bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of germanium-carbon alloys on silicon substrates by molecular beam epitaxy
Authors: Kolodzey J. , O’Neil P. A. , Zhang S. , Orner B. A. , Roe K. , Unruh K. M. , Swann C. P. , Waite M. M. , Shah S. Ismat
Published: July 2002

Metastable Ge1−yCy alloys were grown by molecular beam epitaxy as homogeneous solid solutions having a diamond lattice structure. The substrates were (100) oriented Si wafers and the growth temperature was 600 °C. We report on measurements of the composition, structure, lattice constant, and optical absorption of the alloy layers. In thick relaxed layers, C atomic fractions up to 0.03 were obtained with a corresponding band gap of 0.875 eV. These alloys offer new opportunities for fundamental studies, and for the development of silicon-based heterostructure devices.
Silicon on insulator Mach–Zehnder waveguide interferometers operating at 1.3 μm
Authors: Zhao C. Z. , Li G. Z. , Liu E. K. , Gao Y. , Liu X. D.
Published: July 2002

Mach–Zehnder (MZ) waveguide interferometers integrated on SOI (silicon on insulator) for 1.3 μm operation are studied on the basis of the large cross-section single-mode rib waveguide condition and the free-carrier plasma dispersion effect in Si wafer direct bonding SOI by back-polishing. And the MZ interferometers are fabricated by using KOH anisotropic etching. Their insertion losses and modulation depths are measured to be 4.81 dB and 98%, respectively, at the wavelength of 1.3 μm when a forward bias voltage applied to a p+n junction is 0.95 V and the active zone length of the MZ interferometers is 816.0 μm.
Linear thermal expansivity (1.5–300 K) and heat capacity (1.2–90 K) of Stycast 2850FT
Authors: Swenson C. A.
Published: July 2002

Linear thermal expansivities, α, have been obtained from 1 to 300 K for a sample of Stycast 2850FT epoxy that initially was cured at room temperature (X1), and then at 95 °C (X2). Heat capacities, Cp, were measured from 1 to 95 K for X2. The α’s for X1 and X2 are roughly equal to and twice that of copper near room temperature, but differ by as much as 10% at low temperature. The α’s approach those for copper near 100 K, but have a much smaller temperature dependence at lower temperatures. The different shapes of the α(T) and Cp(T) relations give a Grüneisen parameter, Γ, which decreases from 1.2 at 10 K to 0.5 at 95 K. Both α and Cp show apparent changes in lattice properties near 4.5 K, and a low temperature anomaly (Γ=3.5) that is characteristic of amorphous solids. Sample relaxation (shortening) effects from 300 K to 110 K were more serious for X2 than for X1. Because of differences related to the curing procedure, these data should be used only semiquantitatively in the design of experiments in which Stycast 2850FT will be used as a construction material. Possible Stycast-related problems with a capacitance dilatometer are discussed.
Picosecond dynamic response of nanoscale low-temperature grown GaAs metal-semiconductor-metal photodetectors
Authors: Joshi R. P. , McAdoo J. A.
Published: July 2002

We have successfully fabricated and tested nanoscale metal-semiconductor-metal (MSM) devices with picosecond temporal response for photomixer applications around 830 nm. Previous reports on nanoscale MSM devices were not at this longer wavelength. The ultrafast response resulted from a low capacitance of about 1 fF, the use of low-temperature grown (LT)-GaAs active layers, and ultrashort finger separations. A two-dimensional (2D) Monte Carlo model provided very accurate predictions of the transient MSM response. Values of the full width at half-maxima photocurrents for the 50 and 100 nm devices were found to be 1.4 and 2 ps, respectively, in keeping with experimental observations.
Further evidence for the quantum confined electrochemistry model of the formation mechanism of p−-type porous silicon
Authors: Jia L. , Zang S. L. , Wong S. P. , Wilson I. H. , Hark S. K. , Liu Z. F. , Cai S. M.
Published: July 2002

Two types of p− porous silicon (PS) were formed in HF solutions of different concentrations. One type with nanoscale (NS) dimensions of about 3 nm and the other with dimensions of about 5 nm. PS samples formed in the lower concentration of HF were anodized again in the higher concentration of HF and vice versa. The photoluminescence peak position and, thus, the size of NS units of PS were found to be related to the concentration of HF in which the PS is formed, independent of the forming time. The larger NS units of PS can be further electrochemically etched by anodization, while the smaller ones cannot. These results give a confirming evidence for the quantum confined electrochemistry model of the formation mechanism of PS based on the quantum confinement effect and classical electrochemical theory [S. L. Zhang, K. S. Ho, Y. T. Hou, B. D. Qian, P. Diao, and S. M. Cai, Appl. Phys. Lett. 62, 642 (1993)].
1/f electrical noise in epitaxial thin films of the manganite oxides La0.67Ca0.33MnO3 and Pr0.67Sr0.33MnO3
Authors: Rajeswari M. , Goyal A. , Raychaudhuri A. K. , Robson M. C. , Xiong G. C. , Kwon C. , Ramesh R. , Greene R. L. , Venkatesan T. , Lakeou S.
Published: July 2002

