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Electronic Theses and Dissertations, 2020-

2020

High Performance Micro-scale Light Emitting Diode Display

Fangwang Gou
University of Central Florida

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and Dissertations, 2020-. 222.
https://stars.library.ucf.edu/etd2020/222
HIGH-PERFORMANCE MICRO-SCALE LIGHT EMITTING DIODE DISPLAY

by

FANGWANG GOU
B.S. University of Electronic Science and Technology of China, 2012
M.S. Peking University, 2015

A dissertation submitted in partial fulfillment of the requirements

for the degree of Doctor of Philosophy
in the College of Optics and Photonics
at the University of Central Florida
Orlando, Florida

Summer Term
2020

Major Professor: Shin-Tson Wu

 2020 Fangwang Gou

ii
 ABSTRACT

Micro-scale light emitting diode (micro-LED) is a potentially disruptive display

technology because of its outstanding features such as high dynamic range, good sunlight

readability, long lifetime, low power consumption, and wide color gamut. To achieve full-color

displays, three approaches are commonly used: 1) to assemble individual RGB micro-LED pixels

from semiconductor wafers to the same driving backplane through pick-and-place approach, which

is referred to as mass transfer process; 2) to utilize monochromatic blue micro-LED with a color

conversion film to obtain a white source first, and then employ color filters to form RGB pixels,

and 3) to use blue or ultraviolet (UV) micro-LEDs to pump pixelated quantum dots (QDs). This

dissertation is devoted to investigating and improving optical performance of these three types of

micro-LED displays from device design viewpoints.

For RGB micro-LED display, angular color shift may become visually noticeable due to

mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based

blue/green counterparts. Based on our simulations and experiments, we find that the mismatched

angular distributions are caused by sidewall emission from RGB micro-LEDs. To address this

issue, we propose a device structure with top black matrix and taper angle in micro-LEDs, which

greatly suppresses the color shift while keeping a reasonably high light extraction efficiency. These

findings will shed new light to guide future micro-LED display designs.

For white micro-LEDs, the color filters would absorb 2/3 of the outgoing light, which

increases power consumption. In addition, color crosstalk would occur due to scattering of the

color conversion layer. With funnel‐tube array and reflective coating on its inner surface, the

crosstalk is eliminated and the optical efficiency is enhanced by ~3X.

iii
 For quantum dot-converted micro-LED display, its ambient contrast ratio degrades because

the top QD converter can be excited by the ambient light. To solve this issue, we build a verified

simulation model to quantitatively analyze the ambient reflection of quantum dot-converted micro-

LED system and improve its ambient contrast ratio with a top color filter layer.

iv
To my beloved family.

v
 ACKNOWLEDGMENTS

Five-year Ph.D. study is a challenging and yet enjoyable experience for me. It would not

be possible without the support from those around me. I hold my sincere gratitude towards my

advisor, Prof. Shin-Tson Wu, for his inspirational guidance, continuous encouragement, and strong

support. He will be my lifelong role model of a scientist and teacher. I would also like to thank

Prof. Wu’s better half, our beloved Shimu, Cho-Yan Hiseh, for her love over the years.

I am thankful to my committee members, Prof. M. G. Moharam, Prof. Patrick L. LiKamWa,

late Prof. Boris Y. Zeldovich, and Prof. Yajie Dong. They have been always supportive in

supervising my research and providing strong recommendations for my society award applications.

I would like to thank our senior fellow group members, Dr. Ruidong Zhu, Dr. Fenglin Peng,

Dr. Haiwei Chen, Dr. Yun-Han Lee, Dr. Guanjun Tan and Dr. Juan He, for helping me understand

physical simulations and teaching me to do experiments. I would also like to thank Dr. Yuge Huang,

Tao Zhan, Ziqian He, Kun Yin, Jianghao Xiong, En-Lin Hsiang, Junyu Zou, Yannanqi Li, Zhiyong

Yang and Qian Yang for many valuable discussions and suggestions.

I appreciate the help and support from Victor Yin, Mark Son, Dr. Yi-Pai Huang, Dr. Jun

Qi, Dr. Zhibing Ge, David Suh, Darren So, Dr. Daming Xu, Dr. Yufeng Yan and Dr. Hao Chen at

Apple Inc. during my internship.

Lastly, I would like to thank my parents, my parents-in-law, and my brother, for their care

and love. And most of all, I would like to thank my husband, Ji Chen, for being my rock throughout

all the ups and downs. It is his unconditional love that gives me courage and keeps me moving

forward.

vi
 TABLE OF CONTENTS

LIST OF FIGURES ....................................................................................................................... ix

LIST OF TABLES ....................................................................................................................... xiv

CHAPTER 1 : INTRODUCTION .................................................................................................. 1

1.1 Basis of Micro-LED ...................................................................................................... 1

1.2 Challenges and Motivations .......................................................................................... 4

CHAPTER 2 : RGB MICRO-LED ................................................................................................. 8

2.1 Background ................................................................................................................... 8

2.2 Simulation Model and Experiment ............................................................................... 9

2.3 Emission Spectra and Angular Distributions .............................................................. 11

2.4 Angular Color Shift..................................................................................................... 17

2.5 Discussions ................................................................................................................. 24

2.6 Microstructures for Improvement of Light Extraction Efficiency .............................. 26

2.6.1 Microsphere on Micro-LED Chip ................................................................ 27

2.6.2 Microsphere on Resin .................................................................................. 28

2.6.3 Microsphere on Micro-LED and Resin........................................................ 31

2.7 Summary ..................................................................................................................... 33

CHAPTER 3 : WHITE MICRO-LED .......................................................................................... 35

3.1 Background ................................................................................................................. 35

3.2 Device Structure and Color Crosstalk......................................................................... 36

vii
 3.3 Color Gamut................................................................................................................ 40

3.4 Light Efficiency and Ambient Contrast Ratio ............................................................ 42

3.4.1 Reflective Coating ....................................................................................... 42

3.4.2 Absorptive Coating ...................................................................................... 48

3.5 Summary ..................................................................................................................... 51

CHAPTER 4 : QUANTUM DOT-CONVERTED MICRO-LED ................................................ 53

4.1 Background ................................................................................................................. 53

4.2 Theory and Modeling .................................................................................................. 54

4.3 Ambient Light Excitation of QDs ............................................................................... 57

4.4 DBR for QD-Converted Micro-LED .......................................................................... 59

4.5 Color Filter for QD-Converted Micro-LED ............................................................... 62

4.6 UV Micro-LED with RGB QDs ................................................................................. 67

4.7 Summary ..................................................................................................................... 69

CHAPTER 5 : CONCLUSIONS .................................................................................................. 71

APPENDIX: STUDENT PUBLICATIONS................................................................................. 73

REFERENCES ............................................................................................................................. 79

viii
 LIST OF FIGURES

Figure 1-1 Schematic layout of full-color micro-LED displays. (a) RGB micro-LED; (b) White

micro-LED; (c) QD-converted micro-LED. ................................................................................... 5

Figure 2-1 Refractive indexes of RGB micro-LED epitaxial material. ........................................ 10

Figure 2-2 SEM image of RGB micro-LEDs arrays after transfer. .............................................. 11

Figure 2-3 Measured emission spectra of (a) red, (b) green, and (c) blue micro-LEDs at different

viewing angles. Red and black dashed lines indicate the central wavelength and FWHM at each

viewing angle. ............................................................................................................................... 12

Figure 2-4 Far-field radiation patterns of RGB micro-LEDs. Dots are experimental data and lines

are simulation results. ................................................................................................................... 12

Figure 2-5 Simulated total, top and sidewall emissions of (a) red, (b) green, and (c) blue micro-

LEDs at different viewing angles. ................................................................................................ 14

Figure 2-6 (a) Top view of micro-LED chip with a point-like source located at (x, y). (b, c) Side

views of light emission from the point source with emission angle θi: (a) θi < θc: top emission; (c)

θi > 90°−θc: sidewall emissions. ................................................................................................... 14

Figure 2-7 10 reference colors in CIE1976 color space, with D65 white point and RGB micro-

LEDs primary colors. .................................................................................................................... 17

Figure 2-8 Simulated color triangle of the RGB micro-LED display system and the angular color

shifts of 10 reference colors from 0° to 80° viewing angle. ......................................................... 18

Figure 2-9 RGB spectra of our and Osram’s micro-LEDs. ......................................................... 19

Figure 2-10 RGB micro-LED display with taper angle α and top black matrix........................... 20

Figure 2-11 Relative light intensity as taper angle α changes. ..................................................... 21

ix
Figure 2-12 Simulated color shifts of 10 reference colors from 0° to 80° viewing angle for RGB

micro-LED display with top black matrix and 120° taper angle. ................................................. 22

Figure 2-13 Simulated color shifts of the first 18 colors in Macbeth ColorChecker from 0° to 80°

viewing angle for RGB micro-LED display with top black matrix and 120° taper angle. ........... 23

Figure 2-14 Simulated radiation patterns of (a) red, (b) green, and (c) blue micro-LEDs with top

black matrix and 120° taper angle at different viewing angle θ and azimuthal angle φ. .............. 24

Figure 2-15 The tolerance of angular distribution difference of RGB micro-LEDs when the Δu′v′

of white point D65 is 3.5 JNCD. The radiation pattern of red micro-LED is (a) Lambertian and (b)

Batwing distribution, respectively. ............................................................................................... 26

Figure 2-16 Schematic diagram of micro-LED with microsphere on (a) micro-LED chip, (b)

packing resin, and (c) both chip and resin. ................................................................................... 27

Figure 2-17 Light intensity enhancement ratio of microsphere-hole on (a) green and (b) blue micro-

LEDs. ............................................................................................................................................ 27

Figure 2-18 Light intensity Enhancement ratio of microsphere-hole on packing resin for (a) red,

(b) green and (c) blue micro-LEDs. .............................................................................................. 29

Figure 2-19 Light intensity Enhancement ratio of microsphere-bump on packing resin for (a) red,

(b) green and (c) blue micro-LEDs. .............................................................................................. 30

Figure 2-20 Ray tracing in micro-LEDs: (a) without microsphere, microsphere-hole on (b) chip,

(c) resin and (d) both chip and resin. ............................................................................................ 31

Figure 2-21 Simulated radiation patterns of (a) structure D with microsphere-hole and ( b) structure

E with microsphere-bump. RGB lines represent RGB micro-LED, respectively. ....................... 32

x
Figure 2-22 Simulated color shifts of the first 18 colors in Macbeth ColorChecker from 0° to 80°

viewing angle for RGB micro-LED with microsphere-hole on both chip and resin. ................... 33

Figure 3-1 (a) Schematic diagram for configuration of full color micro-LED display (Device I). (b)

Top view of one pixel. Lx = 30 µm, Ly = 130 µm. ........................................................................ 36

Figure 3-2 Simulated color image of Device I when only one pixel inside the white dashed lines is

turned on. ...................................................................................................................................... 38

Figure 3-3 Simulated color crosstalk of Device I as a function of phosphor thickness................ 39

Figure 3-4 (a) Schematic diagram for configuration of full color micro-LED display with funnel-

tube array (Device II). (b) Cross-sectional views of Device II. The taper angles in x-z plane and y-

z plane are α and β, respectively. .................................................................................................. 40

Figure 3-5 Simulated color image of device II when only one pixel which inside the white dashed

line is turned on. ............................................................................................................................ 40

Figure 3-6 Simulated spectrum of device II when all of the pixels are turned on. ....................... 41

Figure 3-7 Simulated color gamut for Device II with red/green phosphor in CIE 1931 color space.

....................................................................................................................................................... 42

Figure 3-8 Relative light intensity of Device II with reflective coating as a function of taper angle:

(a) α when ß = 80°, and (b) ß when α = 80°. The light intensity is normalized to that of device I.

....................................................................................................................................................... 43

Figure 3-9 Ambient light reflectance (solid lines, left y axis) for RGB subpixels of Device II with

reflective coating and taper angle α = 80° and ß = 80°, and transmittance of RGB color filters

(dashed lines, right y-axis). ........................................................................................................... 44

xi
Figure 3-10 Simulated ambient contrast ratio of Ambient contrast ratio of structure A, B and C.

