Multilayer Quantum Dots Research Articles

Multilayer InAs Quantum Dot with GaNAs Strain Compensation Layers Partly Inserted in a Thin Spacer Layer

Multilayer InAs quantum dot with a thin spacer prepared using a GaNAs strain compensation layer was investigated by metalorganic chemical vapor deposition. The GaNAs tensile strained layer was inserted partly in the spacer without contacting with the compressively strained InAs dot and GaInAs cover layer. The stacking number of up to 5 with a thin spacer of 18 nm was realized at a wavelength of 1.4 µm without severe degradation of optical quality although a slight dot size increase in the upper layer was confirmed. The result indicates the advantageousness of GaNAs for a thin spacer structure of the multilayer quantum dot.

Photoluminescence Imaging of Focused Ion Beam Induced Individual Quantum Dots

We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 μm apart, with a minimum single dot emission line width of 160 μeV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV.

InGaAs self-assembly quantum dot for high-speed 1300 nm electroabsorption modulator

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 1300nm, serving as the active region of EAM. The photocurrent spectrum measurement exhibits a red shift of 15nm in QD transition energy levels on biasing from 0 to 6V. A quadratic relation of energy shift against the reversed bias is extracted, confirming the quantum-confined Stark effect (QCSE) in QD. On fabricating a 300μm long EAM, as high as DC 5dB extinction ratio by 6V voltage swing at 1310nm is observed. As compared with well-developed quantum well (QW) EAM (well thickness ∼10nm) 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 −3dB bandwidth of 3.3GHz 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.

Long-wavelength infrared quantum-dot based interband photodetectors

We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley–Read–Hall generation–recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal–organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 × 10 11 cm −2, 1 × 10 12 cm −2 and 3 × 10 10 cm −2 were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6–12 μm. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II interband transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

The photo-characteristics of (Bi 1− xZn x)S quantum dot complex and multilayer structure for the application to the dye-sensitized solar cell

In recent years, many quantum dot (QD) materials were investigated and applied as the sensitizer replacing organic dyes in the dye-sensitized solar cell (DSC). QD materials can tune their band gap energy level and this characteristic offers new opportunity to improve the light harvest ability. Generally, there is an inverse relationship between the band gap energy and the light absorbance. Therefore, the light absorbance of QD material can expand to the long wavelength region as the band gap energy is decreased. In this study, the light harvest ability is improved as (Bi 1 − x Zn x )S QD complex and multilayer structure are fabricated by using two QD materials (Bi 2S 3, ZnS). The band gap energy is controlled by the amount of the bismuth(Bi) and the zinc(Zn). And then, the light absorbance becomes wide as the amount of Bi is more than that of Zn. In addition, (Bi 1 − x Zn x )S QD multilayer structure is fabricated by using these (Bi 1 − x Zn x )S QD complexes having different amounts of the Bi and Zn. Finally, it is demonstrated that (Bi 1 − x Zn x )S QD multilayer is the appropriate structure to strengthen the light harvest ability and this QD complex is possible to apply to the DSC as sensitizer for the high efficiency.

Kinetic Monte Carlo simulation of quantum-dot nucleation and growth in PbSe/PbEuTe multilayers

We study the influence of different growth conditions on the structural properties of self-organized quantum-dot multilayers PbSe/PbEuTe(111). The kinetic Monte Carlo method is employed and the simulations show that mainly the quality of the dot ordering is affected when the growth conditions are slightly changed.

Morphology and optical properties of single- and multi-layer InAs quantum dots

An understanding of the structural and optical properties of quantum dots (QDs) is critical for their use in optical communication devices. In this study, single- and multi-layer self-organized InAs QDs grown on (001) GaAs substrates by molecular beam epitaxy (MBE) were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images show that the lateral size of multi-layer InAs QDs are larger and flatter than single-layer InAs QDs, which are oval-shaped. The change in shape and size may be attributed to the presence of InGaAs spacer layers in multi-layer InAs QDs. Reciprocal spacer mapping and fast Fourier transformation images clearly show that InGaAs spacer layers present in the multi-layer InAs QDs structures help to release the strain originally existing in the QDs. In addition, the photoluminescence peak of the multi-layer InAs QDs is broader than QD in the single-layer one, which implies that the multi-layer InAs QDs size variation is more random than the single-layer one and this corresponds with the HAADF-STEM images. These results prove that spacer layers release strain influencing the morphology and optical properties of the QDs.

