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Characterizing interface trap states in commercial wide bandgap devices using frequency-based measurements requires unconventionally high probing frequencies to account for both fast and slow traps associated with wide bandgap materials. The C − ψ S technique has been suggested as a viable quasi-static method for determining the interface trap state densities in wide bandgap systems, but the results are shown to be susceptible to errors in the analysis procedure. This work explores the primary sources of errors present in the C − ψ S technique using an analytical model that describes the apparent response for wide bandgap MOS capacitor devices. Measurement noise is shown to greatly impact the linear fitting routine of the 1 / C S ∗ 2 vs ψ S plot to calibrate the additive constant in the surface potential/gate voltage relationship, and an inexact knowledge of the oxide capacitance is also shown to impede interface trap state analysis near the band edge. In addition, a slight nonlinearity that is typically present throughout the 1 / C S ∗ 2 vs ψ S plot hinders the accurate estimation of interface trap densities, which is demonstrated for a fabricated n-SiC MOS capacitor device. Methods are suggested to improve quasi-static analysis, including a novel method to determine an approximate integration constant without relying on a linear fitting routine. [ABSTRACT FROM AUTHOR]

N incorporation mechanisms in GaAs1−xNx alloys are probed using combined experimental and computational Rutherford backscattering spectrometry and nuclear reaction analysis angular yield scans. For xN < 0.025, in addition to substitutional nitrogen, NAs, (N-N)As, and (N-As)As split-interstitials are observed. However, for xN ≥ 0.025, evidence for N tetrahedral interstitials, Ntetra, emerges. We propose a mechanism for stabilization of Ntetra in which the elastic interaction between Ntetra and NAs is induced by the opposite signs of their misfit volumes. This work opens opportunities for exploring the formation of Ntetra and its influence on the properties of a variety of highly mismatched alloys. [ABSTRACT FROM AUTHOR]

The anomalous appearance of a ferromagnetic moment in nominally antiferromagnetic nanoparticles has been known about since Néel, but never well understood. We present proof of the core/shell model of magnetism in antiferromagnetic NiO nanoparticles (NP) using neutron diffraction. Nickel oxide nanoparticles were produced in a large quantity by a novel continuous hydrothermal flow synthesis method. The antiferromagnetic nature of the nanoparticles allowed the structural and the magnetic diffraction peaks to be completely separated. Using both the microstructure option in "Fullprof" microstructure fitting suite and convolution techniques, we determined the NP consisted of an ordered antiferromagnetic core 5.2(2) nm in diameter surrounded by a disordered shell 0.7(2) nm thick. Further magnetic measurements showed that this disordered shell possess a significant polarisable magnetisation, up to a fifth that of pure nickel. They also indicate that two magnetic transitions occur between 400 and 10 K; around 350 K, there is a broad transition from paramagnetic to a form of superparamagnetism, then near 30 K there is a transition to a higher anisotropy state. Differences in field cooled and zero field cooled hysteresis loops were found, though with no evidence of exchange bias effects. [ABSTRACT FROM AUTHOR]

The nonparabolic Schrödinger equation for electrons in quantum cascade lasers (QCLs) is a cubic eigenvalue problem (EVP) which cannot be solved directly. While a method for linearizing this cubic EVP has been proposed in principle for quantum dots [Hwang et al., Math. Comput. Modell., 40, 519 (2004)] it was deemed too computationally expensive because of the three-dimensional geometry under consideration. We adapt this linearization approach to the one-dimensional geometry of QCLs, and arrive at a direct and exact solution to the cubic EVP. The method is then compared with the well established shooting method, and it is shown to be more accurate and reliable for calculating the bandstructure of mid-infrared QCLs. [ABSTRACT FROM AUTHOR]

Discusses the fabrication and electrical performance of an all refractory niobium-nitrogen medium scale integrated circuit process. Fabrication of Josephson junctions; Plasma etching of in situ radio frequency sputter deposited trilayers of niobium-nitrogen/magnesium-oxygen/niobium-nitrogen; Use of a deposited silicon dioxide film for wiring and resistor insulation.

