bluuebunny/crossref_metadata_2025 huggingface dataset:
Metastable and equilibrium phase formation in sputter-deposited Ti/Al multilayer thin films
Authors: Lucadamo G. , Barmak K. , Lavoie C. , Cabral C. , Michaelsen C.
Published: July 2002

The sequence and kinetics of metastable and equilibrium phase formation in sputter deposited multilayer thin films was investigated by combining in situ synchrotron x-ray diffraction (XRD) with ex situ electron diffraction and differential scanning calorimetry (DSC). The sequence included both cubic and tetragonal modifications of the equilibrium TiAl3 crystal structure. Values for the formation activation energies of the various phases in the sequence were determined using the XRD and DSC data obtained here, as well as activation energy data reported in the literature.
Liquid-state theory derivation of surface accessible solvation potential models for proteins
Authors: Freed Karl F.
Published: July 2002

Applying a very simple approximation to the solvation potential theory of Chandler and co-workers enables the derivation of a surface accessible area solvation potential model that has been used to describe the solvation free energies of proteins and small organic molecules in aqueous solution and that has been used as a portion of an implicit solvent approach to simulations of protein dynamics, folding, and aggregation. The derivation indicates that the more general solvation potential theory will provide a fruitful basis for systematically devising significant improvements of the surface accessible area models.
Association–dissociation in solution/Long-time relaxation prediction by a mode coupling approach
Authors: Naumann Wolfgang
Published: July 2002

It is shown that the correct power-law decay predictions for the long-time relaxation of reversible diffusion-influenced reactions, which were recently derived by Gopich, Ovchinnikov and Szabo with a hydrodynamic approach [Phys. Rev. Lett. 79, 922 (2001)] can also be obtained with an appropriate mode-coupling approximation after rephrasing the problem into a memory function approach. This is explicitly demonstrated for association–dissociation in the extreme situations of the target and trapping model. By constructing the memory function approximations via the solution of self-consistent equations for the relevant time-correlation functions, this method paves the way for a systematic study of higher reactant concentration effects. Moreover, the fact that the correlation decay equations derived have the typical form for stochastic approaches facilitates comparison with other theoretical concepts.
Ultraviolet–visible and infrared spectroscopic study of Langmuir–Blogett films of a charge transfer complex based on 2-dodecyl-7,7,8,8-tetracyanoquinodimethane and copper
Authors: Majumder Tapas Pal , Ikegami Keiichi
Published: July 2002

Electronic absorption spectra of Langmuir films of 2-dodecyl-7,7,8,8-tetracyano-quinodimethane (C12TCNQ) mixed with CuI, prepared on pure water, indicated charge-transfer (CT) reactions in these films. The films could be deposited onto solid substrates as Langmuir–Blogett (LB) films and optically characterized. The CT degree of C12TCNQ in these LB films was estimated at −1 from the red shifts of C=C and C≡N stretching modes. The polarized infrared spectra and x-ray diffraction patterns suggested a bimolecular-layer structure for the present films, in contrast to the alkyldicyanoquinonediimine–CuI mixed LB films, for which a monomolecular-layer structure was previously proposed. This distinct difference reflects the effect of the number of cyano groups in acceptor molecules on the structure of LB films based on this kind of transition metal-organic acceptor complexes.
Interband transitions in molecular-beam-epitaxial AlxGa1−xAs/GaAs
Authors: Aubel J. L. , Reddy U. K. , Sundaram S. , Beard W. T. , Comas James
Published: July 2002

Interband transition energies for AlxGa1−xAs layers grown by molecular-beam epitaxy (MBE) techniques have been determined using the electrolyte electroreflectance (EER) technique. The observed data fit quadratic relations for E0, E0+Δ0, E1 and E1+Δ1 to describe variations of energy with composition. Although the x values were not accurately known, the internal consistency of the data is excellent. Given a single bowing parameter we show that accurate values of x can be determined. The EER technique can provide x values with an accuracy better than 0.02 and information on changes in x as small as 0.002. It is thus ideally suited for studying MBE materials.
Laser-intensity-modulation method: A technique for determination of spatial distributions of polarization and space charge in polymer electrets
Authors: Lang Sidney B. , Das-Gupta D. K.
Published: July 2002

A new method for measuring the spatial distribution of combined polarization and space charges in poled polymer electrets has been developed. The technique, called the laser-intensity-modulation method (LIMM), is based upon measurement of the pyroelectric current generated by the electret when it is heated with a laser beam which is intensity-modulated at frequencies varying from 200 to 2500 Hz. A special analytical technique is used to transform the current-frequency measurements to spatial distributions. The method gives high resolution and is very stable with respect to measurement errors. The technique can be easily implemented in a laboratory with conventional equipment. The advantages and limitations of LIMM are illustrated by means of calculations based upon simulated data. Experimental results on a number of laboratory and commercially poled samples are presented.
Effect of nonuniform work function on space-charge-limited current
Authors: Adler E. A. , Longo R. T.
Published: July 2002

The effect of a nonuniformly emitting cathode on a space-charge-limited diode is analyzed. The first part of the analysis is a calculation of the extent to which inhomogeneities in the space charge are reduced by the motion of electrons in directions parallel to the cathode surface. It was found that, in typical cases, outside of a small layer adjacent to the cathode, the space-charge density is virtually constant on surfaces parallel to the cathode. Because of this the total diode current was found to be given by Child’s law even when the cathode has patches whose emission is saturated.
Surface stoichiometry, structure, and chemisorption on silicon nitride studied by direct recoiling, x-ray photoelectron spectroscopy, and Auger electron spectroscopy
Authors: Jo Yang-Sun , Schultz J. Albert , Tachi Shinichi , Contarini Salvatore , Rabalais J. Wayne
Published: July 2002

