1

The Physics of Solid-State Lighting

Fernando A. Ponce

Department of Physics Arizona State University Tempe, Arizona, USA Arizona State University

Nano

& Giga Challenges in Electronics, Photonics & Renewable Energy

14th Canadian Semiconductor Technology Conference

Hamilton, Ontario, Canada, August 10-14, 2009

2

Outline

1. Light production2. The physics of GaN epitaxy3. Overcoming lattice mismatch4. Piezoelectric fields5. Producing white light

3

DEVELOPMENT OF

LIGHTING TECHNOLOGIES

4

Incandescent Lamp

Thomas Edison

1870

Tungsten filaments

early 1900s

Lifetime > 1000 hours

Tungsten-halogen lamp

1970s

a small amount of Hg reduces migration of W,

allows operation at higher T, produces whiter light

http://science.howstuffworks.com/incandesent-lamp2.htm

5

Black Body Radiation

Incandescent lamps follow the black-body radiation characteristics.Their color and brightness depends on the temperature of the filament

3,000K5,000K

10,000K

7,000K

1)exp(

12)( 5

2

−=

kThc

hcTB

λλλUltraviolet Visible Infrared

6

White Light: T= 5500K

The spectrum of white light is that of solar radiation, corresponding to black body radiation at 5500K

Ultraviolet Visible Infrared

7

Human Eye Response

The spectrum of white light is that of solar radiation, corresponding to black body radiation at 5500K

Ultraviolet Visible Infrared

8http://science.howstuffworks.com/fluorescent-lamp2.h

The Fluorescent Lamp

9

HgLine

PHOSPHOR

Fluorescent Lamps

The ultraviolet radiation of mercury is converted into visibleby phosphor deposited inside the glass-tube envelope

10

Earth at Night

Acquired by NASA, Nov 2001 Physics Today, April 2002 IssueNight-time satellite image of the world.

The use of light at night appears to be directly related to wealth and prosperity. The lights also reflect trends in population and energy consumption.

11

Great Challenges for the 21st Century1. Energy consumption

Oil nuclear, wind, and solar

2. Climate changeGlobal heating Melting of the poles, Possible new ice age

• Both are closely linked.• We must be concious of the problem.• It is essential to take drastic steps to reduce the use of

hydrocarbons, and the destruction of our forests.• Kyoto Protocol (199y)• The oil industry questions scientific findings about global climate

change.

12

Concentration of CO2 and Global Mean Temperature

Source: DOE/BES Workshop on “Basic Research Needs for the Hydrogen Economy”

13

SOLID STATE

LIGHTING

14

Solid State Lighting: The Light Emitting Diode

F. A. Ponce and D. P. Bour, Nature 386, 351 (1997)

15F. A. Ponce and D. P. Bour, Nature 386, 351 (1997)

Evolution of Visible LEDs

Fluorescent lighting

16F. A. Ponce and D. P. Bour, Nature 386, 351 (1997)

Compound semiconductors used in visible light emitters

17

Chromaticity Diagram

F. A. Ponce and D. P. Bour, Nature 386, 351 (1997)

Chromaticity Diagram

18

White Light: 3 LEDs

19

White Light: Purple + Phosphor

LEDPHOSPHOR

20

Diff

icul

ty

more indium

more aluminum

High EfficiencyLEDs

CW Lasers

Degree of Difficulty

AlNGaN

InN

Wavelength400 nm 600 nm200 nm

UV | Visible | IR

Nitride Semiconductors for Light Emitters

21

• The main challenge today is in understanding the nature (physics) of these materials.

• An important question is why the internal quantum efficiency is so low.

Challenges

22

GaN 3.4 eV (UV)InN

0.8 eV (IR)

Inx

Ga1-x

N

Visible + UVEg (x) = xEg

InN + (1-x)EgGaN – bx(1-x)

Compositional inhomogeneities phase separation for x >

0.06

a cGaN 3.189 5.185InN 3.545 5.703

InGaN Alloys

11% lattice mismatch

23

THE PHYSICS OF GaN

EPITAXY

24

Columnar Model for III-V Nitride Films

Low-angle domain boundariesBy x-ray rocking curves

C-axis tilt: ~5 arc minC-axis rotation: ~8 arc min

(assymetric

reflexions)F. A. Ponce, MRS Bull, 22, 51 (1997)

Cross-section view --

Nichia blue LEDGaN/GaInN/GaN/Sapphire

Using GaN buffer layers

1010

Dislocations/cm2

F. A. Ponce, D. P. Bour, Nature 386, 351 (1997)

be

bs

bm

Three types of DLs:Edge type, be

= 1/3 <1120>

Screw type, bs

= <0001>

Mixed type, bm

= 1/3 <1123>

F. A. Ponce et al, APL 69, 770 (1996).

Threading dislocations limit the electrical and optical performance of devices.Need to know the electronic charge states at different types of threading DLs.