Presents a study which described the low-level pretunneling leakage currents and the measurement techniques needed to determine the properties of the excess leakage currents associated with the traps generated inside of the oxide. Components of current which were identified; Unstressed current-voltage characteristics; Stressed current-voltage characteristics.

Microarrays and suspension-based technologies have attracted significant interest over the past decade with applications in medical diagnostics and biochemical multiplexed assays. However, the throughput of microarrays will always be limited by the array density and the slow kinetics, while the suspension (or bead)-based technologies are currently limited by the number of distinct codes the beads can carry. A novel digital magnetic tagging technology based on magnetic tags that can be used as encoded microcarriers for biomolecular probes, is presented here. The highly disruptive platform technology can provide a very large number of unique codes, enabling a high degree of multiplexing. The design principles of a novel magnetic laboratory-on-a-chip device comprising microfluidic channels with embedded magnetic tunneling magnetoresistive sensors are also discussed. [ABSTRACT FROM AUTHOR]

Focuses on a study that detailed the measurement of capacitance-time transients on n-type 6H-silicon carbide (SiC) metal-oxide semiconductor (MOS) capacitors. Purpose of conducting the measurement; Result that occurred due to the high band gap energy of SiC; Properties in which SiC is unusual; Key element in MOS devices.

Provides information on a study concerning the fabrication of gate-controlled diodes in the wide band gap semiconductor 6hydrogen-silicon carbon by thermal oxidation and ion implantation of nitrogen. Discussion of the nonequilibrium characteristics of diodes; Room-temperature capacitance-voltage characteristics; Surface state density.

Purcell enhanced Raman scattering (PERS) by means of a doubly resonant Fabry-Perot microcavity (mode volume ≈ 100 µm³ and finesse ≈ 30 000) has been investigated as a technique for isotopic ratio gas analysis. At the pump frequency, the resonant cavity supports a buildup of circulating power while simultaneously enabling Purcell spontaneous emission rate enhancement at the resonant Stokes frequency. The three most common isotopologues of CO2 gas were quantified, and a signal was obtained from 13C16O2 down to a partial pressure of 2 Torr. Due to its small size and low pump power needed (~10 mW) PERS lends itself to miniaturization. Furthermore, since the cavity is resonant with the emission frequency, future improvements could allow it to serve as its own spectral analyzer and no separate spectroscopic device would be needed. [ABSTRACT FROM AUTHOR]

Negative transconductance is reported in a resistive gate metal-semiconductor field-effect transistor (MESFET) for the first time. The negative transconductance arises as a result of the negative differential mobility of electrons in the GaAs channel, and is adequately described by a simple equivalent circuit model. The experimental device is fabricated with a planar ion-implanted MESFET process, and shows promise as a microwave signal source for use in monolithic microwave integrated circuits. [ABSTRACT FROM AUTHOR]

Potassium dihydrogen phosphate (KDP) is widely used for high-power laser frequency conversion. Its applications, however, are limited by a characteristically low laser damage threshold. This damage is associated with extrinsic impurity inclusions, whose exact nature has not been previously identified. We have now demonstrated that KDP grown in solutions containing organic impurities incorporates select organic compounds into the crystal matrix. Furthermore, we have observed a strong correlation between the concentration of these compounds and the crystal laser damage threshold and density. [ABSTRACT FROM AUTHOR]

Presents a study which demonstrated that thermal generation rates in gallium arsenide can increase by more than three orders of magnitude when samples are metallized in E-beam evaporators rather than in thermal evaporators. Device fabrication; Measurement technique; Experimental results; Activation energy of the control sample.

