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Ferroelectric/Dielectric Double Gate Insulator SpinCoated by Using Barium Titanate Nanocrystals for Indium Oxide Nanocrystal–Based Thin-Film Transistor Hien Thu Pham, Jin Ho Yang , Don-Sung Lee, Byoung Hun Lee, and Hyun-Dam Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00109 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 3, 2016

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ACS Applied Materials & Interfaces

Ferroelectric/Dielectric Double Gate Insulator Spin-Coated by Using Barium Titanate Nanocrystals for Indium Oxide Nanocrystal–Based Thin-Film Transistor Hien Thu Pham,1 Jin Ho Yang,2 Don-Sung Lee, 1 Byoung Hun Lee,2 and Hyun-Dam Jeong*, 1 E-mail: [email protected] 1

Department of Chemistry, Chonnam National University, Gwangju, 500-757, Republic

of Korea. 2

Center for Emerging Electronic Devices and Systems, School of Materials Science and

Engineering, Gwangju Institute of Science and Technology, Oryong-dong 1, Buk-gu, Gwangju, Korea

1

ACS Paragon Plus Environment


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Abstract Barium titanate nanocrystals (BT NCs) were prepared under solvothermal condition at 200 °C for 24 h. The shape of the BT NCs was tuned from the nanodot to nanocube upon changing the polarity of alcohol solvent, varying the nanosize in the range 14–22 nm. Oleic acid-passivated NCs showed good solubility in nonpolar solvent. The effect of size and shape of the BT NCs on the ferroelectric properties was also studied. The maximum polarization value of 7.2 µC/cm2 was obtained for BT-5 NC thin film. Dielectric measurement of the films showed comparable dielectric constant value of BT NCs over the 1 kHz- 100 kHz, without significant loss. Further, bottom gate In2O3 NC thin film transistors exhibited outstanding device performance with a fieldeffect mobility of 11.1 cm2/V·s at a low applied gate voltage with BT-5 NC/SiO2 as the gate dielectric. The low-density trapped state was observed at the interface between the In2O3 NC semiconductor and BT-5 NCs/SiO2 dielectric film. Furthermore, the compensation of applied gate field by an electric dipole-induced dipole field within BT5 NC film was also observed. Keywords: barium titanate; ferroelectricity; nanocrystal; indium oxide nanocrystal; nanocrystal dielectric; thin film transistor _____________________________________________________________________________________

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Introduction Barium titanate (BT) is a ferroelectric oxide with the Curie temperature (Tc) of >120 °C. The ferroelectric properties and high dielectric constant attribute BT to be useful in an array of applications such as design for high-k polymer-ceramic nanocomposite film for flexible capacitors in energy storage applications,1–5 multilayer ceramic capacitors,6–8 gate dielectric,9–11 or photovoltaics.12 From the structural point of view, the ferroelectric order is triggered in titanium containing perovskites by the offcenter shift of the Ti4+ ions within the unit cell, leading to the formation of electrical dipoles.13 The ordering of dipoles and the onset of an intrinsic lattice polarization along is accompanied by the lowering of the symmetry of the unit cell from cubic to tetragonal. However, at the nanoscale, the ferroelectric structure exhibits quite different properties from those of the bulk materials. For example, the ferroelectricity including mean polarization, remnant polarization, and area hysteresis loop will become size and shape dependent.14–18 Furthermore, in the thin-film transistor (TFT) applications, gate dielectric is a key component, contributing toward the advancement of TFTs.9–11,19,20 Among high-k material, Cai et al. used BT nanoparticles as the gate dielectric in the bottom gate pentacene TFTs among inorganic colloidal nanoparticle such as TiO2 nanoparticle,19,20 or organic polymer material,21,22 indicating that BT nanoparticles can

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be directly solution-processed into high-k inorganic films without the need for sophisticated nanocomposite synthesis or blending with polymer materials, thus offering simplicity and economic advantages.9 In this study, we first report the controlled synthesis of free-standing BT nanocrystals by the solution chemical process. We demonstrate that not only the shape of nanocrystals was tuned from dots to cubes but also their size was controlled in the range 14–22 nm upon tuning the polarity of the alcohol solvent. Moreover, the ferroelectricity of the nanocrystals was retained. The mechanism of ferroelectricity dependence on the size and shape of nanocrystals is also described. A novel application of BT nanocrystals was proposed for the gate dielectric for low-voltage operating indium oxide nanocrystal (In2O3 NC) TFTs. A BT NCs layer was inserted at the SiO2 dielectric/In2O3 NC semiconductor interface to function as the modifier dielectric gate. Accordingly, the effect of surface modification, namely, the BT-NC/SiO2 gate dielectric, on the bottom-gate contact In2O3 NC TFTs was compared and analyzed. The In2O3 NC TFT-based BT-NC/SiO2 gate dielectric showed surprisingly outstanding device performance in comparison to In2O3 NC TFT-based only SiO2 gate dielectric. Probably attributing to the fact that In2O3 NC/BT NC forms an excellent semiconductor/dielectric interface with lower trapped states density. In addition, strong electric dipoles within the

