1. technology and computing

Electron. Mater. Lett., Vol. 11, No. 1 (2015), pp. 82-87
DOI: 10.1007/s13391-014-4209-0
Rapid Curing of Solution-Processed Zinc Oxide Films by Pulse-Light Annealing
for Thin-Film Transistor Applications
Dong Wook Kim,1 Jaehoon Park,2,* Jaeeun Hwang,3 Hong Doo Kim,3 Jin Hwa Ryu,4 Kang Bok Lee,4
Kyu Ha Baek,4 Lee-Mi Do,4,* and Jong Sun Choi1,*
1
Department of Electrical, Information and Control Engineering, Hongik University, Seoul 121-791, Korea
2
Department of Electronic Engineering, Hallym University, Chuncheon 200-702, Korea
3
Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University,
Young-in 446-701, Korea
4
IT Convergence Technology Research Laboratory, Electronics and Telecommunications Research Institute,
Daejeon 305-700, Korea
(received date: 18 July 2014 / accepted date: 10 September 2014 / published date: 10 January 2015)
In this study, a pulse-light annealing method is proposed for the
rapid fabrication of solution-processed zinc oxide (ZnO) thinfilm transistors (TFTs). Transistors that were fabricated by the
pulse-light annealing method, with the annealing being carried
out at 90°C for 15 s, exhibited a mobility of 0.05 cm2/Vs and an
on/off current ratio of 106. Such electrical properties are quite
close to those of devices that are thermally annealed at 165°C
for 40 min. X-ray photoelectron spectroscopy analysis of ZnO
films showed that the activation energy required to form a Zn-O
bond is entirely supplied within 15 s of pulse-light exposure. We
conclude that the pulse-light annealing method is viable for
rapidly curing solution-processable oxide semiconductors for
TFT applications.
Keywords: thin-film transistor, oxide semiconductor, solution
process, pulsed-light annealing
1. INTRODUCTION
Currently, there is intense and significant interest in
realizing high-performance thin-film transistors (TFTs)
based on oxide semiconductor materials, owing to their
potential to replace amorphous silicon for applications
requiring low-cost, low-temperature manufacturing on largearea flexible substrates.[1-3] For direct patterning capabilities,
numerous studies on solution-processed metal oxide films
present alternative methods to the vacuum deposition
technique, such as inkjet printing.[4] Recently proposed highperformance solution-processed oxide TFTs have mostly
*Corresponding author: [email protected]
*Corresponding author: [email protected]
*Corresponding author: [email protected]
©KIM and Springer
been fabricated at higher annealing temperatures, because
these metal-oxide materials require high activation energy
for condensation. However, in the case of fabricating these
TFTs on flexible substrates, the fabrication process mainly
depends on the annealing temperature. Many fabrication
processes that proposed a decrease in the annealing
temperature use various approaches, such as deep UV
annealing, laser sintering, and microwave-assisted annealing.[5-8]
These processes require relatively long annealing times and
complicated equipment with limited manufacturing space.
In this paper, we propose a pulse-light-induced annealing
system using xenon flash lamps for the rapid fabrication of
high-quality metal-oxide films. A xenon flash lamp is used
for curing and sterilization in processes where high photon
energy is required; further, it is widely used in metal ink
sintering, plastic bonding, and sterilization systems. The
major advantages of pulse-light annealing are related to the
D. W. Kim et al.
process efficiency, such as ultra-high throughput, low substrate
heat, and low power consumption.[9,10] In this study, we
focused on a particular advantage of pulse-light annealing,
wherein pulse-light annealing provides sufficient energy for
the condensation of oxide solutions into a film even in a few
tens of seconds. The electrical properties of zinc oxide
(ZnO) TFTs, together with the morphological and chemical
characteristics of ZnO films, were investigated as functions
of the annealing methods and conditions.
