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We realized the excellent confinement of carriers in single-layer InAs quantum dots (QDs) via introducing two AlGaAs confining layers (CLs), and then fabricated the corresponding rolled-up InGaAs/GaAs QD microtubes by conventional photolithography and wet-etching. Subsequently, the as-fabricated AlGaAs confined QD microtubes were transferred to a Si-based SiOx substrate using a simple liquid-assisted substrate-on-substrate transfer process, thus obtaining the microtube ring resonators. Through micro-photoluminescence (µPL) measurement, optical modes were observed at room temperature and a maximum Q-factor of ~ 550 was demonstrated. In order to clearly show the effect of AlGaAs CLs, we also fabricated and transferred QD microtubes without AlGaAs CLs for comparison. µPL spectra collected at 80 K confirmed that the PL intensity of the central optical mode was increased ~ 10 times with the assistance of AlGaAs CLs. We have confidence that QD microtube ring resonators can be further improved through the incorporation of double-layer QDs together with AlGaAs CLs.

Planar diode structures with small area active region (∼1 μ m2) based on highly doped GaAs/AlAs superlattices (SLs) are investigated. The possibility of effective application of such diodes in the terahertz frequency range is discussed. Monte Carlo simulation of electron transport in 6-period SLs was carried out. The possibility of increasing the operating frequencies of devices on SLs by optimizing their parameters and selecting the appropriate diode design is discussed. This is achieved by special transition near-contact layers, whose thickness is comparable to the thickness of investigated SL.

We report the observation of optically pumped continuous wave lasing in a self-rolled-up InGaAs/GaAs quantum dot microtube at room temperature. Single layer of InAs quantum dots (~ 2.6 ML coverage) in a GaAs well sandwiched by two AlGaAs barriers are incorporated into the tube wall as the gain media. As-fabricated microtube is supported by a 300-nm-thick Au pad, aiming to separate the tube from GaAs substrate and thus to decrease the substrate loss, which finally enables lasing with ultralow threshold power (~ 4 µW) from an microtube ring resonator.

We demonstrate an ultra-compact, room-temperature, continuous-wave, broadly-tunable dual-wavelength InAs/GaAs quantum-dot external-cavity diode laser in the spectral region between 1150 nm and 1301 nm with maximum output power of 280 mW. This laser source generating two modes with tunable difference-frequency (300 GHz–30 THz) has a great potential to replace commonly used bulky lasers for THz generation in photomixer devices.

Formation of semiconductor quantum dots from a metastable wetting layer with the effective thickness well below the critical thickness is observed experimentally in the InAs / GaAs (100) and in the Ge / Si (100) systems. The observed effect is explained within the frame of the kinetic theory of quantum dot formation in mismatched heteroepitaxial systems.

We studied the carrier storage and confinement of electrons and holes in InGaAs/GaAs QDs with deep level transient spectroscopy (DLTS) and time-resolved tunneling capacitance spectroscopy (TRTCS). For InGaAs/GaAs QDs with an additional AlGaAs barrier below the QD layer we observe in charge-selective DLTS experiments a hole ground state activation energy of about 580 meV. This activation energy leads to a storage time of about 5 ms at room temperature. With the TRTCS method we studied in detail the electric field dependence of the electron and hole tunneling time from InGaAs QDs without an AlGaAs barrier. The dependence of the tunneling time on the applied electric field allows us to determine the hole and electron localization energy to EH1loc = (210 ± 20) meV and EE1loc = (260 ± 20) meV, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular beam epitaxy growth of InAs/InGaAs QD structures on GaAs substrates as well as fabrication and performance of long-wavelength QD edge-emitting lasers and VCSELs are discussed. 1.3 pm QD VCSELs were successfully fabricated from the structures with several QD planes inserted into the optical microcavity with AlO-GaAs and AlGaAs-GaAs Bragg reflectors.

InAs/InGaAsN quantum dots have been grown by molecular beam epitaxy on GaAs substrates. Transmission electron microscopy shows the increase in the island size as compared to the InAs/InGaAs quantum dots. Room temperature photoluminescence at 1.55 μm has been demonstrated whose intensity was comparable to that of the InGaAsN/GaAs quantum wells emitting at 1.3 μm. The effect of nitrogen concentration on the PL peak position and intensity has been studied.

