Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique
We have reported the effect of Co doping on structural and optical properties of ZnO thin films prepared by the RF reactive sputtering technique. The composite targets were formed by mixing and pressing ZnO and CoO powders. The thin films were deposited on silica and glass substrates. The structu...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2014
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irk-123456789-1184112017-05-31T03:05:24Z Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique Savchuk, A.I. Stolyarchuk, I.D. Stefanuk, I. Cieniek, B. Sheregii, E. We have reported the effect of Co doping on structural and optical properties of ZnO thin films prepared by the RF reactive sputtering technique. The composite targets were formed by mixing and pressing ZnO and CoO powders. The thin films were deposited on silica and glass substrates. The structures of samples have been studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). With the sensitivity of the XRD instruments, the structural analyses of Co-doped ZnO films reveal formation of predominant (002) reflection corresponding to the hexagonal wurtzite structure without any secondary phase. The AFM study showed that surface morphology of the Zn₁₋xCoxO films is composed of closely packed nanocrystallites with nanorod shape. The optical properties of the samples were studied using UV-vis absorption and PL spectra. The optical absorption spectra show a red shift of the band edge, which indicates that Co²⁺ ions substitute Zn²⁺ ions in ZnO lattice. In the room-temperature photoluminescence spectra, four main peaks were revealed in all the samples, which are attributed to ultraviolet, violet-blue, blue and green emission. 2014 Article Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique / A.I. Savchuk, I.D. Stolyarchuk, I. Stefanuk, B. Cieniek, E. Sheregii // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 353-357. — Бібліогр.: 31 назв. — англ. 1560-8034 PACS 42.25.Bs, 61.05.cp, 78.55.Hx http://dspace.nbuv.gov.ua/handle/123456789/118411 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
| description |
We have reported the effect of Co doping on structural and optical properties
of ZnO thin films prepared by the RF reactive sputtering technique. The composite
targets were formed by mixing and pressing ZnO and CoO powders. The thin films were
deposited on silica and glass substrates. The structures of samples have been studied by
using X-ray diffraction (XRD) and atomic force microscopy (AFM). With the sensitivity
of the XRD instruments, the structural analyses of Co-doped ZnO films reveal formation
of predominant (002) reflection corresponding to the hexagonal wurtzite structure
without any secondary phase. The AFM study showed that surface morphology of the
Zn₁₋xCoxO films is composed of closely packed nanocrystallites with nanorod shape.
The optical properties of the samples were studied using UV-vis absorption and PL
spectra. The optical absorption spectra show a red shift of the band edge, which indicates
that Co²⁺ ions substitute Zn²⁺ ions in ZnO lattice. In the room-temperature photoluminescence
spectra, four main peaks were revealed in all the samples, which are
attributed to ultraviolet, violet-blue, blue and green emission. |
| format |
Article |
| author |
Savchuk, A.I. Stolyarchuk, I.D. Stefanuk, I. Cieniek, B. Sheregii, E. |
| spellingShingle |
Savchuk, A.I. Stolyarchuk, I.D. Stefanuk, I. Cieniek, B. Sheregii, E. Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Savchuk, A.I. Stolyarchuk, I.D. Stefanuk, I. Cieniek, B. Sheregii, E. |
| author_sort |
Savchuk, A.I. |
| title |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique |
| title_short |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique |
| title_full |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique |
| title_fullStr |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique |
| title_full_unstemmed |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique |
| title_sort |
structural and optical properties of zn₁₋xcoxo thin films prepared by rf reactive sputtering technique |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118411 |
| citation_txt |
Structural and optical properties of Zn₁₋xCoxO thin films prepared by RF reactive sputtering technique / A.I. Savchuk, I.D. Stolyarchuk, I. Stefanuk, B. Cieniek, E. Sheregii // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 353-357. — Бібліогр.: 31 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
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| first_indexed |
2025-07-08T13:55:42Z |
| last_indexed |
2025-07-08T13:55:42Z |
| _version_ |
1837087268013080576 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
353
PACS 42.25.Bs, 61.05.cp, 78.55.Hx
Structural and optical properties of Zn1–xCoxO thin films
prepared by RF reactive sputtering technique
A.I. Savchuk1, I.D. Stolyarchuk1, I. Stefaniuk2, B. Cieniek2, E. Sheregii2
1Chernivtsi National University, Department of Physics of Semiconductors and Nanostructures,
2, Kotsyubynsky str., 58012 Chernivtsi, Ukraine, e-mail: istolyarchuk@ukr.net
2Centre for Innovation and Transfer of Natural Sciences and Engineering Knowledge,
University of Rzeszow, 16a Rejtana str., 35959 Rzeszow, Poland
Abstract. We have reported the effect of Co doping on structural and optical properties
of ZnO thin films prepared by the RF reactive sputtering technique. The composite
targets were formed by mixing and pressing ZnO and CoO powders. The thin films were
deposited on silica and glass substrates. The structures of samples have been studied by
using X-ray diffraction (XRD) and atomic force microscopy (AFM). With the sensitivity
of the XRD instruments, the structural analyses of Co-doped ZnO films reveal formation
of predominant (002) reflection corresponding to the hexagonal wurtzite structure
without any secondary phase. The AFM study showed that surface morphology of the
OCoZn xx1 films is composed of closely packed nanocrystallites with nanorod shape.
The optical properties of the samples were studied using UV-vis absorption and PL
spectra. The optical absorption spectra show a red shift of the band edge, which indicates
that Co2+ ions substitute Zn2+ ions in ZnO lattice. In the room-temperature photo-
luminescence spectra, four main peaks were revealed in all the samples, which are
attributed to ultraviolet, violet-blue, blue and green emission.
Keywords: ZnCoO, thin films, RF sputtering, X-ray diffraction, optical absorption,
photoluminescence.
Manuscript received 19.02.14; revised version received 30.07.14; accepted for
publication 29.10.14; published online 10.11.14.
