Woon Song

Fabrication of n-type carbon nanotube field-effect transistors by Al doping

We report the effect of an Al layer, covering the central part of the nanotube channel, on the electrical transport properties of carbon nanotube field-effect transistors (CNFETs). The CNFETs, consisting of single-walled carbon nanotube or double-walled carbon nanotube between two Pd electrodes on top of SiO2 layer, which showed p-type or ambipolar transport behaviors, exhibit clear n-type characteristics after the Al deposition. We ascribe such conversions into n-type behaviors to the electron doping in the Al-covered nanotube region, which results in the bending of the nanotube bands nearby the edges of the Al layer. This technique, Al deposition under a high vacuum, may give rise to a practical fabrication method for the n-type CNFET, which may enable us to develop complementary logic nanotube electronic devices.

Quantum Interference in Radial Heterostructure Nanowires

Core/shell heterostructure nanowires are one of the most interesting mesoscopic systems potentially suitable for the study of quantum interference phenomena. Here, we report on experimental observations of both the Aharonov-Bohm (h/e) and the Altshuler-Aronov-Spivak (h/2e) oscillations in radial core/shell (In2O3/InOx) heterostructure nanowires. For a long channel device with a length-to-width ratio of about 33, the magnetoresistance curves at low temperatures exhibited a crossover from low-field h/2e oscillation to high-field h/ e oscillation. The relationship between the oscillation period and the core width was investigated for freestanding or substrate-supported devices and indicated that the current flows dominantly through the core/shell interface.

Current-carrying capacity of double-wall carbon nanotubes

We have studied electrical transport characteristics of individual double-wall carbon nanotubes (DWNT) under high bias voltages. As the bias voltage applied to the carbon nanotubes increases, the outermost shell of the DWNTs broke down sequentially, which enabled us to determine the current-carrying capacity of each shell. The maximum current-carrying capacity per shell was about 150??A, which is well above that of any other previous reports.

Superconducting junction of a single-crystalline Au nanowire for an ideal Josephson device

We report on the fabrication and measurements of a superconducting junction of a single-crystalline Au nanowire, connected to Al electrodes. The current-voltage characteristic curve shows a clear supercurrent branch below the superconducting transition temperature of Al and quantized voltage plateaus on application of microwave radiation, as expected from Josephson relations. Highly transparent (0.95) contacts very close to an ideal limit of 1 are formed at the interface between the normal metal (Au) and the superconductor (Al). The very high transparency is ascribed to the single crystallinity of a Au nanowire and the formation of an oxide-free contact between Au and Al. The subgap structures of the differential conductance are well explained by coherent multiple Andreev reflections (MAR), the hallmark of mesoscopic Josephson junctions. These observations demonstrate that single crystalline Au nanowires can be employed to develop novel quantum devices utilizing coherent electrical transport.

Short-channel effect and single-electron transport in individual indium oxide nanowires

We have investigated the electric transport properties of individual In2O3 nanowire devices. We have found that the gate modulation characteristics depend strongly on the channel length. If the channel length is greater than 450 nm, the gate modulation curve exhibited field-effect transistor behavior with dominant n-channel current at room temperature. With the decrease of the channel length, the leakage current is increased due to the short-channel effect. For such short-channel devices, the gate modulation curve exhibited quasi-periodic current oscillations at low temperature, which are attributed to the Coulomb blockade of single-electron tunneling. Some devices showed two-fold periodicity in the Coulomb diamonds which may arise from the spin degeneracy of the single-particle energy levels.

Fabrication of n-type nanotube transistors with large-work-function electrodes

The authors found experimentally that carbon nanotube field-effect transistors (CNFETs) could exhibit n-type characteristics even though their electrodes consist of a large-work-function metal such as Co. To explain their result, which is contrary to the general belief that CNFETs with large-work-function electrodes always lead to p-type characteristics, ab initio electronic structure calculation for the metal-carbon nanotube junction was performed, which showed that the Fermi level alignment at the junction could sensitively depend on microscopic structures of the metal-carbon nanotube junction. This suggests that deposition method of electrodes as well as the metal type could be utilized to obtain n-type CNFETs.

Broadband Shot Noise Measurement System at Low Temperature for Noise Thermometry Using a Tunnel Junction

We report our development of a broadband shot noise measurement experiment at Korea Research Institute of Standards and Science for noise thermometry. A precision RF measurement setup in the frequency range near 1 GHz has been installed in a He-3 refrigerator. For noise thermometry, we fabricated an Al-AlOx-Al tunnel junction by the double-angle shadow evaporation technique as a source of noise. Temperature measurement was performed from 0.3 K to room temperature. The influence of each RF component was analyzed and tested to stabilize the measurement system. We compare two different diode detectors for RF power measurement. We describe the thermal anchoring scheme and the influence of tunnel junction imperfections.

A pulse-driven Josephson arbitrary waveform synthesis system as a quantum AC voltage standard in KRISS

We report our progress in the Josephson arbitrary waveform synthesis system in KRISS. The system is based on the pulse-driven Josephson junction array, and currently we routinely produce arbitrary waveforms up to 100 mVrms amplitude with distortions smaller than 100 dBc. Precision waveform synthesis results are presented. Our future works will include AC-DC thermal transfer measurement and quantum noise source for Johnson noise thermometry.

Development of a broadband shot noise measurement system at low-temperature for noise thermometry

We report our experimental setup for the shot noise thermometry system at KRISS. Precision RF measurement setup in the frequency range of about 1 GHz has been installed in a He-3 refrigerator, which can be cooled down to 300 mK. The shot noise of Al-AlOx-Al tunnel junction, fabricated by two angle- evaporation technique, was measured and compared with theoretical curve. The inferred temperature from the noise curve agrees well with the measured temperature.

Single electron pumping through a quantum dot-embedded carbon nanotube using surface acoustic wave

We have studied acoustoelectric current through a quantum dot-embedded carbon nanotube induced by a surface acoustic wave. The measurements were carried out on a same device but in two very different quantum dot charging energy Ec regimes (∼50 and ∼5 meV). The results showed dramatic differences in induced acoustoelectric current depending on Ec. The induced acoustoelectric current showed a polarity reversal around the Coulomb blockade peak when Ec is small (5 meV). For a large Ec (50 meV), however, a current plateau was observed as a function of surface acoustic wave powers as well as gate voltages.