We report measurements of 1/f electrical noise in two hole doped manganite perovskite oxides, La0.67Ca0.33MnO3 and Pr0.67Sr0.33MnO3, which exhibit colossal magnetoresistance. The noise magnitude represented by the Hooge parameter is nearly 8 orders of magnitude larger than that observed in ordinary metals (and semiconductors) and nearly 5–6 orders of magnitude larger than that observed in epitaxial films of the perovskite oxide YBa2Cu3O7 in the normal state. The normalized noise spectral density increases with decreasing temperature below the resistivity peak, suggestive of the presence of additional low energy noise processes in the ferromagnetic metallic state.
Third-order nonlinear optical susceptibilities of the Langmuir–Blodgett films of octa-substituted metallophthalocyanines
Authors: Díaz-García M. A. , Cabrera J. M. , Agulló-López F. , Duro J. A. , de la Torre G. , Torres T. , Fernández-Lázaro F. , Delhaes P. , Mingotaud C.
Published: July 2002

The third-order susceptibilities χ(3)(−3ω:ω,ω,ω) of Langmuir–Blodgett films of a series of octasubstituted metallophthalocyanines have been measured by third-harmonic generation experiments at 1.064 and 1.904 μm. For the latter excitation wavelength a marked enhancement of the nonlinearity for the cobalt-containing compound has been found, and χ(3) values are one order of magnitude larger than for the other systems. The effect has been associated to the role of two-photon allowed metal (possibly d−d) transitions in the cobalt derivative.
An all-polymer charge storage device
Authors: Gofer Yossef , Sarker Haripada , Killian Jeffrey G. , Poehler Theodore O. , Searson Peter C.
Published: July 2002

Conducting polymers that can be switched between a neutral state and a doped state are of interest for charge storage applications. We report on the fabrication of an all-polymer charge storage device incorporating derivatized polythiophene films electropolymerized onto graphite-coated polymeric supports and a polymer gel electrolyte film. The devices exhibited discharge voltages of about 2.4 V and capacities of 9.5–11.5 mAh g−1. The elimination of any metallic components or liquids and the lightweight and flexible construction provides a unique approach for the fabrication of organic power sources.
Microscopic characterization of hot-electron spreading and trapping in SiO2 films using ballistic electron emission microscopy
Authors: Kaczer B. , Im H.-J. , Pelz J. P. , Wallace R. M.
Published: July 2002

Ballistic electron emission microscopy was used to study three-dimensional spreading and trapping of charge in buried, 10 nm thick SiO2 films following hot-electron injection at one location. The trapped charge was found to be distributed approximately uniformly across the thickness of the oxide, and spread laterally ∼50 nm, much more than the oxide film thickness. This large spreading can be explained by a combination of strong hot electron scattering and strong suppression in trapping efficiency at higher injected electron dose.
Single crystal superconductor nanowires by electrodeposition
Authors: Yi Ge , Schwarzacher Walther
Published: July 2002

Superconducting Pb wires (diameter∼50 nm) have been prepared by pulse electrodeposition in nanoporous membranes. Single crystal or polycrystalline nanowires may be grown selectively and reproducibly depending on the pulse parameters. Unexpectedly, the growth of single crystal wires requires a greater departure from equilibrium conditions (greater overpotential) than the growth of polycrystalline ones. The importance of controlling the crystal texture is demonstrated by measurements of the superconducting transition temperature Tc which give significantly different results for polycrystalline and single crystal nanowires.
Quasimagnetic clusters in icosahedral Al–Pd–Mn system
Authors: Nimori Shigeki , Tsai An Pang
Published: July 2002

We have studied the magnetization of the Al–Pd–Mn system using high-quality single crystals. Below room temperature, field-cooled magnetization exceeds zero-field-cooled magnetization. In addition, thermoremanent magnetization has disappeared at the same temperature. The results suggest the formation of quasimagnetic clusters in the Al–Pd–Mn system, although the relaxation of the thermoremanent magnetization was not observed. We discuss this anomalous magnetic phase in the Al–Pd–Mn system and refer to the origin of the behaviors.
High-energy (e, 2e) spectrometer for the study of the spectral momentum density of materials
Authors: Vos M. , Cornish G. P. , Weigold E.
Published: July 2002

A new spectrometer for the study of energy-resolved momentum densities is described. The (e, 2e) spectrometer uses a symmetric configuration and uses incoming energies up to 50 keV. Energy resolution and momentum resolution are 1.8 eV and 0.1 a.u., respectively. Compared to previous spectrometers this spectrometer has rather low levels of multiple scattering, and thus allows for more quantitative analysis of the data and/or the measurement of thicker samples.
Direct ab initio dynamics studies of N+H2↔NH+H reaction
Authors: Zhang Shaowen , Truong Thanh N.
Published: July 2002

Kinetics of the N+H2↔NH+H reaction have been studied using a direct ab initio dynamics method. Potential energy surface for low electronic states have been explored at the QCISD/cc-pVDZ level of theory. We found the ground-state reaction is N(4S)+H2→NH(3Σ−)+H. Thermal rate constants for this reaction were calculated using the microcanonical variational transition state theory. Reaction path information was calculated at the QCISD/cc-pVDZ level of theory. Energies along the minimum energy path (MEP) were then refined at the QCISD(TQ)/cc-pVTZ level of theory. The forward and reverse barriers of the ground-state reaction are predicted to be 29.60 and 0.53 kcal/mol, respectively. The calculated rate constants for both forward and reverse reactions are in good agreement with available experimental data. They can be expressed as k(T)=2.33×1014 exp(−30.83 (kcal/mol)/RT) cm3 mol−1 s−1 for the forward reaction and k(T)=5.55×108T1.44 exp(−0.78(kcal/mol)/RT) cm3 mol−1 s−1 for the reverse reaction in the temperature range 400–2500 K.
Relativistic dynamics of two spin-half particles in a homogeneous magnetic field
Authors: Datta Sambhu N. , Misra Anirban
Published: July 2002

