Simulation of InGaNGaN multiple quantum well light-emitting by broverya73

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									Shanghai Institute of Technical Physics



   Simulation of InGaN/GaN multiple quantum
   well light-emitting diodes with Quantum Dot
   electrical and optical effects

                           C. S. Xia, X. S. Chen, W. Lu
                        Shanghai Inst. Techn. Physics, China

                              Z. M. Simon Li, Z. Q. Li
                              Crosslight Software, Canada
Shanghai Institute of Technical Physics



   Outline:
                 Introduction
                       Origin of luminescence of InGaN based LED
                 Theoretical models for quantum dots
                        Calculation of electronic states of InGaN QD
                        Spontaneous emission
                        Quantum transport mechanism
                 Simulation results
                 Conclusion
Shanghai Institute of Technical Physics


 Introduction

 InGaN based MQW LED : traffic signals
                            full-color displays,
                            back lighting in liquid-crystal displays
                            replacement for conventional incandescent
                            and fluorescent light bulbs in near future
 Blue, green and white LED has a high lumineous efficiency
                  External quantum efficiency : more than 12%

 Threading dislocation density :              108-1012cm-2

 Origin of luminescence of InGaN MQW LED ????????
Shanghai Institute of Technical Physics


 Solid phase immiscibility
      in InGaN alloys



    the large difference in Interatomic
     spacing between GaN and InN

     The binodal and spinodal curve
                                             Ho et al. APL. 69. 2701(1996)



                       InGaN region with high In content
Shanghai Institute of Technical Physics

                              Donnell et al. PRL, 82, 237 (1999)




      EL and PC spectra form green and blue           Stokes shift plotted against emissin peak energy
               inGaN based LED                                      for inGaN based LED


              Large Stokes shift shows origin of luminescence
            comes from InGaN quantum dots with high In content
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            HRTEM images for blue InGaN LED   Musikhin et al APL,80,2099(2002)


            InGaN quantum dots: 3~5 nm
                                In content 35%
Shanghai Institute of Technical Physics



 The mechanism of luminescence in InGaN-based MQW LEDs

       the radiative recombination within the In-rich quantum dots

Numerical simulation is an effective method to study and optimize the
                        characteristics of optoelectronic devices


There is few simulation considering the QD origin of luminescence for
InGaN-based MQW LEDs


                      Simulate In0.22Ga0.78N/GaN MQW green LED
                    by APSYS software based on Quantum Dot model
Shanghai Institute of Technical Physics

 Theoretical models for quantum dots
   1. Quantum dot structure



   InGaN MQW
  with InGaN QDs




        A certain density of QDs is assumed to be embedded in InGaN
                                quantum well
Shanghai Institute of Technical Physics




  QD structure is approximated by a disk-like high indium cylinder surrounded by QW
  material with lower indium composition to form a dot/well complex system

                     InGaN QD parameters:
                       In content:0.56
                       QW In content: 0.22
                       Size: height      1.5nm
                                diameter 3.6nm and 5nm
Shanghai Institute of Technical Physics


 2. Calculation of electronic states of QD
           ⎡ h2 ⎛ 1 ∂ ⎛ ∂ ⎞ ∂ 2 ⎞              ⎤
           ⎢−   ⎜
               *⎜      ⎜ r ⎟ + 2 ⎟ + V (r , z )⎥ϕ (r, z ) = Eϕ (r, z )
                                 ⎟                                                                                               (1)
           ⎢ 2m ⎝ r ∂r ⎝ ∂r ⎠ ∂z ⎠
           ⎣                                   ⎥
                                               ⎦

                                                                1.5


 Disk like shape dot/well system                                1.0

                                                                0.5                                       Ve




                                                Energy ( eV )
                                                                0.0

 cylindrical coordinates to describe QD                         -0.5           Barrier     QW    QD

  potential distribution of electron and                        -1.0


 hole.                                                          -1.5
                                                                                                           Vh
                                                                -2.0

                                                                -2.5
                                                                       0.000   0.002     0.004   0.006   0.008   0.010   0.012
                                                                                          Distance ( micron )



                                                                Energy band diagram of an InGaN
                                                                quantum dot in a quantum well
Shanghai Institute of Technical Physics



                                                          0.8

                                                          0.6

    Confined dot levels for InGaN




                                          Energy ( eV )
                                                          0.4
    quantum dots with height of                           0.2
    1.5nm, diameter of 3.6nm and
    5.0nm                                                                 2.485eV
                                                                                             2.409eV
                                                          -1.6


                                                          -1.8   HH                 HH
                                                                 CH                 CH
                                                          -2.0

