High ‑k Dielectrics for Low Voltage Operations of Organic Thin Film Transistors

Surface Modi fi cation on Solution Processable ZrO

₂

High ‑k Dielectrics for Low Voltage Operations of Organic Thin Film Transistors

Wenqiang He,

^†

Wenchao Xu,

^†

Qiang Peng,

^†

Chuan Liu,

^‡

Guofu Zhou,

^§

Sujuan Wu,

^†

Min Zeng,

^†

Zhang Zhang,

^†

Jinwei Gao,

^†

Xingsen Gao,

^†

Xubing Lu,*

^,†

and J.-M. Liu*

^,†,∥

†Institute for Advanced Materials andGuangdong Provincial Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, China

‡State Key Laboratory of Optoelectronic Materials and Technologies, School of Microelectronics, Sun Yat-Sen University, Guangzhou 510274, China

§Institute of Electronic Paper Displays, South China Academy of Optoelectronics, South China Normal University, Guangzhou 510006, China

∥Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

*^S Supporting Information

ABSTRACT: High quality zirconium oxide (ZrO₂) high-kdielectrics have been fabricated by chemical solution processes. The ZrO₂thinfilms annealed at various temperatures were studied from microstructure properties to electric properties in detail. The dielectricfilm annealed at 700°C features a smooth surface, low leakage current density (1.89×10⁻⁶A/cm²@−3 MV/cm) and high dielectric constant (20). Organic thinfilm transistors (OTFTs) based on ZrO₂with different surface modifications were characterized to investigate the interfacial effects between the high-kdielectric and organic semiconductors. The OTFTs with poly(α-methylstyrene) (PαMS) coated ZrO₂show much higher carrier mobility and on/offratio than those with bare-ZrO₂or with hexamethyldisilazane-treated ZrO₂. The ZrO₂/PαMS layers offer a low surface energy to grow large crystals and benefit the charge transport in organic semiconductors, whereas the dielectric surface roughness and dipole scattering are less important.

The resulting OTFTs show high current on/offratio (1.2×10⁵), low threshold voltage (−0.38 V), and low SS (0.26 V/dec).

Our work has deepened the understanding on the complex interfacial effects between high-k dielectric and organic semiconductor. Finally, we demonstrate low temperature fabrication of ZrO₂ -OTFTs on a flexible substrate, demonstrating solution processable high-kZrO₂dielectricfilms offer great potentials for low-cost organic electronic devices, especially for low voltage organic electronic devices.

1. INTRODUCTION

Different from the conventional thinfilm transistors based on inorganic semiconductors, organic thin-film transistors (OTFTs) have the advantages of being low cost, flexible, lightweight, etc.^1,2In the past two decades, significant progress has been made for applications of OTFTs in organic light- emitting diodes (OLED),³ radio frequency identification (RFID) tags,⁴ sensors,⁵ electronic papers,⁶ portable elec- tronics,⁷ etc. Traditionally the insulating dielectrics in OTFTs are conventional dielectrics (SiO₂ for example) with few exceptions, and yet they are usually of high operation voltage.⁸⁻¹⁰To reduce the operation voltage, high-kinsulators

have been employed to replace the SiO₂layer¹¹so as to afford higher charge densities in OTFTs with low leakage current.

Various high-k insulating oxide materials, polymer, and organic−inorganic multilayer dielectrics have been investi- gated.¹²⁻¹⁴ Deposition methods include radio frequency sputtering,¹⁵atomic layer deposition (ALD),¹⁶ electron beam evaporation,¹⁷ and wet chemical solution deposition.¹⁸ The operation voltages of the OTFTs have been much reduced, and yet the interfacial effects have been found to be rather

Received: April 10, 2016 Published: April 19, 2016

pubs.acs.org/JPCC

complicated, coming from surface roughness, surface energy, surface polarity, dielectric constant, etc.¹⁹⁻²²

Among the candidates, ZrO₂has been regarded as one of the most promising high-kmaterials to replace SiO₂because of its wide band gap (5.8 eV), good thermal stability, and high dielectric constant (∼25).^23,24 Other than the vacuum deposition methods, chemical solution deposition (CSD) has received more and more attention because of its simplicity, low cost, controllability of chemical stoichiometry, and mass productivity in potential printing methods.²⁵⁻²⁷ Yet up to now, quality of thefilm fabricated by solution-processes is still not comparable with those deposited by vacuum methods.

How to obtain high quality dielectricfilm and semiconductor/

dielectric interface by CSD is still a big challenge for its application in future OTFTs.

In this paper, we fabricated high-k ZrO₂ insulators by chemical solution deposition, systematically investigated the electrical properties in OTFT, and modified the surface of the dielectric layer to improve the quality of the interfaces between ZrO₂ and organic semiconductor. By understanding the dominant mechanisms in affecting interfacial charge transport and subsequent processing optimization, we obtained high dielectric constant ZrO₂films and OTFTs with a low leakage current, much reduced operation voltage, and high on/offratio.

