Authorship：Last author,　Corresponding author   Language：English   Publisher：Japanese Journal of Applied Physics  

Hysteresis is usually present in carbon nanotube thin-film transistors exposed to air due to adsorbed water and oxygen molecules. Thus, it is desirable to passivate the device from these environmental effects and provide an air-stable platform for chemical doping to tune the threshold voltages. Here, we demonstrate p- and n-doped carbon nanotube transistors with suppressed hysteresis using bilayer stacking of poly(methyl methacrylate) and aluminum oxide (Al2O3) passivation layers using a low-temperature process suitable for flexible substrates. The results show that the bilayer passivation layers achieved reduced hysteresis to be 2.25% of applied gate voltage at low operation voltage as 2 V.

DOI： 10.35848/1347-4065/ac5264

Scopus

Authorship：Corresponding author   Language：English   Publisher：Applied Physics Express  

Raman scatterings of both pristine and defective single-walled carbon nanotubes were measured. Defects on carbon nanotubes (CNTs) were induced by UV/O3 treatment, and the correlation between the temperature dependence of the Raman shift of the G-band and the crystallinity of CNTs was investigated. In the temperature range of 250-600 K, a gradual negative change in the slope was observed; the linear shift of the Raman G-band frequency with respect to temperature increased as the crystallinity deteriorated. This phenomenon is attributed to the increase in the fourth-order phonon-phonon scattering interaction resulting from the induced defects.

DOI： 10.35848/1882-0786/ac4678

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Authorship：Last author,　Corresponding author   Language：English   Publisher：Applied Physics Express  

Stretchable triboelectric nanogenerators (s-TENGs) are promising power sources for self-powered wearable electronics. Conventional single-electrode s-TENGs require an external ground electrode to form a closed circuit. We propose an in-plane dual-electrode s-TENG based on differential surface functionalization to eliminate the external ground electrode. The s-TENG comprises fully stretchable materials such as carbon nanotube thin films and elastomers. The top surface of the elastomer was functionalized to produce a difference in electronegativity between the two electrode regions. A high output power density of 3.5 W m-2 was obtained by tapping the two electrode regions by hand.

DOI： 10.35848/1882-0786/ac4d07

Scopus

Language：English   Publisher：ACS Applied Nano Materials  

The evolution of carbon nanomaterials can provide tremendous advantages in sensing, computation, and functional materials. A carbon nanotube (CNT) has outstanding thermal and electrical conductivity features and is one of the most promising nanoscale carbon materials. It has a hardness of up to 1 TPa. Exploiting these features, nanomechanical systems with CNTs have been reported to achieve ultrasensitive sensors for mass, force, and electromagnetic waves owing to their outstanding elastic and electric properties. Some research groups have attempted to achieve digital data transfer in potential nanoscale wireless terminals with carbon nanomaterials. Although conceptual demonstrations have been reported, the fundamental capability of the transfer, particularly in the presence of noise, is yet to be explained. Here, we experimentally demonstrate for the first time that an ultrasmall digital receiver with a nanomechanical nanoantenna can transfer a vast amount of digital data, up to 1 Mbit, even in the presence of noise. We successfully transfer a digital image data with 393 216 bits. This demonstration proves that the data-transfer capability is close to the theoretical limit established in information theory and channel capacity. This small but robust nanomechanical receiver will contribute to the forthcoming data-oriented age of Internet of things (IoT)- and artificial intelligence (AI)-based systems.

DOI： 10.1021/acsanm.1c02563

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Authorship：Last author,　Corresponding author   Language：Japanese   Publisher：The Japan Society of Applied Physics  

<p>Triboelectric energy harvesters are expected as a power source for self-powered wearable devices since they can generate electricity from the low-frequency motion of the human body. For wearable device applications, high stretchability is required to follow the dynamic three-dimensional surface of the human body. We have realized a stretchable sheet-shaped triboelectric energy harvester with a high output and elasticity. The high output has been achieved by a fluorine plasma treatment for the surface of the dielectric layer. We demonstrate a self-powered signal transmitter and LED-embedded gloves, exploiting the characteristics of the harvester. Furthermore, we propose a highly efficient intermittent operation circuit to drive electronic devices, using a small amount of power generated by the harvester. Our study can contribute to solve the problem of power source limitations in wearable electronics.</p>

DOI： 10.11470/oubutsu.90.11_684

CiNii Research

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TCSII.2021.3062390

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Language：English   Publisher：Carbon  

DOI： 10.1016/j.carbon.2021.07.001

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Language：English  

DOI： 10.1063/5.0058300

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/ac0261

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1882-0786/abf405

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1882-0786/abe8aa

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acs.jpcc.0c08459

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1882-0786/abd28b

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/aelm.202000674

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/abc3d7

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/chem.202000228

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.nanoen.2019.104370

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.nanoen.2019.104297

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/adfm.201907150

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1361-6528/ab33c8

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/1882-0786/ab369e

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41409-018-0369-0

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5099395

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CiNii Research

Language：Japanese   Publisher：ACS Omega  

Operando analysis of electron devices provides key information regarding their performance enhancement, reliability, thermal management, etc. For versatile operando analysis of devices, the nitrogen-vacancy (NV) centers in diamonds are potentially useful media owing to their excellent sensitivity to multiple physical parameters. However, in single crystal diamond substrates often used for sensing applications, placing NV centers in contiguity with the active channel is difficult. This study proposes an operando analysis method using a nanodiamond thin film that can be directly formed onto various electron devices by a simple solution-based process. The results of noise analysis of luminescence of the NV centers in nanodiamonds show that the signal-to-noise ratio in optically detected magnetic resonance can be drastically improved by excluding the large 1/f noise of nanodiamonds. Consequently, the magnetic field and increase in temperature caused by the device current could be simultaneously measured in a lithographically fabricated metal microwire as a test device. Moreover, the spatial mapping measurement is demonstrated and shows a similar profile with the numerical calculation.

DOI： 10.1021/acsomega.9b00344

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41408-019-0192-x

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.8b19252

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.electacta.2018.10.147

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Language：Japanese   Publisher：Nanoscale Advances  

Carbon nanotube (CNT) thin-film transistors based on solution processing have great potential for use in future flexible and wearable device technologies. However, the considerable variability of their electrical characteristics remains a significant obstacle to their practical use. In this work, we investigated the origins of the variability of these electrical characteristics by performing statistical analysis based on spatial autocorrelation and Monte Carlo simulation. The spatial autocorrelation of the on-current decreased with increasing distance on the order of millimetres, showing that macroscopic non-uniformity of the CNT density was one of the causes of the characteristic variability. In addition, even in the local regime where the macroscopic variability is negligible, the variability was greater than that expected based on the Monte Carlo simulation. The CNT aggregation could be attributed to microscopic variability. We also investigated the variability of the properties of integrated circuits such as inverters and ring oscillators fabricated on flexible plastic film. All of the inverters worked well, and their threshold voltage variations were fairly small. As the number of stages in the ring oscillator increased, the yield decreased, although the oscillation frequency variability improved.

DOI： 10.1039/c8na00184g

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s41061-018-0227-y

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PubMed

Language：Japanese   Publisher：IEEE Access  

The field-emission phenomenon is exploited in a broad variety of applications and systems. Previous studies have reported that the current induced by field emission strongly and inherently depend on the temperature. This dependence enhances the noise in the current, which results in performance degradation in, for example, signal detection and communications in nanoscale receivers. In this paper, a mathematical model is presented for the suppression of the noise based on its probability density. Our experiment and analysis revealed that the density follows a Gaussian distribution, and the dependence on temperature is observed to be exponential. This result is intriguing because in the field of signal processing and communication, the influence of temperature is often considered with a noiseerature model, namely, linear dependence. Using our derived model, we theoretically evaluated the communication performance of a nanoscale receiver; owing to the exponential dependence on temperature, severe performance degradation was found with increasing temperature. This means that, as field-emission technology continues to be developed, the temperature should be kept low, for example, at room temperature, to secure the reliability of nanoscale communication devices.

DOI： 10.1109/ACCESS.2019.2913692

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Language：Japanese   Publisher：IEEE Access  

This paper presents a control method intended to suppress the effects of manufacturing variations on nanomechanical systems. Often, the resonance characteristics of nanoscale devices are inconsistent, due to unavoidable variations in the fabrication process. This is important because resonant vibrations enhance the sensitivities of the devices. As such, the sensitivities of these systems can be degraded if the device characteristics are not identified. To address this fundamental problem, this paper presents a multidisciplinary method based on control theory, nanotechnology, and communication technology. A stochastic optimal feedback controller is employed to enhance an average sensitivity by regarding the variations as stochastic parameters. This method is applied to nanoscale receivers that detect transmitted binary data based on binary phase-shift keying in communication systems. The proposed method controls the vibrations of carbon nanotubes (CNTs) that serve as the antennas of the receiver. The proposed method is demonstrated via a numerical simulation using nanoscale receivers with the manufacturing variation. The simulation based on experimental data obtained from CNTs shows that the average performance of the devices is enhanced.

DOI： 10.1109/ACCESS.2019.2955697

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1039/c8an00528a

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PubMed

Language：Japanese   Publisher：Electronics Letters  

Towards the realisation of reliable communication systems using simple nanoscale transceivers, this Letter presents a method for the non-coherent detection of digitally phase-modulated signals. Without the synchronisation of the phases of the transmitted and received carrier signals, data transmission cannot be reliably achieved. To address this issue, the proposed method is developed using a nanoscale receiver that receives the binary phase-modulated signal through the mechanical vibration of a carbon nanotube cantilever. The present analysis reveals that this receiver has an interesting characteristic: the two decision statistics available in the receiver have a phase difference of p/2, which means that at least one of the statistics is fairly reliable regardless of the synchronisation error. Through the selection of the more reliable of the two statistics, the proposed method achieves low error in the estimation of the transmitted data even without carrier phase synchronisation. This Letter presents the derivation of the proposed rule for selecting the statistic and numerical results that demonstrate the effectiveness of the proposed method. This Letter is expected to contribute to the realisation of future communication applications using nanoscale transceivers.

