Multilayer Quantum Dot Structures Research Articles

Evaluation of In(Ga)As capping in a multilayer coupled InAs quantum dot system: Growth strategy involving the same overgrowth percentage

Strain-coupled multilayer Quantum Dot (QD) structures draw a great attention these days because of their superior optical and device performance. However, these coupled multilayer QD structures have limitations such as non-uniform QD size distribution, defects, and dislocations due to the propagation of strain in QD layers. Furthermore, the carrier relaxation lifetime must be improved in these coupled QD structures for sought-after device performance. This study serves for three main purposes. Firstly, we have utilized a growth strategy to maintain the overgrowth percentage such that the Stranski-Krastanov (SK) QD dimensions in every layer is same for all the multilayer structures. Secondly, these multilayer SK QDs are made to electronically couple with the Sub-monolayer (SML) QDs in such a way that the carrier relaxation lifetime can be increased, that could ameliorate the photoconductive gain and the responsivity. Lastly, the amount of cumulative strain generated inside QDs can be reduced by the incorporation of In0.15Ga0.85As strain reducing layer (capping layer) in a multilayer structure. Five different multilayer structures with single (x1), bi (x2), penta (x5), hepta (x7), and ten-layers (x10) of SK QDs electronically coupled to six stacks of SML QDs are used in this study. Photoluminescence (PL) studies reveal that the SK QD dimensions is maintained to be the same in all multilayer structures due to the proposed growth strategy, which is also observed through transmission electron microscopy (TEM) images. Photoluminescence excitation (PLE) analysis shows the match of excited states of SK QDs to the ground state of the SML QDs, aiding the carrier tunnel phenomena. The ω/2Ө measurements from high resolution X-ray diffraction (HRXRD) is carried out to calculate the overall strain in the grown heterostructures. The optical properties and the strain components obtained from the nextnano simulation tool are in good agreement with the experimental results. Thus, coupled multilayer QD structures with a growth strategy in the current study would be more suitable for the realization of quantum dot infrared photodetectors and intermediate band solar cell applications accommodated with high device efficiency.

Hybrid strain-coupled multilayer SK and SML InAs/GaAs quantum dot heterostructure: Enabling higher absorptivity and strain minimization for enhanced optical and structural characteristics

The size and shape of self-assembled quantum dots (QDs) in the top layer of a strain-coupled multilayer Stranski-Krastanov (SK) type QD heterostructure is decided by the effect of strain from the bottom QD layers along with the compressive strain through InAs/GaAs interface. A novel approach is used in this article to grow the heterostructures such that QDs from one layer to the other are heterogeneously coupled resulting in reduced cumulative strain, which helps in formation of defect free and uniform dots even for 10 strain-coupled SK QD layers. Heterostructures with strain-coupled single- ( × 1), bi- ( × 2), tri- ( × 3), penta- ( × 5), hepta- ( × 7) and deca- ( × 10) layer SK QDs are grown employing a growth mechanism on six stack sub-monolayer (SML) type QDs, considering the individual advantages of SK and SML QDs. Full width half maximum (FWHM) calculated from the low temperature (19 K) photoluminescence (PL) are 50, 52, 54, 55, 56 and 50 nm for single, bi, tri, penta, hepta and deca-layer SK QD stack respectively, which shows uniform dot size distribution up to deca-layer SK QD structure because of the adapted growth mechanism. The activation energies calculated from temperature dependent PL are 244, 281, 299, 275, 288 and 397 meV for single, bi, tri, penta, hepta and deca-layers respectively, which aids that the strain-coupled deca-layer SK QD heterostructure has the highest activation energy compared to other strain-coupled multilayer SK QD structures. These properties make the deca-layer SK QD structure as the optimum device for quantum dot infrared photodetectors (QDIPs), which could operate at higher temperatures with improved performance parameters. High resolution X-ray diffraction (HRXRD) shows a defect free heterostructure with minimized hydrostatic strain and homogeneous dot size distribution for the strain-coupled deca-layer SK QD structure. Thus, the adapted growth mechanism along with heterogeneous coupling of SK and SML QDs facilitates higher absorption efficiency, increased carrier lifetime and better quantum confinement, making it a promising design to be used in intermediate band solar cells (IBSC) and QDIPs, where multilayer QDs are needed so as to improve the device performance.

