Volume 75, Issue 9, 01 May 1994
Index of content:
75(1994); http://dx.doi.org/10.1063/1.355972View Description Hide Description
The paper reviews the electrical and optical mechanisms at work in sulfide‐based thin‐film electroluminescence display devices within the framework of general semiconductor physics. The electrical problem is twofold: (i) charge carriers are sourced at high electric field in a nominally insulating material, the carrier density increasing by almost eight orders of magnitude; (ii) the carriers are transported at high field, with an average energy largely exceeding the thermal one. (i) Carrier sourcing is best understood from direct‐current‐driven ZnS films, and is ascribed to partly filled deep donors transferring electrons to the conduction band by Fowler–Nordheim tunneling. The deep donors also act as carrier sinkers, and evidence for space charge is afforded by small‐signal impedance analysis disclosing a markedly inductive behavior. The conduction picture obtained from dc‐driven films is then used to clarify the operation of alternating‐current electroluminescence structures where the sulfide is sandwiched between two blocking oxide layers. The electrostatics of the ac structure is investigated in detail including space charge and field nonuniformity, and external observables are related to internal quantities. The simple model of interfacial carrier sourcing and sinking is examined. (ii) High‐field electronic transport is controlled by the electron‐phonon interaction, and the modeling resorts to numerical simulations or the lucky‐drift concept. At low electronenergies the interaction with phonons is predominantly polar, while at optical energies it proceeds via deformation potential scattering.
In spite of the uncertainties in transport models in that range, it is likely that ∼50% of the electrons overtake 2 eV at the usual operating fields in ZnS. Light emission is associated with impurity luminescence centers embedded in the sulfide host. They are excited while current is flowing, and the ensuing relaxation is partly radiative. We describe the two ways in which an impurity may be excited electrically, namely, impact excitation (internal promotion of the center to a state of higher energy) or impact ionization (with an electron released to the host conduction band). The actual excitation mechanism depends on the position of the impurity excited level relative to the host energy bands. A calculation of the excitation yield (number of excited centers per transferred electron) is detailed in the case of impact excitation. Lastly, a phenomenological description of the various relaxation channels is given in terms of formal kinetics, and the relative importance of radiative relaxation is assessed by means of the deexcitation yield (fraction of centers decaying radiatively), which is defined in the case of the impulse response.
75(1994); http://dx.doi.org/10.1063/1.355973View Description Hide Description
The inversion formula of Abel’s integral equation has numerous applications in many scientific fields. In practice, its input is experimentally obtained data that contains inevitable measurement noise. Avoiding error amplification is, therefore, a main concern of every algorithm. We present a new fast Abel inversion algorithm that, in contrast to other methods, requires a negligible number of transcendental operations. Storage requirements are reduced as well. The advantage of the algorithm is especially noticeable when multiple data sets need to be inverted. As demonstrated in the paper, the new algorithm attenuates the input errors.
Nondestructive determination of free‐electron concentration and mobility in Hg1−x Cd x Te, n‐type InSb, and n‐type GaAs75(1994); http://dx.doi.org/10.1063/1.355974View Description Hide Description
A method for nondestructive determination of free‐electron concentration and mobility using Faraday rotation and absorption at mid‐infrared wavelengths is presented. Faraday rotationmeasurements were made at four CO2 laser wavelengths between 9 and 11 μm. The component due to free electrons was extracted using its wavelength dependence. Concentration N was determined for N≳1014 cm−3 from calibration plots exploiting Faraday rotation’s linear dependence on free electron concentration. Electron rotation was combined with absorption due to electrons to determine their mobility. These mobilities agreed reasonably well with Hall test mobilities in materials where electron absorption could be determined separately from hole absorption. In materials where hole and electron absorption cannot be determined separately, a reference mobility was calculated using an electron absorption cross section. Samples tested were Hg1−x Cd x Te for 0.20≤x≤0.30, n‐type and intrinsic InSb, and n‐type GaAs at room temperature.
75(1994); http://dx.doi.org/10.1063/1.355975View Description Hide Description
The complete stress field in a polycrystalline sample compressed in a modified Drickamer‐type apparatus was determined from x‐ray diffraction data. The incident x rays, from a synchrotron source, were perpendicular to the compression axis, and the diffracted energy‐dispersive signals were simultaneously determined for two directions relative to the compression axis. The two sets of d values measured by this system were analyzed by making use of a new equation derived by Singh, and the uniaxial stress component σ1−σ3 and the parameter α, which describes the stress and strain conditions across the grain boundaries of the sample, were obtained. This method was applied to NaCl and the results give the important information on the stress state and the pressure determination method under direct compression of a solid sample.