DOI： 10.1049/el.2018.0475

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TNANO.2017.2765310

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Language：Japanese   Publisher：Journal of Applied Physics  

This paper presents a theoretical study of an adaptive microwave phase shifter based on a nanomechanical resonator. The phase shifter can be tuned to provide an arbitrary phase rotation in the range between -90° and 90° by adjusting the bias voltage applied to the resonator. An analytical model is developed to reveal the underlying mechanism of the phase shifter. It is also analytically demonstrated that the operating frequency can be selected by adjusting the bias voltage. In addition, it is found that the bandwidth is tunable via the quality factor for the cantilever.

DOI： 10.1063/1.4992040

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.7b03151

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.56.065102

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Language：Japanese   Publisher：IEEE Transactions on Molecular, Biological, and Multi-Scale Communications  

An adaptive control method of angular sensitivity for a VHF-band nano-antenna is presented. This antenna is composed of a single carbon nanotube cantilever immersed in a DC electric field. The behavior of the antenna is analytically described using a singly clamped metallic sphere model and the image charge method. We derive design guidelines for the direction and shape of angular sensitivity; the amplitude of the DC electric field is a key parameter for controlling the sensitivity. The analysis reveals that the most sensitive direction is tunable in the range of -π/2 to π/2. The shape of sensitivity can be manipulated to be omnidirectional or to correspond to a Hertzian-dipole-like antenna. This investigation is useful for adaptive control to suppress interference signals in nanoscale communication systems.

DOI： 10.1109/TMBMC.2016.2640282

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LAWP.2016.2640272

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.55.080305

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Language：Japanese   Publisher：Carbon  

We report a continuous floating catalyst chemical vapor deposition synthesis of highly individual single-walled carbon nanotubes (SWCNT) for high performance transparent conducting films (TCF). Active feedback dilution of ferrocene-based catalyst vapor leads to an almost complete elimination of SWCNT bundling and a substantial increase in SWCNT lengths via the suppression of bundling-induced growth termination. The fabricated uniform TCFs exhibit sheet resistances of 89 Ω/sq. at 90% transmittance. This was further improved by micro-patterning, resulting in a sheet resistance of 69 Ω/sq. at 97% transmittance - the highest reported for any carbon nanotube TCF - and highly competitive with commercial indium-tin-oxide-TCFs. Furthermore, we demonstrate that thin film transistors fabricated from these highly individual SWCNTs reach charge carrier mobilities up to 100 cm2 V-1s-1 and ON/OFF-ratios up to 106.

DOI： 10.1016/j.carbon.2016.02.099

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Language：Japanese   Publisher：Nanoscale  

Nano-carbon materials (carbon nanotubes, graphene, and graphene oxide) have potential application for photovoltaics because of their excellent optical and electronic properties. Here, we demonstrate that a single-walled carbon nanotubes/graphene oxide buffer layer greatly improves the photovoltaic performance of organo-lead iodide perovskite solar cells. The carbon nanotubes/graphene oxide buffer layer works as an efficient hole transport/electron blocking layer. The photovoltaic conversion efficiency of 13.3% was achieved in the organo-lead iodide perovskite solar cell due to the complementary properties of carbon nanotubes and graphene oxide. Furthermore, the great improvement of photovoltaic performance stability in the perovskite solar cells using carbon nanotubes/graphene oxide/polymethyl methacrylate was demonstrated in comparison with that using a typical organic hole transport layer of 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene.

DOI： 10.1039/c6nr01152g

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.5b10439

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PubMed

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TNANO.2015.2477813

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.carbon.2015.04.033

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/APEX.8.045102

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

<p>Carbon nanotube (CNT) thin films are rare materials with multi-functionality in addition to superior electric, optical and mechanical characteristics. This article introduces all-carbon devices having a semiconductor layer and electrodes, with wiring made of a CNT thin film. They are flexible and transparent as well as having the ability to be thermoformed into a variety of 3D shapes. Taking advantage of these characteristics allows the realization of unobservable devices or 3D or highly designable devices.</p>

DOI： 10.11470/oubutsu.84.2_142

CiNii Research

Language：Japanese   Publisher：Nanoscience and Nanotechnology Letters  

We report electroluminescence (EL) from a carbon nanotube field-effect transistor with split-gates. EL is generated by the electrostatic doping technique. Six EL bands could be observed, with the strongest band peaking between 0.867 and 0.850 eV with a full width at half maximum (FWHM) of 64 to 120 meV, depending on the bias conditions. From the EL peak position we estimate a CNT diameter of ∼ 1.05 nm. We also estimate the power and quantum efficiency of the EL to be around 1 × 10-6and 1 × 10-5 respectively. With a fixed drain voltage, increasing the opposite split gate bias caused the EL to increase monotonically from zero, indicating an ambipolar emission mechanism. With a fixed opposite split gate bias the dependence of EL on drain voltage displays a threshold. Drain current is significantly higher when using equal, rather than opposite split gate biases, which we attribute to the trapping of carries by band bending when using opposite split gate bias.

DOI： 10.1166/nnl.2014.1831

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Language：Japanese   Publisher：Materials Research Express  

We fabricated a Ti/Pd asymmetrically contacted single carbon nanotube (CNT) field-effect transistor (FET) with split-gates. Transfer characteristics can be explained if the Schottky barrier for electrons is lower at the Pd contact than it is at the Ti contact. Strong rectification is observed when the gates are unbiased, and the rectification direction can be inverted with the appropriate gate bias. When operated as an FET the device has an on/off ratio of 1 × 107. Under illumination, photocurrent can only be observed with opposite split-gate bias. Open circuit voltage (VOC) and short circuit current (ISC) increase with increasing opposite polarity split-gate bias, representing the first demonstration of the modulation of VOC and ISC in an asymmetric contact CNT FET.

DOI： 10.1088/2053-1591/1/2/026304

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.53.05FD01

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/ph400133k

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Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/nn4041975

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PubMed

Language：Japanese   Publisher：Proceedings of SPIE - The International Society for Optical Engineering  

Asymmetric contacts or split gate geometries can be used to obtain rectification, electroluminescence (EL) and photocurrent from carbon nanotube field effect transistors. Here, we report devices with both split gates and asymmetric contacts and show that device parameters can be optimised with an appropriate split gate bias, giving the ability to select the rectification direction, modify the reverse bias saturation current and the ideality factor. When operated as a photodiode, the short circuit current and open circuit voltage can be modified by the split gate bias, and the estimated power conversion efficiency was 1×10-6. When using split gates and symmetric contacts, strong EL peaking at 0.86 eV was observed with a full width at half maximum varying between 64 and 120 meV, depending on the bias configuration. The power and quantum efficiency of the EL was estimated to be around 1×10-6 and 1×10-5 respectively.

DOI： 10.1117/12.2036962

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CiNii Research

Language：Japanese   Publisher：Proceedings of the 20th International Workshop on Active-Matrix Flatpanel Displays and Devices: TFT Technologies and FPD Materials, AM-FPD 2013  

Among various kinds of semiconductor materials, carbon nanotubes can provide high-mobility, low-operation voltage, flexible devices on plastic films at low cost. High-mobility thin-film transistors (TFTs) and functional integrated circuits (ICs) have been fabricated by simple transfer process on a transparent plastic substrate. All-carbon ICs have also been realized for the first time, which has better transparency, flexibility, and stretchability. We also demonstrate high-throughput printing fabrication of high-mobility TFT arrays with flexographic printing technique. © 2013 JSAP.

Scopus

Language：Japanese   Publisher：Applied Physics Letters  

A single carbon nanotube diode is reported, with Ti and Pd contacts, and split gates. Without gate bias the device displays strong rectification, with a leakage current (I0) of 6 × 10-16 A, and an ideality factor (η) of 1.38. When the gate above the Ti contact is biased negatively the diode inverts. When positive bias is then applied to the gate above the Pd contact minority carrier injection is suppressed. Configured such I0 and η were 2 × 10-14 A and 2.01, respectively. Electrical characterization indicates that the Schottky barrier height for electrons is lower for the Pd contact than the Ti contact. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4823602

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Language：Japanese   Publisher：Applied Physics Express  

A lack of high-mobility transistors has been one of the most crucial challenges facing the development of printable electronics. In this work, we report on the fabrication of high-mobility carbon nanotube thin-film transistors using a combination of transfer and high-speed flexographic printing techniques. Based on lithography-free nonvacuum processes, a high mobility of 157 cm2 V-1 s-1 with an ON/OFF ratio of 104 was achieved. Our ambient fabrication technique provides not only a promising platform for printed flexible devices but also demonstrates the realistic potential of low-cost manufacturing technology. © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.6.085101

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Language：Japanese   Publisher：Nature Communications  

A variety of plastic products, ranging from those for daily necessities to electronics products and medical devices, are produced by moulding techniques. The incorporation of electronic circuits into various plastic products is limited by the brittle nature of silicon wafers. Here we report mouldable integrated circuits for the first time. The devices are composed entirely of carbon-based materials, that is, their active channels and passive elements are all fabricated from stretchable and thermostable assemblies of carbon nanotubes, with plastic polymer dielectric layers and substrates. The all-carbon thin-film transistors exhibit a mobility of 1,027 cm 2 V -1 s -1 and an ON/OFF ratio of 10 5. The devices also exhibit extreme biaxial stretchability of up to 18% when subjected to thermopressure forming. We demonstrate functional integrated circuits that can be moulded into a three-dimensional dome. Such mouldable electronics open new possibilities by allowing for the addition of electronic/plastic-like functionalities to plastic/electronic products, improving their designability. © 2013 Macmillan Publishers Limited.

DOI： 10.1038/ncomms3302

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PubMed

Language：Japanese   Publisher：Journal of Physical Chemistry C  

We use noninvasive atomic force microscopy to probe the spatial electrical conductivity of isolated junctions of pristine and nitric acid treated single-walled carbon nanotube networks (SWCNT-N). By analyzing the local IV curves of SWCNTs and bundles with various diameters, the resistance per unit length and the contact resistance of their junctions are estimated to be 3-16 kΩ/μm and 29-532 kΩ, respectively. We find that the contact resistance decreases with increasing SWCNT or bundle diameter and depends on the contact morphology, reaching a value of 29 kΩ at a diameter of 10 nm. A nitric acid treatment moderately dopes SWCNTs and reduces their average contact resistance by a factor of 3 while the resistance of the nanotubes remains largely unaltered. Remarkably, the same treatment on an SWCNT-N shows similar reduction in the sheet resistance by a factor of 4. These results suggest that the resistance reduction mechanism is related to the contact modulation with no major impact on conductance of SWCNTs. © 2013 American Chemical Society.