Formation mechanisms of agglomerations in high-density InAs/GaAs quantum dot multi-layer structures

The study explores phenomena that occur during the growth of multi-stacked quantum dots (MQD) InAs layers using thin GaAs spacers. An arrangement of plastically relaxed agglomerations that extend along the MQD structure with diameters of about 70–120 nm and separated every 200–400 nm are observed. These agglomerations hinder the regular development of vertically aligned QD columns, leaving only a regular density of QDs in the first layer. The generation of these agglomerations has been modelled based on two extreme cases: (i) conical-like, it presents a more contrasted base and its nucleation is related to the presence of two coalesced QDs. The progression of these agglomerations can be deactivated in the upper layers, probably to a decrease in surface stress as consequence of misfit dislocation formation in the lower layers; (ii) volcano-like, it presents a crater shape due to the collapse of the upper layers with a higher accumulation of In. It is proposed that the origin of this type of agglomerations is due to the formation of a quantum ring (QR) in the first layer. The initial concavity of the QR increases during successive GaAs/InAs deposition cycles as tendency of Ga to out-diffuse rises.

Nanoscale Patterning of Colloidal Nanocrystal Films for Nanophotonic Applications Using Direct Write Electron Beam Lithography.

The small size of colloidal nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to nanophotonics, including bright, tunable photoluminescence (PL). However, exploring the properties of solid QD assemblies at the nanoscale has proven challenging because of the limitations in the nanoscale QD patterning methods. Generally, the precise placement of QD solids is difficult to achieve, especially for tall structures with multiple QD layers, and when it is achieved the patterns often cannot withstand the further processing steps required for final device construction. Direct electron beam lithography of QDs has emerged as a straightforward patterning process that does not require ligand exchange and results in structures that retain bright PL. Here, we demonstrate that direct patterning QD films on substrates treated with a self-assembled monolayer of octadecyltrichlorosilane allows us to create feature sizes as thin as 30 nm with heights of multiple layers and characterize the pattern resolution, robustness, and placement accuracy. These structures withstand sonication in a variety of solvents, and the structures are placed within 20 nm of their intended location nearly 100% of the time. We further show how this patterning method can be applied to nanophotonics by measuring the complex refractive index of the QD materials to model the absorption and scattering cross sections of QD structures of various sizes and shapes. These simulations reveal that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators.

Control of surface dip diameter in Si-based photonic nanostructures by changing growth temperature of Ge quantum dot multilayer structures and its impact on their optical properties

We fabricate photonic nanostructures by combination of growth of Ge quantum dot (QD) multilayer structures and maskless wet etching, and demonstrate that surface dip diameters in photonic nanostructures can be tuned by changing the growth temperature of QDs. Increase of growth temperature from 700 to 800 °C leads to the enlargement of surface dip diameters by a factor of more than two through the increase of QDs lateral sizes. In addition, the wet etching rate of the stacked Ge QDs depends on the Ge composition in the QDs and the dislocation generation in the Ge QD multilayer structures. The surface dips with larger diameters leads to an enhanced optical absorption in the near-infrared region. These results show that varying the growth temperature of QDs is effective to control the photonic nanostructures geometry and optical properties.

(In,Mn)As multilayer quantum dot structures

(In,Mn)As multilayer quantum dots structures were grown by molecular beam epitaxy using a Mn selective doping of the central parts of quantum dots. The study of the structural and magneto-optical properties of the samples with three and five layers of (In,Mn)As quantum dots has shown that during the quantum dots assembly, the out-diffusion of Mn from the layers with (In,Mn)As quantum dots can occur resulting in the formation of the extended defects. To produce a high quality structures using the elaborated technique of selective doping, the number of (In,Mn)As quantum dot layers should not exceed three.

Optimization of growth conditions of type-II Zn(Cd)Te/ZnCdSe submonolayer quantum dot superlattices for intermediate band solar cells

Intermediate band solar cells (IBSCs) have been predicted to be significantly more efficient than the conventional solar cells, but have not been realized to their full potential due to the difficulties related to the fabrication of practical devices. The authors report here on growth and characterization of Zn(Cd)Te/ZnCdSe submonolayer quantum dot (QD) superlattices (SLs), grown by migration enhanced epitaxy. These QDs do not exhibit formation of wetting layers, which is one of the culprits for the unsatisfactory performance of IBSCs. The ZnCdSe host bandgap is ∼2.1 eV when lattice matched to InP, while the Zn(Cd)Te-ZnCdSe valence band offset is ∼0.8 eV. These parameters make this material system an excellent candidate for a practical IBSC. The detailed structural analysis demonstrates that the process of desorption of Cd and the preferential incorporation of Zn facilitates the formation of unintentional strained ZnSe-rich layer at the QD-spacer interface. The growth conditions have been then optimized so as to obtain high crystalline quality lattice matched SL, by growing intentionally Cd-rich spacers, which strain balanced the SL. The excitation intensity dependent photoluminescence confirmed the type-II nature of these multilayer QD structures, which is expected to suppress nonradiative Auger recombination, and improve the carrier extraction process when implemented in an actual device.