The surface stoichiometry, structure, and chemisorption of O2 and H2O have been investigated for surfaces of silicon nitride prepared by (i) chemical vapor deposition (CVD) and (ii) low-energy N+2 ion nitridation of a Si(100) crystal. The technique of time-of-flight (TOF) analysis of directly recoiled (DR) particles combined with x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Ar+ depth profiling have been used for characterization. The results show that CVD Si3N4 is terminated in a silicon rich layer upon which a one monolayer oxide is formed from O2 exposure. The initial O2 chemisorption rate is fast and saturation occurs at 15 L exposure, with the kinetics fitting a one-site model for which the modified sticking coeffient is S/Nf=3.5×10−16 cm2/atom. Chemisorption of H2O produces a surface H:O ratio of 1:1 as determined by DR. Nitridation of a Si(100) surface by low-energy N+2 produces a thin nitride film with XPS binding energies identical to those of CVD Si3N4. Depth profiling indicates nitride film thicknesses of 3 and 8 Å for 0.5- and 3.0-keV N+2 ion bombardment, respectively, in agreement with ion ranges estimated from LSS theory. Defect controlled incorporation of oxygen and carbon during N+2 bombardment is observed.
Study of Mo-, Au-, and Ni-implanted molybdenum laser mirrors by multiple angle of incidence spectroscopic ellipsometry
Authors: Snyder Paul G. , Rost Martin C. , Bu-Abbud George H. , Oh Jae , Woollam John A. , Poker David , Aspnes D. E. , Ingram David , Pronko Peter
Published: July 2002

Multiple angle of incidence spectroscopic ellipsometric data show that implantation of 150-keV molybdenum ions into polished molybdenum laser mirrors causes microscopic surface smoothing, and that most of the microscopic roughness is removed by a fluence of 5×1015 cm−2. Implantation of Au at 1 MeV significantly increases the microscopic roughness, and also changes the bulk optical properties. 3-MeV Ni ion implantation causes only small changes in the surface and bulk properties. A dielectric film, probably a hydrocarbon, is found to condense on the mirrors in a laboratory atmosphere.
Effects of dopants and excess silicon on the oxidation of TaSi2/polycrystalline silicon structures
Authors: Liu R. , Murarka S. P. , Pelleg J.
Published: July 2002

The oxidation rates of tantalum disilicide/polycrystalline silicon (polysilicon) structures have been studied as a function of dopants in the polysilicon and as a function of excess silicon in the silicide. Our results indicate that the oxidation rate is significantly affected: B in the polysilicon enhances the oxidation while As retards the rate. P doping by ion implantation showed little effect, but doping by PBr3 diffusion retarded the oxidation. Excess silicon in the silicide retards the oxidation. These phenomena can be explained by a mechanism in which the kinetics is controlled by the silicon out-diffusion through the silicide. Analogous to crystalline silicon, the oxidation rate can be enhanced by high concentration of dopants which create point defects that aid the silicon diffusion. The situation is complicated by the presence of grain boundaries. In the case of As and excess silicon, diffusion may be hindered by dopants segregated to the grain boundary, resulting in slower oxidation.
Recombination in solar-grade polycrystalline silicon
Authors: McLarty P. K. , Ioannou D. E. , Shea S. P.
Published: July 2002

Deep level transient spectroscopy (DLTS) and scanning electron microscope-electron beam induced current techniques were used to study recombination in p-type polycrystalline silicon, grown by a casting technique to form ingots with a nominal doping level of 1016 acceptors/cm3. Both Schottky diodes and n+p mesa structures were used for the measurements. In general, very complex DLTS spectra were obtained, and several hole and electron traps were identified. It was found that the hole traps were grain-boundary related. By contrast, the electron traps were found to exist in all regions of the ingot, they were more concentrated in areas near the crucible and at the bottom of the ingot, and they appeared to be growth related.
Diffusion of ion-implanted Sb in SiO2
Authors: van Ommen A. H.
Published: July 2002

A study of the diffusion of ion-implanted Sb in SiO2 by means of Rutherford backscattering spectrometry showed that the major fraction of the implanted Sb is immobile in SiO2 upon annealing at temperatures as high as 1200 °C in N2, O2, or O2/H2O ambients. When the oxide is encapsulated by an Si3N4 film during annealing, no diffusion is observed at all. To mobilize implanted Sb, extra oxygen has to be supplied from the annealing ambient. The diffusion of Sb exhibits a lower activation energy in a N2 or O2 ambient (1.3 eV) than in an O2/H2O ambient (2.3 eV). The present result for the O2 ambient differs spectacularly from the data reported in the literature which gave an activation energy of 8.75 eV and a pre-exponential factor which was 27 orders of magnitude lower than value reported here. The diffusion of implanted Sb exhibits some similarity to that of implanted As. There is also evidence that Sb is incorporated on both silicon and oxygen sites of the SiO2 network, i.e., in a way similar to As.
ZnO transducers on InSb for low-temperature ultrasonics
Authors: Castonguay M. , Fernandez B. , Guillon F. , Cheeke J. D. N.
Published: July 2002

