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Ultralow Temperature Solution-Processed Aluminum-Oxide Dielectrics via Local Structure Control of Nanoclusters Jeong-Wan Jo, Yong-Hoon Kim, Joohyung Park, Jae Sang Heo, Seongpil Hwang, Won-June Lee, Myung-Han Yoon, Myung-Gil Kim, and Sung Kyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09523 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

Ultralow Temperature Solution-Processed Aluminum-Oxide Dielectrics via Local Structure Control of Nanoclusters

Jeong-Wan Jo1†, Yong-Hoon Kim2†, Joohyung Park3, Jae Sang Heo1, Seongpil Hwang4, Won-June Lee5, Myung-Han Yoon5, Myung-Gil Kim3*, and Sung Kyu Park1*

1

School of Electrical and Electronics Engineering, Chung-Ang University, Seoul, South Korea

2

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Advanced Materials Science and

Engineering, Sungkyunkwan University, Suwon, South Korea 3

Department of Chemistry, Chung-Ang University, Seoul, South Korea

4

Department of Advanced Materials Chemistry, Korea University, Sejong, South Korea

5

School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju,

South Korea

†

: Equal contribution

*Corresponding Authors: Prof. Sung Kyu Park and Prof. Myung-Gil Kim Email: [email protected] and [email protected] 1

ACS Paragon Plus Environment


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Abstract 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Oxide dielectric materials play a key role in a wide range of high-performance solid-state electronics from semiconductor devices to emerging wearable and soft bio-electronic devices. Although lots of previous advances are noteworthy, their typical processing temperature still far exceeds the thermal limitations of soft materials, impeding their wide utilization in these emerging fields. Here, we report an innovative route to form highly reliable aluminum-oxide dielectric films using an ultralow temperature (< 60 ˚C) solution process with a class of oxide nanocluster precursors. The extremely low-temperature synthesis of oxide dielectric films was achieved by using low-impurity bulky metal-oxo-hydroxy nanoclusters combined with a spatially controllable and highly energetic light activation process. It was noteworthy that the room-temperature light activation process was highly effective in dissociating the metal-oxo-hydroxy clusters, enabling formation of a dense atomic network at low temperature. The ultralow-temperature solution-processed oxide dielectrics demonstrated high breakdown field (> 6 MV cm-1), low leakage (~1×108

A cm-2 at 2 MV cm-1) and excellent electrical stability comparable to those of vacuum-deposited and high-

temperature processed dielectric films. For potential applications of the oxide dielectrics, transparent metaloxides and carbon nanotube active devices as well as integrated circuits were implemented directly both on ultrathin polymeric and highly stretchable substrates.

Keywords: light activation; low temperature metal-oxide; aluminum oxo cluster; flexible electronics; stretchable electronics 2


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1. INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Dielectric materials are one of the key components for realizing the main functions of various electronic/optoelectronic devices including field-effect transistors1–4, energy storage/harvesting devices5, memory devices and photonic sensors6,7. Previously, thermally grown silicon dioxide, vacuum-deposited oxides and nitrides, or high-temperature-annealed solution-processed oxides have been used as a dielectric material owing to their excellent functional properties and good spatial uniformity. Recently, with the rise of wearable and bio-electronic devices8,9, a variety of low-temperature processable polymeric dielectrics10,11 were also explored taking advantages of their large-area scalability, excellent uniformity, and mechanical flexibility. Although the performance of these polymeric dielectrics was greatly improved during the past decade, the utilization of polymeric dielectrics in high performance electronics is still limited due to their relatively poor reliability, high leakage current, and weak physical endurance as compared to the inorganic counterparts. As an alternative candidate, low-temperature solution-processed oxide dielectrics12–18 have been intensively researched to satisfy these requirements and to utilize them in high performance wearable and bio-electronic devices. Up to now, many researchers have been attempting to develop low-temperature solution-processed oxide dielectrics by engineering the precursor formulation using diverse metal ligands1, non-carbon solvents19–21, dopants22,23 or combustion fuel additives24. In addition, novel post-deposition treatments such as ozone exposure25, laser/microwave annealing26,27, and ultraviolet activation28–32 were also reported. However, these engineered precursor systems with the innovative processing strategies33–37 still required considerably high post-annealing temperature (>400 ˚C) or at least mediocre temperature (>150 ˚C)20,28– 30,32,38

due to the critical energy crest for dissociation, solid-state structural integrity (condensation and

densification), and impurity removal steps for metal-oxygen-metal (M-O-M) lattice formation. Further lowering the processing temperature near to the room-temperature is expected to open a new era especially in the areas of wearable and bio-electronics, by allowing the implementation of high performance oxide materials on various soft substances. In this regard, the metal-oxo-hydroxy nanoclusters have received much 3



