ieee journal of selected topics in quantum … · 2018. 7. 2. · ieee journal of selected topics...

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 23, NO. 5, SEPTEMBER/OCTOBER 2017 2000607

Enhanced Color-Conversion Efficiency of HybridNanostructured-Cavities InGaN/GaN Light-EmittingDiodes Consisting of Nontoxic InP Quantum Dots

Che-Yu Liu, Tzu-Pei Chen, Jhih-Kai Huang, Tzu-Neng Lin, Chia-Yen Huang, Xiu-Ling Li,Hao-Chung Kuo, Senior Member, IEEE, Ji-Lin Shen, and Chun-Yen Chang

Abstract—Color-conversion efficiency enhancement of hybridlight-emitting diodes (LEDs) by cadmium-free colloidal quantumdots (QDs) and a novel selective area nanocavities structure hasbeen demonstrated. Combining nanoimprinting and photolithog-raphy techniques, nanocavities array can be fabricated at desig-nated locations on the LEDs. The color-conversion efficiency of se-lective area nanocavities LED can be enhanced by up to 13%. Thesignificant color-conversion efficiency enhancement is attributed toresonance of InP QDs emission in nanocavities and nonradiativeenergy transfer from LED active layers to InP QDs, which has beeninvestigated and characterized by finite domain time-domain simu-lation, electroluminescence, and time-resolved photoluminescencemeasurements. This hybrid nanostructured device, therefore, ex-hibits a great potential for the applications of multicolor lightingsources and micro-LED.

Index Terms—Light emitting diodes, optoelectronic devices,nanotechnology, energy transfer, colloidal quantum dots.

I. INTRODUCTION

IN RECENT year, colloidal nanocrystals quantum dots(NQDs) have attracted intensive attention due to their high

quantum yield and photo-stability. By using colloidal NQDs,flexible, low-cost, large-area, and easy-processed fabricationsfor optoelectronic devices are enabled. The emission color ofNQDs can be tuned from visible to near-IR spectral rangeby either changing their size or chemical composition [1].Among versatile applications of NQDs, hybrid NQD–GaN

Manuscript received November 17, 2016; revised June 19, 2017 and July 26,2017; accepted August 31, 2017. Date of publication September 7, 2017; date ofcurrent version September 19, 2017. This work was supported by the Ministryof Science and Technology, Taiwan. (Corresponding author: Che-Yu Liu.)

C.-Y. Liu, T.-P. Chen, J.-K. Huang, C.-Y. Huang, and H.-C. Kuo are withthe Department of Photonics, Institute of Electro-Optical Engineering, Na-tional Chiao Tung University, Hsinchu 300, Taiwan (e-mail: [email protected]; [email protected]; [email protected];[email protected]; [email protected]).

T.-N. Lin and J.-L. Shen are with the Department of Physics, Chung-YuanChristian University, Taoyuan City 350, Taiwan (e-mail: [email protected];[email protected]).

X.-L. Li is with the Department of Electrical and Computer Engineering,University of Illinois at Urbana-Champaign, Champaign, IL 61801 USA(e-mail: [email protected]).

C.-Y. Chang is with the Department of Electronics Engineering, NationalChiao Tung University, Hsinchu 300, Taiwan (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2017.2749973

light emitting diodes (LEDs) are becoming a promising can-didate for highly efficient multicolor lighting [2], [3]. How-ever, the emission efficiency of NQDs radiatively pumped byLED is relatively low (<10%) due to the energy loss duringthe energy transfer process, such as waveguide leaky modelosses, light-scattering by the NQDs, and NQDs emission re-absorption by self-assembled NQDs clusters. In contrast to thisoptical pumping channel for radiative energy transfer, LEDscan also pump NQDs by non-radiative energy transfer (NRET),which is also known as Forster-like resonance non-radiative en-ergy transfer (FRET) [4]–[7]. Non-radiative energy transfer isa highly-efficient near-field interaction, which generally onlyoccurs when the distance between energy donor (for instance,LED active layers) and energy acceptor (for instance, NQDs) isless than 10 nm. Also, an overlap of donor emission spectrumand acceptor absorption spectrum is required. For standard hy-brid GaN-based LEDs, NQDs layer is usually capped on theindium tin oxide (ITO) layer, wherein the distant to LED activeregion are far beyond 10 nm.

