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TERM PAPERTERM PAPERTERM PAPERTERM PAPER

InGaN semiconductors for warm white light LEDInGaN semiconductors for warm white light LEDInGaN semiconductors for warm white light LEDInGaN semiconductors for warm white light LEDssss

Sayan BasuSayan BasuSayan BasuSayan Basu

MSc Applied PhysicsMSc Applied PhysicsMSc Applied PhysicsMSc Applied Physics

Student IDStudent IDStudent IDStudent ID---- 11069678110696781106967811069678Dept. Of Physics & EnergyDept. Of Physics & EnergyDept. Of Physics & EnergyDept. Of Physics & Energy

University of LimerickUniversity of LimerickUniversity of LimerickUniversity of Limerick

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IndexIndexIndexIndex

Title Page

1. INTRODUCTION 11.1Physics in LED1.2Colour of LED

2. LED TECHNOLOGY 32.1 Physical Function

2.2 White LEDs & Phosphor-based LEDs

3. InGaN for white light LED 54. Warm White LED & Other LEDs 7

5. APPLICATION OF InGaN 9

6. REFERENCES 14

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1. INTRODUCTIONLight emitting diode is a semiconductor light source. It was introduced in 1962 as a practical

electronic component, early LEDs emitted low-intensity red light, but now at present modern

versions are available with a visible, UV and infrared wavelengths with a very high brightness.

LEDs represent many advantages over incandescent light sources including lower energy

consumption, longer lifetime, improved robustness, smaller size and faster switching. LEDs are

powerful enough for room lighting relatively expensive and require more precious current and

heat management than compact fluorescent lamp sources of comparable output.

LEDs are used in wide range applications like aviation lighting, automotive lighting as well as

traffic signals.

In 1907, a phenomenon called electroluminescence was discovered by British experimenter H.JRound of Marconi lab, using a crystal of silicon and a cats- whisker detector. Rubin Braunstein

of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs)

and other semiconductor alloys in 1955.Braunstein observed infrared emission generated by

simple diode structures using gallium antimonide (GaSb), GaAs, InP, and SiGe alloys at room

temperature and at 77 kelvin. Untill 1968, visible and infrared LEDs were extremely costly, and

had a little practical use. The Monsanto Company was the first organization to mass produce

visible LEDs, using gallium arsenide phosphide in 1968 to produce red LEDs suitable for

indicators. The first commercial LEDs were commonly used as replacements

for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive

equipment such as laboratory and electronics test equipment, then later in such appliances as

TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only

for use as indicators, as the light output was not enough to illuminate an area. Readouts in

calculators were so small that plastic lenses were built over each digit to make them legible.

Later, other colours grew widely available and also appeared in appliances and equipment. As

LED materials technology grew more advanced, light output rose, while maintaining efficiency

and reliability at acceptable levels. The invention and development of the high power white light

LED led to use for illumination, which is fast replacing incandescent and fluorescent lighting.

Most LEDs were made in the very common 5 mm T1 and 3 mm T1 packages, but with risingpower output, it has grown increasingly necessary to shed excess heat to maintain reliability, so

more complex packages have been adapted for efficient heat dissipation. Packages for state-of-

the-art high power LEDs bear little resemblance to early LEDs.

1.1 PHYSICS IN LED:

The LED consists of a chip of semiconducting material doped with impurities to create a p-n

junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or

cathode, but not in the reverse direction. Charge-carrierselectrons and holesflow into the

junction from electrodes with different voltages. When an electron meets a hole, it falls into a

lower energy level, and releases energy in the form of a photon.

The wavelength of the light emitted, and thus its color depends on the band gap energy of thematerials forming the p-n junction. In silicon or germanium diodes, the electrons and holes

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recombine by a non-radiative

indirect band gap materials. T

corresponding to near-infrared

LED development began with

materials science have enabled

variety of colors. LEDs are us

p-type layer deposited on its s

commercial LEDs, especially

Most materials used for LEDlight will be reflected back int

Fig.2: I-V diagram for a diode.

on voltages are 23 volts

transition which produces no optical emiss

e materials used for the LED have a direct

visible or near-ultraviolet light.

