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Solution-processed high-k magnesium oxide dielectrics for low-voltage oxide thin-filmtransistorsGuixia Jiang, Ao Liu, Guoxia Liu, Chundan Zhu, You Meng, Byoungchul Shin, Elvira Fortunato, Rodrigo Martins,and Fukai Shan
Citation: Appl. Phys. Lett. 109, 183508 (2016); doi: 10.1063/1.4966897View online: https://doi.org/10.1063/1.4966897View Table of Contents: http://aip.scitation.org/toc/apl/109/18Published by the American Institute of Physics
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Solution-processed high-k magnesium oxide dielectrics for low-voltageoxide thin-film transistors
Guixia Jiang,1,2,3,a) Ao Liu,1,2,3,a) Guoxia Liu,1,2,3,b) Chundan Zhu,1,2,3 You Meng,1,2,3
Byoungchul Shin,4 Elvira Fortunato,5 Rodrigo Martins,5 and Fukai Shan1,2,3,b)
1College of Physics, Qingdao University, Qingdao 266071, China2College of Electronic and Information Engineering, Qingdao University, Qingdao 266071, China3Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, QingdaoUniversity, Qingdao 266071, China4Electronic Ceramics Center, DongEui University, Busan 614-714, South Korea5Department of Materials Science/CENIMAT-I3N, Faculty of Sciences and Technology, New University ofLisbon and CEMOP-UNINOVA, Campus de Caparica, 2829-516 Caparica, Portugal
(Received 29 June 2016; accepted 20 October 2016; published online 2 November 2016)
Solution-processed metal-oxide thin films with high dielectric constants (k) have been extensively
studied for low-cost and high-performance thin-film transistors (TFTs). In this report, MgO dielec-
tric films were fabricated using the spin-coating method. The MgO dielectric films annealed at vari-
ous temperatures (300, 400, 500, and 600 �C) were characterized by using thermogravimetric
analysis, optical spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and atomic-
force microscopy. The electrical measurements indicate that the insulating properties of MgO thin
films are improved with an increase in annealing temperature. In order to clarify the potential appli-
cation of MgO thin films as gate dielectrics in TFTs, solution-derived In2O3 channel layers were
separately fabricated on various MgO dielectric layers. The optimized In2O3/MgO TFT exhibited
an electron mobility of 5.48 cm2/V s, an on/off current ratio of 107, and a subthreshold swing of
0.33 V/dec at a low operation voltage of 6 V. This work represents a great step toward the develop-
ment of portable and low-power consumption electronics. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4966897]
Oxide based thin-film transistors (TFTs) have been stud-
ied for decades because they are important building compo-
nents in optoelectronic devices. However, a majority of the
oxide TFTs reported to date were integrated on SiO2 and/or
SiNx gate dielectrics, resulting in large operating voltages
due to the weak capacitive coupling between the gate dielec-
tric and channel layer.1–3 Because of the limit capacity of the
rechargeable battery, low power consumption is necessary
for the mobile electronic devices (i.e., smart phone and tablet
personal computer).4 To address this issue, the use of high-k
materials, including the inorganic high-k dielectrics5 and
organic dielectrics,6,7 has been proved to be effective to
enhance the capacitive coupling and to reduce the operating
voltage.
In recent reports, inorganic high-k metal-oxide dielectrics,
such as ZrO2,8,9 Y2O3,10–13 HfO2,14–17 Sc2O3,18 Al2O3,19–21
and MgO,22–24 have been applied in TFTs because they simul-
taneously enable a low leakage current as well as a low-
voltage operation. Although HfO2 and ZrO2 have large k val-
ues of �25, the bandgap is relatively small (5.8 eV). Similarly,
although Al2O3 has a large bandgap of 8.8 eV, the k value is
lower than 9. As for MgO high-k candidate, it possesses a
compromised k value (�9.8) and a bandgap (7.3 eV).25 High-k
MgO thin films have been demonstrated as the possible gate
insulators in TFTs in previous reports. Chen et al. demon-
strated that ZnO TFT based on evaporated MgO dielectric
exhibits a field-effect mobility (lFE) of 78.3 cm2/V s and an
on/off current ratio (Ion/Ioff) of 104.22 Lee et al. fabricated ZnO
TFT with MgO as the gate insulator grown using sputtering
and exhibiting an Ion/Ioff ratio of 105 and a subthreshold swing
(SS) of 1.18 V/dec.24
To date, MgO thin films have been fabricated using
electron-beam evaporation23 and high-pressure sputtering.24
However, the costly vacuum-based techniques undoubtedly
limit their application in low-cost and large-area electronics.
