Quantum dot laser

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Semicond. Sci. Technol. 26 (2011) 014001 (8pp) doi:10.1088/0268-1242/26/1/014001

Quantum dot laserN N Ledentsov1,2,3

1 A. F. Ioffe Physical-Technical Institute, Politekhnicheskaya 26, RU-194021, St Petersburg, Russia2 St Petersburg Physics and Technology Centre for Research & Education of the Russian Academy ofSciences, Khlopina str. 8/3, RU-195220 St Petersburg, Russia3 VI Systems GmbH, Hardenbergstrasse 7, D-10623 Berlin, Germany

E-mail: Nikolay.Ledentsov@v-i-systems.com

Received 3 June 2010, in final form 4 June 2010Published 15 November 2010Online at stacks.iop.org/SST/26/014001

AbstractDiscovery of self-organized epitaxial quantum dots (QDs) resulted in multiple breakthroughsin the field of physics of zero-dimensional heterostructures and allowed the advancement ofoptoelectronic devices, most remarkably, lasers. The most advanced and well-understoodresults are obtained for lasers based on Stranski–Krastanow InGaAs–GaAs three-dimensionalQDs; even significant progress in the understanding of basic lasing properties is also achievedfor QDs made of II–VI materials and ‘native’ QDs formed by nanoscale alloy phase separationin the InGaN–AlGaN material system.

Introduction

Quantum dot (QD) heterostructures with size quantization ofcharged carriers in all three dimensions suitable for advancedresearch and applications were developed significantly laterthan layered quantum well (QW) heterostructures. The latterrepresented essentially the mainstream double heterostructure(DHS) concept [1] complemented by the ultimate reductionof the thickness of a narrow bandgap layer. Neverthelesssome trends in the evolution of both types of size-quantizedstructures are similar. The two-dimensional layer structureswere initially fabricated in non-coherent heterogeneoussystems, such as ultrathin layers of metals or semi-metalson glass substrates [2]. In non-coherent systems eachlayer structure constituting the solid-state phase or materialhas its own lattice parameter and/or crystal orientation.Thus the crystal planes of the constituting materials (orphases of the same material) do not match. Consequentlya lot of defects originate at the interface which hinderthe realization of the intrinsic electric, optical, vibrational,etc properties that could be expected for ideally lattice-matched (or ‘coherent’) heterojunctions. In spite of someprogress in the demonstration of the modifications of electronicand optical properties [2], no serious proofs of strongadvantages for device applications were presented. Thesituation changed remarkably only when coherent singlecrystalline semiconductor heterostructures were applied toQW fabrication [1] and particularly when the advancedtechniques of epitaxial growth such as metal-organic chemical

vapor phase epitaxy (MOVPE) and molecular beam epitaxy(MBE) were developed allowing a precise control of interfacesat a single monolayer (ML) level. Multiple applicationsinvolving two-dimensional heterostructures (exhibiting sizequantization in one direction) arose, including, for examplehigh electron mobility transistors, QW lasers and light-emitting diodes, QW infrared photodetectors (QWIPs),cascade lasers and other devices.

In heterostructure lasers the idea of ‘exploiting quantumeffects in heterostructure semiconductor lasers to producewavelength tunability’ and achieving ‘lower lasing thresholds’via ‘the change in the density of states which results fromreducing the number of translational degrees of freedom ofthe carriers’ was originally introduced by Dingle and Henryin 1976 [3]. The main advantage of using size-quantizedheterostructures in lasers was expected to originate from theincreased density of states for charge carriers near the edgesof the conduction and valence sub-bands. For the activemedium of a laser this results in the concentration of themost of the injected non-equilibrium carriers in an increasinglynarrow energy range near the bottom of the conduction bandand/or the top of the valence band once the dimensionalityis reduced. The concentration of the joint density of statesfor non-equilibrium carriers enhances the maximum materialgain assuming the same homogeneous (or inhomogeneous)broadening. For structures with size quantization in morethan one direction, a singularity in the density of states occurs(see figure 5 in [3]) and the above positive effects should bedramatically enhanced as compared to that in the QW lasercase.

