Technology and Physics of Light Emitting Technology and Physics of Light Emitting Technology and Physics of Light Emitting Technology and Physics of Light Emitting

Diodes Diodes Diodes Diodes

Department of Electronics and Radio EngineeringDepartment of Electronics and Radio EngineeringDepartment of Electronics and Radio EngineeringDepartment of Electronics and Radio Engineering

Kyung Hee UniversityKyung Hee UniversityKyung Hee UniversityKyung Hee University

Prof. Jae Su YuProf. Jae Su YuProf. Jae Su YuProf. Jae Su Yu

2010. 12. 15.2010. 12. 15.2010. 12. 15.2010. 12. 15.

Contents

� Introduction

� Theory of Radiative Recombination

� History of Light Emitting Diodes (LEDs)

� Electrical and Optical Properties of LEDs� Electrical and Optical Properties of LEDs

� Temperature Characteristics of LEDs

� High Internal Efficiency Design

� Light Extraction Efficient Structures

� Packaging

1

� Summary

Applications of LEDsApplications of LEDsApplications of LEDsApplications of LEDsApplications of LEDsApplications of LEDsApplications of LEDsApplications of LEDs

LED TVsLED TVsLED TVsLED TVs

LightLightLightLightgenerationgenerationgenerationgeneration

Traffic Traffic Traffic Traffic signalssignalssignalssignals

휴대전화 모듈Mobile & displaysMobile & displaysMobile & displaysMobile & displays

Car LEDsCar LEDsCar LEDsCar LEDs

Plant growth Plant growth Plant growth Plant growth using LEDsusing LEDsusing LEDsusing LEDs

LED 사인SignsSignsSignsSigns

2

Cost of Light

※ Cost of Light Incorporates• Lifetime• Source efficiency• Energy cost• Replacement cost of lamp/fixture• Labor cost

Metric Metric Metric Metric Cost Cost Cost Cost Lumens Lumens Lumens Lumens Power Power Power Power Lifetime Lifetime Lifetime Lifetime Cost of Light Cost of Light Cost of Light Cost of Light

Incandescent $0.59 630 60W 1000hrs $34.26

Halogen $4.97 700 60W 3000hrs $18.57

CFL $3.97 800 14W 6000hrs $5.70

LED Lamp $50.00 800 11.4W 25000hrs $5.14

Source: PHILIPS

3

•1907 Electroluminescence observed in Carborundum (SiC) –H.J. Round

•1923-1930 Comprehensive study of SiC electroluminescence and discussion of application for communications -O.V. Losev

•1947 Discovery of transistor –Bardeen and Brattain

•1951 Explanation of SiC electroluminescence as carrier injection across a p/n junction–K. Lehovec, et al.

History of LEDs

junction–K. Lehovec, et al.

•1955 Visible electroluminescence inGaP–G.A. Wolff, et al.

•1962 Demonstration of coherent visible light emission from direct bandgap GaAsP alloy semiconductors –N. Holonyak and S.F. Bevacgua

•1962-Present Continuing development and optimization of various direct bandgap ternary (GaAsP, AlGaAs) and quarternary (AlInGaP, AlInGaN) material systems for high performance LEDs –RCA Monsanto, Hewlett Packard, Stanley, Toshiba, Toyoda Gosei, Nichia, and others

4

5

RGB LEDs

� 상용화된 방식

�Red LED: AlGaAs (660 nm), AlInGaP (644 nm)�Green LED: GaP (555 nm, 570 nm), AlInGaP (562 nm), InGaN (525 nm)�Blue LED: InGaN (450 nm)Cf. Orange LED: GaAsP (610 nm), AlInGaP (612 nm)

Yellow LED: GaAsP (585 nm), AlInGaP (590 nm)UV LED: AlInGaN ( < 350 nm)

Source: Light-emitting diodes, Fred Schubert

6

� Commercialization

- Blue LED + Yellow phosphors (YAG:Ce)

- Red LED + Blue LED + Green LED

- ZnSe blue LED + ZnSe substrate

� In progress

- UV LED + White phosphors

- One chip solution (monolithic white LED)

White LEDs

7Source : LG Innotek

Cree

Efficiency of LEDs

- 2009, Nichia, 249 lm/W @20 mA, room temp.- 2010. 2. 4, Cree, 208 lm/W @ 350 mA, room temp. Color temp.: 4579 K 8

LED evolution

1970 - 1992 1997 - 20081993 - 1996 2009 ~

Red LED RGB LED White LED General Lighting

Full Color Display

Years

InGaN, AlGaInP

High power >1,000 lm, High luminous efficiency >100 lm/W, Low cost < $20/klm

20 8040 120

Signal Mobile Phone Back Light Unit, Car LED Lighting

GaAsP, AlGaAsFull Color Display

Lm/W

InGaN, AlGaInP Digital ConvergenceInGaN, AlGaInP

9

Progress in Technology

Efficiency

Color

Integration

Blue active

Bandgap eng

Micro-phosphor

UV active

Nano-phosphor

Multi-color

Micro-pattern

Quantum well

Surface roughening

Nano-pattern

2D Photonic crystal

Quantum dot

Resonant cavity

ⅢⅢⅢⅢ----ⅤⅤⅤⅤ Hetero ⅢⅢⅢⅢ---- N non-sapphire ⅢⅢⅢⅢ---- N Hybrid

Epitaxy

Tunability

Energy saving

Power

Integration

Design

2007 2009 2011

Flip-chip Lift-off Wafer bonding

Large chip Array One-chip Array

Discrete PKG Chip-on-board Plastic optics

Organic PKG Ceramic/Metal PKG Module PKG

Chip

Integration

Source: Samsung Electro-Mechanics

10

Functionality

Flexibility

Cost down

LED Structure

White LED RGB-UV LED

Packaging

Optic apparatus,

system controller

LED module/system

Car headlight/lamp, BLU, displays, etc

Packaging

Package

Wafer

Sapphire, GaN, SiC, Si, GaAs

Epi-wafer

(In,Al)GaN (blue, green, UV), InAlGaP(red, yellow), AlGaAs(red, IR)