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attention as a viable alternative to conventional metal salt or metal alkoxide precursors in the field of low1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperature and high-performance solution-processed electronics due to the minimal impurity content.13,14,34,35 Here, we report a new strategy for obtaining ultralow temperature solution-processed aluminum oxide (Al2O3) dielectrics (Tanneal ~ 60 ˚C), by using aluminum-oxo-hydroxy (Al-13) nanoclusters13,14 and local structure controllable activation process. Several years ago, the class of metal-oxo-hydroxy clusters (nanoclusters) has been developed by Prof. Kessler group at Oregon State University and considered as a very promising building block and a kind of general platform for low temperature and high purity metaloxide functional materials due to their unique skeleton structure. Nevertheless, the extremely high bonding energy of the nanoclusters in solution phase has been one of the main hurdles (process temperature > 500~600˚C) to translate it into smooth and reliable oxide dielectric thin-films which have never been realized before with commercially available large area substrate. Therefore, in this paper, we report a new approach to grow high-performance and large-area oxide gate dielectrics from the nanocluster solution at unprecedentedly low temperatures. More importantly, to ensure the viability of the materials, we use this technique to fabricate electronic devices along with low thermal budget (< 80˚C) stretchable substrates. Additionally, thin-film-transistors (TFTs) and TFT based 7-stage ring-oscillator were implemented as the testbeds for the dielectric properties with their mobility, bias stability, and operational speed. The nanocluster precursor based film formation method is similar to the sol-gel technique, where the coated layer is transformed into a dense inorganic film via a post annealing process. However, in contrast to conventional sol-gel or metal-organic precursors (alkoxides, nitrates, acetates, and halides), the nanocluster precursors have pre-formed M-O-M skeletons, which require much lower activation energy to facilitate the M-O-M bond formation. Also, the inorganic films fabricated from the nanocluster precursors have high chemical purity with minimal organic residues due to small amount of solubilizing ligands or charge balancing anions relative to metal cations in the precursor solution. This in turn leads to a small volume loss during the condensation and densification processes, resulting in formation of highly dense 4


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oxide films. However, despite these structural benefits of the nanocluster precursor systems, the formation of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

uniform amorphous films at low temperature has not been quite successful mainly due to large activation energy required to rearrange the metal-oxygen chemical bonds between the backbone skeleton structures. In this research, a simple and highly energetic photoactivation was utilized to dissociate the internal chemical bonds of the nanocluster precursors, inducing radical mediation at a low temperature. As a result, high performance and large-area-scalable solution-processed oxide dielectric films could be achieved at ultralow temperature as low as 60 ˚C (the minimal temperature we can achieved during the photochemical activation process due to radiation heat from DUV lamp). We believe that all these benefits make the nanocluster based inorganic layers an ideal candidate for versatile ultralow temperature dielectric films that can cope with the various demands of emerging low-temperature required implementations. 2. EXPERIMENTAL SECTION Aluminum-oxo-hydroxy (Al-13) nanocluster synthesis. The Al-13 nanocluster cluster was synthesized following a procedure described in the literature.13 To synthesize the Al-13 nanocluster precursor, initially, 30.01 g of aluminum nitrate nonahydrate (Al(NO3)3·9(H2O)) (Sigma-Aldrich) was dissolved in deionized water (ρ = 18.2 MΩ·cm) to make a 50 mL solution. After dissolving the precursor in the solvent, 2.82 g of a zinc metal powder (Sigma-Aldrich) was added to the solution (Zn:Al molar ratio of 1:2). The solution was stirred for more than 24 h to completely dissolve the zinc powder. The clear solution was then filtered into a dish through a filter paper and then was placed in a fume hood for evaporation of the water. As the solution became concentrated, numerous colorless crystals began to grow, reaching sizes of several millimeters. Forced evaporation of the water resulted in polycrystalline precipitates containing a mixture of the Al-13 nanocluster salt, Zn(NO3)2, and unreacted Al(NO3)3. The product was washed with isopropyl alcohol to remove Zn(NO3)2 and Al(NO3)3 from the Al-13 nanocluster salt. DUV photochemical activation process. A high-density low-pressure mercury lamp that emits radiation at wavelengths of 253.7 nm (90 %) and 184.9 nm (10 %) was used to convert the metal-oxide precursors into a condensed metal-oxide film. After spin-coating the precursor solutions, the samples were 5



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placed under the lamp at a distance of ~1 cm and with controlled processing temperature using glass 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