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BT NC film can be aligned by the gate field, resulting in an electric field induced by these electric dipoles, accumulating excess charge on the semiconductor channel, thus more effectively enhancing TFT performance.

Experimental Chemicals Barium hydroxide octahydrate (≥98%), titanium (IV) n-butoxide (97%), DIwater, oleic acid (≥95%), diethanolamine (≥98%), 1-butanol (≥99.5%), anhydrous 1octanol (≥99%), anhydrous benzyl alcohol (99.8%), anhydrous 2-methoxy ethanol (99.8%), indium (III) acetate (99.99%), 1-octadecene (90%), anhydrous chlorobenzene (99.8%), and acetic acid (≥99.7%) were obtained from Sigma-Aldrich. Diethylene glycol (99%) and 1-octadecanol (97%) were obtained from Alfa Aesar. Ammonium hydroxide solution (25% NH3 in water) was obtained from Acros Organics, and all the chemicals were used without further purification. Acetone (≥99.5%), methanol (≥99.5%), and toluene (≥99.5%) were purchased from Daejung Chemicals (Siheung City, South Korea) and were used as received.

Synthesis of barium titanate nanocrystal A slight modification of the literature procedure of the solvothermal process was used for the synthesis of barium titanate

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nanocrystals (BT NC).23,24 In a typical experiment, 0.012 mol of barium hydroxide octahydrate was dissolved in 8 mL of preheated deionized (DI)-water. In parallel, 0.01 mol of titanium (IV) butoxide was mixed with 2.5 mL of oleic acid and 5 mL of highpurity butyl alcohol, followed by adding 1.75 mL of ammonium hydroxide solution. Then, 2.5 mL of diethanolamine was added to the solution mixture. The aqueous Ba(OH)2 solution was subsequently added to the solution mixture. To control the particle size, various types of alcohol such as diethylene glycol, n-octyl alcohol, benzyl alcohol, or 2-methoxyethanol were added to the solution mixture, instead of butyl alcohol. The final suspension was transferred to a 100 mL Teflon-line stainless-steel autoclave and heated at 200 °C for 24 h. After the reaction, the resultant product was washed repeatedly using high-purity ethanol and dried at 80 °C for 24 h. To eliminate the presence of BaCO3, identified as secondary phase, the powder was washed with dilute (5%) acetic acid solution. The resulting nanocrystals were then collected, washed several times with ethanol, and dispersed in toluene (3 wt%), yielding a stable colloidal solution.

Synthesis of indium oxide nanocrystals Oleic acid-capped In2O3 nanocrystals (In2O3 NC, 5.3 nm size) were synthesized following our previously reported procedure.25,26

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Device fabrication and characterizations Different substrates including highly doped Si substrates and Si/SiO2 (100 nm) substrates were used. Both were cleaned with methanol, acetone, and DI water with sonication before spin-coating. In general, BT NC films were grown on the top of the substrates by a simple spin-coating method at the spin rates in the range 500–3000 rpm/25s. For the preparation of multifold coatings, each coating was dried at 200 °C before forming the next coating and the multicoated thin films was eventually annealed at 500 °C for 3 h in air to achieve full crystallization.

Preparation for measuring ferroelectric polarization-voltage BT NC (~3 wt% in toluene) was grown on the top of highly doped Si substrates, resulting in the formation of a 84-nm thickness film. Subsequently, the gold top contacts were deposited on the nanocrystal film.

Bottom-gate indium oxide nanocrystal thin film transistors Si/SiO2 (100 nm)/BT NC (~46 nm and 2 fold coating, as-fabricated above) and In2O3 NC suspensions in toluene (~3 wt%) were spin-coated on the top of the nanocrystal dielectric substrates then annealed at 350 °C for 3 h. Finally, an aluminum source and a drain were deposited on the top of the indium oxide nanocrystal layer with a channel length and width of 100 and 1000 µm, respectively, to form the thin film transistor (TFT) device.