Relativistic dynamics of two spin-1/2 particles in an external, homogeneous magnetic field is investigated here. The problem is important for a preliminary understanding of the effect of magnetic field on atoms and molecules at the relativistic level. The relativistic Hamiltonian is formulated in three distinct forms which involve the Bethe–Salpeter interaction, generalized Breit interaction and projected Breit interaction. The total pseudomomentum of the two-particle system is conserved in each case, and its components are distinct in the zero-charge sector. This permits the separation of the center of mass motion from the Hamiltonian of the neutral two-particle system. The resulting Hamiltonian operator describes the movement of the two particles in relative coordinates. It is further simplified by using suitable unitary transformations so as to reduce the one-particle operator for the first particle into a diagonal form. The effective equation of motion for the movement of the second particle in relative coordinates is then identified. A second set of transformations convert the two-particle relative Hamiltonian into a form where the one-particle operator for each spin-1/2 particle is completely diagonalized and separable into positive and negative energy states. The correspondingly transformed interaction operators can be written in an order by order expansion from which the odd terms are removable by using suitable Foldy–Wouthuysen type transformations in a systematic way. The resulting Hamiltonian operator reduces to previously known expressions when the magnetic field is switched off. Thus the two sets of transformations which convert the one particle parts completely into separable as well as diagonal forms also transform the interaction operator to generate terms consistently through order v2/c2. The field dependence lies entirely in the diagonalized one-particle parts, which is a consequence of the initial choice of interaction operators. Our results also include expressions corresponding to the interaction operator being projected. The Bethe–Salpeter and projected Breit cases lead to the same interaction operators for a hydrogen atom in the nonrelativistic limit. In the same limit the methodology directly yields the anomalous Zeeman interaction term, some correction to it, and terms which can account for nuclear magnetic resonance. All these terms are embedded in the final two-particle Hamiltonian operator. These, along with the previously known, field-independent, terms which describe the hyperfine interactions, can account for electronic and magnetic resonance spectroscopies on the basis of the same Hamiltonian.
Structural investigations of superconducting multilayers consisting of semiconducting materials
Authors: Erenburg A. I. , Bomze Yu. V. , Fogel N. Ya. , Sipatov A. Yu. , Fedorenko A. I. , Langer V. , Norell M.
Published: July 2002

There are rather exotic semiconductor superlattices (SL) consisting of monochalcogenides of Pb, Sn, and rare-earth metals which exhibit superconductivity at temperature as high as 6 K. Here we report the results of precision x-ray diffractometry and TEM investigations as well as Auger spectroscopy data obtained on some of the epitaxially grown superconducting semiconductor SLs. It is established that the essential features of the SL structure determining the appearance of superconductivity are the perfect single-crystallinity of the samples and the presence of continuous dense grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb which was observed in some cases does not correlate, according to experimental data, with the appearance of superconductivity.
Scanning-tunneling-microscope-assisted assembling of hydrogen-saturated silicon clusters on Si(111)-(7×7) surfaces
Authors: Bolotov Leonid , Uchida Noriyuki , Kanayama Toshihiko
Published: July 2002

Hydrogen-saturated silicon clusters of Si6H12 and Si8H18 deposited on Si(111)-(7×7) surfaces were intentionally assembled as islands 5–50 nm in diameter using scanning tunneling microscopy. Assembling occurs for both clusters when the tip–substrate bias voltage exceeds 3 V, irrespective of voltage polarity. Observed kinetics indicate that the mechanism is directional diffusion caused mainly by interaction between the electric field and the field-induced dipole moment of clusters. We demonstrate that the assembling process is applicable to the formation of artificial cluster nanostructures.
Origin of the unusual dependence of Raman D band on excitation wavelength in graphite-like materials
Authors: Sood A. K. , Gupta Rajeev , Asher S. A.
Published: July 2002

We have revisited the still unresolved puzzle of the dispersion of the Raman disorder-induced D band as a function of laser excitation photon energy EL in graphite-like materials. We propose that the D mode is a combination of an optic phonon at the K point in the Brillioun zone and an acoustic phonon whose momentum is determined uniquely by the double resonance condition. The fit of the experimental data with the double-resonance model yields the reduced effective mass of 0.025 me for the electron-hole pairs corresponding to the A2 transition, in agreement with other experiments. The model can also explain the difference between ωS and ωAS for D and D* modes, and predicts its dependence on the Raman excitation frequency.
The strain rate in evolutions of (elliptical) vortices in inviscid two-dimensional flows
Authors: Vosbeek P. W. C. , van Heijst G. J. F. , Mogendorff V. P.
Published: July 2002

