D. V. Masterov

Thin films of YBaCuO high-temperature superconductor grown in a simplified magnetron sputterer and their microwave application

The physicochemical properties of YBa2Cu3O7−δ high-temperature superconductor exclude designing standard magnetron sputterers and adopting standard film growth conditions that would ensure good and highly reproducible results. A simple and flexible magnetron sputterer making it possible to grow high-quality films (with a critical temperature of 90 K, a critical current density of 4 MA/cm2, and a surface resistance of ≤0.5 mΘ at a frequency of 10 GHz and 77 K) is described. The high quality of the films is proved by successfully applying them in a microwave circuit demonstrating an intrinsic Q factor of 58 000 at a frequency of 70 MHz and 77 K, which is higher than the results obtained by other teams of researchers.

Peculiarities in magnetron sputtering of YBCO epitaxial films for applications in superconductor electronics devices

We consider the main factors determining the growth of YBa2Cu3O7–δ high-Tc superconductor films during magnetron sputtering in the planar axial geometry. Special attention is paid to the increase of the growth rate of the films suitable for superconductor electronics devices. Magnetron sputtering is used for obtaining YBa2Cu3O7–δ films with high structural and electrophysical characteristics for a growth rate up to 200 nm/h, which were used in constructing microwave disk resonators and long Josephson junctions on bicrystal substrates. The unloaded Q factor of cavities exceeds 80000 at a frequency of 7.1 GHz at a temperature of 77 K, which corresponds to the best results in this field. Josephson junction of length 50–350 μm are characterized by critical current density jc = 12–33 kA/cm2 at T = 77 K and jc = 93–230 kA/cm2 at T = 6 K in zero magnetic field. The characteristic voltage IcRn is 0.8–1.96 mV.

Features of technology of high-temperature superconductor films for microwave filters

The characteristics of fourth-order bandpass microwave filters based on high-temperature super-conductor (HTS) films have been numerically simulated and experimentally measured for the devices based on YBCO films purchased from Theva GmbH (Munich) and manufactured at the Institute for Physics of Microstructures (Nizhni Novgorod). Experimental data were used to determine the parameters of the surface impedance model for the HTS films. The results show that HTS-based filters with high performance characteristics can be obtained using HTS films manufactured by different technologies.

Deposition of YBCO films on both sides of substrate by magnetron sputtering

We have studied features of the formation of YBa2Cu3O7 − δ (YBCO) films on both sides of sub-strates by magnetron sputtering in the on-axis geometry. During sputter deposition onto the first (front) side, a thin film is simultaneously formed on the second (rear) side, which can provide a high-quality sublayer for the final coating of preset thickness deposited at the subsequent stage. It is shown that, by monitoring the properties of a sublayer formed on the rear side of substrate during YBCO layer growth on the front side, it is possible to optimize technological parameters of the process.

Surface resistance of an YBaCuO film in the megahertz range determined from the Q factor of a planar inductance coil: Effect of extrinsic factors

High-quality YBaCuO films are used to sequentially prepare a 10-GHz disk resonator and a planar inductance coil. The Q factor of the planar inductance coil at its resonance frequency (64 MHz) is much higher than the values reported for analogous structures. The measurement results are used to estimate the surface resistance of the films at frequencies of 10 GHz and 64 MHz. The surface resistance measured at 64 MHz is more than fourfold of that calculated from the surface resistance measured at 10 GHz by the dependence Rsur ∼ ω2. Our analysis demonstrates that extrinsic factors cannot substantially affect the measurement results; therefore, the deviation from the Rsur ∼ ω2 dependence in the megahertz range is determined by the intrinsic properties of the superconducting strip.

A study of planar structures formed on the modified Al2O3 surfaces determining the topology of superconducting elements during YBa2Cu3O7–d deposition

We investigate the structural and electrical properties of planar superconducting structures based on the YBa2Cu3O7–d (YBCO) epitaxial films obtained by preliminary modification of the substrate surface. A special master mask was formed on the substrates, so that, at the standard YBCO film deposition onto such a substrate, an insulator layer grew in the modified areas and a superconducting film, in the unmodified ones. Thus, the planar superconducting structure of a desired topology was formed, and the YBCO deposition finished the process. Using this technique, YBCO bridges with widths of 4, 10, and 50 μm on films of different thicknesses and a planar inductive coil were formed. The superconducting transition temperature of the bridges was about 90 K, and the critical current density at a temperature of 77 K was up to 3 MA/cm2. The Q factor of the planar inductive coil at a frequency of 85 MHz was 53000 at a temperature of 77 K.

Growth and Formation of the Microstructure of YBCO Films Deposited by Magnetron Sputtering on Fianite Substrates

The dependence of the electrophysical and structural properties of the high-Tc YBa2Cu3O7 − δ superconductor films deposited by magnetron sputtering on fianite on the growth temperature is studied. Scanning electron microscopy and X-ray diffraction demonstrate that the layer-by-layer island growth of an YBa2Cu3O7 − δ film occurs on fianite substrates without an additional buffer layer. An increase in the deposition temperature leads to the division of the growing film into regions with strongly different microstructures. The regions of the first type are represented by a high-quality YBa2Cu3O7 − δ layer, and the regions of the second type consists of an YBa2Cu3O7 − δ layer with well-pronounced granule boundaries, which hinder the passage of an electric current.

Bi3+ cluster primary ions in SIMS depth profiling of YBaCuO high-temperature superconductor films

SIMS depth profiling of YBa2Cu3O7 high-temperature superconductor films was performed using a TOF.SIMS-5 instrument. A new approach was proposed to obtain additional information on the phase composition of YBa2Cu3O7 films based on detection of cluster secondary ions.

Magnetron sputtering of YBaCuO target: Effects of discharge voltage and deposition rate variation

The time variations of the discharge voltage and YBaCuO film deposition rate in an on-axis magnetron sputterer are studied. In the case of an YBaCuO ceramic target, the discharge voltage and the rate of superconducting phase deposition decrease with time, exponentially approaching a quasi-steady regime. At a pressure of 100 Pa, a decrease in the magnetic field induction near the target from 1200 to 600 G leads to a rise in the discharge voltage by 25–30% and increases the deposition rate more than twofold. The deposited films offer high electrophysical parameters, as demonstrated with a high-frequency circuit: the intrinsic Q factor of the circuit at 64.5 MHz is found to be 2.7 × 105.

Investigation of the compositional changes of a Y-Ba-Cu-O HTSC target under ion sputtering

An Auger electron spectroscopy study is reported of the elemental depth profile of Y-Ba-Cu-O HTSC targets subjected to ion-plasma sputtering in a magnetron deposition system and ion-beam sputtering in the Auger spectrometer chamber. It has been established that the process consists in all cases of predominant copper sputtering accompanied by the formation of a modified surface layer and of a copper-depleted region. This region is assumed to originate from intense copper diffusion from the bulk to the modified surface layer driven by a concentration gradient.