability of RTA-induced changes of recombination properties. The recombination properties were evaluated by the method of the Electron-Beam-Induced Current (EBIC) using a Cambridge S 360 scanning Mat. Res. Soc. Symp. Proc. Vol. 262. (P1992 Materials Research Society
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electron microscope, a Keithley 427 current amplifier for monitoring the EBIC signal and a Keithley 485 picoammeter for measuring the beam current lb. To ensure low injection, Ib was kept below 100 pA th
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