InAs Dots Research Articles

Optimisation of the physical properties of InAs/InGaAs/GaAs QDs heterostructures embedded p-i-n GaAs solar cell

In order to enhance absorption at infrared range for GaAs–based solar cell, we have grown multi–stacked InAs/InGaAs/GaAs quantum dots (QDs) heterostructure in the active region by Solid–Source Molecular Beam Epitaxy (SS–MBE). Two different families of dots were observed in the photoluminescence (PL) spectra. Temperature–dependent study was carried out at 10–240 K temperature range. Distinctive, asymmetric shape located in the high energy for the PL spectra of the QD solar cell sample can be deconvoluted in two sub–bands. From the temperature–dependent PL measurement, the two sub–bands are associated with the ground–state emission from the two families of InAs dots with different size. Besides, the spectral response of multi–stacked InAs/InGaAs QD solar cells extends the photo–absorption spectra towards a wavelength longer than the GaAs bandgap of 1280 nm. However, the QDs solar cell shows an enhanced short–circuits current density of 7.8 mA/cm² compared to the GaAs reference cell. The performance of the QD solar cells indicates that the InAs/InGaAs/GaAs QD heterostructures facilitate the fabrication of highly stacked QD layers that are suitable for solar cells devices requiring thick QD layers for sufficient light absorption.

High-performance InAs/GaAs quantum dot laser with dot layers grown at 425 oC

We investigate InAs/GaAs quantum dot (QD) lasers grown by gas source molecular beam epitaxy with different growth temperatures for InAs dot layers. The same laser structures are grown, but the growth temperatures of InAs dot layers are set as 425 and 500 °C, respectively. Ridge waveguide laser diodes are fabricated, and the characteristics of the QD lasers are systematically studied. The laser diodes with QDs grown at 425 °C show better performance, such as threshold current density, output power, internal quantum efficiency, and characteristic temperature, than those with QDs grown at 500 °C. This finding is ascribed to the higher QD density and more uniform size distribution of QDs achieved at 425 °C.

Optical properties of excitons in strained Gax In1-xAs/GaAs quantum dot: ef fect of geometrical conf inement on exciton g-factor

Taking into account anisotropy, nonparabolicity of the conduction band, and geometrical confinement, we discuss the heavy-hole excitonic states in a strained GaxIn1-xAs/GaAs quantum dot for various Ga alloy contents. The strained quantum dot is considered as a spherical InAs dot surrounded by a GaAs barrier material. The dependence of the effective excitonic g-factor as a function of dot radius and Ga ion content is numerically measured. Interband optical energy with and without the parabolic effect is computed using structural confinement. The interband matrix element for different Ga concentrations is also calculated. The oscillator strength of interband transitions on the dot radius is studied at different Ga concentrations in the GaxIn1-xAs/GaAs quantum dot. Heavy-hole excitonic absorption spectra are recorded for various Ga alloy contents in the GaxIn1-xAs/GaAs quantum dot. Results show that oscillator strength diminishes when dot size decreases because of the dominance of the quantum size effect. Furthermore, exchange enhancement and exchange splitting increase as exciton confinement increases.

Probing temperature-driven flow lines in a gated two-dimensional electron gas with tunable spin-splitting

We study the temperature flow of conductivities in a gated GaAs two-dimensional electron gas (2DEG) containing self-assembled InAs dots and compare the results with recent theoretical predictions. By changing the gate voltage, we are able to tune the 2DEG density and thus vary disorder and spin-splitting. Data for both the spin-resolved and spin-degenerate phase transitions are presented, the former collapsing to the latter with decreasing gate voltage and/or decreasing spin-splitting. The experimental results support a recent theory, based on modular symmetry, which predicts how the critical Hall conductivity varies with spin-splitting.

Electron dephasing of a GaAs/AlGaAs quantum well with self-assembled InAs dots

We study the magnetotransport of a GaAs/AlGaAs quantum well with self-assembled InAs quantum dots. Negative magnetoresistance is observed at low field and analysed by weak localization theory. The temperature dependence of the extracted dephasing rate is linear, which shows that the inelastic electron–electron scattering processes with small energy transfer are the dominant contribution in breaking the electron phase coherence. The results are compared with those of a reference sample that contains no quantum dots.

