Transformations among metastable amorphous and crystalline forms of silicon.
Doctoral thesis, UCL (University College London).
This thesis presents experimental and theoretical investigations of two metastable forms of silicon: amorphous silicon and hypothetical clathrate Si46. Amorphous silicon was investigated at room temperature and high pressure by diamond anvil cell Raman scattering, synchrotron x-ray scattering and electrical resistance measurements. The Raman data provide the first direct evidence for a reversible room temperature high pressure low-density to high-density polyamorphic transition in silicon. This transition is also observed by high pressure x-ray scattering. The combined data suggest that the high-density amorphous silicon network is of significantly higher mean coordination number than the familiar tetrahedrally bonded semiconducting low-density amorphous modification of silicon. Complimentary electrical resistance measurements conducted in a diamond anvil cell also suggest that the high-density amorphous modification is metallic. Amorphous silicon at high pressure was also investigated by molecular dynamics simulations conducted with the Stillinger-Weber potential. According to the simulations low-density amorphous silicon networks first become more defective under compression before transforming to a distinct high-density amorphous network. The changes in the simulated Raman spectra and x-ray structure factors across the transition to the distinct high-density amorphous network resemble those observed in the experimental data, again suggesting that the polyamorphic transition in silicon involves significant changes in the local structure of the amorphous network. Si46, a hypothetical metastable crystalline form of silicon, was also investigated by molecular dynamics simulations conducted with the Stillinger-Weber potential. In agreement with previous simulations the Stillinger-Weber potential predicts that Si46 is of slightly higher energy than related silicon clathrate Si136, although the energy difference between these two structures is much smaller according to the Stillinger- Weber potential than in other calculations. The simulations also show that Si46 melts at slightly lower temperature than Si136 at positive pressures but that its melting point becomes equal to that of Si136 at negative pressures.
|Title:||Transformations among metastable amorphous and crystalline forms of silicon|
|Additional information:||Permission for digitisation not received|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry|
Archive Staff Only