Electrical and structural properties of zirconia thin films prepared by reactive magnetron sputtering

Article abstract of DOI:10.1016/j.physb.2007.05.022

Thin films of ZrO₂ were prepared by reactive magnetron sputtering. Annealing of the films exhibited a drastic change in the properties due to improved crystallinity and packing density. The root mean square roughness of the sample observed from atomic force microscope is about 5.75 nm which is comparable to the average grain size of the thin film which is about 6 nm obtained from X-ray diffraction. The film annealed at 873 K exhibits an optical band gap of around 4.83 eV and shows +4 oxidation state of zirconium indicating fully oxidized zirconium, whereas higher annealing temperatures lead to oxygen deficiency in the films and this is reflected in their properties. A discontinuity in the imaginary part of the AC conductivity was observed in the frequency range of tens of thousands of Hz, where as, the real part does not show such behavior.

Full text of DOI:10.1016/j.physb.2007.05.022

ARTICLE IN PRESS
Physica B 399 (2007) 21–26
www.elsevier.com/locate/physb
Electrical and structural properties of zirconia thin films
prepared by reactive magnetron sputtering
a
K.P.S.S. Hembram , Gargi Dutta , Umesh V. Waghmare , G. Mohan Rao
b
b
a,
Ã
a
Department of Instrumentation, Indian Institute of Science, Bangalore 560 012, India
b
Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India
Received 10 September 2006; received in revised form 5 May 2007; accepted 11 May 2007
Abstract
Thin films of ZrO
2
were prepared by reactive magnetron sputtering. Annealing of the films exhibited a drastic change in the properties
due to improved crystallinity and packing density. The root mean square roughness of the sample observed from atomic force
microscope is about 5.75 nm which is comparable to the average grain size of the thin film which is about 6 nm obtained from X-ray
diffraction. The film annealed at 873 K exhibits an optical band gap of around 4.83 eV and shows +4 oxidation state of zirconium
indicating fully oxidized zirconium, whereas higher annealing temperatures lead to oxygen deficiency in the films and this is reflected in
their properties. A discontinuity in the imaginary part of the AC conductivity was observed in the frequency range of tens of thousands
of Hz, where as, the real part does not show such behavior.
r 2007 Elsevier B.V. All rights reserved.
Keywords: Zirconia; Magnetron sputtering; AC conductivity
1
. Introduction
studied on the basis of structural characteristics of the
interface.
Apart from the dielectric applications, ZrO can be used
in optical coatings [7], oxygen sensors [8], ion conductors
[9], heat resistant coatings [10], buffer layers for growing
superconductors [11], etc. In most cases the above
The race for miniaturization of electronic devices has led
to search for high dielectric constant (k) materials such as
TiO , Ta O , barium strontium titanate (BST) and some
2
dopant hosted in BST [1,2]. These materials need to be
compatible with the existing technology of microelectronic
fabrication for commercial implementation. Higher di-
electric constant oxide materials with suitable desired
2
2
5
applications require the ZrO layer on silicon surface and
2
hence require more studies on the structural, optical,
morphological and electrical characteristics of ZrO coat-
ings on silicon.
2
properties are required to replace SiO (kꢀ3.9) for device
2
performance, with ease in fabrication.
Although TiO , Ta O , BST are some of the high k
Conventionally ZrO2 films are prepared by sol gel
[12,13], spray pyrolysis [14], atomic layer deposition [15],
plasma enhanced atomic layer deposition [16], electron
beam evaporation [17], and metal organic CVD [18,19].
However, the advantage of reactive magnetron sputtering
for this purpose is mainly due to the high deposition rate
by sputtering from metallic target [20], which also enables
control of the stoichiometry of the deposited films.
The high sputter rates also reduces the target poisoning
effects which normally affect the reactive sputtering
process. Hence in this study we have chosen reactive
magnetron sputtering process for the deposition of zirconia
thin films.
2
2
5
materials, they are thermodynamically unstable on silicon
at higher temperatures, where as, zirconia (ZrO ) on silicon
is more stable [3]. Thermal stability of ZrO deposited on
2
silicon with respect to silicide formation at higher
temperature, is one of the main factors to be considered
for various applications [4]. Factors like interface engineer-
ing [5], reaction steps for silicide formation [6], are well
2
Ã
Corresponding author. Tel.: +91 80 22932349; fax: +91 80 23604533.
E-mail address: gmrao@isu.iisc.ernet.in (G. Mohan Rao).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2007.05.022

