1. Advisor: Prof. Yen-Kuang Kuo
Advantages of Blue InGaN Light-Emitting Diodes with Slightly-Doped Step-Like Electron-Blocking Layer
Tsun-Hsin Wang
Ph.D. Candidate, Department of Physics, National Changhua University of Education
Advisor: Prof. Yen-Kuang Kuo

2. Tsun-Hsin Wang/BLL/NCUE
Outline
Introduction and Motivation
Device Structure
Simulation Results
Conclusion
Reference
Tsun-Hsin Wang/BLL/NCUE

3. Tsun-Hsin Wang/BLL/NCUE
Introduction
S. Pimputkar, J. S. Speck, S. P. DenBaars, and S. Nakamura, Nat. Photonics 3, 180 (2009).
More than one-fifth of US electricity is used to power artificial lighting.
Light-emitting diodes (LEDs) based on group III/nitride semiconductors are bringing about a revolution in energy-efficient lighting.
Tsun-Hsin Wang/BLL/NCUE

4. Tsun-Hsin Wang/BLL/NCUE
Introduction
E. F. Schubert and J. K. Kim, Science 308, 5276 (2005).
Energy savings and environmental benefits
Spectral power distribution
Spatial distribution
Color temperature
Temporal modulation
Polarization properties
Spontaneous polarization
=>Asymmetric wurtzite
Piezoelectric polarization
=>Lattice mismatch
Tsun-Hsin Wang/BLL/NCUE

5. Tsun-Hsin Wang/BLL/NCUE
Motivation
Development of InGaN LEDs
GaN-InGaN-GaN barriers
InGaN-AlGaN-InGaN barriers
Slightly-doped step-like electron blocking layer (EBL)
Shallow first well
Kuo et al., Appl. Phys. Lett. 99, (2011).
Kuo et al., Appl. Phys. Lett. 100, (2012).
Kuo et al., IEEE Photonics Technol. Lett. 24, (2012).
Wang et al., IEEE Photonics Technol. Lett. (2012).
Tsun-Hsin Wang/BLL/NCUE

6. Tsun-Hsin Wang/BLL/NCUE
Device Structure
p-contact
p-GaN
p-AlGaN
i-InGaN/GaN
n-contact
n-GaN
n-GaN
i-GaN
sapphire
Kuo et al., Appl. Phys. Lett. 95, (2009).
Tsun-Hsin Wang/BLL/NCUE

7. Tsun-Hsin Wang/BLL/NCUE
Device Structure
p-contact
p-GaN
p-AlGaN
i-InGaN/GaN
n-contact
n-GaN
n-GaN
i-GaN
sapphire
Tsun-Hsin Wang/BLL/NCUE

8. Tsun-Hsin Wang/BLL/NCUE
Device Structure
p-contact
p-GaN
p-AlGaN
i-InGaN/GaN
n-contact
n-GaN
n-GaN
i-GaN
sapphire
Tsun-Hsin Wang/BLL/NCUE

9. Tsun-Hsin Wang/BLL/NCUE
Device Structure
Drawbacks of polarization electric field:
Serious tilting of energy band
Severe leakage current of electrons
Insufficient injection efficiency of holes
Nonradiative Auger recombination induced by non-uniform distribution of carriers
=> Efficiency droop!
Tsun-Hsin Wang/BLL/NCUE

10. Tsun-Hsin Wang/BLL/NCUE
Device Structure
p-contact
composition
doping (1018 cm–3)
conventional EBL (original)
Al0.15Ga0.85N
1.2
slightly-doped EBL
0.6
slightly-doped step-like EBL
Al0.075Ga0.925N
GaN
p-GaN
p-AlGaN
i-InGaN/GaN
n-contact
n-GaN
n-GaN
i-GaN
sapphire
Impact ionization
Hole concentration is conventionally 1% of dopant concentration.
Tsun-Hsin Wang/BLL/NCUE

11. Tsun-Hsin Wang/BLL/NCUE
Simulation Results
Effective potential height
Conduction band: electron leakage current
Valence band: hole injection efficiency
Tsun-Hsin Wang/BLL/NCUE

12. Tsun-Hsin Wang/BLL/NCUE
Simulation Results
Effective potential height
Conduction band: electron leakage current
Valence band: hole injection efficiency
Tsun-Hsin Wang/BLL/NCUE

