A study on Light-Emitting Devices on Glass Substrates

Abstract

유리기판의 비정질성과 낮은 융점을 극복하고 발광소자를 유리기판위에 구현하여 대면적, 저가, 투명한 유리기판의 장점을 최대로 살리기 위하여 본 연구가 되었다. 유리기판의 비정질성을 극복하기 위하여 티타늄 박막을 선행 방향층으로 사용하고, 핵 생성위치를 공간적으로 제약함으로서 단결정 수준의 GaN을 피라미드 어레이 형태로 얻었다. 이러한 단결정 수준의 GaN 어레이의 성장 메커니즘은 국부적 이종 에피택시 (local hetero-epitaxy)와 우선성장 (predominant growth)을 포함한다. 이러한 GaN 어레이를 이용하여 600 cd/m2의 휘도 (최종 휘도: 2700 cd/m2 )를 갖고 안정적으로 동작하는 LED를 구현하였다. 글라스의 낮은 공정온도를 극복하기 위하여 유리기판위에 마이크로히터를 형성하여 국부적 가열을 통하여 유리기판은 낮은 온도를 유지하면서 국부 고온에 도달 가능하였고, 이를 이용하여 다양한 형상을 갖는 ZnO 나노구조를 합성하였고, 특히 ZnO 나노로드를 template로하여 GaN를 합성하여 GaN/ZnO 코어쉘 나노구조 합성을 성공하였다. 이는 장차 저융점의 유리기판위에서도 GaN 기반 LED구현까지도 가능함을 의미한다.

The thesis is to realize light-emitting devices on glass substrates, divided into three parts. The first part is devoted to overcoming amorphous nature of glass substrates. Single-crystalline GaN-based light-emitting diodes (s-LEDs) on crystalline sapphire wafers can provide point-like light sources with high conversion efficiency and long working lifetimes. Recently, s-LEDs on silicon wafers have been developed in efforts to overcome the size limitations of the sapphire substrate. However, to create larger, cheaper and efficient flat light sources, the fabrication of high-performance s-LEDs on amorphous glass substrates would be required, which remains a scientific challenge. Here, we report the fabrication of nearly single-crystalline GaN on amorphous glass substrates, in the form of pyramid arrays. This is mainly achieved by high-temperature, predominant GaN growth on a site confined nucleation layer with preferential polycrystalline morphology through local hetero-epitaxy. Especially, relationship between Ti pre-orienting layer (POL)-low temperature GaN (LT-GaN) nucleation layer (NL) will be discussed in detail. Furthermore, shape, in-plane orientation, and crystallinity of GaN pyramid arrays themselves will be discussed in detail. InGaN/GaN multiple quantum wells formed on the GaN pyramid arrays exhibit a maximum internal quantum efficiency of 52%. LED arrays fabricated using these GaN pyramid arrays demonstrate reliable and stable area-type electroluminescent emission with a maximum luminance of 2700 cd m-2. The peak wavelength of EL spectra was modulated from blue to reddish orange by varying the MQW temperature and the thickness of a insulating polymer. The second part is devoted to overcoming temperature limit of glass substrates. First, we report morphology-controlled selective growth of ZnO nanostructures on glass substrates by using catalyst-free metal-organic chemical vapor deposition. For the morphology-controlled selective growth, a microheating method using a series of microheaters was developed, which provided well-controlled local heating based on the microheater geometry and spatial arrangement. ZnO nanostructure morphology depended on the local growth temperature, so various nanostructure morphologies were obtained selectively at specific positions on glass substrates by using local microheating. The monolithic integration of nanostructures with different morphologies will have great potential for applications in multifunctional devices. Then, we report on the fabrication of high-quality GaN on soda-lime glass substrates, heretofore precluded by both the intolerance of soda-lime glass to the high temperatures required for III-nitride growth and the lack of an epitaxial relationship with amorphous glass. The difficulties were circumvented by heteroepitaxial coating of GaN on ZnO nanorods via a local microheating method. Metal-organic chemical vapor deposition of ZnO nanorods and GaN layers using the microheater arrays produced high-quality GaN/ZnO coaxial nanorod heterostructures at only the desired regions on the soda-lime glass substrates. High-resolution transmission electron microscopy examination of the coaxial nanorod heterostructures indicated the formation of an abrupt, semicoherent interface. Photoluminescence and cathodoluminescence spectroscopy was also applied to confirm the high optical quality of the coaxial nanorod heterostructures. Mg-doped GaN/ZnO coaxial nanorod heterostructure arrays, whose GaN shell layers were grown with various different magnesocene flow rates, were further investigated by using photoluminescence spectroscopy for the p-type doping characteristics. The suggested method for fabrication of III-nitrides on glass substrates signifies potentials for low-cost and large-size optoelectronic device applications. The third part will be discussed in Appendix, including fine patterning of organic electroluminescent layer.