Direct formation of vertically coupled quantum dots in Stranski-Krastanow growth

Shchukin V.A., Ledentsov N.N., Grundmann M., Kop'ev P.S., Bimberg D.

The energy of an array of 3D coherent strained islands on a lattice-mismatched substrate equals : E = Δ E V EL + Δ E FACETS RENORM + Δ EL EDGES + E EDGEs + E INTER , where ΔE V EL is the volume elastic relaxation energy, ΔE FACETS RENORM is the change of the surface energy of the system due to the formation of islands, which includes the strain-induced renormalization of the surface energy of the island facets and of the planar surface, ΔE EL EDGES is the contribution of the island edges to the elastic relaxation energy, E EDGE S is the short-range energy of the edges, and E INTER is the energy of the elastic interaction between islands via the substrate. The energy Δ E EL EDGES -L -2 . In L always has a minimum as a function of the size of the islands L, and the total energy E = E(L) may have a minimum at an optimum size L opt . E INTER is the driving force for the lateral ordering of 3D islands. Among different arrays of islands on the (001) - surface of a cubic crystal, the total energy is minimum for the periodic square lattice with primitive lattice vectors along the soft directions [100] and [010]. Thus, a periodic square lattice of equal-shaped and equal-sized 3D islands is, under certain conditions, the stable array of islands which do not undergo ripening. The theory explains the spontaneous formation of ordered arrays of 3D islands in the InAs/GaAs(001) system.

Tersoff J., Teichert C., Lagally M.G.

We investigate the growth of multilayer arrays of coherently strained islands, which may serve as ``quantum dots'' in electronic devices. A simple model reproduces the observed vertical correlation between islands in successive layers. However, the arrangement of islands is not simply repeated from layer to layer. Instead, the island size and spacing grow progressively more uniform. In effect, the structure ``self-organizes'' into a more regular three-dimensional arrangement, providing a possible route to obtain the size uniformity needed for electronic applications of quantum dot arrays.

Bimberg D., Ledentsov N.N., Grundmann M., Kirstaedter N., Schmidt O.G., Mao M.H., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kopev P.S., Alferov Z.I., Ruvimov S.S., Gösele U., Heydenreich J.

Injection lasers based on InAs-GaAs and InGaAs-GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 A are realized. The structures with relatively small dots (80 A) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 A cm -2 ) and ultrahigh characteristic temperatures (To = 350 to 425 K). For temperatures of operation above 100 to 130 K T 0 decreases and the threshold current density increases (up to 0.95 to 3.3 kA cm -2 at room temperature) due to carrier evaporation front QPs. Larger InAs QPs (140 A) providing better carrier localization exhibit saturation of the ground state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size (80 to 140 A) being close to 1.8 to 2 ns. respectively. A significant decrease in radiative lifetime is realized in vertically-coupled quantum clots formed by a QP shape-transformation effect. The final arrangement represents a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. The first injection lasing in such an array is achieved.

Solomon G.S., Trezza J.A., Marshall A.F., Harris, Jr. J.S.

Multilayer, vertically coupled, quantum dot structures are investigated using layers composed of InAs islands grown by molecular beam epitaxy in the Stranski-Krastanov growth mode. Single, 2, 5, and 10 InAs island layers are investigated in which the 40 \AA{} high InAs islands are separated by 56 \AA{} GaAs spacer layers. The InAs islands are vertically aligned in columns and are pseudomorphic. Between 1 and 10 layers of islands, 8 K photoluminescence shows a 25% reduction in PL linewidth, and a peak shift of 92 meV to lower energy, while transmission electron and atomic force microscopy show the island size in different layers remains constant. These effects are attributed to electronic coupling between islands in the columns, and a simple coupling model is used to simultaneously fit the spectral peak position shift and the linewidth changes.

Ledentsov N.N., Grundmann M., Kirstaedter N., Schmidt O., Heitz R., Böhrer J., Bimberg D., Ustinov V.M., Shchukin V.A., Egorov A.Y., Zhukov A.E., Zaitsev S., Kop'ev P.S., Alferov Z.I., Ruvimov S.S., et. al.

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.

Pfeffer P., Zawadzki W.

The spin splitting of conduction subbands in GaAs-${\mathrm{Ga}}_{0.7}$${\mathrm{Al}}_{0.3}$As heterostructures is calculated using a five-level k\ensuremath{\cdot}p model and taking fully into account both bulk and structural inversion asymmetry of the system. The role of the boundary conditions for the structural inversion asymmetry is emphasized and it is shown (in contradiction to previous work) that this mechanism is of decisive importance for the spin splitting. Our theory is in agreement with the recent Raman data.