1. Introduction
Diluted magnetic semiconductors (DMS) are
semiconducting materials doped with magnetic
transition metal (TM) (such as Mn, Fe, Co, Ni etc.) ions
have attracted much interest in recent years because of
their potential application in optoelectronic,
magnetoelectronic and spintronic devices [1-3]. These
are basic semiconductors where the cation sites of the
host lattice are replaced by transition metal atoms.
Recently the family of DMS has been increased by
addition of the semiconducting oxides [4, 5].
Zinc oxide (ZnO), an optically transparent II-VI
semiconductor with hexagonal wurtzite structure of
4
6vC (P63mc) space group, wide direct band gap (Eg
3.37 eV), exciton binding energy (60 meV) has been
identified as a promising host material after theoretical
studies that predicted ferromagnetism above room
temperature in Mn-doped ZnO [6]. Among those
transition metals, cobalt is an important dopant that has
been intensively investigated. Different growth
techniques such as chemical vapor deposition, spray
pyrolysis, pulsed laser deposition, magnetron
sputtering, sol-gel process etc. have been used for
preparing Co-doped ZnO thin films. The magnetic
property of ZnCoO thin films has been the focus of
numerous investigations. Several groups [7, 8] have
observed ferromagnetism in ZnO:Co systems, while in
other groups [9, 10] this magnetic behavior is not
observed in similar samples. These controversial
results indicated that the magnetic properties are very
sensitive to the shape of the samples and to the
preparation conditions.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
354
The present work is devoted to preparation of
ZnCoO thin films and study of their structural,
absorption and photoluminescence properties depending
on the content of cobalt.
2. Experimental
Zn1–xCoxO (where x = 0.02, 0.04, 0.06) thin films were
deposited onto glass and silica substrates using the RF
reactive sputtering technique. The composite targets
were formed by mixing and pressing ZnO and CoO
powders with appropriate ratios of components. The
sputtering was performed under a mixture of argon and
oxygen atmosphere that served as operation and reactive
gases, respectively, through independent mass flow
controllers. The sputtering chamber was evacuated down
to Pa102 4 before Ar and O2 gases were filled with the
flow ratio 4:1. During the sputtering, the RF power,
deposition rate, substrate-to-target distance and substrate
temperature were kept at 300 W, 10 nm/min, 35 mm and
350…400 °C, respectively. Postdeposition annealing
5…8 min in oxygen atmosphere at 500…550 °C was
performed.
The crystallographic studies were performed using
X-ray Diffractometer (D8 ADVANCE X-ray Diffracto-
meter with DAVINCI) using CuKα wavelength (λ =
1.54059 Å) and scanning in 2Θ range from 10° to 70°.
The surface analysis was performed using atomic
force microscopy (AFM) “Nanotec Electronica” in
Dynamic Mode (non-contact) with the force constant
K ~ 40 N/m and f0 ~ 300 kHz. Optical transmission and
absorption spectra between 300 and 700 nm were
measured using a grating monochromator, photo-
detector system and registering computer system. This
setup was also used to register photoluminescence
spectra. For this kind of measurements, the samples were
excited using a 325-nm He-Cd laser with the excitation
intensity 10 mW.
3. Results and discussion
The X-ray diffraction (XRD) patterns of OCoZn xx1
(0 ≤ x ≤ 0.06) films are shown in Fig. 1. All intense peak
positions of the films correspond to the standard
diffraction pattern of ZnO hexagonal wurtzite with a
(002) preferred orientation. With an increase in Co
content (x > 0.02) additional diffraction peaks were
observed, corresponding to (100), (101), and (102) of
ZnO. No peaks corresponding to cobalt metal clusters or
cobalt oxides were observed on the patterns, which
indicates that Co enters the ZnO lattice without changing
the wurtzite structures and systematically substitutes
Zn2+ ions in the lattice. The relative intensity of the (002)
peak of OCoZn xx1 (0 ≤ x ≤ 0.06) reaches higher values
than that of other peaks, indicating the c-axis preferred
texture growth of the Co-doped ZnO films. Further, the
intensity of diffraction peaks of Zn1–xCoxO films
increased with increasing Co concentration, which can
be attributed to a refinement and improvement of
crystalline quality of these films as a result of Co
doping. At the same time, we observed a shift only in the
angular peak position of (002) plane, and no angular
shift was observed for any other plane. This angular
peak shift towards the lower 2Θ value, when transfering
from the undoped sample to the doped up to x = 0.04
one, indicates that films are in a uniform state of stress.
With a further increase in the Co concentration, the peak
shift takes place in opposite direction, i.e. 2Θ value
increases, indicating the change in the direction of stress.
The average crystallite size D was calculated using the
full width at the half maximum (FWHM) of the
diffraction peak and the angle of diffraction Θ in Debye-
Scherer’s formula [11]:
cos
0.9
D , (1)
where λ is the X-ray wavelength, ΔΘ – FWHM, and Θ –
Bragg angle of the diffraction peak. The lattice
parameters a and c for the samples were calculated using
the appropriate formula and depicted in Fig. 2. Co-
doping slightly decreases the lattice parameters of the
ZnO films, considering that the size of Co2+ in
tetrahedral configuration (0.058 nm) is close to that of
Zn2+ in tetrahedral coordination (0.06 nm). Thus, the
Co2+ ions are systematically substituted by Zn2+ ions
without changing the crystal structure.
The surface morphology of the Zn1–xCoxO films
and their crystallite sizes have been studied and
evaluated by AFM. The root mean square of the surface
roughness is measured from the pictures over 5×5 μm
scanning range. The AFM micrograph shown in Fig. 3
indicates that the increase in the surface roughness after
Co doping is caused by closely packed nanorod-like
nanocrystallites oriented non-perpendicularly to the
film-substrate plane over the whole Zn0.98Co0.02O film
surface. The estimated diameter of the observed
nanorods is about 100 nm. Essentially, there is no
obvious change in the microstructure observed in the
films grown with different Co concentrations.