The top surface reflectance is 4.5% without AR (anti-reflection) coating. .................................. 47

Figure 3-11 Simulated ambient contrast ratio of Ambient contrast ratio of structure A, B and C.

The top surface reflectance is 1.5% with AR (anti-reflection) coating. ....................................... 47

Figure 3-12 Relative light intensity of Device II with absorptive coating as a function of taper

angle: (a) α when ß = 80°, and (b) ß when α = 80°. The light intensity is normalized to that of

device I. ......................................................................................................................................... 49

Figure 3-13 Simulated ambient contrast ratio of Device I and Device II with absorptive coating

and reflective coating on the side inner surface of funnel-tube. Taper angle α = 80° and β = 80°.

The top surface reflectance is 4.5% without AR coating.............................................................. 50

Figure 3-14 Simulated ambient contrast ratio of Device I and Device II with absorptive coating

and reflective coating on the side inner surface of funnel-tube. Taper angle α = 80° and β = 80°.

The top surface reflectance is 1.5% with AR coating. .................................................................. 51

Figure 4-1 Device configuration of blue micro-LED array with red/green quantum dots as top color

conversion layer. ........................................................................................................................... 54

Figure 4-2 Simulation parameters. (a) Spectrum of D65 source. (b) Reflectance of micro-LED. 55

Figure 4-3 Absorbance and emission spectra of the employed green and red thick-shell quantum

dots. ............................................................................................................................................... 55

Figure 4-4 Calculated and simulated ambient light excitation spectra in quantum dot-converted

micro-LED displays based on Matlab and LightTools. ................................................................ 57

Figure 4-5 Calculated ambient light excitation and reflection for (a) red, (b) green and (c) blue

subpixel in QD-converted full color micro-LED displays. ........................................................... 58

xii
Figure 4-6 Device configuration of QD-converted micro-LED with DBR. ................................. 60

Figure 4-7 Reflectance spectra of the DBR film at different angles of incidence. ....................... 60

Figure 4-8 Calculated and simulated ambient light excitation spectra in QD-converted micro-LED

displays with a DBR film based on Matlab and LightTools. ........................................................ 61

Figure 4-9 Calculated ambient light excitation and reflection for (a) red, (b) green and (c) blue

subpixel in QD-converted full color micro-LED displays with a top DBR. ................................ 62

Figure 4-10 Schematic diagram for QD-converted micro-LED with a color filter array. ............ 62

Figure 4-11 Transmittance of RGB color filter. ........................................................................... 63

Figure 4-12 Calculated and simulated ambient light excitation spectra in QD-converted micro-

LED displays with a top pixelated color filter based on MATLAB and LightTools. .................. 64

Figure 4-13 Calculated ambient light excitation and reflection for (a) red, (b) green and (c) blue

subpixel in QD-converted full color micro-LED displays with top color filter. .......................... 65

Figure 4-14 Calculated luminous ambient reflectance as a function of QD area ratio. The surface

reflectance is 1.5%. ....................................................................................................................... 66

Figure 4-15 Calculated ambient contrast as a function of QD area ratio at ambient light of (a) 500

lux, (b) 5,000 lux and (c) 10,000 lux. The surface reflectance is 1.5%. ....................................... 67

Figure 4-16 Schematic diagram for configuration of UV micro-LED with RGB QDs................ 68

Figure 4-17 Calculated luminous ambient reflectance of UV and blue micro-LED as a function of

QD area ratio. The surface reflectance is 1.5%. ........................................................................... 69

xiii
 LIST OF TABLES

Table 1-1 Comparisons of LCD, OLED and micro-LED displays. ................................................ 2

Table 2-1 Optical parameters of commonly used AlGaInP-based red micro-LED and InGaN-based

blue and green micro-LEDs adopted in simulations. ...................................................................... 9

Table 2-2 Simulated and calculated sidewall emission ratio for a RGB micro-LED display with

different chip size. ......................................................................................................................... 16

Table 2-3 Light enhancement ratio of RGB micro-LEDs with microsphere................................ 32

Table 3-1 Luminous ambient reflectance RL for RGB subpixels of Device II with reflective coating

and taper angle α = 80° and ß = 80°.............................................................................................. 45

Table 3-2 Relative light intensity and luminous ambient reflectance of structure A, B and C with

reflective coating. .......................................................................................................................... 45

Table 3-3 ACR vs. sunlight readability. ....................................................................................... 48

Table 3-4 Relative light intensity and luminous ambient reflectance of Device I (without funnel

tube) and Device II with absorptive and reflective coatings. The taper angle α = 80° and β = 80°.

....................................................................................................................................................... 49

Table 4-1 Luminous ambient reflectance for RGB subpixels. ..................................................... 59

Table 4-2 Luminous ambient reflectance for RGB subpixels with top DBR. .............................. 62

Table 4-3 Luminous ambient reflectance for RGB subpixels with top color filter. ..................... 65

Table 4-4 Luminous ambient reflectance for UV micro-LED with RGB QDs. ........................... 68

xiv
 CHAPTER 1 : INTRODUCTION

III-nitride-based light emitting diodes (LEDs) have been widely used for various

applications such as lighting [1], signal [2], visible light communication [3,4] and displays [5,6]

owing to the fact that they are reliable and high efficient solid-state light sources. For conventional

LEDs with chip size lager than 200 µm × 200 µm, they provide sufficient brightness for

backlighting [7] in liquid crystal displays (LCDs) [8] and outdoor or indoor public displays. As

the improvement of manufacturing capability, the LED chip size is reduced to between 100 µm to

200 µm, which is called mini-LED [9,10]. It has been widely used as direct-lit backlight for LCDs

with local dimming technology [11–13] to achieve high dynamic range. However, as the

increasing demand of high-resolution displays including mobile phones, head-mounted devices

and wearable displays, it requires to further miniaturize the LED chip size to be less than 100 µm

× 100 µm, which is referred to as micro-scale LED (micro-LED) [14–16].

1.1 Basis of Micro-LED

Nowadays, LCD and organic light emitting diode (OLED) [17] are dominant display

technologies in the market. For traditional LCD, it consists of LED backlight system, aligned liquid

crystal as light switch, polarizers, optical compensation film stack and color filter array to form

red, green, and blue (RGB) subpixels. Although various techniques, including quantum-dot color-

conversion film [18,19], local dimming, new LC materials [20] and modes [21], have been

proposed to improve the performance of LCD in terms of wide color gamut, high dynamic range

and faster response time, it still has limitation in optical efficiency (only ~5%). Different from

LCDs, OLED is self-emissive and each pixel can be switched on and off individually, which

1
enables a true black state. However, it suffers from low brightness and limited lifetime due to its

organic nature. Compared to LCD and OLED, micro-LED is a potentially disruptive display

technology because of its outstanding features such as nanosecond response time, high dynamic

range, low power consumption, and long lifetime [22–24].

Table 1-1 Comparisons of LCD, OLED and micro-LED displays.

LCD OLED Micro-LED

Light efficiency Low Medium High

Peak brightness Medium Low High

Contrast ratio 1000:1 1000,000:1 1000,000:1

Response time 3 ms μs ns

MPRT [25,26]
1 ms 1 ms 1 ms
(90Hz, 0.1 duty ratio)

Image retention No Yes No

Temperature range -40°C ~ 100°C -50°C ~ 70°C -100°C ~ 120°C

Lifetime >10 years ~3 years >10 years

Cost Low Medium High

The investigations of micro-LED start from the early 2000s and it has been proved to have

higher light extraction efficiency, better current spreading and lower self-heating effect compared

to traditional LEDs [27,28]. In recent years, more and more companies have established programs

in micro-LED technology. For example, Sony launched its first 55 inch “Crystal LED” display

with a resolution of 1920 × 1080 in 2012, which consists of about 6 million micro LEDs. In 2017,

2
Sony released the “Crystal LED Display System” with micro-LED chip size of only ~20 μm and

pixel pitch of 1.26 mm [29]. The emitting area is only ~1% and the rest 99% area are black, which

enables high contrast ratio even in a bright environment. In addition, the emission patterns of RGB

micro-LEDs are very close to Lambertian distribution and are well matched, leading to a wide

viewing angle. In 2018, AU Optronics Corp. demonstrated a 12.1 inch full-color micro-LED

display with a resolution of 169 pixel per inch (PPI) by utilizing blue micro-LED with chip size

less than 30 μm to pump red and green color conversion materials [30]. PlayNitride has

demonstrated a 114 PPI transparent “PixeLED” micro-LED display with a transparency of ~60%

based on an active driven LTPS backplane [31]. PlayNitride has also demonstrated a 468 PPI

wearable micro-LED device and a flexible micro-LED display with thickness of only 28 μm. In

2019, Jade Bird Display (JBD) presented a 600 DPI (dots per inch) bi-color Micro-LED display

which implanted JBD’s proprietary transferring technology to assemble red and green chips to

silicon CMOS backplane. Also, JBD exhibited a mono-color Micro-LED module with a pitch size

of only 2.5 µm and a resolution of 10,000 DPI. It achieved a very high brightness of million nits,

which has potential applications in augmented reality (AR). X-Celeprint has demonstrated a 5.1-

inch, 70 PPI Micro-LED display which was made of 8 × 15 μm RGB chips based on active driven

Micro IC [32]. glō Inc. has demonstrated a 0.7 inch micro-LED display with 1000 PPI and a

brightness of 100,000 nits achieved by GaN nanowires technology. Compared to planar micro-

LEDs in which efficiency decreases as chip size shrinks, the efficiency of nanowire micro-LED

improves with decreasing size, which greatly saves the power consumption.

To assemble a micro-LED display, two major processes have been proposed. The first one

is mass transfer, which assembles individual RGB micro-LED pixels from semiconductor wafers

3
to the same driving backplane through pick-and-place approach [33–36]. This approach requires

precise alignment for each pixel and can be used for building of middle- and large-sized displays

including mobile phone, monitor, TV or video wall. The second method is monolithic

integration [37–39], which includes growth of micro-LED chips on a single epitaxial wafer and

bonding with driving circuit. This method is only ideal for small sized displays such as smart

watches and micro-projectors due to the limitation of wafer size for growth of LEDs (usually 4~8

inches) [40,41].

1.2 Challenges and Motivations

Although the science is clear, micro-LED still faces many technical challenges as the chip

size shrinks. For example, to reduce the manufacturing cost, larger-sized epitaxial wafers with

wavelength uniformity are necessary [42]. In addition, it requires that the yield of mass transfer

process reaches to 99.99% to let the following defect testing and repairing become manageable. In

terms of driving, it is still under debating that which technique including CMOS diver IC or thin-

film transistor (TFT) is more suitable. Another challenge is full-color formation. Figure 1-1 shows

the layout of three typical color formation approaches: (a) assemble RGB subpixels with individual

RGB micro-LED chips through mass transfer process; (b) utilize monochromatic blue micro-LED

with a color conversion film to obtain white source first, and then employ color filters to form

RGB pixels [43,44], which is referred to as white micro-LED in this dissertation; (c) use blue or

ultraviolet (UV) micro-LEDs to pump pixelated RG or RGB quantum dots [45,46].

4
Figure 1-1 Schematic layout of full-color micro-LED displays. (a) RGB micro-LED; (b)
White micro-LED; (c) QD-converted micro-LED.

However, for each full-color formation approach, many issues arise as listed as below:

1) For full-color micro-LED display consists of individual RGB LED chips, as the chip

size down to micron scale, although the internal quantum efficiency may droop due to

increased non-radiative recombination from sidewall defects, its light extraction

efficiency is improved because the light emission from sidewall gradually

increases [47–51]. However, the far-field radiation pattern would deviate from ideal

Lambertian distribution, depending on the sidewall emission intensity, which is

5
 determined by the refractive index of the employed semiconductor material and device

structure. For commercial LEDs, the most commonly used epitaxy wafer for red LED

is based on GaInP/AlGaInP multi-quantum wells (MQWs), while blue and green LEDs

are based on InGaN/GaN MQWs [52,53]. Therefore, angular distribution mismatch

among RGB micro-LEDs would occur because the red chip uses different epitaxy

materials and has different structures from the green and blue ones. As a result, the

angular color shift of mixed colors, such as skin tone, by mixing RGB colors with

different ratios may become distinguishable by the human eye [54].

