2, University of Illinois at Chicago, Chicago, Illinois, United States
Strain is a key engineering degree of freedom used to modulate the carrier mobility and tune the bandgap of semiconducting materials. Strain-energy relaxation mechanisms in epitaxial thin-films and heterogeneous devices, including alloy decomposition and dislocation generation and motion, may degrade electronic properties including carrier lifetime and mobility. Herein, we describe the strain-energy release mechanisms of bent silicon (diamond-cubic structure) and GaAs (zinc-blende structure) nanowires, which were elastically strained at ambient temperature, by bending, for different values and then annealed at 920 °C and 450 °C for 4 min and 60 min, respectively. When the nanowires are annealed at 920 °C, dislocations are nucleated at the surfaces of the nanowires and those dislocations align themselves to form grain boundaries, via glide and climb, which reduces the strain energy caused by bending. Using Raman spectroscopy, which is sensitive to local strain, transmission electron microscopy and scanning electron-microscopy we find that the silicon nanowires release their strain-energy by polygonization, which is a known physical-metallurgy phenomenon. Polygonization is the formation of polygon-shaped grains separated by grain-boundaries consisting of edge dislocations. This is the first time that polygonization has been observed experimentally in nanowires. In contrast, GaAs nanowires release their strain-energy by forming nanocracks on the outer portion of a GaAs nanowire, which is under tensile strain, and no polygonization is observed. Our study of the strain-energy release mechanisms of semiconductor nanowires is relevant to possible aging mechanisms and failure modes of flexible electronics.