Thin films of optimally doped(001)-oriented YBa 2 Cu 3 O 7−δ are epitaxially integrated on silicon(001) through growth on a single crystalline SrTiO 3 buffer. The former is grown using pulsed-laser deposition and the latter is grown on Si using oxide molecular beam epitaxy. The single crystal nature of the SrTiO 3 buffer enables high quality YBa 2 Cu 3 O 7−δ films exhibiting high transition temperatures to be integrated on Si. For a 30-nm thick SrTiO 3 buffer, 50-nm thick YBa 2 Cu 3 O 7−δ films that exhibit a transition temperature of ∼93 K, and a narrow transition width (<5 K) are achieved. The integration of single crystalline YBa 2 Cu 3 O 7−δ on Si(001) paves the way for the potential exploration of cuprate materials in a variety of applications.

Layered compounds are extensively studied for applications as superconductors and ionic conductors. These layered structures can be considered as stacking sequences of perovskite layers with or without rock-salt layers separating them and where planar defects such as stacking faults are often present. This anisotropy of defects and the structure of interfaces with the substrates have potentially dramatic effects on the physical properties such as low dimensional transport, diffusion and growth. In this study we focus our attention to the interface structure and defects on YBa 2 Cu 3 O 7-δ thin films grown on different substrates of SrTiO3 (STO) and (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) ultimately used for transport measurements in YBCO-LCMO multilayer [1].Imaging with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (EELS) were carried out with an FEI Titan 80-300 Cubed TEM, equipped with a CEOS-designed hexapole-based aberration corrector for the probeforming lens and imaging lens. The instrument is fitted with a high brightness electron source, a Super-Twin lens in order to achieve sub-Ångstrom resolution both for phase contrast imaging, STEM and EELS mapping [2]. Measurements were realized at 80 and 200 keV in order to reduce knock-on damage. EELS spectra were recorded on 2048 channels with an energy dispersion of 0.5eV. EELS spectrum images were refined using Weighted Principal Component Analysis using the Multivariate Statistical Analysis plugin developed by M. Watanabe [3] and implemented by "HREM Research" within Gatan's DigitalMicrograph. The effects of the MSA on the data is discussed elsewhere [4]. EELS mapping shows that the STO last layer at interface between STO and YBa 2 Cu 3 O 7-δ (YBCO123) terminated by Ti atoms and that the first layer is Ba-O (figure 1). Also from direct atomic EELS imaging, we note that, after the first YBCO unit cell, we detect a shift of the Ba atomic positions suggesting that, in the analyzed area, the YBa 2 Cu 4 O 8 (YBCO124) cell appears. Such complex stacking is exemplified in the growth of YBCO on (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) (figure 2). From the combination of atomic resolved maps and HAADF intensities, we can unambiguously identify the sequence of layers and intergrowths of Cu-O 2 layers. From these images, we identified the local fraction of the YBCO124 phase and the role of this phase on the nature of the interface between YBCO and LCMO ultrathin films. It was found that interfaces have a different layer arrangement on the first few nanometers of the film and that films can present a high fraction of YBCO124 in some growth conditions. Our results also demonstrate that smooth interfaces, at the unit cell level and over several microns, can be obtained and thus very high epitaxial quality of the samples can be grown.

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