Influence of the thickness of CdTe and ZnTe layers on cathodoluminescence spectra of strained CdTe/ZnTe superlattices with quantum-dot layers

The influence of the thickness of ZnTe barrier layers on the cathodoluminescence spectra of strained CdTe/ZnTe superlattices containing layers of quantum dots with an average lateral size of approximately 3 nm has been investigated. In samples with thick barrier layers (30, 15 nm), the cathodoluminescence spectra of quantum dots exhibit one band with a maximum at E = 2.03 eV. It has been revealed that, at a barrier layer thickness of ∼3 nm, the luminescence band is split. However, at a ZnTe layer thickness of 1.5 nm, the luminescence spectrum also contains one band. The experimental results have been interpreted with allowance made for the influence of elastic biaxial strains on the energy states of light and heavy holes in the CdTe and ZnTe layers. For the CdTe/ZnTe heterostructure with quantum dots in which the thickness of the deposited CdTe layer is 1.5 monolayers and the thickness of the barrier layer is 100 monolayers, the cathodoluminescence spectrum contains 2LO-phonon replicas. This effect has been explained by the resonance between two-phonon LO states and the difference between the energy states in the electronic spectrum of wetting layer fragments.     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

Sb/As intermixing in self-assembled GaSb/GaAs type II quantum dot systems and control of their photoluminescence spectra

The intermixing of Sb and As atoms induced by rapid thermal annealing (RTA) was investigated for type II GaSb/GaAs self-assembled quantum dots (QD) formed by molecular beam epitaxy growth. Just as in InAs/GaAs QD systems, the intermixing induces a remarkable blueshift of the photoluminescence (PL) peak of QDs and reduces the inhomogeneous broadening of PL peaks for both QD ensemble and wetting layer (WL) as consequences of the weakening of quantum confinement. Contrary to InAs/GaAs QDs systems, however, the intermixing has led to a pronounced exponential increase in PL intensity for GaSb QDs with annealing temperature up to 875 °C. By analyzing the temperature dependence of PL for QDs annealed at 700, 750 and 800 °C, activation energies of PL quenching from QDs at high temperatures are 176.4, 146 and 73.9 meV. The decrease of QD activation energy with annealing temperatures indicates the reduction of hole localization energy in type II QDs due to the Sb/As intermixing. The activation energy for the WL PL was found to drastically decrease when annealed at 800 °C where the QD PL intensity surpassed WL.     

Bound to continuum intersubband transition optical properties in the strain reducing layer-assisted InAs quantum dot structure

In this paper, the impact of wetting layer, strain reducing layer and dot height on the electronic, linear and nonlinear optical properties of bound to continuum states transitions are investigated in a system of InAs truncated conical shaped quantum dot covered with the In_xGa_1−x As strain reducing layer. The electronic structure, containing two main states of S and wetting layer states (WL), was calculated by solving one electronic band Hamiltonian with effective-mass approximation. The results reveal that the presence of the strain reducing layer in the structure extends the quantum dot emission to longer wavelength which is reported as a red-shift of the photoluminescence (PL) peak in the experimental measurement. This study also highlights the possibility of improving the intersubband optical properties based on the significant size-dependence of the three layer dot matrix by employing the strain reducing and wetting layers. According to this simulation, relatively tall dots on the thick wetting layer introduce the optimized structure size for practical applications to meet the SRL assisted enhanced dot structure.     

Investigation of populated InAs/GaAs quantum dots by photoluminescence and photoreflectance

We studied self-assembled InAs/GaAs quantum dots by contrasting photoluminescence and photoreflectance spectra from 10 K to room temperature. The photoluminescence spectral profiles comprise contributions from four equally separated energy levels of InAs quantum dots. The emission profiles involving ground state and excited states have different temperature evolution. Abnormal spectral narrowing occurred above 200 K. In the photoreflectance spectra, major features corresponding to the InAs wetting layer and GaAs layers were observed. Temperature dependences of spectral intensities of these spectral features indicate that they originate from different photon-induced modulation mechanisms. Considering interband transitions of quantum dots were observed in photoluminescence spectra and those of wetting layer were observed in photoreflectance profiles, we propose that quantum dot states of the system are occupied up to the fourth energy level which is below the wetting layer quantum state.     

