Structural and physical properties of Mg-doped

Vacuum 82 (2008) 1321–1324
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Vacuum
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Structural and physical properties of Mg-doped CuAlO2 thin films
Guobo Dong a, Ming Zhang a, *, Wei Lan a, b, Peiming Dong a, Hui Yan a
a
b
The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China
Department of Physics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 September 2007
Received in revised form 31 March 2008
Accepted 4 April 2008
The CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin films were successfully deposited on quartz substrate
by using the RF magnetron sputtering technique. XRD patterns indicate that the delafossite structure
could be guaranteed for all CuAl1xMgxO2 films. The conductivity measured at room temperature for
CuAl0.98Mg0.02O2 film is three orders of magnitude higher than that of undoped CuAlO2 film and the band
gaps of CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin films decrease with the increase of the doping
concentration, which is related to the formation of impurity energy levels with increasing the doping
concentration.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
CuAlO2 films
Doping concentrations
Band gap
Conductivity
1. Introduction
Transparent conducting oxide (TCO) films are being widely
applied in liquid crystal displays, organic light-emitting diodes,
and solar cells, etc [1–3]. It is well known that TCO films usually are
n-type (electron) semiconductors, no p-type (hole) TCO films were
found until the report of the delafossite-type CuAlO2 prepared by
pulse laser deposition [4]. The discovery of p-type TCO film makes
it possible to fabricate transparent oxide optoelectronic devices
such as transparent p–n junction diodes and transistors using an
appropriate combination of p- and n-type TCO films complementary of Si-based electronics. However, the conductivity of CuAlO2
film was 3–4 orders of magnitude lower than that of more
commonly used n-type TCO like indium-tin oxide. Therefore, the
improvement of electrical property of CuAlO2 film seems a very
exigent and necessary for practical applications. Up to now it is
already known that excess oxygen [4,5] and introduction of some
divalent species [6–8] into the delafossite structure could effectively increase the electrical conductivity of the material. Although
some doped CuMO2 (M ¼ 3d transition trivalent cation) films, i.e.,
CuSc1xMgxO2þy [6], CuCr1xMgxO2 [7], CuY1xCaxO2 [8] films were
already reported and there are theoretical studies that suggest an
improved p-type TCO properties upon Mg doping of CuAlO2 [9,10],
few experimental reports on the improvement of conductivity for
CuAlO2 thin film by doping with divalent cation were found.
* Corresponding author. Tel.: þ86 10 67392733; fax: þ86 10 67392412.
E-mail address: [email protected] (M. Zhang).
0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.vacuum.2008.04.010
In this article, the CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05) thin
films were successfully deposited on quartz substrate by using the
radio frequency (RF) magnetron sputtering technique. The crystal
structures, optical and electrical properties of the deposited films
with different doping concentrations were investigated.
2. Experimental
Polycrystalline targets of CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and
0.05) used for sputtering were synthesized heating a stoichiometric
mixture of Cu2O (99%), Al(OH)3 (99.9%) and MgO (99.9%) at 1373 K
for 10 h [11]. Then the CuAl1xMgxO2 (x ¼ 0, 0.01, 0.02 and 0.05)
powders with the pure delafossite structure were pressed within
the Aluminums holder as the sputtering target. The RF magnetron
sputtering system used in this work is shown in Fig. 1. A mixture of
Ar (4N) and O2 (4N) with a ratio of 3:2 was adopted as the sputtering gas. The substrates were cleaned ultrasonically using
methylbenzene, acetone and ethanol in turn and then fastened to
a heated substrate holder, the distance between substrate and
target was 70 mm. Prior to deposition, the chamber was pumped
down to 4 103 Pa and the target was pre-sputtered for 30 min to
remove contamination from the surface by rotary pump and
molecular pump. After the deposition, all samples were in situ
annealed in the vacuum chamber at 773 K for 1 h and then
annealed under the N2 atmosphere at 1173 K for 5 h. The working
pressure, the sputtering power and the substrate temperature were
around 1.0 Pa, 100 W, and 773 K, respectively. The film thickness
was around 180, 160, 210, and 240 nm for the corresponding
CuAl1xMgxO2 films with x ¼ 0, 0.01, 0.02 and 0.05, respectively.
