Sang-Bae Choi

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.

Efficiency Improvement in InGaN-Based Solar Cells by Indium Tin Oxide Nano Dots Covered with ITO Films

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

Fabrication of a vertically-stacked passive-matrix micro-LED array structure for a dual color display

We report a color tunable display consisting of two passive-matrix micro-LED array chips. The device has combined vertically stacked blue and green passive-matrix LED array chips sandwiched by a transparent bonding material. We demonstrate that vertically stacked blue and green micro-pixels are independently controllable with operation of four color modes. Moreover, the color of each pixel is tunable in the entire wavelength from the blue to green region (450 nm - 540 nm) by applying pulse-width-modulation bias voltage. This study is meaningful in that a dual color micro-LED array with a vertically stacked subpixel structure is realized.

Ag-mesh-combined graphene for an indium-free current spreading layer in near-ultraviolet light-emitting diodes

We have designed and fabricated Ag-mesh-combined graphene current spreading layers (CSLs) for GaN-based near-ultraviolet light-emitting diodes (NUV LEDs). Three different types of the CSLs were fabricated having 300 μm, 150 μm, and 75 μm gaps between the mesh lines. Optical and electrical properties were investigated and the performance of the LEDs having the CSLs and were compared with those of LEDs having indium tin oxide and graphene CSLs. The light output power of the LEDs with the Ag-mesh-combined graphene CSL adopting 150 μm-gap was highest at both the same current injection and at the same input power. This was possible due to the lowering of the sheet resistance via the adoption of an Ag mesh and the networking of the Ag mesh using a graphene sheet with very little transmittance loss.

Optical Characterization of Double Peak Behavior in {10\bar{1}1} Semipolar Light-Emitting Diodes on Miscut m-Plane Sapphire Substrates

{101} semipolar GaN-based light-emitting diodes (LEDs) grown on 1° miscut m-plane sapphires substrates via metal organic chemical vapor deposition showed undulated surface morphology with ridges and valleys. On the ridge regions, two dominant emission peaks, one at a shorter wavelength (~438 nm) and one at a longer wavelength (~490 nm), were observed using electroluminescence and micro-photoluminescence. In the valley regions, the longer peak was observed to be significantly quenched due to the grain boundary. The origin of the longer peak is believed to be not only inhomogeneous distribution of In composition in multiple quantum wells (MQWs) but also strong localization around the ridge region. Moreover, thickness variation of faceted MQWs could be associated with the peak broadening in 101 semipolar LEDs. The results were also confirmed by cathodoluminescence and cross-sectional transmission electron microscopy.

Ag nanoparticles-embedded surface plasmonic InGaN-based solar cells via scattering and localized field enhancement.

Ag nanoparticles are embedded in intentionally etched micro-circle p-GaN holes by means of a thermal agglomeration process to enhance the light absorption efficiency in InGaN/GaN multi-quantum-well (MQW) solar cells. The Ag nanoparticles are theoretically and experimentally verified to generate the plasmon light scattering and the localized field enhancement near the MQW absorption layer. The external quantum efficiency enhancement at a target wavelength region is demonstrated by matching the plasmon resonance of Ag nanoparticles, resulting in a Jsc improvement of 9.1%. Furthermore, the Ag-nanoparticle-embedded InGaN solar cell is effectively fabricated considering the carrier extraction that more than 70% of F.F. and 2.2 V of high Voc are simultaneously attained.

Thickness dependence of terahertz emission in InAs and its application to transmissive source

Terahertz(THz) radiation properties from InAs layers grown on GaAs has been studied as a function of thickness ranging from 0.01 μm to 1.74 μm. The amplitude showed a monotonic increment up to 0.9 μm, followed by a decrement at 1.74 μm. The phase‐shift between 0.07 and 0.37 μm was also observed, which was possibly associated with the transition of the major generation mechanism from drift to diffusion with increasing thickness.