Self-organized InAs Quantum Dots Research Articles

Charge Carrier Localization in InAs Self-Organized Quantum Dots

Charge Carrier Localization in InAs Self-Organized Quantum Dots

On the prospect of identifying visible emissions from optical measurements of self-assembled QDs

The work presented here focuses on the optical properties of molecular beam epitaxy-grown self-organized InAs quantum dots (QDs). Under low excitation power densities, the photoluminescence (PL) spectrum of the structure under study exhibits a double-peak profile, which has been related to the optical emission from the dots. To properly argue for or against explaining a high energy peak as being the result of different-sized QDs or excited state emissions, a comparative study of the optical properties of a set of two grown QD samples was employed. The laser power dependence of the linearity of the PL intensities of the QD transitions was shown to be very useful in determining the possible origins of the PL sub-bands detectable in a double-peak emission from an ensemble of self-organized QDs. Indeed, the change in the linearity of the dot-luminescence intensity with the increase of the excitation density could give helpful information about the active confinement states from which the dots are emitting. On the other hand, despite the use of very low excitation power densities, the optical measurements of the studied structure have revealed an appearance of a PL signal relating to the radiative emission from the first excited state of the dots. A random pathway of carrier capture by the intrinsic levels of the QDs is thought to be the most likely cause of this aspect.

Charge carrier localization in InAs self-organized quantum dots

We considered the problem of localization of electrons and holes taking for instance the pyramidal InAs quantum dots in GaAs. The problem of quantum mechanics was solved for the localizing potential taking into account the geometry, chemical composition and built-in fields of the mechanical stress and strain. We found that the strongest localization of both types of charge carriers can be achieved if the ratio of the pyramid height to its base is about 0.2. Keywords: quantum dots, elastic strain, charge carrier localization.

Charge carrier localization in InAs self-organized quantum dots

We considered the problem of localization of electrons and holes taking for instance the pyramidal InAs quantum dots in GaAs. The problem of quantum mechanics was solved for the localizing potential taking into account the geometry, chemical composition and built-in fields of the mechanical stress and strain. We found that the strongest localization of both types of charge carriers can be achieved if the ratio of the pyramid height to its base is about 0.2. Keywords: quantum dots, elastic strain, charge carrier localization.

On the high characteristic temperature of an InAs/GaAs/InGaAsP QD laser with an emission wavelength of ~1.5 μm on an InP substrate

We report on a study of lasers with an emission wavelength of about 1.5 μm and high temperature stability, synthesized on an InP (001) substrate. Self-organized InAs quantum dots capped with a thin GaAs layer are used as the active region of the laser. A quaternary InGaAsP solid solution with a band-gap width of 1.15 eV serves as the waveguide/matrix layer. A high characteristic temperature of the threshold current, T 0 = 205 K, is reached in the temperature range 20–50°C in ridge-waveguide laser diodes. A correlation between the values of T 0 and the band-gap width of the waveguide layers is found.

General route for the decomposition of InAs quantum dots during the capping process

The effect of the capping process on the morphology of InAs/GaAs quantum dots (QDs) by using different GaAs-based capping layers (CLs), ranging from strain reduction layers to strain compensating layers, has been studied by transmission microscopic techniques. For this, we have measured simultaneously the height and diameter in buried and uncapped QDs covering populations of hundreds of QDs that are statistically reliable. First, the uncapped QD population evolves in all cases from a pyramidal shape into a more homogenous distribution of buried QDs with a spherical-dome shape, despite the different mechanisms implicated in the QD capping. Second, the shape of the buried QDs depends only on the final QD size, where the radius of curvature is function of the base diameter independently of the CL composition and growth conditions. An asymmetric evolution of the QDs’ morphology takes place, in which the QD height and base diameter are modified in the amount required to adopt a similar stable shape characterized by a averaged aspect ratio of 0.21. Our results contradict the traditional model of QD material redistribution from the apex to the base and point to a different universal behavior of the overgrowth processes in self-organized InAs QDs.

Optical investigation of InAs quantum dashes grown on InP(0 0 1) vicinal substrate