LED chip

Chip Fabrication

Epitaxial growth

Epitaxy

Chip

11

GaN-based LED Fabrication Process

SubstrateEpitaxy

Buffer LayerEpitaxy

Active Layer

SiCSapphireSiliconBulk GaNComposite substrates

AlNLow T°GaNAlN/GaN sandwich

GaN

LED epi-wafer

Front-EndBack-End Front-End

Litho./etching/metallization

Back-Endlevel 0

Back-Endlevel 1

Lateral LED structureVertical LED structure

LED dies-on-waferBack-grindingDicing, Flip-chipLaser Lift-Off : LLODie shaping

LED dies

Bonding, Pick-and placePhosphor coatingPackaging, Housing

Packaged LED LED lamp

Source: Yole Development

12

High efficiency LED

High efficiency LED

Chip– Extraction Efficiency

� p-GaN roughening� Chip shaping� LED on patterned sapphire substrate� Vertical chip� Photonic crystal

High efficiency LED

Epilayer– Internal Quantum Efficiency

Packaging/module efficiency – PKG Efficiency

� Micro/Nanostructure� Patterned substrate� Lateral overgrowth� Homo epitaxy� Non-polar epi

� Heat dissipation design� Optics design� Package materials� Phosphor

13

High-purity single crystal semiconductor materials � Development of the vast semiconductor industry

cf) 10 angstroms thick, 12 inch wafer for Si

SemiconductorSemiconductorSemiconductorSemiconductor: conductivity between those of metals and insulators

� Conductivity change over several orders of magnitude by adding controlled amounts of impurity atoms

Semiconductor materials

Ex) AlxGa1-xAs: x is the fraction of the lower atomic number element component

GaN Gallium nitrides

ZnO Zinc oxides

14

Energy band theory

15

For n=3 energy levels

In n=3, begin to interact initially, then n=2, n=1 � If the equilibrium interatomic distance is r0, then we have

bands of allowed energies that the electrons may occupy separated by bands of forbidden energies

Energy band theory

At absolute zero degrees, electrons in the lower energy states (full: valence band), electrons in the upper

energy states (empty: conduction band)

Bandgap, Eg: between the top of the valence band and the bottom of the conduction band is the width of the

forbidden energy band

16

Si: 1s22s

22p

63s

23p

6

N atoms � 2N, 2N, 6N, 2N, 6N states of type 1s, 2s, 2p, 3s, 3p: 2N, 6N states of type 3s, 3p

4 N electrons in the original isolated n=3 shells (2N in 3s states and 2N in 3p states)

As the interacting spacing decreases, this band of 3s-3p levels contains 8N states

As the distance between atoms approaches equilibrium interatomic spacing of silicon, this band splits into two

band contains N states, as does the lower (valence) band

� Qualitatively how and why bands of allowed and forbidden energies formed in a crystal

Energy band theory

17

Metals, insulators, semiconductors

18

Doping and energy levelsDopant atoms and energy levelsDopant atoms and energy levelsDopant atoms and energy levelsDopant atoms and energy levels

Conductivity of a semiconductor varies over approximately 10 orders of magnitude by controlling the

concentration of specifies in the material

Doping � impurity intentionally introduced into the semiconductor � intrinsic material

T=0K T=0K (doping of phosphorus) T=0K T>0K

Group V (eg. phosphorus atom) : five valence electrons � a donor electron

The phosphorus atom without the donor electron is positively charged The phosphorus atom without the donor electron is positively charged

The donor atoms add electrons to the conduction band without creating holes in the valence band

Donor impurity atom � n-type semiconductor

T=0K T=0K (doping of boron) T=0K T>0K

Group III (eg. boron atom) : three valence electrons � an acceptor electron

The boron atom without the donor electron is negatively charged

The acceptor atoms generate holes in the valence band without generating electrons in the conduction band

Acceptor impurity atom � p-type semiconductor

Adding controlled amounts of dopant atoms, either donors or acceptors, creates a material � extrinsic

semiconductor 19

Operation principle of LEDsOperation principle of LEDsOperation principle of LEDsOperation principle of LEDs

Direct bandgapIndirect bandgap

� Energy conservation/Momentum conservation

20

Source: An introduction to semiconductor devices, Donald A, Neamen

Radiative electron-hole recombination

- Electrons and holes in semiconductors recombine either radiatively, i.e.accompanied by the emission of a photon, or non-radiatively (� maximization ofradiative process and minimization of the non-radiative process.

- Any undoped or doped semiconductor has two types of free carriers, electronsand holes. Under equilibrium conditions, i.e. without external stimuli such as lightor current, the law of mass action teaches that the product of the electron andhole concentrations is, at a given temperature, a constant, i.e.

