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Review Cite This: Chem. Rev. 2018, 118, 5690−5754
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High‑k Gate Dielectrics for Emerging Flexible and Stretchable Electronics Binghao Wang,†,‡ Wei Huang,† Lifeng Chi,‡ Mohammed Al-Hashimi,§ Tobin J. Marks,*,† and Antonio Facchetti*,†,∥
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†
Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ‡ Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou 215123, China § Department of Chemistry, Texas A&M University at Qatar, PO Box 23874, Doha, Qatar ∥ Flexterra Corporation, 8025 Lamon Avenue, Skokie, Illinois 60077, United States ABSTRACT: Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for highk (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high-k dielectrics over low-k ones in TFT applications were elaborated. Next, after presenting the design and properties of high-k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high-k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.
CONTENTS 1. Introduction 1.1. Flexible and Stretchable Electronics 1.2. Scope of this Review 2. Properties of TFT Gate Dielectrics 2.1. Dielectric Properties 2.2. Electrical Properties 2.3. Thin-Film and Surface Properties 3. High-k Gate Dielectrics for Fabricating Thin-Film Transistors and Circuits 4. High-k Dielectric Materials 4.1. Inorganic Dielectrics 4.1.1. Group IIIA 4.1.2. Group IIIB 4.1.3. Group IVB 4.1.4. Group VB 4.1.5. Perovskites 4.2. Polymer Dielectrics 4.2.1. Poly(vinyl alcohol) © 2018 American Chemical Society
4.2.2. Cyanoethyl Polymers 4.2.3. Poly(vinylidene fluoride) and Its Copolymers 4.2.4. Other Polymers 4.3. Electrolyte Dielectrics 4.3.1. Polymer Electrolytes 4.3.2. Polyelectrolytes 4.3.3. Ionic Liquids 4.3.4. Ion-Gels 4.4. Hybrid Dielectrics 4.4.1. Self-assembled Nano-dielectrics 4.4.2. Inorganic/Polymer Blends 5. Flexible and Stretchable TFTs Based on High-k Dielectrics 5.1. Mechanical Flexibility of FSE 5.2. Organic TFTs
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Received: January 19, 2018 Published: May 22, 2018 5690
DOI: 10.1021/acs.chemrev.8b00045 Chem. Rev. 2018, 118, 5690−5754
Chemical Reviews 5.2.1. OTFTs Based on Inorganic Dielectrics 5.2.2. OTFTs Based on Polymer Dielectrics 5.2.3. OTFTs Based on Electrolyte Dielectrics 5.2.4. OTFTs Based on Hybrid Dielectrics 5.3. Metal Oxide TFTs 5.3.1. MOTFTs Based on Inorganic Dielectrics 5.3.2. MOTFTs Based on Electrolyte Dielectrics 5.3.3. MOTFTs Based on Hybrid Dielectrics 5.4. Quantum Dot TFTs 5.5. Carbon Nanotube TFTs 5.5.1. CNT TFTs Based on Inorganic Dielectrics 5.5.2. CNT TFTs Based on Electrolyte Dielectrics 5.6. Graphene TFTs 5.6.1. GTFTs Based on Inorganic Dielectrics 5.6.2. GTFTs Based on Electrolyte Dielectrics 5.7. Other 2D Nanosheet TFTs 6. Conclusions and Prospects Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References
Review
Field-effect transistors (FETs) are the key components of most modern electronic circuitry, and they are commonly utilized to amplify or to switch electronic analog and digital signals.17 The development of silicon-based metal-oxide-semiconductor (MOS) FETs for integrated circuits (microprocessors) has forever changed our everyday lives since their inception.18 In addition, low-temperature manufacturing with few constraints on the substrate size, material, and topology has made amorphous silicon (a-Si:H)-based thin-film transistors (TFTs) a mature technology for large-area display backplanes19−24 and provides a transistor architecture (vide infra) compatible with FSE circuits which complement traditional CMOS devices. Furthermore, for emerging FSE applications, TFTs based on semiconductors other than silicon, such as organics and metal oxides, offer more promising features in terms of carrier transport, low-cost solution-based processability, and mechanical stress tolerance. Importantly, for TFTs on flexible/ stretchable substrates, all TFT components including the gate dielectrics, electrodes, and substrates and along with the semiconductors should be designed in a way that the TFT can function under mechanical stress.
