electroluminescent lighting

NON-COHERENT LIGHT SOURCES II
LUMINESCENT RADIATORS
AND LIGHT EMITTING DIODES
Dr. Bálint Pődör
INTRODUCTION
Subjects:
Electroluminescent light sources
Semiconductor light emitting diodes (LEDs)
Organic light emitting diodes (OLEDs)
ELECTROLUMINESCENT LIGHTING
Electroluminescence: excitation by moving electrons driven by electric
field.
Important mechanism: injection (electro)luminescence, ocurring in a
forward biased pn junction. Fundamental mechanism of the operation
of light emitting diodes (LED) and laser diodes (LD).
The original patent of injection electroluminescent lighting:
(Tungsram Rt. Újpest)!
SiC electroluminescent light source (the origin of today’s light emitting
diodes):
György Szigeti and Zoltán Bay
US patent No. 2,254,952 (1942)
ELECTROLUMINESCENT MATERIALS, ZnS
ZnS: One of the most important electroluminescent material, widebandgap semicondcutor.
Forms: fine powder or sputtered layers.
Control of radiated color: Zn-excess or „activators”, i.e. Ag, Cu, or
Mn dopants (0.0001-3% ).
Compound
Melting p.
K
Bandgap
eV
ZnS
2103
3,7
Electron
mobility
m2/Vs
0,014
Hole mobility
m2/Vs
CdS
2023
2,4
0,024
0,005
CdSe
1531
1,8
0,06
0,005
HgSe
1073
0,6
1,8
-
0,0005
ZnS:
ELECTROLUMINESCENCE SPECTRUM
ZnS:Mn orange-yellow
ZnS:Cu green
Electric field strength: 108 V/m
i.e. 100 V across 1 µm thick layer
ELECTROLUMINESCENT CELL
Simplified band diagram of an
electroluminescent crystal.
fénypor
szigetelõ réteg
insulating layer
luminescent powder in insulating matrix
insulating layer
back electrode
szigetelõ réteg
A.C. leads
Cross-section of the ZnS
electroluminescent cell
(Destriaux-cell). Light can
be excited by applying ac
voltage to the cell.
glass plate
transparent electrode
emitted light
ELECTROLUMINESCENT DISPLAY
Al-electrode
insulating layer
phosphor layer
insulating layer
glass substrate
transparent electrode
COLOUR ELECTROLUMINESCENT
DISPLAY
R, G, B szûrõk
RGB filters
Row electrodes
sor elektródák
üveg lemez
Glass
szigetelõ réteg
Insulating layer
fénypor réteg
Phosphor layer
Insulating layer
szigetelõ réteg
Column electrodes
oszlop elektródák
Base
alaplap
CATHODOLUMINESCENCE
Excitation by electron beam, vacuum: TV screens and computer
monitors (however technological change!)
Colour TV picture tube (shadow mask, three e-beams, three
phosphor layers):
Blue:
ZnS:Ag
Green: ZnCdS:Cu,Al
Red:
Y2O3:Eu
LUMINESCENCE IN SEMICONDUCTORS
Conduction band
Impurity/defect centres
Valence band
10
DIRECT AND INDIRECT TRANSITION
Simplified band diagram of direct and indirect bandgap
semiconductors: Energy versus the momentum (wave number) in
the reciprocal lattice.
RADIATIVE RECOMBIBATION
In direct band-structure semiconductors (e.g. GaAs, InP, InGaAs, GaN)
momentum conservation is easily satisfied in the transitions between
the Γ-point (zone centre) band extrema, therefore the probability of
radiation recombination is large.
For indirect band-structure (e.g. (pl. Si, SiC, GaP) an additional particle
(usually a phonon) is necessary because of momentum conservation,
therefore the probability of radiative recombination is much less.
The difference in the radiative recombination rates for these two types
of semiconductors can amount to 3 to 5 orders of magnitude!
12
INJECTION LUMINESCENCE IN A
FORWARD BIASED PN JUNCTION
foton emisszió
kiürülési tartomány
vezetési sáv alja
szabad elektron
p-tip Fermi nívó
n-tip. Fermi nívó
szabad lyuk
vegyérték kötési sáv teteje
az átmenetre helyezett feszültség
13
HETEROJUNCTION LED
b. Simplified energy band diagram. EF
must be uniform.
c. Simplified energy band diagram for
forward bias. There is a large overlap of
electrons and holes.
d. Schematic illustration of photons
escaping reabsorption in the AlGaAs
(window) layer and being effectively
emitted from the device.
BANDGAPS AND LED COLOURS
15
BANDGAPS AND POSSIBLE WAVELENGTH RANGES IN SEMICONDUCTORS
PROGRESS IN LED-S
LED STRUCTURE
LED BASICS
Large wavelength content
Incoherent
Limited directionality
Electron-hole recombination emits photon
Need a location with both electrons and holes
Requires a large applied voltage
Small emission region
Poor optical confinement
LED BASICS
Spontaneous emission dominates
Random photon emission
Broad spectrum (∆λ~0.03 µm)
Not all photons exit the LED
Broad far field emission pattern
Quantum efficiency
Fraction of optical power emitted out of the
LED, caused by material absorption, surface
reflection
Dominant reduction in quantum efficiency is
critical angle
Dome used to extract more of the light
Dome makes LED more directional
LED SPECTRA
LED SPECTRA
Maximum wavelength corresponds to EG + kT.
