Interfacial and optical properties of ZrO2/Si by reactive

Materials Letters 60 (2006) 888 – 891
www.elsevier.com/locate/matlet
Interfacial and optical properties of ZrO2/Si by reactive
magnetron sputtering
L.Q. Zhu a,⁎, Q. Fang a,b , G. He a , M. Liu a , L.D. Zhang a
a
Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanostructure, Institute of Solid State Physics,
Chinese Academy of Science, P.O.Box 1129, Hefei 230031, People's Republic of China
b
Electronic and Electrical Engineering, University College London, Torrington Place, London WCIE 7JE, UK
Received 4 September 2005; accepted 12 October 2005
Available online 9 November 2005
Abstract
ZrO2 dielectric films were deposited on Si substrates by reactive magnetron sputtering technique. Interfacial and optical properties were
investigated. Crystal structure was studied by X-ray diffraction. Fourier Transform Infrared Spectroscopy analysis confirmed the presence of a
low-k interfacial SiO2 layer due to the excited oxygen radicals in the sputtering plasma and the physisorbed oxygen in as-deposited ZrO2 films.
Optical constants were extracted based on the best spectroscopic ellipsometry fitting results. Absorption coefficients near the absorption edge were
also calculated. The absorption tails in the range 4.5–4.75 eV indicated that there was a defect energy level below the conduction band of ZrO2
due to oxygen vacancies.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Dielectrics; Thin films; Spectroscopic ellipsometry; Absorption coefficient
1. Introduction
With the complementary metal-oxide-semiconductor (CMOS)
device scaling, high dielectric constant (high-k) oxides are currently widely investigated as potential candidates for replacement of conventional SiO2 gate oxide, such as SrTiO3, Ta2O5,
TiO2, Al2O3, HfO2, ZrO2, etc. [1–5]. Among these oxides,
zirconia based dielectrics is one of the most promising oxides
because of their good thermal stability [6] and large band-offset
in direct contact with the silicon substrate [7], high dielectric
constant (∼ 25) [2], and large band gap (∼ 5.8 eV) [8]. Based on
these excellent properties, zirconium based oxides have been
studied widely in recent years.
To date, various methods have been employed to prepare
ZrO2 films, such as reactive sputtering [9], metal–organic
chemical vapor deposition [10], atomic layer chemical vapor
deposition [7,11], and pulsed-laser ablation deposition [12].
⁎ Corresponding author. Tel.: +86 551 5591465 424; fax: +86 551 5591434.
E-mail address: qzhuli2003@yahoo.com.cn (L.Q. Zhu).
0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2005.10.039
And the improved electrical and structural properties have
also been obtained by different methods. However, there are
relatively few reports on the optical properties of ZrO2 thin
films. Therefore, the determination of the optical properties for
Fig. 1. XRD patterns of as-deposited (a) and annealed ZrO2 films at various
temperatures for 20 min in Ar/O2 ambient: (b) 700 °C; (c) 800 °C; (d) 900 °C.
L.Q. Zhu et al. / Materials Letters 60 (2006) 888–891
889
Fig. 2. Infrared absorption spectra of as-deposited (a) and annealed ZrO2 films on silicon at different temperature: (b) 700 °C; (c) 800 °C; (d) 900 °C. (1) For 40 nm
and (2) for 20 nm.
ZrO2 thin films in the wide energy range is still necessary. In
this paper, ZrO2 thin films were prepared on n-Si(100) substrate
in a magnetron sputtering system. The interfacial and optical
properties of ZrO2 films on Si were investigated.
2. Experimental
After the modified RCA cleaning, n-Si(100) wafers with a
resistivity of 1–10 Ω cm were put into the deposition chamber
immediately. ZrO2 thin films were prepared using a direct
current (DC) reactive magnetron sputtering. A 99.99% pure
zirconium disk with a diameter of 60 mm was used as the
sputtering target. The distance between the target and substrate was fixed at about 4.5 cm. The base pressure was
about 4.4 × 10− 4 Pa. Ultra-high purity (99.999%) Argon and
(99.999%) oxygen, acted as the sputtering enhancing gas and
reactive gas respectively, were introduced into the vacuum
chamber with flow rates of 17 and 8 sccm. The total working
pressure was held at 0.2 Pa. Prior to ZrO2 film deposition,
the zirconium metal target was pre-sputtered for 10 min in
order to remove the surface contaminants on the target and
stabilize the sputtering. Then ZrO2 layers with different thickness were deposited on silicon substrates with a constant DC
power of 96 W. In order to study the thermal stability, as
deposited ZrO2 films were subjected to post-annealing at
temperature ranging from 700 °C to 900 °C for 20 min in
Ar/O2 ambient.