Low threshold current and widely tunable external cavity lasers with chirped multilayer InAs/InGaAs/GaAs quantum-dot structure

Low threshold and widely tunable InAs/GaAs quantum-dot lasers are implemented with grating-coupled external-cavity arrangement. Throughout the tuning range of 130 nm, from 1160 to 1290 nm, the threshold current density is not more than 0.9 kA/cm2 and no noticeable threshold jump is observed. For a shorter-cavity device, the injection current is kept at a record low value of 90 mA but the tuning range is further extended to 150 nm, from 1143 to 1293 nm. The effect of cavity length on the tuning characteristics is discussed and the strategy for design and optimization of multilayer quantum-dot structure is also proposed.

Optical response of quantum dot multilayer structures

We theoretically study optical response of InAs/GaAs quantum dot multilayered structures. Based on the multi-scale hybrid discrete-continuum description the optical response of an isolated layer of embedded quantum dots has been determined. Using the propagation-matrix approach we express the amplitudes of incident, reflected and transmitted electromagnetic waves in a multilayered structure by the reflection and transmission coefficients of the isolated layers. We study the overall reflectance and transmittance dependencies on the number of layers and distance between consecutive layers. The increase of the number of the layers considerably enhances the overall reflectance of the structures. Interference effects become significant for certain distances between layers in the structures.

Study of InGaN/GaN quantum dot systems by TEM techniques and photoluminescence spectroscopy

InGaN/GaN multilayer quantum dot structures produced by MOCVD techniques on c-plane sapphire were studied by transmission electron microscopy (TEM) and photoluminescence (PL) techniques. Indium fluctuations ranging from 1-4 nm were observed with both energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) scanning TEM. The existence of V-shaped defects with nucleation centres at the termination of threading dislocation were observed in HAADF images. There was also evidence of the formation of large quantum dots at low densities from lattice HRTEM images. This was further confirmed by PL measurements through the observation of a single sharp line at low power with the typical saturation behaviour at higher power excitation.

Three‐dimensional ordering in self‐organized (In,Ga)As quantum dot multilayer structures

AbstractMolecular beam epitaxy (MBE) grown In0.5Ga0.5As/GaAs multilayer structures with quantum dots chains (QDs), obtained under different growth conditions, were investigated by high‐resolution X‐ray diffractometry (HRXRD) and AFM. It was determined that self‐organized epitaxial growth of In0.5Ga0.5As/GaAs can lead to the formation of three‐dimensional quantum‐dot crystals with triclinic (distorted cubic) unit cell. The mechanisms of QDs ordering in dependence on As flux are analyzed.

Raman scattering by porous structures with InAs quantum dots

AbstractIn this paper we search for the possibilities of introducing porosity in InAs/AlAs/GaAs multilayer quantum dot structures as an additional tool for the modification of their phonon spectrum. Raman spectroscopy is used as a primary method of investigation of quantum dot phonon spectrum. Introduction of porosity in quantum dot structures leads to alteration of optical phonon frequencies in quantum dots due to relaxation of built‐in mechanical strain in quantum dots and appearance of Froehlich modes. Magnification of the Raman replicas by optical phonons in quantum dots is explained by multiple light reflection in the porous structure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical reflection from excitonic quantum-dot multilayer structures

We study theoretically and experimentally the optical reflection from excitonic quantum-dot (QD) multilayer structures composed of InAs QDs in a GaAs substrate. Quantum mechanical and finite-difference time-domain numerical calculations indicate that the incident radiation in the optical reflectance measurement photoexcites the InAs QDs which then form excitonic dipoles. The excitonic dipole modifies significantly the dielectric constant of the QD, which results in a reflectance peak in the vicinity of the excitonic energy, as observed experimentally.

Diagnostics of cap layers in InAs(N) quantum-dot multilayer structures on GaAs(001), grown by metal-organic vapor-phase epitaxy

InAs(N) quantum-dot structures grown by metal-organic vapor-phase epitaxy on GaAs(001) substrates were studied. The growth process included deposition of thin InGaAs and GaAs layers above the quantum dots at a lower temperature. Subsequently, a thick GaAs barrier layer was grown at a higher temperature. This procedure resulted in the removal of undesirable large islands while retaining small ones. An indium excess from dissolved islands was transferred to an additional InGaAs layer detected by X-ray diffractometry.