Law of mass action Law of mass action Law of mass action Law of mass action

where n0 and p0 are the equilibrium electron and hole concentrations and ni isthe intrinsic carrier concentration.

- Excess carriers in semiconductors can be generated either by absorption oflight or by an injection current. The total carrier concentration is then given bythe sum of equilibrium and excess carrier concentrations, i.e.

where Δn and Δp are the excess electron and hole concentrations,respectively.

21

Recombination of carriers

The proportionality constant B is called the bimolecular recombinationcoefficient (typical values of 10–11–10–9 cm3/s for direct-gap III–Vsemiconductors)

Bimolecular rate equation Bimolecular rate equation Bimolecular rate equation Bimolecular rate equation

R ∝ n p

- The recombination per unit time per unit volume can be written as

- The recombination rate (R) at which the carrier concentration decreases- The recombination rate (R) at which the carrier concentration decreases

22

Non-radiative recombination in the bulk

During non-radiative recombination, the electron energy is converted tovibrational energy of lattice atoms, i.e. phonons. Thus, the electron energy isconverted to heat. � For obvious reasons, non-radiative recombination eventsare unwanted in light-emitting devices.

In a radiative recombination event, one photon with energy equal to thebandgap energy of the semiconductor is emitted

23

Defects include unwanted foreign atoms, native defects, dislocations, and anycomplexes of defects, foreign atoms, or dislocations � deep levels or traps(luminescence killers) within the forbidden gap of the semiconductor

24

Auger recombinationAuger recombinationAuger recombinationAuger recombination

Energy becoming available through electron–hole recombination (approximately Eg),is dissipated by the excitation of a free electron high into the conduction band, orby a hole deeply excited into the valence band. The highly excited carriers willsubsequently lose energy by multiple phonon emission until they are close to theband edge

Auger recombination

In the high-excitation limit in which the non-equilibrium carriers have a higherconcentration than equilibrium carriers, the Auger rate equations reduce to

where C is the Auger coefficient Auger coefficient Auger coefficient Auger coefficient (a quantum mechanical calculation that takes into account the band structure of the semiconductor)

C: typically 10–28–10–29 cm6/s for III–V semiconductors

Auger recombination reduces the luminescence efficiency in semiconductors onlyat very high excitation intensity or at very high carrier injection currents. At lowercarrier concentrations, the Auger recombination rate is very small and can beneglected for practical purposes. 25

Non-radiative recombination at surface

Substantial non-radiative recombination can occur at semiconductor surfaces. Surfaces are a strong perturbation of the periodicity of a crystal lattice. � This modification includes the addition of electronic states within the forbidden gap of the semiconductor

Some of the valence orbitals do not form a chemical bond. These partially filledelectron orbitals, or danglingdanglingdanglingdangling bondsbondsbondsbonds, are electronic states that can be located inthe forbidden gap of the semiconductor where they act as recombinationcenters� acceptor-like or donor-like states

SurfaceSurfaceSurfaceSurface reconstructionreconstructionreconstructionreconstruction:::: dangling bonds may also rearrange themselves andform bonds between neighboring atoms in the same surface plane. This canlead to a locally new atomic structure with state energies different from bulkatomic states.

The continuity equation for electrons is given by

where Jn is the current density caused by electrons flowing to the surface.

Assume that the illumination causes a uniform steady state generation rate G.

26

Note that unipolar regions of a

Luminescence decreases in the near surface region

Note that unipolar regions of asemiconductor device, e.g. theconfinement regions, are notaffected by surface recombinationdue to the lack of minority carriers.

� Several passivation techniqueshave been developed to reduce thesurface recombination insemiconductors, includingtreatments with sulfur, and otherchemicals

27

Competition between radiative and non-radiative recombination

Just as for surface recombination, non-radiative bulk recombination (Shockley–Read)and Auger recombination can never be totally avoided. Any semiconductor crystalwill have some native defects.

It is difficult to fabricate materials with impurity levels lower than the parts perbillion (ppb) range. T� even the purest semiconductors contain impurities in the1012 cm–3 range. Some elements may form deep levels and thus reduce theluminescence efficiency.

If the radiative lifetime is denoted as τr and the non-radiative lifetime isIf the radiative lifetime is denoted as τr and the non-radiative lifetime isdenoted as τnr, then the total probability of recombination is given by the sum ofthe radiative and non-radiative probabilities:

Probability of radiative recombination or internalinternalinternalinternal quantumquantumquantumquantum efficiencyefficiencyefficiencyefficiency isisisis givengivengivengiven bybybyby

Internal quantum efficiency: the ratio of the number of light quanta emitted insidethe semiconductor to the number of charge quanta undergoing recombination 28

Overall LED PerformanceEpitaxy Front-end Back-end Packaging

Substrate LED epi-wafer Die-on-wafer LED die LED lamp

Internal Quantum Efficiency : ηηηηint.int.int.int.Electrical Losses: ηηηηelectelectelectelect....

Extraction Efficiency: ηηηηextrextrextrextr....Packaging Losses : ηηηηpackpackpackpack....

ηtotal = ηint. x ηelect. x ηextr. x ηpack.

