Transparent Circuits Research Articles

Hybrid-type complementary inverters using semiconducting single walled carbon nanotube networks and In-Ga-Zn-O nanofibers

We constructed complementary inverters utilizing solution-processed semiconducting single-walled carbon nanotube (scSWCNT) random networks and electrospun In-Ga-Zn-O (IGZO) nanofibers as p-type and n-type thin-film transistor (TFT) channels, respectively. The IGZO nanofiber n-type TFT and scSWCNT random network p-type TFTs show an on-off current ratio of 2.82 × 105 and 1.38 × 105, a threshold voltage of −7.60 and 9.50 V, a subthreshold swing of 380.13 and 391.01 mV/dec, and field effect mobilities of 1.96 and 5.67 cm2/V s for electrons and holes, respectively. In addition, these hybrid-type inverters consisting of n-channel TFTs and p-channel TFTs exhibit excellent complementary metal-oxide-semiconductor (CMOS) operation. Therefore, we expect that the hybrid CMOS-type inverters based on scSWCNT random networks and IGZO nanofibers can be innovative electronic devices for transparent and flexible digital logic circuits.

Highly transparent and flexible circuits through patterning silver nanowires into microfluidic channels.

The development of flexible and transparent devices requires completely transparent and flexible circuits (TFCs). To overcome the disadvantages of the previously reported TFCs that are partially transparent, lacking pattern control, or labor consuming, we achieve true TFCs via a facile process with precise pattern control, exhibiting concurrent high transparency, conductivity, flexibility, stretchability, and robustness. A highly transparent and flexible conductive film is first made through spin coating silver nanowires (AgNWs) onto polydimethylsiloxane (PDMS), and demonstrates simultaneous high transparency (90.86%) and low sheet resistance (3.22 Ω sq-1). Taking advantage of microfluidic technology, circuits with ultraprecise and complex patterns from the microscale to milliscale are obtained through spin coating of AgNWs into microfluidic channels on PDMS. Without elaborate processing, this method may be suitable for mass production, which would contribute enormously to potential applications in wearable medical equipment and transparent electronic devices.

Interface Engineering with MoS2 -Pd Nanoparticles Hybrid Structure for a Low Voltage Resistive Switching Memory.

Metal oxide-based resistive random access memory (RRAM) has attracted a lot of attention for its scalability, temperature robustness, and potential to achieve machine learning. However, a thick oxide layer results in relatively high program voltage while a thin one causes large leakage current and a small window. Owing to these fundamental limitations, by optimizing the oxide layer itself a novel interface engineering idea is proposed to reduce the programming voltage, increase the uniformity and on/off ratio. According to this idea, a molybdenum disulfide (MoS2 )-palladium nanoparticles hybrid structure is used to engineer the oxide/electrode interface of hafnium oxide (HfOx )-based RRAM. Through its interface engineering, the set voltage can be greatly lowered (from -3.5 to -0.8 V) with better uniformity under a relatively thick HfOx layer (≈15 nm), and a 30 times improvement of the memory window can be obtained. Moreover, due to the atomic thickness of MoS2 film and high transmittance of ITO, the proposed RRAM exhibits high transparency in visible light. As the proposed interface-engineering RRAM exhibits good transparency, low SET voltage, and a large resistive switching window, it has huge potential in data storage in transparent circuits and wearable electronics with relatively low supply voltage.

Large‐Area Chemical Vapor Deposited MoS2 with Transparent Conducting Oxide Contacts toward Fully Transparent 2D Electronics

Abstract2D semiconductors are poised to revolutionize the future of electronics and photonics, much like transparent oxide conductors and semiconductors have revolutionized the display industry. Herein, these two types of materials are combined to realize fully transparent 2D electronic devices and circuits. Specifically, a large‐area chemical vapor deposition process is developed to grow monolayer MoS2 continuous films, which are, for the first time, combined with transparent conducting oxide (TCO) contacts. Transparent conducting aluminum doped zinc oxide contacts are deposited by atomic layer deposition, with composition tuning to achieve optimal conductivity and band‐offsets with MoS2. The optimized process gives fully transparent TCO/MoS2 2D electronics with average visible‐range transmittance of 85%. The transistors show high mobility (4.2 cm2 V−1 s−1), fast switching speed (0.114 V dec−1), very low threshold voltage (0.69 V), and large switching ratio (4 × 108). To our knowledge, these are the lowest threshold voltage and subthreshold swing values reported for monolayer chemical vapor deposition MoS2 transistors. The transparent inverters show fast switching properties with a gain of 155 at a supply voltage of 10 V. The results demonstrate that transparent conducting oxides can be used as contact materials for 2D semiconductors, which opens new possibilities in 2D electronic and photonic applications.

