Physica B 576 (2020) 411713
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Physica B: Physics of Condensed Matter
journal homepage: http://www.elsevier.com/locate/physb
Effect of oxygen partial pressure during pulsed laser deposition on the
emission of Eu doped ZnO thin films
Vinod Kumar a, b, *, O.M. Ntwaeaborwa c, H.C. Swart b
a
Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, 110016, India
Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA, 9300, South Africa
c
School of Physics, University of the Witwatersrand, Private Bag X3, Wits, 2050, South Africa
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxygen partial pressure
ZnO:Eu3þ
Pulsed laser deposition
Eu3þ doped ZnO (ZnO:Eu3þ) thin films were prepared by pulsed laser deposition at different oxygen partial
pressures. The all ZnO:Eu3þ thin films have a hexagonal structure. The morphology and roughness of the ZnO:
Eu3þ thin film were also dependent on the partial pressure of the oxygen. The strong band to band emission was
observed with the weak emission of both Eu3þ and defects. The intensity of the band emission peaks has
increased with an increase in the oxygen partial pressure.
1. Introduction
Wide band gap semiconductors, such as Zinc oxide (ZnO), doped
with rare earth trivalent (RE3þ) ions, such as Eu3þ ions, have gained
considerable attention in the field of optoelectronic devices because of
their potential use in different applications including light emitting di­
odes (LEDs), flat panel displays (FPDs), plasma display panels (PDPs),
fluorescent lamps, etc [1–3]. ZnO is a wide band gap material with
advantages such as large exciton binding energy, high conductivity, high
thermal stability in hydrogen plasma atmosphere and high chem­
ical/physical stability [3–5]. Different methods have been used to syn­
thesize RE doped ZnO nanophosphors and the emitted light can be tuned
throughout the visible range [3]. Eu3þ doped ZnO presents a great op­
portunity to achieve intense red emission, owing to the advantage of the
4f-4f transition of Eu3þ ions and the excellent properties of ZnO as a
host. Thin films create an opportunity of good adhesion between the
substrate and the film material and therefore provides better lumines­
cence properties, thermal stability and small and uniform particles,
which give high image resolution [6], in some cases the emission in­
tensity is still low and more research needed to improve the efficiency of
the light output. ZnO thin films have been deposited with a number of
different techniques such as pulsed laser deposition (PLD), radio fre­
quency magnetron sputtering, sol-gel spin coating and chemical vapor
deposition [7–12]. Among these techniques, the PLD is preferable and
provides unique growth of oxide material due to the oxygen plasma
created by the pulsed laser, is very energetic and its density is easily
controllable by the oxygen pressure [13]. Doping of ZnO with selective
elements has become an important route for enhancing and tuning op­
tical, electrical, and magnetic properties, which are usually crucial for
their practical applications. It is well known that rare earth (RE) ions are
better candidates as luminescent centres because their special 4f
intra-shell transitions usually have rich spectral lines [14]. The density
of the oxygen vacancy in ZnO was easily controlled by varying the ox­
ygen partial pressure during film deposition [15].
In this work, the effect of oxygen partial pressure during PLD on the
structural, morphological and optical properties of ZnO:Eu3þ thin films
was investigated. A schematic band diagram is provided to understand
the emission mechanism from ZnO:Eu3þ thin films.
2. Experimental details
ZnO:Eu3þ thin films were deposited on Si (100) wafers using a target
in a reactive PLD process. ZnO:Eu3þ (4 mol% Eu) powder synthesized by
the solution-combustion method was used for the preparation of the
target. The optimization (4 mol% Eu) and characterization of ZnO
powder have already been reported elsewhere [16]. A 266 nm Nd:YAG
laser was used for the ablation. The silicon substrates were cleaned ul­
trasonically using acetone, ethanol and deionized water; after that it was
dried by N2 gas. The other parameter for the thin film fabrication by PLD
were already reported in our previous work [3,17,18]. The substrate
temperature was kept at 300 � C during deposition of the thin films. The
laser energy and deposition time were fixed at 40 mJ and 45 min,
* Corresponding author. Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, 110016, India.