Our work demonstrates that the solution-processed high-k ZrO₂films will be promising for applications in future low-cost and high-performance organic electronic devices.

2. EXPERIMENTAL METHODS

2.1. Chemistry of Sol−Gel and Deposition of High-k Zirconium Oxide Films. The precursor solution was synthesized by dissolving metal salt precursors in the strong polar solventN,N-dimethylformamide (DMF) with stabilizing agents. The sol−gel processes include the hydrolysis and condensation process:²⁸

+ + → − + − −

Zr(AcAc)₄ ROH H O₂ AcAc H RO Zr OH (1)

−Zr OH− +HO Zr− − → −Zr O Zr O Zr− − − ··· − +H O₂ (2)

Here AcAc means the acetylacetone and R is the alkyl group which comes from the stabilizing agents. In the hydrolysis reaction,−OH ions were bonded to the metal ions through the loss of a proton by the water molecules surrounding the metal cations. With continuous hydrolysis and condensation reac- tions, the concentration of solution increased due to the oxolation and three-dimensional metal-oxide-metal (M-O-M) grid gel was formed.²⁹The depositedfilms were then annealed so that the organic species were eliminated, and the O−H vibration of the solvent decreased as the annealing temperature increased. The temperature was increased slowly and steadily to reduce the porous density and defects formed in the preannealing and annealing processes.

All of the above chemical materials were purchased from Sigma-Aldrich and used without further purification. The zirconia solution was prepared by dissolving ∼0.48 g of zirconium acetylacetonate (Zr(C₅H₇O₂)₄) (98%) in 10 mL of N,N-dimethylformamide (DMF C₃H₇NO) (99.8%) at a concentration of 0.1 mol/L inside the nitrogen glovebox, with an equivalent mole ratio of ethanolamine (C₂H₇NO) for dispersion. The solution was stirred vigorously at 80°C for 3 h, and then was placed for at least 1 day in the drying cabinet for further aging process. The precursor solutions were filtered through a 0.2μm pore size PTFE membrane syringefilter prior to spin coating. Before coating, the substrates (heavily boron- doped p-type silicon) were cleaned by acetone, isopropanol, and deionized water sequentially. Then the substrates were etched by hydrofluoric acid and cleaned by piranha solution to remove the residual organic contamination. The ZrO₂ films were formed by spin-coating at 3000 rpm for 40s on the precleaned substrates. Subsequently, the spin-coated ZrO₂films were soft-baked at 200 °C for 10 min on a hot plate to evaporate the organic solvent. Three spin coating cycles were adopted to obtain a desired physical thickness of ∼12.0 nm.

Finally, ZrO₂films were annealed at high temperatures (400 to 700 °C) to improve the film quality and reduce the leakage current.

2.2. Fabrication of ZrO2 Based Diode and Transistor Devices. The Cu/ZrO₂/Si (heavily doped p-type silicon) in metal−insulator−metal (MIM) structures were fabricated for Figure 1.Cross-section HRTEM images of the solution processed ZrO₂films annealed at different temperatures as marked.

The Journal of Physical Chemistry C Article

measuring leakage current and capacitance-frequency (C−f) of ZrO₂ films. The Cu electrodes (40 nm thickness) were deposited through a shadow mask (diameter in 100 μm) by vacuum thermal evaporation. Bottom-gate and top-contact (BGTC) OTFTs were fabricated using pentacene as organic semiconductor. Three different dielectric surfaces were used:

bare ZrO₂, ZrO₂modified with hexamethyldisilazane (HMDS), and ZrO₂ coated with poly(α-methylstyrene) (PαMS). The HMDS layer was formed through deposition of HMDS vapor in a chamber at 120°C for 10 min. The PαMS layer (10 nm) was deposited by spin-coating method and annealed in air for 5 min at 120°C. Pentacene (Aldrich, purified with temperature gradient sublimation) was deposited onto the dielectric surface by vacuum evaporation in the thickness of 40 nm. Finally, 40 nm Cu source and drain electrodes were thermally evaporated through a shadow mask.

2.3. Characterizations of Microstructure and Electrical Properties. The microstructures of the ZrO₂ films were investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Surface mor- phologies of the dielectric layer and semiconductor layer are investigated by atomic force microscopy (AFM) AC mode (Asylum Research Cypher S by Oxford Instruments). The dielectric properties of the MIM diodes were investigated through frequency-dependent capacitance (C−f) measurements using a high precision impedance analyzer (Agilent E4980A) with the frequency ranging from 1 kHz to 1 MHz. The current−voltage (I−V) measurements of diodes and OTFTs were carried out by a high precision semiconductor analyzer Agilent B1500A. All the electrical measurements were performed in dark under high vacuum (<5 × 10⁻³ Pa) in a Jannis temperature variable probe station. All electrical results reported in this paper are for the OTFT devices with W/L of 750μm/50μm.