25

THREADING DISLOCATIONS

• They are not necessarily bad• They help in growth morphology, but…• What are they?• Are they electrically active?• Are they charged?• What effect on electrostatic potential?

26

Effect of Charge at Dislocations

-

Majority carriers are not attracted by dislocations. -

Charged dislocations should not adversely affect vertical transport devices such as LEDs

∫−

=r

2 )ρ(c2nr

dxxxπ

n-GaN

Net charge is neutral or negative

p-GaN

Net charge is neutral or positive

Conduction band

Valence band

Conduction band

Valence band

27

Dislocations in Laser Diodes

- Screw dislocations are typically coreless. -

Metal is ionized by high energy photons

- Metal ions migrate towards p-n

junction- Screw dislocations provide a path- Catastrophic failure when metal shorts diode

REDUCTION OF DISLOCATION DENSITY INCREASES LASER DIODE LIFETIME

p-GaN

Conduction band

Valence band

MetalContact

28

LATTICE MISMATCH

29

Lattice Mismatch and Misfit Dislocations

SUBSTRATEInN GaN AlN SiC Sapphire

-10.6% 0 1.9% 12%0 10.6% 13.9% 25.4%

LATTICE MISMATCH

30R. Liu, F. A. Ponce, et al.. Misfit dislocation generation in InGaN epilayers on free-standing GaN. Japan. J. Appl. Phys. 45, L549 (2006).

31

Dislocation punch-out mechanism

Punch-out mechanism results in two edge dislocation with Burgers vector a, in close proximity.

R. Liu, F. A. Ponce, et al.. Misfit dislocation generation in InGaN epilayers on free-standing GaN. Japan. J. Appl. Phys. 45, L549 (2006).

32

On free-standing GaN; [In]~11%

g=11-20Plan-view TEM image

200nm

Arrays of interfacial dislocation half loops are observed around the surface pits.

Basal plane slip from pinholes in free standing GaN

R. Liu, F. A. Ponce, et al.. Generation of misfit dislocations by basal-plane slip in InGaN/GaN

heterostructures . Appl Phys Lett 88

in press (2006)

33

InGaN

GaN

The interface dislocations are induced by the misfit strain.The free surface of the pit facets facilitate the introduction of dislocations.These dislocations are a-type dislocations, they can glide on the basal plane.

x x x x x x x x x x

Dislocation half loops

R. Liu, F. A. Ponce, et al.. Generation of misfit dislocations by basal-plane slip in InGaN/GaN

heterostructures . Appl Phys Lett 88

in press (2006)

34

InN/GaN/sapphire by MOCVD

T. Matsuoka, H. Okamoto, H. Takakhata, T. Mitate, S. Mizuno, Y. Uchyama and T. Makimoto, J. Cryst. Growth 269, 139-144 (2004).

A 5 micron thick GaN epilayer was grown on a c-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD), at ~600C.

Afterwards, a 0.9 micron thick InN epilayer was grown directly on the GaN layer.

100nm

InN

GaN

35

0 5 10 15 20 25 30 35 40600

650

700

750

800

850

900

950

(nm)

inte

nsity

20 nm20 nm

InN

GaN

TEM of InN

GaN Interface

(a) BFTEM [1100] of the InN/GaN interface indicating it is abrupt. (b) Image contrast at the interface outlined by red arrows.Contrast ~ 3nm~ 9 InN unit cells indicates near fully relaxed conditions.The density of the interface defects ~3X1014/cm2

3nm

(a)

(b)3nm

36

Fourier reconstructed image ofInN_GaN

interface

FRTEM image of InN_GaN

interface showing regions where extra (1122) lattice planes are inserted from the substrate side spaced ~ every 2.8nm in the (1-10) direction. Burgers circuit analysis yields the projected component shown in red, note from plane view diffraction patterns the line

direction is expected to be along any of the symmetry equivalent <1120>

10 nm10 nm

bproj

=√3a

⁄2

√2[1100]GaNDLs

InN

GaN

[0001]

[1100]

[1120]

37

5nm

Fig. (a) Plane view image of InN/GaN

with diffraction pattern(inset) (b) Diagram illustrating array of misfit dislocations at InN/GaN

interface down [0001] projection.

