High-density InAs Quantum Dots Research Articles

Molecular beam deposition of high-density InAs quantum dots on SiOx films

InAs quantum dots (QDs) were grown on SiOx films by molecular beam deposition (MBD), and then an SiOx capping layer was deposited by rf magnetron sputtering. High arsenic pressure and As2 molecular beam flux enhanced InAs nucleation and the dot growth. Ultrahigh-density InAs QDs with 8.5 × 1011 cm−2 was obtained for As2 molecular beam of 3.0 × 10−5 Torr. The amount of InAs growth grown by As2 was about three times higher than that by As4. Photoluminescence (PL) properties depended on InAs QD structures. The vacuum transport process between InAs MBD and SiOx sputtering was effective to improve the quality of InAs QDs on a SiOx film. Furthermore, strong PL intensity was obtained from InAs QDs in a SiOx matrix, fabricated by the vacuum transport process.

Coarsening process of high-density InAs quantum dots on Sb-irradiated GaAs

High-density InAs quantum dots were grown on Sb-irradiated GaAs(001) by MBE. From InAs coverage and Sb irradiation time (SIT) dependences, the coarsening process of dots for long SIT (50 s) was classified according to four coverage regions. After the dot density rapidly increased and then reached the maximum (region I), the dot density decreased as the InAs coverage increased (region II). This decrease in dot density was attributed to quantum dot ripening. For a short SIT (7 s), the dot density was maintained without the ripening. In region III, the dot density was nearly constant value [(1.2–1.4) × 1011 cm−2], despite the different SITs. In addition, the dot size was limited by stable {011} facets, and an inhomogeneous broadening of dot size distribution was improved. However, for a long SIT (50 s), small dots remained owing to the ripening. In region IV, coalescence occurred, and the density of giant dots slightly increased.

Antimony mediated growth of high-density InAs quantum dots for photovoltaic cells

We report enhanced solar cell performance using high-density InAs quantum dots. The high-density quantum dot was grown by antimony mediated molecular beam epitaxy. In-plane quantum dot density over 1 × 1011 cm−2 was achieved by applying a few monolayers of antimony on the GaAs surface prior to quantum dot growth. The formation of defective large clusters was reduced by optimization of the growth temperature and InAs coverage. Comparing with a standard quantum dot solar cell without the incorporation of antimony, the high-density quantum dot solar cell demonstrates a distinct improvement in short-circuit current from 7.4 mA/cm2 to 8.3 mA/cm2.

Wideband luminescence of high-density InAs quantum dots on GaAsSb/GaAs layers

In order to achieve wideband luminescence, ultrahigh density InAs quantum dots (QDs) were grown on GaAsSb buffer layers by molecular beam epitaxy under various growth conditions. Ultrahigh density of 3.8–5.2×1011cm−2 and large size fluctuation were obtained. Photoluminescence (PL) spectra of ultrahigh density QD layers were changed by adjusting the growth conditions. Three ultrahigh density QD layers with different PL spectra were stacked on GaAs (001) substrate. The PL spectrum of edge emission revealed strong intensity and a broad linewidth of 194nm. The presented QD structures are expected for development of wideband superluminescent diodes.

Silver Embedded Nanomesas as Enhanced Single Quantum Dot Emitters in the Telecommunication C Band

We use high-density InAs quantum dots, which were grown by molecular beam epitaxy on InP(311)B substrates, as photon sources in the telecommunication C band at approximately 1.55 µm. To select a small numbers of dots, we fabricate sub-micrometer sized mesas by electron beam lithography and reactive ion etching. The benefit of using high-density quantum dot samples is that at least one optically active quantum dot can be expected in every single mesa. We show that the etching rate and resulting mesa shape of the In0.53Al0.22Ga0.25As epitaxial layer can be varied with the chamber pressure during the etching process. Furthermore, under constant pressure and with increasing etching time, the sequential etching of the epitaxial layer and the underneath substrate leads to a significant modification in the mesa shape, too. We demonstrate that the isolation of a small number of quantum dots within one mesa results in the appearance of single quantum dot emission with a narrow line width and minimal spectral overlap between different emission lines. We moreover present significant enhancement of the luminescence collected from single dots in silver-embedded nanomesas when compared with as-etched mesas.

