Optimization of the Czochralski process for the production of monocrystalline silicon with regard to lower costs and thinner wafers - Subproject: Electrical and mechanical characterization of wafers and solar cells
|German title:||Optimierung des Czochralski-Verfahrens zur Herstellung von monokristallinem Silizium in Hinblick auf geringere Kosten und dünnere Wafer - Teilprojekt: Elektrische und mechanische Charakterisierung von Wafern und Solarzellen|
|Duration:||1st July 2009 - 30th June 2012|
|Description:||The further development of the Czochralski process for the production of monocrystalline silicon
wafers is an important way to exploit the cost reduction potentials in the production of
solar power with regard to achieving grid parity. Within the framework
project "CzSil", the aim is to develop new technologies in a network of equipment, component and material
manufacturers - supported by research institutes - to develop both the system technology and the complete
as well as the cycle of the complete crystal growing process. The following
four approaches are being researched:
1) development of post-charging equipment
2) Development of long-term stable quartz crucibles
3) Development of optimized growth and cooling processes
4) Further development of measurement technology and computer simulation
In this subproject, the aim of the CiS research facility is to develop silicon crystals and
wafers, which are used for the development of the long-term stable quartz crucibles (2) and the
growth and cooling processes (3), and to further process them into solar cells.
solar cells, and thus to contribute to the evaluation of these equipment and process innovations.
Within the framework of the collaborative project CZ-Sil, extensive investigations were carried out at CiS regarding the quality of silicon produced by the Czochralski process. In several test series the electrical quality of CZ-Silicon wafers was determined by spatially resolved MWPCD time measurements and injection dependent measurements of the charge carrier lifetime. A prerequisite for these investigations was the prior deposition of a silicon nitride layer to minimize the recombination of charge carriers at the surfaces. This is what makes the electrical quality of the silicon visible in the lifetime measurements in the first place.
A second focus of the silicon characterizations was on the crystallographic quality of the CZ silicon. Here, defect-selective etching techniques were applied and subsequently the surfaces prepared in this way were examined using various measurement methods (microscope, SEM, AFM). Different defect types and dislocation types (e.g. spiral dislocations) could be identified. These investigations have repeatedly shown that the crystallographic quality is closely related to the electrical quality of the silicon. For example, it has been shown that the number of spiral dislocations correlates directly with the charge carrier lifetime. And this in such a way that a high dislocation density of this dislocation type leads to a high electrical quality of the silicon. This could be explained by gettering effect of dislocations on lifetime defects. The effect of gettering of dislocations could also be demonstrated directly using interstitially dissolved iron. Regions with a high dislocation density correlated with regions in which little interstitially dissolved iron could be measured. However, no increase in charge carrier lifetime was observed in this experiment due to the gettering effect of the dislocations. This is probably due to the higher recombination activity of the iron silicides formed along the dislocation line. The results of these investigations were presented in a talk and a paper at the GADEST 2012 (Gettering and Defect Engineering in Semiconductor Technology) conference.
FTIR spectroscopy and 4-peak metrology were used as additional characterization methods. The former allows the determination of the interstitial dissolved oxygen content and the substitutional dissolved carbon content. The second method was used to measure resistivity. Here, a clear correlation between resistivity and MWPCD time was found by spatially resolved measurements in some samples.
Spatially resolved measurements of the electrical quality of the CZ silicon revealed significant differences between photoluminescence measurements and MWPCD measurements. In the MWPCD maps, characteristic rings appeared in the CZ silicon, whereas no rings were visible in the photoluminescence images. A possible explanation could be that these rings are caused by charge carrier traps, which are not visible in the PL image and become visible as trapping artifacts in the MWPCD. However, this assumption could not be confirmed by measurements. Further investigations concerning the ring structures could not be performed within the scope of this project, so that the cause of the rings is still an open problem. Investigations on the development of measurement methods in CZ silicon were published in a paper at the 27th EUPVSEC 2012 (European Photovoltaic Solar Energy Conference).
As part of this project, annealing processes were also performed on CZ silicon and solar cells were fabricated. The annealing experiments showed that in iron-contaminated CZ silicon, the interstitial iron content in the silicon is determined by the dissolution of iron precipitates. It was shown that a slow cooling rate after phosphorus diffusion causes the interstitially dissolved iron to collect in less recombination-active iron precipitates, which increases the electrical quality of the silicon. Results from these experiments were published at the 25th WCPEC 2010 (World Conference on Photovoltaic Energy Conversion). Solar cells were fabricated to evaluate the CZ silicon quality. It was found that regions with multicrystalline structures in CZ silicon show significantly worse luminescence signals than monocrystalline regions.
K. Lauer, A. Laades, A. Lawerenz, K. Neckermann, A. Sidelnicov, IMPACT OF DIFFERENT ANNEALING STEPS ON THE INTERSTITIAL IRON CONCENTRATION IN SOLAR-GRADE CZOCHRALSKI SILICON, Proc. 25th Europ. PVSEC, Valencia (2010) 2344
K. Lauer, M. Herms, A. Grochocki, and J. Bollmann, IRON GETTERING AT SLIP DISLOCATIONS IN CZOCHRALSKI SILICON , Solid State Phenomena 178-179 (2011) 211
M. Turek, C. Möller, K. Lauer, INVESTIGATION OF EXCESS CHARGE CARRIER LIFETIME MEASUREMENTS ON SAMPLES OF ARBITRARY THICKNESS, 27th EPVSEC Frankfurt (2012)
|Partners:||- Bosch Solar Wafers GmbH, 99310 Arnstadt
- PVA Tepla, 35435 Wettenberg
- Forschungszentrum Dresden Rossendorf, 01328 Dresden
- Fraunhofer Center für Silizium Photovoltaik, 06120 Halle/Saale
- Fraunhofer Technologiezentrum für Halbleitermaterialien, 09599 Freiberg
- Heraeus Quarzglas GmbH & Co. KG, 06803 Bitterfeld Wolfen
|Funding code:||03 SF 0379 B|
|Contact:||Contact us about this project via our former business unit Silicon Detectors|
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