DOI： 10.1021/jp403983y

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Language：Japanese   Publisher：Applied Physics Express  

We have investigated the interface charges generated at the interfaces of the high-k gate insulator by Kelvin probe force microscopy, as well as their effects on the properties of carbon nanotube field-effect transistors (CNFETs). It was found that positive charges are concentrated near the interfaces of the gate insulator with Au electrodes and a SiO2 substrate. The device simulation results show that the charges at the interfaces of the gate insulator with the Au electrodes are responsible for the change in the polarity of the conduction carriers in the CNFETs. Copyright © 2013 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.6.024002

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Language：Japanese  

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Language：Japanese   Publisher：Physica Status Solidi (C) Current Topics in Solid State Physics  

We fabricated flexible n -type carbon nanotube thin-film transistors on a plastic film by a simple transfer process and a solution-based chemical doping technique with polyethyleneimine (PEI). There exists an optimal PEI concentration, which is determined by doping level and charge accumulation in the PEI layer. A high electron mobility of 70 cm2/Vs was achieved with an on/off ratio of 105 in ambient air. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.201300231

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CiNii Research

Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.52.266

CiNii Research

Language：Japanese   Publisher：Applied Physics Letters  

Carbon nanotube field effect transistors (CNT-FETs) with graphitic carbon (G-C) contacts were fabricated and the electrical properties of the G-C contacts were studied. The CNT-FETs showed p-type conduction in air. However, the conduction type has changed to ambipolar in vacuum after annealing at 200 °C. This suggests that the p-type conduction in air is attributed to the adsorbed oxygen. The barrier heights at the G-C/CNT contacts in vacuum were ∼400 meV for electrons and ∼310 meV for holes. These values suggest that the Fermi level of G-C contacts is located at slightly below the midgap of the CNTs in vacuum. © 2012 American Institute of Physics.

DOI： 10.1063/1.4737169

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Language：Japanese   Publisher：Nano Research  

The properties of electronic devices based on carbon nanotube networks (CNTNs) depend on the carbon nanotube (CNT) deposition method used, which can yield a range of network morphologies. Here, we synthesize single-walled CNTs using an aerosol (floating catalyst) chemical vapor deposition process and deposit CNTs at room temperature onto substrates as random networks with various morphologies. We use four CNT deposition techniques: electrostatic or thermal precipitation, and filtration through a filter followed by press transfer or dissolving the filter. We study the mobility using pulsed measurements to avoid hysteresis, the on/off ratio, and the electrical noise properties of the CNTNs, and correlate them to the network morphology through careful imaging. Among the four deposition methods thermal precipitation is found to be a novel approach to prepare high-performance, partially aligned CNTNs that are dry-deposited directly after their synthesis. Our results provide new insight into the role of the network morphologies and offer paths towards tunable transport properties in CNT thin film transistors. © 2012 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

DOI： 10.1007/s12274-012-0211-8

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Language：Japanese   Publisher：Japanese Journal of Applied Physics  

We have estimated the height of barriers against carriers formed in the metallic carbon nanotube (m-CNT) grown by plasma-enhanced chemical vapor deposition. The result shows that the heights of the barriers against both electrons and holes are about 300 meV. The existence of the barrier in the m-CNT was confirmed by local current modulation using scanning gate microscopy and by the potential drop obtained by Kelvin probe force microscopy. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.02BN01

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Language：Japanese   Publisher：Japanese Journal of Applied Physics  

The change in the conduction type of carbon nanotube field-effect transistors (CNT-FETs) with Au contacts from p-type to n-type by annealing in vacuum was observed. The result was explained by the local work function change of the Au contacts based on the measurement of the surface potential of the Au/CNT contact by Kelvin probe force microscopy (KFM). This work function change became prominent due to the desorption of oxygen by annealing. The degree of the conduction-type change was found to be dependent on the devices. The CNT-FETs with small OFF current showed clear conduction-type change. However, the devices with large OFF current did not show clear conduction-type change. This device dependent behavior was explained by the energy gap difference among devices, in which suppression of OFF current is not sufficient for the devices with thick CNTs with small bandgap. © 2012 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.51.02BN06

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Language：Japanese   Publisher：Materials Express  

An air-free system which enables experiments from carbon nanotube (CNT) growth to device characterization without an effect of ambient air was constructed. It was clarified by using the system that an origin of instability of n-type conduction of carbon nanotube field-effect transistors (CNTFETs) with Al contacts in air was not attributed to oxidation of the Al contacts, but attributed to adsorption of oxygen. The present system was shown to be useful for investigating the effect of air on intrinsic electrical properties of CNTFETs with Al contacts, investigating the effect of passivation, and fabricating CNT p-n junction with Au(p)/Al(n) asymmetric electrodes. © 2011 by American Scientific Publishers.

DOI： 10.1166/mex.2011.1037

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Language：Japanese   Publisher：Nano Research  

We have developed a process for chemical purification of carbon nanotubes for solution-processable thin-film transistors (TFTs) having high mobility. Films of the purified carbon nanotubes fabricated by simple drop coating showed carrier mobilities as high as 164 cm2V-1s-1, normalized transconductances of 0.78 Sm-1, and on/off current ratios of 106. Such high performance requires the preparation of a suspension of micrometer-long and highly purified semiconducting single-walled carbon nanotubes (SWCNTs). Our purification process includes length and electronic-type selective trapping of SWCNTs using recycling gel filtration with a mixture of surfactants. The results provide an important milestone toward printed high-speed and large-area electronics with roll-to-roll and ink-jet device fabrication. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

DOI： 10.1007/s12274-011-0152-7

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Language：Japanese   Publisher：International Symposium on VLSI Technology, Systems, and Applications, Proceedings  

We report the present status of Carbon Nanotube (CNT) CVD material technologies and their applications for via interconnects and FETs for VLSI. We succeeded in growing multi-walled CNTs (MWNTs) with the highest shell density (as high as 1013/cm2) and in fabricating via interconnects with high robustness against a high current density. We also report a Si-process compatible technique to control carrier polarity of Single-walled CNT (SWNT) FETs by utilizing fixed charges introduced by the gate oxide. High-performance p- and n-type CNT-FETs and CMOS inverters with stability in air have been realized. © 2011 IEEE.

DOI： 10.1109/VTSA.2011.5872210

Scopus

Language：Japanese   Publisher：Nanotechnology  

The electrical properties of carbon nanotube thin-film transistors (CNT-FETs) fabricated using plasma-enhanced chemical vapor deposition (PECVD) were studied by scanning probe microscopy. The measured results suggest the formation of an island structure in the subthreshold regime and the disappearance of the island structure at the ON state. These results were explained by the change in the effective number of CNTs that contributed to the electrical conduction due to the gate-bias-dependent resistance of the semiconducting CNTs. The results obtained by Monte Carlo simulation revealed similar results. The effects of metallic CNTs with defects and the scatter of the drain current in the subthreshold regime were also examined. © 2011 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/22/19/195202

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PubMed

Language：Japanese   Publisher：Nature Nanotechnology  

Carbon nanotube thin-film transistors are expected to enable the fabrication of high-performance, flexible and transparent devices using relatively simple techniques. However, as-grown nanotube networks usually contain both metallic and semiconducting nanotubes, which leads to a trade-off between charge-carrier mobility (which increases with greater metallic tube content) and on/off ratio (which decreases). Many approaches to separating metallic nanotubes from semiconducting nanotubes have been investigated, but most lead to contamination and shortening of the nanotubes, thus reducing performance. Here, we report the fabrication of high-performance thin-film transistors and integrated circuits on flexible and transparent substrates using floating-catalyst chemical vapour deposition followed by a simple gas-phase filtration and transfer process. The resulting nanotube network has a well-controlled density and a unique morphology, consisting of long (∼10 μm) nanotubes connected by low-resistance Y-shaped junctions. The transistors simultaneously demonstrate a mobility of 35 cm 2 V-1 s-1 and an on/off ratio of 6 × 106. We also demonstrate flexible integrated circuits, including a 21-stage ring oscillator and master-slave delay flip-flops that are capable of sequential logic. Our fabrication procedure should prove to be scalable, for example, by using high-throughput printing techniques. © 2011 Macmillan Publishers Limited. All rights reserved.

DOI： 10.1038/NNANO.2011.1

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PubMed

Language：Japanese   Publisher：Journal of Physical Chemistry C  

The present experimental study provides a detailed characterization of the single-wall carbon nanotube (SWNT) length effect on the device characteristics of DNA-wrapped SWNT-network thin-film transistors (TFTs). DNA-assisted dispersion and length separation by high-performance liquid chromatography are used to prepare the SWNTs with average lengths of 400 and 200 nm. The TFTs of both SWNTs exhibit high on/off current ratios of 104-106 and mobilities of 0.4-1.3 cm2/(V s) by optimizing the film density of SWNTs. The optimized density of 200 nm SWNTs is found to be higher than that of 400 nm SWNTs as expected by the percolation theory of two-dimensional random networks, where shorter length SWNTs can maintain high on current despite increasing in the number of SWNT junctions. The present result is an important indicator for realizing high-performance TFTs constructed from SWNTs having desired length. © 2010 American Chemical Society.

