Jeff Tsung-Hui Tsai

On-Chip Fabrication of Well-Aligned and Contact-Barrier-Free GaN Nanobridge Devices with Ultrahigh Photocurrent Responsivity**

In the last two decades, a wide range of semiconductor nanowires have been synthesized and used as building blocks for the development of a new generation of electronic and optoelectronic devices. [1–8] The integration of these nanowires into the thin-film-based microchip has become a critical problem in the practical application of the nanomaterial properties and for industrial manufacture. Single-wire-based devices have been shown to possess novel properties and provide a major platform for fundamental research. [9–13] However, the conventional fabrication of single-wire devices by the ‘‘pick and place’’ method is rather complicated and uneconomic, which is unsuitable for large-scale manufacturing. In contrast, devices based on an ensemble of nanowires are much easier to fabricate, thus reducing the barriers to practical applications. A method based on the bridging concept and showing the potential to directly integrate an ensemble of nanowires onchip was first demonstrated by Haraguchi et al. [14] Different from the traditional ‘‘bottom-up growth and then top-down processing’’, the nanowires are laterally grown across a trench and suspended between two film posts as nanobridges (NBs). [14–26] As the main device architecture and electrodes can be designed and prepared prior to NB growth, nanomaterial deterioration due to the post treatment in the

Effect of graphitic inclusions on the optical gap of tetrahedral amorphous carbon films

High sp3 fraction tetrahedral amorphous carbon (ta-C) films can be prepared using the filtered cathodic vacuum arc (FCVA). A by-product of the deposition process are small micrometer sized graphitic particles which are also incorporated into the film. The particle coverage of FCVA films is typically <5%, and thus the effect of these graphite inclusions have been largely ignored in earlier optical gap measurements of ta-C. By incorporating a better filter design (e.g., S-bend filter), the particle coverage can be reduced to 0.1%. In this article, we show that the effect of these graphitic inclusions is to scatter or absorb light which significantly affects the optical gap measurement and hence reduces the “apparent” optical gap of the ta-C film. By comparing two ta-C films with different particle coverage but the same sp3 content of 85%, we show that we can correct for the effect of these inclusions. Our results confirm that the E04 gap of a 85% sp3ta-C matrix is 3.6 eV. The importance of considering these...

The role of dc current limitations in Fowler–Nordheim electron emission from carbon films

We investigated the electron field emission characteristics of carbon films deposited by filtered cathodic vacuum arc. A special sandwich structure was created using alternating layers of carbon and aluminum. This structure allows both field emission and dc current–voltage characteristics to be measured on the same film. The emission current was found to follow the Fowler–Nordheim law below a critical field, Ecri. Above Ecri, the current deviates from the Fowler–Nordheim law but follows the dc current–voltage characteristics of the film. It is concluded that the frequently observed deviation of electron emission from the Fowler–Nordheim law at high field is due to the current limitation of the resistive carbon film. The important implication of this finding for device fabrication is discussed.

Approach for a self-assembled thin film edge field emitter

A self-assembled thin film edge emitter has been fabricated. The unique fabrication process requires only a tetrahedral amorphous carbon (ta-C) thin film with one photolithography step to generate a three-dimensional structure. A high emission site density was achieved compared with that obtained from a flat ta-C film emitter using the same measurement technique. In the high field regime the emission obtained was based on the Fowler–Nordheim emission mechanism. Using the Fowler–Nordheim equation we calculated the field enhancement factor of this emitter to be 80. The potential of ta-C thin films as hard masks for silicon etching in KOH solution was also investigated.

Highest optical gap tetrahedral amorphous carbon

The deposition and characterisation of tetrahedral amorphous carbon (ta-C) with an E optical gap of 3.5 eV and Tauc gap of 04 3 eV is presented. This is the highest optical gap reported in literature for ta-C and was directly measured using photothermal deflection spectroscopy (PDS) and UV-Vis spectrophotometry. Independent PDS, UV-Vis and electron energy loss spectroscopy (EELS) measurements confirmed the high gap. A large Urbach slope of 600 meV was measured, which indicated that there are many tail states. Electron spin resonance (ESR) measurements confirmed that this material has a high spin density of 7.5=10 20 spinsycm . Post-deposition vacuum annealing of the samples to 500 8C resulted in a small increase of the optical gap, E ;3.6 3 04 eV, and a decrease of the defects to 2.7=10 spinsycm . Post-deposition annealing at this temperature did not significantly 20 3

