ATOMIC-STRUCTURE OF AMORPHOUS NANOSIZED SILICON POWDERS UPON THERMAL-TREATMENT

Amorphous silicon powders prepared by plasma-enhanced chemical vapor d eposition, of 8-24-nm-sized particles agglomerated into larger aggrega tes were annealed in a reducing atmosphere to study the phase transfor mation behavior of these particles. High-resolution electron microscop y 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 tem peratures 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 crystalliza tion is achieved at 700 degrees C, with structures ranging from very s mall crystalline ordered regions of 2.5-3.5 nm in size, to fast-grown multiply twinned crystallites. Rapid progress of crystallization, main ly caused by growth twinning, is observed upon annealing at 800 degree s C. At 900 degrees C, almost completely crystalline particles are for med. The particles having lattice characteristics of diamond cubic sil icon frequently exhibit a faulted structure, because of multiple twinn ing 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 crystalliza tion. Size and surface roughness of the as-prepared powders are widely preserved throughout all stages of heating, and practically no sinter ing occurs up to 900 degrees C.