1. Introduction
Transparent
conducting oxide (TCO) thin films have been widely used in different important
fields of modern civilization such as solar cells, optoelectronics and
different display applications.[1-7] Among all the
transparent conducting thin films, indium tin oxide (ITO) and tin(iv) oxide
(SnO2) were used as anode materials due to their high electrical
conductivity, high optical transmission with low sheet resistance and high
mobility.[8-11]
However, the high cost involved in the production of ITO and SnO2
film, as well as its performance degradation under hydrogen atmosphere made
researcher think of alternative TCO materials. The doped ZnO thin films were
found extremely demanding as alternative TCO materials. ZnO-based films are
nontoxic, low cost and easily available, which make them more appropriate
alternative for ITO based TCO materials.[12-20] It was found that
the doping of low concentration Group-I elements into ZnO enhanced the
optoelectronics properties in the organic electronic device applications,[21,22] and these films
demonstrated comparatively higher mobility and lower work function than bare
ZnO film. In the case of organic solar cells, the performance of the cell can
be enhanced by increasing the charge collection efficiency and reducing charge
recombination rate by introducing Li ions and LiF into ZnO thin film.[23, 24]
Due to high
crystallinity and no-toxic nature, doped ZnO-based TCO materials have been used
for different optoelectronics applications. Both doped and undoped ZnO-based
films do not degrade in hydrogen plasma atmosphere. Different deposition
techniques are used to prepare ZnO thin films such as pulsed laser deposition
(PLD), chemical vapor deposition (CVD), sol-gel, direct current (DC)
sputtering, radio frequency (RF) magnetron sputtering etc.[25-29] Among all the
deposition techniques, RF magnetron sputtering deposited film shows better
crystalline structure due to its high kinetic energy, high ejection and
deposition rate.
In this work, RF
magnetron sputtering was used to prepare the Ga doped ZnO (GZO) films at nearly
room temperature. Their electrical, optical and structural properties were
studied. The efficiency of the synthesized GZO films was tested as anode
materials for OLED devices and their current–voltage characteristic of
GZO-based OLED was compared with ITO-based OLED.
2.
Experimental
Different layers of ZnO:Ga and LED materials were deposited on the glass
substrate. The purchased organic material was sublimated and purified before
deposition by high vacuum purification system. In this experiment, the OLED
material consisted the layers of TCO (180 nm), poly-(N,N'-bis(4-butylphenyl)-N,N'-bis-(phenyl)-benzidine)
(TPD, 50 nm), Tris-(8-hydroxyquinolinato) aluminium (Alq3, 40 nm), lithium fluoride (LiF, 2 nm) and Al (130 nm).
During OLED fabrication for smooth surface of the TCO layer, the process was carried
out by thermal evaporation with a vacuum level of 3×10-6 Torr.
Deposition of every layer of the device was conducted at 0.5−1Å/s. After
deposition of each material, the device was encapsulated with bare glass in the
glovebox without oxygen under the moisture < 0.2 ppm.
GZO thin film was deposited on glass
substrate by dual-target RF magnetron sputtering system at 13.56 MHz frequency 120
W RF-power, and 2.6×10-3 mbar chamber pressure. The substrate
temperature (Ts) varies from 50 ºC to 300 ºC under argon gas
atmosphere. ZnO:Ga (Ga=3 wt%) was used to deposit film on 99.99% pure sintered
ceramic disc of 2 inch diameter and 5 inch thick. During the deposition,
constant argon flow was maintained at 40 sccm
throughout the experiment. At the time of deposition, RF power and base
pressure were maintained at 120 W and 5.6×10-6 mbar, respectively.
Before film deposition, the substrate was thoroughly cleaned, impurities were
removed by ultrasonic-bathing, and the substrate was cleaned with acetone,
de-ionized water, and alcohol sequentially. Finally, the substrate was cleaned
with isopropyl alcohol (2-propanol) for 15 min and then dried in N2
gas atmosphere. The distance between the substrate and target was
maintained 6 cm throughout the experiment.
The electrical properties like
resistivity, mobility, and carrier concentration were measured by the
Ecopia-HMS-3000. The
optical transmission of the film (solid state) was studied by the double beam
UV-Vis spectrophotometer; Perkin Elmer Lamda-35.
