Nanoscale device modeling: the Green’s function method

The non-equilibrium Green’s function (NEGF) formalism provides a sound conceptual basis for the devlopment of atomic-level quantum mechanical simulators that will be needed for nanoscale devices of the future. However, this formalism is based on concepts that are unfamiliar to most device physicists and chemists and as such remains relatively obscure. In this paper we try to achieve two objectives: (1) explain the central concepts that define the ‘language’ of quantum transport, and (2) illustrate the NEGF formalism with simple examples that interested readers can easily duplicate on their PCs. These examples all involve a short n ^+  + – n ⁺ – n ^+  + resistor whose physics is easily understood. However, the basic formulation is quite general and can even be applied to something as different as a nanotube or a molecular wire, once a suitable Hamiltonian has been identified. These examples also underscore the importance of performing self-consistent calculations that include the Poisson equation. The I–V characteristics of nanoscale structures is determined by an interesting interplay between twentieth century physics (quantum transport) and nineteenth century physics (electrostatics) and there is a tendency to emphasize one or the other depending on one’s background. However, it is important to do justice to both aspects in order to derive real insights.     

Laser image measurement of twin bubbles formation in shear-thinning fluids

A laser image system for investigating twin bubbles formation in shear-thinning fluid was established. The process of twin-bubble formation could be directly visualized and real-time recorded through computer by means of He–Ne laser as light source using the beam expanding and light amplification technology. The shape and size of bubbles generating in carboxymethylcellulose (CMC) aqueous solutions were studied experimentally at orifice diameter 1 mm, 1.6 mm and 2.4 mm, the orifices interval 1D_o, 2D_o and 3D_o (D_o: orifice diameter) and the gas flow rate from 0.1 to 1.0 ml/s, respectively. The effects of solution mass concentration, orifice diameter and orifice interval on bubble detachment volume were investigated. The results reveals that twin bubbles gradually touch each other and then deviate from the vertical axis crossing the middle point of the line joining the two orifice during the formation process. However compared with the perfect teardrop terminal shapes in glycerol solution, the bubbles formed in CMC solutions are stretched vertically due to the shear-thinning effect of fluids. The bubble detachment volume increases with the solution mass concentration, whereas decreases with orifice diameter. The detachment volume generated at twin orifices is less affected by orifices interval, but still smaller than that at single orifice.     

Numerical solving of the track wall equation in LR115 detectors etched in direct and reverse directions

Our equation of the track wall was solved numerically by using finite difference method and computer software MATHEMATICA. This method was applied for alpha particle tracks in LR115 detector, assuming both directions of etching, from the top and bottom of the sensitive layer. The equation of the track wall etched in reverse direction was derived, and has the same form as one for direct etching, with some difference in argument of V function. We will analyse the consequences of direct and reverse etching on the shape of alpha particles tracks with energies ranging from 2 to 4 MeV in thin LR115 NTDs.     

A new type of semi-active hydraulic engine mount using controllable area of inertia track

Automotive engine mounts function to constrain the engine shake motion resulting at low-frequencies, as well as to isolate noises and vibrations generated by the engine with unbalanced disturbances at the high frequencies. The property of the mount depends on vibration amplitude and excitation frequency. It means that the excitation amplitude is large in low excitation frequency range and small in high frequency range. In this paper, a new hydraulic engine mount with a controllable area of inertia track is proposed and investigated. Theoretical works with the mount model to isolate the engine-related vibrations were conducted by an optimal algorithm to control the area of the inertia track under shocks and multi-signal force excitations. This research clearly gives an analysis of the considerable changes in the mount dynamic properties according to the changes in the inertia track area. Consequently, when the inertia track area is tuned, the transmissibility of the mount is effectively reduced.     

Verification of Mason and Smythe equations for electrical treeing breakdown at the tip of charged particle tracks in ECE polycarbonate detectors

There are two different equations to describe enhancement of field strength at the track tip in ECE polycarbonate detectors: Mason and Smythe equations. In order to evaluate the equations, PC detectors with different thicknesses were exposed to 1 MeV alpha and/or fast neutron. The response of the detectors was studied as a function of applied voltage and average field strength. The ratio of critical voltages for different detectors was estimated experimentally and compared with calculated values. The experimental results are in good agreement with Smythe's equation. The results show that the track density and mean track diameter in the different detectors have the same behaviour, when the macroscopic field strength increases, and also that the field strength at the tip of tracks is proportional to macroscopic field strength.     