Fig 1: The inner workings of an LED

infrared and red devices made with gallium

making devices with ever-shorter waveleng

ally built on an n-type substrate, with an el

rface. P-type substrates, while less commo

aN/InGaN, also use sapphire substrate.

roduction have very high refractive indices.the material at the material/air surface inter

n LED will begin to emit light when the on-volt

4

ion, because these are

and gap with energies

arsenide. Advances in

ths, emitting light in a

ctrode attached to the

, occur as well. Many

This means that muchace.

age is exceeded. Typical

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1.2 COLOURS OF LEDs:

LEDs are made from a variety of inorganic semiconductor materials. There are some

semiconductor materials and their corresponding colours.

Colour: Infrared; Wavelength- >760 nm; Voltage- < 1.9, Material- GaAs, AlGaAsColour: Red; Wavelength- 610-760 nm, Voltage- 1.63-2.03 V, Material- AlGaAs, GaAsP, GaP

Current bright LEDs are based on the wide band gap semiconductors GaN and InGaN. They can

be added to existing red and green LEDs to produce the impression of white light, though white

LEDs today rarely use this principle. Blue LEDs became very popular in late 1990s.They have an

active region consisting of one or more InGaN quantum wells sandwiched between thicker layers

of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum

wells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of

varying AlN fraction can be used to manufacture the cladding and quantum well layers for

ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological

maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN,instead of alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths

around 350370 nm. Green LEDs manufactured from the InGaN-GaN system are far more

efficient and brighter than green LEDs produced with non-nitride material systems.

There are two primary ways of producing high-intensity white-light using LEDs. One is to use

individual LEDs that emit three primary colours- red, green and blue and then mix all the colours

to form white light. The other is to use a phosphor material to convert monochromatic light from

a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb

works. Due to metamerism, it is possible to have quite different spectra that appear white. But

InGaN can be used too for producing warm white light LEDs.

2. LED TECHNOLOGY:

2.1 PHYSICAL FUNCTION

Like a normal diode, an LED consists of a chip of semiconducting material impregnated, or

doped, with impurities to create a p-n junction. As in other diodes, current flows easily from the

p-side, or cathode, to the n-side, or anode, but not in the reverse direction. Charge-carriers

electrons and electron holes flow into the junction from electrodes with different voltages. When

an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a

photon. The wavelength of the light emitted, and therefore its colour, depends on the band gap

energy of the materials forming the p-n junction. In Silicon or Germanium diodes, the electrons

and the holes recombine by a non-radiative transition which produces no optical emission,

because these are indirect band gap materials. The materials used for an LED have a direct band

gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LEDs are usually built on an n-type substrate, with electrode attached to the p-type layer

deposited on its surface. P-type substrates, while less common, occur as well. Many commercial

LEDs, especially GaN/InGaN, also use sapphire substrate. Substrates that are transparent to the

emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractiveindex of the package material should match the index of the semiconductor, otherwise the

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produced light gets partially reflected back into the semiconductor, where it gets absorbed and

turns into additional heat lowering the efficiency. In 2007 experiments tried to avoid multiple

internal reflection by roughening the chip. Again at the surface from the package to a low

refractive index medium like a glass fiber or air total internal reflection is avoided by using a

sphere shaped package, with the diode in the center, so that the light rays hit the surface quite

perpendicular, and anti-reflection coating may be added. The package may be cheap plastic,

which may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the

color of the packaging does not substantially affect the color of the lightemitted.

2.2 WHITE LEDs & PHOSPHOR BASED LEDs:

Blue LEDs can be added to existing red and green LEDs to produce the impression of white light,

though white LEDs today rarely use this principle. Most "white" LEDs in production today are

based on an InGaN-GaN structure, and emit blue light of wavelengths between 450 nm to 470 nm

blue GaN. These GaN-based, InGaN-active-layer LEDs are covered by a yellowish phosphorcoating usually made of cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals which

have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part

of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the

Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than

a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix

of blue and yellow light gives the appearance of white, the resulting shade often called "lunar

white". This approach was developed by Nichia and was used by them from 1996 for

manufacturing of white LEDs. The pale yellow emission of the Ce3+:YAG can be tuned by

