The growth techniques for Mg_xZn_(1−x)O thin films have advanced at a rapid pace in recent years, enabling the application of this material to a wide range of optical and electrical applications. In designing structures and optimizing device performances, it is crucial that the Mg content of the alloy be controllable and precisely determined. In this study, we have established laboratory-based methods to determine the Mg content of Mg_xZn_(1−x)O thin films grown on ZnO substrates, ranging from the solubility limit of x ∼ 0.4 to the dilute limit of x < 0.01. For the absolute determination of Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x = 0.14, while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based method to determine the Mg content, c-axis length is measured by x-ray diffraction and is well associated with Mg content. The interpolation enables the determination of Mg content to x = 0.023, where the peak from the ZnO substrate overlaps the Mg_xZn_(1−x)O peak in standard laboratory equipment, and thus limits quantitative determination. At dilute Mg contents below x = 0.023, the localized exciton peak energy of the Mg_xZn_(1−x)O films as measured by photoluminescence is found to show a linear Mg content dependence, which is well resolved from the free exciton peak of ZnO substrate down to x = 0.0043. Our results demonstrate that x-ray diffraction and photoluminescence in combination are appropriate methods to determine Mg content in a wide Mg range from x = 0.004 to 0.40 in a laboratory environment

Falson, J.

Kawasaki, M.

Kozuka, Y.

Makino, T.

Segawa, Y.

Tsukazaki, A.

