Stretchable Polymer Gate Dielectric by UV-assisted Hafnium Oxide

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Functional Inorganic Materials and Devices

Stretchable Polymer Gate Dielectric by UV-assisted Hafnium Oxide Doping at Low Temperature for High-performance Indium Gallium Tin Oxide Transistors Jae Seok Hur, Jeong Oh Kim, Hyeon A Kim, and Jae Kyeong Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02935 • Publication Date (Web): 24 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Stretchable Polymer Gate Dielectric by UV-assisted Hafnium Oxide Doping at Low Temperature for High-performance Indium Gallium Tin Oxide Transistors Jae Seok Hur,† Jeong Oh Kim,† Hyeon A Kim,‡ and Jae Kyeong Jeong*,†,‡ †Department

of Information Display Engineering and ‡Department of Electronic Engineering,

Hanyang University, Seoul 133-791, South Korea KEYWORDS: low temperature; UV; stretchability; hybrid dielectric; thin-film transistor (TFT); indium gallium tin oxide (IGTO)

ACS Paragon Plus Environment

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ABSTRACT

This letter reports the fabrication of indium gallium tin oxide (IGTO) thin-film transistors (TFTs) with UV-treated PVP-co-PMMA-based hybrid gate insulators at an extremely low temperature ( 150 °C). Synergetic hafnia loading and UV treatment were used to tailor the mechanical softness and hydroxyl fraction in the polymer dielectric film. The UV-treated hybrid dielectric film had the low hydroxyl concentration, a smoother surface, and a denser packing nature, which can be explained by the high ionicity of hafnium oxide and photon-assisted improvement in the cohesion between organic-inorganic materials. Suitability of the UV-treated hybrid dielectric film as a gate insulator was evaluated by fabricating bottom gate TFTs with sputtered IGTO films as a channel layer which showed high carrier mobility at a low temperature. The resulting IGTO TFTs with a UV-treated hybrid gate insulator exhibited a remarkable high field-effect mobility of 25.9 cm2/(V s), a threshold voltage of -0.2 V, a subthreshold gate swing of 0.4 V/decade, and an ION/OFF ratio > 107 even at a low annealing temperature of 150 °C. The fabricated IGTO TFTs with the UV-treated hybrid dielectric film on plastic substrate was shown to withstand the 100 times mechanical bending stress even under the extremely small curvature radius of 1 mm due to the intrinsic stretchability of the hybrid dielectric film.


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1. INTRODUCTION

Flexible backplane electronics are a topic of intense research due to their emerging applications such as rollable, bendable, and transparent AMOLED displays. Currently, backplane electronics for flexible AMOLEDs mainly use a polycrystalline silicon semiconductor and silicon dioxide (SiO2) insulator on an expensive polyimide (PI) substrate. Despite the high mobility and excellent electrical stability of silicon-based transistors, there are limited by critical issues such as high-cost production, opaque nature, and limited bendability. One of the key factors of flexible electronic fabrication is the ability of integrated devices to resist mechanical deformation such as bending or stretching, while maintaining electrical performance. To implement such flexibility for integrated arrays, all fabrication processing needs to occur at a low temperature ( 200 °C) since conventional flexible plastic substrates such as PI, polyethylene naphthalates (PEN), and polycarbonates (PC) have a low glass transition temperature. Ceramic dielectrics such as high-κ ZrO2,1-6 HfO2,7-9 and Al2O3,10-12 as well as commonly used SiO2 have acceptable insulating and dielectric properties. However, they usually require a high-temperature deposition and/or annealing process (≥ 300 °C), which makes them unsuitable for low-cost plastic substrates.13,14 Moreover, their brittle nature causes uncontrolled cracking and peeling during mechanical stress (such as bending or stretching), leading to adverse defects or device failure.15 For these reasons, polymer-based organic dielectrics have been researched as gate insulators, particularly for flexible, organic thin-film transistors (TFTs). Most polymer-based organic dielectrics can be prepared at a lower temperature compared to inorganic dielectrics. Despite their superior intrinsic flexibility and low-temperature capability, the rather high leakage current and unstable electrical properties limit their use compared to inorganic counterparts.16-21 Recently, organic-inorganic hybrid dielectrics have been studied as feasible gate insulators for high-performance, flexible electronics. The flexibility of organic


