Materials and Technologies for Solid State Lighting
Session FI-1 - Material Growth and Processing
FI-1:IL01 The Transition to Solid State Lighting: Trends and Challenges
B. WESSLER, OSRAM GmbH, Munich, Germany
The global lighting industry is in the middle of a transition towards solid-state lighting, i.e. light-emitting diodes (LED) and organic light emitting diodes (OLED).
The first SSL transition phase mainly driven by energy saving is focussed on the replacement of existing light sources. It faces the challenge of reaching higher performance as the counterparts from the traditional world at affordable initial cost.
Advancements in materials and processing were and are the key enablers for realising the steep performance increase in LED technology but also for bringing cost down to facilitate market entry.
Initially the focus was on brightness and efficacy increase of the LED chip, i.e. III-V semiconductors, phosphors and packaging materials. Today materials along the whole products value chain up to SSL systems are intensely investigated, e.g. novel composites for heat sinks or highly transparent polymers for the optics. Processing covers epitaxial growth of nanostructured LEDs up to injection molding of fixtures.
Materials and processing innovations of the last years will be presented with respect to the whole SSL product value chain as well as an outlook to the future trends and challenges regarding the transformation to intelligent lighting networks.
FI-1:IL03 Fabrication of Nitride Devices by Pulsed Sputtering Deposition
H. FUJIOKA, Institute of Industrial Science, The University of Tokyo, Tokyo, Japan; CREST-JST, Chiyoda-ku, Tokyo, Japan
Group III nitride semiconductors are regarded as promising materials for high performance optical and electron devices. However, it is well known that nitride devices are very expensive because their fabrication process involves low throughput high temperature MOCVD or MBE epitaxial growth. To solve this problem and fabricate low-cost GaN devices, we have to utilize a highly productive crystal growth technique. We have recently developed a new growth technique called PSD (pulsed sputtering deposition) and found that it allows us to obtain device quality III nitrides even at room temperature. PSD has already attracted much attention of industry engineers because its productivity is much higher than that of conventional MOCVD. In this technique, surface migration of the film precursors is enhanced and, therefore, the temperature for epitaxial growth is dramatically reduced. This reduction allows us to utilize various large area low cost substrates such as metal foils and graphene sheets that have never been used for growth of semiconductors due to their chemical vulnerability. In this presentation, we will demonstrate successful epitaxial growth of GaN and operation of various GaN based devices such as RGB full colar LEDs, AlGaN/GaN HEMT, and solar cells on low-cost substrates.
FI-1:IL04 Universal Bipolar Host Materials for Highly Efficient Electrophosphorescent Devices
KEN-TSUNG WONG, Department of Chemistry, National Taiwan University, Taipei, Taiwan
The introduction of heavy metal-based phosphorescent emitters in OLEDs is a major breakthrough for improving the electroluminescence efficiency to 100% internal quantum efficiency (IQE). The high IQE is based on the use of the host-guest strategy with triplet emitter (guest) homogeneously dispersed into a suitable organic matrix (host). This method can suppress the detrimental effects such as aggregation quenching and/or triplet-triplet annihilation of phosphors. As a consequence, the selection of host materials is of great importance for highly efficient OLEDs. Recent research trends have shifted to the development of host materials possessing bipolar property, which can give balance carrier transport, well-defined electron-hole recombination zone within the emitting layer and reduced efficiency roll-off. The physical properties of bipolar host materials can be manipulated by rational molecular design with the judicious selection of hole-transporting (HT) and electron-transporting (ET) subunits and their linking topology. In this conference, our recent efforts on the development of carbazole-based bipolar host materials equipped with various ET-type functional moieties suitable for efficient red, green, blue and even white electrophosphorescent devices will be reported.
FI-1:IL05 Semipolar GaN-based Optoelectronic Structures on Large Area Substrates
F. SCHOLZ, T. MEISCH, M. CALIEBE, Institute of Optoelectronics, University of Ulm, Ulm, Germany; B. NEUSCHL, K. THONKE, Institute of Quantum Matter, University of Ulm, Ulm, Germany
Green light emitting diodes based on group-III nitrides still suffer from fairly low performance. This may be partly due to the formation of huge piezoelectric fields within the strained GaInN quantum wells in such devices separating electrons and holes locally leading to a reduced recombination probability. Hence epitaxial growth in non- or semipolar directions as opposed to the conventional polar c-direction may be a solution. We currently investigate an approach which enables the realization of such structures on cheap, large area foreign substrates like sapphire. In order to take advantage of the well-established c-plane growth, we prepare inclined c-plane side-facets by etching grooves into adequately oriented sapphire wafers as nucleation sites for GaN, which are then put into our MOVPE reactor. After coalescence of these striped nitride structures, they form large area planar semipolar nitride surfaces on which quantum well and LED structures can be grown. The formation of the commonly observed defects like dislocations and stacking faults was reduced by various methods leading eventually to excellent GaInN quantum well properties. The semipolar GaN layers can be further improved by increasing their thickness by hydride vapour phase epitaxial growth.
