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Characterization of Thin ZnO Films by Vacuum Ultra-Violet Reflectometry


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ZnO has a huge potential and is already a crucial material in a range of key technologies from photovoltaics to opto and printed electronics. ZnO is being characterized by versatile metrologies to reveal electrical, optical, structural and other parameters with the aim of process optimization for best device performance. The aim of the present work is to reveal the capabilities of vacuum ultra-violet (VUV) reflectometry for the characterization of ZnO films of nominally 50 nm, doped by Ga and In. Optical metrologies have already shown to be able to sensitively measure the gap energy, the exciton strength, the density, the surface nanoroughness and a range of technologically important structural and material parameters. It has also been shown that these optical properties closely correlate with the most important electrical properties like the carrier density and hence the specific resistance of the film. We show that VUV reflectometry is a highly sensitive optical method that is capable of the characterization of crucial film properties. Our results have been cross-checked by reference methods such as ellipsometry and X-ray fluorescence.
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Mater. Res. Soc. Symp. Proc. Vol. 1494 © 2012 Materials Research Society
DOI: 10.1557/opl.2012.
Characterization of Thin ZnO Films by Vacuum Ultra-Violet Reflectometry
T. Gumprecht1,2, P. Petrik1,3, G. Roeder1, M. Schellenberger1, L. Pfitzner1, B. Pollakowski4, B.
1Fraunhofer Institute for Integrated Systems and Device Technology (IISB), Schottkystrasse 10,
91058 Erlangen, Germany
2Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Strasse 9,
91052 Erlangen, Germany
3Institute for Technical Physics & Materials Science (MFA), Research Centre for Natural
Sciences, Konkoly Thege u. 29-33, 1121 Budapest, Hungary
4Physikalisch-Technische Bundesanstalt (PTB), Abbestr. 2-12, 10587 Berlin, Germany
ZnO has a huge potential and is already a crucial material in a range of key technologies
from photovoltaics to opto and printed electronics. ZnO is being characterized by versatile
metrologies to reveal electrical, optical, structural and other parameters with the aim of process
optimization for best device performance. The aim of the present work is to reveal the
capabilities of vacuum ultra-violet (VUV) reflectometry for the characterization of ZnO films of
nominally 50 nm, doped by Ga and In. Optical metrologies have already shown to be able to
sensitively measure the gap energy, the exciton strength, the density, the surface nanoroughness
and a range of technologically important structural and material parameters. It has also been
shown that these optical properties closely correlate with the most important electrical properties
like the carrier density and hence the specific resistance of the film. We show that VUV
reflectometry is a highly sensitive optical method that is capable of the characterization of crucial
film properties. Our results have been cross-checked by reference methods such as ellipsometry
and X-ray fluorescence.
Key words:
Zink oxide, vacuum ultra-violet reflectometry, spectroscopic ellipsometry, atomic layer
ZnO is a key material in a range of optoelectronic applications, and has promising
properties for numerous other applications. Therefore, it continues to be intensively studied in
the recent years [1]. ZnO layers can be prepared by different methods including sputtering [2],
atomic layer deposition (ALD) [3], pulsed laser deposition [4, 5, 6], spin coating from
nanoparticulates [7], or spray pyrolysis [8]. Highly sensitive optical methods such as
ellipsometry are frequently applied for the quick and non-destructive determination of a range of
material and structural parameters such as thickness, surface nanoroughness, interface quality,
density, homogeneity, band gap and exciton strength. These methods make indirectly also
possible the determination of electrical properties.
The aim of the present work is the investigation of optical modeling of ZnO layers using
a commercial vacuum ultra-violet (VUV) reflectometer, and the cross-checking of the result
using reference metrologies. This study is part of a European project, intended to be a step
towards the establishment of validated reference methodologies for a reliable characterization of
key optoelectronic materials (IND07, "Metrology for the manufacturing of thin films") in the
European Metrology Research Program of EURAMET. Furthermore, our investigations aim for
the development of reference samples with controlled defect concentration and morphology or
methods for elemental depth profiling [9].