X-ray diffraction (XRD) (Philips PW 1710 diffractometer, Cu Kα radiation,
λ=1.54178Å) was used to examine the structural properties of the GZO
films. To study the microstructure morphology of the GZO thin film, the thin
film on glass substrate was placed under the scanning electron microscope (SEM)
and image was taken. The SEM analysis was performed with a Carl Zeiss SMT
Supra 55 instrument.
3. Result and discussion
For Ts at 200 °C,
GZO films exhibit excellent electrical properties (ρ = 2.45×10-4
Ω cm & Rs=19 Ω/sq, high optical transmission (%T >
90%), and high carrier density (4.35×1021 cm-3), and so
it could be the optimum Ts for synthesizing GZO films. It shows an
improved efficiency compared to ITO films, except that the mobility of the GZO
films was 2-3 times lower than ITO.[30-31]
Fig. 1(a) depicts the XRD pattern of a GZO thin
film, and (b) depicts the SEM image of GZO thin film under argon gas atmosphere
(inert). From the figure, it is clear that GZO shows the diffraction peak at
(002) 34.45º. XRD results matched with the popular JCPDS 36-1451 file of the
hexagonal wurtzite structure of ZnO. The full width at half maximum (FWHM)
values decrease as the Ts of the film rises gradually. As the
temperature increases, crystallinity increases, improving the electrical
properties of the film. The high intense (002) peaks indicate that the c-axis
orientation of the crystallite is perpendicular to the substrate. The average
grain size of the TCO thin film was calculated by using the Scherrer’s formula
(D = Kλ/βcosθ),
where λ is the X-ray wavelength (λ=1.54178Å), it is the maximum Bragg
diffraction peak (in radians), and is the XRD peak's full width at half maximum
(β=FWHM), K=0.94 Scherrer’s constant.[32] Table 1 shows the variation of electrical parameters
like resistivity, mobility, and carrier concentration of GZO thin films with
different substrate temperatures. They have been plotted in surface graph (Fig. 2) to understand the variation with different
parameters.
Fig.
1 (a) XRD
pattern of GZO thin film at 200 oC substrate temperature. (b) SEM
image of GZO thin film under argon gas atmosphere (inert).
Table
1. Properties of GZO film under different substrate
temperatures.
|
TS
(ºC)
|
Resistivity
ρ
(Ω cm)
|
Sheet resistance (nm) Rs=ρ/t
|
Mobility
ì
(cm2/V.s)
|
Carrier concentration
(cm-3)
|
T (%)
|
Band gap
(eV)
|
Crystallite size
(nm)
|
Figure of Merit (FOM)
ΦTC = T10/ Rsh
( Ω-1)
|
FWHM
|
|
50
|
8.32×10-4
|
12
|
30.12
|
3.53×1021
|
92.4
|
3.34
|
19.62
|
3.78×1018
|
0.42
|
|
100
|
6.36×10-4
|
16
|
32.23
|
3.71×1021
|
91.6
|
3.28
|
20.22
|
2.59×1018
|
0.38
|
|
150
|
5.75×10-4
|
21
|
33.42
|
4.15×1021
|
91.2
|
3.26
|
21.46
|
1.89×1018
|
0.32
|
|
200
|
2.45×10-4
|
23
|
34.45
|
4.35×1021
|
90.0
|
3.25
|
22.72
|
1.51×1018
|
0.26
|
|
250
|
3.24×10-4
|
25
|
28.21
|
4.21×1021
|
89.6
|
3.24
|
23.62
|
1.33×1018
|
0.22
|
|
300
|
4.31×10-4
|
28
|
16.25
|
3.81×1021
|
88.4
|
3.23
|
24.52
|
1.04×1018
|
0.15
|
It is found that when the Ts
increased from 50 ºC to 200 ºC, the resistivity gradually decreased from 8.32×10-4
Ω cm to 2.45×10-4 Ω cm. Moving
from 250 ºC to 300 ºC, the resistivity showed a slightly increasing trend,
i.e., 3.24×10-4 Ω cm at 250 ºC
and 4.31×10-4 Ω cm at 300 ºC
(Table 1, Fig. 2).
