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LEDs technology
rhombus4
increased substantially after this GaN sample, which has
been grown with low-temperature buffer technology, was
irradiated with electron beams during
cathodoluminescence studies. Although these crystals did
not show p-type conduction, they paved the way to
success. Akasaki’s team switched to developing
magnesium-doping in 1988 and the following year they
produced high-quality, doped samples with two types of
magnesium-based precursors. Irradiating these samples
with an electron beam produced low resistivity p-type
GaN, and the world’s first GaN LED with a p-n junction
followed immediately after.
The efficiency of this device, which emitted in the
blue/ultra-violet region, was only 0.1 percent. But this shot
up to 1.5 percent by 1992, due to improvements in crystal
growth quality. The following year Nichia released a Samsung has
commercial device with 2.7 percent efficiency, and the recently
industry has never looked back since then. Revenues have launched a
grown to billions of dollars a year, and penetrated a range of LCD
diverse range of markets that include mobile phone TVs with LED
displays and backlights, automobile headlamps, torches, backlights.
streetlights and TV backlights. Penetration of
LED-backlight
The first LEDs were a metal insulator semiconductor Efficiencies continue to rise, and the next goal is general technology in
design with very low efficiencies. A p-n junction lighting. This market is being targeted with state-of the-art LCD-TV panels
increases the efficiency LEDs that feature sophisticated light extraction is predicted to
technologies, multiple-quantum-well active regions and increase to
advanced thermal management packages. However, these 39 percent in
concentration was of the order of 10
17
cm
-3
, an devices still tend to share two pieces of DNA with the first 2013, up from
improvement of more than two orders of magnitude over GaN LED with a p-n junction – a sapphire substrate; and 3 percent in
previous films. Dislocation density had fallen from more a low-temperature buffer layer. The advances of Akasaki 2009,
than 10
11
cm
-3
to 10
8
- 10
9
cm
-3
, and electron mobility and his team did not just lead to the making of the first according to
rocketed from 20 cm
2
V
-1
s
-1
to 700 cm
2
V
-1
s
-1
All of commercially viable LED - they created technologies that market analyst
these improvements were down to the role played by have been used to this day for LED manufacture. iSuppli
the low-temperature buffer, which provided a high density
of nucleation centers with the same orientation as
the substrate, and promoted lateral growth of the
subsequent epilayers.
P-type doping in GaN was the next challenge. This
problem had already attracted the interest of many
research groups that were unable to crack it, but
Akasaki’s team had the significant advantage of starting
with a material containing a far lower residual electron
concentration. However, they were unable to realize p-
type GaN using a zinc dopant.
An important advance came in 1987, when they
discovered that the intensity of zinc-related luminescence
Further reading
I. Akasaki J. Crystal Growth 300 2 (2007)
Credit: Samsung
September 2009 www.compoundsemiconductor.net 19
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