Space charge-induced unusually-high mobility of a solution-processed indium oxide thin film transistor with an ethylene glycol incorporated aluminum oxide gate dielectric

Article abstract of DOI:10.1039/c5ra21022d

The incorporation of ethylene glycol (EG) into a high-k aluminium oxide gate dielectric layer was achieved by a solution process, leading to a distinct increase in the mobility of indium oxide TFT. Frequency-dependent capacitance originating from residual EG was examined and the accompanying effects on indium oxide TFT were studied.

Full text of DOI:10.1039/c5ra21022d

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Space charge-induced unusually-high mobility of
a solution-processed indium oxide thin film
transistor with an ethylene glycol incorporated
aluminum oxide gate dielectric†
Cite this: RSC Adv., 2015, 5, 102362
Received 10th October 2015
Accepted 23rd November 2015
a
a
b
a
*
Hyungjin Park,‡ Yunyong Nam,‡ Jungho Jin and Byeong-Soo Bae
DOI: 10.1039/c5ra21022d
www.rsc.org/advances
The incorporation of ethylene glycol (EG) into a high-k aluminium from the semiconductor layer, its structural and electrical
oxide gate dielectric layer was achieved by a solution process, leading properties signicantly affect the TFT performance. Requisites
to a distinct increase in the mobility of indium oxide TFT. Frequency- of the gate dielectric for high performance are a smooth surface
dependent capacitance originating from residual EG was examined and compatibility with semiconductor materials while simul-
and the accompanying effects on indium oxide TFT were studied.
taneously offering good insulating properties such as a wide
band gap and high capacitance. In particular, high capacitance of
the dielectric layer (high-k) readily attracts carrier electrons under
a low gate bias, resulting in a high on-current and mobility.9,10
Zhang et al. showed a high mobility of 40.5 cm2 Vꢁ1 sꢁ1 using
sputtering Ta2O5 as the gate dielectric in a ZnO TFT.11 Park et al.
reported a solution-processed indium oxide (In2O3) TFT with
a boron (B) doped ZrO2 dielectric (k ꢀ 12.1) with a mobility of
39.3 cm2 Vꢁ1 sꢁ1 through 250 ꢂC annealing, whereas a SiO2
Oxide thin lm transistors (TFTs) have been widely studied as
an element of driving and switching circuits for at panel,
exible, and transparent displays. A metal oxide semiconductor
(MOS) as a channel layer has various advantages, including
transparency, large area reproducibility, and high stability, as
well as good electrical characteristics. Although carrier mobil-
ities of MOSs are generally much higher than those of amor-
phous silicon semiconductors (ꢀ1 cm2 Vꢁ1 sꢁ1), in order to
achieve high-denition and high-resolution displays, the MOSs
must afford comparable mobility with low temperature poly-
silicon (over 100 cm2 Vꢁ1 sꢁ1) for high-speed driving.1 In this
regard, various processing techniques have been studied to
improve the electrical properties of the semiconductor layer,
composed of indium zinc oxide (IZO), indium gallium zinc
oxide (IGZO), indium oxide (In2O3), or zinc tin oxide (ZTO).2–7
However, MOS channel layers deposited on the conventional
dielectric (k ꢀ 3.9) based In2O3 TFT showed mobility of 15.95
cm2 Vꢁ1 sꢁ1
. In addition, even oxide TFTs exhibiting mobility
12
over 100 cm2 Vꢁ1 sꢁ1 using solution-processed AlOx and YOx gate
dielectrics have been reported.13,14
However, notably high mobility, especially with reduced
annealing temperature of the dielectric layer seems question-
able, because of the defective nature of solution-processed
metal oxide layers. The similarity found in literature on high
mobility achieved by high-k gate dielectric is that they use
a mixture of solvents containing ethylene glycol (EG). EG
increases the viscosity of the solution resulting in a thicker
gate dielectric layer.13,15–17 Due to a higher boiling point of
around 197 ꢂC than common solvents such as 2-methoxyethanol
(ꢀ124 ꢂC) and acetonitrile (ꢀ81 ꢂC), EG may contribute to
undecomposed residuals in the layer in a low-temperature
process.18 Another suspicious point is that the manner of calcu-
lating mobility from on-current. When a dielectric material
contains impurities, the capacitance varies depending on
frequency.17,19,20 According to the square-law equation, the
capacitance of the gate dielectric is inversely related to the
mobility, which raises the importance of choosing an appropriate
k value, especially for a high-k gate dielectric.
silicon dioxide (SiO2) as the gate dielectric have shown a limited
mobility of up to around 40 cm2 Vꢁ1 sꢁ1
.
