High Dot Density Research Articles

Formation of mid-infrared emissive InAs quantum dots on a gradedInxGa1−xAs/InP matrix with a more uniform size and higher density under safer growth conditions

InAs mid-infrared emissive quantum dots (QDs) grown on a gradedInxGa1−xAs/InP matrix with more uniform size and higher dot density have been successfully prepared by lowpressure metal organic chemical vapour deposition (LP-MOCVD) under safer growth conditions.Low toxic tertiarybutylarsine and tertiarybutylphosphine sources were used to replace the hightoxic arsine and phosphine in the MOCVD growth. To improve the process safety further, inertialN2 instead of the normallyused explosive H2 was used as the carrier gas. Initially, by using a two-step growth method, uniform InAs QDs with a high dotdensity of 1.3 × 1010 cm−2 have been successfully grown on a InGaAs/InP matrix. The emission wavelength of the QDs reaches>2.1 µm. The low temperature photoluminescence spectrum of the QDs grown by the two-stepgrowth has much narrower linewidth and higher intensity than that of the QDs grown byusing normal Stranski–Krastanow (S–K) and atomic layer epitaxy (ALE) growth methods.

1.3-/spl mu/m InAs quantum-dot laser with high dot density and high uniformity

We realized a triple-stacked 1.3-mum InAs quantum dot (QD) with a high density of 2.4times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a high uniformity of below 24 meV that employs an As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> source and a gradient composition (GC) strain-reducing layer (SRL) grown on a GaAs substrate. We demonstrated the 1.3-mum wavelength emission of this triple-stacked QD laser with a 0.92-mm cavity length and a cleaved facet at room temperature. In addition, we realized the highest maximum modal gain yet reported of 8.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> per QD layer at beyond 1.28 mum by using our high-density and high-uniformity QD

Evolution of Microstructure and Microchemistry in Cold-worked 316 Stainless Steels under PWR Irradiation

The evolution of microstructures and microchemistry was examined by transmission electron microscopy in cold-worked SUS 316 stainless steel components irradiated in a pressurized water reactor to 1–73 dpa at 565–596 K. Homogenous nucleation of dislocation loops, helium bubbles and γ′ precipitates was detected. The dislocation loops consisted of a high density of Frank loops and black dots. The black dots are considered to be small Frank loops, some fraction of which could be vacancy-type. The size distribution showed a double peak, which remained unchanged up to 73 dpa, suggesting that the dislocation structure was saturated under a balance of nucleation and disappearance. The helium bubbles were extremely dense and fine, resulting in swelling of less than 0.1%. Such bubble structure was formed under irradiation with a high He/dpa ratio. The γ′ precipitation was detected at doses higher than 4 dpa with an increasing density for higher doses. The measured radiation hardening was almost explained by these visible microstructural features. Radiation-induced segregation at grain boundaries was confirmed to continuously develop up to 73 dpa whereas no significant segregation could be detected at Frank loops.

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 AsH 3 (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.

High-density assembly of nanocrystalline silicon quantum dots

This paper reports on a new bottom–up technique of forming silicon nanostructures based on assembly of nanocrystalline (nc) Si dots from the solution. The nc-Si dots with a diameter of 8 ± 1 nm were fabricated by using VHF plasma decomposition of pulsed SiH 4 gas supply and deposited on the substrate randomly. We first studied the method of making the nc-Si dot dispersion solution by immersing the deposited wafer into various kinds of solvent with ultra sonic treatment. We found that methanol works as a suitable solvent for nc-Si dots. We then add a drop of the nc-Si dot solution onto other substrates and evaporated it. During the evaporation the nc-Si dots were assembled in the solution via the lateral capillary meniscus force which works as an attractive force between the dots. Use of SiO 2 substrate with good surface wettability with the solution was found vital to have the maximum meniscus force and to have two-dimensional assembly of the dots. The evaporation speed was carefully controlled via temperature and evaporation pressure to achieve high dot density assembly. In addition, we examined the assembly of the nc-Si dots on the silicon-on-insulator substrates with various kinds of nanoscale patterning and succeeded in making the nc-Si dots cluster bridging between the nanoelectrodes with a gap of as small as 20 nm.

