Average Dot Research Articles

Honeycomb domain structures in amorphous TbFe thin films

The magnetic properties and domain structure of amorphous TbFe thin films were studied by magnetic measurement and magnetic force microscope. Honeycomb domain structures were found in the as-deposited amorphous film. The domain structures were composed of many small white round dots embedded in the black matrix, which formed an irregular hexagonal pattern with some deformation. The average dot size was about 450 nm, with an average separation of 610 nm. The derived domain wall energy density γ and the exchange constant A were 0.95×10−2 and 1.79×10−11 J/m, respectively.

Optical properties of self-assembled InGaN/GaN quantum dots

Optical spectroscopy under varying temperature is used to investigate samples containing planes of self-assembled Ga 1−xIn xN quantum dots (0.15< x<0.20), embedded in a GaN matrix. The samples have been grown by molecular beam epitaxy on sapphire substrates and the nano-islands have been obtained by the Stranski-Krastanov growth mode transition. Half-widths at half maximum as small as 0.05 eV are obtained for photoluminescence (PL) lines at T=2 K. Increasing the growth time decreases the PL energy and drastically increases the PL decay time, as a result of the increasing of the average dot height. Time-resolved PL measurements with variable temperature allow us to observe the competitive influence of several mechanisms, namely the usual radiative and nonradiative recombination processes, plus the carrier feeding from random fluctuations, which plays a crucial role in the case of the larger dots.

Growth and characterizations of InP self-assembled quantum dots embedded in InAIP grown on GaAs substrates

We report the characteristics of InP self-assembled quantum dots embedded in In0.5Al0.5P on GaAs substrates grown by metalorganic chemical vapor deposition. The InP quantum dots show increased average dot sizes and decreased dot densities, as the growth temperature increases from 475°C to 600°C with constant growth time. Above the growth temperature of 600°C, however, dramatically smaller and densely distributed self-assembled InP quantum dots are formed. The small InP quantum dots grown at 650°C are dislocation-free “coherent” regions with an average size of ∼20 nm (height) and a density of ∼1.5 × 108 mm−2. These InP quantum dots have a broad range of luminescence corresponding to red or organge in the visible spectrum.

Oswald Ripening of Facetted Self-Assembled PbSe Quantum Dots during Annealing

The thermal stability of facetted self-assembled PbSe quantum dots during annealing is investigated. With increasing annealing time the dot density is found to decrease rapidly with a simultaneous increase of the average dot volume. This behavior is well described by Oswald ripening under interface reaction limited mass transfer conditions. Therefore, the islands formed during growth do not represent an equilibrium structure.

Oswald Ripening of Facetted Self-Assembled PbSe Quantum Dots during Annealing

The thermal stability of facetted self-assembled PbSe quantum dots during annealing is investigated. With increasing annealing time the dot density is found to decrease rapidly with a simultaneous increase of the average dot volume. This behavior is well described by Oswald ripening under interface reaction limited mass transfer conditions. Therefore, the islands formed during growth do not represent an equilibrium structure.

Exploiting the difference in lattice structures for formation of self-assembled PbS dots on InP(110)

Via the example of PbS self-assembled nanodots on InP(110) we show that spontaneous self-organization at semiconductor interfaces can be achieved due to different lattice structure of the constituents with close lattice constants. Regularly shaped triangular pyramidal PbS dots almost perfectly oriented on the substrate and distributed reasonably uniform in size can be grown in this way by molecular beam epitaxy. The average lateral dot size can be varied between 15 and 35 nm by appropriate choice of growth temperature and total PbS coverage.