plate/SUS grid plate (150 ˚C) or bulk aluminum block (cooling stage)/SUS grid plate (60 ˚C). Nitrogen was continuously flowed over the samples to prevent ozone formation and create an inert gas atmosphere inside the chamber, allowing for DUV transmission (especially at the 184.9 nm wavelength) without significantly attenuating the DUV intensity. The substrate temperature was monitored with an infrared camera during and after the DUV irradiation. Metal-oxide precursor and sorted semiconducting carbon nanotube (CNT) solution preparation and processing. Two types of precursor solutions were prepared for alumina: conventional aluminum nitrate precursor and the Al-13 nanocluster precursor. The concentrations of the solutions were 0.78 M and 0.06 M, respectively, chosen to exactly match the concentration of aluminum ions in the coating solutions. To prepare the nitrate- and nanocluster-Al2O3 precursor solutions, aluminum nitrate nonahydrate (Al(NO3)3·9(H2O)) metal precursor and Al-13 nanocluster salt were dissolved in 2-methoxyethanol (anhydrous, Sigma-Aldrich) with concentrations of 0.78 M and 0.06 M, respectively. After dissolving the precursors in the solvent, the solutions were sonicated for 2 hours to obtain clear solutions. The a-IGZO solution was prepared as follows. Indium (III) nitrate hydrate (In(NO3)3·x(H2O)), gallium (III) nitrate hydrate (Ga(NO3)3·x(H2O)) and zinc nitrate hydrate (Zn(NO3)2·x(H2O)) (all from Sigma-Aldrich) were dissolved in 5 mL 2-methoxyethanol in concentrations of 0.085 M for indium nitrate hydrate, 0.0125 M for gallium nitrate hydrate, and 0.0275 M for zinc acetate dehydrate. The solution was sonicated for 2 hours after dissolving the precursors in the solvent. The sorted semiconducting CNT solution was prepared with the reported process.41 The suspension of HiPCO SWNT (5 mg) and the P3DDT (10 mg) in toluene (25 ml) was sonicated for 30 min at 500 W power level in cooling bath. The solution was centrifuged at 10,000 rpm for 150 min, and then the supernatant was collected. Alumina dielectric film fabrication and characterization. Solution-processed alumina dielectrics were fabricated as follows. The as-prepared nanocluster or nitrate-Al2O3 precursor solutions were spin6


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coated on a heavily doped p-type silicon wafer at 3000 rpm for 20 sec under ambient. Then, the samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

were annealed by various annealing conditions (DUV or thermal). The HRTEM images were obtained with a JEM-3010 microscope (JEOL), and the samples were prepared by a Helios 650 (NanoLab) focused ion beam (FIB). The X-ray reflectometry (XRR) spectra were obtained with an ATX-G (Rigaku). The X-ray photoemission spectroscopy (XPS) spectra were taken with K-alpha + (Thermo Fisher Scientific) using an Al Kα source at 1486.6 eV and base pressure of 7.83 × 10-9 mbar. The film thickness and AFM images were obtained by using ellipsometry (SpecEL-2000) and XE-100 AFM system (PSIA), respectively. TFT fabrication and electrical measurement. Solution-processed a-IGZO TFTs were fabricated as follows. To fabricate the gate electrode, a 35-nm-thick Au layer and a 5-nm Cr adhesion layer were thermally evaporated on the substrate at 0.1 Å sec-1 deposition rate. The gate electrodes were patterned by conventional photolithography and wet etching. Then, the nanocluster or nitrate-Al2O3 precursor solution was spin-coated on the Cr/Au gate electrode at 3000 rpm for 20 sec under air. Then, the sample was annealed by various annealing conditions (DUV at 60 ˚C / DUV at 150 ˚C). The a-IGZO solution was coated on the gate dielectric by spin-coating at 2000 rpm for 20 sec under air and then DUV-annealed at 150 ˚C for 2 hours. The IZO S/D electrodes were deposited by sputtering and then were patterned using a lift-off process. Solution-processed metal-oxide TFTs and integrated circuits were fabricated on a flexible polyimide (PI) films as follows. First, a polyimide solution (Polyzen 150, PICOMAX) was spin-coated on a glass substrate for the PI film formation. Then, a gate electrode was deposited and then was patterned by the identical processes described above. For the alumina gate dielectric, the nanocluster-Al2O3 precursor was spin-coated on the gate electrode at 3000 rpm for 20 sec. Then, the sample was DUV-annealed for 2 hours under nitrogen at 60 ˚C. The remaining process was conducted by the same method as described above. Solution-processed CNT TFTs were fabricated as follows. To fabricate the gate electrode, a 50-nmthick Cr layer was sputtered on the substrate. The gate electrodes were patterned by conventional photolithography and wet etching. On the Cr gate electrode, nanocluster-Al2O3 gate dielectric was fabricated by DUV annealing of nanocluster-Al2O3 film at 60 ˚C. Next, Au S/D electrodes were deposited by 7



