Abstract X-ray absorption near edge structure spectroscopy has been used to investigate the electronic and atomic structure of vanadium-doped ZnO thin films obtained by reactive plasma. The results show no sign of metallic clustering of Vatoms, +4 oxidation state of V, 4-fold coordination of Zn in the films, and a secondary phase (possibly VO2) formation at 15% V doping. O K edge spectra show V 3d–O 2p and Zn 4d–O 2p hybridization, and suggest that V4+ acts as electron donor that fills the σ* band. © 2006 Elsevier B.V. All rights reserved. Keywords: ZnO; XANES; Vanadium doping; Thin films; DC-sputterin

Faiz, M.

Hussain, Z.

Mekki, A.

Mun, B.S.

Tabet, N.

King Fahd University of Petroleum and Minerals

006) 1377–1379
www.elsevier.com/locate/tsfThin Solid Films 515 (2X-ray absorption near edge structure investigation of
vanadium-doped ZnO thin films
M. Faiz a,⁎, N. Tabet a, A. Mekki a, B.S. Mun b, Z. Hussain b
a Surface Science Laboratory, Physics Department, KFUPM, Dhahran, Saudi Arabia
b Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Received 7 September 2005; received in revised form 8 March 2006; accepted 22 March 2006
Available online 11 May 2006Abstract
X-ray absorption near edge structure spectroscopy has been used to investigate the electronic and atomic structure of vanadium-doped ZnO
thin films obtained by reactive plasma. The results show no sign of metallic clustering of V atoms, +4 oxidation state of V, 4-fold coordination of
Zn in the films, and a secondary phase (possibly VO2) formation at 15% V doping. O K edge spectra show V 3d–O 2p and Zn 4d–O 2p
hybridization, and suggest that V4+ acts as electron donor that fills the σ* band.
© 2006 Elsevier B.V. All rights reserved.Keywords: ZnO; XANES; Vanadium doping; Thin films; DC-sputtering1. Introduction
Zinc oxide is being the object of intense investigation owing
to its attractive applications in optoelectronics. Transition metal-
doped ZnO has the potential to be a multifunctional material
with coexisting magnetic, semiconducting, electromechanical,
and optical properties [1]. Ferromagnetism in diluted magnetic
semiconductors is one of the interesting problems of this century
in condensed matter physics. Vanadium-doped ZnO (Zn1−xVxO)
shows ferromagnetism above room temperature upon electron
doping [2–4]. Such a high Curie temperature is appealing for its
applications in spintronics devices. Understanding the electronic
structure is a prerequisite for understanding the high-tempera-
ture ferromagnetism in these materials.
X-ray absorption near edge structure (XANES) spectroscopy
is a powerful local probe for the electronic structure as well as
the atomic structure of materials. It probes the empty energy
bands by measuring transitions from core levels, and does not
require a crystalline sample. Element specificity and symmetry
selection rules are some of the powerful aspects of the tech-
nique. In this work we have used XANES to investigate the
electronic structure of vanadium-doped ZnO thin films.⁎ Corresponding author.
0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2006.03.0442. Experimental details
Vanadium-doped ZnO films of less than 1-μm thickness
were deposited on unbiased glass substrate at room temperature
by reactive plasma in a direct-current magnetron sputtering
system. The target was a 99.995% pure zinc disk of 5-cm
diameter from Williams Advanced Materials. Various amount
of vanadium stripes (Materials Research Corporation—Marz
grade—99.99% purity) of about 0.15-cm width were fixed
diagonally on the target for the purpose of co-sputtering. The
experimental conditions were as follows: The distance between
the target and the substrate was 3 cm. The initial pressure in the
vacuum chamber was 5×10−5 Pa. Oxygen gas was introduced
through a leak valve up to a pressure=0.1 Pa, and then argon
gas was introduced. The total (Ar+O2) pressure during sput-
tering was 10 Pa. The high voltage was 850 V and the current
was 50 mA. Up to x=15% V-doped samples were obtained by
co-sputtering zinc and vanadium. The film composition was
estimated by X-ray Photoemission Spectroscopy.
The XANES measurements were performed on beamline
9.3.2 at the Advanced Light Source in Lawrence Berkeley
National Laboratory. High-resolution XANES spectra were
taken in the V L2,3, O K, and Zn L3 absorption edge regions,
while the monochromator was set at a resolving power of
