Commercialization of Quantum Dot White Light Emitting Diode technology

Commercialization of Quantum Dot White Light Emitting Diode Technology by

Xinyue Zhao

B. E., Electrical Engineering (2005) National University of Singapore

Submitted to the Department of Materials Science and Engineering in Partial Fulfillment of the Requirements for the Degree of Master of Engineering in Materials Science and Engineering

at the

Massachusetts Institute of Technology MASSACHUSETTS INSI1TUTE OF TECHNOLOGY

September 2006

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@2006 Massachusetts Institute of Technology

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Signature of Author:

Department of Materials Science and Engineering

August 8, 2006

Certified by:

tugene A/itzg rald Merton C. Flemings-SMA Professor of Materials Science and Engineering

Thesis Supervisor

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Samuel M. Allen POSCO Professor of Physical Metallurgy Chair, Departmental Committee for Graduate Students

Commercialization of Quantum Dot White Light Emitting Diode Technology by

Xinyue Zhao

Submitted to the Department of Materials Science and Engineering on August 18, 2006 in Partial Fulfillment of the

Requirements for the Degree of Master of Engineering in Materials Science and Engineering

Abstract

It is well known that the use of high-brightness LEDs for illumination has the potential to substitute conventional lighting and revolutionize the lighting industry over the next 10 to 20 years. However, successful penetration of this extremely large lighting market would require vast improvements in power conversion efficiencies, color index, light output per device and drastic reduction in cost. Quantum Dot white LED (QD WLED) technology may be one of the best choices, due to its higher energy efficiency, larger color render in index, better versatility and more importantly lower cost, compared to conventional blue LED plus YAG: Ce yellow phosphor technology. Due to the fundamental difference of the material structure, QD LEDs will win a steady position among existing white LED patents and a hybrid fabless plus IP business model has the best position to promote this technology to maximize its benefits and potential for the entire LED industry.

Thesis Supervisor: Eugene A. Fitzgerald

Acknowledgements

I am highly indebted to my thesis advisor, Prof. Eugene A. Fitzgerald from MIT and Prof. Soon Jin Chua from NUS. They are both highly accomplished scientists and their help was invaluable for completing this thesis. Prof. Fitzgerald is not only a great researcher but also an enthusiastic entrepreneur. From the interactions that I had with him, I gradually accumulated knowledge on the process of starting up a new high tech business.

Prof Chua has been my advisor for Undergraduate Research and Opportunity Program, my Final Year Project and now my M.Eng thesis from NUS side. I have been his student for the past four years. He has provided me with so much support that I cannot thank him in any form. I only can work harder to continue his passion for research and Singapore industry development.

Secondly, I would like to thank Dr. Aaron Danner and Mr. Huang En Li from Avago. They gave me practical suggestions from an industrial point of view on my thesis topic. I also would like to thank Mr. Hsueh-Shih Chen from Industrial Technology Research Institute Taiwan. He answered my emails and gave me very helpful suggestions on practical issues about QD white LED, despite the fact that we hardly know each other.

I would like to thank my parents for all their unflinching support they have given and continue to give wherever I am. My father has taught me strict self-discipline and strong will to progress in life. My mother has given me strength and capability to face difficulties. With all the character they have imbibed in me, I was able to first complete my undergraduate study abroad in Singapore, then the SMA courses and now, my master's thesis.

I also want to dedicate my great thanks to my SMA AMM&NS 05/06 classmate Rangarajan Vijayaraghavan, who has given me so much help for my thesis. He has induced my active and innovative thinking by giving me his brilliant ideas on both science and business topic. He is my greatest friend in the past year and I believe he will be continuing to be a great friend through out my life.

I want to thank Ms. Jocelyn Sales and Dr. John Desforge from MIT SMA office. They helped me very much in contacting my advisor from MIT side, which was essential for the smooth progress of my thesis.

Finally I want to thank my fellow classmates SMA AMM&NS 05/06. Without their company and encouragement, I would have been subjected to tremendous stress due to the tough coursework. However, I enjoyed my life and study for the past year because of their existence.

I plan to enter the industry as an accomplished engineer having learnt so much from eminent professors and in the coming years put to good use the ideas and concepts they have taught me. Someday I hope to bring about a major contribution to the scientific community that improves the common man's everyday life. That will be the day when I would have left a lasting impression in the sands of time.

Table of content

C hapter 1 Introduction ... 1

1.1 Development of LED technology... 1

1.2 Advantages of LED lighting... 2

1.3 Motivation for development of new white LED technology and business ... 3

Chapter 2 Quantum dot white LED technology ... ... ... 6

2.1. LED basic structure and manufacturing methods... 6

2.2. Conventional m ethods ... 7

2.3. Quantum Dot white LED ... ... 9

2.3.1. Quantum Dot Physics... 9

2.3.2. Quantum Dot phosphor... 10

2.3.3. Fabrication for quantum dot phosphor... ... 11

2.4. Advantages and limitations of quantum dot LEDs... ... 12

2.5. Quantum dot in pnjunction... 16

2.5.1. Theory of white LEDs with QD in pn junction... ... 17

2.5.2. Fabrication of quantum dot in pn junction... ... 17

2.5.3. Advantages and limitations ... 18

Chapter 3 White LED market analysis ... 22

3.1 HB LED market and growth trend ... ... ... 22

3.2 W hite LED in illum ination ... ... 23

3.3 White LED as automobile headlamp... 23

3.4 White LED for medical applications... 25

3.5 White LED market estimation ... 25

Chapter 4 Existing players and intellectual properties ... ... 27

4.1 LED production regions ... ... 27

4.2 M ajor players... 28

4.3 Existing white LEDs patents ... 29

4.4. Q uantum dot LED patents ... 31

4.5 Evident technologies ... ... 33

Chapter 5 Business M odel ... ... 35

5.1. The stage of technology development... ... 35

5.2 D ifferential cost m odeling ... ... 37

5.3. The three basic business models in semiconductor industry... 39

5.3.1. The business structure for semiconductor industry... ... 39

5.3.2. The three business models ... ... ... 40

5.3.3. Business model choice: Hybrid packaging design model and IP model ... 42

5.4 B usiness strategies... 44

5.5. Financial m odel ... ... 47

C hapter 6 C onclusion ... ... 51

Appendix 1: Relevant term inology... ... 52

List of figures

Figure 1-1 Trend of increasing light output per package of LEDs and decreasing of cost... 2

Figure 1-2 Energy consumption in US ... ... ... 4

Figure 2-1 Schematic of the LED and GaN based LED die structure ... 6

Figure 2-2 General types of White-Light LED Devices ... 7

Figure 2-3 Structure of white LED and phosphor conversion ... 8

Figure 2-4 Spectrum of white LED ... ... ... 9

Figure 2-5 Illustration of a buried InAs quantum dot embedded into a GaAs barrier materiall0 Figure 2-6 Quantum Dot phosphor with different positions... ... 12

