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White Paper
Issued date: Sep 2008
New era of Quantum dot lasers with evolution history of
semiconductor lasers
Abstracts
The quantum dot laser is the newly developed semiconductor laser with the ensemble of nano-sized
quantum dots inside the laser cavity as light emitters, in order to revolutionize optical transmitters
for optical communications with its robustness to environments. It includes all of the ingredients for
market success such as extreme temperature insensitivity, low power consumption, high temperature
operation over 100 ℃ , and low cost with stable production, unattainable in conventional
semiconductor lasers. QD Laser, Inc., “QDL”, is leveraging its leading quantum dot technology to
bring cutting edge devices into the optical telecommunication markets.
1. Introduction
Quantum dot lasers are new generation semiconductor lasers including several million nano-sized
crystals called quantum dots in the active region as light emitters, and are expected to revolutionize
optical transmitters for optical communications with their robustness to environments such as
extreme temperature insensitivity, low power consumption, and high temperature operation over
100 ℃. Their high-performance was first predicted by Arakawa and Sakaki in 1982 [1] through
theoretical modeling of semiconductor lasers to clarify the quantum effect on their temperature
characteristic. This prediction has received wide attention as strained quantum well lasers, now
being used as standard light sources for optical telecommunication, suffer from temperature-induced
deterioration of performance. However, in spite of the attractiveness of quantum dot lasers, almost
no progress had been made for a decade after the prediction primarily due to difficulties in
fabricating quantum dots.
Epoch making was the serendipitive finding in 1994 by Fujitsu Laboratories, Ltd., Japan, on InAs
self-assembled quantum dots emitting the light of 1.3 μm, just the wavelength for optical
telecommunication [2]. Since then, Fujitsu laboratories and University of Tokyo have collaborated to
improve quantum dot technology and developed high-performance telecommunication standard
optical devices, i.e., FP (Fabry-Perot) and DFB (Distributed-Feedback) lasers, and SOAs
(Semiconductor Optical Amplifies) under the support of NEDO, Japan. QD Laser, Inc. has
established the world-first mass-production lines of these quantum-dot optical devices to bring them
to the FTTH and Gigabit LAN markets.
2. History of semiconductor lasers
A semiconductor laser diode is technologically one of the most important lasers with widespread
applications, including areas of optical data transmission, optical data storage, material processing,
medical treatments and spectroscopy, etc. This owes to its small size, high reliability,
electrically-pumped high efficiency based on pn junctions, wavelength tunability, and high-speed
modulation capability. In particular, its application to long-distance optical data transmission as a
high-speed light source has given birth to world-wide optical fiber communication networks, over
which Internet has been constructed to globalize human life on the “flat world” at an unexpected
pace.
The first laser diode was realized by R. N. Hall and his team at the General Electric Research Center
in 1962 [3], with many other teams involved in the demonstration of efficient lasing thereafter. The
practical innovation toward room-temperature continuous wave operation was the double
hetero-structure laser with the BULK type active layer demonstrated in 1970. In these devices, a
micron to sub-micron thin layer of low bandgap material is sandwiched between two wide bandgap
layers, confining carriers i.e., electrons and holes, and light to the middle thin layer, and thus,
enabling efficient light amplification. This type of laser was early-stage semiconductor laser for
optical telecommunication in 1980s. For their accomplishment, Alferov and Kroemer shared the
2000 Nobel Prize in Physics.
In the early 1980s Quantum Well structure laser [4,5] in the active region was invented to realize
more efficiency of lasing. If the middle layer is made thin enough, it acts as a quantum well, where
the electron as a wave forms the standing wave with its energy quantized. The efficiency of a
quantum well laser is much greater than that of a bulk laser because the density of states function of
electrons in the quantum well system has an abrupt edge that concentrates electrons in energy states
that contribute to laser action (see Sec. 3).
At present, quantum well lasers are acknowledges as a standard for optical telecommunication,
with satisfactory low operation current as long as the laser temperature is fixed at around room
temperature.
