Elsevier

Carbon

Volume 163, 15 August 2020, Pages 145-153
Carbon

Air-stable N-type printed carbon nanotube thin film transistors for CMOS logic circuits

https://doi.org/10.1016/j.carbon.2020.03.012Get rights and content

Abstract

The lack of long-term air-stable and solution-processed n-doping methods for printed single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) limits their integrations into printed complementary metal-oxide-semiconductor (CMOS) circuits. In this paper, a new chemically modified epoxy amine ink was developed as the chemical dopant and encapsulant to enable the uniform n-type SWCNT-TFTs with long-term air stability (6 months). The epoxy amine inks were dropped onto the printed p-type TFT device channels in a single-step solution process. As a result, printed top-contact n-type SWCNT-TFTs were obtained with well-balanced electrical chararcteristics comparable to their p-type counterparts. The matched p-type and n-type SWCNT-TFTs were thus integrated into the printed CMOS inverters and NAND gates, which have both achieved proper logic operation at supply voltages below 1 V. In particular, the CMOS inverters could operate with VDD down to 0.3V with associated peak power consumption of 0.06 μW, showing full rail-to-rail output swings with voltage gains up to 22, trip voltages of ∼VDD/2, and maximum noise margin of 0.42 V at VDD = 1.1 V (∼76.4% of VDD/2). Furthermore, the static characteristics of CMOS inverters could be maintained for 3 months with negligible changes, proving the feasibility of this long-term air-stable n-doping method.

Graphical abstract

A novel epoxy-amine ink was developed as the chemical dopant and encapsulant to enable the uniform and long-term air-stable n-type SWCNT-TFTs, which showed well-balanced electrical properties comparable to their p-type counterparts. These matched p-type and n-type SWCNT-TFTs could be intergrated into the high air-stable and good-performance printed CMOS inverters and NAND gates. The CMOS inverters could operate with VDD down to 0.3 V with associated peak power consumption of 0.06 μW, showing full rail-to-rail output swings with voltage gains up to 22, trip voltages of ∼VDD/2, and maximum noise margin of 0.42 V at VDD = 1.1 V (∼76.4% of VDD/2). In addition, the NAND gates can work well without any voltage loss at input voltages of 1 V.

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Introduction

Semiconducting single-walled carbon nanotubes (sc-SWCNT) show excellent chemical and physical stability, high carrier mobility, good solubility, and low processing temperatures. Thus, they have become promising candidates to develop printed thin film transistors (TFTs). In particular, printed TFTs based on semiconducting carbon nanotubes have been explored for many applications, such as integrated circuits [[1], [2], [3]], photodetectors [4], neuromorphic devices [5], pressure sensors [6], active matrix for sensors [7] and electrochromic displays [8]. To achieve low static power consumption and noise-resistant devices essential for the realization of large-scale integrated circuits, logic circuits are typically implemented in the complementary metal oxide semiconductor (CMOS) configuration, consisting of two balanced unipolar n-type and p-type transistors.

Although sc-SWCNT are intrinsically ambipolar, transistors made of sc-SWCNTs usually exhibit p-type characteristics in air due to two reasons: one is the easy adsorption of ambient oxygen and moisture [9] and the other is the uneven electrons and holes charge injection at the source and drain electrodes. Therefore, a number of strategies have been developed to enhance the electrical properties and stability of n-type SWCNT-TFTs for high-performance CMOS logic circuits as well as a modern microprocessor [10,11], such as atomic layer deposition (ALD) of high-κ metal oxides [[12], [13], [14]]/nitrides [15] on sc-SWCNT thin films, low work function source/drain contacts [16,17], and chemical doping, i.e. polyethylenimine [18,19](PEI) or reducing small molecules [[20], [21], [22], [23], [24]]. However, the existing n-type doping strategies for SWCNT-TFTs suffer from environmental instability and/or process complexity, which hinders the realization of large-scale integrated circuits. For example, low work function metals can be easily oxidized in ambient condition. ALD deposited metal oxides or nitrides can provide a good encapsulation which lead to highly air-stable systems [14,15]. Nonetheless, it is not easy to selectively achieve printed n-type TFTs and CMOS circuits by the use of ALD technique. In contrast, chemical doping can be easily implemented due to the solution nature of commonly used chemical dopants. Nevertheless, doping agents (PEI [19] and hydrazine [25]) exhibit most of the time poor environmental stability. Although some progress have been made using air-stable molecules, such as benzyl violene [23], β-Nicotinamide adenine dinucleotide, reduced dipotassium salt (NADH) [26] and ethanolamine (EA) [21], the resulting n-type devices still showed significant performance degradation in air due to the surface adsorption of O2 and H2O. To solve this issue, chemical doping combined with ALD encapsulation [19] or top-gate structure [27] have been developed to achieve n-type devices with excellent air stability. Recently, SU-8, an epoxy photoresist [28,29], as both doping and encapsulating material has shown good potential for air-stable n-type SWCNT-TFTs, avoiding the use of complex ALD approach. This n-doping process, originating from crosslinking photoninitiators [28] (Triarylium-solphonium salts), show air stability over 100 days with reasonable ON current degradations for n-type devices [29]. However, it is worth mentioning that long UV exposure might lead to the total conversion of the photon acid generator and thus to deteriorate performance of the n-type SWCNT devices over time.