20 25 30 35 40 45 50 55 60
Co
co
nt
en
t (
%
)
1 x=0
2 x=0.02
3 x=0.04
4 x=0.06
2deg)
0
2
4
6
Fig. 1. X-ray diffraction spectra of Co doped ZnO thin films
with various doping levels.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
355
Fig. 2. Variation of lattice parameters a and c with increasing
the Co content in Zn1–xCoxO thin films.
Fig. 3. 3D AFM images of Zn1–xCoxO thin films with the Co
content x = 0.02.
UV-vis optical measurements were carried out at
room temperature to study the effect of Co doping
concentration on the band gap of ZnO thin films and to
confirm the substitution of Co2+ ions in tetrahedral sites
of the ZnO wurtzite structures. Fig. 4 shows the optical
transmission spectra of undoped and cobalt doped ZnO
thin films deposited on glass substrates. The undoped
ZnO film is more transparent, whereas in cobalt-doped
thin films the transmittance decreases with the increase
of Co-doping concentration. The optical transmittance
spectra showed a shift in the band edge towards the
lower energy side with the increase of Co content in the
films. Additional absorption below the absorption edge
can be seen for the OCoZn xx1 thin films. In particular,
for the Co content x = 0.02, three absorption bands at
about 1.87, 2.03 and 2.19 eV have been revealed, which
are in agreement with already reported absorption peaks
[12, 13]. This absorption structure is associated with d-d
electron transitions of Co2+ ions in a tetragonal crystal
field. According to Hund’s rule and Pauli’s exclusion
principle, the electronic ground state configuration has
L = 3 and S = 3/2. So, the ground state spectral term is
4F and the excited state terms are 4P, 2G, 2F, 2D and 2P.
However, when Co2+ exists in the tetrahedral field, the
4F term splits into 4A2(F), 4T2(F) and 4T1(F), with 4A2(F)
being the lowest in energy and the remaining two having
higher energies. The 4P term corresponding to the first
excited state does not split, but is transformed into
4T1(P). Similarly, 2G splits into 2A1(G), 2E(G), 2T1(G)
and 2T2(G). In the ground state, the atom is in 4A2 state.
When electron has a sufficient energy, it can be excited
to a higher energy state. The observed peaks were
attributed to 4A1(F) 2A1(G), 4A2(F) 4T1(P) and
4A2(F) 2E(G) transitions in Co2+ ions. Thus,
observation of these transitions in transmission spectra
of our OCoZn xx1 films clearly reveals that the added
cobalt atoms have been substituted by Zn2+ cations and
are present in 2+ state.
The optical band-gap energy (Eg) values for these
films were evaluated by plotting (h)2 versus h, where
h is the photon energy, and is the absorption
coefficient that can be calculated from the film thickness
d as well as transmittances T and T0, for the film and
substrate, respectively, using equation =
–(1/d) ln(T/T0). Extrapolation of the linear portion of
(h)2 to zero gives the value of Eg. The estimated Eg
values are decreased with increase of the Co content
(Fig. 5). This result is in contrast with the reported data
[13, 15] where a blue shift of the absorption edge was
observed. On the other side, a similar trend of decrease
of the band edge in OCoZn xx1 films is reported by
many researchers [12, 14, 16]. This low-energy shift of
Eg as a function of the Co content can be explained by
two reasons: (i) it might result from destabilization of
O2– (2p6 orbitals at O2– ions adjacent to Co2+), which
extends the valence band edge by 0.5 eV [17] or point
defects such as nanoclusters of Co, CoO can also lead to
this shift [18] (however, these defects have not been
observed in the present films); (ii) as mainly due to the
s-d and p-d spin-exchange interactions between band
electrons and localized d-electrons of Co2+ ions
substituting Zn2+ ions [7, 19]. The exchange interaction
between transition metal ions and band electrons gives
rise to negative and positive corrections to the
conduction and valence-band edges, leading to
narrowing the band gap [20].
Room temperature photoluminescence (PL) of
Zn1–xCoxO thin films measured by exciting at 325 nm is
shown in Fig. 6. The PL spectra show four peaks
occurring around 386, 426, 443, and 538 nm for all the
samples on glass and silica substrates. The first peak is
in the ultraviolet region, while other three peaks
correspond to the violet-blue, blue and green ones,
respectively, i.e., in the visible region. The UV emission
band is frequently observed in ZnO film and can be
attributed to the near-band-edge exciton emission,
because the emission energy is almost equal to the band
energy of ZnO [21] estimated by UV-vis measurements.
With a Co doping content increase, the UV emission
center shifts to long wavelengths. It can be attributed to
the strong exchange interactions between d-electrons of
doping ion and s-, p-electrons of the host band [22]. This
red shift of near-band-edge emission confirms that the
band gap of Zn1–xCoxO decreases with increasing the Co
content and is in good agreement with data obtained
0.00 0.02 0.04 0.06
5.20
5.22
5.24
5.26
c
a
Co content
L
at
ti
ce
p
ar
am
et
er
c
(
)
3.25
3.26
3.27
3.28
L
at
ti
ce
p
a
ra
m
e
te
r
a
(
)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
356
from optical absorption studies. The violet-blue emission
centered near 426 nm is probably due to radiative
defects related to traps existing at grain boundaries and
emitted due to the radiative transition between this level
and the valence band [23, 24].
With increasing the Co concentration, the peak
position of blue emission shifts slightly to higher
wavelengths from 443 up to 448 nm. The mechanism of
blue emission (~443 nm) in ZnO low-dimensional
structures is still controversial. Xu et al. [25] synthesized
single crystalline ZnO nanoplates by hydrothermal
procedure and attributed blue emission to electron
transitions from the level of ionized oxygen vacancies to
the valence band. Gokulakrishnan et al. [26] studied Zr-
doped ZnO thin films and ascribed the 443-nm centered
emission band to a surface defect in the ZnO films.