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Semicond. Sci. Technol. 26 (2011) 014001 N N Ledentsov

In 1982, a prediction of temperature-insensitive thresholdcurrent of a QD laser was made [4]. A number of negativepredictions were also provided. In 1988, Vahala introduced theinhomogeneous size distribution of QDs into consideration [5].He concluded: ‘for high gain operation a medium composedof quantum boxes does not offer significant advantagesover a conventional bulk semiconductor unless quantum boxfabrication tolerances are tightly controlled’. Benisty et al[6] proposed that the lack of matching energies for phononrelaxation of carriers in QDs (‘the phonon bottleneck effect ina QD’) will inhibit lasing in QD structures.

At that stage the discussions on different opinionshad predominantly a scholastic nature as no benchmarkingexperiment could be performed.

The first QD-like structures suitable for research and firstapplications were synthesized in incoherent heterogeneoussystems, such as semiconductor nanocrystallites in glassmatrices [7], similar to the case of the first size-quantizedlayer structures. These nanostructures certainly provided a lotof important information and stimulated remarkable progressparticularly in the field of theory of the optical properties ofQDs. Nevertheless, the electronic and optical properties ofnanocrystallites were predominantly defined by the interfacestates at the semiconductor–glass interface making evaluationof their intrinsic properties difficult. As also in the caseof QW structures the real progress in QDs was made aftercoherent three-dimensional epitaxial nanoheterostructureswere synthesized and the modern techniques of epitaxialgrowth were applied (MBE, MOVPE).

1. Growth of three-dimensional islands

The formation of three-dimensional islands at crystal surfaceshas long been known in lattice-mismatched epitaxial systems[8–10] representing a quite general phenomenon (Stranski–Krastanow, Volmer–Weber growth mechanisms). In mostof the cases it was believed that the islands are at leastpredominantly dislocated [11], and thus should be avoided.In some publications it was concluded that the islands mayhave a coherent nature [8, 9] particularly in the case when thedeposits are formed by only a few MLs. InAs nanoislands inGaAs was the most extensively studied material combination.The impact of the deposit thickness on defect formation [9]and the role of the underlying stressors on vertically correlatedgrowth of the strained islands [8] was studied.

Photoluminescence (PL) was observed in samples withGaAs-overgrown InAs islands [8]. However no proof of thedirect relation of this luminescence to the appearance of size-quantized zero-dimensional states could be provided at thattime. Other groups claimed that such a luminescence is alwaysobserved in structures with 3D InAs and InGaAs islands andthe ‘energy position is essentially identical to the Ga vacancyin GaAs’ irrespective of the deposition parameters, and theluminescence should be attributed to ‘radiative recombinationvia dislocations, probably those 60◦ dislocations formedduring overgrowth’ [12].

In the early 1990s a new wave of interest toward allepitaxial nanostructures arose. Growth on vicinal, misoriented

Figure 1. Plan-view transmission electron microscopy image ofInAs three-dimensional (3D) islands in a GaAs matrix formed by1.2 nm InAs deposition according to [24]. The marker represents50 nm.

or nanofaceted [13, 14] surfaces was explored. Nanoscalestructures formed by deposition strained submonolayer [15,16] or above ML deposits [17, 18] were investigated. Theresearch indicated that in some cases the nanostructuresformed have a relatively uniform size and shape distribution[19]. The reason for the ordering was attributed to the surfacestress relaxation effects, as was previously understood for thecase of ordered domains of coexisting surface reconstructions.Once a facet edge or a step bunch or a phase boundary betweenthe adsorbate (or surface reconstruction) phases appears on thesurface, the higher surface stress phase may partially relax atthe boundary making it energetically favorable [20–22]. Itwas also understood for the faceted surfaces that if the surfaceenergy of the tilted facets normalized to the average orientationof the crystal surface is lower than the surface energy of theflat surface, surface faceting may take place. An additionalrelief of the surface stress energy at the facet edges makesthe formation of new facets energetically favorable unlessthe contribution from edge-related short-range potential dueto additional dangling bonds becomes significant. Thus theoptimal period of faceting or the optimal size of the surfacedomain structure may exist. In the case of 3D islands, theformation of facets may also result in the surface stress energyrelief. Furthermore, as the strain energy of a 3D island may bestrongly reduced by elastic relaxation, the surface energy mayalso be reduced as compared to that of the strained wettinglayer. Once the surface energy of the 3D island normalizedper unit area of the substrate becomes lower than the surfaceenergy of the wetting layer, no ripening beyond some optimalsize is energetically favorable (see figure 1). It was shown thatthe regime of no ripening of InAs/GaAs dots may be realizedonly in a relatively narrow range of substrate temperaturesand arsenic overpressures [23], which define the necessaryenergy balance. The interaction of the islands through thesubstrate coherently strained in their vicinity results in thelateral ordering of the islands with the symmetry defined bythe Young modulus of the substrate and/or the symmetry ofthe surface reconstructions involved [23]. Depending on the

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lattice mismatch with the substrate, the basis of the islandsmay become square shaped or elongated. Vertical stackingof the islands may cause either vertically correlated or tiltedarrangement [22]. In certain cases the kinetic limitations tothe island growth can also be considered.

