Small Dot Size Research Articles

Optical Properties of Multilayered Staggered SiGe Nanodots Depending on Si Spacer Growth Temperature

The optical properties of the multilayered staggered SiGe nanodots (NDs) embedded in the Si spacers fabricated at varying Si growth temperatures are clarified in conjunction with the effect of strain by Photoluminescence (PL) and Raman spectroscopy. We found that compressive strain is induced in the SiGe NDs, with higher growth temperatures of the Si spacer resulting in stronger compressive strain. PL spectra indicate that the higher growth temperature leads to a wider bandgap. This transition energy behavior is caused by not only the strain in the SiGe NDs but also the Ge segregation. Furthermore, luminescence from the SiGe NDs was observed along with luminescence originated at the Si/SiGe interface in the SiGe NDs with a small dot size. This phenomenon was caused by the small energy difference of the conduction-band minima between the SiGe NDs and the tensile-strained Si spacers and the small volume of the tensile-strained Si spacers.

Shell Filling and Trigonal Warping in Graphene Quantum Dots

Transport measurements through a few-electron circular quantum dot in bilayer graphene display bunching of the conductance resonances in groups of four, eight, and twelve. This is in accordance with the spin and valley degeneracies in bilayer graphene and an additional threefold "minivalley degeneracy" caused by trigonal warping. For small electron numbers, implying a small dot size and a small displacement field, a two-dimensional s shell and then a p shell are successively filled with four and eight electrons, respectively. For electron numbers larger than 12, as the dot size and the displacement field increase, the single-particle ground state evolves into a threefold degenerate minivalley ground state. A transition between these regimes is observed in our measurements and can be described by band-structure calculations. Measurements in the magnetic field confirm Hund's second rule for spin filling of the quantum dot levels, emphasizing the importance of exchange interaction effects.

A Comprehensive Halftone Image Quality Evaluation of First- and Second-order FM Halftones

Halftoning is a crucial part of image reproduction in print. For large format prints, especially at higher resolutions, it is important to have very fast and computationally feasible halftoning methods of good quality. The authors have already introduced an approach to obtain image-independent threshold matrices generating both first- and second-order frequency modulated (FM) halftones with different clustered dot sizes. Predetermined and image-independent threshold matrices make the proposed halftoning method a point-by-point process and thereby very fast. In this article, they report a comprehensive quality evaluation of first-and second-order FM halftones generated by this technique and compare them with each other, employing several quality metrics. These generated halftones are also compared with error diffusion (ED) halftones employing two different error filters. The results indicate that the second-order FM halftoning with small clustered dot size performs best in almost all studied quality aspects than the first-and second-order FM halftoning with larger clustered dot size. It is also shown that the first- and second-order FM halftones with small clustered dot sizes are of almost the same quality as ED halftones using Floyd–Steinberg error filter and of higher quality than halftones generated by ED employing Jarvis, Judice, and Ninke error filter.

Size-dependent phosphorus doping effect in nanocrystalline-Si-based multilayers

Doping in Si nanocrystalline (Si NCs) is currently an interesting issue since it can modulate electronic structures of Si NCs and consequently affect the electric and optical properties. However, it has been reported that doping in Si NCs is quite difficult due to the self-purification effect and the locations of dopants such as phosphorus (P) and boron (B), as well as the influences of dopants on physical properties is still unclear. In the present work, the P-doped Si NCs/SiO2 multilayers with various dot sizes (2–20 nm) were fabricated. It is found that the electronic properties are enhanced significantly after doping. However, for P-doped samples, the conductivity is size-dependent and reduces with decreasing the dot size, which can be attributed to the reduction of activated carrier concentration in the film with small dot size. Moreover, we find P dopants can induce deep-level for sample with small size of 2 nm, and a sub-band near infrared light emission near 1250 nm is detected. The possible mechanism are discussed briefly.

An Electrohydrodynamic (EHD) Jet Printing Method for Increasing Printing Speed

It is well known that drop-on-demand electrohydrodynamic (EHD) jet printing can make small dots and fine line patterns thereby. However, it is believed that drop-on-demand based EHD printing is only limited to very slow printing applications due to small dot size and limited jetting frequencies. In this study, we propose using the change of printed relic shape to enhance printing speed. For this purpose, high molecular polymer was mixed into the silver nanoparticles ink for printing. The proposed method could help to increase the printing speed and reduce the line-pattern width as well. The video of our recent development can be found from website: https://youtu.be/ak6Zl-9yE-c.