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Characterizations

Field-emission transmission electron microscopy (FE-TEM) was performed using a JEM-2100F electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. For the TEM sampling, 0.2 wt% solutions of the as-synthesized NCs in chlorobenzene were drop-casted onto a carbon-coated copper grid, and the solvents were evaporated in vacuum. Field-emission scanning electron microscopy (FE-SEM) was performed using a JSM-7500F (JEOL, Japan) scanning electron microscope. Fourier transform infrared spectroscopy (FTIR) was conducted using a Nicolet 380 spectrometer (Waltham, MA, USA). High-resolution X-ray diffraction (XRD) patterns were collected using an X’Pert PRO Multi-Purpose X-ray diffractometer (PANalytical, Netherlands) equipped with a Cu Kα source operating at 40 kV and 30 mA. Ferroelectric polarization-voltage was measured using Radiant Technologies Precision LC units. The TFT characteristics, including the transfer and output curves, were obtained using an HP4145B semiconductor analyzer. Capacitance–voltage (C–V) of metal–insulator–metal capacitor was measured using a HP4284A precision LCR meter. Dielectric constants (εr) were calculated by the formula C = εrA/d, where C is the capacitance, A is the area of the electrode, d is the film thickness, ε0 is the vacuum permittivity of 8.854 × 10-14 F/cm.

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Results and Discussion Synthesis of size controlled barium titanate nanocrystals

The oleic acid-capped BT NCs were successfully synthesized by the chemical modification of Ti alkoxides (Ti(OR)4) with long chain carboxylic acid (oleic acid), yielding

polynuclear

oxocarboxyalkoxides

(Ti(OR)4-x(OOCR)x)

in

which

the

hexacoordinated Ti4+ ions are surrounded by bulk carboxylate ligands. As reported in several literature, this treatment not only increased the stability of the alkoxides, but hindered the nucleophilic attacking of HO- ions in water to prevent the formation of amorphous TiO2, eventually slowing down the hydrolysis rate of Ti4+.9,24,27,28 In this study, the synthesis of BT oxide nanocrystals was performed from the microemulsion containing transition metal alkoxycarbonxylates generated in situ from the reaction between Ti(OBu)4 and oleic acid, as evidenced by the color change of the reaction mixture from colorless to faint yellow as represented by the following equation.

Ti(OR) 4 +xR'COOH → [Ti(OR) 4-x (OOCR') x ]+xROH

(r1)

Under the solvothermal conditions, Ba2+ ions reacted with titanium alkoxy carboxylates in a mixture of the aqueous with the oil phase, according to the following reaction:

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[Ti(OR) 4-x (OOCR') x ]+Ba 2+ +(x+2)OH - → BaTiO3 +(4-x)ROH+xR'COO- +(x-1)H 2 O (r2)

In addition, the presence of ammonium hydroxide solution as the basic solution, involves the nucleophilic addition of OH- groups under strongly alkaline condition. Diethanolamine (DEA) functioned as the surfactant, playing a role in the formation of hydrogen bonds with the hydrated barium salts and exhibited a strong inclination to take Ba2+ or BaOH+ away from the hydroxide complexes. Figure 1 shows the FTIR spectra of pure DEA, pure oleic acid, and BT NCs powder with different sizes. DEA showed absorption peaks in the range 3000–3500 cm-1, corresponding to stretching mode of H– O and N–H. Besides, the band at 1028 cm-1 was observed corresponding to the stretching mode of the C–N bond. For oleic acid, The characteristic absorption peaks of oleic acid were observed at 3009 cm-1 and in the range 2800–3000 cm-1, coinciding with the signals for the alkenyl C–H (–HC=CH–),alkyl C–H (–CH3), and C=O stretching bands appearing at 1710 cm-1. The IR spectrum of BT NCs exhibited the same characteristic stretching bands for the alkenyl C–H and alkyl C–H with that observed for oleic acid. New strong bands appearing at 1558 and 1435 cm-1 were attributed to the asymmetric and symmetric COO– stretching band, and no C–N stretching band was observed, indicating that oleic acid were successfully modified on the surface of BT NCs. In general, several reaction parameters, including the concentration of precursor,