DOI： 10.1021/jp107361n

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Scopus

Language：Japanese   Publisher：Japanese Journal of Applied Physics  

We have grown thin single-walled carbon nanotubes (SWNTs) with a narrow diameter distribution using the arc-discharge plasma (ADP) technique for Co catalyst metal deposition and a cold-wall chemical vapor deposition (CVD) system for SWNT growth. The diameters of the SWNTs ranged from 0.79 to 1.07 nm. In addition to depositing small and uniform-sized metal catalyst nanoparticles by the ADP technique, decreasing the growth temperature using the cold-wall CVD system seemed to suppress the aggregation of the metal catalyst nanoparitcles leading to SWNTs with small diameters of narrow distribution. © 2011 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.50.015102

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Scopus

Language：Japanese   Publisher：Physica Status Solidi (C) Current Topics in Solid State Physics  

To control the properties of carbon nanotube field-effect transistors (CNFETs), it is important to understand the properties of various interfaces in the devices and their effects on device characteristics. Here, we studied the influence of interface fixed charges introduced between the gate insulator and the substrate on the operation of a top-gate CNFET, using device simulation. The simulation results suggest that with the existence of interface charges, the current modulation mechanism of the CNFET changes from Schottky barrier modulation to channel potential modulation. The minority carrier injection in the OFF state is suppressed by the high-potential region formed near the contact electrodes by the interface charges. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.201000571

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Language：Japanese   Publisher：Physica Status Solidi (C) Current Topics in Solid State Physics  

Aligned growth of single-walled carbon nanotubes (SWNTs), which can be realized on a quartz substrate, is important for electron device applications such as high-frequency transistors. In order to improve the alignment of the SWNTs, we studied pretreatments of the substrate before SWNT growth. When the surface of the quartz substrate was etched using hydrofluoric acid, the alignment and linearity of the grown SWNTs improved. This suggests that it is important to remove the uppermost layer on the surface of as-received substrates, which is damaged due to the polishing process. We also studied an etching process to remove the damaged layer without increasing surface roughness. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.201000551

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Language：Japanese   Publisher：Proceedings of the IEEE  

Carbon nanotubes (CNTs) offer unique properties such as the highest current density, ballistic transport, ultrahigh thermal conductivity, and extremely high mechanical strength. Because of these remarkable properties, they have been expected for use as wiring materials and as alternate channel materials for extending complementary metal oxide semiconductor (CMOS) performance in future very large scale integration (VLSI) technologies. In this paper, we report the present status of CNT growth technologies and the applications for via interconnects (vertical wiring) and field-effect transistors (FETs). We fabricated CNT via and evaluated its robustness over a high-density current. In our technology, multiwalled carbon nanotubes (MWNTs) were successfully grown at temperatures as low as 365 C using Co catalyst nanoparticles, which were formed and deposited by a custom-designed particle generation and deposition system. The density of MWNTs grown at 450 C reaches more than MWNTs were grown in via holes with a diameter as small as 40 nm. The resistance of CNT vias with a diameter of 160 nm was found to be of the same order as that of tungsten plugs. The CNT via was able to sustain a current density as high as at 105 C for 100 h without any deterioration in its properties. We propose a Si-process compatible technique to control carrier polarity of CNFETs by utilizing fixed charges introduced by the gate oxide. High-performance - and type CNFETs and CMOS inverters with stability in air have been realized. © 2006 IEEE.

DOI： 10.1109/JPROC.2010.2068030

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Language：English  

DOI： 10.1166/jnn.2010.3144

PubMed

Language：Japanese   Publisher：Advanced Materials  

[Figure Presented] Controlling the morphology of single-walled carbon nanotubes (SWNTs) is essential to realize their excellent device characteristics in electronics. DNA-wrapped SWNTs provides an effective, scalable way to fabricate the super-uniform networks of highly isolated, structure-sorted SWNTs for thin-film transistors (TFTs). The DNASWNTs are easily formed into uniform, desired-density networks of individual nanotubes. © 2010 WILEY-VCH Verlag GmbH &, Co. KCaA, Weinheim.

DOI： 10.1002/adma.200904006

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PubMed

Language：Japanese   Publisher：Journal of Nanoscience and Nanotechnology  

Fabrication process of the carbon nanotube (CNT) field effect transistors (FETs) for biosensors was studied. Atomic layer deposition (ALD) of HfO 2 was applied to the deposition of the passivation/gate insulator film. The CNT-FETs did not show the drain current degradation after ALD passivation even though the passivation by Si 3N 4 deposited by plasma-enhanced chemical vapor deposition (PECVD) resulted in a significant drain current decrease. This indicates the advantage of the present ALD technique in terms of the damage suppression. The biosensing operation was confirmed using thus fabricated CNT-FETs. Copyright © 2010 American Scientific Publishers All rights reserved.

DOI： 10.1166/jnn.2010.1983

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PubMed

Language：Japanese   Publisher：Nanotechnology  

Thin film transistors with a carbon nanotube(CNT) network as a channel have been fabricated using grid-inserted plasma-enhanced chemical vapor deposition(PECVD) which has the advantage of preferential growth of the CNTs with semiconducting behavior in the I-V characteristics of CNT field effect transistors (CNT-FETs). Taking advantage of the preferential growth and suppression of bundle formation, a large ON current of 170 μ Amm -1, which is among the largest in these kinds of devices with a large ON/OFF current ratio of about 105, has been realized in the relatively short channel length of 10μm. The field effect mobility of the device was 5.8cm2V-1s-1. © 2010 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/21/20/205202

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PubMed

Language：Japanese   Publisher：Nanotechnology  

We study the phenomenon of change in carrier type in carbon nanotube field-effect transistors (CNFETs) caused by the atomic layer deposition (ALD) of a HfO2 gate insulator. When a HfO2 layer is deposited on a CNFET, the type of carrier changes from p-type to n-type. The so-obtained n-type device has good performance and stability in air. The conductivity of such a device with a channel length of 0.7νm is 11% of the quantum conductance 4e2/h. The contact resistance for electron current is estimated to be 14 kΩ. The n-type conduction of this CNFET is maintained for more than 100 days. The change in carrier type is attributed to positive fixed charges introduced at the interface between the HfO2 and SiO2 layers. We also propose a novel technique to control the type of conduction by utilizing interface fixed charges; this technique is compatible with Si CMOS process technology. © 2010 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/21/16/165201

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Language：Japanese   Publisher：Japanese Journal of Applied Physics  

Electrical properties of a carbon nanotube field-effect transistor (CNT-FET) with multiple CNT channels were studied by scanning gate microscopy (SGM), in which the scanning probe tip was used as a local gate. It was possible to distinguish the difference in electrical properties of individual CNT channels by SGM. Spot like SGM images were attributed to the barrier against carriers formed in the metallic CNT, resulting in a current modulation of the CNT-FET. It has also been shown that the barrier in the metallic CNT results in an ambipolar behavior of the CNTFETs. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.49.02BD02

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Language：Japanese  

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Language：Japanese   Publisher：Applied Physics Express  

We have realized high-performance p- and n-type top-gate carbon nanotube field-effect transistors (CNFETs) by controlling the charges introduced at the interface between the gate insulator and the substrate. We also fabricated a complementary metal-oxide-semiconductor (CMOS) inverter on an individual single-walled carbon nanotube (SWNT). This CMOS inverter exhibited a high voltage gain of 26 and a large noise margin of ̃70% of Vdd=2. The proposed technique provides air-stable and high-performance carbon nanotube CMOS devices that are compatible with the Si CMOS process. The effects of interface charges on the mechanisms of CNFET operation have also been investigated on the basis of potential calculations. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.3.105102

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Language：Japanese   Publisher：Applied Physics Express  

Medium scale integrated circuits with more than 100 carbon nanotube (CNT) thin-film transistors (TFTs) have been successfully fabricated. These are the largest CNT-based integrated circuits reported so far. Our fabrication technology features a buried back-gate architecture made by CNT growth using plasma-enhanced chemical vapor deposition, chemical doping and enhancement/depletion mode logic gates. High-speed operation with a switching time of 0.51 μs/gate demonstrated by a 53-stage ring oscillator, which has about two orders of magnitude higher switching speed than previous TFT integrated circuits, confirms the suitability of the present TFT technology for implementing CNT-TFT integrated circuits with various functions. © 2010 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.3.115101

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Language：Japanese   Publisher：2009 International Semiconductor Device Research Symposium, ISDRS '09  

DOI： 10.1109/ISDRS.2009.5378006

Scopus

Language：Japanese   Publisher：Journal of Applied Physics  

The electrical properties of carbon nanotubes (CNTs) grown by plasma-enhanced chemical vapor deposition (PECVD) have been studied by measuring the I-V characteristics of many CNT-field effect transistors. The ratio of modulation current to total current was as high as 97%, with a small nondepletable OFF current component. This suggests that CNTs with semiconducting behavior were preferentially grown in the PECVD process. Raman scattering spectroscopy of the PECVD-grown CNTs, however, revealed several peaks of the radial breezing mode, which correspond to the presence of metallic CNTs. Scanning gate microscopy measurement of the CNT-FET with an ON/OFF ratio of 100 revealed the existence of a potential barrier in the metallic CNTs. These results suggest that observation of the preferential growth of CNTs with semiconducting behavior in the CNT-FETs fabricated via the present PECVD process results from the opening of the band gap due to defects caused by irradiation damage during the PECVD growth. © 2009 American Institute of Physics.

DOI： 10.1063/1.3234389

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Scopus

Language：Japanese   Publisher：Carbon - Science and Technology  

Our study on nickel silicide and gold contacted single-wall-carbon-nanotube field effect transistors (SWCN-FETs) is in sharp contrast to earlier published reports of type conversion in SWCN-FETs (from p- to n-) when cycled between air and vacuum, and indicates that (1) band gap of SWCN (2) the extent to which Fermi level of the metal contact gets shifted due to adsorption/desorption of oxygen and (3) relative position of the Fermi level of the metal contact with respect to the top of the valance band of SWCN (in an oxygen-free environment) are some of the important factors that governs such phenomena. © Applied Science Innovations Pvt. Ltd., India.