Thin-capping-and-regrowth molecular beam epitaxial technique for quantum dots and quantum-dot molecules

A thin-capping-and-regrowth molecular beam epitaxial technique is proposed and demonstrated to be a suitable approach for the growth of lateral quantum-dot molecules (QDMs). By regrowing on top of nanoholes, previously formed from as-grown quantum dots (QDs) via a thin-capping process, nanopropeller QDs are formed. By repeating the thin-capping-and-regrowth process for several cycles at the regrown thickness of 0.6 ML, nanopropeller QDs are linked along the [11¯0] crystallographic direction, leading to the alignment of QDs. The thin-capping-and-regrowth process is repeated for 1, 3, 5, 7, and 10cycles on different samples for comparison purposes. It is found from ex situ atomic force microscopy that at 7cycles of thin capping and regrowth of QDs, the best alignment of QDs is achieved. This is due to the strain having an optimum condition. The samples that undergo three and five thin-capping-and-regrowth cycles show some randomness of QD formation. When the process is repeated for 10cycles⁠, QDs become randomly distributed, but with a higher dot density than the as-grown sample. The high dot density results in a strong photoluminescence at room temperature. It is also shown that when self-aligned QDs are used as templates, aligned QDMs can be obtained at a regrowth thickness of 1.2 ML.

An Essential Function of the C. elegans Ortholog of TPX2 Is to Localize Activated Aurora A Kinase to Mitotic Spindles

In vertebrates, the microtubule binding protein TPX2 is required for meiotic and mitotic spindle assembly. TPX2 is also known to bind to and activate Aurora A kinase and target it to the spindle. However, the relationship between the TPX2-Aurora A interaction and the role of TPX2 in spindle assembly is unclear. Here, we identify TPXL-1, a C. elegans protein that is the first characterized invertebrate ortholog of TPX2. We demonstrate that an essential role of TPXL-1 during mitosis is to activate and target Aurora A to microtubules. Our data suggest that this targeting stabilizes microtubules connecting kinetochores to the spindle poles. Thus, activation and targeting of Aurora A appears to be an ancient and conserved function of TPX2 that plays a central role in mitotic spindle assembly.

Self-assembled quantum-dot molecules by molecular-beam epitaxy

Self-assembled InAs quantum-dot (QD) molecules having high dot density and aligned dot set structure, which is defined by nanotemplates, were realized by thin capping and regrowth technique in a molecular-beam epitaxy process. Thin capping of GaAs on InAs QDs leads to the creation of nanoholes having a camel-like nanostructure due to anisotropic strain fields along the [11¯0] crystallographic direction and anisotropic surface diffusion accompanying the QD collapse. Regrowth of InAs QDs on the nanohole templates initially results in the formation of QDs with good size uniformity in the middle of features with the shape of propeller blades. This takes place at the regrowth thickness of 0.6 monolayer (ML). The strain at propellers’ edge starts to play its role, creating sets of quantum dots surrounding the initial and centered dots at the regrowth thickness of 1.2 ML. The elongated configuration of propellers’ blades defines the pattern of QD sets having five to six dots on each side. The dot density of the QD molecules is 3×1010cm−2, one order of magnitude higher than that of initial dot density (2×109cm−2).

InAs SELF-ORGANIZED QUANTUM DOTS GROWN BY MOLECULAR BEAM EPITAXY USING A "NUCLEATION-AUGMENTED" METHOD

Photoluminescence (PL) measurements performed in a series of InAs self-assembled quantum dot (QD) structures grown by molecular beam epitaxy (MBE) on GaAs (100) substrates using a two-stage "nucleation-augmented" growth method show that an InAs QD "nucleation" layer grown at a fast growth rate, followed by a slowly grown InAs "augmented" layer, dramatically increases the dot density and improves the PL intensity. Besides red-shift in peak wavelength, the PL intensity was found to increase as the growth rate of the InAs augmented layer was reduced. The increased PL intensity was due to a higher dot density arising from the nucleation layer and an improved optical quality caused by the growth interruption in the augmented layer.

