Solution-processed high-k magnesium oxide dielectrics for low-voltage oxide thin-film
transistors
Guixia 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: 10.1063/1.4966897 View online: https://doi.org/10.1063/1.4966897
View Table of Contents: http://aip.scitation.org/toc/apl/109/18
Published by the American Institute of Physics
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Solution-processed high-k magnesium oxide dielectrics for low-voltage
oxide thin-film transistors
GuixiaJiang,1,2,3,a)AoLiu,1,2,3,a)GuoxiaLiu,1,2,3,b)ChundanZhu,1,2,3YouMeng,1,2,3 ByoungchulShin,4ElviraFortunato,5RodrigoMartins,5and FukaiShan1,2,3,b) 1
College of Physics, Qingdao University, Qingdao 266071, China 2
College of Electronic and Information Engineering, Qingdao University, Qingdao 266071, China 3
Lab of New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China
4
Electronic Ceramics Center, DongEui University, Busan 614-714, South Korea 5
Department of Materials Science/CENIMAT-I3N, Faculty of Sciences and Technology, New University of Lisbon 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 dielecdielec-tric films annealed at vari-ous temperatures (300, 400, 500, and 600C) 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 In2O3channel 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 SiO2and/or
SiNx gate dielectrics, resulting in large operating voltages
due to the weak capacitive coupling between the gate dielec-tric and channel layer.1–3Because 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).4To 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,22–24have been applied in TFTs because they simul-taneously enable a low leakage current as well as a low-voltage operation. Although HfO2and ZrO2have large k
val-ues of25, the bandgap is relatively small (5.8 eV). Similarly, although Al2O3has 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).25High-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 cm 2
/V s and an on/off current ratio (Ion/Ioff) of 104.22Leeet al. fabricated ZnO
TFT with MgO as the gate insulator grown using sputtering and exhibiting an Ion/Ioffratio of 105and 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.25For the solution-derived oxide thin films, the physi-cal properties and the chemiphysi-cal 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 construcsolu-tion 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
The detailed experimental section and instrumental analy-sis can be found in thesupplementary 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 162C, which represents the decomposition of volatile nitrate precursor and residual species.27The other exothermic peak at 425C 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 500C, which indicates the complete conversion from xerogel to MgO.
The optical transmittances of MgO thin films annealed at 300, 400, 500, and 600C 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 2shows 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 600C are 1.54, 1.21, 0.95, 4.09 nm, respectively. The MgO thin films exhibit smooth surfaces as the Ta increases to
500C, which is ideal for suppressing the surface roughness induced leakage current and achieving a high carrier mobil-ity in the TFT channel.28,29The significantly increased RMS value of MgO thin film annealed at 600C 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 (RMS3.4 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 80C to 700C.
FIG. 2. The AFM images of MgO thin films annealed at different tempera-tures; (a) 300, (b) 400, (c) 500, and (d) 600C.
the surface at 532.4 eV.29,30 For convenience, OI, OII, and
OIII were used to denote the area of each component, and
Ototaldenoted 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 300C to
600C. 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.12The
areal capacitance densities at 20 Hz were found to be 190, 280, 330, and 370 nF/cm2for 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 C¼eok/d, the dielectric constants were calculated to be 5.8,
6.7, 7.5, and 8.1 for 300, 400, 500, and 600C-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 IDSon VGS. ThelFE
was calculated by the following formula:32
IDS¼
W
2LlFECiðVGSVTHÞ 2
; (1)
FIG. 3. XPS O1s spectra of MgO thin films annealed at 300, 400, 500, and 600C.
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 In2O3TFTs with various MgO dielectrics
(VDS¼5 V). The processing temperature for the In2O3channel was 320C.
where Ciis the areal capacitance of the dielectric and VGSis
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 500C. A reasonable explanation for the increase of thelFEin In2O3TFT can be attributed to the
large areal capacitance of 500C-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-600C TFT, the lFEis 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 (RMS¼4.09 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 VTHof the TFTs based on MgO dielectric thin film
annealed at 300, 400, and 500C 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 VTHis
mainly due to the decreased amount of interfacial defects acting as carrier traps between the In2O3channel 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:
SS¼ dlogðIDSÞ
dVGS 1
: (2)
The SS values of In2O3TFTs based on the MgO dielectrics
annealed at 300, 400, 500, and 600C were 0.97, 0.34, 0.33, and 1.17 V/dec, respectively. The improved swing perfor-mance of the In2O3TFTs based on 400 and 500C-annealed
MgO dielectrics is mainly beneficial from the large areal capacitance of the MgO dielectrics and the smooth interface between the In2O3and MgO layers. Consequently, the trap
density (Dit) can be estimated from SS using the following
equation:
Dit¼
SSlogð Þe kT=q 1
Ci
q; (3)
where k is the Boltzmann’s constant and T is the absolute temperature. The Ditvalues of In2O3TFTs based on the 300,
400, 500, and 600C-annealed MgO dielectrics were calcu-lated to be 2.3, 1.19, 0.9, and 1. 71013/cm2V/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 Ditat the channel/dielectric
interface. This can help us understand why TFT based on 600C-annealed MgO exhibits a small mobility in spite of relatively small Ditvalue.
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 SiO2dielectrics (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 500C 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
500C-annealed MgO dielectric exhibits the largest carrier mobility of 5.48 cm2/V s, a high Ion/Ioffvalue 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.
Seesupplementary materialfor the detailed experimen-tal section and instrumenexperimen-tal analysis. The optical transmit-tances of MgO thin films annealed at 300, 400, 500, and 600C
This study was supported by the Natural Science Foundation of China (Grant Nos. 51472130, 51672142, and 51572135).
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