The 2014 Nobel Prize in Physics went to three scientists
and engineers, Profs Isamu Akasaki, Hiroshi Amano, and
Shuji Nakamura for their invention of energy-efficient and
environment-friendly blue light-emitting diode (LED). A
natural question from readers might be, “Why blue in
particular, but not LEDs in other colors?”
In addition to completing the visible spectrum to make full
color displays, the significance of blue LEDs lies in the fact
that one can create white light for general illumination
using blue light either to combine with red and green light,
or to excite yellow phosphor powders. However, in the
history of LED development, blue LEDs were really missing
for a long time. Gallium arsenide phosphide (GaAsP) is the
material to produce LEDs emitting light from infrared to
green, but not for blue LEDs, in which higher-energy
photons are involved. While Gallium nitride (GaN) was
considered by many labs to be the suitable semiconductor
for emitting blue light, it was in practice extremely difficult
to obtain high quality GaN crystal until the 1990s, let alone
functional GaN-based LEDs.
Lighting up Our Lives - Blue LED
The breakthroughs in creating GaN crystal happened in
late 1980s and early 1990s. In 1986, Prof Isamu Akasaki
and Prof Hiroshi Amano of Nagoya University achieved
mirror-like GaN crystal on a sapphire substrate using an
aluminum nitride (AlN) buffer layer by metal organic
chemical vapor deposition (MOCVD). Simultaneously, Prof
Shuji Nakamura (who was then working at Nichia Chemical
Industries Ltd. and later became Professor at the University
of California at Santa Barbara in 1999 and IAS Visiting
Professor in 2008) managed to grow high quality GaN
crystal by replacing the AlN buffer with a low-temperature
GaN buffer and subsequently through a thermal annealing
method achieved p-type GaN which is essential in
producing p-n junction GaN LEDs.
As a matter of fact, soon after the growth and fabrication
of p-n junction GaN LEDs emitting at 430 nm, InGaN/GaN
multiple-quantum-well (MQW) LEDs were created with
much improved optical output power. The heart of a GaN
LED is made of semiconductor materials in a layer by layer
manner: an active layer (e.g. MQWs) is sandwiched by an
By Xinbo Zou (IAS Junior Fellow) and
Wing Cheung Chong (PhD Student, Department of Electronic & Computer Engineering, HKUST)
0.19 inch 1,700 pixels per inch (PPI) high resolution passive-matrix Light Emitting Diode on Silicon (LEDoS) micro-display for
wearable electronics such as head-mounted display.
Knowledge Corner
Feb 2015
6