An approach to suppress the blue-shift of photoluminescence peaks in coupled multilayer InAs/GaAs quantum dots by high temperature post-growth annealing

Abstract Effect of post-growth annealing on 10 layer stacked InAs/GaAs quantum dots (QDs) with InAlGaAs/GaAs combination capping layer grown by molecular beam epitaxy has been investigated. The QD heterostructure shows a low temperature (8 K) photoluminescence (PL) emission peak at 1267 nm. No frequency shift in the peak emission wavelength is seen even for annealing up to 700 °C which is desirable for laser devices requiring strict tolerances on operating wavelength. This is attributed to the simultaneous effect of the strain field, propagating from the seed layer to the active layer of the multilayer QD (MQD) and the indium atom gradient in the capping layer due to the presence of a quaternary InAlGaAs layer. Higher activation energy (of the order of ∼250 meV) even at 650 °C annealing temperature also signifies the stronger carrier confinement potential of the QDs. All these results demonstrate higher thermal stability of the emission peak of the devices using this QD structure.

Growth and Characterization of Telecommunication-Wavelength Quantum Dots Using Bi as a Surfactant

Telecommunication-wavelength quantum dots (QDs) were successfully grown by metalorganic vapor phase epitaxy using a novel growth method in which trimethylbismuth (TMBi) was supplied during the growth. Supplying TMBi during the growth was confirmed to have a surfactant effect, but did not result in the formation of a bismuth-containing alloy. Using this growth method, the photoluminescence intensity and wavelength of the QDs were much improved. It was found that the QD size was increased during the growth of the InGaAs covering layer; this effect partly resembled activated alloy phase separation reported for molecular-beam-epitaxy-grown QDs. For the realization of high density and multilayer QDs, we confirmed that a much higher V/III ratio than that of usual growth conditions and a strain-compensation structure are effective, respectively.

A novel approach to increase emission wavelength of InAs/GaAs quantum dots by using a quaternary capping layer

In this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590°C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic investigations were carried out to study the dot size and morphology in the different layers of the grown samples. Methods to reduce structural defects like threading dislocations in multilayer quantum dot samples are also studied.

Investigations for InAs/GaAs multilayered quantum-dot structure treated by high energy proton irradiation

This paper focuses on the high energy proton irradiation effect of InAs/GaAs multilayers quantum-dot (QD) wafer and photodetector. With high energy proton path simulation, the releases of proton energy and trap distribution in QD multilayers are predicted well. Treated by 1 and 3 MeV protons, all protons almost penetrate the multilayers of QD structures and stop deeply in GaAs substrate. InAs QD multilayer structures/Infrared photodetector have been irradiated by protons with different energies (1 and 3 MeV) and doses (1 × 10 9∼ 1 × 10 13 protons/cm 2). The photoluminescence (PL) and photoresponsivity (PR) spectrum of samples were measured and discussed with as grown and post irradiation.

Terahertz optical sideband emission in self-assembled quantum dots

A multilayer quantum dot sample has been excited with a strong terahertz (THz) electric field and probed with a near-infrared (NIR) laser. First- and second-order THz optical sidebands are generated on the NIR probe beam by driving quantum dot intersublevel resonances with the THz fields. A conversion efficiency of 3×10−6 was obtained for the conversion of NIR power into sideband emission at 4 K, decreasing by a factor of 20 up to room temperature. The sideband emission wavelength can be tuned over ∼20 nm by selection of appropriate NIR and THz frequencies, due to the inhomogeneous broadening of the dot ensemble.

Controlling blinking in multilayered quantum dots

The fluorescence intermittency of multilayered quantum dots (QDs) is experimentally investigated. The measured blinking statistical probabilities for QDs of different shell structures and at different excitation powers are compared with the diffusion controlled electron transfer model. The results show that the power law statistics for the “on” and “off” events depend strongly on the structure and thicknesses of the shells. Strongly suppressed blinking behavior is observed in the CdSe/CdS/ZnCdS/ZnS QDs.

