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Anti-reflection optical coatings
Anti-reflection coatings are frequently
used to reduce the Fresnel reflection. For
normal incidence, the intensity reflection
coefficient for normal incidence is
(n s  n air ) 2
R
(n s  n air ) 2
For normal incidence, the Fresnel
reflection can be reduced to zero if
 / 4  0 /( 4n AR )
n AR  n s nair
The refractive indices and the
transparency ranges of different
dielectric materials suitable as antireflection coating
Dielectric material
Refractive
index
Transparency
range
SiO2 (Silica)
1.45
> 0.15 μm
Al2O3 (alumina)
1.76
> 0.15 μm
TiO2 (titania)
2.50
> 0.35 μm
Si3N4 (silicon nitride)
2.00
> 0.25 μm
ZnS (zinc sulphide)
2.29
> 0.34 μm
CaF2 (calcium fluoride)
1.43
> 0.12 μm
Epoxy dome
Encapsulation occurs when the epoxy is
in liquid state.
Epoxy reduces the refractive index
contrast between the semiconductor
and air. The refractive index of epoxy is
1.5
A lower index increases the angle of
total internal reflection thereby enlarging
the LED escape cone and extraction
efficiency.
Epoxy usually has the shape of a
hemisphere so that the angle of incidence at the epoxy-air interface is always normal to
the epoxy surface, the total reflection does not occur at the epoxy-air interface.
Planar-surface LEDs are frequently used where the intended viewing angle is close to
the normal incidence or where the LED is intended to blend in with a planar surface.
Epoxy domes provides protection against unwanted mechanical shock and chemicals,
stabilizes the LED die and binding wire, provides the stability to the two metal LEDs and
holds them in place, remains transparent for the longer emission wave lengths and does
not show any degradation over a period of many years. But degrades and loses
transparency in LEDs emitting at shorter wavelengths.
Distributed Bragg reflectors
50% of the light emitted by the
active region is absorbed by the
substrate.
Absorption of light can be avoided
by placing a reflector between the
substrate and the LED active
layers. Light emanating from the
active region towards the
substrate will then be reflected
and can escape from the semiconductor through the top surface.
DBR-multilayer reflector consisting of typically 5-50 pairs of two materials with different
refractive indices. Difference in refractive index the Fresnel reflection will occur at each
of the interfaces.
Thickness of the two materials is chosen so that all reflected waves are in constructive
interference. For normal incidence this condition is fulfilled when both material have a
thickness of a quarter wavelength of the reflected light:
tl ,h  l ,h / 4  0 /( 4nl ,h )
For an oblique angle of incidence Θl,h the optimum thickness for high reflectivity are
given by
tl ,h  l ,h /( 4 cos l ,h )  0 /( 4nl ,h cos l ,h )
DBR must fulfil several conditions:
1. Since DH is grown on top of the DBR, the
DBR must be lattice matched to the DH to
avoid misfit dislocations.
2. To attain high reflectivity DBRs, the DBR
needs to be transparent at the wavelength of
operation unless the DBR has a high-index
contrast. High index contrast DBRs yield
high reflectivity.
3. If the DBR is in the current path, the DBR
must be conductive.
•
Reflectivity of high-contrast DBRs is much
higher than the reflectivity of low index
contrast DBRs .
•
The width of the stop band of the high index
difference DBR is much wider than the stop band width of the low-contrast DBR.
Ll  Bragg /( 4nl )
Lh  Bragg /( 4n h )
Period of the DBR Ll + Lh . Reflectivity of a single interface:
n h  nl
r 
n h  nl
Reflectivity has a maximum at the Bragg wavelength. The reflectivity at the Bragg
wavelength of a DBR with m quarter wave pairs
RDBR rDBR
2
1  (nl / n h ) 2 m 

2m 
1

(
n
/
n
)
l
h


2
The stop band of a DBR depends on the difference in refractive index of the two
materials n h  nl   n
Spectral width of the stop band will be given by
stopband 
2Braggn
n eff
n eff
1 
 1
 2


nh 
 nl
1
effective
refractive index
of DBR
Finite number out of the total number of quarter-wave pairs are effectively reflecting the
wave. The effective number of pairs
meff 