FI-1:IL06 AlGaN Growth for Electron-Beam-Excitation Ultraviolet Light Source
H. MIYAKE1, F. FUKUYO1, 2, K. HIRAMATSU1, Y. KOBAYASHI2, 1Department of Electrical and Electronic Engineering, Mie University, Japan; 2Hamamatsu Photonics K.K., Japan
We performed the growth of Si-doped AlGaN and AlGaN/AlGaN multiple-quantum wells (MQWs) by low-pressure MOVPE. We fabricated Si-doped Al0.60Ga0.40N / Al0.75Ga0.25N MQWs with thickness of 600 nm after sequentially depositing an Al0.85Ga0.15N buffer layer, Al0.80Ga0.20N buffer layer and Si-doped Al0.75Ga0.25N buffer layer on the AlN/sapphire templates. A prototype ultraviolet-light-source tube was fabricated with an AlGaN film used as a target for electron-beam (EB) excitation.
The deep-UV light output power and conversion efficiency of the AlGaN MQW target for EB pumping voltage of 10kV were evaluated. We confirmed that the deep-UV light output power was 16 mW at a wave length of 256 nm, when the EB input power of 2W, and that the conversion efficiency was 1% at the EB input power of 1W.
FI-1:IL07 Novel Blue Fluorescent Organic Emitters Based on Dual Core Chromophores for Highly Efficient OLED Device
JONGWOOK PARK, Department of Chemistry, Catholic University of Korea, Yokkog, Bucheon, Kyunggi, Korea
I will introduce recent OLED material technology and the related issues including TV and lighting applications. The novel chemical structures based on dual core chromophores for blue emission in OLEDs will be also shown. One of the derivatives, TP-AP-TP, exhibits a high luminance EQE value of 7.51% and twice the lifetime of a commercialized material, MADN. The dual core chromophore materials have basically narrower PL and EL spectra and better color purity than single core chromophore compounds. Also, it has higher thermal property than the single core chromophore materials. The molecular design and synthesis as well as the device performance of novel organic molecules for highly efficient blue emission will be discussed.
FI-1:IL08 DERI Method; Possible Approach to Longer Wavelength Light Emitters Based on NitrideSemiconductors
Y. NANISHI1, T. YAMAGUCHI2, T. ARAKI1, 1Ritsumeikan University, Kusatsu, Japan; 2KogakuinUniversity, Hachioji, Japan
Developments of longer wavelength Light Emitting Devices based on InN and In-rich InGaN were hampered mainly due to relatively poor quality of these material systems. We have developed a new RF-MBE growth method called DERI (Droplet Elimination by Radical Beam Irradiation) for growth of these materials. This growth method consists of two series of growth steps with In-rich growth step (MRGP: Metal Rich Growth Process) and consecutive nitrogen radical beam irradiation step (DEP: Droplet Elimination Process). This method enabled us to obtain flat and high quality InN reproducibly without precise control of V/III ratio.
As DERI process is carried out under almost thermal equilibrium condition, as conventional LPE, InGaN tends to make phase separation under highly metal rich growth condition. Using this phenomenon positively, we have successfully obtained InN/InGaN, InGaN/InGaN MQW structures, which emitted strong PL at IR and green wavelength range, respectively.
In order to realize InGaN LED covering full wave length between GaN to InN on one specific substrate, we should suppress electrical or optical adverse effects of generated misfit dislocations due to 11% lattice mismatch. For this purpose, we propose a new way to suppress dislocation effect by using this phase separation phenomenon, growing wider band gap material surrounding dislocation cores.