We investigate with photoluminescence (PL) measurements the optoelectronic properties of self-organized InAs quantum dots (QDs) grown on nominal InP(001) substrate. InAs/InP(001) QDs are grown by Molecular Beam Epitaxy (MBE) method with optimized conditions in Stranski-Krastanov regime. A lateral coupling behavior was shown by photoluminescence spectroscopy. This phenomena is considered as a degradation source of the optoelectronic properties of InAs/InP(001) QDs used in lasers applications. In order to overcome this disadvantage behavior, we have studied the optical properties of InAs quantum islands (QIs) grown on vicinal InP(001) with 2° off miscut angle toward the [110] direction. From Polarized Photoluminescence (PPL) measurements, we have deduced that InAs quantum nanostructures have quantum dashes (QDas) form elongated in [1–10] direction. From excitation density PL measurements, we have evidenced that the different observed PL peaks are attributed to the emission of InAs QDas of different size. The lateral coupling behavior is completely eliminated in the case of this sample. The temperature-dependent PL measurements show a good thermal stability and an emission wavelength at room temperature around 1.55μm of the vicinal sample. All these properties prove that this sample possess favorable characteristics for microlasers based devices functioning at room temperature and for optical telecommunication with long range weapon. The broad emission range observed at 300K of the vicinal sample gives the possibility to use it as an active zone in solar cells and in infrared photodectectors of high optical gain and excellent sensitivity on a wide energy range.

Optically pumped rolled-up InAs/InGaAsP quantum dash lasers at room temperature

We have investigated the fabrication and optical performance of free-standing rolled-up InGaAsP tube optical cavities, wherein self-organized InAs quantum dots or dashes are incorporated as the gain media. Such tubular optical cavities are formed when the coherently strained InAs/InGaAsP quantum dot/dash nanomembrane is selectively released from the host substrate. We have achieved lasing from rolled-up InAs/InGaAsP quantum dash microtubes at room temperature under continuous wave optical pumping. The measured threshold is ∼6 µW, and the Purcell factor and the spontaneous emission coupling factor are estimated to be ∼6.54 and ∼0.87, respectively.

Quantum dot rolled-up microtube optoelectronic integrated circuit

A rolled-up microtube optoelectronic integrated circuit operating as a phototransceiver is demonstrated. The microtube is made of a InGaAs/GaAs strained bilayer with InAs self-organized quantum dots inserted in the GaAs layer. The phototransceiver consists of an optically pumped microtube laser and a microtube photoconductive detector connected by an a-Si/SiO2 waveguide. The loss in the waveguide and responsivity of the entire phototransceiver circuit are 7.96 dB/cm and 34 mA/W, respectively.

A quantum dot rolled-up microtube directional coupler

A rolled-up microtube directional coupler made of twin microtubes is demonstrated. The microtube is made of a InGaAs/GaAs strained bilayer and InAs self-organized quantum dots are inserted in the GaAs layer. The input and coupled microtubes have length and outer diameter of ∼50 and 6 μm, respectively. The coupling characteristics have been analyzed by the three-dimensional finite difference time domain method. The coupling characteristics have also been measured with isopropyl alcohol, instead of air, as the surrounding media to demonstrate the potential of the device as a sensor.

Wetting layers effect on InAs/GaAs quantum dots

Abstract FEM combining with the K·P theory is adopted to systematically investigate the effect of wetting layers on the strain-stress profiles and electronic structures of self-organized InAs quantum dot. Four different kinds of quantum dots are introduced at the same height and aspect ratio. We found that 0.5 nm wetting layer is an appropriate thickness for InAs/GaAs quantum dots. Strain shift down about 3%∼4.5% for the cases with WL (0.5 nm) and without WL in four shapes of quantum dots. For band edge energy, wetting layers expand the potential energy gap width. When WL thickness is more than 0.8 nm, the band edge energy profiles cannot vary regularly. The electron energy is affected while for heavy hole this impact on the energy is limited. Wetting layers for the influence of the electronic structure is obviously than the heavy hole. Consequently, the electron probability density function spread from buffer to wetting layer while the center of hole's function moves from QDs internal to wetting layer when introduce WLs. When WLs thickness is larger than 0.8 nm, the electronic structures of quantum dots have changed obviously. This will affect the instrument's performance which relies on the quantum dots' optical properties.

Temperature-dependent energy band gap variation in self-organized InAs quantum dots

We investigated the temperature-dependent variation of the photoluminescence emission energy of self-organized InAs/GaAs quantum dots (QDs) grown by conventional Stranski-Krastanov (SK) molecular beam epitaxy and migration-enhanced molecular beam epitaxy (MEMBE) and that of MEMBE InAs QDs in a symmetric and an asymmetric In0.2Ga0.8As/GaAs well. The temperature-dependent energy variation of each QD is analyzed in low and high temperature regions, including a sigmoidal behavior of conventional SK quantum dots with the well-known Varshni and semi-empirical Fan models.