Internal quantum efficiency- MQW optimization, n/p-AlGaN/GaN superlattices, low defect epi growth (ELO, PSS, GaN, ZnO, AlN sub.),nonpolar LED

Cf. Internal quantum efficiency x light extraction efficiency= External quantum efficiency

Injection efficiency

Electron-holerecombination� Photon generation rate

Extraction efficiency- LED die shaping, LLO, flip-chip, highly reflecting mirrors, TCO

- Ni/Au, ITO � ohmic improvement, electron mobility improvement material growth, flip-chip

Present: 60% 84% 75% 60%

Expectation: 90% 92% 90% 70%

ηtotal= 23%ηtotal= 52%

Packaging efficiency- Phosphor material efficiency improvement, lens/optics improvement

Source: Yole Development

29

Internal, extraction, external, and power efficiency

The active region of an ideal LED emits one photon for every electron injected� Thus the ideal active region of an LED has a quantum efficiency of unity

The internal quantum efficiency is defined internal quantum efficiency is defined internal quantum efficiency is defined internal quantum efficiency is defined as

where Pint is the optical power emitted from the active region and I is the injection current.

Photons emitted by the active region should escape from the LED die. In an idealPhotons emitted by the active region should escape from the LED die. In an idealLED, all photons emitted by the active region are also emitted into free space.Such an LED has unity extraction efficiency. However, in a real LED, not all thepower emitted from the active region is emitted into free space.

Loss mechanism: 1. Reabsorbed in the substrate of the LED or by a metallic contacts,2. Total internal reflection, also referred to as the trapped light phenomenon� reduces the ability of the light to escape from the semiconductor.

The light extraction efficiency is defined extraction efficiency is defined extraction efficiency is defined extraction efficiency is defined as

where P is the optical power emitted into free space30

The external quantum efficiency is defined external quantum efficiency is defined external quantum efficiency is defined external quantum efficiency is defined as

The external quantum efficiency gives the ratio of the number of useful lightparticles to the number of injected charge particles.

The power efficiency is defined power efficiency is defined power efficiency is defined power efficiency is defined as

where IV is the electrical power provided to the LED. Informally, the powerefficiency is also called the wall-plug efficiency.efficiency is also called the wall-plug efficiency.

31

Three loss mechanism

i) Photon absorption within the semiconductor- Photons in any direction with hν > Eg � reabsorbed within semiconductor

i) ii) iii)

Loss Mechanism in LEDs

ii) Fresnel loss at semiconductor-air interface

- Reflection coefficient: : Fresnel loss

2

12

12

+−

=Γnn

nn

iii) Critical angle loss at semiconductor-air interface

33% are reflected back into the semiconductor

= −

2

11sinn

ncθ

Any photon that is incident at an angle greater than 15.9 ° will be back into the semiconductor

Snell' law �s

32

The emission intensity as a function of energy is proportional to the productof and

Emission Spectrum

Using the requirement that electron and hole momenta are the same, the photonenergy can be written as the joint dispersion relation

Using the joint dispersion relation, the joint density of states can be calculatedand one obtains

The distribution of carriers in the allowed bands is given by the Boltzmanndistribution, i.e. 33

The maximum emission intensity occurs at

The full-width at half-maximum of the emission is

For example, the theoretical room-temperature linewidth of a GaAs LED emittingat 870 nm is ΔE = 46 meV or Δλ = 28 nm.

34

Diode current-voltage characteristics

Depletion region

The space charge region produces apotential that is called the diffusiondiffusiondiffusiondiffusionvoltage,voltage,voltage,voltage, VVVVDDDD....

NNNND D D D

NNNNA A A A

Barrier that free carriersmust overcome in orderto reach the neutralregion of oppositeconductivity type

where NA and ND are the acceptor anddonor concentrations, respectively, and ni

is the intrinsic carrier concentration ofthe semiconductor.

The depletion layer width is given by

where ε = εrε0 is the dielectric permittivity of the semiconductor and V is the diode biasvoltage

The Shockley equation for a diode with cross-sectional area A is given by

where Dn,p and τn,p are the electronelectronelectronelectron andandandand holeholeholehole diffusiondiffusiondiffusiondiffusion constantsconstantsconstantsconstants and the electronelectronelectronelectron andandandand holeholeholeholeminorityminorityminorityminority----carriercarriercarriercarrier lifetimeslifetimeslifetimeslifetimes, respectively.

35

The diode I–V characteristic can be written as

Under typical forward-bias conditions, the diode voltage is V >> kT / e, and thus[exp (eV/kT) – 1] ≈ exp (eV/kT).

Under reverse-bias conditions, the diode current saturates and the saturationcurrent is given by the factor preceding the exponential function in the Shockleyequation.

The exponent of the exponential function illustrates that the current stronglyincreases as the diode voltage approaches the diffusion voltage, i.e. V ≈ VD. Thevoltage at which the current strongly increases is called the thresholdthresholdthresholdthreshold voltagevoltagevoltagevoltage andandandandthisthisthisthis voltagevoltagevoltagevoltage isisisis givengivengivengiven bybybybyVth ≈ VD.