Electronic structure of asymmetric vertically coupled InAs/GaAs quantum dots

In this paper, the electronic structure of an asymmetric self-assembled vertically coupled quantum dots heterostructure has been investigated. The structure consists of two ellipsoidal quantum dot (QDs) caps made with InAs embedded in a wetting layer InAs and surrounded by GaAs. Using the strain dependent k·p theory, the energy of the two lowest states of a single electron/hole which is confined within the coupled QD structure has been calculated. As a result, it can be estimated the energy gap for different geometry parameters and for tuning the external magnetic field. The numerical results show that the energy gap is very sensitive to the size asymmetry of the structure and to the small separation distance of the dots but less sensitive to the existence of an external magnetic field and large interdot distance.     

Influence of nanomechanical force on the electronic structure of InAs/GaAs quantum dots

We show nanomechanical force is useful to dynamically control the optical response of self-assembled quantum dots, giving a method to shift electron and heavy hole levels, interval of electron and heavy hole energy levels, and the emission wavelength of quantum dots (QDs). The strain, the electron energy levels, and heavy hole energy levels of InAs/GaAs(001) quantum dots with vertical nanomechanical force are investigated. Both the lattice mismatch and nanomechanical force are considered at the same time. The results show that the hydrostatic and the biaxial strains inside the QDs subjected to nanomechanical force vary with nanomechanical force. That gives the control for tailoring band gaps and optical response. Moreover, due to strain-modified energy, the band edge is also influenced by nanomechanical force. The nanomechanical force is shown to influence the band edge. As is well known, the band offset affects the electronic structure, which shows that the nanomechanical force is proven to be useful to tailor the emission wavelength of QDs. Our research helps to better understand how the nanomechanical force can be used to dynamically control the optics of quantum dots.     

Finite element analysis of stress and strain distributions in InAs/GaAs quantum dots

In this paper, we perform systematic calculations of the stress and strain distributions in InAs/GaAs truncated pyramidal quantum dots (QDs) with different wetting layer (WL) thickness, using the finite element method (FEM). The stresses and strains are concentrated at the boundaries of the WL and QDs, are reduced gradually from the boundaries to the interior, and tend to a uniform state for the positions away from the boundaries. The maximal strain energy density occurs at the vicinity of the interface between the WL and the substrate. The stresses, strains and released strain energy are reduced gradually with increasing WL thickness. The above results show that a critical WL thickness may exist, and the stress and strain distributions can make the growth of QDs a growth of strained three-dimensional island when the WL thickness is above the critical value, and FEM can be applied to investigate such nanosystems, QDs, and the relevant results are supported by the experiments.     

Optical properties of InAs quantum dots in a Si matrix

We report on the optical properties of nanoscale InAs quantum dots in a Si matrix. At a growth temperature of 400°C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range with a high sheet density. Samples with such InAs quantum dots show a luminescence band in the 1.3 μm region for temperatures up to 170 K. The PL shows a pronounced blue shift with increasing excitation density and decays with a time constant of 440 ns. The optical properties suggest an indirect type II transition for the InAs/Si quantum dots. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.     