Band engineering in nanowires:Ab initiomodel of band edges modified by (111) biaxial strain in group IIIA-VA semiconductors

Quantum dots in nanowires grow on a (111) substrate and it is expected that the modifications of the band structure due to a biaxial strain in the (111) crystallographic plane will determine the confinement of charge carriers in these systems. In this work, we develop an ab initio methodology for the determination of biaxial strain-modified band energies on an absolute energy scale due to a strain in an arbitrary crystallographic plane and apply it to calculate the evolution of band edges in group IIIA-VA zinc-blende semiconductors (InP, InAs, and GaAs) under the (111) biaxial strain. The absolute hydrostatic deformation potentials, a prerequisite for the accurate calculation of the strain-modified band energies within our scheme, are determined. The strain tensor for an InAs dot grown on a (111) GaAs substrate is calculated and the importance of the (111) biaxial strain is demonstrated. The strained band offsets in InAs under a compressive biaxial strain in the (001) and (111) crystallographic planes as well as the local band structure in a InAs/GaAs dot indicates a stronger confinement of holes in the (111) case.

Piezoelectric InAs/GaAs quantum dots with reduced fine-structure splitting for the generation of entangled photons

Polarization-resolved single-dot spectroscopy reveals that the exciton fine-structure splitting in piezoelectric (211)B InAs/GaAs quantum dots is smaller than 10 \ensuremath{\mu}eV in the vast majority of examined dots. These values are significantly reduced compared to as-grown (100)-oriented InAs dots. Time-resolved measurements confirm the high oscillator strength of these dots, in spite of the internal piezoelectric field, suggesting good quantum efficiency at 4 K, comparable with that of (100) InAs/GaAs dots. Lastly, photon correlation measurements demonstrate single-photon emission from exciton levels of these dots. All these features make this intriguing dot system promising for implementing solid-state entangled photon sources.

Effect of proton bombardment on InAs dots and wetting layer in laser structures

The effect of proton bombardment on carrier lifetime and photoluminescence of InAs quantum dots was measured. Optical absorption and transmission electron microscopy show the dots retain their integrity under bombardment. A decrease in ground state photoluminescence with increasing dose is not explained by the decrease in dot carrier lifetime alone, but also by bombardment-induced non-radiative recombination in the wetting layer, which reduces the dot electron population at fixed excitation. To exploit the relative radiation immunity of quantum dots, it is necessary to maximise the dot density and capture probability per dot to minimize the effect of wetting layer recombination.

Temperature dependence of the excitonic energy band gap in In(Ga)As nanostructures

We analyzed the temperature-dependent variation of the peak energies in the photoluminescence of InAs/GaAs quantum dots and InAs/In0.2Ga0.8As/GaAs quantum dots in an asymmetric well. The InAs dots were grown by using migration-enhanced molecular beam epitaxy. The peak emission energies decreased monotonously with increasing temperature over the range of measurements from 13 K to 225 K. The temperature dependence of the peak energy was successfully fitted with two analytic models, namely, the phenomenological Varshni equation and the semi-empirical Fan equation based on electron-phonon statistics, respectively. The physical meaning of the Varshni coefficients is discussed in terms of the phonon energy and the strain in the nanostructures.

Structural properties of InAs‐based nanostructures grown on GaAs(001) and GaAs(111)A by area selective epitaxy

AbstractArea selective epitaxy (ASE) of InAs on patterned SiO2/GaAs(001) and SiO2/GaAs(111)A substrates has been investigated for various In flux intensities and substrate temperatures TS by using migration enhanced epitaxy (MEE). Transmission electron microscopy (TEM) indicates a different behaviour in the strain relaxation mechanism for ASE grown InAs structures on GaAs(001) and those grown on GaAs(111)A. In case of InAs/GaAs(001) Moiré‐like fringes due to strain relaxation are visible up to at least 35 nm in the InAs dot structure, while for InAs/GaAs(111)A the Moiré‐like fringes are confined at the interface. Furthermore, electron dispersion spectroscopy (EDS) showed a relatively high In concentration below the interface for InAs/GaAs(001) indicating the formation of a reaction layer even for structures grown at a low TS of 460 °C. For InAs on GaAs(111)A grown under the same conditions, almost no In was detected below the interface (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Erratum: In situ STM observation of site‐ controlled InAs nano‐dot growth on GaAs(001) during In and As4 irradiations

AbstractThe amounts of As4 flux, scanning time, and collapse time of InAs dot structure at 430 °C were erroneously reported and should be corrected. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hopping conduction and magnetoresistance of a GaAs/AlxGa1−xAs quantum well with embedded InAs dots

Magnetoresistance and temperature-dependent conductance are measured in the sample made of a GaAs/AlxGa1−xAs quantum well with self-assembled InAs dots. Conductance is analyzed by Mott’s hopping theory; the localization lengths have been extracted at various gate voltages. The sample is in the transition from near to metal-insulator to the deeply hopping regime with the combined effect of the longand short-range scattering potentials. The magnitude of the negative magnetoresistance increases with increasing negative gate voltage. The magnetic-field dependence of the resistance can be explained by the theory of the interference model of hopping electrons.

