Journal of Luminescence 260 (2023) 119836

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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin

Full Length Article

Structural and photoluminescence properties of Co-Sputtered p-type

Zn-doped β-Ga2O3 thin films on sapphire substrates
Anoop Kumar Singh a, 1, Chao-Chun Yen a, 1, Kai-Ping Chang a, Dong-Sing Wuu a, b, c, *
a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan
b
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan
c
Innovation and Development Center of Sustainable Agriculture, National Chung Hsing University, Taichung 40227, Taiwan

A R T I C L E I N F O A B S T R A C T

Keywords: This work discusses the growth characteristics, composition, and photoluminescence properties of Zn-doped
Zn-doped Ga2O3 film Ga2O3 (ZnGaO) films. The idea of doping of Zn divalent cation in β-Ga2O3 is to modulate the n-type conduc­
p-type tivity of β-Ga2O3 to p-type. Therefore, a series of ZnGaO films with varying Zn contents have been deposited on
Sapphire
sapphire substrates using co-sputtering of Ga2O3 and Zn targets at the substrate temperature of 400 ◦ C. The X-ray
Co-sputtering
diffraction analysis revealed that divalent Zn dopant is stable up to 8.62% in ZnGaO films. The X-ray photo­
Photoluminescence
First-principles calculation electron spectroscopy defined the increasing amount of Zn content in ZnGaO films. The lowest defect formation
energy per atom by first-principles calculations indicates that the favourable site of Zn atoms is substitutional Ga
tetrahedral site (T-site) in ZnGaO. The photoluminescence (PL) spectra exhibited that the peak emission
wavelength of β-Ga2O3 can be shifted with the inclusion of divalent Zn dopant in Ga2O3 films, which is in
accordance with the energy diagram and charge density distribution, indicating the Zn substituted T-site Ga are
leading to more defect states, and inducing green luminescence in PL spectra. The ZnGaO films exhibited positive
Hall coefficient, which verifies the p-type nature of films. ZnGaO films demonstrate a unique ability to realize p-
type characteristics among emerging wide bandgap semiconductors, extending its applications at the forefront of
contemporary optoelectronics technologies.

1. Introduction focused on the luminescence characteristics of n-type β-Ga2O3 phos­

phors, which leaves behind the scope to investigate the effective
Recently, transparent semiconducting oxides are of particular in­ acceptor dopant for β-Ga2O3 phosphors.
terests due to their wide-bandgap, high chemical and thermal stabilities, The divalent cations such as Zn, Mg, and Cu dopants were believed to
which makes them suitable for gas sensors, deep-ultraviolet photode­ change the characteristics of β-Ga2O3 from n-type to p-type. Among
tectors, and other optoelectronic devices [1,2]. Ga2O3 attracted the them, Zn dopant is often investigated in β-Ga2O3 on Si substrates. Since,
attention of researchers worldwide, which is visible by significant divalent Zn ions has less valence states than that of trivalent Ga ions, the
number of recent publications in various fields [3–9]. The β-Ga2O3 based Zn dopant will behave as a deep-acceptor in β-Ga2O3 [14]. A Zn atom
phosphors can be used in plasma display panels, imaging devices, and has two electrons in its outermost layer, compared to one for a Ga atom.
field emission displays due to their superior luminescence characteris­ By substituting Zn atoms for Ga, they form an acceptor dopant, pro­
tics [10]. The monoclinic β-Ga2O3 possess the optical bandgap of ducing holes. Thus, the Zn dopants in β-Ga2O3 films will produce p-type
4.4–5.1 eV [11,12]. The luminescence of β-Ga2O3 phosphors is first re­ conductivity. The low hole concentration can be increased by further
ported by Herbert et al., in 1969, which revealed the major broad annealing the acceptor Zn dopants. Feng et al. revealed that Zn dopant in
emission peak at 456 nm [13]. Afterwards, the luminescence of β-Ga2O3 Ga2O3 nanowires grown via catalytic chemical vapor deposition method
phosphors is investigated by various dopants such as Si and Sn, and the exhibited p-type conductivity [15]. Other studies revealed the variation
resultant material was n-type β-Ga2O3. As the primary research was in the bandgap as well as different luminescence bands for Ga2O3

