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- Front Matter: Volume 7954
- Special Session on Light and Health: Human Factors for SSL
- High Current Performance and Droop Effect in LEDs I
- LED Applications and SSL
- High Current Performance and Droop Effect in LEDs II
- LED Manufacturing and Applications
- Novel Substrates for LEDs
- UV Emitting LEDs
- Nanomaterials and Nanostructures for LEDs
- OLEDs and OLED Lighting
- Phosphors for LEDs
- Novel Technologies for LED Design and Fabrication
- Poster Session
Front Matter: Volume 7954
Front Matter: Volume 7954
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This PDF file contains the front matter associated with SPIE Proceedings Volume 7954, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Special Session on Light and Health: Human Factors for SSL
Energy efficient lighting for the biological clock
Dieter Lang
Show abstract
Unexpectedly the existence of a formerly unknown type of photoreceptor in the human eye has been proven about 10
years ago. Primarily sensitive in the blue spectral range it is responsible for transducing light signals directly into the
brain, controlling essential biological functions like setting of the circadian clock or daytime activation. Recent scientific
research has enabled beneficial applications.
The paradigms for good lighting design are shifting and standardization activities have been started to build up a sound
base for description and application of biologically effective lighting.
Latest improvements of LED technology are now allowing realizeation of advanced lighting solutions based on SSL.
Optimization of biological effects is possible while demands on good vision are maintained.
As biologically effective lighting is addressing a second system besides vision in the human body a measure beyond
lumen per watt is required for a proper description of energy efficiency.
Human preference in tunable solid state lighting
Jeremy M. Spaulding,
Maria R. Thompson,
Robert E. Levin
Show abstract
Tunable LED illumination systems utilizing digital control can satisfy many user requirements for white and colored
light. Such systems provide a powerful entry point for LEDs into general illumination applications. Parameters such as
light level and color temperature can be used to change space ambience. Variation of spectral distribution can be used to
trade between energy efficiency and color rendition. Studies have been carried out in a controlled laboratory
environment and in a "real life" occupied space. This paper presents an exploration of methodologies used to assess
subjective, preference-based traits.
Human health and well-being: promises for a bright future from solid-state lighting
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Technologies are needed to provide information about the state of our circadian system to our conscious awareness such
that we can take appropriate action to avoid and to correct light-induced circadian disruption. In addition to the implicit
promises of solid-state lighting to improve energy efficiency and the quality of the visual environment, solid-state
lighting also promises to maintain our health and well-being by precisely tailoring light and dark throughout the 24-hour
day.
High Current Performance and Droop Effect in LEDs I
Improvement in efficiency droop of GaN-based light-emitting diodes by optimization of active regions
Show abstract
We had demonstrated several novel methods to improve efficiency droop behavior in GaN-based light-emitting
diodes (LEDs). LEDs with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer
and n-GaN layer were investigated. By using low-temperature (LT, 780°C) n-GaN as IL, the efficiency droop behavior
can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA,
which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices (SPS) layer. The
polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect
density, about 5.3×108 cm-2, in its MQWs region is much higher than that value of 2.9×108 cm-2 in LED with LT n-GaN
layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this
alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of
polarization field. At second part, LEDs with graded-thickness multiple quantum wells (GQW) was designed and found
to have superior hole distribution as well as radiative recombination distribution by simulation modeling. Accordingly,
the experimental investigation of electroluminescence spectrum reveals additional emission from the previous narrower
wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current
density of 30 A/cm2 to 200 A/cm2. Moreover, the light output power is enhanced by 35% at 20 A/cm2.
LED Applications and SSL
Randomized micro lens arrays for color mixing
Show abstract
Color homogeneity is a key issue for LED lighting. To achieve sufficient flux, efficiency and color rendering, LED
luminaires have to use multiple LEDs, whose brightness and color properties differ. To mix greenish white LEDs
using phosphor with red monochromatic LEDs is an especially promising approach to achieve both high effiency
and good color rendering at warm white colors. However, even for luminaires using only white LEDs, color
and brightness of the LEDs varies due to random selection within LED bins, or by using LEDs from different
bins. For diffuse illumination, color mixing is not too difficult, but for collimated light, good color mixing is
a key challenge to the optical system. Micro lens arrays are known to provide extremely good color mixing
while increasing beam divergence only slightly. However, in their standard forms with (i) hexagonal lenslets or
(ii) circular lenslets with blackened triangles, they create beam patterns with (i) sharp edged hexagonal or (ii)
less efficient sharp edged circular distributions, instead of the round, soft edged beam shapes needed for general
lighting. This is due to the fact that the lenslet shapes are imaged to the far field. It is not possible to choose the
lenslet shapes freely: the lenslet edges depend on the lenslet positions, forming a Voronoi diagram. We present
various approaches to perturb the regular lenslet positions, forming a smooth round beam, while keeping the
superior properties of standard micro lens arrays.