By contrast, solution-based processes, such as spin coating,
spray coating, and ink-jet printing, exhibit apparent benefits
such as simplicity, low fabrication cost, and atmosphere proc-
essing.25 For the solution-derived oxide thin films, the physi-
cal properties and the chemical composition of the thin films
strongly rely on the annealing conditions. In this consider-
ation, the fabrication of s MgO dielectric thin films via solu-
tion route will be attractive for the construction of high-
performance and low-voltage electronic devices.
Meanwhile, a desired channel layer also plays an impor-
tant role in achieving high-performance TFT. Among various
semiconducting channel materials, indium oxide (In2O3) has
been widely studied due to its wide band gap (3.6–3.75 eV)26
and the high transmittance in the visible region, making it
suitable for transparent electronics. In this report, the low-
cost spin-coating method was used to fabricate MgO thin
films. The physical and electrical properties of the MgO
thin films annealing at various temperatures were investi-
gated systematically. Finally, the solution-processed In2O3
TFTs on various MgO dielectrics were integrated and
investigated.
a)G. Jiang and A. Liu contributed equally to this work.b)Authors to whom correspondence should be addressed. Electronic
addresses: gxliu@qdu.edu.cn and fukaishan@yahoo.com
0003-6951/2016/109(18)/183508/5/$30.00 Published by AIP Publishing.109, 183508-1
APPLIED PHYSICS LETTERS 109, 183508 (2016)
The detailed experimental section and instrumental analy-
sis can be found in the supplementary material. To investigate
the thermal behavior of the MgO xerogel, thermogravimetric
analysis (TGA-DSC) measurements were carried out, and the
results are shown in Fig. 1. A distinct endothermic reaction
accompanied by a large weight loss was observed below
162 �C, which represents the decomposition of volatile nitrate
precursor and residual species.27 The other exothermic peak at
425 �C indicates the decomposition of the residual nitrates and
the dehydroxylation behavior of the hydrolyzed magnesium
hydroxide. In this stage, the gel film is oxidized, and the metal
oxide lattice is gradually formed and becomes dense. Mass
loss keeps constant at the temperature above 500 �C, which
indicates the complete conversion from xerogel to MgO.
The optical transmittances of MgO thin films annealed
at 300, 400, 500, and 600 �C were shown in Fig. S1 in the
supplementary material. The average transmittances of MgO
films are over 90% in the visible range (400–700 nm). The
transmittance of MgO thin film is found to decrease with
increasing annealing temperature (Ta). This can be attributed
to the increase in surface roughness and/or the elimination of
interstitial oxygen at high Ta. This indicates the potential
applications of the MgO dielectric film in transparent elec-
tronics. In addition, no crystalline peaks are observed in
X-ray diffraction (XRD) spectra, suggesting that these MgO
thin films are amorphous (data not shown here). To be appli-
cable as gate insulator in TFTs, an amorphous structure is
desired because the leakage current would be generated
along the grain boundaries, which certainly results in an infe-
rior dielectric performance. In addition, amorphous thin
films generally exhibited smooth surfaces, which can bring
about an improvement in interface quality.28
Figure 2 shows the surface morphologies of the corre-
sponding MgO thin films. The root-mean-square (RMS)
roughness of MgO thin films annealed at 300, 400, 500, and
600 �C are 1.54, 1.21, 0.95, 4.09 nm, respectively. The MgO
thin films exhibit smooth surfaces as the Ta increases to
500 �C, which is ideal for suppressing the surface roughness
induced leakage current and achieving a high carrier mobil-
ity in the TFT channel.28,29 The significantly increased RMS
value of MgO thin film annealed at 600 �C can be ascribed to
the thermally-derived agglomeration phenomenon. It is
found that the achieved MgO thin films are much smoother
than previous ones fabricated by vacuum-based techniques
(RMS� 3.4 nm).24 This is not only due to the high-speed
spin-coating process but also related to the facile UV-
assisted treatment before the thermal annealing process.5
Figure 3 shows the X-ray photoelectric spectroscopy
(XPS) spectra of the chemical states of MgO thin films. The
binding energies of O 1s peaks were deconvoluted into three
peaks: the peak of lattice oxygen (M-O) in MgO at 529.8 eV;
the peak of oxygen vacancy at 531.6 eV; and the peak of
hydroxyl species (M-OH) or water molecules absorbed on
FIG. 1. TGA and DSC curves of the MgO xerogel from 80 �C to 700 �C.
