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Notes: We present results from a theoretical study of the influence of doping profile variations on the performance of delta-doped AlGaAs/GaAs high electron mobility transistors (HEMTs). An ensemble Monte Carlo simulation coupled with a self-consistent solution of the two-dimensional Poisson equation is used to investigate HEMTs which employ both single and double delta-doped profiles with varying doping configurations. The calculated results reveal that single delta-doped HEMTs designed with identical threshold voltages exhibit improved device behavior when thinner delta-doped layers with more heavily doped concentrations are utilized. For double delta-doped HEMTs with an identical total doping in the AlGaAs layer, improved threshold voltage control is obtained as the spacing between two delta-doped layers increases. However, this increase in spacing also causes a degradation in transconductance, cut-off frequency, and switching time. As gate bias increases, the dependence of device performance (or degradation) on the spacing between doping planes becomes less pronounced due to the upward shift in threshold or "onset'' of parallel conduction in the AlGaAs layer.

Notes: One-dimensional electron-optical-phonon interaction Hamiltonians in a rectangular quantum wire consisting of diatomic polar semiconductors are derived under the macroscopic dielectric continuum model. The scattering rates calculated in a GaAs square quantum wire show that when the quantum wire is free-standing in vacuum, the interaction by the surface-optical phonon modes is very strong and may dominate over other scattering processes, especially with dimensions of about 100 A(ring) or less. When the wire is embedded in a polar semiconductor (AlAs to be specific), the scattering rates by the surface-optical phonon modes become generally smaller, but yet comparable to those by the confined longitudinal-optical modes as the wire dimension shrinks. A considerable decrease in the total scattering rate for optical phonons as a result of simple reduction in dimensionality is not observed in this study.

Notes: The magnetic properties of sintered Nd-(Dy)-Fe-B magnets prepared with different annealing conditions have been investigated. Thermomagnetic analysis was carried out on the sintered and annealed Nd-(Dy)-Fe-B magnets in the temperature range of room temperature to 320 °C. The coercivity of gas-quenched Nd14.1Fe78.3B7.6 magnets after annealing for 1 h at temperatures between 600 and 900 °C is higher by 3 kOe than the furnace-cooled one after annealing. Thermomagnetic analysis found that the sintered Nd14.1Fe78.3B7.6 magnet consists of the Nd2Fe14B phase as well as an unknown magnetic phase (A1) with Tc around 150 °C. The annealed magnets consist of two unknown magnetic phases (A1,A2) and the Nd2Fe14B phase with a Tc of 150, 240, and 305 °C, respectively. Thermomagnetic analysis also showed that the Mr value of the A2 phase in the gas-quenched Nd-Fe-B magnet was higher than that of the furnace-cooled one. These results seem to indicate that increase of coercivity is related to the formation of the A2 phase.

Notes: Many types of magneto-optical structures have been investigated to improve the MO Kerr effect. We have studied the enhancement of the MO Kerr effect in the Tb21Fe79 amorphous film having a multidielectric structure in which the TiO2 layer is sandwiched by SiNx layers. The variations of Kerr angle and reflectivity with the thickness of dielectric layer are calculated by using the reiterative formula [M. Mansuripur, G. A. N. Connel, and J. W. Goodman, J. Appl. Phys. 53, 4485 (1982)]. Experiment and simulation results are discussed.

Notes: A MgO single crystal partly coated with a thin layer of Ag was irradiated with incident electrons while the time evolution of the surface potential and specimen current were recorded. A zero surface potential, Vs, is measured when the uncoated part of the specimen is irradiated, while the potential is very negative on the coated part for primary beam energies in the 2–5 keV range. The results invalidate the usual approach which uses a theoretical total yield curve for evaluating the sign of the surface potential as well as the use of an ohm law Vs = RIs for correlating the surface potential Vs and the specimen current Is. Explanations of the observed phenomena are suggested and their correlation with the secondary electron emission of insulators is indicated. One surprising result is the possibility of injecting (nonohmic) current densities of 1 mA/mm2 or greater into insulators having electrical resistivities of 1015 Ω cm.

