Epitaxial cerium oxide buffer layers and YBa₂Cu₃O_7−δ thin films for microwave device applications

Abstract

CeO2 films with thicknesses ranging from 8.8 to 199 nm were grown on Al2O3 (1102) (R-cut) substrates by off-axis rf magnetron sputtering. X-ray diffraction showed an epitaxial relationship with the CeO2 (001) planes parallel to the Al2O3 (1102) planes for all film thicknesses. Atomic force microscopy (AFM) revealed a rough surface morphology consisting of crystallites with lateral dimensions of 10–90 nm. In the thinnest film, these crystallites were regularly shaped and uniformly distributed on the substrate, while they were rectangularly shaped and oriented mainly in two directions, orthogonal to each other, in the thicker films. The surface roughness of the films increased with increasing layer thickness. Characterization of the microstructure was done by cross-sectional transmission electron microscopy (XTEM) and showed a polycrystalline, highly oriented, columnar structure with a top layer terminated by (111)-facets. High-quality YBa2Cu3O7−δ (YBCO) thin films were deposited directly onto the CeO2 layers. XTEM, rather surprisingly, showed a smooth interface between the YBCO and CeO2 layer. Postdeposition ex situ annealing was carried out on two CeO2 films and evaluated by AFM. Upon annealing samples at 930 °C, a relatively smooth morphology without facets was obtained. Annealing films at 800 °C caused no appreciable change in surface morphology, whereas igniting a YBCO plasma during a similar anneal clearly altered the sample surface, giving facets that were rounded.

Synthesis and Structural Characterization of a Series of Carboxylic Acid Modified Cerium(III) Alkoxides

“…Only a limited number of X-ray crystallographically characterized Ce(OR) 3 [27,28] Ce 4 (µ 4 -O)(µ 3 -OiPr) 2 (µ-OiPr) 4 (OiPr) 8 , [24,29] Ce 3 (µ 3 -OtBu) 2 (µ-OtBu) 3 (OtBu) 5 (NO 3 ), [20] [Ce(µ-OR)(OR) 2 ] 2 where OR = OtBu [23] or OCH(tBu) 2 , [26] Ce[OC(tBu) 3 ] 3 -(CNtBu) 2 , [22] [Ce{OCH(tBu) 2 …”

“…Typically, tailoring hydrolysis and condensation pathways of the M(OR) x precursors is of interest and accomplished by introducing a variety of ligands, including carboxylic acids (H-ORc). We have previously explored the structures obtained for group 4 3 = H-ONc], in an effort to understand how structure affects the final densification of thin films. [1,2] A number of general constructs were isolated for these sys-tems but the partially hydrolyzed smaller trinuclear species proved to be the most useful for the generation of highly dense TiO 2 films and simple dinuclear species for ZrO 2 thin films.…”

“…In addition, CeO x has demonstrated a wide variety of other applications, such as barrier layers, [3][4][5] counter electrodes for electrochromic glasses, [6] catalysts, [7][8][9][10][11] electrolytes for solid oxide fuel cells, [12][13][14] polishing agents for soft and hard optical glasses, [15,16] and numerous other applications. Due to the widespread use of CeO x , it is surprising that only a few structurally characterized cerium alkoxides [Ce(OR) 3 ] have been reported in the literature, [17][18][19][20][21][22][23][24][25][26][27][28][29] and no systematic studies were possible from this eclectic mix of potential precursors. Because it is critical to know the structure of the precursor to be able to optimize the properties of the final CeO x materials, it was necessary to develop a family of wellcharacterized, systematically structurally diverse Ce(OR) 3 precursors.…”

“…Due to the widespread use of CeO x , it is surprising that only a few structurally characterized cerium alkoxides [Ce(OR) 3 ] have been reported in the literature, [17][18][19][20][21][22][23][24][25][26][27][28][29] and no systematic studies were possible from this eclectic mix of potential precursors. Because it is critical to know the structure of the precursor to be able to optimize the properties of the final CeO x materials, it was necessary to develop a family of wellcharacterized, systematically structurally diverse Ce(OR) 3 precursors. Understanding the structural changes available for these precursors is critical prior to generating tailoredproperty CeO x thin films.…”

“…Therefore, we undertook the synthesis and characterization of a series of Ce(OR) 3 that would fill in several gaps in the alkyl and aryl oxide derivatives to develop a systematic family of structurally characterized Ce(OR) 3 complexes which could be used to explore tailored materials. An alcohol amide metathesis reaction pathway was selected to synthesize these compounds [Equation (1)] in toluene (tol), tetrahydrofuran (THF), or pyridine (py).…”