Light Emitting Diodes

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Light Emitting Diodes

• EE 698A
• Kameshwar Yadavalli

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Outline

• Basics of Light Emitting Diodes (Electrical)
• Basics of Light Emitting Diodes (Optical)
• High internal efficiency designs
• High extraction efficiency structures
• Visible Spectrum LEDs
• White-Light LEDs
• The promise of solid state lighting

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LED-Electrical Properties-PN junctions

• PN junction diode in forward bias, the
  electron-hole recombination leads to photon
  emission
• I Is(eeV/kT-1)
• Threshold voltage Vth Eg/e
• I IseeV/?kT
• where ? is the ideality factor

Double Heterostructure is used to confine the
carriers, improving the radiative recombination
rate

• From Light-Emitting Diodes, Fred Schubert.

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LED-Electrical Properties-Hetero junctions
Grading of the heterojunction is done to reduce
the resistance seen by carriers
From Light-Emitting Diodes, Fred Schubert.
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LED-Electrical Properties-Hetero junctions
From Light-Emitting Diodes, Fred Schubert.
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LED-Electrical Properties-Carrier loss

• The confinement barriers are typically several
  hundred meV (gtgtkT)
• Due to Fermi-Dirac distribution of carriers in
  the active region, some carriers will have energy
  higher than that of the barriers
• In AlGaAs/GaAs and AlGaN/GaN the barriers are
  high
• In AlGaInP/GaInP the barriers are lower resulting
  in higher leakage currents

From Light-Emitting Diodes, Fred Schubert.
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LED-Electrical Properties-Blocking layers

• Electron Blocking Layers are required to prevent
  electron escape at high injection current
  densities

From Light-Emitting Diodes, Fred Schubert.
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LED-Optical Properties-Efficiency

• ?int of photons emitted from active region
  per second
• of electrons injected in to LED per
  second
• Pint / (h?)
• I / e
• ?extr of photons emitted into free space per
  second
• of photons emitted from active
  region per second
• P / (h?)
• Pint / (h?)

From Light-Emitting Diodes, Fred Schubert.
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LED-Optical Properties-Emission Spectrum

• The linewidth of an LED emitting in the visible
  range is relatively narrow compared with the
  entire visible range (perceived as monochromatic
  by the eye)
• Optical fibers are dispersive, limiting the bit
  rate X distance product achievable with LEDs
• Modulation speeds achieved with LEDs are
  1Gbit/s, as the spontaneous lifetime of carriers
  in LEDs is 1-100 ns

From Light-Emitting Diodes, Fred Schubert.
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LED-Optical Properties-Light Escape Cone

• Total internal reflection at the semiconductor
  air interface reduces the external quantum
  efficiency.
• The angle of total internal reflection defines
  the light escape cone.
• sin?c nair/ns
• Area of the escape cone 2pr2(1-cos?c)
• Pescape / Psource (1-cos?c)/2 ?c2/4
  (nair2/ns2)/4

From Light-Emitting Diodes, Fred Schubert.
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LED-Optical Properties-Emission Spectrum

• Light intensity in air (Lambertian emission
  pattern) is given by
• Iair (Psource/4pr2) X (nair2/ns2) cosF
• Index contrast between the light emitting
  material and the surrounding region leads to
  non-isotropic emission pattern

From Light-Emitting Diodes, Fred Schubert.
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LED-Optical Properties-Epoxy encapsulants

• Light extraction efficiency can be increased by
  using dome shaped encapsulants with a large
  refractive index.
• Efficiency of a typical LED increases by a factor
  of 2-3 upon encapsulation with an epoxy of n
  1.5.
• The dome shape of the epoxy implies that light is
  incident at an angle of 90o at the epoxy-air
  interface. Hence no total internal reflection.

From Light-Emitting Diodes, Fred Schubert.
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Temperature dependence of emission intensity

• Emission intensity decreases with increasing
  temperature.
• Causes include non-radiative recombination via
  deep levels, surface recombination, and carrier
  loss over heterostucture barriers.

From Light-Emitting Diodes, Fred Schubert.
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High internal efficiency LED designs

• Radiative recombination probability needs to be
  increased and non-radiative recombination
  probability needs to be decreased.
• High carrier concentration in the active region,
  achieved through double heterostructure (DH)
  design, improves radiative recombination.
• 
  RBnp
• DH design is used in all high efficiency designs
  today.

From Light-Emitting Diodes, Fred Schubert.
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High internal efficiency designs

• Doping of the active regions and that of the
  cladding regions strongly affects internal
  efficiency.
• Active region should not be heavily doped, as it
  causes carrier spill-over in to the confinement
  regions decreasing the radiative efficiency
• Doping levels of 1016-low 1017 are used, or none
  at all.
• P-type doping of the active region is normally
  done due to the larger electron diffusion length.
  
• Carrier lifetime depends on the concentration of
  majority carriers.
• In low excitation regime , the radiative carrier
  lifetime decreases with increasing free carrier
  concentration.
• Hence efficiency increases with doping.
• At high concentration, dopants induce defects
  acting as recombination centers.

From Light-Emitting Diodes, Fred Schubert.
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P-N junction displacement

• Displacement of the P-N junction causes
  significant change in the internal quantum
  efficiency in DH LED structures.
• Dopants can redistribute due to diffusion,
  segregation or drift.