13. Tsun-Hsin Wang/BLL/NCUE
Simulation Results
Last barrier
Two dimensional electron gas (2DEG)
Tsun-Hsin Wang/BLL/NCUE

14. Tsun-Hsin Wang/BLL/NCUE
Simulation Results
Tsun-Hsin Wang/BLL/NCUE

15. Tsun-Hsin Wang/BLL/NCUE
Simulation Results
Tsun-Hsin Wang/BLL/NCUE

16. Tsun-Hsin Wang/BLL/NCUE
Conclusion
The advantages of blue InGaN LED with slight-doped step-like EBL are studied numerically.
According to the simulation results, the LED has enhanced carrier concentrations in the QWs due to appropriately modified energy band diagrams which are favorable for the injection of holes without the price of confinement of electrons.
Tsun-Hsin Wang/BLL/NCUE

17. Tsun-Hsin Wang/BLL/NCUE
Reference
T.-H. Wang and Y.-K. Kuo, IEEE Photonics Technol. Lett. accepted (2012).
Y.-K. Kuo, T.-H. Wang, J.-Y. Chang, and J.-D. Chen, IEEE Photonics Technol. Lett. 24, 1506 (2012).
Y.-K. Kuo and T.-H. Wang, IEEE J. Quantum Electron. 48, 946 (2012).
Y.-K. Kuo, T.-H. Wang, and J.-Y. Chang, Appl. Phys. Lett. 100, (2012). 
Y.-K. Kuo, T.-H. Wang, J.-Y. Chang, and M.-C. Tsai, Appl. Phys. Lett. 99, (2011).
Tsun-Hsin Wang/BLL/NCUE

18. Tsun-Hsin Wang/BLL/NCUE
Acknowledgement:
This work was supported by the National Science Council under grant NSC M MY3.
Thank you for your attention!
Tsun-Hsin Wang/BLL/NCUE

19. Tsun-Hsin Wang/BLL/NCUE
Q & A – Physical models
Poisson equation: ∇2V=−ρ /ε, where ρ: volume charge density, ε: dielectric constant.
Continuity equation: ∇J+∂ρ/∂t=0, where J: current density, t: time.
Complex wave equation: ∇2W+k2(ε−β2)W=0, where W: optical wave function, k: wave vector, β: real eigen-value.
Rate equation: ∂S/∂t=c(g−α)/n, where c: speed of light, n: refractive index, g: gain, α: loss, S: photon number.
Gain equation: g=α+[ln(1/R1R2)]2L, where R: reflectance of mirrors, L: cavity length.
APSYS by Crosslight Software Inc.
Tsun-Hsin Wang/BLL/NCUE

20. Tsun-Hsin Wang/BLL/NCUE
Q & A – Physical models
Equations Parameters
Poisson equation: V, n, p, S, W, g
Continuity equation: V, n, p
Complex wave equation: n, p, S, W, g
Rate equation: n, p, W, lambda, g
Gain equation: n, p, lambda, g
V: potential, n and p: electron and hole concentration, S: photon number, W: optical field intensity, lambda: wavelength, g: gain.
APSYS by Crosslight Software Inc.
Tsun-Hsin Wang/BLL/NCUE

21. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Polarization
Vurgaftman et al., J. Appl. Phys. 94, 3675 (2003).
Tsun-Hsin Wang/BLL/NCUE

22. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Polarization
Wu, J. Appl. Phys. 106, (2009).
Tsun-Hsin Wang/BLL/NCUE

23. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Energy band gap
Wu, J. Appl. Phys. 106, (2009).
Tsun-Hsin Wang/BLL/NCUE

24. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Energy band gap
Wu, J. Appl. Phys. 106, (2009).
Tsun-Hsin Wang/BLL/NCUE

25. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Mobility
Kuo et al., IEEE J. Quantum Electron. 46, 1214 (2010).
Tsun-Hsin Wang/BLL/NCUE

26. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Recombination rate
Kuo et al., IEEE J. Quantum Electron. 46, 1214 (2010).
Tsun-Hsin Wang/BLL/NCUE

27. Tsun-Hsin Wang/BLL/NCUE
Q & A – Parameters
Efficiency droop
Tsun-Hsin Wang/BLL/NCUE