Salg, Patrick (2020)
Interfaces in all-oxide thin-film varactors with highly-conducting SrMoO3 electrodes for microwave applications.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00013239
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

In this work, the interfaces and growth by pulsed laser deposition of the all-oxide varactor heterostructurewas investigated. This all-oxide varactor heterostructure consists of SrMoO3/(Ba,Sr)TiO3/Pt/Au layers and has several advantages over thin-film varactors with Pt bottom electrodes such as defect free, epitaxial growth. Furthermore, new interface materials and optimized growth led to an improvement of the all-oxide varactor microwave performance beyond the level of competing state-of-the-art platinum-based technology. A key achievement for this performance increase was the fast and several micrometer thick growth of the bottom electrode SrMoO3. Although grown at a exceptional high rate of 1 µm SMO in only 45 minutes, the bottom electrode exhibits a high conductivity, low defect density, and atomically flat interface to the dielectric layer. The influence of different substrates with varying in-plane lattice constants on the SrMoO3 growth was also investigated, including the industrially relevant substrate silicon. Epitaxial growth has been realized for silicon and a wide range of scandates, proven by reciprocal space maps. In order to unite the the reductive growth conditions of the bottom electrode with the oxidizing background pressure during the growth of the dielectric, a oxygen diffusion barrier is implemented. In order to find the best performing titanate compound, in terms of oxygen diffusion barrier, a novel measurement routine was established utilizing X-ray photoelectron spectroscopy. In addition to comparisons of the different titanate compounds, quantitative diffusion barrier limits for best performing compound, Ba0.5Sr0.5TiO3, such as temperature and background pressure were determined. This enabled the growth of the dielectric at higher oxygen partial pressures leading to a sufficient oxygenation of the dielectric layer and a decrease of the leakage current of several orders of magnitude. The findings were combined in high-performance all-oxide varactor heterostructures both on the GdScO3 and silicon substrates. In summary, fast and micrometer-thick growth of SrMoO3 and a novel oxygen diffusion barrier were established and could raise the performance of all-oxide varactors and demonstrate the feasibility of this technology for microwave applications.

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