E-mail address: vinod.phy@gmail.com (V. Kumar).
https://doi.org/10.1016/j.physb.2019.411713
Received 24 May 2019; Received in revised form 12 September 2019; Accepted 19 September 2019
Available online 19 September 2019
0921-4526/© 2019 Elsevier B.V. All rights reserved.
V. Kumar et al.
Physica B: Physics of Condensed Matter 576 (2020) 411713
Intensity (arb. units)
respectively. The chamber was pumped down to a background pressure
of 5 � 10 5 mbar. It was backfilled with O2 gas during deposition for
different partial pressures, from 50 m Torr to 250 m Torr.
The structural properties were analysed by using an X-ray diffrac­
tometer (XRD) PAN analytical X’pert PRO. Photoluminescence (PL) data
were recorded using a He–Cd laser with a 325 nm excitation wave­
length. The surface morphology of the ZnO:Eu3þ film was examined by
using Shimadzu SPM-9600 atomic force microscope (AFM) in contact
mode. All the characterizations were done at room temperature.
3. Results and discussion
The XRD patterns of the ZnO:Eu3þ phosphor with different concen­
trations of Eu3þ are shown in Fig. 1(a). The ZnO:Eu3þ nanoparticles
were highly crystalline in nature. The strong diffraction peaks at 31.7� ,
34.3� and 36.2� correspond to the (100), (002) and (101) planes of the
hexagonal wurtzite structure of ZnO (ICSD card no. 29272). The lattice
parameter of ZnO, as calculated in a previous report, has increased with
an increase in the doping concentrating due to the large ionic radius of
the Eu3þ (0.95 Å) as compared with the Zn2þ (0.74 Å), indicating that
the incorporated Eu3þ ions occupied the lattice sites of the Zn2þ and lead
to the increase in the inter-atomic distance and the expansion in lattice
constants [16]. It indicated that Eu3þ ions were properly incorporated
into the lattice. A small peak observed at 29.4� (marked with an asterisk)
is assigned to the cubic structure of Eu2O3 (JCPDS Database no.
43–1009) formed at higher Eu3þ concentrations. The PL spectra of ZnO:
Eu3þ phosphors with different Eu3þ concentration are shown in Fig. 1
(b). The broad emission was due to defects related emission of ZnO,
while the sharp emission indicated the emission due to the Eu3þ ions.
The defect related and band emissions in ZnO are dependent on the
synthesis method, annealing temperature, and several other parameters
as summarized in Ref. [3]. The optimization of the doping concentration
of the Eu3þ in ZnO:Eu3þ powder was reported in our previous work [16].
The optimized 4 mol% Eu3þ doping concentration of Eu3þ was used for
making the powder and this optimized powder was used for making the
target for PLD.
The XRD patterns of the ZnO:Eu3þ thin films prepared by the PLD
technique at different oxygen partial pressures are shown in Fig. 2. The
XRD reflections of the ZnO:Eu3þ films prepared by the PLD technique at
different oxygen partial pressures were found to be oriented along the
(002) plane. This is in line with the characteristics of the hexagonal ZnO
wurtzite structure, where the c-axis is perpendicular to the substrate
plane [17]. The intensity of the XRD peaks has increased (from 2000 to
18 000 counts) with an increase in the oxygen partial pressure. ZnO
films usually deviated by the stoichiometry and incorporated of intrinsic
defects, such as zinc interstitials (Zni) and oxygen vacancies (VO),
5 mol%
4 mol%
3 mol%
2 mol%
1 mol%
50 mTorr
25
30
35
40
2 Theta (deg.)
45
50
Fig. 2. XRD patterns of the ZnO:Eu3þ thin films prepared at different oxygen
partial pressures.
especially when they are grown in Zn-rich or O-deficient atmospheres.
The number of intrinsic defects can be reduced with an increase in the
oxygen partial pressure and the film crystallinity may therefore be
improved [15,19].