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1.2. Scope of this Review
There have been several excellent reviews reporting the use of gate dielectrics and semiconductors within the scope of FSE.1,6,10,25−28 However, considering the rapid development of new dielectrics and other electronic materials specifically designed for FSE, it is important to present a comprehensive and up-to-date overview specifically focusing on integrating the dielectric with emerging semiconductor materials. This is particularly relevant considering that TFTs for FSE must operate at low voltages to integrate with mechanically flexible and inexpensive power sources via radiofrequency (RF) fields and flexible batteries.29−31 In section 2, we first summarize the design characteristics and required physicochemical properties of gate dielectric materials. We review the TFT operating mechanism relevant to the gate dielectric properties and extend our discussion to the principal strategies for achieving low-voltage/power TFT operation, given their importance for FSE in section 3. Next, in section 4, we summarize emerging high dielectric constant (high-k) materials families (polymers, metal oxides, electrolytes, and hybrids), and compare/contrast them to silicon dioxide (k = 3.9)32 and the methodologies for fabricating thin dielectric films. In addition, we also discuss the effects of the deposition methodologies on the film morphology, texturing, and electrical and dielectric properties of the gate dielectrics. Section 5 summarizes FSE devices based on organic, metal oxide, quantum dots arrays, carbon nanotube, graphene, and other 2D semiconductors when integrated with the above gate dielectrics. In addition, we discuss the functionality of other components (substrates and electrodes) for FSE. Finally, we conclude the review with an outlook and challenges in section 6.
1. INTRODUCTION 1.1. Flexible and Stretchable Electronics
Flexible and stretchable electronics (FSE) is an emerging technology that aims at fabricating electronic circuits on “soft” substrates to enable mechanically flexible and stretchable optoelectronic devices.1−3 These FSE devices complement rigid ones and are expected to enable the emergence of valueadded products due to their lightweight, conformability, compatibility with textiles, tissues, and organs as well as improved comfort to users.4,5 Potential consumer electronics products include foldable displays (e-books, smartphones, and signage), photovoltaic cells, conformable sensors, soft/smart bionics, robotics, and wearable medical devices and are expected to show remarkable growth.6 Thus, analysis from Market and Market forecasts the global flexible and stretchable electronics market is expected to grow at a compound annual rate of 21.73% and 121.3%, respectively, over the next few years. Besides uniaxial bendable products, more challenging applications of FSE are conformable and deformable epidermal electronics and smart implants.7,8 A successful general strategy to enable FSE devices is to partition them into small functional entities spatially separated by stretchable interconnects (conductive elastomers, pop-up ribbons or serpentine wires).9−14 By employing such a design, even brittle electronics such as silicon complementary metal oxide semiconductor (CMOS) circuits can be integrated with high stretchability (>100%).15,16 Although several of these technologies have shown excellent stretchability as well as biaxial bending capacity, the combination of stretchability with high-density electronic functionality poses great challenges in advanced integrated circuits. Therefore, the use of electronic materials which enable devices that are inherently insensitive to mechanical stress is preferred. Furthermore, if these materials can be fabricated at relatively low temperatures, this could lead to substantial cost reduction since many polymers could serve as substrates.
2. PROPERTIES OF TFT GATE DIELECTRICS 2.1. Dielectric Properties
A dielectric is an electrical insulator that can be polarized by the application of an electric field. When a dielectric is placed in an electric field, electrical charges shift from their average equilibrium positions causing dielectric polarization. Thus, positive charges are displaced toward the field direction and negative charges shift in the opposite direction creating an 5691
DOI: 10.1021/acs.chemrev.8b00045 Chem. Rev. 2018, 118, 5690−5754
Chemical Reviews
Review
internal electric field that reduces the overall field within the dielectric itself.33 In general, a dielectric cannot polarize instantaneously in response to an electric field (E). Thus, a more general formulation of the polarization density (P) as a function of time (t) is
