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CERAMICS
INTERNATIONAL
Ceramics International 41 (2015) 6187–6193
www.elsevier.com/locate/ceramint
Effect of oxygen/argon gas ratio on the structure and optical properties
of sputter-deposited nanocrystalline HfO2 thin films
C.V. Ramanan, M. Vargas, G.A. Lopez, M. Noor-A-Alam, M.J. Hernandez, E.J. Rubio
Department of Mechanical Engineering, University of Texas at El Paso, El Paso, TX 79968, USA
Received 26 November 2014; received in revised form 17 December 2014; accepted 24 December 2014
Available online 3 January 2015
Abstract
Nanocrystalline hafnium oxide (HfO2) thin films have been produced under variable reactive oxygen (O2) fractionation (Г) employing Hf metal
for reactive sputter-deposition. The effect of Г on the HfO2 compound formation, structure, morphology and optical properties has been
evaluated. Without oxygen, the films of hexagonal phase of Hf metal were grown. Films grown at different O2 pressure are nanocrystalline,
monoclinic HfO2 with ð111Þ texturing. The optical properties of HfO2 films have been evaluated using spectroscopic ellipsometry (SE). The
optical constants and their dispersion profiles indicate that the best optical-quality HfO2 films are formed at O2 ratio of Z 0.2. The index of
refraction (n) profiles derived from SE measurements follow the Cauchy dispersion relation. The correlation between oxygen-fraction and optical
properties in HfO2 films is established.
& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Hafnium oxide; Structure; Optical constants; Spectroscopic ellipsometry
1. Introduction
Hafnium oxide (HfO2) is a high temperature refractory ceramic
with excellent physical and chemical properties, which makes it
promising for a wide variety of technological applications [1–24].
The outstanding chemical stability, electrical and mechanical
properties, high dielectric constant, and wide band gap of HfO2
makes it suitable for several industrial applications in the field of
structural ceramics, optics, electronics, magneto-electronics, and
optoelectronics [8–13]. HfO2 nanostructures have the potential to
offer new possibilities for current and emerging technological
applications. For instance, extremely thin (1 nm) overlayer of HfO2
deposited on Si0.8Ge0.2 substrate stabilizes the low oxidation states
of strained germanium oxides (Ge2O3) in comparison to that onto
the SiGe reference substrate [8]. Exceptionally high values of
isothermal compressibility values were reported for Hafnium oxide
doped with nitrogen [6]. HfO2 has been identified as one of the
most promising materials for the nano-electronics industry to
replace SiO2 because of its high dielectric constant and stability
in contact with Si [13–16]. Recently, it has been reported that HfO2
n
Corresponding author.
http://dx.doi.org/10.1016/j.ceramint.2014.12.141
0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
integration with graphene can create hybrid structures, which can
offer the opportunity to combine the versatile functionalities of
oxides with the excellent electronic transport in graphene, for
application in modern, nanotechnology based, high-speed electronics [9].
HfO2 exhibits various polymorphs [16,17,21]. One stable
monoclinic phase and four metastable phases, cubic, tetragonal,
orthorhombic I and orthorhombic II, have been identified for
HfO2 [16,17]. The stable structure of HfO2 is monoclinic under
normal conditions of temperature and pressure [16,17]. It transforms into the tetragonal form when heated to temperatures
higher than 1700 1C [16,17]. Further transformation into the
cubic polymorphic form having the fluorite structure takes place
at 2700 1C [21]. The high density ( 10 g/cm3) makes this
ceramic material attractive as host cell, when activated with rare
earths (Eu3 þ ), for applications as scintillating materials and
waveguides amplifiers [5,7]. A large band gap of HfO2 coupled
with low absorption provides optical transparency over a broad
range in the electromagnetic spectrum. HfO2 dielectrics can,
therefore, operate efficiently down to 220 nm in the ultraviolet
(UV) region and 10 μm in the infrared (IR) region [4]. There are
not many contenders that are stable under UV considerations;
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C.V. Ramana et al. / Ceramics International 41 (2015) 6187–6193
therefore HfO2 is also a preferred high index material when high
laser damage threshold is a requirement. As a result, HfO2 has
been used in optical coating applications, including optical filters,
ultraviolet heat mirrors, antireflection coatings and novel scintillation materials [1–7].
