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An InP based tunnel injection quantum dot (QD) laser and a reference quantum dot laser designed to emit at 1.55 μ m were grown by molecular beam epitaxy. The lattice matched tunnel injection (TI) structures consist of an InGaAs quantum well (QW), an InAlGaAs barrier and multiple InAs quantum dot layers. Quantum well and quantum dot test structures as well as the actual laser structures were treated with rapid thermal annealing (RTA) at varying temperatures (680–780 °C). Photoluminescence measurements were performed at 10 K on all samples and a strong effect of this annealing process on the emission properties is observed. The laser structures were processed into broad area lasers. Power-current characteristics were measured and evaluated. A strong improvement in device performance is observed after annealing for QD lasers while for TI-QD lasers no significant improvement was obtained, which might be caused by detuning of band energy alignment and different impact of the RTA process on the material quality of QW and QD layers.

This work reports on growth and characterization of self-assembled InxGa1−xAs quantum dots grown on GaAs (1 0 0) substrate designed to emit at 1030 nm. All structures were grown by molecular beam epitaxy and investigated by photoluminescence (PL) spectroscopy and atomic force microscopy. The growth parameters, such as the growth temperature, QD layer thickness and indium content were changed systematically. A strong reduction in PL linewidth from 54 meV down to 26 meV measured at 10 K for a single QD layer was achieved as the result of a more uniform QD size distribution. Additionally, a correlation between the linewidth and the temperature dependent wavelength shift of the PL peak was found.

InP based lattice matched tunnel injection structures consisting of a InGaAs quantum well, InAlGaAs barrier and InAs quantum dots designed to emit at 1.55 μ m were grown by molecular beam epitaxy and investigated by photoluminescence spectroscopy and atomic force microscopy. The strong influence of quantum well and barrier thicknesses on the samples emission properties at low and room temperatures was investigated. The phenomenon of a decreased photoluminescence linewidth of tunnel injection structures compared to a reference InAs quantum dots sample could be explained by the selection of the emitting dots through the tunneling process. Morphological investigations have not revealed any effect of the injector well on the dot formation and their size distribution. The optimum TI structure design could be defined.

Two vertically stacked quantum dots that are electronically coupled, so called quantum dot molecules, are of great interest for the realization of solid state building blocks for quantum communication networks. We present a modified gradient approach to realize InAs quantum dot molecules with a low areal density so that single quantum dot molecules can be optically addressed. The individual quantum dot layers were prepared by solid source molecular beam epitaxy depositing InAs on GaAs(100). The bottom quantum dot layer has been grown without substrate rotation resulting in an In-gradient across the surface, which translated into a density gradient with low quantum dot density in a certain region of the wafer. For the top quantum dot layer, separated from the bottom quantum dot layer by a 6 nm thick GaAs barrier, various InAs amounts were deposited without an In-gradient. In spite of the absence of an In-gradient, a pronounced density gradient is observed for the top quantum dots. Even for an In-amount slightly below the critical thickness for a single dot layer, a density gradient in the top quantum dot layer, which seems to reproduce the density gradient in the bottom layer, is observed. For more or less In, respectively, deviations from this behavior occur. We suggest that the obvious influence of the bottom quantum dot layer on the growth of the top quantum dots is due to the strain field induced by the buried dots.

We have demonstrated that in situ annealing effect has to be taken into account in order to realize the 1.31 μm InAs quantum dot (QD) lasers with the dot-in-well (DWELL) structure. The photoluminescence (PL) properties have been investigated for the InAs DWELL samples annealed at different temperatures in situ , simulating the annealing process during the growth of the top cladding AlGaAs layer in the laser structure. The QDs with large size in the DWELL structure are vulnerable to the annealing process at temperatures above 550 °C, revealed by the drastic change in the PL spectra. However, the DWELL structure is stable during the annealing process at 540 °C for three hours. The thermal stability of the QDs in the DWELL structure has to be considered in the growth of QD lasers for long wavelength operation.

We successfully grow vertically aligned quantum ring-dot structures by Multiple Droplet Epitaxy technique. The growth is achieved by depositing GaAs quantum rings in a first droplet epitaxy process which are subsequently covered by a thin AlGaAs barrier. In a second droplet epitaxy process, Ga droplets preferentially position in the center indentation of the ring as well as attached to the edge of the ring in [ 1 1 ¯ 0 ] direction. By designing the ring geometry, full selectivity for the center position of the ring is achieved where we crystallize the droplets into quantum dots. The geometry of the ring and dot as well as barrier layer can be controlled in separate growth steps. This technique offers great potential for creating complex quantum molecules for novel quantum information technologies.