FIG. 2. The AFM images of MgO thin
films annealed at different tempera-
tures; (a) 300, (b) 400, (c) 500, and (d)
600 �C.
183508-2 Jiang et al. Appl. Phys. Lett. 109, 183508 (2016)
the surface at 532.4 eV.29,30 For convenience, OI, OII, and
OIII were used to denote the area of each component, and
Ototal denoted the total area of the O 1s peak. It is found that
the fraction of lattice oxygen (OI) in MgO is increased from
27.0% to 33.1% with an increase in Ta from 300 �C to
600 �C. This indicates that the annealing of MgO thin film at
higher temperatures removes the oxygen-related defects and
improves the metal-oxygen lattice. To be used as the gate
dielectric, the amount of oxygen-related defects should be as
small as possible because the oxygen defects provide trap
states in the bandgap of dielectric thin films, which can
degrade the electrical and dielectric properties.29
The areal capacitance (C) versus frequency (f) curves of
various MgO capacitors are shown in Figure 4(a). The
decreased areal capacitance at high frequency could be attrib-
uted to the limitation of the polarization response time.12 The
areal capacitance densities at 20 Hz were found to be 190,
280, 330, and 370 nF/cm2 for the 300, 400, 500, and 600 �C-
annealed MgO thin films, respectively. The increased areal
capacitance could be attributed to the decomposition of the
residual impurities and the thermally enhanced dehydroxyla-
tion because the areal capacitance of the metal oxide is larger
than that of metal hydroxide. According to the equation
C¼ eok/d, the dielectric constants were calculated to be 5.8,
6.7, 7.5, and 8.1 for 300, 400, 500, and 600 �C-annealed MgO
thin films, respectively. In addition, the insulating properties
of MgO thin films were improved at high Ta, as shown in
Figure 4(b). This can be attributed to the decomposition of
residual species (i.e., nitrate and hydroxyl groups), and a
denser film is formed subsequently.31
To explore the possible application of solution-
processed MgO thin films, bottom gated In2O3 TFTs on
MgO dielectric layers were fabricated and investigated. The
typical transfer characteristics are shown in Fig. 5(a). The
corresponding electrical parameters of the TFTs are summa-
rized in Table I. The field-effect mobility (lFE) and the
threshold voltage (VTH) were determined from linear fits to
the dependence of the square root of the IDS on VGS. The lFE
was calculated by the following formula:32
IDS ¼W
2LlFECi VGS � VTHð Þ2; (1)
FIG. 3. XPS O1s spectra of MgO thin films annealed at 300, 400, 500, and
600 �C.
FIG. 4. (a) Areal capacitance-frequency
curves of MgO capacitors annealed at
various temperatures. (b) Leakage cur-
rent density vs. electric field character-
istics of the corresponding MgO
capacitors.
FIG. 5. (a) Transfer curves of the In2O3 TFTs with various MgO dielectrics
(VDS¼ 5 V). The processing temperature for the In2O3 channel was 320 �C.
(b) Output curves of the In2O3 TFT on 500 �C-annealed MgO dielectric.
183508-3 Jiang et al. Appl. Phys. Lett. 109, 183508 (2016)
where Ci is the areal capacitance of the dielectric and VGS is
the source-gate voltage. The lFE of the device increases
from 0.19 to 5.48 cm2/V s as the Ta of MgO dielectric was
increased from 300 to 500 �C. A reasonable explanation for
the increase of the lFE in In2O3 TFT can be attributed to the
large areal capacitance of 500 �C-annealed MgO dielectric,
which can afford greater surface charge density at the semi-
conductor/dielectric interface. In this case, a large amount of
induced carriers can fill the lower-lying localized states in
energy gap. The additional carriers could occupy the upper-
lying localized states. Then, the carriers could jump to the
percolating-conduction path easily, which leads to an
enhanced carrier mobility.33 However, for the In2O3/MgO-
600 �C TFT, the lFE is only 0.04 cm2/V s. The transport of
the induced carriers is limited in a narrow region at the chan-
nel/dielectric interface. The degraded lFE is mainly due to
the rough surface roughness (RMS¼ 4.09 nm) of the 600 �C-
annealed MgO dielectric layer because the rough surface
morphology of gate dielectric can aggravate the frequent
scattering events, leading to the degradation of the field-
effect mobility.
