Effect of doping with Co and/or Cu on electronic structure and optical
properties of ZnO
M. Xu, H. Zhao, K. Ostrikov, M. Y. Duan, and L. X. Xu
Citation: J. Appl. Phys. 105, 043708 (2009); doi: 10.1063/1.3082023
View online: http://dx.doi.org/10.1063/1.3082023
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v105/i4
Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 105, 043708 共2009兲
Effect of doping with Co and/or Cu on electronic structure and optical
properties of ZnO
M. Xu,1,2,a兲 H. Zhao,3 K. Ostrikov,4 M. Y. Duan,1 and L. X. Xu1
1
Lab for Low-dimensional Structure Physics, Institute of Solid State Physics, Sichuan Normal University,
Chengdu 610068, People’s Republic of China
2
International Center for Material Physics, Chinese Academy of Sciences, Shenyang 110016, People’s
Republic of China
3
Department of Physics and Institute of Condensed Matter Physics, College of Mathematics and Physics,
ChongQing University, ChongQing 400044, People’s Republic of China
4
CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield NSW 2070, Australia
共Received 27 November 2008; accepted 10 January 2009; published online 26 February 2009兲
This paper reports on ab initio numerical simulations of the effect of Co and Cu dopings on the
electronic structure and optical properties of ZnO, pursued to develop diluted magnetic
semiconductors vitally needed for spintronic applications. The simulations are based upon the
Perdew-Burke-Enzerh generalized gradient approximation on the density functional theory. It is
revealed that the electrons with energies close to the Fermi level effectively transfer only between
Cu and Co ions which substitute Zn atoms, and are located in the neighbor sites connected by an O
ion. The simulation results are consistent with the experimental observations that addition of Cu
helps achieve stable ferromagnetism of Co-doped ZnO. It is shown that simultaneous insertion of
Co and Cu atoms leads to smaller energy band gap, redshift of the optical absorption edge, as well
as significant changes in the reflectivity, dielectric function, refractive index, and electron energy
loss function of ZnO as compared to the doping with either Co or Cu atoms. These highly unusual
optical properties are explained in terms of the computed electronic structure and are promising for
the development of the next-generation room-temperature ferromagnetic semiconductors for future
spintronic devices on the existing semiconductor micromanufacturing platform. © 2009 American
Institute of Physics. 关DOI: 10.1063/1.3082023兴
I. INTRODUCTION
Research on enabling effective spin-related degrees of
freedom of charge carriers has recently generated a great
deal of interest and stimulated the development of
spintronics.1 In particular, this research aims to simultaneously increase the speed and the storage capacity in microelectronic devices, which can be achieved by incorporating
additional magnetic features into semiconductor materials.2–4
There are two most widely used approaches to achieve this
goal: one is spin injection into nonmagnetic semiconductors
by placing a ferromagnetic metal adjacent to a conventional
semiconductor;5 the other one is to form diluted magnetic
semiconductors 共DMS兲 by substituting magnetic ions onto
lattice sites of a semiconductor host.3,4 Among the reported
DMS materials, GaN and ZnO are particularly interesting
because they exhibit pronounced DMS properties and have
the Curie temperature 共Tc兲 well above room temperature
共RT兲 共TR兲.3 In recent years, several GaN-based and ZnObased DMS materials with a high Curie temperature have
been reported. Most commonly researched DMS materials
include GaN doped with Mn,6 Cr,6 and Cu,7 and ZnO doped
with Mn, Co, Cr, Cu, etc.8
ZnO-based DMS materials have many attractive aspects
because of the low cost, abundance, and environmental
friendliness of ZnO.8 Among a variety of ZnO-based DMS
a兲
Electronic mail: [email protected].
0021-8979/2009/105共4兲/043708/6/$25.00
materials, Co-doped ZnO is particularly interesting not only
in terms of its RT ferromagnetism3 but also its excellent ferromagnetic transport properties.8 However, the issue of specific magnetic responses of Co-doped ZnO under different
conditions still remains controversial.9–12 For example, Dietl
et al.3 observed room-temperature ferromagnetism only in a
small number of samples while the other samples exhibited a
spin-glass behavior. Meanwhile, Jin et al.12 were unable to
find any ferromagnetism in Co-doped ZnO even with carrier
concentrations of over 1020 cm−3.
