Semiconductor Laser Concepts
Fundamentals of GaAs-based laser designs and the investigated (In)(Ga)As gain media concepts are discussed within this chapter. (Al)GaAs is the material system which is primarily employed for the infrared spectral range. Due to its versatility and ability
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Semiconductor Laser Concepts
Fundamentals of GaAs-based laser designs and the investigated (In)(Ga)As gain media concepts are discussed within this chapter. (Al)GaAs is the material system which is primarily employed for the infrared spectral range. Due to its versatility and ability to form dielectric mirrors for vertically emitting devices, (Al)GaAs forms the basis for a wide range of applications in the near infrared spectrum, and is wellestablished for industrial mass production.
2.1 Evolution of Semiconductor Lasers Since its inception, some of the main goals behind semiconductor laser development have been the creation of new designs to achieve reduction of the lasing threshold, increase in modulation speed, and higher output power. Well known examples in everyday life include the AlGaAs laser diodes operated in continuous-wave mode (CW) at 780 nm employed for compact discs, and at 848 nm for laser computer mice. Optical interconnects driving the Internet rely completely on infrared semiconductor laser technology, and steady demand exists for higher modulation speeds and more cost-efficient devices. All of these examples are based on QW active media. In parallel to this quasi standard in today’s industry, more sophisticated nanostructures such as QDs have been introduced as a step to improving laser performance and to unlocking new application areas. While the evolution of QD lasers started In the 1990s, the first QD devices are just now entering the market. Lithographic techniques and chemical wet etching with subsequent overgrowth were used to fabricate the first QD lasers. These structures showed pulsed lasing at 77 K with extremely high jth of 7.6 kA/cm2 [1]. A significant advance in terms of reducing jth was the use of self organized QD growth in the Stranski-Krastanow growth mode (SK) [2], which allowed for an essential reduction in the defect density within the QD layer. A 942 nm SK-QD laser using MBE growth was first developed by Kirstaedter et al., and demonstrated a significantly reduced jth of 120 A/cm2 at 77 K and 950 A/cm2 at RT [3, 4]. This breakthrough started a series of reports on improved T. D. Germann, Design and Realization of Novel GaAs Based Laser Concepts, Springer Theses, DOI: 10.1007/978-3-642-34079-6_2, © Springer-Verlag Berlin Heidelberg 2012
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2 Semiconductor Laser Concepts
MBE-grown SK-QD lasers with jth down to 19 A/cm2 , realized with aluminum-oxide confinement layers and emission wavelength up to 1.3 µm [5, 6]. While the extremely low threshold characteristics and long wavelength emission around 1.3 µm of these QD lasers were predominantly achieved by MBE-grown devices (an overview can be found in [7]), the first successful MOVPE-based fabrication of SK-QD lasers emerged in 1997 [8]. Steady development of QD devices in the following years enabled success in significantly improved MOVPE-based SK-QD laser processes to close the gap to MBE devices [9, 10].
2.2 Gain Concepts Besides (Al)GaAs itself, the dominant active material for GaAs-based devices are InGaAs quantizati
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