Shchukin V.A., Ledentsov N.N., Kop'ev P.S., Bimberg D.

The energetics of an array of three-dimensional coherent strained islands on a lattice-mismatched substrate is studied. The contribution of the edges of islands to the elastic relaxation energy always has a minimum as a function of the size of an island $L$, and the total energy $E(L)$ may have a minimum at an optimum size ${L}_{\mathrm{opt}}$. Among different arrays of islands on the (001) surface of a cubic crystal, the total energy is minimum for the 2D periodic square lattice with primitive lattice vectors along ``soft'' directions [100] and [010]. This is a stable array of islands which do not undergo ripening.

Grundmann M., Stier O., Bimberg D.

The strain distribution in and around pyramidal InAs/GaAs quantum dots (QD's) on a thin wetting layer fabricated recently with molecular-beam epitaxy, is simulated numerically. For comparison analytical solutions for the strain distribution in and around a pseudomorphic slab, cylinder, and sphere are given for isotropic materials, representing a guideline for the understanding of strain distribution in two-, one-, and zero-dimensional pseudomorphic nanostructures. For the pyramidal dots we find that the hydrostatic strain is mostly confined in the QD; in contrast part of the anisotropic strain is transferred from the QD into the barrier. The optical-phonon energies in the QD are estimated and agree perfectly with recent experimental findings. From the variation of the strain tensor the local band-gap modification is calculated. Piezoelectric effects are additionally taken into account. The three-dimensional effective-mass single-particle Schr\"odinger equation is solved for electrons and holes using the realistic confinement potentials. Since the QD's are in the strong confinement regime, the Coulomb interaction can be treated as a perturbation. The thus obtained electronic structure agrees with luminescence data. Additionally AlAs barriers are considered.

Xie Q., Madhukar A., Chen P., Kobayashi N.P.

Coherent InAs islands separated by GaAs spacer layers are shown to exhibit self-organized growth along the vertical (i.e., growth) direction. The driving force for such vertically self-organized growth is shown to be the interacting strain fields induced by the islands which give rise to a preferred direction for In migration. A model analysis accounting for the mechanochemical surface diffusion gives an island average size and average separation dependent characteristic spacer layer thickness ${z}_{0}$ below which a vertically self-organized growth occurs.

Shchukin V.A., Borovkov A.I., Ledentsov N.N., Kop’ev P.S.

Grundmann M., Christen J., Ledentsov N.N., Böhrer J., Bimberg D., Ruvimov, ‡ S.S., Werner P., Richter U., Gösele U., Heydenreich J., Ustinov V.M., Egorov A.Y., Zhukov A.E., Kop'ev P.S., et. al.

We report ultranarrow $(<0.15\mathrm{meV})$ 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.

Ruvimov S., Werner P., Scheerschmidt K., Gösele U., Heydenreich J., Richter U., Ledentsov N.N., Grundmann M., Bimberg D., Ustinov V.M., Egorov A.Y., Kop’ev P.S., Alferov Z.I.

Morphology evolution of molecular-beam-epitaxy-grown InAs and ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As layers as a function of deposition thickness, range from 1 to 10 ML, is studied by transmission-electron microscopy to characterize the formation and the self-organization of pseudomorphic quantum dots. For deposition (at 450--480 \ifmmode^\circ\else\textdegree\fi{}C) of 3--7 ML of InAs and 5--10 ML of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As, respectively, well-developed and crystallographically perfect dots with typical base length of 12 nm and small size dispersion form, which exhibit short-range ordering on a primitive two-dimensional square lattice along 〈100〉. The luminescence from all samples with coherent dots exhibits high quantum efficiency. For 4-ML InAs dots, coincidence of luminescence and absorption is demonstrated. Arrangement of ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As dots in chains along [110] is the result of ordering during deposition at even lower temperatures (\ensuremath{\sim}320 \ifmmode^\circ\else\textdegree\fi{}C).

Shchukin V.A., Borovkov A.I., Ledentsov N.N., Bimberg D.