* Corresponding author. Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, No. 1 University Rd., Puli Township,
Nantou County 54561, Taiwan,
E-mail address: dsw@ncnu.edu.tw (D.-S. Wuu).
1
A.K. Singh and C.-C. Yen contributed equally to this work.

https://doi.org/10.1016/j.jlumin.2023.119836
Received 17 January 2023; Received in revised form 22 March 2023; Accepted 30 March 2023
Available online 5 April 2023
0022-2313/© 2023 Elsevier B.V. All rights reserved.
A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

depending on the dopants as well as on the different nanostructures. electrical study of ZnGaO films was conducted using van der Pauw Hall
However, these studies were only performed experimentally using measurements (ACCENT, HL-5500PC) at 3200G magnetic field strength.
extrinsic doping of Zn atoms, which lacks of theoretical investigations In order to determine the suitable sites for Zn atoms in Ga2O3 crystal
and the mechanism behind the photoluminescence of Zn-doped Ga2O3 structure, various supercells have been constructed using the Per­
films. Hence, the formation of the Zn dopant on Ga sites in Zn doped dew–Burke–Ernzerhof (PBE) [22] scheme of the generalized gradient
β-Ga2O3 supercells has been studied using first principle investigations approximation (GGA) for describing the exchange correlation in­
in order to find the out the favourable site for Zn dopants in addition teractions. First-principles calculations for the preferred location of Zn
with experimental investigation for Zn-doped Ga2O3 films. Therefore, atoms in Ga2O3 supercells were conducted with the aid of the projector
considering all the facts, a systematic investigation focussing on the augmented wave method and Vienna Ab-initio Simulation Package
structural and photoluminescence properties of ZnGaO films on sap­ (VASP) [23,24]. In the wave function expansion, the energy cut-off and
phire substrates with the verification of experimental results using the force threshold were 600 eV and 10− 5 eV/A, respectively. The
first-principles studies will be useful to provide an insight depth of geometric parameters for a 1 × 2 × 2 supercell ZnxGa32-xO48 (x = 0–9)
ZnGaO films to employ this material in other practical applications. was optimized using Monkhorst-Pack k-points grid with 5 × 5 × 5 mesh.
Many researchers have grown Ga2O3 films using radio-frequency However, the PBE functional level was less precisely to perform the
(RF) magnetron co-sputtering, metal-organic chemical vapor deposi­ electronic structures, we used the hybrid Heyd− Scuseria− Ernzerhof
tion (MOCVD), pulsed laser deposition (PLD), atomic layer deposition (HSE06) functional to describe the electronic properties [25].
(ALD), and Halide Vapor phase epitaxy etc [16–20]. The growth rate of
films using ALD technique is low and the deposited films result in the 3. Results and discussion
amorphous nature, which is a biggest drawback of this technique.
MOCVD and PLD techniques are quite expensive to deposit ZnGaO films. The as-deposited ZnGaO films were amorphous in nature, which
Co-sputtering is widely used for the deposition of alloy films and com­ were not shown here. Fig. 1(a) shows the XRD spectra of 800 ◦ C
posite coatings because it offers flexibility for material combination, annealed ZnGaO films for varying Zn contents. The 0–8.62% ZnGaO
control over material distribution, uniformity over a wide area, and high films represent the (− 201), (− 402), and (− 603) characteristic peaks of
adhesive strength [21]. Additionally, co-sputtering with relatively low Ga2O3 phase, which is verified using JCPDS 43–1012 index. No other
amounts of dopants makes it possible to fabricate doped semiconductors impurity peaks such as ZnO is found in our samples. As the Zn content is