Flexible Ionic‐Electronic Hybrid Oxide Synaptic TFTs with Programmable Dynamic Plasticity for Brain‐Inspired Neuromorphic Computing

Emulation of biological synapses is necessary for future brain-inspired neuromorphic computational systems that could look beyond the standard von Neuman architecture. Here, artificial synapses based on ionic-electronic hybrid oxide-based transistors on rigid and flexible substrates are demonstrated. The flexible transistors reported here depict a high field-effect mobility of ≈9 cm2 V-1 s-1 with good mechanical performance. Comprehensive learning abilities/synaptic rules like paired-pulse facilitation, excitatory and inhibitory postsynaptic currents, spike-time-dependent plasticity, consolidation, superlinear amplification, and dynamic logic are successfully established depicting concurrent processing and memory functionalities with spatiotemporal correlation. The results present a fully solution processable approach to fabricate artificial synapses for next-generation transparent neural circuits.

Highly transparent supercapacitors based on ZnO/MnO2 nanostructures.

The recent rapid development of transparent electronics, notably displays and control circuits, requires the development of highly transparent energy storage devices, such as supercapacitors. The devices reported to date utilize carbon-based electrodes for high performance, however at the cost of their low transparency around 50%, insufficient for real transparent devices. To overcome this obstacle, in this communication highly transparent supercapacitors were fabricated based on ZnO/MnO2 nanostructured electrodes. ZnO served as an intrinsically transparent skeleton for increasing the electrode surface, while MnO2 nanoparticles were applied for high capacitance. Two MnO2 synthesis routes were followed, based on the reaction of KMnO4 with Mn(Ac)2 and PAH, leading to the synthesis of β-MnO2 with minority α-MnO2 nanoparticles and amorphous MnO2 with embedded β-MnO2, respectively. The devices based on such electrodes showed high capacitances of 2.6 mF cm-2 and 1.6 mF cm-2, respectively, at a scan rate of 1 mV s-1 and capacitances of 104 μF cm-2 and 204 μF cm-2 at a very high rate of 1 V s-1, not studied for transparent supercapacitors previously. Additionally, the Mn(Ac)2 devices exhibited very high transparencies of 86% vs. air, far superior to other transparent energy storage devices reported with similar charge storage properties. This high device performance was achieved with a non-acidic LiCl gel electrolyte, reducing corrosion and handling risks associated with conventional highly concentrated acidic electrolytes, enabling applications in safe, wearable, transparent devices.

Ultra-smooth glassy graphene thin films for flexible transparent circuits.

Large-area graphene thin films are prized in flexible and transparent devices. We report on a type of glassy graphene that is in an intermediate state between glassy carbon and graphene and that has high crystallinity but curly lattice planes. A polymer-assisted approach is introduced to grow an ultra-smooth (roughness, <0.7 nm) glassy graphene thin film at the inch scale. Owing to the advantages inherited by the glassy graphene thin film from graphene and glassy carbon, the glassy graphene thin film exhibits conductivity, transparency, and flexibility comparable to those of graphene, as well as glassy carbon-like mechanical and chemical stability. Moreover, glassy graphene-based circuits are fabricated using a laser direct writing approach. The circuits are transferred to flexible substrates and are shown to perform reliably. The glassy graphene thin film should stimulate the application of flexible transparent conductive materials in integrated circuits.

The Current Behaviors of the Amorphous In-Ga-Zn-O Thin-Film Transistor Under Varying Illumination Conditions

In this study, the time response behavior of the amorphous indium-gallium zinc-oxide (a-IGZO) thin-film transistors (TFTs) to the illumination pulse is analyzed. We modified the previously proposed fitting formula by changing the fitting parameters from constant to time dependent. The mechanism for the response behaviors is proposed based on the analysis of the fitting parameters with respect to measurement time and under different light intensities. The single-pulse measurement results and their corresponding fitting parameters is used as the database to predict the current behaviors of the TFTs under multi-pulse illumination. The predicted and measured results fit fairly well. The method to formulize the time response for the a-IGZO TFT under varying situations of illumination is thus developed, which can be important in the design and simulation of transparent electronic circuits.

Transparent megahertz circuits from solution-processed composite thin films.

