Solution processable high quality ZrO 2 dielectric films for low operation voltage and flexible organic thin film transistor applications

1. Introduction

The field of flexible electronics has experienced tremendous developments and has been widely investigated [1]. In par­

ticular, emerging technologies, such as flexible and wearable electronics, provide a remarkable opportunity to commercialize

organic electronic devices [2]. The organic thin­film transistor (OTFT) is a key element for realizing functional large area electronic systems, including displays and sensors, by pro­

viding the capabilities of matrix addressing, current driving, and signal processing [3, 4]. This has motivated considerable effort in the last few decades to develop OTFT technologies.

Journal of Physics D: Applied Physics

Solution processable high quality ZrO ₂ dielectric films for low operation voltage and flexible organic thin film transistor applications

Yanfen Gong¹, Kai Zhao¹, Huixin He¹, Wei Cai², Naiwei Tang¹,

Honglong Ning² , Sujuan Wu¹, Jinwei Gao¹, Guofu Zhou³, Xubing Lu¹ and J-M Liu^1,4

1 Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, and Guangdong Provincial Laboratory of Quantum Engineering and Quantum Materials, South China Normal University, Guangzhou 510006, People’s Republic of China

2 Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China

3 Electronic Paper Displays Institute and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, People’s Republic of China

4 Laboratory of Solid State Microstructures, Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

E­mail: luxubing@m.scnu.edu.cn and liujm@nju.edu.cn Received 19 November 2017, revised 23 January 2018 Accepted for publication 31 January 2018

Published 23 February 2018 Abstract

Low temperature fabrication of high quality dielectric films for high performance flexible electronics is still a big challenge. In this work, we realized low temperature fabrication of high quality amorphous ZrO2 dielectric films via a low­cost solution process. The microstructure and electrical properties, as well as the electronic structures of solution processed ZrO2 films have been investigated systematically. The ZrO2 films with 160 °C annealed showed a low leakage current (3.6 × 10⁻⁵ A cm⁻² at −3 V) and a high band gap (5.4 eV). The flexible organic thin film transistor (OTFT) made by using the solution­

processed amorphous ZrO2 dielectric shows a low operation voltage of 4 V and a high drain current on/off ratio of 2.4 × 10⁵. The frequency response of the ZrO2­OTFT device is up to 51.8 KHz under a low gate voltage of −3 V. Our work demonstrated that a solution processable ZrO2 film is promising for applications in future low power consumption and wearable flexible electronic devices.

Keywords: low­temperature, amorphous ZrO2, low voltage, flexible OTFT (Some figures may appear in colour only in the online journal)

Y Gong et al

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https://doi.org/10.1088/1361-6463/aaac1b J. Phys. D: Appl. Phys. 51 (2018) 115105 (9pp)

Y Gong et al

However, despite both the strong market demand and recent technological advances, there are still many unsolved chal­

lenges associated with the fabrication of high­performance flexible OTFTs, namely to develop low­temperature fabrica­

tion, low­voltage devices and high­frequency operation. In view of the above­mentioned issues, the adoption of high­k dielectrics to replace traditional SiO2 is considered an effec­

tive and important approach to lower the operating voltage and enhance the on/off ratio of the drain current in OTFTs [5, 6]. Over the past few years, several inorganic gate di electrics, such as vapor and solution phase deposited metal oxides (Al2O3 [7], ZrO2 [8], HfO2 [9] and TiO2 [10]), have been investigated, and some of these high­k OTFTs have demon­

strated low­voltage operation below 5 V [2]. Among high­k materials, zirconium oxide is an obvious ideal candidate for OTFT because it uniquely combines excellent thermal, chem­

ical and mechanical stability, as well as a theoretical high­k value (25) [11, 12]. It could thus help suppressing the leakage currents, increasing the breakdown voltage and provide for a relatively large charge response at a small electrical field [13].