Presents a study that examined the photocapacitance-voltage and current-voltage characteristics of p[sup+]-gallium arsenide/i-alimunum arsenide/n-gallium arsenide heterojunction enhancement mode capacitors. Analysis of the constant illumination of the capacitor; Determination of the constant reverse sweep rate; Evaluation of the frequency dependence of the capacitor.

Presents a study which investigated the collisionally induced line shapes corresponding to an electric quadropole transition in neutral calcium for a variety of rare-gas perturber species. Experimental details; Interpretation of the results in terms of molecular potentials; Conclusion.

Focuses on a study which characterized the SiO[sub2]/SiC interface through the high-low capacitance-voltage and conductance-frequency techniques. Experimental details; Results and discussion; Summary.

Presents a study that described electrical measurement on exposed surfaces of n- and p-type gallium arsenide. Features of the n-type surface; Features of the p-type gallium arsenide; Significance of the findings for gallium arsenide devices.

Antiferromagnetic coupling between rare-earth (RE) and transition metals (TM) gives rise to various magnetic ground states in RE/TM heterostructures. Interface structure and morphology tend to play important roles in defining the magnetic properties of heterostructures of these materials. Using spin-dependent polarized neutron reflectivity (PNR), we found a distinct magnetic structure for RE/TM (Gd/Co) multilayers having different interface morphologies. Two Gd/Co multilayers, each consisting of eight bilayers of Gd and Co, were grown at different argon pressures to accomplish variation in the interface morphology. The Gd/Co multilayer, which is grown at a lower argon pressure, exhibits lower intermixing/interdiffusion at the interfaces and shows asymmetric magnetic hysteresis loops below the compensation temperature (Tcomp ∼ 140 K) and antisymmetric magnetoresistance at Tcomp. The other multilayer, grown at two different argon pressures for the top and bottom four bilayers, shows a double hysteresis loop with an exchange-bias-like shift to the field axis at temperatures above Tcomp (∼150 K). From PNR measurements across Tcomp, these behaviors were attributed to interface dependent magnetic helical structures. The PNR results indicate the formation of a twisted helical magnetic structure with planar 2π domain walls and the evolution of magnetic inhomogeneities below Tcomp. The interface dependent twisted helical magnetic structures and the development of highly correlated magnetic inhomogeneities may find applications in all-spin-based technologies. [ABSTRACT FROM AUTHOR]

We have studied the capacitance-voltage (CV) and current-voltage (IV) characteristics of molecular beam epitaxially (MBE) grown p+-GaAs/AlAs/n-GaAs heterostructures as a function of illumination level and temperature. Under illumination we find clear evidence of a hole inversion layer at the AlAs/n-GaAs interface at 77 K, with a maximum inversion layer carrier concentration of about 4.1×1011 cm-2. The samples are essentially free of trapping in the AlAs, as indicated by the almost complete lack of hysteresis in the CV curves. At room temperature under steady-state illumination we see both a voltage shift and a peak in the CV curves. The shift is attributed to the formation of an inversion layer, while the peak is believed due to the ac response of space charge flowing through the AlAs. In contrast to recent reports on similar samples grown by metalorganic chemical vapor deposition, the MBE grown samples appear to be suitable for fabricating field-effect devices. [ABSTRACT FROM AUTHOR]

The physical aging of amorphous polymers induces changes in material properties, which are challenging to detect in situ in industrial settings. Here, we present a nondestructive nonlinear ultrasonic evaluation technique that enables localized measurements of the combined effects of temperature and time in amorphous polymers, without needing to remove materials. The proposed technique is demonstrated using commercial grade poly(vinyl chloride) samples and is supported by analysis of wave–material interactions. The results show that the physical aging of the polymer is described by the Arrhenius equation with an effective activation energy of 103 kJ/mol over the analyzed temperature range. [ABSTRACT FROM AUTHOR]