Solution-processed amorphous oxide semiconductors have attracted considerable interest in large-area transparent electronics. However, due to its relative low carrier mobility (∼10 cm(2) V(-1) s(-1)), the demonstrated circuit performance has been limited to 800 kHz or less. Herein, we report solution-processed high-speed thin-film transistors (TFTs) and integrated circuits with an operation frequency beyond the megahertz region on 4 inch glass. The TFTs can be fabricated from an amorphous indium gallium zinc oxide/single-walled carbon nanotube (a-IGZO/SWNT) composite thin film with high yield and high carrier mobility of >70 cm(2) V(-1) s(-1). On-chip microwave measurements demonstrate that these TFTs can deliver an unprecedented operation frequency in solution-processed semiconductors, including an extrinsic cut-off frequency (f(T) = 102 MHz) and a maximum oscillation frequency (f(max) = 122 MHz). Ring oscillators further demonstrated an oscillation frequency of 4.13 MHz, for the first time, realizing megahertz circuit operation from solution-processed semiconductors. Our studies represent an important step toward high-speed solution-processed thin film electronics.

Fabrication of Transparent Multilayer Circuits by Inkjet Printing.

Conductive microcables embedded in a transparent film are fabricated by inkjet printing silver-nanoparticle ink into a liquid poly(dimethylsiloxane) (PDMS) precursor substrate. By controlling the spreading of the ink droplet and the rheological properties of the liquid substrate, transparent multilayer circuits composed of high-resolution embedded cables are achieved using a commercial inkjet printer. This facile strategy provides a new avenue for inkjet printing of highly integrated and transparent electronics.

Two-Dimensional Atomic Crystals: Paving New Ways for Nanoelectronics

Two-dimensional (2D) atomic crystals are attractive for use in next-generation nanoelectronics, due to their unique performances, which may lead to the resolution of the technological and fundamental challenges in semiconductor industry. Based on the introduction of 2D atomic crystal-based transistors and ambipolar behavior, the review presents a brief summary of 2D atomic crystal integration circuits, including memory, logic gate, amplifier, inverter, oscillator, mixer, switch and modulator. The devices show promising performances for the application in future nanoelectronics. In particular, the 2D atomic crystals, such as graphene, demonstrate good compatibility with the existing semiconductor process. The quaternary digital modulations have been achieved with flexible and transparent all-graphene circuits. Moreover, the heterojunction based on 2D atomic crystals may enable new devices beyond conventional field-effect transistors. The results make us be optimistic that practical 2D atomic crystal technologies with complex functionality will be achieved in the near future. Therefore, 2D atomic crystals are paving new ways for nanoelectronics.

Carbon Nanotube Driver Circuit for 6 × 6 Organic Light Emitting Diode Display.

Single-walled carbon nanotube (SWNT) is expected to be a very promising material for flexible and transparent driver circuits for active matrix organic light emitting diode (AM OLED) displays due to its high field-effect mobility, excellent current carrying capacity, optical transparency and mechanical flexibility. Although there have been several publications about SWNT driver circuits, none of them have shown static and dynamic images with the AM OLED displays. Here we report on the first successful chemical vapor deposition (CVD)-grown SWNT network thin film transistor (TFT) driver circuits for static and dynamic AM OLED displays with 6 × 6 pixels. The high device mobility of ~45 cm2V−1s−1 and the high channel current on/off ratio of ~105 of the SWNT-TFTs fully guarantee the control capability to the OLED pixels. Our results suggest that SWNT-TFTs are promising backplane building blocks for future OLED displays.

Hydrogen centers and the conductivity ofIn2O3single crystals

Transparent conducting oxides, such as In${}_{2}$O${}_{3}$, are optically transparent wide band gap semiconductors with high electrical conductivity and are used in applications such as transparent electrical circuits and flat panel displays. Some previous research has argued that conductivity in In${}_{2}$O${}_{3}$ is caused mainly by oxygen vacancies, as in other transparent conducting oxides. Using infrared absorbtion spectroscopy on single-crystal In${}_{2}$O${}_{3}$, the authors show that, consistent with theory, interstital hydrogen forms shallow donors, which make an important contribution to the $n$-type conductivity.

Transparent capacitors based on nanolaminate Al2O3/TiO2/Al2O3 with H2O and O3 as oxidizers

Transparent capacitors with nanolaminate Al2O3/TiO2/Al2O3 (ATA) hybrid dielectrics have been prepared on quartz glass by atomic layer deposition. The maximal capacitance density of 14 fF/μm2 at 1 kHz was obtained. Moreover, an ultralow leakage current density of 2.1 × 10−9 A/cm2 at 1 V was realized by using O3 as the oxidizer. Fowler-Nordheim tunneling is the main mechanism of the leakage current at high fields, while several conduction mechanisms coexist at low fields. The AlZnO/ATA/AlZnO transparent capacitors exhibit an average optical transmittance of more than 80% in the visible range, which serve as good candidates for integration in transparent circuits.