Theoretical and experimental Raman study of superlattices with GeSi quantum dots

The results of the theoretical and experimental study of Raman scattering in the quantum dot (QD) multilayers are reported. In order to obtain an adequate description of the structure with QDs and correctly explain the experimental Raman spectra, a model is proposed which takes into account the real crystal structure of both the QD and surrounding matrix, as well as the QD-matrix interaction. The secondary quantisation and Green function method were used in the theoretical calculation model. The results obtained show that crystal structures with matrix-embedded QDs can be described as a mixed crystal with specific distribution of “impurities" organised in large "molecules". A qualitative agreement in position and intensity of bands between the calculated and experimental Raman spectra for multilayer Ge/Si QD crystal structure is observed, the doublet character of the bands is explained.

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.

The multilayered spherical quantum dot under a magnetic field

The binding energy of an impurity located at the center of multilayered spherical quantum dot (MSQD) is reported as a function of the dot and barrier thickness for different alloy compositions under the influence of a magnetic field. Within the effective mass approximation, the binding energy has been calculated using the fourth order Runge–Kutta method without magnetic field. A variational approach has been employed if a magnetic field is present. The binding energy in MSQD with equal dot and barrier thickness is calculated. It is shown that the binding energy in MSQD differs from that of a single quantum dot. Also, the geometry is dominant on the binding energy for thin MSQDs, but the magnetic field becomes more effective for thick MSQDs.

A novel approach to increase emission wavelength of InAs/GaAs quantum dots by using a quaternary capping layer

AbstractIn this paper, we present a new approach to obtain large size dots in an MBE grown InAs/GaAs multilayer quantum dot system. This is achieved by adding an InAlGaAs quaternary capping layer in addition to a high growth temperature (590°C) GaAs capping layer with the view to tune the emission wavelength of these QDs towards the 1.3 μm/0.95 eV region important for communication devices. Strain driven migration of In atoms from InAlGaAs alloy to the InAs QDs effectively increases the size of QDs. Microscopic investigations were carried out to study the dot size and morphology in the different layers of the grown samples. Methods to reduce structural defects like threading dislocations in multilayer quantum dot samples are also studied.

Spin resonance of electrons confined by SiGe nanostructures

We performed electron spin resonance measurements at 9.56 GHz on SiGe nanostructures to study coherence and relaxation times of electron spins in Si confined by SiGe quantum dots. Multilayers of self-assembled SiGe quantum dots were grown by molecular beam epitaxy. With illumination below the Si bandgap the intensity of the ESR signal increases strongly. We discuss the relationship between structural parameters of the layers and resulting spin coherence and relaxation times of the confined spins as measured by ESR.

Investigation of strain in self-assembled multilayer InAs/GaAs quantum dot heterostructures

The self-assembled InAs/GaAs multilayer quantum dots (QDs) are formed in a strain-driven process due to the lattice mismatch of the InAs/GaAs system. We have investigated strain interaction in 10 layer QD heterostructure with varying thicknesses of combination capping (InAlGaAs and GaAs) by means of scanning transmission electron microscopy (STEM), high-resolution X-ray diffraction (HRXRD) and Raman scattering. STEM micrographs reveal nice stacking of defect-free dots in all the layers of the sample having a thick combination capping. The periodic satellite peaks in the HRXRD rocking curve show good formation of dots and an indication of reduced compressive strain in the heterostructure with increased capping thickness. We detect an upward phonon frequency shift for InAs QDs in the low-temperature Raman study, which is believed to be due to strain relaxation as the thickness of the capping layer increases. The sample with thick combination capping showed better optical emission properties.

Investigation of structural and optical properties of coupled multilayer InAs/GaAs quantum dots with combinational InAlGaAs/ GaAs capping

The self assembled InAs/GaAs quantum dots (QDs) have been widely researched for their potential use in optoelectronic devices. Most of the studies so far have been confined towards uncoupled QD systems, whereas the coupled multilayer QD systems have remained relatively less unexplored. We have made optical and structural characterization of coupled InAs/GaAs quantum dot heterostructures with varying thicknesses of combination capping of the quaternary InAlGaAs and GaAs by Atomic Force Microscopy (AFM), Double Crystal X-Ray Diffraction (DCXRD) and Raman spectroscopy. The periodic satellite peaks in the XRD rocking curves indicates nice stacking and defect free formation of the dots. In low temperature Raman study we get quite prominent peak of InAs dots with higher thickness of capping layer.