A reproducible method for the fabrication of ZnO transducers on InSb substrates is described. These transducers are being used for ultrasonic attenuation measurements over a wide low-frequency range (90–600 MHz) at very low temperatures(0.1–4.2 K). They have been characterized by their x-ray diffraction patterns, their insertion loss, and their contribution to the acoustic attenuation in InSb at low temperatures.
Spherical lens as a launcher of electromagnetic missiles
Authors: Wu Tai Tsun , King Ronold W. P. , Shen Hao-Ming
Published: July 2002

A uniform dielectric sphere, under transient excitation by a point source, is found to be capable of launching an electromagnetic missile, i.e., there is a direction where the decay of the total energy transmitted to a receiver is slower than r−2. The conditions for the presence of an electromagnetic missile have a natural interpretation, which in turn leads to a classification of electromagnetic missiles according to the singularities of differentiable maps in two dimensions.
Spherical aberrations of multipoles
Authors: Shao Zhifeng , Crewe A. V.
Published: July 2002

It is demonstrated in this article that for an electromagnetic 2n−pole system with n=odd, the only existing symmetric aberration which could be used to correct the spherical aberrations of a round lens is the order of 2n−3. Thus, a 2n−pole corrector can only be used to cancel the aberration of round lenses of order 2n−3.
Modeling of hydrogen implantation into graphite
Authors: Möller W. , Scherzer B. M. U.
Published: July 2002

A new theoretical framework has been developed which is applicable to the implantation and ion-induced release of hydrogen isotopes in graphite. It provides a physical basis and a refinement of the predictions of the simple model of local saturation and mixing. The model treats the trapping at defects and a local release of trapped atoms by nuclear knock-on. Ion deposition and damage functions are taken from trim simulations. The detrapped atoms may become retrapped or recombine to molecules, which then are transported to the surface by fast molecular diffusion, and subsequently released. By the choice of suitable rate constants in the model calculations, different experimental findings for the implantation and high-fluence self-reemission of deuterons in graphite may be explained consistently. Examples cover the saturation as a function of temperature and energy, depth profiles, gas reemission, thermal desorption, and effects of predamage.
Acoustic nonlinearity of a piezoelectric crystal surface
Authors: Solodov Igor Yu.
Published: July 2002

A general approach to acoustic nonlinear problems using partial-mode field decomposition at the piezoelectric crystal surface has been developed in terms of a nonlinear reflection method. A quadratic approximation solution of the reflection problem for a free boundary of a nonlinear piezocrystal shows that the second harmonic field is determined by numerous cross and self-interactions of four fundamental reflected waves. These interactions produce 2ω elastic and electrical sources both in the bulk and at the surface of the crystal because of the second- and third-order elastic, piezoelectric, dielectric, etc., material parameters. Phase matching of the harmonics produced by self-mode interactions is a cause of surface acoustic wave (SAW) second harmonic generation, with the amplitude growing along the boundary. The analytical expression for the SAW second harmonic in a piezocrystal was derived by solving the nonlinear reflection problem. The efficiency of SAW second harmonic generation is determined by the contribution of all four of the partial waves of the fundamental SAW and is proportional to the material acoustic nonlinear parameter values, mode slowness, and their respective amplitudes and phases. Nonlinear SAW steady-state wave patterns in piezocrystals are predicted to be different from conventional N- or ‘‘sawtooth’’ types.
Amplification of surface modes in type II semiconductor superlattices
Authors: Bakshi P. , Cen J. , Kempa K.
Published: July 2002

We study the drift-induced instability of surface modes in type II semiconductor superlattices, including both carrier-phonon and carrier-carrier scatterings. Stability-boundary curves are obtained for typical physical parameters. Amplification is possible for experimentally achievable drift velocities.
Dynamic characteristics of semi-insulating current blocking layers: Application to modulation performance of 1.3-μm InGaAsP lasers
Authors: Cheng W. H. , Renner D. , Hess K. L. , Zehr S. W.
Published: July 2002

The dependence of current-voltage (I-V) characteristics on Fe-doped semi-insulating (SI) InP layer thickness has been investigated experimentally. The I-V characteristics exhibit nonlinear behavior with ohmic, transition, and space-charge-limited regimes. An approximate circuit model of the buried crescent laser which describes the dynamic characteristics of the SI current blocking layers is presented. It is shown that for a 5-μm-thick SI layer, a very high resistivity of 4.9×108 Ω cm and a very low capacitance of 1 pF are obtained at the typical operating voltage for laser diodes of 1–2 V. Thus, semiconductor lasers with Fe-doped SI InP current blocking layers offer great promise for achieving both wide modulation bandwidth and high-power operation.
The effect of nonstoichiometry and polarity of the (111) plane on microtwin formation in ion-implanted GaAs
Authors: Shiota I. , Nishizawa J. , Oyama Y.
Published: July 2002

To study the effect of nonstoichiometry and polarity of the (111) plane on recrystallization of an implantation-induced amorphous layer, arsenic-implanted (100) and (111) A and B GaAs wafers were annealed under vacuum and specified arsenic vapor pressures. It has been found that the formation of microtwins is suppressed by annealing at the specified arsenic pressure. This suggests that microtwin formation is closely related to the arsenic deficiency or arsenic vacancies. Moreover, the result of direct implantation into (111) A (Ga) and B (As) surfaces indicates that the twins would preferentially grow on the (111) A plane, which has a smaller solid-phase regrowth rate than that of the (111) B plane.
Liquid junctions for characterization of electronic materials. V. Comparison with solid-state devices used to characterize reactive ion etching of Si
Authors: Fantini M. C. A. , Shen Wu-Mian , Tomkiewicz Micha , Gambino J. P.
Published: July 2002