Notes: We have employed extended x-ray absorption fine structure (EXAFS) analysis to study the compositional dependence of the atomic structure in Fe–Tb alloy films. Fourier transforms of EXAFS data, relative to both the Fe K and the Tb LIII absorption edges, provide information about the local atomic environments relative to each atom. Results indicate the Fe EXAFS data to be dominated by Fe–Fe correlations, and consists of contributions from two Fe atomic shells at radial distances near 2.47 and 2.66 A(ring) and a Tb shell near 2.91 A(ring). The coordination number of the Fe shells are measured to increase, while radial distances decrease, with increased Fe content. The Tb EXAFS data was found to have an atomic shells of Fe and Tb at 2.91 and 3.47 A(ring), respectively. Analysis suggests that the Fe shell is very disordered and is comprised of approximately 9.5 atoms while the Tb shell has ≈3 atoms.

Notes: The roles of DX centers and surface states associated with the n-AlGaAs layer of δ-doped AlGaAs/GaAs high-electron-mobility transistors have been investigated by employing a two-dimensional, self-consistent ensemble Monte Carlo simulation. It is found that both DX centers and surface states degrade device performance, particularly as gate-to-source voltage increases. This degradation is manifested largely through reduced channel electron concentration, increased intervalley scattering, and enhanced real-space transfer. Of the two defect states, DX centers have more detrimental influence on drain current and transconductance.

Notes: The evolution of magnetic properties and local atomic ordering during thermal annealing has been studied for amorphous Fe75Ni5B15Si5 thin films. Resistivity, vibrating sample magnetometer (VSM), ferromagnetic resonance (FMR), extended x-ray absorption fine structure (EXAFS), and Mössbauer effect (ME) measurements were taken on samples annealed at various temperatures ranging to film crystallization. The as-deposited samples are in a close-packed structure with little short-range order. Samples annealed above 200 °C show ordering of the boron shell, but no indication of long-range ordering. With the exception of the anisotropy and coercive fields, no change in the magnetic or microwave magnetic parameters is observed for these samples prior to crystallization. Samples annealed above 400 °C show indications of crystallization for all measurements.

Notes: Thin-film specimens of amorphous Co74Fe6B15Si5 were subjected to heat-treatment procedures followed by room-temperature magnetic and structural characterization in an effort to quantitatively correlate magnetic property changes to local atomic structure. Fourier transforms of electron-yield extended x-ray-absorption fine structure spectra show second and higher atomic shell ordering about transition-metal ions to occur at Tann≈300 °C. This ordering evolves to a body-centered atomic structure with heat treatments at higher temperatures. Accompanying this structural evolution is an expansion of the nearest-neighbor distance, disruption of the spin-wave-resonance spectra, and general deterioration of the soft magnetic properties.

Notes: Oscillations in light emission and species concentrations, are measured as periodic perturbations are simultaneously applied to the input rates of acetaldehyde and oxygen in the gas-phase combustion of acetaldehyde in a continuous-flow stirred tank reactor for conditions where the autonomous reaction itself is oscillatory. The experimental results are compared with the predictions of a five-variable thermokinetic model. We measure periodic responses in the fundamental entrainment band (ratio of frequency of perturbation to frequency of response equal to unity) for four different values of phase shift between the acetaldehyde and oxygen perturbation wave forms as we vary the frequency and amplitude of the external periodic perturbations. Outside of the entrainment bands we find quasiperiodic response. We determine the phases of the light emission and six species concentrations, as measured with a mass spectrometer, with respect to the periodic perturbation, the variation of these phases across the fundamental entrainment band for different values of reactant phase shift and for different amplitudes of perturbation, and the effects of the phase shift between the two input perturbations on the light emission response of the system for different frequencies of perturbation. Both the experiments and calculations predict a widening of the entrainment band with an increase in perturbation amplitude, and the same variation in bandwidths for the four values of reactant phase shift studied. The experiments and calculations also predict the same general trends in light phase and species phases (difference between the light emission and species concentrations with respect to the perturbing wave form) as the band is traversed for different amplitudes of perturbation and for different values of reactant phase shift. Some discrepancies in the species phases exist; however, the similarities which exist between the experiments and the calculations provide further substantiation for the model. We find that changing the phase shift between reactant inflows from 0° to 360° produces a sharp change in light phase near 180°, in both the experiments and calculations.In general, we also find that the light emission remains entrained near the total flux minimum of the perturbing wave form, and we attribute this correlation to the net heat loss which arises from the reactant flux through the reactor.