FI-1:IL09 Development of AlN-based Technology for Deep UV Light Sources
Z. SITAR1, 2, B. MOODY1, S. CRAFT1, R. SCHLESSER1, R. DALMAU1, J. XIE1, S. MITA1, T. RICE2, J. TWEEDY2, J. LEBEAU2, L. HUSSEY2, R. COLLAZO2, B. GADDY2, D. IRVING2, 1HexaTech, Inc., Morrisville, North Carolina, USA; 2Dept. of Materials Science and Engineering, North Carolina State Univ., Raleigh, USA
For the first time in history of III-nitrides, the availability of low defect density (<103 cm-2) native AlN substrates offers an opportunity for growth of AlGaN alloys and device layers that exhibit million-fold lower defect densities than the incumbent technologies and enable one to assess and control optical end electrical properties in absence of extended defects. Epi-ready AlN wafers are fabricated from AlN boules grown by physical vapor transport at temperatures between 2200 and 2300°C. Gradual crystal expansion is achieved through a scalable, iterative re-growth process in which the high crystal quality is maintained over many generations of boules. Despite the excellent crystal quality, below bandgap optical absorption bands in the blue/UV range affect the UV transparency of wafers. We use density functional theory (DFT) to develop a model to understand the interplay of point defects responsible for this absorption. We show a direct dependence of the mid-gap absorption band with the carbon concentration within the AlN. Low defect density AlN and AlGaN epitaxial films are grown upon these wafers that exhibit superior optical properties in terms of emission efficiency and line width and can be doped with an efficiency that is several orders of magnitude higher than possible in technologies using non-native substrates. UV LED structures and Schottky diodes were fabricated on these materials that exhibit low turn-on voltages and breakdown fields greater than 10 MV/cm. This presentation will review state-of-the-art of AlN-based technology and give examples of potential applications in future devices and contrast these with other wide bandgap technologies.
FI-1:L11 Monolithic White-light LED Based on GaN Doped with Beryllium
H. TEISSEYRE1, 2, M. BOCKOWSKI2, S. GRZANKA2, I. GRZEGORY2, A. KOZANECKI1, B. DAMILAN2, YA. ZHYDACHEVSKII1, M. KUNZER4, K. HOLC4, U.T. SCHWARZ4, 1Institute of Physics PAS, Warsaw, Poland; 2Institute of High Pressure PAS, Warsaw, Poland; 3CRHEA-CNRS, Valbonne Sophia Antipolis, France; 4Fraunhofer Institute for Applied Solid State Physics IAF, Freiburg, Germany
We present a novel concept concerning the building of monolithic white light emitting diode based on gallium nitride substrates doped with beryllium. So far most of the studies on GaN doped with beryllium have mainly concentrated on possible p type doping. Unfortunately the realisation of p type conductivity in GaN:Be appeared to be difficult. It seems, however, that due to very intensive yellow luminescence GaN:Be could be used as white light converters. At present, the most common method of fabricating a white light emitting diode is to cover a blue emitting diode based on InGaN with phosphor (YAG:Ce).
For preliminary studies small crystals doped with beryllium grown by High Nitrogen Pressure Solution Growth method (HNPS) were used. Firstly, optical characterisation of such crystal was performed. It is visible from these results that due to strong beryllium related absorption around 400 nm yellow emission can be effectively pumped. Then we performed characterisation of the diode with GaN:Be substrate as a converter. The high value of the observed Color Rendering Index, gives hope for obtaining an effective light converter based on this method.
FI-1:L12 Development of Organic and Inorganic Phosphors for White Emitting Blue and Near UV LEDs
G. CHADEYRON, R. BOONSIN, J-P. ROBLIN, D. BOYER, R. MAHIOU, Clermont Université, Université Blaise Pascal, Ecole Nationale Supérieure de Chimie, Institut de Chimie (ICCF), Clermont-Ferrand, France
In most WLEDs, rare-earths doped oxides are used as light converting materials in a single diode chip to obtain the targeted emission. Current commercial WLEDs use a 460 nm blue GaN LED chip covered by a Y3Al5O12:Ce3+ yellowish phosphor coating. However, this association suffers from some weaknesses such as a poor color rendering index (CRI) and a low stability of color temperature. Moreover, a too cold color temperature for indoor domestic lighting is reached due to a lack of red contribution. These drawbacks prevent this association "blue LED/phosphor" from penetrating the general public lighting market and in particular highly valued indoor domestic lighting which requires a CRI better than 90, a color temperature comprised between 3000 and 4000 K with a luminous efficiency superior or equal to 150 lumen per watts. Combining deep-ultraviolet (DUV - 200 nm ≤ λem ≤ 300 nm) or ultraviolet (UV - 300 nm ≤ λem ≤ 400 nm) diode chips with a mixture of red, green and blue phosphors to produce white light appear as a promising alternative with several advantages. The potentialities of application of phosphors synthesized by soft chemistry in lighting devices based on each kind of LED/phosphors association will be presented.