Impedance and modulation spectroscopy techniques were used to characterize the damage to Si promoted by reactive ion etching (RIE). We compare in this paper our previous results on liquid junction interfaces with Schottky barrier device configurations Ti/Si and Al/Si and metal-oxide semiconductors Al/SiO2/Si. Important device parameters, such as the barrier height, obtained from the impedance data will be compared with current-voltage measurements. The results cannot be explained only by considering the thermionic emission theory. For the CHF3/Ar and CF4 RIE treatments, the observed barrier lowering confirms our results with liquid junctions, supporting the existence of positive charges on the surface of the damaged Si. For the CClF3/H2 RIE treatment the results are consistent with the existence of a porous polymer layer on the semiconductor surface. In terms of sensitivity of the dielectric properties to the RIE treatment, the liquid junction is the most sensitive, followed by SB devices, with the MOS configuration exhibiting the least sensitivity.
Measurements of twisted nematic cell gap by spectral and split-beam interferometric methods
Authors: Yang K. H. , Takano H.
Published: July 2002

The transmissive Fabry–Perot interference peaks of a filled twisted nematic (TN) cell as a function of wavelength, from 400 to 600 nm and from 1100 to 2000 nm, have been used for the determination of the cell gap. Theoretical formulations have been developed to calculate the effective indices of refraction including the dispersive effect, as well as the twist geometry of the liquid-crystal medium for both the ordinary and extraordinary waves. When the light passes through the TN medium an even number of times caused by the interfacial reflections, the equations governing the effective indices of refraction are well behaved. For an odd number of passes, the validity of these equations is restricted to a region adjacent to the Mauguin limit. The cell gaps were derived from the measured effective optical paths and the calculated indices of refraction, which include the dispersive effect with or without the twisted deformation of the liquid-crystal medium. A general equation has also been obtained to derive the TN cell gap from the effective optical paths, which are measured by using the split-beam interferometric method.
Electrical and optical properties of neutron-irradiated GaP crystals
Authors: Kawakubo T. , Okada M.
Published: July 2002

Infrared absorption, electron paramagnetic resonance (EPR) spectra, and the electrical resistivity of semi-insulating liquid encapsulated Czochralski GaP crystals irradiated by fast neutrons with a dose of 7.6×1018 n cm−2 have been studied. The electrical resistivity decreases with irradiation from 106 Ω cm to 1.5 kΩ cm. The temperature dependence of resistivity at low temperature is fitted to exp(b/T1/4). The strong continuous optical absorption extends to 0.32 eV and its tail spreads until 0.12 eV. The EPR spectrum exhibits a broad singlet at 77 K and a doublet with five line structures at room temperature, which is attributed to antisite defects PGa. The strong infrared absorption begins to be annealed at 150 °C, and the EPR broad singlet decreases with anneals in the same temperature range. The strong infrared absorption is assumed to arise from interstitial phosphorus clusters. A discussion is given concerning the species responsible for the EPR singlet.
The production of vibrationally excited hydrogen molecules
Authors: Zhu Yi-Fei , Huang Yu-Lin , Arepalli Sivaram , Gordon Robert J.
Published: July 2002

The partial pressure of vibrationally excited hydrogen has been measured in a flowing gas using the technique of vacuum ultraviolet laser-induced fluorescence. In most of the studies H2 (v=1) was generated with a microwave discharge. For this source we measured in detail the effects of the partial pressures of H2 and Ar, the flow rate, the microwave power, and the construction of the flow tube. A more limited study was also performed using a thermally heated flow tube.
Detection of ground-state atomic hydrogen in a dc plasma using third-harmonic generation
Authors: Celii F. G. , Thorsheim H. R. , Butler J. E. , Plano L. S. , Pinneo J. M.
Published: July 2002

Third-harmonic generation (THG) was used to monitor ground-state atomic hydrogen H(1s 2S1/2) in a dc plasma system. A 364.6 nm laser beam focused through H2 or CH4/H2 plasmas induced THG at 121.5 nm, near the atomic hydrogen 2p 2PoJ→1s 2S1/2 Lyman-α transition. Both the intensity and frequency shift of the excitation spectra exhibited dependence on the plasma power. Absolute H atom concentration was estimated by comparing the frequency shift to that obtained in a calibrated microwave discharge flow system. The sensitivity was ∼4×1013 cm−3 (100 ppm). The measured atomic hydrogen densities were substantially less than in other diamond chemical vapor deposition methods and may explain the lower diamond deposition rates obtained with dc plasma systems of this type.
Characterization of rf-sputtered InN films and AlN/InN bilayers on (0001) sapphire by the x-ray precession method
Authors: Kistenmacher T. J. , Bryden W. A. , Morgan J. S. , Poehler T. O.
Published: July 2002

