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 diodesAPPLIED PHYSICS LETTERS 101, 231107 (2012)Kishida Takuto-1-Contents.IntroductionLEDPrinciple of LED. Discussion & ResultCauses of the efficiency reduction of LED. Summary. Connection with my research

-2-LED.Introduction

LED is short for Light Emitting Diode.

ApplicationsElectronic devicee.g.) smartphone , large-screen displayTraffic light

AdvantageLow power consumptionLong service lifeSmall size

-3-3

Materials for LEDInxGa1-xN ( - semiconductor )Wurtzite structureLattice parameter GaN : a = 3.189 , c = 5.185 InN : a = 3.584 , c = 5.760 Band gap GaN : Eg = 3.43 eV ( 365 nm) InN : Eg = 0.85 eV ( 1,459 nm)

GaN(365nm)InN(1,459nm)GaNInGaNGaNBase materialGaNGaNInGaN

.IntroductionEnergy-4-4Principle of LED Heterojunction

Heterojunction is created when two layers of semiconductors which have different band gap are placed.RecombinationType Heterojunction is used for LED.

.IntroductionelectronholePhotonhttp://en.wikipedia.org/wiki/Heterojunction-5-5 :overlap , :electron and hole wave functionPrinciple of LED Overlap

The recombination rates of electron and hole depends on electron-hole wave function overlap..Introduction

Recombination rate increases

Recombination rate reducesBecause of electron and hole in the same semiconductor, the overlap increases. -6-6Efficiency reduction of LED Polarization elds The spatial separation of electrons and holes by the polarization elds in the active region.. Discussion & ResultAuger recombination The excess energy given off by an electron recombining with a hole is given to a second electron. Reduction of wave function overlap.Nonradiative recombination that electron-hole recombination processwhich does not produce photons.

-7-7Recombination process. Discussion & Result

() Shockley-Read-Hall recombination (SRH)

The presence of impurities or crystal defects in semiconductors give rise to trap levels.

() Radiative recombination

The photon is emitted.

() 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-8-8Behavior of Overlap and IQE. Discussion & ResultFor c-plane , 0 V

(x = 0.12)

For c-plane , 3.5 V

(x = 0.12)

Polarization elds 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 elds and further reduce the overlap and IQE.

-9-Behavior of Overlap and IQE. Discussion & ResultFor m-plane , 0 V

(x = 0.12)

Electron and hole wave functions are kept apart by the junction eld under 0 V.As the In content increases, the QW connes 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-. SummaryEffect 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-. Connection with my research Solar cellSolar 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.

Solar cellSilicon seriesOrganic seriesCompound seriesClassificationSi : 24.7 % ( Theoretical limitation 29 % ) GaAs : 28.3 % , CIGS : 20.3 % , CdTe : 16.7 %Dye sensitization : 11.2% , Organic thin film : 7.9 % -12-12Principle of Solar cell. Connection with my research In order to increase the efficiency of solar cellUsing the light of various wavelengthsSuppressing the reflection of the incident light.Increasing the incident light by reducing the electrode.Reducing the recombination rate.p-n junctionp-typen-type++++--------++++electric fieldheterojunction

Reducing the wave function overlap.

-13-Thank you for your attention-14-