R. Elilarassi, G. Chandrasekaran [27] and Zeng et al.
[28] attributed this peak to the interstitial Zn level (Zni)
and valence band. F.L. Xian et al. [29] attributed the
blue emission to the states of interstitial cobalt transition
to the valence band. In the latter, it indicates the red shift
and change in intensity of emission band with increase
of the Co content. The green band emission centered at
approximately 538 nm is related to oxygen vacancy, and
it is assigned to transitions from single ionized oxygen
vacancies (VO) to the valence band [30, 31].
4. Conclusions
In conclusion, Zn1–xCoxO thin films with x 0.06 were
deposited by the RF reactive sputtering technique on
glass and silica substrates. XRD analysis of the films
reveals that cobalt ions are successfully doped in ZnO
without changing the hexagonal wurtzite structure. The
grown Co-doped ZnO films show c-axis preferred
orientation with good crystallinity. AFM study shows
that the surface morphology of the OCoZn xx1 films is
composed of closely packed nanocrystallites with
nanorod shape. The optical absorption spectra of the
films demonstrate that the energy band gap was found to
decrease with the Co content increase. Band gap
narrowing effect occurs due to strong exchange
interactions between d-electrons of doping ion and s-, p-
electrons of the host lattice. The room temperature PL
measurements illustrate that UV emission and violet-
blue, blue and green emissions are in the visible region.
The UV emission peak originates from the radiative
recombination of free excitons and their center shifts to
long wavelengths with increasing the Co content. Other
emissions may be attributed to the radiative defects
related to traps existing at grain boundaries for violet-
blue emission, cobalt and zinc interstitial for blue
emission, and singly ionized oxygen vacancies (VO) for
green band emission, respectively.
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1
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
357
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11. B.D. Cullity, Elements of X-ray Diffractions.
Addison-Wesley, Reading, 1978.
12. J.H. Kim, H. Kim, D. Kim, S.G. Yoon, W.K. Choo,
Optical and magnetic properties of laser deposited
Co-doped ZnO thin films // Solid State Communs.
131, p. 677-680 (2004).
13. K.J. Kim, Y.R. Park, Spectroscopic ellipsometry
study of optical transitions in alloys // Appl. Phys.
Lett. 81(8), p. 1420-1422 (2002).
14. K.T.R. Reddy, V. Supriya, Y. Murata, M. Sugiyama,
Effect of Co-doping on the properties of Zn1−xCoxO
films deposited by spray pyrolysis // Surface and
Coating Technology, 231, p. 149-152 (2013).
15. M. Subramanian, M. Tanemura, T. Hihara, V.
Ganesan, T. Soga, T. Jimbo, Magnetic anisotropy
in nanocrystalline Co-doped ZnO thin films //
Chem. Phys. Lett. 487, p. 97-100 (2010).
16. S. Colis, H. Bieber, S. Begin-Colin, G. Schmerber,
C. Leuvrey, A. Dinia, Magnetic properties of Co-
doped ZnO diluted magnetic semiconductors
prepared by low-temperature mechanosynthesis //
Chem. Phys. Lett. 422, p. 529-533 (2006).
17. A. Fouchet, W. Prellier, L. Mechin, Growth and
characterizations of ZnO and Co-doped ZnO films
for their use in spintronics // Superlatt. Microstruct.
42, p. 185-190 (2007).
18. N. Bahadur, A.K. Srivastava, S. Kumar, M. Deepa,
D. Nag, Influence of cobalt doping on the
crystalline structure, optical and mechanical
properties of ZnO thin films // Thin Solid Films,
518, p. 5257-5264 (2010).
19. L. Wei, Z. Li, W.F. Zhang, Influence of Co doping
content on its valence state in OCoZn xx1 (0≤
x ≤ 0.15) thin films // Appl. Surf. Sci. 255, p. 4992-
4995 (2009).
20. J.K. Furdyna, Diluted magnetic semiconductors //
J. Appl. Phys. 64(4), p. R29 (1988).
21. Y.G. Wang, S.P. Lau, H. Lee, S.F. Yu, B.K. Tay,
X.H. Zhang, and H.H. Hng, Photoluminescence
study of ZnO films prepared by thermal oxidation
of Zn metallic films in air // J. Appl. Phys. 94,
p. 354-358 (2003).
22. B. Panigrahy, M. Aslam, and D. Bahadur, Aqueous
synthesis of Mn-and Co-doped ZnO nanorods // J.
Phys. Chem. C, 114, p. 11758-11763 (2010).
23. S. Kumar, S. Basu, B. Rana, A. Barman, S.
Chatterjee, Jha, and A.K. Ghosh, Structural, optical
and magnetic properties of sol–gel derived ZnO:
Co diluted magnetic semiconductor nanocrystals:
an EXAFS study // J. Mater. Chem. C, 2(3), p. 481-
495 (2014).
24. L.J. Zhuge, X.M. Wu, Z.F. Wu, X.M. Yang, X.M.
Chen, Q. Chen, Structure and deep ultraviolet
emission of Co-doped ZnO films with Co3O4 nano-
clusters // Mat. Chem. Phys. 120, p. 480-483 (2010).
25. F. Xu, Z.Y. Yuan, G.H. Du, M. Halasa, and B.L.
Su, High-yield synthesis of single-crystalline ZnO
hexagonal nanoplates and accounts of their optical
and photocatalytic properties // Appl. Phys. A,
86(2), p. 181-185 (2007).
26. V. Gokulakrishnan, S. Parthiban, K. Jeganathan,
and K. Ramamurthi, Investigation on the effect of
Zr doping in ZnO thin films by spray pyrolysis //
Appl. Surf. Sci. 257(21), p. 9068-9072 (2011).
27. R. Elilarassi, G. Chandrasekaran, Influence of Co-
doping on the structural, optical and magnetic
properties of ZnO nanoparticles synthesized using
auto-combustion method // J. Mater. Sci.: Mater.
Electron. 24, p. 96-105 (2013).