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dielectrics and the high permittivity of inorganic dielectrics allow the resulting nanocomposite to have both a reasonable κ value and mechanical bendability.22-27 On the other hand, semiconductors in TFTs are also key ingredients in determining charge carrier transport and switching capability. Metal oxide semiconductors have been intensively investigated for applications in state-of-the-art flat panel displays due to their high carrier mobility, optical transparency, and low-temperature processing. Therefore, the combination of metal oxide semiconductors and hybrid gate dielectrics holds significant promise for low-cost, highperformance flexible electronics. For these reasons, there are a few reports focusing on metal oxide TFTs with an organic-inorganic hybrid gate insulator. In terms of device architecture, the inverted staggered bottom gate configuration is preferred due to its simple structure compared to the selfaligned top gate structure. Process damage during the channel layer formation deteriorates the underlying dielectric film; therefore, the device metrics for flexible oxide TFTs with an organic or hybrid dielectric are still limited and include a field-effect mobility of 4 – 9 cm2/(V s) and an ION/OFF ratio of 104 – 5.28-31 In this study, bottom gate, high-performance metal oxide TFTs with a hybrid gate insulator were developed at an extremely low temperature ( 150 °C). First, a solution-processed hybrid dielectric film consisting of a poly(4-vinylphenol-co-methylmethacrylate) (PVP-co-PMMA) matrix and HfOx was deposited by simply blending organic and inorganic materials, where PVPco-PMMA and poly(melamine-co-formaldehyde) (PMF) were chosen as the polymer backbone and cross-linking agent, respectively. The PVP film has been frequently used as a gate dielectric layer for organic TFTs because it has the reasonable  of ~ 5 and controllability of electrical resistivity via thermal curing with a suitable cross-linker such as PMF.32-34 The residual hydroxyl group on phenol, however, can act as trap states, leading to poor leakage current and/or hysteresis


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problem for the resulting TFTs. Although the introduction of hydroxyl-free poly(methyl methacrylate) (PMMA) into PVP can mitigate these issues, the PVP-co-PMMA film has a lower κ value than PVP due to the inferior polarizability of PMMA (κ ~ 3.5).35,36 High-κ HfOx (κ ~ 20) was added to the polymer to boost the permittivity of the dielectric film.22,37-39 The low temperature processability of HfO2 in solution and UV exposure are anticipated to enhance the cohesion between heterogeneous organic-inorganic interfaces in hybrid dielectric film. Optimized blending of organic/inorganic components and appropriate UV treatment resulted in a hybrid dielectric film with a smooth morphology, excellent leakage characteristics, and good immunity to the sputtering bombardment effect even at a low temperature of 150 °C. Second, amorphous indium gallium tin oxide (IGTO) film (instead of a conventional IGZO film) was adopted as a semiconductor channel layer to achieve high device performance at a low temperature. Recently, a high mobility IGTO film was prepared by dc sputtering at room temperature and subsequent low-temperature annealing of 150 °C.40 Because the electron configuration of Sn4+ ion is the same as In3+, the synergic intercalation of In and Sn 5s orbital in IGTO film is expected to provide the efficient percolation pathways in the channel layer, leading to the enhanced carrier mobility in the resulting TFTs.41,42 The high mobility (~ 35 cm2/(V s)) of the sputtered IGTO TFTs can be attributed to densification of IGTO film via careful control of chamber pressure during channel deposition as well as its efficient percolating property. The fabricated IGTO TFTs with a UV-treated hybrid gate insulator exhibited a respectable high mobility of 25.9 cm2/(V s) and a reasonable ION/OFF ratio > 107, even at a low annealing temperature of 150 °C. The feasibility of the IGTO TFTs with a UV-treated hybrid dielectric films into the flexible, bendable electronics were confirmed by cyclic stretching test of hybrid film on PDMS and bending tests of IGTO TFTs on PI/PDMS film.


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2. EXPERIMENTAL 2.1 Dielectric Film Preparation The organic solution was prepared by dissolving poly(4-vinylphenol-co-methylmethacrylate) (PVP-co-PMMA, 0.502 g, 10 wt.%) and poly(melamine-co-formaldehyde) (PMF, 0.174 g, 3.5 wt.%) in 5mL of propylene glycol monomethyl ether acetate (PGMEA) using PMF as the crosslinking agent. An inorganic precursor solution for HfOx was synthesized using a sol-gel process with HfCl4, HNO3, and H2O dissolved in PGMEA. The concentration of inorganic precursor was 0.2 M, where the molar ratio of HfCl4: HNO3: H2O was 2: 7: 10. The two solutions were stirred for 6 h at 75 °C until each solute dissolved completely. Then, the organic and inorganic precursor solutions were blended at different volumetric ratios (organic solution/inorganic precursor solution, vol.%) of 100/0 (H0), 75/25 (H25), 50/50 (H50), and 25/75 (H75). The blended solutions were stirred again for 6 h at 75 °C to allow for complete dissolution of the solute, followed by filtering through a 0.2 m syringe filter prior to spin-coating. The bare Si substrates were cleaned with acetone, isopropyl alcohol, and deionized water for 10 min each prior to deposition of the dielectrics. The prepared solutions were spin-cast onto the Si substrates at 1500 rpm for 30 s, followed by pre-baking on a hotplate for 10 min at 100 °C to remove residual solvent. For ultraviolet treatment, deep ultraviolet (DUV, 254 nm 90%, 194 nm 10%) photons were irradiated onto the prebaked films for 10 min. Then, the cast films were annealed at 150 °C for 3 h in an air ambient furnace. The fabrication process for PDMS and Al2O3 dielectric films are described in supporting information. 2.2 Material Characterization


