 Operation Mechanisms
 Materials for LEDs
 Construction and Structures of LEDs
 LED Performance Issues
 LED Reliability Issues
 Applications of LEDs
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 forward biased p-n diode
 injection electroluminescence
 strong radiative recombination
 weak non-radiative recombination
 spontaneous emission
 quantum efficiency
– number of photons generated per electron-hole pairs
 internal quantum efficiency
–  i
–  r
–  nr
=
 nr
/(
 r
+
 nr
)
: radiative lifetime
: non-radiative lifetime
 external quantum efficiency
–  e
: the ratio of the power emitted by the surface of the LED to the input power
2


In addition to the band-to-band transition, radiative recombination could be happened via recombination centers generated by impurities.

Some indirect gap materials can have a reasonable efficiency if they are doped with certain impurities.
 isoelectronic center

The isoelectronic center is an efficient radiative recombination center and can largely enhance the probability of radiative processes in an indirect gap material.

An important example is GaAsP:N
–
Nitrogen has the same number of outer shell electrons as phosphorus, hence the term
“isoelectronic”.
–
N replaces P atoms in the lattice sites. It doesn’t introduce either an acceptor donor state, but it creates an electron trap level close to the bottom of the conduction band. A recombination center is thus produced and called an isoelectronic center .
3

•
Defect-related non-radiative recombination
– Defect-related energy levels (states) in the bandgap can arise from chemical impurities or from native defects such as a vacancy or an anti-sites defect (i.e. in compound semiconductors, an atom on the wrong sublattice — Ga on an As site, for example).
– In a LED or LD the non-radiative recombination is not desirable, but it is purposely increased in p-n diodes to increase the speed.

The defected-related non-radiative recombination is also called
Shockley-Read-Hall (SRH) recombination. Such recombination is very important at the surfaces of devices, since there is usually a high density of defects at the surfaces.
• Auger recombination
– Auger process : an electron and a hole recombine, giving up the excess energy to an electron (producing a “hot” electron), and then the hot electron eventually loses its energy by emitting phonons (i.e. giving up heating)

Auger recombination is an important non-radiative recombination, especially in materials with narrow bandgap.
4

• Some III-V compound semiconductors like Al x
Ga
1-x
As and GaAs
1-x
P x at certain compositions. become indirect
 Al x
Ga
1-x
As at x = 0.35 and GaAs
1-x
P x at x = 0.45
5

6

• Infra-red
– for optical communication
•
0.8 ~ 0.95
 m : GaAs or AlGaAs (for LAN)
•
1.3
 m : InGaAsP
•
1.55
 m : InGaAs or InGaAsP
– for impurity monitoring
•
2
 m and above
•
Visible
– red: (1) GaAs
0.6
P
0.4
/ GaAs (660 nm)
(2) GaP:ZnO/GaP:Te (690 nm) p-n diode on GaP
– orange: GaAs
035
P
0.65
:N
– yellow: GaAs
015
P
0.85
:N
– green: GaP:N (565 ~ 570 nm)
– blue: SiC, GaN, ZnS, ZnSe, AlInGaP
• ultra-violet
– BN
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 Minimum attenuation appears at
1.3
 m and 1.55
 m (windows for optical fiber communication )

Typical attenuations are
 3 dB/km at 0.8
 m
 0.6 dB/km at 1.3
 m
 0.2 dB/km at 1.55
 m
8

• (i) absorption in the material
 minimized by making the junction close to the emitting surface
• (ii) absorption in the substrate area
 minimized by using a transparent substrate
• (iii) total internal reflection at the surface
 suppressed by using a dielectric encapsulation dome
• (iv) transmission losses at the interface
 minimized by using a dielectric encapsulation dome
• (v) reflection at the top M/S interface
 minimized by using as small an area as possible of top metal contact
•
(vi) absorption in the top contact metal
 minimized by using as small an area as possible of top metal contact
9

10

Refection Coefficient
R


 n r 2 n r 2

 n r n r 1
1


2
11

 Coupling Efficiency (
 c
): the fraction of the light coupling into the fiber
 c
= sin 2

A
 
A
: the maximum angle of acceptance

A
= sin -1 (n r1
2 - n r2
2 ) 1/2

If the light is incident at an angle

1
<

A so that the light suffers total internal reflection in the fiber, the light is coupled successfully into the fiber.
 Optical waves coming at an angle greater than

A are not able to propagate in the fiber.
12


Basic surface-emitting type
 typical GaAsP red LED
 graded AlGaAs infra-red LED
 diffused and mesa etched green GaP LED
 yellow GaAsP LED