We fabricate self-aligned vertically stacked GaAs quantum dot molecules (QDMs) by filling of self-assembled nanoholes in AlGaAs. The tunable nanoholes are created using local droplet etching (LDE) combining conventional molecular beam epitaxy with self-assembled, lithography-free patterning. The optical emission from single, strain-free QDMs shows clear excitonic features with linewidths below 150 µeV after optimizations of the fabrication process. This allows investigations of the coupling among the individual dots forming a QDM. In electric fields oriented along the axis of the QDM, luminescence emission from direct and indirect transitions can be clearly distinguished. Furthermore, an anti-crossing behaviour demonstrates inter-dot coupling in the QDM.

InAs/GaAs quantum dot (QD) heterostructures with different covering layers (CLs) prepared by MOVPE are compared in this work. The recombination energy of a structure covered only by GaAs depends nonlinearly on CL thickness. Experimental data of photoluminescence (PL) were supported by theoretical simulations. These simulations prove that the strain plays a major role in the structures. InGaAs strain reducing layer (SRL) was studied as well. Due to the strain reduction, the recombination energy is decreased, so the structure has longer PL wavelength. By theoretical simulations it was shown that for high content of In in InGaAs covering layer (approximately 45% and more), the heterostructure is type II, which would normally be unreachable for flat layers. For the structure with GaAsSb SRL, the band alignment is highly dependent on the SRL composition. The type I/type II transition occurs for approximately 15% of Sb; this value also slightly depends on the QD size. All structures were also studied by HRTEM to show different behavior of the CLs on the interface with InAs which highly influences the structure quality.

Quantum dot molecules refer to nanostructures consisting of four quantum dots around a central pit. These are observed to form in Si-Ge heteroepitaxy and are considered as an alternative to quantum dots for strain relaxation. Here we use numerical simulations of a continuum model of strained heteroepitaxy to show the spontaneous formation of quantum dot molecules in thick Si 0.5 Ge 0.5 films grown on pit–patterned Si substrates. The route to their formation first involves a quantum dot in the pit, followed by removal of material from the pit to form quantum dot molecules. These molecules are formed for smaller pit widths, whereas larger pit widths tend to create a ridged surface. A mature single–walled quantum dot molecule has a typical size of about 220 nm. On reducing the wavelength of the original pit–pattern, we notice that multiple–walled quantum dot molecules form, due to the interactions with other nearby molecules. Our results broadly agree with experiment and suggest that QDM formation on patterned substrates does not necessarily have a nucleation barrier and can take place following the usual evolution of the Asaro–Tiller–Grinfeld instability.

The growth front stacking of bilayer quantum dot (BQD) InAs/GaAs heterostructures was studied by reflection high-energy electron diffraction (RHEED). The mean-field theory was employed to describe the quantum dots (QDs) nucleation, which was experimentally monitored during the intensity changes of the (0 0 2)-RHEED diffraction spot along the two- to three-dimensional (2D-3D) InAs growth mode transition. The diffusion parameter obtained from fits of the 2D-3D transition curves was associated to the rate of atoms supply from 2D and 3D islands precursors. The variation of the nucleation parameters during the vertical alignment of QDs associated to the coupling of strain fields were related to the changes of the QDs size and the wetting layer thickness. Numerical simulations indicated that these changes reduce the strain in the BQD heterostructures. Damped oscillatory behavior was observed for the InAs/GaAs critical thickness (Hc) as a function of the number of BQD. The bilayer number after which Hc did not vary significantly, coincided with the one without important variation of the diffusion parameters. The number of BQD layers required to reach this quasi-equilibrium condition depends on the growth parameters of the first layer and the spacer layer thickness, as supported by numerical simulations.

Controlled growth of InGaN quantum dots (QDs) using photoelectrochemically (PEC) etched InGaN QD templates is demonstrated. The InGaN QDs are grown by a self-assembly (SA) method using metal-organic chemical vapor deposition on templates consisting of planar GaN and PEC etched InGaN QDs for comparison. The InGaN QD templates are formed using quantum-size-controlled PEC etching of planar InGaN layers on GaN, which produces controlled QD radiuses with a statistical mean (μ) of 17.3 nm and standard deviation (σ) of 6.2 nm, and densities of 1.2 × 1010 cm−2. The PEC etched QDs are capped with an AlGaN interlayer and GaN barrier layer to recover a planar surface morphology for subsequent SA growth of QDs. The PEC QD templates behave as seeds via localize strain near the PEC QDs which provide improved control of the SA QD growth. The SA grown QDs on PEC QD templates are smaller and have controlled radiuses with μ = 21.7 nm and σ = 11.7 nm compared to the SA QDs on planar GaN templates with radiuses of μ = 37.8 nm and σ = 17.8 nm. Additionally, the dot densities of the SA QDs on PEC QD templates are ~3 times higher and more closely match the underlying densities of the template (8.1 × 109 cm−2). Multiple quantum dots (MQDs) are also grown on both templates that consist of 4 periods of SA QDs and AlGaN/GaN interlayer/barrier layers. The MQDs grown on PEC QD templates better retain their planarized smooth surfaces after barrier layer growth, and exhibit ~3 times stronger PL intensity at room temperature compared to MQDs grown on planar GaN.

