October 2010 - December 2010
Publisher: Chairman Ching-Fuh Lin  Editors: Professor I-Chun Cheng, Ms. Hsiao-wen Lin  January 30, 2011

Congratulations! GIPO Distinguished Research Chair Professor Teresa H. Meng receives NTU’s “Outstanding Alumnus” award.

Congratulations! GIPO Professor Chi-Kuang Sun is awarded Taiwan Photonics Society’s “2010 Photonics Engineering Award”.

Congratulations! GIPO Professor Chung-Chih Wu executes the Interdisciplinary Training Program for Technology Talents in Organic Optoelectronics and Display and receives the Merit Award of “2009 Interdisciplinary Training Program for Talented College Students in Science” from Ministry of Education.


October “Photonics Forum” Lecture Highlights (Compiled by Li-Chi Yao)


October 8th, 2010 (Friday), 12:20 pm
Speaker: Dr. Rick Tsai (General Manager of the New Businesses Department, Taiwan Semiconductor Manufacturing Co., Ltd., TSMC)
Topic: Limitless Opportunities in Moving Toward Green Energy

Dr. Rick Tsai visited GIPO on October 8th (Friday), and delivered his lecture, “Limitless Opportunities in Moving Toward Green Energy” at Auditorium 101, Barry Lam Hall. He introduced the current development of photonics and optoelectronics and their future prospects. The event attracted many GIPO professors and students, and everyone learned much from his lecture.


Speaker Dr. Rick Tsai


October 15th, 2010 (Friday), 3:30 pm
Speaker: Mr. Ching-Wen Cheng (famous writer and alumnus of Department of Business Administration, NTU)
Topic: Writing Experiences
  Mr. Ching-Wen Cheng visited GIPO on October 15th (Friday), and delivered his speech, “Writing Experiences” at Auditorium 101, Barry Lam Hall. Mr. Cheng was a winner of the Kiriyama Pacific Rim Book Prize for fiction, a winner of the 9th National Culture and Arts Award, and was also honored by World Chinese Literature Lifetime Achievement Award. He is adept at depicting turbulent, emotional stories with delicate touches and at revealing vicissitude in eras and in society through the inner lives of average people. GIPO professors and students attended the event with enthusiasm and learned much from the speech.



Speaker Mr. Ching-Wen Cheng

October 22nd, 2010 (Friday), 3:30 pm
Speaker: Mr. Sheng-Yen Lien (Honorary Chairman of Dann-Ru Calligraphy Society)
Topic: Calligraphy and Life
  Mr. Sheng-Yen Lien visited GIPO on October 22nd (Friday), and gave a presentation on the topic of "Calligraphy and Life" at Auditorium 101, Barry Lam hall. Mr. Sheng-Yen Lien is the direct disciple of Dann-Ru Chio-Pu Tsao, renowned modern Taiwanese calligrapher. Sheng-Yen has studied 8 styles of calligraphy thoroughly, and has mastered each. He has devoted himself to the art for more than 40 years. He studies works from the past and takes traditional calligraphy as the foundationto create his own style. He was the principal of Ching-Chwan Commercial High School in Taipei County, a representative of the National Assembly, the chairman of Dann-Ru Calligraphy Society, the chairman of China Calligraphy Society, a member of the appraiser panel of Provincial Fine Arts Exhibition, and a member of the appraiser panel of the National Fine Arts Exhibition. He has dedicated himself to educational activities related to calligraphy, and has been awarded the "Culture and Arts Celebrity Prize" of Taipei County, the "Global Chinese Culture & Calligraphy Art Award" of Republic of China Senior Jaycees Club, and the "Literature and Arts Award, Calligraphy" of the Chinese Literature and Arts Association. He was also invited to hold a retrospective exhibition of his past 70 years' works in April, 2008, at the Chung Shan National Gallery of National Dr. Sun Yat-sen Memorial Hall, which was widely-praised and held in high regard by the calligraphers in the country and worldwide.



