Date of Award
Doctor of Philosophy (PhD)
Metallurgy and Materials Science
SnO₂ films have been prepared by reactive sputtering in an oxygen-containing glow discharge, by ion-beam sputtering in an accelerator, and by anodizing with an ethylen-glycol-based electrolyte. Preparation of SnO₂ films by bombarding metallic tin with oxygen ions has also been attempted. The sputter-deposited films were amorphous if deposited on KCl or Ta at temperatures from -100° to +200°C, though they were crystalline if deposited on SnO₂ at 200°C. The anodic films were variously crystalline or microcrystalline, though clearly not amorphous. Films formed by bombarding metallic tin with oxygen ions turned out to be crystalline α-SnO at all doses from 1 x 10¹⁵ to 3 x 10¹⁷ ions/cm². The crystallization temperatures ranged from <200°C for crystalline SnO₂ substrates, to 250°-300°C for KCl substrates, to 400°-450°C for Ta substrates, to 475°-550°C for unsupported films. The crystallization product was consistently cassiterite, i.e., normal SnO₂. Once crystallinity was attained in unsupported sputtered films, the grain size remained in the vicinity of 400 Å to a temperature of about 1000°C. This is in good agreement with the behaviour of anodic films; which retained their microcrystalline structure in heat treatment again until a temperature of about 1000°C was reached. The thickness of the reactively sputtered films was estimated from the observation of their interference colours when deposited on a Ta substrate, while the thickness of the anodic films was estimated by sputtering the films with 20-keV Kr ions until metal was exposed and noting the weight change. The anodization of Sn has a rather low efficiency (4-21%), and this could be shown to be due to electronic conduction rather than dissolution. Kr-ion bombardment of SnO₂ thin films has shown that this oxide presents a very high value of the sputtering coefficient (21.5 ± 1.5 atoms/ion for 10-keV Kr impact), a result which can be attributed to thermal sputtering (i.e., bombardment induced vaporization). As far as the structure of the target material after bombardment is concerned, the experimental evidence was that SnO₂ undergoes amorphization. The effects of ion bombardment of bulk samples (either SnO₂ sintered powder or natural cassaterite) were investigated using the following techniques: (I) reflection electron diffraction, (II) marker-release spectrometry, (III) dissolution measurements, and (IV) resistivity measurements. Through the use of techniques I and II we have established that the annealing (to the original singlecrystalline phase) of bombardment induced amorphousness occurs in two stages: (i) a homogeneous transformation to a polycrystalline phase showing the normal cassiterite structure; (ii) epitaxial recrystallization to single-crystal cassiterite. Dissolution measurements have established the depth of amorphization due to ion bombardment, while resistivity measurements have indicated that bombardment does not cause SnO₂ to change its stoichiometry. Finally the various results allowed us to expand the formalism on which marker-release spectrometry is based, namely we have identified a previously overlooked release process (Stage IC) due to epitaxial crystallization.
Giani, Enrico, "A Study of SnO₂: Preparation, Characterization and Response to Ion-impact" (1975). Open Access Dissertations and Theses. Paper 3068.