2) For white micro-LEDs, it only needs one type of LED epitaxy wafer and the color

down-conversion layer does not need to be pixelated, which is more feasible for

manufacturing. However, the color filters would absorb 2/3 of the outgoing light, which

increases power consumption. In addition, color crosstalk would occur due to scattering

of the color conversion layer.

3) For QD-converted micro-LED, it can achieve a wide color gamut since QDs have

narrow photo-luminescent emission spectra [55–57]. Moreover, it does not need color

filters compared to white micro-LED, which enables a higher optical efficiency.

However, for complete color down-conversion, the optical density (OD) of the QDs

resin layer must be high, which requires highly absorbing QDs and a relatively thick

layer. Therefore, distributed Bragg reflector is needed to recycle the blue or UV light

to improve the light conversion efficiency. A drawback of this approach is the degraded

ambient contrast ratio. In addition, the top QD layer can be excited not only by the light

from the micro-LED, but also by the short-wavelength component of the ambient light,

6
 which will degrade the ambient contrast ratio of the displays [58]. Therefore, how to

analyze the ambient excitation of QDs quantitatively needs to be explored.

This dissertation will mainly focus on possible solutions to above-mentioned challenges

for these three types of micro-LED displays. In Chapter 2, the angular color shift and light

extraction efficiency of RGB individual micro-LEDs are investigated. In Chapter 3, a funnel-tube

array is proposed to eliminate color crosstalk and triple the optical efficiency in white micro-LEDs.

In Chapter 4, the ambient excitation of color-converted micro-LED is analyzed quantitatively.

7
 CHAPTER 2 : RGB MICRO-LED

2.1 Background

The most straightforward approach to achieve full-color micro-LED display is to assemble

individual RGB micro-LED pixels from semiconductor wafers to the same driving backplane

through pick-and-place approach, which is referred to as mass transfer process. Although it is still

challenging to achieve high manufacturing yield with low cost, large-size displays consisting of

full-color micro-LED modules are emerging, such as Sony Crystal-LED TVs, Samsung micro-

LED video walls and so on.

To fabricate micro-LEDs, III-nitride materials have been proved to be excellent candidates

to achieve high efficiency and high power [59,60]. Although III-nitride materials are considered

to be monochromatic emission sources, in theory, RGB colors can be generated by tuning the

indium composition in the InGaN/GaN multiple quantum wells (MQWs). However, the

luminescence efficiency of InGaN-based red LED are still low due to large lattice mismatch

between the InGaN active layer and GaN buffer for red wavelength. As a results, high efficiency

full-color micro-LED displays cannot be achieved with GaN family alone. Instead, AlGaInP-based

micro-LED has been commonly employed for red emission [61].

For a micro-LED, as the chip size down to micron scale, although the internal quantum

efficiency may droop due to increased non-radiative recombination from sidewall defects [48,62],

its light extraction efficiency is improved because the light emission from sidewall gradually

increases [63]. However, the far-field radiation pattern would deviate from ideal Lambertian

distribution, depending on the sidewall emission intensity, which is determined by the refractive

index of the employed semiconductor material and device structure. For full-color micro-LEDs,

8
because the epitaxy wafer for red LED is based on GaInP/AlGaInP MQWs, while blue and green

LEDs are based on InGaN/GaN MQWs, angular distribution mismatch among RGB micro-LEDs

would occur. As a result, the angular color shift of mixed colors, such as skin tone, by mixing RGB

colors with different ratios may become distinguishable by the human eye.

2.2 Simulation Model and Experiment

Table 2-1 Optical parameters of commonly used AlGaInP-based red micro-LED and
InGaN-based blue and green micro-LEDs adopted in simulations.

Red Thickness Green/Blue Thickness n k

n k
Layers (µm) Layers (µm) Green Blue Green Blue

ITO 0.1 2.07 0 n-GaN 2.5 2.38 2.43 4e-5 4e-5

p-GaP 1.0 3.33 0 MQW 0.1 2.41 2.49 2e-2 2e-2

p-AlGaInP 0.1 3.30 0 p-AlGaN 0.05 2.31 2.35 6e-4 7e-4

p-AlInP 0.6 3.21 0 p-GaN 0.3 2.38 2.43 4e-5 4e-5

MQW 0.4 3.60 0.16 n/p-Metal 0.5 0.44 1.43 2.29 1.85

n-AlInP 0.3 3.21 0 - - - - - -

n-AlGaInP 1.2 3.30 0 - - - - - -

n/p-Metal 0.5 0.15 3.52 - - - - - -

In order to analyze the color shift of RGB micro-LED displays originated from mismatched

angular distribution, firstly we need to examine the emission patterns of RGB micro-LEDs at

different viewing angles. We build a simulation model with ray-tracing software LightTools. Table

2-1 lists the commonly used major structure layers and their thicknesses of flip-chip RGB chips.

9
For AlGaInP-based red micro-LED, it consists of metal contact layer, n-cladding AlGaInP, n-type

AlInP diffusion barrier, GaInP/AlGaInP MQWs, p-type AlInP diffusion barrier, p-cladding

AlGaInP, and p-GaP window layer [61]. For InGaN-based green and blue micro-LEDs, their

structures have metal contact layer, p-type GaN, AlGaN electron block layer, InGaN/GaN MQWs

and n-type GaN. Although only the refractive indices at central wavelength are included in Table

2-1, the wavelength dispersion of refractive index for each material (Fig. 2-1) is also considered

in our simulation [64,65]. The die sizes are all 35 × 60 μm in order to be comparable to some

commercial products. For simplicity, the metal pad is set to be the same size as chip size. The

layout and materials of blue and green micro-LED are similar, but they are quite different from

those of red chip. Light radiation from the multi-quantum wells (MQWs) with uniform angular

distribution travels through each layer and across interfaces according to Snell’s law.

Figure 2-1 Refractive indexes of RGB micro-LED epitaxial material.

10
 2.3 Emission Spectra and Angular Distributions

To validate our model, RGB micro-LEDs with the same chip size 35×60 µm are fabricated

(Fig. 2-2). The measured emission spectra of RGB chips from normal direction to 60° viewing

angle are plotted in Fig. 2-3. For each device, the central wavelength does not shift as viewing

angle increases, indicating the cavity effect inside micro-LEDs is negligible. The central

wavelength and full width at half maximum (FWHM) for RGB chips are [626 nm, 14 nm] for red,

[529 nm, 34 nm] for green, and [465 nm, 16 nm] for blue. It should be mentioned here that organic

LED intentionally utilizes the cavity effect to narrow the emission spectra, but the tradeoff is

noticeable color shift [54].

Figure 2-2 SEM image of RGB micro-LEDs arrays after transfer.

11
 (a) (b) (c)
Viewing Angle (°)

Wavelength (nm)

Figure 2-3 Measured emission spectra of (a) red, (b) green, and (c) blue micro-LEDs at
different viewing angles. Red and black dashed lines indicate the central wavelength and
FWHM at each viewing angle.

Figure 2-4 shows the far-field radiation patterns of our RGB micro-LEDs. Dots represent

the measured data and solid lines stand for simulation results. As can be seen, good agreement

between experiment and simulation is obtained. As viewing angle increases, the light emission

from red chip declines following Lambert's cosine law. In contrast, for green and blue micro-LEDs,

the light intensity gets stronger from normal angle to ~40° and then decreases. The mismatched

angular distributions originate from different materials of RGB chips and will cause angular color

shift of mixed colors.

Figure 2-4 Far-field radiation patterns of RGB micro-LEDs. Dots are experimental data
and lines are simulation results.

12
 For a traditional LED with chip size at millimeter scale, the light emission from top surface

dominates, which is limited by total internal reflection. The critical angle (θc) between the

semiconductor and air interface determines the amount of light escaping from LED and it can be

calculated according to Snell’s Law:

nair
c  sin 1 ( ). (1)
n

In Eq. (1), nair and n represents the refractive index of the air and top semiconductor layer,

respectively. If we neglect the Fresnel loss at semiconductor/air interface, the light extraction

efficiency η of the LED can be estimated using following equation [66]:


 , (2)
4

where Ω is the solid angle of escape cone, which is expressed as:

2 c
   d  sin  d . (3)
0 0

However, as the chip size shrinks, the sidewall emission from micro-LED should also be

taken into consideration. In other words, after photons are generated from the MQWs, they can be

extracted from all of six surfaces of the chip. Figure 2-5 depict the simulated top and sidewall

emissions from RGB micro-LEDs. As can be seen, the top emissions of RGB chips are all

Lambertian distributions, but the sidewall emission from green and blue chips are much stronger

than that from red one. This difference originates from the stronger absorption of red MQW than

that of green and blue MQWs.

13
Figure 2-5 Simulated total, top and sidewall emissions of (a) red, (b) green, and (c) blue
micro-LEDs at different viewing angles.

l
(a)
w-y
w
l-x x

y y

(b) θi h1
z θc
h2
x

(c) θc
z

Figure 2-6 (a) Top view of micro-LED chip with a point-like source located at (x, y). (b, c)
Side views of light emission from the point source with emission angle θi: (a) θi < θc: top
emission; (c) θi > 90°−θc: sidewall emissions.

To analyze the light path inside the micro-LED, we take a point-like source located at

MQW layer with coordinate (x, y) as an example (Fig. 2-6(a)). The chip size is l × w and the

14
distance of MQW from top surface is h1. On the top surface, the light can be extracted for the rays

with θi smaller than θc, as depicted by the black line in Fig. 2-6(b). We can obtain a completed

escape cone due to the short distance h1 between MQW and the semiconductor/air interface. The

top emission ratio to the total emission intensity can be calculated using Eq. (3). In the opposite

direction, the light goes downward and gets reflected by the bottom electrode pad back to top

surface [i.e. yellow line in Fig. 2-6 (b)], which should be also included as top emission. But for the

downward light, both reflectance Rs of bottom metal pad and absorption of MQW need to be taken

into consideration during calculation.

For the emission towards sidewall, the escape cone may not be completed, depending on

the position of point-like sources located at the MQW layer. For example, the point source at a

short distance from sidewall can get a completed escape cone. However, as the distance increases,

the escape cone will become uncompleted because of the small thickness of micro-LED chip (< 4

μm). Fig. 2-6(c) shows the side-view of light emissions from the point source with angle θi

satisfying 90°−θc < θi < 90°. Light within this angle range will escape from the sidewall but

experience different absorption or reflection losses. For example, when the light emission from the

point source reaches the top edge of the sidewall without experiencing reflection [gray line in Fig.

2-6(c)], we can obtain a light escape cone θ1 = tan-1(h1/x) without absorption and reflection loss.

The emission ratio can be calculated using Eqs. (2) and (3). If the emission angle θi satisfies 90°−θc

< θi < 90°−θ1, although the light can still escape from sidewall, it will experience multiple

reflections between top and bottom surfaces as well as absorption of MQW, which are illustrated

by green and orange lines in Fig. 2-6(c). For the light reflected by the bottom surface n times, the

sidewall emission ratio can be estimated by modifying Eqs. (2) and (3) as:

15
 1 n
n  e2 n d Rs n  sin  d . (4)
2  n1

In Eq. (4), α and d represent the absorption coefficient and thickness of MQW layer, and Rs is the

reflectance of bottom metal pad. For simplicity, we have neglected the Fresnel loss at

semiconductor/air interface and the refraction between different layers inside micro-LED. Thus,

the total sidewall emission ratio from four side surfaces can be calculated by adding all the ratios

together.

Table 2-2 Simulated and calculated sidewall emission ratio for a RGB micro-LED display
with different chip size.

Red Green Blue

Chip size
(μm2)
Sim. Cal. Sim. Cal. Sim. Cal.

15×30 10.8% 11.0% 58.9% 58.5% 62.2% 65.7%

35×60 5.3% 5.6% 52.4% 54.2% 55.1% 56.7%

50×100 3.8% 4.0% 47.2% 49.6% 50.1% 52.9%

The calculated sidewall emission ratio by MATLAB and the simulated data using ray-

tracing in LightTools for micro-LEDs with different chip size are listed in Table 2-2. The

agreement between these two methods is very good. As the chip size shrinks from 50×100 µm2 to

15×30 µm2, the sidewall emissions from RGB micro-LEDs increase because of lower absorption

and reflection losses inside the chip. For red micro-LEDs, the sidewall emission ratio is much

smaller than that from green and blue ones due to stronger absorption of red MQW, resulting in a

mismatched angular distribution among RGB micro-LEDs.

