Singly-twinned growth of Si crystals upon chemical modification

Abstract

During crystallization a continuum of patterns could emerge due to the interplay of growth kinetics, material or solution chemistry, and crystallographic defects. The coherent twin boundary is widely known to catalyze growth in pristine crystals including polycrystalline Si. Much remains unknown about the impact of changing the chemical environment of the crystallization process through the deliberate addition of trace metallic species---termed chemical modification. Pristine Si has been reported to grow through the classical model of two parallel twin planes acting in concert to enable steady-state propagation of the solid-liquid interfaces. Here, we achieve a vision on the growth process via in situ synchrotron x-ray microtomography and further corroborated by ex situ crystallographic investigation. We find that steady-state growth is impossible in chemically modified alloys that consist of trace (0.1 wt.%) Sr. This is because the Sr modifier poisons the concave re-entrant grooves, thereby deactivating the advantage of the twin-plane re-entrant edge mechanism and leading to a singly-twinned interface. This study may serve as a proxy to chemically modified crystallization pathways of eutectic Si in Al-Si alloys and, more broadly, as a framework for the crystallization-mediated synthesis of materials.