In this paper, we report a novel doping approach to obtain printed n-type SWCNT-TFTs with long-term stability in air. An epoxy-based ink was developed as both a doping and encapsulating material, and was simply formulated by mixing an epoxy resin with a crosslinking agent containing n-doping amine groups. As-prepared printed top-contact p-type TFT devices with optimized high-k dielectric exhibited good performance with the effective mobility up to 8.9 cm2/V·s, ON/OFF ratio ∼106, small hysteresis (170 mV), and subthreshold swing (SS) as low as 92 mV/dec at low operating voltage (±1 V). With this doping ink, printed bottom-gate/top-contact p-type SWCNT-TFTs could be converted into the air-stable n-type SWCNT-TFTs with well-balanced and symmetric performance (comparable to their p-type counterparts) in a single step of solution process. The n-type SWCNT-TFTs exhibited air stability with negligible ON current degradation and VON shift for over 6 months long period. Matching n-type and p-type TFTs electrical characteristics enable to fabricate low-voltage CMOS inverters and NAND logic gates. The CMOS inverters can operate correctly at a supply voltage (VDD) down to 0.3 V with a voltage gain as high as 22 at VDD = 1.1 V, a trip voltage of ∼VDD/2 and a noise margin of 0.42 V (76.4% of VDD/2). More importantly, CMOS inverters show good air stability after 3 months with negligible static performance degradation. In addition, the NAND gates can work well without any voltage loss at input voltages of 1 V.

Section snippets

Materials and instruments

Arc discharge carbon nanotubes were purchased from Carbon Solutions (USA). PFIID (Isoindigo-based poly(9,9-dioctylfluorene) derivative, synthesized by ourselves [30]. Silver nanoparticles ink was obtained from Advanced Nano Products Co, DGP 45HTG. The epoxy amine ink used for n-doping and encapsulation was purchased from Xuzhou (China) Zhongyan Technology. The ink contains two compounds: the epoxy resin (128) and the cross-linking agent (ZY-F51). Optical absorption measurements were performed

Printed p-type SWCNT-TFTs

Standalone electronic devices are likely to be powered by thin film batteries [32] or solar cells [33], requiring transistors to operate at a fraction of a few volts. To achieve low-voltage and high-performance SWCNT-TFTs and circuits [34,35], high capacitance dielectrics are commonly used and can be obtained with high dielectric constant (high-k) materials and/or ultrathin layers. In this study, two kinds of dielectrics were therefore evaluated to develop p-type and n-type SWCNT-TFTs:

Summary

We have developed an epoxy amine ink as an efficient n-dopant and encapsulant for sc-SWCNT thin films that enables the selective conversion from p-type to n-type transistor behavior. As-prepared printed p-type TFT devices using 10 nm HfOx/AlOx thin films as dielectric layers exhibited good performance with an effective mobility as high as 8.9 cm2/V·s, high ON/OFF ratio (∼106), small hysteresis (170 mV), and small SS (∼92 mV/dec) at low operating voltage (±1 V). For optimized n-doping

CRediT authorship contribution statement

Miaomiao Wei: Formal analysis, Data curation, Writing - original draft. Malo Robin: Formal analysis, Data curation, Writing - original draft. Luis Portilla: Conceptualization, Formal analysis. Yunfei Ren: Investigation, Methodology. Shuangshuang Shao: Data curation. Lan Bai: Formal analysis. Yu Cao: Writing - review & editing. Vincenzo Pecunia: Writing - review & editing. Zheng Cui: Writing - review & editing. Jianwen Zhao: Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Miaomiao Wei and Malo Robin contributed equally to this work. This work was supported by Natural Science Foundation of China (61874132), the CAS President’s International Fellowship for Postdoctoral Researchers (2019PT0020), National Key Research and Development Program of China (2016YFB0401100), Key Research Program of Frontier Science of Chinese Academy of Sciences (QYZDB-SSW-SLH031), and Basic Research Program of Suzhou Institute of Nanotech and Nano-bionics (Y5AAY21001). Cooperation Project

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    Miaomiao Wei and Malo Robin contributed equally to this work.

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