Low loss broadband polarization independent fishnet negative index metamaterial at 40 GHz

We present a polarization independent fishnet negative index metamaterial at 40 GHz. The structure is investigated theoretically using finite element method simulations and experimentally by measuring the amplitude and phase of the S-parameters. The experimental setup for free space measurements of both transmission and reflection is hereby introduced. The internal properties are thereafter retrieved and show the double-negative behavior of the structure. This negative index metamaterial exhibits very high transmission (?0.13 dB), low reflection (?33.1 dB) and a high figure of merit (FOM = |Re(n)/Im(n)| = 42), where the real part of the refractive index is nearly ?1 (Re(n) = ?0.93) at 40 GHz.     

Feasibility study of thin film Al/SnOx/n-Si gate stack for gamma radiation dosimetry

MOS (Al/SnO_x/n-Si) structure is fabricated by thin film deposition technique for gamma radiation dosimetry. A cumulative gamma dose of 480 Gy was given to the devices in steps of 120 Gy each at the dose-rate of 2 Gy/min. Forward bias I–V characteristics and C–V characteristics of the gamma irradiated devices have shown the changes in the current and capacitance values, respectively with gamma irradiation dose. This structure in future will be useful for personal and accidental radiation dosimetry.     

Enhancement in NBE emission and optical band gap by Al doping in nanocrystalline ZnO thin films

Transparent Al doped ZnO nanocrystalline films with a crystallite size less than 19 nm are obtained by spray pyrolysis. Band gap increases monotonically from 3.16 to 3.31 eV with increasing aluminum dopant up to 1.56 at.% facilitating increasing width of a transmission window in addition to the band gap tuning of 4.74% which compares favorably well with literature. UV emission with continuously increasing intensity is obtained which reflects on the good crystalline quality of the films. Also the defect emissions are suppressed remarkably as the dopant Al concentration increases in ZnO. The band gap tuning by quite small increment in dopant amount makes the present films, much attractive for the fabrication of light emitting devices with a much sought-for benefit of large area fabrication. FESEM shows the surface is granular with grain size lying in the range of 20–35 nm and EDX confirms the presence of Al in the doped samples     

CMOS—compatible reconfigurable microring demultiplexer with doped silicon slab heater

We present a 1×4 reconfigurable demultiplexer based on cascaded silicon microring resonators. The device is fabricated on a 0.18 μm complementary metal oxide semiconductor (CMOS) process. A homogeneous doped silicon slab heater is proposed and fabricated directly on the slab region of the microring resonator for thermal tuning. The flows of the heating currents in the heaters are parallel to the ring waveguide through the heavily doped slab regions located on both sides of the ring waveguide without through the waveguide core regions. The proposed doped heaters are experimentally verified with low-voltage operation and tuning efficiency of ～77 pm/mW. Without any tuning or trimming, predicted average channel spacing distribution in the whole free spectral range (FSR) is demonstrated. Full reconfigurability is also demonstrated in the demultiplexer with channel spacing of 2 nm (250 GHz) and 1 nm (125 GHz), corresponding to channel isolation of less than ?21 dB and ?16 dB, respectively. Such a low-voltage operation and reconfigurable demultiplexer is suitable for on-chip optical interconnect.     

Flame propagation of nano/micron-sized aluminum particles and ice (ALICE) mixtures

Flame propagation of aluminum–ice (ALICE) mixtures is studied theoretically and experimentally. Both a mono distribution of nano aluminum particles and a bimodal distribution of nano- and micron-sized aluminum particles are considered over a pressure range of 1–10 MPa. A multi-zone theoretical framework is established to predict the burning rate and temperature distribution by solving the energy equation in each zone and matching the temperature and heat flux at the interfacial boundaries. The burning rates are measured experimentally by burning aluminum–ice strands in a constant-volume vessel. For stoichiometric ALICE mixtures with 80 nm particles, the burning rate shows a pressure dependence of r_b = aPⁿ, with an exponent of 0.33. If a portion of 80 nm particles is replaced with 5 and 20 μm particles, the burning rate is not significantly affected for a loading density up to 15–25% and decreases significantly beyond this value. The flame thickness of a bimodal-particle mixture is greater than its counterpart of a mono-dispersed particle mixture. The theoretical and experimental results support the hypothesis that the combustion of aluminum–ice mixtures is controlled by diffusion processes across the oxide layers of particles.     

