S. N. Cha

Nanoelectromechanical switches with vertically aligned carbon nanotubes

Electromechanical switching devices have been fabricated successfully employing vertically grown multiwalled carbon nanotubes (MWCNTs) from the prepatterned catalyst dots on the patterned device electrodes. The devices show various interesting switching characteristics depending on the length and the number of MWCNTs used. The device design not only simplifies the fabrication process, but also improves the integration density greatly. The device has a great potential in realizing technically viable nanoelectromechanical systems, such as switch, memory, fingers, or grippers.

High performance ZnO nanowire field effect transistor using self-aligned nanogap gate electrodes

A field effect transistor (FET) using a zinc oxide nanowire with significantly enhanced performance is demonstrated. The device consists of single nanowire and self-aligned gate electrodes with well defined nanosize gaps separating them from the suspended nanowire. The fabricated FET exhibits excellent performance with a transconductance of 3.06μS, a field effect mobility of 928cm2∕Vs, and an on/off current ratio of 106. The electrical characteristics are the best obtained to date for a ZnO transistor. The FET has a n-type channel and operates in enhancement mode. The results are close to those reported previously for p-type carbon nanotube (CNT) FETs. This raises the possibility of using ZnO as the n-type FET with a CNT as the p-type FET in nanoscale complementary logic circuits.

Fabrication of a nanoelectromechanical switch using a suspended carbon nanotube

Fabrication and characterization of a nanoelectromechanical switching device consisting of a suspended multiwalled carbon nanotube and self-aligned electrodes is reported. The device has a triode structure and is designed so that a suspended carbon nanotube is mechanically switched to one of two self-aligned electrodes by repulsive electrostatic forces between the nanotube and the other self-aligned electrode. Carbon nanotubes are dispersed on an SiO2 coated Si wafer and their locations recorded using a scanning electron microscope mapping process. Contact electrodes and self-aligned deflection electrodes are formed by a process comprising electron beam lithography, metallic thin film deposition, and lift-off. The electrical measurements show well-defined ON and OFF states with change of gate voltage. The measured threshold voltage for electromechanical switching is ∼3.6V.

Nanoscale capacitors based on metal-insulator-carbon nanotube-metal structures

We report the fabrication process and the electrical characteristics of a nanocapacitor structure using metal-insulator-carbon nanotube-metal layers. The structure shows high capacitance and the possibility of ultrahigh integration density due to the unique nanotube structure. Nanoscale and high-aspect-ratio patterns are achieved by electron beam lithography for the fabrication of these vertical nanostructures. This structure can be substituted for capacitors based on the silicon pillar structure in dynamic random access memory or as a nanoscale capacitor for various nanoelectronic devices.

Nanoelectromechanical switch with low voltage drive

The triode structure vertical carbon nanotube based nanoelectromechanical switch shows excellent low voltage drive (∼4.5 V), owing to its vertical gate and the narrow gap between structural elements. The insulator deposition and the selective etching process steps simplify fabrication through self-alignment. The thickness of the insulator determines the width of the gap and the etching process, used to produce the vertical gate, removes the need for a complicated lithography step. The low drive voltage increases device stability and reliability and allows the device to be deployed in a wide range of applications.

Electrical transport between epitaxial manganites and carbon nanotubes

The possibility of performing spintronics at the molecular level may be realized in devices that combine fully spin polarized oxides, such as manganites with carbon nanotubes. However, it is not clear whether electrical transport between such different material systems is viable. Here, we show that the room-temperature conductance of manganite-nanotube-manganite devices is only one-half of the value recorded in similar palladium-nanotube-palladium devices. Interestingly, the former shows a gap in the conductivity below the relatively high temperature of 200 K. Our results suggest the possibility of new spintronics heterostructures that exploit fully spin polarized sources and drains.

High performance ZnO nanowire field effect transistor

A zinc oxide (ZnO) nanowire field effect transistor having the highest mobility and lowest subthreshold slope obtained to date (from ZnO) is reported. The device consists of a single nanowire with self-aligned gate electrodes having well defined nanoscale spacing, independent of lithographic definition. The fabricated device exhibits a transconductance of 3.06 /spl mu/S, a mobility of 450 cm/sup 2//Vs and a subthreshold swing of 129 mV/decade. The transistor also shows an on/off current ratio of 10/sup 6/.

Nanoscale capacitors based on metal-insulator-carbon nanotube-metal (MICNM) structures

We report on the electrical characteristics and the fabrication process of a nano-capacitor structure using metal-insulator-carbon nanotube-metal (MICNM) layers. The structure shows high capacitance and the possibility of ultra high integration density due to the unique nanotube structure. Nanoscale and high aspect ratio patterns are achieved by electron beam lithography for the fabrication of these vertical nanostructures. This structure can be applied to the substitution of capacitors employing the silicon pillar structure in dynamic random access memory (DRAM) or as a nanoscale capacitor for various nanoelectronic devices.

CNT based mechanical devices for ULSI memory

Nanoelectromechanical (NEM) devices were developed for memory. The concept of a switch unit employing carbon nanotubes (CNT) was extended to random access memory (RAM). The unique vertical structure of these nanotubes allows a high integration density for devices. The easy fabrication process can give a high yield and reliability to device

Nanotube and nanowire transistors

In this paper we report the use of in-situ grown single wall carbon nanotubes (SWCNTs) from pre-patterned catalyst islands to construct nanotube electronics. The SWCNTs were grown via thermal chemical vapour deposition (CVD) on catalysts islands which were prepared by sputtering, initially, alignment marks are patterned simultaneously with the catalysts islands to enable accurate overlay of contact patterns during the top down fabrication approach for SWCNT devices. The gate transfer characteristics of p-channel SWCNTs are presented. The use of pre-patterned catalyst islands allows control of the SWCNT location required for integrated circuits. Characteristics of ZnO nanowire transistors are also introduced. Very high mobilities are measured in n-channel devices in which the gate is defined in a self aligned manner to have a nanoscale air-gap insulator. The characteristics of the ZnO transistor are comparable to those of achieved from SWCNTs. This raises the possibility of using SWCNTs for p-channel and ZnO nanowires for n-channel in complimentary switching devices.