Thin films of InN and AlN/InN bilayers have been deposited on (0001) sapphire at a variety of substrate temperatures by reactive rf-magnetron sputtering. For all films, the c axis of the metal nitride film parallels the c axis of the sapphire substrate, and the transmission x-ray precession method has been employed to study the nature and azimuthal coherence of the in-plane heteroepitaxy. For the InN films, a mixture of textured and epitaxial grains persists up to ∼200 °C and solely epitaxial grains are observed at higher substrate temperatures, to a maximum adherence temperature of 525 °C. In contrast, the AlN/InN bilayers are epitaxial at substrate temperatures of 50 °C and above, and a uniform deposition can be retained up to 650 °C. These fundamental differences in adhesion, structure and morphology of the InN films and the AlN/InN bilayers on (0001) sapphire are also clearly reflected, for example, in the variation in electrical mobility with substrate deposition temperature.
Lattice location and optical activity of Yb in III–V semiconducting compounds
Authors: Kozanecki A. , Groetzschel R.
Published: July 2002

The location of Yb atoms in indium (InP, Ga0.51In0.49P) and gallium (GaAs, Al0.35Ga0.65As, GaAs0.6P0.4) III–V semiconducting compounds was investigated. It has been shown that Yb atoms occupy substitutional lattice positions only in indium compounds. Solid solubility of Yb in InP is of the order of 1020 cm−3, as estimated from Rutherford backscattering spectroscopy (RBS) measurements and is high by a factor of 2.5 than in 50% GaInP alloy. In gallium compounds the substitutional fraction of Yb is much below the detection limit of the RBS method, even in the case of elevated temperature (250 °C) implantation. It is suggested that optical activity of Yb is related to its substitutional location in the lattices and/or to the character of the YbP bond. The reasons for the material dependent behavior of Yb atoms in III–V crystal hosts are discussed.
Observation of hysteresis, transients, and photoeffects in the electrical properties of ZnSe/GaAs heterojunctions
Authors: Ayyar S. G. , Colak S. , Marshall T. , Khan B. , Cammack D.
Published: July 2002

Electronic transport properties of a Au/n-ZnSe/n+GaAs structure are examined by studying the results of current-voltage (I-V) and capacitance-voltage (C-V) measurements at temperatures between 77 and 300 K in the dark and under illumination. C-V data at 77 K in the dark show a large hysteresis indicating long-time constants for reaching a steady state. The data also show a large shift in the peak capacitance towards higher-bias values as compared to room-temperature measurements. All 77-K capacitance values in the dark are considerably lower than both the ones at 77 K under illumination and the ones at room temperature. These results suggest charging of the lattice-mismatch-induced extended defect states in ZnSe near the heterojunction interface as well as macroscopic effects such as barrier-limited electron currents flowing into the ZnSe potential well. Furthermore, as additional evidence, 77-K capacitance transients in the data show two distinct time constants which are due to an initial charge rearrangement in ZnSe followed by a relaxation caused by photogenerated currents. I-V characteristics at 77 K show a steep rise in current at about 2.5 V on the Au, both in the dark and under illumination. This indicates either an effective bias-dependent barrier lowering or field-assisted tunneling/recombination current paths at the heterojunction.
Reverse current-voltage characteristic of almost ideal silicon p-n junctions
Authors: Pellegrini B.
Published: July 2002

The forward current-voltage characteristic of silicon p-n junctions obtained by means of suitable gettering processes is well described by the Shockley diffusion theory. Their reverse characteristic, however, according to the preparation procedure and final annealing temperature, may deviate from both the Shockley model and the generation-recombination (GR) theory of Sah, Noyce, and Shockley owing to an ohmic contribution proportional to the reverse-bias voltage. Such an ohmic current has previously been ascribed to pure-generation centers of the depletion layer. The invoked generation mechanism is a field-assisted thermal emission (FATE) of electron-hole pairs from donor-acceptor twins. Here, we show that both GR and ohmic currents may be ascribed to a single type of defect, namely, four-state traps (FST) which may be empty, or filled by an electron or a hole, or both, and to different Frankel–Pole effects for electrons and holes in their emission process from such traps. The model requires determining the kinetic equations of the FSTs, solving them in the depletion layer for steady state and reverse bias, and computing the Frankel–Pole effect both of a single ion and an ion dipole. In agreement with the experimental results, the new approach also shows (i) that, apparently, the electric field acts and, through the FATE mechanism, induces the ohmic current only when it is greater than a critical field, (ii) that the constant contribution of the reverse current may be much lower than the Shockley saturation current, (iii) that the activation energy is equal for both the GR and the ohmic current, and (iv) that FSTs generate no extra charge in the depletion layer. In more general terms, the model accounts well for all the experimental results of the new ideal and almost ideal silicon p-n junction obtained by means of new gettering processes.
Highly ordered and conducting thin film of pentacene doped with iodine vapor
Authors: Minakata Takashi , Nagoya Ichiro , Ozaki Masaru
Published: July 2002

We have demonstrated the iodine doping of vacuum-deposited pentacene (PEN) film which showed characteristic changes in structure and electrical conductivity. The iodine-doped film exhibited a high electrical conductivity of 110 Ω−1 cm−1, which was 11 orders of magnitude larger than that of as-deposited film, and a high electrical anisotropy of 108. The structural changes by the iodine doping were studied by means of x-ray diffraction method, ultraviolet-VIS absorption spectroscopy, and FT-infrared spectroscopy. These results revealed that iodine molecules were intercalated between the layers of PEN molecules to form charge transfer complex of PEN-iodine with highly ordered structure.
Determination of local magnetic moments of 5d impurities in Fe detected via spin-dependent absorption
Authors: Wienke R. , Schütz G. , Ebert H.
Published: July 2002

