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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183508-5 Jiang et al. Appl. Phys. Lett. 109, 183508 (2016)