ALBERT

Notes: Microcavity effects can apply to intersubband transitions if the polarization selection rules are satisfied. We thus propose to insert a grating in a planar microcavity and to take advantage of both the field enhancement due to the cavity and the coupling with intersubband transitions due to the grating. The electromagnetic coupling is reinforced when both the grating and the cavity are resonant with the light wavelength. Accordingly, the spectral width is reduced where the coupling is efficient, which may provide additional advantages for some applications. Various configurations are envisioned and calculations are carried out for dielectric and metallic gratings. These architectures can be used for infrared detection, emission or modulation. Efficient and realistic devices are proposed and optimized. © 1996 American Institute of Physics.

Notes: A method to calculate the impact ionization rate in submicron silicon devices is developed using both an average energy and an average square energy of electrons. The method consists of an impact ionization model formulated with the average energy and conservation equations for the average square energy in the framework of an energy transport model. Parameters for the transport equations are extracted in such a way that calculated moments based on these equations match Monte Carlo simulation results. The impact ionization generation rate in an n+nn+ structure calculated with this method agrees well with the results obtained from Monte Carlo simulation. The new method is also applied to a submicron n-MOSFET. The calculated distribution of the generation rate is found to be quite different from the results based on a conventional method. © 1996 American Institute of Physics.

Notes: We use a two-dimensional, drift-diffusion calculation to illustrate the physics behind the recently described GaAs metal–semiconductor–metal photodetector with an ohmic backgate provided by a p-doped layer. We calculate the transient response of this structure to a pulse of illumination. According to these simulations, the speed of the falling side of the response is improved by the backgate, which removes photogenerated holes from the active layer, but the degree of improvement depends on the chosen contact. The fastest fall time is found in the current at the cathode when both the cathode and backgate are grounded. We show why this is faster than the response of the current at either the anode or the backgate, and why this advantage is practically lost when the backgate is biased.

Notes: We report on analysis and comparison of the dark current characteristics between InAs p-n and p-i-n diodes at the temperature range from 30 to 300 K. The situation that the dark current is completely due to the bulk instead of the surface leakage is made sure by passivation treatment. The experimental results agree well with a tunnel diode model in which the p-n and p-i-n diodes are described, respectively, with a linear electrical field and a constant electrical field. The diffusion current in our diodes is dominated by the electron diffusion in the p-type material. Through this model and detailed analysis, we conclude that the tunneling current can be suppressed with an intrinsic layer and a low n-type doping density while the diffusion current can be decreased with a high p-type doping density. The advantage of the p-i-n structure is not only to cut down the tunneling current but also to increase the uniformity. In terms of this model, a new p-i-n diode is designed to dramatically increase its zero-bias resistance area product and improve the detector performance. © 1996 American Institute of Physics.

Notes: The photovoltage developed by a p-n junction diode illuminated by an interference pattern through a slit depends both on the distance between fringes and the phase of the interference pattern relative to the position of the slit. For a slit width which accommodates an integer number N of fringes, the voltage is independent of the phase of the pattern but this is no longer the case for a noninteger number of fringes. The maximum dependence is observed for N+1/2 fringes within the slit but the effect decreases as N increases. When the fringe distance is no longer negligible compared to the minority carrier diffusion length the dependence of the photovoltage on the number of fringes becomes more pronounced. A theory is presented which shows how the diffusion length can be obtained from the experimental data. Experiments on GaAs and Si diodes are reported. © 1996 American Institute of Physics.

Notes: A procedure to identify the different components of the oxide space charge created during a Fowler–Nordheim electron injection in metal–oxide–semiconductor capacitors is presented. This procedure is very simple and based on the study of the space-charge relaxation behavior depending on the conditions of polarization and temperature. A model, based on slow states, trapped holes, and fixed charges, is proposed to explain the reversible and irreversible behaviors observed. Their effective numbers per unit area can be evaluated. This procedure is used to compare the influence of different processes on the oxide resistance to electron injections. © 1996 American Institute of Physics.

Notes: Boron diffusion can be used to compensate the n-type layer of a p+nn+ 6H-silicon carbide structure in order to increase its high-voltage capabilities. Measurements under reverse biases for a current range from 10 to 500 μA show that this process is very efficient for working temperatures about 300 K. Indeed we obtained a voltage of 670 V for a reverse current of 10 μA instead of the 120 V calculated for a structure without boron diffusion. Nevertheless, the breakdown voltage decreases rapidly when the temperature increases. Capacitance measurements show that the measured doping level in the n-type layer evolves in the same way as the temperature (it ranges from 1013 cm−3 at 300 K to 1017 cm−3 at 500 K). A great concentration of boron seems to be responsible for this doping variation with temperature. Admittance spectroscopy reveals the presence of D centers at 0.62 eV above the valence band associated to boron at concentration similar or superior to nitrogen concentration in the n-type layer. The increase of the doping level with the temperature is responsible for this decrease of the breakdown voltage. © 1996 American Institute of Physics.

Notes: The average base transit time is computed using a current impulse response technique for three typical abrupt Npn heterojunction bipolar transistors as a function of the emitter-base bias, VBE. This technique is based on a hybrid model of carrier transport incorporating a quantum-mechanical analysis of carrier injection at the emitter-base junction and a Monte Carlo analysis of base transport. For typical AlGaAs/GaAs and InP/InGaAs structures, the base transit time first increases with VBE, reaches a maximum, and then decreases towards a value close to the one predicted using a semi-Maxwellian injection of carriers into the base at an energy equal to the emitter-base conduction band spike. For a typical InAlAs/InGaAs structure, the average base transit time is found to decrease with an increase in VBE. For all structures, we show that there is a correlation between the bias dependence of the average base transit time and the bias dependence of the average number of collisions per carrier (calculated for carriers transmitted across the base). © 1996 American Institute of Physics.

Notes: We provide two qualitative models for the operation of Josephson fluxon–antifluxon transistor based on the pendulum model and image currents induced in the junction. This device, which is a variant of a Josephson vortex flow transistor, utilizes high temperature superconductor long Josephson junctions with a control line on top of the junction. Our models are consistent with the experimental observation that the coupling efficiency increases with this geometry. We also provide a quantitative model using numerical simulations to confirm the static and dynamic characteristics predicted by the intuitive models. © 1996 American Institute of Physics.

Notes: In acoustic imaging and nondestructive testing applications, the quality of images obtained using linear arrays is closely related to the far-field directivity pattern of an elementary transducer bar. But, for some values of width-to-thickness ratio W/T (W being the width and T the thickness) these transducers exhibit parasitic modes with resonance frequencies close to their working frequency. In order to analyze and identify these modes and their parasitic radiations, the finite element method is used with the help of an original extrapolation method to compute the far-field radiation pattern. Measured far-field directivity patterns for (YZw)36° transducer bar of LiNbO3 are presented for a large spectrum of W/T. To get a better understanding of the elementary transducer behavior, an approximate directivity pattern is obtained using a simple method and is compared to the numerical and measured directivities. This simple physical model is based on Sommerfeld's formula. It is shown that for some values of W/T, harmonic vibrations at the surface of transducer can be coupled to the fluid domain and produce a parasitic radiation (for example, for W/T=1, the first harmonic vibration introduces a strong parasitic radiation). © 1996 American Institute of Physics.

Notes: A new approach to simulate electronic transport at high energies in silicon is introduced, which is based on a mixture of evolutionary optimization algorithms and the Monte Carlo technique. The optimization technique of the evolutionary algorithm is used to find electron distributions which are in agreement with a given physical quantity, for example, a measured substrate current. In this way, the evolutionary algorithm can calculate backward the electron distributions from results of measurements. A mutation operator, which is based on the Monte Carlo technique, is used to direct the optimization of the evolutionary algorithm toward physically correct distributions. A comparison of the results of this new approach with electron distributions calculated by a full band Monte Carlo program demonstrates both the backward calculation ability of the evolutionary algorithm and the correctness of the physical model in the Monte Carlo-like mutation operator. It becomes obvious that the electron distribution in silicon is mainly determined by the scattering rates. By suppressing the optimization of the evolutionary algorithm the Monte Carlo-like mutation operator alone was found to provide a powerful new type of Monte Carlo technique. While these methods are not as precise as a full band Monte Carlo approach, they are shown to be numerically efficient and give also a good fit to reliability related hot electron effects. © 1996 American Institute of Physics.

Notes: We present an analytical modeling of the noise temperature associated with velocity fluctuations obtained in the framework of the linear-response theory around a steady state. The expressions are rigorously related to an eigenvalue expansion of the response matrix and are applicable to ohmic as well as to nonohmic (hot-carrier) conditions. Theory requires as input parameters the reciprocal carrier effective mass, the drift velocity, the carrier energy, the variance of velocity fluctuations, and the covariance of velocity-energy fluctuations as functions of the electric field in stationary and homogeneous conditions. The analytical results obtained for the case of holes in Si and electrons in GaAs at T=300 K are validated by comparison with experiments. © 1996 American Institute of Physics.

Notes: Localized-state distributions in copper phthalocyanines dispersed in polymer binders have been investigated with the modulated photocurrent (MPC) technique. The well-defined transport state of holes is observed in the excitation wavelength dependence of MPC. The frequency resolved MPC experiments reveal that the density of the localized states consists of the two exponential distributions having the characteristic temperatures of 500 and 1200 K with the inflection point at 0.53 eV above the transport state. These results are compared with those of evaporated copper phthalocyanine thin films, and no significant difference is found. The origin of the exponential distribution of the localized states is discussed in the light of statistical distribution of charged centers as sources of random potential. © 1996 American Institute of Physics.

Notes: Coherent dynamics of electrons in the triple tunnel-coupled quantum wells after an ultrafast optical excitation is studied. In a collisionless approximation, we consider temporal evolution of the electron distribution for different parameters of structure and pulse duration. Due to mixing of the tunneling frequencies under the three-level tunneling resonance condition, transient evolution of the dipole moment demonstrates different kinds of behavior: harmonic oscillations, beats, and nonperiodic oscillations. © 1996 American Institute of Physics.