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evaporation and then were patterned by a lift-off process. Afterward, the sorted CNT solution was spin1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

coated on top of nanocluster-Al2O3 layer at 3000 rpm for 30 drops. Then, the CNT film was patterned using photolithography and O2 reactive ion etching. Stretchable CNT TFTs were fabricated as follows. To prepare polyurethane (PU) substrates, SG-80A PU (Tecoflex) was dissolved in dimethylformamide (DMF) (anhydrous, Sigma-Aldrich) with concentration of 170 mg/mL. After dissolving the PU in the solvent, the solution was sonicated for 2 hours to obtain clear solutions. The solution was spin coated on a glass substrate and the solvent was evaporated at 120 ˚C for 20 min. To fabricate the gate electrode, a 100-nm-thick IZO layer was sputtered on the substrate. On the IZO gate electrode, nanocluster-Al2O3 gate dielectric was fabricated by DUV annealing of nanocluster-Al2O3 film at 60 ˚C. Next, Au S/D electrodes were deposited by evaporation and then were patterned by a shadow mask with width and length of 700 µm and 70 µm, respectively. Afterward, the sorted CNT solution was spun over the structure at 3000 rpm. The CNT film was patterned using photolithography and O2 reactive ion etching (W/L = 230 µm /70 µm). All the device fabrication process were carried out under the temperature of less than 60 ˚C. All the measurements were performed under air and dark ambient. No encapsulation layer was applied over the channel region. The capacitance and leakage current characteristics were obtained using an Agilent LCR meter 4284A and Agilent 4156C analyzer, respectively. The C-F and leakage current density measurements were performed on Si-insulator-metal (IZO) metal-insulator-metal (MIM) structures with an overlapped area of 100 × 100 µm2. The MIM structures were fabricated using the same method that was used for the TFTs. Electrical characterization of the TFTs and integrated circuit were done by using Agilent 4156C semiconductor parameter analyzer. 3. RESULT AND DISCUSSION Using conventional nitrate type precursors (Al(NO3)3·9H2O), the photochemical activation process allows the formation of highly dense oxide films at temperature below 150 ˚C28–30,32. However, at much lower temperature, particularly below 100 ˚C, typically less densified M-O-M network can be formed possibly due 8


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to the insufficient energy to minimize the impurities such as nitrates and residual solvent molecules. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Moreover, although more stable and denser M-O-M network could be formed at a higher annealing temperature (Tanneal ~ 150 ˚C), distorted and partly porous structure is generally expected due to the decomposition of large amount of impurities. Interestingly, however, using the Al-13 nanocluster ([Al13(µ3OH)6(µ-OH)18(H2O)24]-(NO3)15) as a precursor system, the formation of dense oxide films was possible even at much lower temperature via the activation process (Figure 1a,c). Figure 1a,c shows a schematic, rather than the actual state of aluminum in solution. As well known, the Al3+ in alcohol including 2methoxyethanol shows hexa-coordination, which corresponds to single chemical shift around 0 ppm from 27

Al NMR (6 ppm for 2-methoxyethanol).42 For Al-13 cluster, it was well known that reported that the Al-13

nanocluster was stable in diverse type of solvents, including alcohol, water, and DMSO.43–45 Although the stability of Al-13 in alcohol is well known, to clarify the existence of Al13 during thin-film formation, we have tried 27Al NMR measurement for dried Al-13 cluster after dissolving into 2-methoxyethanol. The 27Al 1D MAS NMR spectra of the dried Al13 cluster powder show distinctive peaks from the AlO4 (63.2 ppm) and AlO6 (3.4 and -30.4 ppm) sites of the Al13 nanocluster (Supporting Information Figure S1).46,47 Although the transformation of Al-13 cluster between different isomers is possible, the cluster nature of Al13 is well preserved from our precursor solution to final precursor film. Considering the stability of Al-13 nanocluster in diverse solvent environments, such as water, methanol and dimethylsulfoxide43–45, the Al-13 nanocluster precursor solution has small amount of solubilizing organic ligands and charge balancing anions (Figure 1a), and therefore, theoretically more dense oxide films could be achieved. As shown in Figure 1b,d, using the Al-13 nanocluster precursor, low-impurity-content and denser alumina films could be obtained at a processing temperature of 60 ˚C. Our previous reports and a large number of photo-activated metal oxide papers showed that the photo-dissociation of nitrates and metal oxide formation require mediocre thermal energy.28–30,32 Because of the lack of kinetic energy due to insufficient heating, the NO3 decomposed by light fails to escape from the condensed film and remained inside the film (Supporting Information Figure S2), thus interfering with the formation of Al-O-Al bonds. However, the nanoclusterbased precursors have a relatively small amount of NO3 and pre-made Al-O-Al skeleton structure, and as a 9



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result even at a low temperature of 60 ˚C, only a small portion of decomposed NO3 is remained in the film 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