GaAs-based vertical cavity surface-emitting lasers (VCSELs) using self-organized quantum dots (QDs) emitting at demonstrate device-acceptable parameters. Threshold currents below , operation voltage below and differential efficiency in excess of 60% are demonstrated. Maximum CW output power of is realized for oxide-confined aperture device. Using fully oxidized top and bottom distributed Bragg reflectors allows reducing the total thickness of the structure to only 5–. Lifetime and temperature cycling tests confirm high reliability of the device. Confinement of nonequilibrium carriers in the QDs facilitates applications in VCSEL arrays with ultrasmall apertures and microcavities. Low homogeneous line width in single QDs makes potentially possible realization of single QD VCSELs.

We report on the fabrication of quantum dot (QDs) heterostructures for applications in optoelectronics. Different kinds of QDs are currently used: (i) three-dimensional quantum dots obtained by Stranski-Krastanow or Volmer-Weber growth in the InAs–GaAs material system, (ii) two-dimensionally-shaped QDs formed by submonolayer insertions in the InAs–GaAs and similar systems, (iii) GaAs QDs formed on a corrugated (311)A AlAs surface, (iv) and QDs obtained by spinodal decomposition and activated spinodal decomposition in InGaAs–GaAs and InGaAsN–GaAs material systems. Formation of uniformly sized and shaped QDs is possible in all of these approaches and is mostly governed by thermodynamics. Ultrahigh modal gain and giant optical nonlinearity can be achieved in dense arrays of very small QDs. Long wavelength (1.3–1.6 µm) emission can be achieved using large InAs QDs. Recent advances in growth have made possible the realization of GaAs 1.3 µm continuous wave (CW) vertical-cavity surface-emitting lasers (VCSELs) with ~ 0.8 mW output power and long operation lifetime.

A new dc plasma source for MBE growth of GaAsN layers is suggested. The efficiency of nitrogen incorporation, crystal perfection, surface morphology, and luminescent properties of the epilayers vs. operation conditions of the source are studied.

Strain-driven decomposition of an alloy layer is investigated as a means to control the structural and electronic properties of self-organized quantum dots. Coherent InAs/GaAs islands overgrown with an InGa(Al)As alloy layer serve as a model system. Cross-section and plan-view transmission electron microscopy as well as photoluminescence (PL) studies consistently indicate an increase in height and width of the island with increasing indium content and/or thickness of the alloy layer. The increasing island size is attributed to the phase separation of the alloy layer driven by the surface strain introduced by the initial InAs islands. The decomposition is enhanced by the addition of aluminum to the alloy layer. The ground-state transition energy in such quantum dots is significantly (up to 200 meV) redshifted compared to the original InAs/GaAs quantum dots, allowing to reach the 1.3 \ensuremath{\mu}m spectral region maintaining the high PL efficiency and the low defect density typical for Stranski-Krastanow growth. The possibility of degradation less stacking of such quantum dot layers enables injection lasing on the ground-state transition with a differential efficiency of 57% and a continuous-wave output power of 2.7 W.

We study 1.3 µm diode lasers based on self-organized InAs quantum dots grown by molecular beam epitaxy on GaAs substrates. Overgrowing the InAs quantum dot array with a thin InGaAs layer allows us to achieve 1.3 µm emission and keep a sufficiently high surface density of quantum dots. Using transmission electron microscopy we show that the main reason for the long-wavelength PL shift in InAs/InGaAs quantum dots is non-uniform distribution of In in InGaAs leading to the increase in effective volume of a quantum dot. Long-stripe lasers showed low-threshold current density ( 50%) and low internal loss (~1-2 cm-1). The maximum output power for wide-stripe lasers was as high as 2.7 W and for single-mode devices 110 mW. The lasing wavelength for VCSELs was 1.3 µm. The threshold current for the device with the 8 µm aperture was 1.8 mA. The output power of 220 µW at a drive current of 2.4 mA was observed under pulsed mode.