FI-1:L13 Steering the Si Doping of the Ultimate Solid-state Semiconductor AlN
A. KAKANAKOVA-GEORGIEVA, R.B. DOS SANTOS, Department of Physics, Chemistry and Biology (IFM), Linköping University, Sweden; R. RIVELINO, F. DE BRITO MOTA, Instituto de Física, Universidade Federal da Bahia, Brazil; G.K. Gueorguiev, Department of Physics, Chemistry and Biology (IFM), Linköping University, Sweden
AlN is being established as the ultimate solid-state semiconductor (wide-band-gap ~ 6 eV) binding the development of AlN-based deep-UV LEDs, which is driven by social (e.g., technologies for portable water/air purification) and market values. We contribute to the understanding of material growth and n-type doping of Al(Ga)N by exploring several approaches: (i) study of common Si donor and major O impurity incorporation kinetics corroborated by electron paramagnetic resonance measurements (e.g., A. Kakanakova-Georgieva et al., Appl. Phys. Lett. 102 (2013) 132113); and (ii) theoretical investigation of pertinent gas-phase chemistry driven by the dopant precursor silane under the conditions of MOCVD of Si-doped AlN (e.g., R.B. dos Santos et al., submitted). Following (i), the presentation will give an introductory example on the complex impact of Si and O on the n-type conductivity of Al(Ga)N. Essential implication for the Si incorporation into AlN and consequently for the n-type doping control is anticipated by the availability of stable dopant species with direct N-Si, alternatively, Al-Si bonding. These species are generated following a network of viable gas-phase reaction routes studied by employing density-functional-theory calculations (ii).
Session FI-2 - Electro-optical Characterization
FI-2:IL01 Advanced Characterization Methods of InGaN-based LEDs
JONG-IN SHIM, HYUNSUNG KIM, DONG-SOO SHIN, Hanyang University, ERICA Campus, Ansan, Korea
During the last two decades, light-emitting diodes (LEDs) based on InGaN material systems have been successfully developed mostly by heuristic approaches. Nowadays, however, their optoelectronic performances are hardly improved by utilizing such methods so that in-depth analysis and characterization revealing operational physics are necessary. For this purpose, we have developed systematic characterization techniques such as the current-voltage (I-V), the current-light power (I-P), the capacitance-voltage (C-V), the photocurrent (PC) and electroreflectance (ER) spectroscopies, the temperature-dependent electroluminescence (TDEL), and the internal quantum efficiency (IQE) measurements. In this work, by comprehensively applying these techniques, we quantitatively investigate how the growth temperature (Tg) of the p-GaN layer in InGaN-based blue LEDs affects optoelectronic characteristics. Experiments with changing Tg confirm that 1) the reduced optical loss via improved crystal quality of the p-GaN layer and 2) the fine control of the Mg diffusion from the p-GaN layer to InGaN/GaN MQW are the key factors for good performances. Moreover, a method estimating the Mg diffusion length is demonstrated by analyzing both the C-V curve and the ER spectra under reverse biases.
FI-2:IL02 Tailoring Electro-optical Properties of Cyclometalated Iridium Complexes
E. BARANOFF, School of Chemistry, the University of Birmingham, Edgbaston, Birmingham UK
Cyclometalated iridium(III) complexes possess unique photophysical properties, namely high phosphorescence quantum yields and relatively short triplet excited state lifetime. As a result of these characteristics, they have attracted considerable interest for use as dopants in organic light-emitting diodes (OLEDs) and virtually any application requiring highly phosphorescent materials.
The possibility to tune the emission maximum over the entire visible spectrum is one major feature of such emitters. Control the color of emission involves not only the control of the emission maximum, but also the control of the broadness of the spectra. In this talk I will summarize our progress in emission color engineering. Furthermore I will briefly present some possibilities offered by the strategies developed for color tuning such as quasi-independent tuning of emission maxima and redox potentials, independent tuning of the onsets of absorption and emission spectra, and advances toward white emitting single center emitters.
FI-2:IL03 Aberration Corrected High Resolution TEM Characterisation of Nitrides Materials and Devices
M. ALBRECHT, T. SCHULZ, T. MARKURT, T. REMMELE, Leibniz-Institut für Kristallzüchtung, Berlin, Germany; A. DUFF, J. NEUGEBAUER, Max-Planck-Institut für Eisenforschung, Germany; F. NIPPERT, G. CALLSEN, A. HOFFMANN, Technische Universität Berlin, Fakultät II, Institut für Festkörperphysik Sekretariat EW 5-1, Berlin, Germany
The role of compositional fluctuations on carrier localization in III-Nitride based alloys is one of the founding myths of nitride semiconductor research. Their role in the radiative efficiency in III-Nitride based heterostructures is debated since first III-Nitride based heterostructures with high dislocation densities have been used in highly efficient devices. Despite huge efforts in theory and experiment it is still controversial whether the In distribution in an InGaN quantum well is truly random or if small In enriched clusters are present.
The progress in aberration corrected transmission electron microscopy made in the last years offers new possibilities to study alloy fluctuations at atomic scale and with high precision. In this presentation we will summarize our research obtained by aberration corrected transmission electron microscopy and scanning transmission electron microscopy. Based on simulated supercells and image simulations we developed an experimental approach that allows under optimized experimental conditions to determine compositions down to the atomic level in single atomic columns. We show that conventional approaches to analyze alloy fluctuations by tetragonal distortions are misleading due to strain interaction within the alloy.