An optical study of self-assembled InxGa1−xAs/GaAs quantum dots embedded in a two-dimensional electron gas

We studied the low temperature (77 K) photomodulated reflection and transmission as well as the photoluminescence at 2.2 K of a self-assembled InxGa1−xAs quantum dot layer grown on a (100) GaAs substrate in the vicinity of a two-dimensional electron gas. The dot layer was grown without rotation of the substrate in order to achieve a gradual variation of the In concentration along the wafer diameter. This resulted in an increase in the density of quantum dots along the In concentration gradient, which is reflected in a characteristic dependence of the relative intensities of the spectral lines. A consistent assignment of the optical structure observed in all spectra leads to an estimate of the average value of the Fermi energy in the conduction band of the wetting layer (EF≃13.4 meV). The variation of this Fermi energy along the composition gradient can be obtained from the spectra, and an estimate of the gradient of the density of quantum dots along this direction can be made. A careful comparison of the variation of the critical energy of the different lines suggests that the average quantum dot size depends on the In molar fraction of the alloy, which is seen to vary more or less linearly across the wafer diameter.

Growth of vertically self-organized InGaAs quantum dots with narrow inhomogeneous broadening

We have fabricated vertically self-organized multiple sets of In0.6Ga0.4As quantum dots (QDs) on GaAs (001) that combines the concepts of variable amount deposition and shape stabilization and size equalization of QDs. The inhomogeneous broadening of optical emission from these dots reached a record low value of 18.4 meV at a wavelength of ∼1185 nm (4 K). The seed layer and the second dot layer have essentially the same dot density of ∼250 μm−2 due to the high degree of dot vertical alignment. The deposition amount for the second dot layer was selected to be 9 monolayers, which resulted in dots with convergent lateral size (∼62 nm) and stabilized facets, close to {011}. The third layer, with the same amount of InGaAs as the second layer, had a dot density of ∼350 μm−2, an average lateral dot size of ∼71 nm, an average dot height of ∼11 nm, and shallower side facets close to {023}.

Origin of size distributions in ZnSe self-organized quantum dots grown on ZnS layers

Main factors which determine the size, the standard deviations which show the degree of the size fluctuations for the average dot height and diameter, and density in ZnSe self-organized quantum dots (QDs) grown on ZnS layers were studied. By lowering the growth temperature the QDs average size and its standard deviation decreased and the density increased due to the slower surface migration. With the application of the scaling theory, it was revealed that the normalized size distributions were uniquely determined by the nucleation process although the apparent standard deviations of the QD sizes were dependent on the growth temperature. The influence of surface roughness of the underneath layer on the formation of the relations of the dot height and diameter was also examined. It was shown that the fluctuation of the surface potential contributes significantly to the apparent standard deviations of ZnSe self-organized QDs sizes.

Single-Electron Tunneling Devices Based on Silicon Quantum Dots Fabricated by Plasma Process

A study of the electrical properties of multiple tunnel junctions (MTJs) formed in a quasi one-dimensional array of randomly deposited silicon nanocrystals is presented. Nanocrystals are deposited by very-high-frequency (VHF) plasma decomposition of silane. The average dot diameter is 8 nm. The source-drain electrode separation is 30 nm. A gate electrode is employed so that the charge states in quantum dots can be controlled. A study of the source-drain current–voltage (I–V) characteristics with various gate voltages is performed. Coulomb blockade, a Coulomb staircase and Coulomb oscillations are observed at temperatures ranging from 40 K to 150 K. Single electron trapping is observed at 40 K.

Growth and luminescence properties of self-organized ZnSe quantum dots

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1×1010 cm−2. Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution.

Effects of vicinal InP (001) surface on InAs dots grown by droplet hetero-epitaxy

We have systematically investigated the effects of InP substrate misorientation on the InAs dot formation by droplet hetero-epitaxy. The density and uniformity of the dots were investigated in the misorientation range between 0 and 4° from (001) toward [100]. Using 2°-off substrate, the density had the highest value of 2.2×10 10 cm −2 and the uniformity was most improved, where average dot height and base diameter were 8.1 and 51 nm. Further, we found that the total volume of the dots increased with misorientation in spite of the other growth conditions remaining unchanged.