In this paper the strain rate in evolutions of elliptical vortices in inviscid two-dimensional flows is considered. Previous work [Vosbeek et al., Phys. Fluids 9, 3315 (1997)] has revealed that there exists a relationship between the (in)stability of an elliptical vortex of uniform vorticity (Kirchhoff vortex) and the spatial distribution of the strain rate. It is examined here how the strain distribution evolves in time for unstable Kirchhoff vortices. Furthermore, it is shown that there also exists a relationship between strain and (in)stability of a vortex consisting of nested elliptical patches representing a more general vorticity distribution.
Electronic states of the phenoxyl radical
Authors: Radziszewski J. George , Gil Michal/ , Gorski Alexandr , Spanget-Larsen Jens , Waluk Jacek , Mróz Bogusl/aw J.
Published: July 2002

The phenoxyl radical and two of its isotopomers were investigated by UV-VIS and IR polarization spectroscopy of molecular samples immobilized in cryogenic argon matrices. Analysis of the combined electronic and infrared linear dichroism data led to determination of absolute transition moment directions and symmetry assignments for four low-lying excited electronic states. The bands observed at 16 000, 25 200, 33 900, and 41 800 cm−1 were assigned to A12, B12, A12, and B12 π–π* states, respectively. A very weak transition observed in the near-infrared close to 8900 cm−1 was assigned to an optically forbidden B22 n–π* state. The electronic transitions predicted by time dependent density functional theory (TD-UB3LYP/cc-pVTZ) were in good agreement with the observed transitions.
Stability, resolution, and tip–tip imaging by a dual-probe scanning tunneling microscope
Authors: Grube Holger , Harrison B. Craig , Jia Jinfeng , Boland John J.
Published: July 2002

A scanning tunneling microscope (STM) comprised of two mechanically and electrically independent probes is described. This dual-probe STM is capable of atomic resolution imaging with either tip. The two probes have five combined degrees of freedom, which allow them to be positioned together at the same surface location without alterations in tip shape. The positioning capabilities of the microscope are demonstrated by obtaining images with each tip of a unique location on a graphite surface. Stable tip–tip imaging is demonstrated by directly recording the tunneling current between probe tips and the sample.
Generation of vorticity and turbulent cooling of “hot channels” in gases
Authors: Kurzweil Yair , Livne Eli , Meerson Baruch
Published: July 2002

“Hot channels” (HC), formed by a rapid energy release in a gas, exhibit turbulent mixing and cooling, and their life time can be several orders of magnitude shorter than the molecular heat conduction time scale. Picone and Boris (1983) suggested that turbulent mixing results from the vorticity generation by the baroclinic mechanism during the early, shock-wave dominated stage of the dynamics. More recently, alternative scenarios of vorticity generation in the HCs were suggested. This work investigates the vorticity generation and turbulent cooling of the HCs by hydrodynamical simulations in two dimensions. Assuming small perturbations of the cylindrical shape of the energy release region, we follow the evolution of several HCs and determine their cooling time. We identify the details of vorticity generation which results in turbulent flow and fast mixing of the cold ambient gas into the HC. The simulations support, with some modifications, the Picone–Boris scenario. The simulated cooling time scale is in good agreement with experimental results, and the cooling process can be described as turbulent diffusion. The width of the mixed region shows dynamic scaling and compares well to experimental data.
An accurate MGGA-based hybrid exchange-correlation functional
Authors: Tao Jianmin
Published: July 2002

An accurate MGGA-based hybrid exchange-correlation functional is proposed based on Becke’s exact mixing approach. The presented functional makes use of the exact Ex and meta-generalized gradient approximations (MGGAs) to Exc containing local density, gradient, Laplacian, and kinetic energy density and is tested on 19 atomization energies. The results show that this functional, which contains only one parameter fit to atomization energies of molecules, improves or competes with the previously proposed accurate hybrid functionals in the literature for atomization energies of the molecules we tested, with an impressively small mean absolute error (mae) of 1.3 kcal/mol. This suggests that chemical accuracy may be achieved within Kohn–Sham density functional formalism without the use of many parameters fitted to atomization energies.
On the structure of polyelectrolyte solutions near the idealized counterion condensation threshold
Authors: Donley James P.
Published: July 2002

In a previous work we derived an equation for the radial distribution function g(r) for molecular liquids. It accounted for density correlations at both the monomer and molecular level. Here, it is shown that the theory can be simplified to a form than allows it to be solved easily by standard numerical methods. The theory is applied to charged, rodlike polymers with explicit counterions in solution near the idealized counterion condensation threshold (λB/b∼1, where λB and b are the Bjerrum and chain bond length, respectively). For densities above chain overlap, ρ*, it is found that the counterion cloud is diffuse about the polymer with a range on the order of the Debye–Hückel screening length. It is shown that the scaling with density of the first nonzero wave vector peak kmax of the polymer–polymer partial structure factor agrees with experiment and previous theory, with kmax∼ρν and ν≈1/2 and 1/3, for densities above and below ρ*, respectively. It is also found that the ratio of the full width at half maximum of the peak, Δk, to kmax is a minimum near ρ*. On the other hand, for the counterion–counterion partial structure factor it is difficult to find any sharp scaling of kmax though the apparent exponent for the semidilute and a large part of the dilute region is roughly approximated by ν=2/5. Asymmetric solutions are also discussed.
Nonlinear relaxation of polarization and optical susceptibility of dielectric particles under sudden change of external field direction
Authors: Jones R. B.
Published: July 2002