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The chemical and structural properties of the dielectric films were characterized by X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo Fisher Scientific Co.). Fourier transform infrared spectroscopy characterization of thin films were recorded using a germanium-attenuated total reflectance (Ge-ATR) mode (FT-IR, Nicolet iS10 FT-IR Spectrometer, Thermo ScientificTM). The surface morphology and roughness of the dielectric films were characterized by atomic force microscopy (AFM, XE-100, Park Systems Co.) in non-contact mode. The thickness of the dielectric films was measured using a spectroscopic ellipsometry (SE, Elli-SE, Ellipso Technology Co.). The densities of the dielectric films were analyzed by high-resolution X-ray reflectivity measurements (XRR, PANalytical, X’pert Pro). The XRR data were fitted using the Philips WinGixa software package. Repeated bending and stretching tests were conducted using a customized universal testing machine (UTM, TM-UMS010K, TESTMATE Co.). 2.3 Device Fabrication and Characterization Metal-insulator-metal (MIM) capacitors were fabricated by sputtering ITO film as the top electrode through a patterned shadow mask with 200-μm-diameter holes onto dielectric film/Si substrates. A heavily-doped p-type silicon wafer itself was used as the bottom electrode and gate electrode for the MIM capacitors and TFTs with PVP-co-PMMA-based dielectric films, respectively. The working pressure and dc power during ITO deposition were 5 mTorr and 50 W under an Ar atmosphere, respectively. The frequency-dependent capacitance was measured using an HP 4284A impedance analyzer ranging from 1 kHz to 103 kHz. The dielectric leakage current density (Jg) was measured using an HP 4140B Picoammeter/DC voltage source. The bottom gate TFTs were fabricated by depositing an IGTO film as a channel layer onto the dielectric film/Si substrates using a dc sputtering system at room temperature. A 3-inch-diameter ceramic IGTO target was used, which was provided by Samsung Corning Advanced Glass Co. The dc power,


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working pressure, and O2/(Ar + O2) ratio were 50 W, 3 mTorr, and 20%, respectively. The gas flow rate for Ar and O2 were fixed at 16 and 4 sccm, respectively, corresponding to an oxygen partial pressure of 20%. Finally, the ITO source/drain electrodes were deposited using the same sputtering system. The channel width (W) and length (L) of the channel region were 100 μm and 150 μm, respectively. The channel and source/drain electrodes were patterned through a shadow mask during deposition. The fabricated TFTs were subjected to thermal annealing at 150 °C for 2 h in an air ambient furnace. The electrical properties of the TFTs were characterized at room temperature in a dark ambient using a Keithley 4200-SCS semiconductor analyzer system.

3. RESULTS AND DISCUSSION Figure 1a shows the wide XPS survey scans of the PVP-co-PMMA-based dielectric films, comprising O-, N-, C-, and Hf-related peaks. The C 1s peak for the C-C bonds was assigned to the peak at 284.5 eV to calibrate the photoelectron binding energy. As expected, the H0 sample contained carbon, oxygen, and nitrogen peaks corresponding to the PVP-co-PMMA and PMF substances. Hybrid films had additional Hf ion-related peaks, such as Hf 4d and Hf 4f, which increased with increasing volumetric ratio of hafnia precursor solution. The chemical states of the polymer and hybrid dielectric films were analyzed using XPS O 1s spectra as shown in Figures 1b-g. The O 1s peak of the H0 and H0UV dielectric films can be de-convoluted into two primary peaks at 533.1 ± 0.3 eV, and 531.8 ± 0.4 eV. The peak at 533.1 ± 0.3 eV was assigned to the O-C or C-OH bonds. The lower peak at 531.8 ± 0.4 eV was assigned to the O=C bonds. In contrast, the O 1s peak of the H25, H50, H75, and H50UV hybrid dielectric films can be de-convoluted into three contributing peaks at 533.1 ± 0.3 eV, 531.8 ± 0.4 eV, and 530.3 ± 0.2 eV. The peak at 530.3