Figure 2-7 Performance of the QD white LEDs ... 12

Figure 2-8 Clear differences of the color rendition in the August Renoir painting ... 14

Figure 2-9 Light traveling pass in LED ... ... 15

Figure 2-10 Schematic diagram for pn junction with quantum dot in between... 17

Figure 3-1 The key applications for HB-LEDs... 22

Figure 3-2 The light output of an LED Headlamp prototype ... 25

Figure 4-1 Deals and disputes in the white LED industry: the key intellectual property relationships as of September 2005 ... 31

Figure 4-2 Two embodiments of an LED according to the invention ... 32

Figure 5-1 Technology development and confidence in chosen market ... 35

Figure 5-2 Semiconductor industry production flow... ... 40

Figure 5-3 Basic business models for semiconductor industry ... 41

List of tables

Table 4-1 Key US patents covering white LEDs... .... ... 30

Table 4-2 M ajor phsphors patented ... 30

Table 5-1 Price comparison of CdSe and YAG precursors ... 37

Chapter 1 Introduction

Until the last decade, LEDs could only produce green, red, and yellow light, which limited their use in only signs, signals and indicators. Then came blue LEDs, which have since been altered to emit white light, which makes the dream of LEDs replacing conventional incandescent and fluorescent lighting within approach. However, there are still many problems with the current technology, which need novel ideas to solve and hence the present white LED performance can be largely improved to the extent that LEDs can be made comparable or even better the current lighting. Quantum dot white LED is one of them and it has the promising potential to pull the dream nearer.

1.1 Development of LED technology

Since the development of the first commercial visible light emitting diodes (LEDs) in 1968, LED technology has under gone a series of both evolutionary and revolutionary changes. For the first 25 years of their history, the materials available for LED fabrication (primarily gallium phosphide (GaP) and gallium arsenide phosphide (GaAsP) were low in efficiency and allowed LEDs to be used primarily as low brightness indicator lamps and alphanumeric displays. Moreover, their spectral range was limited to yellow-green, orange, and red.

In the early to mid-1990s a new generation of LED materials was developed that enabled the fabrication of high-brightness devices across the entire visible spectrum, opening up large new markets that were not addressable by previous material technologies. These materials, indium gallium aluminium phosphide (InGaAlP) and indium gallium nitride (InGaN), have formed the foundation for the large (1.8 billion in 2002) high brightness LED industry that has evolved since 1995.

Presently, LEDs are still in the stage of further improvement in their luminous efficiency and reduction of cost. Figure 1-1, shows how the light output of LEDs has increased 20

fold each decade for the last 40 years, while the cost ($/lumen) has decreased ten fold each decade over that same time period. Figure 1-1 also shows predictions for price and light output over the next two decades. Besides all these, white LEDs attract great interest of researchers and LED manufacturers. It has become more and more important, and has started to occupy larger portion in HB LED applications and may become the main stream in future LED applications. White LEDs are one important research focus in current stage of LED technology. Its performance will become more mature through successive improvement of technology [1].

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Figure 1-1 Trend of increasing light output per package of LEDs and decreasing of cost [1]

Source: Roland Haitz and Lumileds. Note: CAGR = compound annual growth rate. Both lines are on the same numerical scale (however, different units)

1.2 Advantages of LED lighting

The reason for LED technology to keep on evolving is the numerous benefits due to their mode of operation [2]:

Energy Efficiency: LEDs don't emit heat, so they're much more energy efficient. LEDs produce twice as much light as a regular 60 watt bulb and burn for over 50,000 hours.

Long Life: Some LEDs are projected to produce a long service life of about 100,000 hours. For this reason LEDs are ideal for hard-to-reach/maintain fixtures. Much longer life reduces maintenance requirements.

Durable: LEDs are highly rugged. They feature no filament that can be damaged due to shock and vibrations. They are subject to heat, however, and being overdriven by the power supply.

Small Size/Design Flexibility: A single LED is very small and produces little light overall. However, this weakness is actually its strength. LEDs can be combined in any shape to produce desired lumen packages as the design goals and economics permit. In addition, LEDs can be considered miniature light fixtures; distribution of light can be controlled by the LEDs' epoxy lens, simplifying the construction of architectural fixtures designed to utilize LEDs.

Other Benefits, such as lights instantly, can be easily dimmed, silent operation, low-voltage power supply (increased safety).

1.3 Motivation for development of new white LED technology and business

All these advantages make great impact on our life. First, and the most important, is the impact on energy consumption. In the U.S., about one-third of all primary energy is used to produce electricity, and of this electricity about one-fifth is used to produce light. If the displacement of traditional lighting by solid-state lighting can be accelerated by even one year, U.S. consumers would save roughly $35 billion. The use of high-brightness LEDs for illumination has the potential to revolutionize the lighting industry over the next 10 to 20 years [3].

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Figure 1-2 Energy consumption in US [31

The second benefit is the indirect impact of this reduced electricity consumption on the environment. With the current electricity-generation-technology mix in the U.S. approximately 1 Mton of carbon equivalent emission is produced for every 6 TW-h of electricity consumed [3].

The third benefit is the impact on the overall human visual experience of new features associated with solid-state lighting. The compactness, shock-resistance, and low-voltage operation will enable integration with a wide range of architectural and human environments. And the point source nature of SSL will enable the light to be directed and distributed in efficient, innovative ways.

All the benefits from LEDs' superb characteristics are the large driving forces. Successful penetration of this extremely large market would require vast improvements in power conversion efficiencies, color index, and light output per device. It also requires drastic reduction in cost, measured in dollars per thousand lumens.

Reference

[1]. "Solid-State Lighting Research and Development Protfolio", Multi-Year Program Plan FY'07-FY' 12 prepared for: Lighting Research and Development Building Technologies Program Office of Evergy Efficiency and Renewable Energy U.S. Department of Energy

[2]. http://lightingdesignlab.com/articles/LED_fund/led advant.htm

[3]. Jeffrey V.Tsao, "Roadmap projects significant LED penetration of lighting market by 2010", Laser Focus World, May 2003

Chapter 2 Quantum dot white LED technology

2.1. LED basic structure and manufacturing methods

Commercial LED normally includes of three parts: the semiconductor die itself, the mounting substrate and the encapsulant. An LED die (chip) normally is fabricated using LPE, VPE or MOCVD. After leads are put on, the chip is encapsulated. Figure 2-1(a) shows the schematic structure of a commercial LED. Figure 2-1(b) shows an InGaN die structure as an example.