Scopus

Language：Japanese   Publisher：ASDAM 2008 - Conference Proceedings of the 7th International Conference on Advanced Semiconductor Devices and Microsystems  

The electrical properties of carbon nanotube FETs (CNTFETs) have been studied in detail. The conduction type of the CNTFETs was dependent on the work function of the contact metal, which suggests that Fermi level pinning at the metal/nanotube interface is not strong. Chemical doping using F4TCNQ was found to be effective in reducing not only the channel resistance but also the contact resistance. In the CNTFETs fabricated using PECVD-grown nanotubes, the drain current of the most of the devices could be modulated by the gate voltage with small OFF current suggesting the preferential growth of the nanotubes with semiconducting behavior. Multichannel top-gate CNTFETs with horizontally-aligned nanotubes have been successfully fabricated using CNT growth on the ST-cut quartz substrate. CNTFETs with nanotube network have also been fabricated by using grid-inserted PECVD and catalysts formed on the channel area of the FETs. The uniformity of the electrical properties of the network channel CNTFETs was very good. © 2008 IEEE.

DOI： 10.1109/ASDAM.2008.4743290

Scopus

Language：Japanese   Publisher：Proceedings of SPIE - The International Society for Optical Engineering  

The electrical properties of carbon nanotube FETs (CNTFETs) have been studied in detail. The conduction type of the CNTFETs was dependent on the work function of the contact metal, which suggests that Fermi level pinning at the metal/nanotube interface is not strong. Based on the two-probe and four-probe resistance measurements, it has been shown that the carrier transport at the contact is explained by the edge contact model even in the diffusive regime. The chemical doping using F4TCNQ was effective in reducing not only the channel resistance but also the contact resistance. In the CNTFETs fabricated using plasma-enhanced (PE) CVD-grown nanotubes, the drain current of the most of the devices could be modulated by the gate voltage with small OFF current suggesting the preferential growth of the nanotubes with semiconducting behavior. Multichannel top-gate CNTFETs with horizontally-aligned nanotubes as channels have been successfully fabricated using CNT growth on the ST-cut quartz substrate, arc-discharge plasma deposition of the catalyst metal, and ALD gate insulator deposition. The devices show normally-on and n-type conduction property with a relatively-high ON current of 13 mA/mm. CNTFETs with nanotube network have also been fabricated by direct growth on the SiO2/Si substrate using grid-inserted PECVD and using catalyst formed on the channel area of the FETs. The uniformity of the electrical properties of the network channel CNTFETs were very good. Finally, it has been shown that the surface potential profile measurement based on the electrostatic force detection in the scanning probe microscopy was effective in studying the behavior of the CNTFETs such as the transient behavior and the effect of the defects.

DOI： 10.1117/12.794695

Scopus

Language：Japanese   Publisher：Applied Physics Letters  

High-density horizontally aligned single-walled carbon nanotubes are grown on a quartz substrate using Co nanoparticles deposited by arc-discharge plasma method. The Co nanoparticles with a density as high as 6.0× 1010 cm-2 are formed by a single pulse of arc discharge at room temperature. The density of the aligned nanotubes is ∼8 μ m-1 in average. Multichannel nanotube field-effect transistors with a high- k top-gate structure are fabricated with aligned nanotubes. The devices show high-performance, normally on, and n -type conduction property without any doping process. A high on current of 1.3 mA and a large transconductance of 0.23 mS for a channel width of 100 μm are obtained. The normally on and n -type property is attributed to fixed positive charges in the Hf O2 gate insulator and at the interfaces. © 2008 American Institute of Physics.

DOI： 10.1063/1.2969290

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Language：Japanese   Publisher：Japanese Journal of Applied Physics  

Carbon nanotubes (CNTs) are fabricated by adopting plasma-enhanced chemical vapor deposition (PECVD) with a planar microwave plasma source. Plasma is produced by a slot antenna at 2.45-GHz microwave injection in CH 4/H2 mixture. In this study, it is shown that avoiding the exposure of the substrate to the plasma drastically improves the CNT growth. Furthermore, it is found that the CNT quality can be controlled with the optimization of one of the steps in the catalyst treatment, such as the preheating procedure; the treated catalyst is considered to be unaffected by the heating in the high-density microwave plasma treatment during the CNT growth. © 2008 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.47.5652

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

<p>Carbon nanotubes are promising materials for various applications from electron devices to medical equipment. It is necessary to establish a characterization method for the nanotube materials as well as large scale synthesis technique for stable quality. Photoluminescence (PL) spectroscopy of carbon nanotubes is an important technique in addition to Raman scattering spectroscopy for characterizing nanotube materials. Since the optical properties of nanotubes are strongly affected by environmental conditions around them, it is necessary to understand the environmental effects to establish PL spectroscopy as a useful characterization tool. The environmental effects are mainly caused by the dielectric screening of inter-carrier Coulomb interactions. In addition, mechanical interactions with surrounding materials and the interfacial properties are also involved. Utilizing its sensitivity to the environment, PL is also of interest as a nano-scale sensing tool.</p>

DOI： 10.11470/oubutsu.77.6_656

CiNii Research

Language：Japanese   Publisher：Journal of Physics: Conference Series  

The electrical properties of carbon nanotube FETs (CNTFETs) have been studied in detail. The conduction type of the CNTFETs was dependent on the work function of the contact metal, which suggests that Fermi level pinning at the metal/nanotube interface does not happen. Based on the two-probe and four-probe resistance measurements, it was shown that the carrier transport at the contact is explained by the edge contact model even in the diffusive regime. The chemical doping using F4TCNQ was effective to reduce not only the channel resistance but also the contact resistance. It has also been shown that the surface potential measurement based on the electrostatic force detection in the scanning probe microscopy was effective in studying the behavior of the CNTFETs such as the transient behavior and the effect of the defects. Finally, in the CNTFETs fabricated using plasma-enhanced (PE) CVD-grown nanotubes, most of the drain current could be modulated by the gate voltage with little non-depletable drain current suggesting the preferential growth of the nanotubes with semiconducting behavior. © 2008 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/109/1/012002

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Language：Japanese  

Web of Science

Language：Japanese   Publisher：Nano  

We have measured the potential distribution on carbon nanotube (CNT) field-effect transistors (FETs) using electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KFM). Clearer potential profiles were obtained by EFM than by KFM. When the CNT-FET is in the ON state, the EFM image shows uniform potential distribution along the CNT. In contrast, when the CNT-FET is in the OFF state, nonuniform potential image with dark spots are obtained. The dark spots can be attributed to the defects in the CNTs. © 2008 World Scientific Publishing Company.

DOI： 10.1142/S1793292008000812

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Language：Japanese  

Web of Science

Language：Japanese   Publisher：Physica Status Solidi (C) Current Topics in Solid State Physics  

AlGaN/GaN MIS-HEMTs using HfO2 gate insulator with a large dielectric constant have been fabricated. HfO2 gate insulator was deposited by pulsed laser deposition at room temperature. It has been demonstrated that the HfO2 gate insulator is effective in suppressing the gate leakage current keeping a large transconductance. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/pssc.200674769

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Language：Japanese   Publisher：ACS Nano  

Di-erbium and di-erbium-carbide endohedral metallofullerenes with a C 82 cage such as Er2@C82 (isomers I, II, and III) and (Er2C2)@C82 (isomers I, II, and III) have been synthesized and chromatographically isolated (99%). The structures of Er2@C82 (I, II, III) and (Er2C 2)@C82 (I, II, III) metallofullerenes are characterized by comparison with the UV-vis-NIR absorption spectra of (Y2C 2)@C82 (I, II, III), where molecular symmetries of the structures are determined to be Cs, C2v and C 3v, respectively. Furthermore, enhanced near-infrared photoluminescence (PL) at 1520 nm from Er3+ ions in Er 2@C82 (I, III) and (Er2C2)@C 82 (I, III) have been observed at room temperature. The PL intensities have been shown to depend on the symmetry of the C82 cage. In particular, the PL intensity of (Er2C2)@C 82 (III) has been the strongest among the isomers of Er 2@C82 and (Er2C2)@C82. Optical measurements indicate that the PL properties of Er2@C 82 (I, II, III) and (Er2C2)@C82 (I, II, III) correlate strongly with the absorbance at 1520 nm and the HOMO-LUMO energy gap of the C82 cage. © 2007 American Chemical Society.

DOI： 10.1021/nn700235z

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PubMed

Language：Japanese   Publisher：Physica Status Solidi (B) Basic Research  

The dependence of optical transition energies in single-walled carbon nanotubes (SWNTs) on environmental dielectric constant (εenv) have been investigated in the range of εenv from 1.0 to 37, by immersing SWNTs bridged overtrenches in various organic solvents by means of photoluminescence (PL) and the excitation spectroscopies. With increasing εenv, both E11 and E22 exhibited a redshift by several tens meV and a tendency to saturate at a εenv ∼ 5 without an indication of significant (n,m) dependence. The redshifts can be explained by dielectric screening of the repulsive electron-electron interaction. We have also investigated the time-resolved PL in air and in solvent, respectively, utilizing the excitation intensity correlation technique. When the sample was immersed in solvent, the correlation signal collapsed, and the decay time decrease as compared to in air. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/pssb.200776124

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Language：Japanese   Publisher：Nanotechnology  

We have studied the effects of p-type chemical doping with F 4TCNQ (tetrafluorotetracyano-p-quinodimethane) in carbon nanotube field-effect transistors (CNFETs). The transmission-line-model technique using multi-probe CNFETs has been employed to investigate the effects of chemical doping on the channel resistance and contact resistance. It has been found that chemical doping is effective in the reduction of the contact resistance as well as the channel resistance. The device performances of top-gate CNFETs such as transconductance, on-resistance, and on/off ratio were improved by the F 4TCNQ chemical doping on the access regions. © IOP Publishing Ltd.