The effect of partial pressure on the microstructure and surface
morphology of the ZnO:Eu3þ films was studied by AFM. The three
dimension (3D) AFM images of the ZnO:Eu3þ films deposited on the Si
substrates deposited at the different oxygen partial pressure are shown
in Fig. 3. The root mean square (RMS) roughness of the thin film has
increased from 25 to 36 nm with an increase in the partial pressure of
oxygen during synthesis of the film. The surface roughness of the ZnO:
Eu3þ thin film changed severely with a change in the partial pressure
during the deposition of the films by PLD [20]. The roughness were
higher than for example thin films obtained with spin coating, where the
roughness changed from 2.3 to 10.6 nm depending on the experimental
conditions [21]. A ZnO channel layer with a RMS roughness of 0.7 nm
was obtained by using radio-frequency (RF) magnetron sputtering at
350 � C [20]. Their XRD pattern revealed a dense columnar structure of
closely packed ZnO nano grains along the c-axis [22]. At higher oxygen
pressure, the ablated species from the target could undergo much more
collisions with the oxygen molecules than those under lower oxygen
pressure. These collisions decreased the kinetic energy of these species.
(b)
6 mol%
*
150 mTorr
(002)
Intensity (arb. units)
Intensity
(arb.(a.u.)
units)
Intensity
(a)
20
250 mTorr
0 mol%
30
40
50
60
2 Theta
(deg.)
2 (deg.)
70
Wavelength (nm)
Fig. 1. Effect of doping concentration on (a) the XRD pattern and (b) the PL emission of ZnO:Eu3þ phosphors powder [Reproduced from Ref. [16] with permission].
2
V. Kumar et al.
Physica B: Physics of Condensed Matter 576 (2020) 411713
(a)
(b)
(c)
Fig. 3. The effect of oxygen partial pressure on the morphology of ZnO:Eu3þ thin films prepared by PLD at different oxygen partial pressures of (a) 50 m Torr (b) 150
mTorr and (c) 250 mTorr.
Consequently, the deposited ad-atoms do not have the required mobility
in order to diffuse uniformly on the substrate and thus they form clusters
leading to an increase in the roughness of the film [23].
The PL spectra of ZnO:Eu3þ thin films at different oxygen partial
pressure are shown in Fig. 4(a). Generally, ZnO have exhibited two types
of emission: UV and visible emission due to band to band emission, also
called near band edge emission (NBE) and deep level defects (DLE),
respectively [18,24]. Normally ZnO has six kinds of defects, namely zinc
vacancies (Vzn), oxygen vacancies (Vo), zinc interstitials (Zni), oxygen
interstitials (Oi), zinc antisites (OZn) and oxygen antisites (ZnO), The BBE
is very strong in all samples with a small amount of defects. The intensity
of the BBE has increased with an increase in the partial pressure of
oxygen. The oxygen partial pressure influenced the structural and op­
tical properties of ZnO due to the oxygen defects in the ZnO thin films
and is directly correlated to the deposition partial pressure [22]. There is
always a competition between the NBE and the defect emission
recombination mechanisms [25]. Normally the UV emission intensities
decreased when the deep level emission intensities increased [26]. The
increase of the ambient oxygen gas pressure from 50 to 250 mTorr
increased the number of reactive oxygen ions generated by the photon
and electron impact dissociation during the deposition process. Subse­
quently, the increase of the reactive oxygen ions and the interaction of
the Zn ion with the reactive oxygen ion during the transportation to the
substrate lead to the reduction of oxygen vacancies. The increase in the
Fig. 4. (a) Effect of oxygen partial pressures on the Photoluminescence properties of ZnO:Eu3þ thin films prepared by PLD and (b) the energy band diagram of ZnO:
Eu3þ thin films.