The present paper reviews recent achievements in the fabrication of diode lasers for the near-infrared range on GaAs substrates. 1.3??m light emitters are currently widely used in fibre-optic communication systems. GaAs-based devices are potentially advantageous compared to their InGaAsP counterparts in several aspects, such as improvement of thermal stability, possibility to grow vertical-cavity surface-emitting lasers in a single growth run and the use of large-area high-quality inexpensive GaAs substrates. Three main approaches have been suggested so far to achieve the 1.3??m emission from structures grown on GaAs substrates. They are InGaAs and GaAsSb quantum wells, GaInAsN quantum wells and InAs/GaAs quantum dots. In the present paper we discuss all these approaches including material growth, optical properties and laser characteristics. The results obtained by these methods are compared and their potential advantages discussed.

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications.

This paper addresses the issues associated with physics and technology of diode lasers based on self-organized quantum dots (QDs). Theoretically predicted advantages of a QD array as the active region of a semiconductor laser and basic principles of QD formation using self-organization phenomena are discussed. Special attention is paid to relationship between structural and electronic properties of QDs and laser characteristics. Recent ahcievements in controlling these parameters including the effects of vertical stacking, changing the matrix bandgap and the surface density of QDs are reviewed. The threshold and power characteristics of the state-of-the-art QD lasers are demonstrated.

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

Alternate short-period GaAs-InAs deposition following InAs pyramid formation on a GaAs (100) surface leads to the creation of vertically split pyramids. This splitting is driven by the energetics of the Stranski-Krastanow growth mode. The strain energy is reduced due to the successive transfer of InAs from the buried part of the pyramid to the uncovered part. The resulting arrangement represents a laterally ordered array of nanoscale structures inserted in a GaAs matrix, where each structure is composed of several vertically merging InAs parts. Results of optical studies demonstrate the expected electronic coupling in vertical direction. Coupling is found to decrease the radiative lifetime and to result in low-energy shifts of the corresponding peaks in luminescence and absorption spectra. Vertically coupled quantum dots exhibit injection lasing at very low current densities. \textcopyright{} 1996 The American Physical Society.

Elastic relaxation on facet edges, renormalization of the surface energy of the facets, and interaction between i&no3 via the strained substrate are the driving forces for self-organization of ordered arrays of uniform coherent three-dimensional is/a& on crystal surfaces. For a (100) surface of a cubic crystal, two-dimensional square lattice of pyramid-like islands (quantum dots) with the periodicity along the directions of the lowest stiffness (OlO) and (OOI) has the minimum energy among different one-dimen- sional and two-dimensional arrays. For the InAs/GaAs(lOO) system, an equilibrium array of dots of the lateral size _ 120-140 A exists in a fixed range of growth parameters. T'he main luminescence peak at 1.1 eV, as well as peaks of excited states coincide in energy with the peaks revealed in the calorimetric absorption spectra regardless of the amount of InAs deposited (2-5 ML). Raman spectra indicate significant strain in InAs dots. The "phonon bottleneck" effect is bypassed via multi-phonon exciton and carrier relaxation. Ultranarrow lines (< 0.15 meV) are observed in cathodoluminescence spectra up to high temperatures. Low threshold current density operation via zero-dimensional states and ultrahigh temperature stability of the threshold current (T, = 450 K) are realized for a quantum dot injection laser. Increase in the gain and significant reduction in the radiative lifetime are possible via the self-organization of vertically-coupled quantum dots (VECODs) arranged in a well ordered artificial three-dimensional tefragonal lattice.

We report ultranarrow $(l0.15\mathrm{meV})$ cathodoluminescence lines originating from single InAs quantum dots in a GaAs matrix for temperatures up to 50 K, directly proving their $\ensuremath{\delta}$-function-like density of electronic states. The quantum dots have been prepared by molecular beam epitaxy utilizing a strain-induced self-organizing mechanism. A narrow dot size distribution of width $12\ifmmode\pm\else\textpm\fi{}1\mathrm{nm}$ is imaged by plan-view transmission electron microscopy. Cathodoluminescence images directly visualize individual dot positions and recombination from a single dot. A dense dot array $(\ensuremath{\sim}{10}^{11}\mathrm{dots}/{\mathrm{cm}}^{2})$ gives rise to a distinct absorption peak which almost coincides with the luminescence maximum.