From Light-Emitting Diodes, Fred Schubert.
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Doping of the confinement regions

• Resistivity of the confinement regions should be
  low so that heating is minimal.
• High p-type conc. in the cladding region keeps
  electrons in the active region and prevents them
  from diffusing in to the confinement region.
• Electron leakage out of the active region is more
  severe than hole leakage.

From Light-Emitting Diodes, Fred Schubert.
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Non radiative recombination

• The concentration of defects which cause deep
  levels in the active region should be minimum.
• Also surface recombination should be minimized,
  by keeping all surfaces several diffusion lengths
  away from the active region.
• Mesa etched LEDs and lasers where the mesa etch
  exposes the active region to air, have low
  internal efficiency due to recombination at the
  surface.
• Surface recombination also reduces lifetime of
  LEDs.

From Light-Emitting Diodes, Fred Schubert.
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Lattice matching

• Carriers recombine non-radiatively at misfit
  dislocations.
• Density of misfit dislocation lines per unit
  length is proportional to lattice mismatch.
• Hence the efficiency of LEDs is expected to drop
  as the mismatch increases.

From Light-Emitting Diodes, Fred Schubert.
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High extraction efficiency structures

• Shaping of the LED die is critical to improve
  their efficiency.
• LEDs of various shapes hemispherical dome,
  inverted cone, truncated cones etc have been
  demonstrated to have better extraction efficiency
  over conventional designs.
• However cost increases with complexity.

From Light-Emitting Diodes, Fred Schubert.
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High extraction efficiency structures

• The TIP LED employs advanced LED die shaping to
  minimize internal loss mechanisms.
• The shape is chosen to minimize trapping of
  light.
• TIP LED is a high power LED, and the luminous
  efficiency exceeds 100 lm/W.
• TIP devices are sawn using beveled dicing blade
  to obtain chip sidewall angles of 35o to vertical.

Krames et. al, Appl. Phys. Lett., Vol. 75, No.
16, 18 October 1999
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Visible spectrum LEDs
The plot charts the gains made in luminous
efficiency till date.
From Light-Emitting Diodes, Fred Schubert.
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Visible spectrum LEDs

• The emission spectrum of the blue, green and red
  LEDs indicate that the green LED has a wider
  spectrum.
• Alloy broadening leads to spectral broadening
  that is greater than 1.8 kT linewidth.

From Light-Emitting Diodes, Fred Schubert.
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White-light LEDs

• White light can be generated in several different
  ways.
• One way is to mix to complementary colors at a
  certain power ratio.
• Another way is by the emission of three colors at
  certain wavelengths and power ratio.
• Most white light emitters use an LED emitting at
  short wavelength and a wavelength converter.
• The converter material absorbs some or all the
  light emitted by the LED and re-emits at a longer
  wavelength.
• Two parameters that are important in the
  generation of white light are luminous efficiency
  and color rendering index.
• It is shown that white light sources employing
  two monochromatic complementary colors result in
  highest possible luminous efficiency.

From Light-Emitting Diodes, Fred Schubert.
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White-light LEDs

• Wavelength converter materials include phosphors,
  semiconductors and dyes.
• The parameters of interest are absorption
  wavelength, emission wavelength and quantum
  efficiency.
• The overall energy efficiency is given by
• ?
  ?ext(?1/ ?2)
• Even if the external quantum efficiency is 1,
  there is always an energy loss associated with
  conversion.
• Common wavelength converters are phosphors, which
  consist of an inorganic host material doped with
  an optically active element.
• A common host is Y3Al5O12.
• The optically active dopant is a rare earth
  element, oxide or another compound.
• Common rare earth elements used are Ce, Nd, Er
  and Th.

From Light-Emitting Diodes, Fred Schubert.
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White-light LEDs

• Phosphors are stable materials and can have
  quantum efficiencies of close to 100.
• Dyes also can have quantum efficiencies of close
  to 100.

• Dyes can be encapsulated in epoxy or in optically
  transparent polymers.
• However, organic dyes have finite lifetime. They
  become optically inactive after 104-106 optical
  transitions.

From Light-Emitting Diodes, Fred Schubert.
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White LEDs based on phosphor converters
A blue GaInN/GaN LED and a phosphor
wavelength converter suspended in a epoxy resin
make a white Light LED. The thickness of the
phosphor containing epoxy and the concentration
of the phosphor determine the relative
strengths of the two emission bands
From Light-Emitting Diodes, Fred Schubert.
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Promise of Solid State Lighting

• The use of solid state lighting devices promises
  huge savings in energy consumption.
• The electricity for lighting needs is 60GW, over
  24 hrs.
• About 24 GWyear is consumed by incandescent lamps
  with a luminous intensity of 15lm/W.
• 36 GWyear is consumed by FL/HID lamps with a
  luminous intensity of 75lm/W.
• Assuming that by year 2020, they are replaced by
  LEDs with luminous intensity of 150 lm/W, energy
  savings are 40 GWyear.
• That translates to 40 billion in savings.
• At 4Mtons / GWyear of coal consumption, net
  savings lead to 25 less coal consumption,
  leading to lesser emissions of green house gases.
• Global savings are projected to be about 140B.

Roland Haitz, Adv. in Solid State Physics