ALBERT

Notes: This paper describes the application of an adaptive learning system comprising an associative memory to the characterization of ultrasonic phenomena. The mapping of source and waveform data and vice versa can be performed by utilizing the off-diagonal cross-correlation portions of the associative memory. It is suggested that a more general description of the ultrasonic phenomena can be obtained if the diagonal, autocorrelation portions of the memory are also utilized. In this case, the memory is applicable for the autoassociative optimal filtering of experimental data. Experiments are described which utilize such an adaptive system, running on a laboratory minicomputer, to process the signals from a transient ultrasonic source in a plate specimen. It is shown how the system learns from the experimental pattern vectors, formed from the ultrasonic waveforms and encoded information about the source. The source characteristics are recovered by the recall procedure from the detected ultrasonic signals and vice versa. Also, the changes in the wave phenomenon corresponding to changes in the boundary conditions of the specimen can be detected from the discrepancy between the presented and the learned signals.

Notes: A new type of solid-state sensor for the detection of minute concentrations of hydrogen gas has been developed. The sensor was made of thin, commercially available polyvinylidene fluoride (PVDF) pyroelectric film, sputter coated with Pd. An infrared laser beam served to produce alternating temperature gradients on the Pd-PVDF and on reference Al-Ni-PVDF films, which, in turn, generated ac voltages due to the photopyroelectric (P2E) effect. Exposure to hydrogen gas was shown to produce an increased differential signal between the Pd and reference electrodes; this was tentatively attributed to the adsorption and dissociation of hydrogen molecules on the Pd surface, which caused a shift on the Pd-PVDF pyroelectric coefficient, due to interactions at the Pd-PVDF interface. The differential signal was found to be proportional to the square root of the hydrogen partial pressure at very low concentrations (〈1000 ppm). A semiquantitative interpretation of the differential signal has been achieved using simple gas-solid interaction theory and the combination of the Langmuir isotherm with the photopyroelectric theory in the range of 4–200 Pa. For high pressures (〉200 Pa) the paper is limited only to a phenomenological description. The thickness of the palladium layer has been found to play an important role with respect to the signal response. Presently, hydrogen concentrations as small as 40 ppm, in a flowing H2+N2 mixture, have been detected. The influence of gas flow rate has also been studied. Other characteristics such as the response times, the reversibility, and the durability of the Pd-PVDF-P2E hydrogen detector will also be presented.

Notes: The quenching cross sections of Ne metastable levels by N2 and CO were measured to be (in A(ring)2 units): 8.3±0.4, 8.7±0.4 for 3s(3/2)2 level and 8.0±0.4, 9.1±0.4 for 3s(1/2)0 level, respectively, at 443 K. The metastables are produced by a pulsed electron beam and their population was monitored by fluorescence induced by a cw dye laser.

Notes: The generation of quasi-static uniform electric fields by a slotted conducting spherical shell is fully analyzed for an arbitrary number of slots pairs, and optimum surface potential and slots position are determined. Numerical results for spherical shells with up to 14 slots are discussed.

Notes: This paper discusses a theoretical as well as experimental study of the performance of electro-optic sampling using a noncontact electro-optic probe, i.e., external electro-optic sampling. Sensitivity, temporal resolution, and the capacitive loading effect of this system are calculated by analysis of a static electric field coupled to an electro-optic crystal placed in close proximity to transmission lines such as microstrip lines and coplanar strips. Full-wave analysis is also applied to investigate effect of the electro-optic crystal on the transient property of high-speed electrical signals. Based on these analytical considerations, we have developed an external electro-optic sampling system using precise probe-positioning technology, which improves the measurement reproducibility. A temporal resolution of 0.5 ps and a spatial resolution of 1 μm are confirmed with this system.

Notes: Experimental and theoretical studies of a fundamental mode large-orbit gyrotron designed to produce high-power rf radiation in the TE11 mode at about 1 GHz are reported. Theoretical calculations performed as part of the experimental design process are presented. Results of initial experiments are reported in which 500–1000 MW of rf radiation was observed at frequencies 700 and 1200 MHz, consistent with theoretical expectations. The estimated eletronic efficiency of the device is about 10%. Gas cell breakdown patterns produced by the rf pulse are consistent with operation in the desired TE11 mode.

Notes: As a second in a series of theoretical models for the electrical discharge machining (EDM) process, an erosion model for the anode material is presented. As with our point heat-source model in the previous article, the present model also accepts power rather than temperature as the boundary condition at the plasma/anode interface. A constant fraction of the total power supplied to the gap is transferred to the anode. The power supplied is assumed to produce a Gaussian-distributed heat flux on the surface of the anode material. Furthermore, the area upon which the flux is incident is assumed to grow with time. The model is capable of showing, via the determined migrating melt fronts, the rapid melting of the anodic material as well as the subsequent resolidification of the material foation from plasma dynamics modeling could improve substantially our results.

Notes: Designs are proposed, based on a series of one-dimensional calculations, for layered, hollow cylindrical targets to be placed on the axis of an imploding, hollow Z-pinch plasma that can create the approximate plasma conditions, as well as radiation spectrum, for a photoionization pumped, Ne-like recombination laser. The lasant must reach the Ne-like state and be at the appropriate density at the same time that the photoionizing pump radiation is present, placing severe constraints on designs for such targets. Target designs are further constrained by the fact that the 3s-2p resonance line, which depopulates the lower lasing state, must not be highly trapped and by the fact that the upper lasing state must not be collisionally depopulated. We find that hollow, cylindrical targets consisting of a few-micron-thick CH strongback, coated on the inside with a thin layer of Ni lasant and on the outside with an Al converter layer, can be optimized to achieve appropriate conditions for lasing and modest levels of gain.

Notes: The trigatron has been in widespread use as a demand-triggered, high-voltage switch for more than 40 years. In spite of the popularity and maturity of the technology, there persists an uncertainty over the basic physical mechanism(s) responsible for triggering breakdown in the devices. We present the results of an empirical study of trigatrons that directly demonstrates that breakdown is initiated by a streamer launched from the trigger pin, independent of the trigger spark. We compare our results with those of previous workers, and discuss the generality of our conclusions.

Notes: Reaction kinetics for the direct synthesis process of compound GaSb from Ga and Sb metals was studied by observing viscosity changes of the melt during the reaction process. The mole fraction of Ga (cGa) was obtained during synthesis using the equation; log η=cGa(log ηGa +log ηSb)+(1−2cGa)log ηGaSb, where η is the viscosity of the total system, and ηGa,ηSb, and ηGaSb are the viscosities of molten Ga metal, molten Sb metal, and molten GaSb, respectively. The reaction rate was obtained from the variation of the mole fraction of reactant cGa. Assuming a second-order reaction, the activation energy was estimated to be 15.5 kcal/mol.

Notes: A PbTe flux has been used for n-type (Te) doping of GaAs, GaSb, and AlGaSb. The effects of surface accumulation and Te desorption were noticeable in secondary-ion mass spectroscopy profiles of GaAs layers grown at temperatures in excess of 540 °C. Te accumulation was not apparent in GaSb layers grown at temperatures up to 630 °C, but Te desorption occurred from GaSb at temperatures above 540 °C. The donor ionization energy of Te in AlxGa1−xSb is 44 meV for 0.4〈X〈0.5, i.e., significantly lower than the ionization energies of S or Se in similar material.

Notes: The low-frequency excess electrical noise has been measured on carbon fibers with a wide range of crystalline perfection and corresponding electrical and mechanical properties. Fibers include those prepared from ex-PAN and ex-pitch polymers, and a catalytic-chemical vapor deposited filament. The extensional (Young's) moduli of these fibers varied from about 220 to 890 GPa (35–130 Msi), while the electrical resistivities varied from about 19 to 1 μΩ m. The low-frequency electrical noise of each fiber was found to be proportional to I2 and to vary as 1/f α, where f is the frequency and α is about 1.15. The most striking feature of the results was the strong dependence of the normalized noise power on the degree of crystalline perfection.

Notes: The densities of states of the conduction and valence bands of silicon and GaAs have been calculated at 300 K for the case of an electron-hole plasma, which can occur at high-injection levels in bipolar devices or in bulk material under intense optical excitation. The results show considerable narrowing of the band gap, which needs to be included in the analysis of device measurements or the interpretation of photoluminescence data. Furthermore, the band-gap narrowing that results from dopant ions is reduced by excess carriers because of the reduced free-carrier screening radius.

Notes: We investigate the transient response of photogenerated carriers to an external electric field in bulk GaAs. The results of our Monte Carlo simulations indicate that the initial velocity rise times are a strong function of the carrier density. This is caused by a combination of the hot-phonon effect and the enhanced electron-hole scattering within the plasma. Contrary to some previous suggestions, the hot-phonon effect alone is insufficient to explain the initial velocity behavior seen experimentally. The steady-state velocity is limited by the electron-hole scattering.