We propose a method of pinning and propagating domain walls in artificial multiferroic nanowires using electrically induced surface acoustic waves. Using finite-element micromagnetic simulations and 1D semi-analytical modelling, we demonstrate how a pair of interdigitated acoustic transducers can remotely induce an array of attractive domain wall pinning sites by forming a standing stress/strain wave along a nanowire's length. Shifts in the frequencies of the surface acoustic waves allow multiple domain walls to be synchronously transported at speeds up to 50 ms-1. Our study lays the foundation for energy-efficient domain wall devices that exploit the low propagation losses of surface acoustic waves to precisely manipulate large numbers of data bits. [ABSTRACT FROM AUTHOR]

We investigate electron transport in epitaxially grown nitride-based resonant tunneling diodes (RTDs) and superlattice sequential tunneling devices. A density-matrix model is developed, and shown to reproduce the experimentally measured features of the current–voltage curves, with its dephasing terms calculated from semi-classical scattering rates. Lifetime broadening effects are shown to have a significant influence in the experimental data. Additionally, it is shown that the interface roughness geometry has a large effect on current magnitude, peak-to-valley ratios and misalignment features; in some cases eliminating negative differential resistance entirely in RTDs. Sequential tunneling device characteristics are dominated by a parasitic current that is most likely to be caused by dislocations; however, excellent agreement between the simulated and experimentally measured tunneling current magnitude and alignment bias is demonstrated. This analysis of the effects of scattering lifetimes, contact doping and growth quality on electron transport highlights critical optimization parameters for the development of III–nitride unipolar electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

A design of a biological molecule carrier is presented for the application of high throughput multiplexing biological assays. This carrier contains a bit addressable “magnetic barcode” made of either Permalloy or cobalt thin films, sandwiched between two planar SU8 protective layers. We describe how the design of the magnetic carriers is optimized by engineering the coercivity of each barcode element, allowing the number of available signatures to be increased. Fully encapsulated digital magnetic carriers which carry a 5 bit addressable barcode were also fabricated and are presented. Writing and reading of digital carriers were both performed after releasing in dried solution. [ABSTRACT FROM AUTHOR]

We discuss the nature of errors which may occur when the spectra of random signals not obeying Gaussian statistics are measured with typical rf spectrum analyzers. These errors depend on both the noise statistics and the process used to detect the random signal after it has been passed through a narrow bandpass filter within the spectrum analyzer. In general, for random signals not obeying Gaussian statistics, the output of the bandpass filter must be measured with a square law detector if the resulting measurement is to be strictly proportional to the power spectrum of the input signal. We compare measurements of the power spectra of non-Gaussian noise using a commerical spectrum analyzer with its resident envelope detector, with measurements by the same analyzer fitted with a square law detector. Differences of about 5% were observed. [ABSTRACT FROM AUTHOR]

We report the first direct measurement of electron storage time at the AlxGa1-xAs/GaAs interface as a function of Al mole fraction x. Storage capacitors with x=0.4, 0.6, 0.8, and 1.0 were fabricated and their inversion layer electron retention times measured. The optimal mole fraction for electron retention is in the range 0.4≤x≤0.6. [ABSTRACT FROM AUTHOR]

A buried five-period 94 Å Al0.3Ga0.7As/ 56 Å GaAs superlattice is employed to improve the dynamic retention of electrons at an AlGaAs/GaAs heterojunction. The improvement is attributed to the impurity trapping and gettering properties of this superlattice and the amount of time our molecular beam epitaxy system had been closed. Electron retention time constants of 29 s were obtained at a temperature of 77 K. This is more than 200 times longer than previously reported time constants and more than adequate for dynamic logic or memory applications compatible with AlGaAs/GaAs heterojunction field-effect transistors. [ABSTRACT FROM AUTHOR]

We report photosensitive capacitance-voltage characteristics of n+-GaAs/Al0.3Ga0.7As/p-GaAs heterostructures grown by molecular beam epitaxy. Inversion layers are observed due to minority charge retention in the triangular potential well at the AlGaAs/p-GaAs interface. Details of the device structure and experimental evaluation utilizing capacitance-voltage and current-voltage techniques are presented. [ABSTRACT FROM AUTHOR]