these polarizations in Figure 1 are highly dependent on the material types. If applicable, these polarization mechanisms are additive; however, these polarization types do not respond equally to the frequency of the time-varying electric field. Below, we very briefly summarize the origin of these processes. Electronic polarization (Figure 2a), operating at frequencies up to ∼1015 Hz, occurs in atoms when an electric field distorts the negative electronic cloud around the positive atomic nuclei of the material in the direction opposite to the field. In most atomic crystals, with atoms well locked into a lattice structure, the electronic polarization dominates the dielectric properties.41 Ionic polarization (Figure 2b) typically occurs in ionic compounds and responds to field frequencies up to ∼1012 Hz. Upon applying an external field, the cations and anions are displaced in opposite directions, which leads to an induced polarization. Orientational polarization (Figure 2c) arises when there is a permanent dipole moment in the material which rearranges upon the application of an electric field.42 The reorientation of permanent molecular dipole moments and dipolar moieties (Figure 2d) appended to polymers in the direction of the applied field causes polarization. Therefore, for most polymeric dielectrics, the orientation polarization together with chain relaxation dominate the dielectric behavior and their relaxation frequencies are at an intermediate-frequency range (∼108−1010 Hz). The electrolyte dielectric consists of large concentrations of free and condensed counterions (Figure 2e). At frequencies in the MHz range, free counterions respond to the applied field and polarize on the scale of the correlation length. In addition, the polar side chains add a further polarization mechanism that also contributes in this frequency range while the condensed counterions polarize in a nonuniform way along the polyelectrolyte chain backbone in the kHz frequencies range.38 Interfacial and electrode/EDL polarization can only contribute to the dielectric response at low frequencies, with cutoff frequency regions at 400 °C) to enable film densification and impurity removal. Table 1 summarizes the properties of various inorganic gate dielectrics fabricated by various processing methods, and in the following sections, we report details for the most important inorganic families. 4.1.1. Group IIIA. 4.1.1.1. Aluminum Oxide. Aluminum oxide (Al2O3, k ∼ 9) is one of the most widely studied dielectric materials, and atomic layer deposition (ALD) can produce Al2O3 films of excellent quality using a variety of metal−organic precursors and in relatively short cycle times.76 ALD is a mature technique for depositing high-quality thin films, largely adopted
4.1. Inorganic Dielectrics
High-k inorganic dielectrics are essential components of current generation and future electronic circuits.27,70 The most common inorganic TFT gate dielectrics include metal oxides (MOs), nitrides (Si3N4, AlN), perovskites, and hybrids comprising them. The metal elements used in these compositions usually belong to the groups IIA, IIIA, IIIB, IVB, and VB considering that alkalimetal oxides and alkaline earth metal oxides are very hygroscopic and suffer from substantial stability limitations.71,72 Thus, they are seldom studied as single metal components in gate dielectrics. In typical inorganic dielectrics, the band gap (Eg) is inversely proportional to the dielectric constant (Figure 8a); thus, dielectric films having a lower Eg should be preferable to enhance k. However, a large Eg is beneficial to suppress charge injection from the electrodes and to reduce charge generation due to thermal/photoexcitation processes.73,74 Furthermore, to achieve reliable TFTs, the band offsets of the dielectric valence band maximum (VBM) in a p-type transistor and the conduction band minimum (CBM) in an n-type transistor should be >1 eV relative to those of the semiconductor, to keep Schottky emission leakage low enough.73,75 Figure 8b shows the energy bands of semiconductors and the calculated conduction band and valence band offsets of the indicated dielectrics. All the values are normalized to the VBM of ZnO, and the dotted line indicates the minimum of the 1 eV requirement for the conduction band offset. From Figure 8b it is clear that all the oxide dielectrics 5696
DOI: 10.1021/acs.chemrev.8b00045 Chem. Rev. 2018, 118, 5690−5754
Chemical Reviews
Review
Table 1. Summary of Growth Method and Dielectric Properties for Representative Inorganic Dielectrics Materials Al2O3
Nd:Al2O3 Zr:Al2O3 Ti0.45Al0.55Oy Ga2O3 Ga1.34W0.33O3‑δ Sc2O3 Gd0.5Sc1.5O3 Y2O3
Y1.2Sc0.8O3 La2O3 LaAlOx LaLuO3 HfLaOx Pr2O3
Method
Ci (nF cm−2)
k
Jleak (A cm−2) at 2 MV cm−1a −7
Eb (MV cm−1)
σRMS (nm)
Yieldb
Yearref
33 150 100 140 115 44
201 52 60 54 75 625
7.5 8.8 6.8 8.5 9.7 7.1
1 × 10 @1.52 1 × 10−8 5 × 10−
[email protected] 3 × 10−
[email protected] 2.5 × 10−6 @0.26 1 × 10−3
7.5 2.7 − ∼5.0 − −
0.4 3.3 0.3 ∼4.0 0.2 0.3
High Low Low Medium High High
200483 201484 201485 201186 201387 201488
150 350
35 85
180 70
6.1 7.1
1 × 10−8 1 × 10−7
5.4 6.0
0.1 0.3
High High
201589 201646
380 RT 150 120 150 200 400 300 350 300 280 RT RT
16 200 180 420 40 55 80 15 23 15 30 150 100
380 42 180 96 204 193 116 944 450 1239 38 94.4 129
7.9 9.5 7.1 13 9.2 12 10.5 16 11 21 12 16 14.6
1 × 10−8 2 × 10−
[email protected] 1 × 10−8 4 × 10−9 1 × 10−9 2.4 × 10−6 2 × 10−4 6 × 10−9 4.0 × 10−10 1 × 10−8 2 × 10−8 3.5 × 10−
[email protected] 6 × 10−4
6.6 − 7.0 5.0 ∼7.0 − 5.2 >6.7 9.0 5.3 4.5 >3.0 −
High Medium High High High High High High High High Low Low Low
201590 201591 201589 201592 201393 201594 201595 201496 201597 201496 200498 200999 2006100
Spin-coating + Annealing Spray-pyrolysis
400 400
188 107
75 133
15.9 16.2
8.6 × 10−7 −
− −
Medium High
2012101 2011102
Spin-coating + Annealing ALD Spin-coating + Annealing ALD PLD Spin-coating + Annealing MOCVD Sputtering + RTA
500 250 500 225 450 500 300 RT + 700 310 RT RT + 700 RT RT RT 200 250 200 RT 325 600 400 350 350 150 150 350
64 15 137 66 12 60 30 11
133 1239 75 134 2360 178 1000 1432
9.6 21 11.6 10 32 22 26 17.8
5 1 2× 1 3 3 5 1
× 10−6 × 10−8 10−
[email protected] × 10−7 × 10−6 × 10−6 × 10−5 × 10−6
3.8 4.2 1.7 4.5 − 4.7 3.5
