bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of dielectric response functions for conducting materials
Authors: Niklasson G. A.
Published: July 2002

In this paper we study the dielectric response of materials which display a dc conductivity at low frequencies. A framework based on the generalized Master equation and time dependent transition rates is argued to be applicable to disordered materials and exhibits an interesting fractal dynamics. The relationship of this framework to the ‘‘Distribution of transition rates’’ theory and the models put forward by Ngai [Comments Solid State Phys. 9, 141 (1980] and by Dissado and Hill [J. Chem. Soc. Faraday Trans. 2, 80, 291 (1984)] is discussed. Model calculations of the dielectric response by means of the above-mentioned three theories are carried out and are compared to experimental results on Al2O3 and SiOxNy thin films. The theories can be distinguished only when the exponent of the power law at high frequencies is close to unity. The Dissado–Hill model is in better agreement with experiments than the other theories, but important discrepancies can easily be seen.
Plasma-implosion-driven, photoionization-pumped, soft x-ray laser targets
Authors: Hussey T. W. , Matzen M. K. , McGuire E. J. , Dalhed H. E.
Published: July 2002

Designs are proposed, based on a series of one-dimensional calculations, for layered, hollow cylindrical targets to be placed on the axis of an imploding, hollow Z-pinch plasma that can create the approximate plasma conditions, as well as radiation spectrum, for a photoionization pumped, Ne-like recombination laser. The lasant must reach the Ne-like state and be at the appropriate density at the same time that the photoionizing pump radiation is present, placing severe constraints on designs for such targets. Target designs are further constrained by the fact that the 3s-2p resonance line, which depopulates the lower lasing state, must not be highly trapped and by the fact that the upper lasing state must not be collisionally depopulated. We find that hollow, cylindrical targets consisting of a few-micron-thick CH strongback, coated on the inside with a thin layer of Ni lasant and on the outside with an Al converter layer, can be optimized to achieve appropriate conditions for lasing and modest levels of gain.
Characterization of subsurface damage in GaAs processed by Ga+ focused ion-beam-assisted Cl2 etching using photoluminescence
Authors: Taneya M. , Sugimoto Y. , Akita K.
Published: July 2002

Subsurface damage in GaAs processed by a Ga+ focused ion-beam-assisted Cl2 etching is studied by photoluminescence (PL) measurement. The PL intensity of the processed sample decreased to (1)/(30) – (1)/(40) compared to that of the unprocessed sample. The recovery of PL intensity by a step removal of the damaged layer is observed as a function of the removed layer thickness. The removal of a 0.7-μm-thick surface layer enables the PL intensity to be recovered perfectly, which leads to the postulate that the damaged layer thickness is 0.7 μm at least, which is much larger than the ion range (about 0.01 μm). The recovery of PL intensity is analyzed on a one-dimensional model in the direction normal to the sample surface. Computer simulations of PL intensity are carried out. The calculated result fully explains the experimental PL intensity recovery as a function of the removed layer thickness, which gives the profile of subsurface damage in the sample.
Comparison of the χ(3) values of crystalline and amorphous thin films of 4-butoxy-carbonyl-methyl-urethane polydiacetylene at 1.06 and 1.3 μm
Authors: Hsu C. C. , Kawabe Y. , Ho Z. Z. , Peyghambarian N. , Polky J. N. , Krug W. , Miao E.
Published: July 2002

The third-order optical nonlinear susceptibilities of single-crystalline and spin-coated amorphous 4-butoxy-carbonyl-methyl-urethane polydiacetylene (red form) thin films were measured by third-harmonic generation. The third-harmonic intensities were calculated as a function of incident angle by solving the propagation equation of the electromagnetic field for a two-layer system with one nonlinear media. From the best fit of the calculated and experimental data, the ( χ(3)) values of the spin-coated amorphous film (χ(3)p) were found to be (9.6±1.0)×10−12 esu and (2.0±0.2)×10−11 esu at 1.064 and 1.319 μm, respectively, while the χ(3) values of the crystalline ( χ(3)pc) were (4.9±0.5)×10−11 esu and (1.0±0.07)×10−10 esu at 1.064 and 1.319 μm, respectively. A factor of five increase in the χ(3) of the crystalline film is attributed to the orientation effect of the polymer chains. The enhancement of the nonlinearity at 1.319 μm may be due to either three-photon or two-photon resonant effects.
Orientation ratio of sputtered thin-film disks
Authors: Mirzamaani M. , Johnson K. , Edmonson D. , Ivett P. , Russak M.
Published: July 2002

Thin films of CoPtCr, with 15-nm underlayers of Cr, were deposited by dc magnetron sputtering on NiP-plated aluminum substrates that were textured (anisotropic roughness) or chemically etched (isotropic roughness). The relationships between the structure and the in-plane magnetic properties were investigated. Under specific sputtering conditions, the Cr underlayer and the magnetic layer developed 〈100〉 and 〈11.0〉 crystallographic preferred orientations, respectively. The combination of 〈11.0〉 crystallographic preferred orientation and substrate texture resulted in an in-plane uniaxial magnetic anisotropy, i.e., higher squareness and coercivity parallel to the texture lines. The films with the random crystallographic orientation exhibited the in-plane magnetic isotropy on both the textured and the chemically etched Al/NiP substrates. The sputtering parameters and the substrate conditions that can lead to the development of in-plane magnetic anisotropy were specified.
A comparison of the reaction of titanium with amorphous and monocrystalline silicon
Authors: Raaijmakers Ivo J. M. M. , Kim Ki-Bum
Published: July 2002