High Current Performance and Droop Effect in LEDs II
Density-activated defect recombination as a possible explanation for the efficiency droop in GaN-based diodes
Show abstract
A model based on density-activated defect recombination processes is proposed as a possible explanation
for the efficiency droop in GaN-based lasers. The model yields very good agreement with experimentally
measured efficiencies based on fit parameters that indicate the presence of two types of recombination centers
that have different local distributions and recombination rates. The recombination rates of the two types are
found to be very similar for devices operating at 530nm and 410nm.
LED Manufacturing and Applications
Characteristics of linear position sensor based on bi-cell photodiode
Igor Friedland,
Amit Brandes
Show abstract
In this paper we introduce a method allowing theoretical calculation of linearity and scale factor errors in linear position
sensors based on bi-cell photodiodes. Such calculations are applied using three methods to compensate for scale factor
temperature drift. An example based on data of a specific sensor was computed to illustrate how the sensor's dimensions
and drift compensation method influence the scale factor and linearity errors of its volt-displacement characteristic.
A flexible rugged testbed for passive solar collector development
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We report the development of a flexible, rugged test bed for the purpose of testing various passive solar light collector
configurations. An essential aspect in evaluating the performance of such collectors is to analyze its behavior under
various illumination levels and planes of incident light. The test bed consists of a series of white LED assemblies which
are digitally controlled using a micro-controller and is mounted on a rugged frame. The intensity levels of the LED can
be controlled through a computer to emulate the sun's radiation effectively.
Novel Substrates for LEDs
Non-polar GaInN-based light-emitting diodes: an approach for wavelength-stable and polarized-light emitters
Show abstract
In absence of piezoelectric polarization along the growth axis, a- and m-plane green GaInN light emitting diodes
manifest stable emission wavelength -- independent of the injection current density. The shift of the dominant
wavelength is less than 8 nm when varying the forward current density from 0.1 to 38 A/cm2. Furthermore, the light
emitted from the growth surface of such non-polar structures shows a very degree of linear polarization. This is
attributed to a strong valance band splitting in such anisotropically strained wurtzite GaInN quantum wells . Such light
emitting diodes show a high potential for energy efficient display applications.
UV Emitting LEDs
Efficient 350 nm LEDs on low edge threading dislocation density AlGaN buffer layers
Richard Gutt,
Thorsten Passow,
Wilfried Pletschen,
et al.
Show abstract
Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light
emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to
improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the
MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional
facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of
dislocation lines at the growing facets. Secondly, in-situ deposited SiNx interlayers have been used as nano-masks
reducing the dislocation density above the SiNx layers. Both approaches result in reduced asymmetric HRXRD ω-scan
peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with
an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence
measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN
nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN
nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiNx interlayer exhibit a
low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.
Nanomaterials and Nanostructures for LEDs
Nitride nanowire structures for LED applications
Henning Riechert,
Oliver Brandt,
Caroline Cheze,
et al.
Show abstract
This paper discusses some of the advantages of nanowire structures for use in LEDs as well as the challenges that need
to be overcome towards the realisation of real-world devices. Our experimental results pertain to group-III nitride
nanowire structures grown by MBE. We present clear evidence that the catalyst-free growth approach on Si yields best
results with respect to structural and optical material properties. We elucidate the mechanism of nanowire nucleation and
the factors determining the initial nanowire diameter, discuss the issue of InGaN growth in small-diameter nitride
nanowires and review the results reported for nanowire-based group-III nitride LEDs reported so far.
III-nitride nanowires: novel materials for solid-state lighting
Show abstract
Although planar heterostructures dominate current solid-state lighting architectures (SSL), 1D nanowires have distinct
and advantageous properties that may eventually enable higher efficiency, longer wavelength, and cheaper devices.