The theory of thermodynamic faceting is developed for an epitaxial film grown coherently on a lattice-mismatched substrate. The situation is considered where the planar top surface of the epitaxial film in the absence of the lattice mismatch (\ensuremath{\Delta}a=0) is unstable against faceting, and the stable state of the surface is a periodic array of facets. It is shown that, for a finite lattice mismatch (\ensuremath{\Delta}a\ensuremath{\ne}0), the continuous epitaxial film with a periodically faceted top surface is a metastable state of the heterophase system. The global energy minimum corresponds then to a periodic system of coherent strained islands. If attaining the global energy minimum is kinetically forbidden, the metastable continuous epitaxial film with a periodically faceted top surface is formed. In the case where the period of the faceted structure without external stress ${\mathit{L}}_{0}$ exceeds the order of \ensuremath{\approxeq}50 \AA{}, the dependence of the period L on the lattice mismatch is determined by the linear theory of elasticity. The period L of the metastable faceted structure increases with \ensuremath{\Vert}\ensuremath{\Delta}a\ensuremath{\Vert} for both tensile and compressive mismatch-induced strain. The dependence of L on \ensuremath{\Delta}a gives a possibility of controlling the period of faceting by varying \ensuremath{\Delta}a. If the lattice mismatch exceeds a certain critical value [\ensuremath{\Vert}\ensuremath{\Delta}a\ensuremath{\Vert}>(\ensuremath{\Delta}a${)}_{\mathit{c}}$], the breakdown of formation of metastable faceted structures occurs; the metastable state disappears, and the surface shape is governed by kinetic mechanism.In the case where the period of the faceted structure without external stress is ${\mathit{L}}_{0}$\ensuremath{\lesssim}50 \AA{}, the dependence of L on \ensuremath{\Delta}a is determined by nonlinear elastic effects. The period L increases for one sign of \ensuremath{\Delta}a up to the breakdown of formation of metastable faceted structures and decreases for the other sign of \ensuremath{\Delta}a, where the macroscopic faceting transforms gradually into a microscopic surface reconstruction and the surface becomes apparently flat. The typical critical value of the lattice mismatch for nanometer-scale faceting varies from (\ensuremath{\Delta}a/a${)}_{\mathit{c}}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}4}$ for L\ensuremath{\sim}${10}^{3}$ \AA{} to (\ensuremath{\Delta}a/a${)}_{\mathit{c}}$\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}2}$ \AA{} for L\ensuremath{\sim}10 \AA{}. A similar dependence of faceting on externally applied stress occurs for a loaded sample.

Xie Q., Chen P., Madhukar A.

The impact of the strain fields associated with partially strain relaxed InAs islands on GaAs (100) on the evolution of the growth front profile during subsequent GaAs capping layer growth as a function of the growth temperature is examined via placement of very thin AlGaAs marker layers. Transmission electron microscope studies reveal the presence of strain dominated atom migration away from the islands over dynamically evolving length scales of ∼100–400 Å at higher growth temperature whereas at lower growth temperature such an effect is minimal. Anisotropy in the length scale of impact between the [011] and [011̄] directions is observed. Estimates based upon a suitably adapted formulation of the classical theory of grain growth shows the mass transport to be dominantly strain rather than surface curvature driven.

Bressler-Hill V., Lorke A., Varma S., Petroff P.M., Pond K., Weinberg W.H.

Zhao S.

In 2020, the laser, an acronym for “Light Amplification by Stimulated Emission of Radiation” celebrated its 60th anniversary. The concept of stimulated emission, which forms the basis of lasers, was initially proposed by Albert Einstein in 1917 [1]. Einstein’s theory laid the foundation for understanding how electromagnetic fields could be amplified through population inversion. Rudolf Ladenburg’s work in 1928 [2] provided indirect evidence of stimulated emission, although, at the time, physicists referred to this effect as “negative absorption.” In 1951, Charles H. Townes took a significant conceptual leap by proposing that stimulated emission at microwave frequencies could lead to oscillations within a resonant cavity, ultimately producing coherent output. Then, in 1954, Charles H. Townes and his student James P. Gordon demonstrated the first microwave maser.

Grillot F., Duan J., Dong B., Huang H.

Fu W.Y., Choi H.W.

The minimalistic design of InGaN-based MQW microdisk lasers based on whispering gallery mode (WGM) resonances has been attracting research interests in recent years. To compete with the prevalent InGaN-based VCSELs and edge-emitters, microdisk lasers must demonstrate superior performance under electrical injection. Yet, the challenges in the shift from initial optically pumped investigations to studies centered on electrically injected microdisk lasers has posed a barrier to successful commercialization.

