Table Of ContentLIGHT 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
st
May 31 . 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
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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
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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
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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
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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
61
Figure 1.2.1-1 (a) Wurtzite crystal structure of ZnO . (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-
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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
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-
K-M direction with 15 data points in each direction. The vertical coordinate represents the
Bloch mode frequency normalized by c/a……………………………………………….52
Figure 3.2.2-1 Schematic illustration of a PC periodic in one dimension……………….53
Figure 3.2.2-2 (a) Band structure of GaN/InGaN multilayer slab. (b) Band structure of
GaN/air multilayer slab. The right side depicts the energy distribution of the Bloch mode
for band 1 and 2 at the zone edge…………………………………………………………55
Figure 3.2.2-3 Bloch mode profile of hexagonal photonic crystal of pillars for R=0.2a. The
dashed circles represent pillars and the other region is air. (a) the top of band 1 at K point,
f=0.393 c/a. (b) the bottom of band 2 at K point, f=0.6 c/a. TM mode…………………..56
Figure 3.2.3-1 Theoretical and simulation results for transmittivity and reflectivity for
conventional GaN-based blue LEDs. (a) TE mode; (b) TM mode………………………58
Figure 3.2.3-2 Effect of photonic band gap on transmission coefficient. (a) The band
structure of 2D PC slab of pillars at R=0.2a; (b) Possible diffraction options of incident
light; (c) The transmission coefficient of the PC slab with a=207nm, r=0.2a, TM mode for
different wavelength…………………………………………………………………...…59
Figure 3.3.1-1 Illustration for the interference of original top emitting light and the light
reflected by the mirror……………………………………………………………………61
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