as shown in Figure 1b. To examine the degree of M-O-M lattice formation and impurity contents in alumina films, X-ray photoelectron spectroscopy (XPS) analysis and Fourier transform infrared (FT-IR) spectroscopy analyses were performed for various alumina films. Figure 2a-c and 2e-g shows the O1s spectra for Al-13 nanocluster-based alumina (nanocluster-Al2O3) and Al nitrate-based alumina (nitrate-Al2O3) films. Here, the peaks centered at ~531.0 eV and ~532.3 eV correspond to M-O and M-OH bonds, respectively.48,49 As shown in Figure 2h and Supporting Information Table S1, the nanocluster-Al2O3 and nitrate-Al2O3 films (DUV-annealed at 150 ˚C or thermal annealed at 350 ˚C) exhibited significantly high M-O peaks (areal ratio of ~85 %) compared to M-OH peaks (areal ratio of ~15%), which indicates the formation of high quality M-O-M networks regardless of the precursor type. However, when the alumina films were DUVannealed at 60 ˚C, the alumina films showed a large difference in the peak areal ratio depending on the precursor type. Particularly, the nitrate-Al2O3 film exhibited a considerably high M-OH peak ratio (~75 %) than that of a nanocluster-Al2O3 film (~15 %). This indicates that the nitrate-Al2O3 film had incomplete formation of M-O-M network and possessed a large amount of M-OH bonds within the film. Additionally, the FT-IR spectra showed negligible IR absorbance centered at around 3500 cm-1 (O-H stretching) in the nanocluster-Al2O3 film, which suggest the significantly reduced M-OH groups in the structure as shown in Figure 2d. It can be understood that for the nitrate precursor, the ultralow temperature (60 ˚C) photochemical activation lacks complete structural integrity, possibly due to insufficient removal of the chemical impurities and residual solvent molecules which impedes the formation of M-O-M network. Meanwhile, by using the Al-13 nanocluster precursor, the formation of a highly dense alumina film was possible even at 60 ˚C attributing to an efficient dissociation of nanocluster skeleton into a high quality MO-M network via the formation of minimal defect Al-13 polymeric intermediates. Supporting Information Figure S3 shows ultraviolet–visible absorption spectra of solutions for alumina film preparation. For comparison, the absorption spectra of neat solvent (2-ME) and individual 10


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aluminum nanocluster and nitrate solutions are also shown. Unlike 2-ME, which shows minimal absorption 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

at wavelengths of 225–350nm, the solutions of aluminum nanocluster and nitrate in 2-ME exhibit strong light absorption below 350 nm. As the mercury lamp has two main emission peaks at 253.7 and 184.9 nm, the photochemical activation can be facilitated by DUV irradiation from the lamp. Also, the crystalline structure of alumina films was also determined by using X-ray diffraction (XRD) and electron diffraction (ED), as shown in Figure 3a and inset figures. All the alumina films showed no distinctive peak except the silicon peak in the XRD spectra and diffuse ring in ED, indicating their amorphous nature which is preferable for forming highly uniform and minimal leakage dielectric films. Furthermore, the surface roughness of alumina films was identified by using atomic force microscope (AFM) as shown in Supporting Information Figure S4. Most of the alumina films had root-mean-square (RMS) roughness less than 0.2 nm which is comparable to that of thermally grown SiO2 film. For the nitrate-Al2O3 film (DUV-annealed at 60 ˚C), an exceptionally high surface roughness of 14.82 nm was observed reflecting incomplete formation of metal-oxide thin films (or M-O-M network). In addition, X-ray reflectivity (XRR) measurement was carried out to confirm the degree of densification by comparing the film densities of alumina films with different precursor types and annealing methods. From the simulation fitting data of XRR spectra in Supporting Information Figure S5, the bulk densities and thicknesses of alumina films are shown in Figure 3b and Table 1. The thicknesses acquired from the XRR measurements were in good agreement with those determined from high-resolution transmission electron microscope (HRTEM) (inset of Figure 3c) and ellipsometry analysis (Table 1). It was found that, in overall, the nanocluster-Al2O3 films exhibited higher film densities compared to those of nitrate-Al2O3 films annealed by the same method. However, the ultralow temperature (60 ˚C) photochemical activation resulted a huge difference between the nanocluster-Al2O3 and nitrate-Al2O3 films (DUV-annealed at 60 ˚C). While the nanocluster-Al2O3 film had a film density of 2.59 g cm-3, the nitrateAl2O3 film had a much lower density of 1.86 g cm-3. Particularly, the film density of nanocluster-Al2O3 film (DUV-annealed at 60 ˚C) was comparable to those of a nitrate-Al2O3 film DUV-annealed at 150 ˚C and 11



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ALD-deposited alumina film50. Although the high energetic photons could provide sufficient energies for the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