Speaker Mr. Sheng-Yen Lien

October 29th, 2010 (Friday), 3:30 pm
Speaker: Professor Xing-Yi Wu (Professor of Biomedical Engineering, Department of Industrial Engineering, Texas A&M University)
Topic: Photo-acoustic Imaging
  Professor Xing-Yi Wu visited GIPO on October 29th (Friday). He delivered a lecture on the topic of “Photo-acoustic Imaging” in Auditorium 105, EE Building II. GIPO professors and students attended with enthusiasm. Professor Wu’s lecture was fascinating and informative, and he interacted with the audience throughout the event.



Speaker Professor Xing-Yi Wu

November “Photonics Forum” Lecture Highlights (Compiled by Li-Chi Yao)
November 4th, 2010 (Thursday), 10 am
Speaker: Professor Pei-Cheng Gu (Department of Electrical Engineering and Computer Science, University of Michigan)
Topic: Scaling of Semiconductor Lasers and Solar Cells Using Plasmonics
  Professor Pei-Cheng Gu visited GIPO on November 4th (Thursday), and lectured in Auditorium 105, EE Building II. His lecture explored the topic of “Scaling of Semiconductor Lasers and Solar Cells Using Plasmonics”.

November 12th, 2010 (Friday), 3:30 pm
Speaker: Professor Kunji Chen (State Laboratory of Solid State Microstructures and Department of Physics, Nanjing University)
Topic: Silicon Based Photonic Quantum Dots and Photonic Molecules
  Professor Kunji Chen visited GIPO on November 12th (Friday) and lectured in Auditorium 101 of Barry Lam Hall. His lecture was titled “Silicon Based Photonics Quantum Dots and Photonics Molecules”. Professor Chen’s main areas of study include nano-semiconductor materials, nano-quantum electronics and nano-photonics. GIPO professors and students attended the lecture with enthusiasm. The content of Professor Chen’s lecture was exciting and he interacted with his audience throughout the event.
Speaker Professor Kunji Chen

November 30th, 2010 (Tuesday), 10:30 am
Speaker: Professor Ajoy Kar (Physics Department, Heriot Watt University)
Topic: Laser Written Active and Passive Waveguide Devices
  Professor Ajoy Kar arrived at GIPO on November 30th (Tuesday) and lectured in Auditorium 105, EE Building II on the topic of “Laser Written Active and Passive Waveguide Devices”. GIPO professors and students attended the lecture with enthusiasm and everyone benefited greatly from the experience.
Speaker Professor Ajoy Kar

December 1st, 2010 (Wednesday), 10:30 am
Speaker: Professor Min Qiu (KTH Royal Institute of Technology)
Topic: Plasmonic Metamaterial Absorber at Optical Regime
  Professor Min Qui visited GIPO on December 1st (Wednesday) and lectured in Auditorium 105 of EE Building II. The lecture was titled “Plasmonic Metamaterial Absorber at Optical Regime”. Professor Min Qiu’s speech was exciting and he interacted with his audience throughout the event. Everyone benefited greatly from it.
Speaker Professor Min Qiu

