Solution Deposition of HfO2ZrO2 Thin Films on Self Assembled Monolayers

1
Solution Deposition of HfO2/ZrO2 Thin Films on
Self Assembled Monolayers

• Amy C. DeBaillie
• Monmouth College, Monmouth, Illinois
• Chemical Technology Division, Oak Ridge National
  Laboratory, Oak Ridge, Tennessee
• Dr. Michael Z.-C. Hu
• October 13, 1999

2
Abstract Spherical colloidal nanoparticles of
hafnium dioxide have been synthesized via forced
hydrolysis of hafnium sulfate in the presence of
hydrochloric acid. The effects of several
parameters on the characteristics of the
particles such as temperature, concentration of
the inorganic salt, and acidity were studied in
detail through the use of a custom designed,
low-power dynamic light scattering
spectrophotometer. Scanning electron microscopy
was used to further investigate the nucleation
and nanoparticle growth in hafnium sulfate
aqueous solutions.
3
Objective
The main objective of our research is to
synthesize hafnium dioxide nanoparticles by
forced hydrolysis and thin films through the use
of self-assembled monolayers. In particular,
through the deposition of hafnium dioxide
nanoparticles on self-assembled monolayers
(SAMs) with a sulfonate terminal functionality on
single-crystal silicon substrates.
4
Introduction
Hafnium dioxide, due to its thermal, chemical,
and optical properties, has been found to possess
many applications in a variety of areas. For
example, hafnium dioxide particles have potential
applications in ceramics,1 catalysis,2 and the
paint industry,3 while hafnium dioxide thin films
have found significant applications in corrosion
protection4,5,6 and in micro and optoelectronic
devices.7 The useful applications of hafnium
dioxide thin films and particles are a result of
specific properties of hafnium such as high
refractive index, chemical and thermal stability,
high damage threshold, and high melting
point.3,8,15 Hafnium dioxide particles have
been prepared by various techniques such as the
precipitation from solutions of hafnium(IV) salts
by addition of ammonia,9 by the hydrothermal
treatment of hafnium metal chips,10and by the
hydrolysis of solutions of hafnium
oxychloride,11,12 hafnium tetrachloride,13 and
hafnium tert-butoxide.14 Until now, spherical
hafnium dioxide nanoparticles have never been
obtained by the forced hydrolysis of hafnium
sulfate.



The methods for HfO2
thin film fabrication include a variety of
techniques. Hafnium dioxide thin films have been
synthesized by a reactive beam process,15 grown
in electrolytes such as H2SO4, NaOH, H3PO4, and
HNO3 by galvanostatic oxidation,16 by sputtering
a hafnium elemental target in an argon and oxygen
atmosphere,17 by using a dual ion beam sputtering
system,18 and by atomic layer epitaxy (APE).7
Our route for the synthesis of hafnium dioxide
thin films, on the other hand, will involve the
use of aqueous solutions and self-assembled
monolayers. The use of self-assembled monolayers
for the formation of hafnium dioxide thin films
has several advantages over the previously
implemented techniques such as
Ambient Conditions Mimics Nature
More Cost Effective Provides Spontaneous
Formation Chemical Constituency can be
Tailored of a Highly Complex, Uniform,
Potential for Patterned Growth of Films
Large Area Structure



5
Scheme for Synthesis of Hafnium Dioxide Thin
Films Using Self-Assembled Monolayers
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Schematic Illustration of A Dynamic
Light Scattering Spectrophotometer
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Effects of pH on the Formation of HfO2
Nanoparticles
Based on the data from these graphs, it can be
concluded that as the acidity of the medium
increases, the hydrolysis reaction is inhibited.
8
Effects of the Concentration of the
Inorganic Salt on the Formation of HfO2
Nanoparticles
Under some conditions, the particle growth rate
decreases as the concentration of hafnium sulfate
is increased. This affect could be explained by
the acidic inhibition effect. The solution of
higher salt concentration is more acidic than the
lower one.
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Effects of the Concentration of the
Inorganic Salt on the Formation of HfO2
Nanoparticles (cont.)
Significant concentration effect on the particle
nucleation/growth was found under optimized film
growth conditions. With increasing salt
concentration, the induction period decreased and
the growth rate increased.
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Effect of Temperature on the Formation of HfO2
Nanoparticles
As the temperature increases, the induction
period decreases, and the final particle size and
growth rate increases.
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Effect of Shear on The Aggregative Particle Growth
This data affirms the hypothesis that hafnium
dioxide nanoparticles formed by forced hydrolysis
of hafnium sulfate form by aggregation of smaller
clusters/particles. The aggregation of the large
particles (gt50nm) is relatively loose and can be
disrupted by shaking the sample. After the
shaking is stopped, the particles tend to
aggregate again, forming flocculated structures.
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Comparison of HfO2 and ZrO2 Nanoparticle Formation
Even though Zirconium and Hafnium have similar
chemical characteristics, this data shows that
they do not behave the same in terms of
hydrolysis/condensation reactions in solution.
Based on these results, one could conclude that
zirconium species has a lower activation energy
requirement for the reaction to occur than
hafnium species in solution.
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Scanning Electron Microscopy Images
4mM Hf(SO4)2, 0.2N HCl, 80OC, 220 min
4mM Hf(SO4)2, 0.2N HCl, 90OC, 210 min
The affect of temperature can be observed in the
SEM images. The particles synthesized at 90OC are
larger in size than the ones synthesized at 80OC,
which is consistent with the data obtained from
DLS.
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AFM and SEM Images of ZrO2 Thin Film Surfaces
As shown, the film is consisted of nanoparticles
(gt50nm), which is much smaller than the final
size of the particles observed in the bulk
solution.
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Conclusions
Based on the results obtained from the dynamic
light scattering spectrophotometer, it can be
concluded that the aggregation of small primary
particles is a possible mechanism for particle
growth during the synthesis of hafnium dioxide by
forced hydrolysis. DLS results also conclude that
temperature, acidity, and concentration of
hafnium sulfate create a competitive effect that
controls the rate at which the particles form.
With increased temperature, the particles form at
a faster rate, while increased acidity inhibits
hydrolysis/ condensation, and increased hafnium
sulfate concentration either increases or
decreases the reaction kinetics. Overall,
throughout the whole growth process nucleation
occurs constantly. During the formation of the
nanoparticles, larger particles settle out, which
causes the intensity to fall, but smaller
particles are still being formed and detected by
DLS. The ideal condition for film growth was
found to be a solution of 4mM Hf(SO4)2 and 0.2N
HCl at a temperature of 80OC due to the
controllability of the induction period and
particle growth rate by simply varying process
parameters. Future studies will involve the use
of the obtained ideal condition in order to
synthesize hafnium dioxide thin films.
16
Acknowledgements
Dr. Michael Hu
Lubna Khatri
This work is partially supported by the U.S.
Department of Energy, Basic Energy Sciences,
Division of Materials Sciences ACM/ERULF/GLCA
17
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