28. H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, and
W. Cai, Blue luminescence of ZnO nanoparticles
based on non-equilibrium processes: Defect origins
and emission controls // Adv. Functional Materials,
20(4), p. 561-572 (2010).
29. F.L. Xian, L.H. Xu, X.X. Wang, and X.Y. Li,
Crystallographic, optical and magnetic properties of
Co-doped ZnO thin films synthesized by sol gel route
// Cryst. Res. Technol. 47(4), p. 423-428 (2012).
30. S. Chakraborty, and P. Kumbhakar, Observation of
bandgap narrowing effect and photoluminescence
emission characteristics of chemically synthesized
Co doped ZnO nanosheets // Indian J. Phys. 88.3,
p. 251-257 (2014).
31. J. Ding, X. Yana, Q. Xue, Study on field emission
and photoluminescence properties of
ZnO/grapheme hybrids grown on Si substrates //
Mater. Chem. Phys. 133, p. 405-409 (2012).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 353-357.
PACS 42.25.Bs, 61.05.cp, 78.55.Hx
Structural and optical properties of Zn1–xCoxO thin films
prepared by RF reactive sputtering technique
A.I. Savchuk1, I.D. Stolyarchuk1, I. Stefaniuk2, B. Cieniek2, E. Sheregii2
1Chernivtsi National University, Department of Physics of Semiconductors and Nanostructures,
2, Kotsyubynsky str., 58012 Chernivtsi, Ukraine, e-mail: istolyarchuk@ukr.net
2Centre for Innovation and Transfer of Natural Sciences and Engineering Knowledge,
University of Rzeszow, 16a Rejtana str., 35959 Rzeszow, Poland
Abstract. We have reported the effect of Co doping on structural and optical properties of ZnO thin films prepared by the RF reactive sputtering technique. The composite targets were formed by mixing and pressing ZnO and CoO powders. The thin films were deposited on silica and glass substrates. The structures of samples have been studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). With the sensitivity of the XRD instruments, the structural analyses of Co-doped ZnO films reveal formation of predominant (002) reflection corresponding to the hexagonal wurtzite structure without any secondary phase. The AFM study showed that surface morphology of the
0.00
0.02
0.04
0.06
5.20
5.22
5.24
5.26
c
a
Co content
Lattice parameter c (
C
)
3.25
3.26
3.27
3.28
Lattice parameter a (
C
)
films is composed of closely packed nanocrystallites with nanorod shape. The optical properties of the samples were studied using UV-vis absorption and PL spectra. The optical absorption spectra show a red shift of the band edge, which indicates that Co2+ ions substitute Zn2+ ions in ZnO lattice. In the room-temperature photo-luminescence spectra, four main peaks were revealed in all the samples, which are attributed to ultraviolet, violet-blue, blue and green emission.
Keywords: ZnCoO, thin films, RF sputtering, X-ray diffraction, optical absorption, photoluminescence.
Manuscript received 19.02.14; revised version received 30.07.14; accepted for publication 29.10.14; published online 10.11.14.
1. Introduction
Diluted magnetic semiconductors (DMS) are semiconducting materials doped with magnetic transition metal (TM) (such as Mn, Fe, Co, Ni etc.) ions have attracted much interest in recent years because of their potential application in optoelectronic, magnetoelectronic and spintronic devices [1-3]. These are basic semiconductors where the cation sites of the host lattice are replaced by transition metal atoms. Recently the family of DMS has been increased by addition of the semiconducting oxides [4, 5].
Zinc oxide (ZnO), an optically transparent II-VI semiconductor with hexagonal wurtzite structure of
4
6
v
C
(P63mc) space group, wide direct band gap (Eg ( 3.37 eV), exciton binding energy ((60 meV) has been identified as a promising host material after theoretical studies that predicted ferromagnetism above room temperature in Mn-doped ZnO [6]. Among those transition metals, cobalt is an important dopant that has been intensively investigated. Different growth techniques such as chemical vapor deposition, spray pyrolysis, pulsed laser deposition, magnetron sputtering, sol-gel process etc. have been used for preparing Co-doped ZnO thin films. The magnetic property of ZnCoO thin films has been the focus of numerous investigations. Several groups [7, 8] have observed ferromagnetism in ZnO:Co systems, while in other groups [9, 10] this magnetic behavior is not observed in similar samples. These controversial results indicated that the magnetic properties are very sensitive to the shape of the samples and to the preparation conditions.
The present work is devoted to preparation of ZnCoO thin films and study of their structural, absorption and photoluminescence properties depending on the content of cobalt.
2. Experimental
Zn1–xCoxO (where x = 0.02, 0.04, 0.06) thin films were deposited onto glass and silica substrates using the RF reactive sputtering technique. The composite targets were formed by mixing and pressing ZnO and CoO powders with appropriate ratios of components. The sputtering was performed under a mixture of argon and oxygen atmosphere that served as operation and reactive gases, respectively, through independent mass flow controllers. The sputtering chamber was evacuated down to
Pa
10
2
4
-
×
before Ar and O2 gases were filled with the flow ratio 4:1. During the sputtering, the RF power, deposition rate, substrate-to-target distance and substrate temperature were kept at 300 W, 10 nm/min, 35 mm and 350…400 °C, respectively. Postdeposition annealing 5…8 min in oxygen atmosphere at 500…550 °C was performed.
The crystallographic studies were performed using X-ray Diffractometer (D8 ADVANCE X-ray Diffractometer with DAVINCI) using CuKα wavelength (λ = 1.54059 Å) and scanning in 2Θ range from 10° to 70°.
The surface analysis was performed using atomic force microscopy (AFM) “Nanotec Electronica” in Dynamic Mode (non-contact) with the force constant K ~ 40 N/m and f0 ~ 300 kHz. Optical transmission and absorption spectra between 300 and 700 nm were measured using a grating monochromator, photo-detector system and registering computer system. This setup was also used to register photoluminescence spectra. For this kind of measurements, the samples were excited using a 325-nm He-Cd laser with the excitation intensity 10 mW.