The decrease in resistance is due to the increase in both mobility and carrier
concentration (Fig. 2(a) and (b)). The increasing trend of resistivity after 250
ºC is anticipated due to the presence of contamination remaining on glass
substrate even after cleaning. Same phenomenon was observed in case of carrier
concentration and mobility of GZO with respect to increase in Ts (Fig. 2(c)). The increase in carrier concentration (50
ºC to 200 ºC) might be due to more diffusion of Ga atoms from interstitial
positions and grain boundaries into Zn cation sites, and the decreasing trend
beyond 250 ºC might be due to the presence of small contamination on glass
substrate as discussed earlier. Fig. 2(d)
demonstrates that the crystallite size of the GZO film increases smoothly as Ts
increases. Fig. 2(e) demonstrates that the
transparency (%) increases as temperature and crystallite size increase. Fig. 2(f) shows the band gap (eV) in relation to
temperature and resistivity on this surface plot. Here, despite a high
theoretic correlation between these three factors, there is only a very slight
variation in the band gap of GZO film with increasing temperature and
resistivity. Figure of merit (FOM) of the TCO thin film is a parameter to
identify the quality of the TCO films. Good quality of transparent conducting
film can be judged by the higher FOM value. The FOM of a thin film can be
calculated using the Equation: FOM (ΦTC) = T10/Rsh, where T represents total
transmittance and Rsh
represents sheet resistance.[33]
Figure
3 shows the optical transmittance of
GZO and ITO film, scanned from 300 nm to 1100 nm. Both the films show optical
transmission of ~90% in the visible region (for ITO, it starts at 390 nm and
for GZO, it starts from 500 nm) of the solar spectrum. High transmission
spectra indicated that the GZO film would be a better anode material for OLED. In the current study, it is found that FOM values
are extremely high, indicating good quality of the TCO films. There is a
decreasing trend with increasing Ts. The FOM values (1.51×1018
- 1.33×1018 Ω-1) at 200 - 250 ºC are very close
to our previously reported ZnO:Al/Glass (1.16×1018 Ω-1).[17]
Fig.
2 (a). A
surface graph of resistivity (ρ; 1 × 10-4 Ω cm,) w.r.t
substrate temperature (Ts; °C)(X-axis) and mobility (µ; 1 × 10-1 cm2/V.
s) (Y-axis); (b) Surface graph of mobility (µ; 1 × 10-1 cm2/V.
s) w.r.t Ts in °C (X-axis) and resistivity (ρ; 1 × 10-4 Ω
cm,) (Y-axis); (c) Surface graph of
carrier concentration (1×1021 cm-3) of GZO film w.r.t Ts
in °C (X-axis) and resistivity (ρ; 1× 10-4 Ω cm)(Y-axis);
(d) Surface graph of T (%) of GZO film
w.r.t Ts in °C (X-axis) and crystallite size (nm)(Y-axis); (e) Surface graph of
crystallite size (nm) of GZO thin film w.r.t Ts in °C (X-axis) and transparency
T (%)(Y-axis); (f) Surface graph shows the band gap (eV) w.r.t substrate
temperature (Ts; in °C)(X-axis) and resistivity (ρ; 1 × 10-4
Ω cm,)(Y-axis).
4.1.
Organic light-emitting diodes characterization
Organic light-emitting diodes
(OLEDs) were fabricated using GZO
and ITO as anode materials and were finally compared with two configurations.
The device architecture of GZO and ITO-based OLEDs and the Schematic energy
level diagrams are shown in Fig. 4 and Fig. 5, respectively. During the comparison study,
certain parameters were maintained the same for both the ITO and GZO film
(resistivity; 2.1×10-4 Ω cm, sheet resistance; 5.6 Ω/sq,
carrier concentration; 1.25×1021 cm-3, mobility; 25 cm2/V.s.)
The device was configured with TCO (180 nm)/ TPD (50nm)/ Alq3 (40 nm) / LiF (2
nm)/ Al (130 nm).
Fig.
3 UV-vis transmittance spectra of GZO
and ITO thin film.
Figure
4 shows the energy level diagram of a
single layer Alq3 device. It is too complex to analyze through 2-D diagram. Here,
Al acts as cathode and is responsible for the electron emission. Light can be
generated by passing current through them. It requires a lower voltage than
other devices such as LED, LCDs. At first, a glass substrate is coated with ITO
or GZO materials. On the top of this device, a metal is placed as an anode. In
between ITO and metal anode, two very thin low organic materials such as TPD
and Alq3 are placed. The most important one is an electroluminescent layer
consisting of a metal chelate called Alq3 for short. It is a highly fluorescent
semiconducting material of emission wavelength at around 550 nm and it
fluoresces under irradiation with UV-light.
