Materials Science and Engineering B 165 (2009) 178–181
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Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Sol–gel ZrO
2
and ZrO
2
–Al
2
O
3
nanocrystalline thin films on Si as high-k dielectrics
P. Vitanov
a
, A. Harizanova
a
, T. Ivanova
a,∗
, Ch. Trapalis
b
, N. Todorova
b
a
Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, Blvd. Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
b
Demokritos National Centre for Scientific Research, Athens, Greece
article info
Article history:
Received 22 August 2008
Received in revised form 19 August 2009
Accepted 2 September 2009
Keywords:
Aluminium oxide
Sol–gel processing
Electrical properties
XRD
abstract
Currently, the conventional dielectric, SiO
2
or SiON is being replaced by alternative materials to sup-
press high leakage currents observed at low film thickness. In this work, thin layers of ZrO
2
and
(ZrO
2
)
x
(Al
2
O
3
)
1-x
, deposited on Si substrates are obtained by sol–gel method. The structural and electrical
properties as a function of the annealing temperature are studied. Usually, the post-annealing tempera-
ture of 700
◦
C leads to re-crystallization in ZrO
2
layers, which can induce high leakage current and severe
mass transport along the grain boundaries. The purpose is to amorphisize the films by adding aluminium
oxide and in this way improve their electrical behaviour. Structural analysis shows that ZrO
2
films are
crystallized, meanwhile the mixed oxide films remain in amorphous state even after high temperature
annealing. The obtained results are encouraging and determine the (ZrO
2
)
x
(Al
2
O
3
)
1-x
films as a promising
material with good dielectric properties.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
There is recently much interest worldwide in alternative
dielectrics with higher relative permittivity (k) for future gener-
ation of field effect devices.
Zirconium oxide (ZrO
2
) is of particular interest due to its high
thermodynamical stability on silicon and high permittivity com-
pared to other dielectrics (k ∼ 20–24) [1–3]. To minimize electrical
and mass transport along grain boundaries and stabilize the inter-
face between Si and dielectric layer, it is preferable that the gate
oxide remains amorphous throughout processing sequence. The
pure zirconia possesses a low crystallization temperature (close to
400
◦
C) and in order to stabilize its amorphous phase of SiO
2
or
Al
2
O
3
can be added to form a pseudobinary alloy [4,5]. Al
2
O
3
has
many advantages such as high crystallization temperature (above
900
◦
C), low diffusivity of oxygen, large band gap and dielectric
constant higher than that of SiO
2
.
Binary system ZrO
2
–Al
2
O
3
can be able to combine the desir-
able properties of the two single oxides while it diminishes the
undesirable properties of each individual material.
In this work, ZrO
2
and (ZrO
2
)
x
(Al
2
O
3
)
1-x
are studied in respect
to their electrical properties and the effect of the annealing
temperature. The methodology applied here involves a com-
bination of structural (XRD and FTIR analysis) and electrical
(capacitance–voltage (C–V) and current–voltage (I–V)) character-
ization techniques, which previously have been proven successful
∗
Corresponding author.
E-mail address: tativan@phys.bas.bg (T. Ivanova).
in analyzing (ZrO
2
)
x
(Al
2
O
3
)
1-x
layers, prepared by sol–gel method
[3,5].
2. Experimental
Zirconia layers as well as those of the compound system
(ZrO
2
)
x
(Al
2
O
3
)
1-x
were deposited on Si. The Si wafers were
CZ boron doped p-type, with 〈100〉 orientation and resistivity
5–7 cm.
The technology used is a simple and low cost deposition pro-
cess based on sol–gel method. Zr(IV) propoxide was used as a
precursor to form the ZrO
2
films. Acetate modification was per-
formed by introducing glacial acetic acid. The solution was modified
by adding a small amount of water. This causes a hydrolysis and
condensation reaction. Acetylacetone was used as a peptization
agent and stabilizer. For the aluminium component, it was used
AlCl
3
. The Zr/Al molar ratio was 1:1.5. The samples were dried
at 80
◦
C for 30 min to obtain a xerogel film and removed resid-
ual solvents. Firing was carried out at 350
◦
C/30 min to pyrolize
organic components, and it was followed by high temperature
annealings. The samples were thermally treated at temperatures
at 600 and 750
◦
C for 1 h and at 850
◦
C for 15 min in nitrogen ambi-
ent.
The film thickness was measured with a laser ellipsometer at
a wavelength 632.8 nm. The zirconium oxide films are 24 nm thick
for samples treated at 600 and 750
◦
C for 1 h and thickness of 26 nm
after high temperature annealing at 850
◦
C. (ZrO
2
)
x
(Al
2
O
3
)
1-x
films
have film thickness of 42 nm for annealing temperature of 600
◦
C
and 48 and 47 nm after thermal treatment at 750 and 850
◦
C,
respectively.
0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.mseb.2009.09.002