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G. Dong et al. / Vacuum 82 (2008) 1321–1324
Fig. 2. XRD patterns of Mg-doped CuAlO2 films with different doping concentrations
(0, 1, 2, 3 and 5 at.%). The hump around 22 and the peak around 26.7 are due to the
quartz substrate.
Fig. 1. The schematic of the sputtering device.
The structural analysis of Cu–Al–O thin films was carried out by
a BRUKER-AXS D8 X-ray diffraction (Cu Ka, l ¼ 0.154056 nm). The
thickness was determined by the surface roughness automatic
tester (Surfcom 480A). The optical transmittance (T) and the reflectance (R) of the films were measured using UV-3101PC UV–vis
double-beam spectrophotometer in the wavelength (l) range 200–
800 nm. Electrical conductivity was performed by the four-probe
method using the automatic measurement system assembled by
Agilent E5273 and Lakeshore 340.
3. Results and discussion
3.1. The crystal structure of Mg-doped CuAlO2 thin films
Fig. 2 shows the X-ray diffraction (XRD) patterns of the Mgdoped CuAlO2 thin films. All peaks are identified to the reflections
of CuAlO2 with the rhombohedral crystal structure (PDF 35-1401)
and no diffraction corresponding to the second phase is obviously
presented, which clearly indicates that the doping could not induce
any impurity phases except for CuAlO2 films with the delafossite
structure. Although the weak signal of the diffraction, if the results
of the Rutherford Backscattering Spectroscopy measurement (not
shown here) are taken into account, the delafossite structure of
CuAlO2 films might be confirmed. Furthermore, the crystalline
quality of the films declines when the Mg concentration increases
up to 5 at.%, Selim and Youssef reported there was a marked
decrease in the crystallinity of 2% Na doped CuAlO2 after annealing
at 1000 C [12]. Because Mg2þ is chemically close to Naþ, it is the
possibility that substituting Mg2þ for Al3þ results in the decomposition of CuAlO2, which decreases the crystal quality of films.
The hump about 22 and the peak located at 26.7 could be ascribed
to crystalline quartz possibly formed during the annealing process.
As seen from the inset of Fig. 2, the (101) peaks for all films shift
toward lower angles with an increase of the Mg content, which
reflects the fact that the CuAlO2 lattice expands for all doped
samples. According to the data of the ion radii of Mg2þ, Cuþ and
Al3þ (Mg2þ: 0.066 nm, Cuþ: 0.095 nm and Al3þ: 0.055 nm) and the
fact of the cell expansion, it can be concluded that the substituted
Mg is present on the Al site of CuAlO2 lattice to form substitutional
solid solution.
3.2. The optical properties of the Mg-doped CuAlO2 thin films
Fig. 3 presents the optical transmittance and the reflectance
spectra of all Mg-doped CuAlO2 films. The undoped CuAlO2 film
exhibits the relatively high transparency (>80% at around 700 nm)
in the visible light range and the transparency decreases sharply
with increasing the doping concentration, the decline of the crystalline quality may be one of reasons for the strong reduction in
transmittance while increasing the Mg content. Absorption edge
for all samples can be clearly observed and it shifts to the long
wavelength with the increase of Mg concentration.
The optical band gap (Eg) of the Mg-doped CuAlO2 films could be
estimated from the optical transmittance (T) and reflectance (R)
spectra. In detail, optical absorption coefficients (a) of the films
were calculated by the following equation [13]:
1
1R
a ¼ ln
(1)
d
T
Fig. 3. Optical transmittance and reflectance spectra of Mg-doped CuAlO2 films.
