Formation Of InAs Quantum Dots Research Articles

Effects of a thin nitrogen-doped layer on terahertz dynamics in GaAs containing InAs quantum dots

The transient terahertz signal generated by electron diffusion that was excited by an ultrashort pulse was observed to vary with the potential structure around the surface. The formation of InAs quantum dots caused the strain field and the nitrogen atoms to further modify the potential structure around the quantum dots. The terahertz signal that was observed for various pump energies exhibited an interesting phase change. Further, the change in terahertz dynamics originated from the formation of a dilute nitrogen layer around the quantum dots. Furthermore, this change depends on the position of the nitrogen-doped layer. The results of this study provide important information for controlling the optical properties of quantum dots.

Influence of Bi on morphology and optical properties of InAs QDs

We study the tsurface morphology and photoluminescence (PL) property of InAs quantum dots (QDs) on GaAs using bismuth (Bi) in the layer prior to or after the growth of QDs. Incorporating Bi in the layer prior to the QD deposition delays the onset of InAs QD formation resulting in a decrease in QD height and density. As a surfactant, adding Bi in the GaAs capping layer at a high growth temperature reduces the In surface diffusion length leading to uniform and well preserved InAs QDs in terms of height and density. The incorporation of 3% Bi at a low growth temperature, which forms a GaAsBi capping layer, can effectively lower the PL transition energy up to 163 meV and reduce the PL linewidth, leading to an emission wavelength of 1.365 μm at 77 K.

PL of low‐density InAs/GaAs quantum dots with different bimodal populations

Optical response of the indium arsenide (InAs)/gallium arsenide (GaAs) quantum dots (QDs) with a bimodal distribution is investigated through varying an initial GaAs capping layer between 35 and 18 monolayers. Photoluminescence (PL) measurements confirm that a thinner initial capping layer can reduce the QD dimensions and also modify the population ratio between the large QDs and the small QDs. Therefore, PL quenching related to QD dimension and carrier transfer between the bimodal QD populations has been affected. Manipulation of the initial GaAs capping layer provides a feasible approach to tailor the formation and optical performance of InAs QDs for optoelectronic device applications.

Growing InAs/GaAs quantum dots by droplet epitaxy under MOVPE conditions

Results of studying the formation of InAs quantum dots (QDs) on GaAs(100) substrates by droplet epitaxy using trimethylindium and arsine (AsH3) as precursors are presented. The growth process was carried out at temperatures within 230–400°C in a horizontal reactor for metalorganic vapor phase epitaxy (MOVPE) using high-purity hydrogen as the carrier gas. Data on the influence of process temperature on the QD size and the density of QD array and results of investigation of the low-temperature photoluminescence of obtained samples are presented.

Increasing the quantum efficiency of InAs/GaAs QD arrays for solar cells grown by MOVPE without using strain‐balance technology

AbstractResearch into the formation of InAs quantum dots (QDs) in GaAs using the metalorganic vapor phase epitaxy technique is presented. This technique is deemed to be cheaper than the more often used and studied molecular beam epitaxy. The best conditions for obtaining a high photoluminescence response, indicating a good material quality, have been found among a wide range of possibilities. Solar cells with an excellent quantum efficiency have been obtained, with a sub‐bandgap photo‐response of 0.07 mA/cm2 per QD layer, the highest achieved so far with the InAs/GaAs system, proving the potential of this technology to be able to increase the efficiency of lattice‐matched multi‐junction solar cells and contributing to a better understanding of QD technology toward the achievement of practical intermediate‐band solar cells. Copyright © 2016 John Wiley & Sons, Ltd.

Characterization and Effect of Thermal Annealing on InAs Quantum Dots Grown by Droplet Epitaxy on GaAs(111)A Substrates.

We report the study on formation and thermal annealing of InAs quantum dots grown by droplet epitaxy on GaAs (111)A surface. By following the changes in RHEED pattern, we found that InAs quantum dots arsenized at low temperature are lattice matched with GaAs substrate, becoming almost fully relaxed when substrate temperature is increased. Morphological characterizations performed by atomic force microscopy show that annealing process is able to change density and aspect ratio of InAs quantum dots and also to narrow size distribution.