Notes: The transport in free-standing porous silicon layers has been investigated by applying a current step to samples with a four contact geometry and by measuring the voltage time behavior across the voltage contacts. The voltage response showed a transient waveform typical of a network with two time constants and a continuous regime for very long times. The values of the time constants τ1 and τ2 differ noticeably, τ1 and τ2 being in the range 0.1–10 milliseconds and 10–150 milliseconds, respectively. The dependence of the time constants on the current step amplitude and on the temperature has been also investigated. Both τ1 and τ2 showed a temperature activated behavior, which can be related to the resistance behavior vs absolute temperature. © 1996 American Institute of Physics.

Notes: An intense relativistic electron beam injected into dense gas characteristically propagates in a self-pinched mode but is susceptible to the resistive hose instability. This convective instability typically leads to large amplitude beam motion and the disruption of propagation. Theory and computation suggest that, although resistive hose cannot be completely suppressed, its convective growth can be reduced by varying the average betatron oscillation frequency from head to tail in the beam pulse. We report here on experiments designed to implement this variation by tailoring the beam emittance using an ion-focused regime "conditioning'' cell. Conditioning effectiveness is assessed by using measured beam quantities to evaluate a detuning parameter η(t). This information is correlated with beam propagation measurements to determine the optimum conditioning for resistive hose suppression.

Notes: The propagation of intense, relativistic electron beams in air is subject to the resistive hose instability. Conditioning the beam prior to injecting it into the air can extend its range by reducing the hose growth rate and by reducing the initial spatial perturbations that seed the hose instability. Experiments have been performed using the SuperIBEX accelerator (Ipeak=10–30 kA, E=4.5 MeV, 40 ns full width at half-maximum) to develop conditioning cells that suppress the hose. This paper describes the performance of an active wire Bθ cell that is used in conjunction with an ion focused regime (IFR) cell. The IFR cell detunes the instability by producing a head-to-tail radius taper on the beam. The wire cell maintains this radius taper while producing an emittance taper that is necessary to suppress the hose growth. In addition, the wire cell reduces the initial beam perturbations through the anharmonic centering force associated with the wire current and its azimuthal magnetic field Bθ. The ability of the Bθ cell to reduce the beam offset with a minimal increase in the beam radius gives it several advantages over the use of a simple, thick scattering foil to perform the radius taper to emittance taper conversion. The SuperIBEX beam propagation distance, in terms of the betatron oscillation scale length, was extended to ∼10λβ using these cells. © 1996 American Institute of Physics.

Notes: We apply directly a perturbative expansion to the hopping model of Feinberg. The first three harmonics of the space-charge field are presented, considering the contributions from the higher-order terms at large modulation depth. The dependence of the harmonics on spatial frequency, modulation depth and applied field strength are discussed. An analytical solution of the two-wave coupling equations is given in this case. The mixing gain γ is also discussed. © 1996 American Institute of Physics.

Notes: We have directly correlated the electrical behavior, the impurity lattice site location, ion damage, and the local bonding environments of Ge-dopant ions implanted into InP. We have found that after rapid thermal annealing the free electron concentration in the samples implanted at room temperature (RT) are always higher than those implanted at liquid nitrogen temperature (LNT). Although the macroscopic structure seems to be insensitive to the implantation temperature, significantly more local disorder is created in the LNT implanted amorphous layers. Moreover, the amphoteric bonding structure of the Ge atoms is found to be well established already in the as-implanted amorphous InP. After high temperature annealing ((approximately-greater-than)800 °C), the Ge atoms rearrange locally with more of the Ge substituting the In site than the P site resulting in n-type conductivity. The solid solubility of Ge in the InP is measured to be ∼1.4–1.6×1020/cm3 while the free electron concentration is estimated to saturate at ∼3.4×1019/cm3. The relatively low electron concentration can be explained by Ge precipitation and the compensation of GeIn donors by GeP acceptors in the RT implanted case. The further reduction in electron concentration in the LNT implanted samples is believed to be related to the high residual damage found in these samples. The high solubility of Ge in InP can be attributed to the availability of two possible sublattice sites for the dopant and the compensation of the local strains due to the amphoteric substitution of the Ge. The concentration ratio of the GeIn to GeP determined in the heavily implanted material has been used to estimate the difference in the formation energy of Ge substituting those two different sites.

Notes: We have examined proton irradiation damage in high-energy (1–10 MeV) and high-fluence ((approximately-greater-than)1013 cm−2) Si n+-p-p+ structure space solar cells. Radiation testing has revealed an anomalous increase in short-circuit current Isc followed by an abrupt decrease and cell failure, induced by high-fluence proton irradiation. We propose a model to explain these phenomena by expressing the change in carrier concentration p of the base region as a function of the proton fluence in addition to the well-known model where the short-circuit current is decreased by minority-carrier lifetime reduction after irradiation. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has two effects. First, broadening of the depletion layer increases both the generation–recombination current and also the contribution of the photocurrent generated in this region to the total photocurrent. Second, the resistivity of the base layer is increased, resulting in the abrupt decrease in the short circuit current and failure of the solar cells. © 1996 American Institute of Physics.

Notes: Dislocation arrays are investigated in float zone (FZ) grown silicon wafers by the light beam induced current (LBIC) mapping technique at various wavelengths and by deep level transient spectroscopy (DLTS). The LBIC technique allows us to recognize and detect these arrays and to evaluate their recombination strength. Dislocations are found to be less recombining in (100)-oriented FZ samples than in (111) oriented ones. In FZ dislocated wafers, a phosphorus diffusion strongly attenuates the LBIC contrast of dislocations, depending on the duration and temperature of the treatment. Electrical activity of the defects, which are still physically present, as verified by x-ray topography, seems to disappear. Simultaneously, the peak intensities of DLTS spectra related to dislocations are reduced and this reduction depends on the phosphorus diffusion temperature and duration. © 1996 American Institute of Physics.

Notes: Surfaces of fluoride crystals, fractured by a single excimer laser pulse and then covered by a thin conductive layer, are imaged by scanning electron microscopy in the low-voltage secondary electron mode. As a result of charging, at lower primary electron energies a contrast enhancement can be obtained for surface fragments that are no longer tightly attached to the crystal. This differs from high-energy ((approximately-greater-than)10 keV) imaging which only yields topographic contrasts and allows the analysis of the fractured structure by edge and shadowing effects. Even contrast inversion from positive to negative charging of an entire fragment can be achieved, depending on the primary electron energy. It is shown that this effect can be utilized to discriminate between fragments with a good mechanical contact to the bulk and partially detached ones by systematically studying the contrast as a function of electron energy and specimen inclination. © 1996 American Institute of Physics.

Notes: A traveling electric potential hill has been used to generate an ion beam with an energy distribution that is mass dependent from a monoenergetic ion beam of mixed masses. This effect can be utilized as a novel method for mass separation applied to identification or enrichment of ions (e.g., of elements, isotopes, or molecules). This theory for mass-selective acceleration is presented here and is shown to be confirmed by experiment and by a time-dependent particle-in-cell computer simulation. Results show that monoenergetic ions with the particular mass of choice are accelerated by controlling the hill potential and the hill velocity. The hill velocity is typically 20%–30% faster than the ions to be accelerated. The ability of the hill to pickup a particular mass uses the fact that small kinetic energy differences in the lab frame appear much larger in the moving hill frame. Ions will gain energy from the approaching hill if their relative energy in the moving hill frame is less than the peak potential of the hill. The final energy of these accelerated ions can be several times the source energy, which facilitates energy filtering for mass purification or identification. If the hill potential is chosen to accelerate multiple masses, the heaviest mass will have the greatest final energy. Hence, choosing the appropriate hill potential and collector retarding voltage will isolate ions with the lightest, heaviest, or intermediate mass. In the experimental device, called a Solitron, purified 20Ne and 22Ne are extracted from a ribbon beam of neon that is originally composed of 20Ne:22Ne in the natural ratio of 91:9. The isotopic content of the processed beam is determined by measuring the energy distribution of the detected current. These results agree with the theory. In addition to mass selectivity, our theory can also be applied to the filtration of an ion beam according to charge state or energy. Because of this variety of properties, the Solitron is envisioned to have broad applications. The primary application is for the enrichment of stable isotopes for medical and industrial tracers. Other applications include mass analysis of unknown gases (atomic and molecular) and metals, extracting single charge states from a multiply charged beam, accelerating the high energy tail in a beam or plasma with a velocity distribution, and beam bunching.

Notes: We report the observation of an anomalously large electro-optic modulation effect in potassium titanyl phosphate (KTiOPO4) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro-optic coefficient in the waveguides by measuring the phase retardation of an optical mode when a voltage was applied across the substrate. We observed that at low modulation frequency the effective electro-optic coefficient in the waveguides can be higher than the bulk value by a factor up to about 100. A space charge model relating the mobile charge, space charge field, and observed enhancement in the electro-optic modulation is proposed to explain the mechanism of this effect. Calculation for dc steady state, small signal analysis, and large signal simulation is compared to the experimental observation. Possible device application and long term effects on system operation are also discussed. © 1996 American Institute of Physics.

Notes: The origin of second harmonic generation (SHG) of Li3VO4 was investigated from the viewpoint of the band structure by using the tight-binding method. The tight-binding parameters were optimized to reproduce the density of states (DOS) obtained from x ray photoelectron spectroscopy and the optical band gap. Although Li3PO4 has the same crystal structure as Li3VO4, it shows no SHG. To explain the difference in optical nonlinearity we compared the electronic structures of Li3VO4 and Li3PO4, in particular at the bottom of conduction band (CB) and the top of valence band (VB), since they are known to play a primary role in SHG. In Li3PO4, the bottom of CB consists of P 3s and O 2p orbitals and the top of VB is composed of O 2p orbitals. These electronic structures result in a relatively low DOS at the bottom of CB and a wide band gap in Li3PO4. On the other hand, in Li3VO4, both bottom of CB and top of VB are composed of V 3d and O 2p. The preferential contribution of V 3d orbitals to the band edge states leads to a high DOS at the bottom of CB, a narrow band gap and delocalization of electrons on V–O bonds. We conclude that these electronic structures are responsible for the high optical-nonlinearity of Li3VO4. © 1996 American Institute of Physics.

Notes: Variable-dose electron-beam exposure of thin amorphous As2S3 films is studied as a potential fabrication technique of index-modulated diffractive optical elements for the near infrared. The relationship between the electron dose and the phase delay is determined using a coarse grating structure, which eliminates the influence of volume diffraction and proximity effects. The effective refractive index change is determined by comparing experimental and calculated zeroth- and first-order diffraction efficiencies, taking into account the small but detectable surface modulation. © 1996 American Institute of Physics.

