2. ContentsDefinition
History of QD
Power-Current characteristics
Evolution of quantum dot lasers
Fabrication Techniques
Lithography and etching
Self assembling
Disadvantages of Lithography
Structure of quantum dot lasers
Benefits of Quantum dot lasers
2
3. • A 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 ℃.
Definition
3
4. History of QD
• The first laser diode was realized by R. N. Hall and his team at the General Electric Research Centerin
1962 , with many other teams involved in the demonstration of efficient lasing thereafter.
• BULK type active layer demonstrated in 1970. A micron thin layer of low bandgap material is
sandwiched between two wide bandgap layers, confining carriers enabling efficient light
amplification.
• early 1980s Quantum Well structure laser in the active region was invented to realize more efficiency
of lasing.
4
5. History of QD
• The First QDL predicted by Arakawa and Sakaki in 1982 through theoretical modeling of
semiconductor lasers to clarify the quantum effect on their temperature characteristic.
• Almost no progress had been made for a decade after the prediction due to difficulties in
fabricating quantum dots.
5
6. • Threshold current and the slope efficiency deteriorates
sensitively as temperature increases.
• The bias and the modulation current, should be always tuned
in accordance with the environmental temperature by
monitoring and feeding back output power and/or
temperature.
• High cost and low throughput of optical transceivers.
Power-Current characteristics
6
8. 8
• The quantum well consists of a semiconductor thin layer with the thickness of a few to 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.
• Quantum dot has a series of delta function-like energy states.
• 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.
• Enables most of the carriers to participate in the lasing action from the ground state
Evolution of quantum dot lasers
9. 9
Fabrication Techniques
Lithography and etching
• 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.
• Other techniques exploit regrowth of epitaxial layers on a vicinal surface or selective growth on a
patterned substrate
• Artificial structures fabricated in these ways did not work for laser applications.
10. 10
Disadvantages of Lithography
Damage to the
crystals
Defect
formation
Poor interface
quality
Low light
emission
efficiency
Low density
Size
irregularity
11. 11
• Exploits the three dimensional island growth of highly lattice
mismatched semiconductors.
• Growth of InAs on a GaAs substrate where the lattice mismatch
between InAs and GaAs is about 7 %.
• In and As atoms are injected onto the GaAs substrate under the
high vacuum environment of the molecular beam epitaxy “MBE”
reactor, the two dimensional thin layer grows at first.
• Dislocation free high density three dimensional islands of InAs are
self-assembled in order to release the strain energy
Self Assembling
12. 12
Self Assembling
• InAs islands on the GaAs substrate was first reported in 1985 by Goldstein.
• They work as highly efficient light –emitting quantum dots without
dislocations
• Researchers of Fujitsu Laboratories 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.
13. 13
• 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.
• Recently, the QDL, Fujitsu Laboratories, and University of Tokyo have achieved breakthrough to
double the dot density to around 6x1010 cm -2.
Self Assembling
14. 14
• Figure shows the surface AFM images of (a) conventional dots with the density of
3x1010 cm- , and (b) newly developed dots with the density of 6 x1010 cm -2. The
optical gain is now doubled, giving higher slope efficiency and broader modulation
bandwidth, which is now the standard technology of QDL.
Self Assembling
15. 15
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. By modulating the injection current, the laser emits accordingly modulated bit
signals to be transmitted through optical fibers.
16. 16
Benefits of Quantum dot lasers
• Quantum dots are more stable in photonic circuits because they have localized atomlike
energy states.
• Run on less power because they don't need as much electric current.
• Operate at higher temperatures over 100 c.
• Low cost with stable production.
• Can be scaled down to smaller sizes.
17. 17
References
• New era of Quantum dot lasers with evolution history of semiconductor lasers
• Fujitsu (September 10, 2004).
• Quantum Dot Lasers (Huizi Diwu and Betul Arda)
• J. C. Norman, D. Jung, Y. Wan, and J. E. Bowers, “Perspective: The future of quantum dot photonic
integrated circuits,” APL Photonics, vol. 3,
•
↑J. Kwoen, B. Jang, J. Lee, T. Kageyama, K. Watanabe, and Y. Arakawa, “All mbe grown
InAs/GaAs quantum dot lasers on on-axis Si (001),”