In this work, the near-infrared photoluminescence (PL) of InSb/GaSb QD structures grown on GaSb substrate (2° off (100)) using atmospheric pressure Metalorganic Vapor Phase Epitaxy is investigated. The structures are analyzed before capping and after capping using scanning probe microscopy and high resolution transmission electron microscopy (HRTEM), respectively. At 10 K, with an excitation power of 2 mW, a PL peak at ∼ 732 meV is observed. Upon an increase in laser power to 120 mW, a blue shift of ∼ 8 meV is noticed. This emission typically persists up to 60–70 K, after which it becomes weak. An SPM analysis of the size distribution of uncapped dots reveals a mono-modal distribution with an average density of ∼ 5×10 10 cm −2 . However, a HRTEM investigation of the capped dots reveals the formation of an InGaSb quantum well-like structure, ∼ 10 nm thick, which gives rise to the PL signal mentioned above.

Models based on continuum elasticity theory are discussed to calculate the necessary thickness of a strain compensation (SC) layer for a superlattice (SL) of strained quantum wells (QW) or quantum dots (QD). These models are then expanded to cover material systems (substrates, QW or QD, and SC) composed of AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, or InSb, as well as the ternary, quaternary, and higher order material alloys possible in the Al/Ga/In/P/As/Sb systems. SC thickness calculation methods were compared against dynamical scattering simulations and experimental X-ray diffraction measurements of the InAs/GaP/GaAs QD/SC/Substrate superlattices of varying SC thickness. Based on the reduced (but not eliminated) strain present, a further modified strain compensation thickness is calculated to maximize the number of SL repeat units before the onset of misfit dislocations is also calculated. These models have been assembled into a free application on nanoHUB for use by the community.

We have studied the growth of GaSb/GaAs nanostructures by droplet epitaxy with the variation of Ga deposition temperature which is one of the key parameters. With the use of low Sb flux, GaSb quantum rings (QRs) were formed as a result of the outward diffusion of Ga atoms from the droplets during crystallization. An increase of the deposition temperature results in the larger QR size and the lower QR density due to the longer diffusion length of Ga atoms, which gives rise to the larger initial droplets. Interestingly, some portion of QR lobe breaks up and transforms into a quantum dot (QD) so as to reduce the mismatch strain. A nanostructure containing both QR and QD is called a quantum-ring-with-dot structure (QRDS). As the deposition temperature increases, the nanostructure height distribution changes from unimodal to bimodal behaviors owing to the significant difference between QR and QD heights, whereas the nanostructure diameter still exhibits a unimodal distribution. The bimodal height distribution strongly affects the optical properties and the dynamic of thermal-excited carriers.

We performed an intermittent growth of InAs quantum dots (QD) on GaAs(001) at 500 °C. The transition of surface structures during the growth was investigated by using reflection high energy electron diffraction observation. We also performed in situ observation of QD nucleation by using STMBE system in which a scanning tunneling microscope equipped within a molecular beam epitaxy chamber. We have found that the initial QDs formation occurred from 1.15 to 1.38 ML with the intermittent InAs supply. This InAs supply amount was much smaller than that of ordinary continuous deposition ( ∼ 1.66 ML). Moreover, the QDs mainly appeared on the terrace while they mainly appear on step edges by continuous growth at 500 °C. This indicates that the annealing parts of the intermittent growth affected the surface atomic structures of InGaAs wetting layer, uniforming the In fluctuation and stabilized the surface morphology. By the preparation of large ( n × 3 ) and ( 2 × 4 ) area, the QD nucleation occurs with a smaller InAs supply on the surface.

We describe the fabrication, epitaxial growth and photoluminescence (PL) spectra of systems of site-controlled pyramidal quantum dots (QDs) integrated with photonic crystal (PhC) membrane cavities. The spectra of the individual QDs are characterized by scanning micro-PL spectroscopy, and the cavity modes are identified by polarization-resolved PL measurements and modeling. Weak coupling of systems incorporating up to four identifiable QDs with single modes of the PhC cavities is demonstrated. (C) 2014 Elsevier B.V. All rights reserved.

