Page 174 - Photonics Essentials an introduction with experiments

Lasers

168 Photonic Devices

A prominent feature of this model is the wavelength dependence of
the threshold current. GaAs-based lasers emit around 920 nm. A
laser emitting at half this wavelength would be deep blue in color
and have a threshold current four times higher. The wisdom based
on our model might argue that such a laser could not be made to op-
erate continuously at room temperature. I learned this argument in
class. It was used in the 1960s and 1970s by the managers at the
best research laboratories in the world to justify stopping laser de-
vice research on larger-band gap materials such as GaN (E g = 3.48
eV).

7.7 A True Story
In the 1980’s Professor Isamu Akasaki at Nagoya University set his
sights on the growth of GaN materials for optoelectronics. Although
this material was known to have a direct band gap and to emit light
in the blue region of the spectrum, researchers had only been able to
make n-type material. Without p-type material, there could be no p-n
diode and no LEDs or lasers.
Twenty years earlier, a thorough research of possible techniques
had failed to produce p-type GaN, and some scientists published pa-
pers to explain why it would not be possible, ever. However, during
the intervening time many technology changes occurred, including
semiconductor synthesis under ultrahigh purity conditions. These
conditions were developed to solve problems with another material,
AlAs-GaAs alloys, in which residual concentrations of oxygen in the
reactor would combine with Al, rendering it inert. As it turns out, oxy-
gen was part of the problem with GaN, too. Akasaki was able to show
in 1989 that magnesium, which also readily oxidizes, could be used to
make GaN p-type material under conditions of high-purity synthesis.
It was a difficult battle, but this breakthrough set the stage for GaN
optoelectronic devices.
Akasaki and his team knew about Eq. 7.25 and realized that a dif-
ferent kind of laser structure would be necessary to achieve laser op-
eration with practical values of threshold current. The quantum well
laser principle, developed only a few years earlier, was the second im-
portant key that was needed to unlock the door to blue light. The
Akasaki design uses an active region only 2 to 3 nm thick, a 100-fold
reduction over that for the laser shown in Fig. 7.11.
Akasaki took a third crucial step: he encouraged others to work on
these developments in GaN. This was not an easy idea to sell because

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