Nanometer-scale quantum dots are fabricated using the Stranski-Krastanov growth mode of InAs on GaAs(001). For an average coverage of four monolayers, 10 11 cm -2 pyramidal-shaped dots (12±1 nm base along , 5±1 nm high) are formed as observed in plane-view and cross-section transmission electron microscopy. The quantum dots exhibit short-range order, aligning along rows in directions. The three-dimensional confinement of the wave function results in ultrasharp luminescence lines (full width at half maximum

We report on the defect-free (i) nanostructures formation by sub-critical (less than 1 monolayer) inclusions of Ge in a Si matrix and appearance of the new photoluminescence lines from the multilayer structures correlated with the formation of these nanostructures, and (ii) multilayer structures containing close-to-critical Ge insertions in a Si matrix exhibiting strong photoluminescence at room temperature for the optimally grown samples.

We report planar vertical cavity surface emitting lasers with record 15 mW of continuous wave single transverse mode output power that employ spatially displaced multiple aperture and deep oxidation layers in a bottom emitting geometry.

Continuous-wave operation near 1.3 /spl mu/m or a diode laser based on self-organized quantum dots (QD's) on a GaAs substrate is demonstrated. Multiple stacking of InAs QD planes covered by thin InGaAs layers allows us to prevent gain saturation and achieve long-wavelength lasing with low threshold current density (90-105 A/cm/sup 2/) and high output power (2.7 W) at 17/spl deg/C heatsink temperature. It is thus confirmed that QD lasers of this kind are potential candidates to substitute InP-based lasers in optical fiber systems.

InAs self-organized quantum dots inserted in InGaAs quantum well have been grown on GaAs substrates by molecular beam epitaxy. The lateral size of the InAs islands has been found to be approximately 1.5 times larger as compared to the InAs/GaAs case, whereas the island heights and surface densities were close in both cases. The quantum dot emission wavelength can be controllably changed from 1.1 to 1.3 μm by varying the composition of the InGaAs quantum well matrix. Photoluminescence at 1.33 μm from vertical optical microcavities containing the InAs/InGaAs quantum dot array was demonstrated.

New type of carrier distribution among quantum dots — a spatially separated bipolar‐charging state — has been studied experimentally and theoretically. Experimental methods such as pump‐probe spectroscopy of electrically pumped samples and time‐resolved photoluminescenee were used. Theoretical approach based on a detailed statistics of carrier distribution was developed and proposed for interpretation of the experimental results.

Quantum dot (QD) diode lasers attract currently much attention due to their ability to emit light in the advanced near- infrared region at extraordinarily low threshold current densities. A vertical-cavity surface emitting laser (VCSEL), having a superior beam quality, improved temperature stability, low threshold current, and cost-effective planar fabrication, is also an attractive device variant. Here we discuss the state of the art of these lasers intended for the use in 1.3-μm fiber-optic communications. The discussion is centered on an InAs/GaAs semiconductor QD system. Basic issues of the QD synthesis in the system are addressed. The achievement of the control over the 1.3-μm QD emission is demonstrated. Both, wide-stripe and single-mode edge-emitting lasers are described. The lasers designed have a very low threshold current density, high differential efficiency, and a high output power. Narrow-stripe 1.3-μm QD lasers generate in a single mode, have a record-low threshold current, and produce the continuous-wave (CW) power output in excess of 100 mW. Also, we report on QD VCSELs emitting at 1.3 μm. The design of their cavity and active region are described. The room-temperature CW output power of these lasers is as high as 2 mW. Both, the edge- and surface-emitting lasers satisfy the demands of the fiber optical communication technology.

Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.

Annealing at higher temperature (700 °C) of structures with two‐dimensional and three‐dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan‐view transmission electron microscopy (TEM) images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan‐view TEM images taken under [001] zone axis illumination as well as by the cross‐section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time (10–60 min) in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the in...