However, in order to fully realize the potential of nanowire-based SSL, several challenges exist in the areas of controlled
nanowire synthesis, nanowire device integration, and understanding and controlling the nanowire electrical, optical, and
thermal properties. Here recent results are reported regarding the aligned growth of GaN and III-nitride core-shell
nanowires, along with extensive results providing insights into the nanowire properties obtained using cutting-edge
structural, electrical, thermal, and optical nanocharacterization techniques. A new top-down fabrication method for
fabricating periodic arrays of GaN nanorods and subsequent nanorod LED fabrication is also presented.
Enhancement of light extraction efficiency of InGaN quantum wells light-emitting diodes using TiO2 microsphere arrays
Show abstract
The enhancement of light extraction efficiency of InGaN quantum well (QW) light emitting diodes (LEDs) was achieved
by employing the refractive index matched TiO2 microsphere arrays. The optimization studies of the dipping method and
rapid convective deposition (RCD) method were carried out for the deposition of TiO2 microsphere arrays onto LEDs.
The 2-dimensional relatively close-packed and close-packed TiO2 microsphere arrays were deposited by the using
optimized conditions of the dipping method and RCD method, respectively. The light extraction efficiencies of LEDs
under electrical injection were enhanced by 1.83 times by utilizing 520-nm diameter TiO2 microspheres. This
enhancement is primarily attributed to increase in the effective photon escape cone due to the matched index and
spherical shape of TiO2 microstructures arrays.
Nanostructre based antireflection coatings for EO/IR sensor applications
Show abstract
EO/IR Nanosensors are being developed for a variety of Defense and Commercial Systems
Applications. These include UV, Visible, NIR, MWIR and LWIR Nanotechnology based
Sensors. The conventional SWIR Sensors use InGaAs based IR Focal Plane Array (FPA) that
operate in 1.0-1.8 micron region. Similarly, MWIR Sensors use InSb or HgCdTe based FPA that
is sensitive in 3-5 micron region. More recently, there is effort underway to evaluate low cost
SiGe visible and near infrared band that covers from 0.4 to 1.6 micron.
One of the critical technologies that will enhance the EO/IR sensor performance is the
development of high quality nanostructure based antireflection coating. Prof. Fred Schubert and
his group have used the TiO2 and SiO2 graded-index nanowires / nanorods deposited by obliqueangle
deposition, and, for the first time, demonstrated their potential for antireflection coatings by
virtually eliminating Fresnel reflection from an AlN-air interface over the UV band. This was
achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a
minimum value of n = 1.05, the lowest value so far reported
In this paper, we will discuss our modeling approach and experimental results for using oblique
angle nanowires growth technique for extending the application for UV, Visible and NIR sensors
and their utility for longer wavelength application. The AR coating is designed by using a genetic
algorithm and fabricated by using oblique angle deposition. The AR coating is designed for the
wavelength range of 400 nm to 2500 nm and 0° to 40° angle of incidence. The measured average
optical transmittance of an uncoated glass substrate between 1000 nm and 2000 nm is improved
from 92.6% to 99.3% at normal incidence by using a two-layer nanostructured AR coating
deposited on both surfaces of the glass substrate.
OLEDs and OLED Lighting
New concept for in-line OLED manufacturing
Show abstract
A new concept of a vertical In-Line deposition machine for large area white OLED production has been developed. The
concept targets manufacturing on large substrates (≥ Gen 4, 750 x 920 mm2) using linear deposition source achieving a
total material utilization of ≥ 50 % and tact time down to 80 seconds.
The continuously improved linear evaporation sources for the organic material achieve thickness uniformity on Gen 4
substrate of better than ± 3 % and stable deposition rates down to less than 0.1 nm m/min and up to more than 100 nm
m/min.
For Lithium-Fluoride but also for other high evaporation temperature materials like Magnesium or Silver a linear source
with uniformity better than ± 3 % has been developed.
For Aluminum we integrated a vertical oriented point source using wire feed to achieve high (> 150 nm m/min) and
stable deposition rates.
The machine concept includes a new vertical vacuum handling and alignment system for Gen 4 shadow masks. A
complete alignment cycle for the mask can be done in less than one minute achieving alignment accuracy in the range of
several 10 μm.