Tunable dual-wavelength erbium-doped fiber ring laser covering both C-band and L-band for high-speed communications

A simple, continuously tunable dual-wavelength erbium-doped fiber ring laser (TDEDFL) structure for applications in high-speed communication systems is proposed and experimentally demonstrated. The dual-wavelength tuning range is 58 nm covering both the C-band and L-band from 1547 to 1605 nm. We can not only obtain a 45% improvement over previously reported tuning ranges, but also tune the wavelength of each lasing output independently. The power equalization of the dual-wavelength outputs is less than 1.5 dB. We obtain extremely stable power variation and wavelength fluctuation at room temperature. Using this fiber laser, a 10-Gb/s data transmission over a 25-km single-mode fiber (SMF) can be made available with a power penalty of 0.5 dB is demonstrated with this laser.     

First-principles study of small palladium clusters on NiAl(1 1 0) alloy surface

Theoretical calculations focused on the geometry, stability, electronic and magnetic properties of small palladium clusters Pd_n (n=1–5) adsorbed on the NiAl(1 1 0) alloy surface were carried out within the framework of density functional theory (DFT). In agreement with the experimental observations, both Ni-bridge and Al-bridge sites are preferential for the adsorption of single palladium atom, with an adsorption energy difference of 0.04 eV. Among the possible structures considered for Pd_n (n=1–5) clusters adsorbed on NiAl(1 1 0) surface, Pd atoms tend to form one-dimensional (1D) chain structure at low coverage (from Pd₁ to Pd₃) and two-dimensional (2D) structures are more stable than three-dimensional (3D) structures for Pd₄ and Pd₅. Furthermore, metal-substrate bonding prevails over metal–metal bonding for Pd cluster adsorbed on NiAl(1 1 0) surface. The density of states for Pd atoms of Pd/NiAl(1 1 0) system are strongly affected by their chemical environment. The magnetic feature emerged upon the adsorption of Pd clusters on NiAl(1 1 0) surface was due to the charge transfer between Pd atoms and the substrate. These findings may shade light on the understanding of the growth of Pd metal clusters on alloy surface and the construction of nanoscale devices.     

High-pressure shock-tube investigation of the impact of 3-pentanone on the ignition properties of primary reference fuels

Ignition-delay times for pure 3-pentanone, 3-pentanone/iso-octane (10/90% by volume) and 3-pentanone/n-Heptane mixtures (10/90% by volume) have been determined in a high-pressure shock tube under engine-relevant conditions (p₅ = 10, 20, and 40 bar) for equivalence ratios ? = 0.5 and 1.0 and over a wide temperature range 690 K < T₅ < 1270 K. The results were compared to ignition delay times of primary reference fuels under identical conditions. A detailed kinetics model is proposed for the ignition of all fuel mixtures. The model predicts well the ignition delay times for pure 3-pentanone for a wide range of pressure and temperature and equivalence ratios in argon dilution as well as in air. Ignition delay times for 3-pentanone-doped mixtures, especially in the low-temperature range are overpredicted by approx. a factor of 0.5 (at 800 K, 40 bar, ? = 1.0) by the calculation but the model still reproduces the overall trend of the experimental data. For lean conditions, 10% 3-pentanone reduces the reactivity of n-Heptane below 1000 K while for stoichiometric conditions it does not alter the ignition delay by more than 11% at 850 K and 20 bar. In iso-octane the effect is inverse, leading to acceleration of the main ignition. Based on the model, the influence of 3-pentanone on the main heat release in a n-Heptane-fueled HCCI engine cycle is simulated.     

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ = 2 nm around λ = 1565 nm with a group index of n_g = 60. We experimentally demonstrate a relatively low propagation loss, of 130 dB/cm, for waveguides up to 800 μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100 μm that exhibits stitching errors of typically 10–50 nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320 dB/cm to 130 dB/cm at n_g = 60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.     

Low-threshold pure UV electroluminescence from n-ZnO:Al/i-layer/n-GaN heterojunction

Ultraviolet (UV) electroluminescence (EL) of n-ZnO:Al (AZO)/i-layer/n-GaN heterojunctions with different intrinsic layers has been obtained. Rectifying behavior and EL spectra of the heterojunctions are investigated at room temperature. Under positive voltage, a dominant UV emission peak around ～370 nm is observed for both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Nevertheless, the UV emission peak intensity of AZO/i-MgO/n-GaN heterojunction is much stronger than that of AZO/i-ZnO/n-GaN heterojunction at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3 V. The difference of EL spectra and the emission mechanism in these devices are discussed.     

Flux pinning by discontinuous columnar defects in 74 MeV Ag-irradiated YBa2Cu3O7 coated conductors

Linear damage tracks are created in production-quality YBCO coated conductors by irradiation with 61–74 MeV Ag ions. The ion tracks are observed by transmission electron microscopy to be elongated but discontinuous. The in-field transport critical current (I_c) is enhanced significantly for fields applied parallel to the irradiation direction with a broad peak appearing in the magnetic field-angle dependence of the critical current, coinciding with the irradiation direction. The zero-field I_c is typically reduced somewhat, however annealing at 200 °C partially restores this and enhances the in-field I_c even for field parallel to the irradiation direction. Lower energies tend to produce a weaker peak, but also retain the zero-field I_c to a greater extent, consistent with a trend of greater discontinuity in the ion tracks.     