However, “satisfactory“ is not the case,
when it comes to uncooled operation of
lasers
at
severe
high
temperature
environments. Figure １ shows a typical
power-current
characteristics
of
commercially available quantum well lasers
at various temperatures. It is seen that the
threshold current and the slope efficiency
deteriorates
sensitively
as
temperature
increases. Also, high temperature operation
at 100 ℃ or higher is quite hard to be
achieved due to the saturation of output
power. In order to use this kind of
temperature-sensitive device, laser operation
Figure １ Typical power-current characteristics of
current, i.e. the bias and the modulation
conventional
current,
telecommunication standard.
should
accordance
with
be
always
the
tuned
in
quantum
well
lasers
as
environmental
temperature by monitoring and feeding back output power and/or temperature. This not only makes
it hard to guarantee ideal optical signal quality but also requires additional electrical and optical
feedback circuits as well as their testing, leading to high cost and low throughput of optical
transceivers. Moreover, deterioration of laser performance at high temperatures usually ends up with
an increase in the size and consumption power of transceivers. In spite of all this, the reality is that
smaller, faster, and cost-effective optical transceivers are being strongly required year by year, which
makes transceiver manufacturers struggling on how they can radiate heat, and reduce power
consumptions.
The deterioration of laser performance with temperature is more than 30 year problem, being noticed
since the early era of bulk-structure lasers, which quantum well lasers have failed to solve. Advent of
the quantum dot laser has historical and technological significance in its robustness to any high
temperature environments.
Figure 2 shows the history of active layer structure with evolutions.
Figure 2 The history of evolutions and structure of active layers in lasers
3. Evolution of quantum dot lasers
Arakawa and Sakaki predicted in 1982 that semiconductor nano-sized crystals called quantum dots
enable semiconductor lasers to have remarkable temperature insensitivity [1]. Figure 3 shows the
illustration of semiconductor bulk as well as quantum structures, i.e., the quantum well, and the
quantum dot, each of which works as a light emitter when carriers, i.e., electrons and holes, are
injected by the current
through
surrounding
wide-gap layers via the
pn junction. The bulk
means the sandwiched
micron to submicron
layer in the double
hetero-structure
The
laser.
quantum
consists
of
semiconductor
well
a
thin
layer with the thickness
of
a
few
to
Figure3 Evolution of active layer structures from Bulk, Quantum Well
to Quantum Dot with nano-technology
ten
nanometers. The quantum dot is the semiconductor nano-sized crystal. The naming of “quantum”
derives from the fact that the quantum confinement effect works in nano-sized semiconductors to
form the electron standing wave with its energy quantized. Owing to this effect, the freedom of
electron motion as a wave is reduced from three, two, to zero dimension as we move on from the
bulk, the quantum well, to the quantum dot, resulting in abrupt-edge electron density of states that
concentrates electrons in energy states for laser action as seen in Fig. 3.
Note that the quantum dot has a series of delta function-like energy states. As a result, carriers
injected into the quantum dot reside only at the ground state since the separation between the delta
function-like energy states prevent carriers from thermally being excited to upper energy states. This
enables most of the carriers to participate in the lasing action from the ground state, providing highly
efficient lasers even at high temperatures. Based on this mechanism, Arakawa predicted every aspect
of high-performance quantum dot lasers from temperature insensitivity, low power consumption,
high-speed modulation, to narrow spectrum width.
4. Self-assembled quantum dots for 1.3 μm optical telecommunication
Numerous challenges in the fabrication of
quantum dots have been made in 1980s and
90s in order to realize predicted high
performance
lasers.
The
most
straightforward technique is to laterally
pattern the quantum well structure through a
combination of high resolution electron
beam lithography and dry or wet etching [6].
Other
techniques
exploit
regrowth
of
epitaxial layers on a vicinal surface or
selective growth on a patterned substrate [7].
