Hyonwoong Kim

Hydrogen sensing characteristics of semipolar (112¯2) GaN Schottky diodes

The hydrogen detection characteristics of semipolar (112¯2) plane GaN Schottky diodes were investigated and compared to c-plane Ga- and N-polar and nonpolar a-plane (112¯0) GaN diodes. The semipolar GaN diodes showed large current response to 4% hydrogen in nitrogen gas with an accompanying Schottky barrier reduction of 0.53 eV at 25 °C, and the devices exhibited full recovery to the initial current level upon switching to a nitrogen ambient. The current-voltage characteristics of the semipolar devices remained rectifying after hydrogen exposure, in sharp contrast to the case of c-plane N-polar GaN. These results show that the surface atom configuration and polarity play a strong role in hydrogen sensing with GaN.

Nonpolar light emitting diode with sharp near-ultraviolet emissions using hydrothermally grown ZnO on p-GaN

Nonpolar n-ZnO/p-GaN heterojunction light emitting diode has been demonstrated with a-plane (112¯0) ZnO active layer grown by a facile low-cost solution growth method at low temperature of 90 °C. High quality nonpolar ZnO planar film without seed layer was directly formed on a-plane GaN template due to the anisotropic growth rates along the specific crystallographic directions. The turn on voltage of the device was as low as 3 V, and narrow stable UV-blue electroluminescence emissions with peak wavelength of 392 to 420 nm under various forward bias conditions at room temperature were observed.

Investigation of carrier transport properties in semipolar (112¯2) GaN films with low defect density

We report on the anisotropic carrier transport properties of semipolar (112¯2) GaN films with low defect density. We utilized the asymmetric lateral epitaxy to obtain various semipolar (112¯2) GaN films having significantly reduced partial dislocations and basal-plane stacking faults (BPSFs). The directionally dependent carrier transport was observed with the lower sheet resistances (Rsh) along the [11¯00] direction. The Rsh ratios of semipolar (112¯2) GaN films were found to be relatively smaller than those of nonpolar a-plane GaN films, possibly due to low BPSF density and the reduced in-plane electric field induced by BPSF along the [112¯3] direction at wurtzite domain boundaries.

Structural defects in LiCoO2 studied by L7i nuclear magnetic relaxation

Microscopic environments and dynamics in LiCoO2 systems were probed by L7i nuclear magnetic relaxation measurements with regard to the structural defects as revealed by x-ray diffraction and electron spin resonance measurements. Thus, the structural defects of differing degrees, associated with Li vacancies, were well accounted for in the temperature-dependent spin-lattice relaxation rate analysis, and in the structural and dynamical inhomogeneity as derived from the relaxation patterns.

Improved Hydrogen Sensing Performance of AlGaN/GaN Based Sensor with Platinum Nanonetworks

Hydrogen attracts great current interests in alternative energy as a substitute for fossil fuel. Clean energy through the overall water producing reaction can be obtained without any pollutants or carbon dioxide. In spite of this, there is a delay in practical uses of hydrogen energy such as hydrogen fuel cell vehicles due to the risk of explosion in the air. Fast detection of small amount of hydrogen can prevent the explosive accidents and environmental pollutions. Prompt and reliable detection of hydrogen can make it possible to move up the date when the hydrogen is in practical use as an energy source. Also, hydrogen leakage detection is very important in the area of semiconductor fabrication facilities, petroleum distillation towers and chemical plants. Platinum is widely used as a catalyst in the gas phase reaction. Hydrogen molecules are dissociated on the surface of the platinum due to the lowered active energy and chemisorptions of hydrogen ions occurs. Chemisorption area is very closely related to the sensitivity of the sensor. More hydrogen molecules can attach to the larger adsorption area of the sensor and increase the sensitivity. By constructing the active region of the sensor with nanostructure, huge surface area can be obtained. Not only nanostructures increase the surface to volume ratio, but also they are nano-porous, which can induce fast adsorptions and desorption of gas molecules. AlGaN/GaN high electron mobility transistor (HEMT) structure has 2 dimensional electron gas (2DEG) channel due to the piezoelectric effect and spontaneous polarization. This channel is very sensitive to the surface charge, and sensing signal of the device using this structure can be amplified very much. Therefore, AlGaN/GaN based hydrogen sensor with platinum nanonetwork can dramatically improve the sensing performance AlGaN/GaN based hydrogen sensors using platinum nanonetworks synthesized by a simple solution phase method were fabricated and compared to conventional Pt film devices. Nanonetwork sensors show remarkably improved response and good repeatability for cyclic hydrogen exposures. To apply 2–3 nm diameter platinum nanonetworks to the device, spin-coating method which can control the density of nanonetworks, and PR lithography and lift-off which enable the selective patterning of nanonetworks on semiconductor surface were used. Figure 1. SEM image of platinum nanonetwork coated on Si substrate. (inset) TEM image of platinum nanonetwork.