High-speed Synthesis of Zinc-Tin Oxide Nanoparticles and Applications to Optoelectronic devices

Abstract

Light extraction efficiency (LEE) is very important issue for high-efficiency light emitting diodes (LEDs). Even though inherent characteristics such as reliable long life-time, high energy efficiency and eco-friendly of LEDs offer significant advantages over already existing illuminating sources, LEDs still showed low external quantum efficiency (EQE). This phenomenon is caused by the narrow escape cone due to the large refractive index difference between p-GaN of LEDs and air. Therefore, for the high-efficiency LEDs, a relative low EQE compared to internal quantum efficiency must be improved. This thesis proposes that zinc-tin oxide (ZTO) micro/nanostructures, which are synthesized by using sol-gel and ethanol precipitation methods, to apply them to GaN-based LEDs to enhance the LEE. The optical and electrical performance of LEDs are greatly improved by ZTO micro/nanostructures. In addition, the LEE simulations were carried out by Finite-Difference Time-Domain (FDTD) method based on the Maxwell’s law. In this dissertation, the synthesis, the morphology control, characterization, and applications of ZTO nanoparticles have been studied with focusing on the following topics: 1) Methods for synthesis and fabrication of ZTO nanoparticles and microspheres, 2) Detailed investigations of light extraction using layers consisting of synthesized ZTO nanoparticles and microspheres, 3) Characterization of device performance of GaN-based LEDs with fabricated various ZTO layers. Chapter 1 is an introduction of this dissertation. The basic and quantum efficiency of LEDs, limiting factors for LEE of LEDs, and current status of research on light extraction of LEDs are introduced. Overview of Zinc-Tin Oxide (ZTO) is described: crystal structure, morphology, material properties, and applications in ZTO materials properties are described. In addition, the simulation software for calculating light extraction of LEDs is introduced. Chapter 2 describes the experimental such as the synthesis and fabrication methods, its principle and characterization methods and so on. Chapter 3 describes the improvement of light extraction in the LEDs by using ZTO layers with nanoparticles. The ZTO layers on top of LEDs consist of two ZTO layers with different Zn to Sn ratios of 1:5 and 1:1. Effect of ZTO layers on light extraction of LEDs is discussed. Chapter 4 describes improved the optical and electrical performance of GaN-based LEDs using transferrable ZTO microsphere monolayer. The mechanism of device performance improvement is proposed by X-ray photoelectron spectroscopy and FDTD simulations. Finally, Chapter 5 devotes to summarizing and concluding the results.