Elimination of pinholes in epitaxial thin film YBa2Cu3O7x /SrTiO3/PrGaO3multilayersE. Olsson, G. Brorsson, P. Å. Nilsson, and T. Claeson

Citation: Applied Physics Letters 63, 1567 (1993); doi: 10.1063/1.110751 View online: http://dx.doi.org/10.1063/1.110751 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/63/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A novel multilayer circuit process using YBa2Cu3O x /SrTiO3 thin films patterned by wet etching and ionmilling Appl. Phys. Lett. 69, 2752 (1996); 10.1063/1.117700 Growth and properties of YBa2Cu3O7x /(SrTiO3/PrGaO3)/YBa2Cu3O7x trilayers: Optimization of theinsulation J. Appl. Phys. 75, 827 (1994); 10.1063/1.356436 Characterization of a tunable thin film microwave YBa2Cu3O7x /SrTiO3 coplanar capacitor Appl. Phys. Lett. 63, 3078 (1993); 10.1063/1.110238 Laserdeposited PrGaO3 films on SrTiO3 substrates and in YBa2Cu3O7/PrGaO3/YBa2Cu3O7 trilayers Appl. Phys. Lett. 61, 486 (1992); 10.1063/1.107865 Crack formation in epitaxial [110] thin films of YBa2Cu3O7 and PrBa2Cu3O7x on [110]SrTiO3 substrates Appl. Phys. Lett. 58, 1682 (1991); 10.1063/1.105110

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Elimination of pinholes in epitaxial thin film YBa2CU3Q~-,/Sr~i03/PrGa03 multilayers

E. Olsson, G. Brorsson, P. A. Nilsson, and T. Claeson Department of Physics, Chalmers University of TechnoIogv/University of Giiteborg, S-412 96 Giiteborg, Sweden

(Received 6 May 1993; accepted for publication 12 July 1993)

Pinholes have been eliminated in trilayers of YBa~Cu,07~,(YBCO)/PrGaO~(PG0,)/YBC0 by introducing 13-nm-thick SrTiOs( STO) films in the intermediate PGO tilm. The ST0 continuously covers the bottom YBCO layer and promotes a uniform growth of the subsequent PGO films. The individual PGO films are of high quality up to a thickness of about 40 nm where voids start to appear in the films. The voids disrupt the continuity of the subsequent ST0 and PGO layers. The technique of introducing thin layers that establish the growth of pinhole-free films can be used for other similar multilayer systems that suffer from pinholes between individual films.

Techniques for deposition of thin films of the high tem- perature superconductor (HTS) YBa$u30,-x (YBCO) with sufficiently high critical current density, j,, high tran- sition temperature, T,, and low surface resistance, R,, for electronic device applications are presently available. Most applications demand high j, and there is thus a need for epitaxial films where etIects of the highly anisotropic char- acter of the YBCO and the weak-link behavior of grain boundaries are avoided. In addition, most thin film appli- cations of the HTS involve integrated circuits consisting of multilayer structures. The fabrication of integrated circuits is, however, currently limited by difficulties of growing multilayer structures with sufficient insulation between the YBCO layers. The layers most often suffer severely from microshorts and it is difficult to obtain reproducible results.

could affect the microstructure of other related multilayer

The HTS films of multilayer structures are separated by either metals, semiconductors, or insulators, depending on the specific application. In order to obtain high quality YBCO films at all levels of the multilayer, it is necessary to grow epitaxial layers which are devoid of protruding sec- ond phase particles or YBCO particles with a deviating orientation. The particles may disturb the epitaxial orien- tation relationships and can also cause microshorts be- tween the layers. An optimum multilayer structure thus consists of epitaxial layers with atomically flat interfaces without particles.

The present work is concerned with trilayer structures consisting of top and bottom YBCO with an insulat- ing intermediate multilayer of SrTiO,(STO) and PrGaOs( PGO). The PGO has a relatively low dielectric constant and a low loss tangent (tan S) ’ which makes it a suitable material for microwave devices. It has been shown previously that epitaxial trilayer structures of YBCO and PGO can be grown by pulsed laser deposition.’ However, the insulation between the YBCO layers was not complete and superconducting microshorts were detected by resis- tance versus temperature measurements. The aim of this work was to investigate the structure and formation mech- anism of the pinholes with the intention of reducing the interfacial roughness and preventing the formation of mi- croshorts. We were interested ‘in general phenomena that