Effect of antimony irradiation on InAs/Sb:GaAs quantum dots grown by molecular beam epitaxy

In this paper we discuss the fabrication of high-density InAs quantum dots (QDs) on GaAs (1 0 0) substrates by molecular beam epitaxy, via antimony surfactant-mediated growth. The structural and optical properties of the QDs were studied as a function of the amount of antimony irradiation. It was found that with appropriate antimony irradiation, the dot density significantly increases while simultaneously restraining giant QDs. However, when the irradiation is beyond a certain amount, the dot density inversely decreases significantly. Moreover, temperature-dependent photoluminescence (T-PL) measurements reveal that antimony irradiation can cause a larger redshift of the PL peak wavelength at all temperatures, a faster decrease of integrated PL intensity below 100 K and a slower decrease of integrated PL intensity within 100–300 K as compared to measurements taken in the absence of antimony irradiation. It was also found that by increasing the amount of antimony irradiation, the samples' behavior is closer to that of conventional QDs.

Time-Resolved X-ray Diffraction Measurements of High-Density InAs Quantum Dots on Sb/GaAs Layers and the Suppression of Coalescence by Sb-Irradiated Growth Interruption

Self-assembly of high-density InAs quantum dots (QDs) on Sb-irradiated GaAs buffer layers was observed in-situ by a time-resolved X-ray diffraction (XRD) technique using a combination of XRD and molecular beam epitaxy. Evolution of dot height and lattice constant was analyzed during InAs QD growth and subsequent growth interruption (GI), and as a result, dislocated giant dots due to coalescence and coherent dots were separately evaluated. An Sb-irradiated GI (Sb-GI) method to be applied after InAs growth was attempted for the suppression of coalescence. Using this method, the XRD intensity of giant dots decreased, and the photoluminescence intensity of InAs QDs was enhanced. High-density InAs QDs without giant dots were produced by using the combination of the QD growth on the Sb-irradiated GaAs buffer layers and the Sb-GI.

Self-assembled InAs quantum dots on cross-hatch InGaAs templates: Excess growth, growth rate, capping and preferential alignment

Self-assembled InAs quantum dots (QDs) are grown on cross-hatch GaAs/InGaAs templates via molecular beam epitaxy with systematic variations in the degree of excess growth, growth rate and capping of the QD layer. Standard epitaxy (SE) with excess growth and migration-enhanced epitaxy (MEE) at high growth rates result in the formation of uniform, high-density InAs QDs on the InGaAs cross-hatch layer. When overgrown or capped by a thin GaAs layer, these QDs undergo shape transformation where quantum dashes and wires along the [1 1 0]- and [1 −1 0] directions are simultaneously formed as a result of chemical potential gradient and anisotropic strain fields. When the underlying InGaAs cross-hatch layer is covered by a thick GaAs spacer layer, subsequent growth of InAs QDs via SE or MEE results in preferential alignment of QDs along the [1 1 0]- or [1 −1 0] direction, respectively. This is attributed to the asymmetries in the underlying cross-hatch and adatoms mobility. The various size, density, shape and alignment distributions of QDs on cross-hatches provide means for engineering of optoelectronic devices.

Uniform formation of high-density InAs quantum dots by InGaAs capping growth

We studied the capping growth process of an InGaAs on high-density InAs/GaAs(001) quantum dots (QDs) with a bimodal size distribution grown by molecular beam epitaxy. Atomic force microscopy and reflection high energy electron beam diffraction observations revealed that the bimodal size distribution is composed of larger QDs with {110} facets and smaller QDs with {136} facets. In addition, it was found that, during the InGaAs capping growth, the height of larger QDs decreased, while the height of smaller QDs increased due to the incorporation of indium adatoms. As a result, the size fluctuation of the QDs was suppressed as compared to GaAs capping growth. We achieved a narrow photoluminescence (PL) linewidth of 24 meV and an enhanced PL peak intensity.

