Details of Grant 

EPSRC Reference:	GR/H94788/01
Title:	STRUCTURE AND ELECTRONIC PROPERTIES OF POROUS SILICON
Principal Investigator:	Matthai, Dr C
Other Investigators:
Researcher Co-Investigators:
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Department:	School of Physics and Astronomy
Organisation:	Cardiff University
Scheme:	Standard Research (Pre-FEC)
Starts:	01 June 1993	Ends:	31 January 1996	Value (£):	63,941
EPSRC Research Topic Classifications:	Optoelect. Devices & Circuits
EPSRC Industrial Sector Classifications:
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Summary on Grant Application Form
To model the etching process in making porous silicon and to use this structure to calculate the electronic, mechanical and optical properties. To investigate the influence of the internal surfaces (and adatoms) on the luminescence.Progress: We have implemented a modified diffusion limited aggregation model to simulate the the porous silicon structure by electrochemical dissolution. The resulting fractal structures were fully equilibrated using the molecular dynamics method. An analysis of the relaxed structure shows it to be quite stable with the presence of one- two- and three-coordinated atoms as well as the four coordinated atoms found in bulk silicon. We have also calculated the elastic constants for varying degrees of porosity and find that the bulk modulus is surprisingly similar to that in bulk silicon.To determine if coordination effects might in some way be responsible for the observed photoluminescence, we constructed silicon nanoclusters with atomic configurations satisfying the general features of the radial and angular distribution functions of the porous silicon structure and performed electronic structure calculations using the Hartree-Fock-Roothan scheme. Restricted configuration interaction was employed to calculate the excited states and transition rates. We find that both the electronic structure and optical transition strength depend strongly on the cluster size and local environment. Importantly, we also find that many-electron effects are crucial in determining transition rates and so must be taken into account in any theory seeking to explain the observed photoluminescence in porous silicon. We are currently extending our investigations to determine the vibrational density of states as a function of porosity and to study the electron coupling to the vibrational degrees of freedom in the silicon nanoclusters.
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