Notes: We studied the third-order nonlinear optical properties of phthalocyanine and fullerene in solution and in thin film form through third-harmonic generation measurement. X-ray diffraction measurement and scanning electron microscopy observation were also conducted to investigate the relationship between nonlinear optical and structural properties of the thin film. The nonlinearities depend on the π-conjugated length and the introduction of a central metal atom. We clarify the factors for forming suitable thin films by organic molecular beam deposition. It is shown that both the crystal structure and molecular alignment can be controlled to enhance the nonlinearities of the phthalocyanine thin film, thereby providing the maximum third-order susceptibility of 3.0×10−10 esu. We show that nonlinearities of the thin films can be estimated experimentally from those of solutions. © 1996 American Institute of Physics.

Notes: A parameter space approach is used to represent the operation of all liquid crystal displays. The Jones matrix nematic is used to analyze the static optical properties of liquid crystal displays with any twist angle, cell thickness, natural birefringence, and polarizer angles. All the usual display modes such as 90° TN, ECB, OMI, SBE, and STN can be visualized on a simple parameter space diagram. Besides its pedagogical values, this formulation has the advantage of simplicity in showing the physics and operating conditions of supertwist as well as low twist liquid crystal displays. It can also be used to analyze novel situations such as reflective displays which do not require any back polarizer. © 1996 American Institute of Physics.

Notes: Ground state atomic hydrogen (H:1s 2S1/2) concentrations in the CH4-H2 microwave plasma have been measured using actinometry. These measurements have been made over a wide range of plasma conditions including power inputs of 100–800 W and pressures of 0.5–60 Torr. A trace amount of argon was added to serve as an inert reference gas for concurrent optical emission measurements, in which optical emission intensities from the Hγ line (2p2P0–5d2D) at 434 nm and the Ar* line (4s′[1/2]°−4p′[1/2]) at 750 nm were recorded. © 1996 American Institute of Physics.

Notes: Hydrogen incorporation in silicon layers prepared by plasma-enhanced chemical-vapor deposition using silane dilution by hydrogen has been studied by infrared spectroscopy (IR) and elastic recoil detection analysis (ERDA). The large range of silane dilution investigated can be divided into an amorphous and a microcrystalline zone. These two zones are separated by a narrow transition zone at a dilution level of 7.5%; here, the structure of the material cannot be clearly identified. The films in/near the amorphous/microcrystalline transition zone show a considerably enhanced hydrogen incorporation. Moreover, comparison of IR and ERDA and film stress measurements suggests that these layers contain a substantial amount of molecular hydrogen probably trapped in microvoids. In this particular case the determination of the total H content by IR spectroscopy leads to substantial errors. At silane concentrations below 6%, the hydrogen content decreases sharply and the material becomes progressively microcrystalline. The features observed in the IR-absorption modes can be clearly assigned to mono- and/or dihydride bonds on (100) and (111) surfaces in silicon crystallites. The measurements presented here constitute a further indication for the validity of the proportionality constant of Shanks et al. [Phys. Status Solidi B 110, 43 (1980)], generally used to estimate the hydrogen content in "conventional'' amorphous silicon films from IR spectroscopy; additionally, they indicate that this proportionality constant is also valid for the microcrystalline samples. © 1996 American Institute of Physics.

Notes: The effect of SiH4 source gas heating on the properties of hydrogenated microcrystalline silicon film grown by plasma-enhanced chemical-vapor deposition was investigated to improve the crystallinity and inhomogeneities at the early stage of growth on an amorphous substrate such as glass. Optimization of the deposition conditions for μc-Si:H film structure and characteristics was carried out for a film around 1000 A(ring) thickness and as a function of the cathode heating temperature Tc. The grazing incidence x-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, and UV-visible spectroscopic ellipsometry results showed that the SiH4 gas heating significantly improved the crystallinity and inhomogeneities from the early stage of Si thin film growth at Tc(approximately-greater-than)550 °C and Ts of 180 °C condition. The role and effect of the cathode heating in the μc-Si:H growth is discussed. © 1996 American Institute of Physics.

Notes: Epitaxial growth of C60 was found in vapor-deposited crystals on the KI (001) cleavage surface. C60 molecules deposited on the KI surface, which was preheated at 400 °C, then kept at 195 °C during deposition, crystallized in the face-centered cubic form with two types of morphologies. Truncated pyramidal crystals epitaxially grew along the KI 〈110〉 directions, taking its C60 (001) face parallel to the KI (001) surface. This epitaxial nucleation occurred at a corner of the crossing steps along the KI 〈100〉 directions which were caused by thermal etching of the KI surface. Molecular mechanics and dynamics calculations revealed that a three-dimensional (2×2) commensurate lattice matching between the C60 {100} planes and the KI {100} faces at the step corner attributed to the epitaxial nucleation of the pyramidal crystal, in spite of the lower stability of the monolayer interaction of the C60 (001) face with the KI (001) surface. On the other hand, platelike crystals grew, with the C60 (111) face parallel to the KI (001) surface. A stable monolayer nucleation of the close-packed C60 (111) face made this two-dimensional growth of the platelike crystals possible to occur on the (001) terrace of the etched KI surface. © 1996 American Institute of Physics.

Notes: Radiation damage induced by 2 MeV alpha particles in polycrystalline diamond films has been studied as a function of the irradiation dose D (1012≤D≤1017 cm−2). The films were characterized using Raman/photoluminescence spectroscopy and I(V,T) measurements. The results can be summarized as follows. In undoped samples the H3 luminescent center (N–V–N) is observed for D≥1014 cm−2. The Raman diamond peak is broadened and shifted to lower frequencies for D(approximately-greater-than)1015 cm−2. No new graphitic component is detected after irradiation. On the contrary graphitic sp2 defects are annealed by irradiation. For D=3×1016 cm−2 new Raman defect peaks are detected at 1496 and 1635 cm−1. I(V,T) characteristics remain unaffected for D≤1016 cm−2. An increase in the conductivity is observed for D=3×1016 cm−2. At this dose we observe an activation energy of 0.4 eV and thermally stimulated currents related to defect levels at about 0.3 eV. A boron-doped sample (100 Ω cm) has been irradiated at 1017 cm−2 for comparison. After irradiation the conductivity of this sample is reduced and the activation energy of the conductivity is also reduced. Less damage is detected by Raman spectroscopy in the B-doped material. © 1996 American Institute of Physics.

Notes: A thermodynamic equation of state (EOS) is derived which is appropriate for investigating the thermodynamic variations along isobaric paths. By using this EOS, a Hugoniot EOS model with a unified theoretical basis is proposed for predicting the shock compression behavior of porous materials. The model is tested on 2024 aluminum, copper, and tungsten which are the typical materials with low, intermediate, and high shock impedance, respectively, and commonly used as standards. The calculated Hugoniots for these three materials with different initial densities are in good agreement with the corresponding experimental data published previously. It shows that this Hugoniot EOS model can satisfactorily predict the Hugoniot of porous materials over a wide pressure range. © 1996 American Institute of Physics.

Notes: In this article we present a detailed numerical analysis of the problem of diffusion-controlled growth of a solid intermediate phase from two saturated solutions (solid solute and solid or liquid solvent). Spherical geometry is considered and the quasi-steady-state approximation used in earlier analytical models is eliminated. Comparison between our results and those from Larson's model allows us to conclude that the use of a numerical scheme may lead to a significant gain in accuracy except when a solute-rich sphere is converting into a dilute phase. © 1996 American Institute of Physics.

Notes: The depth distribution of sulphur near the Si/GaAs(110) interface has been measured using particle induced x-ray emission (PIXE) in conjunction with Rutherford backscattering spectrometry (RBS); ozone oxidation and a hydrofluoric acid step-etching technique were used for sequential removal of Si/GaAs atomic layers. The depth resolution was also calibrated via 16O(d,p)17O nuclear reaction analysis and x-ray photoemission spectroscopy. PIXE/RBS measurements found a half monolayer of sulphur on the H2Sx passivated GaAs(110) surface. Upon deposition of 15 A(ring) silicon on the S-passivated GaAs(110), the total amount of sulphur remained constant as compared to that before Si deposition. However, no orientated S–Ga bonds were detected via the x-ray absorption measurement and the depth profile revealed that the sulphur atoms diffused into both the GaAs substrate and the Si heterolayer. © 1996 American Institute of Physics.

Notes: An ac calorimetric method is applied to the direct measurement of thermal diffusivity of single fibers. Theoretical analysis is presented, taking into account the two-dimensional ac thermal wave for cylindrical systems and the effect of time delay at the temperature detecting system. Experimental tests are performed using nickel and stainless steel wires about 20 μm in diameter. The thermal conductivity of fibers about 10 μm in diameter of the advanced materials, polyacrylonitrile-based carbon, TORAYCA, and aramide, Kevlar, is determined. The relationship between the thermal conductivity and electric conductivity and the crystalline size of these fibers is discussed. For composite materials made of carbon fibers, the observed thermal conductivity agrees with the thermal conductivity estimated using the values determined by the present method. © 1996 American Institute of Physics.

Notes: Various x-ray techniques have been applied to a study of semiconductor superlattices consisting of 100-period of InxGa1−xAs (15 A(ring))/GaAs (100 A(ring)) grown on GaAs(100) substrates by molecular beam epitaxy. Structural parameters pertaining to the morphology of interfaces and thickness variations were obtained. The interfaces in these superlattices are found to be highly correlated, and the layers all show a high degree of crystallinity. Splittings in the x-ray reflectivity and diffraction patterns in one of the samples provide clear evidence for pronounced thickness modulation, and direct comparison of the diffraction satellite peaks with results of high resolution transmission electron microscopy indicates that there exists a lateral structural ordering in the [110] direction during epitaxial growth. © 1996 American Institute of Physics.

