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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
For normal incidence, the Fresnel reflection can
be reduced to zero if
Dielectric material Refractive index Transparency range
SiO2 (Silica) 1.45 gt 0.15 µm
Al2O3 (alumina) 1.76 gt 0.15 µm
TiO2 (titania) 2.50 gt 0.35 µm
Si3N4 (silicon nitride) 2.00 gt 0.25 µm
ZnS (zinc sulphide) 2.29 gt 0.34 µm
CaF2 (calcium fluoride) 1.43 gt 0.12 µm
The refractive indices and the transparency
ranges of different dielectric materials suitable
as anti-reflection coating
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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.
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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
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For an oblique angle of incidence Tl,h the
optimum thickness for high reflectivity are given
by

• DBR must fulfil several conditions
• Since DH is grown on top of the DBR, the DBR must
  be lattice matched to the DH to avoid misfit
  dislocations.
• 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.
• 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.
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Period of the DBR Ll Lh . Reflectivity of a
single interface
Reflectivity has a maximum at the Bragg
wavelength. The reflectivity at the Bragg
wavelength of a DBR with m quarter wave pairs
The stop band of a DBR depends on the difference
in refractive index of the two materials
Spectral width of the stop band will be given by
effective refractive index of DBR
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Finite number out of the total number of
quarter-wave pairs are effectively reflecting the
wave. The effective number of pairs
For thick DBRs (m ?infinity), the tanh function
approaches unity
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 ? is then given by
Total light intensity reflected by the DBR is
given by
Ii(?)-emission intensity spectrum of the active
region incident on DBR. Ir-Intensity reflected by
the DBR.
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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 ? lt
1.1 µm, i.e for h? gt 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
Material system Bragg wavelength Transparency range
Al0.5In0.5P/GaAs 590 nm 3.13 3.90 0.87 gt 870 nm (lossy)
Al0.5In0.5P/Ga0.5In0.5P 590 nm 3.13 3.74 0.61 gt 649 nm (lossy)
Al0.5In0.5P/(Al0.3Ga0.7)0.5In0.5P 615 nm 2.08 3.45 0.37 gt 576 nm
Al0.5In0.5P/(Al0.4Ga0.6)0.5In0.5P 590 nm 3.13 3.47 0.34 gt 560 nm
Al0.5In0.5P/(Al0.5Ga0.5)0.5In0.5P 570 nm 3.15 3.46 0.31 gt 560 nm
AlAs/GaAs 900 nm 2.97 3.54 0.57 gt 870 nm
SiO2/Si 1300 nm 1.46 3.51 2.05 gt 1106 nm
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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
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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.
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Reflective and transparent contacts Metal ohmic
contacts are practically opaque for thicknesses gt
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.