In situ measurement of spectrum, emission zone, and dipole emitter orientation in OLEDs
Show abstract
The optical features of the internal dipole emission have major impact on the radiation pattern and overall device
efficiency of organic light-emitting diodes (OLEDs). In recent years, the characterization of OLED emitter
properties by optical analysis of far-field radiation patterns of OLEDs in electrical operation was established as an
in situ investigation method. However, in order to observe the internal features of the dipole emission in the
OLEDs far-field accurately, well adapted devices should be utilized to optically enhance the feature of interest.
Although this is a crucial point, the potential of adapted devices to OLED characterization has not been
investigated universally yet.
In our contribution, we provide general directives how the OLEDs layered stack is to be designed in order to
enable for precise measurements of the active optical properties of the emissive material (internal electroluminescence
spectrum, profile of the emission zone and dipole moment orientation) by radiation pattern analyses.
Basically, we utilize the fact that the distance of the emissive sites to the metal cathode is most crucial to enhance
or suppress certain dipole contributions to the far-field. A model layered system is discussed and universal emitter
positions suitable to determine the internal feature of particular interest at most accuracy are deduced.
Phosphors for LEDs
Luminescent features of novel sol-gel derived lanthanide multi-doped oxyfluoride nano-structured phosphors for white LED
Show abstract
Rare-earth doped oxyfluoride 75SiO2:25PbF2 nano-structured phosphors for white-light-emitting diodes were
synthesized by thermal treatment of precursor sol-gel derived glasses. Room temperature luminescence features of Eu3+,
Sm3+, Tb3+, Eu3+/Tb3+ and Sm3+/Tb3+ ions incorporated into low-phonon-energy PbF2 nanocrystals dispersed in the
aluminosilicate glass matrix and excited with UV(395 nm) and blue(405 nm) light emitting diodes was investigated. The
luminescence spectra exhibited strong emission signals in the red(600, 610, 625, 646 nm), green(548, and 560 nm) and
blue(485 nm) wavelength regions. White-light emission was observed in Sm/Tb and Eu/Tb double-doped activated
phosphors employing UV-LED excitation at 395 nm. The dependence of the luminescence emission intensities upon
annealing temperature, and rare-earth concentration was also examined. The results indicated that there exist optimum
annealing temperature and activator ion concentration in order to obtain intense visible emission light with high color
rendering index. The study suggest that the nanocomposite phosphor based upon 75SiO2:25PbF2 host herein reported is
a promising contender for white-light LED applications.
Process to measure particulate down-converting phosphors and create well-correlated software models of LED performance
Show abstract
White light-emitting diodes that use down-converting phosphors have been utilized in the illumination industry for
several years. In many cases, little information needs to be known about the physics and performance of the phosphor
itself to design, optimize, and simulate the light emission of the LED for the purpose of creating secondary optics.
However, the importance of accurately accounting for the effect of the phosphor cannot be overstated when designing
the LED package or when performing a tolerance analysis, for instance. The difficulties in gathering or measuring the
relevant performance metrics of the phosphors are significant barriers to achieving accurate predictions in illumination
software packages.
This paper describes a simple, repeatable process to measure several phosphor performance metrics that are used, in
turn, to create a model of the same phosphor in a commercially-available illumination software package. The measured
values are used either as direct inputs or are used to derive the proper inputs for the software. Derivations and discussion
about the software model are included. The performance of the simulated phosphor will then be compared and
correlated to the physical measurements. Finally, a model of an LED that uses this phosphor model is built in software
and its simulated performance is compared to measured values.
Luminescent ceramics for LED conversion
Show abstract
Many LED-based applications would benefit from more efficient and/or high lumen output devices that enable
usage in both white and single color illumination schemes. In the present article we briefly review the materials
research history leading to optical ceramic converters and discuss their typical characteristics. Recently
demonstrated high performance values in terms of efficacy and external quantum efficiency in orange (amber)
spectral region are described.