Modeling the optical properties of Si-capped germanium quantum dots

We study theoretically the optical properties of Ge Si-capped quantum dots (QDs) with the aim to establish a relation between structure and properties. Specifically we study on how the imaginary part of the dielectric function and absorption coefficient depend on the shape and size of QDs. We have produced model structures with ultra small and very high density of quantum dots. For these models we studied the case of pyramid like or dome like QDs. What we observed is that the pyramid shape influences more the optical properties studied. Also the height-to-base aspect ratio contributes to the properties of the structure. Thus the optical properties can be used to monitor in situ the formation of the QDs.

Fabrication of high density dot lattices with electron beam lithography

We report the fabrication of large area (∼ 5mm2) magnetic dot arrays by electron beam lithography (EBL). After a standard spin on process the PMMA resist mask is exposed in a LEO 1530 scanning electron microscope (SEM) using the advanced SEM/FIB nanolithography System ELPHY plus from Raith. Different exposure modes have been tested to produce dots with minimal sizes and distances. Also, the time to produce a large area dot lattice has been minimized. Magnetic measurements on these dot lattices have been carried out using a SQUID magnetometer with a special sample holder. In this article we present details of the fabrication of the dot lattices and the first preliminary results concerning the magnetic properties of a lattice of dots with perpendicular magnetic anisotropy.

Self-assembled InAs quantum dots on GaSb/GaAs(0 0 1) layers by molecular beam epitaxy

Self-assembled InAs quantum dots (QDs) were grown on one-monolayer-thick GaSb/GaAs(0 0 1) layer by molecular beam epitaxy using Stranski–Krastanov mode. High dot density of about 1 × 10 11 cm - 2 was demonstrated on the GaSb/GaAs buffer layer. In spite of high dot density, coalescence of neighboring dots was effectively suppressed on the GaSb surface. InAs coverage dependence of the dot structure was studied by using atomic force microscopy and reflection high-energy electron-beam diffraction. For large InAs coverage, the {1 1 0} facet appeared on the side wall of the large dots, and, a narrow PL linewidth of 33 meV could be obtained from highly dense InAs QDs.

Metalorganic vapor phase epitaxy growth of AlGaInAs quantum dots on (1 0 0) GaAs substrate

AlGaInAs quantum dots were grown on GaAs (100) substrate with low-pressure metalorganic vapor phase epitaxy. Growth temperature and trimethylaluminum input was varied to investigate their effects on quantum dot density and size. A model was proposed, the results agreed well with experiment. The morphological characteristics were interpreted in terms of adatom migration and nucleus stability, as well as pyrolysis of input precursors. The optimal growth temperature and aluminum composition in the vapor phase were estimated in order to achieve high quantum dot density. Under optimal growth conditions, quantum dot density can be as high as 2×1010cm−2, about 1.5 times of aluminum-free quantum dots grown at the same temperature.

Swelling and post-irradiated deformation structures in 18Cr–10Ni–Ti irradiated with heavy ions

The current work presents irradiation microstructure evolution in 18Cr–10Ni–Ti SS after Cr ion irradiation and tensile deformation. TEM and tensile samples were irradiated to a total dose of 1, 5, 10 and 100 dpa with 2 MeV Cr 3+ ions at 305 °C (8 × 10 −4 dpa/s) and 600 °C (10 −2 dpa), respectively. Damage structure at 305 °C consists of high density of black dots, loops, which are less than 10 nm in diameter (1–5 dpa), at saturation number density 2 × 10 22 m −3. Deformation behavior of irradiated steel is manifested as a transition from twinning to dislocation channeling with test temperature and dose increasing. Damage at 635 °C is a different volumes of swelling: ∼0.2–0.4% at 5 dpa and ∼20% at 100 dpa. Irradiated samples showed a strong role of voids in the matrix and grain boundary fracture processes.