local bond cleavage and formation, the long range structural relaxation and densification by the migration and rearrangement of metal cation and oxygen anion may need critical thermal energy by post-annealing process.51 It can be suggested that the high degree of densification of nanocluster-Al2O3 film at ultralow annealing temperature is attributed to the pre-formed M-O-M skeleton structure, low organic impurities and low charge balancing anion contents in the Al-13 nanocluster precursor, allowing a small volume loss and minimal metal cation rearrangement during the condensation process. Interestingly, when an additional thermal treatment of 500 ˚C was performed after DUV annealing at 150 ˚C, the density of nanocluster-Al2O3 film was further increased to 3.10 g cm-3 which is comparable to that of ALD-deposited alumina film around 300 ˚C.50,52–54 In contrast, the density of the same processed nitrate-Al2O3 films were increased up to ~2.5 g cm-3 possibly due to the porosity generated by a significant volume loss during the film formation process which interferes the formation of a dense M-O-M network as well (Supporting Information Figure S6). Figure 3c,d show the capacitance vs. frequency (C-F) and current density vs. electric field (J-E) characteristics of nitrate- and nanocluster-Al2O3 films measured from metal/insulator/metal structures. As expected, the nitrate-Al2O3 film (DUV-annealed at 60 ˚C) exhibited high leakage current density and highly unstable frequency dependence (Figure 3c and Supporting Information Figure S7a,b). Meanwhile, the nanocluster-Al2O3 film (DUV-annealed at 60 ˚C) exhibited sufficiently low leakage current density (~1×10-8 A cm-2 at 2 MV cm-1) with a high breakdown field (> 6 MV cm-1) and high capacitance (~155 nF cm-2 at 1 kHz) with stable frequency dependence as shown in Figure 3c,d. In fact, these dielectric properties are comparable or even better than those of nitrate-Al2O3 films (DUV-annealed at 150 ˚C or thermal annealed at 350 ˚C). Also, in Supporting Information Figure S8, a plot of J/T2 versus E1/2, ln(J/E) versus E1/2, and ln(J/E2) versus 1/E are depicted for various alumina films to analyze the current conduction mechanism. Typical Schottky emission is observed in both films at the lower electric field regime, possibly due to the not optimized contact and interfacial properties between the electrodes and dielectric films (Figure S8 a,b). At high electric field region (>2.5 MV/cm), the nitrate alumina films clearly reveal large carrier conduction 12


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with good fit to those expected by P-F conduction (can be attributed to the field-assisted thermal de-trapping 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of carriers in localized states of the bulk oxide films)55, while nanocluster films showing less fitted graph which is in good agreement with the temperature dependent J-E characteristics of the films as shown in Figure S8c. Unlike the nitrate alumina films, in nanocluster-based alumina, the F-N tunneling conduction can be confirmed by a plot of J/E2 versus 1/E as shown in Figure S8d. It clearly demonstrates distinct regions at high electric field (> 5 MV/cm) with good fit to F-N tunneling equation plot, which may indicate that the nanocluster alumina films lead to the formation of much less defect or impurity in the bulk oxide structure and thus F-N tunneling can be the dominant carrier conduction mechanism11. In the case of nitrateAl2O3, a large amount of traps can be generated due to inherent impurities and porous structure that have not been properly removed in the DUV process. These traps are not fully removed even with the high temperature annealing process of 500 °C. Since the areal capacitance of the transistor is proportional to the ratio of relative permittivity (εr) to the dielectric thickness, high-k dielectric has been one of key routes to achieve low-voltage-driven electronic devices. Table 1 shows the relative permittivity of various alumina films. Overall, the nanocluster-Al2O3 films exhibited relatively higher permittivity than those of nitrate-Al2O3 films. Despite their low processing temperature, the nanocluster-Al2O3 film (DUV-annealed at 60 ˚C) showed εr of 8.37 which is higher than DUV-annealed nitrate-Al2O3 at 150 ˚C (εr = 7.87). It can be deduced that the rather lower permittivity of nitrate-Al2O3 film resulted from the high porosity of nitrate-Al2O3 films due to a large volume loss during the annealing process. In addition, the DUV 150 ˚C + Thermal 500 ˚C annealed nanocluster-Al2O3 film showed εr of 8.71 which is comparable to high quality ALD-deposited alumina films50,52,53. These observations are in good agreement with the aforementioned observations from XPS and XRR analyses, supporting that the highly energetic photo-annealing of Al-13 nanocluster resulted in the formation of highly dense alumina films with low impurities and minimized structural defects. To demonstrate the versatile utilization of the ultralow-temperature-annealed nanocluster-Al2O3 films, solution-processed amorphous indium-gallium-zinc oxide (a-IGZO) and carbon nanotube (CNT) thin13