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± 0.2 eV corresponds to ionic bonding of oxygen and hafnium (O-M), suggesting formation of HfOx molecules in the hybrid dielectric films. As the inorganic volumetric ratio increased, the portion of O-C or C-OH bond-related peaks decreased, and the subpeak for the O-M bonds increased. This suggests that incorporation of HfOx into the PVP-co-PMMA matrix reduces the residual hydroxyl groups (-OH) in the phenol groups due to an extremely low annealing temperature. Loading of HfOx into the polymer film is expected to exhibit better electrical properties because the hydroxyl group can act as an adverse charge trap site. It would be interesting to discuss the effect of UV treatment on the chemical states of oxygen ions in the hybrid dielectric films. Compared to the H50 sample, the H50UV hybrid dielectric film had higher proportions of O=C (531.8 eV) and O-M (530.3 eV); they increased from 38.0% and 41.7% (H50) to 38.7% and 46.1% (H50UV), respectively. Further, H50UV had the smallest portion of C-O or C-OH bonds of 15.2%, as summarized in Table 1. This indicates that UV treatment further reduces residual trap sites and improves cohesion between organic-inorganic materials within the hybrid dielectric films, which will be discussed later. The consistent trend that the reduction of -OH groups in the hybrid dielectric films with the increasing inorganic volumetric ratio and UV treatment was also observed in the FT-IR spectra as shown in Figure S1. In contrast, the effects of UV treatment were not significant in the H0 polymer dielectric films due to short exposure time and absence of HfOx as shown in both XPS and FT-IR results.


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Figure 1. (a) XPS survey scans of the PVP-co-PMMA-based dielectric films with different inorganic volumetric ratios. (b-g) XPS spectra of the O 1s peak for the (b) H0, (c) H25, (d) H50, (e) H75, (f) H0UV, and (g) H50UV dielectric films. Table 1. De-convoluted O 1s peak results of the PVP-co-PMMA-based dielectric films with different inorganic volumetric ratios and UV treatment.

Sample

O 1s peak (eV) O-C, C-OH (533.1 ± 0.3)

O=C (531.8 ± 0.4)

O-M (530.3 ± 0.2)

H0

51.5%

48.5%

-

H25

27.3%

43.1%

29.6%

H50

20.3%

38.0%

41.7%

H75

16.0%

33.8%

50.2%

H0UV

51.2%

48.8%

-

H50UV

15.2%

38.7%

46.1%


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The surface morphology and roughness of the PVP-co-PMMA-based dielectric films were analyzed by AFM. Figure 2 shows the AFM images with a scan area of 5 μm × 5 μm for the dielectric films with different HfOx fractions and with/without UV treatment. The H0 and H0UV dielectric films had the smoothest morphology; the root-mean-square (rms) values were 0.19 and 0.20 nm, respectively. Comparable rms values of 0.26 and 0.25 nm were observed for the hybrid H25 and H50 dielectric films, respectively. However, a completely different topology appeared for the H75 hybrid film: the rms value increased rapidly to 2.3 nm, suggesting that heavy loading of HfOx into the polymer matrix at a low temperature can cause aggregation of each component and/or creation of pinholes, leading to surface roughening (see Figure 2d). Interestingly, UV treatment on the hybrid film did not deteriorate the surface morphology. That is, the rms value for the H50UV sample was approximately 0.21 nm, as shown in Figure 2f. Considering the statistical variations of AFM analysis, the surface roughness of the UV-treated films was comparable to that of the un-treated films.


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Figure 2. AFM images of the (a) H0, (b) H25, (c) H50, (d) H75, (e) H0UV, and (f) H50UV dielectric films with a scan area of 5 μm × 5 μm. The mass densities of the polymer and hybrid polymer films were evaluated by X-ray reflectivity (XRR) analysis as shown in Figure 3a. The critical angle values for the hybrid dielectric films were larger than those of polymer dielectric films due to loading of heavy hafnium atoms into the polymer matrix and strong ionic bond formation of organic and inorganic materials. Thus, the mass densities of the H0 and H50 dielectric films were 1.22 g/cm3 and 1.45 g/cm3, respectively. Furthermore, the highest mass density of 1.50 g/cm3 was obtained for the H50UV hybrid dielectric film. This demonstrates that UV-assisted improvement in cohesion between organic and inorganic materials results in film densification of the hybrid dielectric film. The vibration decay ratios for H50 and H50UV hybrid dielectric films were larger than those of H0 and H0UV polymer dielectric