substituting the cerium with other rare earth elements such as terbium and gadolinium and can

even be further adjusted by substituting some or all of the aluminum in the YAG with gallium.Due to the spectral characteristics of the diode, the red and green colors of objects in its blue

yellow light are not as vivid as in broad-spectrum light. Manufacturing variations and varying

thicknesses in the phosphor make the LEDs produce light with different color temperatures, from

warm yellowish to cold bluish; the LEDs have to be sorted during manufacture by their actual

characteristics. Philips Lumileds patented conformal coating process addresses the issue of

varying phosphor thickness, giving the white LEDs a more consistent spectrum of white light.

Spectrum of a "white" LED clearly showing blue light which is directly emitted by the GaN-

based LED (peak at about 465 nanometers) and the more broadband stokes shifted light emitted

by the Ce3+:YAG phosphor which extends from around 500 to 700 nanometers. White LEDs can

also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiencyeuropium-based red and blue emitting phosphors plus green emitting copper and aluminum doped

zinc sulfide (ZnS: Cu, Al). This is a method analogous to the way fluorescent lamps work.

However the ultraviolet light causes photo degradation to the epoxy resin and many other

materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This

method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger

and more energy is therefore converted to heat, but yields light with better spectral

characteristics, which render color better. Due to the higher radiative output of the ultraviolet

LEDs than of the blue ones, both approaches offer comparable brightness. The newest method

used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown

zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its activeregion and yellow light from the substrate. A new technique developed by coating a blue LED

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with quantum dots that glow

produces a warm, yellowish-w

Fig.3: Spectrum of a white LE

LED and more broadband stokes

500700 nm.

3. InGaN FOR WHITEInGaN-based light emitting

potential due to their ability of

inaccessible for LED and LD

efficient and long-lasting ro

definition printing (LDs). Th

generated, is an InGaN/GaN o

quantum well structure are al

mechanisms by which thisunderstanding of these mecha

with it, the performance of the

Fig:

hite in response to the blue light from the

hite light similar to that produced by incande

clearly showing blue light which is directly e

shifted light emitted by the Ce3+

:YAG phosphor

LIGHT LEDs:

iodes (LEDs) and laser diodes (LDs) h

working in the short wavelength region, whi

technologies. Their applications vary from l

m lighting (LEDs) to high-density mem

e active region of these devices, in whi

r InGaN/AlGaN quantum well. Even though

ready being mass-produced and are availa

light is generated are poorly understooisms, it should be possible to improve the

devices.

Basic design of InGaN LED

7

LED. This technique

scent bulbs.

itted by the GaN-based

which emits at roughly

ve great commercial

ch has up to now been

arge area displays and

ry storage and high

h the light is being

devices based on this

le in the market, the

. By improving ourstructural design, and

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Polarization Field Effects versus Indium Fluctuations

The analysis of these devices by many researches has brought into focus two different

recombination mechanisms. The first is dominated by indium fluctuations in the indium layer, the

second is dominated by strong polarization fields induced by biaxial strain in the layer. There is

much disagreement between the researches involved in this problem as to which recombinationmechanism is responsible for light emission in these devices.

Fig: Schematic representation of the polarization field model.

Fig: The Indium fluctuation model.

Biaxial Strain Characterization

A unique tool has been developed for the study of the effect of the polarization fields in InGaN

Quantum Wells. A tensile biaxial strain is created in the epitaxial samples by means of a speciallydesigned pressure cell. For a bulk (or even a thin film) semiconductor, this simply results in the

shrinking of the energy-gap - a red shift of the light emitted. However, through the piezoelectric

effect, the tensile strain also reduces the strength of the built-in polarization field. For an LED

structure dominated by the polarization field effect, this results in a blue shift of the emitted light.

Deviation from the model can be explained as screening due to doping, confinement effects, or

localization at indium-rich nano-clusters. Thus the direction and degree to which the colour shifts

informs about the mechanisms that dominate the radiative recombination in structures.