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Figure 2-1 Schematic of the LED and GaN based LED die structure [1,2]

Different colors of LEDs have been achieved from infrared to ultraviolet mainly based on different materials used for the die, as we know the color emitted is directly related to the bandgap of the chip material used. Typically, AlGaAs is for red, AlGaInP is for orange-yellow-green, and AlGaInN is for green-blue. The fact that different material is used for producing different color, may lead to a problem: How to produce white light, a combination of red, blue and green, with a single chip? Since white light has many important applications, such as general illumination, surgical operation and headlights for automobiles and trucks, a lot of effort has been put in to study producing white LED with the most efficient design. In this chapter, the conventional methods and novel methods of

mixing colors to get white light are discussed.

2.2. Conventional methods

There are two common approaches for producing white-light LED: (a) discrete color-mixing (b) phosphor-conversion LEDs (pc-LEDs) [3]. The schematic diagram of these two methods is shown below in Fig. 2-2:

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(a) Phosphor-Conversion LED (b) Color-Mixing Figure 2-2 General types of White-Light LED Devices

Color mixing approach starts with discrete colored sources and uses color mixing optics to blend together the light output from these sources to create white-light emission. The lamp contains a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency. This method also gives good quality white light with high color rendering index (CRI). The spectrum is shown in Fig 2-4 (a). Nichia was the first manufacturer to use this method to produce white-light LED devices on a commercial scale in 1997. It has since been adopted by numerous other manufacturers as the method for white-light LEDs used in display and conspicuity applications [3].

The drawback of this approach is the increased complexity for blending the discrete colors. It would require multi-chip mounting and potentially sophisticated optics for blending the discrete colors. It may also require color control feedback circuitry that could address the different degradation and thermal characteristics of the discrete LED chips. Furthermore, this method is more costly than wavelength conversion method. Due to all these disadvantages, this method is not commonly adopted today. The dominant method to produce white LEDs is pc-LEDs.

Phosphor conversion involves converting some or all of the LED's output into visible wavelengths. From a research perspective, pc-LEDs are often subdivided into two groups: one based on blue LEDs and one on UV LEDs.

The blue LED approach creates white-light by blending a portion of the blue light emitted directly from the chip with light emission down-converted by a phosphor, which is normally a yellow-emitting yttrium aluminum garnet (YAG). The configuration is demonstrated in Figure 2-3 below. Blue LED and yellow phosphor is considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light.

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Figure 2-3 Structure of white LED and phosphor conversion

a)Structure of white LED consisting of a GaInN blue LED chipencapsulated in a phosphor-containing epoxy b)Conversion of blue light to yellow due to phosphorescence [4]

As shown in Fig.2-4 (b), the main problem with this method is the low color rendering index (definition see appendix 1) which is approximately 60 -70 [3], since there is only yellow and blue spectrum without red. To improve color rendering index of white light produced using this phosphor wavelength conversion method, people have been trying to develop new red emission phosphors. The resulting light has a richer and broader wavelength spectrum and produces a higher color-quality light, but at an increased cost. One example of a red phosphor is M2SisN8:Eu2+ (M

Ca, Sr, Ba) [5].

Another example for the red light emitting phosphor used in white LEDs commercially is the divalent Eu ion activated alkaline earth binary sulfides. The conventional alkaline earth sulfides are known to be excellent and versatile phosphor

materials. As members of alkaline earth sulfide families, especially Eu2_{+ doped SrS}

and CaS were considered to be the most promising candidates for red phosphor. However, the binary sulfide-based phosphors have low chemical stabilities and low luminescence [6].

The UV LED approach starts with a UV-emitting LED chip that energizes phosphors designed to emit light in the visible spectrum. All the UV energy is adsorbed and converted into the visible spectrum by the phosphors. A pc-LED using a UV LED chip is similar to the blue LED system, but has some important differences. In this type of pc-LED, the LED radiates energy in the UV (340-380nm) or near-UV (<430nm) that excites phosphors, which down-convert the UV radiation into the visible wavelengths. The discrete emissions from the phosphors combine to produce white light [3].

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(a). Color mixing (b). Wavelength conversion (blue LED and yellow phosphor) [71

2.3. Quantum Dot white LED

2.3.1. Quantum Dot Physics

Quantum dots are nanocrystals with size smaller than 10nm. Each dot contains only 33 or 34 pairs of atoms or from 100 to 1,000 electrons. In a quantum dot (QD), electrons and holes are confined in all three dimensions [8]. They have very different behaviors compared to electrons and holes in bulk semiconductor due to the confinement. They have discretized quantum dot bands and therefore semiconductor quantum dots, e.g., InAs dots embedded in GaAs, behave like non- or weakly

interacting single atoms as shown in Figure 2-5.

Furthermore, the energy separation between the valence and conduction bands (the bandgap) can be altered by changing the QD size. The smaller it is, the larger bandgap it has and hence the more excited it behaves. This is because quantum confinement of both the electron and hole in all three directions leads to an increase in the effective bandgap of the material with decreasing crystallite size [9]. The light emitted by different QD has different wavelength, hence different color corresponding to different QD size. This special optical property makes QD WLED possible.

Figure 2-5 Illustration of a buried InAs quantum dot embedded into a GaAs barrier material (left) and the corresponding schematic quantum mechanical representation of a three-dimensionally confined box structure with conduction (EC) and valence (EV) band barriers and

discretized electron and hole levels (middle). The densities of state functions are 6-like at the transition energies EO and El (right) [8].

2.3.2. Quantum Dot phosphor

Currently, powder phosphors consisting of micron-size particles, hereinafter called "bulk phosphor materials", are used widely in cathode-ray TV tubes, plasma displays panels (PDPs), fluorescent lamps and white LEDs [10]. Quantum dots, which are in the nanoscale, represent a new approach to obtain white light from LEDs. Quantum dot phosphors are integrated with a commercial LED chip that emits in blue or near ultraviolet at 400 nanometers by encapsulating the chip with a dot-filled epoxy, creating a dome. The quantum dots in the dome absorb the invisible 400 nanometer light from the LED and re-emit it in the visible region - a principle similar to that used in fluorescent lighting.

LED light still impinges on phosphor coating composed of quantum dots, but instead of encountering traditional semiconductor energy bands, the LED photons would encounter the discretized energy bands specific to quantum dots. The discretized nature of quantum dot bands means that the energy separation between the valence and conduction bands (the bandgap) can be altered with the addition or the subtraction of just one atom, making for a size dependent bandgap. Predetermining the size of the quantum dots would fix the emitted photon wavelength at the appropriate customer-specified color, even if it is not naturally occurring, an ability limited only to dots.

