Solid State Lighting: The Remaining Challenges

E. Fred SchubertRensselaer Polytechnic Institute, Troy, NY 12180

Presentation at CREOL Industrial Affiliates Symposium, March 2016

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(1) 20 years of white LEDs(2) Challenges in solid-state lighting (3) Efficiency droop – The greatest

challenge in LED efficiency

2015 The International Year of Light

(1) 1996 – 2016 White LEDs – 20 years

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Triplet approach: Red, Green, & Blue• Stinson (1991)

• Commercial devices (2015)

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White LED made with RGB chips?

Multiple choices Phosphor-based White LED

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White LED made with LED chip + Phosphor?

Potter et al. (1970) • Up-conversion Two-photon process Phosphor must be close to chip• GaAs LED chip plus a Fluoride: RE based phosphor (LaF2:Tm)

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White LED made with LED chip + Phosphor?

Tabuchi et al. (1973) • Down-conversion• Proposal: GaN LED chip + phosphor • Full conversion • No demonstration

Full conversion: Reminiscent of fluorescent lighting • Tabuchi (1973) • Stevenson et al. (1974)• Baretz and Tischler (1996)

Partial conversion: More elegant and more efficient • Shimizu (1996)• Schlotter et al. (1996)

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Full and Partial Conversion

September 13, 1996: Announcement in Nikkei Sangyo Shimbun:• White LED• 10 lm/W (R&D), 5 lm/W (production), 50 000 hours • YAG:Ce highly stable phosphor that emits red and green light

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White LEDs – September 1996

Shimizu et al., Nichia Company, Anan, Japan

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White LEDs – November 1996

Radiation intensity 100 greater than the Sun Instead of 1 kW / m2 the radiation intensity is 100 kW / m2

• Organic phosphors:• Inorganic phosphors:

Present efficiency: 100 lm/W available at this time.

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White LEDs – 2011 – 2016

(2) Remaining challenges

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A few years ago: Cost, cost, and cost

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Remaining Challenges – 1st – Cost

Edison LFL & CFL LED Purchase price Energy cost Lamp replace-ment cost

Total US$ Environmental cost

Total

Add intelligence to lighting systems

Controllable light sources enable fundamentally new applications

Use the full potential of lighting systems

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Remaining Challenges – 2nd – Using full potential of LEDs

Droop is the single largest loss mechanism in blue and white LEDs

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Remaining Challenges – 3rd The efficiency droop

Efficiency of LED

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Largest obstacle to high LED efficiency• The efficiency droop

Challenges: Efficiency Droop in GaInN LEDs

Efficiency is high, despite dislocations Efficiency decreases with increasing

injection current Severe obstacle for high-power LEDs

Min Ho Kim et al., 200715

300 K – Systematic trend:• Red devices exhibit no droop • Blue devices exhibit some droop • DUV devices exhibit strongest droop

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Red, Blue, and deep UV LEDs

110 K – Systematic trend:• Red devices exhibit some droop• Blue devices exhibit stronger droop• UV devices exhibit “U-turn”

Correlation with transport properties • High-injection phenomenon has strong correlation with Droop behavior

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Red, Blue, and deep UV LEDs

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Carrier recombination in LEDs

Recombination can be described by

• R = An + Bn2 + Cn3 + f(n)

• A … Shockley-Read-Hall recombination• B … Radiative recombination• C … Auger recombination • f(n) … Leakage

The following trends occur as the bandgap energy increases...• ...the difference between electron and hole mobility can become greater.

Kronig Penney model Large Eg Holes have higher barriers Greater effective mass

• Hydrogenic acceptors acceptor energy gets deeper less acceptor ionization Lower hole concentration

• Compensating native defects Due to the large Eg, compensating native defects are becoming more abundant and indeed limit the hole concentration

These effects lead to a greater asymmetry in transport that affects both carrier concentration as well as carrier mobility.

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Solid-state Lighting – Remaining Challenges – 3rd

The efficiency droop occurs inmultiple material systems

VisibleGaN-based LEDs (300 K and cryogenic temperatures)

VisibleAlGaInP-based LEDs (300 K and cryogenic temperatures)

UVAlGaN / AlGaN (300 K and cryogenic temperatures)

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Droop in LEDs: (1) GaInN / GaN

First found by Mukai et al.Japanese Journal of Applied Physics 38, 3976 (1999)

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Droop in LEDs: (2) AlGaInP / GaAs

First found by Shim et al. Applied Physics Letters 100, 111106 (2012)

In contrast to GaInN, AlGaInP has:• No polarization fields• No dislocations • No In clustering as found in GaInN• No thin active region (MQW with 38 QWs)

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Droop in UV LEDs: (3) AlGaN / AlGaN

First found by Park et al.