This paper describes investigations into the reactions occurring between Ti and monocrystalline Si (x-Si) or sputter-deposited amorphous Si (a-Si). Samples were structured so as to have the same Ti layer in contact with both crystalline and amorphous Si, and were thus ideally suited to compare the reactions in the same sample. Reactions were mainly investigated with cross-sectional transmission electron microscopy, but also with chemical characterization techniques such as Rutherford backscattering and Auger electron spectroscopy. We demonstrated that the reaction between Ti and x-Si or a-Si proceeded very similarly at low temperatures (≤450 °C). In both cases an amorphous silicide layer was observed to grow. Reaction rates were found to be nearly equal, even if some impurities were present on the x-Si surface prior to Ti deposition. One important difference was noted between the reactions, however. The reaction with a-Si was associated with Kirkendall void formation, while these voids were absent in the reaction with x-Si. We argued that the absence of voids in the case of the reaction with x-Si is due to a higher vacancy mobility in x-Si than in a-Si. At higher temperatures (500 °C), marked differences were observed in the reaction of Ti with crystalline or amorphous Si. These differences could be explained satisfactorily from thermodynamical considerations involving the heat of crystallization of a-Si. We postulated the existence of two kinetically competing pathways to go from the low-temperature configuration, where an amorphous silicide grows, to the stationary situation where diffusion-controlled growth of crystalline silicides occurs. The first one is crystallization of the amorphous silicide. The second one is the formation of a compositionally different crystalline silicide at the amorphous-silicide/x-Si interface. This postulate of kinetically competing pathways was used successfully to explain observations of amorphous and crystalline silicide growth in the Ni and Co/Si systems.
Spin-polarized scanning tunneling microscope: Concept, design, and preliminary results from a prototype operated in air
Authors: Johnson Mark , Clarke John
Published: July 2002

We describe the concept of the spin-polarized scanning tunneling microscope. It consists of a ferromagnetic tip with saturated magnetization from which spin-polarized electrons tunnel into a ferromagnetic sample which has its axis of magnetization aligned with that of the tip. When the magnetization of the sample is alternated periodically from parallel to antiparallel, a portion of the tunnel current is predicted to oscillate at the same frequency, with an amplitude linearly proportional to the average tunnel current. The construction of a prototype microscope, operated in air, is described. When the sample magnetization is alternated, a modulation of the tunnel current is observed at the same frequency. This signal satisfies criteria developed to characterize spin-polarized tunneling. Spurious signals are also observed and their probable origins identified. A number of improvements are suggested that should eliminate the spurious effects.
X-ray measurements of ion mixing in amorphous Si/Ge artificial multilayers
Authors: Park B. , Spaepen F. , Poate J. M. , Jacobson D. C. , Priolo F.
Published: July 2002

Amorphous Si/Ge artificial multilayers with a repeat length around 60 Å have been partially mixed with 1.5-MeV Ar+ ions at temperatures in the range 77–673 K. The diffusive component of the square of the mixing length, obtained by subtracting out the ballistic contribution, does not depend on the dose rate at a given dose, and shows an Arrhenius-type temperature dependence with activation enthalpies between 0.13 and 0.22 eV. Possible mechanisms for migration and annihilation processes of defects are discussed to understand these low activation enthalpies.
Mapping induction distributions by transmission electron microscopy (invited)
Authors: Chapman J. N. , McVitie S. , Hefferman S. J.
Published: July 2002

The Foucault and differential phase contrast modes of transmission electron microscopy are described and their uses for mapping induction distributions in magnetic thin films compared. The former is simpler to implement but the latter can provide quantitative information to much higher spatial resolution. Examples are given of instances where each is most suitable. Foucault images recorded during in situ magnetizing experiments provide valuable information on the magnetization process in small regular particles while the detailed magnetization distribution close to walls in soft magnetic films is best investigated using modified differential phase contrast microscopy.
Characterization of a remote hydrogen plasma reactor with electron spin resonance
Authors: Johnson N. M. , Walker J. , Stevens K. S.
Published: July 2002

Gas phase chemical reactions, of importance in the deposition of amorphous semiconductors, were studied in a remote hydrogen plasma reactor with electron spin resonance (ESR). The following reactant pairs (and their observed room-temperature rate coefficients) were characterized: (1) H+SiH4(2.4×1011 cm3 mole−1s−1), (2) D+SiH4 (2.1×1011 cm3 mole−1 s−1), and (3) H+C2H2 (1.2×1010 cm3 mole−1 s−1). The interpretation of these coefficients in terms of primary and secondary gas phase reactions is discussed, and the values are compared where possible with previously published data. In addition, the paramagnetic centers of the silicon-based film that is deposited in situ during the ESR measurement can now be microscopically identified in light of recent studies.
Characterization of interface states at III-V compound semiconductor-metal interfaces
Authors: Burstein L. , Bregman J. , Shapira Yoram
Published: July 2002