For nanophosphors, white light is generated by intermixing red, green, and blue emitting dots homogenously within the phosphor. Thus, quantum dots need only a single excitation source for multiple emission colors, even to the point of producing industry quality white light.

2.3.3. Fabrication of quantum dot phosphor

An example of white LED with quantum dot phosphor was demonstrated by Hsueh-Shih Chen's group [11]. White light-emitting diodes (WLEDs) were fabricated by combining blue InGaN chips with luminescent colloidal core-shell CdSe-ZnSe QDs.

The CdSe QDs were synthesized in supersaturated solution. Experimentally, CdO, TOPO, and HPA/TDPA were loaded in a three-neck flask. At about 3000_{C, reddish}

CdO powder was dissolved and generated a colorless homogeneous solution. Introducing tellurium, selenium, and sulfur stock solutions yield high quality nanocrystals. TEM measurements indicate narrow distribution of these QD size and X-ray powder diffraction shows high crystallinity of these wurtzite nanocrystals. The dot size can be easily controlled by the reaction time [17, 12].

After synthesis, the QD need to be dispersed into some binder and coated on LED chip. Figure 2-6 shows the QD phosphor after dispersion and coating on chip with different phosphor positions relative to the chip. Different dispersion and positioning of the phosphor can greatly affect the efficiency of the LED [13].

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Figure 2-6 Quantum Dot phosphor with different positions [14]

Fig 2-7 (a) shows the spectrum with different wavelength corresponding to different dot size. Color of light emitted can be varied from green to red. And to obtain white light with good CRI, a three-band RGB WLED, containing a blue SMD InGaN chip, green emitting QDs, and red-emitting QDs was fabricated. Fig 2-7 (b) shows this WLED exhibited white light and had an efficiency of 7.21m/W at 20mA. The CRI is 91, which is much higher than the conventional YAG based LEDs [15].

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(a) (b)

Figure 2-7 Performance of the QD white LEDs

(a). PL spectra of the CdSe-ZnSe QDs with various sizes (from left to right, the particle sizes are 2.2, 2.7, 3.2, 3.4, 3.7, 3.8, 4.0, and 4.8 nm). (b). Three-band RGB WLED combined a blue InGaN

chip, green- and performance of the quantum dot LEDs red-emitting CdSe-ZnSe QDs.

Additionally, the fluorescence efficiency and, in particular, the stability of the nanocrystals can be greatly improved by modifying the particle surface. Sandia National Laboratories have also developed solid-state white light-emitting device using quantum dots in year 2003 [16]. The approach is based on encapsulating semiconductor quantum dots and engineering their surfaces so they efficiently emit visible light when excited by near-ultraviolet (UV) light-emitting diodes (LEDs). The quantum dots strongly absorb light in the near UV range and re-emit visible light that has its color determined by both their size and surface chemistry [16].

2.4. Advantages and limitations of quantum dot LEDs I

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QD LEDs have many advantages over LEDs with conventional phosphor. Theoretically they have better CRI at cheaper cost, better luminous efficacy, more flexible and stable. The utilization of quantum dot to produce white light may overcome some problems with current white LED technology [17].

Advantage 1:: High color rendering index and easy color tuning

For nanophosphors, while the optical properties of conventional bulk phosphor powders are determined solely by the phosphor's chemical composition, in quantum dots the optical properties such as light absorbance are determined by the size of the dot. Changing the size produces dramatic changes in color. This also can be applied to QD in pn junctions. QD size will always affect the color of the light emitting. Thus, it provides a convenient way in tuning the emission light color.

More importantly, since different size QDs emit light with different wavelength, when all these lights are mixed together, it gives high CRI. The CRI can be as high as 91 simply achieved by using phosphor with different QD size. And a typical CRI value for currently commercialized white LED is below 70 (appendix 3). Higher CRI with conventional phosphor will have higher cost.

High CRI gives much better visual impact, as shown in Figure 2-8 below. The flowers under light with higher CRI look vivid and colorful, while the flower under the light with lower CRI look dull. High CRI has great commercial benefits, which cannot be easily quantitatively measured.

Figure 2-8 Clear differences of the color rendition in the August Renoir painting (left hand side: high CRI; right hand side low CRI) [7]

Advantage 2: High energy efficiency

The energy efficiency of presently available commercial white LED products is in the range of 40 lumens per watt. The target for solid-state light sources is to reach 150

lumens per watt by 2012. For white LEDs to reach this target, improvements are needed in several areas, including the internal quantum efficiency [18], the light extraction efficiency, and the phosphor efficiency [19]. There are several factors, which affect the phosphor efficiency, the physical shape of the phosphor, position of phosphor, the refractive index mismatch and the photoluminescence efficiency.

There are energy losses when the light is down converted to longer wavelength light. This loss is unavoidable, but there are some other losses that can be reduced or even avoided by substituting conventional phosphor with QD phosphor.

a). The optical property of such QD system is closely related to the nature of excitons, which are the electron-hole pairs that can be created by the absorption of photons. The recombination is much easier to happen for QD than in bulk material. Hence, wavelength conversion loss can be greatly reduced and luminous efficacy can be further improved. According to literature, the quantum efficiency for matrix free QD is between 1% to 10%. We take the average 5% in the following calculation [20].

The typical output of YAG is 8 photons/keV[21]. Assuming the wavelength of incoming photons is 450nm, which is in the blue range, the photon energy is 1.24/0.45= 2.76 eV. The total number of incoming photons will be 362. The rough quantum efficiency is 8/362=2.2%. So we can see that CdSe QD has larger quantum efficiency.

b). For nanophosphors, since QD has a much smaller size than the wavelength of visible light, it eliminates all light scattering and the associated optical losses. Optical backscattering losses using larger conventional phosphors reduce the package efficiency by as much as 50 percent [22, 23]. A method to cut down the loss due to

backward scattering of light would be in bringing particle size down to the nano level where light scattering becomes faint [10]. Physically, the rough surface of QD phosphor reduces backscattering. Nano-sizing not only reduces light scattering but also improves the relative surface area of the material.

c). Total internal reflections (TIR) and Fresnel reflections (FR) occur at both LED/Epoxy and LED/YAG interface due to index mismatch. TIR and FR losses reduce extraction/out-coupling efficiency of blue LED light [24]. One way to reduce the losses is to make the refractive index mismatch smaller. Knowing that the refractive index for InGaN, YAG and CdSe are 2.6, 1.8 and 2.5 respectively [25], we can see that the index of InGaN and CdSe are much closer than InGaN and YAG. Therefore, the losses can be reduced by substituting YAG with CdSe.

