Article pubs.acs.org/JPCC
Surface Modification on Solution Processable ZrO2 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-k dielectrics have been fabricated by chemical solution processes. The ZrO2 thin films annealed at various temperatures were studied from microstructure properties to electric properties in detail. The dielectric film annealed at 700 °C features a smooth surface, low leakage current density (1.89 × 10−6 A/cm2 @ −3 MV/cm) and high dielectric constant (20). Organic thin film transistors (OTFTs) based on ZrO2 with different surface modifications were characterized to investigate the interfacial effects between the high-k dielectric and organic semiconductors. The OTFTs with poly(α-methylstyrene) (PαMS) coated ZrO2 show much higher carrier mobility and on/off ratio than those with bare-ZrO2 or 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/off ratio (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-k ZrO2 dielectric films offer great potentials for low-cost organic electronic devices, especially for low voltage organic electronic devices. have been employed to replace the SiO2 layer11 so 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 investigated.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
1. INTRODUCTION Different from the conventional thin film transistors based on inorganic semiconductors, organic thin-film transistors (OTFTs) have the advantages of being low cost, flexible, lightweight, etc.1,2 In the past two decades, significant progress has been made for applications of OTFTs in organic lightemitting diodes (OLED),3 radio frequency identification (RFID) tags,4 sensors,5 electronic papers,6 portable electronics,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.8−10 To reduce the operation voltage, high-k insulators © 2016 American Chemical Society
Received: April 10, 2016 Published: April 19, 2016 9949
DOI: 10.1021/acs.jpcc.6b03638 J. Phys. Chem. C 2016, 120, 9949−9957
Article
The Journal of Physical Chemistry C
Figure 1. Cross-section HRTEM images of the solution processed ZrO2 films annealed at different temperatures as marked.
complicated, coming from surface roughness, surface energy, surface polarity, dielectric constant, etc.19−22 Among the candidates, ZrO2 has been regarded as one of the most promising high-k materials to replace SiO2 because 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 the film fabricated by solution-processes is still not comparable with those deposited by vacuum methods. How to obtain high quality dielectric film 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 ZrO2 films and OTFTs with a low leakage current, much reduced operation voltage, and high on/off ratio. Our work demonstrates that the solution-processed high-k ZrO2 films will be promising for applications in future low-cost and high-performance organic electronic devices.
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 reactions, the concentration of solution increased due to the oxolation and three-dimensional metal-oxide-metal (M-O-M) grid gel was formed.29 The deposited films 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 0.2 μm pore size PTFE membrane syringe filter prior to spin coating. Before coating, the substrates (heavily borondoped 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 ZrO2 films 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, ZrO2 films 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
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 solvent N,N-dimethylformamide (DMF) with stabilizing agents. The sol−gel processes include the hydrolysis and condensation process:28 Zr(AcAc)4 + ROH + H 2O → AcAc−H + RO−Zr−OH (1)
−Zr−OH + HO−Zr− → −Zr−O−Zr−O···Zr− + H 2O (2) 9950
DOI: 10.1021/acs.jpcc.6b03638 J. Phys. Chem. C 2016, 120, 9949−9957
Article
The Journal of Physical Chemistry C
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 ZrO2 films annealed at different temperatures.
deposited ZrO2 films were postannealed in a rapid thermal furnace. Figure 1 shows the cross-sectional HRTEM images of the ZrO2 films annealed at 400, 500, 600, and 700 °C, respectively. The thicknesses of the ZrO2 films are all around 11.5 nm after three spin-coating cycles, indicating good controllability in the spin-coating processes. We note that an interfacial SiO2 layer exists between the heavily doped p-Si and the ZrO2 layer 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 ZrO2 films start to crystallize for an annealing at 500 °C, and films annealed at 600 and 700 °C show clear poly crystalline structures. The surfaces of the ZrO2 films were investigated by AFM with a scanning area of 4 μm × 4 μm (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-annealed film, the leakage current is ∼3.0 × 10−4 A/cm2 at the electric field of 3 MV/cm. As the annealing temperature increases, the leakage current significantly decreases and down to ∼2.0 × 10−6 A/cm2 at 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 temperature 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 ZrO2 film 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
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, ZrO2 modified 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 morphologies 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 (