8
Recent research in this area has focused on improving TFT
performance by introducing various gate dielectrics. Since the
gate dielectric layer adheres to the electron channel generated
aLaboratory of Optical Materials and Coating (LOMC), Department of Materials
Science and Engineering, Korea Advanced Institute of Science and Technology
(KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea. E-mail: bsbae@kaist.
ac.kr
bMultiscale Hybrid Materials Laboratory (MHML), School of Materials Science and
Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, Korea
† Electronic supplementary information (ESI) available: TGA analyses of
precursor solutions. TEM, XRD and AFM data of Al2O3 lm. See DOI:
10.1039/c5ra21022d
Herein, we investigate the unusual high mobility observed in
metal oxide TFT adopting a solution-processed aluminum oxide
(AO) gate dielectric. Precursor solutions with and without EG
‡ Equally contributed to this work.
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are designed to track the thermal evolution of the aluminum
oxide layer and analyze its electrical properties at discrete
temperature. With an aqueous route indium oxide semi-
conductor which has the lower annealing temperature than the
gate dielectric, the effect of EG in AO gate dielectric on the TFT
performance is examined.
The preparation steps for an AO and EG-AO gate dielectric and
In2O3 TFT are described in Fig. 1. The AO lm, as a reference,
was prepared by dissolving aluminum nitrate nonahydrate
(Al(NO3)3$9H2O, 98%, Sigma-Aldrich) in 2-methoxyethanol (CH3-
OCH2CH2OH, 99.8%, Sigma-Aldrich). For the EG-AO lms,
aluminum nitrate was dissolved in a mixture of ethylene glycol
(HOCH2CH2OH, 99.8%, Sigma-Aldrich) and 2-methoxyethanol
with a 50/50 volume ratio. The molar concentration of the solu-
tions was set to 0.4 M and the solutions were stirred for 6 hours.
The precursor solutions were spincoated at 3000 rpm for 25
seconds and dried for 10 minutes at the nal annealing temper-
ature. AO lm was fabricated by 6 iterative coating and drying
cycles and EG-AO lms were prepared by 4 cycles to obtain the
desired thickness. Finally, spun-on lms were annealed at
250, 350, and 500 ꢂC for 4 hours in air and referred to as AO 250
ꢂC, EG-AO 250, 350, and 500 ꢂC. The thicknesses of the AO 250 ꢂC,
EG-AO 250, 350, and 500 ꢂC layers are 190 nm, 155 nm, 110 nm,
and 100 nm, respectively (Fig. S1a–d†). As the annealing temper-
ature increases, solutions are decomposed and/or evaporated
resulting in dense lms. The AO and EG-AO lms are in an
amorphous phase up to 500 ꢂC (Fig. S1e†) and all the lms show
excellent root mean square roughness (RRMS) of 0.187 nm, 0.198
nm, 0.102 nm, and 0.119 nm, respectively, contributing to high
insulating ability and a smooth surface (Fig. S2†).21
Fig. 2 Output characteristics of solution-processed 250 ꢂC In2O3
TFTs with (a) AO 250 ꢂC, (b) EG-AO 250 ꢂC, (c) EG-AO 350 ꢂC, and (d)
EG-AO 500 ꢂC gate dielectric. Range of gate voltage is 0 V to 6 V with
a step of 1 V. (e) Transfer curves of In2O3 TFTs with AO and EG-AO films
prepared at various temperatures as gate dielectrics.