Influence of Strain-Reducing Layer on Strain Distribution of Self-Organized InAs/GaAs Quantum Dot and Redshift of Photoluminescence Wavelength

A systematic investigation about the strain distributions around the InAs/GaAs quantum dots using the finite element method is presented. A special attention is paid to influence of an Ino.2 Gao.sAs strain reducing layer. The numerical results show that the horizontal- and vertical-strain components and the biaz~ial strain are reinforced in the InAs quantum dot due to the strain-reducing layer. However, the hydrostatic strain in the quantum dot is reduced. In the framework of eight-band k · p theory, we study the band edge modifications due to the presence of a strain reducing layer. The results demonstrate that the strain reducing layer yields the decreasing band gap, i.e., the redshift phenomenon is observed in experiments. Our calculated results show that degree of the redshift will increase with the increasing thickness of the strain-reducing layer. The calculated results can explain the experimental results in the literature, and further confirm that the long wavelength emission used for optical fibre communication is realizable by adjusting the dependent parameters. However, based on the calculated electronic and heavy-hole wave function distributions, we find that the intensity of photoluminescence will exhibits some variations with the increasing thickness of the strain-reducing layer.     

Analysis of wetting layer effect on electronic structures of truncated-pyramid quantum dots

We carry out a theoretical analysis of wetting layer effect on band-edge profiles and electronic structures of InAs/GaAs truncated-pyramid quantum dots, including the strain effect. A combination of an analytical strain model and an eight-band Fourier transform-based k · p method is adopted in the calculation. Strain modified band-edge profiles indicates that wetting layer widens the potential well inside the dot region. Wetting layer changes ground-state energy significantly whereas modifies probability density function only a little. The main acting region of wetting layer is just underneath the base of the dot. Wetting layer redistributes probability density functions of the lowest electron state and probability density functions of highest hole state differently because of the different action of quantum confinement on electrons and holes.     

Optical investigation of InAs/InP(1 0 0) quantum dots grown by gas source molecular beam epitaxy

We report on the optical characteristics of InAs quantum dots based on the InP(1 0 0) substrate grown by gas source molecular beam epitaxy without assisting any other methods. The photoluminescence was carefully investigated by adjusting the thickness of InAs layers and the growth temperature. A wide range of emitting peaks is obtained with the increase in the thickness of InAs layers. In addition, we find that the morphology and shape of quantum dots also greatly depend on InAs layers. The images of atomic force microscopy show that the quantum dots like forming into quantum dashes elongated along the [0 1 ?1] direction when the thickness of InAs layers increased. A critical thickness of formation quantum dots or quantum dash is obtained. At the same time, we observe that the growth temperature also has a great impact on the emission wavelength peaks. High qualities of InAs/InP(1 0 0) quantum dots providing their emission wavelength in 1.55 μm are obtained, and good performances of quantum dots lasers are fabricated.     

Optimising uniformity of InAs/(InGaAs)/GaAs quantum dots grown by metal organic vapor phase epitaxy

A route towards optimisation of uniformity and density of InAs/(InGaAs)/GaAs quantum dots grown by metal organic vapor phase epitaxy (MOVPE) through successive variations of the growth parameters is reported. It is demonstrated that a key parameter in obtaining a high density of quantum dots is the V/III ratio, a fact which was shown to be valid when either AsH₃ (arsine) or tertiary-butyl-arsine (TBA) were used as group V precursors. Once the optimum V/III ratio was found, the size distribution was further improved by adjusting the nominal thickness of deposited InAs material, resulting in an optimum thickness of 1.8 monolayers of InAs in our case. The number of coalesced dots was minimised by adjusting the growth interruption time to approximately 30 s. Further, the uniformity was improved by increasing the growth temperature from 485 °C to 520 °C. By combining these optimised parameters, i.e. a growth temperature of 520 °C, 1.8 monolayers InAs thickness, 30 s growth stop time and TBA as group V precursor, a full-width-half-maximum (FWHM) of the low temperature luminescence band of 40 meV was achieved, indicating a narrow dot size distribution.     