The microstructure of the films was characterized by Xray diffraction. The interfacial layer between ZrO2 thin films
and silicon substrate was investigated with Fourier Transform
Infrared Spectroscopy (FTIR). The thickness and optical
properties of the films were determined by using an ex-situ
phase modulated spectroscopic ellipsometry (Model UVISE
JOBIN-YVON).
3. Results and discussion
Fig. 1 shows a typical series of XRD patterns for as-deposited and
annealed ZrO2 films at various temperatures with 2θ from 20° to 50°.
The diffraction peaks were indexed according to standard JCPDS
patterns for ZrO2 lattice. In Fig. 1(a), the as-deposited ZrO2 films
show a very weak diffraction peak demonstrating an amorphous structure. While for annealed samples from 700 °C to 900 °C (Fig. 1(b)–
(c)), there is an increase in the intensity of (−111) peak for monoclinic
phase. Moreover, the peaks for m(002) and m(−102) were also observed at 800 °C and 900 °C, indicating the polycrystalline structure at
high annealing temperature.
Fig. 2 illustrates examples of infrared absorption spectra at 400–
1200 cm− 1 range obtained for ZrO2 films with different film thickness,
amounting to 40 nm and 20 nm respectively. The absorption bands at
1000–1100 cm− 1 observed for as-deposited and annealed samples are
related to the Si–O–Si asymmetrical stretching mode of the silicon
oxide located at the ZrO2/Si interface. It shows a weak broad absorption band centered about 1037 cm− 1 for as-deposited films regardless
of the film thickness. In fact, the energetic oxygen species in the
plasma can diffuse randomly to the silicon surface and form an
interfacial oxide layer at the initial deposition stage [13]. At the same
time, they not only react with Zr atoms but also penetrate through the
loose ZrO2 matrix and oxidize the silicon substrate because of their
small radius and high energy. High temperature annealing induces the
shift of the peak position and the change of Si–O bond absorption
intensity (Fig. 2). After being annealed at temperatures from 700 °C to
900 °C for 20 min in Ar/O2, the Si–O absorption peak positions show
a constant blue shift from 1037 cm− 1 to 1073 cm− 1 along with the
Table 1
Extracted SE fitting results by using TL dispersion function
Annealing temperature
As-deposited
700 °C
800 °C
900 °C
Interfacial layer (nm)
ZrO2 layer (nm)
Refractive index (n) at 4.0 eV
χ2
1.6
24.0
1.822
1.39
2.5
23.5
1.922
2.67
2.3
22.2
1.944
2.29
2.6
22.8
1.920
2.60
Fig. 3. Calculated refractive indices and extinction coefficients for 20 nm ZrO2
thin films, as deposited and annealed at different annealing temperatures, based
on the results of the TL fitting.
890
L.Q. Zhu et al. / Materials Letters 60 (2006) 888–891
increased intensity of the Si–O absorption band for thick films about
40 nm indicating the constant interfacial growth (Fig. 2(1)). For
thinner films, about 20 nm, both the peak position and the intensity
of the absorption band increases after being annealed at 700 °C (Fig. 2
(2)). While for higher annealing temperature at 800 °C or 900 °C, the
peak position and the intensity have no significant change when
compared to the sample annealed at 700 °C. Based on our FTIR
results, we conclude that oxygen in the annealing system will not
diffuse through the ZrO2 crystalline boundary and react with Si substrate since there is no significant change in Si–O absorption from 700
°C to 900 °C for thinner samples. Hoshino et al. [14] reported that
physisorbed oxygen was incorporated in the HfO2 films during DC
reactive sputtering. It is also right in our case. After the high temperature annealing, such oxygen species can diffuse into the interfacial
region and help to the formation of low-k interfacial layers [15]. So, in
order to control the interfacial layer growth in DC high-k oxides, the
sputtering plasma states should be carefully controlled.