Hopkins-Skellam index and origin of spatial regularity in InAs quantum dot formation on GaAs(001)

We investigate the origin of the spatial regularity of arrays of InAs quantum dots (QDs) grown on GaAs(001). The Hopkins-Skellam index (HSI) is used with a newly developed calculation algorithm to quantify the spatial regularity both of QDs and of nm-sized surface reconstruction territories (SRTs) present in the InxGa1−xAs wetting layer prior to QD nucleation. The SRT is the minimum extent of a surface reconstruction region needed for one QD to nucleate. By computing the evolving HSI of SRTs from sequences of in situ scanning tunnelling microscopy images during growth, we find that the spatial regularity of QDs is traced back to that of the (n × 3) SRTs as early as 0.6 monolayers of InAs coverage. This regularity is disturbed by the (n × 4) SRTs which appear at higher coverage. The SRT approach is discussed in comparison to conventional capture zone theories of surface growth.

Self-assembly of InAs quantum dots on GaAs(001) by molecular beam epitaxy

Abstract Currently, the nature of self-assembly of three-dimensional epitaxial islands or quantum dots (QDs) in a lattice-mismatched heteroepitaxial growth system, such as InAs/GaAs(001) and Ge/Si(001) as fabricated by molecular beam epitaxy (MBE), is still puzzling. The purpose of this article is to discuss how the self-assembly of InAs QDs in MBE InAs/GaAs(001) should be properly understood in atomic scale. First, the conventional kinetic theories that have traditionally been used to interpret QD self-assembly in heteroepitaxial growth with a significant lattice mismatch are reviewed briefly by examining the literature of the past two decades. Second, based on their own experimental data, the authors point out that InAs QD self-assembly can proceed in distinctly different kinetic ways depending on the growth conditions and so cannot be framed within a universal kinetic theory, and, furthermore, that the process may be transient, or the time required for a QD to grow to maturity may be significantly short, which is obviously inconsistent with conventional kinetic theories. Third, the authors point out that, in all of these conventional theories, two well-established experimental observations have been overlooked: i) A large number of “floating” indium atoms are present on the growing surface in MBE InAs/GaAs(001); ii) an elastically strained InAs film on the GaAs(001) substrate should be mechanically unstable. These two well-established experimental facts may be highly relevant and should be taken into account in interpreting InAs QD formation. Finally, the authors speculate that the formation of an InAs QD is more likely to be a collective event involving a large number of both indium and arsenic atoms simultaneously or, alternatively, a morphological/structural transformation in which a single atomic InAs sheet is transformed into a three-dimensional InAs island, accompanied by the rehybridization from the sp 2-bonded to sp 3-bonded atomic configuration of both indium and arsenic elements in the heteroepitaxial growth system.

Distribution of elastic strains appearing in gallium arsenide as a result of doping with isovalent impurities of phosphorus and indium

The distribution of elastic strains in a system consisting of a quantum-dot layer and a buried GaAsxP1 − x layer is studied using geometric phase analysis. A hypothesis is offered concerning the possibility of controlling the process of the formation of InAs quantum dots in a GaAs matrix using a local isovalent phosphorus impurity.

Catalytic influence of a deposition surface on the formation of InAs quantum dots via the pyrolysis of trimethylindium and arsine

Factors affecting the formation of ordered arrays of InAs quantum dots using the drop method on the surface of the GaAs substrate under conditions of MOCVD hydride epitaxy are investigated. The catalytic influence the substrate has on the pyrolysis of trimethylindium and arsine are analyzed by means of quantum chemistry. The energy parameters that characterize the pyrolysis of the investigated compounds under homogeneous and heterogeneous conditions are evaluated quantitatively. The possibility of substantially lowering the temperature of the formation of InAs quantum dots by the considered method on GaAs substrates is shown and confirmed experimentally.