A dilute suspension of particles with permanent electric dipole moment and additional polarizability is subject to an applied electric field which suddenly changes its magnitude and direction. The time dependent behavior of the polarization and of the optical susceptibility is described by solving the Smoluchowski equation for the orientational distribution function. Because of the change of field direction the problem is not axially symmetric but by the proper choice of coordinate system the dynamical equations can be treated in the same manner as in the case of simple field switching on or field reversal. The two independent components of polarization and the nonvanishing elements of the optical susceptibility tensor are shown to be superpositions of exponentially decaying functions of time. A fast numerical method is used to calculate the spectrum of decay rates and the amplitudes of the decaying modes. The time dependence of the difference of principal indices of refraction which produces birefringence is calculated from the difference of characteristic values of the susceptibility tensor. The relaxation behavior of polar particles with additional polarizability is compared with that of simpler polar particles.
Oxygen-related minority-carrier trapping centers in p-type Czochralski silicon
Authors: Schmidt Jan , Bothe Karsten , Hezel Rudolf
Published: July 2002

We investigate minority-carrier trapping centers in p-type Czochralski (Cz) silicon by means of the quasi-steady-state photoconductance method. Boron and gallium-doped Cz silicon wafers of varying resistivities and oxygen contamination levels are examined. A clear correlation of the trap density with the interstitial oxygen concentration as well as with the boron and the gallium concentration is detected. The experimental data suggest that oxygen is a direct component of the defect complex responsible for the trapping, while the role of boron and gallium is not fully resolved. Using a simple single-level trapping model, we determine the energy level and the trapping/detrapping time constant ratio of the oxygen-related trapping center.
Dynamical x-ray diffraction from nonuniform crystalline films: Application to x-ray rocking curve analysis
Authors: Wie C. R. , Tombrello T. A. , Vreeland T.
Published: July 2002

A dynamical model for the general case of Bragg x-ray diffraction from arbitrarily thick nonuniform crystalline films is presented. The model incorporates depth-dependent strain and a spherically symmetric Gaussian distribution of randomly displaced atoms and can be applied to the rocking curve analysis of ion-damaged single crystals and strained layer superlattices. The analysis of x-ray rocking curves using this model provides detailed strain and damage depth distributions for ion-implanted or MeV-ion-bombarded crystals and layer thickness, and lattice strain distributions for epitaxial layers and superlattices. The computation time using the dynamical model is comparable to that using a kinematical model. We also present detailed strain and damage depth distributions in MeV-ion-bombarded GaAs(100) crystals. The perpendicular strain at the sample surface, measured as a function of ion-beam dose (D), nuclear stopping power (Sn), and electronic stopping power (Se) is shown to vary according to (1−kSe)DSn and saturate at high doses.
Formation of primary metal particles in evaporation chambers
Authors: Kim S. G. , Brock J. R.
Published: July 2002

Experimental and theoretical studies are reported on formation of primary particles during evaporation of metals into a host gas in an evaporation chamber. The developed theory explains present and previous experimental observations of this process. The dependence of primary particle size on host gas pressure, host-gas molecular weight and source temperature has been explained. Particle thermophoresis has been shown to cause formation of the so-called ‘‘smoke shell’’ observed surrounding the vapor source. Coalescence of primary particles has been shown not to be an important growth process for the primary particles, although for longer times primary particle coagulation produces the chains of primary particles observed experimentally.
Elastic properties of polycrystals with trigonal crystal and orthorhombic specimen symmetry
Authors: Johnson G. C. , Wenk H. R.
Published: July 2002

Equations for evaluating the effective elastic response of a polycrystalline aggregate exhibiting trigonal crystal symmetry (point groups 3̄m or 32) and orthorhombic sample symmetry (mmm) are presented. The analysis uses the spherical harmonic representation of the orientation distribution function (ODF) to describe the texture. The aggregate elastic constants are determined through the Voigt and Reuss procedures in which the crystallites’ stiffnesses and compliances, respectively, are averaged over all orientations, weighted by the ODF. Results are given in analytic form and require only five coefficients of the ODF for the exact representation of the aggregate characteristics. An example of the application of the results is given for two different textures predicted by the fully constrained Taylor theory for a calcite polycrystal.
On the equation of motion of the undeformed section of a Taylor impact specimen
Authors: Jones S. E. , Gillis Peter P. , Foster Joseph C.
Published: July 2002

In this paper a one-dimensional analysis is presented that leads to the appropriate equation of motion for the undeformed portion of a plastic, rigid rod after impact with a rigid anvil. This equation is used as a basis for deducing material properties of the rod material from post-test measurements.
Barrier height fluctuations in very small devices due to the discreteness of the dopants
Authors: Arnold D. , Hess K.
Published: July 2002

The spatial fluctuation in barrier heights of a planar-doped barrier due to the discreteness of the acceptors and their statistical distribution has been calculated by solving Poisson’s equation in three dimensions. Our model assumes a random distribution of spherical acceptor charges. Nonlinear Thomas–Fermi screening has been used to include the effect of free carriers and to determine their position-dependent concentration. At 4.2 K we find a range in barrier heights of 30 meV for a barrier with an average value of 0.206 eV. The method by which potential fluctuations broaden the energy distribution of ballistic electrons is illustrated by an example.
Ferroelectric microdomain reversal at room temperature in lithium niobate
Authors: Pendergrass L. L.
Published: July 2002