36

In highly doped semiconductors, the separation between the band edges andthe Fermi level is small compared with the bandgap energy, i.e. (EC – EF) << Eg onthe n-type side and (EF –EV) << Eg on the p-type side. Furthermore, thesequantities depend only weakly (logarithmic dependence) on the dopingconcentration as inferred. Thus, the third and fourth summand can be neglected

The difference in energy between the Fermi leveland the band edges can be inferred fromBoltzmannBoltzmannBoltzmannBoltzmann statisticsstatisticsstatisticsstatistics andandandand isisisis givengivengivengiven bybybyby

concentration as inferred. Thus, the third and fourth summand can be neglectedand the diffusion voltage can be approximated by the bandgap energy dividedby the elementary charge

The energy gap and thethreshold voltage indeedagreeagreeagreeagree reasonablyreasonablyreasonablyreasonably wellwellwellwell

37

Deviations from the ideal I-V characteristics

where nideal is the idealityidealityidealityideality factorfactorfactorfactor ofofofof thethethethe diodediodediodediode.... ForForForFor aaaa perfectperfectperfectperfect diode,diode,diode,diode, thethethethe idealityidealityidealityideality factorfactorfactorfactorhashashashas aaaa valuevaluevaluevalue of unity (nideal = 1.0). For real diodes, the ideality factor assumes values oftypically nideal = 1.1–1.5. However, values as high as nideal = 2.0 for III–V arsenide andphosphide diodes. Values as high as nideal = 7.0 for GaN/GaInN diodes.

A series resistance can be caused by excessive contact resistance or by the resistance of theneutral regions. A parallel resistance can be caused by any channel that bypasses the p-njunction, caused by damaged regions of the p-n junction or by surface imperfections.

Parasitic resistances:Parasitic resistances:Parasitic resistances:Parasitic resistances:

Expected theoretical I––––V characteristic of a pV characteristic of a pV characteristic of a pV characteristic of a p----n junctionn junctionn junctionn junction

junction, caused by damaged regions of the p-n junction or by surface imperfections.

The diode I–Vcharacteristicneeds to bemodified inorder to takeinto accountparasiticresistances.

38

For Rp → ∞ and Rs → 0, this equationreduces to the Shockley equation.

Assuming a shunt with resistance Rp(parallel to the ideal diode) and a series resistance Rs(in series with the ideal diode and the shunt)

39

Efficiency Droop

• Light emission intensity not linear with current

• At high driving current, reduction of efficiency: Efficiency droopEfficiency droopEfficiency droopEfficiency droop

• Physical causes of droop still unexplained (many theories and unexplained (many theories and research developed worldwide by all LED manufacturers)

• Current solution: One LED package� multiple chip LED array Or large areachip

• Light output decreases and colour change with Heating: Thermal Heating: Thermal Heating: Thermal Heating: Thermal management necessarymanagement necessarymanagement necessarymanagement necessary

40

Research Institute

Causes of LED Droop Solutions

Samsung/RPIPolarization field - Polarization matching

-> AlGaInN & EBL

Philips LumiledsAuger recombination - Low carrier density for low Auger

recombination-> DH (single QW)

OsramLoss-channel

Carrier overflow- Phonon- or defect-assistedAuger recombination -> LossOsram Carrier overflow Auger recombination -> Losschannel -> Thick SQW/MQW

UCSBElectron overflow, Auger

recombination- Non-polar GaN LED

Virginia CUPoor hole transport - Hole mobility improvement

-> Thin p-barrier

Leti Intrinsic property - New structure-> GaN nanowire

Hanyang Univ. Reduction of radiativerecombination

- Radiative recombinationreduction by In clustering-> Reduction of In clustering- In source treatment

41

Temperature dependence of emission intensity

The emission intensity of LEDs decreases with increasing temperature. Due toseveral temperature-dependent factors including (i) non-radiative recombinationvia deep levels, (ii) surface recombination, and (iii) carrier loss overheterostructure barriers.

the phenomenological equation for LEDs

where T1 is the characteristiccharacteristiccharacteristiccharacteristic temperaturetemperaturetemperaturetemperature.... AAAA highhighhighhigh characteristiccharacteristiccharacteristiccharacteristic temperature,temperature,temperature,temperature,implyingimplyingimplyingimplying aaaa weakweakweakweak temperature dependence, is desirable.

the phenomenological equation for lasersthe phenomenological equation for lasers

where Ith is the threshold current of the laser.

Blue LED has the highestT1 and the red LED hasthe lowest T1. III–Vnitride LEDs havedeeper wells so thatcarrier confinement ismore effective in III–Vnitride structures thanin the III–V phosphidestructures

42

Light Escape Cone

Source: KETI

Escape efficiency of chip (ns= 2.5 for GaN): 8% for air (no=1): 11% for epoxy (no=1.5): 16% for sapphire (no=1.77)

43

Total internal reflection reduces the external efficiency significantly, in particular for LEDs consisting of high-refractive index materials.

Assume that the angle of incidence in the semiconductor at thesemiconductor–air interface is given by φ. Then the angle of incidence of therefracted ray, Φ, can be inferred from Snell’s law

where ns and nair are the refractive indices of the semiconductor and air,respectively. The criticalcriticalcriticalcritical angleangleangleangle forforforfor totaltotaltotaltotal internalinternalinternalinternal reflectionreflectionreflectionreflection isisisis obtainedobtainedobtainedobtained usingusingusingusing ΦΦΦΦ ====90909090°°°°90909090°°°°

The refractive indices ofsemiconductors are usually quite high.For example, GaAs has a refractiveindex of 3.4. Thus, the critical angle fortotal internal reflection is quite small.In this case, we can use theapproximation sinφc ≈ φc.

The angle of total internal reflectiondefines the lightlightlightlight escapeescapeescapeescape coneconeconecone....

θc

n1

n2

Light source44

Surface area of the spherical cone with radius r in order to determine the totalfraction of light that is emitted into the light escape cone.

Let us assume that light is emitted from a point-like source in thesemiconductor with a total power of Psource. Then the power that can escapefrom the semiconductor is given by

where 4πr2 is the entire surface area of the sphere with radius r.where 4πr2 is the entire surface area of the sphere with radius r.