December 2nd, 2010 (Thursday), 11 am
Speaker: Professor Chinlon Lin (Bell Labs and Bellcore (USA, retired); Formerly Professor of Photonics and Director, Center for Advanced Research in Photonics, Chinese University of Hong Kong)
Topic: Fiber-optic Communication and Global Broadband Network --- dedicated to Professor Kuen Gao, 2009 Nobel Physics Prize Laurel Winner
  2010 marks the 40th anniversary of the invention of low-loss optical fibers, which, designed specifically for high-capacity and long-distance communications, has thoroughly changed the world by virtue of its utterly unlimited communication capacity. Nowadays, the use of widespread laid global fiber-optic networks has meant that broadband information infrastructures have been developed in most countries and have unfolded its tremendous, far-reaching and high-speed telecommunication capabilities to the world. For his incredible contribution in expediting the invention of low-loss fiber-optic, Professor Dr. Charles K. Kao, former president of the Chinese University of Hong Kong and a pioneer of fiber-optic communication development shared the 2009 Nobel Physics Prize laurel with Dr. W. Boyle and Dr. G. Smith of Bell Labs, who have invented CCD devices for digital electronic imaging. This seminar covers early stories of photonics and fiber-optic communication developments, and explores the important influence of fiber-optic and global broadband networks on information communication in modern human society. We, herewith, humbly present this seminar to applaud Professor Kuen Gao for his honorable achievement in receiving the Nobel Physics Prize.
Speaker Professor Chinlon Lin
2009 Nobel Physics Prize Laurel Winner
Professor Kuen Gao visits College of EECS

Professor Kuen Gao and Mrs. Gao (third left and fourth left) in the dean’s office, accepting souvenirs from Dean Lin-Shan Lee, EECS (third right).

GIPO Distinguished Research Chair Professor Teresa H. Meng visits (November 16th, 2010)
GIPO Distinguished Research Chair Professor Teresa H. Meng returned to EECS, NTU on November 16th. Dr. Meng gave a lecture titled “Engineer, Educator and Entrepreneur” and spoke with the faculty and students.

GIPO Distinguished Research Chair Professor Teresa H. Meng (middle), with Dean Lin-Shan Lee, EECS (left), Chairman Ching-Fuh Lin, GIPO (right) in the dean’s office


~ 2010 Ph.D. Student Exchange with Nanjing University ~

(The 3rd Cross-Strait Ph.D. Student Forum on Photonic Science and Technology, 2010)

(Time: October 25th to October 26th, 2010; Location: National Taiwan University)

Composed by Po-Chun Yeh, GIPO Ph.D. candidate

This was the 3rd exchange between the Ph.D. students from Nanjing University and from National Taiwan University. GIPO of NTU sponsored this year's event. Professor Shining Zhu, Dean of the College of Physics, NJU, and Professor Zhenlin Wang, Associate Dean, headed the NJU team. They were accompanied by Professor Xiangfei Chen, and Gang Zhao and Liang Sun, two teachers. In recent years, academic exchanges between both sides have been gaining regard and become importance. This year, members of the visiting team have increased to 15 Ph.D. students; they are from Nanjing University, Nanjing Normal University, Nanjing University of Science and Technology and Soochow University respectively.

The exchange began on October 24th and ended on October 30th, 2010. The formal conference was held on October 25th and 26th at 201 conference room, Barry Lam Hall, NTU. 5 teachers and 15 students from the NJU team, Professor Chi-Kuang Sun, Associate Dean of the College of EECS, Professor Ching-Fuh Lin, Chairman of GIPO, Professor Chih-I Wu, Vice Chairman of GIPO, and 10 GIPO doctoral students from NTU attended the opening ceremony. Aside from those mentioned above, there were also several other GIPO professors in attendance. Immediately after the opening ceremony, Dean Shining Zhu and Chairman Ching-Fuh Lin introduced the NJU and NTU teams. In his introduction, Dean Shining Zhu notes that Chien-Shiung Wu and Kwoh-Ting Li were both NJU alumni, which brought us to realize the close relationships between NJU and Taiwan's academic, technology and economic development.

Both the number of schools and participants have increased this year, which made the conference more action-packed than ever before. There were 8 subjects on the agenda, including: nonlinear photonic devices, solid state lighting and display, physics optics & microscopy, integrated optics devices I - waveguide, integrated optics devices II - grating, laser optics & optical devices, nano photonics, and optoelectronic materials. Each student representative had 25 minutes to introduce his laboratory and present his research work to the audience. As the presentation were all given in Chinese, both teams were able to interact more directly and more deeply. Many of the presentations caused fervent discussion among participants, and the discussion continued even during breaks. It could easily be seen that both teams were very interested in each other's research works.