ARTICLE IN PRESS
K.P.S.S. Hembram et al. / Physica B 399 (2007) 21–26
22
The present study deals with the growth of ZrO on
2
400
360
320
silicon substrates at ambient and post deposition annealing
up to 1073 K. The effect of annealing on the structural,
optical, morphological and electrical properties of zirconia
films has been studied.
2
2
2
80
40
00
2
. Experimental
5
1
0 mA
00 mA
Thin films of zirconia were deposited on polished p-type
200 mA
3
4
5
00 mA
00 mA
00 mA
silicon /1 0 0S substrates using reactive magnetron sput-
tering. The substrates were subjected to standard cleaning
procedures before introducing into the deposition cham-
ber. A circular zirconium disk of 99.99% pure and 100 mm
diameter was made the target. Argon and oxygen gas flow
0
2
4
6
8
10
X 10-4 mbar Oxygen Partial Pressure
Fig. 1. Variation of target voltage as a function of oxygen partial
pressure.
was controlled independently and a total pressure of
ꢂ3
6
ꢁ 10 mbar was maintained during deposition. The
details of deposition conditions are listed in Table 1. After
deposition, the samples were annealed in the same chamber
under vacuum for 30 min at a temperature ranging from
the present study, all the oxide films have been deposited
with a discharge current of 450 mA and an oxygen partial
ꢂ4
pressure of 8 ꢁ 10 mbar.
7
73 K to 1073 K. In order to study the AC electrical
conductivity, the electrodes were made by silver contact.
The thickness of the films used in this study was measured
to be 110 nm by stylus profilometer.
3
.2. X-ray diffraction (XRD)
The structure of the samples was characterized by XRD
with CuKa radiation and the diffraction pattern is shown
in Fig. 2. There was no crystallinity in the samples grown at
ambient temperature, but with annealing at 873 K the
monoclinic phase appears as seen from the diffraction
pattern. This is the normally observed phase of bulk
zirconia at room temperature (RT) and pressure. With
annealing at higher temperatures, tetragonal phase is seen
from the hump appearing in the samples annealed at
around 1073 K and simultaneously the intensity of mono-
clinic phase reduces. The average grain size D for the
deposited film was determined to be around ꢀ6 nm using
Scherrer’s formula
3
. Results and discussion
3
.1. Glow discharge characteristics
The glow discharge characteristics during reactive
sputtering were studied under different oxygen partial
pressures, in order to optimize the region where the ZrO2
films can be prepared. Fig. 1 shows the variation of target
voltage as a function of oxygen partial pressure. When the
target current is maintained constant, the characteristics
show that the voltage increases initially, reaches a
maximum at a particular oxygen partial pressure and later
it decreases and after certain pressure, almost remains
constant. This behavior has been explained earlier by
Mohan Rao and Mohan [21] and attributed to the target
poisoning effects. The ideal region for the deposition of
oxide films with reasonable deposition rates is the region
where the cathode voltage starts decreasing. However,
when the target current is low, due to low deposition rate
of the metal, the target gets poisoned even with low partial
pressure of the reactive gas. Based on these results, during
D ¼ ð0:9 l=b cos yÞ,
where y is the Bragg angle, b is the full width at half
maximum of the peak, and l is the wavelength of X-ray
[22].
3.3. UV–visible transmittance spectroscopy
Fig. 3(a) shows UV–visible transmittance spectra of
zirconia thin films. Annealed samples show higher trans-
mittance indicating the improved crystalline nature of the
thin film. The transmittance showed a slight decrease in
case of films annealed at 1073 K. This could be due to the
removal of oxygen due to high temperatures. Fig. 3(b)
Table 1
Deposition conditions for zirconia films
1/2
shows the plot of (ahg) as a function of energy (hg)of the
ꢂ5
incident light. The band gap can be calculated by
extrapolating the linear portion of the curve to the abscissa
intercept. The calculated results are listed in Table 2. The
band gap value shows the insulating nature of the thin film
at all the conditions. While the amorphous nature of the
films deposited at RT exhibits lower band gap value, the
Base pressure
2.5 ꢁ 10 mbar
ꢂ4
Oxygen partial pressure
Total pressure (argon+oxygen)
Substrate to target distance
Discharge current
8 ꢁ 10 mbar
ꢂ3
6 ꢁ 10 mbar
110 mm
450 mA
15 min
Deposition time