Because the critical angle of totalinternal reflection for high-indexmaterials is relatively small, the cosineterm can be expanded into a powerseries. Neglecting higher-than-second-order terms yields

45

Radiation Pattern and Lambertian Emission Pattern

All LEDs have a certain radiationradiationradiationradiation patternpatternpatternpattern orororor farfarfarfar----fieldfieldfieldfield patternpatternpatternpattern.... TheTheTheThe intensity,intensity,intensity,intensity, measuredmeasuredmeasuredmeasuredinininin W/cm2, depends on the longitudinal and azimuth angle and the distance from theLED. The total optical power emitted by the LED is obtained by integration over thearea of a sphere.

where I(λ) is the spectralspectralspectralspectral lightlightlightlight intensityintensityintensityintensity (measured(measured(measured(measured inininin WWWW perperperper nmnmnmnm perperperper cmcmcmcm2222)))) andandandand AAAA isisisis thethethethesurfacesurfacesurfacesurface area of the sphere. The integration is carried out over the entire surfacearea.

The index contrast between the light-emitting material and the surroundingmaterial leads to a non-isotropic emission pattern. For high-index light-emittingmaterials with a planar surface, a lambertian emission pattern is obtained.

46

The total power emitted intoair can be calculated byintegrating the intensity overthe entire hemisphere. Thetotal power is then given by

By using the lambertianemission pattern for Iair and

Lambertian emission pattern given byLambertian emission pattern given byLambertian emission pattern given byLambertian emission pattern given by

air

using cos ΦsinΦ=(1/2)sin(2Φ),the integral can be calculatedto yield

Light power that escapes fromthe semiconductor (Pescape) mustbe identical to the power in air(Pair).

Fresnel reflection at the semiconductorFresnel reflection at the semiconductorFresnel reflection at the semiconductorFresnel reflection at the semiconductor––––air interface air interface air interface air interface has been has been has been has been neglected. At normal incidence, the Fresnel power transmittance is given by 47

Epoxy Encapsulants

The light extraction efficiency can be enhanced by using dome-shapedencapsulants with a large refractive index. As a result of the encapsulation, theangle of total internal reflection through the top surface of the semiconductor isincreased (Nuese et al., 1969)

Ratio of extraction efficiency with and without epoxy encapsulant is given by

where φc,epoxy and φc,air are the critical angles fortotal internal reflection at the semiconductor–epoxyand semiconductor–air interface, respectively

Inspection of the figure yields that the efficiencyInspection of the figure yields that the efficiencyof a typical semiconductor LED increases by afactor of 2–3 upon encapsulation with an epoxy

having a refractive index of 1.5.

light is incident at an angleof approximately 90°at theepoxy–air interface due tothe dome-shape of theepoxy � No total internalreflection losses at theepoxy–air interface.

48

MOCVD System

Vacuum system

Scrubbing system

Exhaust

Glove box

Control

Unit

Reactor with heated suscepter

Gas mixing unit

Gas supply

Gas flow control

49

Horizontal Reactor vs. Vertical Reactor

Turbulent flow Laminar flow

�Horizontal reactor: Hot/cold side walls- Heterogeneous reaction� hot side walls - Condensation: cold side walls

Simple

�Vertical reactor- No reagent pre-reaction - Uniform flow of homogenous mixed reagents- Heater zone temp.: Linear temp. profile- Excellent growth uniformity- Large scale production- High precursor utilization efficiency

“Two flow MOCVD: Vertical reactor”S. Nakamura et al., Appl. Phys. Lett. 58, 2021 (1991)

50

Growth of GaN with/without Buffer Layer

Without buffer layer With buffer layer

Ref. I. Akasaki et al. J. Cryst. Growth 98, 209 (1989)

Reduction of interface energy� Nucleation ↑

51

GaN growth on sapphire (0001)

500-600 oC

1000-1100 oC

Crystallizationat high temp.

LT buffer layer � Strain absorption layerNucleation layerAlN, GaN, InGaN, SiN, SiC etc.

� Nucleation layer theory by Akasaki

52

Growth Process of GaN Epilayer

� GaN on Sapphire substrate: defect density of108 cm-2

� Three step for MOVPE GaN Layer on sapphire

1) High temperature preparation of the sapphire surface2) Deposition of a low-temperature nucleation layer

3) High temperature epilayer growth

1) 3)

53

2)

GaN 성장기술: Low Defects

� Epitaxy: 결정을 갖는 웨이퍼 위에 방향성을 갖는 단결정 막을 성장하는 일

� GaN계 화합물반도체는 동종 기판의 부재로 인해 주로 사파이어 (Al2O3) 기판위에 성장함

Defects � nonradiative centers

� GaN와 사파이어 기판 사이의 격자상수 차이로인한 관통전위 (threading dislocation) 등의 결정결함을 줄이기 위한 방법으로 측면성장 (epitaxial lateral overgrowth: ELOELOELOELO) 법이 널리 활용

Jastrzebski, 1983: ELO Microchannel epitaxy (MCE), 1996

출처 : Sandia National Lab

Substrate

GaN

SiO2 mask

Dislocations

54

�Electron overflow - Poor p-doping of p-AlGaN layer (Ea~ 400 meV)- Increasing Al mole % in p-AlGaN greatly increases Ea

- Small ΔEc offsets between InGaN/GaN QW/barrier- Piezoelectric fields reduce p-AlGaN barrier height

