FREE ELECTRONIC LIBRARY - Dissertations, online materials

Pages:   || 2 | 3 | 4 |

«1. INTRODUCTION AND IMPACT OF WHITE LIGHT EMITTING DIODES The basic structure of an efficient double-heterostructure (DH) light emitting diode (LED) ...»

-- [ Page 1 ] --

Background Story of the Invention of

Efficient Blue InGaN Light Emitting Diodes

Nobel Lecture, December 8, 2014

by Shuji Nakamura

University of California, Santa Barbara, CA, USA.


The basic structure of an efficient double-heterostructure (DH) light emitting

diode (LED) is summarized in Figure 1. This optoelectronic device is composed

of semiconductor materials and is fabricated by sandwiching an active, emit- ting layer between an n-type and p-type layer. The n-type semiconductor layer has an abundance of high-energy electrons, whereas the p-type semiconductor has an abundance of available, empty sites in which the electron may reside at a lower energy level. These sites are also referred to as holes, are positively charged, and are mobile. The energy difference between the high and low energy electron state is referred to as the bandgap of the material. For DH LEDs, the bandgap of the active layer is smaller than that of the n-type and p-type layers.

When forward biasing a DH LED using a battery (or any other direct cur- rent source), electrons and holes are injected into the active layer from n-type and p-type layer, respectively. The electrons and holes recombine radiatively in the active layer, thereby emitting photons. This act is very efficient for DH LEDs as the electrons and holes are confined to the active layer due to the smaller bandgap of the active layer with respect to the n-type and p-type cladding layers (see also Figure 7). The resulting photon has an energy approximately equal to the bandgap of the active layer material. Modifying the bandgap of the active layer creates photons of different energies.

69 70 The Nobel Prizes FIGURE 1. Schematic depiction of a double heterostructure (DH) light emitting diode (LED) in operation while being powered by a 2.8 V battery. Within the active, emitting layer, electrons and holes recombine and emit light equal to the bandgap of said layer.

High-energy electrons are sourced from the negative terminal of the battery and return to the positive terminal after losing their energy to a photon in the active layer.

In the 1980s, all known material systems possessing the necessary material properties for blue light emission had shortcomings negating the possibility of creating an efficient blue LED. Gallium nitride (GaN) was one possible candi- date, though, at the time, no p-type or active layer could be created. These chal- lenges were ultimately overcome, leading to the first efficient blue LED using GaN in 1993 by Nakamura et al. [1]. Figure 2 shows a close-up image of a bare and packaged blue GaN LED.

Using blue LEDs, highly efficient white light sources become possible. This can be achieved by converting part of the blue light emitted from an LED to yellow using a phosphor [2]. To the human eye, the combination of blue and yellow light is perceived as white. A white LED can be created by embedding phosphors in a plastic cap which surrounds a blue LED (see Figure 3). Higher quality white light can also be created by mixing blue light with other colors as well, including red and green [3].

With the availability of white LEDs, a variety of applications can be significantly improved, if not enabled all together. But arguably, the most important impact of the white LED is its ability to generate white light at an efficiency that was previously impossible. The efficacy, a measure of perceived light power relative to the provided electrical power, of white light has improved over the Story of the Invention of Efficient Blue InGaN Light Emitting Diodes 71

–  –  –

centuries, starting with oil lamps (0.1 lm/W) in the 15,000s B.C., incandescent bulbs (16 lm/W) in the 19th century, fluorescent lamps (70 lm/W) in the 20th century, and LEDs (300 lm/W) in the 21st century (see also Figure 15).

With this significant improvement, substantial energy savings are now possible. It is currently estimated that in 2030 approximately 261 TWh of electrical energy will be saved due to widespread use of white LEDs [4]. This corresponds to an electricity savings of approximately 40% in 2030. Furthermore, this reduction in energy usage eliminates the need for at least 30 1-GW power plants by 2030 and avoids generating 185 million tons of CO2.

–  –  –

2. MATERIAL OF CHOICE: ZnSe VS. GaN In the 1980s, there were two materials considered as possible candidates for efficient blue LEDs: zinc selenide (ZnSe) and GaN [5].

ZnSe could be grown on single crystal gallium arsenide (GaAs) substrates, yielding high structural quality material given the very small lattice mismatch of 0.3% between ZnSe and GaAs. For GaN, on the other hand, no lattice-matched substrate was available and researchers were forced to grow it on sapphire. The large lattice mismatch (~ 16%) resulted in heavily defected material with a high density of dislocations.