Novel Technologies for LED Design and Fabrication
Development of patterned sapphire substrate and the application to the growth of non-polar and semi-polar GaN for light-emitting diodes
Show abstract
The light-emitting diodes (LEDs) with high external quantum efficiency (EQE) are usually fabricated on the
patterned sapphire substrate (PSS). The PSS reduces the dislocation density in the GaN layer and enhances the light
extraction efficiency (LEE) from the LED chip by scattering the light confined in GaN layer attributed to the critical
angle between GaN (n=2.4) and sapphire substrate (n=1.7) (or air (n=1.0)). On the other hand, non-polar GaN and semipolar
GaN are attracted much attention to eliminate the quantum confined Stark effect (QCSE). Recently, we have
developed novel technology to grow non-polar or semi-polar GaN on the PSS with high quality and large diameter by
metal-organic vapor phase epitaxy (MOVPE). For example, m-plane GaN grown on a-plane PSS and {112 (see manuscript)} plane GaN
grown on r-plane PSS. The growth of c-plane GaN from the c-plane-like sidewall of the r-plane PSS results in {112 (see manuscript)}
GaN on the r-plane PSS. The full widths at half maximum of X-ray rocking curves (FWHM-XRC) of the {112(see manuscript)} GaN
along the azimuths parallel and perpendicular to the c-direction were 533 and 260 arcsec, respectively. Dislocation
density of the GaN was approximately 2×108 cm-2. These non-polar and semi-polar GaN are expected to be suitable for
novel GaN substrate or GaN template for LEDs.
Novel approaches to realizing chemical lift-off of GaN epilayer from sapphire substrate
Show abstract
Chemical lift-off (CLO) technique has been paid more attention since no damages will be
induced to GaN epi-layer during the epilayer lift-off process. In this study two novel CLO
approaches were used to separate GaN epilayer from sapphire substrate. One is using Ga2O3sacrificial layer deposited by pulsed laser deposition. The other is using a stripe patterned SiO2grown by PECVD. Afterwards, the CLO of GaN epilayers grown on these two templates via metal
organic chemical vapor deposition from sapphire substrate was successfully realized with a
hydrofluoric acid as an etchant.
Novel approaches for high-efficiency InGaN quantum wells light-emitting diodes: device physics and epitaxy engineering
Show abstract
The challenges and approaches for high-efficiency InGaN quantum wells (QWs) light-emitting diodes (LEDs) are
presented. The studies include designs, growths, and device characteristics of 1) InGaN-based QWs LEDs with
enhanced matrix element for realizing green-emitting LEDs with high internal quantum efficiency, and 2) InGaN QW
LEDs device structure with lattice-matched AlInN-barrier to suppress efficiency-droop in nitride LEDs. Other
approaches to improve the efficiency of the nitride LEDs will be discussed as follow: 1) surface plasmon LEDs, 2) new
growth approach for dislocation density reduction in GaN, and 3) novel approaches for light extraction efficiency
improvement of III-Nitride LEDs.
Optimisation of pattern geometry and investigations of physical mechanisms contributing to improved light extraction in patterned substrate LEDs
Show abstract
In this paper we investigate the use of a patterned layer placed at the substrate / GaN interface region of a p-side
up LED to improve light extraction and investigate the optimization of performance geometry by adjustment of
geometrical parameters associated with the shaped structures including: side wall angle, side wall curvature, height and
lattice constant. Performance is in each case evaluated in terms of angular extraction efficiency and far field angular
beam profile. Comparisons are made between conventional large pitch patterned substrate (PSS) designs which have
multiple wavelength length scales, and photonic crystal lattices which have a (sub) wavelength length scale. Physical
mechanisms giving rise to the improvements are identified and discussed in each case. Overall a maximum improvement
in extraction efficiency of 66% was obtained for a 4500nm pitch lattice of truncated cones.
Poster Session
Efficiency enhancement of blue InGaN LEDs with indium composition graded InGaN barriers
Show abstract
In recent literatures, the quantum efficiency of conventional blue InGaN light-emitting diodes (LEDs) is quite limited
under relatively high driving current with conventional GaN barriers due presumably to the poor injection efficiency of
hole. In this study, the efficiency enhancement of blue InGaN LEDs with indium composition graded InGaN barriers is
proposed. The energy band diagram, carrier concentration in the quantum wells, diagram of hole current, radiative
recombination rate, L-I curves, and internal quantum efficiency are investigated numerically. The simulation results
show that the InGaN LED with graded InGaN barriers has better performance over its conventional counterpart with
GaN barriers due to enhanced efficiency of hole injection. The simulation results also suggest that under relatively high
current injection, the internal quantum efficiency and output light power are markedly improved when the traditional
GaN barriers are replaced by graded InGaN barriers. According to the improved optical properties, the new-designed
LED has promising potential in solid state lighting.