High Tc Superconductor Thin Films

863 Pages·1992·40.402 MB·English

by  L. Correra (Eds.)

INTERNATIONAL CONFERENCE
ON
ADVANCED MATERIALS
-ICAM91-
Organised by the European Materials Research Society
Under the auspices of the
International Union of Materials Research Societies
Part of the
EMRS 1991 Spring Meeting
HIGH Tc
ERCONDDC
THIN FILMS
Proceedings of Symposium A1 on
High Temperature Superconductor Thin Films
of the International Conference on Advanced Materials - ICAM 91
Strasbourg, France, 27-31 May, 1991
Edited by:
L. CORRERA
Istituto LAMEL-CNR
Bologna, Italy
NORTH-HOLLAND
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Library of Congress Cataloglng-ln-PublIcatIon Data
Symposium A1 on High Tc Superconductor Thin Films (1991 : Strasbourg,
France)
High Tc superconductor thin films : proceedings of Symposium A1 on
High Tc Superconductor Thin Films of the International Conference on
Advanced Mater 1 a 1 S--ICAM 91, Strasbourg, France, 27-31 May, 1991 /
edited by L. Correra.
ρ. cm.
Includes bibliographical references and index.
ISBN 0-444-89353-9
1. Thin films—Congresses. 2. High temperature superconductors-
-Congresses. 3. Epitaxy—Congresses. I. Correra, L.
II. International Conference on Advanced Materials—ICAM 91 (1991 :
Strasbourg, France) III. Title.
QCi76.82.S86 1991
621 .381'52~dc20 91-44392
CIP
ISBN 0444 89353 9
© 1992 Elsevier Science Publishers B.V. All rights reserved.
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ν
PREFACE
This Proceedings collects the papers presented at the symposium on
High T Superconductor Thin Films, one of the four symposia of the
c
International Conference on Advanced Materials - ICAM 1991 (27-
31 May 1991, Strasbourg).
The symposium focused on interdisciplinary research on super­
conducting oxides, dealing with several aspects of the thin film field,
from fundamental properties to applications. Over 40 contributed
papers, 110 poster contributions and 21 invited talks were presented
during the meeting, averaging 130 attendees. Most of the papers
dealt with the 1-2-3 system, indicating that the Y-Ba-Cu-0 and
related compounds are still the most intensively studied materials
by far. However, interesting results for the Bi system were also
presented.
To develop a better understanding of the relationships between
microstructure and properties, four sessions were devoted to physical
properties and two to structural characteristics. Superconductivity
and two-dimensional transport behavior in Y-Ba-Cu-0 and Bi-Sr-
Ca-Cu-0 thin films were widely discussed, as were optical, magnetic
and microwave properties. Film growth mechanisms were studied
by a variety of techniques, including RBS-channeling, x-ray
diffraction and high-resolution TEM, this latter being a powerful
technique to study layer-by-layer epitaxy.
The main deposition techniques covered were laser ablation,
evaporation, sputtering and chemical vapor deposition. Complete
control over deposition conditions is required for growing complex
structures and for compatability with device fabrication technology.
Considerable progress was reported on high-quality thin film
preparation by CVD and organometallic CVD.
The papers devoted to processing emphasized strategical subjects
such as deposition of epitaxial and smooth films on large area
vi Preface
substrates and control of the film orientation. The importance of
methods for film deposition on silicon was highlighted. Surface and
interface studies, including growth and properties of buffer layers,
were discussed in a session devoted to substrates and multilayers.
Two sessions on application illustrated significant progress in the
fabrication of microwave devices, IR detectors, SQUID magneto­
meters and optoelectronic devices. However, many challenging
problems are still to be solved for junction-based technology.
I would like to thank the Program Committee for the help given in
finding excellent speakers and in undertaking the selection of
papers. I would also like to thank all those who contributed to the
success of the symposium and, in particular, the invited speakers,
the session chairpersons and the referees of the papers. I must
mention that some - not many - papers were published without the
language improvements suggested by the adviser. This was
necessary as it was impossible to contact the authors and have the
revised version in time for publication, and I did not consider it
correct to exclude scientifically valid work from the proceedings.
