Qiu X.G. (Ed.) High Temperature Superconductors
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1.3.3 Characteristic film properties
In spite of the differences in the growth technologies, deposition parameters and
resulting growth mechanisms, a number of characteristic values for the electronic,
structural, and morphologic properties of HTS films can be given, that might
serve as a kind of benchmark for high-quality HTS films. For YBCO these are:
a surface roughness of r
peak-to-peak
≈ 10–20 nm, a FWHM of the XRD rocking
curve of the (005) reflex ∆
ω
≈ 0.1–0.3°, a RBS minimum channelling yield of
χ
min
≈ 2–3 %, a resistivity at 300K of
ρ
(300K) ≈ 3.5–4 µΩm, a resistance ratio of
ρ
(300K)/
ρ
(100) ≈ 3–3.3, a critical temperature (offset, inductive measurement) of
T
c,off
≈ 88–92 K, a transition width (inductive measurement) of ∆T
c
≈ 0.5–0.8 K,
a critical current value (resistive measurement) of J
c
(77K, 0T) ≈1–5MA/cm
2
, and
a microwave surface resistance of R
s
(77K, 10GHz) ≈ 400 µΩ (Fig. 1.15). Similar
sets of data are available for other HTS superconducting films.
1.15 Comparison of the frequency dependence of the microwave
surface resistances of YBCO films on various substrates and Cu at 77K
(Wördenweber, 1999).
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1.4 Concluding remarks
Due to the extraordinary properties of HTS material – especially the extremely
complex structure, small coherence length, and large anisotropy – the preparation
of high-quality HTS thin film requires extremely demanding deposition
technologies. Different technological approaches have been sketched in this
contribution. Each technique has its own advantages and disadvantages, and its
own ultimate potential:
• Evaporation represents the classical approach. The development of rotating
heaters opens the field towards high-pressure and reactive processes. With this
innovation, evaporation turned out to be a reliable tool for the deposition of
HTS films.
• PLD seems to be the most flexible technology. Especially, due to the
development of laser-MBE, it has the potential to produce complex oxide
film systems and multilayers, including layer-by-layer grown HTS films. The
drawback of this technology is the small deposition area and the tendency
to produce droplets due to the explosive-like removal of material at the
target.
• Sputter deposition represents a well established technology. The high energy
of the particles can be overcome via thermalisation. Therefore, this technology
allows for a variation of the particle energy. Layers with optimised Josephson
contacts have been grown with high-pressure cathodic sputtering at a pressure
of several mbar (Divin et al., 2008), whereas the high particle energy has been
utilised in low-pressure sputtering at 0.5 mbar for microwave suitable YBCO
films on sapphire for which the generation of defects leads to an enhancement
of the critical thickness d
crack
(Einfeld et al, 2001). Furthermore, sputtering
has the advantage to be scalable to meter sizes. Its major drawback is the
instability in the reactive mode, which becomes a problem with the size of
the target.
• CSD is probably the technology most suitable for continuous HTS deposition,
e.g. for HTS coated conductors. This technology has recently improved
strongly and the financial argument might be another reason for its use in
large-scale industrial application.
Due to the extreme demands of the HTS material, existing deposition technologies
have been strongly improved and novel technologies have been developed. This
procedure has been beneficial for the deposition technology not only of HTS
material but also for the deposition of complex oxide material.
1.5 Acknowledgement
Valuable support from and discussions with E. Hollmann, J. Schubert, R. Kutzner,
M. Bäcker, A.G. Zaitsev and A. Offenhäuser are greatly acknowledged.
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© Woodhead Publishing Limited, 2011
Evaporation
Conv.
sputtering
PLD
High-pressure
sputtering
Magnetron sputtering
Diode sputtering
PLD
4
3
2
1
0
Particles on substrate (%)
1 Pa
5 Pa
20 Pa
Cu/Y
Source
Source
Plate I Sketch and data of the simulated particle flux from the source to the substrate according to Hollmann et al., 2005. In (a) the
simulated (Cu in Ar/O
2
= 4:1) and literature values (eq. [1.1] in this chapter) of the mean free path are compared. In (b) the pressure
dependence of the percentage of particles arriving at the substrate and the average number of scatter events that these particles
undergo during the process of traversing the distance between source and substrate are shown. The data are simulated for a sputter
process (2” Cu target, 1 cm
2
substrate, target–substrate distance of 5 cm, and gas mixture of Ar/O
2
= 1). The relation between particle
energy and collision parameter b (eq. [1.2]) that is used in the QHS-model is shown in (c). The QHS-model is sketched in the inset
with the impact parameter b, the process gas atom M
g
, the sputtered atom M
s
, and the scattering angle
β
relative to initial direction of
motion of the sputtered atom due to the collision. In (d) the particle flux distribution Cu/Y is given as function of the position above a
YBCO magnetron target. The position of the magnetrons is marked. The gas mixture is Ar/O
2
= 4:1.