systems as well. The multilayers were deposited on (OO1)STO sub-

strates by pulsed laser deposition. An energy density of 1.5 J/cm2 at the target, an ambient O2 pressure of 0.8 mbar and a distance of 5 cm between the target and the substrate resulted in deposition rates of about 0.3, 0.2, and 0.5 A/pulse for YBCO, STO, and PGO, respectively. The dep- osition temperature was 745, 770, and 815 “C for STO, PGO, and YBCO, respectively. The trilayers consisted of two YBCO layers, 100 nm in thickness, that were inter- spersed by an intermediate insulating multilayer, whose to- tal thickness was about 200 nm. The insulating multilayer consisted of thin films of ST0 and PGO. The thickness of the ST0 films was about 13 nm for all samples. Each YBCO/insulating multilayer/YBCO structure was grown with one thickness of the PGO layers. The thickness was varied between the structures from about 10 nm up to about 60 nm. The microstructure of these structures was compared to that of a trilayer where two YBCO layers were separated by one single PC0 film.

The microstructures were characterized using a 200 kV 2000FX transmission electron microscope (TEM/ scanning TEM (STEM) instrument and a CAMSCAN S4-80DV scanning electron microscope (SEM) instru- ment. Cross-section TEM samples provided information about the interfacial microstructure. The integrity of the intermediate layers over relatively large areas (about 50 pm X 50 pm) was investigated by extracting the insulating intermediate layers. The multilayers were then immersed into diluted HCl (4%) which etches the YBCO and leaves the PGO and the ST0 intact. The intermediate layers floated off the substrate and were picked up on Cu grids. These were subsequently examined in the TEM. The top YBCO surface morphology of the multilayers was charac- terized by SEM.

The YBCO films deposited on the (001) ST0 sub- strates were c-axis oriented with [ 1 lO]vn,//[l 10]s,o. The T, of the YBCO films was about 89 K and the critical current density, j,, was about lo6 A/cm2 at 77 K. There were occasional a-axis oriented ([ 100]~nco//[OO llsro and [013]vnco//[100]s~o) YBCO particles. The first and last

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FIG. 1. A cross-section TEM micrograph of a YBCO/STO/PGO/STO/ PGO/STO/PGO/STO/YBCO multilayer structure. The ST0 layers are about 13 nm in thickness and the PGO are 41 nm. The first ST0 layer is continuous and the apparent decrease in thickness as the TEM sample thickness increases towards the right of the micrograph is due to surface roughness of the bottom YBCO layer. Voids appear in the first PGO layer and the density of voids increase with increasing thickness and number of layers in the intermediate insulating multilayer.

layers of the intermediate insulating multilayer were al- ways chosen to be ST0 in order to prevent interdiffusion between PGO and YBCO.’

The ST0 films grew epitaxially on YBCO with [OO1]s-ro//[OO1]vnco and [ 1 10]s-ro//[l 10],nco. TEM of cross-section samples showed that the first 13-nm-thick ST0 film formed a continuous layer with uniform thick- ness on top of the YBCO bottom t?.lm, Fig. 1. Local height variations in the bottom YBCO f?lm did not affect the con- tinuity except for surface morphologies where the surface normal abruptly changes 90”. This particular structure is under further investigation. The discontinuity of the ST0 layer was invariably compensated by the following PGO layer.

Subsequent PGO films were - epitaxial with either c-axis orientation ( [OO1],oo//[OO1]sro, [lOO]roo, or [OIO],oo//[l lO],,), or 110 oriented, i.e., ([l 10]roo// IO0 %TO 3 [oollPGO~~[loolSTO~ or [OlO]sro). The gram boundary planes within the PGO films were perpendicular to the interfaces between the different layers. The presence of PC0 domains with different orientations did not affect the orientation of the subsequent ST0 layers which ob- tained the same orientation as the lirst ST0 film on the bottom YBCO film.

The PGO layers were continuous and uniform up to film thicknesses of 40 nm where voids started to appear. The voids were square in shape with sharp flat walls that were perpendicular to the plane of the film. Subsequent ST0 films were not able to cover or fill the voids and smoothen the surface, see Fig. 1, due to the shape of the voids; the steep walls did not allow a continuous layer of STO. Consequently, the structure of the intermediate mul- tilayer of ST0 and PGO gradually degraded as the thick- ness increased. The presence of square voids in multilayers with PGO thicknesses larger than 40 nm were also re- flected in the surface morphology of the top YBCO layer where square depressions were observed by SEM. The for- mation of voids was eliminated by limiting the thickness of the individual PGO layers to less than 40 nm, see Fig. 2.

