Ju Jin Kim

Electrical transport properties of individual gallium nitride nanowires synthesized by chemical-vapor-deposition

We have synthesized high-quality gallium nitride (GaN) nanowires by a chemical-vapor-deposition method and studied the electrical transport properties. The electrical measurements on individual GaN nanowires show a pronounced n-type field effect due to nitrogen vacancies in the whole measured temperature ranges. The n-type gate response and the temperature dependence of the current–voltage characteristics could be understood by the band bending at the interface of the metal electrode and GaN wire. The estimated electron mobility from the gate modulation characteristics is about 2.15 cm2/V s at room temperature, suggesting the diffusive nature of electron transport in the nanowires.

Schottky diodes based on a single GaN nanowire

On a single GaN nanowire, obtained by chemical vapour deposition, several Schottky-junction diodes were fabricated and their electrical transport properties were studied. Alternately attached metal electrodes of Al and Ti/Au formed a Schottky barrier junction (for Al) or an ohmic contact (for Ti/Au), resulting in several diodes on a single nanowire. The current–voltage measurements exhibited clear rectifying behaviour and no reverse-bias breakdown was observed up to the measured voltage, −5 V. The forward-bias threshold voltage was observed to decrease linearly with temperature, from 0.4 V at 280 K to 1 V at 10 K.

Enhanced Neurite Outgrowth by Intracellular Stimulation

Electrical stimulation through direct electrical activation has been widely used to recover the function of neurons, primarily through the extracellular application of thin film electrodes. However, studies using extracellular methods show limited ability to reveal correlations between the cells and the electrical stimulation due to interference from external sources such as membrane capacitance and culture medium. Here, we demonstrate long-term intracellular electrical stimulation of undamaged pheochromocytoma (PC-12) cells by utilizing a vertical nanowire electrode array (VNEA). The VNEA was prepared by synthesizing silicon nanowires on a Si substrate through a vapor-liquid-solid (VLS) mechanism and then fabricating them into electrodes with semiconductor nanodevice processing. PC-12 cells were cultured on the VNEA for 4 days with intracellular electrical stimulation and then a 2-day stabilization period. Periodic scanning via two-photon microscopy confirmed that the electrodes pierced the cells without inducing damage. Electrical stimulation through the VNEA enhances cellular differentiation and neurite outgrowth by about 50% relative to extracellular stimulation under the same conditions. VNEA-mediated stimulation also revealed that cellular differentiation and growth in the cultures were dependent on the potential used to stimulate them. Intracellular stimulation using nanowires could pave the way for controlled cellular differentiation and outgrowth studies in living cells.

An in-plane GaAs single-electron memory cell operating at 77 K

An in-plane single-electron memory cell operating at 77 K has been fabricated from a Si-doped thin GaAs film. This device utilizes an artificially fabricated floating node as a storage node and detects the charge stored on the floating node using a single-electron electrometer. Charging of the floating node is evidenced by a large peak in source–drain current as a function of control gate voltage, and is further confirmed by a discrete shift in the peak or threshold voltage.

Resolving microscopic interfaces in Si1−xGex alloy nanowire devices

We have fabricated Si(1-x)Ge(x) alloy nanowire devices with Ni and Ni/Au electrodes. The electrical transport characteristics of the alloy nanowires depended strongly on the annealing temperature and contact metals. Ni/Au-contacted devices annealed at 400 degrees C showed p-type transistor behavior as well as a resistance switching effect, while no switching was observed from Ni-contacted alloy nanowire devices. To identify the origin of such a hysteretic resistance switching effect, we constructed nanowire devices on a 40 nm Si(3)N(4) membrane. Transmission electron microscopy analysis combined with electrical transport measurements revealed that devices contacted with Ni/Au, and thereby showing resistance switching, have Au atoms right next to the alloy nanowire.

Performance assessments of vertically aligned carbon nanotubes multi-electrode arrays using Cath.a-differentiated (CAD) cells

In this work, Cath.a-differentiated (CAD) cells were used in place of primary neuronal cells to assess the performance of vertically aligned carbon nanotubes (VACNTs) multi-electrode arrays (MEA). To fabricate high-performance MEA, VACNTs were directly grown on graphene/Pt electrodes via plasma enhanced chemical deposition technique. Here, graphene served as an intermediate layer lowering contact resistance between VACNTs and Pt electrode. In order to lower the electrode impedance and to enhance the cell adhesion, VACNTs-MEAs were treated with UV-ozone for 20 min. Impedance of VACNTs electrode at 1 kHz frequency exhibits a reasonable value (110 kΩ) for extracellular signal recording, and the signal to noise ratio the is good enough to measure low signal amplitude (15.7). Spontaneous firing events from CAD cells were successfully measured with VACNTs MEAs that were also found to be surprisingly robust toward the biological interactions.

The suppression of Coulomb oscillation in a normal-ferromagnet-normal metal single-electron transistor

Abstract The electric transport properties of an Al/Al2O3/Ni/Al2O3/Al single-electron transistor were studied. Compared to the all-normal-metal single-electron transistors, our sample showed enhanced tunnel current inside the Coulomb gap both in the superconducting and the normal state of the Al. The tunnel current showed sharp increase for the magnetic field H≈±1.57 T , which also was evidenced by the change of current–voltage (I–V) characteristics with the magnetic field.

The correlation of the number and the phase uncertainties in the loop-type single-electron tunneling device

Abstract The electric and the magnetic transport properties of the loop-type Al/Al 2 O 3 /Al single-electron tunneling device were studied. Even for the sample with charging energy about one order of magnitude greater than the Josephson coupling energy, the magnetoresistance showed h /2 e -oscillation at low bias voltage, attributed to the phase-coherent pair tunneling. The behavior of the magnetoresistance depends on the gate voltage, showing large and noiseless oscillations with the even number of excess charges but small and noisy oscillations with the odd number of excess charges in the metal islands.

Memory Effect in an Aluminum Single-Electron Floating-Node Memory Cell

We have observed a memory effect in an aluminum single-electron memory cell with a floating node. Electrons were injected to or emitted from the floating node by field emission, which was evidenced by the sudden change in the Coulomb oscillation of a single-electron transistor. With the compensating voltage applied to the back gate, the Coulomb oscillation could be completely suppressed if the gate voltage sweep direction was reversed. We found that the Coulomb-oscillation period changed with the ratio of control-gate to back-gate voltage, being fitted well to the expected formula.