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Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes

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-. Contents. Ⅰ.Introduction LED Principle of LED Ⅱ . Discussion & Result

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Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes

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

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

  3. Ⅰ.Introduction LED • 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. Ⅰ.Introduction Materials for LED InxGa1-xN ( 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: Eg = 3.43 eV ( 365 nm) InN: Eg = 0.85 eV ( 1,459 nm) InN (1,459nm) GaN (365nm) GaN Energy InGaN GaN Basematerial GaN GaN InGaN -4-

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

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

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

  8. Ⅱ. Discussion & Result Recombination process (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. , , : carrier density , , , , • Internal quantum efficiency (IQE) • IQE is a ratio of recombination rates -8-

  9. Ⅱ. Discussion & Result Behavior of Overlap and IQE • 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. Ⅱ. Discussion & Result Behavior of Overlap and IQE • 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. ⭕CO2 is not emitted. ❌Conversion efficiency is not high. ❌Price is high. Classification Silicon series Si : 24.7 % ( Theoretical limitation 29 % ) Solar cell Compound series GaAs : 28.3 % , CIGS : 20.3 % , CdTe : 16.7 % Organic series Dye sensitization : 11.2% , Organic thin film : 7.9 % -12-

  13. Ⅳ. Connection with my research Principle of Solarcell • 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. Reducing the wave function overlap. p-n junction p-type n-type + + + + + + + + - - - - - - - - electric field heterojunction -13-

  14. Thank you for your attention -14-

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