perpendicular to the optical axis to parallel to the optical
axis. This transition occurs for indium content between 5 and
30 percent, and lattice inclinations between 43° of the (101-2)
plane and 62° of the (101-1) plane, and it also affects the optical
gain spectra.
On one hand, birefringence and polarization switching
effects limit the choice of ridge orientation, again narrowing the design freedom for green laser diodes and imposing
constraints—e.g., on laser facet formation by etching or cleaving. On the other, there are more semipolar lattice planes from
which to choose, providing an important degree of freedom
to optimize long wavelength laser diodes. Which semipolar
plane is best is still an open question. While nonpolar planes
can probably be ruled out due to dislocation generation, high-index-lattice planes that are close to the nonpolar plane seem
to produce the best results.
Carrier transport in green (Al,In)GaN laser diodes
The piezoelectric field not only affects the overlap of electron
and hole wave functions inside the QWs, it also generates
potential barriers at the interfaces of the epitaxial layers,
thereby limiting the transport of carriers into the QWs.
Therefore we can expect that carrier injection may be improved
in semipolar laser diodes. In c-plane laser diodes, the number
of QWs is between one and three, since additional QWs will
not contribute to the gain due to inefficient hole injection.
For green-emitting laser diodes, the QWs are so deep that
thermionic reemission (i.e., the heat-induced flow of charge
carriers from a surface or over a potential-energy barrier) into
the transport layer can be neglected.
In a recent experimental and theoretical study, D. Sizov et al.
from Corning demonstrated that the number of QWs can be
increased to five in green-light-emitting semipolar laser diodes,
with optical gain proportional to the number of QWs. The more
uniform carrier distribution is actually caused by a slower carrier
injection into semipolar quantum wells, compared to polar ones.
While in polar QWs, the holes are predominantly cached by the
one or two QWs closest to the p-side, the carriers in semipolar
laser diodes have enough time to redistribute by tunnelling
between more QWs before being captured by a QW.
Outlook
The green laser diode fills the gap between the red and infrared
laser diodes on one side of the spectrum, and blue, violet and
ultraviolet on the other. This gap has been closed—with laser
diodes now forming the complete “rainbow” of the visible
spectrum, admittedly with a 100-nm-wide gap in the yellow-to-orange range. Researchers were able to do this by mastering
the epitaxy of high-indium-containing QWs, balancing strain
and defect formation against quantum-confined Stark effect
and using optimized waveguides.
The first engineering samples of green laser diodes were
done on the standard polar orientation. Now it appears that
crystals of semipolar orientation may offer higher output power
and longer device lifetime. However, progress toward semi-polar laser diodes has slowed because freestanding semipolar
GaN substrates are either very small or exceedingly expensive.
As the price for these exotic substrates drops, the optical community is studying the peculiar physics of semipolar
InGaN QWs, including growth modes, indium and dopant
incorporation, defect generation, polarization switching and
birefringent waveguides.
The initial prototypes of pico-projectors are already
equipped with green laser diodes. In a few years, many cell
phones and PDAs are expected to come equipped with these
projectors, allowing people to share images and presentations
instantaneously. Whether this gadget will succeed in the
marketplace will ultimately be up to the consumer, as it was
with the CMOS camera, which is now part of almost every
cell phone.
Beyond consumer electronics, there are many more applications for the green laser diode in the life sciences. It will act
as an enabling technology—e.g., in spectroscopy, microscopy
and the new field of optogenetics, in which the laser can be
used for selective optical excitation of nerve cells. While the
material system is extremely challenging for high-indium content, we are optimistic that high-power green-light-emitting
(Al,In)GaN lasers will become viable, most likely by using
the semipolar crystal orientation. Ultimately, this may enable
laser modules—for example, single-mode external cavity laser
diodes—which then can be frequency-doubled to 248 nm, in
order to replace excimer lasers, at least in low-power applications such as spectroscopy and lithography, but possibly also
for material processing. t
UTS acknowledges funding by the German Research Society (DFG)
within the research group 957, PolarCon.
Ulrich T. Schwarz ( Ulrich.Schwarz@iaf.fraunhofer.de) and Wolfgang G.
Scheibenzuber are with the Fraunhofer Institute for Applied Solid
State Physics in Freiburg, Germany. Member
[ References and Resources ]
>> Y. Enya et al. “531 nm green lasing of InGaN based laser diodes
on semi-polar {202-1} free-standing GaN substrates,“ Appl. Phys.
Express 2, 082101 (2009).
>> W.G. Scheibenzuber et al. “Calculation of optical eigenmodes and
gain in semipolar and nonpolar InGaN/GaN laser diodes,” Phys.
Rev. B 80, 115320 (2009).
>> A. Avramescu et al. “True green laser diodes at 524 nm with
50 m W continuous wave output power on c-plane GaN,” Appl.
Phys. Express 3, 061003 (2010).
>> P.S. Hsu et al. “InGaN/GaN blue laser diode grown on semipolar
(303-1) free-standing GaN substrates,” Appl. Phys. Express 3,
052702 (2010).
>> T. Lermer et al. “Waveguide design of green InGaN laser diodes,”
Phys. Status Solidi A 207, 1328 (2010).
>> J. W. Raring et al. “High-efficiency blue and true-green-emitting
laser diodes based on non-c-plane oriented GaN substrates,”
Appl. Phys. Express 3, 112101 (2010).
>> D.S. Sizov et al. “Impact of carrier transport on aquamarine–green
laser performance,” Appl. Phys. Express 3, 122101 (2010).