Surface photovoltage spectroscopy (SPS) has been used for direct measurements of the extrinsic surface states within the band gaps of p-InP (110) and (100) and n-GaAs(110) before and after Al and Au deposition. The observed metal-induced surface states are found to pin the Fermi level at monolayer coverages at Ev+0.83 eV for Au/p-InP(110), Ev+1.10 eV for Al/p-InP(110), Ec−0.94 eV for Au/n-GaAs(110), and Ec−0.80 eV for Al/n-GaAs(110). The Au/Al/p-InP(110) structure, studied for the first time using SPS provides evidence of strong Al clustering upon the InP surface. Chemically etched and UHV-cleaved p-InP surfaces and Au interfaces are also compared. The correlation between the observed energy state positions and electrically measured Schottky barrier heights is discussed.
Magnetization reversal in permalloy particles: Micromagnetic computations
Authors: Fredkin D. R. , Koehler T. R. , Smyth J. F. , Schultz S.
Published: July 2002

Magnetization distributions in a rectangular permalloy particle with an 8:1 aspect ratio are presented. When a magnetic field is applied along the easy axis of the particle, there is considerable change in the vortex pattern as the applied field varies, but the moment is nearly constant. Magnetization reversal is sudden. When a field is applied oblique to the easy axis the magnetization is rather uniform in most of the particle. When an in-plane field is applied exactly along the hard axis, a complex domain structure is formed, so that the remanence is zero, in agreement with experiment. Formation of this state requires that the applied field exceed a critical value. Upon application of an easy axis field to this complex state, the domain structure is shifted so that the magnetization increases linearly with the applied field.
Electroreflectance study of a symmetrically coupled GaAs/Ga0.77Al0.23As double quantum well system
Authors: Huang Y. S. , Qiang H. , Pollak Fred H. , Lee Johnson , Elman B.
Published: July 2002

Electroreflectance spectroscopy has been used to study a GaAs/Ga0.77Al0.23As symmetrically coupled double quantum well system as a function of externally applied electric field. Both intra- and inter-well exciton transitions were detected. The energy shifts of the coupled quantum confined levels are in good agreement with a theoretical calculation. Details of the lineshape fit yield important information about the modulation mechanism.
A simple method for reconstructing the density of states of heavily doped silicon from photoluminescence spectra
Authors: Pan Y.
Published: July 2002

A simple method is proposed for reconstructing the density of states of heavily doped silicon from low- and room-temperature photoluminescence spectra. At very low temperatures, the distribution of minority carriers can be approximated by a Dirac δ function, which simplifies the problem significantly. The photoluminescence spectra of boron-doped silicon with an acceptor density 1.7×1020 cm−3 are analyzed. We found that the minority band density of states is lower than the majority band density of states.
Z-discharge transport of intense ion beams for inertial confinement fusion
Authors: Ottinger P. F. , Rose D.V. , Mosher D. , Neri J. M.
Published: July 2002

Ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time-of-flight bunching. This paper evaluates light ion beam transport in a wall-stabilized z-discharge channel, where the discharge azimuthal magnetic field radially confines the ion beam. The ion beam is focused onto the entrance aperture of the transport channel by shaping the diode to achieve beam convergence in a field-free drift region separating the diode from the transport section. Ion orbits are studied to determine the injection efficiency (i.e., the fraction of the beam emitted from the diode which is transported) under various conditions. Ions that are focused onto the channel entrance at too large of an angle for confinement hit the wall and are lost. For a multimodular scheme (10–30 beams), individual transport channels are packed around the target with the exit apertures at some standoff distance from it. The fraction of the beam that is lost in this field-free standoff region is also evaluated under various conditions. The standoff efficiency is then combined with the injection efficiency to give the dependence of the total transport efficiency ηt on diode, focusing, transport and standoff parameters. It is found that ηt can be in the range of 75%–100% for parameter values that appear to be achievable.
Characterization of Te-doped GaSb grown by molecular beam epitaxy using SnTe
Authors: Chen J. F. , Cho A. Y.
Published: July 2002

The use of SnTe as a source of donor impurities in the growth of n-type GaSb by molecular beam epitaxy (MBE) is investigated. Hall carrier concentrations between 1.23×1016 and 3.7×1018 cm−3 have been obtained with room-temperature Hall mobility as high as 5114 cm2/V s for a lightly doped GaSb layer with nH = 3.8 × 1016 cm−3. The temperature-dependent Hall concentrations have been analyzed according to the two-band model to obtain information about the effect of the band structure of GaSb on the electrical properties. In addition, the effects of V-III flux ratio on Te incorporation in GaSb are studied. The measured carrier concentrations are found to be insensitive to the antimony-to-gallium beam equivalent pressures (from 1.5 to 9) at a growth temperature of 500 °C. These results may lead to SnTe being one of the donor dopants of choice in the MBE growth of n-type GaSb.
Surface induced liquid-crystal alignment studied by optical second-harmonic generation
Authors: Barmentlo M. , van Aerle N. A. J. M. , Hollering R. W. J. , Damen J. P. M.
Published: July 2002