3. RESULTS AND DISCUSSION

3.1. Microstructures and Electrical Properties of the ZrO₂Dielectric Thin Films.To obtain high qualityfilms, the

deposited ZrO₂ films were postannealed in a rapid thermal furnace.Figure 1shows the cross-sectional HRTEM images of the ZrO₂ films annealed at 400, 500, 600, and 700 °C, respectively. The thicknesses of the ZrO₂films are all around 11.5 nm after three spin-coating cycles, indicating good controllability in the spin-coating processes. We note that an interfacial SiO₂layer exists between the heavily doped p-Si and the ZrO₂layer in all the four samples, with thickness of 2.6−2.9 nm. The HRTEM images also reveal that the 400°C annealed film has an amorphous structure, while the ZrO₂films start to crystallize for an annealing at 500°C, andfilms annealed at 600 and 700°C show clear poly crystalline structures. The surfaces of the ZrO₂films were investigated by AFM with a scanning area of 4μm×4μm (Supporting Information, Figure S1). The measured root of mean square (RMS) surface roughness data for all the samples are very similar, all within a small range from 0.45 to 0.51 nm regardless of annealing temperatures.

For electrical properties, Figure 2a shows the typical characteristics of leakage current versus voltage for these ZrO₂ films, which are highly dependent on the annealing temperature. For the 400°C-annealedfilm, the leakage current is∼3.0×10⁻⁴A/cm²at the electricfield of 3 MV/cm. As the annealing temperature increases, the leakage current signifi- cantly decreases and down to∼2.0×10⁻⁶A/cm²at the same electric field in the 700 °C-annealed film. Because the thicknesses of the ZrO₂/SiO₂ layers are nearly the same, the lower leakage current in ZrO₂ films annealed at high temperatures may come from their crystalline structures (as revealed in the above HRTEM studies), as well as from their different electronic structures. The higher annealing temper- ature may result in a higher degree of densification, sharper band edges and bigger band offset with respect to the silicon substrate, which help to reduce leakage current. It should be noticed that the present leakage current of 700°C annealed ZrO₂film is acceptably low among solution processed oxides, while it still cannot compare with some of the reported best values for anodic aluminum oxide. For example, Martin Kaltenbrunner et al. had reported a very small leakage current Figure 2.Electrical properties of the solution processed ZrO₂ films annealed at different temperatuers. (a) Leakage current characteristics; (b) frequancy dependent capacitance density; (c) frequency dependent permittivity characteristics; (d) XRD patterns of the ZrO₂films annealed at different temperatures.

The Journal of Physical Chemistry C

density of∼10⁻⁸A/cm²at 2.5 MV/cm for ultrathin anodized AlO_xfilm,³⁰and Petritz et al. showed an extremely low leakage current of 10⁻⁹A/cm² at 3.0 MV/cm of ultrathin anodized Al₂O₃.³¹We are now optimizing the solution preparing process, film coating and annealing processes to further reduce the leakage current level as well as the process temperature. This work is being carried out and some encouraging results have been obtained.

Another important electrical property is the capacitance of the ZrO₂ films, which was characterized by measuring the capacitance-frequency (C-f) relation of the MIM structure (Cu/ZrO₂/p++ Si).The C−f characteristics measured at 0 V biased voltage are shown in Figure 2b, and the calculated permittivity data are shown in Figure 2c. Generally, a high dielectric constant (k) has been obtained and higher annealing temperature leads to a higher dielectric constant. For example, the 400°C annealedfilm exhibits akof∼12 at 1 kHz and∼10 at 1 MHz, while the 700°C annealedfilm exhibits akof∼20 at 1 kHz and∼16 at 1 MHz.

Since the dielectric constant of the materials has been claimed to be closely related to their microstructures,^32,33we investigated the crystallization of the ZrO₂ films by XRD measurement. As shown inFigure 2d, the 400°C annealedfilm exhibits no diffraction peak, while thefilms annealed at higher temperatures show clear peaks of (111), (020), (112), and (121), which indicate coexistence of monoclinic and tetragonal structures. Furthermore, by comparing the (111) and (020) peak intensities, we learn that the crystallization degree generally increases from 500 to 700 °C annealed samples.

Thus, the increased dielectric constant at higher annealing temperature is probably attributed to the better crystallinity.

The physical and electrical properties of the solution processed ZrO₂dielectric films are summarized inTable 1. High quality

thin ZrO₂ film (4 nm/cycle) can be obtained with a good thickness repeatability and thermal stability on Si substrate. The ZrO₂films have highkvalues and low leakage current, which is desirable for their applications as the gate insulator in OTFTs for low voltage operations.

3.2. DC Electrical Properties of the ZrO2based OTFTs.

The ZrO₂ films were applied in bottom-gate top-contact OTFTs with pentacene as the semiconducting layer and the transfer characteristics are shown in Figure 3. The cross sectional structure of the devices and the schematic diagram of the surface modification layer are given in Figure S2 in the Supporting Information.