3
V. Kumar et al.
Physica B: Physics of Condensed Matter 576 (2020) 411713
UV emission was therefore due to the decrease of oxygen vacancies in
the ZnO thin films, which have further decreased with an increase in
oxygen pressure. The crystallinity was improved with deposition at
higher oxygen partial pressures with less oxygen vacancies. In the
powder, the concentration of defects was high, so it has shown very
small NBE, while during the synthesis of the thin films by PLD, the ab­
lated material interact with the oxygen in the system when transferred
from the target to substrate reducing the oxygen vacancies in the pro­
cess. The Eu3þ f-f transitions emission were also observed at 617 and
590. The peaks were observed due to the Eu3þ f-f transitions at 590 and
617 nm attributed to the 5D0→7F2 and 5D0→7F1 radiative transitions,
respectively [27]. In the PL curve of the ZnO:Eu3þ phosphor [Fig. 1(b)]
all transitions of Eu3þ ions were observed although the
5
D0→7F0,5D0→7F3 and 5D0→7F4 were less intense compared to the two
5
D0→7F1 and 5D0→7F2 transitions, but in the PLD films only a low in­
€r
tensity of the two 5D0→7F1 and 5D0→7F2 transitions were observed. Ba
et al. [28] monitored the effect of an amorphous interface layer on the
luminescence of Eu3þ-doped GdVO4 PLD films, where a broadening of
the Eu3þ luminescence peaks was observed. They found additionally, at
the position of the 5D0 →7F1 transition two emission peaks at 593 and
595 nm. These could be explained by the two Stark levels of the 7F1
level, which were separated by 39 cm 1. Some of the PLD films did not
show these sharp emission lines due to inhomogeneous broadening. In
the corresponding excitation spectrum the peaks were strongly broad­
ened and the 7F0 → 5D3 and 7F0 → 5D1 transitions barely exceed the noise
level.
The schematic band diagram of the emissions in the ZnO:Eu3þ was
constructed from the data in Fig. 4. The gap between valence and con­
duction band is ~3.28 eV, constructed from Pl data. The emission peak
of Eu3þ is constructed from PL emission curve. The position of the
theoretical value of the Zni level is at 0.22 eV below the conduction band
[29,30]. The band transitions from Zni to Oi, Zni to Vo and the con­
duction band to Vo are at ~2.06, 1.63, and 1.85 eV, respectively. A
non-radiative decay is possible from the conduction band of ZnO to the
higher state of the Eu3þ and then recombined and provided different
transition emissions. The emission intensity of the Eu3þ ions related
peak was less in the thin films with respect to the phosphor powder.
the preferential orientated ZnO also needs to be expanded.
Acknowledgments
One of the authors (V.K.) is thankful to DST, New Delhi, India, for
support through DST-Inspire faculty award [DST/INSPIRE/04/2015/
001497]. This work is based on the research supported by the National
Research Foundation of South Africa (Grant Numbers 115126). This
work is also based on the research supported by the South African
Research Chairs Initiative of Department of Science and Technology and
National Research Foundation of South Africa (84415).
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4. Conclusion
Eu3þ doped ZnO thin films at different partial pressures of oxygen
were successfully prepared by pulsed laser deposition. The XRD results
confirmed the (002) preferential growth hexagonal wurtzite structure of
ZnO. Band to band and Eu3þ related emission were observed in the PL
results. Both band to band and Eu3þ emission increased with an increase
in oxygen partial pressure, together with an increase in surface rough­
ness. A big differences between the PL and XRD of the powder and thin
films were observed. The concentration of defects was high in the case of
the powder, so it has shown very small NBE with higher defects (DLE)
and Eu3þ emission. While during the synthesis of the thin films by PLD,
the ablated material interact with the oxygen in the system when
transferred from the target to substrate reducing the oxygen vacancies in
the process. With the consequent increase in NBE and less DLE emission.
The Eu3þ emission was also severely quenched for the thin films. The
reasons for the quenching of the Eu3þ emission still need more attention.
The question may arise if the energy transfer to Eu3þ is either more
effective from the defect levels or from the conduction band. It surely
appeared that with less defects, less energy transfer has occurred. Many
more experimental investigations to optimise the Eu3þ emission with
different Eu3þ concentration are needed and the study on the effect of
4