Structure of InAs/GaAs quantum dots grown with Sb surfactant

InAs quantum dots in GaAs, grown under the presence of Sb by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Large flat quantum dots with a truncated pyramidal shape, base lengths between 15 and 30 nm, heights of 1–3 nm, and a rather pure InAs stoichiometry were found for the case of an Sb supply during the InAs deposition. If Sb is already supplied during GaAs stabilization prior to InAs deposition, the dots become even larger and tend to get intermixed with Ga, but remain coherently strained with a reversed cone-like In distribution. Regarding the quantum dot growth Sb acts as surfactant, whereas an incorporation of individual Sb atoms was observed in the wetting layer.     

InSb quantum dots and quantum rings on InAs-rich surface

We report a study of InSb nanoobjects (quantum dots and quantum rings) grown on InAs-rich surface by liquid phase epitaxy. Characterization of the sample surface was performed using atomic force microscopy (AFM). The bimodal formation of the uncapped InSb quantum dots (QDs) was observed for the growing on a binary InAs substrate. Uniform high-density (1 × 10¹⁰ cm⁻²) quantum dots with a height of 3 nm were obtained at T = 420-430 °C, whereas low-density (5 × 10⁸ cm⁻²) big quantum dots were 9 nm in height. As a buffer layer, lattice-matched InAsSb_0.12P_0.25 solid solution was deposed on InAs substrate using metal-organic vapour phase epitaxy. Deposition from the InSb melt on the buffer layer resulted in the formation of InSb nanoobjects with density as high as 3 × 10¹⁰ cm⁻².     

Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3)A substrate

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.     

Photoreflectance study of energy level structure of self-assembled InAs/GaAs quantum dots emitting at 1.3 μm

Photoreflectance and photoluminescence measurements were performed on the ensemble of self assembled InAs/GaAs quantum dots designed to emit at 1.3 μm. As many as six QDs-related optical transitions were observed in PR spectra, the energies of which were confirmed by high-excitation PL results. Numerical calculations allowed estimating the average size of the dots, which is larger than for standard InAs/GaAs QDs. This result is in agreement with structural data. Additionally, the energy level structure for such QDs was derived and compared with the electronic structure of standard InAs/GaAs dots. It was shown that the energy level structure of such large dots qualifies them for the active region of a laser emitting at 1.3 μm.     

Photoluminescence of InAs Self-Organized Quantum Dots Formation on InP Substrate by MOCVD

In this letter, we present results of photoluminescence (PL) emission from single-layer and multilayer InAs self-organized quantum dots (QDs), which were grown on (001) InP substrate. The room temperature PL peak of the single-layer QDs locates at 1608 nm, and full width at half-maximum (FWHM) of the PL peak is 71 meV. The PL peak of the multilayer QDs locates at 1478 nm, PL intensity of which is stronger than that of single-layer QDs. The single-layer QD PL spectra also display excited state emission and state filling as the excitation intensity is increased. Low temperature PL spectra show a weak peak between the peaks of QDs and wetting layer (WL), which suggests the recombination between electrons in the WL and holes in the dots.     

Spectroscopy on single columns of vertically aligned InAs quantum dots

We have investigated the carrier relaxation dynamics in single columns of tenfold stacked vertically aligned InAs quantum dots by micro-photoluminescence measurement. The excitation spectrum in the stacked dots is much different from that in the single dot characterized by the existence of a zero-absorption region and sharp multiple phonon emission lines. We have observed a broad continuum absorption far below the wetting layer band edge in the spectrum of the single columns although we have confirmed the existence of a zero-absorption region in the same sample with reduced number of dot layers to almost single, realized by surface etching. The broad absorption feature suggests the existence of additional carrier relaxation channels through non-resonant tunneling between the dots.     

Picosecond timescale carrier dynamics of InAs quantum dots: The role of a continuum background

We have investigated the ultrafast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots emitting at 1.3 μm by means of time resolved photoluminescence upconversion measurements with a time resolution of about 200 fs. The detection energies scan the spectral region from the energy of the quantum dot excitonic transition up to the barrier layer absorption edge. We found, under high excitation intensity, that the intrinsic electronic states are populated mainly by carriers directly captured from the barrier.