Additionally, majority of studies and applications on color-conversion of hybrid light emitting devices are using NQDssynthesized with group II-VI materials, which usually containscadmium (Cd) metal and is highly toxic that can cause ecologicalissues. Synthesis of NQDs with alternative elements has beeninvestigated, for example, Group III-V (i.e., InP3 and InAs4)or Group IV (i.e., Si5) NQDs. Among the possible materialchoices, InP NQDs, in particular, have gained significant atten-tion, by their wide emission spectrum with tunable range fromthe visible region to the near-infrared (NIR) region [8]–[13],and their non-toxic nature.

In this work, we have demonstrated an innovative nano-cavities LED (NC-LED) and have achieved high color-conversion efficiency using non-toxic InP NQDs. These nano-cavities extend through multiple quantum wells (MQWs) andaccommodate InP NQDs, which enables the near-field FRET be-tween MQWs and InP NQDs close to MQWs. The nano-cavitiescan also enhance light extraction and avoid formation of largeNQDs clusters. The nano-structure can be limited within des-ignated area to prevent form severe loss of LED active region.The use of non-toxic InP NQDs also eliminates the concernof ecological hazards. Additionally, the pulsed-spraying coat-ing method utilized to dispense QD onto LED in this studyprovides a better uniformity of QD distribution comparing to

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2000607 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 23, NO. 5, SEPTEMBER/OCTOBER 2017

Fig. 1. Schematic diagrams. (a)–(d) Brief fabrication flow chart of thenano-cavities structured UV-LED by using NIL and photolithography.(e) Layer structure of a hybrid nano-cavities LED.

conventional methods for QD coating, such as drop-casting andspin-coating, hence the appearance of large size QD clusterscan be reduced. This technique has a great potential to use inhigh performance micro-LED application due to QDs high colorrendering index.

II. EXPERIMENT

Fig. 1(a)–(d) shows the schematic process flow of the fabri-cation of nano-cavities structured UV-LED. The process startswith conventional UV-LEDs, which were grown by metal or-ganic chemical vapor deposition (MOCVD). The respective lay-ers of a UV LED, from the bottom to the top, consists of a c-plane(0001) patterned sapphire substrate (PSS), a 50 nm GaN nucle-ation layer, a 2 μm un-doped GaN buffer layer, a 3 μm Si-dopedn-GaN layer, five pairs of InGaN/GaN MQWs with a centralwavelength of 380 nm and a 0.2 μm Mg-doped p-GaN layer.

Fabrication of nano-cavities arrays at selected area on UV-LEDs are subsequently proceeded using nano-imprint lithog-raphy (NIL) and photolithography techniques. A 400 nm-thickSiO2 layer was deposited on the surface of the UV LED waferby plasma-enhanced chemical vapor deposition (PECVD) fornano-cavities patterning. After that, nano-imprint lithography(NIR) was employed to define the nano-cavities pattern. A360 nm imprint-resist (IR) layer was coated onto the SiO2 layer