≥4000. The photon flux at the sample was about 1011 photons/
1020 1025 1030 1035 1040 1045
x = 0.15
x = 0.11
x = 0.09
x = 0.03
CB
A
Ab
so
rp
tio
n 
(ar
b. 
un
its
)
Photon Energy (eV)
Fig. 2. Room-temperature XANES data of vanadium-doped ZnO thin films at
the Zn L3 edge region as a function of vanadium doping.
1378 M. Faiz et al. / Thin Solid Films 515 (2006) 1377–1379s/0.1% BW. The base pressure in the experimental chamber was
about 2×10¡©8 Pa. XANES measurements were performed in
total electron yield mode by measuring sample current. The
beam flux (I0) signal from a gold grid was used to normalize the
spectra.
3. Results and discussion
Fig. 1 shows room-temperature XANES data of vanadium-
doped ZnO thin films at the V L2,3 and O K edge region as a
function of vanadium doping. The energy scale was calibrated
using absorption peaks of Cr L2,3 edges originated from the
coatings of monochromator [5]. We normalized the spectra by
subtracting a straight line fitted to the region before the peaks
(below 512 eV), and adjusting the intensity of the spectra to 1 at
548 eV.
V L2,3 edge region shows a well-resolved prepeak at about
515.5 eV in addition to the L3 and L2 edges at about 517.7, and
524.7 eV, respectively. The prepeak is a signature of electron
transition from V 2p into the unoccupied V 4s states [6]. Since
the 4s states are full in elemental vanadium, the observation of
the prepeak implies that V is not in the elemental form in the film
down to the detection limit of XANES. In other words, there is
no detectable metallic clustering of V atoms in the film. The L3
and L2 edges are due to electron transition from 2p3/2 and 2p1/2
energy levels to V 3d–O 2p hybridized bands. The decrease in
the apparent spin–orbit splitting of V 2p level in the absorption
spectra (∼7.0 eV) as compared to the splitting observed in XPS
(∼7.7 eV) is a natural consequence of the strong interaction
between the 2p core hole and the 3d electrons in the final state
[7,8]. These interactions are of the same order of magnitude of
the V 2p spin–orbit splitting which causes a large redistribution
of the intensity throughout the entire spectra [8]. There is no
significant shift in the peak positions as a function of V doping.
This observation suggests that the oxidation state of V in the
films remains the same. The peak position of L3 edge suggests
that V is in +4 oxidation state in the film [9]. The peak-intensity510 520 530 540 550
x = 0.15
x = 0.11
x = 0.09
x = 0.03
DCBA
Ab
so
rp
tio
n 
(ar
b. 
un
its
)
Photon Energy (eV)
Fig. 1. Room-temperature XANES data of vanadium-doped ZnO thin films at
the V L2,3 and O K edge regions as a function of vanadium doping.ratio I(L3)/I(L2) is about 2, also indicating +4 oxidation state of
V in the film [10]. Since covalency reduces the number of filled
states with O 2p character, the intensity of the peaks is a measure
of covalency strength; the higher the intensity the higher the
covalency [11]. Note that the peak intensity decreases slightly as
x varies from 0.03 to 0.11 and then markedly increased at
x=0.15. This may be due to the formation of a secondary phase
at x=0.15. In fact, the solubility limit ofVinZnO is about 5mol%,
which is equivalent to x∼0.1. At higher V concentrations,
precipitation of VO2 phase occurs [12].
O K edge region shows three peaks labeled A, B, and C at
about 529.4, 530.8, and 532.3 eV, respectively in addition to a
broad peak (labeled D) centered about 536.3 eV. Peaks A, B,
and C are due to transitions from O 1s to the V 3d–O 2p and Zn
4d–O 2p hybridized bands while Peak D is assigned to V 4sp–
O 2p hybridized bands [7,11]. Only Peak D is observed in pure
wurtzite ZnO [13]. Ishida et al. [3] assigned Peaks A, B, and C
to π*, d//, and σ* bands, respectively based on molecular-orbital
and crystal-field ideas. It is interesting to note that the Peak-C
intensity gradually decreases as V-content increases and finally
disappears at x=0.15. We believe that V in +4 oxidation state
acts as electron donor and fills the σ* band [11]. In fact, Cao et
al. [14] have observed in their XPS study of Al and Mn co-
doped ZnO films that Al3+ and Mn4+ act as electron donors.
Defects can also have considerable influence on XANES spec-
tra as they distort the local symmetry and hence affect the
amount of hybridization. A detailed discussion of O K edge
features would require a symmetry-projected band structure