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film transistors (TFTs) were fabricated using the nanocluster-Al2O3 film as a gate dielectric layer. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Supporting Information Figure S9a,b show the transfer and output characteristics of a-IGZO TFTs with nanocluster-Al2O3 gate dielectrics (DUV-annealed at 60 ˚C). As shown in Supporting Information Figure S9c, the a-IGZO TFTs exhibited saturation field-effect mobility of 3.61 ± 0.41 cm2 V-1 s-1, subthreshold slope (SS) of 0.196 ± 0.016 V decade-1 and threshold voltage (VT) of 2.58 ± 0.14 V (measured from 20 devices). Even compared to the a-IGZO TFTs with nitrate-Al2O3 gate dielectrics (DUV or thermally annealed at higher temperature), improved electrical performance was achieved utilizing the nanoclusterAl2O3 gate dielectrics particularly at low temperature (Supporting Information Figure S10a,c). In addition, Supporting Information Figure S9d,e show the transfer and output characteristics of CNT TFTs with the nanocluster-Al2O3 gate dielectric (DUV-annealed at 60 ˚C). The CNT TFTs exhibited field-effect mobility of ~0.53 ± 0.15 cm2 V-1 s-1 (Supporting Information Figure S9f) and minimal hysteresis behavior (~0.5 V) with negligible interface and bulk defects in nanocluster-Al2O3 gate dielectric.52 In general, the clockwise hysteresis of an n-type device is believed to be due to poor interface traps between the gate dielectric and the semiconductor.18,56,57 However, IGZO and CNT TFTs using nanoclusters Al2O3 exhibit comparatively excellent hysteresis characteristics of ~ 0.5 V without additional process or surface treatment, despite being a low temperature solution processed device. These results also show that the ultralow-temperature-annealed nanocluster-Al2O3 film has sufficient electrical and chemical stabilities to be implemented as a gate dielectric layer for various channel materials. To take a full advantage of the ultralow-temperature-annealed nanocluster-Al2O3 dielectrics, flexible a-IGZO TFT based integrated circuits and CNT TFTs were fabricated directly on ultrathin (< 3 µm) polyimide (PI) (Figure 4), and low-thermal budget stretchable polyurethane (PU) (Figure 5) substrates, respectively with DUV-annealed at 60 ˚C nanocluster-Al2O3 gate dielectrics. The a-IGZO TFTs showed an average mobility of 5.86 cm2 V-1 s-1 (maximum value of ~ 7.95 cm2 V-1 s-1) with a narrow distribution of the performance (standard deviation of 0.21, from 20 devices) (Figure 4b-d). Also, as displayed in Figure 4e and Supporting Information Figure S11, even without device passivation or packaging, the flexible a14


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IGZO TFTs with nanocluster-Al2O3 reveal outstanding operational stability, with a small VT shift (∆VT) of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3.605 V after a gate-bias stress time of 10,000 s. Note that the gate-bias stability is comparable to that of devices with nitrate-Al2O3 annealed at 150 °C (∆VT of 3.601 V) and ALD-deposited Al2O3 (∆VT of 2.896 V) under identical stress condition. It verifies excellent reliability of nanocluster-Al2O3 films even at extremely low processing temperature. More stable TFT characteristics on ALD-deposited Al2O3 may be attributed to the presence of fewer interfacial trap states at the interface between semiconductor and gate dielectric, possibly as a result of the low residual impurity in the dielectric layer. Furthermore, using the IGZO TFTs, 7-stage ring oscillators were also fabricated having a β ratio of 2 (W/Ldrive = 100/10 µm and W/Lload = 50/10 µm for the drive and load transistors, respectively). The overlap distance between the gate and source/drain electrodes was 3 µm. With a supply voltage (VDD) of 20 V, the 7-stage ring oscillator showed an oscillation frequency greater than 1 MHz, and corresponding propagation delay less than 70 ns per stage (Figure 4f,g). Regardless of device area (> 100 micron square) and measurement location, we could obtain the uniform low leakage currents demonstrating defect-free alumina gate dielectric over large area for multiple devices (Figure S12), resulting in excellent operation of the 7-stage ring oscillator circuit (area: 1000×550 µm2) in Figure S13. Stable operation of the circuits such a high frequency above 1 MHz can be a clear demonstration for the stable nanocluster-Al2O3 formation including minimal OH residues throughout the process temperature of 60 °C. For more viable applications of the ultralow-temperature dielectrics, CNT TFTs with nanocluster-Al2O3 dielectrics were built directly on a stretchable PU substrate with bottom-up process and the whole process temperatures below 60˚C. The devices on the PU substrates demonstrated relatively good electrical performance, showing an average field-effect mobility of ~0.2 cm2 V-1 s-1, current on/off ratio of > 104, but the CNT TFTs on the PU substrate exhibit a larger hysteresis than the CNT TFTs on glass substrate because a poor interface created by rough surface of the PU (Figure 5b-d and Supporting Information Figure S14). The nanocluster-Al2O3 film (DUV-annealed at 60 ˚C) exhibited excellent gate dielectric characteristics even on the PU substrate (Supporting Information Figure S15). These results indicate that the ultralow-temperature-annealed nanocluster-Al2O3 film can be applied to various substances which are vulnerable to heat. 15



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4. CONCLUSION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In summary, we explored nanocluster based ultralow-temperature solution-processed metal-oxide films as a way to meet the various demands from emerging, ultra-flexible and bio-inspired implementations. The minimal residual impurity and high density properties of the nanocluster-Al2O3 films can be used as high quality dielectric and insulating layers on various soft substances with ultralow processing temperature, demonstrating their facile applicability to high performance stretchable electronics. Together with possibility for utilization of various functional materials and low-temperature process availability, the aforementioned benefits of the nanocluster based films may open a new pathway toward high performance and reliable solution-processed functional devices under ultralow temperature conditions. Furthermore, we also believe that nanoclusters can be used for surface ceramic coatings, gas barriers, 3d printed ceramic58, and encapsulation resin areas that require high quality ceramic. In addition, the concept of nanoclusters can be extended to oxide semiconductors and conductors as well as alumina insulators.

