High Performance Green LEDs for Solid State Lighting
DOE SSL WorkshopJanuary 29, 2019
PIsShuji NakamuraJames S. SpeckSteve DenBaarsClaude Weisbuch
Materials DepartmentUniversity of CaliforniaSanta Barbara, CA 93106
Core teamCheyenne LynskyRyan WhiteGuillaume LheureuxBastien BonefAbdullah Alhassan
Additional supportYuh-Renn Wu (NTU)
Prime recipient: UCSBAgreement # DE-EE0008204SSL Project Manager: Dr. Joel Chaddock
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Motivation
Fundamental efficiency of RYGB diode direct vs. pc-LED
Goal: WPE (PCE) > 44% to exceed pc-G
DOE 2018 SSL R&D Opportunities
Motivation
WPE = EE x IQE x LEE
EE = Vph/VF (where hν = qVph)
λgreen = 525 – 540 nm … Vph = ~2.25 V
Commercial LEDs c. 2016-2018 … VF (20 or 35 A/cm2) > 3 VEE < 0.75
DOE 2018 SSL R&D Opportunities
Motivation
Estimates for green LED efficiencies …
WPE = EE x IQE x LEE0.27 = 0.68 x 0.47 x 0.85
Three areas of focus:
EE: Identify and engineer all barriers to electron and hole transport
IQE:SRH reduction – material qualityAuger – reduced carrier density in active QW
Engineering Against SRH
Engineering Against SRH
Engineering Against SRH
Engineering against SRH
*High T QW growth via high TMI flow*AlGaN cap at same T as QW*Multistep GaN barrier (higher T, switch to H2 carrier gas)
Assumption: reduced SRH via higher T and high TMI flow
Voltage Reduction
Voltage Reduction
Timeline of VF reduction
Project Kickoff09/01/2017VF = 4.6 V at 20 A/cm2 for 5 QW
Remove EBLY1Q1
Role of QW #Y1Q2
Reduced Al content in capY1Q2
Ohmic p-contactsY1Q4
Role of SLY1Q4
End of Year 110/31/18VF = 3.1 V at 20 A/cm2 for 5 QW
Voltage Reduction
Patterned Sapphire Substrate (PSS)
2.5 μm UID GaN
1.8 nm GaN UID3 nm InGaN QW
5 nm n-GaN
2 nm Al0.10Ga0.90N cap layer
8.3 nm p+-GaN
X QW
27 nm n-GaN
6 nm GaN barrier
1.5 μm n-GaN, [Si] = 4x1018 cm-3
130 nm p-GaN, [Mg] = 5x1019 cm-3
10 nm p+-GaN, [Mg] = 2.5x1020 cm-3
45x SL 2.65 n-In0.04GaN0.96
Achieved low VF green LEDsLow Al content AlGaN capOhmic p-contactsIncreased SL period from 10 to 45p+ layer after last QBReduced GaN QB thickness from 9 to 6 nm
Experimental I-V curve for 1, 3, 5 QW green LEDs
Reduced VF from 4.6 V to 3.1 V for a 5 QW green LED
Voltage Reduction
Patterned Sapphire Substrate (PSS)
2.5 μm UID GaN
1.8 nm GaN UID3 nm InGaN QW
5 nm n-GaN
2 nm Al0.10Ga0.90N cap layer
8.3 nm p+-GaN
X QW
27 nm n-GaN
6 nm GaN barrier
1.5 μm n-GaN, [Si] = 4x1018 cm-3
130 nm p-GaN, [Mg] = 5x1019 cm-3
10 nm p+-GaN, [Mg] = 2.5x1020 cm-3
45x SL 2.65 n-In0.04GaN0.96
Achieved low VF green LEDsLow Al content AlGaN capOhmic p-contactsIncreased SL period from 10 to 45p+ layer after last QBReduced GaN QB thickness from 9 to 6 nm
Experimental I-V curve for 1, 3, 5 QW green LEDs
Reduced VF from 4.6 V to 3.1 V for a 5 QW green LED
Voltage of Green LEDs
*VF >> Vph (best reports ΔV ~0.4 V)*Why?