DOI： 10.1088/0957-4484/18/41/415202

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Language：Japanese   Publisher：Chemical Physics Letters  

We theoretically investigate the environmental effect for optical transition energies of single-walled carbon nanotubes (SWNTs), by calculating the exciton transition energies of SWNTs. The static dielectric constants used in the exciton calculation can be expressed as a function of the dielectric constants of the surrounding material and that of the SWNT, in which the static and dynamic dielectric constants of the SWNT represent the screening effect of core electrons and the valence π electrons, respectively. The calculated results reproduce the environmental effect of the experimental transition energies for various surrounding materials and for various diameters of SWNTs. © 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cplett.2007.06.018

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 2: Letters  

We have investigated the current flow path between the nanotube and the contact electrode in carbon nanotube devices using multiprobe devices. The contact and channel resistances have been evaluated by two methods; transmission-line-model technique and four-probe measurement. By comparing the results, we have found that channel resistance evaluated by the four-probe measurement includes contact resistance. This indicates that the widely used four-probe measurement is not applicable to nanotube devices for the evaluation of channel resistance excluding contact resistance. This finding also implies that electron transport between the nanotube and the contact metal occurs at the edge of the contact electrode. ©2007 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.46.L474

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

The surface potential of carbon nanotube field-effect transistors (CNT-FETs) was measured using Kelvin probe force microscopy (KFM). A clear potential image of the CNT channel with a diameter of 1.1 nm was obtained by measurement in air. The measured potential image was dependent on the sequence of the gate bias, and showed transient behavior with a time constant of several tens of minutes. These behaviors were consistent with the drain current transient and the hysteresis of the current-voltage characteristic of the CNT-FETs. © 2007 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.46.2496

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CiNii Research

CiNii Research

Language：Japanese  

Web of Science

Language：Japanese   Publisher：The Surface Science Society of Japan  

It is important to understand the interfacial properties of the nanotube/metal contacts because, in the case of carbon nanotube field-effect transistors (NT-FETs), the FET action is dominated by the Schottky barrier formed at the interface of the source contact. In this study, we have studied the interfacial properties from the electrical characterization of NT-FETs. First, the conduction pass in the vicinity of the contact electrodes has been investigated using a multi-terminal device. Then, the relation between conduction property of NT-FETs and work function of the contact metals has been investigated. The interfacial band structure has been discussed in terms of the temperature dependence of the drain current.

DOI： 10.1380/jsssj.28.40

CiNii Research

Language：Japanese   Publisher：Nanotechnology  

We have investigated the relation between the conduction property and the work function of the contact metal in carbon nanotube field-effect transistors (NTFETs). The conduction type and the drain current are dependent on the work function. In contrast to NTFETs with Ti and Pd contact electrodes, which showed p-type conduction behaviour, devices with Mg contact electrodes showed ambipolar characteristics and most of the devices with Ca contact electrodes showed n-type conduction behaviour. This indicates that the barrier height of the metal/nanotube contact is dependent on the work function of the contact metal, which suggests that the Fermi-level pinning is weak at the interface, in contrast to conventional semiconductors such as Si and GaAs. We have also demonstrated nonlinear rectification current-voltage characteristics in a nanotube quasi-pn diode with no impurity doping, in which different contact metals with different work functions are used for the anode and the cathode. © 2006 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/17/14/011

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Language：Japanese   Publisher：Physical Review B - Condensed Matter and Materials Physics  

The optical transition energies, E11 and E22, of single-walled carbon nanotubes (SWNTs) suspended in air have been investigated for 20 species by photoluminescence and excitation spectroscopies. We have studied the environmental effects in photoluminescence by comparing our results with those for the SWNTs wrapped by sodium-dodecyl-sulfate (SDS), as reported by Weisman and Bachilo [Nano Lett. 3, 1235 (2003)]. The energy differences between air-suspended and SDS-wrapped SWNTs, Δ Eii = E ii air - E ii SDS, depends on the chiral vector (n,m), specifically on the chiral angle and type of SWNT (type I or type II). The Δ E11 and Δ E22 mostly blueshifted, with the exception of the Δ E22 of some type II SWNTs (that have a small chiral angle), which redshifted. With an increase in the chiral angle, the Δ E11 increased in type I SWNTs and decreased in type II SWNTs. In contrast, the Δ E22 demonstrated opposite dependence on the chiral angle. The differences in Δ E11 and Δ E22 between type I and type II disappeared in the SWNTs with chiral angles close to 30° (near armchair). The (n,m) dependence of the environmental effect on the transition energies can be explained by the difference in the effective mass, which contributes to the energy of Coulomb interactions between carriers. © 2006 The American Physical Society.

DOI： 10.1103/PhysRevB.73.235427

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

Single-walled carbon nanotubes are grown using grid-inserted plasma-enhanced chemical vapor deposition (PECVD). The field effect transistor operation was confirmed using the PECVD grown carbon nanotubes (CNTs). The preferential growth of the semiconducting nanotubes was confirmed in the grid-inserted PECVD by measuring current-voltage (1-V) characteristics of the devices. Based on the measurement of the electrical breakdown of the metallic CNTs, the probability of growing the semiconducting nanotubes has been estimated to be more than 90%. © 2006 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.45.5485

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

Antigen sensors using carbon nanotube field effect transistors (CNT-FETs) were fabricated. In order to avoid the problem of exposing source and drain electrodes directly to the phosphate buffered saline (PBS) solution, source and drain electrodes were covered with evaporated SiO film. The immobilization of antibodies on the device surface was confirmed by the observation of fluorescence. Drain current in the sensor device was decreased by the antibody-antigen binding, which suggests a potential use of CNT-FETs as antigen sensors. © 2006 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.45.5481

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

We have statistically studied the suppression of hysteresis in carbon nanotube field-effect transistors. The effect of the device fabrication process on the hysteresis has been investigated. The results show that contamination induced by the fabrication process impedes the suppression of hysteresis in device passivation. To remove the contamination, we have introduced a cleaning process as a treatment before passivation. The effectiveness of passivation with polymethylmetacrylate has been improved by the cleaning process using H 2SO4/H2O2 solution. © 2006 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.45.5501

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Language：Japanese   Publisher：Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers  

We have studied an inclined-gate-recess structure in order to clarify the effect of increasing the electric field in the channel. Two-dimensional device simulation has revealed that the electric field and the electron velocity in the channel have increased by the inclined-gate-recess structure, which leads to an improvement of gm. AlGaN/GaN high electron mobility transistors (HEMTs) with inclined-gate-recess have been fabricated. Improved gm and fT have been obtained, which confirms the importance of increasing the electric field in the channel. © 2006 The Japan Society of Applied Physics.

DOI： 10.1143/JJAP.45.3368

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Language：Japanese  

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Language：Japanese   Publisher：Nanotechnology  

We have studied the photoluminescence (PL) of individual single-walled carbon nanotubes (SWNTs) placed in field-effect transistor structures. The SWNTs were suspended in the air so that strong PL was obtained. When an external bias voltage was applied to the device, no spectral changes could be detected, but the intensity drastically increased or decreased. This behaviour is explained by the injection/extraction of carriers from/to the electrodes by the electric field. In the case of p-type FETs in the air, PL intensity increased due to hole injection by applying a small negative gate bias. After the device was heated in vacuum, the maximum PL intensity was obtained at zero gate bias. This can be explained by the change in the interface between the SWNT and contact electrodes. The drain field dependence of the PL intensity shows a monotonic decrease, which is explained by a competition between the recombination lifetime and transit time of photo-excited carriers in the SWNT. Using analytical steady-state rate equations on carrier density in the SWNT, the relation between the recombination lifetime and carrier transit time has been evaluated. The carrier saturation velocity has also been estimated. © 2006 IOP Publishing Ltd.

DOI： 10.1088/0957-4484/17/2/035

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CiNii Research

Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.61.1.4.0_805_3

CiNii Research

Language：Japanese   Publisher：Institute of Physics Conference Series  

AlGaN/GaN MIS-HEMTs using ZrO2 gate insulator with a large dielectric constant have been successfully fabricated. The gate leakage current decreased by more than three orders of magnitude compared to that of the conventional AlGaN/GaN HEMTs without the ZrO2 gate insulator. Current collapse has also been suppressed in the MIS-HEMTs. The decrease in g mmax, which was observed in the case of Si3N4 MIS-HEMTs, was suppressed in the present MIS-HEMTs demonstrating the effectiveness of the ZrO2 gate insulator. © 2005 IOP Publishing Ltd.

Scopus

Language：Japanese   Publisher：Institute of Physics Conference Series  

The transient behavior of AlGaN/GaN MIS-HEMTs was studied by drain current DLTS. Two negative peaks with activation energies of 0.49 eV and 0.62 eV were observed in the DLTS spectrum. The location where these levels existed was studied in detail. © 2005 IOP Publishing Ltd.

Scopus

Language：Japanese   Publisher：Institute of Physics Conference Series  

Surface potential of the AlGaN/GaN high-electron-mobility transistors was measured by Kelvin probe force microscopy to study the mechanism of the drain current collapse. The potential after the gate bias stress increased due to the emission of trapped electrons from the surface states. The time constant of the potential increase was 75 s, which was comparable to that of the drain current recovery from the collapsed level. © 2005 IOP Publishing Ltd.

Scopus

Language：Japanese   Publisher：Applied Physics Letters  

Deep levels in unintentionally doped n -type Al0.09 Ga0.91 N and Al0.17 Ga0.83 N films grown on sapphire by hydride vapor-phase epitaxy were characterized using capacitance deep-level transient spectroscopy and were compared to the reported electron traps in GaN grown by various techniques. It was shown that at least three dominant deep levels exist in each sample. The Al mole fraction dependence of their activation energy suggested that each of these three levels has the same origin as the three dominant well known point-defect-related deep levels in GaN, respectively. It is thought that deep levels in GaN change their electric characteristics in low Al content AlGaN. © 2005 American Institute of Physics.

DOI： 10.1063/1.2137901

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Language：Japanese   Publisher：Electronics Letters  

Direct integration of AlGaAs/GaAs HEMTs on AlN ceramic substrates has been demonstrated based on the fluidic self-assembly (FSA) technology. The FSA is a unique technology to arrange small device blocks (around a few tens of microns) onto the other substrates. With this technology the core part of the HEMT having no pad can be mounted on the ceramic substrate. The HEMT core can then be connected electrically with the circuit on a ceramic substrate using a planar wiring process. This eliminates large stray capacitance and inductance in the conventional technology. It has been demonstrated that the good FET characteristics are obtained even after FSA process. © IEE 2005.

