Growth, characterization, and functionalization of wide band gap oxide alloys

• Main Author: Park, Dae-Sung
• Published: University of Warwick 2015
• Subjects: 621.3815; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.682892

In recent years, oxide materials have received great interest due to their potentially superior properties, e.g., high optical transparency and electrical conductivity, for next generation optoelectronics. To realize this, wide band gap semiconducting oxides are key components, not only as passive elements, but also as active components in several device architectures. Therefore, controlling the electrical conductivity, optical transparency, and band gap energy of the oxides is essential for the realization of oxide-based electronics. Moreover, the phase transition of oxides, including atomic diffusion and defect distribution can significantly alter their surface, interface, and bulk properties. In this thesis, band gap modulation and growth features of a ZnO-based alloy, BexZn1-xO, grown on Al2O3 substrates have been systematically studied by Be composition variations in the range of 0 . x . 0.77 using a co-sputtering technique. Continuous lattice shrinkage from 5.23 to 4.80 .A and direct band gap shift from 3.24 to over 4.62 eV were achieved as the Be concentration increased. During the band gap modulation, significant particle pinning effects on the grain growth of the alloy films on highly-mismatched substrate were found together with variations in grain size, orientation, and composition. Such grain boundary decorated by Be particles cause structural fluctuations and compositional inhomogeneity in the alloy films. A correlation between the grain growth driving pressure and the particle pinning pressure was formulated by modelling the Be-induced particle pinning to elucidate the observed phase segregation. Furthermore, a comprehensive understanding of the thermodynamic characteristics of the BexZn1-xO thin films determines the optimized growth condition, e.g., growth temperature of 400◦ C, and sheds light on the effective band gap engineering with compositional homogeneity. The thermodynamic characteristics of the phase transformation of metastable BexZn1-xO alloys have been extensively investigated. The induced phase transition initiates the formation of a highly conductive layer at the interface and secondary phase nanoparticles at the surface. The origin of the highly degenerate interface layers, which were identified by experimental data acquisition and mathematical modellings, has been addressed through a cation counter-diffusion mechanism with respect to the strain relaxation, atomic redistribution, and charge accumulation in the transformed alloy system. The highest interface conductivity, 1.4 × 103 S·cm-1 , is comparable to the highest conductivities recorded for highly-doped ZnO. The concurrence of transient diffusion and segregation of Be is responsible for the evolution of energetically favourable secondary phase nanoparticles as a result of solid-state reactions at the surface. The growth kinetics of the nanoparticles, the associated particle size-distribution, defect-mediated atomic compensations, and their environmentally-robust functionalities (thermal and chemical resistance) are taken into account based on the experimental observations and model calculations. This novel phase transformation has been highlighted as an effective passivation and functionalization of re-active oxide surfaces by the self-assembly of inert nanoparticles. Such spontaneous passivation allows the use of semiconducting oxides in a variety of electronic applications, while maintaining their inherent and intrinsic properties. This oxide phase transformation can provide new insights for the design of environmentally-robust oxide optoelectronics in many transparent device integrations.