The orientational order induced in a liquid crystal (LC) monolayer of adsorbed 4-n-octyl-4′-cyanobiphenyl molecules on rubbed substrates is studied by measuring the second-harmonic response. By systematically varying the rubbing conditions and measuring the induced LC bulk and monolayer alignment, we can conclude that the homogeneity of the LC bulk alignment and the surface in-plane order parameter of the LC monolayer are closely related. The polar and azimuthal ordering of the LC monolayer, however, are found to be decoupled. By rubbing polyimide films twice and studying other rubbed surfaces, we show that two mechanisms are responsible for the LC alignment, depending on the kind of substrate that is used. For rubbed bare glass substrates long-range elastic interactions, which do not affect the monolayer alignment, are dominant. In the case of rubbed polyimide films molecular short-range interactions are responsible for the alignment of both the LC bulk and the LC monolayer.
Surface acoustic wave studies on single-crystal nickel using Brillouin scattering and scanning acoustic microscope
Authors: Mendik M. , Sathish S. , Kulik A. , Gremaud G. , Wachter P.
Published: July 2002

The directional dependence (angular dispersion) of surface acoustic phonons propagating on {100}, {110}, and {111} oriented planes of nickel single crystals has been measured by Brillouin scattering and continuous wave scanning acoustic microscope. All the elastic constants C11, C12, C44 are obtained from the angular dispersion of each plane separately. These are compared with the elastic constants determined with conventional ultrasonic measurements on the same specimens. The difference in the sampling depth or the approximate depth of propagation of the acoustic waves into the specimen in each technique is used to characterize the extent of the polish-induced damage zone in the sample.
Relativistic electron beam neutral gas interaction studies
Authors: Venugopala Rao G. , Iyyengar S. K.
Published: July 2002

A relativistic electron beam of 175 keV, 6 kA, and 40 ns is injected into neutral argon and air in the pressure range of 0.1–20 Torr. The peak beam transport up to 100 cm is found to be maximum, 75% of the input beam current, around 0.3 Torr. The plasma heating level of ≂5×1022 eV/m3 μs is inferred from the net current signals.
Raman scattering from cubic boron nitride up to 1600 K
Authors: Alvarenga A. D. , Grimsditch M. , Polian A.
Published: July 2002

Raman scattering measurements on cubic boron nitride up to 1600 K are reported. The results indicate that at 1600 K the interatomic force constant is only ∼10% weaker than at room temperature and that cubic BN is therefore a good candidate for applications where high-temperature mechanical strength is required. However, contrary to the behavior found for diamond, where the frequency changes with temperature were very nonlinear, it is found that in boron nitride both the TO and LO shift linearly with temperature at a rate of −0.036 cm−1/K.
Franz–Keldysh oscillations of δ-doped GaAs
Authors: Hsu T. M. , Tien Y. C. , Lu N. H. , Tsai S. P. , Liu D. G. , Lee C. P.
Published: July 2002

Si-δ-doped GaAs (N2D ≊ 1011 cm−2) samples grown by molecular-beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ-doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 Å), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.
Instability of a GexSi1−xO2 film on a GexSi1−x layer
Authors: Liu W. S. , Chen J. S. , Nicolet M.-A. , Arbet-Engels V. , Wang K. L.
Published: July 2002

The stability of an amorphous GexSi1−xO2 in contact with an epitaxial (100)GexSi1−x layer obtained by partially oxidizing an epitaxial GexSi1−x layer on a (100)Si substrate in a wet ambient at 700 °C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 °C for 3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 900 °C, the oxide remains amorphous and the amount of GeO2 in the oxide stays constant, but some small crystalline precipitates with a lattice constant similar to that of the underlying GeSi layer emerge in the oxide very near the interface for both x. Similar precipitates are also observed after aging for both x. The appearance of these precipitates can be explained by the thermodynamic instability of GexSi1−xO2 in contact with GexSi1−x. In water at RT, 90% of GeO2 in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28.
XeCl laser ablative deposition and characterization of yttria-stabilized zirconia thin films on glass and CeO2-Sm2O3
Authors: Kokai F. , Amano K. , Ota H. , Ochiai Y. , Umemura F.
Published: July 2002

Thin films have been deposited by XeCl excimer laser ablation of a bulk yttria-stabilized zirconia (YSZ) target. The deposited thin films were characterized in terms of spatial distribution, composition, microstructure, and solid particulates on the film surface through deposition process control. The deposition of dense YSZ films, 0.5–2 μm thick, with an excellent adhesive property at 800 °C was performed on the CeO2-Sm2O3 substrate maintained at 500 °C under an O2 ambient of 5×10−2 Torr. The crystallinity of YSZ thin films was found to be promoted by post-deposition annealing. The YSZ film consisted of a cubic phase with predominant growth along the (200) plane.
Multicomponent structure of electron emission from the Te-related DX center in AlGaAs
Authors: Piotrzkowski R. , Litwin-Staszewska E. , Suski T. , Kończewicz L. , Robert J. L. , Stankiewicz W.
Published: July 2002

Thermal emission of electrons from resonant DX levels introduced by Te in AlGaAs was studied by means of time dependent Hall effect measurements. The levels were filled by applying hydrostatic pressure. Using an original method to analyze the obtained data, up to eight discrete components of the electron emission were clearly separated. This result gives evidence for a large cation displacement, predicted by Chadi and Chang [Phys. Rev. B 39, 10063 (1989)] in the case of group-VI DX center. Moreover, we observed the pressure dependence of barriers for electron emission from Te-related DX states.
Interfacial reactions of ultrahigh vacuum deposited yttrium thin films on (111)Si at low temperatures
Authors: Lee T. L. , Chen L. J.
Published: July 2002