Notes: To determine the influence of interface type on the accumulation of damage and ion mixing in GaAs/AlxGa1−xAs heterostructures, the damage produced by ion implantation at 77 K in single-layer (GaAs/AlxGa1−xAs/GaAs) and double-layer (GaAs/AlxGa1−xAs/GaAs/ AlxGa1−xAs/GaAs) heterostructures has been investigated by using a combination of Rutherford backscattering spectrometry and transmission electron microscopy. In the single-layer geometry, the degree of disorder increases with depth and the mixing is greater at the AlxGa1−xAs on GaAs interface than at the GaAs on AlxGa1−xAs one. The damage distribution in the sample with the double-layer geometry was different in the two layers, but overall it was similar to that in the single-layer geometry. These trends were observed in samples with x=0.6 and 0.2. These results indicate that migration of charged defects due to the presence of an implantation-induced electric field is not responsible for the asymmetry in the damage accumulation across the layer, the interface disorder and ion mixing, and the initiation of amorphization at interfaces. Instead, these effects can be better understood in terms of the depth dependence of the density of cascade events. © 1996 American Institute of Physics.

Notes: A scanned point source-point receiver technique, based on laser generation and detection of acoustic waves, is used to measure the stiffness coefficients of anisotropic materials. The striking effects that anisotropy gives rise to are analyzed and, when possible, advantage is taken of them. The processing developed for recovering the coefficients is presented and applied starting with simulated or experimental signals. A silicon crystal, for which acoustic wave focusing induced by anisotropy is critically sensitive, is first studied. To provide an accurate interpretation of these waves, the two-dimensional problem considering a line source is discussed, before analyzing the point source generation. Secondly, a manufactured composite material is characterized by means of this noncontact technique. By scanning a symmetry plane, four coefficients of the stiffness tensor are then recovered with good reliability. © 1996 American Institute of Physics.

Notes: Young's modulus of porous silicon samples, with porosity ranging from 36% to 90%, is measured by the nanoindentation technique. The analysis of the nanoindentation data, including the specific problem linked with porous materials, is presented. The Young's modulus values Ep thus obtained appear to be drastically dependent on the porosity and on the doping level (p or p+ type). The dependence of Ep versus the relative density (for a series of p+ type samples) is quadratic, in good agreement with the model of Gibson and Ashby developed for cellular materials. This also shows that highly porous silicon layers exhibit very low Young's modulus (for a porosity of 90% it is about two orders of magnitude smaller than that of the nonporous material). © 1996 American Institute of Physics.

Notes: By applying liquid phase epitaxy, we have grown defect-free silicon and silicon–germanium layers on partially oxide-masked Si wafers. The growth of the layers started epitaxially in oxide-free seeding areas and proceeded laterally over the thermal oxide film. Detailed studies by x-ray topography and electron microscopy show that the obtained thin semiconductor-on-insulator layers bend towards the oxide during lateral growth. The bending of the layers can be ascribed to adhesion and interfacial forces. Adhesion operates across a gap between the closely spaced surfaces of the oxide and the epitaxial Si and facilitates lateral growth of high-quality semiconductor layers on dissimilar layers or substrates. The technical potential of adhesion-dependent solution growth on dissimilar substrates is discussed. © 1996 American Institute of Physics.

Notes: A method is presented by which series solutions for the impedance change in an eddy-current test probe due to closed cracks in a non-magnetic, conducting half-space can be derived at low frequency. The series solution is applicable for flaws whose dimensions are much smaller than the electromagnetic skin-depth. The problem is formulated using an approach in which the flaw is represented by an equivalent distribution of current dipoles. The electric field scattered by the flaw is then written as an integral, over the flaw, of the product of the dipole density distribution and an appropriate Green's function. Terms in the series expansion for the dipole density are calculated by solving the integral equation at each order in the chosen small parameter, using perturbation theory and a dual integral equation method. The impedance change due to the crack is then calculated from the dipole distribution using the reciprocity theorem. Example solutions are given for semi-circular surface-breaking cracks and for long, uniformly deep surface-breaking cracks. Results are compared with other analytical solutions and the predictions of an independent numerical scheme, and very good agreement is observed. © 1996 American Institute of Physics.

Notes: High quality epitaxial layers of Al0.22Ga0.78As have been grown on Si substrates by adopting thermal cycle annealing. The quality of the Al0.22Ga0.78As has been assessed by photoluminescence, deep-level transient spectroscopy, and double-crystal x-ray diffraction studies. The emergence of a new luminescence emission (1.718 eV), high concentrations of shallow levels, passivation of a deep level (0.43 eV), and dislocation reduction in the high-temperature thermal cycle annealed samples have been explained by a silicon diffusion mechanism and the formation of complex point defects. Deep-level emission at 0.64 eV has been attributed to disordering in the epitaxial layers. © 1996 American Institute of Physics.

Notes: We have recently found a new phenomenon in the selectivity of growth by molecular-beam epitaxy on patterned GaAs (311)A substrates to form a fast growing sidewall on one side of mesa stripes oriented along the [01−1] direction. Preferential migration of Ga atoms from the mesa top and bottom toward the sidewall forms a smooth convex curved surface profile without facets. Comparison of patterned growth on other high-index (n11)A&B surfaces shows this growth mode to be unique for GaAs (311)A substrates. Lateral quantum wires are realized for step heights in the quantum-size regime. Quantum confinement of excitons in the wires is demonstrated by the transition from two-dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be vertically stacked in the growth direction without increase in interface roughness and wire size fluctuations. © 1996 American Institute of Physics.

Notes: Si-based metal–oxide–semiconductor structure is formed at temperatures as low as 300 °C using the catalytic activity of the platinum (Pt) layer. X-ray photoelectron spectroscopy and transmission electron micrography measurements show that heat treatments of the ∼5 nm-Pt/∼1 nm-chemical oxide/Si(100)〉 devices at 300 °C increase the thickness of the oxide layer to 4–4.5 nm and the oxide layer is present between the Pt layer and the Si substrate, but not on the Pt surface. It is found that the thin chemical oxide layer effectively prevents the Pt diffusion and the silicide formation during the heat treatments. Heat treatments in dry- and wet-oxygen result in nearly the same oxide thickness. Oxygen atoms (or oxygen ions) produced at the Pt surface are suggested to be a diffusing species through the Pt and silicon oxide layers. © 1996 American Institute of Physics.

Notes: The effects of dissipated power and gas dwell time in SiH4+CH4 plasmas on the properties of a-SiC:H films deposited by plasma-enhanced chemical-vapor deposition have been investigated for different methane fractions in plasmas operating in the low-power regime. Optical, structural, and electrical characterizations have been performed in order to investigate the influence of dissipated power and molecule dwell time on the physical properties of a-SiC:H films. It was found that both the investigated deposition parameters can have a remarkable influence on carbon incorporation and on optical properties such as the energy gap. In particular an increase in the dissipated power or in the molecule dwell time leads to an increase in carbon incorporation and in energy gap. The electrical properties and defect density are still those of device quality films grown in standard deposition conditions and are not influenced by variations in dissipated power or gas dwell time. From these results some conclusions regarding the growth mechanisms are drawn. © 1996 American Institute of Physics.

Notes: Within the widely quoted Lipton, Kurz, and Trivedi model for the calculation of dendritic growth velocities [Acta Metall. 35, 957 (1987)], the kinetic and Gibbs–Thomson undercoolings evaluated at the dendrite tip are assumed to apply equally over the whole dendrite surface, approximating the nonisothermal dendrite as an isothermal dendrite with a reduced interface temperature. In a previous article [J. Appl. Phys. 78, 4137 (1995)] we described a finite difference model to calculate the growth velocity of a parabolic, nonisothermal dendrite growing into an undercooled melt, and showed that proper consideration of the nonisothermal interface reduced the growth velocity by ≈35%, relative to that predicted by the analytical model. We report an improved computational scheme which solves the free boundary problem for the shape preserving needle dendrite. At all undercoolings we find that the shape preserving needle dendrite is broadened with respect to the Ivantsov paraboloid with the same tip radius. Moreover, the extent of the broadening increases with undercooling. Thus, unlike the Ivantsov solutions, the form of the dimensionless, shape preserving needle dendrite is not invariant with undercooling. Growth velocities for the shape preserving solutions are found to be within 2% of those of our previous nonisothermal model. © 1996 American Institute of Physics.

Notes: Using photoelectron spectroscopy (PES) we have studied the surfaces of Li1+xMn2−xO4 materials and related surface properties with the electrochemical performance of Li/Li1+xMn2−xO4 cells. The surface seems to be more lithium rich than the bulk as the amount of excess lithium is increased. A Li 1s component with binding energy of about 54.5 eV and a Mn 3p component with binding energy of about 47.6 eV are shown to be correlated with the undesirable 3.3 V discharge plateau in Li/Li1+xMn2−xO4 cells. Using a series of samples quenched from different temperatures we have also studied the surface composition at different temperatures. The PES results of a good performing sample, that cycles well even at a temperature as high as 55 °C are also shown. This shows that PES can be useful to qualify good spinel materials for Li-ion battery applications. © 1996 American Institute of Physics.

Notes: Carbon adsorption to the Si(100) surface in the atmosphere with large or with small concentrations of organic carbons is examined by using thermal desorption spectroscopy or x-ray photoelectron spectroscopy. The number of carbon atoms on a Si(100) wafer stored in the pure water, in which the total organic carbon is 285 ppb, becomes larger until a certain time with increasing time stored in pure water, and it keeps constant over that time. On the other hand, that stored in the air in the clean bench, where the concentration of the organic carbon is thought to be low, becomes small, and the increasing rate of the carbon atoms adsorbed on the Si(100) surface is very small. The difference between these results is due to whether the dominant step is the transportation of organic carbons in the atmosphere or it is the adsorption of organic carbons on the Si surface. The former process occurs for the small concentration of the organic carbons in the atmosphere, and the latter process occurs for the large concentration of them. Adsorption of the organic carbons dominates in the entire process in pure water under the present condition, and the transportation of them dominates in the entire process in the air of the clean bench under the present condition. From the relationship between C1s integral intensity and thickness of the native oxide, it is found that the adsorption of the carbon atoms has a close relationship with the growth of the native oxide on the Si surface. The adsorption site of the carbon atoms is also discussed from the viewpoint of stepped growth of the native oxide. © 1996 American Institute of Physics.

