Impact of different SiO2 diffusion barrier layers on lifetime distribution in multi-crystalline silicon ingots

Three different SiO2-based barrier layers, used in industry for prevention of metallic impurity diffusion into directionally solidified silicon ingots, were investigated in detail by G1 experiments in order to find out the most relevant barrier properties responsible for the blocking behaviour. Minority carrier lifetime and interstitial iron measurements on the grown silicon ingots show significant differences between the barriers regarding the red zone extension as well as the maximum lifetime/minimum iron content in the ingot centre. Structural analysis of the barriers by optical microscopy and Raman spectroscopy reveal a clear correlation to the porosity and thickness of the barrier layers. Further, indications were found that the barriers become effective only during the process and that this should occur as early as possible to obtain an optimal impact of the barrier. Only one of the tested barriers shows an overall positive impact on the lifetime, coming close to using a high purity quartz glass SiO2 crucible instead of the standard ceramic SiO2 crucible.