Ultrahigh contrast light valve driven by electrocapillarity of liquid gallium

This letter describes an ultrahigh contrast valve driven by the electrocapillarity of liquid gallium. We demonstrate that a micrometer-sized gallium droplet can be used to fabricate a prototype backlight transmissive pixel cell by transforming the droplet into a flat thin film. This light valve exhibits significantly high backlight utility (96%), an exceptional contrast ratio (106:1), and fast response time (0.49 ms). The high contrast ratio originated from the exceptional reflectivity of gallium, which can block backlight to prevent any transmission in the off state of our device. Without using any polarizer, the backlight utility can be improved dramatically compared to a conventional liquid crystal display. The backlight utility and switching time obtained from this prototype light valve is higher than that of commercial liquid crystal displays. This concept is also applicable to a wide variety of electro-optical devices.

Fabrication of humidity sensors by multi-walled carbon nanotubes

Humidity sensors have multi-walled carbon nanotubes (MWNTs) as the sensing material is demonstrated. The sensor was fabricated on a silicon dioxide coated silicon wafer with metal electrodes. MWNTs were deposited and interlinked with the electrodes by means of the dielectrophoresis technique. The sensing device has the function of a hygrometer when measuring resistance variations to the local relative humidity percentage (RH%) through MWNTs. By measuring the MWNT resistances, we find that higher RH% results in a decrease of conductivity. The results indicate that electron transports in MWNTs are affected by water molecules adsorption on the outermost nanotube surface. A miniature thermocouple sensor was also fabricated and integrated with the humidity sensor. This allowed us to simultaneously sense environmental humidity and temperature. Hence, accurate humidity measurements were achieved with this prototype by calibrating the electrical resistance and temperature levels to carry out the tests with the humidity percentages.

Defect reduction of multi-walled carbon nanotubes by rapid vacuum arc annealing

A rapid thermal annealing process using a DC vacuum arc discharge system was shown to reduce defects in carbon nanotubes (CNTs). Multi-walled CNTs (MWCNTs) exhibit high-density structural imperfections when deposited via chemical vapour deposition at relatively low temperatures (∼650°C). These defects can be thermally annealed to reconstruct the graphitic structure. A vacuum arc discharge system was used to anneal the MWCNTs through several cycles at high temperatures (∼1800°C) followed by rapid cooling. The annealed MWCNTs were characterised by Raman spectroscopy and transmission electron microscopy. Rapid heating rearranged the imperfect graphitic structure and removed the weakly bonded defects. After eliminating a defect segment, the graphene shell was reconstructed during the cooling process to produce multi-shell perfection. This method effectively reduced MWCNT defects.

Plasma illumination devices enhanced by carbon nanotubes

In this paper, we report the fabrication and testing of a flat visible light source by discharging Ar gas at carbon nanotubes tips. The electrodes comprised of indium tin oxide coated glass plates on which a paste of UV excited phosphor powders blended with the carbon nanotubes were screen printed. When applying voltages on such electrodes, the sharp tips of carbon nanotubes generate very high electric fields at relatively low voltages. The results show that these electrodes reduce the plasma ignition voltages effectively compared to conventional metal electrodes. A prototype light panel was also fabricated. This panel was operated at 180 V DC with a current of 20 mA. The 25cm2 device has a uniform visible light emission with a luminance of ∼500cd∕m2.

Carbon Nanotube Reinforced Conductors for Flexible Electronics

This study demonstrates carbon nanotube (CNT) reinforced conductors for flexible electronics. The supreme mechanical strength of the CNT is toughened by the structure of the metal conductor. Conventional metal lines, fabricated using a screen printing technique on flexible substrates, usually suffer a great deal of stress when bending plastic substrates. These stress-strain educed micro-fractures in the metal line reduce the reliability of the electronic system. We fabricated a durable conductor, integrating CNTs into the conventional metallization process. It was found that the CNT-silver composite can be screen-printed on flexible substrates and the durability of the resulting conduction was enhanced. Compared to CNT-polymer-based materials, the CNT-silver composite exhibited higher conductivities of 104 S/cm. These results show that such an approach offers a promising of enhancing the reliability of flexible electronic systems by utilizing reinforced CNTs in the metallization process.