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Abstract:
Void formation in semiconductors is generally considered to be deteriorating. However, for some systems void formation and evolution are beneficial and can be used for fabrication of novel nanostructures. In either scenario, understanding of void formation and evolution is of both scientific and technical high importance. Herein, using ZnS ribbons as an example, we report real-time observations of void formation and kinetics of growth at nano- and atomic scales upon heating. Direct imaging reveals that voids, created by a focused electron-beam in wurtzite (WZ) ribbons, have a rectangular shape elongated along the <0001> direction. The voids are enclosed by low-surface-energy planes including {01-10} and {2-1-10}, with minor contribution from higher-energy {0001} planes. Driven by thermodynamics to minimize the surface energy, the voids grow straight along [000±1] directions, exhibiting a strong anisotropy. Occasionally, we observe oscillatory kinetics involving periodic void growth and shrinkage, likely due to fluctuation of local chemical potential leading to a transitional kinetic state. We also reveal that the morphology and growth kinetics of voids are highly structural-dependent. Real-time observation during void growth through complex WZ-zinc blende (ZB)-WZ structure shows that the void, with an initial elongated rectangular morphology in the WZ domain, transforms into a different shape, dominated by {110} surfaces, after migrating to a domain of ZB structure. However, when the void moves from the ZB to WZ domain, it transforms back to a rectangular shape followed by fast growth along [0001] direction. Our experimental results together with density functional theory (DFT) calculations provide valuable insights into mechanistic understanding of void formation and evolution in semiconductors. More importantly, our study may shed light on new path ways for morphological modulation of nanostructures by utilizing intrinsic anisotropy of void evolution in WZ semiconductors.