2. EXPERIMENTAL PROCEDURE
Figure 1(a) depicts the pulse-light annealing system
(model: PLA-500, DTX Inc.) that was constructed to
fabricate ZnO TFTs. The xenon lamp emitted white light
over a wide range of wavelengths, from 350 - 950 nm, as
shown Fig. 1(b).[11] A p-doped silicon substrate with a 150nm-thick thermal oxidation layer was prepared using acetone,
iso-propyl ethanol, and DI water sonication cleaning, and
these steps were followed by ultraviolet-ozone (UVO)
83
treatment that lasted for 10 min. A 0.9-wt. % ZnO solution
was synthesized by dissolving zinc oxide in an ammonia
hydroxide solvent and then stirring this solution on a hot
plate for 6 h at 75°C. Subsequently, the ZnO solution (that
had been stirred for 10 min at room temperature (RT) to
prevent aggregation) was spin coated. Thermal annealing
was carried out on a hot plate at 165oC for 40 min in laboratory
ambient air. In detail, the result of thermogravimetric analysis
(TGA) of the prepared ZnO solution in Fig. 2(a) shows that
the solution decomposes to ZnO above 160°C. The isothermal
TGA result shown in Fig. 2(b) also indicates that the
conversion from the ZnO solution to the ZnO film requires
more than 8 min at a constant annealing temperature of
165°C. However, the optimal condition for the TFT
fabrication required a longer annealing time, because the
semiconductor channel in the transistor needed to be densely
solidified from ZnO crystallites or grains during the thermal
annealing process. Meanwhile, spin-coated samples exposed
to the pulse-light annealing system were separated on the
basis of their irradiating condition of 9, 15, 30, or 45 pulses
Fig. 1. (a) Illustration of a pulse-light annealing system, including lamp housing and heating-stage equipment. (b) Spectrum of the xenon flash
lamp. (c) Structural characteristics of ZnO semiconductor-based TFTs.
Fig. 2. (a) TGA and (b) isothermal characteristic curves of the prepared ZnO solution.
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D. W. Kim et al.
(i.e., 3, 5, 10, or 15 s of 3-pulses/s irradiation). It should be
noted that the heating stage was maintained at 90°C during
pulse-light annealing to eliminate solvents. 50-nm-thick Al
source/drain electrodes were thermally deposited under 1 ×
10−6 Torr through a patterned shadow mask, and their fingertyped channel width and length were defined as 2000 µm
and 80 µm, respectively. All electrical characteristics were
measured with a semiconductor analyzer (4200-SCS, Keithley
Inc.); the specific device sizes are illustrated in Fig. 1(c).
exposure duration. In terms of the output current, the device
current that reached ~9.76 µA with pulse-light annealing is
comparable to the current magnitude of ~10.05 µA obtained
by thermal annealing. Furthermore, the transfer characteristics
of the TFTs showed electrical enhancement, including a
comparable mobility of 0.053 cm2/Vs and a subthreshold
swing of 0.66 V/decade. The low mobility values in our
results are thought to be attributed to thin ZnO films.
Similarly, Chung et al. reported an increase in the field effect
mobility of ZnO TFTs from 0.04 to 0.17 cm2/Vs by increasing
3. RESULTS AND DISCUSSION
Figure 3(a) shows a plot of the drain current (ID) against
the drain voltage (VD), and Fig. 3(b) shows a plot of ID
against the gate voltage (VG) of the fabricated TFTs for
different treatments of the ZnO films. In this annealing
process, the reference device was thermally annealed at
165°C for 40 min, and the photo-assisted device was
exposed to high-power pulse light with 9, 15, 30, or 45
pulses on a heating stage at 90°C. It is clearly observed that
when using pulse light, the current increases with the
Table 1. Importance device parameters according to the pulse-annealing condition.
VTH
(V)
Hysteresis
(V)
0.71
12.24
1.41
-
-
-
-
-
-
-
-
0.55
16.21
0.45
0.66
16.86
0.44
# of
pulses
Mobility
(cm2/Vs)
Reference
0.055
>106
9 pulse
Inactive
15 pulse
Inactive
On/off
S/S
ratio (V/decade)
6
30 pulse
0.058
>10
45 pulse
0.053
>106
Fig. 3. (a) Electrical output characteristics and (b) transfer characteristics of fabricated ZnO TFTs using thermal annealing and photo-assisted
annealing as a function of pulse duration.
Electron. Mater. Lett. Vol. 11, No. 1 (2015)
D. W. Kim et al.
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Fig. 4. Comparison of SEM image of fabricated ZnO film between (a) thermal annealing at 165°C for 40 min and (b) pulse-light annealing at
90°C for 15 s. The specific thickness was measured with the vertical SEM image (inset figures).
Fig. 5. (a) O 1-s XPS spectra of ZnO films measured at the etched layer of 5 nm, and (b) comprehensive atomic concentration of fabricated ZnO
film by pulse duration.
the ZnO channel thickness from 30 to 150 nm.[12] The
primary electrical parameters are summarized in Table 1.