GaInZnO (GIZO) samples with a nominal thickness of 50 nm were prepared by dual-
target sputtering on oxidized (10 nm SiO2) single-crystalline silicon wafers used for
reflectometric measurements in comparison to ellipsometry and X-ray. A summary of the sample
properties is shown in Table I. The Ga amount was measured by X-ray fluorescence (XRF)
Table I. Parameters of numbering and preparation of samples investigated in this study. The Ga
content of sample VUV 3 is estimated by quadratic interpolation using the other three points
measured by XPS.
Not annealed Annealed Ga (%)
VUV 1 VUV 9 0.00
VUV 3 VUV 11 0.09
VUV 5 VUV 13 0.19
VUV 7 VUV 15 0.37
Reference 10 nm SiO2 layer, not annealed
Reflectometric measurements were performed with a METROSOL VUV-7000
spectroscopic reflectometer (VUV-R) in the wavelength range of 120 nm to 800 nm with a spot
size of 35 μm by 35 μm. A small spot size can be important to investigate lateral inhomogeneity.
The reference measurements of a single point on the samples were performed using a SOPRA
SE5 multi-channel rotating polarizer spectroscopic Ellipsometer (SE) in the wavelength range of
193 nm to 1690 nm using a spot size of about 4 mm.
The XRF measurements on the Ga- and In- doped Zinc oxides have been carried out at a
four-crystal monochromator (FCM) beamline in the PTB laboratory at the synchrotron radiation
facility BESSY II. This beamline provides monochromatic radiation from a bending magnet in
the energy range between 1.75 eV to 11 keV.
For the measurement two different recipes are used. The first recipe was performed with
a 30 point grid in a step size of 300 μm by 300 μm to measure the uniformity of the layer within
a locally limited area. A repeated measurement of 30 points of a single location on the layer was
used to get an information of a present surface contamination layer [10,11,12]. A surface
contamination layer will affect the measurement results, furthermore the layer will change the
optical properties and will lead to a misleading calculation.
As mentioned above, a possible contamination layer would influence the reflectance
significantly [10]. The spectra of each layer for the static measurement were analyzed at a
wavelength of 130 nm. An absorbing contamination layer changes the intensity of the reflected
light from the sample and this intensity behavior can be seen at the given wavelength. For the
ZnO samples no significant changes in the intensities could be detected. This behavior is plotted
in Fig. 1 in comparison with a thin SiO2 layer as an example for the change in the intensity due
to a VUV light contamination removal [10]. For the analysis of the ZnO samples we decided to
use the third repeated measurement point.
Figure 1. Selection of the measured spectra for analysis by means of a change in the intensity
at a wavelength of 130 nm in comparison to a thin SiO2 as an example for a visible influence
of a contamination removed due to the VUV light exposure
The typical spectra measured by VUV reflectometry and ellipsometry on the ZnO
samples are plotted in Fig. 2. Visible changes in the spectral region of the band gap can be seen
for both measurement methods. Furthermore, there are also changes in the reflectometric spectra
above 4.5 eV, which is not covered by standard spectroscopic hardware. The ellipsometric
measurements were used to calculate the film thicknesses of the different layers, because using
the reflectometric principle it is possible to calculate only one sample parameter (refractive
index, extinction coefficient or thickness). The transparent and opaque photon energy ranges can
clearly be distinguished by the interference oscillations characteristic to the transparent range [9]
in the ellipsometric measurement. Hence, a simple Cauchy dispersion (n=A+B/λ2+C/λ4, where n
denotes the refractive index, and A, B and C are the Cauchy parameters) using an optical model
of c-Si/SiO2/ZnO, whereas the refractive index of ZnO can be described by the Cauchy model,
was used to calculate the layer thickness and surface nanoroughness from the ellipsometric data.
The SiO2 layer thickness was also measured on the reference sample by ellipsometry and a
thickness of 11.5 nm was determined.