To investigate the effects of high temperature annealing on interfacial layers and optical properties of ZrO2 thin films on silicon
substrates, we applied spectroscopic ellipsometry to characterize a
series of samples annealed at different temperatures. In our SE data
analysis, we adopted the widely accepted Tauc–Lorentz (TL) dispersion function [16] to characterize the unknown pseduodielectric function (ε = ε1 + iε2) of the ZrO2 films. According to FTIR analysis, a
simple optical model consisting of an underlying SiO2 interface layer
and top ZrO2 layer has been used. During the simulation, the film
thickness and the TL parameters were fitted through χ2 (goodness of
fit) minimization process. The thickness and optical constants of the
ZrO2 films were extracted based on the best fit between the experimental SE data and the simulated spectra. Table 1 shows the SE fitting
results. There is an increase in interfacial layer thickness after the
additional annealing regardless of the annealing temperatures consistent with FTIR analysis. While there is a slight decrease in high-k
ZrO2 film thickness after high temperature annealing, indicating the
increased packing density.
The optical constants are considered to be a measure of film quality.
Fig. 3 shows the refractive index (n) and extinction coefficient (k) as a
function of energy calculated from the best-fitted parameters. It can be
seen that the optical constants (n, k) are significantly affected by
annealing temperature. There is a gradual increase in refractive index
with the annealing temperatures, attributed to the increased packing
density and the improved crystallinity of the films (Fig. 1). It is
believed that the as-deposited films have a lower packing density
because of a loose arrangement with some voids incorporated during
the sputtering. The high temperature annealing results in the release of
such voids and the increase in the mobility of atoms or molecules of
the films, which favors the formation of more closely packed thin films
leading to an increase in refractive index. While there is a slight
decrease in refractive index for the sample annealed at 900 °C. We
attribute this observation to the surface roughening effects at high
annealing temperature. It can also be seen that there is a decrease in
extinction coefficients after the high temperature annealing indicating
the improved film quality.
In order to study the optical absorption properties, the absorption
coefficients (α) are also calculated using α = 4πk /λ, where λ is the
wavelength of a photon and k is the extinction coefficient. Fig. 4 plots
α vs. hv near the band edge for all the samples. In the spectral region
between 4.4 and 4.75 eV, all the samples display absorption tails of
similar shape. Since the reported bandgap energies were about 5.8 eV
for ZrO2 [8], we attribute the absorption tails to electron transitions
from the valence band to defect energy levels. Such defects were also
reported in HfO2 films [17]. Takeuchi et al. [18] attributed these
Fig. 4. The absorption coefficient α = 4πk/λ, where λ is the wave length of a
photon and k is the extinction coefficient which is calculated from bε = ε1 +
iε2N of as prepared sample.
defects to oxygen vacancies within the HfO2 films. Meanwhile, Venkataraj et al. attributed such defects to oxygen vacancies and the
presence of lattice defects in ZrO2 films [19]. Notably, ZrO2 and
HfO2 are known to have similar electronic structures due to the
similarity in electronic configurations of Zr and Hf atoms [8]. In our
case, we also ascribe the defects to oxygen vacancies since the detected
defects level is about 1.2 eV below the conduction band consistent
with literature reports [20].
4. Conclusions
In summary, high-k ZrO2 films have been prepared by DC
reactive magnetron sputtering technique on the H-passivated
silicon substrate. Interfacial and optical properties in relation
to annealing temperature were studied. XRD analysis indicates
that there is a crystalline growth after the additional high temperature annealing. FTIR measurement indicates that the sputtering plasma states should be carefully controlled in order to
control the interfacial layer growth in DC high-k oxides. Spectroscopic ellipsometry has been used to evacuate the optical
properties. Optical constants are obtained by SE fitting based
on TL dispersion function. The results indicate the increased
packing density and improved film quality after high temperature annealing. The absorption tails in the extracted absorption
coefficients indicate that there is a defect energy level below the
conduction band of ZrO2 due to oxygen vacancies.
Acknowledgements
This work was supported by the National Key Project of
Fundamental Research for Nanomaterials and Nanostructures
(Grant No. 2005CB623603).
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