Nitride LEDs: the red quest. 400. 450. 500. 550. 600. 650. 0.0. 0.2. 0.4. 0.6. 0.8. 1.0. L. E. D efficiency. Wavelength (nm). Phosphide LEDs. IQE ~ 0.9-1. L. D.
34th International Conference on the Physics of Semiconductors (ICPS2018) Montpellier, FRANCE
What is red ? On the chromaticity of orange-red InGaN/GaN based LEDs Room Barthez - July 31st, 2018
Y. Robin, M. Pristovsek and H. Amano Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, JP
F. Oehler, R. A. Oliver and C. J. Humphreys Department for Materials and Metallurgy, University of Cambridge, UK
III-V materials for optoelectronics Phosphide-Arsenide
Nitride
2.5
AlP
Bandgap Energy (eV)
Bandgap Energy (eV)
7
AlAs GaP 2.0 VIS (AlGaInP)
1.5
GaAs 5.25
5.50
IR
5.75
InP 6.00
AlN
6
UV (AlGaN)
5 4
VIS (InGaN)
3
GaN
2
InN
1
IR
0 3.00
3.25
3.50
Lattice Constant (A)
Lattice Constant (A)
⇒ Indirect bandgap…
⇒ Poor crystal quality
3.75
InGaN – AlGaInP based microLEDs ? InGaN
AlGaInP
Display?
+
4
10
PPI, Pixel size (µm)
Xmas fairy light
100µm
Smartphone Watch 3
10
Tablet TV
2
10
Street display Cinema
1
Max PPI Min pixel size
10
(αmin = 1 arcmin) 0
10
1
10
2
10
3
10
Viewing distance (cm)
4K (16:9) ~ 9 millions of pixels !... 1 pixel (RGB) < 100x100 µm
Nitride LEDs: the red quest Nitride LEDs 1.0
LED efficiency
Phosphide LEDs
IQE ~ 0.9-1 LD ~ µms min. size ~ 10s µms
0.8 0.6 0.4 0.2
IQE ~ 0.9-1 LD ~ 100s nm min. size ~ µms
0.0 400
450
500
550
600
Wavelength (nm)
650
Trichromatic Theory of color Vision
=
̅ (λ) (λ) . (λ) λ ⇒ ̅(λ)
=
+
1
+
.
Human eye Cone 2.0
2.0
1.6
1.6
EL intensity (a.u.)
Color matching fct (a.u.)
Stimulus
Rod
1.2
z(λ) 0.8
x(λ) y(λ)
0.4 0.0 400
450
500
550
Wavelength (nm)
600
650
L(λ)
1.2 0.8 0.4 0.0 400
450
500
550
Wavelength (nm)
600
650
CIE xy (1931) diagram Spectral locus (spectral color)
LED (x,y)
⇒ H (hue) S (saturation)
Excitation purity H
100%
x x =0.091 y =0.457 H = 498nm S ~ 74%
(~ saturation)
S D65 White point
Line of purples (non-spectral colors)
0%
Color of InGaN/GaN LEDs λEL=527nm λEL=605nm
x
? x λEL=471nm
RGB target
x
B 465nm G 530-550nm R 610nm
Conventional InGaN/GaN based LEDs FWHM linear with wavelength ( C-plane < (11-22) < A-plane)
PL FWHM (nm)
100
(0001)
(1100)
(11-22)
80 60 40 20 0 400
500
400
500
400
Wavelength (nm)
500
Nitride LEDs from 465 to 610nm c-QWs envelope (Gaussian):
Saturation loss (∆S)
λ =
(λ)
X X sRGB
exp
λ
λ!"#$ (λ)
~590nm
RGB
X Hue shift (∆H)
X
λ!"&'"()"* = λ!"#$ + ∆λ
%
Loss of color saturation (∆S) 465 nm
Adobe RGB
sRGB
610 nm
Color saturation (%)
100
90
80 This work Mukai et al. (Nichia)
70
Damilano et al. (CRHEA) Commercial LEDs
c-QWs envelope 60 450
500
550
Perceived wavelength (nm)
600
Shift of the hue (∆H) 465 nm
530 nm
550 nm
610 nm
Wavelength shift ∆λ (nm)
10 AlGaInP
0 λ!"&' = 610./
-10 -20 -30 -40
c-QWs envelope This work Mukai et al. (Nichia) Damilano et al. (CRHEA) Chen et al. (Taiwan Univ.)