2. Optical properties

The effect of the formation of 3D islands revealed inreflection high-energy electron diffraction (RHEED), scanningtunneling microscopy (STM) studies and transmissionelectron microscopy was accompanied by the appearance ofluminescence in the 1.1–1.25 μm spectral range in the PLspectra of the related structures [8, 23–25]. This observationof the PL emission itself did not evidence the formation ofelectronic QDs and could be attributed to certain artifacts likedefects or local macroscopic InGaAs areas of extended size.Furthermore the related broad luminescence features weretypically observed at low temperatures and at low excitationdensities. The association of these PL features with QDs wasthus questioned [12].

A change happened in the second half of 1993 when asheet of 3D InGaAs islands was introduced into a GaAs–AlGaAs DHS grown on top of a thick AlGaAs claddinglayer. The growth regimes were optimized to achieve ahigh luminescence intensity of the characteristic emissionup to room temperature. A relatively low substratetemperature was found to be optimal [26] for the optimalluminescence efficiency at high excitation densities. As aresult photopumped lasing in the edge-emitting geometry(figure 2) at low and at room temperature was realized [26].

The results achieved demonstrated the significance ofInGaAs nanoislands for future fundamental research anddevice applications because the optical properties and thelasing action revealed allowed the exclusion of dislocationsand other defects as possible sources for the observed PLemission. The injection lasing followed soon [24].

3. Evidence of zero-dimensional states inself-organized QDs

The understanding of the basic physical properties of self-organized QDs required several benchmarking experiments tobe performed.

3.1. Resonant absorption and PL in self-organized InGaAsnanoheterostructures

Historically, before self-organized coherent heterostructurenanoislands were fabricated and studied, II–VI semiconductornanocrystallites in a glass matrix were extensivelyinvestigated. This research revealed the following major trend:(i) the PL of the ensembles of nanocrystals was Stokes shiftedwith respect to the excitation wavelength even in the case of theresonant excitation spectrally within the main emission band,similar to the case of Stokes-shifted luminescence of excitonsin QWs; (ii) a ground-state absorption peak was alwaysresolved in the PL excitation spectra as a clearly resolved

feature; (iii) the PL emission of a single semiconductornanocrystallite represented a relatively broad (about or largerthan 10 meV) emission line even at low temperatures. Therelated features of similar spectral width were revealed in hole-burning experiments.

In contrast, the results were remarkably different forheteroepitaxial 3D structures. Early PL excitation (PLE)studies of the structures with InGaAs insertions [25] revealeda very significant energy shift between the PL observationwavelength and the first feature revealed in the PLE spectra.This fact was attributed to the ‘symmetry-forbidden’ ground-state transition in QDs by the authors [25].

Several results of key importance, which evidenced theQD nature of the InGaAs island-related states and explainedthe basic optical properties of heterostructure QDs, werepresented at the International Conference on the Physics ofSemiconductors in Vancouver, Canada, 15–19 August 1994[24]. It was manifested that very dense arrays of nanoislands∼1011 cm−2 can be realized at moderately low (450 ◦C)substrate temperatures as was revealed in the TEM studies(figure 1). The dots formed by 1.2 nm InAs deposit had alateral size of ∼12 nm with a square base and clearly resolvedcrystallographic orientation with axes parallel to the [0 0 1]and [0 1 0] directions. The islands were aligned with a laterallattice with the main axes in the same directions. The cross-sectional TEM study revealed a height of 2.5–3 nm. Thedots formed by 0.6 nm deposition had a much smaller size<10 nm and a significant size dispersion of ∼3 nm, and nolateral arrangement was resolved. Islands formed by 1 nmInGaAs deposition were predominantly round shaped with alateral size of ∼5–6 nm and were aligned along the [0 −1 1]direction. Deposition at temperatures below 320 ◦C seemed tosuppress the formation of separated nanoislands. A nominal1.6 nm In0.5Ga0.5As deposition at 450 ◦C increased the size ofthe islands to 15–17 nm. High temperature deposition (500 ◦C)resulted in an increase in the lateral size of the nanoislands to30 nm.

