Studies of the electronic and vibrational signatures of the unusual bonding geometries in melt-quenched amorphous silicon

Abstract

Tight-binding molecular dynamics simulations have been performed to investigate the effect of quenching rate of the Si melt on the resulting local structure of amorphous silicon. Different quenching rates were used to cool liquid silicon in the simulations to demonstrate that the choice of quenching rates significantly influences the resulting local structure. The calculated pair correlation functions show that the local structure is sensitive to the thermal processing of the liquid silicon melt. The use of cooling rates higher than 10⁻¹³ K s⁻¹ appears to prevent the activation of the required structural re-arrangements necessary to stabilise the networks, causing unexpected bonding geometries to develop. The electronic signatures of the defects show that only the triangular defect structure contributes resonance states to the conduction band tail. Also, the vibrational signature of the triangular structure shows a high energy transverse optical mode at 95 meV, indicating that the defect is likely to be unstable at 300 K, although both defects contribute minimal states to the mid-gap level.

Keywords:

• positron state
• amorphous silicon
• structural disorder
• molecular simulation
• structural defects

Acknowledgements

This work benefitted substantially from helpful discussions with Professor D.T. Britton. The author acknowledges financial support from the Research Council of the University of Cape Town, South Africa, and the Deutscher Akademischer Austausch Dienst (DAAD), Germany, through scholarship SDV/A/04/29923.