Notes: Solvent reorientational dynamics in concentrated polymer films have been probed with a new experimental method based on a linear electromechanical model and a AT-cut high-frequency quartz resonator. This method is unique in that the viscoelastic characteristics of a composite resonator comprising the viscoelastic film and the quartz resonator are probed at the frequency of minimum resonator amplitude (fYmin) rather than at fYmax, thus permitting measurements under conditions where a linear electromechanical model is most applicable. The method involves measurement of the admittance characteristics of the unloaded quartz resonator and the composite resonator, transformation of the admittance data near fYmin into a linear form that provides accurate determination of the resonant conductance and susceptance, and use of Newton–Raphson numerical iteration to determine the viscoelastic characteristics from these values. This procedure enables real-time investigation of dynamic processes in polymer films, as demonstrated here by the simultaneous determination of the film thickness, storage modulus, and loss modulus during the drying of a spin-coated film containing polystyrene and 2-chlorotoluene solvent. The viscoelastic characteristics are investigated at a resonant frequency near 5 MHz under ambient conditions as the solvent mass fraction continuously decreases from its initial value of 15%. The trends in the measured storage and loss moduli are consistent with a single relaxation process, namely rotational relaxation of the 2-chlorotoluene solvent molecules. The solvent relaxation time increases with decreasing solvent content owing to the increasing influence of the polymer chains on the solvent reorientational dynamics. A plateau in the relaxation time at low solvent content (〈2%) suggests the presence of a solvent glass transition. The results demonstrate that shear mode quartz resonators can be used to investigate solvent dynamics in polymer films at high concentrations that are inaccessible by other experimental methods. © 1996 American Institute of Physics.

Notes: The present uncertainty in the ability of Cu to substitute for Al as the conductor material in very large scale integration arises from the perplexing electromigration behavior of Cu interconnects: the electromigration activation energy in multigrained lines is often about two times lower than for grain-boundary diffusion, while the pre-exponential factor in the electromigration rate expression is several orders of magnitude smaller than that characteristic of electromigration along grain boundaries. Using literature data, in particular those of drift velocity experiments, we show that regardless of these unusual facts, grain boundaries are still most likely the major electromigration diffusion pathways in Cu interconnects. Based upon recent progress in the theory of grain-boundary grooving with an arbitrary grain-boundary flux [Klinger et al. J. Appl. Phys. 78, 3833 (1995)], and the specific model applying the general theory to electromigration [Glickman, Phys. Low-Dim. Struct. 11/12, 69 (1994)], we explain the major features of electromigration in Cu in terms of the extension of slit-like grooves along the interconnect line, followed by their merging. Fitting the electromigration activation energy reported for pure Cu into the model suggests that surface diffusion along freshly created groove walls is slow, with an activation energy above 2 eV. Most likely, this is due to trace surface contaminations. Having this new key element in the grain-boundary grooving model, with surface diffusion acting in effect as a "healing'' mechanism rather than as an independent pathway parallel to grain-boundary diffusion, and using empirical surface diffusion parameters for Cu, enable us to rationalize the major features of Cu electromigration behavior reported in the literature: the values of the electromigration pre-exponents, the island-like morphology of the electromigration displacement region, the current density exponent, and the origin of the Sn effect on electromigration parameters. Our main conclusion is that the full advantages of Cu as the most promising electromigration resistant interconnect material can be realized, provided that trace contaminations responsible for suppressing diffusion along freshly created surfaces are identified, controlled and eliminated. This will allow unalloyed, multigrained Cu interconnects to have an electromigration resistance four orders of magnitude larger than Al(2% Cu) at 100 °C. © 1996 American Institute of Physics.

Notes: We observe the formation of an ordered structure in Si1−xGex films grown on Ge(100) substrates, as well as on Si(100) substrates, by molecular beam epitaxy. The structural characterization of these ordered films is performed. The degree of order in the films is quantitatively measured using x-ray diffraction. The dependence of the degree of order on Ge composition is similar between films on Ge(100) and Si(100) substrates. By careful x-ray diffraction analysis, we find that the degree of order is not equivalent in variants. © 1996 American Institute of Physics.

Notes: In the heteroepitaxial growth of films with large misfit with the underlying substrate (linear mismatch strains in excess of 1%–2%) the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggest that the TD density in relaxed homogeneous buffer layers can be divided into three regimes: (i) an entanglement region near the film/substrate interface corresponding to TD densities of ∼1010–1012 cm−2; (ii) a falloff in TD density that is inversely proportional to the film thickness h, applicable to densities in the range ∼107–109 cm−2; and (iii) saturation or weak decay of the TD density with further increase in film thickness. Typical saturation densities are on the order of ∼106–107 cm−2. In this article, we show that the TD reduction may be described in terms of effective lateral motion of TDs with increasing film thickness. An analytic model is developed that successfully predicts both the 1/h scaling behavior and the saturation of TD densities. Long-range fluctuations in the net Burgers vector content of the local TDs is a cause for saturation behavior. These models are supported by computer simulations. © 1996 American Institute of Physics.

Notes: We have studied the wetting behaviors of eutectic SnPb solder caps on pure Au foils at 200 °C. Surface morphology, wetting angle, and wetting tip stability were examined by scanning electron microscopy and energy dispersion x-ray analysis. In addition, interfacial reaction in the bulk diffusion couple of the solder and Au was studied. Intermetallic compound (AuSn4) was observed on the surface of the solder cap, often called the cold joint, as early as 2 s of reflow at 200 °C. The wetting angle decreased with reflow time but remained constant after 180 s of reflow. However, the side-band width and the diameter of the solder cap continuously increase with the reflow time to 5 min when the entire eutectic SnPb solder cap is fully consumed by intermetallic compound formation. In the bulk diffusion couple of eutectic SnPb and Au, the growth and morphology of the intermetallic compound (AuSn4) was influenced by Au dissolution. Since Au, showing excellent wetting behavior, forms the platelet-type intermetallic compound, we postulate that the wetting rate may be improved with a system containing platelet-type intermetallic compound formation for the eutectic SnPb solder. © 1996 American Institute of Physics.

Notes: Epitaxial growth of the strongly lattice mismatched (6.5%) layered chalcogenides InSe and GaSe on each other is obtained with the concept of van der Waals epitaxy as proven by low-energy electron diffraction and scanning tunnel microscope. InSe/GaSe/InSe and GaSe/InSe/GaSe quantum well structures were prepared by molecular beam epitaxy and their interface properties were characterized by soft x-ray photoelectron spectroscopy. Valence and conduction band offsets are determined to be 0.1 and 0.9 eV, respectively, and do not depend on deposition sequence (commutativity). As determined from the measured work functions the interface dipole is 0.05 eV; the band lineup between the two materials is correctly predicted by the Anderson model (electron affinity rule). © 1996 American Institute of Physics.

Notes: A significant high-frequency magnetic susceptibility was measured both in weakly polarized and nonpolarized samples of barium titanate, lead zirconate titanate, and carnauba wax. Magnetic susceptibility measurements were made from 10 to 500 MHz using a thin film permeameter at room temperature; initial susceptibilities ranged from 0.1 to 2.5. These values are larger than expected for paramagnets and smaller than expected for ferromagnets. It was found that the magnetic susceptibility decreases rapidly with exposure to the exciting field. The origin of the magnetic susceptibility is thought to originate with the applied time varying electric field associated with the susceptibility measurements. An electric field acts to rotate an electric dipole, creating a magnetic quadrupole if the two moments are balanced, and a net magnetic dipole moment if imbalanced. It is thought that local electrostatic fields created at ferroelectric domain discontinuities associated with grain boundaries create an imbalance in the anion rotation that results in a net, measurable, magnetic moment. The origin of the magnetic aftereffect may be due to the local heating of the material through the moving charges associated with the magnetic moment. © 1996 American Institute of Physics.

Notes: In this work, we obtain (Ba0.7Pb0.3)TiO3 materials possessing double critical temperature Tc in resistivity–temperature (ρ–T) behavior by microwave sintering at 1050–1080 °C for 5 min. The cooling-rate control and postannealing processes modify the relative magnitudes of low- and high-Tc resistivity jumps without altering the Tc values. The donor Ed and trap Es levels of those materials are estimated to be Ed(approximately-equal-to)0.05–0.07 eV and Es(approximately-equal-to)1.07–1.32 eV. According to experimental results the voltage sensitivity and transient responsivity of the current passing through double-Tc materials are observed to be superior to those of single-Tc materials prepared by the conventional furnace sintering method. Also, the double-Tc characteristics are attributed to the dual phases with a core-shell structure. Moreover, the change in the relative proportion of the two phases accounts for the influence of post-heat treatment processes on the materials' ρ–T behavior. © 1996 American Institute of Physics.

Notes: In recent work, boundary integral equations and finite elements were used to study the (real) effective permittivity for two-component dense composite materials in the quasistatic limit. In the present work, this approach is extended to investigate in detail the role of losses. We consider the special but important case of the axisymmetric configuration consisting of infinite circular cylinders (assumed to be parallel to the z axis and of permittivity ε1) organized into a crystalline arrangement (simple square lattice) within a homogeneous background medium of permittivity ε2=1. The intersections of the cylinders with the x–y plane form a periodic two-dimensional structure. We carried out simulations taking ε1=3−0.03i or ε1=30−0.3i and ε2=1. The concentration dependence of the loss tangent of the composite material can be fitted very well, at low and intermediate concentrations of inhomogeneities, with a power law. In the case at hand, it is found that the exponent parameter depends significantly on the ratio of the real part of the permittivity of the components. We argue, moreover, that the numerical results discussed here are consistent with the Bergman and Milton theory [D. J. Bergman, Phys. Rep. 43, 377 (1978) and G. W. Milton, J. Appl. Phys. 52, 5286 (1981)]. © 1996 American Institute of Physics.

Notes: Electrostriction is often described by a phenomenological tensor relating a material's deformation to an applied electric field. However, this tensor is not a material parameter; for deformable, weakly compressible materials (e.g., elastomers), the field-induced deformations depend strongly upon boundary conditions. A different approach that relates the deformation to material properties as well as boundary conditions is required. In this article, we describe a linear theory which introduces five material parameters governing electrostriction: the relative dielectric constant, ε0, two derivatives of the dielectric constant tensor, a1 and a2, Young's modulus, Ey and Poisson's ratio, ν. Knowledge of these parameters and appropriate boundary conditions allow one to predict field-induced deformations for arbitrary configurations. We demonstrate an experimental procedure for measuring deformations and permittivity changes, from which the parameters a1 and a2 may be extracted (ε0, ν, and Ey can be measured by a variety of established methods). The linear theory reproduces experimental results for two polyurethane films at small to moderate electric field strengths. We find that the electrostatic force associated with the parameters a1 and a2 is at least ten times larger than the Coulombic attractive force between the electrodes. © 1996 American Institute of Physics.