Controlled growth and manipulation of specific crystal structures at the nanoscale dimensions have important implications for
the design and optical applications of HfO2. However, it is well
known that the optical, electrical and electro-optic properties of
HfO2 are highly dependent on the surface/interface structure,
morphology, and chemistry [1–15]. Recently, considerable attention has been focused towards the sputter-deposition and property
evaluation of HfO2 thin films [2,4,6,12,25–29]. The present work
was performed to determine the effect of oxygen content in the
reactive gas mixture on the structure and optical properties of
HfO2 nanocrystalline films made by reactive sputter-deposition.
Spectroscopic ellipsometry (SE), which is known to be a sensitive
and nondestructive method for structure and optical characterization of thin-film materials, has been employed to determine the
optical constants of HfO2 films as a function of oxygen content.
The qualitative and quantitative information on the structure and
optical constants of HfO2 films grown on Si is reported in
this paper.
2. Materials and methods
2.1. Deposition
HfO2 thin films were deposited onto silicon (Si) (100) wafers by
radio-frequency magnetron sputtering. All the substrates were
thoroughly cleaned and dried with nitrogen before introducing
them into the vacuum chamber, which was initially evacuated to a
base pressure of 10 6 Torr. Hafnium (Hf) metal target (Plasmaterials Inc.) of 2 in. diameter and 99.95% purity was employed
for reactive sputtering. The target to substrate distance was
maintained at 8 cm for all the depositions. The flow of argon
(Ar) and oxygen (O2) and their ratio were varied and controlled
using MKS mass flow meters. The oxygen fraction (Γ), which is
the ratio of O2 to total reactive Arþ O2 gas mixture, is varied over
the wide range of (0.0–0.4) during deposition in order to study the
effect of Γ on the growth, structure and optical constants of HfO2
films. The desired oxygen fraction values were achieved by
carefully controlling the gas flow of high purity Ar and O2 into
the chamber. Before each deposition, the Hf-target was presputtered for 15 min using Ar alone with shutter above the gun
closed. The deposition was carried out with a sputtering power of
100 W and keeping the substrate temperature (Ts) at 300 1C. The
effect of growth temperature on the structural characteristics of
HfO2 films has been reported elsewhere [2]. Briefly, HfO2 films
are completely amorphous for substrate temperatures from room
temperature to 200 1C, the point of structural transformation from
amorphous to monoclinic phase [2]. However, better resolved
XRD peaks related to nanocrystalline, monoclinic phase appear for
HfO2 films grown at 300 1C [2]. Therefore, in the work, the HfO2
films were deposited keeping Ts fixed at 300 1C. The deposition
was made for a constant time of 45 min. The substrates were
heated by a radiative heating source, and the desired temperature
was controlled by a computer programmed PID controller.
2.2. Characterization
The grown HfO2 films were characterized by performing
structural and optical measurements. X-ray diffraction (XRD)
measurements on HfO2 films were performed by using a Bruker
D8 Advance X-ray diffractometer. All the measurements were
performed ex-situ as a function of Г. The XRD patterns were
recorded using Cu Kα radiation (λ¼ 1.54056 Ǻ) at RT. The
coherently diffracting domain size (Dhkl) was calculated from
the integral width of the diffraction lines using the Scherrer
equation after background subtraction and correction for instrumental broadening. The Scherrer equation is as follows [30]:
Dhkl ¼ 0:9λ=β cos θ
ð1Þ
where Dhkl is the size, λ is the X-ray wavelength, β is the width
of a peak at half of its intensity, and θ is the angle of the peak.
Surface imaging analysis was performed using a high-performance
and ultra-high-resolution scanning electron microscope (SEM;
Hitachi S-4800). Secondary electron imaging was performed on
the HfO2 films grown on Si wafers using carbon paste at the ends
of the sample to avoid charging problems.