English

Caltech Authors - Main

J. Appl. Phys. 112, 043515 (2012); https://doi.org/10.1063/1.4748306 112, 043515
© 2012 American Institute of Physics.
Precise calibration of Mg concentration
in MgxZn1−xO thin films grown on ZnO
substrates
Cite as: J. Appl. Phys. 112, 043515 (2012); https://doi.org/10.1063/1.4748306
Submitted: 10 July 2012 . Accepted: 25 July 2012 . Published Online: 29 August 2012
Y. Kozuka, J. Falson, Y. Segawa, T. Makino, A. Tsukazaki, and M. Kawasaki
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Precise calibration of Mg concentration in MgxZn12xO thin films grown
on ZnO substrates
Y. Kozuka,1,a) J. Falson,1 Y. Segawa,2 T. Makino,2 A. Tsukazaki,1 and M. Kawasaki1,2
1Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo,
Tokyo 113-8656, Japan
2Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG),
RIKEN-Advanced Science Institute, Wako 351-0198, Japan
(Received 10 July 2012; accepted 25 July 2012; published online 29 August 2012)
The growth techniques for MgxZn1xO thin films have advanced at a rapid pace in recent years,
enabling the application of this material to a wide range of optical and electrical applications. In
designing structures and optimizing device performances, it is crucial that the Mg content of the
alloy be controllable and precisely determined. In this study, we have established laboratory-based
methods to determine the Mg content of MgxZn1xO thin films grown on ZnO substrates, ranging
from the solubility limit of x 0.4 to the dilute limit of x< 0.01. For the absolute determination of
Mg content, Rutherford backscattering spectroscopy is used for the high Mg region above x¼ 0.14,
while secondary ion mass spectroscopy is employed to quantify low Mg content. As a lab-based
method to determine the Mg content, c-axis length is measured by x-ray diffraction and is well
associated with Mg content. The interpolation enables the determination of Mg content to
x¼ 0.023, where the peak from the ZnO substrate overlaps the MgxZn1xO peak in standard
laboratory equipment, and thus limits quantitative determination. At dilute Mg contents below
x¼ 0.023, the localized exciton peak energy of the MgxZn1xO films as measured by
photoluminescence is found to show a linear Mg content dependence, which is well resolved from
the free exciton peak of ZnO substrate down to x¼ 0.0043. Our results demonstrate that x-ray
diffraction and photoluminescence in combination are appropriate methods to determine Mg
content in a wide Mg range from x¼ 0.004 to 0.40 in a laboratory environment. VC 2012 American
Institute of Physics. [http://dx.doi.org/10.1063/1.4748306]
I. INTRODUCTION
Epitaxial oxide thin films and their interfaces have been
the subject of intensive research for decades in the explora-
tion of material properties beyond conventional semiconduc-
tors.1 Among the oxides, ZnO is one of the most promising
materials for versatile photonic and electronic applications. For
instance, high-intensity ultraviolet light emitting diodes in
ZnO-based p-n junctions have been realized in this material
owing to the rapid development of ZnO thin film growth
technique.2 In the same stream, p-n junctions combined with a
microcavity explore the possibility of realizing room-
temperature exciton-polariton lasing, where coherent light is
expected to emit from exciton-polariton Bose-Einstein conden-
sates under extremely low injection current.3 In a similar heter-
ostructure, a high-mobility two-dimensional electron gas
(2DEG) has been found to form at the interface of MgxZn1xO
and ZnO. Since such a 2DEG is accumulated due to the polar-
ization mismatch between MgxZn1xO and ZnO and without
the aid of intentional doping,4 the electron mobility is
extremely high, exceeding 700 000 cm2 V1 s1 at x  0.01,
which is comparable to the mobility of other well-known two-
dimensional systems, such as AlGaAs/GaAs and SiGe/Si.5
These functional physical properties of ZnO are as a
result of its large band gap (3.37 eV) and exciton binding
energy (60meV), as well as spontaneous polarization along
c-axis due to inversion-asymmetric Wurtzite structure.6
Importantly, these physical parameters may be tuned by the
substitution of Zn ions in the host lattice by isovalent Mg ions,
to form the ternary alloy of MgxZn1xO.
7 In a practical device
utilizing these physical properties of ZnO, MgxZn1xO thin
films should be pseudomorphically grown on single-crystal
ZnO substrates to maintain high crystalline quality. In such a
system, it is crucial that the extent of Mg in the alloy be con-
trollable so that practical device design may be achieved.
However, at present, such systematic work on the methods to
determine Mg composition x in MgxZn1xO thin films grown
on ZnO substrates have not been well established.
In this study, we have investigated various methods to
determine the Mg composition in MgxZn1xO thin films
grown on ZnO substrates. In such a heterostructure, it is not
possible to use methods that are unable to separate the sig-
nals from the film and the substrate, such as inductively
coupled plasma atomic emission spectroscopy and electron
probe microanalysis. In order to determine x, therefore, we
employ Rutherford backscattering spectroscopy (RBS) for
high Mg concentration, and secondary ion mass spectros-
copy (SIMS) for those with low Mg concentration. Analyti-
cally quantified x values in the MgxZn1xO films are then
used as the basis for calibration curves for the values of
c-axis length deduced by x-ray diffraction (XRD)8 and
localized exciton (LE) emission energy revealed by photolu-
minescence (PL).9 With using an almost linear relationship
of these values with x, one can determine Mg content x ina)Electronic mail: kozuka@ap.t.u-tokyo.ac.jp.
0021-8979/2012/112(4)/043515/5/$30.00 VC 2012 American Institute of Physics112, 043515-1
JOURNAL OF APPLIED PHYSICS 112, 043515 (2012)
MgxZn1xO thin films on ZnO substrate with using standard
laboratory techniques such as XRD and PL.