Kim H., Lee S., Ko Y., Ahn J.T., Kim K., Kim D., Geum D., Han W.S.

InAs/GaAs quantum dot laser diodes (QDLDs) on a GaAs substrate grown by utilizing all-metalorganic chemical vapor deposition (MOCVD) technology with a p-AlGaAs cladding layer are systematically investigated to produce 75 oC continuous-wave (CW) operation for the first time. First, the InAs quantum dots (QDs) in a dot-in-a-well (DWELL) structure formed by engineering the strained bottom InGaAs layer are successfully fabricated without detectable clusters, making it possible to increase the number of DWELL stacks effectively. To avoid degradation of the DWELL active layer during the high-temperature growth of the upper p-AlGaAs cladding layer, a detailed analysis of the p-AlGaAs cladding layer grown at low temperature is then carried out. The results of electron-channeling contrast imaging reveal that dislocations in the p-cladding layer are generated due to the accumulative strain of the DWELL and low-temperature growth. The fabricated InAs/GaAs QDLDs showed good electrical and optical characteristics with O-band emission wavelength without high-reflection coating, indicating that the suggested growth technologies and the fabricated devices are promising options for future Si-photonic light source components.

Heindel T., Kim J., Gregersen N., Rastelli A., Reitzenstein S.

The generation, manipulation, storage, and detection of single photons play a central role in emerging photonic quantum information technology. Individual photons serve as flying qubits and transmit the relevant quantum information at high speed and with low losses, for example between individual nodes of quantum networks. Due to the laws of quantum mechanics, the associated quantum communication is fundamentally tap-proof, which explains the enormous interest in this modern information technology. On the other hand, stationary qubits or photonic states in quantum computers can potentially lead to enormous increases in performance through parallel data processing, to outperform classical computers in specific tasks when quantum advantage is achieved. In this review, we discuss in depth the great potential of semiconductor quantum dots in photonic quantum information technology. In this context, quantum dots form a key resource for the implementation of quantum communication networks and photonic quantum computers, because they can generate single photons on demand. Moreover, these solid-state quantum emitters are compatible with the mature semiconductor technology, so that they can be integrated comparatively easily into nanophotonic structures such as resonators and waveguide systems, which form the basis for quantum light sources and integrated photonic quantum circuits. After a thematic introduction, we present modern numerical methods and theoretical approaches to device design and the physical description of quantum dot devices. We then introduce modern methods and technical solutions for the epitaxial growth and for the deterministic nanoprocessing of quantum devices based on semiconductor quantum dots. Furthermore, we highlight the most promising device concepts for quantum light sources and photonic quantum circuits that include single quantum dots as active elements and discuss applications of these novel devices in photonic quantum information technology. We close with an overview of open issues and an outlook on future developments.

Cola A., Leo G., Convertino A., Persano A., Quaranta F., Currie M., Nabet B.

Babichev A.V., Komarov S.D., Tkach Y.S., Nevedomskiy V.N., Blokhin S.A., Kryzhanovskaya N.V., Gladyshev A.G., Karachinsky L.Y., Novikov I.I.

The results of studying the optical properties of InGaAs quantum dots are presented. Single-layer InGaAs quantum dots with a height of 5.3, 3.6 and 2.6 monolayers, as well as three-stacked layers of tunnel-uncoupled quantum dots with a height of 2.6 monolayers were formed by molecular-beam epitaxy according to the Stransky–Krastanov mechanism on GaAs substrates, using the partial capping and annealing technique. A decrease in the size of quantum dots makes it possible to carry out a blueshift of the photoluminescence spectrum maximum from 1200 to 1090 nm, and an increase in the number of QD layers makes it possible to compensate for the decrease in the peak intensity. It is shown that this type of quantum dots is suitable for creating the lasers active regions with a vertical microcavity for neuromorphic computing.

Sargsian T.A., Mantashyan P.A., Hayrapetyan D.B.

Vertically coupled quantum dots are one of the most interesting and promising structures, which find application in a wide range of fields. Particularly, they are noteworthy candidates as a source of single photon and entangled photon pair sources, as qubits and quantum gates in quantum computation, etc. Various external perturbations can act as an instrument for the manipulation of the properties of these structures and an intense laser field is a great example of this. In this work, vertically coupled cylindrical quantum dots made of InAs in a GaAs matrix with a modified Pöschl–Teller potential have been considered under intense laser fields with two laser beam profiles: Gaussian and Bessel. The energy levels and wave functions have been obtained for this structure. Linear and third-order nonlinear absorption coefficients, refractive index changes and second and third harmonic generation susceptibilities have been investigated at different temperatures and for different values of the laser parameters.