Burrus-type LED
 Double heterojunction (DH) LED
 edge emitting type

Burrus-type (surface-emitting)

Blue LED
 White LED
13

1) A Si - (or Te ) doped n-type GaAs substrate
(200
 m, 2 × 10 17 cm -3 , LEC pulled)
2) using VPE to grow a series of layers of
GaAs
1-y
P y
(total 40
 m), with y = 0 ~ 0.4 until a GaAs
0.6
P
0.4
layer is reached
3) growing the active layer of tellurium (Te) doped n -GaAs
0.6
P
0.4
(50
 m, ~ 10 17 cm -3 )
4) forming the diffusion mask layer of silicon dioxide or nitride by CVD and photolithography
5) p-diffusion into the n -GaAs
0.6
P
0.4
layer using zinc ( Zn ) as the p-dopant (junction depth of 5
 m, surface concentration of 10 19 cm -3 ) to form the p-n junction
6) forming ohmic contact: AuGe / Ni for nregion and Al (or AuZn ) for p-region
7) scribed and broken to die
8) soldered onto a lead frame header and wire bonded
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1) LPE-growing a Si-doped n -Al
0.3
Ga
0.7
As layer on the nGaAs substrate (lattice matching)
2) LPE-growing graded composition layers of Si -doped n -Al x
Ga
1-x
As (y = 0.3 ~ 0.08,
50
 m), below 820 o C
3) LPE-growing graded composition layers of Si-doped p -Al x
Ga
1-x
As (y = 0.08 ~ 0,
130
 m), above 820 o C
 Si is a “ amphoteric impurity ” in GaAs
 as a donor if replacing Ga, and as a acceptor if replacing As (above 820 o C)
4) p-type ohmic contact with AuZn
5) GaAs substrate is removed
6) n-type ohmic contact with AuGe/Ni
 the reason for the graded structure: light can emit from the junction to the n -
Al x
Ga
1-x
As (due to its larger E g
)
15

1) A sulfur ( S ) doped nGaP substrate (10 18 cm -3 ,
Czochralski grown)
2) VPE-growing an S -doped nGaP layer (50
 m,
~ 10 17 cm -3 )
3) VPE-growing an S -doped n -GaP:N layer (50
 m, 2 × 10 19 cm -3 )
4) Masked Zn diffusion p-zone
5) forming ohmic contact: Au/Te/Ni for nGaP and Al (or AuZn ) for p -GaP
 brighter LED with a mesa structure can be made using an LPE growth technique
(attributed to the better quality LPE material, with fewer dislocation giving a proportionate reduction in non-radiative recombination)
16


A very similar structure to that of a standard red LED
– Main differences: substrate is
GaP for lattice match, and epi-layer is
GaA
0.15
P
0.85
for yellow emitting

For yellow LEDs, the epitaxial materials is more like GaP than GaAs , it is easier to grow it on a GaP substrate

Orange LEDs, formed in GaA
0.4
P
0.6
, are also grown on GaP substrates
17

 The device is “upside-down” compared to the basic surface-emitting LED.
–
A deeply well in the substrate to allow light to get out by this exit.

The p-type contact is defined in a small area on the axis of the etched well.
– This leads to “current confinement” in a small area of the junction, with the result that it is substantially only this active region that is emitting light.

Use the p-contact as a thermal path to the heat sink to minimize the temperature rise of the junction.
–
Temperature change will leads to a shift in the emission peak to a higher wavelength.
–
Temperature rising will causes a fall in output power.
– Temperature increasing is an important failure factor to shorten the device lifetime.
 The Burrus LED is used for communication type
LEDs where the confined active area and etched well coupled light fairly efficiently into an optical fiber via some refractive index matching epoxy
18

 Problems of the homojunction LED
(1) Strong non-radiative recombination via high density states at the emitting surface
(2) Low carrier injection efficiency
 Advantages of a heterojunction LED
 higher quantum and coupling efficiency
(1) Do not suffer from poor surface conditions since the active region is not near the surface..
(2) Increased carrier injection efficiency due to the low doping active layer.
(3) The photons emitted are also not absorbed in the top or bottom region because the photon energy is smaller than the bandgap of the n- or p-region.
 Material systems
–
GaAs/AlGaAs on GaAs
–
InGaAsP/InP on InP
–
InGaAs/InGaAsP on InP
19


WNW bandgap structure (NnP or NpP)
 Light generated in the GaAs active region will pass unabsorbed through the AlGaAs and be picked up efficiently by the optical fiber.
 Because of the refractive index compositions of GaAs and AlGaAs , the two regions of
AlGaAs also act as optical confinement elements for photons generated in the GaAs region that are incident on the GaAs/AlGaAs interface at an angle.

