An investigation into the properties of nanoporous semiconductors

Markham, Matthew L (2006) An investigation into the properties of nanoporous semiconductors. University of Southampton, Doctoral Thesis.

Abstract

This thesis is concerned with the characterisation of the properties of nanoporous semiconductor films. Thin films of PbTe, Te, CdTe and ZnO were electrodeposited through lyotropic liquid crystals which acted as self-assembled nano-templates. The films had a nanostructure which had pores approximately 3nm in diameter with a pore-to-pore spacing of 6nm in a hexagonal array. The surface morphology, crystallinity, nanostructure, porosity and chemical profile where measured using scanning electron microscopy, X-ray diffraction, transmission electron microscopy, a quartz crystal microbalance and secondary ion mass spectroscopy. The form birefringence of the anisotropic nanostructure was fully characterised using polarised micro-spectroscopy. The birefringence was also modelled using a finite difference simulation, to model the effective dielectric coefficients, and a transmission matrix model to calculate reflection spectra. Photoluminescence measurements performed on CdTe films are also discussed. Electrochemical impedance measurements where used in order to determine semiconductor doping densities and depletion lengths as well as penetration of solution into the pores. Spectroscopic and transient photocurrent measurements were performed using a wet contact method. The spectroscopic photocurrent measurements showed strong adsorption at above band gap energies and allowed accurate determination of the bandgap. The transient photocurrent measurements showed a decay due to surface state recombination and it was found that bulk CdTe films had a smaller number of surface states compared to nanoporous samples.

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