[0001] projection of misfit DL array at InN/GaN

Interface

<1120>

~3nm

(a) (b)[0001]

InN{1010}

GaN{1010}

38

Model for interface

cubic lattice site metastable

wurtzite lattice site

stable GaN

InN

Figure (a) Calculation of number of GaN basal lattice sites required for relaxed commensurate overgrowth.(b) Illustration of the mechanics of non orthogonal misfit dislocation networks, Only at the corners of the SL unit cells are the lattices matched and at the center the In is actually shifted by 1/2[1120] from the underlying GaN lattice. This mechanism would produce an edge component Burgers vector equal to [ ]0011

223]2011[ abproj =

(a) (b)

39

LATTICE MISMATCH: ↓

STRONG INTERNAL PIEZOELECTRIC

FIELDS AND CHARGES

40

Direct determination of the band structure of InGaN/GaN and

AlGaN/GaN by electron holography

41

Schematic of Electron Holography

Consider a free electron traveling through vacuum

Consider a free electron traveling through a thin foil:

Vacuum Material Vacuum

42

Schematic of Electron HolographyThe wavelength of the electron beam is shorter inside the material because the potential energy has changed (dropped) and therefore the kinetic energy has increased (if the total energy of the electron is to remain constant).

Vacuum Material Vacuum

The change in wavelength will cause a phase shift in the signal wave (propagating through the sample) with respect to the reference wave (traveling through vacuum only).

43

Orientation dependence of piezoelectric field

Using rotation transform of coordinate systemto describe the stress, strain, piezoelectric constant and the elastic stiffness coefficients for the epitaxial layer grown along arbitrary direction z’.

T. Takeuchi, H. Amano, I. Akasaki, Jpn. J. Appl. Phys., 39, 413, 2000

The calculated piezoelectric field (longitudinal andtransverse) depends on the polar angle between z’ and z.

Taking the strict constrained condition for substrate and epitaxial layers, to determine the precise lattice mismatch.

The polarization depends on the atomic constitution of the crystalline lattice and the crystalline symmetry.

By changing the crystal orientation intentionally, piezoelectric field can be diminished

44

Orientation dependence of piezoelectric field

0 20 40 60 80-8

-6

-4

-2

0

2

Long

itudi

nal p

iezo

elec

tric

field

(MV/

cm)

Polar angle (degree)

In 10% In 20% In 30% In 40%

0 20 40 60 80-4

-3

-2

-1

0

1

2

3

4

5

Tran

sver

se p

iezo

elec

tric

field

(MV/

cm)

Polar angle (degree)

In 10% In 20% In 30% In 40%

0 20 40 60 800

1

2

3

4

5

6

7

8

Tota

l pie

zoel

ectri

c fie

ld (M

V/c

m)

Polar angle (degree)

In 10% In 20% In 30% In 40%

Polar angle: 0=polar plane, 90=non-polar plane, others=semi-polar plane.For semi-polar planes, the longitudinal piezoelectric field reaches zero at 42 degree while the transverse piezoelectric field reaches zero at 66 degree.

A valley of total piezoelectric field appearsat around 60 degree.

45

Electron Holography and PE Fields

Coherent electron source

Ψref = Ar exp(iθr

) Specimen

Objective lens

Biprism

Ψobj =Ao exp(iθo

)

Philips CM-200 FEGSchematic beam pathInterference fringes

Philips CM-200 FEGSchematic beam path

46

Electron holography study on InGaN single quantum wells

47

5 10 15 20 25

14

12

10

8

6

4

2

Phas

e (r

ad)

[0001]

GaN

GaN

InGaN QW

6.106.146.186.226.266.306.346.386.426.466.506.546.586.626.666.706.746.786.826.866.90

Distance (nm)

Dis

tanc

e (n

m)

Electron Holography of InGaN QW

The phase contour shows that the top GaN barrier has higher value of phase, while thickness contour shows flat plane.Phase and thickness profiles are obtained over a 2nm wide strip.