A high speed electrohydrodynamic (EHD) jet printing method for line printing

Electrohydrodynamic (EHD) jet printing has drawn attention due to its capability to produce smaller dots and patterns with finer lines when compared to those obtained from using conventional inkjet printing. Previous studies have suggested that drop-on-demand EHD-patterning applications should be limited to very slow printing cases with speeds far less than 10 mm s−1 due to the small dot size and limited jetting frequency. In this study, a new EHD printing method is proposed to significantly increase the line-patterning printing speed by modifying the ink and thereby changing the relic shape. The proposed method has the additional advantage of reducing the line-pattern width. The results of the experiment show that the pattern width could be reduced from 20 µm to 4 µm by increasing the printing speed from 10 mm s−1 to 50 mm s−1, respectively.

Exciton binding energy in GaAsBiN spherical quantum dot heterostructures

The ground state exciton binding energies (EBE) of heavy hole excitons in GaAs1-x-yBixNy – GaAs spherical quantum dots (QD) are calculated using a variational approach under 1s hydrogenic wavefunctions within the framework of effective mass approximation. Both the nitrogen and the bismuth content in the material are found to affect the binding energy, in particular for larger nitrogen content and lower dot radii. Calculations also show that the ground state exciton binding energies of heavy holes increase more at smaller dot sizes as compared to that for the light hole excitons.

Formation of uniform high-density and small-size Ge/Si quantum dots by scanning pulsed laser annealing of pre-deposited Ge/Si film

The capability to fabricate Ge/Si quantum dots with small dot size and high dot density uniformly over a large area is crucial for many applications. In this work, we demonstrate that this can be achieved by scanning a pre-deposited Ge thin layer on Si substrate with a line-focused pulsed laser beam to induce formation of quantum dots. With suitable setting, Ge/Si quantum dots with a mean height of 2.9 nm, a mean diameter of 25 nm, and a dot density of 6×1010 cm−2 could be formed over an area larger than 4 mm2. The average size of the laser-induced quantum dots is smaller while their density is higher than that of quantum dots grown by using Stranski-Krastanov growth mode. Based on the dependence of the characteristics of quantum dots on the laser parameters, a model consisting of laser-induced strain, surface diffusion, and Ostwald ripening is proposed for the mechanism underlying the formation of the Ge/Si quantum dots. The technique demonstrated could be applicable to other materials besides Ge/Si.

Exchange bias properties of 140 nm-sized dipolarly interacting circular dots with ultrafine IrMn and NiFe layers

We studied the exchange bias effect in an array of IrMn(3nm)/NiFe(3nm) circular dots (size ~140nm and center-to-center distance ~200nm, as revealed by microscopy analyses), prepared on a large area (3×3mm2) by electron beam lithography and lift-off, using dc sputtering deposition. Hysteresis loops were measured by SQUID magnetometer at increasing values of temperature T (in the 5–300K range) after cooling from 300K down to 5K in zero field (ZFC mode) and in a saturating magnetic field (FC mode). The exchange bias effect disappears above T~200K and, at each temperature, the exchange field HEX measured in ZFC is substantially lower than the FC one. Micromagnetic calculations indicate that, at room temperature, each dot is in high-remanence ground state, but magnetic dipolar interactions establish a low-remanence configuration of the array as a whole. Hence, at low temperature, following the ZFC procedure, the exchange anisotropy in the dot array is averaged out, tending to zero. However, even the FC values of HEX and of the coercivity HC are definitely smaller compared to those measured in a reference continuous film with the same stack configuration (at T=5K, HEX~90Oe and HC~180Oe in the dots and HEX~1270Oe and HC~860Oe in the film). Our explanation is based on the proven glassy magnetic nature of the ultrathin IrMn layer, implying the existence of magnetic correlations among the spins, culminating in a collective freezing below T~100K. We propose, also by the light of micromagnetic simulations, that the small dot size imposes a spatial constraint on the magnetic correlation length among the IrMn spins so that, even at the lowest temperature, their thermal stability, especially at the dot border, is compromised.

Dynamic modeling of superresolution photoinduced-inhibition nanolithography

A dynamical model based on the photo-physics and photo-chemistry processes for superresolution photoinduced-inhibition nanolithography (SPIN) under both single-photon and two-photon excitation is developed and validated by experimental results. Numerical simulation results for the dot fabrication predict that the theoretical single dot size can be infinitely reduced, which shows diffraction-unlimited feature of the SPIN. A small reaction constant of the inhibitor polymerization is crucial to realize a small dot size and high resolution. It is discovered both theoretically and experimentally that the dot minimum size and best resolution occur under different inhibition beam powers because of the influence from the inhibitor polymerization. Moreover, due to the consumption of the photo-inhibitor molecules in the inhibition process, the dot size may vary during the sequential fabrication.