The next morning, we invited professors from both teams to give lectures. Associate Dean Zhenlin Wang of College of Physics, NJU, introduced his team's research which involved creating photonic crystal from microspheres. His abundant and profound presentation, along with his smooth and assured manner of speaking revealed his scholarly style. Vice Chairman Chih-I Wu of GIPO, NTU, represented our team, and gave a lecture on the research of organic light-emitting diodes. His research covers both theory and application. These two professors' lectures set good examples for the students and evoked enthusiasm in students for pursuing research.

Participants outside Barry Lam Hall

We arranged three-day-long visiting activities for the NJU team beginning on October 27th and ending on October 29th. They visited GIPO's laboratories, Taiwan Semiconductor Manufacturing Company Limited (located in Hsinchu Science Park), and AVerMedia Information Inc. (located in Zhonghe District). We also took advantage of this opportunity to visit National Palace Museum, Chiang Kai-Shek Memorial Hall, and Taipei 101 as well. In addition to academic interaction, we also interacted with the mainland teachers and students in culture, life and industry. Both teams learned a lot from each others’ cultures.

During the week, through exciting academic interaction, we have felt the mainland students' dedication to the research. Every student representative has shown his outstanding research accomplishments. We are glad to have interacted with and to have learnt from such outstanding friends. This experience has made us more dedicated to our own work as well. Throughout the week-long interaction, we observed the differences in wording, customs and experiences; however, the friendship and enthusiasm led us to realize that it is not far between the two sides of the strait.

Visiting TSMC

Composed by Yung-Hsiang Lin, GIPO Ph.D. candidate

The 3rd Cross-Strait Ph.D. Student Forum on Photonic Science and Technology, 2010, presided by Ching-Fuh Lin, Chairman of GIPO, NTU, and Shining Zhu, Dean of the College of Physics, NJU, came to a successful close. During the two-day-long forum, held at 201 conference room, Barry Lam Hall, NTU, Professor Chih-I Wu, Vice Chairman of GIPO, NTU, Professor Zhenlin Wang, Associate Dean, College of Physics, NJU, and 25 Ph.D. students from National Taiwan University, Nanjing University, Nanjing Normal University, Nanjing University of Science and Technology and Soochow University, presented the fruits of their research, and learnt from each other. During the presentations, participants listened attentively, raised questions and discussed related subjects enthusiastically afterward. Everyone in attendance said he or she gained a lot from the forum. Among them, Po-Sheng Wang and Wen-Lan Yeh of NTU and Xiang Xiong and Xin-Jie Lv of NJU have each been awarded the "best thesis prize" for their excellent theses and splendid presentations. Both teams exchanged souvenirs after the forum.

NTU and NJU students exchange gifts after the closing ceremony

Aside from academic interactions during the forum, students from both teams made use of the breaks to get to know each other better and exchanged viewpoints concerning international affairs, educational developments, career plans and cross-strait cultures. The event not only helped students grow in their scientific research, but also broadened their horizons and cultivated their global vision.

Doing our best to be the host, GIPO arranged a three-day-tour after the end of the two-day-forum, and entertained our guests with traditional Taiwanese gourmet food. They were very impressed by our hospitality and praised Taiwan as a beautiful island filled with treasures.

A tour of the NTU campus

We arranged a sightseeing tour of NTU campus and visits to GIPO’s related laboratories for our guests. First, we stopped by NTU Heritage Hall of Physics, including the Nuclear Physics Laboratory and the General Physics Homemade Experiments Exhibition Gallery, which enabled our guests to better understand early scientific developments in Taiwan. Afterward we visited GIPO's laboratories, including Photonics Processing Laboratory and several GIPO professors' laboratories. We appreciated the kind assistance of Professor Gong-Ru Lin's laboratory and Professor Chih-I Wu's laboratory in this event.