calculation, which is not available at present. Ishida et al. [3]
have observed in Zn0.95V0.05O, using photoemission spectros-
copy, hybridization of O 2p band with V 3d band that forms in
the band gap region of ZnO. However, they have also observed
suppression of spectral weight at the Fermi level, indicating on-
site Coulomb repulsion of V 3d electrons.
Fig. 2 shows room-temperature XANES data of vanadium-
doped ZnO thin films at the Zn L3 edge region as a function of
vanadium doping. We normalized the spectra by subtracting a
1379M. Faiz et al. / Thin Solid Films 515 (2006) 1377–1379straight line fitted to the region before the peaks (below 1025
eV), and adjusting the intensity of the spectra to 1 at 1098 eV.
The prepeak A at 1030.4 eV is a signature of electron transition
from Zn 2p into the unoccupied Zn 4s states [15]. Peaks B and C
centered about 1035 and 1039.5 eV, respectively are attributed
to the Zn 4d(t2g)–O 2p and Zn 4d(eg)–O 2p hybridized bands
[15]. This is in excellent agreement with the experimental and
calculated spectra of w-ZnO [13,15,16], suggesting 4-fold
coordination of Zn in the film. Note that the spectral weight at
Peak C decreases as V content increases and finally disappears
at the highest V doping (x=0.15). Similar observation was
noted, in Fig. 1, with Peak C of O K edge spectra. This cor-
relation seems to provide additional confirmation for the hy-
bridization of Zn 4d and O 2p bands. We suggest that the
disappearance of Peak C at x=0.15 in both spectra is due to the
formation of a secondary phase, possibly VO2 [12].
4. Conclusion
XANES technique has been used to investigate the electronic
and atomic structure of vanadium-doped ZnO thin films
obtained by reactive plasma. V L2,3 edge spectra show no
sign of metallic clustering of Vatoms in the films and indicate a
secondary phase (possibly VO2) formation at x=0.15. The peak
positions and the peak intensity ratios suggest that the oxidation
state of V in the films is +4. O K edge spectra suggest that V4+
acts as electron donor and fills the σ* band. Zn L3 edge spectra
indicate that Zn in the films is in 4-fold coordination like that in
pure wurtzite ZnO.
Acknowledgement
This work was partially supported by the US Department of
Energy under contract number DE-AC03-76F00098. Faiz, Tabet
and Mekki would like to thank KFUPM for its support.References
[1] R. Janisch, P. Gopal, N.A. Spaldin, J. Phys., Condens. Matter 17 (2005)
R657.
[2] N.H. Hong, J. Sakai, A. Hassini, J. Appl. Phys. 97 (2005) 10D312.
[3] Y. Ishida, J.I. Hwang, M. Kobayashi, A. Fujimori, H. Saeki, H. Tabata, T.
Kawai, Physica. B 351 (2004) 304.
[4] H. Saeki, H. Tabata, T. Kawai, Solid State Commun. 120 (2001) 439.
[5] R.D. Leapman, L.A. Grunes, P.L. Fejes, Phys. Rev. B 26 (1982) 614.
[6] J. Fink, T.M. Heinzerling, B. Scheerer, W. Speier, F.U. Hillebrecht, J.C.
Fuggle, J. Zannen, G.A. Sawatzky, Phys. Rev., B 32 (1985) 4899.
[7] M. Abbate, F.M.F. de Groot, J.C. Fuggle, Y.J. Ma, C.T. Chen, F. Sette, A.
Fujimori, Y. Ueda, K. Kosuge, Phys. Rev., B 43 (1991) 7263.
[8] J. Zaanen, G.A. Sawatzky, J. Fink, W. Speier, J.C. Fuggle, Phys. Rev., B 32
(1985) 4905.
[9] S. Surnev, M.G. Ramsey, F.P. Netzer, J. Phys., Condens. Matter 13 (2001)
11305.
[10] H. Kurata, C. Colliex, Phys. Rev., B 48 (1993) 2102.
[11] F.M.F. de Groot, M. Girioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H.
Petersen, Phys. Rev., B 40 (1989) 5715.
[12] Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Saito, T. Sekiguchi, Y.Z.
Yoo, M. Murakami, Y. Matsumoto, T. Hasegawa, H. Koinuma, Appl.
Phys. Lett. 78 (2001) 3824.
[13] T. Mizoguchi, I. Tanaka, S. Yoshioka, M. Kunisu, T. Yamamoto, W.Y.
Ching, Phys. Rev., B 70 (2004) 45103.
[14] H.T. Cao, Z.L. Pei, J. Gong, C. Sun, R.F. Huang, L.S. Wen, J. Solid State
Chem. 177 (2004) 1480.
[15] J.W. Chiou, J.C. Jan, H.M. Tsai, C.W. Bao, W.F. Pong, M.H. Tsai, I.H.
Hong, R. Klauser, J.F. Lee, J.J. Wu, S.C. Liu, Appl. Phys. Lett. 84 (2004)
3462.
[16] B. Gilbert, B.H. Frazer, H. Zhang, F. Huang, J.F. Banfield, D. Haskel, J.C.
Lang, G. Srajer, G. de Stasio, Phys. Rev., B 66 (2002) 245205.


X-ray absorption near edge structure investigation of vanadium-doped ZnO thin films

https://eprints.kfupm.edu.sa/id/eprint/523/1/FaizTabetetal.pdf

X-ray absorption near edge structure investigation of vanadium-doped ZnO thin films

Abstract

Similar works

Full text

Available Versions

King Fahd University of Petroleum and Minerals