Finally, I am indebted to the Ε-MRS staff for their support in the
organization and smooth running of the symposium. Many thanks
are also due to Elsevier Science Publishers for the assistance and
the timely reproduction of these proceedings.
vii
SYMPOSIUM INFORMATION
This Conference was held under the auspices of:
- The Council of Europe
- The Commission of the European Communities
It is our pleasure to acknowledge with gratitude the financial
assistance provided by:
- Banque Populaire (France)
- Centre de Recherches Nucleaires (France)
- Centre National de la Recherche Scientifique (France)
- Service de Documentation Touristique du Palais des Congres de
Strasbourg (France)
- The Commission of the European Communities
- The Council of Europe
- The European Parliament
- The Brewery Kronenbourg (France)
and in particularly with respect to Symposium Al:
- The Consiglio Nazionale delle Ricerche (Italy)
- The Balzers S.p.A. (Italy)
viii Symposium Information
Chairpersons:
L. Correra (Italy)
T. Kawai (Japan)
T. Venkatesan (USA)
Symposium Programme Committee:
A. Barone (Italy), L. Correra (Italy), J.E. Evetts (UK),
H.-U. Habermeier (Germany), J. Klein (France), L. Shultz (Germany),
A.I. Usoskin (USSR), R. Vaglio (Italy), T. Venkatesan (USA)
ICAM 91 International Organizing Committee:
Chairman: M. Balkansi (France)
K.J. Bachmann (USA), L. Correra (Italy), A.T. di Benedetto (USA),
H.L. Hwang (Taiwan, China), T. Kawai (Japan), L. Nicolais (Italy),
C. Schwab (France), T. Takahashi (Japan), H.L. Tuller (USA),
T. Venkatesan (USA), E. Yasuda (Japan)
High T Superconductor Thin Films
c
L. Correra (Editor)
© 1992 Elsevier Science Publishers B.V. All rights reserved. 3
Recent progress in cuprate superconductors
W. Y. Liang
IRC in Superconductivity, Madingley Road, Cambridge CB3 OHE, U.K.
Abstract
The parent compounds of all copper oxide based superconductors are
antiferromagnetic insulators despite the presence of a single electron in the non-magnetic unit
cell, and charge carriers are introduced by means of partial substitution of the cations or by
changing the oxygen content. Doping produces carriers in either the charge reservoir layers or
in the Q1O2 layers. Increasing the concentration of mobile carriers in the Q1O2 layers initially
induces superconductivity until a maximum T is reached which corresponds to approximately
c
0.2 hole carriers per copper atom. Thereafter T decreases and the non-superconducting
c
metallic state is reached at a carrier concentration which is somewhat below -0.5 hole carriers
per copper atom. We have carried out a study of the relationship between the insulating, the
superconducting and the non-superconducting metallic in detail on a number of systems. A
particular example will be given of the family of (Cai_ Y)Sr2(Tli_PbCu2)C>7 compounds
x x z z
where both χ and ζ can be varied; the highest T is 108 Κ while the non-superconducting
c
metallic member is given by the formula CaSr2(TlCu2)07 and the insulating state is given by
the composition with z=0.5 and y>0.6. Other combinations are also possible, giving this
family of materials their inherently very interesting properties for investigative studies as well
as for practical applications. In addition to the variation in T as a function of carrier
c
concentration and determined from the temperature dependent resistivity, other measurements
on the structure, Hall coefficients, thermo-electric power, magnetisation and specific heat have
revealed further interests relating to superconducting order parameters, critical currents,
dimensionality and other superconductivity properties in these materials. We have found that
variations in the fluctuation superconductivity, thermo-power and magneto-resistance strongly
suggest that two qualitatively different regimes exist within the superconducting range of the
composition. In the low carrier density regime the Cooper pairs may be compared with the
real space spin bi-polarons. In the high carrier density regime strongly interacting BCS
behaviour dominate. The implications on the practical uses of the systematic variation of
superconducting properties with composition will be discussed.
1. INTRODUCTION
High temperature superconductors have made very significant advances in a number of
directions since their discovery nearly four years ago. These include the further discoveries of
many more oxide superconducting systems in addition to the La2Cu04 based family, a better
control of the fabrication conditions for bulk materials to produce single phase, a better control
for the fabrication of thin epitaxial films including those with buffer layers and
4 W.Y. Liang
heterojunctions, a better understanding of the chemical and physical processes, and many
others. This paper will give a brief overview of the materials, and will then use mainly one of
the oxide systems to illustrate some of the remarkable electronic properties. We hope to bring
out those physical and chemical properties, if properly understood, should apply generally to
most if not all the oxide superconductors. Finally the measurements of magnetoresistance
have led us recently to pose an interesting (old) question: Can high temperature
superconductors in their ceramic form be useful in high current applications?
