Wanjun Park

Aligned carbon nanotubes for nanoelectronics

We discuss the central issues to be addressed for realizing carbon nanotube (CNT) nanoelectronics. We focus on selective growth, electron energy bandgap engineering and device integration. We have introduced a nanotemplate to control the selective growth, length and diameter of CNTs. Vertically aligned CNTs are synthesized for developing a vertical CNT-field effect transistor (FET). The ohmic contact of the CNT/metal interface is formed by rapid thermal annealing. Diameter control, synthesis of Y-shaped CNTs and surface modification of CNTs open up the possibility for energy bandgap modulation. The concepts of an ultra-high density transistor based on the vertical-CNT array and a nonvolatile memory based on the top gate structure with an oxide–nitride–oxide charge trap are also presented. We suggest that the deposited memory film can be used for the quantum dot storage due to the localized electric field created by a nano scale CNT-electron channel.

Oxygen-induced p-type doping of a long individual single-walled carbon nanotube

The effect of oxygen adsorption on a nanotube-based field effect transistor have been controversial as to whether it induces p-type doping of the nanotube body or the work function increase in the metal electrode. Here we report a transport measurement showing that a long individual single-walled nanotube can be doped as p-type upon oxygen adsorption. We discuss that, despite the fact that the charge transfer between the nanotube and O2 adsorbator has not been agreed to date, the effect of oxygen adsorption should still be interpreted as inducing p-type doping in the nanotube body. The n-type doping by NH3 adsorption is also measured for the purpose of comparison. Based on these observations, we suggest that, while the Schottky barrier management could be more effective for the transistor with a short nanotube, the doping effect could be more influential in devices with longer nanotubes.

ZnO nanoparticle growth on single-walled carbon nanotubes by atomic layer deposition and a consequent lifetime elongation of nanotube field emission

ZnO nanoparticles were grown on single-walled carbon nanotubes (SWNTs) by atomic layer deposition using diethylzinc (DEZ) and water. The athors discuss that, because of chemical inertness of nanotubes to DEZ and water molecules, such nanoparticles are not likely to grow on the wall of clean and perfect nanotubes. Rather, the growth of ZnO nanoparticles should be attributed to imperfection of nanotubes, such as defects and carbonaceous impurities. Lifetime of field emission from SWNTs with the ZnO nanoparticles is 2.5 times longer than that from the as-grown nanotubes. It is thought that the protection of the defects or impurities by ZnO nanoparticles mainly contributed to the improvement of the field emission lifetime from SWNTs.

Unusual transport characteristics of nitrogen-doped single-walled carbon nanotubes

Electrical transport characteristics of nitrogen-doped single-walled carbon nanotubes (N-SWCNTs), in which the nitrogen dopant is believed to form a pyridinelike bonding configuration, are studied with the field effect transistor operations. Contrary to the expectation that the nitrogen atoms may induce a n-type doping, the electrical transports through our N-SWCNTs are either ambipolar in vacuum or p-type in air. Through the first-principles electronic structure calculations, we show that the nitrogen dopant indeed favors the pyridinelike configuration and the Fermi level of the pyridinelike N-SWCNT is almost at the intrinsic level.

Direct growth of single-walled carbon nanotubes on conducting ZnO films and its field emission properties

Despite the necessity of direct growth of single-walled carbon nanotubes (SWNTs) on conducting films for versatility of designing device architectures for nanoelectronics and optoelectronics, most of SWNT growths have been carried out on insulating films or supporting materials such as SiO2 and Al2O3. Here, the authors report that conducting ZnO films can be used as both an underlying layer for the SWNT growth and an electrode for device operation. ZnO films with a resistivity in the order of 10−3Ωcm were deposited by atomic layer deposition. SWNTs were directly grown on the ZnO film by water plasma chemical vapor deposition. The authors demonstrate field emission properties from the SWNT/ZnO cathode, of which the turn-on electric field for a current density of 10μA∕cm2 and the field enhancement factor are 1.8V∕μm and 3200, respectively.

A flexible graphene touch sensor in the general human touch range

We present a transparent touch sensor based on single layers of graphene that works under a gentle touch. Using the flexible characteristics of graphene, a touching event and a vertical force are measured by a change in the channel conductance. In contrast to the previous graphene gauge sensors, this is an alternative scheme that responds to a vertical force using the contacting properties of two isolated and patterned single graphene layers. This sensor responded to pressures ranging from 1 to 14 kPa, corresponding to the lowest human perception. In addition, we outline the processing methods for handling single layers of graphene for the integration of devices on transparent and flexible substrates.

Adsorption-induced conversion of the carbon nanotube field effect transistor from ambipolar to unipolar behavior

We investigate ambipolar to unipolar transition by the effect of ambient air on the carbon nanotube field-effect transistor. A unipolar transport property of the double-walled nanotube field-effect transistor and its conversion from ambipolar behavior are observed. We suggest that adsorptions of oxygen molecules, whose lowest-unoccupied-molecular-orbital state is around the midgap of the carbon nanotube, could suppress the electron channel formation and, consequently, result in the unipolar transport behavior.

Thin film transistors using preferentially grown semiconducting single-walled carbon nanotube networks by water-assisted plasma-enhanced chemical vapor deposition

Nearly perfect semiconducting single-walled carbon nanotube random network thin film transistors were fabricated and their reproducible transport properties were investigated. The networked single-walled carbon nanotubes were directly grown by water-assisted plasma-enhanced chemical vapor deposition. Optical analysis confirmed that the nanotubes were mostly semiconductors without clear metallic resonances in both the Raman and the UV-vis-IR spectroscopy. The transistors made by the nanotube networks whose density was much larger than the percolation threshold also showed no metallic paths. Estimation based on the conductance change of semiconducting nanotubes in the SWNT network due to applied gate voltage difference (conductance difference for on and off state) indicated a preferential growth of semiconducting nanotubes with an advantage of water-assisted PECVD. The nanotube transistors showed 10(-5) of on/off ratio and approximately 8 cm2 V(-1) s(-1) of field effect mobility.

Magnetization switching and tunneling magnetoresistance effects of synthetic antiferromagnet free layers consisting of amorphous NiFeSiB

MTJs with the synthetic antiferromagnetic (SAF) free layer consisting of CoFeSiB/Ru/CoFeSiB were prepared because a SAF structure can reduce the magnetostatic coupling. Magnetic switching and tunneling magnetoresistance (TMR) ratio of MTJs were investigated by experiment and by simulation. SAF structures show lower TMR ratio and coercivity (H/sub c/) than single free layer due to low net magnetic moment. The CoFeSiB SAF structure was found to have lower exchange energy.

Growth and characterization of nitrogen-doped single-walled carbon nanotubes by water-plasma chemical vapour deposition

Nitrogen-doped single-walled carbon nanotubes (N-SWNTs) are directly grown on SiO2/Si substrates at 450??C with methane and ammonia gases by water-plasma chemical vapour deposition. The strongest radial breathing mode peak in Raman spectra of the grown N-SWNTs, probed with a 633?nm laser excitation, was assigned to (7, 5) semiconducting nanotubes with a diameter of 0.83?nm. As the doped nitrogen content increases, the D-band to G-band ratio in Raman spectra, indicating the imperfection of nanotubes, gradually increases and saturates at around 4%. X-ray photoelectron spectroscopy shows that nitrogen atoms are doped with a pyridine-like configuration in the N-SWNTs.