In addition, we arranged our guests to pay a visit to the National Palace Museum, Chiang Kai-Shek Memorial Hall and Taipei 101. During the tour in National Palace Museum, they were very happy to have the opportunity to appreciate the museum's collections of wonderful Chinese treasures, and praised the beauty of Chinese culture while visiting. At Chiang Kai-Shek Memorial Hall, our guests were greatly impressed by the towering style and grandiose nature of the architecture of the main hall and the National Theater and Concert Halls. The Taipei 101 tower is Taipei's landmark, and our guests certainly did not want to miss it. They have admired the modern beauty of Taipei's luminous night sky and lingered on without any thought of leaving while in the tower's observatory.

This interaction has enriched friendships among professors and students of National Taiwan University, Nanjing University, Nanjing Normal University, Nanjing University of Science and Technology and Soochow University. We are all looking forward to meet each other again in Nanjing next year.



Surface Tension and Concentration Measurement of Sub-mL Solution Using a Cantilever-Based Optical Gauging System

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A cantilever sensor integrated with an on-tip micro spherical reflecting mirror (MSRM) exhibits a larger optical beam displacement than a conventional one, i.e., the system sensitivity/responsivity is enhanced. In our study, it is employed as a surface tension and concentration gauge that only requires 0.5 mL of solution.

The MSRM-integrated cantilever is first set above a sodium chloride-water droplet carried by a glass substrate, and then the droplet is moved up gradually. Once the cantilever is touched by the droplet, it is pulled and bent down as the droplet reshapes. The cantilever deformation amount is related to the surface tension of the solution, which increases with the molar concentration of sodium chloride. According to our experiments, the surface tension varies from 72.1 to 77.7 mN/m as the molar concentration of sodium chloride in water increases from 0 to 3.13 M (Fig. 1). Therefore, by measuring the bending amount of the cantilever, the surface tension as well as the concentration of the NaCl-water solution can be determined. We also perform the experiments on the alcohol (ethanol)-water mixture, whose surface tension, conversely, reduces from 71.4 to 57.5 mN/m as the alcohol molar concentration increases from 0 to 0.81 M (Fig. 2).

Fig. 1

Fig. 2

© 2010 IEEE

C. D. Liao, K. H. Chao, and J. C. Tsai, "Surface tension and concentration measurement of sub-mL solution using a cantilever-based optical gauging system," IEEE Journal of Quantum Electronics, Vol. 46, No. 9, pp. 1268-1274, September 2010.


Arbitrary-Order Interface Conditions for Slab Structures and Their Applications in Waveguide Analysis

Professor Yih-Peng Chiou

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Convergence of truncation error is one of critical factors in finite-difference simulation. Since step-index structure is a common feature in recent photonic devices design, traditional formulation based on graded-index (GI) and index averaging (IA) scheme cannot accurately model field behavior near abrupt interfaces between different materials. We derive generalized interface conditions of arbitrary orders combined with Taylor series expansion in homogeneous region for TE and TM mode calculation of step-index waveguide. We also adopt generalized Douglas (GD) scheme for further convergence order without demanding more reference points.

We model a multiple-quantum-well (MQW) waveguides with 56 barriers and 55 wells as an assessment. Refractive indices of cladding, barrier, and well are 3.2224, 3.2874, and 3.3704, respectively. Widths of each barrier and well are 12nm and 7nm, respectively. The relative propagation constant error of fundamental TE and TM modes illustrated in Fig. 2 shows that our proposed scheme using (2N+1)-point without GD yields convergence between O(h2N-1) and O(h2N), or O(h2N+1) and O(h2N+2) if GD is adopted. The high-order convergence can greatly reduce computation effort of waveguide analysis. Formulation can also be applied to other simulation methods such as beam propagation method.

Fig. 1 Illustration of field continuity and sample points.

(a)                                                                                               (b)

Fig. 2 Relative propagation constant error of fundamental (a) TE and (b) TM modes.