Inorganic molecules have seldom been included into cyclodextrin crystal. Here in this paper we report the first example of including CdS quantum dots into cyclodextrin crystal by simple rapid crystallization. A CdS quantum dot/cyclodextrin composite has been obtained, in which quantum dots of CdS are embedded in β-cyclodextrin (β-CD) crystal. Experiments have proven that it is the rapid crystallization of cyclodextrin in acetone that results in the formation of abundant cyclodextrin nuclei, which include CdS dots into them forming the CdS quantum dot/cyclodextrin compostie. This research opens the new research field of inorganic species/cyclodextrin inclusion complex.

Self-assembled InAs quantum dots (QDs) grown on an InP(311)B substrate were embedded using lattice-matched InAlAs/InGaAs superlattice with the digital embedding method. The thickness of quantum wells and barriers of the superlattice varied from 2 to 16 monolayers. The six layer stacking structures were successfully grown without any degradation of the QD and superlattice structure. The cross-sections of QDs embedded within the superlattice were visualized by scanning transmission microscope. The emission wavelength of the QDs was measured by photoluminescence and could be changed by changing the thickness of the superlattice.

The effect of GaAs host matrix on excitonic behaviour in AP-MOVPE grown GaSb/GaAs quantum dots (QDs) was investigated. Room temperature (RT) photoluminescence (PL) emission was achieved from single layers of quantum dots by controlling the GaAs host matrix growth temperature. Samples were prepared using a GaSb dot growth temperature of 530 °C, followed by growth of a thin GaAs ‘cold’ cap, before depositing the final part of the GaAs capping layer at either 550 °C, 600 °C or 650 °C. PL measurements at 10 K revealed QD emission peaks for all the samples at around 1.1 eV. However, variable temperature PL revealed different thermal quenching rates of the emission, with the rates of quenching reduced with increasing GaAs growth temperature. This was ascribed to reduced defect densities in GaAs grown at higher temperature, which resulted in QD emission even at RT. The hole localisation energy determined for these samples at RT was approximately 470 meV.

We report on the InAs quantum dot (QD) external cavity laser (ECL) using a digital mirror device (DMD) as the key component for wavelength tuning. The InAs QD laser diode was grown by gas source molecular-beam epitaxy, which had a broad gain profile. Single mode operation was achieved with the side mode suppression ratio of 21 dB when the optical feedback was provided by a mirror pattern consisting of 9 micromirrors. Moreover, two-color lasing was demonstrated with two laser lines having frequency difference in the THz range. The incorporation of DMD in the ECL enables great flexibility and many unique features, such as high tuning speed independent of the tuning step, two-color or multicolor lasing, and adjustable intensity for individual laser lines. (C) 2015 Elsevier B.V. All rights reserved.

Broadband In x Ga 1− x As quantum dot-in-a-well (DWell) solar cells are grown by stacking layers of composition-tailored In x Ga 1− x As ( x =1, 0.75, and 0.65) quantum dots on p-type In 0.1 Ga 0.9 As quantum wells (QWs). Doping concentration and growth temperature for the Be-doped quantum wells are optimized to enhance the conversion efficiency ( η ). The broadband DWell solar cell of Be: 2×10 17 cm −3 QWs grown at 570 °C shows the best photovoltaic characteristics of η =10.86%, which is 3% higher than that of the GaAs baseline solar cell.

Blue-and green-emitting quantum dots have been characterized and ridge waveguide lasers incorporating such quantum dots into the active region have been realized. The laser heteroscturctures were grown by plasma assisted molecular beam epitaxy. Injected carrier lifetimes in the quantum dots have also been measured by temperature dependent and time resolved photoluminescence. A threshold current density of 930 A/cm 2 in the blue-emitting lasers was measured under pulsed bias. A tunnel injection scheme to inject holes has been incorporated in the design of the green quantum dot lasers, and a threshold current density of 945 A/cm 2 in the green-emitting lasers has been measured under pulsed bias. Slope efficiencies of 0.41 W/A and 0.25 W/A have been measured, corresponding to differential quantum efficiencies of 13.9% and 11.3%, in the blue and green lasers, respectively.

Synchrotron X-ray diffraction has been applied to the in situ monitoring of the molecular beam epitaxial growth of self-assembled InAs/GaAs(001) quantum dots (QDs). As well as the strain distribution inside QDs, the lateral and vertical size of QDs was determined as a function of growth time. In combination with post-growth atomic force microscopy, the evolution of the total volume of QDs was evaluated. It was found that the QD volume increases at a similar rate over the temperature range of 450–480 °C. A significant mass transport from the wetting layer and the substrate was confirmed.

