Hoonil Jeong

Effect of indium composition on carrier escape in InGaN/GaN multiple quantum well solar cells

The influence of indium composition on carrier escape was studied considering recombination in InGaN/GaN multiple quantum well solar cells with indium compositions of 17% and 25%. Competition between tunneling and recombination turned out to act as a crucial role for the short-circuit current density (Jsc) and fill factor (FF). To enhance the Jsc and the FF, the tunneling-dominant carrier decay rather than recombination is required in the operating range of the solar cells which is possible by optimizing the band structures for a shorter tunneling time and by improving the crystalline quality for a longer recombination time.

Nexus between directionality of terahertz waves and structural parameters in groove patterned InAs

We performed terahertz (THz) time-domain spectroscopy in various geometries, for characterizing the directivity of THz waves emitted from groove patterned InAs structures. We first identified two transient transport processes as underlying THz emission mechanisms in InAs epilayers with different thicknesses. Carrier drift around the surface depletion region was predominant for the THz wave generation in the thin sample group (10–70 nm), whereas electronic diffusion overrode the drift currents in the thick sample group (370–900 nm) as revealed by the amplitude change and phase reversal. Through a combination of electron-beam lithography and inductively coupled plasma etching in 1 μm-thick InAs epilayers, we could further periodically fabricate either asymmetric V-groove patterns or symmetric parabolic apertures. The THz amplitude was enhanced, particularly along the line-of-sight transmissive direction when the periodic groove patterns act as microscale reflective mirrors separated by a scale of the diffus...

Directional terahertz emission from corrugated InAs structures

The terahertz (THz) radiation from transient dipoles, formed by distinct diffusion coefficients between oppositely charged carriers as often observed in low band gap semiconductors, propagates with an anisotropic amplitude distribution perpendicular to the dipole axis along the diffusive motion. By directionally adjusting the electronic diffusion, we conceptualize groove-patterned THz emitters based on (100) InAs thin films and demonstrate the unidirectional radiation. Line-of-sight emission along the surface-normal direction is greatly enhanced in a distributed asymmetric trapezoid with its period similar to the electronic diffusion length of InAs. This directional enhancement is in clear contrast to the constant emission amplitude along the lateral direction, regardless of pattern scale, which manifests the role of groove patterns as microscale reflectors in laterally corrugating the carrier density. In contrast to the rather limited nonlinearity in (100) plane, the azimuthal angle dependence of the THz field amplitude in corrugated samples shows a combined effect of diffusive transport and second-order nonlinearity, whose compositional contributions varies in different structures.

Spatially-resolved measurement of arc flash event based on transmission loss of plastic optical fiber

A novel scheme of plastic optical fiber (POF) based arc flash sensor capable of tracing arc event locations is presented. Incident position of flash light can simply be known by measuring the ratio of intensities at both fiber-ends, since the intensity of the flash light assisted by side-coupling of the fiber is generally attenuated a the fiber length. The arc flash sensor which can cover a wide range up to 10 m with a high spatial resolution of ±10 cm is experimentally demonstrated using the POF. Arc flash intensity can also be known by analyzing the coupled light intensity level at both fiber ends.

Fiber-optic arc flash sensor based on plastic optical fibers for simultaneous measurements of arc flash event position

Abstract. We present an arc flash sensor that can trace the arc event position as well as intensity by utilizing conventional plastic optical fibers (POFs). In order to check the possibility as a light-receiving sensor, we experimentally confirm that the externally irradiated flash light can be coupled into the fiber core through the surface of POF without any additional treatment. After the incident light is divided in two optical paths toward opposite directions, they have the different attenuation values determined by the propagation distance. Since the optical transmission loss of a POF is constant regardless of the irradiated energy, the intensity ratio for two signals measured at both fiber ends is given as a function of position. The experimental results show that we can successfully trace the event position from this intensity ratio. In addition, it is possible to define the illuminated energy by comparing the absolute value of the intensity measured from one side. According to the experimental results, the proposed sensor has a relatively fine spatial resolution, ±10  cm, despite having a simple structure.

Method for Locating Arc-events by Utilizing Transmission Loss of Plastic Optical Fiber

For an arc-flash protection system, the selection of arc-flash sensor in implementation is largely dependent on the coverage area and the spatial resolution. Typically, a point sensor is used to accurately measure an arc event within a very narrow region; whereas, a loop or a line sensor can cover several electrical compartment at the same time, but with a poor resolution. In this work, a novel scheme for an arc-flash sensor was developed by making use of the transmission loss of plastic optical fibers (POFs) to cover a broad range with a high spatial resolution. By relating the amplitude ratio of the arcsignals at the ends of the POF with the arc-location, arc events could be located with a resolution of ~5 cm within a spatial range of 10 m, which has not been reported yet.

Arc-Flash Detection Sensor Based on Surface Coupling of Plastic Optical Fiber

Abstract In this work, a loop sensor for Arc-Flash detections has been developed in order to trip a circuit breaker within 2.5 ms after an Arc-Flash event. For an efficient capturing of the flash light, plastic optical fibers, where light attenuations are larger than those in silica-based ones, with different diameters and surface conditions were utilized. The performance was comparatively analyzed with those ofa point sensor and a commercialized product. The point sensor module was designed for hemisphere-like capturings of Arc-Flasheslarger than 3 kA at 2 meters from the sensor. On the other hand, the loop sensor allowed 360-degree-detections around the fiber axisand the measurement range was dependent on the length of the fiber connected to the sensor module. The trip-level-dependent bright-ness measurement results showed that the fabricated point sensor and loop sensor satisfied a brightness condition, 10~40 klux, and theresponses of the system to Arc-Flashes were completed within 2.5 ms.Keywords: Arc-Flash sensor, Plastic optical fiber, Protective relay

Mode switching of acoustic phonons by external bias

We experimentally manifested a new type of transverse acoustic (TA) phonon generation and its amplitude control by external reverse bias in c-axis grown from InGaN/GaN diode structures, in clear contrast to the well-established conditions for longitudinal acoustic (LA) phonon generations. The generation and propagation dynamics were investigated by analyzing the dynamic Fabry-Perot interferences and, based on the twisted energy band structure, we have confirmed that the lateral electric fields under reverse bias broke the in-plane isotropy of c-GaN resulting in TA mode generation.

Comparative study on THz radiation from various semiconductors

We have measured THz radiation from various semiconductor groups: InGaSb, GaAsSb, InAsSb, InAlSb, InSb, InAs, and InGaAs. The InAs series was the most intense radiation source whereas GaAsSb was the weakest one. In terms of the emission time, InGaAs (GaAsSb) was the fastest (slowest) material.