Porous silicon nanowires phase transformations at high temperatures and pressures

Abstract

Porous silicon nanowires (NWs) with homogenous lateral dimensions of 90 nm are investigated by Raman scattering experiments along isothermal pressure cycles in a diamond anvil cell. Experiments were performed at variable temperatures up to 400 °C for maximal pressures of about 30 GPa comparing directly with transformations in bulk Si and porous NWs. Scanning electron microscopy demonstrates the persistence of one-dimensional morphology after high pressure investigation. The diamond phase in porous nanowires persists upon compression up to around 20 GPa at room temperature (25 °C) and to about 14 GPa at 200 °C and 400 °C. However, the β−Sn high pressure phase is seen to coexist with the diamond phase above 12 GPa at 25 °C and above 6 GPa at 200 °C and 400 °C. The coexistence region of the two phases is found to be considerably enlarged as compared with crystal silicon at each temperature. Upon decompression from 30 GPa, nucleation to the β−Sn, followed by formation of amorphous structures, is observed for porous NWs. Returning to ambient pressure and temperature, amorphous silicon is the dominant form with a residual contribution of β−Sn. At higher temperatures, nucleation back to the diamond structure is triggered although coexistence of amorphous and crystalline phases is observed up to 400 °C.