LIGHT EXTRACTION EFFICIENCY IN III- NITRIDE LIGHT-EMITTING DIODES AND PIEZOELECTRIC PROPERTIES IN ZNO NANOMATERIALS by JUNCHAO ZHOU Submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Electrical Engineering and Computer Science CASE WESTERN RESERVE UNIVERSITY August, 2016 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the thesis of Junchao Zhou candidate for the Master of Science degree*. Committee Chair Dr. Hongping Zhao Committee Member Dr. Christian A. Zorman Committee Member Dr. Philip Feng Committee Member Dr. Roger H. French Date of Defense May 31st. 2016 *We also certify that written approval has been obtained for any proprietary material contained therein. Acknowledgements I would like to express my sincere gratitude to my advisor Dr. Hongping Zhao, for her dedicated help on my Master’s study and related research, for her patience, motivation, and immense knowledge. Her guidance helped me throughout my research and writing of this thesis. Then, I would like to thank Dr. Zorman for his constructive advice for my thesis work as well as his help on my thesis project experiment. His insights helped me overcome the difficulties in my experiment. I would also like to thank Dr. French, Dr. Feng and Dr. Zorman for being on my thesis committee member and providing insightful comments. Also, I would like to thank Dr. Ming-chun Huang for the discussion on the applications of ZnO in piezoelectric devices. In addition, I would like to thank my group members for their advice on my research projects. I would like to thank Lu Han for her guidance on using the experimental equipment and Subrina Rafique for providing the materials for experiment. Finally, I would like to thank my family for their unlimited support and encouragement. Table of Content List of Tables……………………………………………………………………….……..4 List of Figures…………………………………………………………………………5 Abstract……………………………………………………………………………..12 Chapter 1: Introduction……………………………………………………………..13 1.1. InGaN Quantum Wells Light-Emitting Diodes: Problems and Solutions.13 1.1.1.Light Emitting Diodes for Solid State Lighting………………………….14 1.1.2.Problem of Light Extraction in Planar Light-Emitting Diodes (LEDs)…….16 1.1.3.Approaches to Enhance Light Extraction Efficiency for III-nitride LEDs…18 1.1.4.Electromagnetic Guided Modes in Periodic Dielectric Medium………..18 1.1.5.Band Structure and Guided Modes of Photonic Crystals………………..20 1.1.6.Bloch Modes and Light Extraction……………………………………..21 1.2. ZnO Piezoelectric Devices……………………………………………………..24 1.2.1.Piezoelectric Property of ZnO…………………………………………...25 1.2.2.ZnO Piezoelectric devices………………………………………………….26 1.3. Thesis Organization……………………………………………………………27 Chapter 2: Finite-Difference Time-Domain (FDTD) Method for Calculating Light Extraction Efficiency of Light-Emitting Diodes………………….28 2.1. FDTD Method………………………………………… ……………...……….28 2.1.1. Introduction…………………………………………………………….28 2.1.2. Three-Dimensional FDTD Method and Yee’s Mesh……………………29 2.2. Computational Model…………………………………………….……………34 2.2.1. Light Extraction Efficiency Calculation Method………………………..34 1 2.2.2. Photonic Crystal Band Structure Simulation Methodology…………….37 Chapter 3: Analysis of Light Extraction Efficiency for Thin-Film-Flip-Chip (TFFC) InGaN Quantum Wells (QWs) Blue Light-Emitting Diodes with Different Structural Design………………………………………………40 3.1. Introduction of InGaN Quantum Wells Blue LEDs…………………………..40 3.1.1. Structure of InGaN QWs Blue LEDs…………………………………….40 3.1.2. Thin-Film-Flip-Chip Technology……………………………………...42 3.1.3. Emission Polarization of InGaN QWs Blue LEDs…………………….....45 3.1.4. Fabrication of Photonic Crystals on TFFC LEDs…………………….46 3.2. Band Structure of 2D Photonic Crystals……………………………….……….49 3.2.1. 2D Simulation of Hexagonal PC…………………………………………49 3.2.2. Physical Meaning of Photonic Band Gap………………………………..53 3.2.3. Transmittivity and Reflectivity of 2D PC Slab………………………….56 3.3. Effect of P-GaN Layer Thickness on Light Extraction Efficiency for Conventional TFFC InGaN LEDs…………………………………………………………….60 3.3.1. Emission Enhancement by Constructive Interference: Micro-Cavity Effect……………………………………………………………………..60 3.3.2. FDTD Analysis of the Effect of P-GaN Layer Thickness…………….63 3.4. Effect of Photonic Crystals on Light Extraction Efficiency of Blue LEDs……65 3.4.1. The Simulation Model……………………………………………...……65 3.4.2. Effect of Photonic Crystal Depth d………………………………………70 3.4.3. Effect of the Filling Factor f………………………………………….…..74 3.4.4. Effect of the Lattice Constant a………………………………………….77 2 3.4.5. Effect of Dipole Source Position…………………………………………80 3.5. Effect of Cone-Shaped Periodic Nanostructure on Light Extraction Efficiency of Blue LEDs…………………………………………..