| dc.contributor.author | Alema, Fikadu Legesse | |
| dc.description.abstract | In recent years there has been enormous progress in the development of barium strontium titanate (BST) films for tunable microwave applications. However, the properties of BST films still remain inferior compared to bulk materials, limiting their use for microwave technology. Understanding the film/substrate mismatch, microstructure, and stoichiometry of BST films and finding the necessary remedies are vital. In this work, BST films were deposited via radio frequency magnetron sputtering method and characterized both analytically and electrically with the aim of optimizing their properties.
The stoichiometry, crystal structure, and phase purity of the films were studied by varying the oxygen partial pressure (OPP) and total gas pressure (TGP) in the chamber. A better stoichiometric match between film and target was achieved when the TGP is high (> 30 mTorr). However, the O2/Ar ratio should be adjusted as exceeding a threshold of 2 mTorr in OPP facilitates the formation of secondary phases. The growth of crystalline film on platinized substrates was achieved only with a lower temperature grown buffer layer, which acts as a seed layer by crystallizing when the temperature increases.
Concurrent Mg/Nb doping has significantly improved the properties of BST thin films. The doped film has shown an average tunability of 53%, which is only ~8 % lower than the value for the undoped film. This drop is associated with the Mg ions whose detrimental effects are partially compensated by Nb ions. Conversely, the doping has reduced the dielectric loss by ~40 % leading to a higher figure of merit. Moreover, the two dopants ensure a charge neutrality condition which resulted in significant leakage current reduction. The presence of large amounts of empty shallow traps related to NbTi localize the free carriers injected from the contacts; thus increase the device control voltage substantially (>10 V).
A combinatorial thin film synthesis method based on co-sputtering of two BST sources doped with Mg/Nb and Ce, respectively, was applied. The composition and the dielectric properties of the deposited film were correlated and the optimal concentration of dopants corresponding to high tunability and low dielectric loss was determined in a timely fashion. | en_US |
| dc.publisher | North Dakota State University | en_US |
| dc.rights | NDSU policy 190.6.2 | |
| dc.title | Multicomponent Doped Barium Strontium Titanate Thin Films for Tunable Microwave Applications | en_US |
| dc.type | Dissertation | en_US |
| dc.type | Video | en_US |
| dc.date.accessioned | 2018-01-18T19:16:07Z | |
| dc.date.available | 2018-01-18T19:16:07Z | |
| dc.date.issued | 2014 | |
| dc.identifier.uri | https://hdl.handle.net/10365/27258 | |
| dc.rights.uri | https://www.ndsu.edu/fileadmin/policy/190.pdf | |
| ndsu.degree | Doctor of Philosophy (PhD) | en_US |
| ndsu.college | Graduate and Interdisciplinary Studies | en_US |
| ndsu.department | Materials and Nanotechnology | en_US |
| ndsu.program | Materials and Nanotechnology | en_US |
| ndsu.advisor | Pokhodnya, Konstantin | |