and can tune the physical characteristics of the host material. increased higher 8.62% then the crystal structure of Ga2O3 is found
Herein, we propose the use of different targets to deposit ZnGaO distorted, which can be ascribed to the presence of the high amount of
films using RF magnetron co-sputtering. This work sheds the light on the Zn content in the films. The XRD peaks were found to be shifted
structural and photoluminescence properties of ZnGaO thin films on continuously at smaller 2θ angles with increasing Zn content, indicating
sapphire substrate systematically. Besides, the experimental results were that the Ga2O3 lattice is expanded along the c-axis because Zn2+ has a
verified using first-principles calculations. larger large ionic radius (0.74 Å) than Ga3+ (0.62 Å), which is in
accordance with literature [26]. As a result of this, d-spacing values is
2. Experimental found to be increased from 2.3468 to 2.3856 Å, as shown in Table S1.
The crystalline β-Ga2O3 plane (− 402) is used as a criterion for calcu­
The deposition of ZnGaO films (200 nm thick) were performed on 2- lating the d-spacing and the grain size using Scherrer equation as D =
inch sapphire substrates (c-plane) using the 3-inch ceramic Ga2O3 Kλ/(β cosθ), where D indicates the grain size of the films, K the constant
(99.99% purity) and 3-inch metallic Zn (99.995% purity) targets. The RF (0.9), λ the wavelength of the incoming X-ray beam source, β the FWHM
power of 100 W is kept fixed for Ga2O3 target whereas DC power is of the Ga2O3 (− 402) plane, and θ the Bragg diffraction angle [10]. Be­
varied from 0 to 50 W to ignite the plasma. The sapphire substrates were sides, the grain size is found to be decreased from 25.64 to 7.51 nm with
properly cleaned prior to transfer in the sputtering chamber. The base the increase in Zn content. The detailed structural parameters for ZnGaO
pressure of 5 × 10− 7 Torr was attained prior to the deposition of films. films are shown in Table S1 for reference.
The working pressure of 5 × 10− 3 Torr was kept maintained during the Fig. 2(a–d) shows the top view SEM micrographs of annealed ZnGaO
deposition. The rotation speed of substrate was 6 rpm to maintain the films for different Zn contents According to SEM micrographs, the
uniform thickness of films. These films were grown at the fixed substrate
temperature of 400 ◦ C, followed by the annealing treatment at tem­
perature of 800 ◦ C using the furnace tube for 60 min in air ambience.
The Ar and O2 gases were flowed in the ratio of 5:1.
The α-step Tencor surface profiler is used to measure the thickness of
films. The crystal phase of ZnGaO films was characterised using X-ray
diffraction diffractometer (HR-XRD, X’Pert Pro MRD, PANalytical). The
top-view and cross-sectional SEM micrographs were evidenced using
field-emission scanning electron microscope (SEM, JEOL JSM-6700 F).
The surface roughness is revealed using Atomic Force Microscope (AFM,
Dimension 5000, Bruker). The transmittance spectrum is obtained using
n&k analyser (model: 1280, n&k Technologies). X-ray photoelectron
spectrometer (XPS, PHI 5000 Versa Probe, ULVAC-PHI). ULVAC-PHI’s
XPS (5000 Versa Probe, pHI-5000) was used to characterize the chem­
ical state of elements as a result of the monochromatized AlKα source
(1486 eV). The pass energy, X-ray beam size, and take-off angle were
58.7 eV, 100 μm, and 45◦ , respectively. By averaging 50 scans per
element, high resolution scans were obtained, and surface charging was
minimized using an electron flood gun operating at 3 V. The photo­
luminescence (PL) measurements were obtained using LabRAM HR800n
single monochromator (Horiba Jobin Yvon, France) having a He–Cd
laser (excitation source: 325 nm). These PL measurements were
measured in the range from 320 to 550 nm at the room temperature. An Fig. 1. XRD patterns of 800 ◦ C annealed ZnGaO films for varying Zn contents.

