Chang-Mo Kang

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.

Color tunable monolithic InGaN/GaN LED having a multi-junction structure

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.

Monolithic integration of AlGaInP-based red and InGaN-based green LEDs via adhesive bonding for multicolor emission

In general, to realize full color, inorganic light-emitting diodes (LEDs) are diced from respective red-green-blue (RGB) wafers consisting of inorganic crystalline semiconductors. Although this conventional method can realize full color, it is limited when applied to microdisplays requiring high resolution. Designing a structure emitting various colors by integrating both AlGaInP-based and InGaN-based LEDs onto one substrate could be a solution to achieve full color with high resolution. Herein, we introduce adhesive bonding and a chemical wet etching process to monolithically integrate two materials with different bandgap energies for green and red light emission. We successfully transferred AlGaInP-based red LED film onto InGaN-based green LEDs without any cracks or void areas and then separated the green and red subpixel LEDs in a lateral direction; the dual color LEDs integrated by the bonding technique were tunable from the green to red color regions (530–630 nm) as intended. In addition, we studied vertically stacked subpixel LEDs by deeply analyzing their light absorption and the interaction between the top and bottom pixels to achieve ultra-high resolution.

InGaN/GaN microcolumn light-emitting diode arrays with sidewall metal contact

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.

Blue to Green Tunable GaN-based LEDs Having a Dual-Junction Structure

Blue to green tunable GaN-based LEDs having a dual-junction structure have been fabricated. We observed that the color of the dual-junction LEDs can be tuned by controlling both current path and density through three terminals.