Meanwhile, it appears that the ZnO films that were annealed
with 9 pulses and 15 pulses were electrically inactive, even
when the substrate was maintained at 90°C, whereas the
required electrical properties were mostly achieved for
Electron. Mater. Lett. Vol. 11, No. 1 (2015)
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D. W. Kim et al.
thermal annealing of 30 and 45 pulses, which required 10 s
and 15 s, respectively. It is thought that the pulsed-light
exposure expedites the decomposition of the ZnO solution
and thus contributes to the rapid crystallization into the ZnO
film. Consequently, these electrical results prove that pulselight annealing is reasonably capable of achieving rapid
annealing.
The SEM images of the ZnO film with respect to the
annealing process are shown in Figs. 4(a) and 4(b). From the
insets of these figures, it is observed that the thickness of the
thermally annealed and pulse-light-annealed ZnO films is
approximately 20 nm and 17 nm, respectively. Considering
the dependence of the TFT performance on the semiconducting
film thickness,[13] we find that the electrical characteristics of
our TFTs are expected to be further improved by adopting
thick ZnO films. It should be noted that the decrease in the
channel resistance with an increase in the thickness
contributes to a higher flow of electrons that pass through the
source and drain electrodes.[12,14] On the other hand, there is
no noticeable interconnectivity or orientation irregularities
for either annealing treatment. Although this pulsed light
apparently transfers high power to the coated ZnO layer, the
film is not structurally deformed. We believe that the films
are uniformly formed (regardless of the annealing process)
because the nanopores and the crystallization of the ZnO
films are dominated by both the residual precursor and gas
escape through densification.[8]
We analyzed the x-ray photoelectron spectroscopy (XPS)
spectra with three-peak fittings using 85% of a Gaussian and
Lorenzian function. Figure 5(a) shows the O 1-s spectra of
the ZnO films at the surface and at the 5-nm-etched layer. A
low and dominant OM-O peak (529.9 eV) represents the Zn-O
bond that contributes to the concentration of the zinc-oxygen
lattice due to full oxidation. The OV peak (531.2 eV)
represents oxygen vacancy. The OM-OH peak (532.0 eV) is
determined by residual Zn-OH bonds in the film. The ZnOH bonds correspond to higher binding energy than Zn-O
bonds owing to the smaller negative charge of the hydroxyl
ion, largely depending on the Zn(OH)x concentration.[15-17]
Additionally, comprehensive atomic concentration extracted
from the three-peak analysis is summarized in terms of
pulse-light duration in Fig. 5(b).
On the basis of the analytical effect of pulse-light
annealing on the ZnO film, two possible reactions can be
inferred as taking place during densification. First, the
formation of well-constructed solution-based ZnO films in
this process requires a minimum thermal energy to evaporate
the residual solvents. It should be noted that we demonstrated a
pulse-light treatment at RT for sufficient time (40 s, which
represents 120 pulses); however, the ZnO TFT remains
inactive. Without post-treatment to eliminate solvents in the
solution, photo-assisted annealing is seemingly inoperative
for ZnO annealing.[5] Second, pulsed light provides higher
levels of activation energy to oxidize zinc hydroxide to zinc
oxide from the surroundings. Figure 5(b) shows this result
clearly. OM-O concentration (which corresponds to the ZnO
lattice bonds) substantially promoted the suppression of the
Zn(OH)x bond concentration. We conclude that these
increased ZnO lattices remarkably improve the electrical
performance, and therefore, similar to thermal annealing,
pulse-light annealing efficiently transfers energy in an
extremely short duration.
4. CONCLUSIONS
In summary, we demonstrated a novel pulse-light annealing
method for the rapid fabrication of solution-processed ZnO
TFTs. The present pulse-light annealing procedure enables
us to achieve simple deposition of a dense and uniform layer.
Electrical properties of the ZnO TFTs fabricated with the
pulse-light annealing process are comparable to those of the
devices fabricated using conventional thermal annealing.
These results are attributed to increased atomic concentrations
of ZnO lattice bonds upon pulse-light exposure. We believe
that pulse-light annealing in a solution process can contribute
to realizing metal-oxide TFTs on a flexible substrate with
short-duration and low-temperature annealing. Further studies
on various compositions, such as ternary and quaternary
metal-oxide systems, remain to be carried out.
ACKNOWLEDGEMENTS
This research was supported by Hallym University
Research Fund, 2014 [HRF-201406-006]. This work was
also financially supported by by the research project
(10041808) funded by the Ministry of Knowledge Economy
(MKE) of the Korean Government.
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