We compare our results on the alloy distribution of InGaN quantum wells to optical properties measured by cathodoluminescence, photoluminescence excitation and time resolved photoluminescence. Especially we show that we are able to coherently describe the optical properties of staggered quantum wells and GaN quantum dots embedded in AlN barriers.
FI-2:IL04 Novel Microlenses for Light Extraction in OLED Lighting
MAO-KUO WEI, Department of Materials Science and Engineering, National Dong Hwa University, Taiwan, ROC; JIUN-HAW LEE, HOANG-YAN LIN, Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taiwan, ROC
Microlenses with different contours and bottom shapes are used to extract light from the substrate waveguide in organic light-emitting devices (OLEDs). It was found that the efficiency of the OLEDs can be effectively improved with spherical microlenses having a large lens sag and fill factor.
Spherical microlenses can be easily made using thermal reflow. Hemispherical microlenses can be produced after thermally reflowing photoresist disks with a proper height. The fill factor of microlenses is normally defined by the Cr patterns on the photomask, but can be adjusted to some extent by controlling the heating temperature carefully. The luminous current efficiency and power efficiency of the OLEDs can be increased up to ~70% and ~50%, respectively, when the OLEDs attached with a highly dense and hemispherical microlens array.
Both spherical and non-spherical microlenses can be manufactured by using diffuser lithography. Cone-like and ellipsoid-like microlenses, having an aspect ratio greater than unity and a fill factor approaching 100%, can be easily made by adjusting the haze of the diffuser and the UV exposure dose. Attaching these non-spherical microlens arrays, the luminous current efficiency and power efficiency of the OLEDs can be improved up to ~60% and ~50%, respectively.
FI-2:IL05 Terahertz Spectroscopy Study of Semiconductor Nanostructures
XINHAI ZHANG, Department of Electrical and Electronic Engineering, South University of Science and Technology of China, Shenzhen, China
Terahertz (THz) wave refers to electromagnetic radiation in the frequency interval from 0.1 to 10 THz. THz time-domain spectroscopy (THz-TDS) is a powerful technique to study materials properties such as complex dielectric response and conductivity in the far-infrared spectral region, with the advantages of high signal-to-noise ratio, noncontact optical probe, and measuring the amplitude and phase of electric field simultaneously thus no requirement for Kramers-Kronig transformation. The optical pump-THz probe (OPTP) spectroscopy has recently emerged as a powerful technique to study the carrier dynamics of materials. In OPTP, the photon energy of THz probe is in the range of meV (1 THz = 4.1 meV), naturally matching the energy scale of elementary excitation in solids (i.e., excitons, phonons). THz spectroscopy shows its advantage for study the conductivity of nanostructures, because of no requirement for electrical contacts.
In this presentation, we will present THz spectroscopy study of carrier dynamics and transient photoconductivity in a number of semiconductors nanostructures, which are technologically important.
FI-2:L06 Hydrogen Trap Mediated Cathodoluminescence Kinetics in Mg Doped p-type GaN
M.R. PHILLIPS1, C. NENSTIEL1, 2, M. HOFFMANN2, 3, G. CALLSEN1, 2, M. WINTREBERT-FOUQUET4, C.TON-THAT1, A. HOFFMANN2, 1Microstructural Analysis Unit, University of Technology, Sydney, Broadway, Australia; 2Technische Universität Berlin, Department of Solid State Physics, Berlin, Germany; 3Material Science and Engineering, North Carolina State University, Raleigh, NC, USA; 4BluGlass Limited, Silverwater, NSW, Australia
The defect structure of Mg acceptor doped p-type GaN remains unclear despite years of intense research and the successful commercialisation of a vast array of opto-electronic devices based on nitrides. Recent theoretical reports have indicated that Mg can exist as both a shallow effective mass like acceptor as well as a deep level state. While the latest experimental work on very high quality GaN:Mg have confirmed the presence of a number of additional unstable luminescence peaks that either intensify or quench during electron and UV laser irradiation. These emission bands have been attributed to a variety of acceptor and donor bound excitons, DAPs and free-to-bound optical transitions involving hydrogen related donor and acceptor complexes. In this work GaN:Mg epilayers were grown by MOCVD with Mg doping concentrations from 5 x 10^17 to 2 x 10^19 cm-3. Time-resolved cathodoluminescence (CL) spectral data has been collected between 10 K to 500 K from GaN:Mg "as-grown" and annealed at 820 K to elucidate the identity of the unstable CL peaks. These data strongly suggest that the observed Mg associated CL electron irradiation kinetics are mediated by the presence of additional defects centres, such as gallium vacancies and threading dislocations, which trap mobile hydrogen.