However, artificial structures fabricated in
these
ways
did
not
work
for
laser
applications. For one thing, lithography and
etching based technology caused damage to
Figure 4 Growth process of self-assembled InAs
quantum dots on GaAs.
the crystals such as impurity contamination,
defect formation, and poor interface quality, resulting in low light emission efficiency. For the other,
there were serious problems in the fabricated structures themselves, such as low density and size
irregularity. While millions of the structure should be packed with the atomic-scale size uniformity
and with high density over 1 x 1010 cm-2 for laser application, no techniques could meet with this
standard unfortunately.
Self-assembling, a novel way to fabricate quantum dots, is now being welcomed as the most
promising approach in overcoming the above mentioned problem of the previous techniques. This
process exploits the three dimensional island growth of highly lattice mismatched semiconductors.
The growth of InAs on a
GaAs substrate is a typical
example, where the lattice
mismatch
between
InAs
and GaAs is about 7 %. As
seen in Fig. 4, when In and
As atoms are injected onto
the GaAs substrate under
the
high
environment
molecular
vacuum
of
beam
the
epitaxy
Figure 5 TEM photograph of the first quantum dots emitting at
the wavelength of 1.3μm
“MBE” reactor, the two
dimensional thin layer grows at first, and then, dislocation free high density three dimensional
islands of InAs are self-assembled in order to release the strain energy. This is analogical to our daily
experience that we see water droplets on the waxed body of a car. InAs islands on the GaAs
substrate was first reported in 1985 by Goldstein [8], and then, were noted by Tabuchi that they work
as highly efficient light –emitting quantum dots without dislocations [9].
Epoch making was the serendipitive finding in 1994 by Fujitsu Laboratories, Ltd., Japan, on InAs
self-assembled quantum dots emitting the light of 1.3 μm, just the wavelength for optical
telecommunication like FTTH and Gigabit LANs [2]. Researchers of Fujitsu Laboratories, Ltd.
Japan, K. Mukai, N. Ohtsuka, and M. Sugawara, found these new quantum dots when they tried to
fabricate GaAs/InAs short-period superlattices on GaAs substrates by atomic layer epitaxy. The
purpose of their research was to realize materials that emit at 1.3 μm on GaAs with an expectation
that lasers on GaAs substrates would have high temperature stability since high potential barriers
like AlGaAs and AlGaInP on GaAs can prevent carrier leakage from the active region. Simply
growing InGaAs quantum wells on GaAs substrates does not work because the large lattice
mistmatch causes mifit dislocation as the thickness goes over its critical level, severely damaging the
crystal quality. Their belief was that short period superlattices can break through the limit set by the
critical thickness to reach a highly efficient 1.3 μm emission. Actually they succeeded in realizing
highly efficient 1.3 um emission. However, the grown materials were not short period superlattices
but had the bizarre structures as seen in the transmission electoron microcipe (TEM) photograph of
Fig. 5, where spherical dark regions are buried in a quantum well. Having done detail diagnostics of
the materials, we finally concluded that structures are quantum dots [10].
This finding motivated research institutes all over the world to challenge 1.3 μm quantum dot lasers.
After several years of research, the Texas University group realized pusled operation at room
temperature [11], followed by Fujitsu group’s continuous wave operation in 1999 [12]. It was
confirmed that lasing operation occurred just from the top of the ground state of quantized energy
states. The key to this success was a technology to enhance the optical gain by increasing the
quantum dot density as well as by stacking dot layers repeatedly in the growth direction. In 2004, the
modulation speed reached 10Gbps with its power-current characteristics almost temperature
insensitive [13]. This was done by stacking the dot layers up to ten in order to further increase the
optical gain and also by p-type doping to make the lasers immune to temperature [1].
Recently, the QDL, Fujitsu Laboratories, and University of Tokyo have achieved breakthrough to
double the dot density to around 6x1010cm-2 [14]. Figure 6 shows the surface AFM images of (a)
conventional dots with the
density of 3x1010cm-, and
(b) newly developed dots
with the density of
6
x1010cm-2. The optical gain
is now doubled, giving
higher slope efficiency and
broader
modulation
bandwidth, which is now
(a)
(b)
the standard technology of
Figure 6 Surface AFM images of (a) conventional dots with the
QDL.
density of 3x1010cm-2, and (b) newly developed dots with the
density of 6x1010cm-2.