Conventional methods previously reported for the pro­

cessing of inorganic dielectric materials mainly rely on vacuum­based deposition (e.g. magnetron sputtering [14], chemical vapor deposition [15], atomic layer deposition [16]

and e­beam evaporation [17]). These methods have proven to be able to deposit very thin films with good electrical and mechanical properties, thus efficiently resolving the limita­

tions concerning scaling issues [18]. However, most vacuum­

based deposition techniques require high initial costs for equipment set­up and complex fabrication processes, both of which undoubtedly limit practical applications where cost effi­

ciency is a priority. To address this issue, solution­processing of high­k materials has attracted much interest, since it has the advantages of low­cost, high­efficiency, easy chemical composition control and compatibility for large­scale roll­to­

roll production using spin­coating or high­throughput printing [19, 20]. Rim et al [21] fabricated 100 nm thick ZrO2 di electric films by spin­coating, followed by an annealing at 450 °C.

The corresponding In2O3/InZnO bilayer TFTs showed high mobility (~40 cm² V^−s·s⁻¹) and low­voltage operation (3 V).

However, a high annealing temperature (~500 °C) is required to fabricate high­quality oxide films [22–24]. Apparently, a high­temperature treatment is a persistent issue which usually makes the process incompatible with cheap plastic substrates [19]. Therefore, further studies on low­temperatures are nec­

essary before being able to control the fabrication of high quality dielectric films at room temperature.

In this work, we successfully demonstrated that high­

quality solution­processed ZrO2 films can be obtained via low­temperature annealing, thus realizing the low temperature fabrication of flexible pentacene OTFTs with high frequency and low­voltage operation. We carried out a systematic study on the impact of thermal annealing, which allowed us to con­

clude how temperature affects the microstructure and the elec­

trical performance of solution processed ZrO2 films. Flexible pentacene OTFTs with low operation voltage were finally fab­

ricated. Our work clearly shows that a low temperature treat­

ment is sufficient to achieve a high quality amorphous ZrO2

films, so that it will be promising for future low cost, flexible, low power consumption device applications.

2. Experimental details

2.1. Fabrication of ZrO2 thin films by solution processing.

All precursors and solvents, namely zirconium acetylaceto­

nate (Zr(C5H7O2)4)(98%), N,N­dimethylformamide (DMF C3H7NO) (99.8%), poly­α­methyl styrene (PαMS), penta­

cene (C22H14) were purchased from Sigma­Aldrich and were used without further purification. P­type lightly­doped single­

crystal silicon (1 0 0) wafers and polyethylene terephthalate (PET) flexible substrates were obtained from the Electronics and Materials Corporation.

Before the deposition, the substrates (commercial light­

doped p­type silicon substrate) were cleaned with consecu­

tive rinses of acetone, isopropanol and deionized water. Then, they were etched by hydrofluoric acid to remove the native oxide covered on the surface of the Si substrate, and cleaned by a piranha solution (H2SO4:H2O2 = 4:1) to remove the residual organic contamination and improve the wettability.

The 0.15 mol l⁻¹ ZrO2 dielectric precursor solution was pre­

pared by dissolving ~0.7 g of zirconium (IV) acetylacetonate powder into 10 ml N, N­dimethylformamide solvent in a glove box. Then, the solution was stirred at 90 °C for 32 h. During this stirring process, the hydrolysis and condensation reac­

tions occurred in the ZrO2 precursor solution [17]. The pre­

cursor solution was filtered through a 0.2 µm pore size PTFE membrane syringe filter before solution casting. Immediately following this procedure, the ZrO2 film was deposited by spin­

coating at 500 rpm for 5 s and 2000 rpm for 40 s on the pre­

cleaned substrates. After the spin coating, the thin films were treated by thermal annealing methods to evaporate the solvent and promote the crystallization. The thermal annealing pro­

cess consisted at first of baking on a hot plate at 160 °C for 10 min. After several coating cycles, the films were finally annealed at 160, 200, 300, 400 and 500 °C for improving the densification and the crystallization, respectively, and each fabrication step was performed in ambient conditions.