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4. WARM WHITE LED & OTHER LEDs:

There are two primary ways of producing high intensity white-light using LEDs. One is to use

individual LEDs that emit three primary colorsred, green, and blueand then mix all the

colors to produce white light. The other is to use a phosphor material to convert monochromaticlight from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent

light bulb works.

Due to metamerism, it is possible to have quite different spectra that appear white.

RGB systems

Combined spectral curves for blue, yellow-green, and high brightness red solid-state

semiconductor LEDs. FWHM spectral bandwidth is approximately 2427 nm for all three colors.

White light can be produced by mixing differently colored light, the most common method is to

use red, green and blue (RGB). Hence the method is called multi-colored white LEDs (sometimes

referred to as RGB LEDs). Because its mechanism is involved with electro-optical devices to

control the blending and diffusion of different colors, this approach is little used to produce white

lighting. Nevertheless this method is particularly interesting in many applications because of the

flexibility of mixing different colors, and, in principle, this mechanism also has higher quantum

efficiency in producing white light.

There are several types of multi-colored white LEDs: di-, tri-, and tetrachromatic white LEDs.

Several key factors that play among these different approaches include color stability, color

rendering capability, and luminous efficacy. Often higher efficiency will mean lower color

rendering, presenting a trade off between the luminous efficiency and color rendering. For

example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest

color rendering capability. Conversely, although tetrachromatic white LEDs have excellent color

rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in

between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.

What multi-color LEDs offer is not merely another solution of producing white light, but is a

whole new technique of producing light of different colors. In principle, most perceivable colors

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can be produced by mixing di

to produce precise dynamic c

technique, multi-color LEDs s

we use to produce and control

on the market, several technic

of LEDs emission power d

substantial change in color s

Therefore, many new packag

their results are now being rep

Phosphor-based LEDs

Spectrum of a white LED

based LED (peak at about 46Ce

3+:YAG phosphor which em

This method involves coating

phosphor of different colors to

white LEDs. A fraction of t

shorter wavelengths to longe

different colors can be emplo

emitted spectrum is broadened

given LED.

Phosphor based LEDs have a

Stokes shift and also other pho

still the most popular techniq

production of a light source

conversion is simpler and chea

white LEDs presently on the

The greatest barrier to high eff

much effort is being spent on

temperatures. For instance, thby using a more suitable type

ferent amounts of three primary colors, and

lor control as well. As more effort is devot

hould have profound influence on the fund

light color. However, before this type of LE

l problems need to be solved. These certainl

cays exponentially with increasing tempe

tability. Such problems are not acceptable

designs aimed at solving this problem ha

oduced by researchers and scientists.

learly showing blue light which is directly

nm) and the more broadband Stokes-shifteits at roughly 500700 nm.

an LED of one color (mostly blue LED

produce white light, the resultant LEDs are

e blue light undergoes the Stokes shift be

r. Depending on the color of the origina

yed. If several phosphor layers of distinct

, effectively increasing the color rendering i

lower efficiency than normal LEDs due to

sphor-related degradation issues. However, t

e for manufacturing high intensity white

or light fixture using a monochrome e

per than a complex RGB system, and the ma

arket are manufactured using phosphor light

iciency is the seemingly unavoidable Stokes

ptimizing these devices to higher light outp

efficiency can be increased by adapting bef phosphor. Philips Lumileds patented con

10

this makes it possible

d to investigating this

mental method which

D can truly play a role

include that this type

rature, resulting in a

for industrial usage.

e been proposed and

emitted by the GaN-

d light emitted by the

ade of InGaN) with

called phosphor-based

ing transformed from

l LED, phosphors of

olors are applied, the

ndex (CRI) value of a

the heat loss from the

he phosphor method is

EDs. The design and

mitter with phosphor

ority of high intensity

conversion.

energy loss. However,

t and higher operation

tter package design orormal coating process

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addresses the issue of varying phosphor thickness, giving the white LEDs a more homogeneous

white light. With development ongoing, the efficiency of phosphor based LEDs is generally

increased with every new product announcement.

Technically the phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor

coated epoxy. A common yellow phosphor material is cerium-doped yttrium aluminium garnet

(Ce3+

:YAG).