High-density InAs quantum dots on GaNAs buffer layer

Characteristics of InAs quantum dot density on a GaNAs buffer layer grown by metalorganic chemical vapor deposition were investigated using atomic force microscopy, focusing on the growth rate of the buffer layer as a significant growth parameter. High dot density of 1.7×10 11 cm −2 was achieved by using the GaNAs buffer with the growth rate of 1.45 μm/h. It is found that surface roughness of the buffer layer is a key factor of increasing dot density. Suppression of coalescent dots that are one of the causes of emission intensity degradation was also quantitatively obtained. These results are effective for quantum dot lasers with high modal gain.

In situ determination of Sb distribution in Sb/GaAs(0 0 1) layer for high-density InAs quantum dot growth

An Sb-adsorbed GaAs(0 0 1) substrate that serves as a template for high-density InAs quantum dot (QD) growth was investigated using in situ X-ray diffraction. The Sb distribution in the top eight layers from the surface was determined by crystal truncation rod scattering analysis. It was found that Sb atoms penetrated to the eighth layer when GaAs(0 0 1) came in contact with an Sb environment. The amounts of Sb in the first and second layers were, however, saturated at 1 3 atomic layer (AL) and 2 3 AL, respectively. A comparison between the X-ray results and atomic force microscopy observations of the QD density showed that the formation of high-density QDs is correlated with the total amount of Sb in the surface and subsurface layers.

Improved quantum confinement of self-assembled high-density InAs quantum dot molecules in AlGaAs∕GaAs quantum well structures by molecular beam epitaxy

Self-assembled, multistack InAs quantum dot molecules (QDMs) were grown by a modified molecular beam epitaxial (MBE) technique, which involves multiple stacking and multiple cycling of the thin-capping-and-regrowth process, so as to obtain a large volume density of quantum dots on the sample. Furthermore, the high-density InAs QDMs were also grown sandwiched either between a double heterostructure (DHS) or between a quantum-well (QW) structure. It was found from microphotoluminescence (μ-PL) measurements that the QDMs sandwiched between these structures give broader PL spectra than those of the as-grown QDMs. The broadening of the PL spectra is associated with the poorer dot size uniformity, which arises from the long and complicated MBE growth processes. However, comparing between the QDMs in the DHS and in the QW structure, the latter give narrower PL spectra. The narrower PL spectra for the QDM-in-QW structure is attributed to the improved quantum confinement effect arising from the use of the QW.

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.

High-density and narrow size-distribution InAs quantum dots formed by a modified two-step growth

We develop a modified two-step method of growing high-density and narrow size-distribution InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. In the first step, high-density small InAs QDs are formed by optimizing the continuous deposition amount. In the second step, deposition is carried out with a long growth interruption for every 0.1 InAs monolayer. Atomic force microscope images show that the high-density (∼5.9×1010 cm−2) good size-uniformity InAs QDs are achieved. The strong intensity and narrow linewidth (27.7 meV) of the photoluminescence spectrum show that the QDs grown in this two-step method have a good optical quality.

In-plane self-arrangement of high-density InAs quantum dots on GaAsSb∕GaAs(001) by molecular beam epitaxy

InAs quantum dots (QDs) were grown on GaAsSb∕GaAs(001) buffer layers by molecular beam epitaxy using a Stranski-Krastanov mode. A high QD density of about 1×1011 cm−2 was obtained for an Sb flux ratio of more than 0.14. In the case of high-density QD growth on the GaAsSb layers, many nanoholes were observed on the surface and located beside the QDs. In addition, one-dimensional QD chains were formed near nanogrooves and step edges. As the InAs growth proceeded, an in-plane arrangement of the InAs QDs was partially formed along ⟨010⟩ directions. It is considered that the in-plane arrangement of the QDs originates from the QD chains and the nanogrooves, aligned along ⟨010⟩ step edges. From transmission electron microscopy and photoluminescence measurements, it was found that the high-density InAs QDs on the GaAsSb buffer layer reveal high crystal quality.

