2.5

2.4

2.3

2.2

2.1

AlInP-Based LEDs for E˜cient Red and Amber Emission Kirstin Alberi1, Chris Stender2, Phil Ahrenkiel3

1National Renewable Energy Laboratory, Golden CO 2MicroLink Devices Inc, Niles IL

3South Dakota School of Mines and Technology, Rapid City SD

Background and Importance

Present EQE trend (Al Ga ) In POverview x 1-x 0.5 0.5 Red and amber phosphide-based LEDs have e˜ciencies well below blue InGaN LEDs, in part due to fundamental

intervalley transfer

E c

E v cladding claddingactive

MQWs

electron leakage

ΓA - ΓC

XA - ΓΑ

Band

gap

(eV

) 620

nm

590

nm

10

20

30

40

50

60

0 550 650

Projected trend 2.5

limitations of the materials from which they are made. The e˜ciencies of these LEDs must be improved to enable next-generation color-mixed white LED architectures.

We are addressing the limitations of phosphide-based LEDs by modifying the materials that are used for the active and cladding layers. The objective of this project is to demonstrate the feasibility of fabricating red and amber LEDs from ordered/disordered Al In P heterostructures and to quantify the performance improvements this design x 1-x can yield over incumbent phosphide LEDs.

Exte

rnal

Qua

ntum

E˜

cien

cy (%

)

2.0

1.5

!

Color- ./* Wavelength Emitter Type Material Spectral Half PCE (%) PCE (%) Hot/Cold Width @ 25 C @ 85 C Factor

1.0 Blue 459 nm Direct Ga1-xInxN 20 nm 66 63 0.95

Direct Indirect

AlP

GaP

GaAs InP

InAs

Al x In1-xP}Δ

5.6 6.0

Green 530 nm Direct Ga1-xInxN 30 nm 24 22 0.90 Peak Wavelength (nm) Lattice Constant (A) Amber 595-605 nm Phosphor-Converted Ga1-xInxN/Phosphor 80 nm 23 18 0.80 Methodology: Power Conversion E˜ciency (PCE)

values at 25 C and hot/cold factors are reported for Amber 585-595 nm Direct (Al Ga ) In P 20 nm 19 8 0.40 x 1-x 0.5 0.5 best-known commercially-available devices in each (Al Ga ) In P LEDs su°er from internal intervalley transfer losses and external carrier leakage losses. These losses are fundamental to the x 1-x 0.5 0.5 Red 615 nm Direct (Al Ga ) In P 20 nm 44 24 0.55 x 1-x 0.5 0.5 color range; PCE values at 85 C are computed from (Al Ga ) In P material system and can only be overcome by using another material. x 1-x 0.5 0.5 Deep Red 650-670 nm Direct (Al Ga ) In P 20 nm 49 39 0.80 the PCE @ 25 C and hot/cold factors. x 1-x 0.5 0.5 Direct bandgap Al In P has a higher direct-indirect bandgap energy crossover, which helps to reduce intervalley transfer. We combine Al In P x 1-x x 1-x with control of spontaneous atomic ordering to increase barriers to electron leakage to improve the e˜ciencies of red and amber LEDs.

Al x In1-xP Properties and Growth Luminescence Results Laser Power (mW) Properties Amber Red 0 5 10 15 20

2.6 0.4 10 K

10 K

295 K

295 K

Modulated reˆectance Photoluminescence

adapted from D. Beaton, et al., J. Appl. Phys., 114, 203504 (2013)

590 nm

measured datapoints

target range

X-Γ Conduction Band O°set Γ X

Ener

gy

0.180.11 0.20 0.29 25 C 85 C

125 C

3kBT

AlInP

AlGaInP

PL In

tens

ity (a

.u.)

disordered AlInP clad

disordered AlInP clad

ordered AlInP active

metamorphic bu°er

GaAs substrate

160 meV clad

active

1.0

[111]Disordered Ordered

Ener

gy (e

V) ΔEG

Γ4V, Γ5

V

ΔEV

Γ C 6 Γ C

6

Al

In

VΓ8 P

VΓ Γ6(2)

7

k = 0 k = 0

33 36 39 42 45 48 51 54 Al Concentration (%)

Al and In atoms can order along alternating [111] planes depending on the growth conditions. Ordering causes the bandgap to decrease via a reduction in the conduction band edge energy.

O°s

et E

nerg

y (e

V)

O°s

et E

nerg

y (e

V)

0.3

0.2

0.1

Γ6V(1) 0 PL

Inte

nsit

y N

orm

aliz

ed to

10

K

PL Intensity (a.u.)

8 0.8

0.6

0.4 Ln C

ount

s 6

4

0.2

2 0

25 C 85 C

125 C

3kBT

Cladding-Active Layer O°set

Ener

gy AlInP

AlGaInP

0.190.08 0.14 0.2

active

clad clad 0 50 100 150 200 250 300

Temperature (K) 540 560 580 600 620 640 0 1 2 3 4

Wavelength (nm) Time (ns) 0.2

Al In P emits at 590 nm a compositions ~ 36-43% Al, depending on the ordering. We demonstrated x 1-x

Photoluminescence (PL) measurements as a function of temperature and Internal Quantum E˜ciency

excitation density provide information about electron loss pathways. 1.0

0.1 PL from the cladding layer is observed at low temperatures, giving an indication of the energy barrier o°set.

Time-resolved PL shows two characteristics decay times associated with radiative and non-radiative recombination that can be used to extract