by spin coating at a rotational speed of 3000 r.p.m. Then, theIR was imprinted by an intermediate polymer stamp (IPS) at ahigh pressure of 40 bar, and exposed under a UV light with 10second for curing. After that the curing, the IPS can be removedby annealing at 65 °C leaving the transferred nano-cavities pat-tern on the IR layer. Next, the IR layer and SiO2 layer weredry-etched by reactive ion etching (RIE), respectively with O2plasma and CF4 plasma. SiO2 dielectric nano-cavities patternwith a 400 nm diameter were subsequently formed on the wafersurface. Then, a photo-resist (PR) layer with a thickness of2 μm was coated on the SiO2 dielectric layer by a spin coaterat a rotational speed of 3000 r.p.m. A standard photolithogra-phy method was utilized to fabricate a pattern of micro-cavitiesarrays on the PR layer. The LED wafer with a micro-patternedPR layer and a nano-patterned dielectric layer was etched byan inductively coupled plasma reactive ion etching (ICP-RIE)system with mixed process gases of Cl2 and BCl3 (Cl2 /BCl3 =20/10 sccm) at a bias power of 80 W and ICP power of 100 W.After the dry etching process, the residual PR layer and dielec-tric layer were removed by H2SO4 and H2O2 (H2SO4 : H2O2 =3:1). Finally, the nano-cavities structure uniformly distributed infive circular patterned areas were completed. The diameter anddepth of nano-cavities are 450 nm and 1.2 μm, respectively. Thepitches of nano-cavities array and the diameter of micro cavitiesare 750 nm and 50 μm, respectively [14], [15]. Previous studyalready shows the better efficiency of different etching depth, toget the better non-radiative energy transfer efficiency, the etch-ing depth need to approach the active region [19]. In this work,patterned micro-hole areas with nano-cavaties structured havebeen fabricated, combined with non-toxic InP quantum dots de-posit by pulsed-spraying methods have a potential to get betterefficiency and easier to be mass production.

To complete the LED chip, the mesa isolation with an areaof 300 × 300 μm2 was defined by using standard photolithogra-phy method and dry etching. In addition, a 0.24 μm indium tinoxide (ITO) thin film as contact layer was deposited by e-beamevaporation on top of the nano-cavities LED wafer, followedby deposition of 1.4 μm Cr/Pt/Au metal p-pad and n-pad elec-trodes onto the ITO layer and n-GaN layer surface, respectively.To fabricate nano-cavities structured UV LED with InP QDs(hybrid NC-LED), InP QDs were dispensed on the chip surfaceby pulsed-spraying (PS) coating, the central wavelength of InPQDs is about 530 nm. The monodisperse InP/ZnS core-shellnanocrystals were synthesized by using standard air-free proce-dures, and the InP/ZnS quantum dots were dispersed in toluenewith 10 mg/mL [11]. The pulsed-spraying method used to coatQDs was a promising method for the purpose to get uniformQDs layers. In traditional spraying techniques, spin coating ordeposited by pipette, the viscosity of the spraying mixtures canconsiderably affect the uniformity of the QDs layers, and theinteraction between the particles in the premixed solution cancause a gathering of the materials, and self-clustering can blockthe passage of the spray. PS method using two special designsto avoid this issue, that is, using the air-injection mechanismin the nozzle and intermittent spraying frequency (5 to 10 Hz),and using the constant stirring system. Thus, the target particles(QDs) can separate in the suspending solution more effectively,

LIU et al.: ENHANCED COLOR-CONVERSION EFFICIENCY OF HYBRID NANOSTRUCTURED-CAVITIES InGaN/GaN LEDs CONSISTING 2000607

Fig. 2. The calculate results for QDs under different emission wavelength.(a) The absorption coefficient of InP QDs varies with wavelength and the InPemission peak position. (b) The simulation results of relative output powerversus the emission wavelength from 350 nm to 450 nm. (c) The calculatetotal LEE ηtota l LEE (λ) of InP QDs versus the emission wavelength of thenano-cavities structure LED.

reducing the chances of QDs self-assembly, improved the con-version efficiency [3].

In this study, three kinds of samples had been fabricated forcomparison: conventional LED (C-LED), micro-cavities LED(MC-LED), and nano-cavities LED (NC-LED).

