Physics of Blue Light Emitting Diode

1. " Every decade, for a given wavelength of light, the cost per lumen falls by a factor of 10 and the amount of light
generated per LED package increases by a factor of 20. "
~ Haitz's Law

2. 1 Blue Light Emitting Diode
▪ 1.1 Fundamentals.
▪ 1.2 Materials
▪ 1.3 Structure
▪ 1.4 History.
▪ 1.5 Applications (III/V LED benefits and
implications.)
2

3. ▪ An Electromagnetic radiation.
▪ A section of radiation contains in the
electromagnetic spectrum so called visible light.
▪ High frequencies (Thz) range have shorter
wavelengths.
3
Source:Wikipedia images

4. ▪ A typical thermal source, where the radiation
emitted by the sun (@ 6000K) due to the black
body radiation is in range of visible spectrum.
▪ Wien's displacement law
“black body radiation curve for different
temperature peaks at a wavelength is
inversely proportional to the temperature.“
4
Source:Wikipedia

5. ▪ An incandescent lamps so called bulb where a
light emitted by heating a filament. Heat is also
generated with light.
▪ Tungsten due to the highest known melting
temperature at 3680k of the pure metals.
▪ Low pressure inert gas, which has a high
molecular weight, to reduce evaporation of the
tungsten.
▪ Electrons will collide with the atoms that
generate collision and produce heat.
5
Source: R. Kane and H. Sell, Revolution in Lamps: A Chronicle of 50Years of Progress. 2nd Ed

6. ▪ Electron relay from electrode (typically
tungsten ), pass into Hg vapour and photons
emitted (UV light) due to the collision of
electron to neutral gas atoms and released.
▪ Coated with phosphor to emit visible light.
▪ Emission spectrum depends on the noble
gas, pressure and other variables.
▪ Emitting light of short to longer wavelength.
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X 15% heat
✓ 85% visible light.
Source: http://large.stanford.edu/courses/2014/ph240/dikeou2/
Transition of an electron from
energy states.

7. ▪ Consist of PN junction and emit spontaneous
radiation in ultraviolet, visible, or infrared regions .
▪ Emission of photons due to the recombination of
electron/hole pair, known as injection
electroluminescence.
▪ Wavelength determines the color and depends on
energy bandgap of material.
7
Source: Semiconductor Physics by Naeman

8. ▪ Illumination of photons in case of the direct
band-band process due to the recombination.
▪ A need of wide band gap materials due to the
shorter wavelengths of blue color.
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Thealloycompositionsshowncorrespond
tored(y=0.4),orange(0.65),yellow
(0.85),andgreenlight(1.0).

9. ▪ Key to create white light.
▪ Compound Semiconductors GaN, ZnSe, and SiC, a
better candidate.
▪ Challenges for bright light leads to the development
of high power blue LED.
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10. ▪ IV-IV covalently bonded semiconductor with indirect
bandgaps at room temperature of 3C-SiC, 6H-SiC, and
4H-SiC are 2.2 eV, 2.86 (or 3.02) eV, and 3.2–3.3 eV,
respectively *.
▪ Well known for mechanical stress and thermal stability.
Advantage for fast, High Temperature/Voltage electronic
devices.
▪ Indirect band transition leads to the inefficient light
source.
10* The letter ‘‘C’’ in ‘‘3C-SiC’’ denotes the cubic crystal structure and ‘‘3’’ refers to the number of double-atomic layers in one repeating unit (ABC). 3C-SiC is the only
polytype with the cubic crystal structure

11. ▪ Promising material for optoelectronics since the last
century.
▪ Direct transition band structure with wide band gap of
3.39ev.
▪ Difficult to grow high quality of crystalline structure
(Defect Free).
▪ Can be doped with silicon (Si) or with oxygen to n-
type and with magnesium (Mg) to p-type.
▪ Less sensitive to radiations and thermal stable.
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Small defects (less than 10 nm) in near
surface epitaxial p-GaN layers
Source: Focused Ion Beam Methods for Research and Control of HEMT Fabrication,

12. 12

13. ▪ P-GaN layer using Mg-doped.
▪ Saphire substrate used for the growth of single
crystalline GaN.
▪ Thin GaN Buffer Layer, for lattice mismatch and
difference in the thermal expansion between GaN
and Sapphire Substrate, which leads to better quality
of GaN.
▪ Low-Energy electron beam irradiation, decreases
resistivity and increase hole mobility.
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14. ▪ Illumination of the blue light due to radiative
recombination in P-GaN layer.
▪ Of course,Typical diode IV characteristic.
▪ With the presence of thin GaN layer, electro
luminescence of the photon appears to be
20mA at ~3.8V which is approximately the
band gap of the .
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15. ▪ 1980s Isamu Akasaki, Hiroshi Amano, Shuji
Nakamura successfully created efficient blue
light-emitting diodes for the first time.
▪ Nobel prize for the invention of efficient blue
light-emitting diodes which has enabled bright
and energy-saving white light sources.
▪ Sales of lighting products are $60 billion each
year worldwide
▪ Blue Light was the key for a variety of
applications and was a challenge to create
bright blue light.
15Haitz's law

16. 16
• Physical deposition method to
produce single- or polycrystalline thin
films.
• Take place at moderate pressures (10
to 760 Torr).
• First Substrate Heated to high Temp ~
1050 C
• Temp Lowered to 510 C to grow.
• Maintain flow rates of the gases

17. 17

18. ▪ Stimulated emission
▪ Population inversion
▪ Double Hetero PN junction for the
confinement of photon.
▪ Feedback and mirrors to increase
the population inversion
18Source: Semiconductor Physics SM Sze,

19. ➢ Typical ways to generate light waves starting from Sun.
➢ A need to move from traditional to solid state electronic light sources.
➢ Recombination's implications on emission for Direct, In-direct band gap for semiconductor.
➢ Blue is the key to create white light.
➢ Secret behind the generation of blue light (wide band gap) .
➢ Challenges to grow GaN, especially P-doped .
➢ Current Applications fulfilled by this inventions.
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