Notes: Data obtained on a set of GaAs/AlGaAs double-barrier quantum-well resonant tunneling structures are compared with model calculations of the ideal case where scattering is negligible and tunneling is coherent throughout the entire structure. The comparison points to interface roughness in the well as the most likely cause for the observed large valley currents. The currents at low biases, before resonance sets in, are also studied. Their magnitude is found to be consistent also with the sequential tunneling picture.

Notes: The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x-ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi-rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ-Bi2O3 and less interpenetration between α-Bi2O and δ-Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi-rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3 were asymmetrical with respect to the polarity of the applied voltage. It was found that α-Bi2O3 and δ-Bi2O3 give rise to different breakdowns for electrons traveling from the Bi2O3 into an adjacent ZnO grain.

Notes: Isothermal anneal experiments have been conducted over a wide temperature range from 300 to 900 °C for polycrystalline YBa2 Cu3 O7−x samples in air. The phase transformation and oxygen contents of the annealed samples were studied and determined by x-ray diffraction and iodine titration. Oxygen diffusion coefficients at different temperatures were determined by a novel technique based on direct observation of the oxygen diffusion fronts by polarized light microscopy. The activation energy for the diffusion of oxygen in YBa2 Cu3 O7−x was obtained by an Arrhenius plot of the diffusion coefficients at different temperatures. The tetragonal-to-orthorhombic phase transformation controlled by oxygen diffusion is discussed.

Notes: In the limit of negligible exchange and anisotropy (except uniaxial), certain shapes of magnetic samples in certain subsaturating field configurations can be analyzed analytically to obtain magnetization and domain wall patterns. Here we consider two cases: (1) generalized cylindrical objects of arbitrary cross section (with several specific examples given) and (2) a general ellipsoid. We also discuss a "tipping instability'' which might occur in an ellipsoid with uniaxial anisotropy, resulting in a sudden transition from a "curling'' state to a "transverse vortex'' state.

Notes: Fe multilayered films with various intermediate layers are formed. Their magnetic properties and film structures are examined to understand the mechanism originating soft magnetic properties. The soft magnetic properties change with the lattice mismatches between Fe and intermediate layers, showing low coercive force and high relative permeability at lattice mismatches from 0.4 to 1.2%. It is thought that the lattice mismatches above 0.4% decrease Fe crystallite size and improve soft magnetic properties. Although Fe crystallite size is small at the lattice mismatches above 1.2%, the soft magnetic properties are poor. This is because the large lattice mismatch increases internal stress and magnetic anisotropy energy. This report indicates that the good soft magnetic properties are obtained when both Fe crystallite size and internal stress are small.

Notes: The advantage of a film with high saturation magnetic flux density for perpendicular magnetic recording is investigated through computer simulations and read/write measurements. A single-pole head using Fe-C/Ni-Fe multilayered film with a saturation magnetic flux density of 2.0 T is fabricated. Read/write tests are performed with the Co-Cr double-layer media. It is shown that the recording ability of this head is very high and does not change with increasing Co-Cr layer coercivity. Computer simulations show that the recording ability also increases when using film with high saturation magnetic flux density for the back layer of a double-layer medium. The upper limit of saturation magnetization of the recording layer can be extended by increasing coercivity. Thus, the higher saturation magnetization medium, which offers higher output voltage and resolution, can be used by applying film with high saturation magnetic flux density for the main pole and the back layer.

Notes: In this paper we study the dielectric response of materials which display a dc conductivity at low frequencies. A framework based on the generalized Master equation and time dependent transition rates is argued to be applicable to disordered materials and exhibits an interesting fractal dynamics. The relationship of this framework to the "Distribution of transition rates'' theory and the models put forward by Ngai [Comments Solid State Phys. 9, 141 (1980] and by Dissado and Hill [J. Chem. Soc. Faraday Trans. 2, 80, 291 (1984)] is discussed. Model calculations of the dielectric response by means of the above-mentioned three theories are carried out and are compared to experimental results on Al2O3 and SiOxNy thin films. The theories can be distinguished only when the exponent of the power law at high frequencies is close to unity. The Dissado–Hill model is in better agreement with experiments than the other theories, but important discrepancies can easily be seen.

Notes: Band-gap narrowing of GaAs as a function of doping concentration has been measured using photoluminescence spectroscopy on samples grown by molecular beam epitaxy. Both n- (Si) and p- (Be) doped samples with concentrations varying from 3×1017 to 3×1018 cm−3 have been measured. The experimental results obtained from a line-shape analysis of the spectra taking tailing effects into account are in good agreement with recent theoretical calculations. A simple expression for the band-gap narrowing as a function of concentration for both n-and p-doped GaAs is given.

Notes: We propose that short-period doping superlattices are suitable for the enhancement of a third-order susceptibility arising from free carriers in nonparabolic energy subbands. The inherent advantage lies in the ability to simply engineer the superlattice potential profile, yielding control of miniband energy dispersion. We consider short-period GaAs doping superlattices composed of uniformly doped donor and acceptor layers, and planar-doped n- and p-type monolayers separated by intrinsic regions. Calculations of the electronic structure of compensated and n-type noncompensated n-i-p-i superlattices incorporating miniband dispersion at nonzero temperature are reported. We show that small modulations of the superlattice potential lead to large subband nonparabolicities and we calculate a twentyfold improvement in the third-order susceptibility over bulk GaAs at room temperature, comparable to that predicted for GaAs/AlGaAs compositional superlattices.

Notes: The physical mechanism of photostimulated luminescence (PSL) of RbBr:Tl, resulting from x-ray irradiation, has been investigated. Spectroscopic methods have been utilized to identify the bromine F center as the occupied electron trap. By means of absorption studies the charge state of the dopant Tl was found to be monovalent. The spectral emission showed the characteristic Jahn–Teller splitting of the 3P1 state of the Tl+ and its known temperature dependence. PSL lifetime and efficiency experiments have been conducted in the temperature regime from liquid helium to 500 K to understand the charge transport from the F center to the activator and have been compared with corresponding data obtained from direct optical excitation into the A band of the Tl+. The observed temperature dependencies of both lifetimes could be explained by the properties of the s2 ion Tl+. As a resulting model for the PSL process a tunneling from the relaxed excited state of the F center to an excited state of the Tl2+ ion following photostimulation is suggested. The temporal, thermal, and spectral behavior of the subsequent radiative fraction of the deexcitation process is solely determined by the properties of the Tl+ ion in the RbBr matrix.

Notes: Experiments were conducted in which a high-power CO2 TEA laser interacted with metallic cathode in a high-vacuum (10−8 Torr) diode. For power densities lower than 5×107 W/cm2, no current was detected. For power densities in the range of 5×107–5×108 W/cm2, the Cu cathode emitted a maximum current of 40 mA. At a higher power density level, a circuit-limited current of 8 A was detected. The jump of a few orders of magnitude in the current is attributed to breakdown of the diode gap. The experimental results are similar to those of a triggered vacuum gap, and a thorough comparison is presented in this paper. The influence of the pressure in the vacuum chamber on the current magnitude shows the active role that adsorbed gas molecules have in the initial breakdown. When the cathode material was changed from metal to metal oxide, much lower laser power densities were required to reach the breakdown current region.

Notes: The intrinsic magneto-optical readout performance in reflection is calculated for bismuth and cobalt-substituted iron-garnet films on a multilayer interference mirror at 800-, 633-, 488-, and 420-nm wavelengths and is compared with that of a trilayer medium composed of an antireflection layer, a rare-earth transition-metal film, and a metallic mirror. It is found, when disregarding inhomogeneities, like irregular domain shape, ripple of the magnetic anisotropy, and surface roughness, that iron garnets are superior to rare-earth transition-metal films at blue to near-ultraviolet wavelengths if operated at thicknesses where optical interference occurs in the magnetic layer. Optical transmittance at these thicknesses is sufficiently high so that multilevel recording media can be conceived. In contrast, the optical absorption of rare-earth transition-metal alloys is much higher so that only thicknesses much above interference conditions are feasible, thus precluding them from multilevel recording.This comparative study is supplemented by calculating the magneto-optical performance in reflection of a recently reported multilayer medium composed of an antireflection coating and a periodically repeated sandwich of 4-A(ring) Co and 9-A(ring) Pt layers. In contrast to conventional rare-earth transition-metal films, the magneto-optical Kerr effects of this material do not degrade when decreasing the wavelength from 800 to 400 nm, but still do not reach the performance of bismuth-iron garnets in the green to ultraviolet spectrum. For the garnet system Y3−xBixFe5O12 the spectra of the real and imaginary parts of the diagonal and off-diagonal component of the dielectric tensor εij are reported in the range of photon energies between 1 and 5 eV, i.e., 1240- and 248-nm wavelengths and a bismuth concentration up x=1.4 Bi3+ atoms per garnet formula. In addition, the off-diagonal components ε'12 and ε(large-closed-square)12 are parametrized in terms of paramagnetic optical transitions, taking the spectra for x=1.25 as a typical example. Furthermore, optical and magneto-optical spectra are presented for Co2+- and Co3+- substituted iron garnets and barium hexaferrite BaFe12O19.Finally, the spectral dependence of the magneto-optical figure of merit 2aitch-thetaF/α of (Y,Bi)3Fe5O12 and amorphous TbFe is compared. Furthermore, high-resolution transmission electron micrographs and x-ray double-crystal diffractograms are presented that elucidate the perfect epitaxial alignment of single-crystalline iron-garnet films and the columnar morphology of polycrystalline iron-garnet films prepared by rf magnetron sputtering. The initial nucleation period of polycrystalline garnet films can be influenced by low-energy ion bombardment for improving the film texture. Under favorable sputtering conditions single- and polycrystalline bismuth-iron garnet films develop a perpendicular magnetic anisotopy. It is not yet clear whether sputtered iron-garnet films can meet the critical requirements on magnetic wall coercivity and magnetic remanence.