The drain currents were observed to be saturated under −5 V (ON state) and 1 V (OFF state) gate voltage in transfer characteristics. Thefield-effect hole mobility (μ) and threshold voltage (V_T) of the ZrO₂ OTFTs were extracted from the saturation regime and are summarized in Table 2. Compared with that of the OTFTs with SiO₂dielectrics, the OTFTs with high-k ZrO₂ show a much reduced working voltage, which is

desirable for the portable electronic device applications. The hysteresis in the transfer curves is generally small, indicating the good quality of the ZrO₂/pentacene interface without mobile ions. Yet a small carrier mobility and low ON/OFF ratio are observed for all the devices with bare ZrO₂surface. To improve the device performance, the ZrO₂surface was modified with an HMDS SAM layer or a PαMS layer (10 nm). The typical transfer curves of the OTFTs with HMDS- or P-modified ZrO₂ are shown inFigure 4a,b, respectively. The values ofμandV_T under saturation mode are summarized inTable 2. The typical output characteristics for PαMS-modified devices are shown in Figure S3, and the drain current under−2 V drain voltage has been found to be saturated under various gate voltages. For HMDS modified devices, no significant improvement on the electrical properties is observed andμis still below 0.1 cm²/(V s). The reason for the low hole mobility in bare ZrO₂ and HMDS modified devices will be discussed in the following section. In contrast, the PαMS-modified devices show much higher on/offratio up to 8.4×10⁴(600°C annealed) and∼1.2

× 10⁵ (700 °C annealed). The field effect mobility also significantly increases, as the values are 0.29, 0.40, and 0.51 cm²/V s for the 500, 600, and 700 °C annealed devices, respectively. Although one 10 nm low permittivity (∼2.3) PαMS layer was inserted between ZrO₂and pentacene, values of V_T are still generally within −1 V to afford a low working voltage. FromFigure 4(b), a small subthreshold slope of 0.26 V/dec can be observed for 700°C annealed device. The ZrO₂− OTFTs with PαMS modified interface exhibit good electric performances, compromising a low working voltage, small subthreshold slope, and a high on/offratio of drain current.

3.3. Insulator (ZrO₂)/Semiconductor (Pentacene) In- terface.The above experimental results show that ZrO₂films annealed at varied temperatures share similar surface roughness but induce different hole mobility. To understand the different electrical performances as shown above, we further investigate the impact on hole transport coming from surface energy and dielectric constant.

First, the surface energy of dielectric layers affect perform- ances of OTFTs by affecting crystal growth, as reported in some previous studies.^20,34,35 To characterize the surface energy, the contact angles of two test liquids, deionized water and ethylene glycol (EG C₂H₆O₂), are measured (DataPhysics OCA) for ZrO₂films with different surface modifications. The surface energy is calculated according to the methods proposed by Wu model (harmonic mean).^35,36The total surface energy consists of dispersion and polar components. For ZrO₂ films annealed at 700 °C, the contact angles of bare ZrO₂ film, HMDS-modified, and PαMS-modified ZrO₂ surfaces with water and EG are shown in Figure 5a−f, the PαMS-ZrO₂ surface shows the highest contact angles, whereas the bare- ZrO₂ surface shows the lowest contact angles. The calculated surface energy are 43.9, 37.8, and 35.5 mJ/m²for bare-ZrO₂, HMDS-ZrO₂, and PαMS-ZrO₂, respectively. The surface energy is clearly decreased after surface modification, especially in PαMS modified surfaces. Similar results on contact angles and surface energy have been observed for ZrO₂films annealed at other temperatures, as shown in Table S2 in theSupporting Information. The low surface-energy surfaces can enhance the interconnection between organic semiconductor grains.²⁰

Figure 5g−i shows the surface morphologies of the pentacene films grown on the different surfaces with the underneath ZrO₂films annealed at 700°C. Small grain sizes of pentacene are observed on the bare or HMDS modified Table 1. Summary of the Physical and Electrical Properties

of the Solution Processed ZrO2Films

temperature (°C)

thickness of SiO₂

(nm)

thickness of ZrO₂ (nm)

dielectric constant (at 1 kHz)

leakage current (A/cm²) at 3

MV/cm

400 2.6 11.4 12.17 2.79×10⁻⁴

500 2.9 11.2 13.52 8.12×10⁻⁵

600 2.8 11.6 16.78 5.19×10⁻⁵

700 2.9 11.3 19.70 1.89×10⁻⁶

The Journal of Physical Chemistry C Article

Figure 3.TypicalIDS−VGScharacteristics of the OTFTs with bare ZrO2annealed at different temperatures of 400, 500, 600, and 700°C.