The improved control of lattice strain in the quantum-dot (QD) region of p – i – n structures using a modified epitaxial growth procedure has been observed and analyzed. Strain in the QD region was managed by (a) inserting a correction layer (CL) with a lattice constant that was intermediate between the lattice constant of the QD region and the lattice constant of the underlying substrate, (b) capping the QD islands with a layer that had the same lattice constant as the CL, and (c) the utilization of only three atomic elements in the growth of the QD intrinsic region. These results were demonstrated in In x Ga 1− x As/InAs/In x Ga 1− x As p–i–n devices with five InAs QD layers and then compared with In x Ga 1− x As p–n (HOM) devices with identical ternary alloy compositions and no quantum dot layers. The layers in all of the devices were grown by molecular beam epitaxy. X-ray diffraction (XRD) measurements showed the interface dislocations in the QD samples were fewer than in the HOM samples and were isolated at the In x Ga 1− x As–GaAs interface, away from the optically active QD region. Cross-sectional, high-resolution transmission electron microscopy (HRTEM) images showed no evidence of threading dislocations in the QD region. Post-growth calculations of the average lattice constant of the QD region, using atomic force microscopy, XRD, and HRTEM data, indicated the QD region experienced a ∼3× reduction in its lattice misfit while increasing its critical thickness by more than 3×. Although the total misfit in the QD samples increased with the insertion of the CL and the average lattice constant of the QD region was not matched to the CL, the strain energy nevertheless was absorbed successfully without creating deleterious dislocations as seen in QD devices exhibiting lower dark current densities than in HOM control devices.

The phase transformation from planar to quantum dot growth is driven by strain energy reduction at the cost of surface energy. By calculating and comparing the strain energies of monolayer thick GaSb and InAs films on GaAs(001), a critical thickness for the 2-dimensional to 3-dimensional phase transformation of about 1.2 ML was derived for the GaSb/GaAs quantum dot system. This value is in agreement with the direct observation of the effectively deposited amount of material using cross-sectional scanning tunneling microscopy. Deviating experimental literature values can be traced back to the neglect of the Sb-for-As exchange process.

Ever since self-organized In(Ga)As/Ga(Al)As quantum dots were realized by molecular beam epitaxy, it became evident that these coherently strained nanostructures could be used as the active media in devices. While the expected advantages stemming from three-dimensional quantum confinement were clearly outlined, these were not borne out by the early experiments. It took a very detailed understanding of the unique carrier dynamics in the quantum dots to exploit their full potential. As a result, we now have lasers with emission wavelengths ranging from 0.7 to 1.54 μm, on GaAs, which demonstrate ultra-low threshold currents, near-zero chip and α-factor and large modulation bandwidth. State-of-the-art performance characteristics of these lasers are briefly reviewed. The growth, fabrication and characteristics of quantum dot lasers on silicon substrates are also described. With the incorporation of multiple quantum dot layers as a dislocation filter, we demonstrate lasers with J th = 900 A/cm 2 . The monolithic integration of the lasers with guided wave modulators on silicon is also described. Finally, the properties of spin-polarized lasers with quantum dot active regions are described. Spin injection of electrons is done with a MnAs/GaAs tunnel barrier. Laser operation at 200 K is demonstrated, with the possibility of room temperature operation in the near future.

This paper reports on an in-situ growth process used to optimize InAs/GaAs quantum dot (QD) multilayer structures grown on (001) GaAs substrate by chemical beam epitaxy (CBE). Defects related to the incoherently relaxed InAs clusters are found to alter the QD nucleation mechanism on the subsequent layers, leading to reduced QD density and photoluminescence intensity. The formation of poor crystalline quality clusters is avoided by growing the GaAs spacer layers in a two-step process. The technique consists in covering the InAs QD layer with a 10 nm-thick GaAs layer grown at 465 °C, and then removing the excess indium contained in the uncapped portion of the clusters by increasing the temperature to 565 °C for 10 min before the deposition of the remaining GaAs spacer layer. Morphological investigation shows that the QD density and size distribution obtained in the first layer are preserved up to the tenth layer. The QD integrated photoluminescence intensity is found to increase linearly with the number of stacked layers. These results are very promising for chemical beam growth of high performance intermediate-band solar cells.