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

Fig. 2. SEM micrographs of 800 ◦ C annealed ZnGaO films for various Zn contents.

annealed 0% ZnGaO had smooth and homogeneous surfaces. However, with the XRD analysis. The cross-sectional SEM micrographs of ZnGaO
the ZnGaO films exhibited non-homogeneous surface when the Zn is films are shown in Fig. S2, which verifies the thickness of films.
doped in the Ga2O3 films up to 8.62%. When the Zn atomic content is Fig. 3(a–d) shows the morphology of annealed ZnGaO films for
increased higher than 8.62%, the SEM micrographs revealed the dislo­ different Zn contents using AFM. It is observed that root-mean-square
cation of grains and column type structure for all the films. The grains as (RMS) surface roughness of ZnGaO films continuously increased from
observed in ZnGaO film are dense and the grain size are in accordance 1.12 to 1.50 nm with the increase in Zn contents. The AFM micrographs

Fig. 3. AFM micrographs of annealed ZnGaO films for varying Zn contents.

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

revealed the domelike structure for ZnGaO films, which became sharper luminescence band at 525 nm were observed for 8.62% ZnGaO film,
with increase in Zn contents. The increased surface roughness can be these regions were denoted as (I), (II), (III), and (IV), respectively. The
ascribed to the coalescence of the grains during the thermal treatment at UV luminescence band at 372 nm for 8.62% ZnGaO film is likely due to
temperature of 800 ◦ C, which is beneficial for phosphors [27]. Gener­ the incorporation of the dopant (Zn) into the ZnGaO lattice. This
ally, Zn-doping in Ga2O3 enhances the surface roughness of films. incorporation can result in the formation of a shallow acceptor level,
Fig. 4 shows the transmittance spectra of annealed ZnGaO films for which can trap excited electrons and produce UV emission upon relax­
varying Zn contents. The average transparency of ~80% were found for ation. This is because Zn is known to be a p-type dopant in Ga2O3,
ZnGaO films, which significantly reveals their potential in numerous meaning it can introduce holes into the conduction band and create
optoelectronic applications. The optical energy gap of these ZnGaO films acceptor levels. These acceptor levels can then trap excited electrons,
is shown inset of Fig. 4. The energy gap of these ZnGaO films is found as leading to a buildup of holes and the emission of UV light upon relax­
5.10, 5.07, 5.01, and 4.99 eV for 0, 4.41, 8.62, and 11.39% Zn doped ation. For the low dopant concentration of 4.41%, it is possible that the
Ga2O3 films. It was interesting to note that the optical energy gap of dopant concentration is not high enough to form a significant number of
ZnGaO films is found red-shifted. The energy gap of these ZnGaO films is shallow acceptor levels, leading to a reduced UV luminescence band or
found significantly decreased from 5.10 to 4.99 eV with the increase in no UV luminescence band at all. On the other hand, for the high dopant
Zn content, which can be ascribed to the presence of defects, stress, and concentration of 11.39%, it is possible that the dopant concentration is
lattice mismatch in the ZnGaO films. These results are consistent with too high and leads to the formation of deep acceptor levels. These deep
the previous report [15]. acceptor levels can give more defect states to excited electrons, sup­
Fig. 5 shows the Zn 2p3/2 and Ga 2p3/2 core-level XPS spectra of pressing the UV luminescence band or no UV luminescence band at all.
ZnGaO films for varying Zn contents. The intensity of Zn 2p3/2–1022 eV In addition to this, the high Zn dopant concentration can distort the
is found to be increased with the increasing Zn content. The binding ZnGaO lattice, due to which there might be the suppression/decrease in