Phase contour image Thickness contour image

J. Cai and F. A. Ponce, J. Appl. Phys. 91, 9856 (2002)

5 10 15 20 25

14

12

10

8

6

4

2

[0001]

GaN

GaN

InGaN quantum well

Thic

knes

s (n

m)

Dis

tanc

e (n

m)

60.0

80.0

100.0

120.0

140.0

160.0

180.0

Distance (nm)

48

Energy Profile across InGaN QW

50 1000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Distance (Å)

Thic

knes

s (n

m)

Phas

e (R

ad)

0

50

100

GaN InGaN GaN

[0001]

50 100-0.8

-0.6

-0.4

-0.2

0.0

Ener

gy (e

V)Distance (Å)

Observed Curve A Curve B

The slope of energy profile indicates the electric field in the quantum well is -2.2 MV/cm. Charge distribution is analyzed following two approaches, curve A and B.

Phase and thickness profiles Energy profile

J. Cai and F. A. Ponce, J. Appl. Phys.

91, 9856 (2002)

)y,x(t)y,x(VC)y,x( E=θΔ

49

Device Structure

(4.6nm)

50

Phase and amplitude image derived from the holograms

(b) phase image (c) amplitude image

51

Electrostatic potential across the active region and the charge distribution

Piezoelectric polarization charges

:Bottom 3 QWs: 0.018 C/m2

(corresponding to PE fields of ~1.95 MV/cm)Fourth QW: 0.007 C/m2

Fifth QW: 0.0005 C/m2

Drastic reduction in the internal fields for the top two QWs

(1) The average variation of the potential corresponds to the p-n

junction (~3 eV).

(2) The bottom three wells exhibit strong PE fields. that oppose the p-n

junction built-in field, while the top two QWs

only show weak PE fields.

52

e-

hνe-

hν

Depth-Resolved Cathodoluminescence

Bethe Range:

H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, Japan. J. Appl. Phys. 28, L2112 (1989).

Maximum energy absorption depth increases with beam energy

53

Interpretation of depth resolved CL spectra

(1) electron beam voltage 5-14 kV, top two QWs with weak piezoelectric field are mainly excited, 2.35eV peak dominates

(2) electron beam voltage >17 kV,bottom three QWs with strong piezoelectric field are mainly excited, 2.23eV peak dominates

54

No misfit dislocations observed for InGaN QWs

What could be the reason?

pure edge

g = 0002Mixed TD

g=1120

55

Compensation of piezoelectric charge by H ions

InGaN InGaNInGaN

p GaN

• Decay of internal fields in InGaN QWs observed.• Possible passivation of the PE dipole moment by H ions.

56

Determination of the electronic band structure for an

AlGaN/AlN/GaN 2-DEG

57

AlGaN/AlN/GaN heterostructure

Sapphire

2um GaN bulk

1nm AlN

30 nm nid AlGaN

2um GaN bulk

High resolution electron micrograph of the AlGaN/AlN/GaN heterostructure. The <0001> growthdirection is from bottom to top. The AlN layer has a bright contrast.

Layer structure

58

Phase and amplitude of a electron hologram

(a) Phase and (b) amplitude hologram images showing the edge of thesample at the lower right corner. The <0001> growth direction is indicated.

59

Phase and thickness profile across the AlGaN/AlN/GaN heterostructure

Electrostatic potential (lower curve) and corresponding thickness (upper curve) profilesderived from the reconstructed phase and amplitude images, assuming an inelasticmean-free path of 60 nm. All the profiles are along the growth direction. The position of the AlN/GaN interface is at the origin.

60

Experimental potential profile

Potential energy profile across the AlGaN/AlN/GaN heterostructure, derived from the electron hologram data.

61

Charge distribution

(a) Functional fit of the potential profile over the 2DEG region and the AlGaN barrier layer. The AlN layer is not being considered. (b) Charge density distribution over the 2DEG region

and the AlGaN barrier layer, obtained using Poison’s equation by twice differentiating the functional fit of the potential profile.

62

Determination of the electronic band structure for a graded modulation-doped

AlGaN/AlN/GaN superlattice

63

Layer structure

Sapphire

2um GaN bulk0.5 nm AlN

12 nm n- AlGaN

2um GaN bulk

2um GaN bulk0.5nm AlN

12 nm n-AlGaN

30 nm GaN

20 nm GaN

0.5nm AlN12 nm n-AlGaN

One period

X 11

AlGaN is graded from30% to 0%, and doped as2*1019cm-3

64

Transmission electron microscope image of the SL structure

Bright-field transmission electron micrograph of the AlGaN/AlN/GaN superlattice. The <0001> growth direction is from bottom to top. (a) Multibeam image showing the superlattice, (b) enlargement of the rectangular box showing the graded natureof the barrier. The AlGaN layer is the brighter layer.

