Impact of silicon feedstock contamination by gas phase diffusion on material quality of cast silicon ingots

The impurity level of cast silicon ingots for photovoltaic application has become more and more crucial over the last years with respect to the competitiveness to the high quality monocrystalline Czochralski material. Typical contaminants diffusing into the silicon melt and ingot during the casting process are metals, dopants and light elements like C, O and N. They originate from the consumables like crucible, crucible coating and silicon feedstock as well as the furnace atmosphere. If high quality electronic grade or solar grade silicon feedstock is used, it exhibits several orders of magnitude lower impurities than the other consumables and therefore is mostly neglected during optimization processes in industrial environment. However, the pure feedstock is exposed to the furnace atmosphere during the heating process for long timescales at high temperatures even before melting and solidification. This can lead to a pronounced contamination of the feedstock particles, which negatively influences the resulting material quality of the silicon ingots. In this work, this phenomenon is investigated in laboratory scale by carrying out several two-step G1 experiments containing a pre-annealing step of the feedstock in different crucible setups followed by a regular crystallization step in a virgin crucible. Through lifetime, interstitial iron, resistivity and C-/O-measurements, the effect of feedstock contamination by the surrounding environment on the ingot quality was evaluated. The results show that a significant feedstock contamination by boron, iron and carbon species takes place if the vacuum phase during heating phase is too long. In contrast, no significant oxygen contamination could be observed. Further, the main sources for iron and boron contamination via gas phase diffusion were identified to be the silica crucible and the Si<inf>3</inf>N<inf>4</inf> coating, respectively. In contrast, the contamination by the furnace itself (e.g. graphite parts) plays only a minor role concerning these elements, but is mainly responsible for carbon contamination.