On the other hand, doping ZnO with reasonably small
amounts of Cu atoms made it possible to achieve DMS features 共with Tc ⬎ TR兲 and avoid a troublesome problem of
metal cluster precipitation common to conventional ferromagnetic materials.13 However, the origin of ferromagnetic
properties of Cu-doped ZnO still remains essentially
unclear.14
Furthermore, additional doping with Cu has been utilized to stabilize and enhance ferromagnetic response of Codoped ZnO. Recently, Lin et al.,15 Chakraborti et al.,16 and
Zhang and Li17 reported the enhancement of ferromagnetic
properties in ZnO codoped with Co and Cu at a certain concentration; however, the origin of room-temperature ferromagnetism and optical properties have not been discussed.
To date, most of the relevant efforts have been focused
on the studies of the magnetic properties of ZnO doped with
transition metals and some other elements. However, this
doping may also significantly modify its electronic and opti-
105, 043708-1
© 2009 American Institute of Physics
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043708-2
J. Appl. Phys. 105, 043708 共2009兲
Xu et al.
O 2p
50
Zn 3d
(a)
O2s
EF
0
50
FIG. 1. 共Color online兲 Schematic of one Co atom and one Cu atom substituting Zn ions on the neighboring cation sites connected by an O atom in
ZnO共2 ⫻ 2 ⫻ 1兲 supercell.
TDOS
(b)
0
50
(c)
cal properties, controlling that which is particularly important for future applications of ZnO in hybrid nanoelectronic,
optoelectronic, and spintronic on-chip device platforms. This
is presently the missing link in the literature, which we aim
to fill by ab initio predictive modeling18 of the effect of
differential or combined doping by Co and Cu atoms on the
electronic and optical properties of the ZnO material. Comparison with the results obtained in the case when only one
of these atomic species is incorporated reveals several unusual properties that arise only when both Co and Cu are
used as doping elements.
II. NUMERICAL METHOD
Our simulations are based on the density functional
theory 共DFT兲 and use the Cambridge Serial Total Energy
Package 共CASTEP兲 code.19 The core region and valence electrons of the atoms in the supercell of pure and doped ZnO
are described by generalized gradient approximation 共GGA兲
employing Perdew-Burke-Enzerh 共PBE兲 of functional
form.20 Vanderbilt-type pseudopotential developed by
Vanderbilt21 has been used in optical properties calculations.
The valence-electron configuration for the O, Zn, Co and Cu
atoms is chosen as 3s23p4, 3d104s2, 3d74s2, and 3d104s1, respectively. The numerical integration of the Brillouin zone is
performed using a 4 ⫻ 4 ⫻ 5 Monkhorst-Pack k-point sampling; the cutoff energy of the plane wave has been assumed
to be 380 eV. The numerical calculations have been performed by considering a 2 ⫻ 2 ⫻ 1 ZnO supercell that contains eight atoms of each Zn and O elements.
Two different options of atomic substitution have been
considered: 共1兲 two Co atoms 共or two Cu atoms, or one Co
and one Cu atom兲 substitute Zn ions on the neighboring cation sites connected by an O atom; 共2兲 two Co atoms 共or two
Cu atoms, or one Co and one Cu atom兲 substitute Zn ions on
the cation sites separated by two O atoms and one Zn atom.
In Fig. 1, we display a schematic of one Co atom and one Cu
atom substituting Zn ions on the neighboring cation sites
connected by an O atom in ZnO共2 ⫻ 2 ⫻ 1兲 supercell. Note
that this atomic substitution scheme is quite different from
the process of doping of ZnO with carbon or nitrogen atoms,
which have also been shown to lead to ferromagnetic properties of ZnO.22,23 The lattice constants of pure wurtzite ZnO
before the geometry optimization have been chosen as
3.249 27 Å for a and 5.205 44 Å for c, respectively. In the
optimization process, the energy change, as well as the maxi-
0
50
(d)
0
-25 -20 -15 -10 -5
0
5
10 15 20 25
Energy [eV]
FIG. 2. 共Color online兲 The total density of states 共TDOS兲 of 共a兲 intrinsic
ZnO, 共b兲 Co-doped ZnO, 共c兲 ZnO codoped with Co and Cu, and 共d兲 Cudoped ZnO 共2 ⫻ 2 ⫻ 1兲 supercells.
mum tolerances of the force, stress, and displacement have
been set as 1 ⫻ 10−5 eV/ atom, 0.03 eV/Å, 0.05 GPa, and
0.001 Å, respectively.
