High Performance Green LEDs for Solid State Lighting

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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

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

*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

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

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

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)

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]

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 QW 2 QWs 3 QWs 5 QWs 7 QWs

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

Current (A/cm2)

0 200 400 600 8000

Doped Undoped

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

40 7.5 nm SQW Screened Reference MQW LED

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

Thank You!

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

High Performance Green LEDs for Solid State Lighting

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