� Solution: EBL (Carrier injection efficiency↑]

Electron Blocking Layer (EBL)

Cf. Hole Blocking layer� n-AlGaN

EC

Confinement layer

EBL

MQWActive region

Confinement layer

�Prevent the carrier overflow

EF

EV

EC

EF

EV n-type

i-type

p-type

AlGaN GaN GaInNAlGaN AlGaN

Undoped sturucture

Doped structure

Source : Light-emitting diodes, Fred Schubert

Prevent the injection of hole into active region � Highly doped p-AlGaN (difficult)[Solution) Superlattices

55

Polar/Non-polar GaN substrate

� Non-polar or semi-polar substrate

a or m-planePolarizations � electrostatic field�QCSE � Internal eff. , redshift of spectrum,Vth ↑, blueshift with increasing current

solutionBand bending

Wurtzite [1000]

Ga

N

Ga-facedN-faced

+

-

Sponta

neous p

ola

rizatio

n

+

-GaN

Ref. P. Waltereit et al. Nature 406, 865 (2000)Heterostructure� Piezoelectric polarization (Large lattice mismatch in group III)

56

Nichia

(0001) Sapphire Substrate(0001) Sapphire Substrate

결함 밀도 감소 � 내부양자효율 ↑발광면적 증가 � 광출력↑

PSS � 기판표면 난반사 � 광추출 효율↑

Patterned Sapphire Substrate (PSS)Nichia AlInGaN patterned substrate and meshed electrode LED

삼성종기원PSS위에 GaN 성장- Conventional: ηext: ~ 28%- Strip pattern: ηext: ~ 40% (1.43 배 증가)- Rectangular pattern: ηext: ~ 58% (2.1 배 증가)- Hemispherical pattern: ηext: ~ 63% (2.25 배 증가)

�Wet etching based on H3PO4 at 300 oC� pyramidal PSS�Dry etching

Ni

57

Why Vertical LEDs ?

1111stststst Generation (Lateral)Generation (Lateral)Generation (Lateral)Generation (Lateral)2222ndndndnd GenerationGenerationGenerationGeneration

(Flip Chip)(Flip Chip)(Flip Chip)(Flip Chip)3333rdrdrdrd Generation (Vertical)Generation (Vertical)Generation (Vertical)Generation (Vertical)

Structure

Sapphire

n-GaN

p-GaN

p-pad

n-pad

P-electrode

QW(s)

Sapphiresubstrate

Base

n-contact

p-reflector

Solder

p-GaN

n-GaN QW(s)

Metalalloy

p-GaN

n-GaN

n-pad

QW(s)

p-reflector

(+) ESD(+) ESD(+) ESD(+) ESD

((((----) ESD) ESD) ESD) ESD

� Loss in mesa etched area� Local current crowding� Low reliability� Low thermal conductivity of sapphire � Low power LED

� Uniform current distribution� Improved ESD� Thermal conductivity� High power LED

Sapphiresubstrate

Base

materialp-contact

alloy

58

Photolithography Process

Photomask patternsUV Light

PROxide layer

Silicon

GaN with oxides

Photoresist (Spin coater)

SoftbakeHotplate or oven

HardbakeHotplate or oven

Development of patterns

Etching region

Patterned oxides

GaN with oxides 1) PR coating 2) Exposure

3) Development 4) Oxides etching 5) Photoresist removal

HMDS (hexamethyldisilane)� adhesion

59

Sapphire substrate

u-GaN

n-GaN

InGaN/GaN

MQW

p-metal contact

p-GaNITO

n-metal contact

p-metal contact

n-metal contact

Transparent ITO contact

Dry Etching for Mesa

� Mesa etching � Sapphire: nonconductive substrate- Etching gases: Cl/Ar/BCl3 plasma, Mask: SiO2 or Si3N4

� Plasma defect (Additional wet etching in HCl:H2O (1:1))

GaN LED

Current Current Current Current spreadingspreadingspreadingspreadinglayerlayerlayerlayer

Topconfinement

Bottomconfinement

Active

Emission region

n-contact

Current flowCurrent flowCurrent flowCurrent flowInsulating substrate

p-contact

n type GaN

p type GaNpnpnpnpn junctionjunctionjunctionjunction

Ion bombardment + Chemical reaction

GaN LED

Conventional LED

++++ RIon Volatile productOhmic contact

region

Pump Gas

RF signal

Insulator

Lower electrode wafer holder

Diffuser nossles

Plasma

Gas

WafersUpper electrode

ICP (Oxford Plasmalab system 100) 60

Overview of Vertical LED Process using LLO

PR 코팅Scribing or 식각부분 선택

PR 코팅 제거

보호층의Spin coating

제거할 epi층선택

PR코팅 제거 도체 기판 위에본딩

Sapphire

GaN GaN GaN

Sapphire Sapphire Sapphire

Si or 금속 박막

본딩

레이저 scribing으로Si or 금속 박막 scribing

Laser pulse-> vaporization in interface (GaN/Sapphire)-> increasing Ionized vapor-> temporal evolution of plasma->explosive shock wave ->separationSapphire

Si or 금속 박막

Si or 금속 박막 Excimer laserLift-off

GaN(3.3 eV)

248 nm excimer Laser (5 eV)

(9.9 eV)248 nm투명함

61

Chip Shaping (I)

Lumileds TIP (truncated inverted pyramid) chipLumileds TIP (truncated inverted pyramid) chipLumileds TIP (truncated inverted pyramid) chipLumileds TIP (truncated inverted pyramid) chip

p-GaN

n-GaN

AlGaInp

M. O. HOLCOMB et al., Compound Semicinductor 7, 59 (2001)

� 측벽면 기울임 구조: 칩의 벽면으로 입사한 광자들이 반사를 할 때 진행방향이 불규칙하게 변하게 되어 광자가 첫번째 벽면에서 전반사를 하여 칩 밖으로 빠져나가는데 실패한다고 하더라도 다음 단계에서의 벽면에 대한 입사각이 임계각 이하로 주어질 기회가있기 때문에 광자가 빠져나갈 수 있는 확률이 커지게 되어 광추출 효율이 향상.