III. RESULTS AND DISCUSSION
Our calculations indicate that the ZnO system in which
one Co atom and one Cu atom substitute Zn ions on the
neighboring cation sites connected by an O atom is more
stable than the configuration in which one Co atom and one
Cu atom substitute Zn ions on the cation sites separated by
two O atoms and one Zn atom. This is why the main focus of
this work is on the electronic structure and optical properties
of the former system. The calculated electronic band structure shows that the direct band gap of intrinsic 共undoped兲
ZnO is about 0.94 eV at symmetric ⌫ point, in agreement
with the previously reported results of calculations 共0.5–1.0
eV兲,24 which is smaller than the experimental value 共3.37
eV兲. Generally, the underestimated value of the band gap
共Eg兲 is due to the choice of the exchange-correlation energy
within the CASTEP simulation module. The underestimate of
the Eg of ZnO appears to be quite significant because of
difficulties to quantify the hybridization between the Zn 3d
and O 2p levels. From Fig. 2, one can see that the valence
band 共VB兲 of intrinsic ZnO mainly consists of the 2p and 2s
states of O, and 3d states of Zn. In the uppermost VB, 2p
states of oxygen are predominantly found between ⫺6.3 and
0 eV while the O 2s states appear in the range from ⫺18 to
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J. Appl. Phys. 105, 043708 共2009兲
Xu et al.
6
4
(I)
EF
2
0
6
4
(II)
2
DOS
⫺16 eV, which is consistent with the results of other authors.
It can be seen that Zn 3d states give rise to some bands in the
energy range of 3–6.3 eV, which partially overlap with the
bands attributed to the O 2p states. The 3d states contribute
little to the band structure due to the repulsion of p and d
electrons. In addition, the lowest conduction band 共CB兲 is
dominated by the Zn 4s states; the available experimental
results also indicate that the CB is primarily derived from the
O 2p and Zn 4s states.
An interesting observation from our simulations is that
the energy band gap disappears completely after the substitution 共by Co or Cu atoms兲 of 25% of Zn ions on the neighboring cation sites connected by an O atom. It is also noted
that the uppermost VB broadens into higher and lower energies. The expansion of the VB to the higher energy is caused
by the 3d states of the doping elements Co or Cu, whereas
the observed shift of the VB to the lower energy can be
attributed to the 2p states of oxygen atoms bonded to Co or
Cu atoms.
On the other hand, the CB of ZnO doped with Co or Cu
shows a slight shift to the lower energy due to the incorporation of Co 4s or Cu 4s electrons. In this case, the band gap
of ZnO doped with Co or Cu is lower than Eg of intrinsic
ZnO. Strong localization of the 2s states of O bonded to Co
or Cu atoms causes the peak located between ⫺18 and ⫺16
eV in the VB to slightly shift toward lower energies.
The extent of the observed shifts strongly depends on the
doping elements and specific locations of the substituted
sites. The shift of the conduction and VBs is largest when
ZnO is doped with Co, is intermediate in the case of codoping with Co and Cu, and is smallest when Cu is used. Quite
similar conclusions hold if Co or Cu atoms substitute Zn ions
separated by two or more O atoms.
In Figs. 3 and 4, we show the density of states 共DOS兲 of
Zn in intrinsic ZnO, as well as the DOS of Co and Cu atoms
substituted into the same Zn site under conditions of separate
and combined dopings of ZnO with Co and Cu elements.
One can see that, in comparison to the upper VB created by
the Zn 3d states, the upper VB attributed to the Co or Cu 3d
states shifts toward higher energies; moreover, some energy
levels within the bands overshoot the Fermi level. It is interesting to note that the DOS of Co in ZnO codoped with Co
and Cu is larger than the DOS of Co or Cu inserted into the
same lattice sites of ZnO doped by Co or Cu separately. On
the contrary, the DOS of Cu in ZnO codoped with Co and Cu
decreases as compared with the DOS of Co or Cu atoms in
the same lattice sites of Co-doped or Cu-doped ZnO. This
suggests that some electrons transfer between the Cu to Co
atoms when both elements are inserted into the ZnO lattice.
This results in the observed increase in the DOS of Co atoms
and the decrease in the DOS of Cu atoms above the Fermi
level.