Inserting an antiferromagnetic layer of NiO between Pt and Y3Fe5O12 (YIG) changes the positive sign of the spin Hall magnetoresistance (SMR) in Pt/YIG to a negative sign at low temperature. Here, we use polarized neutron reflectometry to explore the coupling between NiO and YIG to understand the mechanism of the negative SMR. A weak uncompensated magnetic moment is observed in the NiO and the direction of this moment is perpendicular to YIG. Therefore, we infer that the spin axis of NiO is perpendicular to YIG. This result directly supports the explanation that the negative SMR results from the pure spin current reflected back by the NiO layer and the spin-flop coupling between NiO and YIG. [ABSTRACT FROM AUTHOR]

Presents results of experiments probing the effect of collisions on forbidden lines. Classes of transitions of low oscillator strength in many atoms; Description of collisions between radiators and perturbers via an interaction; Blue wing of the collisionally induced signal for the calcium/argon collision complex.

We demonstrate that the magnetic state of a superconducting spin valve, that is normally controlled with an external magnetic field, can also be manipulated by varying the temperature which increases the functionality and flexibility of such structures as switching elements. In this case, switching is driven by changes in the magnetostatic energy due to spontaneous Meissner screening currents forming in the superconductor below the critical temperature. Our scanning Hall probe measurements also reveal vortex-mediated pinning of the ferromagnetic domain structure due to the pinning of quantized stray fields in the adjacent superconductor. The ability to use temperature as well as magnetic field to control the local magnetisation structure raises the prospect of potential applications in magnetic memory devices. [ABSTRACT FROM AUTHOR]

Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light-current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs. [ABSTRACT FROM AUTHOR]

We report an investigation of the long term retention characteristic of a hole inversion layer at the interface between an AlAs/GaAs superlattice and GaAs in the dark. The retention time constant at 77 K is measured to be 34 h. From the temperature dependence of the retention time constant, an effective valence-band discontinuity of 0.17 eV is obtained. [ABSTRACT FROM AUTHOR]

A multitasking extension has been developed for computers based on the Z80A CPU; it enables the Sinclair ZX Spectrum 48 K to be used for data acquisition and analysis simultaneously. [ABSTRACT FROM AUTHOR]

We describe the design and fabrication of nanostructured silver split-ring resonators and demonstrate their application as dichroic sensors for multipurpose visible wavelength molecular spectroscopy. By producing arrays of these identical nanostructures, with critical dimensions of 30–150 nm, we demonstrate the controllable generation of plasmonic resonances at two common visible laser wavelengths, produced as a consequence of the polarization of the light with respect to structural geometry. We show the application of these devices in surface enhanced Raman spectroscopy, using their dichroic properties to perform molecular sensing of a self-assembled monolayer of 2-mercaptopyridine at both 532 and 633 nm. [ABSTRACT FROM AUTHOR]

Top-gated, few-layer graphene field-effect transistors (FETs) fabricated on thermally decomposed semi-insulating 4H-SiC substrates are demonstrated. Physical vapor deposited SiO2 is used as the gate dielectric. A two-dimensional hexagonal arrangement of carbon atoms with the correct lattice vectors, observed by high-resolution scanning tunneling microscopy, confirms the formation of multiple graphene layers on top of the SiC substrates. The observation of n-type and p-type transition further verifies Dirac Fermions’ unique transport properties in graphene layers. The measured electron and hole mobilities on these fabricated graphene FETs are as high as 5400 and 4400 cm2/V s, respectively, which are much larger than the corresponding values from conventional SiC or silicon. [ABSTRACT FROM AUTHOR]