High-injection phenomenon

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Theoretical current-voltage (I-V) characteristic for p-n junctions (“Shockley equation”):

Maximum voltage that can drop across the depletion region of a p-n junction:

Beyond this voltage, the junction voltage saturates and an additional voltage will drop in part across the neutral regions of the diode.

The onset of high injection can be easily identified:When the I-V characteristic starts to deviate from the exponential dependence.

See paper by Sah, Noyce, and Shockley, Proceedings of the IRE (1957)25

High Injection in p-n junctions

1ep0n

nn0

p

p

kTeVnDp

DeJ

2i

ADbi ln)/(

n

NNekTV

Energy band diagram for low-injection and high-injection

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High Injection in p-n junctions

Shockley theory: • Low currents: Exponential I-V characteristic• Onset of high injection: Transition region • High currents Linear I-V characteristic

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Identification of “Onset of high injection”

Onset of the efficiency droop is the peak-efficiency point

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Identification of “Onset of efficiency droop”

Onset of high injection Onset of the efficiency droop Voltage difference = 0.3 V

• Part of this voltage drops across the p-type region

• Electric field• Drift of electrons • Drift-induced leakage

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Universal Phenomenon

Cree EZ900 Sanan S-45BBMUP

Nichia 50mil FC Osram ODB40UX3

Y. Wang, M. Pan, and T. Li, Proc. SPIE 9003, 90030D (2014)

Analytic model based on high-injection phenomenon

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Analytic model of efficiency droop

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High injection condition in pn junctions

Conventional low-injection condition (Sah, Noyce, Shockley, 1957):

∆np(0) << pp0

In the presence of a large mobility difference, the low-injection condition can be generalized:

∆np(0) µn << pp0 µp

High-level injection more severe in GaN due to large asymmetry in pn junction

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Analytic Model for Efficiency Droop

Recombination can be described by • R = An + Bn2 + Cn3 + f(n)

Analysis shows• f(n) = CDL nQW

3 + DDL nQW4

Using pp0 = 5 1017 cm–3, p = 2.5 cm2/Vs, n = 300 cm2/Vs, B = 10–10

cm3/s, and = 0.1%, we obtain:

C = 2.4 10–29 cm6/s

D = 5.7 10–48 cm9/s

Agreement with experimental values.

Bpμ

D2

p0p

nDL

B

pμC

p0p

nDL

Under high injection, an electric field develops in the p-type region Radiative 10 ns and Drift << 1 ns Drift << Radiative

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Why does droop occur despite good carrier confinement?

PropertiesExperiments

Drift-induced leakage model

Magnitude of Ccoefficient

C = 10–29 cm6/s factor of 10 ~ 10–29 cm6/s

Onset-of-droop current density 1 – 10 A/cm2 Temperature dependence of the droop

T C Symmetry of the EQE-vs.-n curve Asymmetric Asymmetric

Droop in different materials

GaInN, AlGaInP, InSb, and AlGaN

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Testing models for droop

Recent measurements:“U-turn” feature

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“U-turn” is helpful in further understanding the droop phenomenon “U-turn” is found on high efficiency devices with EQEmax > 60% Y. Wang paper (SPIE, 2014), IQE = 69.5% at 35A/cm2 at 300 K (Cree

EZ900 LED) as determined by temperature-dependent EL method.

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New feature: “U-turn” in the droop regime

Cause of U-turn:• U-turn due to an increase in free hole concentration, pp0

• If this is the case, then the p-type layer becomes more conductive and the conductivity of the device will change

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Onset of high injection; Onset of droop; “Minimum of U-turn”

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Correlation between (1) conductivity and (2) “U-turn”

Correlation of conductivity & “U-turn” (!) This demonstrates that efficiency is limited by

hole concentration, hole injection, or pp0

How can the efficiency droop be overcome?

Acknowledgements

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Future

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Reduce n• Thinner QBs; Reduction of n-type doping in active region

Increase pp0• New dopants; p-type GaInN; p-type superlattices

Increase p• New material compositions; strain; modulation doping

Decrease • Polarization-matched EBL; polarization-matched MQW; Reduction of

polarization charge at spacer-EBL interface

Reduce E (Polarization field)

• Non-polar growth; Polarization matching concept

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Future

3

p0p

n3DL nB

pμnC

New p-type materials • Wide energy gap (optical transparency)• Better p-type characteristics than GaN• Candidates ?

Field-ionization of acceptors • More complete ionization of acceptors • This would increase hole concentration by about a factor of 10• External voltage ?• Compositional grading ?

Increase in hole mobility • Strain ? • Use of p-type GaInN alloy rather than p-type GaN ?

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Overcoming the efficiency droop