The distributions of valence electrons of all atoms in the
intrinsic or doped ZnO are listed in Table I. The bond population is a common characteristic used to analyze ionic or
covalent bonds.25,26 A positive value of the bond population
is indicative of covalent bonding while a negative population
value corresponds to an ion crystal. One can see that the net
charge of Co or Cu in doped ZnO is significantly lower than
0
4
(III)
2
0
4
(IV)
2
0
-25 -20 -15 -10 -5
0
5
10 15 20 25
Energy [eV]
FIG. 3. 共Color online兲 The DOS of a Zn atom at the Co site of Fig. 1 in
intrinsic ZnO 共I兲; DOS of a Co atom at the same lattice site of ZnO doped
with only Co 共II兲, and codoped with Co and Cu 共III兲; DOS of Cu at the same
lattice site for Cu-doped ZnO 共2 ⫻ 2 ⫻ 1兲 supercells 共IV兲.
the charge of Zn in intrinsic ZnO. Consequently, the population of the Co-O and Cu-O bonds in Co-doped or Cu-doped
ZnO increases. It is remarkable that, in ZnO codoped with
Co and Cu, the population of the Co-O bond is higher than in
ZnO doped only with Co. On the other hand, the population
of the Cu-O bond in ZnO codoped with Co and Cu is smaller
6
(I)
EF
4
2
0
6
(II)
4
DOS
043708-3
2
0
6
(III)
4
2
0
4 (IV)
2
0
-25 -20 -15 -10
-5
0
5
10
15
20
25
Energy (eV)
FIG. 4. 共Color online兲 The DOS of a Zn atom at the Cu site of Fig. 1 in
intrinsic ZnO 共I兲, DOS of Cu at the same site of ZnO codoped with Co and
Cu 共IV兲, Cu-doped ZnO, and 共II兲 Co at the Cu site of Fig. 1 for Co-doped
ZnO.
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043708-4
J. Appl. Phys. 105, 043708 共2009兲
Xu et al.
TABLE I. Valence electron distributions of Zn, Co, Cu, and O atoms in
doped ZnO
Doping
Intrinsic
Co+ Co
Co+ Cu
Cu+ Cu
Species
s
p
d
Zn1
Zn2
O
Co1
Co2
O
Co
Cu
O
Cu1
Cu2
O
0.49
0.49
1.85
0.42
0.42
1.86
0.43
0.51
1.86
0.54
0.54
1.86
0.72
0.72
4.97
0.47
0.47
4.85
0.46
0.54
4.85
0.55
0.55
4.85
9.98
9.98
0
7.54
7.54
0
7.46
9.57
0
9.45
9.45
0
Total Charge
11.18
11.18
6.82
8.43
8.43
6.71
8.34
10.62
6.71
10.53
10.53
6.72
0.82
0.82
⫺0.82
0.57
0.57
⫺0.71
0.66
0.38
⫺0.71
0.47
0.47
⫺0.72
Bond
Population
Zn1-O
0.39
Zn2-O
0.40
Co1-O
0.43
Co2-O
0.43
Co-O
0.45
Cu-O
0.40
Cu1-O
0.44
Cu2-O
0.44
than in ZnO doped only with Cu. This is caused by the
electron transfer from Cu to Co atoms when both are incorporated into the ZnO lattice; this is evidenced by the net
charges of Co and Cu listed in Table I.
Recent calculations27–29 suggest that Co-doped ZnO has
a weak preference for antiferromagnetic ordering, yet its
stable ferromagnetic response can be achieved through electron 共n-type兲 doping. Experimentally, Lin et al.,15
Chakraborti et al.,16 and Zhang and Li17 demonstrated that
room-temperature ferromagnetism can be achieved in
Zn1−xCoxO by incorporating Cu atoms. In combination with
the results of our computations, the enhancement of ferromagnetism in Zn1−xCoxO by adding Cu can be well understood since electrons prefer to transfer from the Cu to the Co
atom in ZnO codoped with Co and Cu atoms which substitute Zn ions on the neighbor cation sites connected by an O
atom. In other words, the doped Cu ion introduces additional
electrons into the Co-doped ZnO system, and these additional electrons enable the electron 共n-type兲 doping of the
Zn1−xCoxO material. This is why stable ferromagnetism is
observed in ZnO codoped with Co and Cu.