Semi-transparent silver electrodes for flexible electronic devices prepared by nanoimprint lithography

The preparation of mechanically flexible and optically transparent electronic circuits plays a key role in the development of next-generation display technologies. Silver nano-gratings are of particular interest due to their excellent conductivity and adjustable transmittance. Printed on polymeric substrates they are suitable for an application in flexible opto-electronic devices. Here, we present the preparation of a smart silver precursor system combining both the ability of cheap and scalable nanoimprint patterning and simple thermal silver reduction. Homogeneous silver line and grid patterns with line widths down to 400 nm are prepared using poly(dimethylsiloxane) stamps in a thermal nanoimprint lithography process. Relatively low process temperatures allow the film formation on polymeric substrates. Semi-transparent silver electrodes with a resistance of 2.8 ohm are patterned on polyimide foils to prepare flexible electro-luminescence devices. A detailed investigation of the precursor's thermal decomposition behaviour as well as the resulting electrical and optical properties of the films is offered.

Doped polymer electrodes for high performance ferroelectric capacitors on plastic substrates

Flexible ferroelectric capacitors with doped polymer electrodes have been fabricated on plastic substrates with performance as good as metal electrodes. The effect of doping on the morphology of polymer electrodes and its impact on device performance have been studied. Improved fatigue characteristics using doped and undoped poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) electrodes versus metal electrodes are observed. It is shown that the polymer electrodes follow classical ferroelectric and dielectric responses, including series resistance effects. The improved device characteristics obtained using highly conducting doped PEDOT:PSS suggest that it may be used both as an electrode and as global interconnect for all-polymer transparent circuits on flexible substrates.

Metal organic chemical vapor deposition of ZnO from β‐ketoiminates

ZnO is a high‐mobility electron conductor being considered for high‐throughput electronics in flexible and transparent formats. We demonstrated the Zn β‐ketoiminate system, based on acetylacetimine with N‐propyl, isopropyl, and butyl groups, as a vehicle for preparing ZnO thin films for electronic applications. Surface carbon was a primary impurity, and the precursors studied afforded films with the lowest surface carbon contamination at deposition temperatures near 400°C. Thermal annealing of the films reduced the surface carbon content and afforded semiconducting materials. Annealing also gave larger‐grained, better connected films. Thinner films were associated with semiconducting as opposed to ohmic behavior; such films will be adaptable for transparent logic circuits. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Flexible and transparent all-graphene circuits for quaternary digital modulations

In modern communication systems, modulation is a key function that embeds the baseband signal (information) into a carrier wave so that it can be successfully broadcasted through a medium such as air or cables. Here we report a flexible all-graphene modulator circuit with the capability of encoding a carrier signal with quaternary digital information. By exploiting the ambipolarity and the nonlinearity in a graphene transistor, we demonstrate two types of quaternary modulation schemes: quaternary amplitude-shift keying and quadrature phase-shift keying. Remarkably, both modulation schemes can be realized with just 1 and 2 all-graphene transistors, respectively, representing a drastic reduction in circuit complexity when compared with conventional modulators. In addition, the circuit is not only flexible but also highly transparent (~95% transmittance) owing to its all-graphene design with every component (channel, interconnects, load resistor and source/drain/gate electrodes) fabricated from graphene films.

Light and Temperature Stability of Fully Transparent ZnO-Based Inverter Circuits

The stability of the figures of merit of transparent inverter circuits at temperatures up to 150°C and under illumination by light in the visible spectral range is investigated. The inverter circuits consist of two transparent metal-semiconductor field-effect transistors. For temperatures up to 150°C, the inverters remain operational; the gate electrode degradation affects the voltage transfer characteristic (VTC) for temperatures above 90°C. Except for blue light, which slightly alters the VTC, visible light does not influence the device operation.

Structure and stability of N–H complexes in single-crystal ZnO

Zinc oxide (ZnO) is semiconductor with a wide band gap of 3.4 eV. It continues to gain more attention not only for its versatile use in industry but also its potential for further application in electronics, optics, spintronics, and transparent circuits. Many of these applications require p-type ZnO. Nitrogen substituting for oxygen is a possible acceptor for such applications. In this paper, we report a study of nitrogen-hydrogen (N–H) complexes grown into single-crystal ZnO, using seeded chemical vapor transport in an ammonia ambient. An infrared (IR) absorption peak arising from N–H complexes was observed at 3150.6 cm−1 at liquid-helium temperatures. The assignment of this peak was confirmed by nitrogen and hydrogen isotope substitution. Polarized IR spectroscopy shows that the N–H dipole is oriented at an angle ∼114° to the c axis, in agreement with previous first-principles calculations. To probe the stability of the N–H complexes, samples were annealed in air, oxygen, and argon. Samples annealed in oxygen at 725 °C showed a significant increase in resistivity, due to outdiffusion of hydrogen and compensation by nitrogen acceptors.