Notes: Chemical analysis of the surface of an oxidized silicon-germanium mixture by Rutherford backscattering reported recently showed that the silicon is oxidized, but not the germanium. This result provides evidence that the effective oxygen concentration at the silica-silicon interface is low, which is contrary to a slow interface reaction. Strain in the oxide film can result in linear-parabolic oxidation kinetics without a slow interface reaction.

Notes: Low energy inelastic electron tunneling spectra are presented for thermally formed oxides on copper films. Thinner oxide tunnel barriers are observed to contain interfacial gradients as evidenced by spectral intensity asymmetries between forward and reverse bias, whereas thicker oxides appear to be more uniform in composition. Spectral results are in good qualitative agreement with reported studies of bulk cuprous oxides, and the semiconducting nature of the oxide layers is reflected in a negative temperature coefficient of resistivity of the tunnel junctions.

Notes: Optical waveguide profiles of He+ ion implanted polymethylmethacrylate have been analyzed. The surface index increases by up to 3% and the depth profile of the index is consistent with a diffusion limited decomposition of the target. This was confirmed by 77- or 300-K implants with a capping layer to inhibit decomposition.

Notes: Infrared absorption spectroscopy was used to study the oxidation of hydrogenated amorphous silicon carbide (a-Si:C:H) films prepared by the glow-discharge decomposition of gaseous mixtures of silane and methane. It has been found that carbon-rich samples incorporate oxygen when exposed to air, as detected by an increased absorption of the Si-O-Si stretching vibration band. The analysis of the infrared spectra of samples annealed in air at room temperature and at 200 °C indicates that, except for their oxidation rate, no appreciable difference exists in the mechanisms of oxygen incorporation in the films at the two temperatures. The oxidation kinetics suggests an open porous structure for these carbon-rich films. On the contrary, samples having a low carbon content appear to oxidize on the surface only, in a way similar to amorphous silicon.

Notes: Thin-film transistors have been made using nickel phthalocyanine (PcNi) as active component. The characteristics of these transistors are studied with positive and negative gate-source and drain-source polarizations. The channel conductivity was p type. Very different thicknesses of PcNi layer are used, showing that in the two polarizations the effects are interfacial.

Notes: A technique for measuring the field-effect mobility of a gateless In0.53Ga047As/In0.52Al0.48As heterojunction has been developed by using the nonpersistent photoconductive effect. Subband depopulation in a parallel magnetic field is consistent with the "universal'' depopulation observed in InAs, InSb, and HgCdTe metal-oxide-semiconductor field-effect transistors.

Notes: Transmission versus voltage operating characteristics of a GaAs/AlGaAs multiple-quantum-well light modulator have been measured at temperatures between 4.3 and 290 K. At 4.3 K, the linewidth of the heavy-hole exciton absorption is 1.3 meV, and the modulation ratio is greater than 1000 at a 9-V bias. At room temperature, the exciton resonance is broadened to 5.3 meV by phonon scattering and the maximum modulation ratio is 4.93 at 12 V. The measured temperature and voltage dependence of the exciton absorption energies, linewidths, and intensities have been used with a simple model for the modulator's optical transmission to calculate the operating characteristics as a function of material quality. The upper limit on the room-temperature transmission modulation ratio is estimated to be 5.4 for a device fabricated from 100 quantum wells of width 100 A(ring).

Notes: Adhesion-induced deformations of a polyurethane substrate in contact with cross-linked polystyrene spheres, having diameters ranging from less than 2 μm to approximately 12.5 μm were observed using scanning electron microscopy. The diameters of the contact areas were measured from the micrographs. It was found that the contact radius varied as the particle radius raised to the 0.75±0.05 power. Experimental results are compared to the predictions of various adhesion models. The results are also discussed in terms of the Dupré work of adhesion.

Notes: We have observed very rapid migration of silicon atoms along the grain boundaries of electron-beam-annealed polycrystalline silicon film in a silicon-on-insulator (SOI) island structure with a capped layer consisting of Si3N4 and SiO2 films. A considerable amount of mass transport was observed in the SOI structure at a temperature high enough to melt grain boundaries but not high enough to melt the entire polycrystalline silicon film. Our results support the notion that electromigration of Si+4 ions in molten silicon at the grain boundaries is responsible for the rapid migration of silicon atoms.

Notes: Traditionally, the chemical composition of surfaces prepared by various chemical, thermal, and plasma oxidation are analyzed by x-ray photoelectron spectroscopy. In this work, both x-ray wide-angle and glancing angle diffraction techniques were used, not only to investigate chemical composition, but also to characterize the structural features of the chemical oxide grown on an InP (100) surface in both HCl and HNO3 etches. These oxides all showed different degrees of preferred orientation with respect to the substrate. In 1M–8M HCl etches, the gradient composition, identified as crystalline hydrous InPO4, was the predominant phase and the rest was mainly composed of hydrous InPO4-based oxides. Coherent hydrated layers, also identified as hydrous InPO4, were obtained in HNO3 etches. In addition, the observation that InI -InIII mixed valence and InI chloro compounds are produced helped clarify the understanding of the dissolution mechanism of InP.

Notes: To understand the behavior of a YBa2Cu3O7−x weak link (constriction), a simple model of two Josephson junctions in series with each other is proposed. Electronic simulation of this two-junction array under various conditions generates dc and ac IV curves. Some of these curves duplicate very well what has been observed on a real YBa2Cu3O7−x weak link, while others predict phenomena that have not yet been observed. These results suggest the possible applicability of this model to explain the complicated experimental features of a YBa2Cu3O7−x weak link.

Notes: The superconductive properties in the Bi(Pb)-Sr-Ca-Cu-O system with nominal composition of Bi2−x Pbx Sr2 Ca2 Cu3 Oy (x=0, 0.2, 0.4, and 0.6) and annealed at 860 °C for 240 h were studied using dc resistivity and ac susceptibility measurements. For x=0.2 and 0.4, the bulk zero resistivity temperatures were found to be ∼103 K. For x=0 and 0.6, these temperatures were found to be ∼73–75 K. The ac susceptibility data show enrichment of the volume fraction of the high-Tc phase in compositions with x=0.2 and 0.4. Tc of this phase is constant at 107 K for all values of x. The enhancement of the bulk zero resistivity temperature for x=0.2 and 0.4 is due to the improved intergranular coupling of the high-Tc phase, although the resulting weak-link behavior reflects a low current carrying capacity in these materials. The presence of the low-Tc phase (∼65 K) is still visible in the susceptibility data. For x=0.6, the intergranular coupling of the high-Tc phase is almost destroyed; however, the coupling of the low-Tc phase is better than when x=0. Powder x-ray diffraction and scanning electron microscopy data show the formation of large grains of the calcium plumbate, Ca2PbO4 phase with increasing Pb up to x=0.6. The large grains of the Ca2 PbO4 phase may incorporate other impurity phases within it when x=0.6, cleaning up the remaining grain boundaries, thereby improving the intergranular coupling within this material. Energy dispersive x-ray analysis indicates that some Pb is incorporated into the structure of both superconducting phases.

Notes: A possible pairing mechanism involving the collective oscillations ("dipolons'') of the crystal-field-generated dipoles of the oxygen ions in the CuO2 planes in the superconductor YBa2Cu3O7−δ has been used to estimate, in the framework of the BCS theory, the variation of Tc as a function of δ, which compares very well with the experimental observations.

Notes: The demagnetizing energy of thin magnetic films can be written in terms of the Fourier components of the magnetization distribution. This formalism is used to investigate the structure of jagged domain walls in both perpendicular and in-plane media. Also introduced is a set of correlation functions and their relationship to demagnetizing energy density in media with random magnetization distribution.

Notes: The resonance spectrum of composite plates made of alternating piezoelectric ceramic and epoxy elements is studied both theoretically and experimentally in the frequency range below the thickness resonance. The transmissivity of the finite structure to plate modes is calculated, taking into account the effective plate velocities of the two constituent materials and the finite number of elements in the structure. The results are in excellent agreement with the electrically excited acoustical resonances of samples with different geometry and number of elements. In particular, the effects of the finite size on the number and frequency of modes, as well as their selection rules in the given experimental setup, are successfully interpreted. New "surface'' resonances, attributed to the end elements, are observed in the stopbands when the surface elements have lower impedance.