FI-2:L07 Human- and Artwork-friendly Lighting Sources Based on Candle Light-style OLED
JWO-HUEI JOU, Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu, Taiwan, ROC
White light is not safe to use as a lighting source after dusk due to its notorious effect on suppressing the generation of the oncostatic hormone, melatonin, and the blue light within may impose an irreparable damage on the retina of human eyes as well as discolor the masterpieces of van Gogh and Cezanne. To echo the call for a human- and artwork-friendly light source that is low in blue emission or low in color temperature to safeguard human health and masterpieces, we demonstrate herein a candle light-style organic light emitting diode (OLED) with a color temperature varying around 1,900 K with an efficacy at least 300 times that of candles or 3 time that of incandescent bulbs. Most importantly, a sensationally warm candle light-style emission is driven by electricity in lieu of the hydrocarbon-burning candles invented 5,000 years ago. This candle light-style OLED may serve as a safe measure for illumination especially after dusk. It may also be used to replace the traditional metal halide lamps or white LED lamps currently used in many museums and galleries that would potentially speed up artwork deterioration.
Session FI-3 - Device Structures and Manufacturing
FI-3:IL01 Nano Technologies for Light Extractions in OLEDs
JIN-WOOK SHIN, DOO-HEE CHO, CHUL WOONG JOO, JAEHYUN MOON, SEUNK KOO PARK, JONGHEE LEE, JUN-HAN HAN, NAM SUNG CHO, JOOHYUN HWANG, JIN WOO HUH, HYE YONG CHU, JEONG-IK LEE, ETRI, Daejeon, Korea
In OLEDS, a great advancement in the synthesis of organics and energy level designed electrodes has enabled the achievement of 100% in the internal quantum efficiency. However, OLEDs still suffer from low external quantum efficiency, which is explained by the fact that the outward traveling light is confined due to the difference in refractive indices of the device constituting layers.
Scattering using nano structures is useful way to extract the light from OLEDs. We have applied scattering layers between the substrate and the anode of OLEDs as well as the substrate and air. The nano scattering structures have been fabricated by thermal dewetting or self assembled process. We have achieved more than 30% efficiency improvement from the OLED with the scattering layer between the substrate and the anode and the efficiency improvement reached 100 % with MLA(Micro Lens Array) for the external light extraction. As for the scattering layer between the substrate and air, more than 50 % efficiency improvement has been achieved. In this paper, the detailed processes for nano scattering layers and OLED characteristics will be presented.
FI-3:IL02 Microstructured GaN Light Emitting Diodes for Visible Light Communications and Solid State Lighting
M.D. DAWSON, Institute of Photonics, University of Strathclyde, Wolfson Centre, Glasgow, UK
Specialised forms of gallium nitride light-emitting diode (LED) technology involving electronically-interfaced arrays of micro-sized LEDs ('micro-LEDs') offer new regimes of operation in terms of device physics and also enable novel applications. We will describe the fascinating characteristics of these devices, which include operation per pixel at high optical power densities (>100W/cm2) and high current densities (>10kA/cm2). The high optical power density enables applications requiring pattern-programmable image projection, for example in mask-free photolithography and photostimulation. The high current density leads to a short differential carrier lifetime which enables high modulation capability for data communications at >1Gb/s per pixel. We discuss the implications of these capabilities in terms of new forms of bioinstrumentation, hybrid inorganic/organic optoelectronics, nanolithography, and visible light communications. The latter includes exploitation, for the first time, of spatial multiplexing at visible wavelengths to provide aggregated data rates of potentially Tb/s/mm2.
FI-3:IL03 InGaN/GaN Quantum Dot and Nanowire LEDs and Lasers
P. BHATTACHARYA, T. FROST, A. BANERJEE, S. JAHANGIR, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA
Light sources spanning the visible range are required for solid-state lighting, full color mobile projectors and laser displays, heads-up displays, optical data storage, and medical applications. Nitride-based light sources currently use InGaN/GaN quantum wells in the active region, which pose problems at longer wavelengths. In this context self-organized Stranski-Krastanaw (S-K) quantum dots present distinct advantages since they are formed by strain relaxation and the built-in polarization field is much smaller than those existing in equivalent quantum wells. Similiarly, GaN nanowires and InGaN/GaN dot-in-nanowire heterostructures can be grown on Si substrates in the wurtzite form. There is a radial relaxation of strain during growth and they are free of extended defects. As a result, the polarization field in the InGaN dots in the GaN nanowires is also small. By varying the growth parameters it is possible to grow S-K quantum dots and dots-in-nanowires that emit at long wavelengths, upto red (λ=630nm). In this talk we will present the characteristics of red-emitting S-K quantum dot lasers with low threshold and high differential gain. The modulation characteristics of the lasers will also be presented. We will also present the characteristics of red-emitting nanowire quantum dot light-emitting diodes with high efficiency and small efficiency droop. The peak efficiency in the LEDs occur at low injection currents. The material characteristics of the two types of InGaN/GaN nanostructures will also be presented.
