Showing papers by "Bell Labs published in 1977"

Nonradiative capture and recombination by multiphonon emission in GaAs and GaP

Abstract: Data are presented for 9 capture cross sections of deep levels in GaAs and 4 in GaP which can be interpreted as capture by multiphonon emission (MPE). At high temperatures the cross sections have the form $\ensuremath{\sigma}={\ensuremath{\sigma}}_{\ensuremath{\infty}}{e}^{\frac{\ensuremath{-}{E}_{\ensuremath{\infty}}}{\mathrm{kT}}}$ where ${\ensuremath{\sigma}}_{\ensuremath{\infty}}={10}^{\ensuremath{-}14}\ensuremath{-}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ and ${E}_{\ensuremath{\infty}}=0\ensuremath{-}0.56$ eV. A simple theory of MPE capture is presented in which vibrations of a single lattice coordinate modulate the depth of the potential well binding the carrier. In this model capture results from lattice vibrations causing the crossing of free- and bound-carrier levels. The breakdown of the adiabatic approximation near the crossing is discussed. The calculated cross sections have the form $\ensuremath{\sigma}=Af(0)$ where $f(h\ensuremath{ u})$ is the normalized line shape for radiative capture. The lattice relaxation properties of the center determine $f(0)$. The temperature dependence of $f(0)$ correctly accounts for the thermally activated behavior of the cross sections at high temperatures. Classical and quantum treatments of the lattice motion give the same expression for $\ensuremath{\sigma}$ at high temperature. A detailed calculation of $A$ is made for the capture of a carrier by an attractive neutral impurity in the case where both the free-carrier and bound-carrier wave functions are describable in a one-band effective-mass approximation. The theoretical value of $A$ leads to ${\ensuremath{\sigma}}_{\ensuremath{\infty}}\ensuremath{\approx}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, the same order of magnitude as the experimental values. However, many of the experimental cross sections involve complexities not accounted for in this simple model such as charged impurities and transitions between free states and bound states of different symmetry. The lattice relaxation parameters are experimentally determined for the Zn-O and O centers in GaP. Lattice relaxation is found adequate to explain the large cross sections for electron capture by the Zn-O center and hole capture by the two-electron state of O. The studies of the O and Zn-O centers also provide evidence for nonlinear changes in the impurity energy level with lattice displacement which decrease the electron capture cross sections and greatly enhance the hole recombination cross sections. The source of this nonlinearity is discussed.

A unified approach to short-time Fourier analysis and synthesis

Abstract: Two distinct methods for synthesizing a signal from its short-time Fourier transform have previously been proposed. We call these methods the filter-bank summation (FBS) method and the overlap add (OLA) method. Each of these synthesis techniques has unique advantages and disadvantages in various applications due to the way in which the signal is reconstructed. In this paper we unify the ideas behind the two synthesis techniques and discuss the similarities and differences between these methods. In particular, we explicitly show the effects of modifications made to the short-time transform (both fixed and time-varying modifications are considered) on the resulting signal and discuss applications where each of the techniques would be most useful The interesting case of nonlinear modifications (possibly signal dependent) to the short-time Fourier transform is also discussed. Finally it is shown that a formal duality exists between the two synthesis methods based on the properties of the window used for obtaining the short-time Fourier transform.

Short term spectral analysis, synthesis, and modification by discrete Fourier transform

Abstract: A theory of short term spectral analysis, synthesis, and modification is presented with an attempt at pointing out certain practical and theoretical questions. The methods discussed here are useful in designing filter banks when the filter bank outputs are to be used for synthesis after multiplicative modifications are made to the spectrum.

Offset masks for lift‐off photoprocessing

Abstract: We describe a technique using photolithography to produce submicron‐scale thin‐film structures and simple multilevel structures by single‐mask lift‐off processing. The technique employs masks offset from the substrate and oblique angle thin‐film deposition. It provides a simple means of making small‐area Josephson junctions and varying‐thickness superconducting bridges and is suitable for the inclusion of these devices in circuits. The examples we show emphasize such applications in superconductivity; however, the technique may find uses in other fields as well.

