bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of heat treatment in H2-N2 atmosphere on the performance of screen-printed CdS/CdTe solar cells
Authors: Yoshida Tomoyuki , Matsui Noboru
Published: July 2002

Heat treatments in a 50% H2+50% N2 atmosphere have been applied to the fabrication process of screen-printed CdS/CdTe solar cells for improving cell performance. The heat treatment at 450 °C after the sintering of a C (carbon) electrode on a CdTe film was found to be most effective in improving conversion efficiency. The efficiencies increased from about 8% to about 10% because of a large increase in fill factor and a slight increase in open-circuit voltage. Current versus voltage characteristics, short-circuit current versus open-circuit voltage characteristics, and spectral responses were analyzed. The above improvement was found to result from a decrease in CdS film resistivity, a small change in the diode characteristic of the junction under illumination, and the reduced voltage dependence of the light-generated current arising from the increased minority carrier diffusion length in the CdTe. The heat treatment at 450 °C after the sintering of the CdTe film also increased fill factor, resulting in efficiencies of about 9%. The improvement was caused by a decrease in CdS film resistivity. A stability test under sunlight conditions showed that the efficiency of the cells heat treated after the C-electrode sintering decreased gradually because of a degradation in the CdTe-C contact. However, the CdS film resistivity and junction properties of the cells were stable. On the other hand, the cells heat treated after the CdTe film sintering were stable under the same conditions.
Electronic excited-state dynamics in the transient grating signal: Absolute quantum yields of fluorescence and triplet formation
Authors: Terazima Masahide , Hirota Noboru
Published: July 2002

The time dependence of a transient grating (TG) signal after photoexcitation of a photochemically stable molecule to an excited singlet state is presented including the decay from the excited singlet state as well as from the excited triplet state. By analyzing the time dependence, the absolute quantum yields of fluorescence and the triplet formation, the decay rates of the excited singlet and triplet states can be determined at once. The time dependence of the TG signal after the excitation of naphthalene is studied to confirm the validity of the present method.
Surface morphology of Si on Si(100) grown below 500 °C using H/Cl exchange chemistry
Authors: Koleske D. D. , Gates S. M.
Published: July 2002

Changes in surface structure for thin epitaxial Si layers (Si*) grown on Si(100) below 500 °C using alternating exposures to Si precursors are presented. Two different methods for growing Si* are compared, which involve a self-limiting exposure to Si2Cl6 followed by removal of the Cl layer with either Si2H6 or atomic hydrogen. Scattering and recoiling spectroscopy is used to depth profile a N marker layer as Si* is grown using Si2Cl6 and Si2H6 at TS=465 °C. The decrease in the N marker intensity proves that a Si* film is grown, and reflection high-energy electron diffraction (RHEED) patterns show that the Si* is epitaxial. RHEED patterns from Si* grown using alternating exposures of Si2Cl6 and atomic hydrogen are nearly identical to clean Si(100), again indicating epitaxial growth. Both methods suppress formation of three-dimensional islands, which are observed for Si growth from Si2H6 below TS=600 °C.
X-ray diffraction measurement of partially correlated interfacial roughness in multilayers
Authors: Phang Y. H. , Savage D. E. , Kariotis R. , Lagally M. G.
Published: July 2002

The existence of partially correlated roughness in multilayer thin films is demonstrated using x-ray diffraction diffuse-intensity distribution measurements. The method is generally applicable and produces, in addition to values of magnitudes of interfacial roughness and its lateral correlation length, a measure of the cross correlation between interfaces separated by intermediate ones. A simple phenomenological model can describe roughness in W/C multilayers prepared under standard conditions. A cumulative roughness function is used to show that the wavelength range in which the interfacial roughness predominates in these layers lies between 50 Å and 2000 Å and that the long-wavelength roughness replicates better than the short-wavelength roughness.
Simulation of the transient indiffusion-segregation process of triply negatively charged Ga vacancies in GaAs and AlAs/GaAs superlattices
Authors: You Horng-Ming , Gösele Ulrich M. , Tan Teh Y.
Published: July 2002