ARTICLE IN PRESS
K.P.S.S. Hembram et al. / Physica B 399 (2007) 21–26
23
annealed films exhibited higher band gap and a maximum
4.83 eV) was seen in the films annealed at 873 K. Further
increase in annealing temperature did not show much
variation in the band gap. It is well known that the
sputtered films have lower packing density and annealing
improves the packing density. Thus the increase in the
band gap with annealing could be a combined effect of
improved packing density and improved crystallinity.
Approximations have been taken in the computation of
n and k as the oscillations of the amplitude of transmit-
tance is not observed due to small thickness of the films [23,
24]. The variation of the refractive index and extinction
(
t(002)
1073K
m(022)
m(310)
973K
m(221)
m(112)
873K
t(111)
773K
RT
10
20
30
40
Theta (Degree)
50
60
2
Fig. 2. X-ray diffraction of the ZrO
2
thin films grown at different conditions.
1
1
200
000
1
00
1
073K
973K
73K
RT
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
8
800
600
4
2
00
00
0
1
073K
973K
73K
RT
8
100
200
300
400
500
600
700
800
900
0
1
2
3
4
5
6
7
Wave Length (nm)
hν (eV)
2
.5
.4
.3
.2
.1
.0
.9
.8
1
9
073 K
73 K
0.20
1
9
073 K
73 K
2
2
2
2
2
1
1
873 K
RT
873 K
RT
0.15
0
0
.10
.05
0.00
100
100
200
300
400
500
600
700
800
900
200
300
400
500
600
700
800
900
Wave Length (nm)
Wave Length (nm)
1
/2
Fig. 3. (a) Transmittance spectra of ZrO
2
thin film with and without annealing; (b) (ahg) as a function of energy (hg); (c) Refractive index as a function
wavelength; (d) extinction coefficient as a function of wavelength.

ARTICLE IN PRESS
K.P.S.S. Hembram et al. / Physica B 399 (2007) 21–26
24
Table 2
Band gap at different conditions
coefficient as a function of wavelength at different sample
preparation conditions is shown in Fig. 3(c) and (d). From
this it can be seen that k is nearly 0.05 and hence the films
are fully transparent in the visible range. The refractive
index of the films is nearly the same (about 1.95) for all the
films in the visible range. Higher value of the refractive
index is observed at the lower wave length regime.
Our values compare well with the results obtained by
Venkataraj et al. [25].
Samples prepared
RT
Band gap (eV)
4.37
4.83
4.70
4.78
8
9
1
73 K
73 K
073 K
3.4. Atomic force microscopy (AFM)
Fig. 4(a) shows the two dimensional surface morphology
2
of the zirconia film with scan size 5 ꢁ 5 mm . The
corresponding three dimensional image is shown in
Fig. 4(b). The columnar growth with dense structure is
observed from the top and lateral view. The mean height of
the surface morphology is 188.92 nm obtained from the
linear fit of the scan height taken over 256 points with
standard deviation of 0.00532 and correction coefficient
ꢂ0.08169. The mean of root mean square (RMS) rough-
ness of the film is found to be 5.75 nm as shown in Fig. 4(c).
The image clearly shows a dense microstructure and is
reflected in the improved properties of the annealed films as
discussed above.
3.5. X-ray photoelectron spectroscopy (XPS)
The as deposited and annealed (873 K) films were studied
using XPS. The wide range survey scan spectra of the
annealed film is shown in Fig. 5(a). The more specific
scanning of Zr 3d spectra, which splits into its two
spin–orbit components 3d5/2 at 183.3 eV and 3d3/2 at
1
1
85.7 eV separated by 2.4 eV and at the intensity ratio of
0:7 as shown in Fig. 5(b). This can be explained by
considering complete oxidation of zirconium to zirconia in
its +4 oxidation state [4,26,27]. The specific scanning O1s
spectra, which is at 531.2 eV is primarily due to Zr–O
bonding which is shown in Fig. 5(c). The shift in peak
position for zirconium and oxygen for the as deposited
sample may be due to partial bonding of zirconium atoms
with the oxygen. The XPS data thus show that the
annealed films are completely stoichiometric resulting in
better optical characteristics.
3.6. AC conductivity
Fig. 6(a) shows the real part of the AC conductivity of
the as prepared and annealed samples measured at
different temperatures. The low frequency conductivity of
the as deposited samples differs from that of annealed
samples. At higher frequencies all the samples show similar
behavior for the real part. But the imaginary part gives
discontinuity in the range of tens of thousands of Hz. This
may be due to the reason that the thermal fluctuations of
the conducting species are suppressed at low temperature,
reducing the scattering probability. (At the RT the data
Fig. 4. (a) 2D surface morphology of ZrO
b) 3D surface morphology of ZrO thin film annealed at 873 K; (c) RMS
surface morphology of ZrO thin film annealed at 873 K.
2
thin film annealed at 873 K;
(
2
2