16
 2.4 Angular Color Shift

To analyze the color shift induced by subpixels’ angular distribution mismatch, a more

representative way is to calculate the color shifts of the mixed colors by RGB color mixing. We

have defined 10 reference colors in total in order to evaluate color shift throughout the entire color

gamut. These reference colors include three primary colors, white point D65, and six mixed colors

(three 100% saturated colors and three 50% saturated colors), which are plotted in CIE 1976 color

space [67,68] , as shown in Fig. 2-7.

Figure 2-7 10 reference colors in CIE1976 color space, with D65 white point and RGB
micro-LEDs primary colors.

17
Figure 2-8 Simulated color triangle of the RGB micro-LED display system and the angular
color shifts of 10 reference colors from 0° to 80° viewing angle.

Figure 2-8 depicts the color triangle of the RGB micro-LED display and CIE coordinates

of 10 reference colors at viewing angles from 0° to 80° with 10° intervals. For primary colors, no

color shift is observed at fixed driving current due to weak cavity effect inside micro-LEDs. While

for mixed colors and white point, color shift becomes worse as viewing angle increases as expected.

The average color shift Δu′ν′ of all colors at 80° is 0.061 and the maximum value is 0.169 for

magenta channel, which exceeds the just-noticeable level (Δu′ν′ < 0.02). This issue will get worse

as the micro-LED size decreases because of increased sidewall emissions from green and blue

chips, as listed in Table 2-2. Therefore, it is necessary to improve the color performance of RGB

micro-LED display system, especially for high resolution applications.

18
Figure 2-9 RGB spectra of our and Osram’s micro-LEDs.

In terms of color gamut, our display device can cover 97% of DCI-P3 standard and 78%

of Rec. 2020 standard, as indicated by the color triangle in Fig. 2-8. The relatively narrow color

gamut coverage results from the spectral crosstalk between green and red chips, as Fig. 2-9 shows.

Compared to our device, the Osram’s green LED has a slightly shorter central wavelength and

narrower FWHM. As a result, it has less crosstalk with the red LED and its RGB LED system can

achieve wider color gamut. The calculated color gamut coverage is 99% of DCI-P3 standard and

90% of Rec.2020 standard. To achieve a desired color gamut, we can tune the electroluminescent

(EL) spectrum of green micro-LED by controlling the well/barrier width and indium composition

of MQW [69].

To reduce color shift, a straightforward method is to obtain matched RGB radiation

patterns, i.e. Lambertian distribution, which means the sidewall emissions from green and blue

micro-LEDs should be eliminated. Figure 2-10 illustrates our proposed display configuration

19
consisting of micro-LED array and top black matrix outside the emission region. The gap between

micro-LEDs are filled with resin or other dioxide materials with refractive index of ~1.5. As a

result, the sidewall emissions from green and blue chips can be completely absorbed by the black

matrix, allowing only emissions from top surface of RGB micro-LEDs with matched Lambertian

distributions. Thus, a negligible color shift can be obtained, but the trade-off is ~2x lower optical

efficiency, because the sidewall emission ratios of green and blue chips contribute to ~50% of the

total emission intensity, as listed in Table 2-2.

Figure 2-10 RGB micro-LED display with taper angle α and top black matrix.

Since power consumption is another key performance metric for displays, we introduce a

taper angle α in the micro-LED structure (Fig. 2-10) to improve the light intensity from top

surface [70–72]. The simulated light intensity is normalized to the device with normal angle and

without black matrix as Fig. 2-11 shows. As the taper angle α changes from 90° to 140°, the light

intensity from green and blue chips first increases and then saturates, while that from red LED

almost keeps steady. The color shift becomes severe even if the presence of black matrix when α

is larger than 130°. This is because a larger taper angle causes a narrower angular distribution of

top emissions for green and blue LEDs but it has little impact on the red counterpart. Thus, the

mismatched angular distribution causes a severe color shift. By considering device fabrication,

light efficiency as well as color shift, taper angle α = 120° is an optimal choice. With this

20
configuration, the light efficiency of green and blue micro-LEDs can be boosted to 83% and 86%,

respectively, compared to the device with 90° taper angle but without black matrices, while

keeping unnoticeable color shift.

Figure 2-11 Relative light intensity as taper angle α changes.

The simulated color shifts for RGB micro-LED display with top black matrix and 120°

taper angle is plotted in Fig. 2-12. The average Δu′ν′ of all 10 reference colors at 80° is 0.005 and

the maximum value is 0.014 for magenta channel, which is below 0.02 and is acceptable for

commercial applications.

21
Figure 2-12 Simulated color shifts of 10 reference colors from 0° to 80° viewing angle for
RGB micro-LED display with top black matrix and 120° taper angle.

Besides these 10 reference colors, we also evaluate the color shift using the first 18 colors

in Macbeth ColorChecker [73], which is commonly used in color tests and reproductions to mimic

the colors of natural objects such as human skin, foliage and flowers. Figure 2-13 depicts the

simulated results. The color shifts of all 18 reference colors within 80° viewing cone are below

0.01 and the maximum average Δu′ν′ is 0.007, which remains visually unnoticeable by human eye.

22
Figure 2-13 Simulated color shifts of the first 18 colors in Macbeth ColorChecker from 0°
to 80° viewing angle for RGB micro-LED display with top black matrix and 120° taper
angle.

In the above analysis, we discuss the color shift of RGB micro-LED displays when the

viewing angle θ (polar angle) changes along a fixed azimuthal angle φ. It is noteworthy that the

sidewall emission from a bare LED without black matrix will not stay the same when the azimuthal

angle changes because it has a rectangular shape. As a result, the angular distributions would be

different, leading to different levels of color shifts of RGB micro-LED display as the azimuthal

angle changes. With the presence of black matrix, the color shift can be greatly suppressed at all

polar angles and azimuthal angles because all the sidewall emissions are effectively absorbed and

matched angular distributions for both RGB colors can be obtained, as shown in Fig. 2-14. In

addition, the black matrices help to absorb the ambient light reflected from the micro-LED chips

and metal wires, which in turn improves the ambient contrast ratio [10,74]. Other approaches to

reduce color shift include making asymmetric subpixel arrangement [75] or employing a patterned

23
scattering film on top of the display panel to achieve matched angular distributions [76]. Each

approach has its own merits and demerits.

Figure 2-14 Simulated radiation patterns of (a) red, (b) green, and (c) blue micro-LEDs
with top black matrix and 120° taper angle at different viewing angle θ and azimuthal
angle φ.

2.5 Discussions

As discussed in above sections, the angular color shift of RGB micro-LED display

originates from angular distribution mismatch. To better understand their relationship, we

investigate the maximum tolerable angular distribution mismatch while keeping indistinguishable

color shift within the ±85° viewing zone. At normal viewing angle, the RGB ratio [a, b, c] to obtain

a white point D65 can be calculated using the following equation [68]:

XR XG X B  a   X t 
Y YG YB   b    Yt  ,
 R (5)
 Z R ZG Z B   c   Z t 

In Eq. (5), X, Y, Z are tristimulus values for colored stimuli and can be obtained by multiplying

the color matching functions x() , y() and z( ) by the amount of energy I(θ) in the stimulus at

each wavelength and integrating across the spectrum S(λ) as shown in Eq. (6) [68].

24
 X   I ( ) S ( ) x( )d  ,
Y   I ( ) S ( ) y ( )d  , (6)
Z   I ( ) S ( ) z ( ) d  ,

After obtaining RGB ratio [a, b, c] at normal viewing angle, we can use it to calculate the

color difference Δu′v′ at oblique viewing angle. While in our analysis, we keep Δu′v′ of white point

is 0.014 which is corresponding to 3.5 Just Noticeable Color Difference (JNCD), and then calculate

the tolerable angular distribution mismatch among RGB emissions by using Eq. (7):

S ( ,  )  a  I R ( )  S R ( )  b  I G ( )  SG ( )  c  I B ( )  S B ( ). (7)

The calculated results are depicted in Fig. 2-15. Here we assume the blue and green chips have the

same angular distributions, and red chips have Lambertian (Fig. 2-15(a)) or Batwing distribution

(Fig. 2-15 (b)). From the results, we conclude that only if the angular distributions of blue and

green micro-LEDs are within the marked blue region, the color shift of white point is below 0.014.

In other words, the angular distribution of RGB chips are not necessary to be exactly the same.

This tolerance provides a useful guidance for further optimizing the display structure in order to

reduce color shift and improve light efficiency simultaneously. For instance, microstructures can

be employed helps improve the light efficiency and modify the angular distribution.

25
Figure 2-15 The tolerance of angular distribution difference of RGB micro-LEDs when the
Δu′v′ of white point D65 is 3.5 JNCD. The radiation pattern of red micro-LED is (a)
Lambertian and (b) Batwing distribution, respectively.

2.6 Microstructures for Improvement of Light Extraction Efficiency

Surface roughness and microstructures have been used to break the total internal reflection

at the semiconductor-air interface and improve the light extraction efficiency of traditional

LEDs [77,78]. They can be realized through different etching processes. In this dissertation, we

investigate how the microstructures affect the light output when they are on micro-LED chip, on

packing resin, and on both surfaces. Figure 2-16 illustrates three device structures with

microsphere. For the microsphere on the resin, it can be fabricated as either hole or bump, which

are referred to as microsphere-hole and microsphere-bump in this chapter. The reference micro-

LED chip has a taper angle of 120° and reflector on the sidewall to improve the light output

efficiency as discussed in Section 2.4.

26
Figure 2-16 Schematic diagram of micro-LED with microsphere on (a) micro-LED chip,
(b) packing resin, and (c) both chip and resin.

2.6.1 Microsphere on Micro-LED Chip

Figure 2-17 Light intensity enhancement ratio of microsphere-hole on (a) green and (b)
blue micro-LEDs.

First, we investigate the effect of microsphere only on micro-LED chip by using ray tracing

in LightTools. Light enhancement ratio to the structure without microsphere are calculated and

results are plotted in Fig. 2-17. The radius and pitch of microsphere are chosen as variables.

Considering the thickness of top n-GaN layer is only 2.5 μm in our chip, the radius of the

microsphere changes from 1 to 2 μm. Besides, no microstructures are applied to the red micro-

LED due to its thin epitaxial layer. As it can be seen, smaller pitch size and larger radius leads to

27
higher light enhancement ratio. For a certain pitch, the highest enhancement ratio can be obtained

when the pitch to radius ratio equals to 2, which means the microsphere with larger density is

preferred. Here, we select pitch = 2 μm and radius = 1 μm in the following simulation, and the

corresponding light intensity enhancement ratio is 1.35 and 1.16 for the green chip and the blue

chip, respectively.

2.6.2 Microsphere on Resin

Microsphere on micro-LED helps to extract light outside of chips, however, the total

internal reflection also occurs at the resin-air interface, which reduces the overall light output

efficiency. Therefore, in this section, we calculate the light enhancement ratio by optimizing the

radius and pitch of microsphere-hole on the packing resin. Results for RGB micro-LED devices

are depicted in Fig. 2-18. As pitch length increases, the light enhancement ratio first decreases

sharply and then saturates at 1, indicating the number of microspheres is too small to help with

light extraction. In addition, a larger radius is preferred to obtain higher light enhancement ratio.

This trend is very similar to that microsphere on micro-LED discussed in the above section. Again,

when the pitch to radius ratio equals to 2, highest light enhancement ratio can be achieved, which

are 1.16 and 1.10 for green and blue chips, respectively. While for red micro-LED, the efficiency

improvement is negligible because of the high refractive index (n ≈ 3.3) of epitaxial material and

large absorption loss of GaInP/AlGaInP MQWs. As a result, most of the emission is still trapped

inside the red chip.

28
Figure 2-18 Light intensity Enhancement ratio of microsphere-hole on packing resin for (a)
red, (b) green and (c) blue micro-LEDs.

