Investigation on graphene growth for InGaN/GaN light emitting diodes

Abstract

Graphene is a two-dimensional carbon structure with great attraction for future-optoelectronics due to its excellent electrical, optical, and mechanical properties to apply for optoelectronics. Recently, many research groups have tried to apply graphene to optoelectronics. Especially transferred graphene released from Cu, Ni metal catalyst substrate is one of the most intensively studied materials in the context of optoelectronic devices. However, although graphene looks attractive to replace indium tin oxide (ITO) in optoelectronic devices, the luminous efficiency of light emitting diodes (LEDs) with graphene transparent conducting electrodes has been limited by degradation in graphene taking place during device fabrication. In this study, I have tried to develop InGaN/GaN blue LEDs with single layer graphene released and transferred from Cu catalyst substrate. Chapter 1 and 2 introduce motivation of this study and describe various measurement, respectively. In chapter 3, I will report the high quality single layer graphene grown method by plasma enhanced chemical vapor deposition (PECVD) and analyze its resulting graphene with Raman spectroscopy, AFM, TEM, and hall measurement. In chapter 4, I will report the problem of previous fabrication method for LEDs with graphene films as transparent conductive electrode, and demonstrate the high performance InGaN/GaN blue LEDs with single layer graphene by avoiding graphene damages during LED fabrication process. Properties of graphene after the device fabrication were a critical factor affecting the performance of GaN-based LEDs. It was found that during the fabrication graphene was severely damaged and split into submicrometer-scale islands bounded by less conducting boundaries when graphene was transferred onto LED structures prior to the GaN etching process for p-contact formation. On the other hand, when graphene was transferred after the GaN etch and p-contact metallization, graphene remained intact and the resulting InGaN/GaN LEDs showed electrical and optical properties that were very close to those of LEDs with 200 nm thick ITO films. The forward-voltages and light output powers of LEDs were 3.03 V and 9.36 mW at an injection current of 20 mA, respectively. In chapter 5, directly grown graphene growth method and its resulting LEDs will be introduced in detail. I will demonstrate graphene growth method to directly define it without metal catalyst such as Cu or Ni substrate and additional transfer process to remove the Cu or Ni metal substrate. Direct graphene growth without a catalyst is possible on a variety of substrates including transparent and flexible ones at temperatures as low as 500 °C using plasma enhanced chemical vapor deposition. The film consists mainly of few-layer polycrystalline graphene, confirmed by transmission electron microscopy and Raman spectra. With the few-layer graphene directly grown on GaN substrates, light emitting diodes could be fabricated without transfer process and resulted in uniform output power over tens of devices. These show that direct graphene growth provides advantage of uniform interface and reliable performance in addition to transferfree device fabrication, which can be adopted easily in the industrialization of graphene