Spontaneous ferroelectric domain reversal in lithium niobate has been observed on a microscopic scale (∼0.1 μm) on standard surface acoustic wave (SAW) 128°-rotated Y-cut material due to pyroelectricity. Scanning electron micrographs are shown which clearly indicate the crystallographic shape of these reversed domains. Effective steps have been taken to eliminate this cause of domain reversal, and experiments were performed to measure the dc and rf fields necessary for this phenomenon to occur. In this paper we present our observation of spontaneous polarization reversal at room temperature. We discuss domain reversal in light of studies into the stress induced on the substrate by thin metal films. Theory behind this spontaneous polarization reversal and the threshold electric fields required to cause this reversal in the presence of thin metal film stresses is presented. From this discussion it is made clear that the threshold for ferroelectric microdomain reversal is within the bounds of the fields created by the pyroelectric effect (during standard thin film processing) but outside the normal operating range for SAW devices.
Optimization of the energy resolution of deep level transient spectroscopy
Authors: Nolte D. D. , Haller E. E.
Published: July 2002

We address the problem of optimizing the energy resolution of deep level transient spectroscopy, emphasizing techniques which are applicable for signals from defect levels which are either alloy broadened or stress broadened and which therefore produce nonexponential or multiexponential transients. We discuss the possibility of using a numerical inverse Laplace transform for which there is, in principle, no limit to the resolution. The sensitivity of the inversion algorithm to noise and to round-off error, however, practically limits the energy resolution to several percent of the deep level defect binding energy. This resolution is an improvement by about a factor of 2 over the best resolution possible from the standard rate window techniques for which there is a finite lower bound to the temperature resolution. This lower bound depends on the capture cross-section of the defect and the effective time constant of the rate window, but is independent of the defect energy.
Equivalent dipole moments of helical arrangements of small, isotropic, point-polarizable scatters: Application to chiral polymer design
Authors: Varadan Vasundara V. , Lakhtakia Akhlesh , Varadan Vijay K.
Published: July 2002

Electromagnetic waves can discriminate between objects of different handedness due to their transverse nature, which implies that the origin of chirality necessarily need not be molecular as in the case of optically active media. By embedding chiral microstructures in a host medium, effectively chiral composites can be constructed. The simplest microstructures are polymer strands which conform to left- or right-handed helices. Using a simple multiple-scattering theory, the electromagnetic signature of a polymer model, consisting of tiny point-polarizable spheres strung on helical strands, is examined. From the computed scattered field in the far zone, the equivalent electric and magnetic dipole moments of the helical ensemble are computed for different plane-wave incidence cases. Based on parametric studies of the equivalent moments, specific recommendations on the helical geometry are made for use in designing chiral composites.
Distribution functions and fluid variables in a semiconductor
Authors: Liboff Richard L. , Schenter Gregory K.
Published: July 2002

Fluid dynamic variables for charge-carrier transport in a semiconductor in the presence of an electric field are constructed from a recently derived distribution. This distribution is relevant to processes in a semiconductor where the deformation-potential interaction dominates. Fluid variables thus found are compared to those obtained from a shifted Fermi–Dirac distribution. In the limit of zero electric field both distributions give identical results. Analytic corrections to Fermi–Dirac variables are obtained by expanding the new variables about small electric field. Corrections at higher electric field are found numerically. Among other results it is found that at sufficiently high electric field, drift velocity grows insensitive to charge-carrier concentration. A discussion is included of the appropriate expression for electron temperature in a semiconductor.
Beam-propagation model of distributed-feedback lasers: Below-threshold behavior
Authors: Federighi Marco
Published: July 2002

We present a self-consistent model of distributed-feedback (DFB) laser diodes, including both axial and lateral variations of the optical field and the carrier concentration. The model is based on the beam-propagation method, and is used here to investigate the subthreshold properties of various DFB structures, with particular emphasis on the way the lateral waveguiding affects the lasing frequency and the threshold. We show how the lateral waveguiding affects the effective refractive index inside the laser cavity, thereby causing a shift in the lasing wavelength and breaking the symmetry of the spectrum. We also show that the presence of higher-order lateral modes in the optical field gives origin to a maximum in the dependence of the modal gain on the lateral refractive index step.
Electron wave optics in semiconductors
Authors: Gaylord T. K. , Brennan K. F.
Published: July 2002

Starting from fundamental principles, quantitative analogies between quantum mechanical electron waves in semiconductor materials and electromagnetic optical waves in dielectrics are presented. This, in turn, suggests many new classes of electron wave optical devices such as narrow-band superlattice interference filters. Phase effects associated with an electron wave are incorporated using an ‘‘electron wave phase refractive index’’ that is proportional to the square root of the product of the electron effective mass and the electron kinetic energy. It is shown that the amplitude of an electron wave is analogous to the electric field of a TE polarized electromagnetic wave (or to the magnetic field of a TM polarized electromagnetic wave) in a dielectric. Amplitude effects associated with an electron wave are incorporated using an ‘‘electron wave amplitude refractive index’’ that is proportional to the square root of the ratio of the kinetic energy to the effective mass. A simple expression for the critical angle for total internal reflection of an electron wave is developed. By analogy to the electromagnetic optical case, the total electron transmissivity and reflectivity of a semiconductor superlattice is presented. For illustration, an electron wave interference filter that is the counterpart of a multilayer quarter-wave stack thin-film optical interference filter is designed as a variable band gap and variable thickness semiconductor superlattice. Numerous new electron optical devices are suggested.
Rocking curve peak shift in thin semiconductor layers
Authors: Wie C. R.
Published: July 2002