Growth and Characterization of InAs Quantum Dots on GaAsSb

The growth details of strained GaAsSb layers on GaAs(001) substrates were studied by reflection high energy electron diffraction (RHEED) beam intensity oscillations as a function of both substrate temperature and Sb/As flux ratio. Both the RHEED intensity and RHEED oscillation cycles are reduced with decreasing substrate temperature and Sb/As flux ratio. InAs QDs with high dot density, small dot size and narrow size distribution have been achieved on strained GaAs / GaAsSb buffer layer. The average lateral size of dots shows a trend toward to smaller size and dots’ density shows a trend toward to higher density as the surface Sb composition increasing. The QDs with higher density and smaller size distributions at high Sb composition, indicates that the Sb plays an important role in the dot formation under this growth condition. The lattice mismatch of InAs layer with the GaAsSb buffer layer is reduced with increasing of Sb composition in the GaAsSb interlayer. This result indicates that the density, size and size distribution of self-assembled quantum dots (QDs) can be controlled through the manipulation of the Sb-mediated strain field in the lattice mismatched system.

Effects of hydrostatic pressure and external electric field on the impurity binding energy in strained GaN/Al x Ga1−x N spherical quantum dots

The binding energy and Stark effect energy shifts of a shallow donor impurity state in a strained GaN/AlxGa1−xN spherical finite-potential quantum dot (QD) are calculated using a variational method based on the effective mass approximation. The binding energy is computed as a function of dot size and hydrostatic pressure. The numerical results show that the binding energy of the impurity state increases, attains a maximum value, and then decreases as the QD radius increases for any electric field. Moreover, the binding energy increases with the pressure for any size of dot. The Stark shift of the impurity energy for large dot size is much larger than that for the small dot size, and it is enhanced by the increase of electric field. We compare the binding energy of impurity state with and without strain effects, and the results show that the strain effects enhance the impurity binding energy considerably, especially for the small QD size. We also take the dielectric mismatch into account in our work.

Hydrogenic donor impurity in a cubic quantum dot: effect of position-dependent effective mass

In this paper, we intend to study the effect of variable mass on the binding energy. In this regard, we apply an analytic expression for position-dependent effective mass in a cubic quantum dot. Then, we obtain the binding energies of a shallow donor in the quantum dot of GaAs/AlxGa1−xAs using a variational procedure within the effective mass approximation. Calculations are presented with a constant effective mass and position-dependent effective mass. It is found that (i) the binding energy decreases as the dot length increases in both the cases of constant and variable masses, (ii) an increase of binding energy is observed when the spatially varying mass is included, and (v) the binding energy shows complicated behavior when the position-dependent mass is included for the small dot size L ≤ 130 A.

Exciton bound to an ionized donor impurity in GaAs/Ga 1− xAl xAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga 1− x Al x As ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.

Multilevel Charge Storage in a Multiple Alloy Nanodot Memory

A multilevel charge storage in a multiple FePt alloy nanodot memory is investigated for the first time. It is demonstrated that the memory structure with multiple FePt nanodot layers effectively realizes a multilevel state by the adjustment of gate voltage. Metal oxide semiconductor (MOS) capacitors with four FePt nanodot layers as a floating gate are fabricated to evaluate the multilevel cell characteristic and reliability. Here, the effect of memory window for a nanodot diameter is also investigated, and it is found that a smaller dot size gives a larger window. From the results showing good endurance and retention characteristics for the multilevel states, it is expected that a multiple FePt nanodot memory using Fowler–Nordheim (FN) tunneling can be a candidate structure for the future multilevel NAND flash memory.

Theory of excitons in cubic III-V semiconductor GaAs, InAs and GaN quantum dots: Fine structure and spin relaxation

Exciton fine structures in cubic III-V semiconductor GaAs, InAs and GaN quantum dots are investigated systematically and the exciton spin relaxation in GaN quantum dots is calculated by first setting up the effective exciton Hamiltonian. The electron-hole exchange interaction Hamiltonian, which consists of the long- and short-range parts, is derived within the effective-mass approximation by taking into account the conduction, heavy- and light-hole bands, and especially the split-off band. The scheme applied in this paper allows the description of excitons in both the strong- and weak-confinement regimes. The importance of treating the direct electron-hole Coulomb interaction unperturbatively is demonstrated. We show in our calculation that the light-hole and split-off bands are negligible when considering the exciton fine structure, even for GaN quantum dots, and the short-range exchange interaction is irrelevant when considering the optically active doublet splitting. We point out that the long-range exchange interaction, which is neglected in many previous works, contributes to the energy splitting between the bright and dark states, together with the short-range exchange interaction. Strong dependence of the optically active doublet splitting on the anisotropy of dot shape is reported. Large doublet splittings up to 600 $\ensuremath{\mu}$eV, and even up to several meV for small dot size with large anisotropy, are shown in GaN quantum dots. The spin relaxation between the lowest two optically active exciton states in GaN quantum dots is calculated, showing a strong dependence on the dot anisotropy. Long exciton spin relaxation time is reported in GaN quantum dots. These findings are in good agreement with the experimental results.