Electrical and structural properties of stepped,partially reconstructed Au(11n) surfaces in HClO4 and H2SO4 electrolytes

We have studied the interface capacitance and the voltammograms of Au(11n) (n = 5, 7, 11, 17) and of Au(100) electrodes in 5 mM HClO₄ and 5 mM H₂SO₄ after immersion into the electrolyte solution at ?0.4 V versus a saturated calomel electrode. The minima of the capacitance curves measured in positive sweeps continuously shift towards positive potentials as function of 1/n. All voltammograms, even that of Au(1 1 5), display peaks that are characteristic for lifting of surface reconstructions, albeit at different potentials. Thus, all vicinal surfaces appear to have at least sections that allow reconstruction. This inference is consistent with STM-profiles of an Au(1 1 9) surface which displays a wide range of local inclination angles corresponding to local (11n)-orientations with 3.5 < n < ∞. A numerical analysis of the voltammograms shows the existence of three different ranges of transition potentials for the lifting of the reconstruction. The transition potentials are assigned to three different structures of the reconstructed phase as either observed by experiment or proposed by theory.     

Arsenic dopant mapping in state-of-the-art semiconductor devices using electron energy-loss spectroscopy

Knowledge of the dopant distribution in nanodevices is critical for optimising their electrical performances. We demonstrate with a scanning transmission electron microscope the direct detection and two-dimensional distribution maps of arsenic dopant in semiconductor silicon devices using electron energy-loss spectroscopy. The technique has been applied to 40–45 nm high density static random access memory and to n–p–n BiCMOS transistors. The quantitative maps have been compared with secondary ion mass spectrometry analysis and show a good agreement. The sensitivity using this approach is in the low 10¹⁹ cm^?3 range with a spatial resolution of about 2 nm.     

Resonances in the final Rydberg state population of multiply charged ions ArVIII,KrVIII, and XeVIII escaping solid surfaces

The appearance of resonances (pronounced maxima at n_A = n_res) in the probability distributions for the population of the Rydberg state (n_A, l_A, m_A) of multiply charged ions (Z ? 1) escaping solid surfaces at intermediate velocities (v ≈ 1 a.u.) is discussed. Within the framework of the time-symmetrized two-state vector model, in which the state of a single active electron is described by two wave functions Ψ₁ and Ψ₂, the resonances are explained by means of an electron tunneling in the very vicinity of the ion–surface potential barrier top. To include this specific feature of electron transitions into the model, the appropriate etalon equation method is used in the calculation of the function Ψ₁. We consider the ions ArVIII, KrVIII, and XeVIII with the same core charges Z = 8 a.u., but with different core polarizations. The effect of the ionic core polarization is associated with the function Ψ₂. The population probabilities for n_A ≈ n_res are complemental to those obtained recently for n_A < n_res, and in sufficiently good agreement with available beam-foil experimental data. The pronounced resonances in the final population distributions are recognized only in the case of ArVIII ion and for the lower values of the solid work function (argon anomaly).     

Photoionization studies of the atmospherically important species N and OH at the Elettra synchrotron radiation source

In this paper angularly resolved photoelectron spectra (PES) and constant-ionic-state (CIS) spectra are presented for the atmospherically important species N and OH.The natural width Γ, line shape parameters q and ρ² and discrete oscillator strengths f have been measured for the members of the N*[2s2p³(⁵S), np] (⁴P) ← N(⁴S) autoionizing resonances for n = 5–10. The n = 5 parameters calculated in this work are in good agreement with the values obtained previously whereas for the resonances with n = 6–10 the values of these parameters are reported for the first time. The asymmetry parameter (β) for the first band of N atoms, the N⁺(³P) ← N(⁴S) ionization, has also been measured in the photon energy range of the above autoionizing resonances.For OH, CIS spectra have been recorded for the first photoelectron band corresponding to the ionization OH⁺(X³Σ⁻, v⁺ = 0) ← OH(X²Π, v″ = 0). In these spectra, rotationally partially resolved bands associated with OH*(a¹Δ3d, v′ = 0) ← OH(X²Π, v″ = 0) resonances have been observed. Suggestions for their assignment are made on the basis of their positions and band simulations which use rotational line strength calculations.