GaN based blue LED
Joonas Leppänen
Emma Kiljo
Jussi Taskinen
Niklas Heikkilä
Alexander Permogorov
13.5.2016
Photonics
School of Electrical Engineering
13.5.2016
Group 3
LED
Content
1.
2.
3.
4.
5.
6.
Introduction
Theoretical Background
Manufacturing
Applications
Future Prospects
Conclusions
Photonics
Group 3: LED
21.4.2016
Introduction
• First LEDs from late 50’s to early 60’s
– Expensive at first, nowadays quite cheap
• Green and red LEDs easier to create than blue ones
– Higher energy (gap) needed
• By combining red, green and blue LEDs, it is possible to create
white light
– Nobel prize for blue LED in 2014
• Phosphorus coating of blue-LED also provides white light
• By replacing incandecent lighting with LEDs, energy can be saved
Photonics
Group 3: LED
13.5.2016
Theoretical Background
• The operation princible of LED is based on semiconductor pnjunction
• Heterojunction structure is a typical structure used in LEDs
– Confinement of the charge carriers in active layer
– Reduction of reabsorption
• Manipulation of bandgaps by alloying
compound semiconductors
Photonics
Group 3: LED
13.5.2016
Theoretical Background
• The most important materials for blue LEDs are currently GaN
based compound semiconductors
• Zinc compounds and silicon carbide may also be utilized but they
are indirect bandgap semiconductors
• GaN is direct band gap semiconductor, which provides better
efficiency.
• GaN and most substrates have a large lattice mismatch, sapphire is
often used as substrate
• P-type GaN can be produced by doping with Mg or Zn, and n-type
by doping with silicon
Photonics
Group 3: LED
13.5.2016
Manufacturing
On foreign substrate
MOVPE (Metal-Organic Vapour Phase Epitaxy)
•
Poor quality (TDD = 106 - 109 cm-2)
MBE (Molecular Beam Epitaxy)
•
Better quality
MBE
MOVPE
(TDD = 105 - 106 cm-2)
Pros
Cons
Control of doping
Good crystalline quality
Slower growth
Expensive
Composition at the
monolayer level
Control of doping
Faster growth ~ 2µm/h
Easier maintenance
Widely used in industry
Ultra high vacuum
required
Poisonous precursor gases
C and H from precursors
are incorporated into layers
Photonics
Group 3: LED
13.5.2016
Manufacturing
Bulk Growth
•
MOVPE and MBE are not cost effective since they are slow when compared to
Czochralski for Si
•
Quality is not that good due to lattice mismatch and other things related to the growth
on top of foreign material
–
Low luminous flux
•
Bulk growth of GaN may overcome these issues using GaN seeds which may be
grown on foreign substrates
•
Possibility of higher growth rates for cost effective growth
Photonics
Group 3: LED
13.5.2016
Manufacturing
Bulk Growth
HVPE (Hydride Vapour Phase Epitaxy)
Process
•
High quality
•
Extremely high growth rate (300 µm/h)
•
Can be used to grow GaN seeds on sapphire or •
silicon, which can be removed by a lift-off
technique after the growth
•
Seeded growth is possible but requires high
quality seed
•
–
(TDD = 104 - 106 cm-2)
•
FEOL (Front-End of line) process
–
1000 ◦C
Based on a reaction between
GaCl and NH3 to form GaN
crystals
GaCl is provided via reaction
between solid Ga and HCl
the quality is seed quality dependent
•
HVPE-GaN can itself be used as a seed
•
Wafer bow/bending causes difficulties for larger
wafers
•
Widely used for bulk GaN growth at the moment
Photonics
Group 3: LED
13.5.2016
Manufacturing
Bulk Growth
Ammonothermal growth
•
Higher quality (TDD = 103 - 104 cm-2)
•
Difficult to handle safely!
•
Growth rate only 20 µm/h
•
No bending or cracking issues
Process
•
BEOL (Back-End of line) process
– 400 - 500 ◦C
– 3500 atm pressure
•
Grown from polycrystalline GaN grains in a
supercritical ammonia solution
•
Can be grown on HVPE seeds but the quality
depends on the seed quality
Photonics
Group 3: LED
13.5.2016
Manufacturing
Bulk Growth
Sodium (Na) Flux
•
Extremely high quality (TDD = 102 - 103 cm-2)
•
High growth rate of 100 µm/h
•
No cracking
•
Still on its early stages!
Process
•
FEOL process
– 800 ◦C
• Grown in Ga-Na melt on top of seed
–
•
MOCVD-GaN or HVPE-GaN on sapphire,
which can be removed afterwards
Necking technique decreases the TDD
even if the seed is poor quality!
–
Reason behind this are unknown
Photonics
Group 3: LED
13.5.2016
Why LED chosen
• Incandescent: 17 lm/W
• Fluorescent: 90 lm/W
• LED: up to 425 lm/W
Photonics
Group 3: LED
13.5.2016
LED in illumination
Blue LED to white light: phosphors
• Ce3+:YAG
• Excited by LED radiation
• Blue + Yellow = White
• Highest efficiency – 425 lm/W
• Reasonable Color Rendering
Index (CRI)
Blue LED to white light: color mixing
• Efficiency lower than for
phosphors – 300 lm/W
• Highest CRI
• Possibility of color changing
Future Prospects
Driving Forces of the blue-LED technology
• US DOE plan that 70 % of public lights will be replaced by LED’s til
2030
• Pressure to lower price and increasing efficiency and lumens.
Photonics
Group 3: LED
13.5.2016
Conclusions
• First LEDs in the 60’s
• Blue LEDs enabled white light
• Current manufacturing methods can be used
– A need for better quality and more cost-effective wafers
– Bulk growth may be the answer
• HVPE+MOVPE-seed is currently used but the TDD must be decreased
• Na-flux seems most promising in the future but size must be increased
• Superior lumen per watt production
– Major possibilities for energy saving
Photonics
Group 3: LED
21.4.2016
Thanks!
Photonics
Group 3: LED
21.4.2016