In this thesis, a systematic study of thin film preparation of p- and n-type infinite
layer (IL) compounds and artificially layered structures, as well as their properties (e.g.,
structural and electrical) are presented. Pulsed laser deposition (PLD) was effectively
used for the epitaxial growth of these oxides. Different substrates (e.g., NdGaO3, SrTiO3
or KTaO3) were used to epitaxially stabilize these metastable structures. The role of the
compressive or tensile strain in stabilization of p- or n-type IL phase is also discussed.
The IL compounds are ideal candidates for studies of the formation of artificially layered
structures. Pulsed laser interval deposition (PLiD) growth and properties of such
artificially layered structures on NdGaO3 substrates, constructed by sequentially
ablating IL targets, is also presented and discussed. A critical parameter in the film
growth process is the substrate-film interface, determining the morphology and
structural properties of the film. To improve the substrate surface morphology different
chemical or thermal treatment are developed for NdGaO3, SrTiO3, LSAT, SrLaAlO4, and KTaO3 substrates and described here.
The infinite layer (IL) compounds ACuO2 (A is an alkaline earth metal) have the
simplest structure among the HTSc materials. It consists of corner-shared CuO2 layers
and A-metal(s) planes, alternately stacked along the c-axis. With the exception of
Ca0.85Sr0.15CuO2, the IL compounds are metastable phases that require high-pressure
(2-6 GPa) and high-temperature (≤ 1400 oC) conditions for their synthesis.
Superconductivity can be induced in these compounds by electron or hole doping. Hole
doping (p-type) can be obtained by introduction of excess oxygen in the A-plane, creating
alkaline earth deficient defect layers in the structure, or by monovalent ion (e.g., Na+,
Li+) substitution. Electron doping (n-type) of IL phases is obtained by partial
substitution of alkaline earth atoms with lanthanide Ln3+ (Ln = La, Pr, Nd, Sm, Gd)
atoms. Since the need of very high external pressures impedes technological
applications, simulating such effects by the use of internal strain in thin films offers an
alternative way. The stability effect of the substrate can be used to grow metastable
phases with structural characteristics similar with that of the bulk samples, but with no
impurity phases. Using substrates with different in plane cell parameters (e.g., SrTiO3
and KTaO3) the misfit between the substrate and film is used here to induce the desired
p- or n-type doping.
The high deposition rates attained in PLD during the deposition pulse and the
variable range of the kinetic energy of the deposited particles is exploited to improve the
film properties (e.g., crystallinity and surface smoothness). Taking advantage of the
growth kinetics at the high supersaturation reached during the deposition pulses, a new
approach is used, called pulsed laser interval deposition (PLiD). This method enables
2-dimensional (2-D) growth and, therefore, a smooth film surface. It is used in imposing
a layer-by-layer growth of ACuO2 phases and artificially layered structures (build by
sequential deposition of different IL blocks).
Control of the oxygen content is one of the requirements for tuning the valence of
copper that can result in superconductivity in the IL materials. The oxygen pressure
used during deposition of the IL films results in structural modifications that have
substantial influence on the electrical transport properties. The oxidation power during
growth should be sufficiently high to enable growth of stable phases. This is achieved
using relatively high oxygen pressures (i.e., 10-3-0.4 mbar). High-pressure RHEED is
used for growth control and monitoring.
Besides the control of the deposition conditions, the fabrication of artificially
structured films using layer-by-layer growth requires optimal surface smoothness of the
substrate. The chemistry and morphology of the terminating atomic layer(s) influences
the interface properties and, consequently, the structural and physical properties of the
film. Therefore, a substrate with good lattice matching and an atomically smooth
surface with a known composition are essential. In order to control the chemistry and
morphology of the surface terminating layer(s), different chemical and thermal
treatment methods were developed for (001) and (110) NdGaO3, (001) SrTiO3, (001)
LSAT, (001) SrLaAlO4 and (001) KTaO3 single-crystal substrates. They were selected for their chemical and structural compatibility with the phases under investigation in this
thesis. For KTaO3 substrates, a BaTiO3 buffer-layer is also used in order to improve the
surface crystallinity. These substrates have a perovskite or related structure, consisting
of alternating layers of AO1+δ (A = Sr, La or Nd) and BO2-δ (B = Ti, Al, Ta or Ga) in the c direction. By considering their layered structure and reactivity with mineral acids,
selective removal of one of the surface oxides was achieved by a thermal treatment or
though chemical etching, followed by annealing. The resulting surface morphology was
studied by AFM and high-pressure RHEED. A-site single terminated surface is obtained
after annealing at optimum conditions (i.e., annealing temperature, time, and ambient
gas). Chemical etching in solutions with different reactivity (i.e., NH4F + HF + H2O,
NH4Cl + HCl, HCl + HNO3, and HCl), followed by thermal treatment is used to obtain a
B-site single terminated surface, free of etch pits. There is a strong dependence of the
final surface morphology on the properties of the substrate (e.g., surface chemistry,
vicinal angle and its orientation), on the characteristics of the etching solution, and on
the annealing conditions.
