  The nature of the conducting filaments in many resistive switching systems
has been elusive. Through in situ transmission electron microscopy, we image
the real-time formation and evolution of the filament in a silicon oxide
resistive switch. The electroforming process is revealed to involve the local
enrichment of silicon from the silicon oxide matrix. Semi-metallic silicon
nanocrystals with structural variations from the conventional diamond cubic
form of silicon are observed, which likely accounts for the conduction in the
filament. The growth and shrinkage of the silicon nanocrystals in response to
different electrical stimuli show energetically viable transition processes in
the silicon forms, offering evidence to the switching mechanism. The study here
also provides insights into the electrical breakdown process in silicon oxide
layers, which are ubiquitous in a host of electronic devices.