2. MATERIALS
We are concern only with the p-type oxide superconductors whose charge carriers are
hole-like. These are presented generally either as families of La-Cu-O, Y-Ba-Cu-O, Tl-Ba-
Ca-Cu-O and Bi-Sr-Ca-Cu-O, or as compounds containing one, two or more of Q1O2 active
layers in the formula unit. There are also further divisions according to the number of atomic
layers which separate the CuU2 active layers. The latter forms the charge reservoir which can
provide a means of moderating the number of carriers in the active layers without introducing
scattering centres for carriers but it can also affect the degree of electrical and magnetic
anisotropy which may bring the unwelcome effect of increased superconducting fluctuations.
Figure 1 shows our view of the structural division between the active layer block and the
charge reservoir block for a few typical compounds.
Charge
Reservoir CCuuOO
Block
BaO
Active La/SrO Cu02
Layer Cu0 Y
2
Block
La/SrO Cu0
2
BaO
(a) (La,Sr) Cu0 (b) YBa Cu 0 (c)Tl Ba Ca Cu Oi (d) Pb (Y,Ca)Sr Cu 08
2 4 2 3 7 2 2 2 3 0 2 2 3
Figure 1. Block view of oxide superconductors in terms of the active layer block which
contains the superconducting Cu0 layers, and the charge reservoir block which may be
2
conducting, insulating or highly disordered. (La,Sr) Cu04 does not possess charge
2
reservoir.
The distribution of charge and the effects on the doping efficiency will be discussed in the next
section. In Table 1 we list typical oxide systems together with the range of T reported for
c
each of the system. This T range includes all the values obtainable by means of cation
c
doping, by varying oxygen content, by hydrogen annealing [1] as well as by the application of
high pressure [2]. One of the more interesting developments concerning materials preparation
in recent years has been the discovery that certain phases could be considerably stabilized by
the addtion of a small amount of tracer element. This way not only the high T phase can be
c
Recent progress in cuprate superconductors 5
preserved but also it iseasier to fabricate single phase materials, for examples In stabilizes the
60K phase of (Tl,V,In)Sr2(Y,Ca)Cu2C>7 [3] and Bi stabilizes the 120K phase of
(Tl,Pb,Bi)SrCaCu30io [4].
2 2
Table 1
Families of p-type oxide superconductors and the range of T observed.
c
No. of CuU2 No. of atomic High
layers in layers in Examples -est
active block reservoir block T ,K
C
0 (La,Sr)2CuU4 45
1 1 Tl(Ba,La)2Cu05_a, Bi(Sr,La)2Cu05_a 42
2 TlBa2Cu06, Bi Sr Cu06 85
2 2 2
0 (La,Sr)CaCu206 60
2
1 YBa2Cu3<>7, TlBa2CaCu20s, BiSr2CaCu20s 110
2 2 YBa2Cu40s, Tl2Ba2CaCu08, Bi SrCaCu208 115
2 2 2
3 Pb (Y,Ca)Sr Cu30 80
2 2 8
1 TlBa2Ca2Cu309, BiSrCa2Cu309 120
2
3 2 TlBa2CaCu30io, (Bi,Pb)2Sr2Ca2Cu30io 125
2 2
3. DISTRIBUTION OF CHARGE CARRIERS
Superconductivity in the layered cuprates requires the presence of carriers in the active
Cu02 layers but prefers not to have impurity ions into these layers. The carriers can be
introduced via the charge reservoir block or the partial substitution between Ca and Y in the
active block as shown in Figure 1. This provides the freedom to select specific sites in which
to place the doping atoms in order to optimise the effect, similar to the process of modulation
doping in semiconductor hetero-junction multilayers. However unlike doping in
semiconductors, the amount of impurity ion substitution is usually large leading to changes in
the lattice potential. This will have the effect of shifting the band density of states of the layer
with respect to the Fermi level and to that of the other layers. It is well known that doping
does not always lead to a direct increase in the number of carriers in the Cu02 layers. We note
2+
here that adding oxygen ions or a partial substitution of a less positive ion such as Ca for a
3+
more positive ion such as Y can provide hole carriers but also raises the energy of the
receiving layer or block as illustrated in Figure 2 for YBa2Cu306- This example shows that
although YBa2Cu306.22 and (YjsCa 25)Ba2Cu306.i both have about 0.45 holes per formula
unit, the former is a long way from becoming superconducting while the latter has a T of 15K
c
[5]. Another more extreme example is provided by the compound Pb2(Y,Ca)Sr2Cu30s which
does not superconduct without Ca substitution despite increasing the oxygen concentration to
9.8, yet a modest 0.25 Ca substitution for Y will produce a T of ~80K [6]. The explanation
c
in both examples is that when oxygen enters the charge reservoir block, it tends to first oxidize
+ 2+
the local atoms such as Cul in the case of YBa2Cu30o and Pb in the case of
Pb2(Y,Ca)Sr2Cu308 before releasing the 'holes' to the active block.