Notes: Thirty-seven of 97 AlxGa1−xAs laser diodes from three tests conducted in vacuum (10−6 Torr) at a −20 °C heat sink temperature (Tj∼0 °C) failed catastrophically. Causes of failure varied from testing error to defects in the metallization. In all cases, p-side anomalies allowed indium solder (and probably gold in addition to indium in one case) to migrate into the GaAs bulk, causing the observed failures. The results of this research show that the integrity of the p-side metallization is crucial for reliable operation. Stringent controls have been implemented in the design and processing of the p-side metallization and catastrophic failures due to p-side anomalies have not been observed in subsequent life tests. A correlation was made between shifts in the forward voltage at 1 mA (Vf at 1 mA) and the specific failure mechanisms found in this research. In all cases Vf at 1 mA was found to decrease. The amount of decrease depended on the failure mechanism. A quantitative model is presented that explains the shift in Vf as a function of the type of defect in the active region.

Notes: To accurately evaluate the integrity of the interfaces of a multilayered structure using ultrasonic pulse echo methods, we must understand the wave propagation in the structure and identify the appropriate interface signal. However, the signal is often complicated by the material and defect characteristics of the preceding interfaces. We have derived generalized mathematical formulas for the reflected mode time and the pulse amplitude response of an arbitrary n-layered structure for use in modeling the wave propagations in multilayered structures and identifying the critical structural interfaces. The time and echographic amplitude responses are calculated by computer using routines programmed for a given specimen. A comparison of the results of laboratory measurements of various glass and aluminum specimens with the results from computer-simulated experiments have verified the validity of the formulas.

Notes: Acceleration instability occurs when a body is accelerated by surface tractions. This situation resembles classic Rayleigh–Taylor instability, but differs due to the temporal and spatial variation of the stress field in the accelerated body caused by wave propagation and the time dependence of the accelerating forces. These factors produce phenomena in acceleration instability which are without precedent in classical Rayleigh–Taylor analyses. An extensive numerical study of acceleration instability using a Lagrangian finite-difference wavecode has determined the influence of various parameters including amplitude and wavelength of initial surface perturbations, material yield strength, and time dependence of the driving force. The nature of the criteria determining stability or instability is established, and the fundamental physical quantity controlling perturbation growth at an interface is shown to be the local stress gradient.

Notes: We present a new model (including both temperature and electron-hole effects) of two-beam coupling in photorefractive semiconductors under an external dc field E0. This model predicts that the exponential gain Γ can exhibit an intensity-dependent resonant behavior, yielding a π/2 phase shift of the space-charge field with respect to the incident fringe pattern. This optimum intensity strongly depends on crystal temperature but it is practically independent of the grating period. As an illustration this model is applied to InP:Fe. In this case the resonance occurs when the hole photoionization rate and the electron thermal emission rate are equal. Values of Γ as high as 20 cm−1 at 1.06 μm, for a fringe spacing of 15 μm and E0 =10 kV/cm, are predicted. The comparison between theory and experimental data requires taking into account the variation of the pump intensity throughout the sample (due to optical absorption), which reduces the volume where the resonance condition is fulfilled; in this way a satisfactory fit of the experimental gain versus intensity curve has been obtained. The characteristics of this new resonance mechanism suggest several gain enhancement techniques which are briefly discussed. In particular, an increase of Γ by using an additional incoherent backward propagating beam is predicted and experimentally demonstrated.

Notes: Despite their increased safety, alkylarsine compounds have not generally replaced arsine (AsH3) in the metalorganic vapor phase epitaxy (MOVPE) of GaAs because of carbon incorporation and high background doping levels. We have studied the thermal decomposition of AsH3 and its alkyl derivatives (methyl, ethyl, and butyl compounds) to determine the impact of the thermochemistry on growth processes. The thermal stability of the As-precursor compounds was found to decrease in the order AsH3〉MenAsH3−n 〉Et3As〉t-BuAsH2. We report the first evidence for production of diarsine (As2H4) from t-BuAsH2 and for formation of lower substituted methylarsine homologs from Me3As and Me2AsH. The presence of these species is strong evidence that decomposition of the alkylarsines occurs via a free-radical mechanism. Formation of carbon-free arsenic products appears to be the key difference between t-BuAsH2 and the more highly substituted alkylarsines in attaining high quality films by MOVPE.

Notes: A new type of application of porous silicon formation is proposed, which does not deal with the material properties, but with the characteristics of the involved electrochemical silicon dissolution reaction. The anodization potential of p-type silicon in concentrated hydrofluoric acid under galvanostatic conditions has been shown to be characteristic of the silicon doping concentration at the interface and independent of the porous silicon-layer thickness. This close correlation between potential and doping level is used to determine concentration profiles of p-type implanted dopants. During anodization of silicon presenting dopant concentration variations, the anodization potential varies according to silicon doping concentration. The potential values can be converted to dopant concentration, and the electrolysis time scale to a depth scale, leading then to the doping impurity profile. This method, which is used as well for deep and shallow diffused impurities, exhibits a very good agreement with spreading resistance measurement and secondary-ion mass spectrometry analysis. Simultaneous anodization of regions of different doping concentration has also been investigated. It allows the characterization of the porous silicon formation selectivity by a selectivity coefficient, which is measured on a large range of silicon doping concentration. The selectivity is found to be strongly dependent on the doping level: it is nearly infinite for samples presenting highly doped regions (greater than 3×1018 cm−3), but decreases sharply when the doping concentration is below 1018 cm−3.

Notes: The complete reduction of oxides on the chemically polished surface of (100)InP has been obtained by exposure at 300 °C to low-pressure ammonia. The whole process was monitored by various surface techniques. In a first step, exposure to ammonia removes the weakly bonded oxygen atoms and stabilizes the one-monolayer oxide, i.e., turns the initial wet oxide to a thinner, well-defined, and more stable oxide, which should prove to be a better base for further processing. Excitation of the ammonia gas by an ion gauge, i.e., use of highly active radicals, is necessary for the second step, where the oxygen atoms strongly bonded to InP are finally completely removed. The final surface is free of all contaminants and of nitrogen, its crystal structure displays a 4×1 reconstruction, and its Fermi level is pinned at 0.8 eV above the top of the valence band, i.e., 0.2 eV lower than on the clean InP surface. It is mostly InP covered by adsorbed hydrogen, with a small coverage of indium atoms liberated by the reduction of the native oxides and grouped as metallic clusters.

Notes: GaAs layers grown by organometallic chemical vapor deposition (OMCVD) on Si substrates oriented 2° off (100) toward [011] have been studied by 10-K filtered cathodoluminescence, deep-level transient spectroscopy (DLTS), electron-beam-induced current (EBIC), and secondary ion mass spectrometry (SIMS) measurements. The mapping of the near-band-edge emission intensity shows the existence of zones of lower defect density, whose relative volume increases with the layer thickness. Four deep-level energy peaks have been characterized at 1.33, 1.20, 1.15, and 1.02 eV. The 1.33 and 1.20 eV energy peaks are attributed to Si in GaAs. Strong inhomogeneities in the intensity of these two emissions have been observed with depth of the layers, and this behavior has been related to the silicon distribution. From measurements on structures grown with different buffer layers, we show that a GaAs/AlAs supperlattice located at 1 μm from the Si substrate is efficient in decreasing the width of a highly Si-doped interface region. DLTS measurements confirm the decrease in the deep-level concentration with increasing thickness. Furthermore, by EBIC characterization, we measure a minority-carrier diffusion length of 2.7–3 μm on a 5-μm-thick layer. This corresponds to a dislocation density of 8×106 cm−2.

Notes: Polycrystalline barium titanate that has been doped to give a positive temperature coefficient of resistance (PTCR) effect is an inhomogeneous material electrically. Analysis of ac impedance data using the complex impedance plane representation gives the dc resistance of PTCR ceramics. By additional use of the complex electric modulus formalism to analyze the same data, the inhomogeneous nature of the ceramics may be probed. This reveals the presence of two, sometimes three elements in the equivalent circuit. Grain-boundary and bulk effects may be distinguished from capacitance data: grain-boundary effects have temperature-independent capacitances, whereas bulk effects show a capacitance maximum at the Curie point and Curie–Weiss behavior above the Curie point. Both grain-boundary and bulk effects appear to contribute to the PTCR effect. These results reveal limitations in current theories of the PTCR effect.