Optical properties of HfO2 films were evaluated using
spectroscopic ellipsometry (SE) measurements. SE measurements were performed ex-situ on the films grown on silicon wafers
by utilizing a J. A. Woollam V-VASE instrument. Measurements
were performed in the range of 300–1350 nm with a step size of
2 nm and at angles of incidence of 651, 701, and 751, near the
Brewster angle of silicon. The ellipsometry data analysis was
performed using commercially available WVASE32 software [31].
3. Results and discussion
The XRD patterns of HfO2 films grown at different Γ are
shown in Fig. 1. Samples grown without any oxygen (Γ ¼ 0.0)
in the reactive gas mixture are the Hf metal films (not shown).
HfO2 compound formation with monoclinic phase began to
appear with oxygen content in reactive pressure. It is evident
that the HfO2 films grown under different oxygen reactive
pressure are nanocrystalline, monoclinic. The XRD peak at
28.11 corresponds to diffraction from ð111Þ planes. This peak
is rather broad for HfO2 films grown at Γ ¼ 0.2–0.4 indicating
the average crystallite size is small. In the case of Γ ¼ 0.1,
however, the intense peak corresponds to HfO2, and some
contributions from Hf-metal also present. Furthermore, the
ð111Þ peak is relatively sharp compared to that of the films
grown at Γ ¼ 0.2–0.4. Perhaps, the Hf-oxide phase began to
appear at Γ ¼ 0.1 but may not be able to form fully the high
end compound i.e., HfO2. Evidence for such a mixed Hfþ HfOx
phase formation is more clear from SE analyses as discussed later
below. The d( 111) (lattice spacing) value derived from XRD data
for all the films is 3.185 Å, which is in good agreement with the
reported value [14,15]. No lattice expansion of HfO2 films is
observed at any Γ values. This indicates that the lattice expansion,
as observed in some ionic solids, is certainly sensitive to the
C.V. Ramana et al. / Ceramics International 41 (2015) 6187–6193
(002)
(020)
(-211)
(211)
(-122)
♦
(221)
Intensity (a.u.)
(-111)
0.3
Hf-O
0.2
Hf-O
0.1
20
30
40
50
2θ (degrees)
60
70
Fig. 1. XRD patterns of HfO2 films grown at various oxygen ratio values.
HfO2 films grown with variable oxygen content exhibits the monoclinic phase
while films grown with Ar alone exhibits Hf hexagonal (metallic) phase.
60
55
Grain size (nm)
50
45
40
35
30
25
20
0.0
Thickness (nm)
150
140
130
120
110
100
90
0.0
0.1
0.2
0.3
0.4
Fig. 3. Thickness variation of HfO2 films with Γ. Line is provided as a guide
to the eye. It is evident that the film thickness decreases significantly with the
initial oxygen content in the sputtering gas mixture and becomes more or less
constant for Γ¼ 0.2–0.4.
Si
Hf-O
0.4
Hf-O
160
♦
growth temperature but not oxygen pressure. For instance, we
noted lattice expansion coupled with strain in HfO2 films grown
as a function of variable growth temperature [2]. The average
grain size variation with Γ is shown in Fig. 2. It can be noted that
the grain size in pure Hf metallic film is higher (57 nm) to that of
monoclinic HfO2 films grown at Γ¼ 0.2–0.4. The grain size
decreases from 35 to 25 (72) nm with increase in O2 content.
The significant decrease in grain size occurs for the variation of Γ
from 0.0 to 0.2 but not significantly afterwards. A similar trend is
observed for film thickness variation with Γ indicating that the
underlying physical mechanism is similar.
