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 ^∘{}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.