High LightExtraction Efficiency in GaInN LightEmitting Diode with Pyramid Reflector

1
High Light-Extraction Efficiency in GaInN
Light-Emitting Diode with Pyramid Reflector
J.-Q. Xi, Hong Luo, Alyssa J. Pasquale, Jong Kyu
Kim, and E. F. Schubert Future Chips
Constellation, Rensselaer Polytechnic Institute,
110 8th Street, Troy, NY
Simulation Model
GaInN LED with Pyramid Reflector
Escape Cone
Escape Cone from Total Internal Reflection

• Pyramid dimension lb 3.5 ?m s 2 ?m
• Find out optimal ?

• Escape-cone ?c  sin?1(n0/ns)
• Encapsulant n0 ?1.5
• Air n0 1.0
• GaN ns ? 2.5
• Air/GaN ?c 23.6?
• 91.6 of light is trapped
• Encapsulant/GaN ?c 36.9?
• 80.0 of light is trapped

• Light-escape cone is caused by total internal
  reflection
• Ray 1 Can be extracted to the free space
• Ray 2 Trapped inside semiconductor

• Additional escape cone, ??2, is obtained from
  pyramid reflector
• Bottom emission is enhanced

Simulation Results
Current-Spreading Length
Current-Spreading Effect
Fabrication

• Current-spreading length, LS
• with
• Measured ideality factor nideal  4.5
• Hole concentration p  4?1017/cm3
• Hole mobility ?p  14.2 cm2/(V s)
• Area of a device S  300 ?m ? 300 ?m
• P-type GaN thickness t  150 nm

• GaN has absorption, ?? 150 cm-1

• ? ? 25?
• ? is determined by wet etch isotropicity

I0 (mA)
LS (?m)

• Bottom emission is dramatically enhanced
• ? 30? ? Maximum total enhancement

Device Structure
Measurement Results
Conclusions
Acknowledgements

• GaInN with pyramid reflector has 13.9
  enhancement on light output compared to a
  conventional GaInN LED with a planar Ag
  reflector.
• Ray tracing simulation shows 14.1 enhancement
  for total light output and 27.6 enhancement for
  bottom emission.
• The enhancement is mainly attributed to an
  additional escape cone for light extraction
  enabled by pyramid reflector.
• Current-spreading length is approximately equal
  to the base dimension of SiO2 pyramids ? Good
  current spreading beneath the SiO2 pyramids

• GaInN with pyramid reflector
• Reference GaInN with planar Ag reflector

• Crystal IS Corporation
• Samsung Advanced Institute of Technology (SAIT)
• Army Research Office (ARO)
• New York State Office of Science, Technology and
  Academic Research (NYSTAR)
• National Science Foundation (NSF)
• Department of Energy (DOE)

• Bottom emission is enhanced by 13.9 at I 20 mA
• Consistent with simulation results