The variation of HfO2 film thickness with Γ is shown in
Fig. 3. The film thickness was determined by ellipsometry and
verified by cross-sectional SEM for a set of representative
films. The agreement between the film thickness values using
SEM and SE analyses is remarkably good for the Hf-oxide
formation region, which is Γ ¼ 0.2–0.4. As shown in Fig. 3, the
6189
0.1
0.2
0.3
0.4
Fig. 2. Grain size–Γ relationship in HfO2 films. Line is provided as a guide to
the eye. It is evident that the grain size decreases significantly with the initial
oxygen content in the sputtering gas mixture and becomes more or less
constant for Γ ¼0.2–0.4.
film thickness or deposition rate is high when Γ ¼ 0.0. It is
evident that film thickness decreases with increasing oxygen
gas fraction in the total gas mixture. The reduction in film
thickness and/or deposition rate is the result of the formation of
higher order Hf-oxide compound. The reduction of the deposition
rate associated with the formation of Hf-oxide is believed to be a
direct result of the decreased sputter yield. Specifically, a decrease
in film thickness with increasing oxygen content in the gas
mixture indicates that the effective number of species ejected
from target surface and the effective number of particles reaching
and attaining the substrate decreases with increasing Γ values. It
can be seen that increasing oxygen concentration reduces the
energy that the particles attain to the substrate and their mobility
[32]. As such, it is more difficult for the sputtered species to
bombard the substrate and, thus, leads to a decreasing growth rate
that in turn reduces film thickness [32,33]. It must be pointed out
that, during sputtering process, the target species are subjected to
collisions with ambient gas molecules and other ejected atoms.
This behavior results in a partial loss of energy and direction on
their respective paths, which makes it more difficult to attain the
substrate [32–34]. The reactive sputter-gas, therefore, impedes the
mobility and trajectory of sputtered species which also accounts
for the grain size reduction. However, it is quite interesting to
compare the trend observed for HfO2 with GeO2, similar
materials with comparable band gaps. In GeO2, the increase in
deposition rate and film thickness with Γ was noted until it
reaches 0.25 and continuous to decrease thereafter [34].
The SEM images of HfO2 films grown at various Γ are
shown in Fig. 4. The effect of Γ on the surface morphology of
HfO2 films is remarkable (Fig. 4). The SEM images of HfO2
films grown at Γ ¼ 0.2–0.4 show the fine microstructure and
uniform distribution of dense particles spherical in shape.
However, at Γ ¼ 0.0, the formation of Hf metal films with a
different morphology can be clearly seen (Fig. 4a). As
discussed above, variation in growth rate due to increased
oxygen content in the reactive gas mixture accounts for the
reduced mobility of the sputtered species and, hence, the
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C.V. Ramana et al. / Ceramics International 41 (2015) 6187–6193
Г=0.1
Г=0.0
500nm
Г=0.2
500nm
Г=0.4
500nm
500nm
Fig. 4. SEM images of HfO2 films grown at various Γ. Morphology changes are significant for films grown with and without oxygen content while morphology
changes are not visible for Γ ¼ 0.2–0.4.
reduced grain size for films grown at Γ ¼ 0.2–0.4, compared to
that grown at lower Γ or without oxygen.
Optical constants of the HfO2 films were primarily probed
by SE, which measures the relative changes in the amplitude
and phase of the linearly polarized monochromatic incident
light upon oblique reflection from the sample surface. The
experimental parameters obtained by SE are the angles Ψ
(azimuth) and Δ (phase change), which are related to the
microstructure and optical properties, defined as follows [35–39]:
ρ ¼ Rp =Rs ¼ tan Ψ expðiΔÞ
ð2Þ
where Rp and Rs are the complex reflection coefficients of the light
polarized parallel and perpendicular to the plane of incidence,
respectively [30–33]. In general, the fundamental equation of
ellipsometry that relates the measurable with the accessible optical
information is as follows:
ρ ¼ tan ψ expðiΔÞ ¼ ρðN 0 ; N 1 ; N 2 ; L1 ; Φ0 ; λÞ
ð3Þ
where the middle term contains the measurable and the last term on
the right contains all the accessible parameters of the measurement,
namely, film thicknesses, optical properties, the wavelength of
light, and the angle of incidence [36]. The spectral dependences of
Ψ and Δ determined for HfO2 films on Si are shown in Fig. 5 for
representative samples. The curves obtained for HfO2 films
indicate a reasonable agreement between experimental and simulated data. The spectral dependences of ellipsometric parameters Ψ
(azimuth) and Δ (phase change) can be fitted with appropriate
models to extract film thickness and the optical constants i.e., the
refractive index (n) and extinction coefficient (k), based on the best
fit between experimental and simulated spectra [35,36]. In the
present case, the Levenberg–Marquardt regression algorithm was
Fig. 5. The spectral dependencies of Ψ and Δ for representative HfO2 films
grown at variable oxygen ratio. The experimental data obtained and modeling
curves are shown.