The spectroscopy of spin-dependent absorption using circularly polarized synchrotron radiation is a new method to study the magnetic structure of multicomponent systems. We demonstrate for substitutional 5d transition-metal impurities in an iron host that from L2,3 spin-dependent absorption signal information on the local magnetic moment, carried by the d electrons of the absorbing atom, can be deduced. The experimentally determined local magnetic moments of the 5d impurities in Fe are in good agreement with the theoretical results based on the spin-polarized relativistic Korringa–Kohn–Rostoker–Green function method of band-structure calculation. The presented data are−to our knowledge−the first experimental proof of the change from antiferro- to ferromagnetic coupling between Os and Ir, which has been predicted by the above-mentioned calculations.
Optical transmission in undoped crystalline α-Al2O3 grown by several techniques
Authors: Evans B. D.
Published: July 2002

Transmission spectra from 1.5 to 8.8 eV of undoped crystalline α-Al2O3 synthesized by five growth techniques are compared. Salient features in this below-band-gap region are attributed to extrinsic effects of absorption by impurities and color centers, and to Rayleigh scattering. Analysis suggests material with substantially less absorption is possible.
The determination of exciton levels in superlattices by the photovoltaic effect
Authors: Hlávka J. , Ploog K.
Published: July 2002

A possibility to determine exciton energy levels in superlattices in a simple way based on the contactless photovoltaic effect measurement is demonstrated. Good agreement between results obtained from the photovoltaic effect and from the optical reflectivity measurements was found.
Laser spectroscopy of ground-state scandium ions in a high-pressure metal halide discharge II. Optogalvanic spectroscopy
Authors: Kramer Jerry
Published: July 2002

Pulsed laser irradiation of ground-state scandium ions in a vertically burning 60-Hz high-pressure metal halide discharge decreased the voltage required to sustain the discharge. The scandium ions were excited with a pulsed dye laser at 363.1 nm from the a3D2 spin-orbit component of the ground state (68 cm−1 above the ground state) to the z3F03 state. The optogalvanic Sc+ signals were compared with the optogalvanic signals from excitation of an excited state of neutral Hg at 365.0 nm (6 3P2 → 6 3D3). The Sc+ and Hg optogalvanic signals had the same polarity and very similar temporal response. The optogalvanic signals from Sc+ and Hg had a similar nonlinear dependence on laser energy. The comparison between Sc+ and Hg suggests that a common mechanism produces the optogalvanic signal for ions and neutrals in this high-pressure discharge. The radial and axial dependencies of the Sc+ optogalvanic signals were compared with saturated laser-induced fluorescence (LIF) measurements. The radial optogalvanic profiles were compressed relative to the LIF profiles, but the axial optogalvanic profiles showed good agreement with the LIF values. From 355 to 370 nm there was good correspondence between the optogalvanic and emission spectra. The transitions in this wavelength range are from Sc+ and Hg. The optogalvanic signals for Sc+ were coupled with LIF data for Sc+ and Sc to calculate the shape of the electron density profile and as an independent check of the temperature dependence of the Saha equation.
Optimized selective mixing of a GaAs/GaAlAs quantum well for the fabrication of quantum wires
Authors: Vieu C. , Schneider M. , Benassayag G. , Planel R. , Birotheau L. , Marzin J. Y. , Descouts B.
Published: July 2002

We present the structure of a GaAs/GaAlAs quantum well, which can promote strong mixing rates upon high-dose implantation, with good recovery of the electronic properties after annealing. This structure is employed to fabricate quantum-well wires by Ga+ masked implantation. Low-temperature photoluminescence measurements reveal large lateral modulations of the effective band gap (≳178 meV), and small lateral interdiffusion lengths (10 nm). A simple calculation shows that one-dimensional quantization energies between 11 and 20 meV can be expected in these structures.
Electric-field dependence of acceptor-level binding energies in strained SiGe and InGaAs quantum-well structures
Authors: Loehr John P. , Singh Jasprit
Published: July 2002

The acceptor energies for strained SiGe (on Si) and InGaAs (on GaAs) quantum wells are calculated from a 4×4 k⋅p band structure that includes the effects of strain and electric fields. Both center- and edge-doped cases are examined. The theory shows marked changes in the acceptor energies with both strain and electric field. The wide variation in binding energy for the edge-doped quantum wells may provide a mechanism for tunable far-infrared detectors.
Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In0.2Ga0.8As/GaAs multiple quantum wells
Authors: Rich D. H. , George T. , Pike W. T. , Maserjian J. , Grunthaner F. J. , Larsson A.
Published: July 2002

The spatial distribution of the long-wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) has been investigated using cathodoluminescence (CL) imaging and spectroscopy. The CL spectra show defect-induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy showed that [110]-oriented misfit dislocations occur primarily at the substrate-to-MQW and GaAs capping layer-to-MQW interfaces. The large spatial variation of the luminescence intensities indicates that the DLDs observed in CL images are caused by the presence of nonradiative recombination centers occurring in the MQW region located between the interface misfit dislocations. This study provides new information describing the origin and nature of DLDs and differs from previous models which have regarded the electronic nature of dislocation cores as the primary mechanism for inducing DLD radiative contrast in luminescence imaging of strained InGaAs/GaAs.
Disordering in 69GaAs/71GaAs isotope superlattice structures
Authors: Tan T. Y. , You H. M. , Yu S. , Gösele U. M. , Jäger W. , Boeringer D. W. , Zypman F. , Tsu R. , Lee S.-T.
Published: July 2002

