Claude Weisbuch

1
Light extraction from semiconductors

• Claude Weisbuch
• Materials Department, UCSB, USA, Genewave, Ecole
  Polytechnique, Palaiseau, Lab PMC, Ecole
  Polytechnique, Palaiseau, France

2
Light extraction from semiconductors

• ? Introduction to light extraction
• ? GaN microcavities expectations, achievements,
  challenges
• ? Photonic Crystals LEDs
• principles
• photo and electroluminescence
• advanced concepts
• Tailoring of guided modes
• Archimedean tilings
• Laser lift-off Photonic Crystal LEDs
• ?Conclusions what remaining challenges

3
Microcavity and photonic crystal LEDs The basic
idea
6 of light extracted 94 trapped in
high-index material
Geometrical solutions Use the wave nature
of light
1.
2.
Modification of emission pattern MCLEDS
Diffraction of trapped light Photonic Crystals
4
Solid state lighting so far, geometrical
solutions, quite good
maximum in-plane wavevector for escape is
outside air wavector
5
Microcavity LEDs starting from the Fabry-Pérot
cavity
Output mirror
Fabry-Perot cavity Interference effects ? Modify
the directions of emission
z
Back Mirror
Important parameters for the FP effect -
mirror reflectivities (r1, r2) - position of the
source (z) - size of the cavity (L)
Solid-State Lighting Display Center
6
Microcavity LEDs Requirements from the
Fabry-Pérot cavity
- size of the cavity (L)
Solid-State Lighting Display Center
7
Microcavity LEDs Effect of cavity order and
tuning on emission pattern
mc 2
mc 8
Efficiency h 2/mc Low mc needed ? small
cavity
Solid-State Lighting Display Center
8
Microcavity emission directionality Perfect
bottom mirror top mirror R0.2
????????3 zero detuning
?????????? zero detuning
????????3 optimal detuning
????????? zero detuning
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Microcavity emission directionality Perfect
bottom mirror top mirror R0.2
top mirror R0.2
top mirror DBR 3 pairs R0.2
???????? ??????????
???????? ??to air
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Microcavity LEDs Effect of cavity order on
efficiency
Strongest effects for Lcav ?/n i.e. Lcav 200
nm here !
Solid-State Lighting Display Center
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Microcavity LEDs Various cavity designs
1. Metallic mirrors
Requires lift-off of Sapphire substrate and
flip-chip technique Requires mirror with good
reflectivity and electrical properties Enables
very small cavity length
L
Metallic mirror
Solid-State Lighting Display Center
12
Microcavity LEDs Ultimate goal
Rabbits ears
Only possible with perfect control of L, z and
mirrors
Solid-State Lighting Display Center
13
Results of GaN microcavity emitter modeling It is
better to have poor reflectivity but well
localized mirror by using a single interface
Metal mirror DBR (3 periods)
DBR (13 periods) DBR (3 periods)
r0.3
L /2
? /4n
r0.98
mc1.5 meff5 31 43
mc2 meff8.5 23 27
Metal mirror interface
DBR (13 periods) interface
L /2
? /4n
mc1.5 31 44
mc2 meff5 24 27
- Only in-plane dipoles - Monochromatic emission
in air in epoxy
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GaN microcavity emitter modeling
????????l??????????????????????????????????/??????
???????????????????? ??? Extracted intensity as
function of cavity length and QW position in
cavity. ?
Quantum well must be 100nm from top and 150 from
bottom with 20nm accuracy(?/8n)
15
Characterization of GaN microcavity
LEDs Angle-resolved luminescence
not yet enough to get very high efficiency
should already exhibit microcavity behavior
Laser Excitation
Solid-State Lighting Display Center
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Characterization of GaN microcavity
LEDs Angle-resolved luminescence
???????????????????? But high order (15) No
microcavity enhancement
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Microcavity LEDs limited in efficiency and
difficulties to fabricate
An optimal GaN-based MCLED 30 emitted in
air (44 in epoxy)
Still 60-70 guided in the GaN
In a periodic structure, the guided mode can be
diffracted
Bouncing light forms a guided mode
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How to have efficient emission in planar
structures ?
Extracted In air
Emitted in substrate
Guided in GaN
Quantum Wells
12 extracted 66 trapped in GaN
? guided modes
4?nm
GaN n2.5
Sapphire n1.7
500?m
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LED designs involving Photonic Crystals
1/2 Photonic bandgaps for propagation
Photonic Bandgaps PBGs
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LED designs involving Photonic Crystals
2/2 Photonic crystals as diffractive elements
Emission Extraction on the whole surface
Diffractive
Emission, separated extraction region
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Diffraction by a 1D grating
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Diffraction of a guided mode by a 1D grating
???????a?
?
Diffraction condition
lt
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Light extraction by a 2D periodic media
Emission
Extraction
light line
Modes in air
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Figures of merit of PhCs for efficient extraction
1. Extraction length 2. Isotropy (or
omnidirectionality) 3. Current injection 4.
Air/substrate competition 5. Coupling of
incoming light into PC
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Towards high efficiency LEDs by Photonic
Crystal-assisted extraction
? Introduction to light extraction by Photonic
Crystals ? Photoluminescence cavities and PhCs
experiments in GaN ? Tailoring of guided
modes ? Omnidirectional extraction Archimedean
tilings ? Laser lift-off Photonic Crystal LEDs
26
Analysis of a GaN layer by angle-resolved
photolum.
GaN layer Fabry-Pérot interferences
27
Analysis of a GaN PhC by angle-resolved PL
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Angle-resolved PL on photonic crystals
Appl. Phys. Lett. 87 (2005)
Observed intensity I spectral shape I(?) x
extraction efficiency ? ??) Divide I by I ?)
gives ? ??)
Angle (deg)
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Band structure of GaN Photonic Crystals
?
k
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Band structure of GaN Photonic Crystals
 High-order  modes are observed
200 nm
4 ?m
 Low-order  modes are strongly confined in
GaN ? They do not  feel  the effect of the PC ?
25 of the total emitted light is lost ! This is
the case in all current shallow-PhC approaches
31
Is deep etching a solution for efficient
extraction ?
a
d
4a
Increasing extraction efficiency
32
Towards high efficiency LEDs by Photonic
Crystal-assisted extraction
? Introduction to light extraction by Photonic
Crystals ? Photoluminescence experiments in GaN
cavities and PhCs ? Tailoring of guided modes ?
Omnidirectional extraction Archimedean
tilings ? Laser lift-off Photonic Crystal LEDs
33
Tailoring of the guided modes distribution
Excited mode
GaN n2.5
Sapphire
Low-order modes
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Tailoring of the guided modes measurements
New structure (LED)
Original structure
A new set of intense photonic bands appears due
to the surface mode ? 25 of the total emitted
light can be recovered
Appl. Phys. Lett. 88, 061124 (2006)
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Diffraction directionality by 2-D Photonic
Crystals
Real lattice/ real space
Reciprocal lattice/ Reciprocal space
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4.2 Increase of the lattice constant
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Theoretical extraction in air of GaN guided modes
Limited by other diffractions in sapphire
Solid angle
extraction
Extraction in air
?? a/??