The growth of gold catalyzed ZnSe nanowires, with CdSe insertions, by molecular beam epitaxy is investigated. In situ reflection high energy electron diffraction and ex-situ transmission electron diffraction reveal that both during, the gold dewetting and the nanowire growth, the gold particles remain always in the solid phase. The nanowire growth proceeds by ledge flow at the gold/nanowire interface as observed ex-situ by the presence of two monolayers high steps at the interface. The nanowire diameters present a high homogeneity corresponding to the low dispersion of the gold droplets. Finally, a rather abrupt interface, of less than 1 nm thick, is observed between the ZnSe barrier and the CdSe quantum dot allowing a high confinement of the excitons. All the above observations are compatible with a Vapor–Solid–Solid growth mode.

The controlled fabrication of self-assembled InGaAs nanostructures i.e., quantum ring (QR) and quantum-dot-in-ring (QDIR) by droplet epitaxy is reported. The effects of crystallization temperature (170–260 °C) on the nanostructure shape, dimension, density, and depth profile are investigated. The QRs transform to QDIRs when the crystallization temperature is increased. At transformation state, the QRs with distorted nanohole profile along the [1–10] crystallographic direction are observed. The formation mechanism can be explained by the competitive crystallizations in and around the nanodroplet and strain relaxation in the nanohole.

We study the temperature dependence of the photoluminescence (PL) from InAs quantum dot chains (QDC) grown by MBE on [011]- and [01 1 ¯ ]-oriented UV nanoimprint lithography processed groove patterns. We observe an increase of PL intensity from the [01 1 ¯ ]-oriented QDCs within the temperature range from 20–70 K, which is attributed to thermally activated carrier transport from small quantum dots accumulated on the sidewalls of the [01 1 ¯ ]-oriented grooves to the quantum dots at the bottom of the groove. We utilize a rate equation model to quantitatively analyze the carrier transfer mechanism. Furthermore, we show that the defect related carrier loss mechanism, which accounts for weak PL quenching at low temperatures, is similar for QDCs and self-assembled quantum dots (SAQD) that were used as a reference. The carrier loss mechanism that causes the rapid quenching of SAQD PL at high temperatures is identified as exciton escape, while for the QDCs it is either single carrier escape or escape of uncorrelated electron–hole pairs. This result reveals a significant difference in the carrier dynamics of site-controlled QDCs and SAQDs.

We report on the InAs/GaAs quantum dot (QD) lasers grown by gas-source molecular beam epitaxy. Ridge waveguide lasers were processed on the grown structure consisting of five-stacked InAs QD layers formed by InAs layers with slightly different thicknesses. Continuous-wave operation was achieved up to 80 °C and more than 50 mW optical power was collected from one facet at 20 °C. The characteristic temperature of the QD laser was measured as high as infinity in the temperature range of 80–185 K. In addition, the lasing lines covered the wavelength window as wide as 26 nm at 20 °C and 52 nm at 80 K, respectively. By adjusting the cavity length, the lasing wavelength can be tuned in the range of 1.05 – 1.10 μ m . Both of the broad tuning wavelength and the wide lasing spectrum indicate that the QD lasers have very broad gain profile, which is highly desirable for making widely tunable lasers.

Self-assembled lateral InAs quantum dot molecules (QDMs) are grown by solid-source molecular beam epitaxy (MBE) using the thin-capping-and-regrowth MBE process. Thin capping of GaAs on as-grown InAs quantum dots (QDs) at low temperatures leads to nanohole templates. The shape, size and depth of nanoholes are controlled by capping thickness. Subsequent regrowth with different amounts of InAs on the templates result in nano-propeller QDs with different blades’ dimensions. We showed that the length of the propeller blades is controlled by either the capping temperature or the thickness of capping layer. When the regrowth thickness reaches 1.2 ML, different self-assembled lateral QDMs are formed depending on the shape of nanoholes. The dot uniformity and the dot size of all QDM samples are confirmed by photoluminescence (PL) measurements at 77 K. In addition, by ramping the regrowth temperature while the In shutter is open after the deposition of the capping layer, we are able to investigate the formation of nano-propeller QDs at the beginning phases and to propose a model to explain the evolution.

We report the molecular beam epitaxial growth and characteristics of 1.45 μm metamorphic InAs quantum dot lasers grown on GaAs. By detailed investigation of the growth kinetics of the metamorphic quantum dot heterostructures, we have achieved high-quality 1.45 μm metamorphic quantum dot layers that exhibit intense and narrow photoluminescence linewidths (∼30 meV) at room temperature. Utilizing the techniques of p-doping and tunnel injection, we have also realized high-performance 1.45 μm lasers that exhibit ultra-low threshold current density (⩽70 A/cm 2 ), very high temperature stability ( T 0 =556 K) in the temperature range of 263–305 K, and large frequency response ( f −3dB =8 GHz).