Computer-aided analysis of electronic circuits: Algorithms and computational techniques

Abstract: Preparing the books to read every day is enjoyable for many people. However, there are still many people who also don't like reading. This is a problem. But, when you can support others to start reading, it will be better. One of the books that can be recommended for new readers is computer aided analysis of electronic circuits algorithms and computational techniques. This book is not kind of difficult book to read. It can be read and understand by the new readers.

On the use of autocorrelation analysis for pitch detection

Abstract: One of the most time honored methods of detecting pitch is to use some type of autocorrelation analysis on speech which has been appropriately preprocessed. The goal of the speech preprocessing in most systems is to whiten, or spectrally flatten, the signal so as to eliminate the effects of the vocal tract spectrum on the detailed shape of the resulting autocorrelation function. The purpose of this paper is to present some results on several types of (nonlinear) preprocessing which can be used to effectively spectrally flatten the speech signal The types of nonlinearities which are considered are classified by a non-linear input-output quantizer characteristic. By appropriate adjustment of the quantizer threshold levels, both the ordinary (linear) autocorrelation analysis, and the center clipping-peak clipping autocorrelation of Dubnowski et al. [1] can be obtained. Results are presented to demonstrate the degree of spectrum flattening obtained using these methods. Each of the proposed methods was tested on several of the utterances used in a recent pitch detector comparison study by Rabiner et al. [2] Results of this comparison are included in this paper. One final topic which is discussed in this paper is an algorithm for adaptively choosing a frame size for an autocorrelation pitch analysis.

Large-Lattice-Relaxation Model for Persistent Photoconductivity in Compound Semiconductors

Abstract: A new model, based on an extremely strong coupling between the electronic and vibrational systems of certain defect centers, is proposed to explain the phenomenon of persistent photoconductivity observed in some compound semiconductors. The model is supported by data on donor-related defects in $n$-type ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ which exhibit the features characteristic of this effect: a very large Stokes shift (thermal depth, \ensuremath{\sim}0.1 eV; optical depth, \ensuremath{\sim}1.2 eV); and a very small (${10}^{\ensuremath{-}30}$ ${\mathrm{cm}}^{2}$), thermally activated, electron-capture cross section at temperatures below 77 K.

Observation of the Two-Dimensional Plasmon in Silicon Inversion Layers

Abstract: The two-dimensional plasmon of an $n$ inversion layer of (100) $p$-type Si is observed at a fixed wave vector as a function of electron density. The position, width, and strength of the resonance agree with existing theory at electron densities \ensuremath{\gtrsim}${10}^{12}$/${\mathrm{cm}}^{2}$. At lower densities, the resonance position is below the predicted value, implying a substantial increase in the electron mass.

Adiabatic bond charge model for the phonons in diamond, Si, Ge, and α-Sn

Abstract: An adiabatic bond charge model (BCM) for the lattice dynamics of diamond-type crystals is developed. Our BCM unites elements of earlier models by Phillips and Martin, Keating, and Cochran. Four types of interactions are used: (a) central ion-ion forces, (b) Coulomb interactions of the ions and bond charges (BC's), (c) central ion-BC forces, and (d) bond-bending forces. These interactions represent the metal-like (a) and covalent (b)-(d) part of the crystal bonding. The phonon dispersion curves for Si, Ge, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ are calculated using only four disposable parameters; for diamond, five parameters are employed. For all crystals, very good agreement with experiment is obtained. In particular, the typical flattening of the transverse acoustic phonons in the semiconducting materials is understood as a consequence of the adiabatic motion of the BC's, when the effective ion-BC coupling (b)+(c) is weak compared to the bond-bending forces (d). In an alternative representation of the BCM, the interactions (b) and (c) are replaced by central and noncentral ion-BC-ion potentials along one bond. The remaining long-range part of the Coulomb forces is unimportant; therefore, all essential interactions of the BCM are of very short range. Furthermore, the interaction parameters follow clear trends from diamond to $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$: type (a) increases, whereas types (b)-(d) decrease, especially the ion-BC coupling tends to vanish toward $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$.

Perovskite oxides: materials science in catalysis.