energy of metallic Zn is 1021 eV whereas metallic Ga is 1116.6 eV. The the intensity of luminescence bands is possible. Therefore, the absence
binding energy of Ga 2p3/2 peaks were observed at 1118 ± 0.09 eV. of a UV luminescence band for the low and high dopant concentrations
Therefore, it can be concluded that Zn and Ga are present in the com­ could be attributed to the insufficient or excessive concentration of
pound form in the films. The decrease in the intensity of Ga 2p3/2 is also dopant atoms, respectively, which affects the formation of shallow
observed for ZnGaO films, indicating the inclusion of Zn in Ga2O3 lat­ acceptor levels in the ZnGaO lattice [30–32]. The PL emission peak at
tice. The binding energy of Zn 2p3/2 corresponds to the presence of 415 nm results from electrons recombining with holes supplied by the
divalent Zn2+ in Ga2O3 lattice, which further supports that Zn (2+) has acceptor level at the donor level near the valence band. The recombi­
lower valence than Ga (3+). The elemental content of Zn is 0, 4.41, 8.62, nation of electrons given by oxygen vacancies (VO) with holes produced
and 11.39% and Ga is 45.10, 40.81, 34.32, and 33.59% observed for by gallium vacancies (VGa) or gallium-oxygen vacancy pair (VGa-VO)
ZnGaO films. The stoichiometric Zn/(Zn + Ga) ratio 0, 0.09, 0.20, 0.25 defect levels produces the PL emission peaks at 450 and 466 nm. The
and O/(Zn + Ga) 1.22, 1.21, 1.32, 1.22 were observed in the ZnGaO green luminescence band can be ascribed to the ZnGa and gallium va­
films, which play a significant role in enhancing the luminescence of cancies in ZnGaO films, and it could be a signature of p-type β-Ga2O3. In
phosphors. The stoichiometric Zn/(Zn + Ga) ratio 0.20 and O/(Zn + Ga) contrast, the intensity of green luminescence band at 525 nm increased
ratio 1.32 are found optimized for ZnGaO film, which is near stoichio­ with the increase in Zn content up to 8.62%. The presence of concen­
metric for ZnGaO phosphors on sapphire substrates [28,29]. tration quenching effect can be identified due to the decreasing intensity
Fig. 6 shows the gaussian fitted PL spectra of ZnGaO films for varying of luminescence bands located at 455 nm and at 525 nm upon increasing
Zn contents (normalized PL spectra of ZnGaO films for varying Zn the Zn content higher than 8.62%. These results suggest that 8.62% Zn
contents is shown in Fig. S1). The 0% ZnGaO film exhibited a broad doping is stable in β-Ga2O3 phosphors.
luminescence band centred around 450 nm, which is generally seen in To systematically understand the optimized doping amount of Zn
Ga2O3 phosphors. Zn-doping in Ga2O3 can induce more defects and as a and emission mechanism in ZnGaO films, first-principles calculations
result of this, ultraviolet (UV) luminescence band at 372 nm, the broad were discussed onwards. Fig. 7 shows β-Ga2O3 1 × 2 × 2 supercell
blue luminescence band at 450 nm, 466 nm as well sharp green containing four monoclinic unit cells was modelled with 80 atoms, in
which one Zn atom was incorporated into the supercell in order to
determine the favourable site for Zn atoms in Ga2O3 supercell. The
crystal structure of Ga2O3 has two distinct Ga sites, denoted as tetra­
hedral (T site) and octahedral (O site) site. β-Ga2O3 belongs to space
group C2 = m with two-fold rotation axis b. The defect formation energy
of supercells was calculated considering one Zn atom at two different
interstitial sites (inter1 and inter2) as well as at Ga substitutional
tetrahedral and octahedral sites, as shown in Fig. 7(a–d). The defect
formation energy per atom (EAd ) and per number of Zn (EZd ) were
calculated using following equation:
Interstitial type: interstitial site 1 (inter1) and interstitial site 2
(inter2)
1 ( )
Zn1 Ga32 O48 : EdA = EZn1 Ga32 O48 − EGa32 O48 + μZn (1)
N

