Kyu-Tae Kim

OBSERVATION OF A HEXAGONAL BN SURFACE LAYER ON THE CUBIC BN FILM GROWN BY DUAL ION BEAM SPUTTER DEPOSITION

Cubic-boron nitride (c-BN) thin films were grown by dual ion beam sputter deposition and the growth mechanism was studied using angle resolved in-situ XPS (x-ray photoelectron spectroscopy) analysis. The π bond shake-up satellite of the B 1s peak, which is observed only in a hexagonal boron nitride (h-BN) phase, appeared in the XPS spectrum obtained for the surface layer of the c-BN film. This can be a clear evidence that the c-BN phase grows via the transformation of an initially formed h-BN phase and the transformation is induced by the compressive stress accumulated in the subsurface region during ion bombardment.

Analog-to-digital conversion for low-frequency waveforms based on the Josephson voltage standard

A waveform synthesizer adopting a superconductor–normal metal–superconductor junction array has been developed, which can generate arbitrary stepwise waveforms with a number of quantum-voltage steps up to 1 V level amplitude. As an application of the synthesizer, we have built a sampling voltmeter that measures the differential voltages between a sinusoidal waveform produced by a semiconductor-based ac source and the Josephson waveforms. We carried out extensive sampling measurements for a 50 Hz sine wave with 1 V amplitude, applying sampling apertures in the range of 55 µs ≤ta ≤ 130 µs and using Josephson waveforms with 32, 60, 80 and 100 quantum steps. From the measurements, the amplitude of the ac waveform was determined with a type A uncertainty (k = 2) of 0.15 µV. Also, we elucidated how the phase jitter in the ac waveform is reflected in the overall uncertainty for the measurements. The type B uncertainty due to the jitter is at least one order of magnitude smaller than the type A uncertainty.

A whole-head magnetoencephalography system with compact axial gradiometer structure

We have fabricated a whole-head superconducting quantum interference device (SQUID) gradiometer system for measuring the magnetoencephalography (MEG) of the human brain. Major technical features of the system are the compact structure of the gradiometer and compact readout electronics. As the gradiometers, first-order gradiometers of 50xa0mm baseline were used to reduce environmental noises. To simplify the fabrication process of the gradiometers, and to increase the refill interval of liquid He, the superconductive connection between the pickup coil wires and input coil pads was done by direct bonding of Nb wires. Therefore, bulky superconducting blocks or superconducting screws were not used for the superconductive connection, and superconducting shielding was not used around the SQUID module, resulting in no distortion of external field uniformity. The distance between the compensation coil of the gradiometer and SQUID input coil pad was reduced to 10xa0mm, and the total length of the gradiometer is 70xa0mm. A sensor helmet having 128 gradiometers was cooled inside a helmet-shape liquid He dewar. The average boil-off rate of the MEG system is 10xa0lxa0d−1 and the refill interval is 7 days when the 128-channel system is in operation every day. To simplify the readout electronics of the SQUID system, double relaxation oscillation SQUIDs (DROSs) having large flux-to-voltage transfer coefficient were used. The magnetically shielded room (MSR) has a wall thickness of 200xa0mm, and consists of two layers of Permalloy and one layer of aluminum. When the 128-channel system was operated inside the MSR, the average magnetic field noise level of the 128 channels was about 3.5xa0fTrmsxa0Hz−1/2 at 100xa0Hz. Spontaneous and evoked brain magnetic fields were measured using the developed system.

Origin of variation in switching voltages in threshold-switching phenomena of VO2 thin films

We investigated the origin of the variation in switching voltages in threshold-switching of VO2 thin films. When a triangular-waveform voltage signal was applied, the current changed abruptly at two switching voltages, i.e., VON (insulator-to-metal) and VOFF (metal-to-insulator). VON and VOFF were measured by changing the period of the voltage signal, the temperature of the environment, and the load resistance. We observed that either VON or VOFF varied significantly and had different dependences with respect to the external parameters. Based on the mechanism of the metal–insulator transition induced by Joule heating, numerical simulations were performed, which quantitatively reproduced all of the experimental results. From the simulation analysis, the variation in the switching voltages for threshold-switching was determined to be thermal in origin.

Microwave-induced constant voltage steps in surface junctions of Bi 2 Sr 2 CaCu 2 O 8 + δ single crystals

We have observed the zero-crossing steps in a surface junction of a mesa structure microfabricated on the surface of a

Strain distribution and interface modulation of highly lattice-mismatched InN/GaN heterostructure nanowires

{mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}}

Microwave distribution in stacked Bi2Sr2CaCu2O8+x intrinsic Josephson junctions in a transmission-line geometry

single crystal. With the application of microwave of frequencies 76 and 94 GHz, the current-voltage characteristics show clear voltage steps satisfying the ac Josephson relation. Increasing the microwave power, the heights of the steps show the Bessel-function behavior up to step number

Development of decay energy spectroscopy using low temperature detectors

n=4.

A high-sensitivity magnetocardiography system with a divided gradiometer array inside a low boil-off Dewar

We confirm that the intrinsic surface junction meets the criterion for the observation of zero-crossing steps.

GaAs delta‐doped layers in Si for evaluation of SIMS depth resolution GaAs

The structural properties of InN/GaN heterostructure nanowires (NWs) were studied using transmission electron microscope techniques to determine strain behavior. A great quantity of the misfit strain between InN and GaN was relaxed through the introduction of misfit dislocations along the interface. Geometric phase analysis revealed a strain-concentration phenomenon in the strain map of the out-of-plane components and a gradual lattice recovery in that of the in-plane components over the InN/GaN interface. Interface structures that were modulated at the atomic-scale were observed in several InN/GaN heterostructure NWs. Complex strain distributions were identified in both InN and GaN.