Notes: It is shown that voltage shifts in the hysteresis response of SrBi2Ta2O9 (SBT) thin-film capacitors can be induced using both thermal and optical methods. These voltage shifts are important since they can lead to imprint failure in ferroelectric memory devices. It is suggested that the voltage shifts in the hysteresis curve of SBT are caused by trapping of electronic charge carriers near the film/electrode interfaces, as has been previously reported for the Pb(Zr,Ti)O3 (PZT) system. In addition, a direct correlation is established between the magnitude and sign of remanent polarization (Pr) and the thermally induced voltage shifts (Vi), where Vi=αPr+β. It is also found that, unlike the PZT system, the thermally induced voltage shifts in SBT are smaller than those optically induced. One possible implication of this result is that the contribution of defect–dipole complexes to the voltage shifts in SBT is negligible. We suggest that the smaller contribution of defect-dipole complexes to the voltage shifts in SBT may be related to a smaller oxygen vacancy concentration in the perovskite sublattice of SBT as compared to that of PZT. © 1996 American Institute of Physics.

Notes: This article presents a study of the effect of temperature on the defect band photoluminescence (PL) of moderately phosphorous doped amorphous silicon thin films deposited by magnetron sputtering. We have identified two types of recombination processes responsible for the observed temperature dependence of the defect PL band produced by subgap excitation. One of the processes is similar to that observed in intrinsic hydrogenated amorphous silicon and may originate from the recombination of carriers at band tail states and dangling bonds. The donor-defect pairs at nearest neighbor sites may be responsible for the second recombination process. © 1996 American Institute of Physics.

Notes: The optical processes in AlInP/GaInP/AlInP quantum wells free from long-range ordering are examined by photoluminescence (PL), photoluminescence excitation (PLE), and photoreflectance (PR) measurements. The PL method observes lower transition energy than the PLE and the PR methods which observe the space-averaged transition energy. This is because PL originates from localized lower-energy states to which an exciton state relaxes. By analyzing these measurements carried on 20- and 60-A(ring)-wide wells, the reason for this deviation is ascribed to the fluctuation of the transition energy due to the local variation of the well width by one molecular layer. The plausible share of the band offset for the conduction band against the energy gap difference at the Γ point is obtained by comparing the experimentally obtained relative position of the energy levels in AlInP barriers and the 20 A(ring) GaInP well with the calculated ones. This is found to be 0.75 (±0.06). © 1996 American Institute of Physics.

Notes: Using spectroscopic ellipsometry, we measured the pseudodielectric function of Si1−x−yGexCy alloys (0≤x≤0.48,0≤y≤0.05) grown on Si(001) using molecular beam epitaxy. For pseudomorphically strained layers, the energy shifts of the E1, E1+Δ1, E0′, and E2 transitions are determined by line shape analysis and are due to alloy composition effects, as well as hydrostatic and shear strain. We developed expressions for hydrostatic and shear shift from continuum elasticity theory, using deformation potentials for Si and Ge, for biaxial stress parallel to the (001) growth plane in a diamond or zinc blende-type crystal and applied this to the ternary Si–Ge–C alloy. The energies of E1 and its spin-orbit split partner E1+Δ1 agree fairly well with theory. The E2 transitions in Si1−xGex at around 4.3 eV depend linearly on Ge concentration. In case of relaxed layers, the E1 and E1+Δ1 transitions are inhomogeneously broadened due to the influence of misfit and threading dislocations. For a silicon cap on top of a dislocated, relaxed SiGe layer, we recovered the bulk Si dielectric function. © 1996 American Institute of Physics.

Notes: A pseudo-Rayleigh wave traveling along the edge of a thin plate is excited by a pulsed laser and detected by optical interferometry. The field confinement feature at the top of the plate edge is investigated by means of acoustic dilatation measurement. A nearly dispersionless behavior and little diffraction loss are observed during the guided mode propagation which agree with theoretical prediction. The possibility of using this wave for viscosity sensing is also discussed. © 1996 American Institute of Physics.

Notes: A novel approach for enhancing the selectivity of the desired isotope in the molecular laser isotope separation (MLIS) process is presented. The scheme consists of simultaneously applying two laser beams with frequencies corresponding to those between the ground and the first energy excitation level and the ground and the second energy excitation level, respectively. Practical relations on the properties of the spherical-top molecules are derived and a semiclassical analysis of the electromagnetic interaction within the limits of the experimental conditions applied in actual MLIS experiments shows that the selectivity, defined as the ratio of the absorption cross sections of the two isotopes, increases by a factor of 10–20 times in the case of the uranium isotopes. In addition, it is demonstrated that during the multiphoton absorption process energy-level splittings due to induced magnetic dipoles and induced electric quadrupoles are by no means negligible. They become significant during multiphoton processes where two or more photons are lost during the interaction process. At high pumping powers they become dominant and inhibit selectivity. They cancel out during interaction processes where there is no change in the total number of photons, such as scattering. These effects can be avoided by applying the laser beams to the molecular gas in arrangements which in principle are equivalent to a Mach–Zehnder interferometer with the molecules substituted for the reuniting beam splitter. Moreover, the induced electric quadrupoles (E2) are fully exploited. The application of the results and the concepts described herein can render the MLIS process the most economic and practical method for the commercial separation of the uranium isotopes. © 1996 American Institute of Physics.

Notes: Employing impedance analysis of quartz resonators, we have monitored the complex viscoelastic compliance of a 1.67-μm-thick film of poly(isobutylene) during swelling in toluene vapor. The optical thickness was simultaneously measured by ellipsometry. The acoustic model used for analysis is discussed. We can quantitatively derive the shear modulus. The results show a decrease of the glass transition temperature of about 60 °C. We suggest that the method is of high value for the investigation of sorption and drying kinetics in the regime of low solvent content. © 1996 American Institute of Physics.

Notes: Optical emission spectroscopy is used to investigate trends with changes in processing parameters for Ar/Cu plasmas in an electron-cyclotron-resonance (ECR) plasma deposition system. The primary motivation for this work is to monitor trends in ionization fractions for copper deposition plasmas using a noninterfering diagnostic tool. The system, which consists of a solid copper sputter target coupled to a permanent magnet ECR microwave plasma system, is operated in the range of 1–6 mTorr argon with net microwave input power of 500–1500 W. Emission from the following excited states is monitored: Ar neutrals (696.5 nm); Ar ions (488 nm); Cu neutrals (521.8 and 216.5 nm); and Cu ions (213.6 nm). Cu ion emission and Cu neutral emission monotonically increase with net microwave input power but at slightly different rates for different pressures, while argon-ion emission as a function of pressure shows a broad peak around 4 mTorr. The ratio of Cu ion emission to Cu neutral emission is used as an indicator of the relative ionization efficiency for Cu and peaks near 5 mTorr. Spectroscopic estimates of electron temperature differences between pure Ar and Ar/Cu plasmas are also presented.

Notes: The competition between band gap and the 2.2 eV (yellow) luminescence of epitaxial GaN is studied for excitation densities ranging from 5×10−6 to 50 W/cm2. The ratio of the peak intensities of the band gap-to-yellow luminescence changes from 4:1 to 3000:1 as the excitation density is increased by 7 orders of magnitude. At room temperature, the band gap luminescence linewidth is 2.3kT, close to the theoretical minimum of 1.8kT. A model is developed describing the intensity of the two radiative transitions as a function of the excitation density. This model is based on bimolecular rate equations and takes into account shallow impurities, deep levels, and continuum states. The theoretically predicted dependences of the two different luminescence channels follow power laws with exponents of 1/2, 1 and 3/2. Thus the intensity of the yellow luminescence does not saturate at high excitation densities. These dependences are in excellent agreement with experimental results. The relevance of the results for optoelectronic GaN devices is discussed. It is shown that the peak intensity of the yellow luminescence line is negligibly small at typical injection currents of light-emitting diodes and lasers. © 1996 American Institute of Physics.

Notes: We investigated the influence of electron temperature and rf bias on charge build-up caused by electron shading in inductively-coupled plasmas (ICP) at 2 to 40 mTorr in Ar. We used Si substrates covered with a 500-nm-thick SiO2 film which had a line-and-space pattern. We measured the electron and ion currents going into the Si substrate through the dielectric structure. When the pattern size decreases, the electron current through the dielectric structure is suppressed and the floating potential increases. We also measured the change in the floating potential of the sample as the electron temperature was increased. As the electron temperature is increased from 2 eV to 4 eV by controlling the gas pressures, the floating potential difference increases between samples with different pattern sizes. To investigate the influence of rf bias (13.56 MHz) on charge build-up, we measured differences in the dc self-bias voltage between samples with different pattern sizes. dc self-bias voltage differences increase with increasing rf bias voltage. This cannot be explained by the slight increase (3 eV to 5.5 eV) in the electron temperature near the rf electrode due to sheath oscillation heating. Therefore, the dc self-bias voltage difference is mainly caused by a decrease in the ion flux impinging on the sidewall of the trench because of the increase in the ion energy. © 1996 American Institute of Physics.

Notes: We have developed a computational model to optimize the design of a surface reflection neutralization source of hyperthermal neutrals for charge-free processing. For the deployment of this technique to production (≥8 in. wafers) processing, a system design study has been completed. A Monte Carlo model is used to determine the energy and angular distributions of reflected hyperthermal neutrals at the surface of a wafer, as well as flux uniformity. A simple form for the plasma profile is chosen to allow simulation of various profiles reported in the literature. Neutrals are launched from the reflector consistent with the relative plasma density at each position on the reflector with angular and energy distributions consistent with experimental observations. These neutrals are then followed through interactions with thermal background atoms and the plasma. Charge exchange, ionization, and elastic scattering processes are considered for argon as the main feedstock gas. The results show the trade off between a high density plasma for substantial wafer neutral flux and losses of the neutral stream due to ionization and charge exchange in the plasma stream. © 1996 American Institute of Physics.