One possible solution for modifying the carrier density concentration is to apply a
different level of strain on the CuO2 planes. NdGaO3 single-crystal substrate is used as
template layer in order to study the role of the compressive strain and of the deposition
conditions on the structural and electrical properties of these structures. Pulsed laser
deposition was used to grow single-phase thin films of ACuO2 phases and artificially
layered structures of these copper oxides. Using (001) or (110) NdGaO3 as atomic
template these metastable phases can be stabilized in their tetragonal symmetry. It is
shown that by growth manipulation all ACuO2 phases can be grown with different
critical thickness on bare NdGaO3 substrates without using a buffer layer. The use of
single-terminated NdGaO3 substrates plays a crucial role in yielding films with smooth
surface morphology. The films are epitaxial, with complete in-plane crystalline
alignment with the substrate. The artificially layered superlattices showed a
dependence of their structural and electrical properties on the growth conditions
(deposition pressure and temperature, as well as on the deposition rate for each infinite
layer constituent block).
Growth manipulation by means of pulsed laser interval deposition relies on the
correct determination of the deposition rate, which can be accurately determined by
means of in situ high-pressure RHEED, as well as the properties of the interface
between the constituent blocks. These aspects determined the final structure, defects
network, and the morphology of the films and, therefore the final transport properties.
The use of NdGaO3 helps stabilizing the right tetragonal ACuO2 phases and their
superlattices by the induced compressive strain. The desired superconducting properties
of these structures require deposition conditions that can assure incorporation of
enough excess oxygen to reach the charge carrier density needed to dope the CuO2
planes from the IL blocks.
Sr1-xLaxCuO2±δ (x = 0.1-0.2) thin films are epitaxially grown on (001) SrTiO3 and
KTaO3 substrates (with and without a BaTiO3 buffer layer) by means of PLD with in
situ monitoring using high-pressure RHEED. BaTiO3 is used as buffer layer with the
aim of i) reducing the epitaxial compressive strain in films grown on SrTiO3 and ii) to
improve the surface morphology of the KTaO3 substrates. The deposition parameters for
the films were optimised for the La concentration. For the compressively strained n-type
IL films, Tc is reduced due to difficulty in removing the excess oxygen from the Sr(Ln)
planes. Improved Tc values can be obtained by inducing tensile strain in the CuO2
planes. Here, the role of the substrate-film misfit on the structural and electrical
properties of Sr1-xLaxCuO2 films is studied for SrTiO3 and KTaO3 cases.
Cooling procedure was found to be one of the critical factors for yielding
superconductivity. Annealing in an oxygen atmosphere degraded or suppressed
superconductivity, as well as cooling and annealing in vacuum. An increase of Tc was
observed for decreasing oxygen partial pressure during cooling, down to certain values
(about 10-5 mbar), an indication of n-type doping in Sr1-xLaxCuO2. Annealing in vacuum resulted in a slightly orthorhombic distortion, most probably due to partial removal of
oxygen from CuOx planes at the annealing temperatures (400-550oC). The time scale for
cooling procedure was also found to be crucial for presence of superconductivity.
In this thesis, PLD (or PLiD) is used for epitaxial growth of p- and n-type infinite
layer thin films and of artificially layered structures. The combination of PLD (or PLiD)
with in situ monitoring of the growth front with high-pressure RHEED enabled
accurate control of the growth. Chemical and thermal treatments were developed and
used in order to improve the surface morphology of the substrate materials used for the
epitaxial growth of these structures.