III. RESULTS & DISCUSSION

In order to enhance the emission of QD on a hybrid nano-cavity LED, the wavelength of the LED emission has to matchthe wavelengths of guiding mode of the nano-cavity arrays, anddeeply overlap with the absorption spectrum of QD. Fig. 2(a)shows the absorption coefficient of InP QD, in which it showsstronger absorption at wavelengths in deep UV region, whereinLED emission wavelength should be to obtain high color conver-sion efficiency. For the selection of LED emission wavelength,three dimensional (3D) finite domain time domain (FDTD)simulations using FullWAVETM software was conducted to cal-culate the electric field intensity of nano-cavity LED with dif-ferent LED emission wavelength. In the FDTD simulation, az-directional dipole source is positioned in the middle of theMQW active region. The diameter and the height of GaN nano-cavities are about 400 nm and 1.2 μm, while the period and thespacing are 700 nm and 300 nm, respectively.

As shown in Fig. 2(b), the relative output power is calculatedas a function of the emission wavelength spanning 350 nm to450 nm. Quite interestingly, the relative output power shows aperiodic behavior with emission wavelength dependence. Thisperiodic variation is attributed to the resonant modes of nano-cavities array [16]. The confinement of photons under resonancemode of nano-cavities array results in the valley of the relativeoutput power. The resonance also leads to peak relative output

Fig. 3. Cross-section scanning electron microscopy (SEM) and Scan-ning transmission electron microscopy (STEM) images of nano-cavities.(a) SEM image before and (b) after dispensing QDs by pulsed-spraying method.(c) shows STEM images of hybrid NC-LED that the QDs are successfully in-jected into the nano-holes with the zoom-in image shown in inset, same regionas the red frame indicated in (c). (d) shows the hybrid C-LED which couldclearly revealed that InP QDs were remote from active region.

power at emission wavelengths of 380 nm, 405 nm, and 440 nm.Therefore, with any of these three emission wavelengths, nano-cavities LED structure can perform the best light extractionefficiency (LEE).

However, this result stands only as the NC-LED is solely con-sidered, without the participation of QDs. We should practicallyconsider the effect of QDs properties as the NC-LED will be in-tegrated with QDs. Given that the absorption coefficient of InPQDs varies with wavelength, as shown in Fig. 2(a), the relativeLEE ηLEE(λ) of hybrid NC-LEDs should be expressed as:

ηLEE (λ) × α (λ) ∝ ηtotal LEE (λ) (1)

where ηLEE(λ) is the calculated light extraction efficiency ofnano-cavities structured LED, and α(λ) is the InP QDs ab-sorption spectrum. In Fig. 2(c), total light extraction efficiencyηLEE(λ) of the nano-cavities structured LED is plotted as afunction of the emission wavelength. Due to the fact that InPQDs have a higher absorption at UV region, and that ηLEE hasthe largest value at 380 nm, the nano-cavities structure LEDwith 380 nm emission wavelength can optically pump InP QDsmost effectively. As a result, a 380 nm NC-LED integratingwith pulsed-spraying InP QDs have been choice to achieve highcolor conversion efficiency hybrid NC-LED. In this research,LED with 380 nm wavelength have been growth for furtherexperiment, due to the simulation results shows in Fig. 2.

Fig. 3(a) and (b) shows cross-section scanning electron mi-croscopy (SEM) images of nano-cavities structure before andafter InP QDs deposition.


Fig. 4. Photoluminescence (PL) and time resolved results. (a) shows theintensity was enhanced by up to 30.14% by usnig nano-cavities structure.(b) C-LED with/without InP QDs, and (c) NC-LED with/without InP QDs.As the results, the time decay rate decrease in hybrid NC-LED, these resultsshow the hybrid MHNR-LED produce another decay channel.

The GaN nano-cavities are found to be 460 nm in diameterand 1.1 μm in depth. The diameter of the nano-cavities decreaseswhen the etching depth goes deeper than 600 nm, as shown inFig. 3(a). This is due to the fact that the depth of nano-cavitiesis too deep for the CF4 etching gas to reach the bottom ofnano-cavities, which causes inadequate etch at the bottom ofnano-holes and hence the tooth shape cavity profile.