Notes: The etch rate of GaAs during reactive ion etching (RIE) in a CCl2F2:O2 discharge (4 mTorr, 0.56 W cm−2) shows a strong temperature dependence, increasing from ∼500 A(ring) min−1 at 50 °C to 2800 A(ring) min−1 at 400 °C. Arrhenius plots of the etch rate show two activation energies (0.17 eV from 50 to 150 °C and 0.11 eV from 150 to 400 °C). There is no significant plasma power density dependence of the etch rate at elevated temperatures (≥100 °C) in contrast to the strong dependence at 50 °C. The surface morphology undergoes smooth-to-rough-to-smooth-to-rough transitions at ∼150, 250, and 400 °C, respectively, although TiPtAu Schottky diodes exhibit near-ideal behavior on GaAs etched at 150 °C. The As-to-Ga ratio in the first 100 A(ring) from the surface increases with increasing RIE temperature, with chloride residues absent above 150 °C. Fluorocarbon residues were present on all samples, but were limited to the first 10–15 A(ring). As determined by x-ray photoelectron spectroscopy, fluorine was present almost exclusively as metallic gallium fluorides, while oxygen was present as both Ga and As oxides. Ion channeling detected lattice disorder to depths of ∼200 A(ring) for etch temperatures from 50 to 250 °C, while there was less damage for etching at 400 °C. No new RIE-induced deep levels were observed by capacitance transient spectroscopy in any of our samples, although the concentration of the midgap donor EL2 increased with increasing temperature during the RIE process.

Notes: Thin films of III-V nitride semiconductors (AlN, GaN, InN), mixed-crystalline films (AlxIn1−xN), and multilayered films (GaN/InN)n were grown by rf magnetron sputtering at low substrate temperatures below 500 °C. These films were characterized by x-ray diffraction, Raman scattering, optical absorption, and electrical measurements; it was proved that they have high crystal quality comparable to the previously reported data obtained by other growth methods. Dependence of the band-gap energy of AlxIn1−xN on composition x was determined. Multilayered films of (GaN/InN) were prepared for the first time; these films showed the characteristic diffusion property to make an ordered-alloy superlattice.

Notes: Hg1−xZnxTe-CdTe strained layer superlattices were grown by molecular-beam epitaxy. Their structural properties and interplay with a CdTe buffer layer were investigated with transmission electron microscopy. There is an order of magnitude reduction in the density of threading dislocations in the superlattice compared with the CdTe buffer layer. The reduction is accomplished by using the lattice-mismatch-induced strain to bend threading dislocations over into the superlattice-buffer layer interface. The magnitude of the reduction agrees well with predicted values for this system.

Notes: Concentration depth profiles of carriers (or of electrically active defects) and/or deuterium in GaAs following exposure to a hydrogen plasma (or, in one case, to molecular hydrogen) are fit using a simple diffusion model with second-order reactions. We find that (1) the activation energy for hydrogen diffusion and the dissociation energies of hydrogen-defect complexes are dependent on the concentration of hydrogen, (2) there is no molecular hydrogen formation and no fast-diffusing species of hydrogen away from the near-surface region, and (3) atomic hydrogen can in-diffuse and passivate EL2 defects when semi-insulating GaAs is annealed at a high temperature in a molecular hydrogen ambient.

Notes: Reaction between Cu and PtSi is studied over 200 to 700 °C. Using structures of Cu on a preformed PtSi, and of Cu on Pt/Si, Cu-PtSi reaction is observed below 300 °C for both structures, resulting in increasing sheet resistances above this temperature. The low thermal stability of the Cu/PtSi metallurgy, compared with that of Al/PtSi, is attributed to the ready formation of Cu silicides at low temperatures. Cu3 Si and Cu4 Si are observed for both Cu/PtSi and Cu/Pt/Si structures heated to 300–400 °C. Little improvement is obtained using an 1000 A(ring) barrier layer of Ti, W, or Cr.

Notes: High-power microwaves (20–50 MW, 8.8 GHz) have been produced in repetitive pulses at a rate of 10 Hz in a joint experiment by Harry Diamond Laboratories and Sandia National Laboratories. These microwaves were produced with Harry Diamond Laboratories' backward-wave oscillator (BWO), driven by a 600 to 800-kV, 2-kA electron beam, which was guided by a magnetic field of variable strength from 0.75 to 1.05 T. The beam was produced with Sandia National Laboratories' repetitive high-voltage pulser, RAVEN. Microwave diagnostic instrumentation included a directional coupler, heterodyne receiver, dispersive waveguide delay line, and calorimeter. In a train of 10 pulses, the shape, duration, and amplitude of the injection voltage and beam current remained unchanged for each individual pulse. The intensity of the final microwave pulse was 40% of the first pulse, and the final pulse was 80% as long as the first. The basic pulse shape remained unchanged. More intense microwave radiation should be obtained with a higher magnetic field. Computer simulations using the code magic predicted the characteristic dip in the magnetic field strength versus microwave power output curve. Code predictions of microwave power output with a warm beam agree very well with experimental results. Finally, cyclotron resonance is presented in a descriptive fashion to account for the power output dip. This is the first experiment with a repetitively pulsed high-power BWO driven by a relativistic electron beam performed outside the USSR.

Notes: The behavior of trap-related transfer loss in acoustic charge transport devices is characterized experimentally through measurements of pulse degradation and frequency response rolloff. A theoretical loss analysis which incorporates a physical model of the charge packets in the device and the characteristics of electron traps in the GaAs epitaxial layers is presented and correlated with experimental results. It is found that the trap-related loss cannot be described by a proportional loss model. Good qualitative agreement is found between theoretical predictions and experimental results; however, the time constants observed experimentally in pulse measurements are 50–100 times faster than the theoretical predictions. Additional loss due to traps with long time constants is predicted theoretically but not observed experimentally.

Notes: We present a theoretical study of semiconductor T-structures which may exhibit transistor action based on quantum interference. The electron transmission through a semiconductor quantum wire can be controlled by an external gate voltage that modifies the penetration of the electron wavefunction in a lateral stub, affecting in this way its interference pattern. The structures are modeled as ideal two-dimensional electron waveguides and a tight-binding Green's function technique is used to compute the electron transmission and reflection coefficients. The calculations show that relatively small changes in the stub length can induce strong variations in the electron transmission across the structure. Operation in the fundamental transverse mode appears to be important for applications. We also show that a bound state of purely geometrical origin nucleates at the intersection between waveguide and stub. The performance of the device can be improved by inserting additional stubs of slightly different lengths. Taking into account the applicable scaling rules, we give estimates of the experimental parameters that optimize the transmission characteristics and speed of operation of the proposed transistor.

Notes: We have developed a new fabrication process of as-grown Y-Ba-Cu-O thin films using a reactive coevaporation method specially with the rf-plasma cooling in the low oxygen pressure of 0.4 mTorr. By this O2 plasma cooling process, the transition temperature Tc is improved from 40 to 81 K for the film with a thickness of 1000 A(ring). The x-ray diffraction analysis shows that the activated oxygen species generated by the rf plasma make Y-Ba-Cu-O films oxidize sufficiently even in the low pressure of oxygen. In addition, we have studied the thickness dependence of Tc for as-grown films with various thicknesses of 60–2000 A(ring).

Notes: Optical gain measurements on the donor-acceptor (D-A) photoluminescence band of CdIn2S4 revealed maximum gain coefficients varying from 1.5 to 3.5 cm−1 at 300 K, in an optical excitation energy range of 2.016–2.101 eV. The threshold pumping intensity required for the onset of gain was found to be dependent on the excitation energy. Such characteristics were used to calculate the energy distribution of the acceptor levels in that energy range, which turned out to be exponential.

Notes: The radiation hardness of fluorinated SiO2/Si interface in metal-oxide-semiconductor field-effect transistors has been found to depend strongly on the amount of fluorine introduced. In this study, the fluorine was introduced by low-energy F implantation onto the surface of the polycrystalline silicon gate electrode, followed by annealing at 950 °C to diffuse F into the gate SiO2 toward the SiO2/Si interface. The improved radiation hardness is attributed to the strain relaxation near the SiO2/Si interface by fluorine incorporation.

Notes: Temporal laser pulse distortion during amplification in the presence of nonsaturable absorbers is analyzed. The laser pulse duration ranges between the inverse of the laser bandwidth and the lasing state lifetime. It is found that nonsaturable absorption in the amplifying medium has two effects: (i) it limits the maximum pulse fluence and (ii) it reduces the pulse-width growth.

Notes: The indentation-induced cracks in brittle coating/substrate systems are examined. The crack paths induced by indentation with different applied loads are exploited by a post-bending test. Sandwiched bending bars are designed to conduct this post-bending test. The influence of the mechanical properties of each component and the interface and the thickness of coating on the paths of the indentation cracks are addressed.

Notes: Photoluminescence (PL) spectra have been investigated at liquid-He temperature in the ZnTe layers grown by metalorganic vapor-phase epitaxy. The epitaxial layer exhibits the spectra characterized by a neutral acceptor bound-exciton line in the near-band edge and the donor-acceptor pair emission in the wavelength range of 550–580 nm. At the low substrate temperature or low transport rate ratio of diethyltelluride to dimethylzinc, the spectrum of layer becomes dominated by this donor-acceptor pair emission. From the results of secondary-ion mass spectrometry analysis and PL property, it is concluded that Na and Cl are important impurities in the layer.

Notes: 4H and 6H silicon carbide (SiC) crystals are piezoelectric and can be excited to resonant vibrations with quality factors up to Q≈100 000 and an electromechanical coupling factor of k31≈0.03 for 6H material. The resonance frequencies are used for the determination of elastic constants from 4 to 320 K. One finds a Poisson's ratio σ=0.212 and low-temperature values s11=2.035×10−12 m2/N and c33=55.12×1010 N/m2 for 6H, s11=2.114×10−12 m2/N, and c33=60.52×1010 N/m2 for 4H crystals. All elastic constants show a nonmonotonic dependence from the temperature at about 50 K.