3. Results and discussion
The X-ray diffraction (XRD) patterns of
O
Co
Zn
x
x
1
-
(0 ≤ x ≤ 0.06) films are shown in Fig. 1. All intense peak positions of the films correspond to the standard diffraction pattern of ZnO hexagonal wurtzite with a (002) preferred orientation. With an increase in Co content (x > 0.02) additional diffraction peaks were observed, corresponding to (100), (101), and (102) of ZnO. No peaks corresponding to cobalt metal clusters or cobalt oxides were observed on the patterns, which indicates that Co enters the ZnO lattice without changing the wurtzite structures and systematically substitutes Zn2+ ions in the lattice. The relative intensity of the (002) peak of
O
Co
Zn
x
x
1
-
(0 ≤ x ≤ 0.06) reaches higher values than that of other peaks, indicating the c-axis preferred texture growth of the Co-doped ZnO films. Further, the intensity of diffraction peaks of Zn1–xCoxO films increased with increasing Co concentration, which can be attributed to a refinement and improvement of crystalline quality of these films as a result of Co doping. At the same time, we observed a shift only in the angular peak position of (002) plane, and no angular shift was observed for any other plane. This angular peak shift towards the lower 2Θ value, when transfering from the undoped sample to the doped up to x = 0.04 one, indicates that films are in a uniform state of stress. With a further increase in the Co concentration, the peak shift takes place in opposite direction, i.e. 2Θ value increases, indicating the change in the direction of stress. The average crystallite size D was calculated using the full width at the half maximum (FWHM) of the diffraction peak and the angle of diffraction Θ in Debye-Scherer’s formula [11]:
Q
Q
D
l
=
cos
0.9
D
,
(1)
where λ is the X-ray wavelength, ΔΘ – FWHM, and Θ – Bragg angle of the diffraction peak. The lattice parameters a and c for the samples were calculated using the appropriate formula and depicted in Fig. 2. Co-doping slightly decreases the lattice parameters of the ZnO films, considering that the size of Co2+ in tetrahedral configuration (0.058 nm) is close to that of Zn2+ in tetrahedral coordination (0.06 nm). Thus, the Co2+ ions are systematically substituted by Zn2+ ions without changing the crystal structure.
The surface morphology of the Zn1–xCoxO films and their crystallite sizes have been studied and evaluated by AFM. The root mean square of the surface roughness is measured from the pictures over 5×5 μm scanning range. The AFM micrograph shown in Fig. 3 indicates that the increase in the surface roughness after Co doping is caused by closely packed nanorod-like nanocrystallites oriented non-perpendicularly to the film-substrate plane over the whole Zn0.98Co0.02O film surface. The estimated diameter of the observed nanorods is about 100 nm. Essentially, there is no obvious change in the microstructure observed in the films grown with different Co concentrations.
202530354045505560
Co content (%)
1 x=0
2 x=0.02
3 x=0.04
4 x=0.06
2deg)
0
2
4
6
Fig. 1. X-ray diffraction spectra of Co doped ZnO thin films with various doping levels.
O
Co
Zn
x
x
1
-
Fig. 2. Variation of lattice parameters a and c with increasing the Co content in Zn1–xCoxO thin films.
Fig. 3. 3D AFM images of Zn1–xCoxO thin films with the Co content x = 0.02.
UV-vis optical measurements were carried out at room temperature to study the effect of Co doping concentration on the band gap of ZnO thin films and to confirm the substitution of Co2+ ions in tetrahedral sites of the ZnO wurtzite structures. Fig. 4 shows the optical transmission spectra of undoped and cobalt doped ZnO thin films deposited on glass substrates. The undoped ZnO film is more transparent, whereas in cobalt-doped thin films the transmittance decreases with the increase of Co-doping concentration. The optical transmittance spectra showed a shift in the band edge towards the lower energy side with the increase of Co content in the films. Additional absorption below the absorption edge can be seen for the
O
Co
Zn
x
x
1
-
thin films. In particular, for the Co content x = 0.02, three absorption bands at about 1.87, 2.03 and 2.19 eV have been revealed, which are in agreement with already reported absorption peaks [12, 13]. This absorption structure is associated with d-d electron transitions of Co2+ ions in a tetragonal crystal field. According to Hund’s rule and Pauli’s exclusion principle, the electronic ground state configuration has L = 3 and S = 3/2. So, the ground state spectral term is 4F and the excited state terms are 4P, 2G, 2F, 2D and 2P. However, when Co2+ exists in the tetrahedral field, the 4F term splits into 4A2(F), 4T2(F) and 4T1(F), with 4A2(F) being the lowest in energy and the remaining two having higher energies. The 4P term corresponding to the first excited state does not split, but is transformed into 4T1(P). Similarly, 2G splits into 2A1(G), 2E(G), 2T1(G) and 2T2(G). In the ground state, the atom is in 4A2 state. When electron has a sufficient energy, it can be excited to a higher energy state. The observed peaks were attributed to 4A1(F) ( 2A1(G), 4A2(F) ( 4T1(P) and 4A2(F) ( 2E(G) transitions in Co2+ ions. Thus, observation of these transitions in transmission spectra of our
O
Co
Zn
x
x
1
-
films clearly reveals that the added cobalt atoms have been substituted by Zn2+ cations and are present in 2+ state.
