Atomic structure of amorphous nanosized silicon powders upon thermal treatment

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Abstract
Amorphous silicon powders prepared by plasma-enhanced chemical vapor deposition, of 8–24-nm-sized particles agglomerated into larger aggregates were annealed in a reducing atmosphere to study the phase transformation behavior of these particles. High-resolution electron microscopy revealed a very rough surface, with structural details of 1 to 2 nm, of the as-prepared single powder particles. Upon 1 h annealing at temperatures between 300 and 600 °C circular contrast features, 1.5–2.5 nm in size, are observed in the amorphous particles, hinting to the formation of a medium-range order. A distinct onset of crystallization is achieved at 700 °C, with structures ranging from very small crystalline ordered regions of 2.5–3.5 nm in size, to fast-grown multiply twinned crystallites. Rapid progress of crystallization, mainly caused by growth twinning, is observed upon annealing at 800 °C. At 900 °C, almost completely crystalline particles are formed. The particles having lattice characteristics of diamond cubic silicon frequently exhibit a faulted structure, because of multiple twinning events. They are covered by an amorphous oxide shell of a 1.5 to 2 nm thickness, which is found to develop with the onset of crystallization. Size and surface roughness of the as-prepared powders are widely preserved throughout all stages of heating, and practically no sintering occurs up to 900 °C. © 1996 The American Physical Society.