Reference: Y.-P. Chiou and C.-H. Du, OSA Optics Express, Vol. 18, No. 5, pp. 4088-4102, Mar. 2010.


Virtual Optics: Reconstruction of Light by Means of Optical Phase Conjugation

Professor Snow H. Tseng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We simulate the optical phase conjugation (OPC) phenomenon of light multiply scattered through a macroscopic scattering medium.  The pseudospectral time-domain (PSTD) technique is employed to enable a virtual optics experiment for analysis.  Specifically, we analyze the OPC reconstruction phenomenon from various categories of scattered light and determine the contribution.  We found that an unbiased ensemble of scattered light is required to reconstruct the original incident light.

Fig. 1.  Depicting the Poynting vector field of light multiply scattering through a scattering medium.  As light scatters through the scattering medium, the Poynting vector field in each region bears different characteristics: (a) randomly oriented, (b) in general pointing away from the scattering medium and anti-parallel to the incident light, (c) uniformly pointing away from the scattering medium, and (d) also pointing away from the scattering medium, but not as uniform as in (c).

Fig. 2. OPC refocused light from selected local regions shown in Fig. 1(a-d).  A cross-correlation of each OPC reconstructed light and the original incident light profile is calculated.  (a) diffuse light within the scattering medium, correlation coefficient cc = 2.6´10-4, (b) enhanced backscattered light, cc= 4.2´10-5, (c) specularly reflected light, cc = 4.3´10-6, (d) diffuse light outside the scattering medium, cc=3.2´10-5.  As shown in 2(c), the reconstruction falls short without an unbiased collection of scattered light.


Plasmon Field Enhancement in Silver Core-Protruded Silicon Shell Nanocylinder Illuminated with Light at 633 nm

Professor Ding-Wei Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have shown, to the best of our knowledge, the first simulation result of the strong plasmonic field coupling and enhancement at the Ag/Si interface of a silver core/protruded silicon shell nanocylinder by using the finite element method. The strong plasmon field with a slow effective phase velocity accumulated at Ag/Si interface which results from the large effective index of surface plasmon due to the nearly identical real parts with opposite signs of the permittivities of silver and silicon at 633 nm is analyzed. When the silicon shell has shallow protrusions of proper periodicity to meet the phase matching condition between the incident light and the surface plasmon wave at the Ag/Si interface, a higher scattered electric field and a higher sensitivity to the refractive index change of the surrounding medium can be achieved. The highest scattering electric field can be determined by the optimized average thickness of the protruded silicon nanoshell and silver core radius. Our novel concepts may provide a guideline for designing silver core/protruded silicon shell nanocylinder for biosensing applications.

Fig. 1 Schematic diagrams of (a) silver core-silicon shell nanocylinder, (b) silver core-protruded silicon nanoshell nanocylinder, and (c) solid silver nanocylinder. Fig. 2 Scattered electric field as a function of the surrounding medium for three cases with the inner radius 100 nm at 633 nm.

Fig. 3 Scattered electric field distributions for Case 2 at 633 nm with different incident angles.


Two Dimensional Electron Gases in Polycrystalline MgZnO/ZnO
Heterostructures Grown by RF-Sputtering Process

Professor I-Chun Cheng's group

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We study the formation of two-dimensional electron gas (2DEG) in rf-sputtered defective polycrystalline MgZnO/ZnO heterostructure via the screening of grain boundary potential by polarization-induced charges. The sheet resistance reduces rapidly and then saturates as the MgZnO thickness increases. Both the mobility and the sheet carrier concentration in saturation increase as the Mg content is raised and reach maxima of 39.4 cm2V-1s-1 and 1.15x1014cm-2 at x=0.4, confirmed by low temperature Hall measurement. Monte Carlo method by including grain boundary scattering effect as well as 2D finite-element-method Poisson and drift-diffusion solver is applied to analyze the polycrystalline heterostructure. The experimental and simulated saturation sheet resistance show good agreement. Both carrier concentration and mobility are temperature independent, indicating the formation of 2DEG with roughness scattering at the MgZnO/ZnO interface. Our result suggests that polycrystalline MgZnO/ZnO material system can be a potential candidate for the application of low-cost large-area high electron mobility transistor (HEMT) devices.