……………………….82 3.5.1. Effect of Sharp-tip Cones………………………………………………..83 3.5.2. Effect of Truncated Cones…………………………………………….86 3.5.3. Effect of Dipole Source Position………………………………..………88 3.6. Summary of Light Extraction Efficiency Enhancement for InGaN Blue TFFC LEDs…………………………………………………………………………89 Chapter 4: Piezoelectric Properties in ZnO Nanomaterials……………………...….90 4.1. Simulation of ZnO Nanostructured Materials……………………………….90 4.2. Transfer of ZnO Nanostructures Grown by Chemical Vapor Deposition ……94 4.3. Wet Etching of PDMS………………………………………………………99 4.4. Dry etching of PDMS……………………………………………………….104 4.5. Summary of ZnO Piezoelectric Force Sensor…………………………….…106 Chapter 5: Conclusions and Future Work……………………………………..….107 5.1. Conclusions ………………………………………….……………………….107 5.2. Future Work……………………………………..…………………………108 Appendix………………………………………………...……………………………109 References…………………………………………………………………...………111 3 List of Tables Table 3.1 Comparison between FDTD simulation and experimental result………………69 Table 3.4.5-1 Weighted average of light extraction efficiency………………………….81 Table 3.5.3-1 Weighted average of LEE for LEDs with truncated cones……………….88 4 List of Figures Figure 1.1.2-1 The illustration of the total internal reflection. amb=ambient, SC=semiconductor. k0 is the wave number in air………………………………………....17 Figure 1.1.5-1 Dispersion relation for TE mode of a squared lattice photonic crystal of air holes computed by FDTD and Effective index method…………………………………..21 Figure 1.1.6-1 Left: Two-dimensional photonic crystal using a square lattice. Vector r is an arbitrary vector. Right: The Brillouin zone of the square lattice, centered at the origin (Γ). k is an arbitrary in-plane wave vector. The irreducible zone is the light blue triangular wedge. The special points at the center, corner, and face are conventionally known as Γ, Μ, and Χ………………………………………………………………………………….22 Figure 1.1.6-2 Ewald construction for a Bloch mode. The wave vector k|| of the main harmonic is coupled to other harmonics k|| + G by the RL (gray dots). Here, one of the RL points is in the air circle (inner circle) and radiates to air, while two are in the substrate circle (outer circle)………………………………………………………………………………23 Figure 1.2.1-1 (a) Wurtzite crystal structure of ZnO61. (b) The formation of electric dipole under external strain……………………………………………………………………...25 Figure 2.1.2-1 A unit cell of Yee’s lattice with specified position of the field components………………………………………………………………………………32 Figure 2.2.1-1 Calculating light extraction efficiency using FDTD method: (a) simulation region setting; (b) Determining the total power emitted from a dipole source using a power box……………………………………………………………………………….……….36 5 Figure 2.2.2-1 Simulation region settings for calculating photonic crystal band structure using Lumerical’s FDTD solutions. The orange square is the simulation region of one unit cell. Yellow cross is the field monitor…………………………………………………….38 Figure 2.2.2-2 Recorded signals of a guided mode in a two-dimensional hexagonal photonic crystal. (a) The recorded time signal; (b) Fourier transform of (a)……………..39 Figure 3.1.1-1 A typical structure of InGaN-based LED grown on sapphire substrate…..42 Figure 3.1.2-1 A typical structure of thin-film flip-chip GaN-based LED on a metal mirror…………………………………………………………………………………….44 Figure 3.1.2-2 A brief schematic diagram of the fabrication process for the LLO-LEDs. (a) laser processing, (b) separation, (c) etching of undoped GaN, (d) TFFC LED…………..44 Figure 3.1.3-1 The edge-emitting spectrum of blue InGaN/GaN MQWs LED at 455 nm..46 Figure 3.1.4-1 Illustration of processing flow for the formation of photonic crystals on LEDs. (a) e-beam resist by spin coating or deposition; (b) patterning by direct write e-beam lithography; (c) Dry etching of GaN surface; (d) e-beam resist lift-off………………….48 Figure 3.1.4-2 Nanoimprint process for the formation of photonic crystals on LEDs. (a)form a NIP polymer resist layer by spin coating; (b) the pattern of the stamp is transferred to polymer resist by imprinting; (c) dry etching of GaN surface; (d) NIP polymer resist lift-off…………………………………………………………………….49 Figure 3.2.1-1 TE (left) and TM (right) mode light in photonic crystals. Here the two- dimensional photonic crystals are considered as infinite in the vertical direction……….50 Figure 3.2.1-2 Band structure for photonic crystals of pillars. The left side is for TE mode and the right side is for TM mode. Photonic crystals of different R/a ratio were analyzed. The horizontal coordinate is Bloch wave vector in the first Brillouin zone along M-Gamma- 6 K-M direction with 15 data points in each direction. The vertical coordinate represents the Bloch mode frequency normalized by c/a……………………………………………….51 Figure 3.2.1-3 Band structure for photonic crystals of air holes. The left side is for TE mode