Arrays of semiconductor quantum dots grown by selective area epitaxy, a process in which the size and position of the dots is determined by a lithographically patterned mask, can have a high degree of uniformity in both size and position. However, non-uniformity in the initial stages of growth causes broadening of the energy states of GaN/InGaN heterostructures grown using this technique, limiting their practical utility for device applications. A phase-field model was developed to simulate selective area epitaxy, accounting for a crystallographic-orientation-dependent deposition rate. Model parameters were varied to optimize the uniformity of the InGaN active layers. Conditions that led to the most uniform active layers included low total deposition rate, high surface diffusivity, low deposition of surface adatoms from the mask, and smaller contact angle at the mask–vapor–quantum dot interface. Other factors that improved uniformity were growth on (0001) substrates, which is the fastest growth direction, and more vertical orientation of the sidewalls of the mask holes.

We grew a GaSb/Al0.3Ga0.7Sb multiple-quantum-well (MQW) structure on a two-inch Si(1 0 0) substrate using a 100-nm-thick heteroepitaxial GaSb thin-film buffer with a nucleation layer of GaSb dots by molecular beam epitaxy (MBE) and evaluated the surface morphology and the photoluminescence (PL) and X-ray diffraction spectra of the MQW structure. The full width at half maximum of the PL spectrum of the GaSb/Al0.3Ga0.7Sb MQW structure was very narrow, although the buffer thickness was much lower than those for previously reported GaSb-based QW structures on Si(1 0 0) substrates. This indicated that the surface of the 100-nm-thick GaSb thin-film buffer was sufficiently flat to form heterostructures and MQWs. The surface roughness and crystalline quality of the GaSb buffer and MQW structure were strongly dependent on the growth temperature; high-performance devices were realized by optimizing the growth temperature. These results showed the advantage and potential applicability of the GaSb/Al0.3Ga0.7Sb MQW structure and the GaSb thin-film buffer with GaSb dots as a nucleation layer grown on Si(1 0 0) substrates.

The molecular beam epitaxy (MBE) growth of GaAs and InAs quantum dots on etched mesas has been studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The [0 1 1]-oriented mesas are etched into (1 0 0) GaAs substrates, exposing (5 3 3)B sidewall facets. At a substrate temperature of 610 °C a top (1 0 0) plane is seen to evolve on a ridge mesa structure. Alternatively, if the overgrowth is carried out at 630 °C no such facet is seen, and the top ridge remains unchanged during GaAs growth. By controlling the mesa shape, either ordered lines of dots can be grown or the dot density can be varied from 8 cm −2 to >1×10 11 cm −2 on the same substrate in pre-defined regions. The dot distribution observed on the mesa sidewalls and top is discussed in terms of net migration of adatoms from different facets, underlying step density, step height and surface curvature of the mesa top.

A study on the influence of the growth conditions on the optical and structural properties of In 0.6 Ga 0.4 As quantum dots (QDs) grown on GaAs(100) substrates is presented. We investigated the impact of the substrate temperature, the growth rate, and V/III flux ratio on the full-width of half-maximum (FWHM) of the photoluminescence (PL) spectra and on the density and height of quantum dots. Our results show that the dot density depends strongly on the growth parameters. From them the substrate temperature has the strongest influence on the density and the height of QDs as well as on the PL properties. By varying only this parameter a direct control of the dot density and the height is possible over more than one order of magnitude (from 6×10 10 cm -2 at 480°C to 1×10 8 cm -2 at 530°C). The smallest PL linewidth of 36 meV was observed by QDs deposited at low substrate temperatures (480 °C).

Lateral InGaAs quantum dot molecules (QDMs) formed by thin-cap-and-regrowth molecular beam epitaxial technique comprise large, central quantum dots (cQDs) and small, satellite quantum dots (sQDs) in close proximity. Temperature-dependent photoluminescent (PL) measurements show that the bimodal size distribution gives rise to bimodal optical characteristics: the cQDs ground-state (GS) emissions vary slowly with temperature while the full-width at half maximum (FWHM) remains almost constant; the sQDs GS emissions, on the other hand, exhibit a sigmoidal temperature shift while the FWHM shows an anomalous temperature behaviour. The bimodal optical characteristics are well described in the existing framework of spatially localised excitons in QDs and inter- and intramolecular carrier redistributions in each and among the QDMs via non-resonant multi-phonon assisted mechanisms.