2.2. Fabrication of flexible pentacene OTFTs

Transistors were fabricated by firstly thermally evaporating on a flexible PET substrate a 40 nm thick gold (Au) electrode acting as the bottom gate. Then, the ZrO₂ film was fabricated and annealed by using the above­mentioned coating and annealing methods. The ZrO₂ film was exposed to UV­ozone plasma for 5 min to enhance its surface wettability. Then, poly­α­methyl styrene (PαMS) with ~10 nm thickness was spin coated to improve the interfacial quality between the ZrO₂ dielectric layer and the organic semiconductor. The PαMS layer is used to reduce the surface energy and enhance the ordered growth of pentacene film. Furthermore, the PαMS layer will also passivate the OH group on the surface of ZrO₂ film, and eliminate the hysteresis in the I_DS­V_GS curve, since PαMS itself contains no OH groups and has a non­polar sur­

face. A pentacene film with 40 nm thickness was thermally

J. Phys. D: Appl. Phys. 51 (2018) 115105

evaporated on the top of the PαMS/ZrO2 surface to form the organic semiconductor layer. The choice of the 40 nm thick­

ness of pentacene film is due to the fact that the saturation values of Id and the mobility of OTFT occurs when the pen­

tacene film is around 40 nm [25]. The exact reason is not clear, and Guo et  al argued that the maximum Id thickness was closely related to the potential and charge distribution along the surface normal according to Kelvin force micros­

copy measurement. It should be noticed that the 40 nm pen­

tacene film was grown at a substrate temperature of 50 °C.

The reason is as follows: Guo et  al demonstrated that only low temperature annealing at 45 °C caused improved mobility compared to the value before annealing, while annealing with a temperature >50 °C decreased the mobility [26]. In our pre­

vious work, a moderate growth temperature around 50–60 °C should be favorable to form a well­ordered crystalline struc­

ture and large grain size [27]. Finally, a 40 nm thick Cu source/drain (S/D) electrode was thermally evaporated on the pentacene layer with a shadow mask, which finally defined a bottom­gate and top­contact OTFT device with channel width/length of 750 µm/50 µm. The main consideration for us to choose Cu as the S/D electrodes is that it can provide a reduced contact resistance between the S/D electrode and pentacene and enhanced carrier mobility in the channel when compared with that of Au electrodes [28, 29].

2.3. Microstructure investigation and characterization of electrical properties

The surface morphology of different samples was observed with a Nanoscope multimode atomic force microscope (AFM) in the taping mode. The microstructure of the ZrO2 films was

investigated by x­ray diffraction (XRD) and high­resolution transmission electron microscopy (HRTEM). The energy bands of zirconia thin films were characterized by x­ray pho­

toelectron spectroscopy (XPS). Film thicknesses were evalu­

ated by HRTEM. The capacitance of the ZrO2 dielectric layers and the AC frequency response characteristics of the OTFTs were measured using a four­probe measurement system with a high precision impedance analyzer (Keysight E4990A). The insulating properties of the films and the transistor properties of the OTFTs were investigated by using an Agilent B1500A high­precision semiconductor analyzer. All the electrical measurements were performed in the dark under vacuum (~5 × 10⁻³ Pa) in a Janis temperature variable probe station.

3. Results and discussion

3.1. Microstructure and dielectric properties of ZrO2 films by thermal annealing

The annealing process for solution­processed oxide dielectric is significant, since during the annealing process unneces­

sary constituents are removed and the oxide is developed. We applied thermal annealing to density ZrO2 films and inves­

tigated their microstructures and electrical properties. To identify how the thermal annealing temperature affects film quality, we gradually decreased the annealing temperature from 500 °C down to 160 °C with 1 h annealing time in atmos­

phere. Why we chose a 160 °C temperature for annealing on the flexible substrate is due to the following consider­

ation. The boiling point of the DMF solvent is 153 °C and the melting point of the PET flexible substrate is 250 °C, so 160 °C is the lowest temperature for removing the residual

Figure 1. (a) XRD patterns for the Si/ZrO₂ structure. (b) Areal capacitance versus frequency, (c) capacitance­voltage and (d) leakage current characteristics for the Si/ZrO₂/Cu capacitor structure annealed at different temperature.