Abstract: In a time of growing need for catalysts, perovskites have been rediscovered as a family of catalysts of such great diversity that a broad spectrum of scientific disciplines have been brought to bear in their study and application. Because of the wide range of ions and valences which this simple structure can accommodate, the perovskites lend themselves to chemical tailoring. It is relatively simple to synthesize perovskites because of the flexibility of the structure to diverse chemistry. Many of the techniques of ceramic powder preparation are applicable to perovskite catalysts. In their own right, they are therefore of interest as a model system for the correlation of solid-state parameters and catalytic mechanisms. Such correlations [See figure in the PDF file] have recently been found between the rate and selectivity of oxidation-reduction reactions and the thermodynamic and electronic parameters of the solid. For commercial processes such as those mentioned in the introduction, perovskite catalysts have not yet proven to be practical. Much of the initial interest in these catalysts related to their use in automobile exhaust control. Current interest in this field centers on noble metalsubstituted perovskites resistant to S poisoning for single-bed, dual-bed, and three-way catalyst configurations. The formulations commercially tested to date have shown considerable promise, but long-term stability has not yet been achieved. A very large fraction of the elements that make up presently used commercial catalysts can be incorporated in the structure of perovskite oxides. Conversely, it is anticipated that perovskite oxides, appropriately formulated, will show catalytic activity for a large variety of chemical conversions. Even though this expectation is by no means a prediction of commercial success in the face of competition by existing catalyst systems, it makes these oxides attractive models in the study of catalytic chemical conversion. By appropriate formulation many desirable properties can be tailored, including the valence state of transition metal ions, the binding energy and diffusion of O in the lattice, the distance between active sites, and the magnetic and conductive properties of the solid. Only a very small fraction of possible perovskite formulations have been explored as catalysts. It is expected that further investigation will greatly expand the scope of perovskite catalysis, extend the understanding of solid-state parameters in catalysis, and contribute to the development of practical catalytic processes.

Destruction of Superconductivity at the Onset of Long-Range Magnetic Order in the Compound ErRh 4 B 4

Abstract: The compound ErRh4B4 becomes superconducting at a critical temperature T c1 of 8.7 K followed by a return to the normal state at a second critical temperature T c2 of 0.9 K. The return to the normal state at T c2 is coincident with the occurrence of long-range ordering of the magnetic moments of the Er 3+ ions which completely occupy a set of equivalent lattice sites.

New method for the calculation of atomic phase shifts: Application to extended x-ray absorption fine structure (EXAFS) in molecules and crystals

Abstract: The scattering of electrons of kinetic energy up to 1000 eV by an atom is of special interest in the understanding of extended x-ray absorption fine-structure (EXAFS) spectra. An important physical feature is the reduction of the exchange and correlation potential as the kinetic energy of the electron increases. This is taken into account by replacing the atom by an electron gas with spatially varying density and calculating the self-energy using the plasmon pole approximation. This results in a set of complex phase shifts which is then applied to the EXAFS problem. Comparison is made with phase shifts extracted from experimental EXAFS spectra and excellent agreement is obtained. Direct comparison of the theoretical and experimental spectra again shows excellent agreement in both the amplitude and the phase. We also analyze the EXAFS spectra by a Fourier-transform technique which first removes the amplitude and phase shift using the calculated result. The importance of a proper choice of zero of energy ${E}_{0}$ is emphasized. We choose ${E}_{0}$ by the requirement that the imaginary part and the absolute value of the Fourier transform should peak at the same distance, thus assuring that the absolute phase is given correctly. Using this procedure the nearest-neighbor distances in ${\mathrm{Br}}_{2}$, Ge${\mathrm{Cl}}_{4}$, and crystalline germanium are determined. In all cases the results are within 0.01 \AA{} of the known distances. Several shells in germanium are also determined, with accuracy of better than 1%. Application of our method to crystalline copper indicates that the outer shells are more seriously affected by multiple-scattering problems and our procedure permits us to discard peaks that are spurious or unreliable. The present determination of the nearest-neighbor distance in copper is found to be in error by 0.014 \AA{}. Results of the application of this method to the determination of the bond lengths of a variety of compounds are summarized.