We report on optical studies of relaxation processes in self‐organized InAs/GaAs quantum dots (QDs). Near resonant photoluminescence excitation spectra reveal a series of sharp lines. Their energy with respect to the detection energy does not depend on QD size and their energy separations are close to the InAs LO phonon energy of 32.1 meV estimated for strained pyramidal InAs QDs. The shape of the PLE spectra is explained by multiphonon relaxation processes involving LO phonons of the QD as well as of the wetting layer, an interface mode, and low frequency acoustical phonons.

Extensive mode-locking investigations are performed in InGaAs/InAs/GaAs quantum dot (QD) lasers. Monolithic mode-locked lasers are fabricated using QD material systems grown by MOCVD and MBE techniques and emitting at 1.1μm and 1.3μm, respectively. The mode-locking performance is evaluated using a variety of laser designs, with various ridge waveguide geometries, cavity and absorber lengths. Passive and hybrid mode-locking are studied and compared in 3.9mm long devices emitting at 1.1μm and operating at a repetition rate of 10GHz. Using 2.1mm long devices emitting at 1.3μm, 18GHz passive mode locking with 10ps Fourier transform limited pulses is demonstrated. This confirms the potential of quantum dot laser for low chirp, short optical pulse generation. Preliminary investigation of the timing jitter of QD passively mode-locked lasers and the behaviour of the QD absorber are also presented. Finally, we report 36GHz passive mode-locking with 6ps optical pulse obtained using 1.1mm long QD lasers emitting at 1.3μm.

We analyse the sensitivity of quantum dot semiconductor lasers to optical feedback. While bulk and quantum well semiconductor lasers are usually extremely unstable when submitted to back reflection, quantum dot semiconductor lasers exhibit a reduced sensitivity. Using a rate equation approach, we show that this behaviour is the result of a relatively low but nonzero line-width enhancement factor and strongly damped relaxation oscillations.

The molecular beam epitaxy of self-assembled quantum dots (QDs) has reached a level such that the principal advantages of QD lasers can now be fully realized. We overview the most important recent results achieved to date including excellent device performance of 1.3 μm broad area and ridge waveguide lasers (J th 2 , I th =1.4 mA, differential efficiency above 70%, CW 300 mW single lateral mode operation), suppression of non-linearity of QD lasers, which results to improved beam quality, reduced wavelength chirp and sensitivity to optical feedback. Effect of suppression of side wall recombination in QD lasers is also described. These effects give a possibility to further improve and simplify processing and fabrication of laser modules targeting their cost reduction. Recent realization of 2 mW single mode CW operation of QD VCSEL with all-semiconductor DBR is also presented. Long-wavelength QD lasers are promising candidate for mode-locking lasers for optical computer application. Very recently 1.7-ps-wide pulses at repetition rate of 20 GHz were obtained on mode-locked QD lasers with clear indication of possible shortening of pulse width upon processing optimization. First step of unification of laser technology for telecom range with QD-lasers grown on GaAs has been done. Lasing at 1.5 μm is achieved with threshold current density of 0.8 kA/cm 2 and pulsed output power 7W.

Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids. We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz. GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2. Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers. Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained. Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 ´m, probably up to 1.55 μm.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The effect of nitrogen composition on structural parameters of intrinsic quantum dots (QDs) has been studied in GaAs1-yNy and InxGa1-xAs1-yNy alloys (y∼0. 015–0.03) using low-temperature near-field scanning optical microscopy (NSOM) combined with magneto-photoluminescence spectroscopy. We used measurements of the diamagnetic shift (magnetic field strength 0–10T), temperature dependent spectra (temperature range 5–300K) and near-field monochromatic images for the estimation of the size, nitrogen excess and density of QDs. The obtained values (size ∼10–30 nm, nitrogen excess ∼0.005 and density ∼100 /μm-3) suggest spontaneous formation (phase separation) of QDs. Strong lateral inhomogeniety of the QD distribution on a micron length scale was observed.

This chapter provides an historical overview of the development of quantum dot edge-emitting lasers from their first demonstration to the present status. Issues of gain saturation and temperature instability in initial lasers based on a single plane of self-organized quantum dots are discussed. Several approaches to improve the performance of InGaAs quantum dot lasers are discussed in detail: increase of the QD density of states by stacking of quantum dots; use of matrix material having a higher bandgap energy; seeding of quantum dots to increase their density; and sub-monolayer formation of quantum dots to increase their density and improve uniformity. At the end of the chapter, experimental results for representative quantum-dot lasers grown on non-GaAs substrates are presented.