65

Low magnification electron holography

(a) Phase and (b) amplitude images derived from the low magnification electron hologram, used to analyze the nature of superlattice.

66

Potential profile derive from low magnification electron holography

The distribution of the 2DEG over different periods are uniform.

2DEG signature

67

High resolution potential profile for the top two periods of the SL

Potential energy profile across the AlGaN/AlN/GaN heterostructure for the top two periods, derived from a high magnification electron hologram.

68

Comparison with the self-consistent Schrodinger- Poisson calculations

A good match can be obtained by a parabolic Al grading, where t is the distance from AlN/AlGaN interface, and the total AlGaN layer thickness (12 nm in this work).

2( ) 0.4( / ) 0.1 / 0.3AlGaN AlGaNx t t t t t= − + +

69

Summary• Polarity in nitride semiconductors play a strong

role in optoelectronic devices • Electron holography in the TEM allows direct

measurement of polarization fields• Piezoelectric fields with strengths of 2 MV/cm are

commonly observed in QWs• H-passivation effects are observed at GaN-based

p-n junctions• Properties of 2DEG AlGaN/GaN superlattices

can be discerned

70

Nano-Challenges: Phosphor

• Use UV emission of InGaN/GaN with low indium composition ( λ≈

410 nm)

• Use phosphor to down convert and produce desirable white irradiation - using specially designed phosphors

• Multiple photon down conversion - high energy electrons transfer all their energy into light using nanoscale phosphor materials

71

Nano-Challenges: InGaN• Understand the origins of the Green Gap • Synthesis of InGaN

- High temperature needed for N ( > 1100 C) - Low temperature needed for In ( < 600 C) - Hydrogen induced volatility of indium Possible solution – High pressure reactors

• Polarization and piezoelectric fields - Overcome by using non-polar orientations - Take advantage of their localization effects

• Lattice mismatch - Suitable substrate - Controlled introduction of misfit dislocations

72

PRODUCTION OF WHITE LIGHT

73

White Light: Purple + Phosphor

LEDPHOSPHOR

74

White Light: 3 LEDs

75

A true white light emitter

Growth on c-plane substrates:

Strong piezoelectric fields:• Quantum confined Stark effect.• Reduced quantum efficiency due to carrier separation

Limited avenues for strain relaxation:• Slip along basal plane is inactive.• Pseudomorphic growth.

Sapphire

GaN

GaN

(0001)

Conventional epitaxy

{0001}<11 0>2

J. Cai and F. A. Ponce, J. Appl. Phys 91, 9856 (2002).

S. Srinivasan et al., Appl. Phys. Lett.

83, 5287 (2003).

InGaN QW

76

Non-planar growth of InGaN QWs

K. Nishizuka et al. Appl. Phys. Lett., 2004 {1122}P.R. Edwards et al. Appl. Phys. Lett., 2004 {1011}

S. Khatesevich et al. J. Appl. Phys., 2004 {1101}

Motivation:• Reduced piezoelectric fields.

• Different slip plane for strain relaxation.

• Differences in indium incorporation.

{1122}

Conventional Epi (Reference) This study

Sapphire

(0001)

3 nm InGaN QW

4 μm GaN

35 nm GaN

Previous studies on non-planar QWs

77

350 400 450 500 550

500 nm QW

CL

Inte

nsity

(cou

nts)

Wavelength (nm)

GaN-related peaks

InGaN QW emission

CL Integral Spectrum

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

QW emission is seen as a flat spectrum over a wide wavelength range

350 400 450 500 550 600

Wavelength (nm)

Non-planar QW C-plane QW

78

CL Images

350 400 450 500 550

C

L In

tens

ity (c

ount

s)

Wavelength (nm)

5 μm

λ

= 397 nmλ

= 417 nmλ

= 439 nmλ

= 486 nm

SE

CLI

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

79

Sapphire

GaN

InGaN QW

<1 00>1

SE

λ = 397 nm

λ = 417 nm

λ = 439 nm

λ = 486 nm

5 mμ

(a)

(b)

(c)

(d)

(e)

(f)

CL Images

Each wavelength emission arises from a well-defined narrow band.