Observation of Resonances in the Radiation Pressure on Dielectric Spheres

Abstract: We report an experimental check of the Mie-Debye theory for the variation of radiation pressure on dielectric spheres with wavelength and size using optical-levitation techniques. Sharp resonances are observed which are shown to be related to dielectric surface waves. They permit particle-size measurement to a precision of 1 part in ${10}^{5}$ to ${10}^{6}$.

Stationary Spectrum of Strong Turbulence in Magnetized Nonuniform Plasma

Abstract: A stationary spectrum in the frequency range much below the ion cyclotron frequency is obtained for a strongly turbulent nonuniform plasma. The $\ensuremath{\omega}$-integrated $k$ spectrum is given by ${k}^{1.8}{(1+{k}^{2})}^{\ensuremath{-}2.2}$, while the width of the frequency spectrum is proportional to ${k}^{3}{(1+{k}^{2})}^{\ensuremath{-}1}$, where $k$ is normalized by $\frac{{c}_{s}}{{\ensuremath{\omega}}_{\mathrm{ci}}}$. The result compares well with the recently observed spectrum in the ATC tokamak.

Neutron scattering study of the charge-density wave transitions in 2 H − Ta Se 2 and 2 H − Nb Se 2

Abstract: We have used the triple-axis neutron-scattering technique to study $2H\ensuremath{-}\mathrm{Ta}{\mathrm{Se}}_{2}$ and $2H\ensuremath{-}\mathrm{Nb}{\mathrm{Se}}_{2}$ which undergo charge-density wave transitions at ${T}_{0}=122.3$ K and ${T}_{0}=33.5$ K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of ${\ensuremath{\Sigma}}_{1}$ symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude ${q}_{\ensuremath{\delta}}=\frac{1}{3(1\ensuremath{-}\ensuremath{\delta}){a}^{*}}$, with $\ensuremath{\delta}\ensuremath{\sim}0.02$. The Nb${\mathrm{Se}}_{2}$ superlattice remains incommensurate to 5 K but Ta${\mathrm{Se}}_{2}$ undergoes a first-order lock-in transition where $\ensuremath{\delta}\ensuremath{\rightarrow}0$ at 90 K. The temperature dependence of the superlattice wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ in the incommensurate phase and the lock-in transition are discussed using a free energy involving third-order "umklapp" terms and a secondary order parameter. The secondary lattice distortion which is predicted in this model is observed experimentally. Most phonon branches having energies less than 10 meV with propagation vectors in the [$\ensuremath{\zeta}00$] and [$00\ensuremath{\zeta}$] directions have been measured at 300 K. Strong anomalies are found in the ${\ensuremath{\Sigma}}_{1}[\ensuremath{\zeta}00]$ phonon branches in both materials near the wave vector ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{c}=(\frac{1}{3},0,0)$ characteristic of the low-temperature superlattices. Substantial softening of this phonon is observed as the transition is approached. In addition, the spectral profile exhibits a central peak which is not measurably inelastic.

Adaptive transform coding of speech signals

Abstract: This paper discusses speech coding systems based upon transform coding (TC). It compares several transforms and shows that the cosine transform leads to a nearly optimum performance for almost all speech sounds. Various adaptive coding strategies are then investigated, and a coding scheme is proposed that is based on a nonadaptive discrete cosine transform (DCT), on an adaptive bit assignment, and on adaptive quantization. The adaptation is controlled by a short-term basis spectrum that is derived from the transform coefficients prior to coding and transmission and that is transmitted as side information to the receiver. The main result is that this adaptive transform coder performs better than all known nonpitch-tracking coding schemes; it extends the range of speech waveform coding to lower bit rates and closes the gap between vocoders and predictive waveform coders.