For a deeper look into the spatial distribution of nano-cavitiesand PS InP QDs, scanning transmission electron microscope(STEM) images have been obtained as shown in Fig. 3(c) and(d). As shown in Fig. 3(c), it can be clearly seen that InP QDshave covered the entire side-wall area of the nano-cavities, in-cluding active region of the LED, which allows InP QD to dis-tribute next to MQW that enables NRET. For the conventionalLEDs, on the contrary, the distance between InP QDs and activeregion is nearly 150 nm as shown in Fig. 3(d) and allows onlyradiative energy transfer. The other benefits of this hybrid struc-ture is the enhancement of light extraction. It is well known thatthe GaN-based LED has a poor light extraction efficiency dueto the significant refractive index difference between GaN andair. The additional QDs filling up the nano-holes have refractiveindex with value between those of air and GaN therefore cangreatly help extract the emission light from the active region ofa hybrid NC-LED.

Photoluminescence (PL) measurement had been used toinvestigate the optical color-conversion efficiency enhance-ment. The results are shown in Fig. 4(a). The intensity ofInP QDs luminescence of nano-structured LED is enhancedby up to 30.14% comparing to those of planar LED. Thisenhancement is attributed to the light extraction enhance-ment caused by nano-cavities, and FRET between MQWs andthe adjacent QDs, which has been verified by time-resolved

photoluminescence (TRPL) measurement illustrated as the fol-lows. The laser source used in this experiment was tunablefento-second pulse Ti:sapphire laser with triple-frequency mod-ules on the optical path, the wavelength used in this experimentwas 266 nm, and the power was 5 mW. The photons was col-lected by photomultiplier tube (PMT).

Owing to the addition route to transfer the energy, the carrierdecay time should be different for both donor (MQWs) andacceptor (QDs) that undergo NRET. The lifetime of MQWsshould be faster as the additional FRET process accelerate tototal rate of energy pumping to QDs. The lifetimes of MQWs forboth C-LED and NC-LED were investigated. Fig. 4(b) shows thedecay time of C-LED without InP QDs (purple square) and withInP QDs (green triangle). It is found that the decay time of theC-LED has not changed after coated with InP QDs, revealing noadditional energy transfer channel. On the contrary, an obviouschange between the decay time of the NC-LED before (purplesquare) and after (green triangle) coated with InP QDs has beenobserved, indicating the existence of FRET as shown in Fig. 4(c).To calculate the non-radiative energy transfer rate, the lifetimesof the PL decays have been obtained by the curve fitting usingthe following equations [17].

IQW (t) = Ae−kQ Wt (2)

IHQW (t) = Ae−kQ Wt + Be−(kQ W +kE T )t (3)

where IQW (t) and IHQW (t) are the intensities of UV emission

peak for LED and hybrid LED, respectively. kQW and kET

are the rates coefficients of MQWs emission decay and non-radiative energy transfer, respectively. A and B are the con-tributions of the dipole pairs that do not participate and doparticipate the energy transfer, respectively. Form the aboveequations, the obtained kQW and kET are about 0.331 ns−1 and0.599 ns−1, respectively. These values can lead to the efficiencyof non-radiative energy transfer (ηET ) by the following equation[18]–[20].

ηET ≈ QYN CkET

kQW(4)

where QYN C is the quantum yield of the InP QDs and is nearly18% in this case. The non-radiative energy transfer efficiencyis calculated to be around 32.6% for hybrid NC-LED. The en-hancement of QDs color-conversion efficiency were caused byhigh non-radiative energy transfer efficiency achieved by FRETand the nano-cavities which enhanced the light extraction.