FI-3:L04 Electrically Pumped Low-threshold Random Lasing from Hydrothermal ZnO Film
CANXING WANG, YANG YANG, XIANGYANG MA, DEREN YANG, State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China
Random lasing (RL) in polycrystalline ZnO films is an intriguing research subject. Here, we report the low-threshold electrically pumped RL from high quality ZnO films with near-micron-sized grains grown on silicon substrate by a convenient, low-temperature hydrothermal process. The morphological characterization shows that the ZnO film is closely packed of hexagonal grains with an average size of about 800 nm. The photoluminescence characterization indicates the ZnO film is well crystallized and has few defects. The electrically pumped RL from the ZnO film is achieved by using the pumping scheme of metal-insulator-semiconductor structure. It is demonstrated that the device exhibits fairly small threshold current (~6 mA). In the ZnO film with near-micrometer sized grains, the optical loss caused by the boundaries and defects is less than that in the ZnO film with smaller grains. Therefore, it is believed that the less optical loss contributes to the improved performance of the electrically pumped RL from the device using the hydrothermal ZnO film. We believe that the present work provides a strategy to develop low-threshold ZnO-based random lasers and is signiﬁcant for the practical applications of random lasers.
FI-3:IL05 White LEDs on Flexible Substrates
M. WILLANDER, Dept. of Science and Technology, Linköping University, Norrköping, Sweden
Growing large area inorganic white light emitting diodes (LEDs) on flexible substrates is of technical and fundamental aspects interesting. In this talk I will discuss flexible substrates as graphene, paper, textile and plastic. The inorganic active part is zinc oxide nanowires. We have hybrid structures and the second active part are different polymers. The properties of the LEDs (as well as some other devices) will be discussed.
FI-3:IL07 Quantum Dots Based on Nitrides Candidates for Single Photon Sources and Light Emitting Devices
A. HOFFMANN, Technical University of Berlin, Institute of Solid State Physics, Berlin, Germany
Nitride quantum dots (QDs) are promising candidates for the realization of electrically triggered sources of single photons or light emitting devices in the yellow-green spectral range. In the first part, we review the fundamental processes of photon emission by excitonic complexes confined in single III-N QDs. Experimental results from time-integrated and time-resolved single-QD spectroscopy are evaluated as well as theoretical results obtained by Hartree-Fock calculations based on realistic eight-band k.p wave functions. Furthermore, we demonstrate that single GaN/AlN QDs exhibit a huge exciton brightstate FSS of up to 7meV, larger than in any other QD system. Thus, the results of this work open the possibility to tailor the FSS in GaN/AlN QDs in order to develop high-temperature emitters of polarized single photons or entangled photon pairs on demand. In the second part, we review about the use of InGaN quantum dots embedded in GaN nanowires (NW), and active regions containing InGaN quantum dots with different indium compositions. Further development of these materials and devices require systematic studies of their optical and electrical properties to optimize their performance and then establish their potential for device applications.
FI-3:L08 Laser Diode Arrays as a Source of Solid State Lighting
M. LESZCZYNSKI, P. PERLIN, R. CZERNECKI, E. GRZANKA, S. GRZANKA, A. KAFAR, G. TARGOWSKI, M. SARZYNSKI, T. SUSKI, Institute of High Pressure Physics UNIPRESS and TopGaN, Warsaw, Poland
GaN single crystals grown by us and AMMONO SA have an ultra low dislocation density (10000/cm2) distributed evenly over the whole wafer. Such substrates enable us to construct large area devices, as laser diode arrays (up to 30 stripes tested successfully so far) emitting in a wide range of wavelengths (380-480 nm) with powers of a few watts. Such arrays may serve as powerful light source for solid state lighting both in RGB approach and in phosphor one.
In the presentation, we will discuss a number of epi-growth parameters which we are optimizing having a target of the highest possible power and life-time of devices. Such issues are: i) substrate off-orientation, ii) H2 flow during the MQW growth, iii) p-type growth temperature, and some others.
We will show how useful is substrate lateral patterning, that enables us to obtain local variation of off-orientation, and to prevent cracking of AlGaN cladding layers.
Finally, we will discuss future of the GaN-based laser-diode-array technology with some commercial aspects.