Notes: The magnetic field induced by a current which flows in a semiconductor during charge redistribution when photoeffect relaxation takes place is measured in a contactless way. The experimental results presented verify the above explanation of the contactless measurement and illustrate how to utilize it.

Notes: We have found that at least four kinds of oxygen-related thermal donors are distinguishable by their deep-level transient spectroscopy peaks and generation kinetics around 520 °C for 10–106 min in phosphorus-doped Czochralski silicon. Each kind of thermal donors successively appeared with its preceding kind decaying. The first appearing kind is identified as a family of thermal donors that have been most studied by many investigators. It is suggested that several kinds of thermal donors arise from various oxygen clusters with different sizes and thermal stabilities.

Notes: Transparent ultrathin films (〈100 nm) of lead lanthanum zirconate titanate (PLZT) with good dielectric properties were successfully grown on a platinum plate by rf magnetron sputtering and following an anneal in PbO atmosphere. The sputtering conditions were controlled by monitoring the plasma emission from the sputtering species. The PLZT thin films were deposited at an extremely low growth rate of 36 nm/h at 400 °C and annealed at 800 °C for 3 h in PbO atmosphere. The obtained thin films are oriented to [110] of the perovskite-type structure and have relatively low dc conductivity (∼10−7 Ω−1 cm−1). The evaluated dielectric constants are anomalously large around 5000 at room temperature and 1 kHz.

Notes: The optical properties of AlxGa1−xP alloys of various compositions have been investigated by electrolyte electroreflectance. E0 and E1 optical transitions have been observed for the first time in the spectral region between 3000 and 4500 A(ring). Both empirical relationships, E0=2.78+1.05x and E1=3.77+0.26x, between the transition energies and the alloy composition were found. The extrapolated values for x=1 were the first semiempirical values for the E0 and E1 transition of AlP at room temperature. The indirect energy gap of the sample with x=0.30 was obtained from photoelectrochemical measurements to be 2.43 eV, a value slightly higher than that reported in the literature for the same composition.

Notes: This work is the first which clearly demonstrates, by deep-level transient spectroscopy analysis in a Schottky structure, the presence of excimer laser-related defects in both n- and p-type virgin monocrystalline silicon. The detected point defects are correlated to those observed after solid-state laser (ruby, YAG) annealing in silicon. The main part of these traps could be attributed to the melt-related defects produced by the fast quenching rate in pulsed laser treatment.

Notes: Thin films of Y-Ba-Cu-O were deposited by rf magnetron sputtering from a single stoichiometric target and then altered in composition by ion implantation. Under optimum deposition conditions the films are deficient in Ba and Cu. Ion implantation of Cu was performed using a metal-vapor vacuum-arc source having high current and a broad energy spectrum for good depth distribution. Composition was determined by Rutherford backscattering spectrometry. The zero-resistance temperature was greatly increased after implantation and reannealing. This method could be used to write superconducting patterns on insulating material.

Notes: Velocity distributions of molecular species ejected by ∼80 mJ/cm2, 266-nm laser ablation of polycarbonate, polyimide, poly(ethylene terephthalate), and poly(α-methylstyrene) are presented and discussed. Time-of-flight mass spectroscopy in conjunction with both 248- and 193-nm laser ionization was used to probe the escaping vapor. Up to three distinct waves of material pass through the ionization zone. The fastest wave (6–8×105 cm/s) appears to consist of highly degraded species such as C3; the arrival profiles are well fit by a velocity offset Maxwell–Boltzmann distribution with offsets typically 3–6×105 cm/s and transverse temperatures above 10 000 K. The second wave has a characteristic velocity of 1–2×105 cm/s, and, except with the poly(α-methylstyrene) target, the associated material is not cleanly ionized to parent ions under our typical conditions. It is hypothesized that this wave consists of hot, fairly heavy (up to a few hundred amu) radicals. The slow wave (2–5×104 cm/s) is composed of stable molecules which do not readily condense on the chamber walls. Its arrival profile is too broad to be described by a simple Maxwell–Boltzmann velocity distribution. A mechanism involving a thermal velocity distribution combined with laser-associated background vapor might explain the broad profiles. Problems related to the largely unknown and highly variable ionization cross sections of diverse organic molecules with 193- and 248-nm light are briefly discussed.

Notes: Schottky-barrier characteristics and chemical reactions of contacts between oxygen-contained W films and GaAs substrates annealed up to 800 °C have been investigated. Phase transformation from β-W to α-W of the oxygen-contained W films (3 at. % oxygen), sputter deposited in Ar and O2 gas mixture, is observed at temperatures around 650 °C. Electrical degradation of the contacts, especially in capacitance-voltage (C-V) characteristics, is found after annealing above 650 °C. Oxygen accumulation at the W/GaAs interfacial region and Ga and As atom diffusion into the W films are found by secondary-ion mass spectrometry during annealing. Formation of a Ga2O3 layer at the interfacial region is also detected by x-ray photoemission spectroscopy and transmission electron microscopy. These results suggest that Ga2O3-layer formation is related to the electrical degradation. The C-V characteristics can possibly be explained by series capacitance of an interfacial oxide layer and a depletion layer together with an assumption of nonuniform carrier distribution in the GaAs substrate.

Notes: Minority-carrier diffusion length of a highly Ga-doped Pb0.78Sn0.22Te (n=1.5×1019 cm−3) liquid-phase epitaxy grown layer was measured between 10–80 K using a light spot scanning technique on a beveled diode. The carrier lifetime, derived from the observed diffusion length, and the measured mobility is shown to be dominated by Auger recombination. The effect of Ga and In heavy doping on the spectral response of homostructure PbSnTe diodes is demonstrated. The advantage of Ga over In as an n-type dopant of the cladding layer of PbSnTe homostructure lasers is discussed.

Notes: We present a fully self-consistent model for the current-voltage characteristic of quantum well tunneling structures. Our approach is based on the independent particle approximation and incorporates both screening of the carrier potential and the response from the contacts. It is shown that a self-consistent treatment of charge supply from the contacts is essential for meaningful description of the system far from equilibrium. Results are presented for GaAs-AlGaAs double-barrier structures. Compared to standard calculations, this approach leads to strong quantitative changes in the I-V characteristic. In particular, peak and valley currents increase and current peaks shift to higher voltages. Doping effects on the I-V characteristic are discussed.

Notes: Interface formation for Ti, Co, Pd, and Au overlayers grown on thin epitaxial CaF2 layers on Si(111) have been investigated with photoelectron spectroscopy and low-energy electron diffraction. In general, metal overlayer growth leads to rapid attenuation of emission from the substrate CaF2 core levels, but the low-energy electron diffraction (LEED) results demonstrate the persistence of surface order to high coverages, indicating the growth of small clusters followed by coalescence. The photoemission results showed that Au deposition produces sharp metal/insulator interfaces. In contrast, signs of disruption were found for Ti, Co, and Pd deposition. For Au and Pd, metal-induced energy shifts of ∼1 eV in the CaF2 core levels and valence bands were observed; Ti and Co deposition induced smaller shifts. This indicates that the Fermi level in the CaF2 layer depends on the electronegativity of the metal overlayer. Annealing at 200–300 °C produced dramatic changes in the overlayer morphology with reappearance of the photoemission signals of CaF2 and its LEED pattern as large metal clusters were formed and the CaF2 surface was exposed. The photoemission results also indicate that metal-induced reaction with Si occurs upon annealing.

Notes: The formation, interaction, and propagation of misfit dislocations in molecular-beam epitaxial InGaAs/GaAs heterointerfaces have been studied by transmission electron microscopy. With the lattice mismatch less than 2%, most of the interfacial dislocations are found to be 60° mixed dislocations introduced by glide processes. Sessile edge-type dislocations can also originate from the combination of two 60° mixed dislocations. The ratio of densities of edge dislocations to 60° dislocations was increased during the later part of the elastic strain relaxation. These sessile edge dislocations may be generated in appreciable numbers through a climb process. For large lattice-mismatched systems, the majority of the misfit dislocations are pure edge dislocations and high threading dislocation density is generally found. The interfacial dislocation network is found to contain regions of dislocations with the same Burgers vector that extend over several micrometers. The results support a mechanism that involves misfit dislocation multiplication during the molecular-beam epitaxial growth process.

Notes: Under a mixed hydrogen/argon atmosphere epitaxial osmium silicide has been successfully grown on (111)Si for the first time. A new process of electroless deposited osmium film on Si subsequent with a suitable two-step hydrogen annealing scheme was developed for the formation of epitaxial osmium silicide. Polycrystalline silicides of Os2Si3 and OsSi2 were found in all cases of single annealing, and the grain size increased with annealing temperature. While in the developed two-step annealing scheme single-crystalline silicide can form; the first step annealing was found to be critical to determine epitaxy. The epitaxial silicide was identified to be OsSi2 of orthorhombic structure. The orientation relationships between epitaxial OsSi2 and Si were found to be (040)OsSi2(parallel)(22¯0)Si, and [102¯] OsSi2(parallel) [111] Si.