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Figure 2-9 Light traveling pass in LED [24]

2 reflectivity = n, +-n2

Reflectivity of InGaN and QD CdSe system R, = 3.84 x 10-4

Reflectivity of InGaN and YAG system R2 = 3.3 x 10-2

Since R1 is really small, we assume there is no reflection due to index mismatch of

InGaN and QD CdSe nanophosphor. For the case of YAG, there is 3.3% reflection.

The above analysis is only true when the distance between the dots is much less than the light wavelength. In this case, the light sees an effective wavelength that is mostly dot index plus a little matrix index. But if the interdot spacing is much greater than the wavelength, the above argument does not hold, as there are 2 interfaces reflecting

-the semiconductor/matrix when it leaves -the LED, and -the semiconductor/matrix 15

when it enters the dot. In this case, the index of the matrix comes into the picture. The index mismatch of matrix/LED, matrix/CdSe becomes important. Another effect that can influence the light path is that if the dot spacing is on the order of the wavelength of light, a lot of scattering will occur. The scattering may increase the chances of light absorbance, or it may also scatter the light away and reduce the absorbance. Therefore, dispersion configuration in the matrix and the matrix material has great influence on the energy efficiency. A good configuration and suitable material has the potential to reduce reflection and increase energy efficiency [26].

Advantage 3: Low cost

QD phosphor has lower cost mainly due to cheaper precursor and an easier process. The details on the estimates of cost will be discussed in chapter 5.

Advantage 4: Better versatility

For nanophosphors, the extremely small size and versatility of form for quantum dots would allow them to be inserted into any medium necessary to accommodate any underlying light emitting source. For QDs inserted in pn junction, their extremely small size and versatility of form allows them to be inserted into any medium necessary - paint, water, plastics and more [17].

Limitation & obstacles

According to Appendix 2, commercialized white LED has a luminous efficacy 20 Im/W. The current QD LED only has a luminous efficiency around 7 Im/W. This may be due to poor dispersion and the material that is used for the binder. The QDs photoluminescence is strongly affected by its binders, which also influence QDs stability and dispersion. A good binder in which QDs disperse well may be found but the QD emission intensity reduces. After dispersing, QDs need to be coated onto the LED chip. Poor coating technique may be another reason for low luminescence. Another main problem with this technology is thermal stability especially when QDs loaded on high-power GaN chips [28].

2.5.1. Theory of white LEDs with QD in pn junction

Current LED technology produces electroluminescent, non-tunable light emission through the use of a p-n semiconductor junction. As an electrical current is driven through the junction, electrons are excited across the bandgap into the conduction band. From there, the electrons diffuse away from the junction, and proceed to decay back across the bandgap to the valence band, accompanied by the emission of light with a wavelength corresponding to the energy of the bandgap.

Quantum dot material was used as LED phosphor in the earlier part of this chapter. In fact, there is another kind of QD LEDs, where QDs are used in between pn junction as shown in Figure 2-10. Electrical current would still be driven through the quantum dot network, but instead of encountering traditional semiconductor energy bands, the current would encounter the discretized energy bands specific to quantum dots. Recombination will also occur in QDs but by electroluminescence mechanism instead of photoluminescence mechanism. Predetermining the size of the QLED's dots would fix the emitted photon wavelength at the appropriate customer-specified color, even if it is not naturally occurring. By carefully controlling the size of quantum dot inserted, white light can be created.

Figure 2-10 Schematic diagram for pn junction with quantum dot in between

2.5.2. Fabrication of quantum dot in pn junction

Quantum dot structures became possible by the introduction of self-organized growth. Both molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used. Self-assembled QD forms during Stranksi-Krastanov epitaxial growth, the surface of a film becomes unstable after a few layers of pseudomorphic growth, resulting finally in the formation of three-dimensional islands. Similar to the previous method, mixture of QD with different sizes give rise to white light. This method can be referenced with US patent No. 6,645,885. In this

patent, Indium Nitride (InN) and Indium-rich Indium Gallium Nitride (InGaN) quantum dots embedded in single and multiple InxGal-xN quantum wells (QWs) are formed by using TMIn and /or Triethylindium (TEIn) as antisurfactant during MOCVD growth.

Another way of incorporating QD into the p-n junction conductive layer is to mix colloidally produced quantum dot with a transparent and electrically conductive host matrix, and coat in between the junction. This way requires less cost than epitaxy [9].

2.5.3. Advantages and limitations

Advantage 1: Even higher energy efficiency

There are always energy losses in terms of pc-LED. As mentioned earlier in this chapter, the energy losses are associated with the conversion from shorter wavelength to longer wavelength. Some of the photons may just be absorbed by the phosphor instead of being converted to yellow light. The loss is also related to the reflection and backscattering between LED and phosphor interface. However, for this QD in pn junction method, there is no phosphor, so all the losses can be avoided. Below is the estimation of the amount of energy that can be saved compared to the conventional yellow.

From blue light to red light, the amount of energy saved is: Energy of blue light: 1.24/0.46 = 2.7eV

Energy of red light: 1.24/0.70 = 1.77eV

Percentage of engery losses: 0.93/2.7 * 100% = 34%

So 34% energy due to wavelength conversion can be saved by adopting this method.

Advantage 2: Even lower cost

Since there is no phosphor, the cost for phosphor is totally reduced. As we analyzed in part 2.4, the cost of phosphor is roughly 1/6 of the total white LED cost or the 1/5 of a blue LED cost.

According to the above mentioned patent, growth of the active layers of blue and green LEDs can be achieved, but it is not efficient in producing red light from GaN pn junction. A possible reason is, in industry, the material system Ga, In and N is used to produce blue or green light rather than yellow light due to the natrual bandgap range of these material. It is very difficult to achieve red light emission bandgap using this material system. Due to this reason, the red light emitting quantum dot is hard to grow.

Limitations and obstacles 2

Another possible reason it is hard to get good control of quantum dot size and position [27]. Although theoretically it should work and give different color light with different dot sizes, in production line there may not be a workable die among 100 wafers. Since the repeatability is very low, so far none of the fabs are willing to invest in this technology.

Quantum dot is a very new research field, some phenomena associated with it are not well understood so far and its growth control is also in a lab research stage. QDs is a broad field and it has many applications. QD LED is just a technology overlap of Quantum dot and LEDs. To finally achieve QD in pn junction LED, QD technology needs to be developed to a more mature stage.

Limitations and obstacles 3

The growth of InxGal-xN alloys and quantum wells is extremely difficult mostly due to the trade-off between the epilayer quality and the amount of InN incorporation into the alloy. Lowering the growth temperature results in the indium content at the expense of reduced crystalline quality. The lattice mismatch and different thermal stability of the two constituents InN and GaN, also complicate the growth of InxGal_

xN. The lattice mismatch can lead to a miscibility gap [7] which causes fluctuations of In content across the film.