ARTICLE IN PRESS
K.P.S.S. Hembram et al. / Physica B 399 (2007) 21–26
25
873K
RT
O1S
OKLL
Zr 2P 3/2
Zr 3d
Zr 2P 1/2
Zr 2S
C 1S
Zr 4P
0
200
400
600
800
1000
180
182
184
186
188
190
Binding Energy (eV)
Binding energy (eV)
873K
RT
525
530
535
540
545
Binding Energy (eV)
Fig. 5. (a) Wide scan XPS spectrum of ZrO
ZrO
2 2
thin film annealed at 873 K; (b) XPS spectrum of Zr 3d 5/2 and Zr 3d 3/2 of ZrO ; (c) XPS spectrum of O1s of
2
.
points are not consistent and we are not able to get clear
data). The same explains the behavior in the imaginary part
of the AC conductivity, shown in Fig. 6(b).
correlated barrier hopping (CBH) model [28,30] can be
used to describe sAC (o) of these oxides and is described
elsewhere [31–34].
The measured conductivity can be related by the
equation sAC ¼ sDC(0) +sAC (o) [28,29], where sDC(0)
is a zero-frequency (DC) conductivity. sAC(o) is a
frequency-dependent component of the conductivity,
4. Conclusions
Zirconia thin films were prepared by reactive magnetron
sputtering. The electrical and optical properties of the films
were found to show improvement after annealing. Anneal-
ing at 873 K resulted in a higher band gap of 4.83 eV for the
films and further annealing showed no significant change in
s
expressed as a power law sAC (o) ¼ Ao , (where A is
the proportionality constant that depends on the tempera-
ture and s is the frequency exponent) which describes the
conduction by hopping of charge carriers [28,29]. The

