Material properties of Cd1 x Mg x O alloys synthesized by radio ...

Material properties of Cd1 x Mg x O alloys synthesized by radio frequency sputtering
Guibin Chen, K. M. Yu, L. A. Reichertz, and W. Walukiewicz
Citation: Applied Physics Letters 103, 041902 (2013); doi: 10.1063/1.4816326
View online: http://dx.doi.org/10.1063/1.4816326
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APPLIED PHYSICS LETTERS 103, 041902 (2013)
Material properties of Cd 12x Mg x O alloys synthesized by radio frequency
sputtering
Guibin Chen, 1,2 K. M. Yu, 1,a) L. A. Reichertz, 1,3 and W. Walukiewicz 1
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2 Department of Physics and Jiangsu Key Laboratory for Chemistry of Low Dimensional Materials,
Huaiyin Normal University, Jiangsu 223300, People’s Republic of China
3 RoseStreet Labs Energy, Phoenix, Arizona 85034, USA
(Received 27 February 2013; accepted 2 July 2013; published online 22 July 2013)
We have studied structural, electrical, and optical properties of sputter deposited ternary CdMgO
alloy thin films with total Mg concentration as high as 44%. We found that only a fraction
(50%–60%) of Mg is incorporated as substitutional Mg contributing to the modification of the
electronic structures of the alloys. The electrical and optical results of the Cd 1 x Mg x Oalloysare
analyzed in terms of a large upward shift of the conduction band edge with increasing Mg
concentration. With the increase of the intrinsic bandgap, appropriately doped Cd-rich CdMgO alloys
can be potentially useful as transparent conductors for photovoltaics. VC 2013 AIP Publishing LLC.
[http://dx.doi.org/10.1063/1.4816326]
Metal oxides have been extensively studied over the last
two decades. 1–6 The reason for the wide interest in these
materials is that they can be highly conductive and transparent
over a wide wavelength region at the same time. Wide gap
oxides such as In 2 O 3 :Sn (ITO) 7–10 or ZnO:Al (AZO) 11–13 are
widely used as transparent contacts on thin film solar cells.
An important limitation in the applications of these oxides is
that they have significant long wavelength absorption and
thus cannot be used for photovoltaics that utilize the infrared
part of the solar spectrum. The poor infrared transmittance of
these materials can be attributed to the relatively strong ionized
impurity scattering resulting in low mobility and large
free carrier absorption. In addition, due to the high electron
concentration in these materials (10 21 /cm 3 ) their plasma
reflection edge typically occurs at wavelengths

041902-2 Chen et al. Appl. Phys. Lett. 103, 041902 (2013)
FIG. 1. (220) x-ray diffraction peaks from a series of Cd 1 x Mg x O alloys
with different substitutional Mg concentrations.
substitution of Cd with Mg. The amount of substitutional
Mg in Cd 1 x Mg x O can be derived from the shift of the diffraction
peak using Vegaard’s law. Here we use the values
of 4.212 Å and 4.6958 Å for the lattice parameter of MgO
and CdO, respectively.
Figure 2(a) compares the Mg concentrations determined
by XRD with those measured by RBS. We note that RBS
measures the total Mg concentration whereas peak shifts in
XRD arise from lattice parameter change due to the substitution
of Cd with Mg. Since the electronic band structure is
affected by the composition of substitutional Mg, therefore,
in the following, the samples will be identified by the composition
obtained from the XRD. Figure 2(a) shows that a
significant fraction of the total Mg is incorporated on nonsubstitutional
sites, most likely in the forms of Mg interstitials
and Mg or MgO precipitates. In ZnMgO Mg interstitials
can increase the lattice parameter of the alloy and hence partially
compensate the reduction of the lattice parameter due
to Mg substitution. As a result, if significant amount of Mg
interstitials are present the XRD measurements may underestimate
the substitutional Mg. However, we found that as the
FIG. 2. (a) Dependence of the substitutional Mg concentration on the total
Mg concentration. (b) Grain size of the Cd 1 x Mg x O films as a function of
Mg concentration. The dashed line connecting the data is a polynomial fit to
the data.
FIG. 3. Carrier concentration (n) and mobility (l) as a function of Mg concentration,
x, in the Cd 1 x Mg x O films.
Mg composition increases, the electron concentration
decreases in the alloy (Fig. 3), and this does not support the
presence of significant amount of Mg interstitial donors.
This suggests that Mg interstitials are not the dominating
defects (