Fig. 2 shows the output and transfer characteristic curves of
the In2O3 TFTs with the AO and EG-AO gate dielectric. An
aqueous indium oxide precursor solution was synthesized by
dissolving 0.2 M indium nitrate hydrate in de-ionized water.2
The synthesized solution was spin-coated on the prepared AO or
EG-AO/p + Si substrate at 5000 rpm for 30 s. The annealing
results, exhibiting an on-current (ION) of 2.5 ꢃ 10ꢁ3 A, an on/off
ratio (ION/OFF) of around 105, and a sub-threshold swing (S.S.) of
0.28. The eld-effect mobility (msat) of the TFTs can be calculated
in the saturation region using the following equation:
ꢂ
process was carried out at 250 C for 4 h in air. The Al source
and drain electrodes were then deposited on the semiconductor
layer by e-beam evaporation with a shadow mask. The thickness
of the Al source/drain electrode was 100 nm and the channel
width and length were 1000 mm and 100 mm, respectively. All of
the In2O3 TFTs exhibited clear pinch-off behavior in the output
curve. The drain current (ID) of the In2O3 TFT with EG-AO 250 ꢂC
in the output curve is one order higher than that of the other
TFTs. Its transfer characteristics also show superior electrical
W
2
IDS
¼
mCiðVG ꢁVTÞ ;
(1)
2L
where IDS is the drain to the source current, W and L are the
width and length of the channel, respectively, Ci is the capaci-
tance per unit area of gate dielectric, VG is the gate voltage, and
VT is the threshold voltage.
To calculate the mobility of the TFT with each gate dielectric,
the capacitance was measured using a metal–insulator–metal
(MIM) structure as a function of frequency from 20 Hz to 1 MHz
(Fig. 3a). In addition, dielectric constant was obtained using the
equation; C ¼ 30kA/d, where C is the capacitance of the gate
dielectric layer, 30 is the vacuum permittivity (8.85 ꢃ 10ꢁ14
F
cmꢁ1), A is the area of the capacitor, and d is the thickness of the
ꢂ
ꢂ
dielectric (Fig. 3b). The AO 250 C, EG-AO 350 C, and EG-AO
500 ꢂC exhibit a nearly at capacitance–frequency curve (C–f
curve) and constant capacitance over the entire range of
frequency. According to the conventional manner in which the
capacitance at 1 MHz is used for the mobility calculation, the
Fig. 1 Schematic illustration for fabrication of (a) MIM device structure
using aluminum oxide (AO) or ethylene glycol-incorporated aluminum
oxide (EG-AO) gate dielectric and (b) indium oxide TFT based on AO
and EG-AO gate dielectric.
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Fig. 3 (a) Capacitance and (b) dielectric constant curves according to frequency of MIM devices fabricated using AO and EG-AO films annealed
at various temperatures as insulators. TGA results of (c) AO and (d) EG-AO precursor solutions after annealing at various temperatures for 4 h. The
heating rate was 5 ꢂC minꢁ1. FT-IR spectra of (e) AO and (f) EG-AO films annealed at various temperatures.
In2O3 TFTs with AO 250 ꢂC, EG-AO 350 ꢂC, and 500 ꢂC dielectrics EG in the solution, the noticeable drop in weight is attributed to
show mobilities of 23.1 cm2 Vꢁ1 sꢁ1, 30.5 cm2 Vꢁ1 sꢁ1, and 23.3 EG, which has a high onset of decomposition temperature
cm2 Vꢁ1 sꢁ1, respectively. On the other hand, the EG-AO 250 ꢂC (240 ꢂC) and boiling point (197.6 ꢂC).18 Moreover, discrete
gate dielectric experiences a frequency-dependent capacitance, weight loss over 330 ꢂC indicates that high thermal energy is
especially at a low frequency region. Since the characterization necessary to completely eliminate undecomposed residuals in
of TFT device is conducted in a static condition, the capacitance the EG-AO lm.