ARTICLE IN PRESS
K.P.S.S. Hembram et al. / Physica B 399 (2007) 21–26
26
Acknowledgement
1
1
1
1
03
d
c
b
a
The authors are grateful to Prakash of Department of
Physics, Indian Institute of Science, for assistance in
electrical characterization of samples.
02
References
[
1] M.C. Gilmer, T.Y. Luo, H.R. Huff, M.D. Jackson, S. Kim,
G. Bersurker, P. Zeitzoff, L. Vishnubhotla, G.A. Brown, R. Amos,
D. Brady, V.H.C. Watt, G. Gale, J. Guan, B. Nguyen,
G. Williamson, P. Lysaght, K. Torres, F. Geyling, C.F.H. Gondran,
J.A. Fair, M.T. Schulberg, T. Tamagawa, Mater. Res. Soc. Symp.
Proc. 567 (1999) 323.
01
[2] H.F. Luan, A.Y. Mao, S.J. Lee, T.Y. Luo, D.L. Kwong, Mater. Res.
Soc. Symp. Proc. 567 (1999) 481.
00
[3] K.J. Hubbard, D.G. Schlom, J. Mater. Res. 11 (1996) 2757.
[4] T.S. Jeon, J.M. White, D.L. Kwong, Appl. Phys. Lett. 78 (2001) 368.
104
105
1
03
[
[
[
5] H. Wantable, Appl. Phys. Lett. 78 (2001) 3803.
6] K. Muraoka, Appl. Phys. Lett. 80 (2002) 4516.
7] H. Wendel, H. Holzschuh, H. Suhr, G. Erker, S. Dehnicke, M. Mena,
Mod. Phys. Lett. B 4 (1990) 1215.
Fequency (Hz)
05
1
1
1
1
1
1
[
[
8] A. Bastianini, G.A. Battiston, R. Gerbasi, M. Porchia, S. Daolio,
J. Phys. IV 5 (1995) 525.
04
9] G.Z. Cao, H.W. Brinkman, J. Meijerink, K.J. De Vries,
A.J. Burggraaf, J. Am. Ceram. Soc. 76 (1993) 2201.
03
[10] K. Izumi, M. Murakami, T. Deguchi, A. Morita, J. Am. Ceram. Soc.
2 (1989) 1465.
[11] Y. Komatsu, T. Sato, S. Ito, K. Akadi, Thin Solid Films 341 (1999) 132.
7
02
[
[
12] M.A. Villegas, Thin Solid Films 382 (2001) 124.
13] A. Mehner, H.K. WestKamp, F. Hoffmann, P. Mayr, Thin Solid
Films 308 (1997) 363.
01
00
[
[
14] P. Peshev, I. Stambolova, S. Vassilev, P. Stefanov, V. Blaskov,
K. Starbova, N. Starbov, Mater. Sci. Eng. B 97 (2003) 106.
15] J. Aarik, A. Aidla, H. Mandar, T. Uustare, V. Sammelselg, Thin
Solid Films 408 (2002) 97.
a
b
c
d
0-1
1
1
[16] J. Koo, Y. Kim, H. Jeon, Jpn. J. Appl. Phys. 41 (2002) 3046.
[
[
[
17] S. Shao, Z. Fan, J. Shao, H. He, Thin Solid Films 445 (2003) 59.
18] J.H. Hong, W.J. Choi, J.M. Myoung, Microelectron. Eng. 70 (2003) 35.
19] M.S. Kim, Y.D. Ko, J.H. Hong, M.C. Jeong, J.M. Myoung, I. Yun,
Appl. Surf. Sci. 227 (2004) 387.
0-2
1
03
104
105
Frequency (Hz)
[
20] P. Yashar, J. Rechner, M.S. Wong, W.D. Sproul, S.A. Barnett, Surf.
Coat. Technol. 94 (1997) 333.
Fig. 6. (a) Real part of AC conductivity as a function of frequency; (b)
imaginary part of AC conductivity as a function of frequency. ((a) as
prepared sample measured at 200 K; (b) as prepared sample measured at
[
[
21] G. Mohan Rao, S. Mohan, J. Appl. Phys. 69 (1991) 6652.
22] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley
publishers, London, 1978.
100 K; (c) annealed sample measured at 200 K; (d) annealed sample
measured at 100 K).
[
[
[
23] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214.
24] E. Nichelatti, J. Opt. A: Pure Appl. Opt. 4 (2002) 400.
25] S. Venkataraj, O. Kappertz, H. Weis, R. Dress, R. Jayavel,
M. Wuttig, J. Appl. Phys. 92 (2002) 3599.
the characteristics of the films. Annealing at higher
temperatures (1073 K) resulted in lower transmittance due
to oxygen deficiency. The mean roughness of the annealed
films is about 5.75 nm comparable to the average grain size
of the thin film. The film annealed at 873 K shows the +4
oxidation state, indicating the complete oxidized nature of
zirconium. The imaginary pat of the AC conductivity gives
discontinuity in the frequency range of tens of thousands of
Hz, where as, the real part does not show such behavior.
The conduction mechanism in the oxide film is due to the
hopping of the conducting species.
[
[
[
26] Y.M. Sun, J. Lozano, H. Ho, H.J. Park, S. Veldman, J.M. White,
Appl. Surf. Sci. 161 (2000) 115.
27] N.L. Zhang, Z.T. Song, Q. Wan, Q.W. Shen, C.L. Lin, Appl. Surf.
Sci. 202 (2002) 126.
28] S.R. Elliott, Adv. Phys. 36 (1987) 135.
[29] A. Ghose, Phys. Rev. B. 41 (1990) 1479.
[
[
[
[
[
30] A.R. Long, Adv. Phys. 31 (1982) 553.
31] A.A. Dakhel, Thin Solid Films 476 (2005) 366.
32] A.A. Dakhel, Solid State Electron. 49 (2005) 1996.
33] A.A. Dakhel, Chem. Phys. Lett. 393 (2004) 528.
34] R.O. Ndong, G. Ferblantier, F.P. Delannoy, A. Boyer, A. Foucaran,
Microelectron. J. 34 (2003) 1087.