Volmer–Weber InAs quantum dot formation on InP (113)B substrates under the surfactant effect of Sb

We report on Sb surfactant growth of InAs nanostructures on GaAs0.51Sb0.49 layers deposited on InP (001) and on (113)B oriented substrates. On the (001) orientation, the presence of Sb significantly favors the two-dimensional growth regime. Even after the deposition of 5 mono-layers of InAs, the epitaxial film remains flat and InAs/GaAs0.51Sb0.49 type-II quantum wells are achieved. On (113)B substrates, same growth runs resulted in formation of high density InAs islands. Microscopic studies show that wetting layer is missing on (113)B substrates, and thus, a Volmer-Weber growth mode is concluded. These different behaviors are attributed to the surface energy changes induced by Sb atoms on surface.

On the size distribution in three-dimensional quantum-dot crystals

The size distribution function of nanodots in artificial three-dimensional (Si)Ge/Si and In(Ga)As/GaAs quantum dot crystals grown using templates with perfect periodicity is calculated. Pyramidal nanodots were modeled by cone-shaped clusters for which the Thomson formula was derived, which is necessary to determine the growth (dissolution) rate of clusters during Ostwald ripening. A comparison of the calculated curve with experimental histograms shows that the size distribution itself is formed during Ostwald ripening and is caused by features of Ge and InAs quantum-dot formation on preliminarily textured Si and GaAs substrates.

Droplet epitaxial growth of highly symmetric quantum dots emitting at telecommunication wavelengths on InP(111)A

We demonstrate the formation of InAs quantum dots (QDs) on InAlAs/InP(111)A by means of droplet epitaxy. The C3v symmetry of the (111)A substrate enabled us to realize highly symmetric QDs that are free from lateral elongations. The QDs exhibit a disk-like truncated shape with an atomically flat top surface. Photoluminescence signals show broad-band spectra at telecommunication wavelengths of 1.3 and 1.5 μm. Strong luminescence signals are retained up to room temperature. Thus, our QDs are potentially useful for realizing an entangled photon-pair source that is compatible with current telecommunication fiber networks.

Modeling InAs quantum-dot formation on the side surface of GaAs nanowires

We have theoretically studied the formation of InAs quantum dots (QDs) on the side surface of GaAs nanowires (NWs). The effective energies of formation of a thin InAs layer and QDs on the NW side surface are compared with allowance for elastic stresses at the radial heteroboundary of two materials with lattice mismatch. The concept of a critical thickness of the external (wetting) layer is introduced, at which the mechanical stresses stimulate three-dimensional growth of QDs. The dependence of the critical layer thickness on the NW diameter and elastic constants of the system is determined. The phenomenon of partial filling of the NW side surface by QDs is explained by a decrease in the thickness of a deposited InAs layer with increasing height. The results of modeling agree well with the available experimental data.

Growth of InAs quantum dots on Si-based GaAs nanowires by controlling the surface adatom diffusion

InAs/GaAs dots-on-nanowire (NW) hybrid heterostructures are grown on Si substrate by metal organic chemical vapor deposition. The formation of InAs quantum dots (QDs) is intimately associated with the NW density as well as the substrate surface properties, both of which strongly affect the surface adatom diffusion. InAs QDs are realized with a short deposition time by reducing the NW density. The QDs exhibit specific facets and pure zinc blende structure, residing on a wetting layer of several nanometers. Photoluminescence emission from the QDs is observed at room temperature, with a linewidth of 186meV. The results are promising for future integration of III–V NW hybrid devices, especially photovoltaic cells on Si.

Effects of growth conditions on the formation of self-assembled InAs quantum dots grown on (1 1 5)A GaAs substrate

Effects of growth conditions on the formation of InAs quantum dots (QDs) grown on GaAs (115)A substrate were investigated by using the reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy (PL). An anomalous evolution of wetting layer was observed when increasing the As/In flux ratio. This is attributed to a change in the surface reconstruction. PL measurements show that QDs emission was strongly affected by the InAs deposited amount. No obvious signature of PL emission QDs appears for sample with 2.2 ML InAs coverage. Furthermore, carrier tunneling from the dots to the non-radiative centers via the inclination continuum band is found to be the dominant mechanism for the InAs amount deposition up to 4.2 MLs.