used for minimizing the mean-squared error (MSE)
2 (
)2 (
)2 3
n
Ψ
Ψ
Δ
Δ
1 X
exp
calc
exp
calc
4
5
MSE ¼
þ
2N M i¼1
σ exp
σ exp
Ψi
Δi
ð4Þ
C.V. Ramana et al. / Ceramics International 41 (2015) 6187–6193
where Ψexp, Ψcalc and Δexp, Δcalc are the measured (experimental)
and calculated ellipsometry functions, N is the number of measured
Ψ, Δ pairs, M is the number of fitted parameters in the optical
model and σ are standard deviations of the experimental data points
[31,35]. A noteworthy characteristic of the curves is the behavior
as a function of oxygen ratio during fabrication of HfO2 films.
In order to extract optimal data from SE experimental and
simulated measurements, the construction of a multilayer optical
model is essential. The model representation accounts for a
number of distinct layers with individual optical dispersions and
the interfaces between these layers are optical boundaries at
which light is refracted and reflected according to the Fresnel
relations. The dispersion relations of the optical constants of
HfO2 films are derived using a stack model shown in Fig. 6. The
model is composed of the Si substrate, interfacial SiO2 layer,
and HfO2 film; the surface roughness was also considered to
obtain precision during experimental fitting. Succeeding the
construction of the optical layer model, the HfO2 films were
modeled with a conventional Cauchy dispersion model, because
the films are transparent in the visible region the Cauchy model
is optimal. The Cauchy equation can be expressed approximately as a refractive index n as a function of wavelength λ:
nðλÞ ¼ a þ b=λ2 þ c=λ4
ð5Þ
where A, B, and C are the Cauchy coefficients and specific to
the material, A is the constant that dominates n(λ) for long
wavelengths, B controls the curvature of n(λ) in the middle of
the visible spectrum, and C influences n(λ) to a greater extent in
shorter wavelengths [31]. The significance of the optical model
of the samples presented in this work can be understood on the
fact that it was able to accurately account for the effect of
growth temperature on the HfO2 films and accurately reproduce
the X-ray reflectivity data as reported elsewhere [37]. Therefore,
the present work also confirms the validity of the optical model
since it reasonably fits and generates the optical spectra of HfO2
films grown under different oxygen fraction.
The dispersion profiles of n(λ) determined from SE data for
the HfO2 films are shown in Fig. 7. The n(λ) curves also
indicate a sharp increase at shorter wavelengths corresponding
to fundamental absorption of energy across the band gap. The
effect of oxygen pressure during deposition is evident in the n
(λ) profiles (Fig. 7). There is an increase in ‘n’ values with
Fig. 6. Stack (optical) model of the nanocrystalline HfO2 sample constructed
for ellipsometry data analysis. Various layers and surface/interface characteristics utilized are as shown in the model.