Edge-emitting LED
20


A surface-emitting LED

A heterostructure LED has an optical fiber butt coupled to it with an epoxy resin

The light generated in the active p-
GaAs is fed into the fiber directly.

In advanced designs, a microlens is used to improve the coupling.
21


Carrier and optical confinement
– In an edge-emitting LED, the wide gap cladding layers confine not only the electrons and holes to the active layer, but also cause the emitted photons to travel along the LED axis and emerge from from the edge of the device

Improved coupling efficiency
– Due to the superior collimation of the edge-emitting LED (~30 o width perpendicular to the layer and ~120 o parallel to the layer), the coupling efficiency to a fiber is greatly improved.
22


An edge-emitting LED has a very similar structure as that of a striped laser diode. (In the latter exceptional care is taken to produce a high quality optical cavity to ensure optical feedback.

The active region is In
0.47
Ga
0.53
As
(Eg = 0.8eV) surrounded by confining layers fabricated from InGaAsP
(Eg ~ 1.0eV).

The confining layers cause the light to be coupled out through the edge of the devices.
23


For development of high-brightness blue
LED (0.455 ~ 0.492 μm), the II-V-based devices with short lifetimes and the indirect bandgap SiC devices with low brightness are not suitable.

GaN (3.44 eV) and related III-V nitride such as AlGaInN , which has direct bandgaps ranging from 1.95 to 6.2 eV (0.2 ~ 0.63 μm) are the most promising candidates for highbrightness blue LED.

High-quality GaN can been grown on sapphire ( Al
2
O
3
) using low-temperature grown AlN as a buffer layer.

Because the sapphire is an insulator, ohmic contacts are formed on the top surface.
 The active region is In
1-x
Ga x
N layer which is sandwiched between two larger bandgap semiconductors: a p-type Al x
Ga
1-x
N and an ntype GaN layer.
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25

26

27


An InGaN high brightness LED chip coated with a yttrium aluminum garnet
(YAG) inorganic phosphor

The 472 nm wavelength provides high energy photons to excite the white phosphor (though a shorter wavelength such as UV light is more efficient)

Future development
 high efficient phosphors

UV LED for excitation

30 ~ 40 lm /W
 innovative packaging
 replacing the incandescent bulbs for room lighting

Features:
– High luminescence intensity:
• 80 mcd (white emission)
– High efficiency :
•
5 lm/W for Nichia
– Emission color:
• x = 0.29, y = 0.30,
• color temperature: 8000 K
28


Development of a growth procedure including buffers layers grown at low temperature to grow high quality GaN on sapphire substrate, and its improvement

Demonstration of p-type GaN using low energy electron bean irradiation

Elucidation of the annealing process, and demonstration of ptype doping without electron irradiation by thermal annealing

Two gas flow MOCVD technique
29


Full-color Display (Red, Green and Blue)
 Blue, Green LEDs
 High Density Data Storage
 Blue, UV LDs
 High-Temperature Devices

Diodes, FETs, HBTs
 Solar-Blind Detectors

UV Detectors

Communication

Light Sources, Photodetectors

Chemical/Biological Reagent Detection
 UV Detectors
30

• Light-current characteristics
– the light-current of the LED is essentially linear except for high drive current when the light output saturated.
• Spectral purity
– the LED output is quite broad (~ k
B
T ), since all of the occupied e-h states participate in photon emission with no reference.
– the spectral purity of LEDs is rather poor, being about 200 ~ 300 Å
• Temporal response
– the time response of the LED is controlled by e-h recombination. LEDs can be operated at

1 GHz
• Temperature dependence
– high temperature response of the LED suffers because of leakage current and
Auger recombination
31

•
LEDs normally have a MTTF (Mean Time To Failure) of up to 10 6 hours for GaAs based devices and up to 10 9 hours for InP based 1.3
 m devices.
32




Display Elements
1) active display (unlike the LCD)
2) tails lights of automobiles, daytime programmable displays for advertisements
3) full-color display
4) single lamps or in arrays (indicators such as sevensegment array to generate alphanumerial characters.
5) LED-based flat panel display for laptop computers (?)
6) replace the light bulbs (?) used in automobile brake lights and traffic signals
–
Communication
LAN (Local Area Network)
 Laser diodes are used in long distant communication system
Opto-isolators (opto-couplers)
1) light emitter (LED) + light detector (photodiode or phototransistor)
2) provides very high degree of isolation

In some medical devices, such as the electrocardiograph amplifiers , very high degree of isolation is need to ensure patient safety during medical treatment.
33