The VTH of the TFTs based on MgO dielectric thin film
annealed at 300, 400, and 500 �C is 1.21, 0.89, and 0.72 V,
respectively. Because the channel layers of the TFTs com-
posed in this study are the same, the negative shift of VTH is
mainly due to the decreased amount of interfacial defects
acting as carrier traps between the In2O3 channel and MgO
dielectric layers. As a result, a smaller voltage is required to
induce carriers to prefill the traps, leading to a decrease of
VTH. Meanwhile, the enhanced interface quality also helps to
improve the swing performance of the as-integrated TFTs.
The subthreshold swings (SS) can be calculated using the
following formula:
SS ¼ d log IDSð ÞdVGS
� ��1
: (2)
The SS values of In2O3 TFTs based on the MgO dielectrics
annealed at 300, 400, 500, and 600 �C were 0.97, 0.34, 0.33,
and 1.17 V/dec, respectively. The improved swing perfor-
mance of the In2O3 TFTs based on 400 and 500 �C-annealed
MgO dielectrics is mainly beneficial from the large areal
capacitance of the MgO dielectrics and the smooth interface
between the In2O3 and MgO layers. Consequently, the trap
density (Dit) can be estimated from SS using the following
equation:
Dit ¼SS log eð Þ
kT=q� 1
� �Ci
q; (3)
where k is the Boltzmann’s constant and T is the absolute
temperature. The Dit values of In2O3 TFTs based on the 300,
400, 500, and 600 �C-annealed MgO dielectrics were calcu-
lated to be 2.3, 1.19, 0.9, and 1. 7� 1013/cm2 V/dec, respec-
tively. In this work, the mobility variation is attributed to the
synergistic effects from the surface scattering caused by sur-
face roughness and the estimated Dit at the channel/dielectric
interface. This can help us understand why TFT based on
600 �C-annealed MgO exhibits a small mobility in spite of
relatively small Dit value.
The output curves for In2O3 TFT based on 500 �C-
annealed MgO dielectric are shown in Fig. 5(b). The device
exhibits a typical n-channel transistor behavior with the clear
pinch-off and current saturation. It is noted that these TFTs
based on MgO dielectrics exhibited a low operating voltage
of 6 V. As a result, the fabricated transistors dissipate a much
lower power when compared to those TFTs based on con-
ventional SiO2 dielectrics (typically higher than 30 V), which
is important for low-power electronic devices. This work can
undoubtedly fill the vacancies in the high-k research field
and make contribution to the development of the low-
voltage electronics and circuits.
In summary, we have demonstrated the solution-
processed MgO high-k thin films and explored their applica-
tion possibilities as gate dielectrics in TFTs. The physical
properties of MgO thin films annealed at various tempera-
tures were investigated by using various techniques. The
transmittances of the as-fabricated MgO thin films are over
90% in the visible range. The MgO thin film annealed at
500 �C shows a smooth surface with a small RMS value of
0.95 nm, a low leakage current density of 0.3 nA/cm2 at 1
MV/cm, and a large areal capacitance of 330 nF/cm2 at
20 Hz. As a result, the as-integrated In2O3 TFT based on
500 �C-annealed MgO dielectric exhibits the largest carrier
mobility of 5.48 cm2/V s, a high Ion/Ioff value of 107, and a
small SS value of 0.33 V/dec, respectively. In particular, all
these performances were obtained at a low operation voltage
of 6 V, which represent great achievements towards the
development of low-voltage, high-performance transistors.
See supplementary material for the detailed experimen-
tal section and instrumental analysis. The optical transmit-
tances of MgO thin films annealed at 300, 400, 500, and
600 �C
This study was supported by the Natural Science
Foundation of China (Grant Nos. 51472130, 51672142, and
51572135).
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