II. EXPERIMENTAL
Samples used in this study are listed in Table I with the
Mg concentration determined as discussed below. The
MgxZn1xO thin films were grown on Zn-polar ZnO sub-
strates (Tokyo Denpa Co.)10 by molecular beam epitaxy at a
substrate temperature of 750 C using 7N Zn and 6N Mg
sources. Distilled pure ozone (Meidensya Co.) was utilized
as the oxygen source due to its extremely low impurity
level.5 The typical film thickness was 200–700 nm. The
XRD measurement was carried out with a lab-based x-ray
source with four-bounce Ge (220) monochrometer (X’Pert
MRD, Panalytical Co., and SmartLab, Rigaku Co.). For PL
measurement, we used a He-Cd laser (325 nm) for x 0.10
and a Nd:YVO4 laser (266 nm) for x 0.14, as a result of the
shifting band-gap energy of the MgxZn1 xO layer. The light
intensity was 20 mW/cm2 at the sample surface.
III. RESULTS AND DISCUSSIONS
A. Surface morphology
In order to eliminate the possibility of structural defects
affecting lattice constant such as granular structure, the sur-
face morphology was examined by an atomic force micros-
copy as shown in Figs. 1(a)–1(c) for the representative
samples. The surfaces exhibit a step-and-terrace structure
with the root-mean-square roughness of 0.1 nm, regardless
of Mg content investigated, which ensures the variation of
the lattice constant purely originates from the alloying of
MgO in ZnO.
B. High Mg concentration
First, RBS was utilized in determining the Mg content
of the four most concentrated samples of this study. This
technique is known to provide the absolute concentration
without the need of calibration by standard samples. Figure 2
shows an example of the RBS spectra for a 200 nm-thick
MgxZn1xO film grown on ZnO substrate with the best-fit
simulation curve of x¼ 0.40. The insets indicate the magni-
fied spectra for Zn and Mg signals from MgxZn1xO layer to-
gether with the simulation curves of x¼ 0.406 0.04 and
x¼ 0.406 0.10. It is apparent from the data that a smaller
TABLE I. MgxZn1xO/ZnO samples used in this study. The x values and
their measurement methods used to determine x are shown. x values in
brackets have not been analytically quantified but interpolated through the
x-ray diffraction calibration curve formulated.
Methods to determine Mg
concentration
Sample No. x in MgxZn1xO Absolute value Calibration
79 0.0042 SIMS PL
66 0.0056 SIMS PL
77 0.0073 SIMS PL
74 0.010 SIMS PL
76 0.011 SIMS PL
81 0.015 SIMS PL
28 (0.023) XRD, PL
23 (0.044) XRD, PL
22 (0.056) XRD, PL
14 (0.069) XRD, PL
12 (0.090) XRD, PL
11 (0.10) XRD, PL
153 0.14 RBS XRD, PL
147 0.20 RBS XRD, PL
152 0.27 RBS XRD, PL
168 0.40 RBS XRD, PL
FIG. 1. Atomic force microscopy images of the
surface morphology of MgxZn1xO films for (a)
x¼ 0.010, (b) x¼ 0.056, and (c) x¼ 0.27.
FIG. 2. Rutherford backscattering spectrum for a 200 nm-thick MgxZn1xO
film on ZnO substrate. Dots are experimental data, and the solid curves
indicate the simulation. The onsets of the signals from Zn and Mg, and O
are also indicated by arrows with the indication of the host layers in the
brackets. Top and bottom insets are the magnifications around Zn and Mg
signals, respectively, from the MgxZn1xO layer together with the best-fit
simulation curves, for x¼ 0.40 (bold), x¼ 0.406 0.04, and x¼ 0.406 0.10.
043515-2 Kozuka et al. J. Appl. Phys. 112, 043515 (2012)
fitting error is achieved when x is estimated from Zn signal
(Dx¼60.01) than from Mg signal (Dx¼60.04). Below, we
refer to this x value estimated from RBS as xRBS.
Knowing this absolute value of xRBS, we then employed
laboratory-based XRD, to correlate the measured c-axis
length of the MgxZn1xO layer with the analytically deter-
mined Mg content. Figure 3 shows h-2h diffraction patterns
around the ZnO (0004) peak. For the high Mg region, a sec-
ond peak, corresponding to the MgxZn1xO layer is clearly
observed together with Laue fringes, which reflect the
thickness of the film. The c-axis length difference
(Dc) between the ZnO substrate (c¼ 520.4 pm) and the
MgxZn1xO thin film is plotted as a function of xRBS in
Fig. 4 for the four samples. The best-fit line was found to be
Dc (pm)¼0.069 x. This relation is significantly different
from that for relaxed MgxZn1xO films grown on Al2O3 sub-
strates,7 as a result of the films being under epitaxial strain
for this study; the in-plane lattice is coherently connected
with that of ZnO substrate and is extended in comparison
with the strain-free state. Here, we note that the present
results also deviate from those reported by Nishimoto et al.,8
where MgxZn1xO thin films are pseudomorphically grown
on ZnO (0001) substrates as in the present study. In the pre-
vious work, Auger electron spectroscopy was used to probe
the absolute value and depth profile of the Mg content of
films. However, the calibration curve used for such quantifi-
cation was originally formulated from MgxZn1xO films
grown on Al2O3 substrates and hence brings into question
possible errors in the original calibration due to differences
in the sticking coefficient of Mg for films grown on ZnO as
opposed to Al2O3.
11 We speculate that this gave significant
error in the previous data and we insist to revise the relation
by the data given in this paper. By using the relation shown
in Fig. 4, interpolation gives estimates of Mg concentration,
which we refer to as xXRD, down to xXRD¼ 0.023. This value
is as a result of the resolution limit of a regular lab-based
monochrometer in XRD equipment, where ultimately the
peak of the MgxZn1xO and ZnO film cannot be separated,
as displayed in Fig. 3, for x¼ 0.011 (determined by SIMS as
explained below). This limit may also be affected by the
thickness or the quality of the film, which broadens the
diffraction peaks.
C. Low Mg concentration
Although we have thus demonstrated that the Mg con-
tent can be determined from XRD, this is not applicable
below x  0.02 as the XRD peak from MgxZn1xO layer is