In addition to microsphere-hole, we also study microsphere-bump on resin and results are

plotted in Fig. 2-19. As expected, similar trend that smaller pitch length and lager radius enable

higher light enhancement ratio is observed. Moreover, the ratio value is slightly higher compared

to that with microsphere-hole. The reason is that microsphere-bump helps to converge the forward

emission thus more light is collected by the receiver. However, besides light extraction efficiency

29
improvement, matched RGB angular distributions also need to be taken into consideration in order

to keep an indistinguishable angular color shift, which will be discussed later.

Figure 2-19 Light intensity Enhancement ratio of microsphere-bump on packing resin for
(a) red, (b) green and (c) blue micro-LEDs.

30
 2.6.3 Microsphere on Micro-LED and Resin

Figure 2-20 compares the raytracing results in structure without microsphere, microsphere

on chip, on resin and on both chip and resin. The preview ray number is 100. When no microsphere

is on micro-LED (Fig. 2-10(a) and (c)), it can be observed that the light is trapped inside the chip

due to the total internal reflection at the LED-resin interface. For comparison, in Fig. 2-10(b) and

(d), more light is extracted into the resin with the help of microsphere. With microsphere on resin,

the light extraction efficiency is further improved.

Figure 2-20 Ray tracing in micro-LEDs: (a) without microsphere, microsphere-hole on (b)
chip, (c) resin and (d) both chip and resin.

Table 2-3 lists the light enhancement ratio of different devices. The pitch = 2 μm and radius

= 1 μm for microsphere on micro-LED, and pitch = 12 μm and radius = 6 μm for that on resin. For

structures D and E with microsphere on both chip and resin, the light output efficiency for green

31
and blue is improved by more than 50% and 35% compared to the structure without microsphere,

which is promising for saving the power consumption of micro-LED displays.

Table 2-3 Light enhancement ratio of RGB micro-LEDs with microsphere.

Structure Red Green Blue

A: Hole on LED 1.00 1.35 1.16

B: Hole on resin 1.01 1.16 1.10

C: Bump on resin 1.08 1.26 1.18

D: Hole on LED + Hole on resin 1.02 1.54 1.35

E: Hole on LED + Bump on resin 1.07 1.69 1.50

Figure 2-21 Simulated radiation patterns of (a) structure D with microsphere-hole and ( b)
structure E with microsphere-bump. RGB lines represent RGB micro-LED, respectively.

For structures D and E, we calculate the radiation patterns of RGB device and results are

shown in Fig. 2-21. It can be observed that although structure D has a lower light enhancement

ratio, its RGB angular distributions are more matched than those of structure E, which enable a

lower color shift at large viewing angle. Figure 2-22 depicts the simulated color shift of structure

32
D from 0° to 80° viewing angle using the first 18 colors in Macbeth ColorChecker. The average

color shifts of all reference colors within 80° viewing cone are 0.007, which is visually

unnoticeable by human eye.

Figure 2-22 Simulated color shifts of the first 18 colors in Macbeth ColorChecker from 0°
to 80° viewing angle for RGB micro-LED with microsphere-hole on both chip and resin.

2.7 Summary

In this chapter, we built an experiment-valid simulation model to analyze the color shift of

RGB micro-LED displays. Results show that the sidewall emission causes mismatched angular

distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts

due to material difference, which further leads to visually noticeable color shift at larger viewing

angle. To mitigate the color shift while keeping a high light extraction efficiency, a device structure

with top black matrix and taper angle in micro-LEDs was proposed. After optimization, the color

33
shift Δu′v′ of the RGB micro-LED display with 120° taper angle is suppressed to below 0.01 within

80° viewing cone for the first 18 reference colors in Macbeth ColorCheker, and the efficiency

keeps ~85% of the device without black matrix. To further improve the efficiency, we proposed

microspheres with optimized size on both micro-LED chip and packing resin. For the green and

blue devices, their light extraction efficiency is enhanced by 54% and 35%, respectively. Although

only microsphere is discussed in this chapter, microstructure with other shape such as cones or

pyramid can also be applied.

34
 CHAPTER 3 : WHITE MICRO-LED

3.1 Background

To achieve full color micro-LED display while avoiding the challenging mass-transfer

fabrication process, one approach is to utilize single color micro-LEDs to excite color converters,

such as phosphors or quantum dots (QDs). For example, UV LED array with pixelated RGB

quantum dots can achieve high efficiency and wide color gamut because no color filter is needed.

However, to achieve complete color down-conversion, QDs with a high optical density and a

relatively thick layer are required. Therefore, light recycling components such as distributed Bragg

reflector are usually employed to improve the light conversion efficiency, which would degrade

the ambient contrast ratio [79]. To overcome this problem, another approach is to employ blue

micro-LEDs to excite color converters in order to obtain white light first, and then utilize color

filters to achieve RGB subpixels [44], which is referred to as white micro-LEDs in this dissertation.

In this device configuration, the color down-conversion layer does not need to be pixelated, which

is more favorable from device fabrication viewpoint. However, the color filters would absorb 2/3

of the outgoing light. In addition, color crosstalk would occur due to scattering of the color

conversion layer.

In this chapter, we propose a funnel-tube array for two-color phosphor based micro-LED

displays to reduce color crosstalk and improve light conversion efficiency simultaneously. In

addition, the issue of ambient light reflection from the device is addressed.

35
 3.2 Device Structure and Color Crosstalk

Figure 3-1 (a) Schematic diagram for configuration of full color micro-LED display (Device
I). (b) Top view of one pixel. Lx = 30 µm, Ly = 130 µm.

Figure 3-1(a) illustrates the device structure of color-converted micro-LED displays

(Device I). An array of monochromatic blue micro-LEDs with a LED driving backplane are

formed on the bottom substrate. Above the micro-LED array, a layer of green and red phosphor is

coated to obtain a white light source. On top of the phosphor, a color filter array is used to form

RGB subpixels. The pitch length of the display panel is set to be 150 µm, and the color filter size

is Lx = 30 µm and Ly = 130 µm (Fig. 3-1(b)). The chip size of blue micro-LED is Wx = 15 µm and

Wy = 30 µm. The system is simulated using ray-tracing software LightTools. For simplicity, we

assumed that all the micro-LEDs having the same central wavelength of 448 nm with Lambertian

angular emission distribution. The phosphor photoluminescence is simulated using LightTools

Advanced Physics Module [80]. The absorption spectrum, color conversion efficiency, and

emission spectrum are all taken into account during calculations [81]. The phosphor particle size

in the simulation is set to vary from 0.25 µm to 5 µm due to the small pixel size [82]. The refractive

index of phosphor is 1.8 at all wavelengths. The concentrations of green and red phosphors are

36
adjusted to obtain a white point for the display system. Simulation process starts with the emission

of blue light from the micro-LED array. When the light encountering phosphor particles, the blue

light is partially absorbed or scattered by the phosphor particles. The absorbed blue light will be

down-converted to green and red light with isotropic radiation, while the scattered light has

intensity distributions calculated by Mie theory [83]. Re-absorption of green and red light by the

phosphor particles is also considered in the modeling. Compare to QDs, phosphors have the

advantages of over 80% quantum yield, high thermal stability, resistance to moisture and stable

quantum yield and spectral properties [84].

In Device I, when only one pixel emits light and all other pixels are turned off, light leakage

may come from the surrounding pixels due to light scattering of the phosphor film, which is

referred to as color crosstalk. Figure 3-2 shows the simulated color image. Although only the RGB

subpixels inside the white dashed lines are turned on, light leakage outside this region is clearly

observed. This problem becomes more severe as the thickness of the phosphor film (h)

increases [85].

37
Figure 3-2 Simulated color image of Device I when only one pixel inside the white dashed
lines is turned on.

To evaluate the color crosstalk quantitatively, we define the color crosstalk ratio as Rcrosstalk,

which can be calculated using Eq. (8):

Itotal  I pixel
Rcrosstalk  , (8)
Itotal

where Itotal represents the total intensity of the whole panel (>> 10 pixels), and Ipixel stands for the

light intensity from the turned-on pixel. Figure 3-3 plots the calculated color crosstalk as a function

of the phosphor thickness h in Device I. When h increases from 15 µm to 100 µm, the optical path

inside the phosphor film becomes longer, leading to a more severe light scattering. Therefore, the

color crosstalk of RGB subpixels increases accordingly.

38
Figure 3-3 Simulated color crosstalk of Device I as a function of phosphor thickness.

To eliminate color crosstalk, we propose Device II with funnel-tube array and taper angles

α (x-z plane) and β (y-z plane) as illustrated in Fig. 3-4. It is formed above micro-LED layer while

the tube region aligned with each subpixel. The inner surface of the funnel-tube can be either

absorptive or reflective. The two-color phosphors are filled inside the funnel-tube to obtain white

light. On top of the funnel-tube array, the color filters with RGB subpixels are aligned with each

tube region. The simulation parameter is set to be the same as in Device I: Wx = 15 µm, Wy = 30

µm, Lx = 30 µm and Ly = 130 µm. In the system, the phosphors for each subpixel region are

designed to be totally isolated. Thus, the color crosstalk will be eliminated. Figure 3-5 shows the

simulated color image of Device II. As it can be seen, there is no light leakage outside the turned-

on pixel region.

39
Figure 3-4 (a) Schematic diagram for configuration of full color micro-LED display with
funnel-tube array (Device II). (b) Cross-sectional views of Device II. The taper angles in x-z
plane and y-z plane are α and β, respectively.

Figure 3-5 Simulated color image of device II when only one pixel which inside the white
dashed line is turned on.

3.3 Color Gamut

Vivid color is a key metric for display devices, as it enables a more realistic viewing

experience. Generally, light sources with a narrower full width at half maximum (FWHM) would

40
lead to a wider color gamut [56]. In our modeling, we use a combination of green phosphor (β-

sialon:Eu2+, FWHM ~ 50 nm) and red phosphor (CaAlSiN3:Eu2+) [86] to improve the color

performance. Figure 3-6 depicts the spectrum of blue micro-LED pumped the two-color phosphors

and the simulated color gamut in CIE 1931 color space is shown in Fig. 3-7. This system covers

92% of DCI-P3 standard [87] and 67% of Rec. 2020 standard [88,89]. Alternative color converters

such as CdSe red/green QDs with FWHM ~ 25–30 nm can also be used to obtain wider color

gamut [19,57]. However, the device structure, especially the thickness of the funnel-tube array,

needs to be optimized according to the optical density of the color converters.

Figure 3-6 Simulated spectrum of device II when all of the pixels are turned on.

41
Figure 3-7 Simulated color gamut for Device II with red/green phosphor in CIE 1931 color
space.

3.4 Light Efficiency and Ambient Contrast Ratio

3.4.1 Reflective Coating

In addition to color crosstalk, power consumption is another critical performance metric

for displays. To improve the light efficiency, the inner surface of funnel-tube can be deposited

metallic reflectors to recycle light and elongate the effective optical path inside the phosphors.

Furthermore, the taper angles (α and β) of the funnel-tube need to be optimized. The simulated

relative light intensities of Device II with reflective coatings normalized to that of Device I are

plotted in Fig. 8. Specifically, in Fig. 3-8(a), the taper angle α varied from 76° to 108°, while β

was fixed at 80°; in Fig. 3-8(b), the taper angle β varied from 31° to 108° when α was set to be

42
80°. As can be seen from these figures, the light intensity decreases as the taper angle increases.

When taper angle is larger than 90°, more light will be reflected downwards by the inner reflector

and then absorbed or scattered by the backplane. On the other hand, when taper angle is less than

90°, more light could be reflected upwards and then escape from the device to the air. Therefore,

taper angle less than 90° is preferred to obtain higher light efficiency. However, the ambient light

reflectance also increased as illustrated in Fig. 3-8 (right scale), which would degrade the ambient

contrast ratio of the displays.

Figure 3-8 Relative light intensity of Device II with reflective coating as a function of taper
angle: (a) α when ß = 80°, and (b) ß when α = 80°. The light intensity is normalized to that
of device I.

In order to further understand the ambient light reflection, we plot the reflectance for RGB

subpixels of structure with reflective coating and α = 80° and β = 80° as well as transmittance of

color filters in Fig. 3-9. The reflectance spectral profiles are very similar to the transmittance of

color filters except for the lower intensity of the blue light. For the blue subpixel, the blue

component of the ambient light transmitted through the color filter and entered the funnel-tube,

some of the light was absorbed and then converted by the phosphor, and other light was scattered

43
and reflected inside the funnel tube. A part of the unconverted blue light could escape from the

color filter. For green and red subpixels, the ambient light was scattered by the phosphor particles

or reflected by the funnel-tube back to the air, leading to higher reflectance.

