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.

In recent years, research into implementing microdisplays for use in virtual reality and augmented reality has been actively conducted worldwide. Specifically, inorganic light-emitting diodes (LED) have many advantages in microdisplays, so much effort has been made to implement them in various ways. However, it is still challenging to realize a display with high resolution using only inorganic LEDs without color conversion layers because a typical LED chip is designed to emit only one color on a single wafer. In this study, we integrated high-efficiency red, green, and blue (RGB) LED material systems on the same sapphire substrate. Since the blue and green LED structures comprised the same GaN semiconductor, a metalorganic chemical vapor deposition method was used to integrate them. Meanwhile, the red LEDs made from another semiconductor were incorporated into the blue/green LEDs using a wafer-bonding technique. The fabricated hybrid RGB LEDs were able to cover a wide color space. In addition, the RGB LED material systems consisting of a high-quality single crystal were stably combined on the sapphire substrate without any structural defects. We show the possibility of their use in displays by integrating the RGB LEDs on one chip and suggest that their utilization could range from large-area LED displays to ultrahigh-resolution small displays.

We demonstrate a cost-effective top-down approach for fabricating InGaN/GaN nanorod arrays using a wet treatment process in a KOH solution. The average diameter of the as-etched nanorods was effectively reduced from 420 nm to 180 nm. The spatial strain distribution was then investigated by measuring the high-resolution cathodoluminescence directly on top of the nanorods. The smaller nanorods showed a higher internal quantum efficiency and lower potential fluctuation, which can subsequently be exploited for high-efficiency photonic devices.

In this study, we have fabricated a blue-green color-tunable monolithic InGaN/GaN LED having a multi-junction structure with three terminals. The device has an n-p-n structure consisting of a green and a blue active region, i.e., an n-GaN / blue-MQW / p-GaN / green-MQW / n-GaN / Al₂O₃ structure with three terminals for independently controlling the two active regions. To realize this LED structure, a typical LED consisting of layers of n-GaN, blue MQW, and p-GaN is regrown on a conventional green LED by using a metal organic chemical vapor deposition (MOCVD) method. We explain detailed mechanisms of three operation modes which are the green, blue, and cyan mode. Moreover, we discuss optical properties of the device.

Endowed structural colors in some living organisms provide inspiration for alterable colors depending on the surrounding environment. Although many structural designs for surrounding sensitive colors for various analytes have been introduced, practical applications in a variety of environments require a unified design that can cover the key optical influences of external environmental changes. A bimodal approach to colorimetric sensor design for optimization on different environmental stimuli is presented, based on a single platform composed of a highly lossy medium and a metal. A dual-mode colorimetric facilitator (DMCF) is optimally designed on the basis of the spectral response for each mode depending on the refractive index and thickness, which are optical key elements of changes in the surrounding environment. DMCFs with aqueous solutions and oxide layers are experimentally verified for different refractive indices and thicknesses in nanometer scale. As a biomaterial application, minute in nanometer-thick changes of the virus coated on DMCF are distinguished by color differences. For intuitive recognition of environmental change, the colorimetric indicator is designed as a separate insensitive/sensitive area to reveal hidden patterns beyond a certain thickness of the coated virus in nanometer thick. For scalability/flexibility, a large-area flexible sample is also fabricated on a wafer scale.

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 J_sc 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 V_oc are simultaneously attained.

We introduce ITO on graphene as a current-spreading layer for separated InGaN/GaN nanorod LEDs for the purpose of passivation-free and high light-extraction efficiency. Transferred graphene on InGaN/GaN nanorods effectively blocks the diffusion of ITO atoms to nanorods, facilitating the production of transparent ITO/graphene contact on parallel-nanorod LEDs, without filling the air gaps, like a bridge structure. The ITO/graphene layer sufficiently spreads current in a lateral direction, resulting in uniform and reliable light emission observed from the whole area of the top surface. Using KOH treatment, we reduce series resistance and reverse leakage current in nanorod LEDs by recovering the plasma-damaged region. We also control the size of the nanorods by varying the KOH treatment time and observe strain relaxation via blueshift in electroluminescence. As a result, bridge-structured LEDs with 8 min of KOH treatment show 15 times higher light-emitting efficiency than with 2 min of KOH treatment.

In this study, we produce InGaN/GaN microcolumn LED (MC-LED) arrays having nonpolar metal sidewall contacts using a top-down method, where the metal contacts only with the sidewall of the columnar LEDs with an open top for transparency. The trapezoidal profile of the as-etched columns was altered to a rectangular profile through KOH treatment, exposing the nonpolar sidewalls. While the MC-LED with no treatment emitted no light because of the etch-damaged region, the MC-LEDs with KOH treatment exhibited much improved the electrical properties with the much higher shunt resistance due to the removal of the etch-damaged region. The optical output power was strongest for the MC-LED with a 5-min treatment indicating an almost complete removal of the damaged region.