A strong correlation between the optical and structuralproperties of the nanoislands was found. Dense arrays of InAs(1.2 nm) and In0.5Ga0.5As (1 nm, 320 ◦C) QDs demonstrateda good match between the absorption peak revealed in thecalorimetric absorption studies and the PL peak. In contrast,the dots with a broad size distribution did not show clearabsorption resonances. 0.6 nm InAs nanoislands demonstrateda broad PL peak at 1.37 eV. Well-developed QDs formedby 1.2 nm InAs deposition demonstrated absorption andluminescence features at 1.1 eV at low temperature, and thefeature shifted in accordance with the GaAs bandgap when thetemperature was increased.

3.2. Photoluminescence excitation spectroscopy of quantumdots

It was understood that the resonant excitation of an InGaAsnanoisland in its ground state is possible only with a photonenergy exactly matching the δ-function-like exciton absorptionpeak as expected for an ideal QD [24]. In case there is nofurther energy transfer possible to parasitic interface states,the dot will emit radiatively with exactly the same photon

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Figure 2. Optical properties of a structure with InGaAs nanoislands(‘quantum clusters’ [26]) inserted into a GaAs waveguide layerconfined by AlGaAs cladding layers. PL excitation spectra at 77 K(a) shows a broad feature at 1.32 eV and a feature at 1.42 eV. Athigh excitation densities a broad emission peak is revealed in astructure with islands formed by 1 nm InGaAs deposition at 460 ◦C.The same nominal layer thickness at 430 ◦C did not result in theformation of well-resolved islands, and the PL peak was observed at1.42 eV. Photo-pumped lasing in the direction parallel to the surfaceof the wafer was realized in the structure with the 3D islands at 77 Kand at 300 K at much longer wavelengths as compared to those forthe layer-like distribution of InGaAs (according to [26], submittedDecember 29, 1993).

energy. Thus only the excited states can be revealed in thePLE spectra of the ground-state QD transition. Dots having thesame ground-state energy may have different excited states dueto nonuniformities in the shape and strain. This was proposedto result in broadened PL and PLE peaks (figure 3).

3.3. Ultranarrow luminescence lines

Using spatially resolved spot-focus cathodoluminescence(CL) at low observation temperatures to excite a small fractionof nanoislands it became possible to resolve ultranarrowluminescence lines each originating from single InAs island[24] (figure 4). This was directly confirmed by the intensitydistribution of the CL intensity. The minimum line separationof 0.1 meV corresponded to a change in the island size by oneInAs molecule [24].

3.4. Injection lasing

In the same paper injection lasing has been reported in shallowmesa structures with InGaAs QDs at 95 K and at roomtemperature with threshold current densities of 120 and 950 Acm−2, respectively [24].

As the clear proof of zero-dimensional electronicspectrum was provided and the basic structural and

Figure 3. PL and PLE spectra of InAs nanoisalnds. The maininhomogeneously broadened PL emission peak is resonant to thepeak revealed in the calorimetric absorption spectra (CAS). There isno ground-state feature revealed in the PLE spectra, which shows asignificant intensity only at photon energies corresponding to theinhomogeneously broadened excited state transitions. Phononresonances revealed in the PLE spectra indicated that the excitonrelaxation in QDs occurs faster when the energy difference betweenthe ground and the excited states of the QD matches the longitudinaloptical (LO) phonon energy (according to [24]). However, thebroadening of the phonon resonances by strain, the co-existence ofdifferent kinds of phonons and the contribution of interface acousticphonons makes the fast relaxation possible for all QD excitons in aself-organized InGaAs QD irrespective of the precise energymatching to particular bulk phonons.

(b)(a)

Figure 4. Intensity distribution map in scanned spot-focus CLimaging of a sample with InAs QDs taken for particular wavelengthwithin the CL spectrum (a). The spectrum recorded at the maximumintensity position (A) clearly shows a δ-function-like emission lineoriginating at a single nanoisland (b) [24]. The size of the brightspot in the intensity map image is defined by the diffusion length ofnonequilibrium carriers generated by the electron beam and theelectron–hole pair generation volume.

optical properties of InGaAs–GaAs self-organized 3Dnanoheterostructures became well understood, the nature ofthe QD lasing mechanism was fully manifested.