To observe far field light emission pattern of the emissionpeak of QDs on both devices under current injection, the beamview measurement has been obtained and shown in Fig. 5. Dur-ing the measurement, long pass filter has been used to filterout the 380 nm UV LED emission light. Therefore, the lightfield distribution of QDs could be revealed under UV LED ex-citation. The measured chip size is about 300 × 300 μm2, andthe operation current was 20 mA. The beam view images inFig. 5(a)–(c) show separated-area of high intensity QD emis-sion on nano-cavities LED, majorly concentrated at the nano-cavities arrays, while C-LED and MC-LED generally showsweaker QD emission. Fig. 5(d) shows cross-sectional intensity


Fig. 5. Light emission pattern images of InP QDs of (a) hybrid C-LED,(b) hybrid MC-LED and (c) hybrid NC-LED with UV filter, showing the inten-sity difference between three samples, and (d) cross-section intensity profile ofbeam view images of hybrid C-LED, MC-LED and NC-LED.

Fig. 6. The electroluminescence emission peak of (a) C-LED (back solid line)and hybrid C-LED (red solid line), (b) MC-LED (back solid line) and hybridMC-LED (red solid line), and (c) NC-LED (back solid line) and hybrid NC-LED(red solid line) under 20 mA operating currents.

profile along the centers of the circular pattern areas. QD emis-sion shows the strongest intensity on NC LEDs. It is due to thefact that nano-cavities structure can effectively increase the con-tact area between QDs and the NC-LED active region, whereFRET is enabled. The highest intensity shown in the edge ofnano-cavities area was attributed to the edge of ITO where hadmore QDs, but the highest intensity do not means the higherconversion efficiency. In particular, a decrease in QD emissionhas been observed in the pattern area of MC-LED. This is causedby the self-assembled QD clusters formed in the micro-cavities[19]–[21].

To investigate the color-conversion efficiency of hybridC-LED, MC-LED and NC-LED, electroluminescence (EL)spectra have been obtained and shown in Fig. 6(a)–(c). Withthe EL intensity of the LEDs, color-conversion efficiency canbe calculated by using the following equation [17], [24]:

ηconversion = αout−coupling ×∫ IH

QDs

∫ IUV−LED − ∫ IHUV−LED

(5)

where ηconversion is the color-conversion efficiency,αout−coupling is the coefficient of photons escaping froma LED after photons being generated, ∫ IH

QDs is the integratedQDs emission peak area of the EL spectrum of hybrid LED,∫ IUV LED is the integrated UV emission intensity peak areaof LED without QDs, and ∫ IH

UV LED is the integrated UVemission intensity peak area of hybrid LED. The calcu-lated color-conversion efficiencies of hybrid C-LED, hybridMC-LED and hybrid NC-LED are 8.4%, 7.1% and 11.2%,respectively.

Compared with C-LED and MC-LED, the color-conversionefficiency of NC-LED shows enhancement of 13% and 22.4%,respectively. The higher color-conversion efficiency of NC-LEDreveals improved extraction of QDs emission, which is achievedby the decreased size of QD clusters to avoid re-absorption, andthe enhanced QDs absorption by extract LED emission withinthe nano-cavities layer. Above all, the most dominant enhance-ment mechanism is FRET between InP QDs and active regionof LED. However, relative conversion efficiency enhancementdecreases with increasing injection current, which could beenunderstood by the increasing heat and Coulomb screening athigh injection current, which leads to exciton dissociation intofree electron–hole pairs. The decreased of relative conversionefficiency enhancement under high current injection also revealsthe saturation of QDs excitation [22]. Although InP QDs com-bined with NC-LED didn’t had long life time due to InP QDsefficiency decrease when it exposed to air, it still had potentialfor future development of environmentally friendly optoelec-tronic devices when QDs synthesize technique improve.