A simple x-ray diffraction method for determining layer composition and mismatch is by measurement of the separation of peaks in a rocking curve. This method can only be used for layers with a thickness above a certain value. This minimum thickness can be significantly large for layers with a small lattice mismatch as in AlGaAs/GaAs or isoelectronic-doped III-V semiconductor layers. We give such an example and show that the interference between the diffraction amplitudes of the thin layer and that of the substrate is responsible for the peak shifting of the layer Bragg peak. When this peak shifting is significant, the kinematical diffraction theory and the peak separation method should not be used for the mismatch measurement, and only the dynamical diffraction theory simulation should be used. We present a criterion on the layer thickness, below which the dynamical theory simulation must be used. This thickness is inversely proportional to the lattice mismatch and does not depend on the diffraction geometry, wavelength, and substrate material.
The properties of the electromagnetic missile
Authors: Shen Hao-Ming , Wu Tai Tsun
Published: July 2002

The electromagnetic field from the current in a circular disk and its Poynting vector off the axis of propagation have been investigated both analytically and experimentally. These studies show that the electromagnetic (EM) missile has the following properties: (1) The waveform of the EM field when propagating remains similar in shape but diminishes in size. (2) The transverse distribution of energy around the axis is stable; i.e., when the longitudinal distance increases, the transverse pattern of the energy remains the same. This ‘‘plane-wave beamlike’’ property is different from continuous-wave (cw) radiation, in which the energy pattern is like that of a spherical wave. (3) The transverse energy pattern has a cusp on the axis. This property is also different from cw radiation. With cw, the radiation has a flat top on the axis of the main lobe.
Gettering of donor impurities by V in GaAs and the growth of semi-insulating crystals
Authors: Ko K. Y. , Lagowski J. , Gatos H. C.
Published: July 2002

Vanadium added to the GaAs melt getters shallow donor impurities (Si and S) and decreases their concentration in the grown crystals. This gettering is driven by chemical reactions in the melt rather than in the solid. Employing V gettering, we were able to grow reproducibly semi-insulating GaAs by horizontal Bridgman and liquid-encapsulated Czochralski techniques, although V did not introduce any midgap energy levels. The compensation mechanism in these crystals was controlled by the balance between the native midgap donor EL2 and residual shallow acceptors. Vanadium gettering contributed to the reduction of the concentration of shallow donors below the concentration of acceptors. The present findings clarify the long-standing controversy on the role of V in achieving semi-insulating GaAs.
An effective medium treatment of the transport properties of a Voronoi tesselated network
Authors: Vrettos Nikolas A. , Imakoma Hironobu , Okazaki Morio
Published: July 2002

The present work stresses the significance of the effective medium theory in the computation of the macroscopic transport coefficients from the microgeometry of porous media. The porous ‘‘material’’ is simulated as a two-dimensional network of interconnected slits of irregular shape and a random distribution using the Voronoi–Delaunay tesselation technique. The calculation procedure for the macroscopic transport coefficients is based on two concepts, the first one being the approximation of the microscopic field by a smooth field (SFA), and the second one being the average of the network random parameters to a mean/effective value by the effective medium theory (EMT). For the latter we apply an improved version of the EM equation derived for regular lattices by Kirkpatrick [Rev. Mod. Phys. 45, 4 (1973)]. This equation takes into account the irregularity in the slit-length distribution and is applicable on both regular and irregular lattices. The EMT/SFA results of the improved version for ordinary diffusion (apparent diffusivity) are in very good agreement with the numerical ones.
Quantitative separation of mechanisms for power dissipation in solar cells by photoacoustic and photovoltaic measurements
Authors: Flaisher Harvey , Wolf Martin , Cahen David
Published: July 2002

Photoacoustics is used as a calorimetric method in conjunction with electrical measurements to determine which mechanisms are involved in the conversion of most of the absorbed radiation to thermal energy in (mainly Si p-n) solar cells. The major mechanisms that are identified and quantified include local cooling, near the junction of the cells. Quantification is made possible by the use of a model for internal energy fluxes in a photovoltaic cell, which takes into account the different spatial distributions of heat generated by photogenerated and injected carriers. The experimental results agree well with calculations based on the model also in the case of thin-film CdS/CuInSe2 cells.
Thermodynamics and kinetics of intracellular ice formation during freezing of biological cells
Authors: Toner Mehmet , Cravalho Ernest G. , Karel Marcus
Published: July 2002

A physicochemical model based on the modified classical heterogeneous nucleation theory was proposed to analyze the ice formation inside biological cells during freezing. According to this model, intracellular ice formation (IIF) may be catalyzed either by the plasma membrane via the effects of the external ice on the plasma membrane, called surface-catalyzed nucleation (SCN), or by the intracellular particles, called volume-catalyzed nucleation (VCN), depending on the freezing conditions. The model for IIF was coupled with the model describing the kinetics of water transport to predict the thermodynamic state of the cytoplasm.