Interfacial reactions of ultrahigh vacuum deposited yttrium thin films on atomically clean (111)Si at low temperatures have been studied by both conventional and high-resolution transmission electron microscopy, Auger electron spectroscopy, and x-ray diffraction. A 10-nm-thick yttrium thin film, deposited onto (111)Si at room temperature, was found to completely intermix with Si to form an 11-nm-thick amorphous interlayer. Crystalline Y5Si3 and Si were observed to nucleate first within the amorphous interlayer in samples annealed at temperatures lower than 200 °C. Epitaxial YSi2−x was found to be the only phase formed at the interface of amorphous interlayer and crystalline Si in samples annealed at temperatures higher than 250 °C. In as deposited 20- to 60-nm-thick Y thin films on silicon samples, crystalline Y5Si3, Si, and YSi and a 2.5-nm-thick amorphous layer were found to be present simultaneously. Good correlations were found among difference in atomic size between metal and Si atoms, the calculated free energy of mixing as well as the critical and maximum amorphous interlayer thickness for the Y/Si and a number of refractory metal/Si systems. The Y/Si system is the only system found up to date among all metal/Si systems that the a interlayer can be grown to a thickness exceeding 10 nm during deposition at room temperature.
Effect of thin-film disk texture on magnetic recording signals
Authors: Klaassen K. B. , Eaton R. E. , van Peppen J. C. L.
Published: July 2002

The effect of texturing on the readback signal of Co-alloy thin-film disks is investigated. Instantaneous pulse area (PA) traces, representative of the magnetic signal strength, and instantaneous pulse width (PW50) traces, indicative of the transition resolution, are measured. The low-frequency, ‘‘twice-around’’ modulation is removed from these traces. The resulting PA fluctuations are small; the PW fluctuations are larger and closely correlated to the texture. Calibration of PW vs flying height allows comparison of the texture-induced head clearance modulation and the actual disk surface roughness. The texture causes signal resolution (PW) noise which, for these head/disk combinations, is predominantly due to the spacing loss variations during the readback process. The write process and any texture-related Hc or Mrt fluctuations appear to contribute negligibly.
Plasma generation in an organic molecular gas by an ultraviolet laser pulse
Authors: Zhang Y. S. , Scharer J. E.
Published: July 2002

Plasmas are generated in a low ionization potential gas, tetrakis(dimethylamino)ethylene (TMAE) vapor through a one-photon ionization process by an ultraviolet laser beam at a 193 nm wavelength. The TMAE plasma characteristics are studied by means of a Langmuir probe and microwave scattering. A new method is used to measure the 193 nm ultraviolet (UV) photon absorption cross section in TMAE. It is determined to be 1.1±0.3×10−17 cm2 from the axial profile of electron density. The temporal evolution of electron temperature is measured. A peak electron density of ne=5×1013/cm3 and peak electron temperature of Te=1 eV are measured at 500 mTorr TMAE pressure. The plasma decay process is studied, and the electron-ion recombination coefficient is measured to be 5.4±0.5×10−6 cm3/s. A theoretical model is derived to describe the photon flux and the one photon ionization process. An application of a TMAE plasma as a mirror for microwave reflections is proposed.
Exploratory observations of post-breakdown conduction in polycrystalline-silicon and metal-gated thin-oxide metal-oxide-semiconductor capacitors
Authors: Nafría M. , Sun̄é J. , Aymerich X.
Published: July 2002

The post-breakdown conduction of thin-oxide metal-oxide-semiconductor structures with different gate electrodes and substrates is studied. Due to the extreme localization of the breakdown, many breakdown events can be produced in one capacitor during a constant voltage stress. In some cases, these events have been found to be reversible and this suggests that the breakdown is a reversible phenomenon (i.e., that the breakdown is a reversible switching between two conduction states of different conductivities). This reversibility is further supported by the observation of bistable conduction in the post-breakdown I-V characteristic when the breakdown current is externally limited. The experimental results are interpreted assuming that the breakdown is a three-stage process (degradation-breakdown-thermal effects), and a simple phenomenological model is presented. The role of the gate electrode (chromium, aluminum, or polycrystalline-silicon) and that of the substrate doping are analyzed within this framework. The presented results show that the analysis of the post-breakdown properties is a powerful technique to investigate the physics of the breakdown.
Modeling of zinc-indiffusion-induced disordering of GaAs/AlAs superlattices
Authors: Zimmermann H. , Gösele U. , Tan T. Y.
Published: July 2002