Jia H., Yang J., Tang M., Li W., Jurczak P., Yu X., Zhou T., Park J., Li K., Deng H., Yu X., Li A., Chen S., Seeds A., Liu H.

Abstract In this work, we investigate the epitaxial growth of InAs quantum dots (QDs) on Ge substrates. By varying the growth parameters of growth temperature, deposition thickness and the growth rate of InAs, high density (1.2 × 1011 cm−2) self-assembled InAs QDs were successfully epitaxially grown on Ge substrates by solid-source molecular beam epitaxy and capped by Ge layers. Pyramid- and polyhedral-shaped InAs QDs embedded in Ge matrices were revealed, which are distinct from the lens- or truncated pyramid-shaped dots in InAs/GaAs or InAs/Si systems. Moreover, with a 200 nm Ge capping layer, one-third of the embedded QDs are found with elliptical and hexagonal nanovoids with sizes of 7–9 nm, which, to the best of our knowledge, is observed for the first time for InAs QDs embedded in a Ge matrix. These results provide a new possibility of integrating InAs QD devices on group-IV platforms for Si photonics.

Holewa P., Gawełczyk M., Maryński A., Ryczko K., Liverini V., Beck M., Faist J., Sęk G., Syperek M.

Lasers, light-emitting diodes, and other optoelectronic devices employing $\mathrm{In}\mathrm{As}$/$\mathrm{In}\mathrm{P}$ quantum dots (QDs) instead of quantum wells (QWs) as their active parts benefit from the quasi-zero-dimensional (0D) density of states while maintaining the emission at the communication-relevant range of $1.55\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. However, for certain application purposes, the substitution of QWs with QDs is advantageous only if QDs can either be treated as isolated objects or exhibit quantum-mechanical coupling between deeply confined states. Here, we compare two material systems of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ and $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$/$\mathrm{In}\mathrm{P}$ elongated QDs (quantum dashes, QDashes) of comparable surface densities and interdash distances. We investigate the presence and type of coupling between QDashes, focusing on the direct tunnel coupling for both types of carriers manifested already at low temperature. In the time-resolved photoluminescence (PL) experiment, we observe a significant dispersion of the PL decay time for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes, resulting from the migration of carriers from high- to low-energy QDashes due to substantial tunnel coupling. In the case of $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes, the dispersionless time dependence points toward the absence of such coupling. We confirm this interpretation with multiband $\mathbit{k}\ensuremath{\cdot}\mathbit{p}$ calculations. We check that the proposed coupling scenario is possible and show its much higher probability for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ than for $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDashes. Finally, in temperature-dependent PL, we observe the redistribution of holes among QDashes via thermal excitation to wetting-layer states for both systems, constituting an additional interdash coupling channel at elevated temperatures. Our results indicate that the $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$)$\mathrm{As}$ QDash system is preferential for optoelectronic applications where QD isolation is highly sought after, whereas $\mathrm{In}\mathrm{As}$/($\mathrm{In}$,$\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ QDashes exhibit quantum-mechanical coupling between deeply confined states.

Yao Z., Jiang C., Wang X., Chen H., Wang H., Qin L., Zhang Z.

Owing to their high integration and functionality, nanometer-scale optoelectronic devices based on III-V semiconductor materials are emerging as an enabling technology for fiber-optic communication applications. Semiconductor quantum dots (QDs) with the three-dimensional carrier confinement offer potential advantages to such optoelectronic devices in terms of high modulation bandwidth, low threshold current density, temperature insensitivity, reduced saturation fluence, and wavelength flexibility. In this paper, we review the development of the molecular beam epitaxial (MBE) growth methods, material properties, and device characteristics of semiconductor QDs. Two kinds of III-V QD-based lasers for optical communication are summarized: one is the active electrical pumped lasers, such as the Fabry–Perot lasers, the distributed feedback lasers, and the vertical cavity surface emitting lasers, and the other is the passive lasers and the instance of the semiconductor saturable absorber mirrors mode-locked lasers. By analyzing the pros and cons of the different QD lasers by their structures, mechanisms, and performance, the challenges that arise when using these devices for the applications of fiber-optic communication have been presented.