1 ( )
Zn1 Ga32 O48 : EdZ = EZn1 Ga32 O48 − EGa32 O48 + μZn (2)
x
Substitutional type: T site and O site
1[ ( )]
Znx Ga32− x O48 (x=1,4,7,9) : EdA = EZnx Ga32− x O48 − EGa32 O48 +x⋅ μZn − x⋅ μGa
N
(3)
Fig. 4. Transmittance spectra of annealed ZnGaO films for varying Zn contents.
Optical energy gap is shown inset.

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

Fig. 5. The core-level XPS spectra of annealed ZnGaO films for varying Zn contents.

Ga atoms were substituted by various Zn atoms at tetrahedral and

octahedral Ga coordinated sites equivalent to their doping contents,
respectively. In order to determine the optimized Zn atoms substitution
in Ga2O3 supercell (ZnGaO), the EAd atom was calculated for various Zn
dopants. The cell volume, (− 402) d-spacing, EAd , and EZd were listed in
Table 1. Since the ionic radius of Zn2+ is larger than that of Ga3+, the cell
volume increases when Zn atoms will substitute Ga atoms at tetrahedral
and octahedral sites. Although the tendency of optimized cell volume of
ZnGaO supercells increased linearly with the increasing Zn content, the
tendency of (− 402) d-spacing is different between tetrahedral and
octahedral sites. In the T-site, the tendency of optimized (− 402) d-
spacing is Ga32O48 (2.3887) < Ga31Zn1O48_T (2.3913) < Ga28Zn4O48_T
(2.3947) < Ga25Zn7O48_T (2.4116) < Ga23Zn9O48_T (2.4283), which are
in good agreement with (− 402) d-spacing from XRD results. On the
contrary, this argument is untenable when we look at the results of the
O-site in Table 1. For the T-site or O-site ZnxGa32-xO48 supercells (x =
0–9), as the Zn increases from 0 to 9, the EAd was increasing, which means
that the supercell will be more unfavourable. On the other hand, as the
Zn increases from 0 to 9, the EZd was decreasing, which means that each
Zn atom will easier to substitute Ga site in each specific composition.
Because EAd and EZd are competitive relationships, the Zn atoms have the
Fig. 6. Room temperature PL spectra of 800 ◦ C annealed ZnGaO films for
varying Zn contents. best-optimized composition for ZnGaO. Besides, the T-site EAd and EZd are
all less than O-site ones. Therefore, the ZnxGa32-xO48 supercells are more
1[ ( )] favourable at Ga substitutional T-sites than Ga substitutional O-sites due
Znx Ga32− x O48 (x =1,4,7, 9) : EdZ = EZnx Ga32− x O48 − EGa32 O48 +x⋅ μZn − x⋅ μGa to the tendency of (− 402) d-spacing in XRD results, lower EAd , and EZd , as
x
(4) listed in Table 1.
A simple model of energy band diagram for Ga25Zn7O48_T supercell
Chemical potential:
using first-principles studies have been shown in Fig. 10 where Zn
1 [ ] created the various defect levels upon Ga substitution. The (I) 3.07 eV as
μZn = EZn16 O16 − 8EO2 (5)
16 well as 3.69 eV, (II) 2.63 eV, (III) 2.73 eV, and (IV) 2.23 eV match
Fig. 6’s region (I), (II), (III), and (IV), respectively. Although their value
1 [ ]
isn’t identical, the simulation supercell is the perfect structure only with
μGa = EGa16 O24 − 12EO2 (6)
16 T-site defects, which ignore the other impacts, such as crystallinity,
vacancy, dislocation, grain boundary, etc. These different defect levels
where EZn1 Ga32 O48 , EZnx Ga32− x O48 , EZn16 O16 , and EGa16 O24 represents the total
were indicated for spin up and spin down characteristics, which were in
energy of Zn1Ga32O48 (inter1 and inter2), ZnxGa32-xO48 (T site and O
good agreement with Fig. 6. In contrast, the Fermi level is found near
site), Zn16O16, and Ga16O24 systems, respectively. Besides this, EO2 is the
valence band, which indicated that Zn doping in GaO film is p-type.
total energy of a single oxygen molecule, μZn is the chemical potential of
Furthermore, compared to conduction band, the energy levels of the
Zn atoms, and μGa is the chemical potential of Ga atoms. The EAd of Ga2O3
defect states are close to the valence band relatively. Such a scenario can
supercell were 7.331, 8.718, 1.119, and 1.322 eV for Zn1Ga32O48
be attributed to the characteristics of the p-type doping. In order to
(inter1), Zn1Ga32O48 (inter2), ZnxGa32-xO48 (T site), and ZnxGa32-xO48
verify this p-type nature for 8.62% ZnGaO film, we have utilized Hall
(O site), respectively. The lowest EAd indicates the favourable site of Zn measurements in Van der Pauw configuration, which gives the positive
atoms in Ga2O3 supercell, which is substitutional Ga tetrahedral sites. Hall coefficient as shown in Table S2. This positive sign of Hall coeffi­
Figs. 8 and 9 show the Zn-doped Ga2O3 1 × 2 × 2 supercells con­ cient indicates the p-type nature of film, which is in accordance with the
taining four monoclinic unit cells was modelled with 80 atoms, in which results obtained using first-principles investigations.

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

Fig. 7. Ga2O3 supercells for one Zn atom at (a) interstitial site 1, (b) interstitial site 2, (c) substitutional tetrahedral Ga, and (d) substitutional octahedral Ga site.

Fig. 8. Ga2O3 supercells for various Zn atoms at substitutional tetrahedral Ga sites (a) 1 Zn atom, (b) 4 Zn atoms, (c) 7 Zn atoms, and (d) 9 Zn atoms.