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38
2
Transport properties of high-T
c
cuprate thin
films as superconductive materials
J. VANACKEN and V. V. MOSHCHALKOV, Leuven Catholic
University, Belgium
Abstract: In this chapter, the systematic evolution with doping of the normal
state properties (resistivity and Hall effect) of the La
2 – x
Sr
x
CuO
4
thin film
cuprates is analyzed. This analysis reveals convincing evidence in favor of the
presence of the charge-rich 1D features (stripes), which eventually determine
the 1D character of the transport properties at temperatures below the so-called
‘spin-gap’ temperature T*(x). Measurements of the fluctuation conductivity
show how stripe coupling close to T
c
recovers the 2D character of the Cu-O
planes which is responsible for the 2D Aslamasov–Larkin fluctuations at T~T
c
and the superconducting state itself at T < T
c
. Stripes decoupling at low
temperature, via strong magnetic fields, induces again a 1D stripe behavior,
which, in the presence of impurity centers, leads to blocking of the 1D
intra-stripe transport and consequently to the formation of the insulating state in
underdoped La
2 – x
Sr
x
CuO
4
at T →0.
Key words: in-plane resistivity, scattering mechanisms, scaling behavior, stripe
coupling, Drude model, Hall effect.
2.1 Introduction
The close connection between the electrical resistivity and the scattering
mechanisms of charge carriers is already well known from the Drude model,
which treats the electrons in a metal as free non-interacting particles:
[2.1]
where m* represents the effective mass, n the density and
τ
the scattering time of
the electrons. Expression [2.1], based on a simple single electron picture, implies
that the study of the resistivity of a material can give key information about the
nature of the electronic scattering. According to the Bloch theory, electrons in a
periodic scattering potential of a perfect crystal, are not scattered at all. The finite
resistance to electron flow in normal metals is only exhibited because of the
thermal vibrations of the ions (phonons), resulting in a resistivity
ρ
ph
, and crystal
defects, giving a residual resistivity
ρ
D
. While
ρ
D
remains constant,
ρ
ph
varies
with temperature. The Debye temperature
Θ
D
, which is a measure for the stiffness
of a crystal, separates two temperature regions where
ρ
ph
(T) reveals a different
temperature dependence (Ashcroft 1976). At low temperatures, T <<
Θ
D
, phonon
modes with a high excitation energy are frozen out. By increasing the temperature,
Transport properties of high-T
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© Woodhead Publishing Limited, 2011
more and more phonon modes get excited, thus strongly enhancing the
electron–phonon scattering. In this temperature region, the resistivity
ρ
ph
increases
fast with increasing temperature, obeying a T
5
law. At high temperatures, T >>
Θ
D
,
all phonon modes are already excited and the resistivity
ρ
ph
varies linearly with
temperature. Finally, when the inelastic mean-free path approaches the lattice
spacing at even higher temperatures, the resistivity
ρ
ph
saturates (Ioffe-Regel
criterion). Aside from the scattering mechanisms due to deviations from perfect
periodicity, another source of scattering arises from the interactions among
electrons, giving rise to a resistivity
ρ
el
. However, this electron–electron scattering
usually plays a minor role. At high temperatures, it is much less important than
the phonon scattering, and at low temperatures, it is usually dominated by impurity
or defect scattering. We conclude that the total resistivity
ρ
in ordinary metals is
given by:
ρ
=
ρ
D
+
ρ
ph
+
ρ
el
[2.2]
with
ρ
ph
the dominant term.
In high-T
c
cuprates, however, the nominally metallic temperature dependence
of the resistivity
ρ
(T) has been explored at various levels of hole doping and for
different geometrical configurations between the applied current and the
crystallographic axes (Takagi et al. 1992a, Kimura et al. 1992, Bucher 1994, Ito
et al. 1993, Batlogg et al. 1994, Hwang et al. 1994, Wuyts et al. 1996, Ando et al.
1996, Trappeniers 2000, Vanacken et al. 2001). From the slightly doped up to the
strongly overdoped regime, the resistivity of these materials reveals a very
anomalous behavior.
We will focus in this chapter on the in-plane resistivity
ρ
ab
(T) of c-axis oriented
epitaxial films, which impose by their geometry, a configuration where the current
is applied parallel to the ab-plane. The shape of the
ρ
ab
(T)-curves for the different
doping regimes is, in fact, universal for all high-T
c
compounds. As a result, the
analysis of the transport properties of a particular cuprate system, La
2 – x
Sr
x
CuO
4
,
is also valid for other high T
c
cuprates.
2.2 Temperature dependence of the zero-field
resistivity in superconducting La
2 – x
Sr
x
CuO
4
thin films
A clear insight into the scattering processes, which are responsible for the non-
Fermi liquid behavior of the normal state in high-T
c
cuprates, may provide a clue
to the understanding of high-T
c
superconductivity itself. The normal state
electronic transport properties have been investigated because they give direct
information about the scattering mechanisms, which are relevant for the systems.
In the following section, the temperature dependence of the in-plane resistivity at
zero magnetic field is presented for La
2 – x
Sr
x
CuO
4
thin films with different doping
levels. A remarkable scaling behavior of the temperature dependent in-plane