75(1994); http://dx.doi.org/10.1063/1.355976View Description Hide Description
A technique for evaluating second‐order nonlinear optical materials in powder form is described. The power of the second‐harmonic wave generated with the evanescent wave (SHEW) loosely depends on the phase matchability of material. The largest element of the nonlinear optical coefficient can be estimated from the SHEW power, regardless of the phase matchability of the material in a bulk crystal, even when powder crystals are used as the sample. The sample setting condition that gives appropriate repeatability is obtained, and the accuracy of this measurement is also discussed. Several well‐known materials are measured using this technique, and the results are compared with other works.
Angular and energy distributions of surface produced H− and D− ions in a barium surface conversion source75(1994); http://dx.doi.org/10.1063/1.355977View Description Hide Description
The production of negative hydrogen and deuterium ions is studied in the foundation for fundamenteel Onderzoek der Materie surface conversion experiment. In this experiment a Barium surface is used to convert positive ions into negative ions. We have developed a detector able to analyze the characteristics of the surface produced negative ion beam. Measurements with this detector show that in barium surface conversion sources negative ions can be formed by recoiling and direct reflection of positive ions from the barium surface. The broad angular distribution of the surface produced negative ions is determined to extend over 16°.
Planar 1.3 and 1.55 μm InGaAs(P)/InP electroabsorption waveguide modulators using oxygen ion mixing and the photoelastic effect75(1994); http://dx.doi.org/10.1063/1.355978View Description Hide Description
Efficient 1.3 and 1.55 μm InP‐based electroabsorption waveguide modulators with planar device structures have been demonstrated. Elevated temperature oxygen ion implantation and/or the photoelastic effect induced by W metal stressor stripes deposited on the semiconductor surface have been used to produce these self‐aligned planar guided‐wave devices. The oxygen ion mixing process has been used to simultaneously achieve compositional disordering and electrical isolation of superlattice material while the photoelastic effect has been used to improve the lateral mode confinement. A 1.3 μm Franz–Keldysh modulator with a ≳10 dB extinction ratio at 2 V and a 1.55 μm device with a ≳10 dB extinction ratio at 7 V are reported. These single growth step planar processing techniques have also been used to fabricate relatively low‐loss (<4 dB/cm) double heterostructure InGaAs(P)/InP single‐mode optical waveguides which demonstrate their usefulness in developing InP‐based photonic integrated circuits.
75(1994); http://dx.doi.org/10.1063/1.355979View Description Hide Description
The dynamics of thermoplastic‐photoconductor films is reviewed. A theory of thermoplastic film deformation is presented for a three‐layer structure in air with modified boundary conditions. The amplitude of the deformation A versus the spatial frequency k is of bandpass type (A‐k curve). The influence of the thickness of the layers and the heating and electric parameters on the A‐k curve are analyzed. The predictions of the model are compared to others previously reported and with experimental data. The validity and limitations of the model are also discussed.
75(1994); http://dx.doi.org/10.1063/1.355981View Description Hide Description
Reflection effects in oblique incidence grooved gratings in the presence of propagating surface acoustic waves are analyzed. The particle displacements associated with these waves are described by a modal expansion including second‐order evanescent and propagating bulk waves. The reflection coefficient and velocity shift are derived to first and second order (in the overlay height) using the boundary condition matching method. It is shown that the same second‐order effects predicted for normal incidence groove gratings will exist for oblique incidence groove gratings. In addition, the reflection coefficient and velocity shift behavior are examined as functions of incidence angle and metallization ratio.
75(1994); http://dx.doi.org/10.1063/1.355982View Description Hide Description
The Green’s function temperature expressions formulated by McGahan and Cole [J. Appl. Phys. 72, 1362 (1992)] are modified into a form suitable for solving the heat conduction problem encountered in magneto‐optical (MO) disk recording situations. The temperature distribution within MO multilayer media heated by a pulsed scanning Gaussian laser beam is calculated by using Fourier‐transformed Green’s functions. The linear heat conductionequation is solved exactly not in real space but in frequency space. The temperature in real space is efficiently recovered by the inverse fast Fourier transform; numerical integrations are unnecessary. Optical absorption in MO media is calculated exactly. Realistic and piecewise‐linear models of the laser pulse’s time dependence are incorporated directly into the formalism. Elliptically shaped laser‐beam cross sections are also easily included. At the same time, the extended method still preserves the conceptual simplicity and computational efficiency of the original theory. This paper describes the extended method, discusses some numerical issues arising from the modifications, and presents comparisons with previously published finite‐difference calculations.
75(1994); http://dx.doi.org/10.1063/1.355983View Description Hide Description
Calculated photothermal contrast functions arising from lateral scanning of buried stripe‐ and disk‐shaped deviations in thermal conductivityk are presented. From the theoretical findings the measuring conditions are derived in terms of the modulation frequency for which optimal photothermaldiscrimination of subsurface thermal inhomogeneities can be achieved. The theoretical results have been verified experimentally be measuring model samples, in which localized inhomogeneities were embedded.