6191
Fig. 7. The index of refraction profiles of HfO2 films grown at various oxygen
ratio values. The effect of oxygen content in the reactive gas mixture is evident
in the dispersion curves; ‘n’ values increase with oxygen content.
increasing oxygen ratio. The metallic Hf films (grown with Ar
sputtering gas alone) are not of interest and eliminated from
the scope of the present study/discussion. Therefore, the data
are shown only for the films grown with Ar þ O2 (reactive gas
mixture). It is clearly evident in that data set that the n(λ)
profiles are distinctly different and can be divided into two
groups. As indicated by the arrow (Fig. 7), the ‘n’ values increase
with increasing oxygen ratio and achieve saturation for the film
set grown with an oxygen ratio of Z0.2. In order to further
understand the mechanistic aspects and associated effect of
oxygen content on the optical constants, the refractive index
variation at λ¼ 500 nm with oxygen content ratio in the reactive
gas mixture is analyzed. The ‘n’ increases from 1.86 to 2.12
(70.02) with increasing oxygen ratio from 0.0 to 0.2 and then
becomes more or less constant. Note that the ‘n’ value increases
sharply with O2 ratio initially and begins to depend less on the
oxygen content increase in the reactive gas mixture at higher
values. However, the values obtained for the films are slightly
less than the reported bulk value for HfO2 which can be attributed
to the method of growth; the sputtered films are well known to
have a higher defect density than bulk HfO2. While the effect of
oxygen content in the reactive gas mixture is evident from SE
analyses, most remarkable effect is the fact that ‘n’ increases
drastically at the beginning which accounts for most significant
chemical and structural changes in the films.
A simple model can be formulated to explain the effect of
oxygen pressure on the optical constants of nanocrystalline
HfO2 films. As it is evident from the SE results and analysis, the
optical quality of the HfO2 films depends on the oxygen content
in the reactive gas mixture and, hence, the film chemistry. The
XRD measurements demonstrated that the HfO2 films grown in
Ar sputtering gas alone i.e., without any oxygen in the reactive
gas mixture, were the Hf metal films and are not interesting for
discussion. The nanocrystalline HfO2 films crystallize in monoclinic structure. It is well known that the refractive index is
closely related to the physical properties, chemistry and density
of the films. The observed increase in ‘n’ values when HfO2
films grown at oxygen ration of Z0.2 can be attributed to the
complete oxidation leading to the formation of nanocrystalline
HfO2 films coupled with improved structural order. At lower
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C.V. Ramana et al. / Ceramics International 41 (2015) 6187–6193
values of oxygen ratio, films grown may not be completely
HfO2. While the films grown without oxygen were fully
metallic Hf, the chemical stoichiometry of the films undergoes
the transition from Hf to a mixture of Hf and HfOx and, then, to
fully stoichiometric HfO2 phase. Thus, based on SE data and
analyses, we believe that the films formed at an oxygen ratio
o0.2 were nonstoichiometric and contained a mixture of Hf
and HfOx phases, which results in lower n values. However, at
oxygen ratio Z0.2, the formation of monoclinic HfO2 phase
occurs. Perhaps, the oxygen ratio of 0.2 is critical to promote the
stoichiometric HfO2 growth. Further increase in oxygen content
in the range of 0.2–0.4 may not yield significant differences in
either crystal structure, phase, morphology or chemical stoichiometry. This results in the observed saturation of ‘n’ values
and/or dispersion profiles (Fig. 7) for HfO2 films grown with an
oxygen ration of Z0.2.
4. Conclusions
Nanocrystalline HfO2 thin films were deposited by sputterdeposition keeping the growth temperature fixed at 300 1C and
by varying the oxygen ratio in the reactive mixture in a range
of 0.0–0.4. The effect of oxygen ratio is significant on the
structure and optical constants of HfO2 films. HfO2 films grown
without oxygen in the reactive gas are Hf metal films in
hexagonal phase. The SE data analyses indicate clear transition
of the index of refraction profiles when oxygen ratio is set to 0.2
or higher. The observed trend of ‘n’ values suggest the evolution
from pure Hf to Hfþ HfOx (xo2) mixed phase and then finally
to HfO2 with increasing oxygen ratio from 0.0 to 0.4. The
transition from Hf to HfO2 is with a characteristic change in
refractive index (λ¼ 500 nm). The ‘n’ profiles of nanocrystalline
HfO2 films follow a Cauchy dispersion relationship.
Acknowledgments
MV, GAL and CVR acknowledges with pleasure the support
from National Science Foundation (NSF); NSF-PREM Grant ♯
DMR-1205302.
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