A model for the effect of Zn indiffusion on enhancing the GaAs/AlAs superlattice (SL) disordering process, which combines recently proposed models for Ga self-diffusion and Zn diffusion in GaAs, is presented. Four coupled partial differential equations describing the process were solved numerically. Satisfactory agreement between the simulated results and experimental data available in the literature is obtained. At a given temperature, the used values for the diffusion coefficient and the thermal equilibrium concentration of the responsible point defect species, the doubly positively charged Ga self-interstitials IGa2+, are a consistent splitting of the known Ga self-diffusion coefficient dominated by IGa2+. Quantitatively, the SL disordering enhancement is mainly due to the Fermi-level effect while an IGa2+ supersaturation also makes a small contribution. Because of p-doping by Zn acceptor atoms, the IGa2+ concentration is increased tremendously via the Fermi-level effect. An IGa2+ supersaturation also develops because the IGa2+ generation rate is higher than its removal rate. The enhanced SL disordering process mainly proceeds under the Ga-rich SL composition conditions. The Zn-indiffusion-enhanced Al-Ga interdiffusion coefficient shows an apparent dependence on the Zns− concentration differing slightly from a quadratic relationship.
Electron drift velocities in argon-boron trichloride gas mixtures
Authors: Mosteller Donald L. , Andrews Merrill L. , Clark Jerry D. , Garscadden Alan
Published: July 2002

Electron drift velocities were measured for argon gas mixtures containing 25 ppm to 0.5% boron trichloride additive using a pulsed-Townsend drift tube. These results show a marked sensitivity to mixture ratio and the gas mixtures also show negative differential conductivity and strong attachment at low electric field/gas number density.
Magnetotransport and magneto-optical properties of δ-doped InSb
Authors: Heremans J. , Partin D. L. , Morelli D. T. , Thrush C. M. , Karczewski G. , Furdyna J. K.
Published: July 2002

Results of Shubnikov–de Haas (SdH), cyclotron resonance (CR), and Hall-effect measurements on δ-doped InSb:Si films grown by molecular-beam epitaxy on insulating InP substrates are reported. The investigation covers samples with sheet densities of Si dopant atoms ranging from 1×1011 to 1×1013 cm−2, temperatures from 4.2 to 300 K, and fields from 0 to 7 T. The SdH oscillations show that the samples contain electrons of two-dimensional nature, occupying multiple subbands. The effective masses obtained from the CR data correspond well to the subband occupation densities. The Hall measurements as well as the CR experiments also give evidence for the presence of additional electrons, with the conduction-band-edge mass m*=0.014m0 of bulk InSb, which exist presumably in the bulk of the films.
Enhancement of magnetic moment of iron atoms in the Fe/Au nanomultilayers
Authors: Pan F. , Tao K. , Liu B. X.
Published: July 2002

Fe/Au multilayers were prepared by alternative vapor deposition. The periodicity, thickness, chemical composition, microstructure, and the magnetic moment of the films were determined and measured by various methods. The magnetic moment per Fe atom in Fe/Au multilayers was considerably enhanced when the Fe layer thickness was thinner than 8 nm and it was up to 2.59 μB, i.e., about 1.2 times of that of bulk Fe, at an Fe layer thickness of 4.5 nm. The experimental results also revealed that as the thickness of the Fe layer decreased, there was an increasing tendency towards perpendicular magnetization in the Fe/Au multilayers. The possible mechanism responsible for the modification of magnetic properties is also discussed.
Structural and magnetic behavior of Ar+-implanted Co/Pd multilayers: Interfacial mixing
Authors: Schelp L. F. , Carara M. , Viegas A. D. C. , Vasconcellos M. A. Z. , Schmidt J. E.
Published: July 2002

The magnetization behavior of Co/Pd multilayers has been analyzed as a function of the degree of interfacial mixing among the Co and Pd layers. Controlled atomic mixing was induced by low-dose and low-flux ion implantation and a follow-up of the structural status of the samples was made by simulation of the high-angle x-ray-diffraction data. Values of the saturation magnetization as a function of the broadness of Co concentration profile are presented and explained by a simple model based on the parameters obtained from the x-ray simulations.
Electron density determination using phase measurement in a pulsed surface wave discharge at high pressure
Authors: Rousseau V. , Boisse-Laporte C. , Leprince Ph. , Marec J.
Published: July 2002

This paper deals with the determination of electron density in a pulsed surface wave discharge (frequency ω/2π=1.1 GHz, pulse duration τ∼10 μs), in an air-like mixture at about 10 Torr in a capillary tube (inside diameter 1.5 mm). A time-resolved diagnostic based on microwave interferometry is developed. It is shown that it provides both the collision frequency ν and the electron density ne except when ν/ω exceeds a certain value (5 for these operating parameters).
Magnetization reversal in CoFe/Ag/Fe/ZnSe thin layer sandwiches
Authors: Bilic D. , Dahlberg E. Dan , Chaiken A. , Gutierrez C. , Lubitz P. , Krebs J. J. , Harford M. Z. , Prinz G. A.
Published: July 2002