Indocyanine green angiographic features in ocular sarcoidosis

Objective To determine indocyanine green (ICG) angiographic features and evaluate the extent of choroidal involvement in proven cases of posterior ocular sarcoidosis. Design Nonrandomized controlled trial. Participants Nineteen patients (14 females, 5 males; average age, 56 ± 4 years) with clinically typical posterior sarcoidosis (biopsy-proven in 6 cases and fulfilling the other diagnostic criteria in 13 cases) participated, with 10 control subjects (average age, 48 ± 7 years). Criteria for the diagnosis of sarcoidosis were a positive biopsy result or the presence of at least three of the following four criteria: elevated serum angiotensin-converting enzyme, elevated lysozyme, cutaneous anergy, and hilar lymph node enlargement. Intervention Indocyanine green angiography was performed according to a standard angiographic protocol used in inflammatory disorders. Main outcome measures Indocyanine green angiographic features and proportion of choroidal inflammatory involvement were measured. Results Indocyanine green angiographic features could be classified into four main patterns. The first pattern is hypofluorescent choroidal lesions in the early and intermediate phases, irregularly distributed, invisible on funduscopy or fluorescein angiography, and localized in the midperiphery (63% of patients), in the macula (11%) or in both regions (26%) with an average dot diameter of 0.31 ± 0.03 disc diameters. These lesions either became isofluorescent in the late phase of the angiogram (Type 1, present in all patients) or remained hypofluorescent (Type 2, present in 84% of patients). The second pattern is focal hyperfluorescent pinpoints visible in the intermediate and late phases (in 89% of patients). The third pattern is fuzzy choroidal vessels with leakage in the intermediate phase of the angiogram, and the fourth pattern is diffuse late zonal choroidal hyperfluorescence with staining in the late phase of the angiogram, both features being present in all patients. Conclusions Indocyanine green angiography allowed the authors to assess and quantify the hitherto unknown extent of choroidal involvement in ocular sarcoidosis. Furthermore, characteristic ICG findings might represent an additional valuable tool for diagnosing and monitoring this disease.

A thermal inkjet printhead with a monolithically fabricated nozzle plate and self-aligned ink feed hole

A monolithic thermal inkjet printhead has been developed and demonstrated to operate successfully by combining monolithic growing of a nozzle plate on the silicon substrate and electrochemical etching of silicon for an ink feed hole. For the monolithic fabrication, a multiexposure and single development (MESD) technique and Ni electroplating are used to form cavities, orifices, and the nozzle plate. Electrochemical etching, as a back-end process, is applied to form an ink feed hole through the substrate, which is accurately aligned with the frontside pattern without any backside mask. The etch rate is nearly proportional to the current density up to 50 /spl mu/m/min. Experiments with a 50-/spl mu/m-diameter nozzle show ink ejection up to the operating frequency of 11 kHz with an average ink dot diameter of about 110 /spl mu/m for 0.3-A, 5-/spl mu/s current pulses.

Quantum Confinement of Above-Band-Gap Transitions in Ge Quantum Dots

AbstractA number of research efforts have been focused on self-assembled germanium quantum dots in which indirect optical transitions take place across the band gap. However, many questions regarding the confined electronic state transitions of Ge quantum dots still remain unanswered. In the present report, we have deposited ten alternating layers of crystalline Ge quantum dots embedded in an Al2O3 or an AIN matrix on sapphire substrates by pulsed laser deposition. The average dot sizes (73 Å to 260 Å) were controlled by the laser energy density, deposition time and substrate temperature. The spectral positions of both the E1 and the E2 transitions in the absorption spectra at room temperature and 77 K shift toward higher energy (ΔE1=1.19 eV, ΔE2 =0.57 eV) as the Ge dot size decreases (73 Å). Structural analysis using transmission electron microscopy and atomic force microscopy and the interpretation of optical absorption measurements in terms of quantum confinement of carriers in both transitions are presented.