                                                                 Diameter=3.6nm
                                                                                     Diameter=5nm




     the interband transition between confined dot levels close to the
     bottom of dot potential is responsible for the LED emission
                                                                 2
                                      2
                           q 2 n r EM b N qd ( s )
  sp
rqd ( E )   =   ∑∑ πε m h c t
                s   i, j       0
                                   2 2 3
                                   0   0 cmplx
                                                     ϕ is ϕ js       G s ( E − Eijs ) f c (1 − f v )



 Shanghai Institute of Technical Physics


       3. Spontaneous emission
                    In dot/well system, spontaeous emission comes from two part:
       QD
                                                                                 2           2                            2
                                                                             q       nr EM b N qd ( s )
                                 sp
                               rqd ( E )             =   ∑∑ πε
                                                           s         i, j            0
                                                                                         2
                                                                                       m0 h 2 c0 t cmplx
                                                                                               3
                                                                                                                ϕ is ϕ js Gs ( E − Eijs ) f c (1 − f v ) (2)

      QW                                                                               ⎛ 2π ⎞
                                                                            ∑
                                                                                                                 2
                                   sp
                                r qw        (E ) =                                     ⎜    ⎟ H            ij        f j' ( 1 − f i ' ) D ( E ) ρ   ij   (3)
                                                                            i= j       ⎝ h ⎠

 Total spontaneous emission determined by:

                                                      r2sp ( E ) = rqw rqw ( E ) + rqd ( E )
                                                         d
                                                                         sp          sp
                                                                                                                                                         (4)
Shanghai Institute of Technical Physics


4. Quantum transport mechanism


     Non-equilibrium quantum transport model

          1) fly directly over the small QDs

          2) escaping from the deep QD potential before
             being thermalized
Shanghai Institute of Technical Physics


 Simulation results
                                                                          1.2
 Stokes shift is 370meV                                                          Experimental EL




                                               EL and PC ( arb.units. )
                                                                                 Experimental PC
                                                                                 Simulated absorption spectrum based on MQW model
 QD emissin in our green LED                                              0.9                        370 meV



                                                                          0.6




 photocurrent is from the                                                 0.3


 inter-subband transition in
                                                                          0.0
 InGaN quantum wells                                                            1.5    2.0     2.5     3.0     3.5     4.0     4.5
                                                                                             Energy      ( eV )



                                          EL, PC spectrum and simulated absorption
                                          spectrum based on MQW model with In content
                                          of 0.22
Shanghai Institute of Technical Physics



                                                                          Experimental EL
                                                                          Simulated EL based on MQW Model
                                                                          Simulated EL based on QD Model




                                          Intensity ( arb. units )
Experiment          2.35-2.40eV

MQW model             2.67eV

QD model
good agreement with experiment
                                                                 2.0       2.2        2.4        2.6        2.8   3.0
                                                                                 Wavelength     ( nm )



                                                                     Calculated and experimental EL spectrum
Shanghai Institute of Technical Physics



                                                              30
                                                                       Experimental
                                                                       Simulated based on MQW Model
                                                              25       Simulated based on QD Model with Q.Trans.
                                                                       Simulated based on QD Model without Q.Trans.




                                             Current ( mA )
                                                              20


 QD model with Quantum Transport                              15

     close to experiment                                      10


                                                              5


                                                              0
                                                                   0    1         2         3         4         5     6
                                                                                      Voltage ( V )




                                          The I-V characteristics of InGaN-based LED
Shanghai Institute of Technical Physics




 MQW model                                                                  1.0




                                              Internal Quantum Efficiency
 QD model without Q.Trans                                                   0.8

                                                                                        Experimental
     90%~100% which is overestimated                                        0.6         Simulated based on MQW Model
                                                                                        Simulated based on QD Model with Q.Trans.
                                                                                        Simulated based on QD Model without Q.Trans.
                                                                            0.4
 QD model with Q.Trans
                                                                            0.2

     close to experiment
                                                                            0.0
                                                                                  0      5          10          15          20         25
                                                                                                    Current ( mA )




                                                                                      The IQE of InGaN-based LED

  It indicates that quantum transport mechanism plays an important role in the
  InGaN-based MQW LED
Shanghai Institute of Technical Physics



   Conclusion
  1) QD model with Q.Trans. accurately accounts for the experimental data of InGaN
     based LED

  2) Quantum dot emission and non-equilibrium quantum transport played very
     important roles in the InGaN-based MQW LEDs

  3) The simulation allows us to understand better for the quantum states effect in the
     device performance

  4) With our more delicate model, one may be able to optimize the InGaN-based
      LEDs performance
Shanghai Institute of Technical Physics




               Thank you for your attention!

								
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