value at a low frequency qualies for extracting representative
In order to determine the quantity of the undecomposed
mobility. While the capacitance at 1 Hz is 110 nF cmꢁ2, where 1 portion in the lm aer the nal annealing process, the AO and
Hz capacitance is assumed by extrapolating to a frequency of 1 EG-AO precursor solutions were annealed at 250 ꢂC, 350 ꢂC, and
Hz from the C–f curve, it signicantly decreases to one third 500 ꢂC for 4 hours and collected for TGA measurement. As
(35.7 nF cmꢁ2) as the frequency increases to 1 MHz.17,19,20 When presented in Fig. 3c and d, all samples show negligible weight
ꢂ
using capacitance of 1 MHz, the mobility of the EG-AO 250 C loss up to each annealing temperature. Beyond the annealing
based TFT is 136.4 cm2 Vꢁ1 sꢁ1. Such high mobility is adjusted temperatures, the weights gradually decrease up to 700 ꢂC. For
to 46.2 cm2 Vꢁ1 sꢁ1 by calculating from k at 1 Hz. The electrical the samples annealed at 350 ꢂC and 500 ꢂC, the nal weight
properties of each device is summarized in Table 1.
losses are around 10%, and 5%, respectively, regardless of
The frequency-dependent capacitance is clearly observed whether EG is incorporated. On the other hand, 250 ꢂC
when EG is included in precursor solution and annealed at annealed AO and EG-AO show a large disparity in weight loss
temperature lower than 350 ꢂC. TGA analysis of EG-AO over 300 ꢂC. The EG-AO 250 ꢂC sample ends up having less than
ꢂ
precursor solution dried at 120 C displays a large amount of 60% of its initial weight, whereas the AO 250 ꢂC sample retains
weight loss compared with AO precursor solution and the around 80% of its weight up to 700 ꢂC. Even though most of the
thermal decomposition terminates over 450 ꢂC (Fig. S3†). Since weight loss occurs in a range of 300 ꢂC to 400 ꢂC for EG-AO 250
the only difference between the two samples is the existence of ꢂC, weight loss of EG-AO 350 ꢂC and AO 350 ꢂC is almost
Table 1 Summary of electrical properties of AO and EG-AO dielectric layers annealed at various temperatures and In2O3 TFTs prepared using
those films as gate dielectrics
Capacitance (nF cmꢁ2
)
Dielectric constant
Mobility (cm2 Vꢁ1 sꢁ1
)
Dielectrics
ata 1 Hz
at 1 MHz
ata 1 Hz
at 1 MHz
atb 1 Hz
at 1 MHz
ION (A) at 10 V
ION/OFF
S.S. (V decꢁ1
)
AO 250 ꢂ
C
40.5
110
48.3
54.9
27
8.7
19.2
6
5.8
6.25
5.3
5.4
19.4
46.2
25.7
19.7
23.1
136.4
30.5
2.9 ꢃ 10ꢁ4
2.5 ꢃ 10ꢁ3
3.8 ꢃ 10ꢁ4
3.7 ꢃ 10ꢁ4
ꢀ105
ꢀ105
ꢀ104
ꢀ103
0.35
0.28
0.99
1.2
EG-AO 250 ꢂ
EG-AO 350 ꢂ
EG-AO 500 ꢂ
C
C
C
35.7
42.6
47.8
6.2
23.3
Obtained from extrapolated capacitance curve. b Calculated from capacitance at 1 Hz.
a
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identical, indicating that EG-related undecomposed residuals
are decomposed with sufficient annealing time.
The chemical species of the residuals in the lms were
investigated by FT-IR (Fig. 3e and f). The vibration peaks related
to NO3 are assigned to 1200–1700 cmꢁ1, and they originate from
aluminium nitrate in both the AO and EG-AO precursor solu-
tions.21,22 Nitrate groups are observed in lms annealed at below
250 ꢂC and removed aer 350 ꢂC due to thermal decomposition.
A broad peak in the range of 3000–3500 cmꢁ1 indicates hydroxyl
groups (–OH) derived from hydrolysis of the precursor solu-
tions. As the annealing temperature increases, condensation
and dihydroxylation reactions become more active leading to
a decrease in intensities of the hydroxyl-related peaks from the
AO and EG-AO lms.23 A distinction between the AO and EG-AO
lms appears in the carbon-related peaks. The peaks at 2800–
2900 cmꢁ1 and 1000–1100 cmꢁ1 correspond to the C–H and C–O
vibration peaks, respectively.23 The peak at 900–1000 cmꢁ1 is
attributed to the C–C stretching vibration.24 Sharp and strong
C–H, C–O, and C–C peaks are observed only in the EG-AO lms
ꢂ
ꢂ
ꢂ
Fig. 4 Hysteresis behaviors of the In2O3 TFTs measured under the
forward and reverse sweeping with (a) AO 250 ꢂC, (b) EG-AO 250 ꢂC,
(b) EG-AO 350 ꢂC, and (c) EG-AO 500 ꢂC gate dielectric. Insets of (c)
and (d) are enlarged pictures of each transfer curve.