G. Dong et al. / Vacuum 82 (2008) 1321–1324
1323
where the T was corrected for reflection losses at the air–film and
film–substrate interface, and d is the film thickness. Then Eg could
be deduced by the formula [14]:
ðahnÞ1=n ¼ A hn Eg
(2)
where hn is the incident photon energy, A is a constant and the
exponent n depends on the type of transition, n ¼ 1/2 and 2 for
direct and indirect transitions, respectively. Thus, the indirect (Egi)
and direct (Egd) optical band gaps of Mg-doped CuAlO2 films could
be deduced from the extrapolation of the linear portion of the plots
of (ahn)1/2 vs. hn and (ahn)2 vs. hn to a ¼ 0, respectively. The results
for undoped CuAlO2 film are illustrated in Fig. 4, Egd and Egi are
around 3.6 and 2.0 eV, respectively, which are basically consistent
with ones of polycrystalline CuAlO2 thin film (Egd and Egi are around
3.5 and 1.8 eV, respectively) deposited by pulsed laser deposition if
taken into account the error induced by the measurement [4,15].
Fig. 5 shows the Egd and Egi deduced from Fig. 3 and for Mg-doped
CuAlO2 films with different concentrations. It is clear that the Egd
and Egi of Mg-doped CuAlO2 films decrease with increasing doping
concentration, which is probably due to the increase of the amount
of impurities and/or defects with increasing the heavy acceptor
doping concentration and finally formation of the impurity energy
levels that effectively reduces the optical band gaps.
3.3. The electrical properties of Mg-doped CuAlO2 films
Fig. 6 presents ln s vs. 1000/T plot of the Mg-doped CuAlO2 films
measured from 150 to 300 K. The conductivity of all Mg-doped
CuAlO2 thin films is higher than that of the undoped film and especially the conductivity of CuAl0.98Mg0.02O2 is about three orders
of magnitude higher than that of the undoped film. Nevertheless,
the conductivity decreases when the doping concentration is
further increased up to 5%, which might be ascribed to the excess
Mg atoms acting as defects in the CuAlO2 lattice. Although all
CuAl1xMgxO2 thin films annealing in a N2 environment and it is
already proved that N dopant can improve the electrical conductivity of CuAlO2 thin film (from 3.8 102 to 5.4 102 Scm1 with
N-doping effect) [16], in this paper, the deposition and the
annealing condition for the CuAl1xMgxO2 thin films are uniform, it
can be ensured the main difference in electrical properties between
the undoped and doped samples is mainly due to the Mg-doping
effect. Above 235 K, the linear dependence relationship is shown in
the Arrhenius plot of ln s vs. 1000/T, indicating that CuAl1xMgxO2
Fig. 4. Plots of the direct and indirect optical band gaps for an undoped CuAlO2 film
(inset: determination of indirect band gap).
Fig. 5. The direct and indirect band gaps of Mg-doped CuAlO2 films with different
doping concentrations (0, 1, 2, 3 and 5 at.%).
thin films are of semiconducting thermal-activation type, the activation energy of CuAl1xMgxO2 is evaluated as 0.11, 0.053, 0.005
and 0.095 eV with x ¼ 0, 0.01, 0.02 and 0.05, respectively. On the
other hand, ln s decreases linearly as a function of 1/T1/4 (not
shown here) below 235 K. These observations indicate that temperature dependence above 235 K is of a thermal-activation type,
but below 235 K a variable-range hopping mechanism [17] becomes dominant. Hall measurements confirm p-type conductivity
and the carrier densities estimated from the Hall measurement are
on the order of 1015–1016 cm3 for all films.
4. Conclusions
The Mg-doped CuAlO2 thin films were firstly successfully deposited on quartz substrate by RF magnetron sputtering, all the
films exhibit the delafossite structure. Additional, the decline of
the crystalline quality possibly caused by Mg-doped effect induces
the strong reduction in transmittance while increasing the Mg
content. The band gaps of Mg-doped CuAlO2 thin films decrease
with increasing the doping concentration. The temperature dependence of conductivity is of semiconducting type at around room
temperature and the conductivity has a notable improvement after
Mg-doped and reaches 8.3 102 Scm1 for x ¼ 0.02 measured at
Fig. 6. Temperature dependence of the conductivity of Mg-doped CuAlO2 films with
different doping concentrations (0, 1, 2, 3 and 5 at.%).
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G. Dong et al. / Vacuum 82 (2008) 1321–1324
room temperature, which is around three orders of magnitude than
that of the undoped CuAlO2 film, Hall measurements confirm the
p-type conduction of all films.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (No. 60576012) and the Excellent Persons in Science
and Engineering of Beijing (20061D0501500199).
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