Matrix-embedded silicon quantum dots for photovoltaic applications: a theoretical study of critical factors

Si Quantum dots (QD's) are offering the possibilities for improving the efficiency and lowering the cost of solar cells. In this paper we study the PV-related critical factors that may affect design of Si QDs solar cell by performing atomistic calculation including many-body interaction. First, we find that the weak absorption in bulk Si is significantly enhanced in Si QDs, specially in small dot size, due to quantum-confinement induced mixing of Γ-character into the X-like conduction band states. We demonstrate that the atomic symmetry of Si QD also plays an important role on its bandgap and absorption spectrum. Second, quantum confinement has a detrimental effect on another PV property – it significantly enhances the exciton binding energy in Si QDs, leading to difficulty in charge separation. We observe universal linear dependence of exciton binding energy versus excitonic gap for all Si QDs. Knowledge of this universal linear function will be helpful to obtain experimentally the exciton binding energy by just measuring the optical gap without requiring knowledge on dot shape, size, and surface treatment. Third, we evaluate the possibility of resonant charge transport in an array of Si QDs via miniband channels created by dot-dot coupling. We show that for such charge transport the Si QDs embedded into a matrix should have tight size tolerances and be very closely spaced. Fourth, we find that the loss of quantum confinement effect induced by dot-dot coupling is negligible – smaller than 70 meV even for two dots at intimate contact.

Exciton states in zinc-blende GaN/AlGaN quantum dot: Effects of electric field and hydrostatic pressure

Abstract Based on the effective-mass approximation, the effects of the electric field and hydrostatic pressure on exciton states in a cylindrical zinc-blende (ZB) GaN/AlGaN quantum dot (QD) are investigated variationally. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy and interband transition energy in the case of any hydrostatic pressures. However, the hydrostatic pressure increases the exciton binding energy and interband transition energy in the case of any electric fields. In particular, the electric field has a remarkable influence on the exciton binding energy in the QD with large dot size and small hydrostatic pressure; moreover, the hydrostatic pressure obviously affects the exciton binding energy in the QD with small dot size and weak electric field.

Objective Measurement of Distance Visual Acuity Determined by Computerized Optokinetic Nystagmus Test

To investigate the efficacy of a computerized optokinetic nystagmus (OKN) test in evaluating the objective distance visual acuity and to determine the correlation between subjective and objective visual acuities. This prospective, noninterventional study included 83 eyes of 83 volunteers. Objective visual acuity was defined as the smallest size stripe that evoked the OKN response (induction method) or as the smallest dot size that suppressed the OKN response (suppression method). Distance visual acuity was measured by computerized OKN and infrared oculography at distance. The reproducibility of the test was evaluated by intraclass correlation coefficient (ICC). The correlation between measured objective and subjective visual acuity was then evaluated with linear regression analysis. Subjects were grouped according to their objective visual acuity, and the mean subjective visual acuities were compared with those of the objective visual acuity groups. There was a significant correlation between distance objective and subjective visual acuity (correlation coefficient R(2), induction method:suppression method = 0.566:0.832, P < 0.05). The mean subjective visual acuity was significantly different in the objective visual acuity groups (Welch's ANOVA, P = 0.000 for induction and suppression methods). The objective visual acuity test showed good reproducibility (ICC; induction method:suppression method = 0.945:0.988, P < 0.05). The computerized OKN test could serve as an objective and reliable tool for assessing distance visual acuity.

Ultra-high detectivity room temperature THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD)

In this paper, a novel structure for THZ-IR photodetector based on resonant tunneling spherical centered defect quantum dot (RT-SCDQD) operating at room temperature is proposed. The proposed structure includes a quantum dot with centered defect following a resonant tunneling double barrier. It is shown that inserting a centered defect leads to considerable enhancement in absorption coefficient at long wavelength in small dot size (1.05 × 10 6–7.33 × 10 6 m −1 at 83 μm). This effect guarantees large responsivity of the proposed system for THZ-IR photodetector. In this proposal, intersublevel transitions in related states positioned at mid energies of large conduction-band-offset materials (GaN/AlGaN) are used to depress the thermal effect in dark current. Adding the resonant tunneling double barrier to the quantum dot resolves the basic problem of collecting electrons from deep excited state without applying large bias voltage. Also, employing the RT double barrier reduces the ground state dark current term. Reduction of the dark current and increasing the responsivity yields ultra-high detectivity, 5 × 10 16 and 2.25 × 10 9 cm Hz 1/2/W at 83 μm, at 83 and 300 K, respectively. Analysis of the proposed structure is done analytically.