Y Gong et al

solvent effectively and being compatible with the flexible PET substrate. Figure 1(a) shows the microstructural properties of ZrO2 films analyzed by XRD. ZrO2 thin films show a typical amorphous structure when annealed below 400 °C, while a weakly crystalline phase appears in ZrO2 films annealed at above 400 °C, as testified by the (0 1 1) peak. To investigate the electrical properties of the ZrO2 thin films, we fabricated and characterized Cu/ZrO2/Si structured (metal­insulator­

semiconductor MIS) capacitors. Figure 1(b) shows the areal capacitance of ZrO2 films in the frequency range between 100 Hz and 1 MHz. The areal capacitance of ZrO2 films increases with the temperature: the capacitance at 1 KHz is ~117 nF cm⁻² for 160 °C and ~399 nF cm⁻² for 500 °C. According to the thicknesses of ZrO2 films determined from HRTEM, the relative dielectric constants (k) of ZrO2 are 7.8 for 160 °C and 9 for 500 °C. The relatively low k value of 160 °C­annealed film was ascribed to the presence of organic residuals in the film, the low atomic density hence results in small electronic and ionic polarizations [30, 31]. Obvious accumulation and depletion regions are clearly observed in the C–V curves

(figure 1(c)), the ΔVFB shift results smaller than 0.1 V and the negligible hysteresis shift voltage indicate that the fixed charge density and the trapped charge density in the ZrO2 or at the interface are relatively low [13]. Nevertheless, the C–V hysteresis width increases when decreasing the annealing temperature, suggesting the presence of residual solvents and defects in the interface or films. Figure 1(d) shows the typical leakage current characteristics obtained in ZrO2 films.

It is ~3.6 × 10⁻⁵ A cm⁻² at −3 V for the 160 °C­annealed film, while it is greatly reduced to ~3.0 × 10⁻⁷ A cm⁻² for the 500 °C­annealed film. The current density decreases regularly with the annealing temperature, which agrees well with what was previously reported in MIS diodes [32].

Table 1 summarizes the physical and electrical properties of the ZrO2 films for different temperatures. The values of the leakage current and the areal capacitance demonstrate that the high temperature could effectively promote the formation of high quality ZrO2 films. The surface morphology of ther­

mally­annealed ZrO2 films was investigated by AFM, showing that films have a smooth surface and that a higher annealing

Table 1. Summary of the physical and electrical properties of solution­processed ZrO₂ films annealed at different temperature.

Temperature Thickness ZrO2 (nm) Leakage current

(A cm⁻²) at −3 V Capacitance

(nF cm⁻²) at 1 KHz Dielectric

constant (at 1 KHz) Roughness (nm)

500 °C 20.0 2.8 × 10⁻⁷ 398.8 9 0.38

400 °C 33.6 6.1 × 10⁻⁷ 295.2 11 0.32

300 °C 40.3 1.8 × 10⁻⁶ 219.1 8.8 0.25

200 °C 55.2 2.8 × 10⁻⁶ 161.5 10.1 0.24

160 °C 59.0 3.6 × 10⁻⁵ 117.1 7.8 0.21

Figure 2. High­resolution cross sectional TEM micrograph of (a) 160 °C annealed amorphous ZrO₂ films. AFM image for (b) 160 °C annealed ZrO₂ films (roughness: 0.212 nm), (c) pentacene based on PαMS­modified ZrO₂ surface.

J. Phys. D: Appl. Phys. 51 (2018) 115105

temperature is associated with a slightly rougher surface. This reveals that the sintering of precursor into a dense oxide film is more sufficient with the increasing of annealing temperature.

Furthermore, low­temperature processing reduces the inter­

diffusion of species between the dielectric and the substrate, hence minimizing the effect of the substrate on the properties

of the dielectric stack. As shown in figure 2(a), solution­pro­

cessed ZrO2 annealed at 160 °C has a very dense amorphous structure, which is consistent with the XRD diffraction pat­

tern, and the ZrO2 film thickness is ~59 nm. Figure 2(b) indi­

cates pinhole free and smooth surfaces in thermally annealed at 160 °C (roughness: 0.212 nm), which helps the subsequent

Figure 3. (a) XPS spectra of survey scan, (b) O 1s electron energy loss spectroscopy, (c) spectra of O 1s and (d) Zr 3d for ZrO₂ films with different temperature treatment.

Figure 4. (a) The actual photograph of flexible transparent OTFT sheet and schematic illustration of bottom­gate, top­contact OTFT device structure. The OTFT sheet is fully transparent in the visible light region. (b) I_DS–V_GS curves recorded under V_DS = −2 V, (c) IDS–V_DS curves of the FET recorded for various top­gate voltages at 1 V step.