When I joined the field in 1989, ZnSe was grown on GaAs with dislocation densities less than 103 cm–2. It was very popular among scientists, given the high crystal quality and the prevailing notion that a dislocation density below 103 cm–2 is needed to achieve optically functional LEDs with a high efficiency and a long lifetime [5]. Most researchers worked in this field. GaN, however, was grown on sapphire, yielding dislocation densities on the order of 109 cm–2.

Unsurprisingly, few researchers were working in this field except, most notably, fellow Nobel Laureates Professor Isamu Akasaki and his graduate student at the time, Hiroshi Amano.

A striking example to highlight the popularity of ZnSe, as compared to GaN, is provided by looking at the attendance of researchers at the most popular conference for applied physics in Japan. At the Japan Society of Applied Physics (JSAP) conference in 1992, there were approximately 500 individuals attending the ZnSe sessions, whereas for GaN, there were around 5, including the chair Professor Isamu Akasaki, speaker Hiroshi Amano and myself, as a member of the audience. Not only was ZnSe more popular at the time, GaN was actively discouraged with researchers stating “GaN has no future” and “GaN people have to move to ZnSe material.”


My entry into the field started in April of 1988, when I went to the University of Florida as a visiting researcher. The main purpose of my visit was to learn how to use a MOCVD (Metal Organic Chemical Vapor Deposition) system to growth GaAs crystals on a silicon substrate, as I had no experience in how to use a MOCVD. During my stay there, I worked together with graduate students and they all asked me if I had a Ph.D. I said no. At the time, I only had a Master’s.

Next, they asked me if I had published any scientific papers. Again, I said no, I had never published a single paper. Consequently, they treated me as a technician. In the U.S., this meant one has to help the researcher and one’s name would Story of the Invention of Efficient Blue InGaN Light Emitting Diodes 73 not appear on papers or patents. Gradually, I became very frustrated with this arrangement.

One year later, in March of 1989, I came back to Japan. It was my dream to get a Ph.D. degree. In Japan, at the time, it was possible to be awarded a Ph.D.

if one published five scientific papers. This type of degree was called a paper degree and one did not need to go to the university to get the degree. It was therefore my ultimate dream to publish at least five papers and get a Ph.D.

With this in mind, I noted that the ZnSe field was publishing lots of papers.

As I had never published a paper, I had no confidence in publishing a paper. In the GaN field, only very few papers had been published, mainly from Professor Isamu Akasaki and Hiroshi Amano. I was therefore confident that I could publish lots of papers, though had no confidence that I could actually invent the blue LED. My only objective was to get a Ph.D. That’s it.

So, after returning to Japan in March of 1989, I wanted to grow GaN using a MOCVD reactor. I purchased a commercially available MOCVD reactor for 2 million U.S. dollars. But this MOCVD reactor was designed for growth of GaAs. At the time, Professor Akasaki and his student Amano had developed a novel, research-scale MOCVD reactor for growth of GaN [6]. Their design required exceptionally high carrier gas velocities (around 4.25 m/s) yielding GaN, though the high carrier velocities presented challenges pertaining to uniformity, scalability and reproducibility. Furthermore, their reactor design could only be used for small area growths, thereby lacking the necessary properties for commercialization. Since I was working for a company, I had to find a way to grow high quality GaN on large area, 2-inch diameter sapphire substrates.

Another challenge related to growing high quality GaN was the use of high concentrations of aluminum in the MOCVD reactor. While the development of the aluminum nitride (AlN) buffer layer by Akasaki and Amano was a major breakthrough providing high quality GaN film growth with a mirror-like surface morphology [6], the use of aluminum caused significant problems to the MOCVD reactor resulting in poor reproducibility in subsequent GaN growths.

Eliminating the use of high concentrations of aluminum during growth was strongly desired.

After my purchase of a MOCVD reactor, I attempted a significant number of growths over the course of a few months, but consistently failed. Either no growth of GaN occurred or the grown layer was black. GaN should be transparent. I realized this was a big problem, especially considering the substantial investment in the tool. That is when I decided I had to modify the reactor.

For the next 1.5 years, I modified the reactor design. In the morning, I would go to work and modify the reactor. In the afternoon, I would perform a couple 74 The Nobel Prizes of growths and analyze the results. I would repeat this pattern for 1.5 years until I invented a novel MOCVD reactor design with a low carrier gas flow which I called a two-flow MOCVD (Figure 4 a) [7]. Using this reactor, I was able to get very uniform and high quality 2-inch GaN growth. The main breakthrough of this reactor was the introduction of a subflow (Figure 4 b) which gently pushed the carrier gases down to the substrate, thereby also improving the thermal boundary layer.