Thus within only a few months between the autumn of1993 and the summer of 1994, a novel type of heterostructureswas investigated and confirmed to behave as true QDs andresulted in QD lasers. An era of QD lasers started. I mustappreciate a very close cooperation of my colleagues at theA. F. Ioffe Physical-Technical Institute in St Petersburg andthe Institute of Solid State Physics at the Technical Universityof Berlin. I am grateful for the cooperation of scientistswho contributed strongly to the development of the fieldof QD lasers during this period: Dieter Bimberg, Jurgen

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Christen, Marius Grundmann, Niels Kirstaedter and others.No success would be reached without excellent efforts madein the epitaxial growth and characterization of QDs by mycolleagues at the laboratory of Zhores Alferov at the A. F. IoffeInstitute in St Petersburg: Anton Egorov, Mikhail Maximov,Victor Ustinov, Alexey Zhukov and other colleagues. SergeyIvanov and his colleagues contributed a lot to the developmentof type-II GaSb/GaAs QDs [27] and II–VI submonolayer QDsand QD superlattices [28] also at the A. F. Ioffe Institute. Ourcooperation with S Ruvimov, P Werner and U Gosele fromthe Max-Planck Institut fur Mikrostrukturphysik in Halle,Germany, in structural characterization of QDs also playedan extremely important role. The work would not be sosuccessful without stimulating support provided by ZhoresAlferov who immediately recognized the role of self-organizednanoheterostructures since they were first realized.

The investigation of self-organized III–V QDs led to re-examination of the optical properties of II–VI nanocrystallites[29]. It was found that the optical properties of the crystallitesare controlled by the interface states trapping the excitonsand prohibiting them from showing a true QD nature. Thetrapping of excitons of the same nanocrystallite by differentsurface-related bound states causes broadening of the PL line,blinking of the PL emission and hopping of excitons betweenthe different surface (or interface) states. Similar effectswere responsible for the spectral diffusion effects revealed inthe hole-burning experiments performed on nanocrystallites.Later with dot-shell technology, which enabled coherentheteroepitaxial covers isolating the nanocrystallite excitonsfrom the surface, II–VI nanocrystals with quasi-ideal QD-likeproperties have also been demonstrated.

4. QD lasers

After the first breakthrough results on photopumped lasingin self-organized QDs [26] a lot of research has followed.Injection lasing was first observed in shallow mesa structureswith InGaAs QDs at 95 K and at room temperaturewith threshold current densities of 120 and 950 A cm−2,respectively [24]. A high characteristic temperature of thethreshold current density was reported initially up to ∼150 K[30]. The threshold current density remained relatively highat room temperature, and the devices tend to lase via QDexcited states at short and moderate cavity lengths. Anotherachievement came from applying vertically coupled QDs toheterostructure lasers to overcome gain saturation in QDs [31].This resulted in stable ground-state lasing in QDs at very lowthreshold current densities (∼60 A cm−2 [31]) matching theperformance of the best QW devices. Interestingly, verticallycoupled QDs provided an efficient way of developing verticalquantum wire-like structures enabling polarization control inlasers and semiconductor optical amplifiers. In the followingperiod the threshold current density at room temperature inQD lasers was steadily decreased as follows from figure 5 andpresently approaching 10.4 A cm−2 [32] and even 6 A cm−2

when detected by the appearance of stimulated emission. Thedevices operate at the wavelength 1.22 μm delivering 2 W ofpower from a 1.6 cm long cavity with uncoated facets [32]. Thelaser diode’s internal waveguide loss was extracted from cavity

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length measurements to be ∼0.25 cm−1. Shorter-wavelengthlasers based on QDs formed by submonolayer deposits [27] inII–VI and III–V systems were demonstrated to exhibit a highmodal gain due to the very high density of the islands.