Self-assembled quantum dots in a liquid-crystal-tunable microdisk resonator

GaAs-based semiconductor microdisks with high quality whispering gallery modes ( Q > 4000 ) have been fabricated. A layer of self-organized InAs quantum dots (QDs) served as a light source to feed the optical modes at room temperature. In order to achieve frequency tuning of the optical modes, the microdisk devices have been immersed in 4-cyano-4 ′ -pentylbiphenyl (5CB), a liquid crystal (LC) with a nematic phase below the clearing temperature of T C ≈ 34 ∘ C . We have studied the device performance in the temperature range of T = 20 – 50 ∘ C , in order to investigate the influence of the nematic–isotropic phase transition on the optical modes. Moreover, we have applied an AC electric field to the device, which leads in the nematic phase to a reorientation of the anisotropic dielectric tensor of the liquid crystal. This electrical anisotropy can be used to achieve electrical tunability of the optical modes. Using the finite-difference time domain (FDTD) technique with an anisotropic material model, we are able to describe the influence of the liquid crystal qualitatively.

Selective growth of stacked InAs quantum dots by using the templates formed by the Nano-Jet Probe

We have demonstrated the selective area growth of stacked self-assembled InAs quantum dot (QD) arrays in the desired regions on a substrate and confirmed the photoluminescence (PL) emission exhibited by them at room temperature. These InAs QDs are fabricated by the use of a specially designed atomic force microscope cantilever referred to as the Nano-Jet Probe (NJP). By using the NJP, two-dimensional arrays with ordered In nano-dots are fabricated in the desired square regions on a GaAs substrate and directly converted into InAs QD arrays through the subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs are stacked. These stacked QDs exhibit the PL emission peak at a wavelength of 1.02 μm.

Site-selective growth of self-assembled InAs quantum dots on focused ion beam patterned GaAs

A site-selective growth of self-organized InAs quantum dots (QDs) employing a combination of in-situ focused ion beam (FIB) implantation and self-organized molecular beam epitaxy (MBE) growth has been successfully demonstrated. First, a buffer layer of GaAs was grown by MBE before a square lattice of shallow holes with a pitch of 1–2 μm was fabricated by FIB implantation of Ga and In ions. Before InAs deposition, different procedures to remove the implantation damage were tested. The best results were obtained after re-evaporation of 3–5 nm GaAs. Another critical parameter that was carefully optimized is the deposited InAs amount. For the optimized process it was possible to induce growth of single QD in the hole with more than 50% probability. Between the shallow holes of 2 nm depths, no QD growth was observed. The optical quality of positioned dots was investigated by photoluminescence spectroscopy, employing a separate sample where the QDs were overgrown by GaAs cap layer. Excited-state interband transitions up to n=5 were observed from positioned QDs.

The impact of thermal effects on emission characteristics of asymmetrical AlGAO-waveguide microdisks based on quantum dots

Microphotoluminescence studies of AlGaO-based asymmetric waveguide microdisk structures incorporating a single sheet of self-organized InAs quantum dots (QD) under photoexcitation are performed. The authors have observed a strong redshift and broadening of the QD optical transition energy at high excitation densities due to excitation-density-induced overheating and many-particle effects and monitored the impact of these effects on the whispering gallery modes (WGMs) in spontaneous and stimulated emission regimes. They have observed strongly nonmonotonic dependencies of the linewidth and intensity of the WGMs with excitation density increase and underline possible reasons for the observed behavior.

Molecular beam epitaxial growth of site-controlled InAs quantum dot arrays using templates fabricated by the Nano-Jet Probe method

We have demonstrated the selective area growth of self-assembled InAs quantum dots (QDs) in the desired regions by using a template composed of InAs QD arrays. These InAs QDs were fabricated by the use of a specially designed atomic force microscope cantilever, referred to as the Nano-Jet Probe (NJP). Using the NJP, two-dimensional arrays of ordered In nano-dots were fabricated in the selected square regions on a GaAs substrate and directly converted to InAs QD arrays by subsequent annealing by the irradiation of As flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected square regions. These InAs QD arrays were used for the further stacking of QDs, and photoluminescence emission was confirmed from the fabricated stacked QD structure.

Defects around self-organized InAs quantum dots measured by slow positron beam

Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy. The authors try to use a slow positron beam to detect defects in and around self-organized QDs, and point defects are observed in GaAs cap layer above QDs. For the self-organized InAs QDs without strain-reducing layer, it is free of defects. However, by introducing a strain-reducing layer, the density of point defects around larger sized InAs QDs increased. The above results suggest that low energy positron beam measurements may be a good approach to detect depth profiles of defects in QD materials.

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal–organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.

One-electron spin-dependent transport in split-gate structures containing self-organized InAs quantum dots

The paper presents the results obtained in a study of electron transport in split-gate structures prepared from heterostructures with self-organizing InAs quantum dots situated close to a two-dimensional electron gas. Coulomb oscillations of current through InAs quantum dots depending on the voltage on the gate were observed. Coulomb current oscillations persisted up to about 20 K. The Coulomb energy ΔE C = 12.5 meV corresponding to theoretical estimates for the p-states of quantum dots in our structures was determined.