Y Gong et al

deposition of the pentacene layer. As shown in figure 2(c), pentacene molecules form terrace­like grains because of the interaction between the adsorbed molecules and the underlying surface. The obtained grain size of pentacene is

~16 µm², which is considerably larger than what has typi­

cally been reported [27, 33]. The very large grain of the pen­

tacene film is attributed to the optimized growth parameters including three aspects: (1) a comparatively low deposition rate of 0.2–0.3 Å s⁻¹; (2) a substrate temperature of 50 °C; (3) a low surface energy modified by a PαMS layer. All three fac­

tors will contribute to the ordered growth of pentacene films.

3.2. Electronic structure of thermally annealed ZrO2 films To further understand the different electrical properties of ZrO2 films by thermal annealing, their chemical composition and electronic structures were investigated by XPS. As shown in figure 3(a), the ZrO2 film annealed at 500 °C or 160 °C shows obvious zirconium and oxygen peaks but no obvious nitrogen peak. Since the solutions from the precursor have a large amount of carbon and nitrogen elements, a small amount of nitrogen indicates the removal of the solvent. Figure 3(b) shows the O 1s spectra of ZrO2 films, from which their band gap can be extracted by using the methods reported by Xu et al [34]. The band gap of ZrO2 films are 5.4 eV (160 °C) and 5.6 eV (500 °C), respectively. Figures 3(c) and (d) show the O 1s and Zr 3d XPS spectra, respectively. By using a Gaussian distribution function, the O 1s spectrum was well fitted by two components. One is centered at ~530.0 eV representing the oxygen ions (O²⁻) combined with metal cations in the ZrO2 film. Another is centered at ~531.4 eV representing the O²⁻ ions vacancies and other bonded oxygen species such as O2, OH − at the film surface [30]. For convenience of discus­

sions, we defined O1A and O2A to denote the area of the O 1s peak centered at 530.0 eV and 531.4 eV, respectively. The 500 °C­annealed ZrO2 film sample shows a higher O1A (82%) with respect to the 160 °C­annealed sample (35%), which is assumed to be an important reason for the better electrical per­

formances observed in 500 °C­annealed ZrO2 film. In other words, the higher the percentage of M–O–M species in O 1s

XPS spectrum and the lower the percentage of M–OH species results in a denser film with less defect state, which is good for the electric performance for gate insulator as it decreases access for leakage current and improves the on/off ratio for TFT device. Figure 3(d) shows the Zr 3d spectra characterized by a spin–orbit doublets (d5/2 and d3/2) separated by ~2.4 eV, which implies the formation of ZrO2 films [35]. According to the XPS results, the 0.2 eV difference of the band gap between the 500 °C and 160 °C annealed ZrO2 films implies that the band structure is closely related to the oxygen content in the ZrO2 films. A smaller leakage current for the 500 °C annealed film is attributed to its larger band gap, as well as the lower defect level in the film.

3.3. Low temperature fabrication of ZrO2-OTFTs and their electrical properties

After the successful fabrication of high­quality amorphous ZrO2 films by the low temperature of 160 °C treatment with 1 h, we investigated the application of the solution­processed ZrO2 film in a flexible device. We fabricated a flexible OTFT device with a bottom­gate/top­contact configuration on a PET substrate. Figure 4(a) shows the photograph of the OTFT device on the flexible PET substrate under bending and the schematic diagram of the device structure used in our work,

Figure 5. Transfer characteristics of (a) ZrO₂­OTFTs with different channel lengths; (b) ZrO₂­OTFT measured from 1 V to −20 V with the same device.

Figure 6. Channel­length dependent variation of mobility and threshold voltage.