The optical band-gap energy (Eg) values for these films were evaluated by plotting ((h()2 versus h(, where h( is the photon energy, and ( is the absorption coefficient that can be calculated from the film thickness d as well as transmittances T and T0, for the film and substrate, respectively, using equation ( =
–(1/d) ln(T/T0). Extrapolation of the linear portion of ((h()2 to zero gives the value of Eg. The estimated Eg values are decreased with increase of the Co content (Fig. 5). This result is in contrast with the reported data [13, 15] where a blue shift of the absorption edge was observed. On the other side, a similar trend of decrease of the band edge in
O
Co
Zn
x
x
1
-
films is reported by many researchers [12, 14, 16]. This low-energy shift of Eg as a function of the Co content can be explained by two reasons: (i) it might result from destabilization of O2– (2p6 orbitals at O2– ions adjacent to Co2+), which extends the valence band edge by 0.5 eV [17] or point defects such as nanoclusters of Co, CoO can also lead to this shift [18] (however, these defects have not been observed in the present films); (ii) as mainly due to the s-d and p-d spin-exchange interactions between band electrons and localized d-electrons of Co2+ ions substituting Zn2+ ions [7, 19]. The exchange interaction between transition metal ions and band electrons gives rise to negative and positive corrections to the conduction and valence-band edges, leading to narrowing the band gap [20].
Room temperature photoluminescence (PL) of
Zn1–xCoxO thin films measured by exciting at 325 nm is shown in Fig. 6. The PL spectra show four peaks occurring around 386, 426, 443, and 538 nm for all the samples on glass and silica substrates. The first peak is in the ultraviolet region, while other three peaks correspond to the violet-blue, blue and green ones, respectively, i.e., in the visible region. The UV emission band is frequently observed in ZnO film and can be attributed to the near-band-edge exciton emission, because the emission energy is almost equal to the band energy of ZnO [21] estimated by UV-vis measurements. With a Co doping content increase, the UV emission center shifts to long wavelengths. It can be attributed to the strong exchange interactions between d-electrons of doping ion and s-, p-electrons of the host band [22]. This red shift of near-band-edge emission confirms that the band gap of Zn1–xCoxO decreases with increasing the Co content and is in good agreement with data obtained from optical absorption studies. The violet-blue emission centered near 426 nm is probably due to radiative defects related to traps existing at grain boundaries and emitted due to the radiative transition between this level and the valence band [23, 24].
0.00
0.02
0.04
0.06
5.20
5.22
5.24
5.26
c
a
Co content
Lattice parameter c (
C
)
3.25
3.26
3.27
3.28
Lattice parameter a (
C
)
With increasing the Co concentration, the peak position of blue emission shifts slightly to higher wavelengths from 443 up to 448 nm. The mechanism of blue emission (~443 nm) in ZnO low-dimensional structures is still controversial. Xu et al. [25] synthesized single crystalline ZnO nanoplates by hydrothermal procedure and attributed blue emission to electron transitions from the level of ionized oxygen vacancies to the valence band. Gokulakrishnan et al. [26] studied Zr-doped ZnO thin films and ascribed the 443-nm centered emission band to a surface defect in the ZnO films. R. Elilarassi, G. Chandrasekaran [27] and Zeng et al. [28] attributed this peak to the interstitial Zn level (Zni) and valence band. F.L. Xian et al. [29] attributed the blue emission to the states of interstitial cobalt transition to the valence band. In the latter, it indicates the red shift and change in intensity of emission band with increase of the Co content. The green band emission centered at approximately 538 nm is related to oxygen vacancy, and it is assigned to transitions from single ionized oxygen vacancies (VO) to the valence band [30, 31].
4. Conclusions
In conclusion, Zn1–xCoxO thin films with x ( 0.06 were deposited by the RF reactive sputtering technique on glass and silica substrates. XRD analysis of the films reveals that cobalt ions are successfully doped in ZnO without changing the hexagonal wurtzite structure. The grown Co-doped ZnO films show c-axis preferred orientation with good crystallinity. AFM study shows that the surface morphology of the
O
Co
Zn
x
x
1
-
films is composed of closely packed nanocrystallites with nanorod shape. The optical absorption spectra of the films demonstrate that the energy band gap was found to decrease with the Co content increase. Band gap narrowing effect occurs due to strong exchange interactions between d-electrons of doping ion and s-, p-electrons of the host lattice. The room temperature PL measurements illustrate that UV emission and violet-blue, blue and green emissions are in the visible region. The UV emission peak originates from the radiative recombination of free excitons and their center shifts to long wavelengths with increasing the Co content. Other emissions may be attributed to the radiative defects related to traps existing at grain boundaries for violet-blue emission, cobalt and zinc interstitial for blue emission, and singly ionized oxygen vacancies (VO) for green band emission, respectively.
References
1.
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T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in zinc-blende magnetic semiconductors // Science, 287, p. 1019-1022 (2000).
7.
X. Xu, C. Cao, Hydrothermal synthesis of Co-doped ZnO flakes with room temperature ferromagnetism // J. Alloys Compound. 501, p. 265-268 (2010).
8.
K.P. Bhatti, V.K. Malik, S. Chaudhary, Cobalt substituted ZnO thin films: a potential candidate for spintronics // J. Mater Sci.: Mater. Electron. 19, p. 849-854 (2008).
9.
Y. Belghazi, M. Ait Aouaj, M. El Yadari, G. Schmerber, C. Ulhaq-Bouillet, C. Leuvrey, S. Colis, M. Abd-lefdil, A. Berrada, A. Dinia, Elaboration and characterization of Co-doped ZnO thin films deposited by spray pyrolysis technique // Microelectronics J. 40, p. 265-267 (2009).
10.
A.S. Risbud, N.A. Spaldin, Z.Q. Chen, S. Stemmer, and R. Seshadri, Magnetism in polycrystalline cobalt-substituted zinc oxide // Phys. Rev. B, 68, 205202 (2003).
11.
B.D. Cullity, Elements of X-ray Diffractions. Addison-Wesley, Reading, 1978.
12.
J.H. Kim, H. Kim, D. Kim, S.G. Yoon, W.K. Choo, Optical and magnetic properties of laser deposited Co-doped ZnO thin films // Solid State Communs. 131, p. 677-680 (2004).
13.
K.J. Kim, Y.R. Park, Spectroscopic ellipsometry study of optical transitions in alloys // Appl. Phys. Lett. 81(8), p. 1420-1422 (2002).
