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UBC Theses and Dissertations

Electron localisation in non-stoichimetric films of aluminum nitride produced by reactive sputtering Fortier, Normand


Sputtering is a very versatile process for the fabrication of thin solid Gims. The subject of this thesis concerns the study of thin films of non-stoichiometric aluminum nitride fabricated by voltage-controlled reactive sputtering. Using the cathode voltage on the sputtering target, the relative arrival rates of nitrogen and aluminum atoms on a substrate are precisely controlled. The method allows the deposition of films over a wide range of composition. This deposition technique is shown to be an extremely useful tool in the study of electron localisation in solids. This thesis will provide a contribution toward a more complete understanding of these localisation effects. As the composition of the deposited films is gradually varied from essentially pure aluminum to nearly stoichiometric aluminum nitride, the structural disorder of the resulting films is smoothly varied. This technique has permitted the observation of three distinct transport regimes. These regimes are: The Boltzmann regime, the regime of moderate disorder, and finally the regime of strong disorder. The results obtained in this work are consistent with the interpretation that an enhancement of the structural disorder is accompanied by a change in the nature of the wave functions. To each of the three transport regimes observed is attributed a specific type of wave function. In the Boltzmann regime the wave functions are the familiar Bloch waves. These wave functions are extended. In the regime of moderate disorder the wave functions are thought to be power-law localised, the envelope of the wave function decaying as a power law. Finally in the strong disorder limit the wave functions are thought to be exponentially localised. A model proposed by Kaveh and Mott in which the electron wave function is assumed to be power-law localised is shown to accurately describe the regime of moderate disorder. A simple extention of the model is shown to account for the observed free-electron behavior of the Hall effect and the thermoelectric power. A regime of strong disorder in which the electronic wave functions are thought to be exponentially localised is observed. In this regime the conductivity proceeds by variable-range-hopping: Precise measurements of the temperature dependence of the conductivity support the relation: [See Thesis for Equation]. According to the variable-range-hopping model, the form of the exponential temperature dependence should be a consequence of the relatively constant density of states at the Fermi level. The results of optical and thermoelectric power measurements support this hypothesis.

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