Presently the highest continuous wave power from aQD laser reached 16 W continuous wave (CW) operationper chip at room temperature [33]. An extremely reliableand temperature-insensitive operation at 1.3 μm up tohigh temperatures and transmission rates of ∼10 Gb s−1

was achieved [34] using QD laser wafers from NL-Nanosemiconductor GmbH. High-performance high-powerlasers operating at high power well above 1.3 μm werealso demonstrated [35]. In this paper a modal gain over20 cm−1 in the 1315–1345 nm wavelength range was revealedby the Hakki–Paoli technique at a low current density of500 A cm−2. 1.5 μm range QD metamorphic lasers havebeen realized [36] and a good reliability of the devices hasbeen demonstrated [37]. As was underlined in [37] andthe references given therein, a specially developed defectreduction technique must be applied to achieve degradation-free operation of metamorphic QD lasers. This technique isbased on (i) the deposition of a plastically relaxed metamorphicInxGa1−xAs layer on top of the lattice-mismatched substrate,e.g. GaAs or Si; (ii) the growth of an additional layerwith a higher indium content with the local formation ofInAs-enriched regions predominately decorating the regionsaround the plastically relaxed zones in the vicinity of thedislocations; (iii) the deposition of a ML-thick AlAs layerwhich undergoes a repulsive interaction with the decoratedregions due to the larger lattice parameter mismatch with them;(iv) selective evaporation of the uncovered InGaAs regionsby high temperature annealing and (v) further overgrowthwith the InxGa1−xAs layer. In this approach local defectssuch as clusters or dislocation loops can be removed, whilethreading dislocations can be tilted. With a few cycles of defectreduction, most of the threading dislocations can be bent intothe plane parallel to the interface and will not propagate into theactive region of the device. Furthermore in the active region,each sheet of the QDs can be subjected to AlAs depositionand annealing to remove both large dislocated InAs clusters

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and smaller InAs islands which may become dislocated dueto the surface imperfections or statistical fluctuations andimperfections of the deposition process. The technique wassuccessfully applied to different material systems includingGaN–InGaN structures on sapphire substrates.

5. Applications of QD lasers

QD lasers are gradually extending their field of application.Presently QD lasers take about a 100% share of the devicesin the spectral range of ∼1.15–1.25 μm important for medicaland display applications. In this spectral range, InP-basedlasers do not provide good performance at wavelengths below1.3 μm, while the InGaAs QW GaAs-based lasers cannot beapplied at wavelengths above ∼1.15 μm due to the plasticrelaxation of the InGaAs layers.

Most of the commercial success is presently coming fromwide gap InGaN–AlInGaN QD lasers. InGaN QDs originatefrom the spontaneous phase separation effect activated bythe elastic relaxation of the local layer strain originatingfrom statistical compositional fluctuations allowed at thecrystal surface. As the III–V alloys are known to bestrain stabilized by lattice matching to the substrate (incase of layers) or the average lattice parameter (bulk), thepossibility of the elastic strain relaxation at the crystalsurface stimulates the phase separation effect. The relatedcomposition domain structures are observed in many III–Valloys (InGaP, InGaAlSb, InGaAsP, InGaAsN, etc). In thecase of the InGaN layers the InN-rich domains typically havea very small lateral size (∼2–5 nm depending on the depositionconditions) and a disc-like shape. In contrast to the narrowgap III–V materials, a high exciton effective mass and a smallBohr radii in GaN, combined with the giant bandgap offsetbetween GaN and InN binary materials, make the confinementof excitons at small InN nanodomains extremely efficient. Theeffect can be further enhanced by using the activated phaseseparation approach, seeding of QDs and other ideas borrowedfrom the InGaAs–GaAs material system. These approachesare particularly important for further extension of the lasingwavelength toward the true green and yellow spectral ranges.Presently, the leading manufacturers of GaN-based LEDs andlasers acknowledged the importance of QDs in their devices[38–44].

Nichia was the first to state QD lasing in their InGaNlasers [38–41]. Recently, a detailed structural characterizationof Nichia’s QDs was reported [42]. The authors have examinedboth the electrostatic potential and relative atomic density ofIn0.13Ga0.87N 10 nm wide layers containing compositionalfluctuations and electrostatic potential fluctuations, usingelectron holography (EH) and electron tomography (ET). EHhas shown that the potential fluctuations are localized near theGaN interface, are 2–3 nm in size and have negatively chargedcores. The ET 3D density maps of In distribution revealed ‘theexistence of QD-like regions (QDs) of high In% also localizedto within 3 nm of the substrate, 3–5 nm in width and spaced∼8 nm apart’.

Lumileds co-authored a paper on structural characteri-zation of InGaN [43] where real-time x-ray reciprocal space

mapping revealed the development of strain and compositiondistributions during MOCVD of InxGa1−xN on GaN. It wasfound that strong, correlated inhomogeneities of the strain stateand the In fraction x arose during growth in a manner consistentwith the models for instabilities driven by strain relaxation.