DOI： 10.1049/el:20052840

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Scopus

Language：Japanese   Publisher：The Japan Society of Applied Physics  

In this paper we discuss the properties of InGaAs/AlAs metamorphic resonant tunneling diodes (RTDs) on GaAs substrates. The current–voltage, surface roughness, and low-frequency noise characteristics were investigated to clarify the effect of step-grading buffer layers. No degradation of the current–voltage characteristics were observed despite the large surface roughness due to a lattice mismatch. Moreover, low-frequency noise measurements showed only 1⁄<I>f</I> noise without special peak structures, and the amplitudes of which were comparable to those of the lattice-matched ones. Next, we fabricated resonant tunneling frequency divider ICs, which is based on the bifurcation phenomenon in a chaos system, on metamorphic substrates. A maximum operation frequency of 88 GHz and good phase noise properties were obtained, which are similar to those of the lattice-matched ones. These results demonstrate the effectiveness of the step-grading buffer layers even for the RTDs that consist of very thin layers and are sensitive to crystal qualities.

DOI： 10.1143/JJAP.44.4790

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

The effects of the fabrication process on the current–voltage (<I>I</I>–<I>V</I>) characteristics of carbon nanotube field effect transistors (CNT-FETs) were studied in detail. A large hysteresis observed in the <I>I</I>–<I>V</I> characteristics of the CNT-FETs having no passivation film became small by passivating the device surface with a SiN<I><SUB>x</SUB></I> film. The conduction behaivior of the CNT-FETs changed from p-type to ambipolar characteristics by SiN<I><SUB>x</SUB></I> passivation. Ambipolar characteristics were also observed in the top-gate CNT-FETs with a gate-source spacing of 2 μm.

DOI： 10.1143/JJAP.44.1599

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

The effect of a grid inserted between the anode and the cathode in plasma-enhanced chemical vapor deposition (PECVD) on carbon nanotube (CNT) growth was studied. The optimum grid-cathode distance and grid-cathode voltage were 4 mm and 2–10 V, respectively. Long small-diameter nanotubes were grown under these conditions. The smallest nanotube diameter obtained was 3 nm. However, when the grid-cathode voltage was higher than 10 V, short large-diameter nanotubes were grown. The field emission characteristics of CNTs with a low ON voltage and a high current density were explained by Fowler-Nordheim (FN) tunneling.

DOI： 10.1143/JJAP.44.2600

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

Photoresponse of carbon nanotube field-effect transistors (FETs) is investigated using microscopic measurements. The nanotube FETs, with an isolated single-walled carbon nanotube (SWNT) for the channel, were fabricated by means of the position-controlled nanotube growth technique. An increase in the off-state current and the threshold-voltage shift of the FET were caused by laser illumination. The increase in the off-state current is attributed to photocurrent due to carriers excited in the SWNT channel. The excitation spectrum of the photocurrent had a peak corresponding to optical absorption by the third interband gap of the van Hove singularity of the semiconducting SWNT with a diameter of ∼2 nm. The photocurrent increased in proportion to incident laser power with a dynamic range over four orders of magnitude. The external quantum efficiency was 2×10<SUP>−7</SUP>. An inverter action to optical-signal input was observed near the threshold voltage of the FET. The responsivity was as high as 2×10<SUP>−3</SUP> A/W for a single SWNT channel. This high responsivity is explained by the field-effect amplification phenomenon.

DOI： 10.1143/JJAP.44.1592

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

Vertically aligned long multi-walled carbon nanotubes (MWNTs) have been successfully grown by grid-inserted plasma-enhanced chemical vapor deposition (PECVD). A thin Fe (1 nm)/Ti (1 nm) double-layer catalyst was effective in growing the long MWNTs. The length of the MWNTs increased to 1 mm for the growth time of 120 min without growth termination. A thick Fe catalyst or thick Ti underlayer resulted in a low growth rate. Transmission electron microscopy, energy-dispersive X-ray analysis and thermo-gravimetric analysis (TGA) measurements suggest that the MWNTs were grown in a root growth mode. Moreover, the TGA measurements suggest that MWNTs of high quality/purity were successfully grown by grid-inserted PECVD with a double-layer catalyst.

DOI： 10.1143/JJAP.44.1554

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CiNii Research

Language：Japanese  

DOI： 10.1063/1.1865343

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DOI： 10.1063/1.1849835

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

A fine tuning of the band gap of single-wall carbon nanotubes (SWNTs) has been achieved by filling various types of endohedral metallofullerenes into the SWNTs, the so-called nanopeapods. We report various electronic transport properties of fullerene peapods used as the channels of field-effect transistors (FETs) and demonstrate that the metallofullerene peapods can provide the tunable band gaps of the FET channels depending on the type of metallofullerene inserted in the SWNTs. All of the metallofullerene peapods FETs exhibit p- and n-type, the so-called ambipolar carrier transportation by variable gate bias. The ranges of the off state regions of the FET fabricated highly sensitivity with respect to the amount of charge transfer in metallofullerenes, which results in band-gap engineering. Metallofullerene peapods can be used to manipulate the electronic structure of SWNTs in nanometer scale. In such a highly functionalized SWNT, metallofullerene peapods, might be a key material for fabricating and developing sophisticated electronic devices in the future.

DOI： 10.1143/JJAP.44.469

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

We have successfully fabricated carbon nanopeapod field-effect transistors (FETs) using peapods synthesized directly on a SiO<SUB>2</SUB>/Si substrate. Single-walled carbon nanotubes (SWNTs) for pods were synthesized on the substrate by utilizing position-controlled chemical vapor deposition. The end caps of the SWNTs were removed by oxidation by baking in air. Insertion of fullerenes into the cap-opened SWNTs was performed by the vapor phase doping method. Fabricated Gd@C<SUB>82</SUB> peapod FETs showed ambipolar characteristics. We have estimated the bandgap of the Gd@C<SUB>82</SUB> peapod to be ∼100 meV from the temperature dependence of the drain current.

DOI： 10.1143/JJAP.44.L1341

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DOI： 10.1117/12.580040

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DOI： 10.1016/j.carbon.2004.11.044

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DOI： 10.1063/1.1835551

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

We have studied the fluidic assembly (FA) technique for heterogeneous integration. In this technique, device blocks, the size of which ranges from a few tens of microns to a few hundreds of microns, are arranged onto another material substrate in fluid. Here, we propose a novel technique to improve FA, called fluid dynamic assembly (FDA), which employs fluid dynamic effects. In the FDA, a special structure is fabricated on the block, which stabilizes the posture of the blocks falling in the fluid. This can be used to control the face of assembled blocks. In this paper, the feasibility of the FDA is discussed on the basis of fluid dynamic simulation and experiments. The numerical simulations revealed that the face control can be carried out for the block having ringlike (O-type) structures on it. Then we carried out the FDA experiments using thin disk-shaped GaAs blocks with O-type structure, and demonstrated an 87% face control ratio for the blocks with 7.5-μm-thick structure. Moreover, the less symmetric C-type structure was examined, and demonstrated even a high face control ratio of 96%.

DOI： 10.1143/JJAP.43.5951

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

The direct observation of the chaos signal patterns in a microwave frequency range was demonstrated on the resonant tunneling chaos generator circuit. It is difficult to observe such high-frequency chaos because one must use a sampling oscilloscope. To overcome this problem, we devised a special circuit controlling the chaos to output identical signals repeatedly. This permits us to observe (seemingly)random patterns. This control scheme may lead to various applications.

DOI： 10.1143/jjap.43.5235

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CiNii Research

Language：Japanese  

DOI： 10.1109/LED.2004.832788

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

We have proposed a technique for fabricating carbon nanotube single-electron transistors using a focused ion beam (FIB). Wiring to a multiwalled carbon nanotube was formed by the ion-beam-assisted deposition of tungsten. Tunneling barriers were formed in the nanotube by ion beam etching. A drain current oscillation originating from the Coulomb blockade effect was observed at 23 K.

DOI： 10.1143/JJAP.43.5669

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CiNii Research

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DOI： 10.1063/1.1689404

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DOI： 10.1063/1.1687983

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DOI： 10.1016/j.physe.2003.11.197

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DOI： 10.1063/1.1650554

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Language：Japanese  

DOI： 10.1002/pssa.200303464

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

We proposed and successfully fabricated a GaAs-based semiconductor-on-insulator (SOI) metal–semiconductor-filed-effect-transistor (MESFET) using a low-temperature wafer-bonding technique with a spin-on low-<I>k</I> dielectric film, FO<I>x</I>, as a bonding layer. In this structure, the channel is isolated from the semi-insulating (SI) GaAs substrate by a FO<I>x</I> layer. The fabricated 1-μm-gate-length SOI MESFETs showed good pinch-off and saturation characteristics. One of the most important improvements expected for this structure is a reduction of the drain-source capacitance, <I>C</I><SUB>DS</SUB>, which dominates the microwave switching property (on/off ratio). It was demonstrated that the <I>C</I><SUB>DS</SUB> of the SOI MESFET is much smaller than that of the conventional GaAs MESFET. It was also confirmed that the drain current of the SOI MESFET is negligibly affected by the side-gate voltage.

DOI： 10.1143/JJAP.42.6839

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

The capacitor-coupled monostable-bistable transition logic element (C<SUP>2</SUP>MOBILE) is an extension of the high-speed logic gate MOBILE based on the negative differential resistance of the resonant tunneling diodes. The C<SUP>2</SUP>MOBILE uses a transient current through the coupling capacitor to transmit signals between gates, and is expected to operate with an extremely short gate delay time. In this paper, basic operations of the C<SUP>2</SUP>MOBILE are demonstrated with the circuits fabricated on InP substrates. The circuits tested here are the 2- and 3-stage logic circuits and the AND/OR gate. The gate delay time was estimated to be around 1 ps for these circuits. A much shorter gate delay time close to 100 fs should be possible with the sophisticated circuit design and fabrication process.

DOI： 10.1143/JJAP.42.6766

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DOI： 10.1049/el:20030972

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DOI： 10.1109/TED.2003.816549

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

Position-controlled carbon nanotube field effect transistors (FETs) have been fabricated by using patterned catalysts and chemical vapor deposition. A double-layer metal of platinum and cobalt was used as the catalyst. The use of a mixture of ethanol and argon as the source gas was effective for obtaining FETs with good characteristics. Coulomb oscillation was observed at room temperature. 76% of the fabricated devices showed FET operation. The characteristics of FETs with metallic nanotubes were improved by applying a high voltage.