Photoluminescence and magnetophotoluminescence of vertically stacked InAs/GaAs quantum dot structures

We report on photoluminescence of a series of vertically stacked multilayer quantum dot structures. Our analysis of the data taken at different excitation powers and in the magnetic field up to 24T yields a useful insight into the electronic structure of the InAs/GaAs quantum dot structures. The data are compared with results of model electronic–structure calculations for flat dots, including the effects of the strain field. The calculations suggest that the difference between the energies of the ground and first excited transitions is mainly determined by the lateral dimensions of the dots. A comparison of the experimental results with the theoretical ones reveals an increase of the dot dimensions (both height and diameter) and a decrease of the aspect ratio (height/diameter) with increasing number of the quantum dot layers. For seven-layer samples, the wavelength of 1.3μm has been achieved. The increase of the thickness of the spacing layers between adjacent dot layers leads to a decrease of the lateral dimensions of the dots, accompanied by an increase of the aspect ratio and of the energy separation between the ground and first excited transitions.

Defect-free 100-layer strain-balanced InAs quantum dot structure grown on InP substrate

A high quality 100-layer InAs quantum dot (QD) structure was successfully grown on InP substrate. The overall compressive strain caused by InAs QDs on InAlGaAs∕InP is effectively balanced by inserting tensile-strained InGaAs strain-balance layers immediately above QD layers. The cross-sectional transmission electron microscopy images show a low defect density of less than 106cm−2 and a smooth interface between QD layers throughout the whole structure. In addition, the intense room temperature photoluminescence indicates a good optical quality of the multilayer QD structure.

Theoretical analysis of strain and strain decay in InAs∕GaAs(001) multilayer quantum dot growth

The results of analytically based calculations of the various strain components within and outside InAs quantum dots (QDs) in a GaAs matrix are presented. The calculations performed here take into account cubic crystal strain anisotropy and spatial grading of the indium composition. The assumptions regarding the shape and compositional profile of the QDs have been refined and reflect experimental findings from previous morphological studies. Generally, cone-shaped QDs are modeled with and without truncation, and the composition is either pure InAs or is assumed to change linearly from 50% at the bottom to 100% at the top. The exact QD dimensions—height and base diameter—have been obtained from scanning tunneling microscopy and atomic force microscopy. The first part of the calculation addresses structures containing a single QD layer. Particular emphasis is placed on evaluating the decay of strain in the growth direction, as this is known to affect QD nucleation and growth in subsequent layers. In the second part the calculations are expanded to structures containing two layers of QDs with separations of 10, 20, and 30nm. It is shown that the biaxial strain component decays more rapidly in the case of an isolated QD compared with a QD in the second layer of a structure with 10nm spacing. In this bilayer structure, the hydrostatic strain within the first layer QDs is significantly smaller compared with that in the upper QDs and the implications for the electronic band structure are discussed. Our calculations provide insight into trends in (multilayer) QD structures that are not easily observed experimentally.

Growth of high quality InAs quantum-dot multilayer structures on InP for infrared photodetector applications

We present a growth technique to improve the structural property of InP-based multilayer quantum-dot (QD) structures. A thin layer of AlGaInAs grown under a group-III stabilized condition can effectively smooth out the three-dimensional growth front caused by the QD formation. Thus, the AlGaInAs barrier layers with high crystal quality and smooth interfaces can be achieved. Using this technique, an InP-based QD infrared photodetector structure containing ten-period QD layers has been grown using molecular beam epitaxy, and its high structural and optical quality was confirmed by x-ray diffraction and photoluminescence measurements.

Characterization of structure and defects in dot-in-well laser structures

Recent progress in the development of 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, a consequence of the finite dot density, and to carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of using thin InAlAs capping layers and high temperature buffer layers. Both effects are observed to strongly modify the structure of the QDs producing significant improvements in the InAs QDs optical properties at room temperature. Initial attempts at multilayer QD structures showed substantial degradation in optical and electrical properties compared to single layer structures. Analysis by Transmission Electron Microscopy (TEM) has identified the presence of defects arising from the complex interaction of QDs, which propagate through the QD layers into the upper regions of the structure as being the primary cause of the poor electronic device characteristics. The use of high growth temperature spacing layers (HGTSLs) have recently allowed us the fabrication of a defect free five layer-stacked structure with record low threshold current density.

Chirped multiple InAs quantum dot structure for wide spectrum device applications

A chirped multiple quantum dot (QD) structure is introduced as a means to controllably broaden the gain spectral width of InAs QDs. The In composition and thickness of InGaAs strain-reducing layers (SRLs) can be used as tuning parameters to adjust the emission wavelength of each layer in the chirped stack. The spectral linewidth depends on the span of the In composition and thickness of the InGaAs SRL. By varying the In composition of each InGaAs SRL, a ground-state photoluminescence spectral width of 78 nm was achieved. The spectral width can be further increased by Be modulation -doping. The chirped multi-layer QD structure was used as the active region of a laser diode, exhibiting a lasing spectrum as wide as 30 nm.