In situ observation of the solidification interface and grain boundary development of two silicon seeds with simultaneous measurement of temperature profile and undercooling

Abstract

Grain boundaries in multi-crystalline silicon are crucial to the minority carrier lifetime, and thus, solar cell efficiency. Therefore, further understanding on the grain boundary development during crystal growth is needed. This in situ observation study is focused on the solidification interface behavior and the grain boundary development between different orientation crystals – considering both stable and unstable growth conditions. Silicon seeds with 〈1 0 0〉 and 〈1 0 0〉 + 20° growth direction orientations were partially melted and then solidified inside an observation furnace, and a digital and an infrared (IR) microscopes were utilized for the visualizations and temperature profile measurements, respectively. The unstable growth was found to exhibit continuous fluctuations with increasing amplitudes in the growth velocity which were related to the buildup of undercooling and the growth of facets. The Electron backscatter diffraction (EBSD) analysis showed that twin nucleation occurred from the valley of the faceted groove for the low-undercooling stable growth, whereas random grain nucleation occurred from the tip of the facets for the high-undercooling unstable growth. The measured negative temperature gradient inside the groove was used to explain the growth behavior under the unstable condition.