• assuming 8 PC (in practice, should be 200 ?m
  long)

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Directionality in normalized far-field patterns
Classic LED ( Lambertian)
PCLED a215 nm
Crystal lattice constant ? Directionality tuning
PCLED a200 nm
PCLED a190 nm
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Kinematic model accounts well for
behaviour (mainly 1st-order diffractions)
(Spectra integrated over wavelength)
-90
90
0
-90
90
0
theory
experiment
40
Diffraction change with Increase of the lattice
constant
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Towards high efficiency LEDs by Photonic
Crystal-assisted extraction
? Introduction to light extraction by Photonic
Crystals ? Photoluminescence experiments in GaN
cavities and PhCs ? Tailoring of guided modes ?
Omnidirectional extraction Archimedean
tilings ? Laser lift-off Photonic Crystal LEDs
42
Archimedean tiling
7 atoms (holes) per unit cell ? Constructive
interference on some diffraction orders
G
?
atoms
43
LEDs with Archimedean tilings band structure
A7
a/?
k//
Triangular
a/?
k//
44
Photonic crystals and current injection
Drawbacks of surface photonic crystals
Injection in the patterned region is difficult
(no current spreading for holes)
Dry etching ? induced damage in the doped regions
Surface PCs ? inefficient extraction
45
Towards high efficiency LEDs by Photonic
Crystal-assisted extraction
? Introduction to light extraction by Photonic
Crystals ? Photoluminescence experiments in GaN
cavities and PhCs ? Tailoring of guided modes ?
Omnidirectional extraction Archimedean
tilings ? Laser lift-off Photonic Crystal LEDs
46
Flip-chip and laser lift-off
Light generation
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LLO-PC LEDs band structure
a/l
1?m GaN
400nm GaN
k//
k//
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LLO-PC LEDs Metal losses
Au
SiO2
Au
SiO2
Ag
49
Microcavity LEDs
What did we learn? Work as expected Could
lead to LEDs with efficiency in the 40 range
Some directionality Require very good
fabrication process (Laser lift off and
thinning) Full planar process
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Photonic crystal LEDs
What did we learn? Many types of Bloch modes
are extracted Some guided modes are however
not extracted A large energy window for high
extraction efficiency Shallow etch enough for
good outcoupling ( 200 nm) Emission can be
highly directional Good designoptimization
essential for high integrated emission So far,
significant enhancement (70), still to be
optimized
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Photonic crystal LEDs
Where are we? Optimization should bring
extraction efficiency of guided mode to 90
With 12 direct extraction, this brings us in the
60 plus range with sapphire substrate (laser
lift-off 80 range)

Will we ever have PhC-LEDs on the market? Is
fabrication competitive with geometrical optics
LEDs? Beware In the end, efficiency limited
by second order effects