Chang Hoon Choi

Tungsten nanowires and their field electron emission properties

We report the fabrication of tungsten nanowires, by simple thermal treatment of W films, that behave as self-catalytic layers and their excellent electron field emission properties as well. The obtained nanowires have a diameter ranging from 10 to 50 nm, showing perfect straightness and neat appearance. Typical turn-on field for the electron emission is about 5 V/μm, and the field enhancement factor β becomes 38 256, which is very close to that of the high efficient single-wall carbon nanotube emitters. The most exciting result is the possibility of easy fabrication of perfectly straight nanowires as promising building blocks for terabit-level interconnection and nanomachine components without the intentional use of any heterogeneous catalysts.

Fabrication and characteristics of field emitter using carbon nanotubes directly grown by thermal chemical vapor deposition

We experimentally present the effects of vertical alignment and density of carbon nanotubes on the emission current level. For practical display application, we have fabricated the triode type emitter using directly grown nanotubes as emission tip, and characterized their basic field emission properties. The triode type emitter exhibited a turn-on voltage of 37 V and an anode current density of 1.7 μA with gate voltage at 47 V. The vertical alignment of nanotubes does not play a key role in improving the emission properties in triode type nanotubes emitter.

Direct nano-wiring carbon nanotube using growth barrier: A possible mechanism of selective lateral growth

In this work we report the direct nano-bridging of carbon nanotubes (CNT) between micro-sized islands using conventional photolithography technique necessary for the nanomachining and the molecular device applications compatible with the Si-based process. The most distinct feature in this work is to use a growth barrier of Nb metal or insulating layer on the top of the catalytic metal to prevent the growth of CNT from the vertical direction to the substrate. As a result, CNTs of either “straight line” or a perfect “Y shape” were selectively grown between lateral sides of the catalytic metals or pre-defined electrodes without any trace of vertical growth. The length of the CNTs was 500–1000 nm and the diameter thinner than about 20 nm. We suggest that magnetic and crystallographic characteristics due to the unique interaction between the Nb overlayer and ferromagnetic Ni catalysts and nano-granulation of Ni layer during the growth process are important for the lateral (i.e., parallel to the substrate) CNTs...

Suppression of leakage current via formation of a sidewall protector in the microgated carbon nanotube emitter

In this work, we have fabricated a triode field emitter using directly grown carbon nanotubes (CNTs) as an electron emission source. Vertically aligned CNTs have been grown in the centre of the gate hole, to the size of 1.5µm in diameter, using thermal chemical vapour deposition. A comparison of the field emission characteristics of two types of microgated nanotube emitter with and without a sidewall protector for the gate hole is made. A sidewall protector formed by the method of tilting the substrate can enhance the electrical characteristics by suppressing the problem of short circuits between the gate and the CNTs. The leakage current of an emitter with a sidewall protector is approximately sevenfold lower than that of an emitter without a sidewall protector at a gate voltage of 100 V.

Coulomb blockade devices of Co dot arrays on tungsten-nanowire templates fabricated by using only a thin film technique

A simple fabrication of gate-controlled nanodevices made of the Co nanodots on tungsten (W)-nanowire templates by using conventional photolithography and a thin film technique is reported. The combined multiple grain nanobridge allows the observation of the Coulomb gap up to a temperature as high as 200 K and shows nonlinear current–voltage characteristics up to 250 K. The combination of Co dots with straight W-nanowire templates opens possibilities of reproducible blockade devices.

Growth and electrical properties of multidimensional tungsten nano-buliding blocks

We report the demonstration of tungsten nanoblocks such as nanorods, nanoblocks, and nanocylinders through a simple thermal process of sputtered W films. The formation of various types of tungsten nanostructures was based on the difference in the magnitude of the residual film stress and in its spatial distribution, which depends on the deposition pressure during the sputter deposition of self-catalytic W thin film. The resulting shapes displayed well-controlled structures that have characteristic dimensionalities such as zero-dimensional dot, one-dimensional wire, and two-dimensional nanosheet with the deposition pressure. The results demonstrate the possibility of construction of in situ multidimensional achitectures through the one-step thermal process which uses a self-catalytic function of each thin film.

Gated field emitter using carbon nanotubes for vacuum microelectronic devices

We have fabricated a gated field emitter using directly grown carbon nanotubes (CNTs) as an electron emission source. Vertically aligned CNTs were grown at the center of the gate hole using thermal chemical vapor deposition. In order to reduce the leakage current due to flowing current along CNTs grown on the sidewalls of the gate hole, we adopted a new process scheme such as sidewall protector. The leakage current of gated CNT emitter with sidewall protector shows a decrease of 85.8 % compared to that of a conventional structure.

Selective lateral nano-bridging of carbon nanowire between catalytic contact electrodes

We report selective lateral nano-bridging of carbon nanowire (CNW) between microsized islands using conventional photolithography technique necessary for the nanomachining and the molecular device applications compatible with Si-based process. Most distinct feature in this work is to use a growth barrier of Nb metal or insulating layer on the top of the catalytic Ni metal to prevent the growth of CNW from vertical direction to the substrate. As a result, CNWs of either “straight line” or a “Y-junction” were selectively grown between lateral sides of the catalytic metals or pre-defined electrodes without any trace of vertical growth. These results clearly indicate that this method would be one of the most feasible fabrication techniques for the nanomachines or the electronic applications with high integration level through process optimization.

A novel micro-gas sensor using laterally grown carbon nanotubes

We experimentally present the multiwalled carbon (MWNT) based gas sensor by a direct nano-wiring between Nb electrodes. Upon exposure MWNT to NH/sub 3/, the electrical resistance of the MWNTs is found to increase. The increase of temperature and gas concentration actually reduces the response time and increases the sensitivity to introduction of gas, respectively. Our results show a promising technique for forming CNT bridges between electrodes for chemical gas sensor at room temperature.