Table 2. Carrier Mobility, Threshold Voltage andIon/IoffValues of the Different OTFTs

mobility (cm²/V s) threshold voltage (V) I_on/I_offratio

temperature (°C) bare ZrO₂ HMDS-ZrO₂ PαMS-ZrO₂ bare ZrO₂ HMDS-ZrO₂ PαMS-ZrO₂ bare ZrO₂ HMDS-ZrO₂ PαMS-ZrO₂

400 0.06 0.10 0.08 −0.97 −1.01 −1.61 1.7×10³ 1.1×10³ 1.6×10⁴

500 0.06 0.11 0.29 −0.80 −1.01 −1.01 2.8×10³ 1.9×10⁴ 1.8×10⁴

600 0.04 0.09 0.40 −0.90 −0.87 −0.80 4.1×10³ 6.5×10⁴ 8.4×10⁴

700 0.02 0.07 0.51 −0.73 −0.47 −0.36 1.9×10⁴ 7.2×10⁴ 1.2×10⁵

Figure 4.TypicalIDS−VGScharacteristics of the OTFTs with modified ZrO2surfaces, in which the ZrO2films annealed at different temperatures of 400, 500, 600, and 700°C. (a) HMDS-modified; (b) PαMS-modified.

The Journal of Physical Chemistry C

surfaces, whereas the grain size is much increased on the PαMS modified surfaces. Similar surface morphology results are found on ZrO₂films annealed at other temperatures (Figure S4). The present results prove that the ZrO₂ surface energy is an important factor to affect the morphology of pentacene films.

PαMS-modified ZrO₂surface has a reduced surface energy, and correspondingly a large pentacene grain size. HMDS- modification can decrease the surface energy of ZrO₂ film, however it is not effective to promote the increase of the pentacene grain size. As to the relationship between dielectric surface energy and the pentacene grain size, it is still under controversy. For example, Yang et al. reported that the pentacene grain size increases with the increase of surface energy of poly(imide-siloxane) dielectrics.³⁷Depressed mobility of OFETs is observed on high surface energy dielectric layers even with larger-grain morphology. The reason is still not clear, and it needs to be clarified in the future work. Referring to Table 2, we find that the carrier mobility on the PαMS- modified ZrO₂ devices is nearly 10 times higher than that in bare or HMDS-modified ZrO₂. We propose that large pentacene grain size with much less grain boundaries is responsible for the high mobility in PαMS-modified ZrO₂ devices. Therefore, it can be concluded that the surface energy of the dielectric that plays a critical role in determining the carrier mobility in ZrO₂−OTFTs with different surface treatments.

Figure 6 summarizes the relationship between the carrier mobility and the surface energy of the ZrO₂ dielectrics. It clearly shows the strong dependence of the carrier mobility on the surface energy of the dielectrics. It should be noticed that the carrier mobility exhibits a big difference for PαMS-modified ZrO₂ films with different annealing temperatures, even their

surface energies are nearly the same. This means that surface energy is not the only factor to determine the carrier mobility in the pentacene channel. Considering that the effects from dielectric constant are complicated for the interface between the high-k insulator and the organic semiconductor. Despite inducing a low-working voltage, high-k inorganic materials can deteriorate device performance through introducing a combi- nation of surface trap sites associated with−OH groups, and/or inducing ionic polarization between charge carriers and the high-kionic lattice.³⁸Therefore, we consider that the dielectric constant may be also important mechanism to affect the charge transport in pentacene channel.

The relation between the measured permittivity and mobility is shown inFigure 7. For devices with bare ZrO₂and HMDS- modified ZrO₂, the mobility nearly remains constant even though the permittivity increases, whereas the mobility of the devices with PαMS-modified ZrO₂increases remarkably with Figure 5.Water contact angles of bare-ZrO₂surface (a); HMDS modified surface (b); PαMS modified surface (c); the EG contact angles of bare- ZrO2surface (d); HMDS modified surface (e); PαMS modified surface (f); AFM surface morphologies of pentacenefilms grown on bare-ZrO2

surface (g); HMDS modified surface (h); PαMS modified surface (i). All of the ZrO2film were annealed at 700°C.

Figure 6.Carrier mobility dependence on the surface energy of ZrO₂ films with different surface modification and annealing temperatures.

The Journal of Physical Chemistry C Article

increasing the permittivity. Note that the surface energy and pentacene grain size of the PαMS-modified device were almost the same (Table S2 and Figure S4), and thus the surface energy is not a major mechanism here. It has been theoretically and experimentally demonstrated that coupling between the charged carriers and the ionic lattice of the dielectric will lead to lower carrier mobility.^20,38Stassen et al. had reported that the decrease in mobility with the increasing dielectric constant is attributed to the localized nature of charge carriers in the materials and their interaction with the induced polarization in the gate insulator. Such a dipole scattering effect expect a low mobility with a high permittivity.³⁹Yet in a different way, the carrier mobility increases with the increasing permittivity for PαMS modified devices (Figure 7a). The results probably point to the mechanism that mobility increases with carrier densities.⁴⁰The relation between mobility and carrier densities were shown inFigure 7b, which are calculated byQ=C(V_G− V_T)/e, whereQis the carrier density andC is the capacitance density. With a higher carrier concentration, the immobile subgap states induced by structural disorders are sufficiently filled and more mobile states are occupied by carriers at the

same gate voltage. Consequently the overall average mobility are larger as compared with the case with a low carrier concentration.