DOI： 10.1143/JJAP.42.4116

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) using Si<SUB>3</SUB>N<SUB>4</SUB> film as a gate insulator have been successfully fabricated. The gate leakage current decreased by about three orders of magnitude compared to that of the conventional AlGaN/GaN HEMTs without the Si<SUB>3</SUB>N<SUB>4</SUB> gate insulator. The low-frequency noise of the MIS-HEMTs was smaller than that of the conventional HEMTs. Current collapse has also been suppressed in the MIS-HEMTs.

DOI： 10.1143/JJAP.42.2278

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

Metamorphic In<SUB>0.52</SUB>Al<SUB>0.48</SUB>As/In<SUB>0.53</SUB>Ga<SUB>0.47</SUB>As high electron mobility transistors (MHEMTs) were fabricated on GaAs substrates. A step-graded InAlAs buffer layer was used to relax the lattice mismatch between the active layers and the substrates. The electrical characteristics of the MHEMTs were compared with those of HEMTs on InP substrates (InP HEMTs) and pseudomorphic HEMTs (PHEMTs). Present MHEMTs have exhibited superior characteristics in the low-frequency (LF) noise and side-gate effect to InP HEMTs and PHEMTs. A sufficiently high current-gain cutoff frequency of 210 GHz was obtained for the MHEMTs with gate length of 0.1 μm.

DOI： 10.1143/JJAP.42.2219

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

Current-voltage characteristics of micro-Schottky contacts fabricated on Epitaxial lateral overgrowth (ELO) GaN have been measured. The obtained characteristics are classified into two groups; good contacts with small ideality factor (<I>n</I>-value) and poor contacts with large <I>n</I>-value. By comparing the distribution of <I>I</I>-<I>V</I> characteristics of the micro-Schottky contact array with the distribution of dislocations revealed by molten KOH etching, it was concluded that the dislocations are not responsible for the poor <I>I</I>-<I>V</I> characteristics of the Schottky contacts. Regarding the origin of the poor <I>I</I>-<I>V</I> characteristics, the high doping density area formed at the top surface of the as-grown wafer was suggested.

DOI： 10.1143/JJAP.42.2250

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CiNii Research

Language：Japanese   Publisher：The Japan Society of Applied Physics  

We have studied the fluidic assembly technique for heterogeneous integration. In this technique, small device blocks, the size of which ranges from a few tens of microns to a few hundreds of microns, can be assembled into the recesses formed on the Si substrate. First, we used thin disk-shaped GaAs blocks to study the fundamental process of the fluidic assembly. A 100% yield was achieved with an appropriate strength of sonic vibration. The assembled blocks were confirmed to have no strain by using Raman scattering spectroscopy even if they were annealed at 320°C for 30 min. In addition, the effects of the shape of blocks are discussed based on the experimental results for disk- and square-shaped blocks.

DOI： 10.1143/JJAP.42.2226

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DOI： 10.1007/s00339-002-2039-7

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

Frequency dispersion of the drain conductance was observed in AlGaN/GaN high electron mobility transistors (HEMTs). The transition frequency shifted to higher frequencies with increasing temperature. The activation energy for the change of the transition frequency was 0.47 eV, which was almost the same as that obtained by the measurement of the temperature dependence of the low-frequency noise.

DOI： 10.1143/JJAP.42.424

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.58.2.4.0_760_3

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DOI： 10.1002/pssc.200303405

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Publisher：Japan Society for Software Science and Technology  

DOI： 10.11309/jssst.20.537

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DOI： 10.1063/1.1522482

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DOI： 10.1002/ecjb.10055

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A large gate leakage current in AlGaN/GaN high electron mobility transistors (HEMTs) was observed. Temperature dependence of <I>I</I><FONT SIZE="-1"><SUB>g</SUB></FONT>-<I>V</I><FONT SIZE="-1"><SUB>g</SUB></FONT> characteristics revealed that tunneling current is dominant in the leakage current. By introducing ECR plasma treatment before the gate metal deposition, the gate leakage current was reduced by two to three orders of magnitude.

DOI： 10.1143/JJAP.41.5125

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Electroluminescence (EL) in GaN high electron mobility transistors (HEMTs) biased at high drain-source voltages was investigated. The electroluminescence was observed at the drain edge where the high-field region is formed. This is quite different from the case of GaAs HEMTs where luminescence was observed at the gate edge. EL spectroscopy was also performed. The luminescence with the wavelength of visible to near-infrared light, the energy of which was less than the band gap energy, was observed with polarization in the direction parallel to the drain current.

DOI： 10.1143/JJAP.41.1990

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DOI： 10.1049/el:20020222

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Language：Japanese   Publisher：The Japan Society of Applied Physics  

This paper demonstrates high-frequency operations of the frequency divider IC composed of a resonant tunneling diode and a high electron mobility transistor. The maximum frequency dividing operation of 6.3 GHz was confirmed. This circuit is based on the long-period behavior of the nonlinear circuits generating chaos. We investigate the effects of the input frequency, the bias voltage, and the input amplitude on the operation to discuss the operating margins. It is also shown that the dividing ratio can be selected by changing the input amplitude.

DOI： 10.1143/JJAP.41.1150

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Temperature distributions in AlGaN/GaN high-electron-mobility transistors under bias voltage have been measured by micro-Raman scattering spectroscopy. The temperature was estimated from the Raman shift of E<FONT SIZE="-1"><SUB>2</SUB></FONT> phonons of GaN. The spatial and temperature resolutions are 1 μm and 10 K, respectively. When the power dissipation was 248 mW at a drain voltage of 40 V, the peak temperature of 443 K was observed at the gate edge on the drain side at the center of the channel. This position corresponds to the high-field region at the gate edge.

DOI： 10.1143/jjap.41.l452

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A novel delta-sigma (ΔΣ) analog-to-digital converter (ADC) using resonant tunneling diodes (RTDs) is proposed. A ΔΣ modulator circuit, which is the key element of the ΔΣ ADC, can be designed in a very simple form using a monostable-bistable transition logic element (MOBILE). The operation of this ΔΣ modulator circuit is confirmed by numerical simulation.

DOI： 10.1143/jjap.40.l1005

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We have investigated resonant-tunneling-injection photoluminescence (PL) of single InAs self-assembled quantum dots (QDs) which were embedded in the barrier layer of an n-GaAs/i-AlGaAs/n-GaAs tunneling diode. A triangular structure in the PL intensity-voltage characteristics of a single PL line was observed. This reflects the electron tunneling from a three-dimensional emitter to the zero-dimensional quantum level of the QD. In addition, three subpeaks and a broad bulge of the PL were superimposed on the triangular structure. The subpeaks were ascribed to the resonant tunneling of holes into valence quantum levels of the QD. The origin of the excess PL bulge which is obtained at off-resonance bias condition is also discussed.

DOI： 10.1143/jjap.40.2065

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We studied photoluminescence (PL) of a resonant tunneling transistor with a p<FONT SIZE="-1"><SUP>+</SUP></FONT>/n-junction gate. The excitation energy was selected to be close to the band-gap energy of the GaAs collector layer 1) so as not to excite the barrier layer and quantum well directly, and 2) in order to obtain a simple luminescence spectrum. The PL signal shows strong correlation with the resonant tunneling current. The PL peak position shows a redshift with increasing collector voltage, indicating the existence of the quantum-confined Stark effect. The collector voltage dependence of the PL linewidth suggests the existence of charge accumulation in the quantum well. The accumulated electron density and the charging time were estimated. It was also shown that the PL intensity could be controlled by the gate voltage without affecting the resonant tunneling behavior of electrons.

DOI： 10.1143/jjap.39.35

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We studied electroluminescence (EL) intensity distributions in the direction of gate width of n<SUP>+</SUP> self-aligned gate GaAs metal-semiconductor field-effect transistors (MESFETs). Nonuniform EL was observed along the gate-width direction, suggesting the nonuniform high-field formation in the direction of the gate. It has also been found that the EL at the source side shows gentler distribution than that at the drain side. These features suggest that the nonuniform hole distribution generated by impact ionization at the drain side becomes gradual during the process of their flow into the source and gate electrodes.

DOI： 10.1143/jjap.38.1363

CiNii Research

Language：English   Publisher：The Japan Society of Applied Physics  

A novel optoelectronic resonant tunneling logic gate, OE MOBILE, is proposed and its basic logic functions such as NOT, NOR, NAND, OR, and AND are confirmed for optical inputs. The fundamental idea is to use optical inputs to determine the logic level of the gate which consists of two serially connected resonant tunneling transistors (RTTs) with a p⁺/n-junction gate. The OE MOBILE is operated by oscillating bias voltage to employ monostable-bistable transition. At the time when the transition occurs, the optical inputs are applied by illuminating the RTTs to control their peak currents and consequently to determine the output logic levels. The power of an optical input for logic operations is as small as 5 µW owing to the operation based on monostable-bistable transition. It should be pointed out that the functions of an OE MOBILE can be changed between NOR and NAND by changing the control terminal voltage.

DOI： 10.1143/jjap.38.2586

CiNii Research

Language：English   Publisher：The Japan Society of Applied Physics  

We report ultranarrow (≤60 µeV) photoluminescence (PL) lines originating from single InAs quantum dots (QDs) sandwiched by Al<SUB>0.35</SUB>Ga<SUB>0.65</SUB>As barrier layers, demonstrating their δ-function-like density of states (DOS). The temperature dependence of the full-width at half maximum (FWHM) was studied between 10 and 90 K. It linearly increased with increasing temperature from 40 to 80 K (~7 µeV/K), suggesting the existence of an excitonic dephasing mechanism. Pure dephasing of excitons due to exciton-phonon interactions in acoustic phonon mode was suggested as a possible mechanism for the PL line broadening.

DOI： 10.1143/jjap.38.546

CiNii Research

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.22661/AAPPSBL.2020.30.4.10

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41409-019-0469-5

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

DOI： 10.1016/j.carbon.2017.09.079

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jid.2017.06.028

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-017-10322-0

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DOI： 10.1038/ncomms7305

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DOI： 10.1038/ncomms7305

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