Longer wavelengths from higher regions of triangle

350 400 450 500 550

500 nm QW

CL

Inte

nsity

(cou

nts)

Wavelength (nm)

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

80

1

2

3

4

5

{1122}

Gradient in emission

350 400 450 500 550

500 nm QW

CL

Inte

nsity

(cou

nts)

Wavelength (nm)

350 400 450 500 550

CL

Inte

nsity

(cou

nts)

Wavelength (nm)

Local Spectra

1

2 3 4 5

Integral Spectrum

• Local emission wavelength increases continuously from base to apex of triangle.

• Gaussian shaped peak from each region.

• Polychromatic emission.

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

81

Quantum well width

10nm

0 5 10 150

1

2

3

4

5

Distance from substrate (μm)

Qua

ntum

wel

l wid

th (n

m)

0

10

20

30

40

50

Cap

laye

r thi

ckne

ss (n

m)

• QW width increases continuously

from base to apex.

• GaN capping layer follows similar trend.

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

82-5 0 5 10 15 20 25

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

distance(nm)

rela

tive

pote

ntia

l(V

)Electron Holography

0 2 4 6 8 10 12-1.0

-0.5

0.0

0.5

1.0

Capping LayerUnderlayer

Rel

ativ

e Po

tent

ial

(V)

Base

Middle

Apex

Distance (nm)

Measured EH Potential

C-plane; 3 nm QW; x = 0.13

Measured potential profile: ELOG QW

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

750 850 950 1050 1150 1250

(angstroms)

(V)

apex

mid

base

Calculated potential profile for square QW

• No net piezoelectric fields across ELO QW.

• Wider wells have a bigger potential drop across QW.

• Calculations show that the potential profiles correspond to a square QW.

•The depth of the potential is associated with increased free carrier trapping in wider QWs.

M. Stevens et al., Appl. Phys. Lett. 85, 4651 (2004).

83

No PE field across QW on inclined facet

Seoung-Hwan Park, J. Appl. Phys. 91, 9904 (2002).

θ= 55o

{1122} = 58.4o

1 µm1 µm 58.4o

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

84

5 10 15380

400

420

440

460

480

500

Calculatedx = 0.27

MeasuredS3

Emis

sion

Wav

elen

gth

(nm

)

Distance from Substrate (μm)

Wavelength Calculation

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

• No significant variation in composition detected along the {1122} facet.

• Polychromatic emission appears to be due to well width variation alone.

85K. Hiramatsu, J. Phys.: Condens. Matter 13, 6961 (2001)

GaN

InGaN

Facet formation• InGaN is grown at a lower temperature than the ELO stripe.

• The shape of the equilibrium facet is different under these conditions.

• Capping layer follows the same trend.

86

True White LED?

87M. Yamada et al., Jpn. J. Appl. Phys. 41, L246 (2002).

White Light

Polychromatic EmissionWhite

RGB MQWs

J. Y. Tsao, IEEE Circuits and Devices 20, 28 (2004).

S. Srinivasan et al., Appl. Phys. Lett., in press, (2005)

Phosphor Conversion

White

Blue LED

Yellow Phosphor

Multi-chip

White

RGB LEDs

88

• Polychromatic light emission has been obtained from single InGaN

QWs

grown on pyramidal GaN planes using ELOG.

• A steady gradient is observed in the local emission characteristics with position along the {11-22} facet.

• This gradient is found to be related to a continuous spatial variation in the quantum well width.

• Different ranges in wavelength have been achieved by changing the growth temperature.

• White light can be obtained with a single quantum well.

Polychromatic light emission

89

Summary

• Nitride semiconductors have triggered a new generation of lighting technologies

• There is much activity worldwide• It promises high energy efficiency in

lighting applications, and long lifetimes.• The nature of these materials is

surprisingly different and challenging• The combination of microscopic techniques

allows simultaneous structural, electronic, and optical characterization

90

1

10

100

1000

400 500 600 700 800 900

Peak Wavelength (nm)

Lum

inou

s Pe

rfor

man

ce(lu

men

s/W

att)

High PressureSodium (1kW)

Fluorescent (40W)Mercury Vapor (1kW)

Halogen (30W)Tungsten (60W)

Red-FilteredTungsten (60W)

AlGaInP

InGaN AlGaAs

Eye Response Curve(CIE)

Bes

t

Luminous efficiency

Courtesy of D. P. Bour,

91

Nitride LEDsE

xter

nal Q

uant

um E

ffic

ienc

y (%

)

Wavelength (nm)

Underlying layer：GaN

Underlying layer :AlGaN

Source: Prof. H. Amano, Sept’04