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films, suggesting that loading of inorganics into an organic matrix may lead to deterioration of surface morphology, as shown in Figure 2. Figures 3b-d depict the changes in chemical structure of the polymer-based dielectric films with hafnium oxides and UV treatment. The polymer structure was simplified to focus on the crosslinking reaction of organics and/or inorganic materials and densification of the dielectric film. Figure 3d shows an illustration of the chemical structure of the UV-treated hybrid dielectric film, which consists of more O-M bonds, fewer residual hydroxyl groups, and denser film properties compared to the other two illustrations (see Figures 3b, c). It is noted that the appropriate exposure of UV into PVP-co-PMMA can produce the reactive radicals (for example, from -OH) via either photo-ionization or weak bond breaking, which would facilitate the formation of stable chemical bond with the adjacent cross-linker and, thus, the densification of thin film.43-45 Simultaneously, the hydrolysis and oxidation of hafnium precursor can be enhanced by the UV-induced photochemical activation, which also constitutes the reason for the denser film formation. However, the over-exposure of UV to organic ingredient may cause the random ruptures of C-C bonds in the organic backbone where the created defect sites can deteriorate the structural and electrical insulating property of the resulting film.43,46 Therefore, the optimal exposure time of UV for the hybrid film to exhibit the suitable structural property needs to be carefully determined.


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Figure 3. (a) Measured XRR data for the PVP-co-PMMA-based H0, H50, H0UV, and H50UV dielectric films. (b-d) Schematic illustration of the chemical structures of the (b) H0, (c) H50, and (d) H50UV polymer-based dielectric films. MIM capacitors and IGTO TFTs with PVP-co-PMMA-based dielectric films were fabricated to evaluate the suitability of the dielectric films as gate insulators. Figure 4a shows the frequencydependent capacitance variations for the MIM capacitors with different dielectric films. The areal capacitance values evaluated at 100 kHz for the fabricated MIM capacitors increased with HfOx loading. The extracted relative dielectric constants (κ) for the H0, H25, H50, and H75 dielectric films were ~ 5.1, 6.4, 7.5, and 6.3, respectively, which reflects the loading effect of high-κ hafnium oxide into the polymer matrix, as shown in Figure 4b. The κ value for the HfOx dielectric film annealed at 150 C was estimated to be ~14 (see Figure S2). The UV-treated H50UV sample had the highest  value of ~ 8.8, as shown in Figure 4b and Table 2. Figure 4c shows the electric field (E)-dependent leakage current density (Jg) of the fabricated MIM capacitors. The Jg values for the MIM capacitors were plotted as a function of thickness-normalized E for a fair comparison. An excellent low Jg value of 1.1 × 10-8 A/cm2 at the 1 MV/cm was observed for the MIM capacitor with the H0 dielectric film. However, it suffered from a rather low breakdown E-field (Ebr) (~ 1.9 MV/cm) due to the softness of the chemical bonds and film density. Breakdown E-field values were improved to 2.9 – 3.2 MV/cm for the H25 and H50 hybrid dielectric films with intermediate loadings of HfOx, while low leakage current densities were maintained (1.4 – 5.4 × 10-8 A/cm2). Conversely, the H75 dielectric film with the largest loading of HfOx suffered from an unacceptable high Jg value of 7.7 × 10-4 A/cm2 at 1 MV/cm, which was attributed to its rough morphology and the presence of many pinholes, which can act as leakage paths. The inorganic HfOx dielectric film had the similar high Jg value ( ~10-4 A/cm2) because the low temperature annealing of 150 C


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caused the considerable impurity residues such as H, N, Cl as well as the incomplete Hf-O lattice bonding in the hafnia film (Figure S2a). Notably, the UV-treated H50UV dielectric film exhibited the lowest Jg value (5.8 × 10-9 A/cm2) and highest breakdown E-field (~ 3.8 MV/cm), as shown in Figure 4d and Table 2, suggesting its suitability as a flexible gate dielectric film.

Figure 4. (a) Areal capacitance, (b) corresponding dielectric constant versus frequency characteristics, (c) gate leakage current density (Jg) versus applied electric field (E) characteristics, (d) breakdown E-field (Ebr) characteristics of the capacitors with PVP-co-PMMA-based dielectric films with different inorganic volumetric ratios and UV treatment. Table 2. Film thickness and insulating properties of the PVP-co-PMMA-based dielectric films with different inorganic volumetric ratios and UV treatment. Sample

Thickness (nm) Jg (A/cm2)

Ebr (MV/cm)

Ci (nF/cm2)