This was the most important breakthrough in my life and was instrumental toward all future breakthroughs in GaN research. One significant advancement (a) (b) FIGURE 4. (a) Schematic of a two-flow MOCVD for GaN growth and (b) schematic of the effect of the newly introduced subflow on the carrier gases. [7] (Reprinted with permission. Copyright 1991, AIP Publishing LLC.) Story of the Invention of Efficient Blue InGaN Light Emitting Diodes 75 this tool immediately enabled was the development of a GaN buffer layer which was superior to the AlN buffer layer, in part due to the elimination of aluminum from the growth system. With the invention of the two-flow MOCVD and the GaN buffer layer, it was possible to achieve the highest quality GaN material in the world. One measure for crystal quality is the value of the electron mobility in a crystal. Fewer defects result in fewer scattering events, which enhances overall mobility of the electrons. Mobilities for GaN grown directly on sapphire (no buffer layer) by Akasaki and Amano resulted in values around 50 cm2/Vs [6], whereas use of the two-flow MOCVD yielded 200 cm2/Vs [7]. Use of an AlN buffer layer improved the mobility to values as high as 450 cm2/Vs for Akasaki and Amano [8]. Use of a GaN buffer layer and the two-flow MOCVD values as high as 600 cm2/Vs were measured at room temperature (see Figure 5) [9].

This was a clear sign that the two-flow MOCVD was producing GaN material of higher quality on larger area substrates, a key step towards commercialization of GaN based devices.

The next significant development in creating an efficient blue LED occurred in 1992 when I was able to clarify why p-type GaN had remained so elusive for 20 years. While Akasaki and Amano achieved a major breakthrough in 1989 by demonstrating local p-type GaN after treating magnesium doped GaN (GaN:Mg) with low-energy electron beam irradiation (LEEBI) [10], its origin

–  –  –

was not understood for another three years. In 1992, I clarified that hydrogen was the source of passivating p-type GaN [11]. A few years later, theoretical computations by Jörg Neugebauer and Chris Van de Walle confirmed hydrogen passivation in Mg-doped GaN [12].

For MOCVD growth of GaN, ammonia (NH3) is used as the nitrogen source. Ammonia dissociates during growth and atomic hydrogen is introduced into the GaN crystal. If Mg is present in the crystal, the hydrogen atom forms a magnesium hydrogen complex (Mg-H), thereby preventing Mg from acting as an acceptor [11]. Thermal annealing of the GaN:Mg sample in a hydrogen-free environment above approximately 400 °C permits hydrogen to diffuse out of the crystal, thereby breaking up the Mg-H complex [13]. As thermal annealing can be performed quickly and simultaneously on multiple substrates of any size in parallel (an act not achievable using LEEBI), it has become the industrial standard process for p-type activation of GaN. The formation of local p-type GaN using LEEBI treatments can be explained by local heating of the GaN:Mg by the electron beam, causing the hydrogen to locally diffuse out of the crystal and permitting the affected Mg atoms to act as acceptors yielding p-type GaN.

Pages:   || 2 | 3 | 4 |

Similar works:

«The Russian Aid to Byzantium during the Turkish Siege of Constantinople, 1394–1402 Jan Brandejs After their first successful conquest of Gallipoli in 1354 and more importantly of Karasi emirate in 1361, the Ottoman Turks took advantage of their position on frontiers between Christian and Muslim world and started conducting invasions on European soil, enjoying the instability of many Balkan states. Indeed, within just one lifetime OPEN ACCESS the Ottomans became a force to be reckoned with,...»

«2006-2007 ASUCSD COUNCIL MEETING # 17 Wednesday, January 24, 2007 Price Center, Ballroom A, 6:30 p.m. Minutes I. CALL TO ORDER Matthew Bright, Muir Senior Senator, calls 2006-2007 ASUCSD Council Meeting # 17 to order at 6:35pm. II. ROLL CALL Present for the first roll call were: Voting Council Members: Alan Roof Harry Khanna Ellen Almirol Kyle Samia Marco Murillo Matthew Bright Adi Singer Conrad Ohashi Meghan Clair Rabia Paracha John Cressey Non-Voting Council Long Pham Ashwin Dighe Members:...»

«St Nicholas Annual Report 2014 Contents Page Christianity Explored.. 2 Training... 2 Evangelism... 2 Grace Church... 2 4 o’clock Church.. 3 Report of the Creation Care Committee.. 3 Stewards... 6 Babies Club... 6 Mum & Co.... 7 Dad & Co.... 7 Crèche... 7 Children’s Ministry Review.. 7 Centrepoint... 8 Contact... 8 Ministry Associates.. 9 Retired Men’s Lunches.. 9 Men's Breakfasts.. 9 Women’s Breakfasts.. 10 Central Focus... 10 Women’s Daytime Study Groups.. 10 Home...»