IV. CONCLUSION

In this work, we have successfully demonstrated a highcolor-conversion efficiency hybrid NC-LED fabricated bynano-imprint lithography (NIL), photolithography and pulsedspraying techniques. The nano-cavities structure could greatlyenhance the light extraction efficiency at specific wavelengthsdecided by the diameter and pitches of the structure. Based onthe results of numerical calculation, LED emission wavelengthof 380 nm was chosen due to the high light extraction efficiencyat 380 nm by the nano-structure, and strong absorption of InPQDs. Beam view images reveal that the nano-cavities can in-crease the light output of InP QDs through the decrease of totalinternal reflection at the device surface. The color-conversionefficiency enhancement of hybrid NC-LED is found to be about13% and 22.4% compared with that of hybrid C-LED and hy-brid MC-LED. The result of TRPL measurement shows thatnon-radiative energy transfer (NRET) plays an important rolein the scheme of color-conversion efficiency enhancement of hy-brid NC-LED, while NRET is absent on C-LED. The fact thatNRET is only observed on NC-LED is due to that, in the hybridNC-LED, the QDs in the nano-cavities are tightly close to theMQWs active layers, increasing the near-field FRET coupling.Our first proof-of-concept demonstration of using nontoxic InPQDs in hybrid NC-LED to enhance color-conversion efficiencycan provide a useful new route for the future developmentof environmentally friendly optoelectronic devices with highperformance.


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Che-Yu Liu received the B.S. degree in electrical engineering from NationalCentral University, Taoyuan, Taiwan, in 2010, and the M.S. degree in 2012from the Department of Photonics, National Chiao Tung University, Hsinchu,Taiwan, where he is currently working toward the Ph.D. degree. His researchinterests include the epitaxy of III-V compound semiconductor materials byMOCVD and analysis for GaN-based light-emitting diodes.

Tzu-Pei Chen received the B.S. degree from the Department of Physics, Na-tional Chung Hsing University, Taichung, Taiwan, in 2013. Her research in-terests include photoluminescence measurement and materials analysis, opticalsimulation, and characterization for high-power light-emitted diodes.

Jhih-Kai Huang received the B.S. degree from the Department of ElectricalEngineering, National Central University, Taoyuan, Taiwan, in 2007, and theM.S. degree in electro-optical engineering from National Chiao Tung University,Hsinchu, Taiwan, in 2009. He is currently working toward the Ph.D. degree atthe Institute of Electro-Optical Engineering, National Chiao Tung University.His research interests include GaN-based device fabrication and nano-imprinttechnology and nanostructure process for GaN-based light-emitting diodes.

Tzu-Neng Lin was born in 1984, Taichung, Taiwan. He received the B.S. degreein physics in 2008, the Master’s degree in physics in 2011, and the Ph.D. degreein physics in 2015 from Chung Yuan Christian University (CYCU), Taoyuan,Taiwan. He is currently a Postdoctorate Associate at CYCU. His research interestinclude the synthesis, characterization, and applications of nanomaterials and/orquantum dots.

Chia-Yen Huang received the Ph.D. degree in University of California, SantaBarbara, CA, USA. He is currently working in the Department of Photonics,National Chiao Tung University, Hsinchu, Taiwan. His research interests includenitride-based material, semiconductor lasers, and light-emitting diodes.

Xiu-Ling Li received the B.S. degree form Peking University, Beijing, China,and the Ph.D. degree from the University of California at Los Angeles, LosAngeles, CA, USA. Following Postdoctoral positions at California Instituteof Technology and University of Illinois, as well as industry experience atEpiWorks, Inc., she joined the faculty of the University of Illinois in 2007 as anAssistant Professor in the Department of Electrical and Computer Engineering.She was promoted to Associate Professor with tenure in 2012, and to Professorin 2015. Her research interests include the area of nanostructured semiconductormaterials and devices for applications in electronic, photonic, and biomedicalapplications. She has published more than 120 journal papers and holds morethan 20 patents. Her honors and awards include NSF CAREER award, DARPAYoung Faculty Award, ONR Young Investigator Award, and IEEE Fellow. Sheserved on the Board of Governors of the IEEE Photonics Society, and technicalprogram committees of several international conferences. She is also a DeputyEditor of Applied Physics Letters.