J. Phys. D: Appl. Phys. 51 (2018) 115105

indicating its flexibility and transparency. The ZrO₂ film (~60 nm) was deposited by solution processing and annealed at 160 °C. The flexibility of ZrO₂ film was first investigated by measuring its leakage current after it received bending with a 10 mm radius for 0, 50, 250, 500 and 1000 times. After bending 1000 times, we find that the leakage characteristics show only a slight increase. It is ~1.2 × 10⁻⁶ A cm⁻² at −3 V for the unbending ZrO₂ film, while it slightly increased to

~2.9 × 10⁻⁵ A cm⁻² after bending 1000 times. These results indicate that the ZrO₂ film can withstand bending at least 1000 times, and demonstrates potential applications for flexible wearable electronics. Figures 4(b) and (c) show the typical transfer and output curves of the flexible ZrO₂­OTFT with a channel width/length of 750 µm/50 µm. The low temperature fabricated ZrO₂­OTFT exhibited excellent device performance including a high field­effect mobility µ of 0.85 cm² (V · s)⁻¹, a low threshold voltage of −0.81 V, and a high on/off drain cur­

rent ratio of 2.4 × 10⁵. The negligible hysteresis between for­

ward and backward voltage scans indicates that the interface between the dielectric and the channel layer has a high quality with a low density of traps [36]. The carrier mobility of around 0.85 cm² V⁻¹ s⁻¹ is not so excellent when compared with that of the highest reported values. However, it is still comparable or even higher than most of the reported values for pentacene­

OTFTs. For example, the ZrO₂­OTFT in our work even shows a slightly higher hole mobility (0.85 cm² (V · s)⁻¹) than that of SiO₂­OTFT (0.6 cm² (V · s)⁻¹) [16]. The results show that the hole mobility of pentacene­OTFT is not mainly determined by the dielectric layer, and it is significantly affected by the interface quality and interface modification. Another reason is that the grain boundaries in the pentacene film greatly limit their fast transport in the channel, which is essentially respon­

sible for the mobility value of pentacene film. Figure 4(c) shows the typical output characteristics of the ZrO₂­OTFT.

Well­saturated I_DS–VDS curves are observed under the small operation voltage of 4 V. The linear behavior of the output curve at a low drain voltage (V_D < −2.5 V) indicates a small contact resistance between the pentacene channel and the S/D electrodes [37].

Figure 5(a) shows the typical drain current­gate voltage (I_DS–VGS) characteristics of the flexible pentacene OTFT with different channel lengths ranging from 50 to 300 µm. In voltage sweeping between 1 and −4 V, well­saturated IDS–VGS curves

were observed in each device, indicating the low operation voltage ability of flexible ZrO₂­OTFTs. Furthermore, I_DS–VGS

curves acquired at a gradually increasing gate voltage (figure 5(b)) show that breakdown does not occur even for negative bias voltage up to −20 V, demonstrating the high quality of 160 °C­annealed ZrO₂ films. According to the results shown in figures 4 and 5, we successfully realized the low opera­

tion voltage at least down to −4 V for flexible OTFT devices.

This is attributed to the successful fabrication of a high quality ZrO₂ film through low temperature annealing. In addition, low temperature fabrication can make the film effectively compat­

ible with flexible substrates and improve the interfacial adhe­

sion, which significantly enhances the viability of flexible electric devices.

Carrier mobility is the most important parameter deter­

mining the practical application of OTFTs. We investigated the devices with a channel length ranging from 50 µm to 300 µm for 160 °C treated devices. The mobility in the linear­

regime is calculated using the following equation:

µ= L

WCiVSD

∂I_SD

∂VG

(1),

where ISD is the source and drain current, W is the channel width, L is the channel length, Ci of 60 nm ZrO2 and 10 nm PαMS is 168 nF cm⁻², and VG is the gate voltage. The channel length dependent variation in mobility and threshold voltage is shown in figure 6. Threshold voltage is nearly independent of L, while µ increases with the channel length. This implies that contact resistance is an important parameter that affects the carrier mobility of the present OTFTs.

In addition to the static DC characteristics, the dynamic AC frequency response is also an important figure of merit to describe the overall OTFTs performance. Various mech anisms, such as contact resistance, carrier mobility, threshold voltage, charge trapping and channel length, have been proposed to affect the frequency response of OTFTs [38]. However, the effects of the dielectric layer, especially for the solution­

processed high­k oxide materials, have been rarely reported.

Figures 7(a) and (b) show the AC capacitance­frequency (C–f) characteristics of the OTFTs with ZrO2 and SiO2, respectively, from which the cutoff frequency fc under different bias gate voltages can be extracted by using the method proposed in our previous work [38]. The cutoff frequency fc was estimated

Figure 7. Capacitances versus frequency characteristics of (a) ZrO₂­OTFTs and (b) SiO₂­OTFTs under different gate bias voltages. The inset of figure (a) shows the schematic diagram of the comb­shaped OTFTs.