Fig. 4 Device architecture of GZO and ITO based anode.
Figure
5 depicts the
schematic band diagram of GZO and ITO-based OLED. Since the positively biasing
ITO or GZO layer acts as ‘hole’ generator layer while due to the application of
negative biasing on Al, it acts as an electron emitter layer in the device. The
work function for ‘hole’ conduction is 4.7 eV in ITO and 4.3 eV for GZO and it
is 5.8 eV in AlQ3 material. It is not easy to transport hole from 4.7 eV (ITO)
or 4.3 eV (GZO) to 5.7 eV easily and there will be a quantum inefficiency of
hole and electron. So, another material was needed, whose work function resides
in between 4.6 eV and 5.8 eV (such as TPD, whose work function is 5.7 eV) to
remove the mismatch between hole and electron concentration from both sides of
Alq3 layer.
As Al is on the cathode side (work
function 4.2 eV), the electrons are easily transported to Alq3 conduction layer
(3 eV) through LiF layer (conduction band at 2.4 eV) absorbing 1.8 eV of energy
by electron on Al layer (as shown in the figure) from the power supply. This
LiF layer is also known as hole blocking layer. On Alq3, when electrons
recombine with hole coming from ITO or GZO, light emission takes place from
this organic material and makes it as OLED. The luminescence occurs between
HOMO-LUMO levels in Alq3 material. Evac is considered as the zero
potential energy level.
Fig.
5 Schematic band diagram of GZO and
ITO-based OLED.
In case of ITO anode-based OLED, the
current increased rapidly due to its relatively high work function (Fig. 6) compared to GZO. The performance of the OLED depends on the work function of the
TCO materials. The barrier height in the anode between GZO and TPD was about
1.43 eV. However, the ITO work function increased to 4.7 eV when the barrier
height was about 1.03 eV. To get a clearer explanation, further study is needed
to enhance the work function with an appropriate surface modification process
by improving the carrier injection properties at the GZO/TPD interface.
Generally, low work function anode materials are appropriate for OLED to
achieved high electron injection efficiency. Here, it was found that the GZO
film showed lower work function compared to ITO, and hence would be a better
TCO material to be used as an anode in OLED [34-36]
and different optoelectronics device applications. The current density (J, A/cm2)
for the I-V curve (Fig. 6) shows that the
current of the OLED with the GZO anode gave better results when compared to the
ITO-based OLED.
Fig.
6 Current–voltage characteristic of
GZO and ITO-based OLED.
4. Conclusion
A GZO thin film
was deposited on a glass substrate by RF magnetron sputtering deposition
techniques. The electrical, optical, and work functions of GZO film were
studied and analyzed. Finally, 200 °C
was found to be the optimum substrate temperature, where a low resistance
(2.45×10-4 Ω cm) and high mobility (34.45 cm2/V.s) GZO film was achieved. It is found that FOM values are
extremely high, indicating the good quality of the TCO films, and at 200 - 250
ºC the GZO film can be treated as alternative to ZnO:Al/Glass and ZnO:Al/PEN
thin films.[17]
GZO anode-based OLED was fabricated and also compared with an ITO anode-based
OLED. Finally, when compared to their current voltage characteristics, GZO
anode-based OLED shows quite promising results and near 500 nm, the observed
optical transmission of ~90% suggests that it could be used for optoelectronics
applications.
Acknowledgment
The authors
sincerely acknowledge the R&D and instrumental support provided by the
Centre of Excellence for Green Energy and Sensor Systems, Indian Institute of
Engineering Science and Technology (IIEST), Shibpur, Howrah; and School of
Electronics Engineering, Kalinga Institute of Industrial Technology (KIIT), Bhubaneswar;
Department of Chemistry, Raghu Engineering College, M.P., India. Author HSD
sincerely acknowledges the research facility provided by the Department of
Applied Sciences, Haldia Institute of Technology for his post-doctoral research
project. GR wants to thank DST-SERB, Govt. of India for the fanatical support
under the TAR/2022/000162 project grant.
Conflict of
Interest
There is no
conflict of interest.
Supporting
Information
Not applicable.
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