Very long wavelength quantum dot infrared photodetector using a modified dots-in-a-well structure with AlGaAs insertion layers

We report a modified dots-in-a-well (DWELL) infrared photodetector by inserting some very thin GaAs or AlGaAs layers into the InAs dots. The photoluminescence (PL) measurements indicate that the modified DWELL structure with the insertion layers (ILs) of GaAs has a larger peak intensity and a narrower PL linewidth than that without the ILs. For the modified DWELL detector with AlGaAs ILs, the peak detection wavelength reaches very long infrared window of 14.1 μm. The peak detectivity D∗ is 1.1×108 cm Hz1/2/W at 77 K under normal incidence infrared irradiation.

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.

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.

Strain Effects on Optical Properties of (In,Ga)As‐Capped InAs Quantum Dots Grown by Molecular Beam Epitaxy on GaAs (113)A Substrate

We have investigated the optical properties of InAs/GaAs (113)A quantum dots grown by molecular beam epitaxy (MBE) capped by (In,Ga)As. Reflection high‐energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A broadening of the PL emission due to size distribution of the dots, when InAs dots are capped by GaAs, was observed. A separation between large and small quantum dots, when they are encapsulated by InGaAs, was shown due to hydrostatic and biaxial strain action on large and small dots grown under specifically growth conditions. The PL polarization measurements have shown that the small dots require an elongated form, but the large dots present a quasi‐isotropic behavior.

In situ CBrCl 3 etching to control size and density of InAs/GaAs quantum dots

The effects of in situ CBrCl 3 etching of InAs quantum dots (QDs) grown on GaAs by metal-organic vapor phase epitaxy have been studied in detail for control of the size and density of InAs dots. The width and height distributions of InAs dots after etching for various etching times and CBrCl 3 flow rates suggest that the CBrCl 3 etching is ineffective in decreasing the width but effective in decreasing the height. The density of the InAs dots are controlled over more than 2 orders of magnitude by changing the flow rate from 0 to 22 μmol/min. Although the atomic force microscopic (AFM) image for 22 μmol/min does not show any dots because of a detection limit, the PL peak associated with the QDs evidences presence of dots whose density is less than 2×10 6 cm −2. Such a low density is suitable for single QD device applications. The PL spectra for various CBrCl 3 flow rates suggest less effective etching of the Ga-rich InGaAs wetting layer and the decreased size of QDs with the increased flow rates.

Temperature-dependent carrier tunneling for self-assembled InAs/GaAs quantum dots with a GaAsN quantum well injector

We have investigated the carrier tunneling process in a quantum-dot (QD) tunnel injection structure, which employs a GaAs1−xNx quantum well (QW) as a carrier injector. The influence of the barrier thickness between the GaAs1−xNx well and InAs dot layer has been studied by temperature-dependent photoluminescence. Although the 2.5 nm barrier sample exhibits the best tunneling efficiency, a 3.0 nm thickness for the barrier is optimum to retain good optical properties. The carrier capture time from the GaAs1−xNx QW to QD ground states has been evaluated by time-resolved photoluminescence. The result indicates that efficient carrier tunneling occurs at temperatures above 150 K due to the temperature dependent nature of phonon-assisted processes.

Probing two-dimensional metallic-like and localization effects at low magnetic fields

The metallic-like regime characterized by Shubnikov–de Haas (SdH) oscillations is investigated between localization-induced weak insulator and quantum Hall liquid in a gated two-dimensional GaAs electron system containing InAs dots. Multiple SdH crossing points are observed in the longitudinal resistivity before the appearance of the critical point of a plateau transition with increasing perpendicular magnetic field. In conductivities, however, there is no corresponding crossing in the metallic-like regime because of the T-dependent Hall slope under electron–electron interaction, which provides an explicit way to distinguish SdH crossing points from the critical point in our study.

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.