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture

of high efficiency europium-based red and blue emitting phosphors plus green emitting copper

and aluminium doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way

fluorescent lamps work. This method is less efficient than the blue LED with YAG:Ce phosphor,

as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with

better spectral characteristics, which render color better. Due to the higher radiative output of the

ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness. Another

concern is that UV light may leak from a malfunctioning light source and cause harm to human

eyes or skin.

Other white LEDs

Another method used to produce experimental white light LEDs used no phosphors at all and was

based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously

emitted blue light from its active region and yellow light from the substrate.

5. APPLICATION OF InGaN:Some years ago, the color range of Light Emitting Diodes (LEDs) on the market was limited to

the red to green spectrum. Then, blue LEDs were developed and introduced into the market.

These blue devices made it possible to build so called single-chip white LEDs, using a yellow

converter material in combination with a blue die. Most of the blue and white LEDs use IndiumGallium Nitrite (InGaN) as an epitaxial layer. The wavelength (chromaticity coordinates) of the

generated light of these InGaN-based LEDs shows a strong dependency on the driving current.

This special property of InGaN based LEDs must be considered well in advance for new

application solutions. This application Note is intended to enable the reader to avoid some

common design mistakes when using InGaN-LEDs. To obtain white light, a blue light-emitting

die (wavelength 450 nm to 470 nm) is covered with a converter material that is stimulated by blue

light and emits a yellow light. The human eye detects the mixture of blue and yellow light as

white. Because this mixture cannot be described by a simple dominant wavelength (there are two

peaks in the spectrum, as shown in Figure 1), color coordinates must be used.

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Figure: 2

The two main impacts on the color coordinates of the generated white light are:

The wavelength of the blue die

The concentration of the converter material.

Therefore, if oneor bothof these parameters changes, the color coordinate change

accordingly. Figure 2, top shows the area within the CIE diagram in which the color coordinates

of white Osram Opto Semiconductor LEDs typically vary. To avoid the problem of different

whites in an application using more than one LED, OSRAM Opto Semiconductors (OSRAM OS)

LEDs are grouped into three bins (see Figure 2, right). As well as this production-relatedvariation of the color coordinates, the driving condition in an application may also have an impact

on the color coordinates of the generated white light. Because the wavelength of an InGaN based

LED (chromaticity coordinates) shifts against the forward current (see Figure 3), there is a color

shift in the following instances:

Dimming of InGaN-based LEDs by varying the forward current

Using parallel circuits to drive more than one InGaN-based LED.

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Fig. 3 : Chromaticity coordinate vs. Forward Current

Using Parallel Circuits to Drive More Than one InGaN-based LED

In contrast to commonly-used standard LED types, InGaN-based LEDs cover a wider variation of

forward voltage. Using LEDs with different forward voltages in a parallel circuit causes different

forward currents for each LED. This may lead to a remarkable variation in brightness as well as a

shift in chromaticity coordinates. Figure 4 shows the I-V curves of some randomly selected white

LEDs. It is quite apparent that using these devices in a parallel circuit results in differences in

brightness as well as a color shift.

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REFERENCES:

1. A. Yasan, T. McClintock, K. Mayes, S. R. Darvish, H. Zhang, P.Kung, M. Razeghi, S.K.

Lee and J. Y. Han, Appl. Phys. Lett., 81, pp2151, 2002.

2. M. Yamada, Y. Narukawa and T. Mukai, Phosphor Free High-Luminous-Efficiency

White Light-Emitting Diodes Composed of InGaN Multi-Quantum Well, Jpn. J. Appl.Phys., vol. 41, pp. L246-L248, 2002.

3. S. Nakamura, Tukai and M. Senoh, Candela-class high-brightness InGaN/AlGaN

double-heterostructure blue-lightemitting diodes, Appl. Phys. Lett., vol. 64 pp.1687-1689,

1994.

4. T. Mukai and S. Nakamura, Ultraviolet InGaN and GaN Single-Quantum-Well-Structure

Light-Emitting Diodes Grown on Epitaxially Laterally Overgrown GaN Substrates, Jpn.

J. Appl. Phys., vol. 38, pp. 5735-5739, 1999.

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