In recent work [J. Appl. Phys. 70, 10 (1991)] the spin valve effect (SVE) was measured in CoxFe1−x/Ag/Fe(x<0.7) thin layer sandwiches grown by molecular beam epitaxy. The field dependence of the SVE was correlated with VSM magnetization data taken on the samples. It was found that only at low fields was there a correlation between the SVE resistance and the magnetization data. These results indicated a more complex reversal mechanism that included substantial magnetizations in directions not measured in traditional VSM measurements. In order to more accurately determine the field-dependent magnetization of the samples during reversal, the longitudinal and transverse Kerr effects were measured on the layers. The particular geometry used to measure the Kerr effects was for the light scattering plane to be perpendicular to the applied magnetic field. The transverse Kerr effect data, which in this geometry are sensitive to the magnetization parallel to the applied magnetic field, replicated the VSM data. The longitudinal Kerr effect, which is sensitive to the net magnetization perpendicular to the applied field (in the plane of the film), indicated a substantial perpendicular magnetization component. Based upon previous work on epitaxial iron films, it appears that the magnetization reversal process proceeds by transitions between easy axes. From these measurements, the anomalous resistances observed as a function of the applied magnetic field in J. Appl. Phys. 70, 10 (1991), are explained by having one of the films soft, and two easy axes in the plane of the other film.
Preparation of Ni–Fe/Cu multilayers with low coercivity and GMR effect by ion beam sputtering
Authors: Naoe Masahiko , Miyamoto Yasuyoshi , Nakagawa Shigeki
Published: July 2002

Multilayers composed of 30 [Ni–Fe/Cu] bilayers deposited on a 50-Å-thick Fe buffer layer were prepared by ion beam sputtering. The magnetoresistance (MR) ratio Δρ/ρ0 of the multilayers took high value at the Cu layer thickness δCu of 10, 20, and 32 Å for the multilayers with Ni–Fe layer thickness δNi–Fe of 10 Å. It was observed that (100) orientation of Ni–Fe and Cu crystallites were enhanced at δCu of 20 and 32 Å. The multilayered film with δCu of 10 Å exhibited Δρ/ρ0 as high as 12%. The multilayered film possessed good soft magnetic properties, and exhibited coercivity Hc and relative permeability μr of about 4 Oe and 800, respectively. The saturation magnetic field Hs was about 300 Oe. Δρ/ρ0, Hc, and Hs at δCu of 20 Å, i.e., at the ‘‘second peak,’’ were 6%, 10 Oe, and 25 Oe, respectively. The δCu dependence of Δρ/ρ0 seemed to be correspondent to that of the preferential orientation of the (100) plane. Ni–Fe/Cu multilayers with the same construction but without an Fe buffer layer did not reveal apparent (100) preferential orientation and did not exhibit Δρ/ρ0 as high as detectable. Then, Δρ/ρ0 was strongly dependent not only on interlayer thickness, such as δCu, but also (100) crystallite orientation.
Response of interferometer based probe systems to photodisplacement in layered media
Authors: Liu M. , Suddendorf M. B. , Somekh M. G.
Published: July 2002

This paper presents a detailed theory which describes the response of a focused interferometer when a modulated laser source is incident upon a layered sample. In order to solve this problem it is necessary to solve the thermoelastic equations for a layered sample. The paper presents an analytical solution obtained using a symbolic manipulation program (reduce). In particular, we point out that the analytical form of the solution is necessary to achieve numerically stable results. In addition to calculating the actual displacement of the sample surface, the response of the instrument is considered in detail, in terms of the refractive index changes of the air above the sample and the optical transfer function of the probe optics.
Investigation of silicon delta-doped gallium arsenide using the Shubnikov–de Haas effect and theoretical modeling
Authors: Wilks S. P. , Cornish A. E. , Elliott M. , Woolf D. A. , Westwood D. I. , Williams R. H.
Published: July 2002

We report a detailed study of electron sub-band occupancies and saturation effects in silicon delta-doped gallium arsenide samples, using Hall and Shubnikov–de Haas (SdH) measurements in conjunction with numerical modeling. This study extends previous work in two respects. First, the samples, produced by molecular beam epitaxy with a nominal delta doping density of 1×1013 cm−2, were examined over a wide range of growth temperature (395–710 °C) to allow the influence of broadening of the doping profile to be examined. Second, the numerical modeling method, based on a self-consistent solution of Poisson’s and Schrödinger’s equations, included directly the influence of DX-like donor levels, located at 200 meV above the conduction band edge. Excellent agreement with the individual sub-band occupancies determined by SdH was found for all samples up to a growth temperature of 605 °C, with the total silicon doping density kept constant and dopant broadening as the only adjustable parameter in the fit. Despite the evidence for inclusion of DX-like donor levels based on our modeling, all samples showed only a weak persistent photoconductivity effect. This is in contrast to uniformly doped bulk GaAs, indicating the different nature of the DX level in two and three dimensional doping. Above 605 °C it was not possible to model sub-band occupancies using a constant total doping density, showing that another deactivation mechanism such as autocompensation becomes important.
Surface reaction probability in hydrogenated amorphous silicon growth
Authors: Nuruddin A. , Doyle J. R. , Abelson J. R.
Published: July 2002

We deposit hydrogenated amorphous silicon (a-Si:H) on a novel ‘‘macroscopic’’ trench substrate using both remote hollow cathode (HC) silane discharges and reactive magnetron sputter (RMS) deposition sources. Both methods produce state of the art optoelectronic quality a-Si:H. We analyze the surface coverage profiles in terms of the surface reaction probability β, using a Monte Carlo simulation to correct for particle reflection and loss. We also measure the deposited film quality as a function of position in the trench. For low power silane HC deposition, we find β=0.28±0.05, whereas for the RMS case β=0.97±0.05. In contrast to the prevailing thinking in the a-Si:H field, this result demonstrates that β is not universally correlated with film quality. We discuss the role of energetic particle bombardment in RMS that permits high quality films to be deposited despite the high precursor reactivity.
Roughness dependent magnetic hysteresis of a few monolayer thick Fe films on Au(001)
Authors: He Y.-L. , Wang G.-C.
Published: July 2002