Figure 3-9 Ambient light reflectance (solid lines, left y axis) for RGB subpixels of Device II
with reflective coating and taper angle α = 80° and ß = 80°, and transmittance of RGB color
filters (dashed lines, right y-axis).

Luminous ambient light reflectance can be defined as [90]:

2

RL 

1
V (  ) S ( ) R ( ) d 
, (9)
2
1
V (  ) S ( ) d 

where V(λ) is the photopic human eye sensitivity function, R(λ) is the spectral reflectance of the

display device, and S(λ) is the spectrum of ambient light. In our modeling, we used CIE standard

D65 as light source. Table 3-1 lists the luminous ambient light reflectance for each subpixel of

44
structure with reflective coating and α = 80° and β = 80°. Due to human eye sensitivity, ambient

reflection from green subpixel was dominant.

Table 3-1 Luminous ambient reflectance RL for RGB subpixels of Device II with reflective
coating and taper angle α = 80° and ß = 80°.

Pixels RGB Blue Green Red

RL 1.61% 0.05% 1.02% 0.54%

To analyze how taper angle affects the optical performance, we chose three typical

structures: A (α= 108°, β = 108°), B (α = 90°, β = 90°) and C (α= 76°, β = 31°). The relative light

intensities Iemission (which are normalized to the light intensity of Device I) and luminous ambient

reflectance of these three structures are listed in Table 3-2. Compared to Device I without funnel-

tube array, the light intensities of structure A, B and C are all improved, indicating higher optical

efficiency. Specifically, the light intensity of structure C is ~3X larger, but the luminous ambient

light reflectance also increases, which would degrade the ambient contrast ratio.

Table 3-2 Relative light intensity and luminous ambient reflectance of structure A, B and C
with reflective coating.

Structure (α, β) A (108°, 108°) B (90°, 90°) C (76°, 31°)

Iemission 1.20 1.60 2.93

RL 1.12% 1.34% 2.64%

The ambient contrast ratio (ACR) of these three structures is calculated using following

equation [74,90]:

45
 Lon  Lambient  RL
ACR  . ( 10 )
Loff  Lambient  RL

In Eq. (10), Lon (Loff) represents the on-state (off-state) luminance value of a display, Lambient is the

ambient luminance and RL is the luminous reflectance of the display panel.

To calculate ACR, we assumed Lon of Device I (without funnel-tube) to be 600 nits, while

Lon of structures A, B and C can be calculated according to the relative intensity and the results are

720 nits, 906 nits and 1758 nits, respectively. For all structures, Loff is 0, the surface reflectance is

assumed to be 4.5%. The calculated ACR of structures A, B and C are plotted in Fig. 3-10. As the

ambient light gets stronger, the ACR decreases dramatically first and gradually saturates. Among

these three structures, although structure C has the most severe ambient light reflectance, it

achieves the largest ACR under different ambient light conditions due to its highest brightness.

However, under strong ambient light conditions such as full daylight (~20,000 lux), the ACR of

three structures are small (≤ 5:1, inset of Fig. 3-10), which is barely readable under sunlight

according to Table 3-3. This relatively low ACR originates from both light scattering by the

phosphor particles and surface reflection. To improve ACR, commercial anti-reflection

coating [91] with surface reflectance about 1.5% can be employed, and results are shown in Fig.

3-11. When Lambient = 20,000 lux, the ACR can be improved to above 5:1, which is adequately

readable in sunlight [92].

46
Figure 3-10 Simulated ambient contrast ratio of Ambient contrast ratio of structure A, B
and C. The top surface reflectance is 4.5% without AR (anti-reflection) coating.

Figure 3-11 Simulated ambient contrast ratio of Ambient contrast ratio of structure A, B
and C. The top surface reflectance is 1.5% with AR (anti-reflection) coating.

47
Table 3-3 ACR vs. sunlight readability [92].

ACR Sunlight readability

1-2 Unreadable in sunlight

3-4 Adequately readable in shade; barely readable in sunlight

5-9 Adequately readable in sunlight; looks OK

10 Very readable in sunlight; looks good

15 Outstanding readability; looks great

20 Totally awesome; excellent readability; can't be improved

3.4.2 Absorptive Coating

Another approach to reduce the ambient light reflection is to use absorptive material for

the funnel-tube array, but the tradeoff will be reduced light efficiency. We calculate the relative

light intensities of Device II with absorptive coatings normalized to that of Device I. Results are

plotted in Fig. 3-12. Specifically, in Fig. 3-12(a), the taper angle α varied from 76° to 108°, while

α was fixed at 80°; in Fig. 3-12(b), the taper angle β varied from 31° to 108° when α was set to be

80. Different from the results for the reflective coating as shown in Fig. 8, for absorptive coating

the light intensity slightly increased as taper angle increased because more forward emission was

absorbed. The relative intensity is only 0.62 even for α = 108° and β = 80°, which is >2X lower

compared to the reflective coating structure. However, the ambient light reflectance becomes

lower as expected, which may help to improve the ambient contrast ratio.

48
Figure 3-12 Relative light intensity of Device II with absorptive coating as a function of
taper angle: (a) α when ß = 80°, and (b) ß when α = 80°. The light intensity is normalized to
that of device I.

Table 3-4 lists the relative light intensity and luminous ambient reflectance of Device I

without funnel tube and Device II with absorptive and reflective coatings. Although absorptive

coating helps to reduce ambient reflectance by about 2X compare to reflective coating, the light

intensity is sacrificed by more than 5X, which causes much larger power consumption.

Table 3-4 Relative light intensity and luminous ambient reflectance of Device I (without
funnel tube) and Device II with absorptive and reflective coatings. The taper angle α = 80°
and β = 80°.

Device W/O funnel tube Absorptive coating Reflective coating

Iemission 1.00 0.41 2.12

RL 0.96% 0.85% 1.61%

To compare the ACR of different coatings, we choose Device II with absorptive and

reflective coatings with taper angle α = 80° and β = 80°. Their Lon are calculated to be 246 nits and

49
1272 nits according to the relative intensity to Device I (without funnel-tube) with Lon = 600 nits.

The surface reflectance was assumed to be 4%. Figure 3-13 plotted the calculated ACR. Although

funnel-tube with reflective coating had the most severe ambient reflection, it achieved the largest

ACR under different ambient light conditions due to its highest brightness. Under strong ambient

light conditions such as full daylight (~20,000 lux), the ACR of Device I and Device II with

absorptive coating were all < 5:1 (inset of Fig. 3-13), which is barely readable under sunlight. To

improve ACR, we also employed AR coating with a surface reflectance of 1.5% and results are

shown in Fig. 3-14. When Lambient = 20,000 lux, the ACR of Device II with reflective coating could

be improved to ~8:1, which is adequately readable in sunlight. While for Device II with absorptive

coating, the improvement of ACR is minor and the readability still keeps poor. Therefore,

reflective coating is preferred for the funnel-tube to improve the light efficiency while keep a good

sunlight readability.

Figure 3-13 Simulated ambient contrast ratio of Device I and Device II with absorptive
coating and reflective coating on the side inner surface of funnel-tube. Taper angle α = 80°
and β = 80°. The top surface reflectance is 4.5% without AR coating.

50
Figure 3-14 Simulated ambient contrast ratio of Device I and Device II with absorptive
coating and reflective coating on the side inner surface of funnel-tube. Taper angle α = 80°
and β = 80°. The top surface reflectance is 1.5% with AR coating.

3.5 Summary

In this chapter, we proposed a funnel-tube array to eliminate the color crosstalk of a two-

color phosphor-converted white micro-LED displays. By depositing reflective or absorptive

coating on the inner surface of the funnel-tube, a clear boundary was formed between the subpixels

to prevent light leakage, thus a near zero optical crosstalk was achieved.

To improve the light output efficiency, the taper angle of the funnel-tube was also

optimized. With reflective coating on the inner surface, the light efficiency of the device was

enhanced by ~3X as the taper angle [α, β] equals to [76°, 31°]. Compare with the absorptive coating,

the ambient contrast ratio with reflective coating is also improved due to higher light intensity

despite the increased ambient reflectance. In addition, by utilizing two-color phosphors, which

51
have high thermal stability and resistance to moisture, the color gamut of this device covers 92%

of DCI-P3 standard.

52
 CHAPTER 4 : QUANTUM DOT-CONVERTED MICRO-LED

4.1 Background

In recent years, quantum dots (QDs) have been widely used as color conversion layers in

LCD backlight system due to their large absorption cross-section and narrow emission spectra

(20~40 nm), which help to create vivid color image on displays [55,93–95]. These nanometer-

sized (2~10 nm) semiconductor particles consist of core-shell structure and organic ligands and

are mainly governed by the quantum confinement effects [96]. The emission spectrum of QDs can

be tuned by varying their particle size with the same material system. This property enables a

design freedom for display applications to achieve wider color gamut. Compared to typical

phosphors, QDs with nanometer scale also have potential to improve luminance uniformity due to

reduced light scattering. In addition to LCD backlight, QDs have also been used as color down-

converters for monochromatic micro-LED display to achieve full color [18,97]. By employing

monolithically fabricated blue or UV micro-LED array to pump QD material, we can avoid the

challenging mass transfer process and only need one type of LED epitaxy wafer. Besides, this

approach has potential to achieve high resolution density, especially for small-size display

applications such as micro-displays or digital watches.

Different from application in LCD backlight in which QD layer is underneath a stack of

optical films, for QD-converted micro-LED display, the QD materials are on the top surface. As a

result, the QDs can not only be excited by light from micro-LED, but also the short-wavelength

component of ambient light, which will degrade the ambient contrast ratio of displays. In order to

solve this issue, it is necessary to explore how to analyze the ambient light excitation of QDs

53
quantitatively at first. However, until now, there is no detailed discussion on ambient contrast ratio

evaluation of QD-converted micro-LED displays.

4.2 Theory and Modeling

Figure 4-1 illustrates the device structure of blue micro-LED array with red and green QD

conversion layer. When the ambient light illuminates on the device, the ambient contrast ratio may

degrade due to the QD excitation. For simplicity, we divide the ambient excitation of QDs into

following three parts. 1) The first-time excitation. When the ambient light first hits the QDs, some

of the incident light is absorbed and converted according to the QD’s absorption and emission spectra,

and then emits back to air. 2) The second-time excitation. This part refers to the unabsorbed ambient

light when first passing through the QDs but is absorbed and converted after being reflected by the

bottom micro-LEDs and traversing through the QD at the second time. 3) Reflection. This is the

reflected ambient light by the micro-LED layer without being absorbed or converted by the QDs.

Figure 4-1 Device configuration of blue micro-LED array with red/green quantum dots as
top color conversion layer.

54
Figure 4-2 Simulation parameters. (a) Spectrum of D65 source. (b) Reflectance of micro-
LED.

Figure 4-3 Absorbance and emission spectra of the employed green and red thick-shell
quantum dots.

To quantitatively evaluate the ambient excitation of QDs, we use white light D65 as the

ambient light source and its spectrum is plotted in Fig. 4-2(a). The reflectance of blue micro-LED

is calculated with mixed-level simulation by combing Fresnel equation and ray tracing. For

55
simplicity, the micro-LED structure is assumed to be consisted of a 4-µm-thick GaN layer and a

100-nm-thick bottom gold electrode layer. The calculated reflectance is shown in Fig. 4-2(b). Fig.

4-3 shows the absorption and emission spectra of green and red QDs [98]. They are CdSe/CdS

thick-shell QDs with low self-absorption and film photoluminescence quantum yield (PLQY) as

high as ~70%. The absorbed number of photons (Nabs) by the QDs can be calculated by using Eqs.

(11) and (12):

Nabs ( )  h  I D65 ( )  AQD (), ( 11 )


N abs   I D 65 ( )  AQD ( )  d , ( 12 )
hc

where ID65 is the intensity of ambient light, AQD is the absorbance of QDs, λ is the wavelength, h

is the Planck constant, and c is the speed of light in vacuum. Then the emitted number of photons

(Nemit) can be obtained according to the PLQY of the QD layer as:

N emit  PLQY  N abs . ( 13 )

Therefore, the emitted light intensity from the QD layer can be calculated from following

equation:


PLQY   I D 65 ( )  AQD ( )  d
I emit  I PL ( )  hc . ( 14 )

I PL ( ) 
hc
d

In Eq. (14), IPL is the emission spectra of QD materials.