14.
K.T.R. Reddy, V. Supriya, Y. Murata, M. Sugiyama, Effect of Co-doping on the properties of Zn1−xCoxO films deposited by spray pyrolysis // Surface and Coating Technology, 231, p. 149-152 (2013).
15.
M. Subramanian, M. Tanemura, T. Hihara, V. Ganesan, T. Soga, T. Jimbo, Magnetic anisotropy in nanocrystalline Co-doped ZnO thin films // Chem. Phys. Lett. 487, p. 97-100 (2010).
16.
S. Colis, H. Bieber, S. Begin-Colin, G. Schmerber, C. Leuvrey, A. Dinia, Magnetic properties of Co-doped ZnO diluted magnetic semiconductors prepared by low-temperature mechanosynthesis // Chem. Phys. Lett. 422, p. 529-533 (2006).
17.
A. Fouchet, W. Prellier, L. Mechin, Growth and characterizations of ZnO and Co-doped ZnO films for their use in spintronics // Superlatt. Microstruct. 42, p. 185-190 (2007).
18.
N. Bahadur, A.K. Srivastava, S. Kumar, M. Deepa, D. Nag, Influence of cobalt doping on the crystalline structure, optical and mechanical properties of ZnO thin films // Thin Solid Films, 518, p. 5257-5264 (2010).
19.
L. Wei, Z. Li, W.F. Zhang, Influence of Co doping content on its valence state in
O
Co
Zn
x
x
1
-
(0≤ x ≤ 0.15) thin films // Appl. Surf. Sci. 255, p. 4992-4995 (2009).
20.
J.K. Furdyna, Diluted magnetic semiconductors // J. Appl. Phys. 64(4), p. R29 (1988).
21.
Y.G. Wang, S.P. Lau, H. Lee, S.F. Yu, B.K. Tay, X.H. Zhang, and H.H. Hng, Photoluminescence study of ZnO films prepared by thermal oxidation of Zn metallic films in air // J. Appl. Phys. 94, p. 354-358 (2003).
22.
B. Panigrahy, M. Aslam, and D. Bahadur, Aqueous synthesis of Mn-and Co-doped ZnO nanorods // J. Phys. Chem. C, 114, p. 11758-11763 (2010).
23.
S. Kumar, S. Basu, B. Rana, A. Barman, S. Chatterjee, Jha, and A.K. Ghosh, Structural, optical and magnetic properties of sol–gel derived ZnO: Co diluted magnetic semiconductor nanocrystals: an EXAFS study // J. Mater. Chem. C, 2(3), p. 481-495 (2014).
24.
L.J. Zhuge, X.M. Wu, Z.F. Wu, X.M. Yang, X.M. Chen, Q. Chen, Structure and deep ultraviolet emission of Co-doped ZnO films with Co3O4 nanoclusters // Mat. Chem. Phys. 120, p. 480-483 (2010).
25.
F. Xu, Z.Y. Yuan, G.H. Du, M. Halasa, and B.L. Su, High-yield synthesis of single-crystalline ZnO hexagonal nanoplates and accounts of their optical and photocatalytic properties // Appl. Phys. A, 86(2), p. 181-185 (2007).
26.
V. Gokulakrishnan, S. Parthiban, K. Jeganathan, and K. Ramamurthi, Investigation on the effect of Zr doping in ZnO thin films by spray pyrolysis // Appl. Surf. Sci. 257(21), p. 9068-9072 (2011).
27.
R. Elilarassi, G. Chandrasekaran, Influence of Co-doping on the structural, optical and magnetic properties of ZnO nanoparticles synthesized using auto-combustion method // J. Mater. Sci.: Mater. Electron. 24, p. 96-105 (2013).
28.
H. Zeng, G. Duan, Y. Li, S. Yang, X. Xu, and W. Cai, Blue luminescence of ZnO nanoparticles based on non-equilibrium processes: Defect origins and emission controls // Adv. Functional Materials, 20(4), p. 561-572 (2010).
29.
F.L. Xian, L.H. Xu, X.X. Wang, and X.Y. Li, Crystallographic, optical and magnetic properties of Co-doped ZnO thin films synthesized by sol gel route // Cryst. Res. Technol. 47(4), p. 423-428 (2012).
30.
S. Chakraborty, and P. Kumbhakar, Observation of bandgap narrowing effect and photoluminescence emission characteristics of chemically synthesized Co doped ZnO nanosheets // Indian J. Phys. 88.3, p. 251-257 (2014).
31.
J. Ding, X. Yana, Q. Xue, Study on field emission and photoluminescence properties of ZnO/grapheme hybrids grown on Si substrates // Mater. Chem. Phys. 133, p. 405-409 (2012).
� EMBED Origin50.График ���
� EMBED Origin50.Graph ���
Fig. 4. Transmittance spectra of Zn1–xCoxO thin films with various contents of Co in the spectral region near the absorption edge.
� EMBED Origin50.Graph ���
Fig. 5. The energy band gap of Zn1–xCoxO thin films as a function of the Co content.
� EMBED Origin50.Graph ���
Fig. 6. The room temperature PL spectra of Zn1–xCoxO thin films with various Co contents.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
353
2.2
2.4
2.6
2.8
3.0
3.2
0.0
0.2
0.4
0.6
0.8
4
3
1
2
Transmittance
Photon energy (eV)
1
x=0
2
x= 0.02
3
x= 0.04
4
x= 0.06
5
x= 0.08
5
0.00
0.02
0.04
0.06
0.08
2.9
3.0
3.1
3.2
3.3
3.4
Band gap (eV)
Co dopant content
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
3
2
PL intensity (arb. un.)
Photon energy (eV)
1
2% Co
2
4% Co
3
6% Co
1
_1476160625.unknown
_1479629603.bin
_1479629748.bin
_1479629847.bin
_1476188850.bin
_1476160473.unknown
_1476160558.unknown
_1476159977.unknown
_1476159988.unknown
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