We have studied quantitatively the surface and interface roughness of Fe films in a few monolayer regime on an Au(001) surface using the high-resolution low-energy electron diffraction technique. The hysteresis loops of those films were also measured in situ by the surface magneto-optical kerr effect technique. A correlation between the shape of hysteresis loops and the roughness of films was observed. The hysteresis loops are consistently squarelike for films with a two-dimensional (2D) smooth structure. For films with isolated three-dimensional (3D) islands structure, the hysteresis loops are typically stripelike. For films with a combination of 2D/3D structure, the loop shape is squarelike for longitudinal loops and stripelike for polar loops.
Analysis of phase shifts due to etching in photoreflectance spectra of a GaAs/AlGaAs single-quantum well structure
Authors: Hosea T. J. C. , Hughes P. J. , Weiss B. L.
Published: July 2002

Room-temperature photoreflectance measurements of a GaAs/Al0.2Ga0.8As single-quantum well structure showed well defined Franz–Keldysh oscillations in the neighborhood of the GaAs and Al0.2Ga0.8As band-edge energies. That experiment investigated the origin of the Franz–Keldysh oscillations by sequential etching and photoreflectance analysis of the grown layers and showed that the phase of the Franz–Keldysh oscillations shifted as the upper Al0.2Ga0.8As barrier was etched, with eventual phase reversal when roughly half of the upper barrier was removed. Here, these phase shifts are determined accurately using a novel Kramers–Kronig approach and they are interpreted in terms of optical interference effects using both a simple two-ray model and a multiple-reflection treatment incorporating a calculation of the Seraphin coefficients. The results also enable the thickness of the layers removed to be determined.
Partially ionized beam deposition of Ag films on insulating substrates
Authors: Soss S. R. , Cook C. A. , Lu T.-M.
Published: July 2002

A partially ionized beam deposition technique which utilizes a small fraction of self-ions during deposition is used to grow thin Ag films (∼1200 Å) on glass substrates. It is shown that due to substrate charging the Ag film is discontinuous and has inferior structural and electrical properties. This difficulty was overcome by using an oscillatory substrate bias so that electron showers are directed toward the surface to neutralize the substrate during deposition. We show that this scheme of deposition can produce thin and continuous Ag films on glass with bulk-like resistivity with a substrate bias of ≳1.3 kV. This deposition technique opens up the possibility of coating high-quality metal films on insulators for optical and electronic applications.
Optical investigations on isovalent δ layers in III-V semiconductor compounds
Authors: Schwabe R. , Pietag F. , Faulkner M. , Lassen S. , Gottschalch V. , Franzheld R. , Bitz A. , Staehli J. L.
Published: July 2002

In contrast to usual quantum wells or barriers having a thickness of some lattice constants, spatially well-separated, electronically uncoupled monolayers of group-III or V elements are considered as isovalent δ doping or δ layers. Similar to the case of randomly distributed nitrogen dopants in GaP bulk material, it is shown that the two-dimensional arrangement of isovalent atoms brings forth a new quality of III-V semiconductor compounds: The optical emission and absorption properties near the fundamental band gap of indirect-gap, and even of direct-gap, host material, where the isovalent layers are incorporated, are drastically improved. Low-temperature luminescence and transmission experiments on metal-organic vapor-phase epitaxially grown InAs δ layers in GaAs, AlAs δ layers in GaAs, and GaAs monolayers in AlAs are dealt with.
Investigation of 90° coupling in Fe/Ag/Fe structures: ‘‘Loose spins’’ and fluctuation mechanism
Authors: Schäfer M. , Demokritov S. , Müller-Pfeiffer S. , Schäfer R. , Schneider M. , Grünberg P. , Zinn W.
Published: July 2002

Results are outlined of experimental work concerning Slonczewski’s theories explaining 90° coupling: ‘‘loose spin’’ theory and thickness-fluctuation mechanism. The loose spin theory suggests that 90° coupling originates from paramagnetic impurities in the interlayer of an exchange-coupled layered structure. The influence of these paramagnetic impurities on the magnetic exchange coupling was studied using the wedge technique. High-quality single-crystalline Fe/Ag/Fe samples were prepared in UHV and each sample consisted of two wedges: one wedge with additional Fe deliberately inserted during growth into the Ag interlayer and one wedge with a pure Fe/Ag/Fe structure. The detailed analysis of magneto-optic hysteresis loops revealed quantitatively the bilinearly and the 90° coupling strength. As predicted by loose spin theory additional Fe in the Ag spacer lead to a strong temperature dependence of the 90° coupling. According to the theory an increase of 90° coupling strength proportional to impurity concentration was detected while the strength of bilinear coupling decreased. For the pure Fe/Ag/Fe structure, a linear increase of the 90° coupling strength with decreasing temperature was observed. This result can be explained within the fluctuation mechanism which creates 90° coupling through a combination of interface roughness and intralayer ferromagnetic exchange.