Hao-Chung Kuo (S’98–M’99–SM’06) received the B.S. degree in physicsfrom the National Taiwan University, Taipei, Taiwan, in 1990, the M.S. degreein electrical and computer engineering from Rutgers University, Camden, NJ,USA, in 1995, and the Ph.D. degree in electrical and computer engineeringfrom the University of Illinois at Urbana-Champaign, Urbana, IL, USA, in1999. He has an extensive professional career both in research and industrialresearch institutions. From 1995 to 1997, he was a Research Consultant withLucent Technologies, Bell Lab, Holmdel, NJ. From 1999 to 2001, he was anR&D Engineer with the Fiber-Optics Division, Agilent Technologies. From2001 to 2002, he was the R&D Manager with LuxNet Corporation. SinceSeptember 2002, he has been a member of the faculty at the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan. He hasauthored or coauthored more than 60 publications. His current research interestsinclude the epitaxy, design, fabrication, and measurement of high-speed InPandGaAs-based vertical-cavity surface-emitting lasers, as well as GaN-based light-emitting devices and nanostructures.

Ji-Lin Shen received the Ph.D. degree in Physics from National Taiwan Uni-versity, Taiwan in 1994. From 1995 to 1997, he was a post-doctoral researchin Electrical Engineering Department at UCLA, USA. In 1998, he joined thefaculty of the Physics Department, Chung-Yuan Christian University, Taoyuan,Taiwan, and is currently a Professor. His current research interests include op-tical characterization of semiconductors and nanomaterials.

Chun-Yen Chang was born in Feng-Shan, Taiwan. He received the B.S. degreein electrical engineering from the National Cheng Kung University (NCKU),Tainan, Taiwan, in 1960, and the M.S. and Ph.D. degrees from the National ChiaoTung University (NCTU), Hsinchu, Taiwan, in 1962 and 1969, respectively. Hehas profoundly contributed to the areas of microelectronics, microwave, and op-toelectronics, including the invention of the method of low-pressure MOCVDusing triethylgallium to fabricate LED, laser, and microwave devices. He pi-oneered works on Zn incorporation (1968), nitridation (1984), and fluorineincorporation (1984) in SiO2 for ULSIs, as well as in the charge transfer insemiconductor–oxide–semiconductor system (1968), carrier transport acrossmetal–semiconductor barriers (1970), and the theory of metal–semiconductorcontact resistivity (1971). In 1963, he joined the NCTU to serve as an In-structor, establishing a high vacuum laboratory. In 1964, he and his colleaguesestablished the nation’s first and state-of-the-art Semiconductor Research Cen-ter, NCTU, with a facility for silicon planar device processing, where theymade the nation’s first Si planar transistor in April 1965 and, subsequently, thefirst IC and MOSFET in August 1966, which strongly forms the foundationof Taiwan’s hi-tech development. From 1977 to 1987, he single-handedly es-tablished a strong electrical engineering and computer science program at theNCKU, where GaAs, α-Si, and poly-Si research was established in Taiwan forthe first time. He consecutively served as the Dean of Research (1987–1990),the Dean of Engineering (1990–1994), and the Dean of Electrical Engineeringand Computer Science (1994–1995). Simultaneously, from 1990 to 1997, heserved as the Founding President of the National Nano Device Laboratories,Hsinchu. Since August 1, 1998, he has been the President with the Institute ofElectronics, NCTU. In 2002, to establish a strong system design capability, heinitiated the “National program of system on chip,” which is based on a strongTaiwanese semiconductor foundry. He is a Member of Academia Sinica (1996)and a Foreign Associate of the National Academy of Engineering, USA (2000).He received the 1987 IEEE Fellow Award, the 2000 Third Millennium Medal,and the 2007 Nikkei Asia Prize for Science category in Japan and regarded as“the father of Taiwan semiconductor industries.”

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