49 research outputs found
Fabrication and nano-scale characterisation of ferroelectric thin films
PhD ThesisThis thesis focuses on the fabrication and characterisation of BaTiO3 thin films.
One of the aims is to deposit amorphous BaTiO3 films on conductive thin films
through sputtering at temperatures compatible with semiconductor manufacturing,
followed by post deposition annealing to crystallise these films. However, rapid thermal
processing (RTP) is known to create pinholes and cracks due to thermal mismatches
between the electrode and insulator, causing degradation of the film quality. Initial focus
was to develop thin film electrodes which can withstand process temperatures above
800 C. Deposition conditions, including the nitrogen flow rate relative to that of argon
during deposition were optimised to obtain TiNx with least resistivity and excellent
material properties through reactive sputtering. TiNx films deposited at various nitrogen
flow rates were then annealed in a non-oxidising condition and their properties were
thoroughly studied. Films deposited at the highest nitrogen flow rate (95%) showed
least variation in resistivity and showed excellent material properties even after a high
temperature anneal.
BaTiO3 films of varying thicknesses were deposited on TiNx using RF-sputtering
and subjected to RTP at various temperatures. It was found that there exists a
critical thickness for each RTP temperature below which BaTiO3 films are pinhole
free. A process was then developed by depositing and annealing multiple layers of
BaTiO3 films, with the thickness of each deposition less than the critical thickness. It
was observed that the multi-layered films are stable and pinhole free with a smooth
surface while the single layers of equivalent thicknesses showed cracked surfaces.
Current-atomic force microscopy studies showed leakage current through large pinholes
in single-layered films, whereas the pinholes were not the leakage path for multi-layered
films. Metal-insulator-metal capacitor structures were also fabricated using BaTiO3
with TiNx top and bottom electrodes and the fringing effects in leakage characteristics
were studied.
Finally, the polarisation reversal mechanism in BaTiO3 was investigated using piezoresponse force spectroscopy (PFS). It was experimentally demonstrated that
the polarisation reversal in these materials is a two-step process, which involves
polarisation rotation and switching when the applied electric field is not parallel to the
crystallographic orientation of the grain. However, it is a single step switching when
the polarisation and the electric field are parallel, as widely perceived. The two step
polarisation reversal was found to help [101] and [111] oriented grains to switch at a
lower electric field compared to [001] grains.Engineering and Physical Sciences
Research Council (EPSRC), UK:
Intel Ireland
TEM Investigation of Hydrothermally Synthesised Ba(Mg1/3Ta2/3)O-3 Powders
Perovskite Ba(Mg1/3Ta2/3)O-3 powders have been produced by hydrothermal synthesis at less than or equal to 200 degrees C. It was shown that the perovskite particles are irregular in shape and magnesium deficient, and that the excess magnesium was precipitated separately as the hydroxide. An improved method was developed and the resulting powder had a high proportion of rounded near-stoichiometric particles, although some irregular shaped;magnesium-deficient particles were also present. Thus, a link has been found between the stoichiometry and morphology of particles, which suggests that the mode of growth is affected strongly by particle stoichiometry