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 Shanghai Institute of Technical Physics 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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