Atomic structure of amorphous nanosized silicon powders upon thermal treatment

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 degrees 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 degrees 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 degrees C. At 900 degrees 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 degrees C.