FI:P03 Tunable Upconversion Luminescence of Scheelite Structural Crystals
JUNG-IL LEE, SUNG-LIM RYU, JEONG HO RYU, Department of Materials Science and Engineering, Korea National University of Transportation, Chungju, Chungbuk, Korea; JOON HWANG, Department of Aeronautical and Mechanical Design Engineering, Korea National University of Transportation, Chungju, Chungbuk, Korea; CHANG WOO HONG, Department of Civil Engineering, Korea National University of Transportation, Chungju, Chungbuk, Korea
In recent years, research on upconversion (UC) materials, particularly the lanthanide-doped UC phosphors, have attracted much attention because of their unique properties that can convert low energy such as IR radiation to high energy such as visible or UV light. In particular, white UC luminescence has been extensively studied, because white luminescent UC phosphors can replace conventional lighting sources in optical devices and three-dimensional backlighting for color displays. Also, white UC phosphors can be used as bio-medical probes with extended simultaneous detection, a capability unmatched by conventional bio-labeling. In this work, we report the intense white UC luminescence of scheelite structural tungstate and molybdate crystals. Yb3+ ions are co-doped as sensitizer ions for improving the UC emission efficiency. The color index of white UC emission is modulated by controlling the blue, green, and red components depending on the Tm3+ and Ho3+ ions. Polycrystalline Tm3+ and Ho3+ co-doped scheelite structural tungstate and molybdate phosphors were prepared by a solid state reaction method. The effects of Ho3+, Tm3+, and Yb3+ ions on the photoluminescece (PL) of the scheelite structural materials and possible UC mechanism are discussued in detail.
FI:P04 Electrical and Optical Properties of Ge1-ySny Grown on Si Substrates
MEE-YI RYU1, YUNG KEE YEO2, T. HARRIS2, J. KOUVETAKIS3, R. BEELER3, 1Kangwon National University, Korea; 2Air Force Institute of Technology, USA; 3Arizona State University, USA
A significant breakthrough has recently been achieved in fabricating Si- or Ge-based direct bandgap semiconductors such as Ge1-ySny and Ge1-x-ySixSny alloys using the ultra-high vacuum chemical vapor deposition (UHVCVD) and molecular beam epitaxial methods. The Ge1-ySny alloys grown on Si substrates are expected to undergo an indirect to direct bandgap transition near x=0.06. In spite of recent progress in crystal growth of these newly developed Ge1-ySny and Ge1-x-ySixSny alloys, many significant fundamental challenges still exist such as the controllability of accurate alloy compositions, Sn segregation, dislocations, as well as undesirable residual impurities and defects. Furthermore, definite knowledge of various optical and electrical properties of these alloy systems is still lacking due to the lack of systematic studies of these materials. In this paper, the electrical properties of unintentionally doped p-Ge1-ySny (y=0.06%) grown on Si substrate by UHVCVD have been investigated as a function of temperature. The temperature-dependent Hall-effect measurements show that there exists a p-type degenerate parallel conducting layer in Ge1-ySny/n-Si, which affects significantly the electrical properties of the Ge1-ySny layer. Also, a conductivity type conversion from p to n was observed at around 370 K for this sample. This type-conversion temperature is low enough that it may affect the properties of electronic and optoelectronic devices and their operations above room temperature. Temperature-dependent photoluminescence (PL) has been investigated for both p- and n-type Ge1-ySny alloys and p-Ge grown on Si substrates. All these three samples show both direct and indirect bandgap optical transitions, but show different temperature-dependent photoluminescence intensity. Moreover, they clearly show competitiveness between the direct and indirect PL transitions as temperature changes.
FI:P08 Yttrium Disilicate Stability in Aqueous Medium
S.C. SANTOS, C. YAMAGATA, S. MELLO-CASTANHO, Nuclear and Energy Research Institute, IPEN-CCTM, Sao Paulo, SP, Brazil; W. ACCHAR, Federal University of Rio Grande do Norte, UFRN, Natal, RN, Brazil
Recently Brazil has been considered as ascending potency from South America as 6th biggest economy in the world, whose international political is seen as "Soft power", which means diplomacy, economic development and common interests. To enhance the progress of Brazilian nation is extremely important to establish a sustainable energy security program, wherein the processing of materials to save energy plays an important role. Yttrium disilicate (Y2Si2O7) presents considerable luminescent properties making it a potential substitute of rare earth oxides in gas burners technology. In this work Y2Si2O7 was synthesized by a simpler hydrothermal method and its stability in aqueous media was evaluated by measuring electrophoretic velocity of particles, zeta potential determination and flow curves in controlled shear rate mode. As a result, yttrium disilicate particles were more stable at alkaline pH range, showing a minimum viscosity at pH 11.