Realization of Stranski–Krastanow InAs quantum dots on nanowire-based InGaAs nanoshells

We investigate the formation and optical properties of InAs quantum dots on an InGaAs nanosubstrate. We find that Stranski–Krastanow InAs quantum dots are hardly formed on Au-catalyzed InGaAs nanowires due to the phase separation as well as stacking faults. High-quality Stranski–Krastanow InAs quantum dots are realized on a pure zinc blende InGaAs shell radially grown on a GaAs nanowire core. The quantum dots have a large size and regular shape, residing on a wetting layer of several nanometers. For optical characterization, we fabricate a “dot-in-well” structure by capping the quantum dots with InGaAs/GaAs double layers. Photoluminescence from the quantum dots is observed at 77 K, with a peak wavelength of 954 nm, which is distinctly redshifted compared with that of InAs quantum dots directly grown on GaAs nanowires. This work shows the potential of growing Stranski–Krastanow QDs on more types of NWs and obtaining longer wavelengths for wider applications.

Effect of vicinal substrates on the growth and device performance of quantum dot solar cells

During quantum dot (QD) growth, substrate misorientation has been shown to play a role in the QD growth mechanism, changing their size, shape and density. Since various misorientation angles are used in production of solar cells, this work investigates QD enhanced GaAs p-i-n solar cells grown using the Stranski–Krastanov (SK) growth method on substrates misoriented either 2° or 6° off the (100) in the [11¯0] direction. Results of this work show that 2° misoriented samples have a lower critical thickness for InAs QD formation as compared to the 6° misorientation: ∼1.7 monolayers (ML) versus∼1.8ML, respectively. In addition, the 6° substrates showed a more uniform QD density and size distribution of QDs without significant QD coalescence. Photoluminescence of both substrate types shows that the QD ground state transitions are similar in wavelength. Results of the solar cells under one sun illumination show that QD cells grown on both 2° and 6° substrates have higher short-circuit current density than comparable cells without QD and maintain a high open-circuit voltage. The QD contributed short-circuit current density was normalized for QD size and density for both the 2° and 6° samples. Values of 360A/cm2 per cm3 of InAs and 400A/cm2 per cm3 of InAs were found for the 2° and 6° samples, respectively. For both substrate types, the reduced number of coalesced QDs promoted effective strain balancing, while the increased QD density lead to strong QD absorption.

Formation and morphological evolution of InAs quantum dots grown by chemical beam epitaxy

In this work, we study the formation and the morphological evolution of InAs quantum dots (QDs) grown by chemical beam epitaxy on GaAs (001) substrate. A series of samples having different nominal InAs thicknesses has been investigated using atomic force microscopy (AFM) and low-temperature photoluminescence (PL) experiments. AFM results show that large two-dimensional (2D) clusters evolve into three-dimensional (3D) islands that change in size and density as the quantity of deposited InAs material increases. The 2D–3D growth mode transition occurs at an InAs thickness of 1.6 monolayer (ML). The QD density reaches a maximum value of about 8 × 1010 cm−2 at 2.4 ML and dot coalescence is observed for larger InAs thicknesses. These results are consistent with PL measurements performed on samples having an additional GaAs cap layer. A broad QD PL band appears when the InAs thickness reaches 1.6 ML and this emission band is redshifted for thicknesses above 2.4 ML.

Self-assembled InAs quantum dots on anti-phase domains of GaAs on Ge substrates

The authors report the formation of self-assembled InAs quantum dots (QDs) grown on GaAs/Ge substrates having anti-phase domains (APDs) by molecular beam epitaxy. The AFM images of InAs QDs grown on different GaAs thicknesses are shown and compared. The samples with InAs coverage of 1.80MLs with GaAs thickness of 300 and 700nm show non-uniformed size distribution of the dots. Due to anisotropic property of quantum dots, ellipsoidal quantum dots appear. Unexpectedly, most of InAs quantum dots align perpendicularly to anti-phase boundary (APB) and to quantum dot alignment formed in adjacent domains. Photoluminescence spectrum excited by 20mW 476-nm Ar+ laser at 20K does not show emission peak of InAs QDs. This is due to defects in the GaAs buffer layer.