Y Gong et al

from the intersection point of the extrapolated fitting lines and the capacitance in the low­frequency region. For the low­

temperature fabricated high­k ZrO2 dielectric, we measured a cutoff frequency fc of 51.8 KHz at −3 V gate voltages. While for the SiO2­OTFTs, the cutoff frequency fc was only 20.2 KHz at −40 V gate voltages, indicating the high­k ZrO2 di electric allows us to both drastically reduce the driving voltage and achieve a high cutoff frequency. These advantages will be very favorable for its applications in low power consumption and high frequency operations in the future flexible OTFTs.

4. Conclusions

We carried out a systematic study on the low­temperature fab­

rication of high­quality ZrO₂ films by using a facile, scalable, patternable and inexpensive method based on a solution pro­

cess. The obtained ZrO₂ films exhibited a low leakage current down to 3.6 × 10⁻⁵ A cm⁻² at −3 V and a band gap to 5.4 eV.

The flexible ZrO₂­OTFTs showed excellent electrical perfor­

mances, such as low operation voltage (down to −4 V), high carrier mobility µ of 0.85 cm² (V · s)⁻⁾, and high on/off drain current ratio of 2.4 × 10⁵. In addition to these excellent static DC properties, the 160 °C annealed ZrO₂ films resulted in a cutoff frequency up to 51.8 KHz at a low bias gate voltage (−3 V). XPS measurements of the electronic energy band structure of the ZrO₂ films showed that the post­deposition treatment strongly affects the electronic structure of the ZrO₂ film, and consequently results in their different electrical per­

formances. Our work will contribute to the future development of the low power consumption and high frequency operations of flexible electronic devices.

Acknowledgment

This work is supported by the National Natural Science Foun­

dation of China (Contract No. 51472093, 51431006). XB Lu acknowledges the support of the Project for Guangdong Prov­

ince Universities and Colleges Pearl River Scholar Funded Scheme (2016). This work was also supported by Science and Technology Planning Project of Guangdong Province (No. 2016B090907001), the Guangdong Innovative Research Team Program (No. 2013C102), the Guangdong Provincial Key Laboratory of Optical Information Materials and Tech­

nology (Grant No. 2017B030301007) and the 111 Project.

ORCID iDs

Honglong Ning https://orcid.org/0000­0001­9518­5738 Xubing Lu https://orcid.org/0000­0002­3741­0500

References

[1] Kim J, Hassinen T, Lee W and Ko S 2017 Organ. Electron.

42 361–6

[2] Kim K, Song H, Shin K, Kim S and Park C E 2016 J. Phys.

Chem. C 120 5790–6

[3] Guo X, Xu Y, Ogier S, Ng T, Caironi M, Perinot A, Li L, Zhao J, Tang Wei and Radu A 2017 IEEE Trans. Electron Devices 64 1906–21

[4] Yan H, Chen Z, Zheng Y, Newman C, Quinn J, Dotz F, Kastler M and Facchetti A 2009 Nature 457 679–86 [5] Liu G, Liu A, Shan F, Meng Y, Shin B C, Fortunato E and

Martins R 2014 Appl. Phys. Lett. 105 113509 [6] Beaulieu M, Baral J, Hendricks N, Tang Y, Briseno A

and Watkins J 2013 ACS Appl. Mater. Interfaces 5 13096–103

[7] Yang C, Yoo E, Lee S, An T K and Kim S H 2016 Chin.

J. Phys. 54 471–4

[8] Gao Y, Zhang J and Li X 2015 J. Disp. Technol. 11 764–7 [9] Nishizawa R, Naka S, Okada H, Suzuki K and Kato K 2011

J. Appl. Phys. 50 01BC02

[10] Lee W­H, Wang C­C, Chen W­T and Ho J­C 2008 J. Appl.