Undoped 69GaAs/71GaAs isotope superlattice structures grown by molecular beam epitaxy on n-type GaAs substrates, doped by Si to ∼3×1018 cm−3, have been used to study Ga self-diffusion in GaAs by disordering reactions. In the temperature range of 850–960 °C, the secondary ion mass spectrometry (SIMS) measured Ga self-diffusivity values showed an activation enthalpy of 4 eV, and are larger than previously compiled Ga self-diffusivity and Al-Ga interdiffusivity values obtained under thermal equilibrium and intrinsic conditions, which are characterized by a 6 eV activation enthalpy. Characterizations by SIMS, capacitance-voltage (C-V), and transmission electron microscopy showed that the as-grown superlattice layers were intrinsic which turned into p type with hole concentrations of ∼2×1017 cm−3 after annealing, because the layers contain carbon. Dislocations of a density of ∼106–107 cm−2 were also present. However, the factor responsible for the presently observed larger Ga self-diffusivity values appears to be Si out-diffusion from the substrate, which was determined using C-V measurements. Out-diffusion of Si decreases the electron concentration in the substrate which causes the release of Ga vacancies into the superlattice layers where they become supersaturated. This Ga vacancy supersaturation leads to enhanced Ga self-diffusion in the superlattice layers.
Magnetic anisotropy in epitaxial fcc Co/Cu (1 1 13)
Authors: Oepen H. P. , Schneider C. M. , Chuang D. S. , Ballentine C. A. , O’Handley R. C.
Published: July 2002

Magnetization behavior of ultrathin (3 ML) epitaxial Co(001) films on Cu(1 1 13) terraced surfaces was studied in situ using magneto-optic Kerr effect. The films show easy magnetization along the steps. Unusual hard axis Kerr loops (H perpendicular to the steps and in the surface plane) are observed near room temperature and evolve at higher temperature (127 °C) toward the soft magnetic behavior more typical of the easy axis loops observed on the same films. A model is postulated for the behavior that includes a fourfold in-plane anisotropy and a uniaxial in-plane anisotropy of possibly magnetoelastic (ME) or magnetostatic (MS) origin.
Analysis of defect energy levels in the limit of low emission rates
Authors: Lamp C. D. , Yang S. , Gangopadhyay S.
Published: July 2002

A new deep level transient spectroscopy (DLTS) analysis method is presented. This method operates in the limit of large emission times which leads to a major simplification of standard DLTS calculations. This analysis can be applied as data is collected, and yields defect energy levels completely independent of the thermally activated cross section. This modification is especially useful in separating closely spaced energy levels not resolvable by standard analysis methods. Specifically, in the modified technique the shallower energy level is reliably determined. As an example, the new technique is applied to the energy level splitting of EL2 in n-type gallium arsenide induced by uniaxial stress. Our preliminary results for stress applied along <100≳ indicates that C3v and Td symmetries are incompatible with this defect.
Shock wave analysis of laser assisted particle removal
Authors: Lee S. J. , Imen K. , Allen S. D.
Published: July 2002

Laser assisted particle removal (LAPR) is an innovative laser cleaning technique which can remove various particles from solid surfaces via laser induced explosive evaporation of a chosen energy transfer medium, e.g., water. An Ar+ ion continuous-wave laser (488 nm) was used to study the CO2 laser pumped explosive evaporation of water adsorbed on a Si substrate. The probe laser beam was parallel to the sample surface at different displacements and interacted with the ejected material upon pulsed CO2 laser irradiation in analogy with the time resolved laser beam deflection experiments on laser induced vaporization of copper by Guo et al. [Opt. Commun. 77, 381 (1990)]. Using CO2 laser energies which are much greater than the LAPR thresholds, we observed the generation and propagation of a shock wave at supersonic speeds followed by a water vapor/aerosol/particle cloud at a much slower speed. From the evolution of the shock wave, the total conversion efficiency of the incident laser beam into the shock wave has been determined using a self-similar approximation.
Interlayer coupling and spin polarization of the nonmagnetic layers in Fe/Cu and Fe/Ag CMFs
Authors: Xu Y. B. , Lu M. , Jin Q. Y. , Hu C. , Miao Y. Z. , Zhai Y. , Bie Q. S. , Zhai H. R. , Dunifer G. L. , Naik R. , Ahmad M.
Published: July 2002

Compositionally modulated films (CMFs) of Fe/Cu and Fe/Ag, made by RF sputtering, were investigated with ferromagnetic resonance (FMR), nuclear magnetic resonance (NMR), and magneto-optical (MO) spectroscopy. Evidence of spin polarization of the nonmagnetic layers, related to the interlayer coupling, was obtained. Direct evidence of the penetration of an exchange field and the spin polarization of the conduction electrons in the nonmagnetic Cu layers was obtained in Fe/Cu CMFs by NMR. Spin-wave resonances were observed in Fe/Cu CMFs in FMR experiments, and the spin wave was found to be sustained by both the Fe and Cu sublayers. The MO spectra of Fe/Ag CMFs indicated that the conduction electrons of Ag are spin polarized and give an additional MO activity.
Growth and magnetic dynamic scaling of ultrathin ferromagnetic films: Fe/Au(001)
Authors: He Y.-L. , Liew Y.-F. , Wang G.-C.
Published: July 2002