For the colloidally produced quantum dot in electrically conductive layer, the devices require a transparent, electrically conductive host matrix, which severely limits the available materials for producing LEDs by this method [9].

Reference

[1]. Cheng-Huang KUO, Jinn-Kong SHEU, Shoou-Jinn CHANG,

"n-UV+Blue/Green/Red White Light Emitting Diode Lamps", Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 2284-2287

[2]. http://www.nanotechnology.bilkent.edu.tr/research%20areas/ documents/LEDs.html

[3]. "Solid-State Lighting Research and Development Protfolio", Multi-Year Program Plan FY'07-FY' 12 prepared for: Lighting Research and Development Building Technologies Program Office of Evergy Efficiency and Renewable Energy U.S. Department of Energy

[4]. http://www.mse.berkeley.edu/classes/matscil02/F01 reports/whiteled.pdf [5]. Y.Q. Li, J.E.J. van Steen, J.W.H. van Krevel, "Luminescence properties of

red-emitting M2SisN8:Eu2+ (M = Ca, Sr, Ba) LED conversion phosphors", Journal

of Alloys and Compounds 417 (2006) 273-279

[6]. Chongfeng Guo, Dexiu Huang, Qiang Su,"Methods to improve the

fluorescence intensity of CaS:Eu2+ _{red-emitting phosphor for white LED",}

Materials Science and Engineering B 130 (2006) 189-193

[7]. Institute of Material Research and Engineering Singapore Prof. Chua SJ QD White light 29 Sept 05 presentation on White light LED

[8]. Johann Peter Reithmaier, Alfred Forchel,"semiconductor quantum dots" IEEE Circuits & Devices magazine November 2003

[9]. US patent no. 6,501,091

[10]. http://www.nitto.com/company/release/05_ 10_1 8/index.html

[11]. Hsueh Shih Chen, Shian Jy Jassy Wang, " White light emission from organics capped ZnSe quantum dots and application in white-light-emitting diodes", Applied Physics Letters 86, 131905 (2005)

[12]. http://www.aist.go.jp/aist_e/latest_research/2006/20060601/20060601 .html [13]. Jong Kyu Kim, Hong Luo, Eric Red Schubert, "Strongly Enhanced Phosphor

Configuration and Diffuse Reflector Cup", Japanese Journal of Applied Physics, Vol. 44, No.21, 2005, pp. L649-L651

[14]. Shankar M. Venugopal, "Recent advances in the development of Quantum Splitting Phosphors and White LED Phosphors", GE India Technology Centre, Bangalore

[15]. Hsueh-Shih Chen,Cheng-Kuo Hsu,and Hsin-Yen Hong, "InGaN-CdSe-ZnSe Quantum Dots White LEDs", IEEE Photonics Technology Letters, Vol.18, No. 1 ,January 1,2006

[16]. www.physlink.com/News/071403QuantumDotLED.cfm [17]. www. evidenttech.com

[18]. http://www.answers.com/topic/quantum-efficiency

[19]. Nadarajah Narendran, "Improved Performance White LED", Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY 12180

[20]. F. Gindele, R. Westpha ling, and U. Woggona, "Optical gain and high quantum efficiency of matrix-free, closely packed CdSe quantum dots", Appl. Phys. Lett. 71 (15), 13 October 1997

[21]. Saint-Gobain Crystal YAG data sheet

[22]. E. Fred Schubert and Jong Kyu Kim, "Solid-State Light Sources Getting Smart", 27MAY2005VOL 308 SCIENCE, 1274

[23]. http://www.netl.doe.gov/ssl/portfolio05/EnhancedOpticalEfficiencyPackage.htm [24]. http://www.nsf.gov/eng/sbir/SECTOR/Devices%20II/PhosphorTech.pdf

[25]. B. Jensen and A. Torabit, "Refractive index of hexagonal II-VI compounds CdSe, CdS, and CdSexS_-x", Vol. 3, No. 6, June 1986, J. Opt. Soc. Am. B [26]. Jong Kyu KIM, Hong LUO, "Strongly Enhanced Phosphor Efficiency in

GaInN White Light-Emitting Diodes Using Remote Phosphor Configuration and Diffuse Reflector Cup", Japanese Journal of Applied Physics Vol. 44, No. 21, 2005, pp. L 649-L 651

[27]. M. Noemi Perez-Paza and Xuecong Zhou, "Single layer and stacked CdSe self-assembled quantum dots with ZnCdMgSe barriers for visible and white light emitters", J. Vac. Sci. Technol. B 23,,3..., May/Jun 2005

[28]. Email communicated with Prof. Chua, Prof. Fitzgerald, Dr. Aaron Danner, Mr. Huang en li and Hsueh-Shih Chen

Chapter 3 White LED market analysis

3.1 HB LED market and growth trend

In 2003, the total high brightness (HB) LED market was 2.71 billion, mainly distributed in six application areas [1]:

Mobile Appliance: backlight for LCD screens and keypads in mobile phones, camera flashes

Signs and display: Single-color moving message panels, full-color video displays Automotive: Car, truck and bus exterior lighting (stop, turn etc), car interior

Illumination: Architectural lighting, machine vision, channel letters, decorative and accent lights

Signal: traffic signals, railroad, aviation

From Figure 3-1, we can see that HB LED market increased from 2.71 billion in year 2003 to 4 billion in year 2005, and will increase to 8.2 billion in year 2010. Also, inside the HB LED market, illumination, which mainly utilizes white LED, is one of the fastest growing applications. It accounted for just 5% of the high-brightness market in 2003, 6% in year 2005 and it is predicted to be 13% in year 2010. Therefore, illumination is the main market for white LED [2].

Another important application of white LED is automotive headlamps, in the near term, it is also an important market which is being targeted by white LED.

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3.2 White LED in illumination

The key to the near-term adoption of high brightness LEDs in illumination applications is not to target applications where traditional lighting (fluorescent, halogen, incandescent) is firmly entrenched, or where lower cost is desired, and where maintenance and replacement costs are low. It takes a great deal of effort, time and money to convince lighting manufacturers and their customers to shift away from applications in which traditional lightings have been successfully used for many years. What will help the illumination market for LEDs the most is to use LEDs in new applications where it is not possible or at least very undesirable) to use traditional lighting, or in applications that place a high premium on a particular attribute of LEDs, such as reliability. Currently, the major uses of LEDs in lighting applications

fall primarily within seven applications [3]:

--Channel letter/contour lighting --Architecture/retail/theme --Industrial/machine vision

--Consumer portable/novelty (such as flashlights and key lights); --Maitainess retrofit (socket-compatible replacement lamps); --Safety/security (exit signs and emergency lighting);

--Specialty illumination (such as aircraft interior lighting and task lights).