3.4. Low Temperature Fabrication of OTFT on Flexible Substrate.To probe their applications of solution-processable ZrO₂films inflexible electronic devices, we further investigated the low temperature fabrication processes of ZrO₂films. High quality ZrO₂films have been fabricated at a very low processing temperature of 160°C by using optimized processes, especially for surface ozone treatment and step-temperature treatment during each coating cycle. The details about the optimized processing parameters of low-temperature fabrication of high quality ZrO₂film will be reported elsewhere. OTFTs have been fabricated on flexible polyethyleneterephthalate (PET) sub- strate by using a 12 nm solution processed ZrO₂film.Figure 8a shows a schematic diagram of theflexible OTFT device. Au is deposited by vacuum evaporation as the bottom gate. The 12 nm solution processed ZrO₂film is annealed at 160°C, and a 10 nm PαMS layer was used as the interfacial modification layer. Figure 8b shows the photograph of the actual OTFT devices on theflexible PET substrate in our experiment.Figure Figure 7.(a) Field effect hole mobility against ZrO₂permittivity for devices with different surface modifications. (b) Thefield effect hole mobility against carrier density in the channel for devices with different surface modifications, calculated withV_G=−5 V and capacitance density underf= 1 kHz.

Figure 8.(a) Schematic diagram of theflexible OTFT fabricated on PET substrate; (b) the actual photograph of theflexible OTFTs; (c)I_DS−V_GS characteristics and (d)IDS−VDScharacteristics of a ZrO2−OTFT fabricated on PETflexible substrate. The W/L of the transistor is 750μm/50μm.

The Journal of Physical Chemistry C

8c shows the typical I_DS-V_GS characteristics of the flexible ZrO₂−OTFT, from which a∼10⁵on/offratio of drain current can be observed between +1 V and−5 V scanning. The well- behaved drain current versus drain voltage (I_DS-V_DS) character- istics of the same device can be also observed, as shown in Figure 8d. The results shown inFigure 8demonstrate that high quality ZrO₂ films can be processed at low temperature, and they are promising for low voltage operations in futureflexible electron devices.

4. CONCLUSIONS

In summary, we have fabricated high-quality ZrO₂ dielectric films by using solution processes. A high dielectric constant of

∼20 and low leakage current of 2×10⁻⁶A/cm²@ 3MV/cm were obtained for the ZrO₂ films annealed at 700 °C. The surface modification effects on the electrical performance of the ZrO₂ OTFTs have been systematically explored. The PαMS- modified devices exhibit higher carrier mobility and on/offratio than that of the devices with bare ZrO₂and HMDS-modified surfaces, because the low surface energy induces large crystal sizes. Also, a higher annealing temperature of ZrO₂induces a higher permittivity and thus high carrier concentrations, resulting in both small operation voltage and good carrier mobility. In addition to clarify the interfacial effects between ZrO₂ and organic semiconductor, the low temperature fabrication of high-quality ZrO₂ film and high performance ZrO₂−OTFT on flexible substrate have been also observed.

Our work will help understand the complex interfacial effects in the OTFTs with high-k dielectrics. The solution processable high-k ZrO₂ dielectric film is promising for applications in future low cost organic devices, especially for low voltage and low power organic devices.

■

ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acs.jpcc.6b03638.

AFM surface morphology of ZrO₂ dielectric films, ZrO₂−OTFT device structure, typical output character- istics of PαMS-modified devices, surface contact angles of the ZrO₂surfaces, surface energy and corresponding surface morphologies of the pentacenefilms at different ZrO₂annealing temperatures (PDF)

■

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected](X.L.).

*E-mail: [email protected](J.-M.L.).

Notes

The authors declare no competingfinancial interest.

■

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Contract Nos. 61271127, 51472093, 51431006, and 61504173) and the program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1243). X.B.L. and J.W.G. acknowledge the support of Science and Technology Planning Project of Guangdong Province (Nos. 2014B090915004 and 2014B090915005).

■

(1) Wei, Q.; You, E.; Hendricks, N. R.; Briseno, A. L.; Watkins, J. J.^REFERENCES Flexible Low-Voltage Polymer Thin-Film Transistors Using Super- critical CO₂-Deposited ZrO₂ Dielectrics.ACS Appl. Mater. Interfaces 2012,4, 2322−2324.

(2) Reeder, J.; Kaltenbrunner, M.; Ware, T.; Arreaga-Salas, D.;

Avendano-Bolivar, A.; Yokota, T.; Inoue, Y.; Sekino, M.; Voit, W.;

Sekitani, T. Mechanically Adaptive Organic Transistors for Implant- able Electronics.Adv. Mater.2014,26, 4967−4973.