0 Δ(Γ -Γ ) and Δ(X-Γ) o°sets > 140 meV and are moving the composition toward the targeted range. dis ord

600 650 information about the internal quantum e˜ciency. Emission Wavelength (nm)

Given the o°sets demonstrated here and in the literature, Al In1-xP devices, the barriers to electron loss at We initially used the ratio of PL intensity at 295 K/10K to estimate the

x amber emission wavelengths are expected to be similar to those in (Al Ga ) In P red LEDs. x 1-x 0.5 0.5

internal quantum e˜ciency of simple Al In P double heterostructures x 1-x PL In

tens

ity

Nor

mal

ized

to 1

0 K

state of the art

0.4 Al x In P

0.8

0.6

1-x

0.2

(Al Ga ) In P x 1-x 0.5 0.5 and compared to (Al Ga ) In P samples with similar structures. We are x 1-x 0.5 0.5

0 590 600 610 620 630 640 650

continuing to improve our Al In P material to increase luminescence. Wavelength (nm) x 1-x

Metamorphic Growth Electroluminescence Current and Potential Limiting Factors Growth Structure Cross-Sectional Transmission Electron Microscopy (TEM) Image

GaAs substrate

Al x In1-xP active layer Al y In1-yP cladding layer

Al z Ga y In1-y-zAs MBL

thic

knes

s

Metamorphic bu°er

ÿnal bu°er

AlInP Α

Current-voltage measurements of double 1. Oxygen impurities heterostructure devices indicate potential Deep traps associated with oxygen impurities increase non-radiative improvements in series resistance. recombination. SIMS measurements indicate our devices have 1017 cm-3

oxygen impurities due to reactor-speciÿc issues. While we cannot remedy 50

40

(Alx Ga1-x)0.5In0.5P

Al x In1-xP

disordered AlInP clad

disordered AlInP clad

ordered AlInP active

metamorphic bu°er

GaAs substrate

InGaAs contact layer Au top contact

Au bottom contact

the reactor design, we are looking into the use of higher V/III ratios to prevent oxygen inclusion.

2. Dislocations

lattice constant Ordered Domains Di˛raction Pattern High Resolution TEM Image PL Image

To prevent threading dislocations, a 0

20 μm

ΑAl In P (x < 0.43) has a larger lattice x 1-x

Curr

ent (

mA

)

30 We have limited the threading dislocations in our devices to 105 cm-2.

20 However, we ÿnd that the pattern of misÿt dislocations within the bu°er layer a°ects the luminescence. We are currently exploring whether this is

constant than GaAs. 10 due to strain, composition modulation or non-radiative recombination.

3. Ordered domains and composition modulation

metamorphic bu°er layer is used to 0 1 2 3 4 Domain boundaries between ordered variants and composition

extend the lattice constant before Voltage (V) modulation on a short scale are observed. We do not yet know the extent to which domain boundaries or composition modulation impact Al In P growth. x 1-x We are currently performing non-radiative recombination.

This approach reduces the electroluminescence measurements as a dislocation density to 104 - 105 cm-2. 4. Multiple quantum wells function of temperature and drive current to

Project Metrics and Milestones

determine how ordered/disordered Al In P LEDs The devices we have tested to-date mainly consist of double x 1-x di°er from incumbent (Al Ga ) In P devices in heterostructures without multiple quantum wells. We will begin to add in x 1-x 0.5 0.5 terms of emission e˜ciency. optimized MQW structures to improve radiative recombination.

Milestones and Progress Indicators

Maximize ordering in Al In P (˛ = 590 nm) to be η > 0.45. x 1-x

Demonstrate internal -̋X o°sets of 130 meV in ordered Al In P (˛ = 590 nm).x 1-x

Demonstrate electron conÿnement potentials of 130 meV in ordered/disordered Al In P heterostructures (˛ =590 nm). x 1-x

Demonstrate ordered Al In P (˛ = 590 nm) with IQE = 75%.x 1-x

Timing (Months) 0 3 6 9 12 15 18 21 24 Results Summary, Future Research and Considerations

We have shown that we can achieve large bandgap energy shifts with ordering. Our experiments suggest that we can achieve up to 200 meV direct/indirect and active/clad conduction band o°sets, producing higher barriers to electron loss than incumbent (Al Ga ) In P LEDs. We have also shown that we can metamorphically grow high x 1-x 0.5 0.5 quality Al In P heterostructures on GaAs substrates. Luminescence results indicate areas for improved x 1-x performance by reducing non-radiative recombination sites and optimizing the device structure.

Demonstrate (Al Ga ) In P (˛ = 590 nm) wtih IQE = 65%. x 1-x 0.5 0.5 Going forward, we will continue to optimize the growth conditions to improve Impact analysis. the Al In P material, incorporate multiple quantum well structures and analyze x 1-x

the potential of this approach to improve amber LED e˜ciency. Demonstrate Al In P heterostructure LEDs (˛ =590 nm) with a MQW active layer x 1-x that exhibit higher EQE than equivalent (Al Ga ) 1In P LEDs (˛ = 590 nm).x 1-x 0.5 0.49

Characterize loss channels in Al In P MQW structures. Evaluate the advantages and x 1-x limitations of this approach.

This work was authored°in part°by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by°the De-

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36 In

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33 In

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w

ell

Al 0.

43 In

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Al 0.

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partment of Energy, O˜ce of Energy E˜ciency and Renewable Energy, Solid-State Lighting Program. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains NREL Publication: PO-5K00-75897 and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.