In order to verify above calculation model with MATLAB code, we also build a simulation

model using commercial ray-tracing software LightTools [80]. Because the size of QD is in

nanoscale, its photoluminescence (PL) is simulated using the mean free path, which is defined as

56
the average distance a ray travels inside the QD film before striking a QD nanoparticle [99,100].

The value of mean free path is adjusted to let all the blue light from micro-LEDs be absorbed. The

absorption spectrum, PLQY (70%), and emission spectrum are all considered during simulations

(Fig. 4-3). The refractive index of QDs is set to be 1.5 for all the wavelengths employed.

Simulation process starts with the emission of ambient light source D65. When the light

encountering QD particles, the ambient light is partially absorbed and converted to red or green

light with isotropic radiation. While the unabsorbed light will continue to travel without changing

its path [80].

4.3 Ambient Light Excitation of QDs

Figure 4-4 Calculated and simulated ambient light excitation spectra in quantum dot-
converted micro-LED displays based on Matlab and LightTools.

Figure 4-4 depicts the calculated and simulated ambient light excitation spectra and they

agree very well. These results show that the blue component of ambient light is nearly absorbed

57
and converted to red and green counterparts. Especially for the red component, the ambient

excitation even exceeds 100%.

Figure 4-5 Calculated ambient light excitation and reflection for (a) red, (b) green and (c)
blue subpixel in QD-converted full color micro-LED displays.

To further understand the ambient excitation of QDs, we plot the emitted light for RGB

subpixels in Fig. 4-5. The area ratio of each subpixel to a whole pixel is assumed to be 1/3. For the

red and green subpixels, the blue component of the ambient light is absorbed and converted by the

QDs through first-time and second-time excitations, and other light is reflected by the bottom

micro-LEDs. While for blue subpixel, only reflection needs to be considered because there is no

QD presented. The luminous ambient reflectance RL of the device is calculated by using Eq. (9).

The calculated and simulated luminous ambient reflectance for RGB subpixels are listed in Table

4-1. Due to the reflectance of micro-LED, the luminous ambient reflectance from blue subpixel is

dominant.

58
Table 4-1 Luminous ambient reflectance for RGB subpixels.

RGB
Pixels Red Green Blue
Cal. Sim.

RL 15.2% 17.0% 26.4% 58.6% 55.0%

4.4DBR for QD-Converted Micro-LED

Besides ambient excitation, another important parameter for QD-converted micro-LED is

the optical efficiency, which determines the color performance of the display. Currently, it is still

challenging to fabricate QD films with sufficient thickness and conversion efficiency by using

existing QD materials and fabrication methods including ink jet printing and photolithography. As

a result, the blue micro-LED emission cannot be completely absorbed by QD films and the color

gamut is degraded. To solve this issue, some prior arts have proposed to use distributed Bragg

reflector (DBR) to enhance the optical efficiency by recycling the blue light [79,101]. However,

very few investigations about the ambient light refection by the DBR have been reported.

Figure 4-6 illustrates the system configuration of QD-based micro-LED with DBR. As

discussed above, the DBR film reflects the blue light while transmits the green and red lights.

Therefore, the blue light can pass through QDs several times to be completely absorbed. We design

a wide viewing angle DBR structure with TiO2/SiO2 pairs by using commercial thin film coating

software TFCalc and its reflectance spectra of DBR structure is shown in Fig. 4-7. As Fig. 4-7

shows, the reflected/transmitted band does not shift even when the angle of incidence increases to

40°.

59
Figure 4-6 Device configuration of QD-converted micro-LED with DBR.

Figure 4-7 Reflectance spectra of the DBR film at different angles of incidence.

To investigate the ambient reflectance of micro-LED display with top DBR, similarly we

do the simulation with home-made MATLAB code and LightTools software. Results are plotted

in Fig. 4-8 In comparison with the structure without DBR (Fig. 4-4), the ambient reflectance of

from DBR increases sharply in the blue range (400~500 nm). Surprisingly, the ambient excitation

in the green and red range (500 ~ 700 nm) is still strong even though most of the blue light is

reflected by the DBR film. One reason is that the green and red QDs have a broad absorption

spectrum from 400 nm to 600 nm as shown in Fig. 4-3, which means they still can be excited to

60
emit green and red lights. Another reason is because although the reflection band of DBR structure

is optimized from 0°~40°, part of the blue light at angle of incidence larger than 40° can still pass

through the DBR film and be absorbed by the QD particles.

Figure 4-8 Calculated and simulated ambient light excitation spectra in QD-converted
micro-LED displays with a DBR film based on Matlab and LightTools.

Figure 4-9 shows the relative ambient reflected spectra for RGB subpixel. Compared to the

structure without DBR film, the first-time and second-time excitations are reduced while the

reflection increases greatly because of the DBR film, which is consistent with the above discussion.

Table 4-2 lists the luminous ambient reflectance for RGB subpixels. The results are very close to

those listed in Table 4-1, indicating that DBR film does not help to reduce the ambient reflection.

61
Figure 4-9 Calculated ambient light excitation and reflection for (a) red, (b) green and (c)
blue subpixel in QD-converted full color micro-LED displays with a top DBR.

Table 4-2 Luminous ambient reflectance for RGB subpixels with top DBR.

RGB
Pixels Red Green Blue
Cal. Sim.

RL 16.2% 15.6% 26.4% 58.2% 52.7%

4.5 Color Filter for QD-Converted Micro-LED

Figure 4-10 Schematic diagram for QD-converted micro-LED with a color filter array.

62
 To improve the display performance and reduce the ambient light excitation from QDs, a

layer of RGB color filter is added on top of the QD as shown in Fig. 4-10. The transmission spectra

of the color filter are shown in Fig. 4-11 [43]. Figure 4-12 shows the calculated and simulated

ambient light excitation spectra. Compared to the results without color filters, the ambient

excitation of QDs is greatly suppressed.

Figure 4-11 Transmittance of RGB color filter [43].

63
Figure 4-12 Calculated and simulated ambient light excitation spectra in QD-converted
micro-LED displays with a top pixelated color filter based on MATLAB and LightTools.

Figure 4-13(a) shows the calculated results for red subpixel, although the reflected red light

is still strong, the first-time and second-time excitations are greatly reduced. This is because with

the color filters, only the red component in the ambient light can transmit through and enter the

QD layer and it can hardly be absorbed according to the absorption spectra in Fig. 4-3. While for

green subpixel, because the green QDs can still absorb the green light, the first-time excitation still

exists. For the blue subpixel, the blue light is simply reflected by the micro-LEDs and other light

is absorbed by the color filter. Table 4-3 lists the luminous ambient reflectance for RGB subpixels.

Compared to the results listed in Table 4-1, the reflectance is reduced by ~87% with color filters.

Besides, the reflectance from green subpixel is dominant because of human eye sensitivity.

64
Figure 4-13 Calculated ambient light excitation and reflection for (a) red, (b) green and (c)
blue subpixel in QD-converted full color micro-LED displays with top color filter.

Table 4-3 Luminous ambient reflectance for RGB subpixels with top color filter.

RGB
Pixels Red Green Blue
Cal. Sim.

RL 2.8% 4.4% 0.52% 7.68% 5.8%

In the above ambient light excitation calculation, we assume the area ratio of each subpixel

to one pixel is 1/3, which is not practical for real display applications because the black matrix is

always needed to reduce the color crosstalk. Therefore, we need to investigate the luminous

ambient reflectance as a function of the QD area ratio, which is defined as the QD area to one

subpixel area. Besides, the surface reflectance of 1.5% with commercial antireflection coating is

applied. Results are plotted in Figure 4-14. As the QD area ratio increases, the luminous ambient

65
reflectance increases linearly. In addition, the luminous ambient reflectance is greatly suppressed

with the top color filter.

Figure 4-14 Calculated luminous ambient reflectance as a function of QD area ratio. The
surface reflectance is 1.5%.

To reduce the ambient reflectance, ambient contrast ratio (ACR) is calculated by using Eq.

(10). Figure 4-15 depicts the calculated results at different ambient conditions: office lighting (500

lux), overcast sky (5,000 lux) and full daylight (10,000 lux). The on (off) state luminace Lon (Loff)

are assumed to be 1000 nits (0 nits). The results show that the ACR decreases as the QD area

increases because of the increased ambient reflectance. Under the office lighting condition, the

ACR of the device without color filter is larger than 10:1 when QD area ratio is smaller than 0.3,

which means the display looks great as indicated in Table 3-3 [92]. As the QD area ratio increases

to 0.8, the device is still adequately readable (ACR > 5:1). While with the color filter, the ACR

becomes much larger than 10:1 even with QD area ratio equal to 1. Under a stronger ambient

66
lighting condition such as overcast sky as shown in Fig. 4-11(b), the display is barely readable

without color filters (ACR ≤ 5:1). But if the QD area is reduced to 0.4, with the help of color

filter, the ACR keeps larger than 5:1. However, under full daylight the QD area ratio needs to be

further reduced to 0.1 in order to make the display adequately readable. The trade-off is decreased

optical efficiency. To address this issue, the employed AR coating can be optimized to reduce the

surface reflectance to 0.2% so that the QD area ratio can be kept at 0.3 [91].

Figure 4-15 Calculated ambient contrast as a function of QD area ratio at ambient light of
(a) 500 lux, (b) 5,000 lux and (c) 10,000 lux. The surface reflectance is 1.5%.

4.6UV Micro-LED with RGB QDs

Figure 4-16 illustrate the layout of UV micro-LEDs with RGB QDs. Compared to blue

micro-LEDs with red and green QDs, this device has several advantages: 1) It can balance the

RGB colors by adjusting the amount of three QD primaries, leading to better color performance.

2) The viewing angle can be improved because QD particles enable isotropic emission. 3) The

angular color shift can be minimized due to matched angular distributions from RGB QDs. 4) The

67
DBR film optimized for recycling UV light does not need to be patterned, which is more feasible

in fabrication. However, the external quantum efficiency (EQE) of UV micro-LED is still much

lower than that of blue micro-LED at present.

Figure 4-16 Schematic diagram for configuration of UV micro-LED with RGB QDs.

To investigate the ACR of UV micro-LED with RGB QDs, we calculate the luminous

ambient reflectance (RL) and results are listed in Table 4-4. Similar to the results obtained from

above sections, the top color filter layer helps to reduce RL of the UV micro-LED device from

58.4% to 7.6%. Figure 4-17 compares the RL of QD-converted UV and blue micro-LED systems

with surface reflectance of 1.5%. As expected, these two devices have similar luminous reflectance

as the QD ratio changes.

Table 4-4 Luminous ambient reflectance for UV micro-LED with RGB QDs.

Device Red Green Blue RGB

W/O CF 15.2% 17.0% 26.2% 58.4%

With CF 2.8% 4.3% 0.4% 7.6%

68
Figure 4-17 Calculated luminous ambient reflectance of UV and blue micro-LED as a
function of QD area ratio. The surface reflectance is 1.5%.

4.7 Summary

In this chapter, we mainly discussed the ambient light excitation of QD-converted micro-

LED displays. Different from conventional color conversion application in LCD backlight in

which QD layer is underneath a stack of optical films, the QDs are on the top surface of micro-

LED, which will degrade the ambient contrast ratio of displays. To study this issue quantitatively,

we built a simulation model by dividing the ambient excitation into first-time excitation, second-

time excitation, and reflection. The luminous ambient reflectance was calculated to be as high as

58.6% for the 100% QD area ratio, indicating a poor ambient contrast ratio. The calculated results

agreed very well with the simulated ones based on commercial software LightTools. In addition,

a DBR film with wide viewing angle was optimized to enhance the QD conversion efficiency.

However, it did not help to improve the ambient contrast ratio. In order to address this issue, a top

layer of color filter was employed to absorb the ambient light, which reduced the luminous ambient

69
reflectance by ~8X. Another method was to reduce the QD area ratio to 0.1 of one subpixel, and

the ambient contrast ratio was improved to 5:1 under full daylight, which was adequately readable.