Grazing incidence x-ray diffraction and atomic force microscopy investigations of germanium dots grown on silicon (001) by successive depositions of germanium through a thin silicon oxide layer

Structural properties of high density, nanometric size germanium dots grown on a silicon (001) surface covered by a very thin (1.2 nm thick) silicon oxide layer have been investigated by in-situ grazing incidence x-ray diffraction (GIXD) and ex-situ atomic force microscopy. Growth under molecular nitrogen partial pressure of 10−5 Torr yielded a high density (∼4×1010/cm2) of dots. The dot size can be progressively increased by successive depositions of germanium. GIXD investigations show that dots grow in epitaxial relationship to the silicon (001) substrate and that after few successive depositions, the dots are composed of pure and fully relaxed germanium.

Proposal of New Nonvolatile Memory with Magnetic Nano-Dots

In this study, a new nonvolatile memory with magnetic nano-dots (MNDs) was proposed. A relatively large anisotropic change of gate current–voltage characteristics by the magnetization in magnetic non-volatile memory with Co nano floating gate and Ni-Fe control gate was obtained. A Co magnetic nano-dot film with very high dot density of 2×1013 cm-2 was successfully formed by sputtering with optimized target composition. A small anisotropic change of gate current–voltage characteristics by the magnetization in MND memory with Co MNDs and Ni-Fe control gate was observed.

Optical and transport studies in coupled InAs quantum dots embedded in GaAs

We studied optical and electron transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. Photoluminescence (PL) from the high dot density samples indicated asymmetry in the PL spectra when the ambient temperature is lower than about 50 K . Comparing this result with theoretical calculations, it is shown that this phenomenon is explained by the inter-dot electronic coupling effect. In the photo-conductance measurement, resonance peaks in the current–voltage characteristics were observed in the low-temperature region. The dependence of the resonance voltage on the magnetic field intensity was studied to extract the g-factor. It is also shown that the resonances are attributed to the current corresponding to the electron transport through QDs. According to these results, it is concluded that the inter-dot electronic coupling in the self-assembled InAs/GaAs QD systems occurs when the inter-dot spacing is as low as several nanometers and the ambient temperature is less than about 50 K .

Effect of InGaAs capping layer on the properties of InAs/InGaAs quantum dots and lasers

We report the effects of In0.33Ga0.67As capping layers on the structural and optical properties of InAs self-organized quantum dots grown by gas-source molecular-beam epitaxy. With different deposition methods for the InGaAs capping layer, the quantum-dot density can be adjusted from 2.3×1010 to 1.7×1011 cm−2. As-cleaved 3.98-mm-long diode laser using triple stacks of InAs quantum dots with the capping layer grown by GaAs/InAs sequential binary growth demonstrates an emission wavelength of 1305 nm and a threshold current density of 360 A/cm2. A ground-state saturation gain of 16.6 cm−1 is achieved due to the high dot density.

Formation trends in quantum dot growth using metalorganic chemical vapor deposition

We discuss the results of a growth matrix designed to produce high quantum dot (QD) density, defect-free QD ensembles, which emit at 1.3 μm using metalorganic chemical vapor deposition (MOCVD). In our study, we balance the nucleation rate and adatom surface migration to achieve high surface densities (1×1011 dots/cm2) and avoid QD coalescence or defects that commonly characterize MOCVD-grown QD ensembles designed for longer wavelength emission. Room-temperature photoluminescence (PL) spectra from corresponding surface QDs depend on QD size and density and show an emission wavelength up to 1600 nm. Ground-state PL from capped QDs is measured at 1.38 μm with a 40 meV linewidth. We demonstrate the ground-state 1.3 μm electroluminescence from a QD light-emitting diode structure grown on n-type GaAs.

Photoluminescence of self-assembled InAs/AlAs quantum dots as a function of density

We report photoluminescence data from three InAs/AlAs quantum dot samples with different densities and separations, respectively. For medium density the quantum dot related emission peak shows a blueshift with increasing excitation power whereas for very high dot density a redshift is observed. These opposite peak shifts are explained by different carrier transfer processes amongst lateral coupled QDs.