κ

H0

510 ± 7.9

(1.1 ± 1.7) × 10-8

1.9 ± 0.6

8.8 ± 0.7

5.09 ± 0.39

H25

305 ± 5.2

(1.4 ± 1.6) × 10-8

2.9 ± 0.4

18.6 ± 1.0

6.43 ± 0.35


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H50

170 ± 3.6

(5.4 ± 3.7) × 10-8

3.2 ± 0.5

38.8 ± 1.0

7.46 ± 0.20

H75

80 ± 1.6

(7.7 ± 5.0) × 10-4

2.3 ± 0.7

69.7 ± 1.8

6.30 ± 0.17

H50UV

160 ± 3.1

(5.8 ± 2.1) × 10-9

3.8 ± 0.4

48.9 ± 1.2

8.84 ± 0.21

The functionality of the synthesized dielectric films as a gate insulator was evaluated by fabricating and characterizing IGTO TFTs with a bottom-gate and top-contact configuration. Figure 5 shows the representative transfer characteristics of the IGTO TFTs with different gate insulators. The field-effect mobility (μFE) was extracted from the maximum incremental slope of the IDS1/2 versus VGS plot in the saturation region using the following equation (see Figure S3): IDS = (WCi/2L)μFE(VGS - VTH)2 where L is the channel length, W is the width, Ci is the gate capacitance per unit area of the insulator layer, and VTH is the x-axis intercept in the IDS1/2 versus VGS plot. The VGS-dependent mobility variations for all devices confirmed that the used gate bias condition to calculate the maximum saturation mobility satisfy the saturation region of TFT operation (VDS  VGS - VTH), as shown in Figure S4. The subthreshold gate swing (SS = dVGS/d(logIDS) (V/decade)) was extracted from the linear part of the log(IDS) versus VGS plot. Both VTH and SS were extracted when a forward gate bias was applied at VDS = 5.1 V. The device with the control H0 dielectric film had a reasonable μFE of 12.8 cm2/(V s), presumably due to the efficient percolation pathway for electron carriers in the IGTO active layer. However, it suffered from a large SS of 2.8 V/decade, a negative VTH of 6.5 V, and high IOFF current of 1  10-9 A, which could be explained by the energetic ion bombardment effect. The in-diffusion of energetic In, Ga, and Sn cations into the polymer dielectric film during IGTO sputtering could induce image electron carriers in the channel layer, causing a negative VTH and an unacceptably high IOFF value.28,47-49 Simultaneously, the surface of


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the polymer dielectric film can be attacked by energetic particles, including high energy electron and charged ions, creating adverse defect states near the channel/polymer dielectric interface due to the low density of the polymer dielectric film, as mentioned in the discussion of the XRR analysis. This is partially responsible for the large SS and high IOFF values. The existence of residual hydroxyl groups as trap sites in the H0 dielectric film (which was mentioned in the XPS analysis discussion) was an additional origin for these deteriorated properties. The large clockwise hysteresis (~ 6 V) for the control device clearly indicates the existence of many temporal trapping sites due to the hydroxyl groups and bombardment-induced defects. Substantial improvements in μFE, SS, and VTH were observed for the IGTO TFTs with H50 and H50UV hybrid gate insulators (see Figures 5c, e). The device with a H50 hybrid gate insulator exhibited a high μFE of 19.2 cm2/(V s), an SS of 2.2 V/decade, and a VTH of -5.9 V. It can be inferred that the hybrid dielectric film was less affected by the energetic ion bombardment or ion migration toward the dielectric film because the Hf-O (with strong ionicity) in the polymer matrix enhanced the mechanical hardness of the resulting hybrid dielectric film. The positive effect can be reflected in the high carrier mobility and low defect density of the device with a H50 hybrid gate insulator. The switching capability, which is one of the critical parameters for the TFTs, can be further enhanced by using the UV-treated H50UV dielectric film: IGTO TFTs with H50UV hybrid gate insulators had a high ION/OFF ratio of ~ 107 as a result of reduction in IOFF value (see Figure 5e and Table 3). In addition, the highest μFE of 25.9 cm2/(V s), lowest SS of 0.4 V/decade, a VTH of -0.2 V, and ION/OFF ratio > 107 for this device correspond to the state-of-the-art characteristics for any sort of metal oxide transistor with a polymer and hybrid gate insulator (Tmax  150 °C) (see Table 4). It should be noted that the nonnegligible gate leakage current can cause the overestimation of μFE value for the given TFTs in Figure 5. When the μFE values were extracted from the gate leakage current-corrected drain current,


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the maximum error range of μFE values was less than 0.6%. The problematic hysteresis (~1.2 V) was also reduced for the IGTO TFTs with a H50UV hybrid gate insulator. These superior performances can be attributed to i) the lowest concentration of hydroxyl groups in the hybrid dielectric film, ii) loading effect of Hf-O with a large ionicity, and iii) better interface matching between IGTO- and Hf-O-based hybrid dielectric films. However, a heavy loading of HfOx in the polymer matrix deteriorated the drain leakage current: the IOFF value for the device with the hybrid H75 insulator was degraded to ~ 10-6 A, leading to the smallest ION/OFF ratio of 102. This degradation comes from the rough surface morphology and the existence of many pinholes in the H75 hybrid dielectric film, as described above. Figures 6a and b show representative output characteristics of the IGTO TFTs with the control H0 and H50UV gate insulators, respectively. Standard pinch-off phenomena were seen for both devices. However, the saturation IDS value for the IGTO TFTs with the H50UV gate insulator was greatly enhanced from 28 A (H0 gate insulator) to ~ 200 A at VGS = 12 V due to the superior transport properties of the device with the H50UV gate insulator.


