The Physics of Solid-State Lighting - ASDN
Transcript of The Physics of Solid-State Lighting - ASDN
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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
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Outline
1. Light production2. The physics of GaN epitaxy3. Overcoming lattice mismatch4. Piezoelectric fields5. Producing white light
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DEVELOPMENT OF
LIGHTING TECHNOLOGIES
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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
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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
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White Light: T= 5500K
The spectrum of white light is that of solar radiation, corresponding to black body radiation at 5500K
Ultraviolet Visible Infrared
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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
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HgLine
PHOSPHOR
Fluorescent Lamps
The ultraviolet radiation of mercury is converted into visibleby phosphor deposited inside the glass-tube envelope
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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.
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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.
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Concentration of CO2 and Global Mean Temperature
Source: DOE/BES Workshop on “Basic Research Needs for the Hydrogen Economy”
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SOLID STATE
LIGHTING
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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
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Chromaticity Diagram
F. A. Ponce and D. P. Bour, Nature 386, 351 (1997)
Chromaticity Diagram
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White Light: 3 LEDs
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White Light: Purple + Phosphor
LEDPHOSPHOR
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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
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• 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
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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
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THE PHYSICS OF GaN
EPITAXY
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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.
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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?
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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
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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
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LATTICE MISMATCH
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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).
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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).
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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)
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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)
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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
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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
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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]
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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}
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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)
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LATTICE MISMATCH: ↓
STRONG INTERNAL PIEZOELECTRIC
FIELDS AND CHARGES
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Direct determination of the band structure of InGaN/GaN and
AlGaN/GaN by electron holography
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Schematic of Electron Holography
Consider a free electron traveling through vacuum
Consider a free electron traveling through a thin foil:
Vacuum Material Vacuum
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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).
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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
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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.
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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
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Electron holography study on InGaN single quantum wells
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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)
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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
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Device Structure
(4.6nm)
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Phase and amplitude image derived from the holograms
(b) phase image (c) amplitude image
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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.
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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
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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
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No misfit dislocations observed for InGaN QWs
What could be the reason?
pure edge
g = 0002Mixed TD
g=1120
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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.
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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.
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Determination of the electronic band structure for a graded modulation-doped
AlGaN/AlN/GaN superlattice
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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
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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
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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
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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
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PRODUCTION OF WHITE LIGHT
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White Light: Purple + Phosphor
LEDPHOSPHOR
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White Light: 3 LEDs
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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
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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)
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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).
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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
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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