Notes: Spatially resolved, line integrated, excited state densities, and neutral and ion temperatures have been measured in inductively coupled argon plasmas. Absorption spectroscopy was used to measure the line integrated density and temperature of the argon 1s5, 1s4, 1s3, and 1s2 energy levels. Laser-induced fluorescence was used to confirm the neutral temperatures and to measure argon metastable ion temperatures. For rf powers between 50 and 300 W and pressures of 4–50 mTorr, the line integrated density of the 1s5 energy level varied between 1×1016 and 2×1016 m−2. The densities of the 1s4, 1s3, and 1s2 levels were approximately 4–10 times smaller. In the center of the plasma, the ion and neutral temperatures were identical, between 550 and 1000 K for plasma powers between 30 and 240 W and pressures between 4 and 50 mTorr. The neutral temperature had a maximum in the center of the discharge and decreased towards the edge of the discharge. However, the ion temperature increased to between 3000 and 4000 K at the edge of the discharge. Ion drift velocity in the radial direction was between 1×105 and 2×105 cm/s at the edge of the plasma. No significant changes in the spatial density distribution or temperature were observed when either a rf bias was applied to the lower electrode or when the stainless-steel lower electrode was covered with a bare silicon wafer. The addition of nitrogen to the argon discharge resulted in the density of the 1s5 state decreasing by a factor of 2 and the density of the 1s4 state decreasing by a factor of 10. Implications of these measurements on the radial electric fields, radiation trapping, and the energy transport in the plasma are discussed.

Notes: The elastic deformation of the surface of thin MgO films grown on Fe(001) induced by the presence of misfit dislocations at the Fe/MgO interface has been put into evidence using low-energy electron diffraction. Satellite spots surrounding the fundamental Bragg reflections appear once the critical thickness is exceeded and undergo a shift in position as thickness of MgO increases, gradually blending in with the fundamental reflections. The evolution of the position of the spots is in good agreement with kinematical diffraction calculations in which the surface deformation is determined from the displacement field of a single misfit dislocation using both isotropic and anisotropic elasticity theory. © 1996 American Institute of Physics.

Notes: This paper shows ion channeling images of the strain field produced by precipitate particles in a crystal matrix. Images have been produced by mapping the energy of 3 MeV protons transmitted through a thinned silicon crystal containing colonies of copper silicide particles, with the incident beam at or close to planar channeling directions of the lattice. Features of the precipitate contrast observed as a function of beam tilt angle away from channeling alignment are qualitatively explained using a model based on symmetrical plane rotation of the crystal lattice around the colonies. © 1996 American Institute of Physics.

Notes: The amorphous alloy Fe81.5P12C3Cu0.5Mo0.5Si2.5 has been prepared and the crystallized alloy exhibits an ultrafine structure with a grain size of about 20 nm and excellent soft magnetic properties. The coercivity and the core loss as low as 6.7 A/m, 0.26 W/kg, respectively, and the maximum permeability as high as 10.2×104 at an optimal annealing temperature of about 360 °C were obtained. By means of x-ray diffraction, transmission electron microscope, and Mössbauer spectroscopy measurements, microstructures of the alloy were investigated as a function of the annealing temperature. The primary crystallization produces ultrafine grains of α-Fe(Si) solid solution with a grain size of about 20 nm precipitated in the residual amorphous matrix. The Si in the α phase was estimated near to be completely disorder ranged and the Si concentration was determined to be about 2%–5%. The Fe3P phase appears in the residual amorphous phase upon annealing at 420 °C. © 1996 American Institute of Physics.

Notes: Combined Brillouin light-scattering (BLS) and microwave pumping techniques were used to measure the second-order spin-wave instability threshold microwave field amplitudes and characterize the critical modes for ferromagnetic resonance (FMR) saturation in 4.15 μm-thick yttrium iron garnet films at 8.47 GHz with the static magnetic field in-plane. To the best of our knowledge, this work represents the first BLS measurements for second-order FMR saturation processes in ferrite materials. Several new results were obtained: (1) A new type of butterfly curve for second-order processes. (2) Strong scattering from modes at low wave numbers in the 104 rad/cm range. (3) An observed in-plane propagation direction for these low wave number modes at 90° to the in-plane static field direction. These results differ significantly from the behavior expected from the bulk sample instability theories of Suhl and Schlömann, which predict critical modes at wave numbers and propagation directions in the 105–106 rad/cm and 0° ranges, respectively, and a sharp cusplike response centered at FMR. These results indicate that significant modifications of the bulk theory, which take into account the real modes supported by the film, are needed to realistically model second-order instability processes in films. © 1996 American Institute of Physics.

Notes: Accurate stopping powers of polyimide have been determined for mean energies (E) in the range of 200 keV≤E≤3200 keV for 4He ions and 550 keV≤E≤1550 keV for 1H ions, using the transmission method for foils with areal densities of 30–225 μg/cm2. The overall uncertainties of ∼2% are mainly due to the foil thickness determination. The present data have been parametrized and compared to results obtained by previous authors in the upper energy range and to various parametrizations found in the literature. The best description of the data is given by Bragg's rule calculations using parametrized elemental stopping powers that were derived from large data sets (deviations 〈2%). No support is found for the cores-and-bonds model showing deviations of 3%–5%, that may be due to the neglect of phase state effects. © 1996 American Institute of Physics.

Notes: Modified rhodamine 6G molecules with polymerizable double bonds have been copolymerized with methacrylic monomers and the resulting polymers have been pumped at 337 nm with a N2 laser in a transversal configuration. The preparation of these new materials is described in detail and their lasing properties are evaluated. Important increases in photostability, with lasing efficiencies similar to those found for the parent dye rhodamine 6G in ethanol solution, have been obtained for some of these materials. Lifetimes (measured as an 80% efficiency drop) in excess of 20 000 shots at repetition rates of 2 Hz have been demonstrated. Strong dependence on pump repetition rate was observed. Possible mechanisms and processes responsible for the behavior of these materials are discussed. By using a rotating system where the sample is scanned in a continuous way, the laser output remained stable, with no sign of degradation, after 500 000 shots. © 1996 American Institute of Physics.

Notes: A technique of impurity-free vacancy-induced disordering of GaAs/AlGaAs multiquantum wells (MQW) that is area selective, very reliable, and highly reproducible, has been developed. The localized compositional disordering is induced by rapid thermal annealing of the sample after it has been coated with a thin film of "spin-on'' glass and prebaked at 400 °C in a high purity nitrogen:oxygen (78:22) atmosphere. In order to self-consistently determine the diffusion coefficient of the Al and Ga atoms, the photoluminescence peak is fitted to the n=1 electron to heavy hole transition that corresponds to an error function potential profile caused by the diffusion. The process has been used to integrate two optical devices on a MQW structure. One is a nonlinear directional coupler all-optical switch, and the other is an integrated Mach–Zehnder all-optical modulator. The switching characteristics of the devices were measured using the conventional pump-probe measurement technique. © 1996 American Institute of Physics.

Notes: We present theoretical results for acoustic band structures for cubic arrays of spherical water balloons surrounded by a mercury host. Multiple, complete acoustic band gaps (i.e., stop bands) are found for the face-centered cubic (fcc), body-centered cubic (bcc), and simple-cubic (sc) lattices. These stop bands are widest for a volume fraction of ∼24% and the corresponding gap/midgap ratios are about 0.83, 0.77, and 0.62, respectively, for fcc, bcc, and sc lattices. We stress that such a simple three-dimensional (3D) inhomogeneous system of liquids exhibits the largest stop bands ever reported for 3D elastic as well as for dielectric composites. For mercury balloons surrounded by water the gaps obtained are surprisingly small. © 1996 American Institute of Physics.

Notes: The effect of porosity on the complex dielectric permittivity of microwave sintered zinc oxide at room temperature and 2.45 GHz is reported. The predictions of conventional Maxwell–Garnet theory and the effective medium approximation are in poor agreement with the experimental results. Various methods are employed to investigate the system in an effort to come up with new mixing laws, including combinations of these two analytic theories and finite difference electromagnetic simulations of representative microstructures. A model that assumes the existence of dielectrically inactive, fractal-geometry boundaries between ceramic grains provides an excellent description of the results with no free parameters. It gives physical insight into the experimentally observed mixing law. © 1996 American Institute of Physics.

Notes: The antiferroelectric (AFE)-ferroelectric (FE) phase transformation under dc bias and hydrostatic pressure conditions in tin-modified lead zirconate titanate ceramics [Pb(Zr,Sn,Ti)O3, i.e., PZST] was investigated. The shifting of transformation temperature under these conditions and an electric field induced lattice softening are reported. Depending on the symmetry of external applied fields, the thermal stability region of one phase can be expanded at the expense of another. Experimental results indicate that a symmetric external field (such as hydrostatic pressure) tends to stabilize the AFE phase region, whereas an asymmetric external field (such as electric field) tends to extend the FE phase region. These observations are found to be generally consistent with many ferroelectric and antiferroelectric materials exhibiting a displacive structural phase transformation. Results are compared with the Clausius–Clapeyron relationship, and fundamental issues underlying the thermodynamic relationship and field-induced lattice softening behavior are discussed from the perspective of lattice dynamics theory. © 1996 American Institute of Physics.

Notes: We report a simple technique for fabricating a layer of isolated Si quantum dots on SiO2 glass substrates. This technique uses conventional low-pressure chemical-vapor deposition for an extremely short deposition time in the early stage of poly-Si film growth. The layer after a deposition time of 60 s has isolated Si nanocrystals 5–20 nm in diameter and 2–10 nm in height. The measurements of optical absorption coefficient α show that the absorption edge for Si nanocrystals shifts to higher energies compared to that of bulk Si, indicating a widening of the energy gap caused by quantum size effects. The linear relationship (αhν)1/2 against hν suggests that the Si nanocrystal, whose diameter is as small as 10 nm, basically maintains the properties of an indirect band-gap semiconductor. Special attention must be paid to the Brownian migration of Si nanocrystals for fabricating Si quantum dots. © 1996 American Institute of Physics.

Notes: Optical and electrical properties of SiGe strain-adjusted superlattices have been studied. Diode structures were processed into waveguide geometries to investigate the role of optical confinement and the lowering of cubic symmetry with regards to the polarization properties of interband absorption and emission. The polarization anisotropy of the absorption coefficient suggests that the heavy-hole band of strain-adjusted Si6Ge4 superlattices is the top valence band. © 1996 American Institute of Physics.