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Figure 5. Representative transfer characteristics of the IGTO TFTs with PVP-co-PMMA-based gate insulators of (a) H0, (b) H25, (c) H50, (d) H75, and (e) H50UV.

Figure 6. Corresponding output characteristics of the IGTO TFTs with PVP-co-PMMA-based gate insulators of (a) H0 and (b) H50UV.


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Table 3. Electrical characteristics of the IGTO TFTs with PVP-co-PMMA-based gate insulators with different inorganic volumetric ratios and UV treatment. Sample

µFE (cm2/(V s))

SS (V/decade)

VTH (V)

ION/OFF

H0

12.8 ± 0.6

2.8 ± 0.5

-6.5 ± 0.4

> 104

H25

15.4 ± 0.5

2.4 ± 0.6

-5.3 ± 0.4

> 103

H50

19.2 ± 0.7

2.2 ± 0.2

-5.9 ± 0.3

> 105

H75

31.6 ± 0.6

3.0 ± 0.3

-2.8 ± 0.8

> 102

H50UV

25.9 ± 0.3

0.4 ± 0.2

-0.2 ± 0.1

> 107

As mentioned before, it is evident that the device metrics in this study (such as VTH, SS, and ION/OFF ratio) are comparable to the previously studied inorganic-assisted, polymer-based gate insulators, as shown in Table 4. This was due to minimized residual defect states, smooth surface properties, and film densification, which were achieved by optimizing the amount of inorganic material incorporated along with UV treatment. Moreover, choosing an IGTO system instead of a conventional IGZO system resulted in a high field-effect mobility at a low annealing temperature of 150 °C. Table 4. Comparison of insulating properties and electrical characteristics for various metal oxide TFTs with different inorganic assisted polymer-based gate dielectric materials. Dielectric Material

Tmax (°C)

κ

Jg (A/cm2)

Channel Material

μFE (cm2/(V s))

VTH (V)

SS (V/decade)

ION/OFF

REF

m-YHD

150

5

10-5

ZnO

0.318

0.89

0.56

~ 106

25

PVP / Al2O3

150

-

-

SIZO

8.84

-1.5

1.09

~ 105

26

PVP / ZrO2:B

200

8.5

10-8

In2O3

0.44

1.7

-

~ 105

27

PVP-co-PMMA, PMF, ZrO2

250

5.6

10-7

IZO

28.4

-2.0

0.7

~ 107

28

GTPMS, HfO2

150

11.4

10-7

IGZO

4.74

0.3

-

~ 104

29


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PVP, PMF, Al2O3

100

6.1

10-9

IGZO

5.13

2.3

0.84

~ 105

30

PVP / Al2O3

200

-

10-6

IGZO

8.39

7.03

0.68

~ 105

31

PMMA, GPTMS, Al2O3

150

6.2

10-7

ZnO

4.5

0.7

-

~ 107

50

GO, PVA

-

-

10-9

IGZO

42

0.79

0.106

~ 107

51

PMMA, ZrHfO2

200

9.4

10-6

IGZO/ ZnO

2.45 / 12.8

1.2 / 2.5

0.68 / 3

~ 107 / ~ 103

52

Siloxane

300

3.78

IGZO

22

5.6

0.32

~ 106

53

PVP-co-PMMA, PMF, HfOx

150

8.84±0.21

IGTO

25.9±0.3

-0.2±0.1

0.4±0.2

~ 107

This work

10-9

Next, the resistance to mechanical deformation of the hybrid dielectric films was examined by a stretching test. Both H50UV and Al2O3 dielectric films were coated onto intrinsically stretchable polydimethylsiloxane (PDMS) films as shown in Figure 7a. Al2O3 was coated with a thickness over 100 nm at a deposition temperature of 150 °C for a fair comparison. As shown in Figure 7f, the Al2O3 dielectric film suffered from large microscale cracks due to thermal expansion of the PDMS during dielectric film deposition, even before detaching it from the substrate. After detaching it from the substrate, numerous additional microscale cracks were formed for Al2O3 dielectric film in random directions due to their rigid characteristics (see Figure 7g). On the other hand, H50UV hybrid dielectric films showed excellent physical resistance, even after detaching from the substrate without crack formation (see Figures 7b, c). After cyclic stretching at 10% strain, linear cracks in the vertical direction of stretching were formed for both dielectric films. Still, it was evident that polymer-based dielectric films are fine candidates for stretchable electronics due to their intrinsic softness compared to inorganic materials.