Variations in quantum dot density across 16 different 2 in quarter wafers have been studied. The InAs/GaAs quantum dots (QDs) were grown on n-type GaAs(0 0 1) substrates by molecular beam epitaxy and studied by scanning electron microscopy (SEM). The SEM study reveals large variations in QD density and size homogeneity across the 16 quarter wafers. A representative sample was studied in great detail. For this sample the QD density is lower in the middle of the 1/4-wafer than along the rims, and also considerably higher in one of the corners compared to the other two corners. The QD diameters and size homogeneity are reduced in areas with high QD density compared to areas with low QD density. The above variations are believed to be mainly due to temperature variations across the 1/4-wafer during growth of the QDs.

In this paper, the reflection high energy electron diffraction of the transition from a two-dimensional growth mode to a three-dimensional growth mode of InP ring-shaped quantum-dot molecule (QDM) formation in the matrices of In 0.5 Ga 0.5 P on semi-insulating GaAs(0 0 1) substrates was reported. All samples were grown by solid-source molecular beam epitaxy using the droplet epitaxy technique under different crystallization temperature conditions. The surface morphologies of InP ring-shaped QDMs were examined by atomic force microscopy and the photoluminescence (PL) spectra were obtained by the 478 nm line of an Ar + laser with an InGaAs detector. The dependence of the PL ground-state peak energies as the function of power and temperature with the tendencies of PL peak and full width at half maximum were investigated and discussed.

Terahertz (THz) technology has attracted vast interests due to its wide applications. Quantum dot (QD) system is proposed to be the most suitable candidate for compact THz sources based on intraband transitions. However, transition energies of reported results still fall outside the THz range (with corresponding energy of ∼0.4–41 meV). In this study, we investigate the effect of growth temperature and monolayer coverage on the transition energies of InAs/InGaAs/GaAs QDs and InAs/GaAs bilayer QDs, respectively. Consequently, the obtained energy difference was less than 40 meV, thus demonstrating the feasibility of varying the QD growth parameters for bandgap engineering towards the THz emission range.

In this paper, a new type of high-speed electroabsorption modulator (EAM) based on quantum dot (QD) p–i–n heterostructure is demonstrated. The QD layers sandwiched by p-AlGaAs and n-AlGaAs are grown by multilayer InGaAs self-assembled QD with luminance wavelength of 1300 nm, serving as the active region of EAM. The photocurrent spectrum measurement exhibits a red shift of 15 nm in QD transition energy levels on biasing from 0 to 6 V. A quadratic relation of energy shift against the reversed bias is extracted, confirming the quantum-confined Stark effect (QCSE) in QD. On fabricating a 300 μm long EAM, as high as DC 5 dB extinction ratio by 6 V voltage swing at 1310 nm is observed. As compared with well-developed quantum well (QW) EAM (well thickness ∼10 nm) of the same length, the lower density of states still shows the same order of magnitude in extinction ratio, suggesting strong QCSE in such 3-dimensional confined QD. An electrical-to-optical conversion with −3 dB bandwidth of 3.3 GHz is also attained in such QD EAM, where the speed is mainly limited by the parasitic capacitance on substrate. It implies that through optimization of QD and device structures, the advantages of QD properties are quite promising to be used in high-speed optoelectronic fields.

The present work enables the understanding of the porous system of porous silicon (PS) in terms of growth of dots and wires (quantum Si nanostructures) under certain conditions. When it comes to optical properties, PS should be recognized in terms of growth of the dots and wires instead of its porosity as ∼2–5 nm sized Si-nanostructures are responsible for luminescence. In present work, the photoluminescence (PL) spectra of PS samples of thickness 1 μm were used to estimate the percentage, average diameters and corresponding variances of dots and wires nanostructures by Singh and John (John-Singh) model. We have explicitly included the role of surface states on the fitting of John-Singh model. As a result, the plotted analytical curve revealed that an increase of current density, for fabricating the same thickness of the PS systems, results in the homogeneous formation of relatively thinner wires with a simultaneous growth of dots. It is a useful clue towards the fabrication of silicon quantum dots for photonic applications.

InAs/GaAs quantum dot (QD) properties can be significantly influenced by the growth conditions of the QD capping layer. We have studied the effect of a group III partial pressure in the reactor on the QD capping process and on the QD photoluminescence when the capping layer is grown under the kinetically limited regime. Two types of capping layers were prepared: GaAs and InGaAs. The GaAs capping layer growth rate decrease did not influence QD dissolution, but increased the dissolution of big hillocks. Influence of the GaAs capping layer thickness on QD photoluminescence is also demonstrated. The composition of the ternary strain reducing InGaAs capping layer can be considerably changed depending on the V/III ratio under kinetically limited growth.