This chapter considers the quantum dot active region as a possible candidate for high-power laser applications. Thermal rollover, catastrophic optical mirror damage, and other mechanisms of power limitation in diode lasers are discussed, taking into account specific aspects of quantum dot lasers. Power conversion efficiency in a QD laser is analytically treated, and influence of laser structure and QD parameters is also discussed. Internal loss and other parameters affecting slope efficiency of a QD laser are considered in connection with inherent properties of self-organized quantum dots. The chapter also addresses issues associated with the realization of a continuous-wave lasing regime in QD lasers. Device characteristics are summarized for state-of-the-art high-power quantum dot lasers operating in the 0.9-1.1 and 1.2-1.3 spectral ranges.

This chapter is devoted to laser structures on GaAs substrates, which are capable of operating near the 1.3-um spectral window. Firstly, motivation for long-wavelength emitters on GaAs is discussed and possible semiconductor materials, suitable for 1.3-um application, are compared. The main part of the chapter is focused on long-wavelength quantum dot lasers. Various approaches for epitaxial deposition of long-wavelength QDs are described. The device characteristics of diode lasers comprising quantum dots formed either with atomic layer epitaxy or dots-in-a-well method are then compared. Efficiency, threshold, and temperature characteristics of long-wavelength QD lasers are also discussed. For the sake of comparison, data on non-QD laser structures are presented. InGaAsN quantum wells and diode lasers based on them are also discussed in detail.

This introductory chapter considers the basic principles of operation of diode lasers. The condition of inverse population in semiconductors is derived from Fermi statistics. Light confinement in a laser cavity, including transverse and longitudinal optical modes, is discussed. Main device characteristics of a diode laser, such as light-current and current-voltage curves as well as power conversion efficiency are described in their relation with the internal parameters of the active region. Optical gain and its relationship with laser threshold are considered. Electronic structure of solid state and microscopic theory of optical gain are briefly summarized in the chapter. Size quantization in semiconductors caused by energy barriers at heterointerfaces is considered. Density of states for various types of size dimensionality is presented. Effect of the density of states in the laser active region on the gain characteristics is discussed with attention to the ideal quantum dot array.

This chapter first considers the basic principles of molecular beam epitaxy (MBE) of III-V materials. It describes a typical MBE system and its components with special attention to the reflection high-energy electron diffraction system for in-situ monitoring of a growth surface. Metal organic chemical vapour deposition (MOCVD), chemical reactions in MOCVD process and typical reactor scheme are then described. The chapter also describes different methods of in situ formation of one- or zero-dimensional quantum size objects, including formation of quantum wires on a V-grooved substrate, quantum wires and dots on high-index surfaces, and selective epitaxial growth of low-dimensional structures. Special attention is paid to a method of quantum dot formation in Stranski–Krastanow growth mode.

This chapter presents some concluding remarks. Remarkable progress in the development of self-organized quantum dot lasers is pointed out and the most notable results, e.g. a record-low threshold current density, are briefly summarized. The main parameters of QD arrays affecting the laser’s threshold characteristics are emphasized. It is recognized that device performance of existing QD lasers is still inferior when compared with the theoretical predictions for the ideal quantum dots. Inhomogeneous broadening associated with the size non-uniformity of the islands is the main problem that still remains. An expectation of bright prospects is expressed for device application of quantum dots emitting in wavelength ranges beyond the spectral limit of a quantum well.

This book is devoted to the physics and technology of diode lasers based on self-organized quantum dots (QD). It addresses the fundamental and technology aspects of QD edge-emitting and vertical-cavity surface-emitting lasers, reviewing their current status and future prospects. The theoretically predicted advantages of an ideal QD array for laser applications are discussed and the basic principles of QD formation using self-organization phenomena are reviewed. Structural and optical properties of self-organized QDs are considered with a number of examples in different material systems. The book includes recent achievements in controlling the QD properties such as the effect of vertical stacking, changing the matrix bandgap and the surface density of QDs. The book is also focused on the use of self-organized quantum dots in laser structures, fabrication and characterization of edge- and surface-emitting diode lasers, their properties and optimization. Special attention is paid to the relationship between structural and electronic properties of QDs and laser characteristics. The threshold and power characteristics of the state-of-the-art QD lasers are also demonstrated. Issues related to the long-wavelength (1.3-um) lasers on a GaAs substrate are also addressed and recent results on InGaAsN-based diode lasers presented for the purpose of comparison.

