1. Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes APPLIED PHYSICS LETTERS 101, 231107 (2012) Kishida Takuto -1-

2. Contents Ⅰ.Introduction LED Principle of LED Ⅱ. Discussion & Result Causes of the efficiency reduction of LED Ⅲ. Summary Ⅳ. Connection with my research -2-

3. LED Ⅰ.Introduction LED is short for Light Emitting Diode. Applications Electronic device e.g.) smartphone, large-screen display Traffic light Advantage Low power consumption Long service life Small size -3-

4. Materials for LED In x Ga 1-x N ( Ⅲ - Ⅴ semiconductor ) Wurtzite structure Lattice parameter GaN : a = 3.189 Å, c = 5.185 Å InN : a = 3.584 Å, c = 5.760 Å Band gap GaN : E g = 3.43 eV ( 365 nm) InN : E g = 0.85 eV ( 1,459 nm) GaN (365nm) InN (1,459nm) GaN InGaN GaN Base material GaN InGaN Ⅰ.Introduction Energy -4-

5. Principle of LED Heterojunction Heterojunction is created when two layers of semiconductors which have different band gap are placed. Recombination Type Ⅰ Heterojunction is used for LED. Ⅰ.Introduction electron hole Photon http://en.wikipedia.org/wiki/Heterojunction -5-

6. :overlap, :electron and hole wave function Principle of LED Overlap The recombination rates of electron and hole depends on electron-hole wave function overlap. Ⅰ.Introduction Recombination rate increases Recombination rate reduces Because of electron and hole in the same semiconductor, the overlap increases. -6-

7. Efficiency reduction of LED Polarization fields The spatial separation of electrons and holes by the polarization fields in the active region. Ⅱ. Discussion & Result Auger recombination The excess energy given off by an electron recombining with a hole is given to a second elec tron. Reduction of wave function overlap. Nonradiative recombination that electron-hole recombination process which does not produce photons. -7-

8. Recombination process Ⅱ. Discussion & Result ( ⅰ ) Shockley-Read-Hall recombination (SRH) The presence of impurities or crystal defects in semiconductors give rise to trap levels. ( ⅱ ) Radiative recombination The photon is emitted. ( ⅲ ) Auger recombination (AR) The excess energy given off by an electron recombining with a hole is given to a second electron. Internal quantum efficiency (IQE) IQE is a ratio of recombination rates,,,,,, : carrier density -8-

9. Behavior of Overlap and IQE Ⅱ. Discussion & Result For c-plane, 0 V (x = 0.12) For c-plane, 3.5 V (x = 0.12) Polarization fields keep the electron and hole wave functions at opposite ends of the quantum well (QW) under 0 V and 3.5 V. Increasing In concentrations in the QW enhance the strain and polarization fields and further reduce the overlap and IQE. -9-

10. Behavior of Overlap and IQE Ⅱ. Discussion & Result For m-plane, 0 V (x = 0.12) Electron and hole wave functions are kept apart by the junction field under 0 V. As the In content increases, the QW confines electrons and holes more effectively. For m-plane, 3.5 V (x = 0.12) Flat-band conditions created by 3.5 V bias and the resulting overlaps approach unity. IQE is independent of In content. -10-

11. Ⅲ. Summary Effect of polarization fields and Auger recombination reduces LED efficiency. When m-plane and 3.5 V bias, flat band conditions is created and the value of overlap approach unity. When c-plane and high In content, spatial separation of electrons and holes by the polarization fields reduces overlap. -11-

12. Ⅳ. Connection with my research Solar cell Solar cells have attracted attention as clean energy for the next generation. Solar energy that reaches the earth in one hour is comparable to the energy humanity consumes in a year Energy source is the sun. CO 2 is not emitted. Conversion efficiency is not high. Price is high. Solar cell Silicon series Organic series Compound series Classification Si : 24.7 % ( Theoretical limitation 29 % ) GaAs : 28.3 %, CIGS : 20.3 %, CdTe : 16.7 % Dye sensitization : 11.2%, Organic thin film : 7.9 % -12-

13. Principle of Solar cell Ⅳ. Connection with my research In order to increase the efficiency of solar cell… Using the light of various wavelengths Suppressing the reflection of the incident light. Increasing the incident light by reducing the electrode. Reducing the recombination rate. p-n junction p-type n-type ++++++++ -------- -------- ++++++++ electric field heterojunction Reducing the wave function overlap. -13-

14. Thank you for your attention -14-