ASSOCIATED CONTENT Supporting Information NMR spectrum, XPS analysis, Light absorption characteristic, AFM data, XRR spectra, gate dielectric properties and TFT characteristics.

ACKOWLEDGEMENT This research was partially supported by the Engineering Research Center of Excellence (ERC) Program supported by National Research Foundation (NRF), Korean Ministry of Science & ICT (MSIT) (Grant No. NRF-2017R1A5A1014708), by Development of excellent waterproof and impact resistance mobile phone bezel adhesive tape with thickness less than 80 µm (2016-10067433) funded by the Korea government 16


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Ministry of Trade, Industry and Energy, and by the Industrial Strategic Technology Development Program 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[10045269, Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV] funded by MOTIE/KEIT.

17



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Figure captions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Ultralow temperature photoactivation of ceramic solution-processed dielectrics by DUV. (a) Schemes of alumina formed by DUV photochemical activation at 60 ˚C (precursor types: nanocluster and conventional nitrate). (b) Comparison of nitrogen anion content in alumina films obtained from XPS data. (c) Schemes showing degrees of densification according to nanocluster/conventional aluminum precursors. (d) Film thickness variations of nanocluster-Al2O3 and nitrate-Al2O3 films with different annealing conditions.

Figure 2. XPS O1s spectra of nanocluster- and nitrate-Al2O3 films. O1s spectra of nanocluster-Al2O3 films annealed by; (a) thermal 350 ˚C, (b) DUV 150 ˚C and (c) DUV 60 ˚C. (d) FT-IR spectra of nanoclusterAl2O3 films annealed by DUV 60 ˚C, nitrate-Al2O3 films annealed by DUV 60 ˚C and nitrate-Al2O3 films annealed by DUV 150 ˚C: here, the O-H stretching vibration mode is centered around 3500 cm-1. XPS O1s spectra of nitrate-Al2O3 films annealed by; (e) thermal 350 ˚C, (f) DUV 150 ˚C, and (g) DUV 60 ˚C. (h) Summary of areal ratios of M-O and M-OH bonding in nanocluster-Al2O3 and nitrate-Al2O3 films with different annealing conditions (Thermal 350 ˚C, DUV 150 ˚C and DUV 60 ˚C).

Figure 3. Structural and dielectric property analyses of alumina films. (a) X-ray diffraction (XRD) spectra and Electron diffraction (inset) of nanocluster- and nitrate-Al2O3 films showing no distinctive peaks, indicating amorphous states. (b) Bulk film densities of nanocluster-Al2O3 and nitrate-Al2O3 films depending on annealing conditions (Thermal 350 ˚C, DUV 150 ˚C and DUV 60 ˚C). (c) Capacitance vs. frequency data for various alumina films. Inset figures show the HRTEM images of DUV-annealed nanocluster-Al2O3 and nitrate-Al2O3 films at 150 ˚C (left and center) and a DUV-annealed nanocluster-Al2O3 film at 60 ˚C (right), respectively. (d) Leakage current density vs. electric field plots for various alumina films.

Figure 4. Flexible a-IGZO TFTs and circuits using an ultralow–temperature-annealed nanocluster-Al2O3 26


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gate dielectric. (a) Photographs and a schematic cross-section of a flexible a-IGZO TFT array fabricated on 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a PI substrate. (1) Ultra-flexible a-IGZO devices attached on a stem of tree, and (2) on a curved surface (bending radius ~ 1 mm). (b) Transfer and (c) output characteristics, and (d) the statistical distribution of aIGZO TFTs on a PI substrate. (e) The VT shift of un-passivated the a-IGZO TFTs with various alumina gate dielectrics under positive gate-bias stress (VGS = +5 V). (f) Oscillation frequency (red) and per-stage propagation delay (blue) of seven-stage ring oscillator as a function of supply voltage (VDD). (g) Output waveforms of the seven-stage ring oscillator operating with supply voltages of 1 V and 20 V, and oscillation frequency of 6.6 kHz and 1.03 MHz, respectively.

Figure 5. Stretchable CNT TFTs using an ultralow–temperature-annealed nanocluster-Al2O3 gate dielectric. (a) Photographs and a schematic cross-section of a stretchable CNT TFT array fabricated on a PU substrate. (b) Statistical distribution of saturation mobility and (c) transfer, and (d) output characteristics of CNT TFTs on PU substrate with an ultralow-temperature-annealed nanocluster-Al2O3 gate dielectric (Tanneal = 60 ˚C).

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Table caption 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1. A summary of thicknesses, dielectric constants and film densities of various alumina films (data were obtained from ellipsometry and XRR analysis).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1.

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Ultralow-Temperature Solution-Processed ... - ACS Publications

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