A method to estimate the nucleation frequency from the observed probability of nucleation was suggested. This method was based on the assumption that each cell has the same heteronucleating particles with identical properties to alter the nucleation kinetics in an identical way. This allowed the correlation of the experimental probability of IIF with the nucleation rate. It was suggested that the uncertainties in the experimental conditions could be minimized if cells were cooled at rapid rates to prevent water transport during freezing. The model was applied to mouse oocytes at metaphase II. Correlation of the theory with the experimental kinetics of the cumulative fraction of IIF revealed preexponentional factors of 3.56×108 (m2 s)−1 and 1.84×1050 (m3 s)−1, and exponential factors of 4.6×109 and 1.08×1012 K5 for SCN and VCN, respectively. The contact angle between the internal surface of the plasma membrane and the ice cluster for SCN, θ, was determined to be 35° for freezing in isotonic solutions (285 mosm) increasing linearly to 47° in hypertonic solutions (1035 mosm). SCN was active between ∼−5 and −20 °C in the presence of external ice. It is suggested that the presence of external ice alters the plasma membrane through its chemical, electrical, mechanical, ionic, and thermal effects, to make it an energetically favorable site for heterogeneous nucleation. For VCN, lower and upper limits for θ and R were found to be 60°<θ<100° and 10−7<R<4×10−7 cm, respectively. VCN was active between ∼−30 and −35 °C.
Elimination of slips on silicon wafer edge in rapid thermal process by using a ring oxide
Authors: Cho Byung-Jin , Kim Choong-Ki
Published: July 2002

The slips on a silicon wafer edge which normally occur during rapid thermal process have been eliminated by forming an oxide ring on the wafer-back surface, thereby reducing the light reflectance from the silicon wafer edge. A simple model which gives the optimum ring oxide thickness and width has been proposed. From the model, it has been found that about 3000 Å thickness of ring oxide gives the best thermal compensation effect. The effect of nonuniform irradiation on the wafer has been incorporated in the model by introducing light power uniformity (LU) factor. In case of 3300-Å-thick, 7-mm-wide ring oxide and 0.86–0.88 of LU, the temperature variation on a 3-in. wafer is within ±4 °C at 1137 °C, and the slip is completely eliminated.
Optical constants of AlxGa1−xAs
Authors: Jenkins David W.
Published: July 2002

A simple, analytical, semiempirical model is presented for the optical constants of AlxGa1−xAs including the index of refraction. The model is tailored to the energy range of 1.0–3.0 eV, the operating range for many AlxGa1−xAs-based devices and incorporates the important electronic transitions in the solid which affect light propagation and absorption. The parameters of the model are simple functions of composition allowing for ready computation of any of the optical constants, n, k, α, ε1, or ε2, for any energy E (or wavelength λ) of light between 1.0 and 3.0 eV (400–1200 nm) and any composition x.
Defect mechanisms in degradation of InGaAsP long-wavelength edge-emitting light-emitting diodes
Authors: Chu S. N. G. , Nakahara S. , Luther L. C. , Krautter H. W.
Published: July 2002

We report on degradation mechanisms in 1.3-μm wavelength edge-emitting light-emitting diodes aged for 1000 h at 150 °C and 150 mA studied by transmission electron microscopy. During this degradation, three types of defect structures are generated at the interface between InGaAsP active layer and n-InP buffer layer along the light emitting stripe: (i) 1/2〈100〉{100} faulted extrinsic dislocation loops formed by condensation of point defects, similar to those previously observed in degraded channeled substrate buried heterostructure lasers, (ii) long 1/2〈101〉 dislocation clusters developed along the active stripe, and (iii) 1/3[111] and 1/3[11̄1] faulted Frank loops developed from condensation of point defects onto the 1/2[101] dislocation through dissociative reactions: 1/2[101]→1/6[12̄1]+1/3[111] and 1/2[101]→1/6[121]+1/3[11̄1]. The driving force for these reactions is discussed.
Influence of probe absorption in the photothermal deflection technique
Authors: Bertolotti M. , Fabbri L. , Fazio E. , Li Voti R. , Sibilia C. , Leakhou G. , Ferrari A.
Published: July 2002

The photothermal deflection technique has been used to study thermal nonlinearity in samples of glasses doped with CdSxSe1−x microcrystals. The effect of the influence of probe beam absorption on deflection signal has been investigated from a theoretical point of view. Measurements have been performed showing the possibility of detecting very small changes of the probe beam absorption coefficient produced by the pump intensity.
Model for dry etching of silicon
Authors: Kojima Masahiko , Kato Hisao , Gatto Mitsuru , Morinaga Shigeyoki , Ito Nobuyoshi
Published: July 2002

An etch rate equation for the dry etching of silicon in the absence of ion bombardment is derived from a model in which transport of atomic fluorine is rate determining. This equation indicates that the etch rate follows the Arrhenius rule and is proportional to the fluorine-atom concentration in the gas phase. The theoretical etch rate is shown to be in good agreement with experimental data.