Spectral width (FWHM) typically 2.5kT - 3kT (on energy scale)
Spectral width on wavelength scale
if ∆Eph ≈ 3kT then
hc
λ = ——
Eph
and
dλ
hc
—— = - ——
dEph
Eph2
3kT
∆λ ≈ λ2 ——
hc
λ = 870 nm
λ = 1300 nm
λ = 1550 nm
∆λ = 47 nm
∆λ = 105 nm
∆λ = 149 nm
InGaAsP/InP LED SPECTRA
InGaAsP/InP LED series (1100-1700 nm).
The half widths of the spectra are ∼65 nm (at 1100 nm), and ∼120 nm
(at 1700 nm).
COMMERCIAL LED SPECTRA
Data from thee LED types (Hewlett-Packard) (from data sheets):
Type
Material
λp
nm
∆λ1/2
nm
Ep
eV
Red
GaAsP
635
41
1.953
25
Yellow
GaAsP
583
30
2.127
22
Green
GaP
565
20
2.195
20
∆λ1/2
nm
(calc. with 3kT)
Diode voltage (at 10-20 mA) 2.2 – 2.4 V.
The last column is the theoretical estimation with 3 kT.
Quantum mechanics adequately describes operational parameters!
LED STRUCTURES AND
OPERATIONAL CHARACTERISTICS
LED LIGHT-CURRENT CHARACTERISTICS
The ”internal” optical power is proportional to the number of injected
charge
Pint = ηint (hν
ν) (I/q)
The optical power
Popt = ηext Pint = ηext ηint (hν
ν) (I/q)
The(idealized) light flux-current characteristics of LED is linear. In
practical units
Popt [mW] = ηextηint (1,24/λ
λ[µ
µm]) I [mA]
LED QUANTUM EFFICIENCY
An other characteristic parameter of a LED is the total quantum
efficiency ηtot, which equals the ratio of the emitted light power to the
input electrical power. Pel = IUd, where Ud is the voltage drop on the
diode, Ud ≈ hν/q = hc/λq.
ηtot = ηextηint (hν
ν/qUd) ≈ ηextηint
E.g. for a 1.3 µm LED,
ηextηint ∼5 %,
Popt/I = ∼0,05 mW/mA
SURFACE EMITTING LED
EDGE EMITTING LED
STRUCURE OF BLUE GaN LED
Aktív réteg
Kontaktus réteg
P
kontaktu
s
GaN:Mg
~4 µm
GaN:Si
GaN: Si
Leválasztó réteg
Zafír alap
( ~ 100 µm)
n
kontaktus
NEW APPLICATION OF
PHOTOLUMINESCENCE: ”WHITE LED”
Producing white light:
Blue LED + phosphor
Yellow or green + red phosphor
UV-LED + phosphor
Stokes shift (energy shift between the exciting and
emitted photon)
THREE METHODS OF
GENERATING WHITE LIGHT
BLUE LED + YELLOW PHOSPHOR: WHTE
Cross section of a blue LED with
the light emitting phosphor
powder in the package
(Y3Al5O12:Ce3+)
pcLED
60
White light is obtained in a
such way that a fraction of the
LED’s light excites the
phosphor’s yellow band
radiance, a.u.
50
40
30
20
10
0
400
500
600
700
nm
800
TWO-PHOSPHOR LED: WHITE LIGHT
Blue: InGaN/GaN LED (460 nm)
460nm+TG:Eu+SrS:Eu
50
Green: SrGa2S4 = TG:Eu2+
7560 K
2930K: Ra=93
45
3060K: Ra=93
40
Red: SrS:Eu2+
2930 K
3350K: Ra=93
35
3970K: Ra=93
radiance
30
5620K: Ra=93
25
7560K: Ra=90
20
15
10
5
0
400
500
600
700 nm
800
ORGANIC LIGHT EMITTING DIODE: OLED
OLED ENERGY LEVEL STRUCTURE
singlet
triplet
singlet
Level scheme and colour depends on the chemical structure of the
polimer.
OLED STRUCTURE
< 1µm thick
< 1 µm thick
EXERCISES
AlGaAs LED emitter. An AlGaAs LED emitter for use in optical fiber network has
the output spectrum shown in the figure. It is designed for a peak emission at
820 nm at 25 oC.
a. What is the linewidth ∆λ between half power points at the three indicated
temperatures? What is the empirical relationship between ∆λ and T and how
does this compare with ∆(hν) ≈ (2.5-3)kBT ?
b. Why does the peak emission wavelength increase with temperature?
c. What is the bandgap of AlGaAs in this LED ?
d. The bandgap EG of the ternary alloy AlxGa1-xAs follows the empirical
expression
EG(eV) = 1.424 + 1.266x + 0.266x2
What is the composition of AlGaAs in this LED ?
EXERCISES
Relative spectral output power
The output spectrum from an AlGaAs LED. Values normalized to a peak
emission at 25 oC.
END OF LECTURE

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