A Quantitative Model for the Diffusion of Phosphorus in Silicon and the Emitter Dip Effect

Abstract: A consistent model of P diffusion in Si is presented which accounts quantitatively for the existence of electrically inactive P, the "kink" and the tail regions of the P profile, and the emitter dip effect. In this model it is shown that three intrinsic P diffusion coefficients exist, each one associated with the diffusion of P with vacancies in three different charge states. In the so‐called "anomalous" high concentration region of the profile , it is shown that equilibrium concentration of P+ V= pairs dominates P diffusion and P electrical activity. At lower electron concentrations when the Fermi level is ∼0.11 eV below the conduction band, the V= vacancy gives up an electron, and the 0.3 eV lower binding energy of the resulting P+ V− pairs enhances the probability for pair dissociation by a factor of 10–35, depending on the temperature. This effect creates a steady‐state excess concentration of V−vacancies which flow away from the point of pair dissociation. The concentration of excess V− vacancies created is proportional to the number of P+ V=pairs created at the Si surface times the enhanced probability for pair dissociation. These vacancies in the V− charge state interact with P to create the enhanced tail diffusion. In a npn structure, the charge state of the excess vacancies becomes V+ in the base region, thus enhancing the diffusivity of the base dopant and causing the emitter dip effect. The magnitude by which the P tail diffusivity and the base dopant diffusivity are enhanced is the same and may reach a factor of 135 for a 900°C diffusion.

Electron states in α-quartz: A self-consistent pseudopotential calculation

Abstract: Self-consistent pseudopotentials are used to investigate the electronic structure of $\ensuremath{\alpha}$-quartz. We present calculations for the band structure, electronic density of states, optical response functions, pseudocharge densities, and x-ray emission spectra. We find excellent agreement between theory and experiment with respect to photoemission and uv absorption data. The chemical bonding present in $\ensuremath{\alpha}$-quartz as determined from pseudocharge density contour maps is consistent with other theoretical calculations. The theoretical x-ray emission spectra, as obtained from an orthogonalized-plane-wave scheme, are compared with experimental data. The calculated silicon and oxygen $K$ spectra agree very well with experiment; however, the $\mathrm{Si} {L}_{2,3}$ spectrum exhibits substantial disagreement with the data. An explanation is proposed based upon the formation of amorphous elemental Si in Si${\mathrm{O}}_{2}$ during electron irradiation.

Sharpening the response of a symmetric nonrecursive filter by multiple use of the same filter

Abstract: When processing data by filters, we often find it necessary to improve the performance of the filter, either by increasing the out-of-band rejection (loss) or by decreasing the error in the passband, or both. A first approach is to process the data by repeated passes through the same filter. Each pass, while increasing the out-of-band loss, also increases the passband error, often to an undesirable level. It also increases the length (order) of the equivalent filter. How can we do a better job of filtering by suitably combining the results of several passes through the same filter? By "better" we mean both less passband error and greater out-of-band, or stopband, loss. This process is called filter sharpening. A simple, powerful method for doing this is described in detail, and its computational efficiency is compared to the best possible filter designs meeting the same specifications. The design method, based on the idea of the amplitude change function, is restricted to symmetric nonrecursive (finite impulse response) filters with piecewise constant pass- and stopbands. Several illustrative examples are given.

The low energy charged particle (LECP) experiment on the Voyager spacecraft

Abstract: The Low Energy Charged Particle (LECP) experiment on the Voyager spacecraft is designed to provide comprehensive measurements of energetic particles in the Jovian, Saturnian, Uranian and interplanetary environments. These measurements will be used in establishing the morphology of the magnetospheres of Saturn and Uranus, including bow shock, magnetosheath, magnetotail, trapped radiation, and satellite-energetic particle interactions. The experiment consists of two subsystems, the Low Energy Magnetospheric Particle Analyzer (LEMPA) whose design is optimized for magnetospheric measurements, and the Low Energy Particle Telescope (LEPT) whose design is optimized for measurements in the distant magnetosphere and the interplanetary medium. The LEMPA covers the energy range from ∼10 keV to > 11 MeV for electrons and from ∼15 keV to ≳ 150 MeV for protons and heavier ions. The dynamic range is ∼0.1 to ≳ 1011 cm−2 sec−1 sr−1 overall, and extends to 1013 cm−2 sec−1 sr−1 in a current mode operation for some of the sensors. The LEPT covers the range ∼0.05 ≤ E ≳ 40 MeV/nucleon with good energy and species resolution, including separation of isotopes over a smaller energy range. Multi-dE/dx measurements extend the energy and species coverage to 300–500 MeV/nucleon but with reduced energy and species resolution. The LEPT employs a set of solid state detectors ranging in thickness from 2 to ∼2450 μ, and an arrangement of eight rectangular solid state detectors in an anticoincidence cup. Both subsystems are mounted on a stepping platform which rotates through eight angular sectors with rates ranging from 1 revolution per 48 min to 1 revolution per 48 sec. A ‘dome’ arrangement mounted on LEMPA allows acquisition of angular distribution data in the third dimension at low energies. The data system contains sixty-two 24-bit sealers accepting data from 88 separate channels with near 100% duty cycle, a redundant 256-channel pulse height analyzer (PHA), a priority system for selecting unique LEPT events for PHA analysis, a command and control system, and a fully redundant interface with the spacecraft. Other unique features of the LECP include logarithmic amplifiers, particle identifiers, fast (∼15 ns FWHM) pulse circuitry for some subsystems, inflight electronic and source calibration and several possible data modes.