Notes: We present experimental results concerning optical transitions and carrier dynamics (capture and relaxation) in self assembled InAs/GaAs quantum dot structures grown by metalorganic vapor phase epitaxy. Photoluminescence (PL) measurements at high excitation level reveal optical transitions above the ground state emission. These transitions are found to originate from occupied hole states by solving the quantum dot eigenvalue problem. Time-resolved studies after non-resonant pulse excitation exhibit a relaxation ladder of the excited carriers from the GaAs barrier down to the ground state of the quantum dots. From both the continuous-wave measurements and the PL-decay curves we conclude that the carrier relaxation at non-resonant excitation is mediated by Coulomb interaction (Auger effect). PL-decay curves after resonant pulse excitation reveal a longer rise time compared to non-resonant excitation which is a clear indication of a relaxation bottleneck inside the quantum dots. We interpret the rise time (≈ 400 ps) in this case to originate from relaxation via scattering by acoustic phonons. The PL-decay time of the ground state emission ≈700 ps is interpreted as the excitonic lifetime of the quantum dot. © 1996 American Institute of Physics.

Notes: A simple technique has been used to measure the two polarization-dependent absorption edges in a single quantum well around 1.5 μm. The broadband spontaneous emission of an AR-coated semiconductor laser chip was used as the spatially coherent light source in conjunction with a grating spectrometer. Absorption edges for both TE and TM polarized light have been measured in single quantum well InGaAs/InGaAsP waveguides and modeled by the k⋅p method. By pumping the devices with a 1.3 μm semiconductor laser, absorption saturation was also studied. The resulting changes in the refractive index were calculated from a Kramers–Kronig transformation of the absorption changes. © 1996 American Institute of Physics.

Notes: The photovoltaic effect in tunnel-coupled double quantum wells (DQWs) due to resonant infrared (IR) excitation of the electrons from the lower ground state to the excited state is considered. Intra- and interwell relaxation of electrons are studied self-consistently, taking into account modification of the electron states in DQWs due to the upper ground state occupation under IR pumping. The photoinduced potential is calculated for different energies of IR excitation, and the correlation between this potential and modification of the ground states in DQWs is discussed. © 1996 American Institute of Physics.

Notes: Near band edge absorption spectra of the narrow-gap semiconductor alloys InxTl1−xP, InxTl1−xAs, and InxTl1−xSb were calculated and compared with those of HgxCd1−xTe. To test accuracy, we compared the calculated absorption spectra in GaAs with experimental results and found good agreement. Within 50 meV from the absorption edge, the absorption coeffi cient of InxTl1−xP is found to have about the same magnitude as that in HgxCd1−xTe and GaAs, whereas that in InxTl1−xAs and InxTl1−xSb is much smaller. This result and other merits found from previous studies indicate that InxTl1−xP has a potential to compete favorably with HgxCd1−xTe for long-wavelength infrared applications. © 1996 American Institute of Physics.

Notes: Interference pattern due to standing polariton waves have been observed in thin planparallel ZnSe layers. The samples consist of pseudomorphic ZnSe/ZnCdSe quantum well structures grown by molecular beam epitaxy with cap layers of various thicknesses. The interference pattern appearing in reflectance and photoluminescence excitation spectra were fitted to calculated spectra applying a simple theoretical model. This fit results in the determination of the deadlayer thickness and the evaluation of the Luttinger parameter γ1 and γ2. © 1996 American Institute of Physics.

Notes: Several multiquantum wells of InP/GaxIn1−xAsyP1−y grown by chemical-beam epitaxy have been studied by high-resolution x-ray diffraction, low-temperature photoluminescence, and Raman scattering to characterize interfacial layers between the barriers and the wells. These interfacial layers are created during the initial stage of growth of the quaternary material as a result of the longer transient for the saturation of the group-III elements flux. The combination of x-ray diffraction and photoluminescence allows a precise determination of the interfacial layer thickness and composition grading and shows that interface roughness is of the order of 1 monolayer. Raman scattering confirms these results and is used to determine values of the sound velocity and of the index of refraction in the quaternary alloy material.

Notes: The effect of sulfur (S) treatments on InP is investigated by low-temperature photoluminescence (PL) measurements. For both n- and p-InP, the PL intensity is observed to increase about four times in magnitude if the scattering by the S overlayer is relatively small. Some PL bands are observed to disappear after S treatments and then reappear if the S-treated surface is heat treated at 220 °C in a vacuum of 10−3 Torr. By observing their dependence on the excitation power density, the doping level of the samples, and measurement temperature, these PL bands are ascribed to the optical transitions via surface states. Our results thus indicate that the S-treated InP surface may not be stable at a subsequent processing temperature of about 250 °C. © 1996 American Institute of Physics.

Notes: Raman spectra of GaN films grown by molecular-beam epitaxy and hydride vapor-phase epitaxy on GaAs and Al2O3 substrates have been studied. It was found that longitudinal phonon modes disappear from the spectra of n+ films due to screening by free carriers, but coupled plasmon phonon modes of the higher-energy branch are not observed because of strong damping of plasmons. Precise values for phonon frequencies and linewidths are presented. No differences in phonon frequencies for the films of different thicknesses grown on different substrates have been found which indicates that the strain due to lattice and thermal-expansion mismatch is relaxed by the formation of the dislocations very close to the substrate–film interface. © 1996 American Institute of Physics.

Notes: Thin film light emitting diodes based on multibilayer combinations of the conjugated polymer poly(phenylene vinylene) (PPV) and different polyanions were fabricated via the use of a simple layer-by-layer molecular-level processing scheme. Molecular-level manipulation of the type and sequence of PPV bilayers used to construct the films was found to produce devices with dramatically improved luminance levels and efficiencies. It was also found that device efficiency could be improved by controlling the nature of the thin film/electrode interface. © 1996 American Institute of Physics.

Notes: Raman scattering from longitudinal acoustic phonons in W/Ti Fibonacci superlattice is studied. Doublet peaks which result from the folded longitudinal acoustic phonons appear in the Raman spectra as expected. Based on the Kronig–Penney model or the Rytov model, both the normal-mode frequencies and scattering intensities are numerically calculated by using a transfer-matrix method. Noticeable agreement is obtained between the results of quantitative calculations and experimental data. © 1996 American Institute of Physics.

Notes: The Young's modulus E and the modulus of rigidity G of ZrHx (0≤x〈0.9) were measured as functions of temperature between 300 and 1300 K for Zr and of hydrogen concentration at 941 and 1001 K in the previous paper. The abrupt change of the observed G for Zr at a critical temperature determines all the unknown coefficients of the free energy (LM free energy) derived by Lindgård and Mouritsen, who discussed the structural phase transition between the α (a hexagonal closed-packed structure) and β (a body centered cubic structure) phases in a Landau theory [Phys. Rev. Lett. 57, 2458 (1986)]. LM free energy is modified so as to include effects of interstitial hydrogen atoms at low hydrogen concentration. A modified LM free energy which represents the transition from the α to β phase through the α+β phase is obtained by introducing an order parameter proportional to hydrogen concentration. The order parameter multiplied by a lattice constant is the displacement between the position of Zr atom in the α phase and its position in the β phase. © 1996 American Institute of Physics.

Notes: Ability to control decomposition path and products for addition polymers is important in many applications ranging from thin film fabrication and basic understanding of reaction kinetics to evaluating the feasibility of thermal routes for the recycling of addition polymers. Laser ablation, a technique widely used to deposit thin films of a variety of inorganic materials, can also be used as a simple and highly versatile method for the deposition of thin polymer films of materials that are difficult to process and studying decomposition kinetics as well. In situ studies of the products formed by the laser/polymer interaction suggest that, as in standard pyrolysis, decomposition of poly(tetrafluoroethylene), poly(methylmethacrylate), and poly(α-methyl styrene) proceeds via chain unzipping leading to monomer. Unexpectedly, high monomer yields are also observed for addition polymers that pyrolyze by random scission producing little or no monomer. Examples of such materials presented here include poly(vinylfluoride) and polypropylene. © 1996 American Institute of Physics.

Notes: InGaP/InGaAlP single quantum wells (SQWs) were fabricated on (100)GaAs substrates misoriented towards the [011] direction at different growth rates. Photoluminescence (PL) measurements were carried out at 4.2 K to investigate dependence of the interface smoothness on substrate misorientation and growth rate for InGaP/InGaAlP SQWs. It has been found that the PL linewidth of the SQWs becomes a minimum value in the 10°–40° substrate misorientation range. This means the smoothest InGaP/InGaAlP heterointerface can be obtained on these misoriented substrates. It has also been found that the heterointerface can be improved with reduced growth rate. © 1996 American Institute of Physics.

Notes: The minimization of the radial release effects in the recovery configurations of plate impact experiments is essential for accurate postmortem microstructural investigations. The present study evaluates the results of several three-dimensional finite-element simulations involving different plate geometries. The study examines combinations of circular, square, and star-shaped plate geometries, with no lateral momentum traps or guard rings. Experiments and simulations are conducted on brittle specimens. A simple, but fairly successful, combination for the pressure–shear recovery experiment is reported, which makes use of a single square and three circular plates. In the case of the normal impact recovery configuration, it is found that the star-shaped flyer in combination with the square target and momentum trap gives good results at locations away from the axis of the specimen. Crack patterns observed experimentally in the conventional nonrecovery pressure–shear mode, and in recovery pressure–shear and normal impact modes are discussed in relation to the simulation results. © 1996 American Institute of Physics.

Notes: A kinetic model is developed to study the process of phase separation that occurs when a polymer-dispersed liquid-crystal diffraction grating is produced by a process of photopolymerization. We find that the type of system produced depends sensitively on the rate of polymerization and on whether the spatially periodic illumination used to produce the grating is followed by a period of uniform illumination. While the average liquid-crystal concentration is always increased in the region of least irradiation, in some cases small inclusions of liquid-crystal form by spinodal decomposition at the minima in the irradiating intensity, while in others they form at the maxima. The simplest version of the theory combines the Flory-Huggins and Hillert models of phase separation. In some circumstances it must be augmented by including the effects of interdiffusion of polymer molecules having different degrees of polymerization. © 1996 American Institute of Physics.

Notes: Iron-silicide has been formed by ion implantation of iron into silicon (111). In the as-implanted sample, a new type of orthorhombic FeSi2 phase was found. Although the lattice parameters of the new phase are the same as those of the known semiconductor β-FeSi2, its point group and space group were different and determined to be mmm and Pbca, respectively. © 1996 American Institute of Physics.