«Městský úřad Litoměřice Odbor životního prostředí Vaše značka: Ze dne: Č. j.: 0070427/12/ŽP/PGr Sp. zn.: 0061155/12/ŽP Dle rozdělovníku Vyřizuje: Ing. Pavel Gryndler Telefon: +420 416 916 179 Fax: +420 416 916 211 E-mail: pavel.gryndler@litomerice.cz Litoměřice 25.10. 2012 ROZHODNUTÍ Městský úřad Litoměřice, odbor životního prostředí, rozhodující jako věcně a místně příslušný orgán podle § 60 zákona č. 449/2001 Sb., o myslivosti (dále jen,,zákon o...»

«Junior School Handbook 2014/2015 www.sibfordschool.co.uk INTRODUCTION I would like to take the opportunity of welcoming you and your child to Sibford Junior School. A move to a new school presents an exciting and challenging time and this booklet is designed to give new pupils and their parents some of the important information that you will need to know when you arrive and to keep as reference throughout your time at the school.Other useful sources of information are: Our website:...»

«THE EFFECT OF PARTICIPATION IN READ 180 ON SIXTH GRADE STUDENTS’ READING ACHIEVEMENT Christopher M. Miller B.A., Southwest Missouri State University, 1998 M.S., Southwest Missouri State University, 2002 Submitted to the Graduate Department and Faculty of the School of Education of Baker University in partial fulfillment of the requirements for the degree Doctor of Education in Educational Leadership May 1, 2014 Copyright 2014 by Christopher M. Miller ii Dissertation Committee Major Advisor...»

«H Moving to a hospital or skilled nursing facility What to expect when you have MRSA (Methicillin-resistant Staphylococcus aureus) A booklet for patients, residents, family members, and caregivers About this booklet You are getting this booklet because you are moving between a hospital and a skilled nursing facility. Hospitals and skilled nursing facilities care for you at different stages of your illness or injury. Because of this, they don’t always do things the same way. This booklet will...»

«SECURITIES AND EXCHANGE COMMISSION 17 CFR Parts 200, 229, 230, 232, 239, 240, and 249 [Release No. 33-9186; 34-63874; File No. S7-18-08] RIN 3235-AK18 SECURITY RATINGS AGENCY: Securities and Exchange Commission. ACTION: Proposed rule. SUMMARY: This is one of several releases that we will be considering relating to the use of security ratings by credit rating agencies in our rules and forms. In this release, pursuant to the provisions of Section 939A of the Dodd-Frank Wall Street Reform and...»

«Mike Gallagher (WI-08) Research Book The following report contains research on Mike Gallagher, a Republican candidate for Wisconsin’s 8th Congressional District. Research for this book was conducted by the Democratic Congressional Campaign Committee’s Research Department between April 2016 and May 2016. By accepting this report, you are accepting responsibility for all information and analysis included. Therefore, it is your responsibility to verify all claims against the original...»

«Standard Eurobarometer European Commission EUROBAROMETER 71 PUBLIC OPINION IN THE EUROPEAN UNION Spring 2009 Standard Eurobarometer 71 / Spring 2009 – TNS Opinion & Social NATIONAL REPORT EXECUTIVE SUMMARY LITHUANIA This survey was requested and coordinated by Directorate-General Communication. This report was produced for the European Commission’s Representation in Lithuania. This document does not represent the point of view of the European Commission. The interpretations and opinions...»

«CLEANSHIP Clean Baltic Sea Shipping Task 3.4 STUDY OF ENERGY LOGISTICS AND STRUCTURE IN PORTS ENVIRONMENTAL DEVELOPMENT ASSOCIATION, PP17 CLEANSHIP Task 3.4 DOCUMENT INFORMATION Task 3.4 STUDY OF ENERGY LOGISTICS AND STRUCTURE IN PORTS ZANNA SAVCUKA Author(s): ENVIRONMENTAL DEVELOPMENT ASSOCIATION Issuing entity: CLEANSHIP TASK 3.4 Document Code: Pages 110 Figures 56 Tables 31 Annexes 0 CLEANSHIP Task 3.4 Disclaimer This publication is part of the Clean Baltic Sea Shipping project and it is...»

«A Photo Illustrated Guide to the Moss Families of the Island of Dominica Steven Paul Martinez Dominica Undergraduate Research Program 2007 Dr. Jim Woolley Dr. Bob Wharton Abstract: No complete identification has ever been documented of Dominica’s mosses. The last known research was done in 1914 by Miss E. F. Noel. My main goal was to identify as many of the island’s mosses down to family. I collected the specimens while on the various hikes we took as a group around the island and was able...»

<<  HOME   |    CONTACTS
2016 www.dissertation.xlibx.info - Dissertations, online materials

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.