041902-3 Chen et al. Appl. Phys. Lett. 103, 041902 (2013)
FIG. 4. Composition dependence of the position of the conduction band
edge (solid line) and the Fermi level () of the Cd 1 x Mg x O samples with
respect to the vacuum level.
FIG. 5. (a) Room-temperature absorption spectra of Cd 1 x Mg x O films for
various magnesium concentration, x. (b) Dependence of the optical gap
E opt
g () and the intrinsic bandgap E g () on Mg concentration. The dashed
curve shows the fit to the bandgap energies using a bowing parameter
b ¼ 2.17 eV.
below the vacuum level. This constant level crosses the CBE
and enters the bandgap for x > 0.15. This behavior could be
explained assuming that the electrons in those undoped films
originate from native donor defects with the energy level
located at about 5.4 eV below the vacuum level. As the concentration
of electrons depends on the position of this level
relative to the CBE it varies from 2 10 20 cm 3 when the
Fermi energy is at E CBE þ 0.5 eV in CdO to low concentrations
when the Fermi energy falls well below CBE for larger
x values. Additionally, at higher compositions the electron
concentration is further reduced due to localization effects
arising from fluctuations of the alloy composition. This picture
appears to be consistent with the results of the mobility
measurements shown in Fig. 3. The decrease of the mobility
at low Mg concentration most likely originates from the
alloy disorder scattering although it is possible that it is further
reduced by scattering from the grain boundaries since as
it has been shown above the grain size is decreasing with
increasing Mg concentration.
The absorption coefficients a of films with different Mg
concentration are shown in Fig. 5(a). A clear blueshift in the
absorption edge is observed with increasing Mg concentration.
The absorption edge energy or optical gap, E opt
g ,of
Cd 1 x Mg x O films was obtained by linear extrapolations of
a 2 to the base line in the energy region. The optical gaps,
, as functions of x are shown as square symbols in Figure
5(b). However, because of large electron concentrations in
some of our samples these optical gaps are expected to be
higher than the intrinsic direct gaps E g . Several corrections
have to be taken into account to obtain E g (Ref. 6)
E opt
g
E g ¼ E opt
g DE F Hall ðnÞþDE Ie e ðnÞþDE Ie i ðnÞ; (1)
where the Burstein Moss shift 23,24 DE F-Hall is determined by
the position of the Fermi level in the conduction band and
can be calculated directly from the electron concentration
measured by Hall effect, DE Ie-e (n) and DE Ie-i (n) are bandgap
renormalizations due to electron-electron and electron-ion
interactions, respectively. The bandgap renormalizations
were calculated using the formulism in Berggren and
Sernelius 25 and Walukiewicz. 26 The nonparabolic dispersion
relation for the conduction band was derived from Kane’s
two band kp model. 27
The solid triangle symbols in Fig. 5(b) represent the
intrinsic direct gap of Cd 1 x Mg x O as a function of Mg concentration
x obtained from Eq. (1), using the conduction
band edge effective mass of 0.21m 0 . The bandgap energy
corrections become insignificant, and values of E opt
g and E g
are very similar for alloys with high Mg concentrations
where the electron concentration falls below 10 18 /cm 3 .
Fitting the composition dependence of this intrinsic bandgap
to the standard second order formula yields a bandgap bowing
parameter b ¼ 2.17 eV. This bowing parameter is reasonable
comparing to the linear bandgap dependence of Mg in
ZnMgO alloys. 28 Using the same formulism, a much larger
bandgap bowing parameter of 5.3 eV is obtained if the RBS
measured Mg contents are used.
An ideal transparent conductor on photovoltaic devices
that utilizes most of the solar spectrum requires highly conducting
materials (r > 5000 S/cm) with high transparency
from 340 to1300 nm. In our prior work on In and Ga doped
CdO we have achieved thin films with r 15 000 S/cm and
with excellent infrared transparency down to 0.8 eV
(1500 nm) but with too low UV absorption edge at 3.1 eV
(400 nm). 15 Here we show that alloying 10% of MgO with
CdO increases the intrinsic bandgap by 400 meV, resulting
in a desirable shift of the optical absorption edge to higher
energies. However, we note that without any intentional doping,
alloying of CdO with MgO results in a reduced electron
mobility and electron concentration resulting in a conductivity
of

041902-4 Chen et al. Appl. Phys. Lett. 103, 041902 (2013)
doping of Cd-rich Cd 1 x Mg x O alloys will be required to
determine if similar effects occur in these alloys too.
In summary, we have synthesized and characterized a
series of Cd 1 x Mg x O thin films with substitutional Mg x up
to 0.28 by radio frequency magnetron sputtering method. We
found a significant decrease in the grain size with increasing
Mg concentration. The reduction of the electron concentration
and mobility in these undoped samples is explained by a
rapid upward shift of the conduction band edge with increasing
Mg concentration. Cd-rich CdMgO alloys offer a potential,
as materials, for transparent conductors. We expect that
the electron concentration and mobility of Cd 1 x Mg x O can
be further increased by intentional doping with In or Ga.
Hence, further systematic studies of intentional doping and
post-growth annealing will be required to optimize this material
for PV applications.
This material was based upon work supported by the
Department of Energy through the Bay Area Photovoltaic
Consortium under Award No. DE-EE0004946. G.B.C.
acknowledges the support provided by the National Natural
Science Foundation of China (No. 11174101), Natural
Science Foundation of Jiangsu Province (NSFJS, Nos.
BK2010499, BK2011411, and HAG2011006).
This report was prepared as an account of work sponsored
by an agency of the United States Government. Neither
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or favoring by the United States Government or
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This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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