The importance of InN-rich nanoscale compositionaldomains revealed in image-processed high-resolutiontransmission electron microscopy was underlined in OSRAMpresentations [44] also in relation to the necessity of abetter control of the nanoscale compositional modulationfor its further advancement in reaching the improved deviceperformance.

QD lasers presently cover a significant part of the near-infrared spectrum and the complete visible range. TheInGaAlP material system appeared to be particularly importantfor QD lasers emitting in red. For QD edge-emitting lasersin this system an about twice-higher power density to reachcatastrophic optical mirror damage was demonstrated ascompared to the otherwise similar QW devices [45]. Thisadvantage may become crucial for laser display applications asfacet degradation is particularly important for InGaAlP lasers.Furthermore, ultralow current density and room temperatureoperation is realized in red vertical-cavity surface-emittinglasers based on InGaAlP QDs [46].

6. Future applications

Ultrahigh material gain and a 3D confinement ofnonequilibrium carriers in QDs are extremely important forhigh-speed devices. At first, the high material gain allowsus to achieve a higher resonant frequency at lower currentdensities, thus enabling faster performance at a lower currentdensity, the property vital for device reliability. Then at thesame current density, QDs can provide a much higher operationlifetime by the suppression of the diffusion of nonequilibriumcarriers, which causes nonradiative recombination-inducedgrowth of defects. High-speed lasers are particularly importantfor optical interconnects. Modern supercomputers already usetens of thousands of vertical-cavity surface-emitting (VSEL)-based optical links. Power 7 IH computing system (IBM),for example, is based on 420 thousands optical modules at10 Gb s−1 each. Next generation supercomputers will bebased on millions of 25 Gb s−1 VCSEL-based links. Exaflop-scale supercomputers (∼2015–2016) are expected to use up to800 million optical links with the share of optics approaching40% in the total system cost (∼USD 0.5B) and in energyconsumption.

In figure 6(a) we show the dependence of the resonantfrequency on the square root of current above the threshold ofQD VCSEL [47]. The electroluminescence spectra are shownin (b).

As follows from figure 6 QD VCSEL approaches resonantfrequencies of ∼17–18 GHz in spite of significant overheatingdue to non-optimal resistance of the device [47]. This resonantfrequency indicates that the transmission bit of ∼40 Gb s−1 canbe realized in QD lasers, while the 3D confinement of carriersmust ensure sufficient reliability even at a high temperature ofthe device of ∼100 ◦C needed for its application in intrasystemlinks.

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Figure 6. The dependence of the resonant frequency on the square root of current above the threshold of QD VCSEL (a) and theelectroluminescence emission spectra of the device at different currents (b). The inset shows a high-resolution cross-sectional transmissionelectron microscopy image of the QDs taken under chemically-sensitive conditions.

7. Conclusions

QD lasers are continuing their way toward future mainstreamapplications. The most successful implementation of QDswill continue in applications where traditional QW technologyeither cannot be applied or can give rise to strong reliabilityproblems. Industrial introduction of QD lasers has begunin medical applications in the 1200–1300 nm spectral range,in UV and visible lasers for Bluray and microdisplayapplications. The next targets may be Raman amplifiers at12XX nm for seamless fiber-to-the-home (FTTH) networks,semiconductor optical amplifiers for 1300 nm range for FTTHand datacom [48], 1300 nm QD GaAs VCSELs for datacommunications, FTTH and radio-over-fiber applications.Very recently 1.3 μm InAs/GaAs QD Fabry–Perot laserssuitable for the 25 Gbps direct modulation have beendemonstrated [49]. Broad and flat gain spectrum can begenerated in QD lasers by stacking of QDs having differentsizes. In combination with pronounced gain saturation effectand reduced cross-talk between the QD-induced states, asimultaneous lasing via a very large number of longitudinalmodes of the Fabry-Perot resonator with uniform intensitydistribution (comb spectrum) can be achieved. Each QD comblaser may provide hundreds of low noise channels, which canbe separately-modulated in intensity, being well suited forapplications in on-chip photonics and communications [50].Mode-locked QD lasers have shown dramatic advantages overQW lasers and may be applied for optical clocking [51], andthus may strongly contribute to the field of silicon photonics.Recent advances in InGaN QD technology may allow us toextend the spectral range of the InGaN lasers toward deepgreen and yellow spectral ranges, and thus extending theapplication field of GaN-based light-emitting devices and thedevelopment of entirely GaN-based white LEDs.

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