According to Figure 3-1, in year 2010, illumination market will reach 13%* $8.2 million =US$lbillion, which represents a near term market size.

Today, lighting product sales in the U.S. are worth approximately $11.9 billion annually. Of this, approximately $2.65 billion is associated with lamps while the remaining sales are divided between fixtures, components (including ballasts and controls) and associated services such as design and maintenance. Therefore, the long term illumination market is approximately US$2.65 billion.

LEDs have had a reasonably successful history in penetrating the automotive lighting market over the past 20 years. Conventional LEDs have long been used for various indicator-lamp functions inside the car. The first significant use for LEDs as exterior signaling is the stop lamp on Nissan 280Z in 1998. Automotive signaling got substantial growth in early 1990s. And by 2004, approximately 40% of all automobiles and light trucks produced worldwide featured LED-based center high-mounted stop lamps. LEDs are also used for rear-stop, turn, and tail lamp but only for high-end luxury cars. To varying degrees, high-brightness LEDs are being adopted for most forms of automotive lighting. Now, LEDs are poised to move into the one remaining, and potentially most lucrative, automotive-lighting application: headlamps [4].

In the first quarter of 2005, three major automotive-lighting companies announced LED-based headlamps that would be ready for adoption into production automobiles in the next few years: Stanley Electric, Visteon (Van Buren Township, MI), and Hella KTaA Hueck&Co. (Lippstadt, Germany). Standley announced that it would start producing white LED-based headlamps in 2007. It stated that its production capacity would be sufficient to supply 5000 vehicles per month in 2007 and 600,000 vehicles per month in 2010. As shown in Figure 3-2, Hella has developed a prototype headlamp assembly for the Volkswagen Golf 5 and Visteon's LED headlamp assembly was designed for the Cadillac STS. For near term, white LED headlamps will be only for luxury cars due to its high cost. This market has been driven by a combination of the higher performance (for example, shorter turn-on time), higher reliability, and more flexible styling possibilities offered by LEDs as compared to the standard incandescent lamp alternative [3].

Figure 3-2 The light output of an LED Headlamp prototype

The light output of an LED Headlamp prototype developed by Hella KGaA Hueck & Co. (left) and Visteon's LED headlamp assembly for the Cadillac STS (right)

We can expect to see an increasing number of cars on the road with LED headlamps in 2007 and beyond. This application will be one of the most promising new markets for HB LEDs in the second half of this decade, and will be a strong component of an expected $1 billion market for HB LEDs in automotive lighting in 2009. 20% of the total amount may go for headlamps, so it is approximately $200 million business for the near term market.

3.4 White LED for medical applications

A special application for white LED is medical applications, especially for surgical operation, which requires the highest quality and quantity of lighting. Common ceiling surgical halogen lighting system cannot provide an adequate amount of beams because the surgeons' heads hinder the illuminations from reaching the operation field. A solution is the doctor wears goggles with white LED. The light should have high color rendering index in order to render inherent color of raw flesh such as skin, blood, fat tissue and internal organs.

Since the white LEDs used were composed of InGaN-blue-emitters and YAG-yellow-phosphors, the color rendering property was not sufficient in the reddish colors [4]. So far there are not much consolidated data on this area, but it shows that high light quality is a very important issue for LED development.

3.5 White LED market estimation

* Present

white LED market = $240 million (mainly from illumination)

white LED market = $1,200 million/year (from illumination and automotive headlamps)

* Future

white LED market = the entire illumination market ~ 3 billion

(US market only)

Reference

[1]. "Solid-State Lighting Research and Development Protfolio", Multi-Year Program Plan FY'07-FY' 12 Prepared for: Lighting Research and Development Building Technologies Program Office of Evergy Efficiency and Renewable Energy U.S. Department of Energy

[2]. www.ledmagzine.com

[3]. Rovert V.steele, "High-brightness LEDs open new illumination markets", Laser Focus World, May 2003

[4]. Robert V.Steele, "LED automotive headlamps move closer to market", Laser Focus World, November 2005

Chapter 4 Existing players and intellectual properties

4.1 LED production regions

The major LED production regions are United States, Japan, South Korea, China, Taiwan and Europe. Governments in these regions support LED development, since LED technology can save huge amount of energy if it at last becomes mature. There are also LED consortiums that help promoting LED industries in these regions [1].

Japan

A consortium of companies and universities in Japan are developing efficient white LEDs and fixtures for solid-state lighting applications. Japan's Light for the 21st Century program was initiated with the goal of developing UV LEDs for solid-state white lighting. The project, which ended in 2004, brings together 13 companies and four universities. Research is proceeding in five main areas, namely substrates, epitaxy, devices, lamps and fixtures.

Taiwan

Taiwan is investing in solid-state lighting at the national level. The "Next Generation Lighting project" involves a consortium of 11 companies. Predicted earlier between 2003 and 2005, approximately NT$ 383 million would be invested in the technology. The goal is to achieve 50 Im/W output products and 100 Im/W in the "Taiwanese companies ramp up MOCVD capacity" (Compound Semiconductor, June 2004) and "Formosa Epitaxy forms white LED consortium" (Global Sources, January 2004) respectively.

China

China has also made public its intentions to support the development of solid state lighting. Called the "Semiconductor Lighting Project," four industrial bases for solid-state lighting will be developed with government support: Xiamen city ( Fujian province ), Shanghai, Dalian ( Liaoning province ), and Nanchang ( Jiangxi province ). Roughly $12 million total is estimated as the government investment.

South Korea

Korea has identified that solid-state lighting is an important technology. Korea's Photonics Technology Institute (KOPTI) works with LED developments. In addition, Korea apparently has an initiative funded for about $20M per year aiming to produce an 80 lm/W white LED in 2008.

Europe and USA

The European Union is also investing in programs that either directly or indirectly support Solid-State Lighting.

4.2 Major players

All over the world there are over 300 LED manufacturers and distributors distributed in these areas mentioned above [2]. Some of the companies are listed below:

US: Cree, Avago, Vishay Europe: Osram, Lumileds

Japan: Nichia, Toyoda Goesi, Standley, Rohm, Intematix, Citizen

Taiwan: Everlight Electroics, Cotoo (HK), Kingbright, Opto tech, Harvatek, Lite-on Korea: Samsung SEM

Among all, the "big five" LED manufacturers are Nichia, Osram, Toyoda Gosei, Cree and Lumileds.

Nichia's LED products include compound semiconductor materials (e.g. as LED infrared epitaxial wafers), Packaged LEDs and ultra violet LEDs. There are lamp shape white LED under packaged LEDs category. Their chromaticity coordinate equals to (0.41,0.39) and luminous intensity is from, 1250 to 9,200 mcd.