(3) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D.

Organic Transistors in Optical Displays and Microelectronic Applications.Adv. Mater.2010,22, 3778−3798.

(4) Cantatore, E.; Geuns, T. C. T. A 13.56-MHz RFID System Based on Organic Transponders.IEEE J. Solid-State Circuits2007,42, 84−92.

(5) Lin, P.; Yan, F. Organic Thin-Film Transistors for Chemical and Biological Sensing.Adv. Mater.2012,24, 34−51.

(6) Dodabalapur, A. Organic and Polymer Transistors for Electronics.Mater. Today2006,9, 24−30.

(7) Franklin, A. D. Nanomaterials in Transistors: From High- Performance to Thin-Film Applications. Science 2015, 349, No. aab2750.

(8) Newman, C. R.; Chesterfield, R. J.; Panzer, M. J.; Frisbie, C. D.

High Mobility Top-Gated Pentacene Thin-Film Transistors.J. Appl.

Phys.2005,98, No. 084506.

(9) Di, C. A.; Liu, Y.; Yu, G.; Zhu, D. Interface Engineering an Effective Approach toward High-Performance Organic Field-Effect Transistors.Acc. Chem. Res.2009,42, 1573−1583.

(10) Yoon, M. H.; Kim, C.; Facchetti, A.; Marks, T. J. Gate Dielectric Chemical Structure-Organic Field-Effect Transistor Performance Correlations for Electron, Hole, and Ambipolar Organic Semi- conductors.J. Am. Chem. Soc.2006,128, 12851−12869.

(11) Facchetti, A.; Yoon, M.-H.; Marks, T. J. Gate Dielectric for Organic Field-Effecttransistor: New Opportunities for Organic Electronics.Adv. Mater.2005,17, 1705−1725.

(12) Kim, C. S.; Jo, S. J.; Lee, S. W.; Kim, W. J.; Baik, H. K.; Lee, S. J.;

Hwang, D. K.; Im, S. High-k and Low-k Nanocomposite Gate Dielectrics for Low Voltage Organic Thin Film Transistors.Appl. Phys.

Lett.2006,88, 243515.

(13) Ortiz, R. P.; Facchetti, A.; Marks, T. J. High-k Organic, Inorganic, and Hybrid Dielectrics for Low-Voltage Organic Field- Effect Transistors.Chem. Rev.2010,110, 205−239.

(14) Veres, J.; Ogier, S.; Lloyd, G. Gate Insulators in Organic Field- Effect Transistors.Chem. Mater.2004,16, 4543−4555.

(15) Liao, M.; Ishiwara, H.; Ohmi, S. I. Excellent Current Drivability and Environmental Stability in Room-Temperature-Fabricated Penta- cene-Based Organic Field-Effect Transistors with HfO₂ Gate Insulators.IEEE Trans. Electron Devices2014,61, 569−575.

(16) Koo, J. B.; Lim, J. W.; Kim, S. H.; Yun, S. J.; Ku, C. H.; Lim, S.

C.; Lee, J. H. Pentacene Thin-Film Transistors and Inverters with Plasma-Enhanced Atomic-Layer-Deposited Al₂O₃ Gate Dielectric.

Thin Solid Films2007,515, 3132−3137.

(17) Kim, J.-M.; Lee, J.-W.; Kim, J.-K.; Ju, B.-K.; Kim, J.-S.; Lee, Y.- H.; Oh, M.-H. An Organic Thin-Film Transistor of High Mobility by Dielectric Surface Modification with Organic Molecule. Appl. Phys.

Lett.2004,85, 6368.

(18) Adamopoulos, G.; Thomas, S.; Wobkenberg, P. H.; Bradley, D.

D.; McLachlan, M. A.; Anthopoulos, T. D. High-Mobility Low-Voltage ZnO and Li-Doped ZnO Transistors Based on ZrO₂High-k Dielectric Grown by Spray Pyrolysis in Ambient Air. Adv. Mater. 2011, 23, 1894−1898.

(19) Shtein, M.; Mapel, J.; Benziger, J. B.; Forrest, S. R. Effects of Film Morphology and Gate Dielectric Surface Preparation on the Electrical Characteristics of Organic-Vapor-Phase-Deposited Penta- cene Thin-Film Transistors.Appl. Phys. Lett.2002,81, 268−270.

(20) Sun, X.; Di, C. A.; Liu, Y. Engineering of the Dielectric- Semiconductor Interface in Organic Field-Effect Transistors.J. Mater.

Chem.2010,20, 2599−2611.

The Journal of Physical Chemistry C Article

(21) Shao, W.; Dong, H.; Jiang, L.; Hu, W. Morphology Control for High Performance Organic Thin Film Transistors.Chem. Sci.2011,2, 590−600.

(22) Ma, H.; Yip, H. L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371−1388.