-50 -60 450
Commercial LEDs
500
550
600
Peak wavelength (nm)
650
Abney effect (1909)
+
=
White: not neutral Abney, Proc. Royal Soc. Lond. 83, 120-127 (1909)
=
+
Bandwidth ~ dilution w/ white Mizokami et al., J. Vis. 6, 996–1007 (2006)
Conventional InGaN/GaN LEDs This work
(0001)
c-QWs envelope
700
Abney effect
500 λperc
x λEL, FWHM
(1100)
550
Seo et al. (KETI)
(1122)
600
Sato et al. (UCSB)
(0001)
Wavelength (nm)
Damilano et al. (CRHEA) Hwang et al. (Toshiba)
650
450
Mukai et al. (Nichia)*
Shojiki et al. (Tohoku Univ.)
400 0
50
100
150
200
FWHM (nm)
250
Phosphor InGaN (OSRAM)
InGaN/Al(Ga)N QWs 629nm Data from: Jpn. J. Appl. Phys. 38, 3976-3981 (1999)
RED
and: Appl. Phys. Express 7, 071003 (2014)
53nm
430nm < 5%
100
EQE at 20 mA (%)
~655nm 470nm ~30%
10
80nm
Pink (redish) (x=0.442, y=0.235)
1 Nichia (InGaN/GaN)
RED
Toshiba (InGaN/AlGaN)
0.1 400
450
500
550
?? 600
Peak wavelength (nm)
650
700
x
Summary Color of LEDs is NOT defined by the emission peak wavelength λ012314516 = λ0178 + ∆λ
Abney Effect
For a single symmetric peak: ∆λ = f (FWHM) FWHM ∝ λ0178 (empirical)
MicroLED display consideration: Chromaticity of blue and green LEDs relatively unaffected FWHM < 30 nm
∆: ≈ 0 (whatever the FWHM)
Red LEDs are prone to strong hue shift above 575 nm Red target: λ012314516 = 610 nm
Acknowledgment This research was supported by : JST, Strategic International Collaborative Research Program, SICORP JSPS and RFBR under the Japan - Russia Research Cooperative Program “Knowledge Hub Aichi” Priority Research Project from Aichi Prefectural Government
Thank you for your attention
Extra sildes
InGaN/AlGaN QWs ∆λperc ~ 35-40nm !
Data from Appl. Phys. Exp. 7, 071003 (2014)
650
70
640 65 630 60 620
FWHM (nm)
Peak wavelength (nm)
Red
75
660
“Amber”
580nm 100A/cm2
600nm
50A/cm2 25A/cm2
55
610 600 0
25
50
75
100 2
Current density (A/cm )
50 125
RED (Adobe)
10A/cm2 1A/cm2
620nm 640nm 660nm
0.5A/cm2
HUE of red InGaN/AlGaN LEDs unstable with current
InGaN/InGaN QWs RED
Lund University approach
Bandgap Energy (eV)
4
3
GaN
Nano-imprinted mask/GaN
εs~5% Dislocation-free relaxed InxGa1-xN
465 nm
εs~2-3% 2
x
p-InGaN
610 nm
SQW
1
InN 0 3.0
n-InGaN
3.2
3.4
Lattice Constant (A)
3.6
Color drift with current: white LEDs Cool
White LED (OSRAM)
Warm
InGaN
1 mA 0.370 0.360
yCIE
Correlated Color Temperature
Phosphor
0.350
5500K
2 mA
5300K
White point (E)
5 mA 10 mA
0.340
20 mA 0.330
Planckian locus (Black body)
0.320 50 mA
30 mA
5000K
40 mA
0.330 0.332 0.334 0.336 0.338 0.340 0.342 0.344
xCIE
CCT of white LEDs unstable with current ⇒ Dimming by PWM