In-plane polarized intraband absorption in InAs/GaAs self-assembled quantum dots

In-plane polarized photoinduced intraband absorption is reported in InAs/GaAs self-assembled quantum dots in the 90--600-meV energy range. This in-plane absorption mainly originates from bound-to-continuum transitions in the conduction and valence bands and lies in the 4--8-\ensuremath{\mu}m spectral range. The continuum is constituted either by the dot quasibound states hybridized with the wetting layer subbands or by the delocalized states of the barriers. A weak in-plane polarized bound-to-bound hole transition is also observed at 11 \ensuremath{\mu}m. Its in-plane absorption cross section is estimated to be around $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2}.$ The energy position of the p-polarized and in-plane polarized intraband transitions is analyzed as a function of the average quantum dot size. An experimental energy diagram of the quantum dots is presented, as deduced from the interband and intraband measurements. The existence, directions, and lengths of the intraband transition dipoles are compared to the ones deduced from the numerical resolution of the three-dimensional effective mass Schr\odinger equation taking into account the flat lens-shape geometry of the dots.

Atomic force microscopy studies of ZnSe self-organized dots fabricated on ZnS/GaP

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. In situ reflection high-energy electron diffraction studies reveal that growth interruption is required for the formation of the dot structure. Atomic force microscopy (AFM) images of the dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. A density of 18 μm−2 was achieved with a coverage of 8.0 ZnSe monolayers. AFM images taken at later times (up to six months later) show ripening effects. The average dot size measured at various times after growth is consistent with the prediction of the Ostwald ripening model with a growth time constant of 4±1 days for the structure with a coverage of 8.0 ZnSe monolayers. The dot size and density in the fully ripened state are essentially independent of the initial ZnSe coverage.

Carbon-induced germanium dots: Kinetically-limited islanding process prevents coherent vertical alignment

Pregrowth of a small amount of C on a Si(001) substrate induces small Ge quantum dots. We present a structure where an initial layer of these C-induced Ge dots is followed by five layers of four monolayers Ge, each layer separated by a 2 nm thick Si spacer. Although the critical thickness for planar growth of a single Ge layer is not exceeded, a vertically aligned stack of Ge islands is formed. If we substitute the pure Ge layers by a fivefold stack of C-induced Ge dots, no vertical island correlation is observed. The phenomenon is explained by the strongly kinetically limited process of C-induced Ge island formation, itself. Photoluminescence experiments on stacks of 0.16 monolayers C/2.2 monolayers Ge dots, where we systematically varied the Si spacer thickness and the number of dot layers, suggest that the average dot size increases if the Si spacer is kept thinner than 10 nm.

Growth of self-assembled InAs and InAs xP 1− x dots on InP by metalorganic vapour phase epitaxy

The formation of self-assembled InAs and InAs x P 1− x dots on InP has been studied, in particular with deposition conditions under which mainly coherent dots are developed. The samples were grown by metalorganic vapour phase epitaxy. Morphological investigations were performed by atomic force microscopy, with the instrument working in the contact mode as well as in the noncontact mode. Surface densities and height distributions were extracted, as a function of growth conditions. In addition, photoluminescence was used for investigations of the optical properties of capped InAs dots, formed under equivalent conditions. Comparisons between the two characterization techniques show a qualitative agreement with respect to the density of dots as well as their size homogeneity. It is also indicated that dots of binary InAs can be formed at deposition temperatures not higher than about 500°C. Elevated deposition temperatures in this process result in an unintentional alloying mechanism due to exchange reactions at the interface, leading to the formation of ternary InAs x P 1− x dots, which can be seen as a simultaneous increase in the energy of the light emission and the average dot size, indicating the widening of the energy gap in the quantum dots, which counteracts the decreased energy quantization in the larger dots formed at higher temperatures.

Light emission from individual InAs/GaAs self-assembled quantum dots excited by tunneling current injection

Emission from individual InAs quantum dots by tunneling current injection using a scanning tunneling microscope (STM) is reported. By scanning the STM tip above the self-assembled InAs quantum dot structures, a spatially resolved scanning tunneling luminescence (STL) image was measured, which contained some peaks originating from InAs quantum dots. The width of these peaks are of the same size as the average dot size, suggesting that the resolution of this method is much higher than 20 nm.