not clearly resolved from the peak of the ZnO substrate.
RBS is not also applicable for the absolute determination of
Mg concentration in this range because of the large error of
Dx 0.01. As another physical parameter which is depend-
ent on Mg content, we focused on the exciton energy
observed by PL, which is conventionally used to determine
Al composition in (Al,Ga)As thin films grown on GaAs sub-
strate.12 For absolute calibration of the Mg content, we uti-
lized SIMS measurements that were calibrated with a Mg
ion-implemented standard sample. A series of depth profiles
of the SIMS spectra are shown in Fig. 5(a), with the depth
normalized by the thicknesses of the films. Due to finite
inhomogeneity of Mg concentration along the depth, the
peak of the histogram in Mg concentration is taken as a rep-
resentative value as shown in Fig. 5(b).
Knowing the absolute Mg content of the dilute films, the
energy dependence of the localized exciton (LE) lumines-
cence on Mg concentration was investigated by PL for all
samples at 100K and 10K as shown in Figs. 6(a) and 6(b),
FIG. 3. h-2h x-ray diffraction around ZnO (0004) peak. The asterisks
indicate the peaks corresponding to MgxZn1xO layers. The methods to
determine x depend on Mg concentration ranges as indicated.
FIG. 4. c-axis length difference (Dc) between the ZnO substrate and the
MgxZn1xO layer estimated from XRD as a function of Mg content deter-
mined from RBS. Previous results using ZnO substrate (Nishimoto et al.)8
and Al2O3 substrate (Ohtomo et al.)
7 are also shown for comparison.
043515-3 Kozuka et al. J. Appl. Phys. 112, 043515 (2012)
respectively. In the PL spectra, a peak corresponding to free
excitons (FE) is clearly visible in ZnO (x¼ 0) at T¼ 100K,
while only weak intensity from FE was observed at T¼ 10K
as indicated by the asterisks. The intense peaks at lower
energies originate from bound excitons, of which were
assigned by observing the temperature dependence of such
peaks (not shown). In the case of MgxZn1xO films, the LE
peak (indicated by asterisks in Fig. 6) appeared in addition to
the FE peak from the ZnO substrate. At 10K, additional
broad peaks appeared at lower energies than the LE peaks,
the origin of which is not clear at present because of its
nonsystematic dependence on Mg content.
The energy difference (DE) between LE emission
energy from MgxZn1xO layers and FE emission energy
from ZnO as a function of x is plotted on a log-log scale
(Fig. 7(a)). The overall feature indicates that DE has a quite
good linear dependence on Mg content. In order to see the
applicable range of the linear fitting, the relation is plotted in
Fig. 7(b) on a linear scale, which indicates a nontrivial devia-
tion of the LE exciton energies toward lower energy for high
Mg content films from the extrapolated fitting line. This
tendency is interpreted as stronger localization with higher
Mg concentration. Thus, c-axis length from XRD is more
appropriate to estimate Mg content at high Mg region due to
its relative insensitivity to localized Mg concentration fluctu-
ations compared to that of PL peak energies. Therefore, we
provide a fitted relation of DE (eV)¼ 2.2 x, only valid for
that of x 0.023. This LE energy dependence on Mg content
is similar to previous results of MgxZn1xO films grown on
Al2O3 substrate.
9 The error bars of x in Fig. 7 reflect the
FIG. 6. Photoluminescence spectra measured (a) at 100K and (b) at 10K for
representative MgxZn1xO thin films. The asterisks correspond to emission
peaks from FE for undoped ZnO and these from LE for MgxZn1xO layers.
The method to determine x is also indicated (see Table I).
FIG. 5. (a) Depth profile of Mg content measured by SIMS and (b) its histo-
gram for low Mg samples. The dashed curves in (b) are the Gaussian fits to
the histogram data.
FIG. 7. (a) Log-log and (b) linear plots of the energy difference (DE)
between LE emission from MgxZn1xO films and FE emission from ZnO as
a function of x at 100K and 10K. The FE energy of ZnO is 3.377 eV at 10K
and 3.368 eV at 100K. The methods to determine x are also indicated (see
Table I). The dashed lines are the fitting for the data below x¼ 0.015 at
100K. The error bars in x indicate the full width at half maximum in the his-
togram of Fig. 5(b), while those in DE are the full width at half maximum of
LE peaks at 100K.
043515-4 Kozuka et al. J. Appl. Phys. 112, 043515 (2012)
inhomogeneity of Mg concentration shown in the histogram
of Fig. 5(b), which is negligible in our discussion.
IV. CONCLUSION
In summary, we have established comprehensive means
to determine the Mg content in MgxZn1xO films grown on
ZnO substrates by using standard lab-based XRD and PL
techniques. For the high Mg content region of x 0.14, the
c-axis length is estimated by XRD and is well associated
with x, where x is calibrated by RBS. On the other hand, for
the dilute region, a linear dependence between x and LE
exciton energy was obtained below x 0.015, where x is
calibrated by SIMS. Extrapolation of x using these two meth-
ods gives smooth connection of x values in the range of
0.03 x 0.10. This result can be widely used to determine
Mg content in MgxZn1xO films on ZnO substrates using
XRD and PL in combination, depending on the Mg concen-
tration range, and may form the infrastructure for continued
research for the application of this promising material.
ACKNOWLEDGMENTS
This work was partly supported by “Funding Program
for World-Leading Innovative R&D on Science and Tech-
nology (FIRST)” Program from the Japan Society for the
Promotion of Science (JSPS) initiated by the Council for Sci-
ence and Technology Policy.
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Precise calibration of Mg concentration in Mg_xZn_(1−x)O thin films grown on ZnO substrates

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Precise calibration of Mg concentration in Mg_xZn_(1−x)O thin films grown on ZnO substrates

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