Cree's LED product families include LED chips, packaged LEDs and LED backlighting solutions. The packaged LEDs, which are labeled as Xlamp series, are mainly for lighting purpose. Cree has its distributors all over the world, such as Europe, USA, Australia, New Zealand, Mainland China and Taiwan.

Lumileds is a fully integrated manufacturer of LED dice, packaged LEDs, and high-brightness LEDs -Luxeon designed for integration into general lighting products. The company markets LED solutions designed specifically for automotive applications, LCD displays, general lighting, portable applications, signage, traffic signals, and other segments.

Osram is a lighting company. It has lighting product from conventional incandescent lamps to the new LED lighting solution. Its LED design suits into different purposes. For example, Dragonlight is for high luminous intensity applications. Linearlight is for Contour lighting and injecting light into diffused or transparent light guides.

Toyoda Gosei's nature of business is research, development, manufacture and sales of: Parts for automobiles, conveyors, ships and various other transportation equipment; rubber, plastic and urethane components for agricultural, construction and machine tool equipment etc. LEDs is just 2.9% of total sales for year 2005. However it still has the capability to produce high brightness white LEDs.

4.3 Existing white LEDs patents

The history of white-light LEDs is surprisingly complicated, given the fact that Nichia first produced white LEDs commercially in 1996. Key patents, mainly US, are listed in table 4-1 below in order of priority filing date [3].

Company Filing date Patent No. Contents

Bell Lab Jan-17-1970 3,691,482 The use of a screen containing one or more phosphors, illuminated with a laser source to give white or colored light

Nichia Nov-25-1991 -- Florescent dye added to the resin molding Cree Mar-26-1996 6,600,175 A single LED with a down converting

phosphor

Osram Aug-29-2000 6,245,259 A blue, green or UV LED with a cerium or terbium doped garnet or sulphur substituted garnet phosphor

HP(Agilent) July-14-1997 5,847,507 A wide range of phosphors included Nichia July-14-1997 5,998,925 GaN LED with a garnet-based phosphor Toyoda Dec-28-2000 6,809,347 Blue or UV LEDs with alkaline-earth

Table 4-1 Key US patents covering white LEDs

Among these patents, a key difference is the choice of phosphor, or "down-converting" material. The major phosphors are listed in table 4-2 below:

Yttrium aluminum garnet (YAG) doped with cerium, excited at about 460 nm and with a broad emission peak centre at 550 nm.

Terbium aluminum garnet (TAG), licensed by Osram to several manufacturers.

Sulphide phosphors such as strontium thiogallate doped with europium, excited at 460 nm and emitting in the green (550 nm), or strontium sulphide doped with europium and emitting in the red.

Silicate-based structures such as those patented by Toyoda Gosei and Tridonic, and also by Intematix.

Organic phosphors or dyes. It is not clear if a "fluorescent dye" would cover the first two categories.

Table 4-2 Major phsphors patented

Since there are numerous patents and most of them are about the phosphors used for conversion of blue and UV LEDs, there are extraordinarily complicated series of overlapping and apparently conflicting US patents. Hence, disputes and cross agreements happen mainly among the five major players. Since the big companies own the important patents, they also license it to smaller LED chip manufacturers. An overview of the IP relationship about white LED patents is shown in Figure 4-1 below.

Figure 4-1 Deals and disputes in the white LED industry: the key intellectual property relationships as of September 2005

Andrew Phillips of phconsult Ltd reports on a situation: the white LED area is a minefield of patents, cross-licensing agreements and infringement lawsuits involving the big five manufacturers. This can prove extremely daunting for new players entering the field.

However, apparently QD WLED can avoid these disputes about patents, since QD phosphor, which has a nanocrystal form, is totally different from conventional phosphors described by these patents. This is another advantage of QD WLED technology. It gives a steady position for the newly started business among the competitive market and complicated IP network.

4.4. Quantum dot LED patents

There are also patents on quantum dot LEDs. Below are two important patents on QD phosphor and QD in between pn junction.

US Patent No. 6,501,091

This patent is about QD phosphor and its assignees are Massachusetts Institute of Technology and Hewlett-Packard Company. Date of patent is Dec. 31, 2002. It patented an electronic device comprising a population of quantum dots embedded in a host matrix and a primary light source which causes the dots to emit secondary light of a selected color, and a method of making such a device. The size distribution of the quantum dots is chosen to allow light of a particular color to be emitted therefrom. The light emitted from the device may be of either a pure (monochromatic) color, or a mixed (polychromatic) color, and may consist solely of light emitted from the dots themselves, or of a mixture of light emitted from the dots and light emitted from the primary source. The dots desirably are composed of an undoped semiconductor such as CdSe, and may optionally be overcoated to increase photoluminescence. In one embodiment of this aspect, the quantum dots comprise CdS, CdSe, CdTe, ZnS or ZnSe and optionally be overcoated with a material comprising ZnS, XnSe, CdS, CdSe, CdTe, or MgSe. The host matrix may be any material in which quantum dots

may be dispersed in a configuration in which they may be illuminated by the primary light source [4]. 12 22 -16 in * fD "o,• •

Figure 4-2 Two embodiments of an LED according to the invention 22-red emitting QD; 18--green emitting QD; 12-matrix; 10--light source

Another relevant patent is US Patent No. 6914265, which is done by the same group of inventors.

US Patent No. 6,645,885

This patent is about QD grown in pn junction. Its assignees are National University of Singapore (NUS) and Institute of Material Science and Engineering (IMRE). Date of Patent is Nov 11, 2003. In this patent, Indium Nitride (InN) and Indium-rich Indium Gallium Nitride (InGaN) quantum dots embedded in single and multiple InxGal-.N/ InyGal.yN quantum well (QWs) are formed by using TMIn and/or Triethylindium (TEIn), Ethyldimethylindium (EDMIn) as antisurfactant during MOVCD growth. Controlled amounts of TMIn and/or other Indium precursors are important in triggering the formation of dislocation-free QDs, as are the subsequent flows of ammonia and TMIn. This method can be readily used for the growth of the active layers of blue and green light emitting diodes (LEDs) [5].

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Although white LED mentioned in this patent, but blue and green light is achieved by growing QDs inside a pn junction. It shows the possibility of producing white light if following the same routine.

4.5 Evident technologies

Evident technologies is a company that specializes in quantum dot product development. Its product can be used in many applications: life science quantum dot conjugate and label, LEDs, Photovoltaics, inks, telecommunication etc. QD white LED is one of the applications. The method they use is very similar to what is described in Chapter 2.

Through my research, I found Evident technologies is the only company that emphasize on QD white LEDs. The president of this company Ballinger expects Evident to start impacting on the LED market in 2005, initially with a purple LED. This color is produced by mixing the emission from a blue emitter with the red light

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