The Loading Effect in Plasma Etching

Abstract: The dependence of etch rate on the quantity of material being etched, often referred to as the loading effect, for plasma etching is analyzed quantitatively with the aid of some simplifying assumptions. The etch rate is related to the quantity of material being etched through three phenomenological parametersβ, τ, and , which are related to the affinity of the etching material for the active species created in the plasma, the lifetime of the active species, and the volume generation rate of the active species, respectively. It is shown analytically that the reciprocal of the etch rate, when identical wafers are etching simultaneously, is directly proportional to . Data are presented for the plasma etching of Si in a plasma which confirm the analytical prediction. The significance of loading with respect to the measurement of etch rate, end‐point detection, and the interpretation of the temperature dependence of etch rate is discussed. A brief discussion of the physical meaning of the phenomenological parameters and their relation to process parameters subject to control such as power, pressure, and temperature is also included.

Adaptive quantization of picture signals using spatial masking

Abstract: Visual sensitivity of human observers decreases at and adjacent to large luminance changes: a fact well known to psychophysicists but not yet fully utilized for picture coding. In this paper we present a systematic investigation of these changes in visual sensitivity and apply it to adapt the quantizer of a predictive coder. The paper consists of three parts. In the first part, earlier psychophysical work in related areas relevant to picture coding is briefly reviewed. Certain simple measures of luminance activity (called the masking functions) are constructed. Subjective experiments which obtain fidelity measures related to the masking functions, using complex scenes (real-life pictures, head and shoulders view), are described. These relationships, called the visibility functions, express relationship between the relative amplitude accuracy required by the viewer and the masking functions. Perceptual, statistical, and contextual properties are inherent in them. Relationship between the visibility functions and some earlier measurements of psychovisual weighting functions are pointed out. In the second part, several adaptation strategies for the quantizer of a predictive DPCM coder are discussed. These include uniform as well as nonuniform quantizers with or without entropy constraints. These strategies are simulated on a computer, and the results are presented in the third part. The simulations indicate that, for the same picture quality, by using adaptive strategies, entropy reductions of about 30-50 percent are possible over nonadaptive techniques.

Positron-annihilation momentum profiles in aluminum: Core contribution and the independent-particle model

Abstract: The momentum profiles from positron annihilation with core electrons in Al have been observed with a recently developed two-detector Doppler-broadening technique. Independent-particle-model (IPM) calculations are in good agreement with the experimental results beyond twice the Fermi momentum and suggest the absence of strong positron-electron correlation effects in this region. Comparisons of calculations with properly normalized momentum profiles indicate that high-momentum core contributions may provide information on the nature of vacancy-type defects and self-trapping effects.

Pulse delay measurements in the zero material dispersion wavelength region for optical fibers.

Abstract: Subnanosecond pulses in the 1120-1550-nm region are generated by multiple-order stimulated Raman scattering in a small core single-mode silica fiber pumped by a Q-switched and mode-locked Nd:YAG laser at 1064 nm. These near ir pulses are injected into various km long test fibers, and relative time delay changes between different wavelengths are used to determine dispersion in a region where fiber material dispersion is small. Zero material dispersion has been observed in germanium and boron-doped single-mode and multimode est fibers.