Defect dynamics during the faceted growth of salol

Abstract

Understanding faceted growth mechanisms is of major interest in materials science, as it is encountered during the formation of most semiconductors and oxides. Defects appear frequently in solidification processes, associated to the presence of impurities, dislocations, grain boundaries or stresses. Studying the formation and dynamics of these defects is essential, as they directly influence material properties such as strength, conductivity, and optical behavior. Directional solidification of thin samples of salol, an organic transparent material, were conducted to investigate the evolution of a faceted solid-liquid interface. Different conditions, including thermal gradients of 1 and 2 K/mm and pulling velocities ranging from 2.5 to 10 µm/s, were applied, allowing to characterize the resulting facet dynamics. For the highest thermal gradient, increasing the pulling velocity resulted in a decrease in the facet tip angle, whereas for the lowest gradient, the angles remained similar across all pulling velocities. These observations are influenced by the initial conditions, such as variations in seed crystal orientation and quality, which affect the evolution of the solidification front. The complex dynamics of the interface reveals the formation and evolution of several defects at different scales, including bubbles, growth striations, facet splits and distortions, and twin boundaries. In situ observations allows a detailed analysis of defect origins. Bubbles, systematically observed, are most likely attributed to nitrogen contamination in melt during sample filling. The stress-induced dislocation movement has been identified as one of the mechanisms for defect formation. Additionally, crystal misorientation leads to stress redistribution, which increases the likelihood of defects.