Sala E.M., Godsland M., Na Y.I., Trapalis A., Heffernan J.

Abstract InAs quantum dots (QDs) are grown on an In0.53Ga0.47As interlayer and embedded in an InP(100) matrix. They are fabricated via droplet epitaxy (DE) in a metal organic vapor phase epitaxy (MOVPE) reactor. Formation of metallic indium droplets on the In0.53Ga0.47As lattice-matched layer and their crystallization into QDs is demonstrated for the first time in MOVPE. The presence of the In0.53Ga0.47As layer prevents the formation of an unintentional non-stoichiometric 2D layer underneath and around the QDs, via suppression of the As-P exchange. The In0.53Ga0.47As layer affects the surface diffusion leading to a modified droplet crystallization process, where unexpectedly the size of the resulting QDs is found to be inversely proportional to the indium supply. Bright single dot emission is detected via micro-photoluminescence at low temperature, ranging from 1440 to 1600 nm, covering the technologically relevant telecom C-band. Transmission electron microscopy investigations reveal buried quantum dots with truncated pyramid shape without defects or dislocations.

Park Y., Roh J., Diroll B.T., Schaller R.D., Klimov V.I.

Semiconductor nanocrystals represent a promising class of solution-processable optical-gain media that can be manipulated via inexpensive, easily scalable colloidal techniques. Due to their extremely small sizes (typically <10 nm), their properties can be directly controlled via effects of quantum confinement; therefore, they are often termed colloidal quantum dots (CQDs). In addition to size-tunable emission wavelengths, CQDs offer other benefits for lasing applications, including low optical-gain thresholds and high temperature stability of lasing characteristics. Recent progress in understanding and practical control of processes impeding light amplification in CQDs has resulted in several breakthroughs, including the demonstration of optically pumped continuous-wave lasing, the realization of optical gain with direct current electrical injection and the development of dual-function electroluminescent devices that also operate as optically pumped lasers. The purpose of this Review is to assess the status of the field of CQD lasing and discuss the existing challenges and opportunities. A particular focus is on approaches for suppressing nonradiative Auger recombination, novel optical-gain concepts enabled by strong exciton–exciton interactions and controlled CQD charging, effects of nanocrystal form factors on light amplification and practical architectures for realizing electrically pumped CQD lasers. This overview suggests that the accumulated knowledge, along with the approaches developed for manipulating the optical-gain properties of colloidal nanostructures, perfectly position the CQD field for successfully addressing a long-standing challenge: the realization of CQD-based laser diodes. Colloidal quantum dots are promising materials for realizing versatile, wavelength-tunable, solution-processed lasers. This Review surveys recent advances in colloidal quantum dot lasing, provides an in-depth analysis of outstanding challenges and discusses a path forward to implementing technologically viable lasing devices.

Norman J.C., Mirin R.P., Bowers J.E.

We describe the initial efforts to use molecular beam epitaxy to grow InAs quantum dots on GaAs via the Stranski–Krastanov transition and then discuss the initial efforts to use these quantum dots to demonstrate quantum dot lasers. We discuss the developments in quantum dot lasers over the past 20 years and the future prospects for these lasers for scientific and commercial applications.

Ruiz-Marín N., Reyes D.F., Braza V., Flores S., Stanojević L., Gonzalo A., Utrilla A.D., Ulloa J.M., Ben T., González D.

The implementation of GaAs0.8Sb0.2 as CL to obtain type-II strain-coupled InAs MQD structures has been examined and compared to similar structures without Sb or without strain coupling. First, it has been demonstrated that capping with GaAsSb prevents the formation of In-rich agglomerations that hampered the QD formation as it has been observed in the sample without Sb. Instead, it promotes the vertical alignment (VA) of almost all QDs with a high density of QD columns. Second, there is a preferential Sb accumulation over the dots together with an undulation of the growth front, contrary to the observed in the uncoupled structure. In case of a deficient covering of GaAsSb, as occurs for giant QDs, In-rich agglomerations may develop. Each VAQD column consists of a sequence of alternating quantum blocks of pyramid-shaped In(Ga)As separated by GaAsSb blocks that rest over them. These Sb-rich blocks are not homogeneous accumulating around the pyramidal apex like a collar. Between the columns, there is an impoverishment of In and Sb compared to the uncoupled sample. These columns can behave as self-aligned nanowires with type II band alignment between self-assembled InAs and GaAsSb quantum blocks that opens new opportunities for novel devices.