To investigate the optical properties of the Co-doped
and/or Cu-doped ZnO system, it is necessary to calculate the
imaginary part of the dielectric function ␧2共␻兲. As is known,
the interaction of a photon with the electrons can be described in terms of time-dependent perturbations of the
ground-state electronic states. Optical transitions between
occupied and unoccupied states are caused by the electric
field of the photon. The spectra arising from the excited
states can be described by densities of states neither pertaining to the VB or CBs. The function ␧2共␻兲 can be calculated
straightforwardly using the momentum matrix elements between the occupied and the unoccupied states, and applying
the appropriate selection rules. The real part of the dielectric
function ␧1共␻兲 can be calculated from ␧2共␻兲 using the
Kramers-Kronig relationship. Finally, the other optical constants such as the absorption coefficient ␣共␻兲, reflectivity
R共␻兲, refractive index n共␻兲, and electron energy loss spectrum L共␻兲 can be derived straightforwardly from ␧1共␻兲 and
␧2共␻兲.24
Since ␧2共␻兲 is a critical characteristic of the optical
properties of any materials, Fig. 5共a兲 shows its dependence
on the photon energy for intrinsic and Co-doped and/or Cudoped ZnO systems. For intrinsic ZnO, there are three main
peaks in ␧2共␻兲, which are 2.15, 6.90, and 10.50 eV. The peak
at 2.15 eV is mainly caused by the electron transitions between the O 2p states in the highest VB and the Zn 4s states
in the lowest CB. These states are close to the direct band
gap of 0.94 eV, which is measured between the minimum of
the CB and the maximum of the VB. The peak at 6.90 eV
comes from the electron transitions between the Zn 3d and
O 2p orbitals, in a good agreement with the available ab
initio calculations of other authors. The peak at 10.50 eV is
mainly attributed to electron transitions between the Zn 3d
and O 2s states. For the Co-doped and/or Cu-doped ZnO
systems, the first and second peaks are significantly shifted
toward low energies while the third peak shows a slight blueshift. One can also note that the first peak for ZnO codoped
by Co and Cu atoms is much higher than in ZnO doped with
either Co or Cu. This indicates that the probability of the
electron transfer from the O 2p states in the highest VB to
the Zn 4s states in the lowest CB is highest when ZnO is
simultaneously doped with Co and Cu.
The reflectivity R共␻兲, refractive index n共␻兲, and electron
energy loss spectrum L共␻兲 as functions of the photon energy
for intrinsic and Co-doped and/or Cu-doped ZnO systems are
shown in Figs. 5共b兲–5共d兲. The refractive index and reflectivity of Co-doped and/or Cu-doped ZnO sharply increase in
the low energy range in comparison to those of the intrinsic
ZnO. On the other hand, the increase in the refractive index
and reflectivity in the low energy range indicates that the
band gap of Co-doped and/or Cu-doped ZnO decreases, also
noticeable in the optical absorption spectra discussed below.
One can also observe the peaks between 20 and 25 eV in the
energy loss spectra, which are related to the excitation of the
plasmon resonance, in agreement with the results of previous
calculations by Sun et al.24 These peaks correspond to the
trailing edges in the reflection spectra and represent the transition points from the metallic to the dielectric properties for
intrinsic and Co-doped and/or Cu-doped ZnO.
Figure 6 shows the optical absorption spectra of the intrinsic and Co-doped and/or Cu-doped ZnO calculated by
using the scissor operation 共which is the established and
commonly used procedure for this type of calculations; note
that an even more effective modification can be implemented
through DFT+ U computations兲, which is effective for the
ZnO material system.30 Since the CASTEP module significantly underestimates the value of the energy band gap, we
have applied the energy correction of 2.3 eV to fit the absorption edge. The optical band gap calculated using the
above correction is approximately 3.24 eV, which is consistent with the experimental value of 3.37 eV. One can notice
that doping with Co or Cu atoms causes a very clear redshift
of the optical absorption edge. After doping, the optical band
gap decreases from 3.24 to 2.0 eV for Cu-doped ZnO and to
1.9 eV for Co-doped ZnO. It is noteworthy that the optical
band gap of ZnO codoped by Co and Cu is smaller than Eg
of ZnO doped with either Co or Cu. The absorption coefficient,
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043708-5
J. Appl. Phys. 105, 043708 共2009兲
Xu et al.