Figure 2. Measured reflectometric (static point 33) and ellipsometric Ψ and Δ spectra for
the as deposited and annealed ZnO samples. The rectangles show the most relevant
A summary of the sample properties measured by ellipsometry and reflectometry is
compiled in Table II. The Cauchy parameters A, B and the layer thickness are used as start
values for the regression to calculate the sample properties after the reflectometric
measurements. The Cauchy parameter C was fixed to zero for a more stable analysis. A good
correlation between the ellipsometric and reflectometric measurements was revealed. In Table III
the resulting values are summarized. The thickness and nanoroughness of the ZnO layers
determined by reflectometry are to a small extent but systematically thinner compared to the
ellipsometer measurements. The optical properties show also a small deviation. A possible
explanation could be an offset of the measurement location on the sample as well as the
difference of the spot size between methods. The spot size of the reflectometric system is much
smaller than the spot of the ellipsometer.
Table II. Reference data of the ZnO samples measured by spectroscopic ellipsometry, dZnO and
dr denote the thicknesses of the ZnO layer and the surface nanoroughness, respectively.
Nr. A B
n dZnO
VUV1 1.934 0.038 2.030 50.7 9.2
VUV5 1.933 0.035 2.026 51.6 8.4
VUV7 1.932 0.038 2.033 46.6 8.9
VUV9 1.922 0.038 2.017 50.7 8.0
VUV13 1.916 0.040 2.018 51.3 8.0
VUV15 1.926 0.041 2.032 46.3 8.1
Table III. Data of the ZnO samples measured by VUV reflectometry, dZnO and dr denote the
thicknesses of the ZnO layer and the surface nanoroughness, respectively.
Nr. A B
n dZnO
VUV1 1.897 0.040 1.941 49.1 8.4
VUV5 1.824 0.042 1.961 47.6 6.9
VUV7 1.901 0.036 1.997 43.6 5.6
VUV9 1.851 0.037 1.968 49.7 7.7
VUV13 1.869 0.045 1.943 49.3 7.6
VUV15 1.831 0.045 1.946 45.6 6.7
The last step of our investigations was the calculation of the gap energies in comparison
of both methods. A Tauc-Lorentz model with one oscillator was used and the value of the layer
thickness was fixed to the measured thickness for each method. The analysis of reflectometry
was performed in the photon energy range from 2.5 to 3.1 eV, because the decaying excitonic
line shape at higher photonic energies can not be described by the Tauc-Lorentz model. Both
ellipsometry and reflectometry show a systematic increase of Eg with annealing, though with a
slight offset (Table IV). The reason of the offset can partly be explained similar to that for the
results of Tables II and III. Furthermore, the different implementation of the Tauc-Lorentz
equation by two equipment manufacturers (Woollam and Metrosol) cannot be ruled out.
Table IV. Gap energies in eV calculated using the Tauc-Lorentz parameterization measured by
spectroscopic ellipsometry and vacuum ultra-violet reflectometry.
As deposited Annealed
2.95 3.02
2.87 2.88
2.82 2.85
2.63 2.75
2.34 2.47
2.41 2.59
In this study it has been shown that thin ZnO films can be characterized by vacuum ultra-
violet reflectometry with results in agreement with ellipsometry. The effect of small changes in
the optical film properties can be traced by optical methods. The results show that the effects of a
surface nanoroughness must be taken into account during the optical modeling in order to
determine correct layer thicknesses and optical properties. Hence, because of the smaller amount
of directly measured information by reflectometry, the layer thickness must be measured with
ellipsometry to have a starting value for the reflectometric analysis. Using this careful modeling
approach, the reflectometric results show a good correlation with the ellipsometric ones. The
band gap energy (which is consistent with decreasing specific resistance [13]) increase as a result
of annealing and decrease with increasing Ga content. The refractive index decreases with
annealing and increases with increasing Ga content. The surface nanoroughness was slightly but
systematically smaller on the annealed samples. Finally, comparable physical properties between
ellipsometry and reflectometry could be found.
Support from the European Community's Seventh Framework Program, European Metrology
Research Program (EMRP), ERA-NET Plus, under Grant Agreement No. 217257 as well as
from OTKA grant Nr. K81842 is greatly acknowledged.
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