70
 CHAPTER 5 : CONCLUSIONS

In this dissertation, we mainly focus on the challenges of three approaches to form full-

color micro-LED displays: 1) individual RGB micro-LED, 2) white micro-LED and 3) quantum

dot-converted micro-LED.

For individual RGB micro-LEDs, in order to analyze its angular color shift, we built a

simulation model and validated it with experiment. Results show that the sidewall emission causes

mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based

blue/green counterparts due to material difference, which further leads to visually noticeable color

shift at larger viewing angle. In order to mitigate the color shift while keeping a high light

extraction efficiency, a device structure with top black matrix and taper angle in micro-LEDs was

proposed. After optimization, the color shift Δu′v′ of the RGB micro-LED display with 120° taper

angle is suppressed to below 0.01 within 80° viewing cone for the first 18 reference colors in

Macbeth ColorCheker, and the efficiency keeps ~85% of the device without black matrix. To

further improve the efficiency, we proposed microspheres with optimized size on both micro-LED

chip and packing resin. For green and blue device, their light extraction efficiencies are enhanced

by 54% and 35% respectively. Although only microsphere is discussed in this chapter,

microstructure with other shape such as cones or pyramid can be also applied.

For the white micro-LED, we proposed a funnel-tube array to eliminate the color crosstalk.

By depositing reflective or absorptive coating on the inner surface of the funnel-tube, a clear

boundary was formed between the subpixels to prevent light leakage, thus a near zero optical

crosstalk was achieved. To improve the light output efficiency, the taper angle of the funnel-tube

71
was also optimized. With reflective coating on the inner surface, the light efficiency of the device

was enhanced by ~3X as the taper angle [α, β] equals to [76°, 31°]. Compare with the absorptive

coating, the ambient contrast ratio with reflective coating is also improved due to higher light

intensity despite the increased ambient reflectance. In addition, by utilizing two-color phosphors,

which have high thermal stability and resistance to moisture, the color gamut of this device covers

92% of DCI-P3 standard.

For the color-converted micro-LED, we mainly discussed the ambient light excitation of

QD. Different from conventional color conversion application in LCD backlight in which QD layer

is underneath a stack of optical films, the QDs are on the top surface of micro-LED, which will

degrade the ambient contrast ratio of displays. To study this issue quantitatively, we built a

simulation model by dividing the ambient excitation into first-time excitation, second-time

excitation, and reflection. The luminous ambient reflectance was found to be as high as 58.6% for

the 100% QD area ratio, indicating a poor ambient contrast ratio. The calculated results agreed

very well with the simulated ones based on commercial software LightTools. In addition, a DBR

film with wide viewing angle was optimized to enhance the QD conversion efficiency. However,

it did not help to improve the ambient contrast ratio. To address this issue, a top layer of color filter

was employed to absorb the ambient light, which reduced the luminous ambient reflectance by

~8X. Another method was to reduce the QD area ratio to 0.1 of one subpixel, and the ambient

contrast ratio was improved to 5:1 under the full daylight, which was adequately readable.

72
APPENDIX: STUDENT PUBLICATIONS

73
 Journal publications

1. E. L. Hsiang, Z. He, Y. Huang, F. Gou, Y. F. Lan, and S.T. Wu, “Improving the power

efficiency of micro-LED displays with optimized LED chip sizes,” Crystals 10, 494 (2020).

2. L. Wu, F. Gou, S. T. Wu, and Y. Wang, “SLEEPIR: Synchronized Low-Energy

Electronically-Chopped PIR Sensor for True Presence Detection,” IEEE Sensors Lett. 4(3),

2500204 (2020).

3. Z. He, F. Gou, R. Chen, K. Yin, T. Zhan, and S. T. Wu, “Liquid crystal beam steering

devices: principles, recent advances, and future developments,” Crystals 9, 292 (2019).

4. F. Gou, E. L. Hsiang, G. Tan, P. T. Chou, Y. L. Li, Y. F. Lan, and S. T. Wu, “Angular

color shift of micro-LED displays,” Opt. Express (Accepted, 2019).

5. F. Gou, E. L. Hsiang, G. Tan, Y. F. Lan, C. Y. Tsai, and S. T. Wu, “High performance

color-converted micro-LED displays,” J. Soc. Inf. Display 27, 199-206 (2019).

6. Y. Huang, G. Tan, F. Gou, M. C. Li, S. L. Lee, and S. T. Wu, “Prospects and challenges

of mini-LED and micro-LED displays,” J. Soc. Inf. Display 27, 387-401 (2019).

7. T. Zhan, Y.H. Lee, G. Tan, J. Xiong, K. Yin, F. Gou, J. Zou, N. Zhang, D. Zhao, J. Yang,

S. Liu, and S. T. Wu, “Pancharatnam-Berry optical elements for head-up and near-eye

displays,” J. Opt. Soc. America B, 36(5), D52-D65 (2019).

8. F. Gou, E. L. Hsiang, G. Tan, Y. F. Lan, C. Y. Tsai, and Shin-Tson Wu, “Tripping the

optical efficiency of color-converted micro-LED display with funnel-tube array,” Crystals,

9(1), 39 (2019).

74
9. F. Gou, R. Chen, M. Hu, J. Li, J. Li, Z. An, and S. T. Wu, “Submillisecond-response

polymer network liquid crystals for mid-infrared applications,” Opt. Express 26(23),

29735-29743 (2018).

10. F. Gou, H. Chen, M.C. Li, S.L. Lee, and S. T. Wu, “Motion-blur-free LCD for high

resolution virtual reality displays,” J. Soc. Inf. Display 26(4), 223-228 (2018).

11. G. G. Liu, K. Wang, Y. H. Lee, D. Wang, P. P. Li, F. Gou, Y. Li, C. Tu, S. T. Wu, and H.

T. Wang, “Measurement of the topological charge and index of vortex vector optical fields

with a space-variant half-wave plate,” Opt. Lett. 43(4), 823-826 (2018).

12. Y. H. Lee, D. Franklin, F. Gou, G. Liu, F. Peng, D. Chanda, and S. T. Wu, “Two-photon

polymerization enabled multi-layer liquid crystal phase modulator,” Sci. Rep. 7, 16260

(2017).

13. F. Gou, F. Peng, Q. Ru, Y. H. Lee, H. Chen, Z. He, T. Zhan, K. L. Vodopyanov, and S. T.

Wu, “Mid-wave infrared beam steering based on high-efficiency liquid crystal diffractive

waveplates,” Opt. Express 25(19), 22404-22410 (2017).

14. G. Tan, Y. H. Lee, F. Gou, H. Chen, Y. Huang, Y.F. Lan, C.Y. Tsai, and S. T. Wu, “Review

on Polymer-Stabilized Short-Pitch Cholesteric Liquid Crystal Displays,” J. Phys. D: Appl.

Phys. 50, 493001 (2017).

15. Z. He, Y. H. Lee, F. Gou, D. Franklin, D. Chanda, and S. T. Wu, “Polarization-independent

phase modulators enabled by two-photon polymerization,” Opt. Express 25(26), 33688-

33694 (2017).

16. Y. H. Lee, G. Tan, T. Zhan, Y. Weng, G. Liu, F. Gou, F. Peng, N.V. Tabiryan, S. Gauza,

and S. T. Wu, “Recent progress in Pancharatnam-Berry phase optical elements and the

75
 applications for virtual/augmented realities,” Optical Data Processing and Storage 3(1), 79-

88 (2017).

17. G. Tan, Y. H. Lee, F. Gou, M. Hu, Y. F. Lan, C. Y. Tsai, and S. T. Wu, “Macroscopic

model for analyzing the electro-optics of uniform lying helix cholesteric liquid crystals,”

J. Appl. Phys. 121(17), 173102 (2017).

18. F. Gou, H. Chen, M. C. Li, S. L. Lee, and S. T. Wu, “Submillisecond-response liquid

crystal for high-resolution virtual reality displays,” Opt. Express 25(7), 7984-7997 (2017).

19. F. Peng, H. Chen, F. Gou, Y. H. Lee, M. Wand, M.C. Li, S.L. Lee, and S. T. Wu,

“Analytical equation for the motion picture response time of display devices,” J. Appl.

Phys. 121(2), 023108 (2017).

20. H. Chen, F. Gou, and S. T. Wu, “A submillisecond-response nematic liquid crystal for

augmented reality displays,” Opt. Mater. Express 7(1), 195-201 (2017).

21. H. Chen, F. Peng, F. Gou, Y. H. Lee, M. Wand, and S. T. Wu, “Nematic LCD with motion

picture response time comparable to organic LEDs,” Optica 3(9), 1033-1034 (2016).

22. Y. H. Lee, F. Gou, F. Peng, and S.T. Wu, “Hysteresis-free and submillisecond-response

polymer network liquid crystal,” Opt. Express 24(13) 14793-14800 (2016).

23. F. Peng, F. Gou, H. Chen, Y. Huang, and S. T. Wu, “A submillisecond-response liquid

crystal for color sequential projection displays,” J. SID 24(4), 241-245 (2016).

24. F. Peng, Y. Huang, F. Gou, M. Hu, J. Li, Z. An, and S. T. Wu, “High performance liquid

crystals for vehicle displays,” Opt. Mater. Express 6(3), 717-726 (2016).

76
 Conference proceedings

1. F. Gou, E. L. Hsiang, G. Tan, P. T. Chou, Y. L. Li, Y. F. Lan, and S. T. Wu, “High-

efficiency micro-LED displays with indistinguishable color shift,” Proc. SPIE 11304,

113040I (February 2020, San Francisco, California).

2. F. Gou, E. L. Hsiang, G. Tan, Y. F. Lan, C. Y. Tsai, and S. T. Wu, “High performance

color-converted micro-LED displays,” SID Symp. Digest 50(1), 22-25 (May 2019, San

Jose, California).

3. F. Gou, H. Chen, M.C. Li, S. L. Lee, and S.T. Wu, “Blur-free LCD for high resolution

virtual reality displays,” SID Symp. Digest 49(1), 577-580 (May 2018, Los Angeles,

California).

4. F. Gou, Y. H. Lee, G. Tan, M. Hu, Y. F. Lan, C. Y. Tsai, and S. T. Wu, “Submillisecond

grayscale response time of a uniform lying helix liquid crystal,” SID Symp. Digest 48(1),

1822-1825 (May 2017, Los Angeles, California).

5. G. Tan, Y. H. Lee, F. Gou, M. Hu, Y.-F. Lan, C.-Y. Tsai, and S. T. Wu, “Figure of Merit

for Optimizing the Performance of Uniform Lying Helix Cholesteric Liquid

Crystals,” SID Symp. Digest 48(1), 490-493 (May 2017, Los Angeles, California).

6. F. Peng, H. Chen, F. Gou, Y. H. Lee, S. T. Wu, M. Wand, M. C. Li, and S. L. Lee, “A LCD

with submillisecond motion picture response time,” SID Symp. Digest 48(1), 1826-1829

(May 2017, Los Angeles, California).

7. H. Chen, F. Gou, and S. T. Wu, “Submillisecond-response nematic LC for wearable

displays,” SID Symp. Digest 48(1), 377-380 (May 2017, Los Angeles, California).

77
8. H. Chen, F. Peng, F. Gou, M. Wand, and S. T. Wu, “Fast-response LCDs for virtual reality

applications,” Proc. SPIE 10125, 101251E (February 2017).

9. F. Peng, H. Chen, F. Gou, Y. Huang, and S. T. Wu, “A submillisecond-response liquid

crystal for color-sequential projection displays,” SID Symp. Digest 47(1), 1025-1028 (May

2016, San Francisco, California).

10. F. Peng, Y. Huang, F. Gou, S. T. Wu, M. Hu, J. Li, and Z. An, “High-performance liquid

crystals for vehicular displays,” SID Symp. Digest 47(1) 754-756 (May 2016, San

Francisco, California).

Patents

1. Y. H. Lee, T. Zhan, G. Tan, F. Gou, F. Peng, S. T. Wu, “Optical Display System with

Enhanced Resolution, Methods, and Applications,” US Patent 10,115,327 B1 (Oct. 30,

2018).

78
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