a
20
ZnO
ZnO -Co
ZnO -CoC u
ZnO -Cu
0.5
0.4
R
ε2
15
b
0.6
ZnO
ZnO -C o
ZnO -C oCu
ZnO -C u
10
0.3
0.2
5
0.1
0
0.0
0
5
10
15
20
25
35
40
45
0
5
10
15
20
25
30
35
40
45
d
c
8
7
ZnO
ZnO -Co
ZnO -C oC u
ZnO -Cu
5
ZnO
ZnO -C o
ZnO -C oCu
ZnO -C u
5
Loss
6
n
30
4
3
2
1
0
0
0
5
10
15
20
25
30
35
40
Photon energy (eV)
45
0
5
10
15
20
25
30
35
40
45
Photon energy (eV)
FIG. 5. 共Color online兲 共a兲 Imaginary part of the dielectric function ␧2, 共b兲 reflectivity, 共c兲 refractive index, and 共d兲 loss function of pure and doped ZnO共2
⫻ 2 ⫻ 1兲 supercells. Solid lines correspond to intrinsic ZnO, dashed lines to Co-doped ZnO, dotted lines to ZnO codoped with Co and Cu, and dash-dotted
lines to Cu-doped ZnO.
␣共␻兲 = 冑2␻关冑␧21共␻兲 + ␧22共␻兲 − ␧1共␻兲兴1/2 ,
is a complex function of ␧1共␻兲 and ␧2共␻兲.23 However, in the
frequency range of interest, ␧1共␻兲 is much larger than ␧2共␻兲;
hence, the variation in ␣共␻兲 will be mainly determined by
the product ␻ · ␧2共␻兲. From Fig. 4共a兲 one can notice that, for
ZnO codoped with Co and Cu, ␧2共␻兲 experiences a much
stronger increase in the low energy range than in any other
Absorption [a.u.]
2x10
5
P u re Z n O
C o -d o p e d
C u -d o p e d
codoped
1x10
5
case. Therefore, the product ␻ · ␧2共␻兲 will decrease more
slowly in ZnO codoped with Co and Cu. One can thus conclude that the optical absorption edge of ZnO codoped with
Co and Cu atoms shifts to lower energies. This rather uncommon codoping can be realized experimentally through
plasma-assisted magnetron cosputtering in reactive
plasmas31,32 or exposure of ZnO to Co+ and Cu+ ions which
can be generated in vacuum arc or other plasma-based
systems.33–35
It should be pointed out that, since the magnetism is very
important for DMS materials, it will be particularly interesting to investigate the magnetic properties of doped ZnO
DMS systems by placing Co and/or Cu atoms on different
lattice sites. This work is in progress and relies on advanced
calculation tools that are specifically tailored for the analysis
of magnetic properties.
IV. CONCLUSIONS
0
0
1
2
3
4
5
P h o to n E n e rg y [e V ]
FIG. 6. 共Color online兲 Optical absorption spectra of pure and doped
ZnO共2 ⫻ 2 ⫻ 1兲 supercells.
To conclude, we have computed the electronic structure
and optical properties of ZnO doped with Co and/or Cu
based upon the PBE GGA of the DFT. It has been revealed
that the electrons located around the Fermi level tend to
transfer between the Cu and Co ions when Co and Cu sub-
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043708-6
stitute Zn ions on the neighbor sites; this is not the case when
Co and Cu are separated by a larger number of atoms. Our
results support the experimental evidence that addition of Cu
helps achieving stable ferromagnetism of Co-doped ZnO.
Most important optical constants such as the dielectric function, reflectivity, refractive index, and electron energy loss
function also experience significant changes when ZnO is
doped with Co and/or Cu. In particular, the optical absorption edge of ZnO codoped with Co and Cu shows a redshift
compared to the case when either Co or Cu atoms are used
for doping. The computed DOS of Zn, Co, and Cu under
different doping conditions enabled a simple explanation of
this phenomenon based on electron transfer between the Co
and Cu atoms. Finally, our results suggest that ZnO simultaneously doped with Co and Cu holds an outstanding promise
as an advanced and flexible diluted magnetic semiconducting
material that features room-temperature ferromagnetism as
well as excellent semiconducting and optical properties.
ACKNOWLEDGMENTS
This work was partially supported by Education Bureau
of Sichuan Province, People’s Republic of China 共Project
No. 2006C020兲, the Australian Research Council, and
CSIRO.
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