Notes: Polycrystalline silicon (polysilicon) films are deposited on thermally oxidized silicon above 250 °C by pyrolyzing silane by a tungsten heater. The deposition rate and the structure of the polysilicon films are a function of substrate temperature, heater current, and total pressure. Polysilicon films deposited at 340 and 450 °C have 〈110〉 and 〈100〉 preferred orientations. Films deposited at other temperatures have no preferred orientations. The resistivity varies in a range of 4×107–5×10−4 Ω cm as the heater current increases from 19 to 24 A. Lower resistivities are caused by the fact that tungsten evaporating from the heater during deposition is doped into the polysilicon.

Notes: The perturbed γγ angular correlation (PAC) technique was used to study the interaction of implanted 111 In probe atoms with the donor atoms P, As, and Sb in Si. Nearest-neighbor pairs of In-P, In-As, and In-Sb atoms, characterized by νQ1 =179(1), 229(1), and 271(1) MHz, respectively, and having trigonal symmetry about a 〈111〉 axis (η1 =0), were observed after annealing the samples between 540 and 1170 K. These results indicate a strong interaction between acceptor and donor atoms in Si, thus explaining the present and earlier Rutherford backscattering-channeling results that the In atom solubility in Si was enhanced by the addition of As. The In-donor atom binding energy was about 0.5 eV. For increasing As concentrations, PAC data showed the appearance of small In-As atom clusters characterized by νQ2 (As)=238(1) MHz, η2 = 0.65(1); they were probably In-As2 complexes produced when mobile In atoms were trapped by As2 pairs.

Notes: We report the first study, to the best of our knowledge, of the nonresonant third-order optical nonlinearity of quantum-confined CdS clusters using third harmonic generation from 1.91 to 0.64 μm. We observe that the nonlinearity increases with the increase in the cluster size. The intrinsic, nonresonant χ(3)CdS for ∼30, ∼15 A(ring) thiophenolate-capped CdS clusters and [Cd10S4(SPh)16]−4 molecular clusters are 3.3×10−10, 2.5×10−11, 4.7×10−12 esu, respectively. To utilize this large nonresonant nonlinearity it is necessary to incorporate these clusters into a sample in high concentrations.

Notes: Surface impedance techniques are useful means of predicting fields in eddy current problems since they circumvent the need to model the conducting regions themselves. Thus, with their use, two-and three-dimensional field predictions can be made using only scalar potentials. Their use is normally confined to (1) problems where the skin depth is small relative to the other dimensions of the problem and (2) steady-state problems where the skin depth itself is well defined. In this regard, the technique is approximate at best. Presented here is a formulation which can realize an exact prediction of the field, both in steady-state and transient problems. The technique is exact for those problems where knowledge is known as to the nature of the field variation tangential to the conductor shell interface; otherwise, an iterative numerical scheme must be employed to converge on the correct tangential variation. Surface impedances are determined generically and expressed in terms of transfer functions for shell-type structures in three different geometries. The surface impedances happen to be trigometric functions, Bessel functions, and spherical Bessel functions in planar, cylindrical, and spherical shell structures, respectively. Their use is easily implemented in finite difference, finite element, and boundary integral formulations; in this paper, the surface impedances are coupled into a boundary integral approach to verify their use in both two-dimensional cylindrical and a three-dimensional spherical problem. The results are compared to analytical expressions and are shown to disagree by no more than 0.01%.

Notes: In two previous papers [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)], we used a highly accurate numerical model to solve the problem of equilibrium barrier formation in graded Hg1−xCdxTe heterojunctions. However, we concentrated solely on narrow-gap p on wide-gap n heterojunctions in which the barrier forms in the conduction band. The present calculation is an extension of that work, but with emphasis on wide-gap p on narrow-gap n heterojunctions. We perform a full parametric study, varying doping concentrations (symmetric and asymmetric), cadmium compositions, and cadmium grading widths. All band profiles are calculated with and without a valence-band offset. Clear trends are observed. Without the valence-band offset, for the larger grading widths, there is a tendency for the valence band to bow down on the n side. The band bowing acts as a potential barrier for the minority carriers. When the valence-band offset is included though, the existence of a barrier to minority carriers depends on the grading width. For the most narrowly graded junctions, a potential well for the minority carriers is present. In the present study, we assume a valence-band offset of 300 meV for HgTe:CdTe. The effects of elevated temperatures and interdiffusion of dopants are also studied, but appear to be inconsequential for the present cases.

Notes: Thin films of hydrogenated amorphous silicon-germanium (a-Si1−xGex@B:H) alloys have been prepared by reactive ion beam sputtering of a composite target of silicon and germanium. The dependence of the deposition rate, conductivity-temperature variation, optical absorption coefficient, refractive index, imaginary part of the dielectric constant, hydrogen content, and infrared (IR) absorption spectra on germanium content (x) are reported and analyzed. For a typical composition—a-Si28Ge72:H (x=0.72), the effect of beam voltage, H2:Ar flow ratio, and substrate temperature on the material properties have also been investigated. For the films prepared with increasing x, the expected behavior of a decrease in both hydrogen content and band gap and an increase in the electrical conductivity have been observed. The films prepared at x〉0.80 are found to be more homogeneous than the films deposited at 0.0〈x〈0.80, due to the compositional disorder introduced by the random mixing of Si and Ge atoms in the a-Si1−xGex@B:H network in the latter case. The a-Si28Ge72:H films exhibiting minimum conductivity (1.7×10−7 Ω−1 cm−1) have been obtained for an H2:Ar flow ratio of 10:1 and a beam voltage and substrate temperature of 1500 V and 300 °C, respectively. These films contain a hydrogen concentration of 10.2 at. % and show an optical band gap of 1.25 eV. The IR studies have shown that a-Si28Ge72:H films prepared both at low beam voltages and at low substrate temperatures show the unusual preferential attachment of hydrogen to Ge rather than to Si.

Notes: We have modeled the generation, diffusion, and recombination of photoexcited electrons and holes for the case of Czochralski Si wafers having a defect-free-zone (DFZ) device layer of thickness d above a highly precipitated wafer core and having a finite surface recombination velocity, S. The incident photoexcitation source has a Gaussian power distribution and is focused to a small spot on the sample surface. When the source is sinusoidally modulated at frequency ν, the intrinsic band-edge photoluminescence (PL) emission displays modulations at the fundamental and first overtone of the modulation frequency. The PL signals at frequencies ν and 2ν are delayed in phase, with respect to the source modulation by angles φ2(ν) and φ2(2ν). We relate these phase angles to material properties such as d, S, the optical absorption coefficient α at the incident wavelength, and to the effective carrier lifetimes τ1 and τ2 in the DFZ and precipitated wafer core, respectively. We show that when τ1 and τ2 are independently measured and S(approximately-less-than)100 cm/s, as is common for a Si surface passivated with a thermally grown oxide layer, it is possible to deduce d from a measurement of φ2(ν) or φ2(2ν).

Notes: We have performed photoluminescence, photoexcitation, and transmission electron microscopy measurements on boron nitride films grown by chemical vapor deposition and later reacted with potassium. After reaction, the potassium atoms were found to intercalate the BN host and to form a (2×2)R0° in-plane structure which is commensurate with the pristine BN lattice. Optical transitions with ∼2.7 eV onsets were found to occur within the ∼5-eV BN band gap and have been interpreted as Γ-point transitions between the K(4s) band and the BN(2p) bands. The absence of an appreciable shift in the E2g2 phonon frequency of the pristine and reacted films suggests that the charge transfer between the K and BN bands is very small.

Notes: Thin films for optical second-harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p-nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second-order nonlinear coefficient, d33=31 pm/V, measured by the Maker-fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen-in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a stabilized value of 19 pm/V for d33.

Notes: Mercuric-iodide (HgI2 ) photodetectors with sputtered indium-tin-oxide (ITO) entrance electrodes were studied using low-temperature photoluminescence spectroscopy. The photoluminescence spectrum obtained on each photodetector was found to differ for points beneath the ITO contact and points adjacent to it, indicating that the contact fabrication process introduces new carrier traps and radiative recombination centers within the ITO-HgI2 interfacial region. In particular, a new broad band was observed in the spectra taken from points beneath the ITO electrode. Photocurrent-versus-position measurements showed that the intensity of this broad band was enhanced in regions having relatively poor photoresponse.

Notes: We propose a new method to create a medium with an intensity-dependent optical activity. The medium consists of a dilute suspension of nonlinear, but optically inactive particles, such as crystallites of CdSxSe1−x, in a linear but optically active host. We derive formulas for calculating the intensity dependence of the optical rotatory power in the dilute limit, and apply these formulas to calculate the optical rotation of a model dilute composite.