To understand the various defect states upon Ga substitution by Zn, calculations, as shown in Fig. 11. The charge density distributions of the
the charge density distributions of the conduction band minimum CBM and VBM is distributed uniformly, as shown in Fig. 11(a) and (b).
(CBM), valence band maximum (VBM), and first, second high defect On the other hand, Fig. 11(c) and (d) show charge density distribution
states for Ga25Zn7O48 supercell at T-sites were investigated by the DFT only localized on the oxygen, which bonded with the Zn atom. The

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

Fig. 9. Ga2O3 supercells for various Zn atoms at substitutional octahedral Ga sites (a) 1 Zn atom, (b) 4 Zn atoms, (c) 7 Zn atoms, and (d) 9 Zn atoms.

Table 1
The optimized parameters of ZnGaO supercell for various Zn contents at tetra­
hedral and octahedral Ga coordinated sites.
Supercell Cell vol (Å3) (-402) d-spacing (Å) EAd (eV) EZd (eV)

Ga2O3 890.7 2.3887 – –

Ga31Zn1O48_T 893.3 2.3913 0.0134 1.1196
Ga28Zn4O48_T 901.1 2.3947 0.0530 1.0606
Ga25Zn7O48_T 912.0 2.4116 0.0853 0.9745
Ga23Zn9O48_T 918.5 2.4283 0.1078 0.9581
Ga31Zn1O48_O 893.7 2.3898 0.0165 1.3216
Ga28Zn4O48_O 903.0 2.3893 0.0631 1.2624
Ga25Zn7O48_O 910.5 2.3914 0.1095 1.2519
Ga23Zn9O48_O 913.4 2.3921 0.1397 1.2415

characters #1 and #2 represent the first and second high energy levels of
defect states, respectively. This implies the relative electron deficient
property of ZnGaO, indicating the Zn substituted T-site Ga are leading to
more defect states, and inducing more peaks in PL spectra. These results
have a good agreement with Fig. 6 (room temperature PL spectra) and
Fig. 10 (energy level diagram of ZnGaO film).

4. Conclusions

ZnGaO films were successfully deposited on sapphire substrates

Fig. 10. Schematic energy diagram of Ga25Zn7O48_T supercell using first-
using the co-sputtering of Ga2O3 and Zn targets. The 8.62% Zn content is
principles studies. The up, down, yellow, and white arrows represent the spin
found as optimized to grow ZnGaO films. The optical bandgap exhibited
up, spin down, occupied state, and unoccupied state, respectively.
the red shift for ZnGaO films with the increase in Zn content. The surface
roughness is found to be increased with the increase in Zn content,
which is in good agreement with the energy diagram and charge density
which is beneficial for phosphors. The core-level XPS spectra of Zn
distribution by first-principles investigations, indicating the Zn
indicated the inclusion of Zn in Ga2O3 films. We demonstrated that the
substituted T-site Ga are leading to more defect states, and inducing
ZnxGa32-xO48 supercells are more favourable at Ga substitutional T-sites
green luminescence in PL spectra. The positive Hall coefficient verified
than Ga substitutional O-sites due to the tendency of (− 402) d-spacing in
the p-type nature of ZnGaO film. The results presented here demon­
XRD results, lower EAd , and EZd . The PL characteristics of ZnGaO films
strates the deposition of ZnGaO film using different targets, which can
exhibited the sharp peak maximum at 525 nm for 8.62% ZnGaO film,
pave the way to apply these films in phosphors, deep-ultraviolet

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A.K. Singh et al. Journal of Luminescence 260 (2023) 119836

Fig. 11. The charge density distributions of the (a) conduction band minimum, (b) valence band maximum, and (c) defect state #1, and (d) defect state #2 for
Ga25Zn7O48 supercell at T-sites. The yellow region represents the electron distribution of each states.

photodetectors, and other optoelectronic applications. Appendix A. Supplementary data

Credit author statement Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jlumin.2023.119836.
Anoop Kumar Singh: Investigation, Methodology, Data curation,
Validation, Writing – original draft, review & editing. Chao-Chun Yen: References
Data curation, Validation, Writing – original draft, review & editing.
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