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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. Legal Information Copyright Copyright 2008 QD Laser, Inc. All rights reserved. The information in this document is proprietary and confidential to QD Laser, Inc., and for its customers’ internal use. In any event, no part of this document may be reproduced or redistributed in any form without the express written consent of QD Laser, Inc. Disclaimer None of the information contained in this document constitutes an express or implied warranty by QD Laser, Inc. as to the sufficiency, fitness or suitability for a particular purpose of any such information or the fitness, or suitability for a particular purpose, merchantability, performance, compatibility with other parts or systems, of any of the products of QD Laser, Inc., or any portion thereof, referred to in this document. QD Laser, Inc. expressly disclaims all representations and warranties of any kind regarding the contents or use of the information, including, but not limited to, express and implied warranties of accuracy, completeness, merchantability, fitness for a particular use, or non-infringement. In no event will QD Laser, Inc. be liable for any direct, indirect, special, incidental or consequential damages, including, but not limited to, lost profits, lost business or lost data resulting from any use of or reliance upon the information, whether or not QD Laser, Inc. has been advised of the possibility of such damage. Trademarks For a complete list of QD Laser’s trademarks and registered trademarks, visit [email protected] Patents The technology discussed in this document may be protected by one or more patent grants. References [1] Y. Arakawa and H. Sakaki, Appl. Phys. Lett., 40, 939 (1982). [2] K. Mukai, N. Ohtsuka, M. Sugawara and S. Yamazaki, Jpn. J. Appl. Phys.,33,L1710 (1994). [3] R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson, Phys. Rev. Lett. 9, 366 (1962). [4] W. T. Tsang, Appl. Phys. Lett. 39, 786 (1981). [5]P. J. A. Thijs and T. Dongen, Ext. Abs. 22nd Int. Conf. Solid State Devices and Materials, Sendai, Japan 1990. [6] P. M. Petroff, A. C. Gossard, R. A. Logan, and W. Wiegmann, Appl. Phys. Lett. 41, 635 (1982). [7] E. Kapon, S. Simhony, R. Bhat, and D. M. Hwang, Appl. Phys. Lett., 55, 2715, (1989). [8] L. Goldstein, F. Glas, J. Y. Marzin, M. N. Charasse, and G. Le Roux, Appl. Phys. Lett. 47, 1099 (1985). [9] M. Tabuchi, S. Noda, and A. Sasaki, J. Cryst. Growth, 99,315 (1990). [10] “Self-Assembeld InGaAs/GaAs Quantum Dots” Semiconductors and Seminetals Vol 60, edited by M. Sugawara, Academic Press, San Diego, 1999. [11] D. L. Huffaker, G. Park, Z. Zou, O. B. Shchekin, and D. G. Deppe, Appl. Phys. Lett. 73, 2564 (1998). [12] K. Mukai, Y. Nakata, K. Otsubo, M. Sugawara, N. Yokoyama, H. Ishikawa, IEEE Photon. Technol. Lett., 11, 1205 (1999). [13] K. Otsubo, N. Hatori, M. Ishida, S. Okumura, T. Akiyama, Y. Nakata, H. Ebe, M. Sugawara, and Y. Arakawa, Jpn. J. Appl. Phys., Part 2 , 43, L1124 (2004). [14] M. Ishida, K. Watanabe, N. Kumagai, Y. Nakata, N. Hatori, H. Sudo, T. Yamamoto, M. Sugawara, and Y. Arakawa, 19th International Conference on Indium Phosphide and Related Materials (IPRM2007) FrB1-3, Matsue, Japan (2007)