Notes: Room-temperature photoluminescence (PL) and electrolyte electroluminescence (EL) spectra of polycrystalline CdS samples, in air and in contact with aqueous electrolytes, have been studied as a function of the atmosphere and temperature of annealing, light excitation intensity and wavelength, applied bias, electrolyte composition, and photoetching treatments. Both luminescence spectra are characterized by two broad bands centered at about 770 and 920 nm, associated with a red (R) and an infrared (IR) emission, respectively. According to our experimental results, the R band can be attributed to radiative recombination between electrons trapped at sulfur vacancies, localized at about 0.79 eV below the bottom of the conduction band, and valence-band free holes. The IR emission, on the other hand, is associated with a radiative recombination process involving electrons trapped at sulfur vacancies and holes trapped at cadmium vacancies located at about 0.26 eV above the top of the valence band. The luminescence spectrum shape is shown to depend on the rate of hole injection into the semiconductor and on the concentration of cadmium and sulfur vacancies. From the analysis of the R and IR band intensities, in both PL and EL spectra, valuable information about the generation and spatial distribution of vacancies under different annealing treatments can be obtained. Such information is of interest for the fabrication of high-performance liquid-junction solar cells based on CdS polycrystalline electrodes.

Notes: ZnSe and ZnTe thin films and ZnSe-ZnTe strained-layer superlattices (SLS) have been successfully grown by atomic layer epitaxy (ALE) using molecular-beam epitaxy. The ALE growth of ZnSe and ZnTe was achieved at substrate temperatures between 250 and 350 °C and between 240 and 280 °C, respectively. The Zn, Se, and Te beam intensities necessary to achieve ALE growth were evaluated. Under optimum growth conditions, the surfaces of ZnSe and ZnTe thin films were very specular without any surface structure. Modulation-doped n-type SLSs have been prepared by ALE using Cl as a n-type dopant. The carrier concentration of superlattices grown by ALE (3.8×1016 cm−3) was three orders of magnitude higher than those of superlattices grown by conventional molecular-beam epitaxy.

Notes: A simple pyroelectric detector has been constructed and used to measure the ablation velocity for various polymers as a function of laser (248 nm) fluence. The advantage of this approach over other techniques is that a full sampling of the products in the ablation plume is obtained. Results for polymethyl methacrylate (PMMA) ablation can be interpreted in terms of a single explosive decomposition process in which numerous small fragments such as C2, CO, and CO2 propel small segments of the polymer at velocities which are scaled to the square root of their molecular weights. Data for polymide suggest that this polymer is decomposed by laser photons principally into small molecules. The solid carbon that is obtained on ablation in air is probably derived by reactions between carbon clusters. This technique is suitable for the study of the angular distribution of the ejected material which is useful for modeling and in schemes for the deposition of material by laser ablation.

Notes: Cathodoluminescence (CL) performed at low temperature has been used for the characterization of the fabrication technology of circuits implemented with heterojunction bipolar transistors. The cathodoluminescence signal from each GaAs layer has been identified. Different ion implantation masks, which protect the structure and the integrated resistors during the ion-implantation step used for device insulation, have been investigated. Thin bilayers have to be used for the fabrication of the integrated resistors to obtain uniformity adequate for integrated circuits operation. Thicker bilayers are necessary in order to achieve an effective protection of the bottom and top layers of the mesa. CL results have been compared with electrical measurements showing a very good correlation. Its ability to obtain different penetration depths and to perform CL images as well as secondary- or backscattered-electron images makes of the CL technique a powerful microcharacterization method.

Notes: n-type silicon-doped epitaxial layers of gallium arsenide grown by molecular-beam epitaxy (MBE) or metal-organo chemical vapor deposition (MOCVD) have been investigated by measurements of the Hall effect and the strengths of the localized vibrational modes (LVM) of silicon impurities using both Fourier transform absorption spectroscopy and Raman scattering at an excitation energy of 3 eV close to the E1 band gap. Lines from Si(Ga) donors, Si(As) acceptors, Si(Ga)-Si(As) pairs, and Si-X, a complex of silicon with a native defect, were detected and correlated for the two techniques. The maximum carrier concentration [n] found for samples grown under standard conditions was 5.5×1018 cm−3. At higher doping levels Si-X becomes dominant and acts as an acceptor, so reducing [n]. An integrated absorption of 1 cm−2 in the Si(Ga) LVM line corresponds to 5.0±4×1016 atoms cm−3: a similar calibration applies to the Si(As) line, but for Si-X, an absorption of 1 cm−2 corresponds to only 2.7±1.0×1016 defects cm−3. Possible structures for Si-X are discussed but a definitive model cannot yet be proposed. MBE samples grown at 400 °C had values of [n] close to 1019 cm−3, and a negligible concentration of Si-X. On annealing, [n] decreased and Si-X defects were produced together with site switching of Si(Ga) to Si(As). These results are important to the understanding of the mechanism of silicon diffusion at low temperatures. The infrared absorption and Raman measurements are complementary. Absorption measurements made at a resolution of 0.1 cm−1 require layers greater than or equal to 1 μm in thickness doped to a level of 3×1017 cm−3 but require the prior elimination of free-carrier absorption. Raman measurements can be made on as-grown layers only 10 nm in thickness doped to a level of 2×1018 cm−3, but with a spectral resolution of only 5 cm−1.

Notes: The thermal diffusivity values of La2.7Sm0.3S4 , La2.3Sm0.7S4 , La2.7Eu0.3S4 , La2.2Eu0.8S4 , La2.8Yb0.2S4 , and La2.3Yb0.7S4 were measured by the flash diffusivity method from 400 to 1000 °C. The values ranged from 0.007 to 0.018 cm2 /s. The thermal conductivities of the six ternary rare-earth sulfides were calculated from the thermal diffusivity data and ranged from 10.7 to 31.6 mW/cm °C. Of the alloys measured, La2.2Eu0.8S4 had the highest thermoelectric figure of merit at 1000 °C (0.52×10−3 °C−1). All of the samples had an oxysulfide present at the grain boundaries which degraded their high-temperature performance.

Notes: We deal with the hysteretic behavior of partial cycles in the two-phase region associated with the martensitic transformation of shape-memory alloys. We consider the problem from a thermodynamic point of view and adopt a local equilibrium formalism, based on the idea of thermoelastic balance, from which a formal writing follows a state equation for the material in terms of its temperature T, external applied stress σ, and transformed volume fraction x. To describe the striking memory properties exhibited by partial transformation cycles, state variables (x,σ,T) corresponding to the current state of the system have to be supplemented with variables (x,σ,T) corresponding to points where the transformation control parameter (−σ and/or T) had reached a maximum or a minimum in the previous thermodynamic history of the system. We restrict our study to simple partial cycles resulting from a single maximum or minimum of the control parameter. Several common features displayed by such partial cycles and repeatedly observed in experiments lead to a set of analytic restrictions, listed explicitly in the paper, to be verified by the dissipative term of the state equation, responsible for hysteresis. Finally, using calorimetric data of thermally induced partial cycles through the martensitic transformation in a Cu-Zn-Al alloy, we have fitted a given functional form of the dissipative term consistent with the analytic restrictions mentioned above.

Notes: The effect of Cr as a barrier between PtSi and Al is studied by analyzing the reactions of the Al/Cr/Pt/Si (2000 A(ring)/1000 A(ring)/1000 A(ring)/Si substrate) structures. Electrical measurement shows a rapid rise in sheet resistance after annealing at 400 °C. Structural analysis shows extensive reactions between Al and Cr at such temperatures. The PtSi formed remains little changed until 550 °C, and is converted to PtAl2 at higher temperatures. The results are compared with those using W, Ti-W, and carbon barriers, and the roles of barrier materials on the stability of the Al/PtSi metallurgy are discussed.

Notes: A method is described for calculation of the optical constants (the refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity) of Si and Ge in the entire range of photon energies (0–0.6 eV). The imaginary part of the dielectric function [ε2(ω)] is derived first from the joint-density-of-states functions at energies of various critical points (CPs) in the Brillouin zone; then its real part [ε1(ω)] is obtained analytically using the Kramers–Kronig relation. The indirect-band-gap transitions are also assumed to provide a gradually increasing absorption spectrum expressed by a power law of ((h-dash-bar)ω−EIDg)2, where (h-dash-bar)ω is the photon energy and EIDg is the indirect-band-gap energy. The optical dispersion relations are expressed in terms of these model dielectric functions. The present model reveals distinct structures in the optical data at energies of the E0, E0+Δ0 [three-dimensional (3D) M0 critical point (CP)], E1, E1+Δ1 [3D M1 or two-dimensional (2D) M0 CP], E2 [a mixture of damped harmonic oscillator (DHO) and 2D M2 CP], E'0 triplet (DHO), and E1 (DHO). Calculated optical spectra are in satisfactory agreement with the experimental data over a wide range of the photon energies.

Notes: Data on the hydrogen release from a-C:H induced by ion irradiation have been analyzed using a statistical model based on bulk molecular recombination. Good fits were obtained for all available data by adjusting a single parameter which is correlated to the stopping power of the projectile in the material.