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Figure 7. (a) Schematic illustration of the fabrication and stretching processes of dielectric films on a PDMS film. The effects of strain on the structure and morphology of (b-e) H50UV hybrid dielectric film and (f-i) Al2O3-coated PDMS film under 10% strain. Finally, the electrical stability of the IGTO TFTs with H50UV hybrid gate insulator under a bending stress was evaluated by fabricating on plastic PI/PDMS substrate using a transfer method. Figure 8a shows the images of the as-fabricated IGTO TFTs with H50UV hybrid gate insulators on the PI/glass substrate, detached free-standing devices, and transferred devices on PDMS film. Generally, it is known that metal oxide TFTs with a ceramic gate insulator such as Al2O3 film on plastic substrates lose their switching functionality under the bending stress less than the bending radius of 3 mm.54 The variations in the transfer characteristics of the IGTO TFTs with H50UV hybrid gate insulator on the PI/PDMS film were shown in Figure 8b. The bending test of IGTO


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TFTs on PI/PDMS film was performed at the bending radius of 1 mm. The excellent switching capability of the IGTO TFTs was maintained even after 100 cycles of the bending tests at an extremely small bending radius of 1 mm, which was comparable to that of the as-fabricated IGTO TFTs on PI/glass substrate (see Figure 8b). This intriguing result can be explained by the inherent bendability and stretchability of the hybrid polymer dielectric film. The superior mechanical stability of flexible IGTO TFTs demonstrates the feasibility of the polymer-based hybrid gate dielectric layer and the IGTO system for high performance bendable electronics.

Figure 8. (a) Images of the as-fabricated IGTO TFTs with H50UV hybrid gate insulators on the PI/glass substrate, detached free-standing devices, transferred devices on PDMS film and their bending test procedure. The schematic device cross-sections are included in the inset figure. (b) Evaluation of transfer characteristics for the IGTO TFTs with H50UV gate insulator on PI/PDMS substrate under cyclic bending tests.

4. CONCLUSION


23


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We examined the effects of hafnium oxide loading and UV treatment on the electrical performance of PVP-co-PMMA-based hybrid gate insulators for low-temperature-processed metal oxide TFTs. Insulating properties of the hybrid gate dielectric films were drastically improved due to the reduced residual hydroxyl groups resulting from incorporation of hafnium oxide (with its high ionicity) along with UV treatment. We found that the UV-treated hybrid dielectric film had a smoother surface roughness and better interface matching with an IGTO channel layer. Interestingly, the UV-assisted hybrid dielectric films were able to withstand energetic ionic bombardment during sputtering deposition of the IGTO channel layer, and this was attributed to its denser film nature. As a consequence, the bottom gate IGTO TFTs with a UV-treated hybrid gate insulator exhibited a high μFE of 25.9 cm2/(V s), a VTH of -0.2 V, a low SS of 0.4 V/decade, and an ION/OFF ratio > 107 even at a low annealing temperature of 150 °C. Furthermore, the IGTO TFTs with a UV-treated hybrid gate insulator on PI/PMDS film was shown to withstand the 100 times cyclic bending test under the curvature radius of 1 mm. Therefore, we concluded that UVtreated hybrid dielectrics are promising candidates as gate insulators for low-temperatureprocessed flexible and stretchable electronics. Their integration with a high mobility IGTO channel system enabled to successfully achieve state-of-the-art characteristics for a metal oxide TFT with a polymer-based dielectric film.


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ASSOCIATED CONTENT Supporting Information FT-IR spectra of the PVP-co-PMMA-based dielectric films; Gate leakage current and C-f characteristics of the HfOx dielectric film; Transfer characteristics of the IGTO TFTs with a plot of VGS vs IDS1/2; Mobility extraction of the IGTO TFTs; Fabrication process of PDMS films and Al2O3 dielectric films. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Funding Sources MKE/KEIT through the Industrial Strategic Technology Development Program under Grant 10079971 and Samsung Research Funding Center for Future Technology through Samsung Electronics. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by MKE/KEIT through the Industrial Strategic Technology Development Program under Grant 10079974 and Samsung Research Funding Center for Future Technology through Samsung Electronics.


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Stretchable Polymer Gate Dielectric by UV-assisted Hafnium Oxide

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