This chapter includes an overview of the basic principles of quantum-dot formation using self-organization phenomena in lattice-mismatched materials. A relationship between lattice mismatch and critical thickness for dislocation formation and island growth mode is discussed. Main structural and optical properties of self-organized quantum dots are then discussed and how they correlate to each other. Special attention is paid to the methods that allow control of the emission wavelength, localization energy, and surface density of QDs. A majority of experimental examples relates to InGaAs/GaAs material system. In addition, quantum dot formation driven by the elastic strain relaxation is reviewed for a wide class of alternative material systems, including GaSb/GaAs, InP/GaP, InGaAsN/GaAs, III-N, III-V/Si, and Ge/Si. The specific aspects of QD formation in these systems as well as the structural and optical properties are discussed.

This chapter discusses the fabrication and performance of QD vertical-cavity surface-emitting lasers (VCSELs) as well as their possible applications. The physical fundamentals of optical microcavities and VCSELs are briefly reviewed. Some fundamental issues and specific features of QD VCSEL design and fabrication are considered. Specifically, scalability properties of QD VCSELs in comparison with quantum well structures are discussed. The effects of realistic (existing) QD active media on VCSEL design are examined. A technique of selective wet oxidation of AlGaAs alloys is described, and its use in QD VCSEL technology is justified. The current status of QD VCSELs is presented, and their possible device applications are also discussed. Advantages of InGaAs quantum dots for GaAs-based long-wavelength VCSELs are considered.

1.3 μm GaAs-based quantum dot (QD) lasers demonstrate parameters improved over InP-based devices. They exhibit lower threshold current densities and losses, higher differential efficiencies and improved temerature stability. Highspeed operation is demonstrated. Reduced linewidth enhancement factor advantageous for low-chirp operation makes it possible to suppress dramatically filamentation effects destroying lateral far-field pattern. GaAs-based QD 1.3 μm VCSEL with 8 μm oxide aperture wavelength emits up to 1.2 mW CW multimode.

Quantum dot (QD) is one of the most perspective candidates to be used as an active region of temperature-insensitive 1.3-micron GaAs based lasers for optical networks. However, the limited optical gain achievable in QD ground state hindered their practical use. In this work we have demonstrated that using of high number of QDs stacks grown under proper conditions by MBE is an effective way to considerably increase the optical gain of QD lasers. Ridge waveguide laser diodes with width of 2.7 μm and 4.5 μm based on various numbers of QD layers ( N =2, 5, 10) were fabricated and studied in this work. Ultra-low threshold current of 1.43 mA was achieved for 2-stack QD. Regime of simultaneous lasing at ground- and excited-states was discovered. This effect was accounted for the finite time of carriers capture to the ground-state in QD. Multi-stack QD structures enabled to maintain continuous work ground-state lasing up to the current density of 10 kA = 100xJ th . Enhanced optical gain allowed us to unite very high differential efficiency (>75%) with low threshold current ( 2 ) and characteristic temperature (T 0 >100K). For example, laser diode of 1-mm cavity length has shown single mode output power of 100mW at operating current of 195 mA and at high operation power demonstrated insensibility to the changes of temperature. The combination of parameters achieved is quite competitive to all technologies currently used for 1.3-micron lasers including traditional InP-based lasers and makes QD gain medium very promising for VCSEL and telecom laser applications.

Electroluminescent study of heterolasers based on vertically coupled self-assembled quantum dots has been done. Luminescent parameters were measured in the 77 ÷ 300 K temperature range. Lasing via ground state of quantum dots up to room temperature has been shown. Temperature independence of the electoluminescent peak position, which corresponds to the second excited state in quantum dots, has been explained.