62

OSRAMOSRAMOSRAMOSRAMOSRAMOSRAMOSRAMOSRAM의의의의의의의의 기술기술기술기술기술기술기술기술� 옆면을 scribing하는 기술로 측면 각도의 변화는 반도체 내에서 발생한 빛이 측면으로의 추출이 원활하게 한함.

� 단점: 질화물 반도체의 측면 내부각도의 세밀한 조절이 질화물 반도체가 가지는 물리적, 화학적 성질 때문에 쉽지 않음.

� Thin GaN는 sapphire 기판 위에 결정 성장시킨 InGaN와 Ge 등의 캐리어 재료를 접합하여 sapphire 기판을 레이저 광조사법으로 lift-off 하여 제작

Chip Shaping (II)

OSRAM Opto Semiconductors 자료참조

63

Filp-chip LED에서의 빛의 경로

~~ ~~

p-GaN

n-GaN

MQW

Ni/Ag

Sapphire

air

~~ ~~

p-GaN

n-GaN

MQW

Ni/Ag

Sapphire

airlight

Ti/Al

Non textured

Flip Chip LED + Texture

Flip-chip LED의 표면에 요철을형성하였을 때의 빛의 경로

Flip-chip LED에 요철 적용 후의 광 출력

광결정 구조로 발전

� Sapphire 표면에 형성된 요철의 깊이를 달리하며 건식 식각을 하면 요철의 깊이에 따라 광추출 효율이 달라지며, 요철의 깊이가 400 nm 일 때 가장 큰 광추출효율을 얻음.

출처: 물리학과 첨단기술 (2008)

~~ ~~

p-GaN

n-GaN

MQW

Ni/Ag

Ti/Al

~~ ~~

p-GaN

n-GaN

MQW

Ni/Ag

Ti/Al

light

64

1. Cr or SiO2 증착 및 PR 도포

Resist

Cr or SiO2p-GaN

QW active layern-GaN

Sapphire

Cr or SiO2p-GaN

QW active layern-GaN

Sapphire

Cr or SiO2

Resist

p-GaNQW active layer

n-GaN

Sapphire

2. PR nano patterning 3. Cr or SiO2 식각 및 PR 제거

LED with Nanostructures

p-GaNQW active layer

n-GaN

Sapphire

4. p-GaN 식각 및 mask 제거

Daniel L. Barton et al., 17 April 2006, SPIE Newsroom

65

LED Packaging 개요

LED package

SMD type Lamp type

고분자 접착제

SMD type

경제적칩보호 및 빛 투과 (고출력: Si계)

Lamp type

Plastic dome

pn junction

Electrodes

� 패키지: 칩의 보호, 에폭시를 통한 전반사 감소, 광학설계에 의한 빛의 제어, 열방출

� LED 패키지: 칩, 접착제, 봉지재, 형광체 및 방열 부속품 등으로 구성

� Lamp type: 주로 투명한 몰드 (mold)로 쌓여져 있으며 내부에 LED칩이 들어있음

� SMD (surface mount device) type : 부품의 다리를 인쇄회로기판 (PCB)의 구멍에끼워서 납땜하지 않고 부품을 회로 기판에 얹어 놓은 상태로 납땜하여 사용� 소형화가 가능해 주로 휴대폰 등 모바일 기기에 사용

물리학과 첨단기술, Nov. 2008, pp. 16-21

고분자 접착제E-beam, X선, 자외선 � 가시광선 (고상, 액상, 기상)

칩보호 및 빛 투과 (고출력: Si계)

� 강제대류, 강제전도: 수냉식, 공랭식, TEC, 자연대류: heatsink, slug

66

LED Grinding/Lapping/Polishing/Dicing

� LED dicing: GaN scribing and substrate cutting- GaN scribing with high precision by laser or diamond techniques- Substrate cutting with less precision by diamond saws and (laser or diamond),

break techniques

� LED grinding/lapping/polishing

출처: λ LOGITECH

Laser Scriber (diode-pumped solid state (DPSS) laser)출처: JPSA

Diamond saws

Laser

67

LED Package 열 특성

� 고출력 LED: 소비전력이 높아 많은 열을 dissipation하기 위해 방열이 필수적발생된 열 � 소자의 온도 ↑ � thermal stress � 소자 degradation, 파장 변화 � reliabilty, 수명 ↓

� 칩본딩, 패키지 물질, 고출력 멀티칩, 환경/구동조건에 따른 열설계 필요Cf. AlN �180 W/mK

Cu � 393 W/mK

SiC � 270 W/mK

Si � 140 W/mK

Al � 240 W/mK

In� 87 W/mK

AnSn � 57 W/mK

출처: OSRAM

AnSn � 57 W/mK

68

Thank you for your attentionThank you for your attention