Effect of time‐of‐flight bunching on efficiency of light‐ion‐beam inertial‐confinement‐fusion transport schemes75(1994); http://dx.doi.org/10.1063/1.355984View Description Hide Description
The Laboratory Microfusion Facility (LMF) has been proposed for the study of high‐gain, high‐yield inertial‐confinement‐fusion targets. The light‐ion LMF approach uses a multimodular system with applied‐B extraction diodes as ion sources. A number of ion‐beam transport and focusing schemes are being considered to deliver the beams from the diodes to the target. These include ballistic transport with solenoidal lens focusing, z‐discharge channel transport, and wire‐guided transport. The energy transport efficiency η t has been defined and calculated as a function of various system parameters so that point designs can be developed for each scheme. The analysis takes into account target requirements and realistic constraints on diode operation, beamtransport, and packing. The effect on η t of voltage ramping for time‐of‐flight beam bunching during transport is considered here. Although only 5 mrad microdivergence calculations are presented here, results for bunching factors of ≤3 show that transport efficiencies of ≳50% can be obtained for all three systems within a range of system parameters which seem achievable (i.e., for diode microdivergence within 5–10 mrad, for diode radius within 10–15 cm, and for diode‐ion‐current density within 2–10 kA/cm2). In particular, the point design for the baseline LMF system using ballistic transport with solenoidal lens focusing and a bunching factor of 2 was calculated to have η t =84%. Other factors affecting the overall system efficiency, but not included in the analysis, are also identified and estimated.
75(1994); http://dx.doi.org/10.1063/1.355985View Description Hide Description
An entirely physical model is proposed to explain a wide range of seemingly conflicting observations of plasma‐charging damage. Unlike other authors who largely ignored the role of substrate potential, we carefully track both the gate and the substrate potentials to explain the origin of the electric field developed across the thin oxide during plasma exposure. Central to this model is the fact that the surface floating potential tracks the plasma potential. Thus a nonuniform plasma drives the gate potential to a nonuniform distribution. Another important idea of this model is the continued adjustment by both the gate and the substrate of their potentials to satisfy the charge balance requirement of plasma system. The interaction between saturated ion‐current, asymmetric electron‐current, and the Fowler–Nordheim tunneling current produces a complex dynamic for the movements of the gate and the substrate potentials. This complex behavior of the gate and the substrate potential allows many of the reported observations in the literature to be explained logically. We explored three types of charging effect and their damage characteristics. They are dc effect, ac effect, and transient effect. The separation of dc and ac effect is artificial. They exist simultaneously and add to each other to cause more serious damage than by themselves. The model predicts antenna effects from all three types of charging effects. Unique to the acantenna effect are saturation behavior, the oxide thickness ratio dependence, and the rf bias‐frequency dependence. The effect of ON/OFF transient is explored quantitatively using the concept of effective exposed substrate area. The combination of large antenna ratio and rapid turn‐off of plasma causes severe damage to gate‐oxide. Magnetic field in general will worsen the charging damage. The reason for that is explained. Plasma uniformity is the most important factor governing charging damage, but is not the only factor. Plasma uniformity extending beyond the edge of the wafer is important. Mere showing that the plasma is uniform across the most part of the wafer is misleading.
75(1994); http://dx.doi.org/10.1063/1.355986View Description Hide Description
Critical parameters for the development of the plasma‐source ion implantation process are the ion implantation current and the sheathexpansioncharacteristics. Recently, Xia and Chan [J. Appl. Phys. 73, 3651 (1993)] have investigated these parameters for small spherical electrodes inserted in a two component positive ion‐electron plasma. This investigation is extended to a plasma that consists of three components: positive ions, negative ions, and electrons.
Characterization of electron beams generated in a high‐voltage pulse‐line‐driven pseudospark discharge75(1994); http://dx.doi.org/10.1063/1.355957View Description Hide Description
Emittance and energy measurements have been performed on a high‐brightness electron beam (≳1010 A/m2 rad2) with diameter in the range 1–3 mm and energy in the range 150–170 keV. This electron beam is generated by the mating of a hollow‐cathode discharge device operating in the pseudospark regime to the output of a high‐power pulse line accelerator. The measured effective emittance lies in the range between 30 and 90 mm mrad and increases with axial distance. Electron energy measurements indicate that the high‐energy electrons are generated during the first 20–30 ns of the discharge. Both the emittance and energy experiments were performed at two different ambient argon gas pressures (92 and 152 mtorr). Beam expansion as a function of axial position has also been studied and a lower bound on the beam brightness has been obtained.