5. Basic structure of quantum dot lasers
Quantum dot FP lasers are the
lasers with the ensemble of
self-assembled
quantum
dots
inside the cavity as shown in Fig.
7. By modulating the injection
current,
the
laser
emits
accordingly modulated bit signals
Figure 7
Quantum dot FP lasers
to be transmitted through optical fibers. Their applications are in the field of GE-PON for use in the
FTTH access network (IEEE802.3ah 1000BASE-PX10-U), which supports symmetrical 1Gbps rates
and is based on Ethernet and IP protocols, as well as in the field of LRM (IEEE P802.3aq
10GBASE-LRM), Fibre Channels……
Figure 8 is a comparison of typical power-current characteristics of FP lasers. The quantum dot
FP laser not only provides optical power enough to meet the regulation of GEPON with high slope
efficiency but also temperature-insensitive operation with almost constant threshold current and
slope efficiency.
QD - FP Laser
10
Conventional FP Laser A
Conventional FP Laser B
10
- 40ºC
10
- 20ºC
Output Power (mW)
+ 40ºC
+ 60ºC
6
8
+ 90ºC
+ 100ºC
4
8
- 40ºC
Output Power (mW)
+ 20ºC
+ 70ºC
+ 80ºC
Output Power (mW)
+ 0ºC
8
+ 25ºC
6
+ 85ºC
4
2
2
0
10
20
30 40 50
Current (mA)
60
70
+ 25ºC
6
+ 85ºC
4
2
0
0
- 5ºC
0
0
10
20
30 40 50
Current (mA)
60
70
0
10
20
30 40 50
Current (mA)
60
Figure 8 Comparison of quantum dot laser and conventional quantum well lasers
QDL also has recently realized the 10Gbps version in 100℃ environment for its application to IEEE
802.3aq 10GBASE-LRM, demonstrating extreme high temperature insensitivity of quantum dot
lasers.
6.
Quantum dot DFB lasers and their applications.
DFB Lasers to emit a single longitudinal mode using the grating inside at the wavelength of 1.3μm
are the light sources for long-reach GEPON (IEEE802.3ah 1000BASE-PX20-U) and GPON in the
FTTH access network, optical LAN and other long distance applications. To stabilize the lasing
wavelength, a diffraction grating is etched close to the p-n junction of the diode. This grating acts
like an optical filter, causing a single wavelength to be fed back to the gain region, leading to single
mode lasing.
QDL has developed the quantum dot DFB lasers with the same temperature insensitivity as seen in
70
the quantum dot FP lasers.
7. Conclusion
At present QDL differentiates itself from other enterprises and research institutes all over the world
working quantum dots by its ultra-high density dots to guarantee communication level lasers and by
its world-first mass production line for the commercialization coming in the mid of 2009. The
production capacity of laser chips will reach several millions a year by the end of 2009.
QDL strongly believes QD technologies and devices will be well-accepted in the telecommunication
industry such as FTTH, LAN, etc., and would contribute to the Eco-world with lower power
consumption, realizing small form factors of high speed transceiver systems even under high
temperature environments. Not limited to the telecom industry, QDL will expand quantum dot
technologies and devices into other industries such as life science, auto-mobile and consumer
electronics, etc., in the future.
Information is available at
www.qdlaser.com and [email protected] b y email
About QD Laser, Inc.
QD Laser, Inc. (QDL) is the curve-out venture company launched on April 24 2006 from Fujitsu
Ltd., under the joint investment of Fujitsu and Mitsui ventures, and is a leading provider of
quantum-dot based optical devices for optical communication including quantum dot lasers and
optical amplifiers. QDL has established the world-first mass-production line of GaAs-based FP and
DFB quantum dot lasers, and is ready to offer these devices to optical communication markets. The
joint research collaboration with University of Tokyo and Fujitsu Laboratories Ltd. plays a key role
in the R&D of the state-of-the-art quantum dot technology.
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Patents
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References
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