Figure 1.  Sheet resistance of polycrystalline MgxZn1-xO/ZnO heterostructure versus thickness of MgxZn1-xO cap layer for x=0.15, 0.2, 0.3, 0.4, and 0.5. The lines are guides to the eyes.

Figure 2. (a) Hall mobility and sheet carrier concentration of MgxZn1-xO/ZnO heterostructure in saturation as functions of Mg content x. (b) Comparison of experimental and calculated saturation sheet resistances as functions of Mg content x. The lines are guides to the eye.

Figure 3. Hall mobility and carrier concentration of Mg0.3Zn0.7O/ZnO heterostructure with Mg0.3Zn0.7O capping layer thickness of ~30 nm as functions of temperature.


Study of Carrier Dynamics and Radiative Efficiency in InGaN/GaN LEDs
with Monte Carlo Method

Professor Yuh-Renn Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

In a LED, carriers are injected from the contacts and diffuse laterally in the quantum wells (QWs). In this paper, we apply the Monte Carlo (MC) method to study the carrier dynamics in the InGaN QW. The radiative recombination rate is calculated by a self-consistent Poisson, drift-diffusion, and Schrodinger solver. We set different dislocation densities and trap cross sections of threading dislocation, and study the influences to the device efficiency by MC method. The effects of alloy scattering, charged dislocation scattering, interface roughness scattering, and e-e scattering are included in our MC model. The locations of dislocations are arranged randomly and are depending on dislocation densities. The electron will diffuse randomly by scattering in the QW before recombination. If the electron incidents into the location of the threading dislocation, it is captured by the threading dislocation and recombines non-radiatively. The diffusion ranges and the captured chances are influenced by many scattering mechanisms and will be discussed in this paper.

Our simulation result shows a strong dependence on the indium composition because of the longer radiative lifetime in the longer wavelength due to QCSE. The carrier diffusion length with higher In composition increases because of a longer radiative lifetime. However, the diffusion length in a higher In composition QW will decrease if the interface roughness scattering is very strong. The larger diffusion length leads to a higher capture rate. To estimate the internal quantum efficiency (IQE), we need to define the carrier capturing cross section diameter, δ. The published TEM results show the threading dislocation has a carrier trapping diameter around a few nanometer. Within these ranges, we start to estimate the expected IQE. We find that with a larger interface roughness, the IQE can be improved by a few percent. If the δ changes from 2nm to 4nm, the radiative efficiency will decrease 20% to 30%. If the dislocation density drops to 106 cm-2, the IQE can be improved significantly. Since most LEDs are the lateral LED, there would be a lateral electric field in QW with a larger applied bias. Our results show the efficiency will drop 30% to 70% depending on the In composition when applying a small lateral electric field. The carrier is easier to be captured if it is drifting with a certain direction rather than diffusing in a random direction. This indicates that non-radiative coefficient, A, is not a constant and should be strongly dependent on the bias, δ, and dislocation density. Our estimated IQE is compared with published experimental results and shows a good agreement in the trend. Our results show that for the longer wavelength source, we should try to decrease the radiative lifetime by reducing the QCSE with a nonpolar/semipolar structure. The other way is to reduce the dislocation density to be lower than 106 cm-2. However, both should be achieved to make the IQE reaching 100%.

Fig. 1 (a) Estimated maximum efficiency versus indium composition for different dislocation density values when n2d is 8x1012 cm-2. Also shown is a comparison with experimental results. (b) Estimated e±ciency versus indium composition with different dislocation densities for the nonpolar plane cases.


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