*Need to identify barriers to carrier transport
Advanced Characterization and Simulations
Numerical Tools
APT In concentration map of an In0.29Ga0.71N QW (courtesy of B.Bonef)
10 nm
Alloy fluctuations play a major role in nitride devices
Need to be taken into account
Major computation issue in semiconductor physics
Requires solving Schrodinger equation for electrons and holes in a random, disordered potential
[0001]
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𝑯𝑯𝑢𝑢 =ħ2
2𝑚𝑚∆ 𝑢𝑢 + 𝑉𝑉𝑢𝑢 = 1 (2)
𝑯𝑯ψ =ħ2
2𝑚𝑚∆ψ + 𝑉𝑉ψ = 𝐸𝐸ψ (1)
Idea: Replace Schrodinger equation with the landscape equation
1/u acts as an effective confining potential
Replacing ψ by (u 𝜙𝜙) in (1) leads to
−ħ2
2𝑚𝑚1𝑢𝑢2𝑑𝑑𝑑𝑑𝑑𝑑(𝑢𝑢2𝛻𝛻𝜙𝜙) +
1𝑢𝑢 𝜙𝜙 = 𝐸𝐸𝜙𝜙
M. Filoche and S. Mayboroda, PNAS 109, 14761 (2012)D. Arnold et al. PRL 116, 056602 (2016)
1/u describes the localization energies for localized state
Can be used to predict local DOS
Landscape Theory
Poisson-landscape-drift-diffusion solverSelf-consistent algorithmFast convergence Enables 3D simulation of nitride devices
Blue LED simulationsExperimental parameters are usedExcellent agreement with commercial blue LEDs
C.K. Li et al., PRB 95, 144206 (2017)M. Filoche et al., PRB 95, 144204 (2017) 15
Landscape Theory
Poisson-landscape-drift-diffusion solverSelf-consistent algorithmFast convergence Enables 3D simulation of nitride devices
Blue LED simulationsExperimental parameters are usedExcellent agreement with commercial blue LEDs
C.K. Li et al., PRB 95, 144206 (2017)M. Filoche et al., PRB 95, 144204 (2017) 16
Landscape Theory
3D Landscape-Poisson Solver
*100X – 1000X faster than 3D Schrodinger-Poisson*Facilitates 3D simulations including natural alloy disorder
*Marked improvement in device I-V prediction
*Alloy fluctuations (experiment and theory)Percolative paths for carrier transportPockets for locally high carrier density and enhanced Auger
0 1 2 3 4 5 6
1
2
3
4
5
4
8
12
16
20
1 QW 2 QWs 3 QWs 5 QWs 7 QWs
Cur
rent
den
sity
(A/c
m2 )
Curre
nt (m
A)
Voltage (V)
Experimental Series• 3 nm In0.24Ga0.76N QWs• 2 nm Al0.30Ga0.70N/ 7 nm GaN QB
Experimental
Simulation
Simulation Series• 3 nm In0.24Ga0.76N QWs• 7 nm GaN barriers• No sheet resistance, Ohmic
contact
Good Agreement
The turn-on voltage increases with the number of QWs !
Voltage Drop due to QWs
• Each QW adds a voltage penalty• Unbalanced e-h injection• One active (top) QW
J=0 A.cm-2 J=10 A.cm-2
3.34 V3.7 V
4.12 V4.58 V
5.08 V
How to overcome this in Green LED ?
• Polarization screened QBs• InGaN QBs
Two main issues in Green LED• Large Barriers due to polarization charge• Hole injection
Voltage Drop due to QWs
Two main effects can explain the improved turn-on
• Balanced e-h injection• Larger e-h overlap
Simulation of a green SQW LED with 0% and 100% polarization coefficients
1/uC and 1/uV band diagram
Motivation – Field Screening
Polarization charge:InGaN/GaN interface
QB doping: screen QW electric field
VF=3.41 V at 100 A/cm2
Advanced Design
Simulation1 QW
By increasing the doping up to 7x1019 cm-3,the electric field can be fully screenedThe turn-on is improved by 0.7 V
Advanced Design
Simulation1 QW
By increasing the doping up to 7x1019 cm-3,the electric field can be fully screenedThe turn-on is improved by 0.7 V
Advanced Design
Field Screening
*Eliminates polarization barriers for carrier transport
*Allow wide QWsReduced carrier density … reduced droop
Challenge:SRH due to growth of screening layer
Demonstration of Field Screened Blue SQW LEDs
Nathan Young (Ph.D. Dissertation … 2015)
Field Screening in Blue:Simulated Device Structure
Structure designed for low droop: Wide SQW active region for low carrier density &
Heavily doped layers surrounding SQW for polarization screening
Field Screening in Blue:Simulated Device Structure
Demonstration of Screening in Blue
0 200 400 600 8000
2
4
6
8
Volta
ge (V
)
Current (A/cm2)
0
100
200
300
400
Lig
ht O
utpu
t Pow
er (m
W)
0 200 400 600 8000
5
10
15
Doped Undoped
EQE
(%)
Current (A/cm2)
• Better voltage w/ doping: lower injection barriers• Much higher power and EQE, especially at low current: poor
overlap, poor radiative efficiency• Apparent low carrier density in unscreened LED: excited state
emission
7.5 nm SQW
200 400 600 8000
10
20
30
40 7.5 nm SQW Screened Reference MQW LED
EQE
(%)
Current Density (A/cm2)
Challenge for Screened Structures: SRH
Should have nearly same peak EQE
Increased A coefficient in SQW
No “conditioning” QWs or underlayers, heavily doped barrier regions
SRH
Thank You!
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Osram Opto SemiconductorsOSCONIQ ® P 2226Mid-power, true green LED
Version 1.0 (2017-08-10) at 100 mALuminous efficacy = 93 lm/WForward voltage = 3.35 V
Version 2.1 (2018-05-11) at 100 mALuminous efficacy = 127 lm/WForward voltage = 2.75 V
Increase in luminous efficacy is due to lower VF!
State-of-the-art comparison: industry green LEDs
𝐖𝐖𝐖𝐖𝐖𝐖 = 𝐖𝐖𝐄𝐄𝐖𝐖 ×𝐡𝐡𝛖𝛖/𝐪𝐪𝐕𝐕𝐅𝐅
Voltage Excess
𝑾𝑾𝑾𝑾𝑾𝑾 = 𝑾𝑾𝑬𝑬𝑾𝑾 ×𝒉𝒉𝝊𝝊/𝒒𝒒𝑽𝑽𝑭𝑭
Typical forward voltage in Green LED are still far away from the photon energy
Origins of this voltage excess in Green LEDs ?
Alhassan, Abdullah I., et al. "High luminous efficacy green light-emitting diodes with AlGaN cap layer." Optics express 24.16 (2016): 17868-17873.
https://www.osram.com/os/press/press-releases/longer-battery-life-for-fitness-trackers-osram-increases-the-efficiency-of-green-leds-by-40-percent.jsp
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