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Outline of My PhD Project
At The Department
of Physics
Queen's University of Belfast
Northen Ireland, UK
High Tc Superconductor then films by laser ablation techniques
Introduction
Since
1984 the scientific world was excited by the discovery of high Tc
superconductors above the temperature of liquid nitrogen in copper oxide-based
systems.
The
materials must be made into usable forms such as wire, thick films, thin films
and bulk ceramics. In the fabrication of bulk superconductors, problems with
the brittleness of the ceramic make it difficult to machine the material into
complicated shapes. Also it turned out that the high Tc superconductivity in
the bulk material suffered from low current density.
The
availability of these superconducting materials in the form of thin films will
be important from both scientific and industrial view points, especially for
microand opto-electronics applications, and they could be used as interconnects
in integrated circuits.
A
variety of film forming techniques have been tried to produce high Tc
superconducting films, such as electron beam evaporation, sputtering, molecular
beam epitaxy and laser ablation. However, for preparation of high quality thin
film there are three main parameters (1) stoichiometry (2) crystallinity (3)
substrate interaction.
Among
these high temperature thin film deposition techniques being tried, the laser
ablation is very effective, very reliable and have numerous advantages such as
fast, cheap and have better control over the film stoichiometry and deposition
rate, where in many cases an amorphous film is deposited and a post annealing
step involving temperature up to 900oC
is necessary to form the crystalline superconducting phase, however, the films
made by laser ablation technique does not need high temperature annealing, this
will prevent the substrate material from diffusing into the deposited film, thus
more interesting substrate from industrial point of view can be used such as
semiconductors.
In
the laser ablation technique, thin films are formed by condensing material
ablated from the surface of superconducting target (Y-Ba-Cu-O) onto a heated
substrate (720oC)
in an oxygen partial pressure, by focusing 20ns
excimer laser pulses (5-15j/cm2)
at 10Hz for several thousands laser shots. The vaporized and ejected material
from the target surface condenses onto the substrate forming the film.
The aim of the project
The
aim of the project is twofold, first to make the films and study their
superconducting characteristics, second is the spectroscopic study of the laser
induced emission and evaluate it as a possible method for process control in
producing high quality superconducting thin films.
Film characteristics
Several deposition parameters can affect the film quality such as laser fluence,
target-to-substrate distance, substrate temperature substrate material, laser
wavelength and oxygen partial pressure, therefore the effect of each parameter
on the film quality was examined by varying one at the time.
These
films were subjected to different analysis techniques listed below:
1) Resistance vs temperature (RvT) was performed by the four-point
probe.
2) Energy
Dispersive X-ray analysis (EDX) to obtain the film stoichiometry
,contaminants and interfacial reaction.
3)
Scanning
Electron Microscope (SEM) to assess the overall morphology of the
films.
4)
X-ray Diffraction (XRD) to investigate the film crystallinity.
5)
Film thickness was obtained by using mechanical stylus (AlfaStep).
Plasma spectroscopy studies
As we
are have been concentrating on optimizing the quality of films produced, we have
been concerned with the ablation process itself in order to gain a fundamental
understanding of the process and how it relates to the quality of the resulting
films. The laser induced plasma plume from the irradiated Y-Ba-Cu-O samples
provide information on the nature of the ablated species, their expansion
velocities and extent of ionization in the plume.
The
plasma plume was studies by several spectroscopic techniques, such as
optical emission and absorption spectroscopy,
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