Formation of gold nano‐particles during pulsed laser deposition of YBa₂Cu₃O_7–δ thin films

Abstract

The mobility of flux vortices in high temperature superconductors in general, and YBa 2 Cu 3 O 7-δ (YBCO) thin films in particular, is an essential limiting factor determining the performance of superconducting devices, such as Josephson junctions and magnetic field sensors. Flux noise properties and critical current density can be improved by adding artificial defects serving as pinning centers. A simple and effective technique to obtain such defects is decorating the substrate with prefabricated nano-particles before YBCO film deposition. A number of materials, such as CeO 2 [1], Y 2 O 3 [2, 3] as well as MgO, BaZrO 3 and Ir [4][5][6], have successfully been tested in this regard.In this work we utilized the tendency of thin gold (Au) layers to self-assemble nano-scale particles at elevated temperatures during deposition of the YBCO film. A similar technique has been presented for vortex pinning in YBCO thin films by Mikheenko et al. [7]. It allows to control the particle size over a range of a few nanometers to one micron and can be utilized to lower the contact resistance to bonding wires [8,9], which often is the source of current noise. The aim of this work is to analyze particle formation and size distribution as well as crystallographic properties of the particles and the circumjacent YBCO film.Prior to the growth of the YBCO films a thin gold layer was deposited on SrTiO 3 (STO) single crystal substrates under high vacuum conditions (p 0 < 5 × 10 -4 Pa). For layer thicknesses up to 20 nm this was accomplished by pulsed laser deposition using a KrF excimer laser (λ = 248 nm) at a repetition rate of 10 Hz and a laser fluence of 1.3 J/cm 2 at the target. Under these conditions the growth rate was 4.4 nm/min. Thicker Au layers were grown by dc-sputtering to limit the deposition time.Subsequently, the substrate was heated to 750 °C. Again using the KrF excimer laser a 150 nm thick YBCO film was then grown in an atmosphere of pure oxygen 2 O ( p = 50 Pa). The repetition rate was set to 5 Hz and the laser fluence to 2.2 J/cm 2 . After deposition the film was slowly cooled to room temperature at a rate of 50 K/min in an oxygen atmosphere of 2 O p = 8 × 10 4 Pa. During film growth small particles evolve from the Au layer.Transmission electron microscopic (TEM) images show that particles having diameters much smaller than the YBCO film thickness preferentially grow at the YBCO In order to improve transport and noise properties of YBa 2 Cu 3 O 7-δ thin films, we studied a technique to produce gold nano-particles in these films. We report on the formation of these particles from thin gold layers during pulsed laser deposition of YBa 2 Cu 3 O 7-δ . Depending on initial gold layer thickness we analyzed their size distribution and structural properties. Gold particles preferentially grow at the YBa 2 Cu 3 O 7-δ surface and the SrTiO 3 /YBa 2 Cu 3 O 7-δ interface and show an epitaxial alignment parallel to the YBCO 001 direction. We discuss the influence of these particles on the crystal structure of YBa 2 Cu 3 O 7-δ in...

Engineering of YBa2Cu3O7–δ grain boundary Josephson junctions by Au nanocrystals

“…1) we do not find any additional agglomeration of such particles. XRD and TEM data of our films prove a highly oriented crystalline nature of the Au nanoparticles grown under our preparation conditions 8. This is a completely different behavior than reported in Pinto et al 9 for Au‐added YBCO films.…”

“…The distribution of these Au nanocrystals within the YBCO film is not fully clarified yet. We found such particles not only at the surface of the YBCO film, but also near the substrate surface and inside the YBCO film itself 8.…”

Gold nanocrystals in high-temperature superconducting films: creation of pinning patterns of choice

“…Due to the high temperatures during YBCO thin film deposition one can observe a dewetting of the Au layer and the formation of crystalline Au nanoparticles within the growing YBCO matrix. In this way the growth conditions of the YBCO thin film are modified [31]. Owing to their distribution over the whole YBCO layer the Au nanocrystals create inhomogeneous strain at the nanoscale which efficiently creates an additional pinning force density and thus increases the YBCO critical current density [25].…”

Electron backscatter diffraction analysis of gold nanoparticles on Y1Ba2Cu3O7−δ

“…8 We have found a relaxation of the film tensions during growth on STO near the nanoparticles, and bending is clearly observed in the environment of the nanoparticles. 6 In conclusion, the result of structural changes is a modification of superconducting properties. Critical temperature T C and critical current density j C but also the London penetration depth k L can be influenced.…”

“…[3][4][5] However, in our experiments single crystalline Au nanoparticles were used, which can be incorporated in the thin film matrix during the growth process of the YBCO films. 6 In this case, single crystalline Au-nanoparticles improve the crystalline quality of YBCO films in a significant manner. The interaction between epitaxial growth of YBCO films and the development of the nanoparticle ensemble can reduce the length of the c-axis of YBCO and the full half width of the characteristic YBCO peaks in XRD measurements.…”