Selective growth of InAs quantum dots using In nano-dot arrays formed by nano-jet probe method

We have demonstrated the selective area growth of high density InAs quantum dots (QDs) by using site-controlled InAs dots that were formed in the desired regions as templates. The fabrication of the InAs dots for the templates was enabled by the use of a specially designed atomic-force-microscope (AFM) cantilever, referred to as the nano-jet probe (NJP). Using the NJP, two-dimensional (2D) arrays of ordered In nano-dots were reproducibly fabricated in the desired regions on a GaAs substrate. These In nano-dots were directly converted to InAs QD arrays by subsequent annealing with irradiation of arsenic flux. By using the converted QD arrays as strain templates, self-organized InAs QDs were formed in the selected regions.

Effect of antimony on the density of InAs/Sb:GaAs(1 0 0) quantum dots grown by metalorganic chemical-vapor deposition

We report the fabrication of high-density InAs quantum dots (QDs) grown on GaAs(1 0 0) substrate by metalorganic chemical-vapor deposition (MOCVD), obtained by antimony surfactant-mediated growth. We achieved InAs/Sb:GaAs QDs with dot density ranging from 2×10 10 cm to 2–11×10 11 cm −2, with complete suppression of coalescence. We studied the dependence of the total dot density and the density of coalesced dots on the growth conditions, such as antimony irradiation time, growth temperature and growth rate. Strongly enhanced PL intensity at room temperature (RT) was obtained from InAs/Sb:GaAs QDs with density above 4×10 10 cm −2, compared to reference InAs/GaAs QDs.

Realization of 1.3 μm InAs quantum dots with high-density, uniformity, and quality

We propose a 1.3 μm high-density InAs quantum dot (QD) structure for optical devices that uses an As 2 source and a gradient composition strain reducing layer (GC-SRL). The temperature dependence of the photoluminescence (PL) peak intensity for a temperature increase from 11 K to room temperature was very low at 1 3 . Moreover, the PL peak intensity was ten times greater than that of low-density QDs because of an increase in the QD number, although the PL bandwidth was the same. For the first time, we realized QDs with high-density, uniformity, and quality. This is a promising result since optical devices with QDs exceeding 1.3 μm on a GaAs substrate are useful in fiber communications.

Self-Formation of High-Density and High-Uniformity InAs Quantum Dots on Sb/GaAs Layers by Molecular Beam Epitaxy

Self-assembled InAs quantum dots (QDs) grown on Sb/GaAs buffer layers by molecular beam epitaxy were investigated to achieve high-density and high-uniformity QDs. The density of InAs QDs on Sb/GaAs layers was markedly higher than that of InAs QDs on conventional GaAs layers. The high-density nucleation of three-dimensional (3D) InAs islands was attributed to the formation of two-dimensional (2D) wirelike structures prior to 3D islanding. QD density was controlled from 3×1010 to 1×1011 cm-2 by changing Sb supply amount. In addition, the density of very large dots due to coalescence increased with increasing Sb supply amount. On Sb (0.6 atomic layers)/GaAs buffer layers, we obtained high-density and high-quality InAs QDs, which revealed a narrow photoluminescence linewidth of about 30–35 meV.

Sb原子による表面・界面改質効果を利用したInAs量子ドットの自己形成制御

We demonstrated a fabrication method for high-density and high-uniformity InAs quantum dots (QDs) on Sb-terminated GaAs(001) buffer layers using molecular beam epitaxy (MBE) of Stranski-Krastanov (SK) mode. The high-density formation of QDs is caused by the initial formation of 2-dimensional wire structures and small islands, which are induced by a surface exchange reaction of Sb atoms and As atoms. The surface segregation of Sb atoms also suppresses coalescence between neighboring QDs. In addition, the non-radiative recombination process is effectively suppressed by developed Sb-termination conditions. Furthermore, we found a new in-plane self-arrangement of high-density InAs QDs on the GaAsSb/GaAs(001) buffer layers.