Dynamic responses of the magnetic hysteresis of a few monolayer thick smooth Fe films on a Au(001) surface were measured by the surface magneto-optic Kerr effect technique. The Fe films were prepared by molecular beam epitaxy technique and their growth and structures were characterized in situ by high resolution low energy electron diffraction with angular profile measurement. The shape of the hysteresis loop changes with the frequency Ω and the amplitude H0 of the applied periodic magnetic field. Over a range of frequency (<225 Hz) at a fixed amplitude and at room temperature, the area of the hysteresis loops follows a scaling form. The best fit of a scaling function to the frequency-dependent area gives exponents a=0.58±0.03 and b=0.24±0.02. For low values of frequency (<25 Hz) and amplitude (<400 Oe), the area of hysteresis loops measured at room temperature reduces to a power law A∝Ha0Ωb with a=0.59±0.07 and b=0.26±0.05. Our measured exponents and the scaling function are consistent with recent results of theory and Monte Carlo simulations.
Site-selective spectroscopy of Ho3+:KYF4
Authors: Peale R. E. , Weidner H. , Summers P. L. , Chai B. H. T.
Published: July 2002

Site-selective photoluminescence spectroscopy of Ho3+ ions in single-crystal KYF4, measured with a Fourier-transform spectrometer at 80 and 2 K sample temperatures, gives evidence for two dissimilar classes of crystal-field sites. Three similar sites contribute to each class giving a total of six sites in agreement with recent crystallographic results. Analysis of crystal-field splittings indicates that only one of the classes can contribute to Yb→Ho upconversion green lasing in (Yb,Ho):KYF4.
Crystallization-induced stress in phosphorus-doped amorphous silicon thin films
Authors: Miura Hideo , Okamoto Noriaki
Published: July 2002

The effect of phosphorus doping on the crystallization-induced stress of silicon thin films is investigated experimentally using a scanning laser microscope. Though the intrinsic stress of the phosphorus-doped amorphous silicon films and the crystallization-induced stress of the films do not change, regardless of the doped phosphorus concentration, the final residual stress of the film after full annealing depends on the phosphorus concentration. The final stress decreases by increasing the dopant concentration. In addition, phosphorus doping lowers the crystallization temperature of the amorphous silicon thin films. The existing phosphorus at the interface of the film and the base oxide film is found to change the crystallization mechanism and the magnitude of the stress developed.
Interface roughness characterization using x-ray standing waves
Authors: Kawamura Tomoaki , Takenaka Hisataka
Published: July 2002

An x-ray standing wave technique is used to characterize the interface roughness of multilayer structures. To include the interface roughness effect, standing wave fields of x-rays in multilayers are described in terms of a modified optical matrix. Analysis of a Ni/C multilayer with a period of about 54 Å by the x-ray standing wave technique showed a ratio changing of the nickel layer, suggesting the diffusion of nickel atoms and formation of a nickel-carbon complex. Interface roughness was estimated from the calculation at bulk density of nickel to be about 10 Å, suggesting the diffusion of nickel.
Thin-film multilayer interconnect technology for YBa2Cu3O7−x
Authors: Wellstood F. C. , Kingston J. J. , Clarke John
Published: July 2002

The construction of microelectronic circuits from high-transition-temperature (Tc) superconductors requires techniques for producing thin-film wires, insulating crossovers, and vias (window contacts) between wires. Together, these three components form a superconducting interconnect technology. The challenges encountered in developing such a technology for high-Tc superconductors involve factors associated with the materials, the circuits and the fabrication techniques. The use of pulsed laser deposition in conjunction with shadow mask patterning, photolithographic pattern definition, acid etching, ion-beam etching, and surface cleaning to produce multilayer interconnects from YBa2Cu3O7−x (YBCO) is discussed. These processes have been used to construct a variety of passive high-temperature superconducting components and circuits, including crossovers, window contacts, multiturn coils, and flux transformers. Integrated magnetometers incorporating superconducting quantum interference devices, multichip modules with semiconductor die bonded to YBCO interconnect structures, and analog-to-digital converters have also been successfully demonstrated.
A model for electrical conduction in metal-ferroelectric-metal thin-film capacitors
Authors: Sudhama C. , Campbell A. C. , Maniar P. D. , Jones R. E. , Moazzami R. , Mogab C. J. , Lee J. C.
Published: July 2002

Time-zero current-voltage characteristics and time-dependent current behavior of metal-ferroelectric-metal (Pt-PZT-Pt) capacitor structures have been studied. Under constant-voltage stressing, the current density through the 1500-Å-thick lead-zirconate-titanate (PZT) film exhibits a power-law dependence on time, with the exponent (∼0.33) independent of temperature and voltage. Electrode material dependence of current density indicates that the conventional model of trap-limited single-carrier injection over nonblocking contacts is inadequate to explain the time-zero current. A change in top electrode material from Pt to In leads to the observation of work-function-driven Schottky contacts between the metal and ferroelectric. The current-voltage characteristics fit a two-carrier injection metal-semiconductor-metal model incorporating blocking contacts, with distinct low- and high-current regimes (PZT is assumed to be p-type and trap-free in this model). Temperature-dependent I-V measurements indicate a Pt-PZT barrier height of 0.6 eV and an acceptor doping level of ∼1018 cm−3 in PZT. The implications of this model on the optimization of ferroelectric capacitors for dynamic random access memory applications are discussed.