75(1994); http://dx.doi.org/10.1063/1.355958View Description Hide Description
The formation, structure, and the crystallization of Al85Y x Ni15−x are studied using x‐ray diffraction and differential scanning calorimetry. The results show two distinct glasses depending on composition. Y‐rich glasses (x≥8) are homogeneous with a well‐defined glass transition. The x‐ray diffraction patterns have a single main peak. These glassescrystallize through a nucleation and growth process. Y‐poor glasses (x<8) do not show a glass transition and have a shoulder on the high‐angle side of the main peak in their x‐ray diffraction patterns. We show that the shoulder peak is due to quenched‐in Al nuclei. These glasses are shown to crystallize through the growth of these nuclei. Y‐rich glasses (x≥8) are more stable as demonstrated by the presence of the glass transition and their higher crystallization temperature, enthalpy, and activation energy. The occurrence of a prepeak for all compositions is attributed to Y‐Y pairs.
75(1994); http://dx.doi.org/10.1063/1.355959View Description Hide Description
A theoretical model for predicting the kinetics of icecrystallization inside cells during cryopreservation was developed, and applied to mouse oocytes, by coupling separate models of (1) watertransport across the cell membrane, (2) icenucleation, and (3) crystal growth. The instantaneous cell volume and cytosol composition during continuous cooling in the presence of glycerol were predicted using the watertransport model. Classical nucleationtheory was used to predict icenucleation rates, and a nonisothermal diffusion‐limited crystal‐growth model was used to compute the resulting crystallization kinetics. The model requires knowledge of the nucleation rate parameters Ω and κ, as well as the viscosity η of a water‐NaCl‐glycerol solution as a function of both the composition and temperature of the solution. These dependences were estimated from data available in the literature. Cell‐specific biophysical parameters were obtained from previous studies on mouse oocytes. A sensitivity analysis showed that the model was most sensitive to the values of κ and η.
The coupled model was used to study the effect of cooling rate and initial glycerol concentration on intracellular crystal growth. The extent of crystallization, as well as the crystal size distribution, were predicted as functions of time. For rapid cooling at low to intermediate glycerol concentrations, the cells crystallized completely, while at high concentrations of glycerol, partial or total vitrification was observed. As expected, the cooling rate necessary for vitrification dropped with increasing glycerol concentration; when cells initially contained ∼7.5 M glycerol, vitrification was achieved independent of cooling rate. For slow cooling protocols, watertransport significantly affected the results. At glycerol concentrations greater than ∼3 M, the final intracellular ice content decreased with increasing glycerol concentration at a fixed cooling rate. In this regime, the cooling rate at which a critical amount of ice was formed increased as more glycerol was used. When less than ∼3 M glycerol was initially present in the cell, an increase in glycerol concentration was predicted to cause an increase in the final intracellular ice content at a given cooling rate. In this regime, the critical cooling rate decreased with increasing glycerol concentration. These predictions clarify previous empirical observations of slow freezing phenomena.
A simplified procedure for obtaining relative x‐ray intensities when a texture and atomic displacements are present75(1994); http://dx.doi.org/10.1063/1.355960View Description Hide Description
The orientation problem in polycrystalline cubic materials has been simplified, using fundamental relationships, so that the determination of a quantitative interrelationship between the various Bragg peak intensities is no longer a formidable task. This is demonstrated with a cubic Cu‐Be‐Co alloy having a fiber texture, and a conventional focusing diffractometer. Because data are required which extend over a larger range in d spacings, extinction, thermal, and static atomic displacements must be included into the analysis of intensities. The displacement terms and the extinction parameters may be of primary interest or used as a correction. Seventeen diffraction peaks are used in the example. These must be internally consistent with a crystallite orientation function, the cubic symmetry of the sample, extinction effects influencing the two strongest peaks, and attenuation due to atomic displacements. Tabulated coefficients are presented which greatly reduce the task of calculating the orientation function. A correction is given for instrumental smearing which should be considered for stronger textures than the intermediate case examined or for intermediate textures and nonfocusing instrumental conditions.
75(1994); http://dx.doi.org/10.1063/1.355961View Description Hide Description
A simplified method of calculating thermal diffuse scattering (TDS) for materials at or above their Debye characteristic temperature is presented. This method is based upon the properties of the lattice Green’s function and eliminates the need of solving the equations of lattice dynamics. This simplifies the calculation procedure. The general scheme of evaluating the diffuse scattering for polycrystallinematerials with a texture is discussed. A sample calculation of the TDS from a polycrystallinematerial with a fiber texture is given using the simplified procedure just presented.
75(1994); http://dx.doi.org/10.1063/1.355935View Description Hide Description
Dynamic ion‐beam mixing (i.e., simultaneous ion irradiation and layer deposition), allowing better control of intermixing and stoichiometry than conventional mixing methods, has been simulated by a Monte Carlo method. Reasonable agreement between simulation and measurements for the systems TiNSi and TiNFe was achieved.