No microshorts between the bottom and top YBCO layers were observed by TEM. TEM of cross sections gen- erally provides information from relatively small areas and

FIG. 2. Cross-section TEM micrograph of a trilayer where the insulating intermediate layer consisted of five ST0 films (each 13 nm in thickness ) and four PGO films (each 22 nm in thickness). The void formation was inhibited by choosing a thickness less or equal to 40 nm of the individual PGO layers.

efforts were, therefore, made to investigate large areas by successively thinning the cross sections in order to obtain further electron transparent regions. The results were also compared to plan view samples, where the intermediate layers had been extracted from the multilayers by etching away the YBCO films. These samples allowed TEM inves- tigations of areas that were about 50 X 50 ,um. A compar- ison between an intermediate multilayer and a single layer of PGO of similar thickness is shown in Fig. 3.

The plan views of the intermediate layers showed that the STO/PGO multilayers were pinhole-free while the sin- gle PGO layers had a high density of pinholes;The pin- holes in the single PGO layer caused direct microshorts between the two YBCO layers and the size of the pinholes was about 50-100 nm in diameter. The pinholes were often connected forming strings of perforations in the PGO, see Fig. 3. These observations were confirmed by resistivity measurements as a function of temperature: the STO/PGO intermediate layer exhibited an exponentially increasing re- sistivity with decreasing temperature while the PGO inter- mediate layer had superconducting shorts between the YBCO films.

The morphology of the first ST0 film of the STO/ PGO multilayers indicates that the ST0 initially is grow- ing layer-by-layer on top of the YBCO. This assumption is supported by previous observations of two-dimensional growth of YBCO on (001) ST0 substrates.3 The full cov- erage of PGO on ST0 is indicating an initial layer-by-layer growth of PGO on top of STO. The layer-by-layer growth is subsequently interrupted and voids appear at a thickness of about 40 nm. The morphology of the PGO films is sim- ilar to the expected morphology of sputtered amorphous films where growth instabilities occur at a certain thick- ness.475 Protrusions then develop and grow upward and outward. Shadowing may subsequently give rise to a fine void network. The laser ablation process can be compared to the sputter process in the sense that the film is grown from species impinging on the surface. Surface diffusion will smooth the surface but for the rapid and nonequilib-

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directly on YBCO and the structure of the pinholes are strong indications of an island growth mechanism. The observation is somewhat surprisifig since the lattice mis- match between the PGO and YBCO is significantly smaller (0.3% for c-axis oriented PGO and 0.1% for 1 lo-oriented PGO) at the deposition temperature’ than between YBCO and ST0 ( 1%) .6 Other factors are obviously also affecting the growth modes of these systems.

In summary, we have been able to reproducibly elim- inate pinholes in YBCO/msulating multilayer/YBCO where the intermediate insulating multilayer consists of ST0 and PGO films. The ST0 provides a continuous cov- erage of the bottom YBCO film and prevents interdiffusion between the PGO and ihe YBCO. Subsequent PGO films grow epitaxially with high quality up to a thickness of 40 nm whereafter voids form in the films. The voids deterio- rate the quality of the subsequent films and the quality decreases with increasing thickness of the intermediate multilayer. The technique of interspersing a layer with thin layers that eliminate pinhole formation can be applied to other systems as well. A layer-by-layer growth mode favors high quality multilayers with large critical thicknesses. The multilayer technique also opens possibilities to continu-, ously vary the physical properties of the intermediate lay- ers by combining materials with different behavior.

The authors acknowledge stimulating discussions with Z. G. Ivanov and Yu. Boikov. Financial support from TFR (Swedish Research Council for Engineering Sciences), NUTEK (National Board for Industrial and Technical Development), and NFR (Swedish Natural Science Re- search Council) is also gratefully acknowledged.

rium deposition technique used here shadowing may easily occur. The present system is complicated to model since we are concerned with crystalline surfaces and multicom- ponent structures while simulations of the film evolution have been carried out for amorphous films. Another pos- sible explanation for the void formation is an accumulation of defects in the PGO films as the thickness increases. The detailed mechanism for the void formation is currently un- der investigation.

The high density of pinholes in the PC0 film grown

FIG. 3. Plan view TEM micrographs of the intermediate insulation lay- ers. (a) A single PGO film about 200 nin in thickness. (b) A multilayer consisting of four ST0 films (each 13 nm in thickness) and three PGO films (each 41 nm in thickness).

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