1 n h  nl
n h  nl 

tanh  2m
2 n h  nl
n h  nl 

For thick DBRs (m →infinity), the tanh function approaches unity
meff 
1 n h  nl
2 n h  nl
Reflectivity of the DBR depends on the polar angle of incidence and on the wavelength.
Reflected intensity can be obtained by integration over all angles. The reflectance at a
certain wavelength λ is then given by
 /2
R  
 R( , )2 sin  d
0
 /2
 2 sin  d
1

2
 /2
 R( , )2 sin  d
0
0
Total light intensity reflected by the DBR is given by
I r   I i ( ) R( ) d

Ii(λ)-emission intensity spectrum of the active region incident on DBR.
Ir-Intensity reflected by the DBR.
Ideally the layers comprising the DBR are transparent, which have negligible absorption
loses.
Absorbing DBRs have a maximum reflectivity of less than 100%.
Si absorbs the light for λ < 1.1 μm, i.e for hν > Eg. High reflectivities can be attained at
1.0 μm, where Si is absorbing, this is due to the high index contrast between Si and SiO2
n high n
Material system
Bragg
wavelength
Al0.5In0.5P/GaAs
590 nm
3.13
3.90
0.87
> 870 nm (lossy)
Al0.5In0.5P/Ga0.5In0.5P
590 nm
3.13
3.74
0.61
> 649 nm (lossy)
Al0.5In0.5P/(Al0.3Ga0.7)0.5In0.5P
615 nm
2.08
3.45
0.37
> 576 nm
Al0.5In0.5P/(Al0.4Ga0.6)0.5In0.5P
590 nm
3.13
3.47
0.34
> 560 nm
Al0.5In0.5P/(Al0.5Ga0.5)0.5In0.5P
570 nm
3.15
3.46
0.31
> 560 nm
AlAs/GaAs
900 nm
2.97
3.54
0.57
> 870 nm
SiO2/Si
1300 nm
1.46
3.51
2.05
> 1106 nm
n low
Transparency
range
The table shows that the absorbing Al05In0.5P/GaAs DBRs have the advantage of
a high index contrast. High contrast DBRs have a wider stop-band width.
The transparent Al0.5In0.5P/(AlGa)0.5In0.5P DBRs have the advantage of negligible
optical losses.
SiO2/Si is a high index-contrast system, it can not be used for current
conduction due to the insulating nature of SiO2.
AlAs/GaAs is used in resonant cavity LEDs and vertical cavity surface emitting
lasers.
Different strategies are used to optimize DBRs.
One of them is to use composite DBR in an AlGaInP LED, that is, two types of
DBRs stacked on top of each other, namely a non-absorbing
(Al0.4Ga0.6)0.5In0.5P/Al0.5In0.5P DBR resonant at the peak emission wavelength of
590 nm and an additional high-contrast absorbing AlAs/GaAs
Current-blocking layers
Current-Blocking layer will deflect
the current away from the top
contact, thus allowing for much
higher extraction efficiency.
The current blocking layer has
n-type conductivity and is
embedded in material with p-type
conductivity.
Owing to the p-n junction
surrounding the current blocking
layer, the current flows around the current-blocking layer.
Reflective and transparent contacts
Metal ohmic contacts are practically opaque for thicknesses > 50 nm. Thus light
incident on the top or bottom contact of an LED will not be transmitted through
the contact.
Annealing and alloying forms low-resistance ohmic contacts.
During the annealing process, the metal surface changes from a smooth to a
rough appearance and a concomitant decrease in the optical reflectivity results.
Non-alloyed contacts are just deposited on the semiconductor without annealing.
Very thin metals are semitransparent, i.e. a small percentage of the incident light
is transmitted through the metal.
Most metal contacts have a transmittance of approximately 50% at a metal film
thickness of 5-10 nm.
Very thin metallic contacts may form an island structure rather than a single
continues film. The electrical resistance of thin metal films can be large, in
particular if an island structure is formed.