Papers by Author: Hans Joachim Möller

Abstract: The influence of the CO concentration in the gas phase on the distribution of carbon in Bridgman-grown, multicrystalline silicon is studied. The growth experiments were conducted in a high-vacuum induction furnace either under a CO enriched atmosphere or under CO free conditions. Furthermore, thermodynamic calculations in the system silicon/oxygen/carbon were done. In crystal growth under a CO enriched atmosphere a SiC-containing layer is formed on the top surface of the melt in agreement with the calculated phase diagram. In this case, the level of substitutional carbon in the cystal was found to be almost constant, whereas the axial carbon concentration in crystals grown under CO free conditions increases monotonously according to Scheil's law.

Abstract: This work introduces two different approaches to explain the growth of silicon carbide (SiC) filaments, found in the bulk material and in grain boundaries of solar cells made from multicrystalline (mc) silicon. These filaments are responsible for ohmic shunts. The first model proposes that the SiC filaments grow at the solid-liquid interface of the mc-Si ingot, whereas the second model proposes a growth due to solid state diffusion of carbon atoms in the solid fraction of the ingot during the block-casting process. The melt interface model can explain quantitatively the observed morphologies, diameters and mean distances of SiC filaments. The modeling of the temperature- and time-dependent carbon diffusion to a grain boundary in the cooling ingot shows that solid state diffusion based on literature data is not sufficient to transport the required amount of approximately 3.4  1017 carbon atoms per cm2 to form typical SiC filaments found in grain boundaries of mc-Si for solar cells. However, possible mechanisms are discussed to explain an enhanced diffusion of carbon to the grain boundaries.

Abstract: The content of interstitially solved oxygen (Oi) in heavily boron doped silicon (9- 29 m
cm) were measured by low temperature Fourier transform infrared (FTIR) spectroscopy. Therefor an alternative thinning technique for silicon is used: by alkaline potassium hydroxide etching (KOH) prepolished silicon specimens are thinned down to 8 - 60 microns. The optimal end thickness depends on the boron concentration which specifies the free carrier concentration. Specimens with three different boron concentrations (9/19/29 m
cm) were examined. The results are compared with gas fusion analysis (GFA) measurements. Furthermore the precipitated oxygen Oi was measured for a RTA process (20s@1250°C) with subsequent growth steps (4h@780°C + 16h@1000°C).