X-ray Study of Polymorphism in SnO₂ Thin Films Synthesized via RF Powder Sputtering

Abstract

This dissertation investigates the polymorphic behavior of tin dioxide (SnO₂) thin films grown on single-crystal substrates via radio-frequency (RF) powder sputtering. Particular attention is paid to the crystallization mechanisms, phase competition, and epitaxial stabilization of the rutile-type (R–SnO₂) and columbite-type (C–SnO₂) polymorphs under varying growth and annealing conditions. In the first part of the study, amorphous SnO₂ films deposited at room temperature on Al₂O3(0001) were subjected to in situ annealing, revealing the coexistence of R and C phases during solid-phase epitaxy. High-resolution X-ray diffraction (HRXRD) and off-specular scans demonstrated that both polymorphs nucleate competitively under strain and thermal activation, with their relative volume fractions modulated by film thickness and interfacial strain. Building on these findings, direct epitaxial growth was achieved by heating the substrate during deposition. While similar R/C phase coexistence was observed, detailed off-specular diffraction and reciprocal space mapping enabled the unambiguous identification of phase-specific Bragg peaks and domain orientations. Unexpectedly, room-temperature-deposited films also exhibited partial crystallinity, and thickness- dependent domain evolution was tracked using rocking curve analysis and atomic force microscopy. These results highlighted a critical role of surface roughness, domain alignment, and strain relaxation in determining crystalline quality and phase distribution. In contrast, SnO₂ films grown on YSZ(001) under identical sputtering conditions exhibited stabilization of the C–SnO₂ phase across all thickness examined. Off-specular diffraction and azimuthal ϕ-scans confirmed multi-domain epitaxy involving C[100] and C[010] domains. The absence of R–SnO₂ was attributed to unfavorable lattice matching and anisotropic bonding incompatibility with the cubic YSZ substrate. Comparative analysis suggests that the choice of substrate governs the polymorph selection via strain accommodation and interface energetics, rather than bulk thermodynamic stability. Taken together, this study provides a comprehensive framework for understanding and controlling SnO₂ polymorphism in epitaxial thin films. The results offer new insights into how strain, symmetry, and thermal conditions interplay to govern phase stability and domain orientation in complex oxide systems. These findings contribute to the rational design of polymorphic thin films for advanced electronic and optoelectronic applications.