prepared at 200 C and 250 C, and disappear aer 350 C. In
contrast, none of the AO lms shows carbon-related peaks.
Wheꢂn the precursor solution containing EG is annealed under
250 C, EG is not fully decomposed and leaves carbon-related
species, such as C–O, C–H, and C–C, in the lms. Combining
the results of the TGA and FT-IR analyses, it is concluded that
the EG-AO lm annealed at 250 ꢂC contains abundant EG-
related carbon species.
restricted responding speed, i.e. slow polarization, retaining
higher capacitance formed at high gate bias.28 The higher
capacitance generates higher current (eqn (1)) during reverse
sweep than in the case of forward bias, which leads to counter-
clockwise hysteresis.
Even though several literatures reported that hydroxyl and
nitrate group induce the frequency-dependent capacitance, AO
250 ꢂC containing those shows constant capacitance over the
entire range of frequency.17,19 Thus, EG residuals in EG-AO
ꢂ
250 C are the main reason for increase of capacitance at low
Conclusions
frequency. Under external applied bias, EG residuals are
polarized and polar groups located at the end of EG residuals
easily capture charge carriers, producing space charges inside
AO lm.14,25 At a low frequency, the increase in capacitance due
to space charge polarization results in a high current level of
In2O3 TFT. As the frequency increases, the rotation of EG
residual is interrupted due to friction against the AO matrix
and charge carriers constructing space charges undergo
macroscopic migration to change the orientation of polariza-
tion. Eventually, they cannot follow the eld and, as a conse-
quence, remain randomly oriented, resulting in a decrease in
capacitance.
Observation of the hysteresis of the transfer curves further
claries the EG residual in the AO layer acting as the source of
the space charge polarization. Fig. 4 displays the hysteresis of
the transfer curves of the samples under forward and reverse
gate bias sweeps. The EG-free AO based In2O3 TFTs (see Fig. 4a,
c and d) show clockwise hysteresis during the forward–reverse
sweep; this is related to charge trapping at the channel/
dielectric interface because the trapped electrons screen the
applied gate eld.26,27 In contrast, counter-clockwise hysteresis
is observed in the EG-AO based In2O3 TFTs. During the forward
In summary, we have investigated unusual high mobility of
metal oxide TFT with solution-processed aluminum oxide gate
dielectric. According to TGA and FT-IR analyses, EG in
precursor solution is not fully decomposed and incorporated in
the aluminum oxide layer at a low annealing temperature. EG
residuals act as space charges, inducing frequency-dependent
capacitance and leading to high capacitance at
a low
frequency. In this case, if the channel mobility is calculated with
the conventional capacitance value at 1 MHz, it represents the
severely overestimated value. Since TFTs operates in static
condition, it is plausible to adopt the capacitance at 1 Hz for the
calculation of the channel mobility. Our conclusions may not
apply to all studies on TFTs in which solution-processed Al2O3
gate dielectric with EG solvent is used. The mobility determined
by frequency-dependent capacitance also depends on the
maximum annealing temperature used during TFT processing,
ꢂ
especially below 250 C.
Acknowledgements
gate bias sweep, the EG residuals are polarized according to the This work was supported by the Materials Original Technology
gate bias. As the reverse bias sweep begins, the change of the Program (10041222) funded by the Ministry of Trade, Industry
polarization lags behind the decreasing gate bias due to its and Energy of Korea and the National Research Foundation of
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Korea (NRF) grant funded by the Korea government (MSIP) 14 K. Song, W. Yang, Y. Jung, S. Jeong and J. Moon, J. Mater.
(CAFDC 5-3, NRF-2007-0056090).
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