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the nature of solvent,24 surfactant,25,29 and the reaction time,29 played a significant role on the morphology of the BT NCs. Table 1 lists the shape and average particle size, and lattice parameters of BT NCs obtained from the mixture in which 1-butanol was replaced with other alcohols with different polarities (ε). The size of BT NCs were 14, 15, 17, 18, and 22 nm in 1-octanol (ε = 3.4), benzyl alcohol (ε = 13.1), 2methoxyethanol (ε = 16.9), DEG (ε = 3.2), and butyl alcohol (ε = 17.8), respectively. Interestingly, replacing 1-butanol with other alcohols, not only controlled the size but also the shape, as shown in Figure 2. Uniform regular nanodots were produced using 1octanol and 1-butanol (Figure 2(a), (e)) as the alcohol solvent. Uniform regular nanocubes were obtained using DEG (Figure 2(d)). In comparison, a mixture of nanodots and nanocubes were observed with benzyl alcohol and 2-methoxyethanol (Figures 2(b) and (c)). Our experiment strongly suggests that the polarity of the solvent was also an important parameter in the growth mechanism of BT NCs. The addition of alcohol solvents with a relatively high dielectric constant led to the bigger particle size. The purity and crystallinity of BT NCs were studied by X-Ray powder diffraction. The XRD patterns shown in Figure 3(a) confirm the high crystallinity with no impurity peak appearing in the diffraction of the BT NC samples. The diffraction peaks were assigned to the (100), (110), (111), (002, 200), (210), (211), and (220) planes. The XRD pattern

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of tetragonal phase was identified by the splitting of peak around 45° - 46° into two peak, which correspond to the (hkl) Miller index (002) and the (200 + 020) reflection, respectively,30 as shown in Figure 3 (b). In cubic system there is no splitting of the 46° peak and one single peak at 45° corresponding to the (002) plane. BT-1 and BT-5, as well as BT-4 show obvious peak splitting, however BT-2 and BT-3 showed the peak splitting were not very distinct, probably because of the peak broadening effect of nanocrystalline structures or the cubic dominant BT structures. The grain size obtained from X-ray peak broadening was calculated. From the XRD patterns, the lattice distortion was calculated as a function of grain size, as the grain size increases, the c/a ratio also increase, as shown in Table 1.

Ferroelectricity in barium titanate nanocrystal thin film Figure 4 shows the three dimensional surface topography images for BT NCs on the Si substrate. BT NCs layer seems relatively undulating and has interstices between the granules. Figure 5 (a) shows the polarization versus drive voltage (P–V) hysteresis loops of our BT NC films after fivefold coating (thickness details are summarized in Figure S3 (Supporting Information)), and it shows constant ferroelectric hysteresis loops and can be stable up to an external voltage of 3 V without any sign of breakdown. All the five films exhibited some hysteresis when sweeping the drive voltage across the

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films. Hysteresis loops were found to change significantly with the shape and size of NCs. The maximum remnant polarization value (Pr) of 7.2 µC/cm2 was observed for BT-5 NC thin film, and it decreased with decreasing nanosize. Moreover, the conversion ratio of nanodots into nanocubes increased, and no polarization saturation was observed with increasing driving voltage, which due to the increased chance of leakage at higher applied voltage and incomplete ferroelectric properties, compared to that reported for single crystal value of 24 µC/cm2

31

and the ceramic value of 8 µC/cm2.32 Moreover, as

shown in Supporting Information Figure S4, the local ferroelectric characterization of BT NC films were performed by electrostatic force microscopy (EFM). First, a 10 x 10 µm area was scanned with applying a bias voltage of 0V. Then a 7 x 7 µm area was scanned for + 5V, subsequently, 3 x 3 µm area was scanned with reversed bias of -5V. Finally, 0V was applied for 10 x 10 µm area. We observed that only our BT-5 NC showed a slight polarization switching (EFM image). Unfortunately, for BT-1 NC film up to BT-4 NC film, no polarization switching was observed. Dielectric constant was also studied over the frequency range of 1 – 100 kHz on those electrodes which shows ferroelectric hysteresis curves. Figure 5 (b) also showed the significantly changing of dielectric constant with the shape and sizes of BT NCs. BT-5 NC film showed the dielectric constant is about 1416 at 1 kHz which was comparable with the reported bulk

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BT ceramic value 1500 at 1 kHz.32 Moreover, as frequency is below 100 kHz, the dielectric constant increases very slightly in the whole frequency region. Suggesting that a few conducting carriers such as ionic space charge carrier exist in our BT films, as promising candidate in thin film capacitance applications. The theoretical tetragonal asymmetry of BaTiO3 is 1.011 (a = 3.992 Å, c = 4.036 Å);32 however, as listed in Table 1, in our BT NCs, the tetragonal structure was distorted by 1.009 for BT-5 NCs. Subsequently, a drastic decrease in the tetragonal value from 1.009 to 1.001 was observed when the particle size was

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