Graphene is a hexagonally bonded sheet of carbon atoms that exhibits superior transport properties with a velocity of 108 cm/s and a room-temperature mobility of >15 000 cm2/V s. How to grow gate dielectrics on graphene with low defect states is a challenge for graphene-based nanoelectronics. Here, we present the growth behavior of Al2O3 and HfO2 films on highly ordered pyrolytic graphite (HOPG) by atomic layer deposition (ALD). To our surprise, large numbers of Al2O3 and HfO2 nanoribbons, with dimensions of 5–200 nm in width and >50 μm in length, are observed on HOPG surfaces at growth temperature between 200 and 250 °C. This is due to the large numbers of step edges of graphene on HOPG surfaces, which serve as nucleation sites for the ALD process. These Al2O3 and HfO2 nanoribbons can be used as hard masks to generate graphene nanoribbons or as top-gate dielectrics for graphene devices. This methodology could be extended to synthesize insulating, semiconducting, and metallic nanostructures and their combinations. [ABSTRACT FROM AUTHOR]

Electron beam lithography was used to fabricate gold crescent shaped split-ring resonators with 30 nm minimum feature size. By varying the crescent’s arc length over a range of nanometer-scale dimensions the authors demonstrate the tuneability of visible resonances within such structures. Results, which correlate closely with those predicted using finite-difference time-domain modeling, open the way for these devices to be used in near-field biological sensing. [ABSTRACT FROM AUTHOR]

The authors describe the integration of low-loss optical waveguides with lab-on-a-chip structures to produce an integrated optical-microfluidic platform for time-correlated single-photon counting of fluorescent molecules. Waveguides were fabricated using electron beam densification of planar silica on silicon, eliminating any requirement for depositing upper cladding silica layers. Microfluidic channels were dry etched directly through the waveguides and the device was sealed using a poly(dimethylsiloxane) gasket. Time-resolved fluorescence lifetime measurements of the fluorophore nile blue were used as a model system to demonstrate the operation of the microfluidic device, with dye concentrations as low as 1.5 nM (equivalent to <6000 molecules) being measured. [ABSTRACT FROM AUTHOR]

Understanding and controlling plasmon resonances from metallic nanoscale structures have been the focus of much attention recently, with applications including local surface plasmon resonance sensing, surface enhanced Raman spectroscopy, and negative refractive index materials. In this letter the authors demonstrate the fabrication of uniform arrays of split rings from gold and show that such structures are capable of supporting multiple plasmon resonances. The authors show that up to five plasmon resonances can be identified and use finite difference time domain modeling and absorption spectroscopy to fully characterize and identify each resonance. The implications of higher order surface plasmon resonances for sensing are discussed. [ABSTRACT FROM AUTHOR]

An ultrasensitive nanocalorimeter for use with pl-scale biological samples using silicon microfabrication technology has been developed in which a 720 pl reaction vessel, a calibration heater, and a thermoelectric transducer of 125 µK sensitivity were integrated into a single multilayer thin-film configuration. The resolution of the system ranged from 10 to 25 nW depending on the heat capacity, conductance and power density of the samples studied. The device has been used in heat conduction measurements of the energy released from the enzyme catalyzed hydrolysis of hydrogen peroxide using purified catalase, and for the determination of the catalase activity within a single mouse hepatocyte. The nanocalorimeter has the potential for integration in a high-density array format, where the change in temperature from ultralow volume cellular assays could be used as a generic analytical tool for high throughput screening of bioactive compounds. [ABSTRACT FROM AUTHOR]

Experimentally we compare the densification induced by electron beam irradiation of Ge-doped silica, produced by flame hydrolysis deposition with the densification of thermally produced SiO[sub 2]. By comparing these results to the predictions made by elasticity theory, we find good agreement for the thermal SiO[sub 2] by considering a single region of electron beam damage. For the Ge-doped flame-hydrolysis-deposited silica, we need to include in the model a second, shallow region, which densifies to a greater extent. X-ray photoelectron spectroscopy measurements suggest that the thickness of this additional region is comparable to a layer that was found to be depleted of Ge. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]