(23) Robertson, J. High Dielectric Constant Oxides. Eur. Phys. J.:

Appl. Phys.2004,28, 265−291.

(24) Robertson, J. High Dielectric Constant Gate Oxides for Metal Oxide Si Transistors.Rep. Prog. Phys.2006,69, 327−396.

(25) Park, Y. M.; Daniel, J.; Heeney, M.; Salleo, A. Room- Temperature Fabrication of Ultrathin Oxide Gate Dielectrics for Low-Voltage Operation of Organic Field-Effect Transistors. Adv.

Mater.2011,23, 971−974.

(26) Beaulieu, M. R.; Baral, J. K.; Hendricks, N. R.; Tang, Y.; Briseno, A. L.; Watkins, J. J. Solution Processable High Dielectric Constant Nanocomposites Based on ZrO₂ Nanoparticles for Flexible Organic Transistors.ACS Appl. Mater. Interfaces2013,5, 13096−13103.

(27) Park, Y. M.; Desai, A.; Salleo, A.; Jimison, L. Solution- Processable Zirconium Oxide Gate Dielectrics for Flexible Organic Field Effect Transistors Operated at Low Voltages.Chem. Mater.2013, 25, 2571−2579.

(28) Harizanov, O.; Ivanova, T.; Harizanova, A. Study of Sol-Gel TiO₂and TiO₂-MnO Obtained from a Peptized Solution.Mater. Lett.

2001,49, 165−171.

(29) Jeong, S.; Moon, J. Low-Temperature, Solution-Processed Metal Oxide Thin Film Transistors.J. Mater. Chem.2012,22, 1243−1250.

(30) Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwödiauer, R.; Graz, I.; Bauer- Gogonea, S. An Ultra-Lightweight Design for Imperceptible Plastic Electronics.Nature2013,499, 458−463.

(31) Petritz, A.; Wolfberger, A.; Fian, A.; Griesser, T.; Irimia-Vladu, M.; Stadlober, B. Cellulose-Derivative-Based Gate Dielectric for High- Performance Organic Complementary Inverters.Adv. Mater.2015,27, 7645−7656.

(32) Anderson, J. T.; Munsee, C. L.; Hung, C. M.; Phung, T. M.;

Herman, G. S.; Johnson, D. C.; Wager, J. F.; Keszler, D. A. Solution- Processed HafSOx and ZircSOx Inorganic Thin-Film Dielectrics and Nanolaminates.Adv. Funct. Mater.2007,17, 2117−2124.

(33) Jiang, K.; Anderson, J. T.; Hoshino, K.; Li, D.; Wager, J. F.;

Keszler, D. A. Low-Energy Path to Dense Hfo2thin Films with Aqueous Precursor.Chem. Mater.2011,23, 945−952.

(34) Yan, Y.; Huang, L. B.; Zhou, Y.; Han, S. T.; Zhou, L.; Sun, Q.;

Zhuang, J.; Peng, H.; Yan, H.; Roy, V. A. Surface Decoration on Polymeric Gate Dielectrics for Flexible Organic Field-Effect Transistors Via Hydroxylation and Subsequent Monolayer Self- Assembly.ACS Appl. Mater. Interfaces2015,7, 23464−23471.

(35) Hutchins, D. O.; Weidner, T.; Baio, J.; Polishak, B.; Acton, O.;

Cernetic, N.; Ma, H.; Jen, A. K. Effects of Self-Assembled Monolayer Structural Order, Surface Homogeneity and Surface Energy on Pentacene Morphology and Thin Film Transistor Device Performance.

J. Mater. Chem. C2013,1, 101−113.

(36) Wu, S. Calculation of Interfacial Tension in Polymer Systems.J.

Polym. Sci., Part C: Polym. Symp.1971,34, 19−30.

(37) Yang, S. Y.; Shin, K.; Park, C. E. The Effect of Gate-Dielectric Surface Energy on Pentacene Morphology and Organic Field-Effect Transistor Characteristics.Adv. Funct. Mater.2005,15, 1806−1814.

(38) Coropceanu, V.; Cornil, J.; Filho, D. A. S.; Olivier, Y.; Silbey, R.;

Bredas, J. Charge Transport in Organic Semiconductors.Chem. Rev.

2007,107, 926−952.

(39) Stassen, A. F.; de Boer, R. W. I.; Iosad, N. N.; Morpurgo, A. F.

Influence of the Gate Dielectric on the Mobility of Rubrene Single- Crystal Field-Effect Transistors.Appl. Phys. Lett.2004,85, 3899.

(40) Xia, Y.; Cho, J. H.; Lee, J.; Ruden, P. P.; Frisbie, C. D.

Comparison of the Mobility-Carrier Density Relation in Polymer and Single-Crystal Organic Transistors Employing Vacuum and Liquid Gate Dielectrics.Adv. Mater.2009,21, 2174−2179.

The Journal of Physical Chemistry C