Phys. 47 8955–60

[11] Dutta S, Pandey A, Yadav I, Thakur O P, Kumar A, Pal R and Chatterjee R 2013 J. Appl. Phys. 114 014105

[12] Li X, Xin E and Zhang J 2013 IEEE Trans. Electron Devices 60 3413–6

[13] Wang W, Fu K, Hu C, Li F N, Liu Z C, Li S Y, Lin F, Fu J, Wang J J and Wang H X 2016 Diam. Relat. Mater.

69 237–40

[14] Liu A, Liu G­X, Shan F­K, Zhu H­H, Shin B C, Lee W J and Cho C R 2013 Chin. Phys. Lett. 30 127301

[15] Fan C­L, Chiu P­C, Yang Y­H and Lin C­C 2010 Semicond.

Sci. Technol. 25 075006

[16] Kim L H, Kim K, Park S, Jeong Y J, Kim H, Chung D S, Kim S H and Park C E 2014 ACS. Appl. Mater. Interfaces 6 6731–8

[17] Kim C S, Kim W J, Jo S J, Lee S W, Lee S J and Baik H K 2006 Electrochem. Solid-State Lett. 9 G96–9

[18] Park S, Kim C­H, Lee W­J, Sung S and Yoon M­H 2017 Sci.

Eng. R 114 1–22

[19] Xu X, Cui Q, Jin Y and Guo X 2012 Appl. Phys. Lett.

101 222114

[20] Meng Y, Ling C, Xin R, Wang P, Song Y, Bu H, Gao S, Wang X, Song F and Wang J 2017 npj Quantum Mater.

2 1–5

[21] Rim Y S, Chen H, Kou X, Duan H S, Zhou H, Cai M, Kim H J and Yang Y 2014 Adv. Mater. 26 4273–8

[22] Wang B, Yu X, Guo P, Huang W, Zeng L, Zhou N, Chi L, Bedzyk M J, Chang R P H and Marks T J 2016 Adv.

Electron. Mater. 2 1500427

[23] Liu A, Guo Z, Liu G, Zhu C, Zhu H, Shin B, Fortunato E, Martins R and Shan F 2017 Adv. Electron. Mater.

3 1600513

[24] Pang H, Zhang F, Zeng M, Gao X, Qin M, Lu X, Gao J, Dai J and Li Q 2016 npj Quantum Mater. 1 16015

[25] Guo D, Miyadera T, Ikeda S, Shimada T and Saiki K 2007 J. Appl. Phys. 102 023706

[26] Guo D, Ikeda S and Saiki K 2006 J. Appl. Phys. 99 094502 [27] Shao Y, Zhang Y, He W, Liu C, Minari T, Wu S, Zeng M,

Zhang Z, Gao X and Lu X 2015 Semicond. Sci. Technol.

30 035005

[28] Wang S D, Minari T, Miyadera T, Tsukagoshi K and Aoyagi Y 2007 Appl. Phys. Lett. 91 203508

[29] Chen X, Wei X, Zhang H and Wang J 2017 Phys. Status Solidi a 214 1700064

[30] Liu G X, Liu A, Meng Y, Shan F K, Shin B C, Lee W J and Cho C R 2015 J. Nanosci. Nanotechnol. 15 2185–91 [31] Park Y M, Desai A and Salleo A 2013 Chem. Mater.

25 2571–9

[32] Zhu C, Liu A, Liu G, Jiang G, Meng Y, Fortunato E,

Martins R and Shan F 2016 J. Mater. Chem. C 4 10175–721 [33] Zhang Q, Xia G, Xia W, Zhou J and Wang S 2015 Synth. Met.

210 282–7

[34] Xu W et al 2017 Nanoscale Res. Lett. 12 270

J. Phys. D: Appl. Phys. 51 (2018) 115105

[35] Hwang S M, Lee S M, Park K, Lee M S, Joo J, Lim J H, Kim H, Yoon J J and Kim Y D 2011 Appl. Phys. Lett.

98 022903

[36] Na J Y, Kang B, Sin D H, Cho K and Park Y D 2015 Sci. Rep.

5 13288

[37] Kim B, Jang S, Prabhumirashi P L, Geier M L,

Hersam M C and Dodabalapur A 2013 Appl. Phys. Lett.

103 082119

[38] Lu X, Minari T, Liu C, Kumatani A, Liu J M and Tsukagoshi K 2012 Appl. Phys. Lett. 100 183308