In GaAs and AlAs/GaAs superlattice crystals containing n-type regions, several sets of recent experimental results obtained from diffusion studies require the interpretation that the responsible point defect species, the triply negatively charged Ga vacancy (VGa3−), has attained its thermal equilibrium concentration (CVGa3−eq) at the onset of an experiment. This could be due to either the fact that under heavy n-doping conditions CVGa3−eq is fairly temperature independent, or the fact that the transient process of populating VGa3− from an undersaturated to the appropriate CVGa3−eq value via indiffusion from the surfaces to the interior of the crystals is extremely rapid. We have simulated the transient process of populating VGa3− to the crystal interior. The experiments use crystals consisting of adjacent intrinsic and n-type regions for which CVGa3−eq values are different, leading to the simultaneous occurrence of VGa3− diffusion and segregation phenomena. A diffusion-segregation equation has been derived and subsequently used in the simulation calculations. The simulation results showed that, as long as n-type regions are involved, such transient processes are ineffective and therefore cannot explain the experimental requirement that VGa3− is already present in the appropriate CVGa3−eq(n) value at the onset of an experiment. On the other hand, the transient process is sufficiently rapid for the purely intrinsic crystal cases. These simulation results support our recent finding that the CVGa3−eq(n) values are essentially temperature independent, obtained via a thermodynamic treatment.
A quantitative study of impurities in photoluminescent and nonphotoluminescent porous silicon layers
Authors: Grosman A. , Ortega C. , Siejka J. , Chamarro M.
Published: July 2002

The visible photoluminescence of porous silicon layers has been attributed to energy confinement of carriers in the Si microcrystallites, but this hypothesis is currently challenged by models in which the photoluminescence originates in silicon based molecules, possibly formed during the etching process, such as siloxene, siloxene derivatives with fluorine substitution, hydride species. To study the possible relation between the photoluminescence phenomenon and such molecules we have performed quantitative analysis of hydrogen, oxygen, and fluorine and of carbon which is also present in porous silicon layers. Oxygen, fluorine, and carbon content have been measured by nuclear reaction analysis. To measure the hydrogen content we used elastic recoil proton detection analysis. We found similar impurity content in photoluminescent and in nonphotoluminescent layers, which seems to indicate that the photoluminescence does not originate in silicon based molecules present in porous silicon layers. In freshly prepared samples, the amount of hydrogen we found is higher than that expected from a monolayer of hydride species. The excess of hydrogen is probably due to the presence of carbon bearing species from the atmosphere, the quantity of which corresponds to about one monolayer. A small fraction of the Si surface (10%–20%) is covered by SiO2 or/and O-Si-H species.
Time-of-flight mass spectroscopic studies on the laser ablation process
Authors: Fukushima Kensuke , Kanke Yukio , Morishita Tadataka
Published: July 2002

The time-of-flight (TOF) measurements for ions in the expanding plume during laser ablation of Y1Ba2Cu3Ox are performed. The TOF spectra consist of peaks of only the monatomic ions. The drift velocities for these ions are revealed to be (2–4)×106 cm/s at the laser fluence of 2.0 J/cm2 and an oxygen pressure of 10−3 Torr. These velocities are affected by the ablation conditions, and are found to increase with increasing laser fluence and decrease with increase of oxygen pressure. The velocity distributions for the ions at high oxygen pressure are described by the supersonic expansion model with a distribution width narrowing with decreasing oxygen pressure.
Generation of response maps of gas mixtures
Authors: Lundström Ingemar , Sundgren Hans , Winquist Fredrik
Published: July 2002

Large area field effect structures with thin catalytic metal gates can be used to generate two dimensional images of the response of the metal to gas mixtures. This is done by using a scanning light pulse technique to determine polarization changes occurring in the thin metal film due to chemical reactions on the metal. Such images appear to be a new concept in chemical sensing. The present contribution is a first attempt to predict the shape of these images. We show that the occurrence of maxima in the response at certain points on the surface is most likely due to temperature dependent chemical rate constants and to the consumption of molecules on the catalytic metal surface. Experimental results of the detection of methanol, ethanol, and hydrogen on palladium and platinum, with a temperature gradient along the metals, are used to support the theoretical arguments.
High field magnetization measurements of Sm2Fe17, Sm2Fe17N3, Sm2Fe17D5, and Pr2Fe17, Pr2Fe17N3 (invited)
Authors: Isnard O. , Miraglia S. , Guillot M. , Fruchart D.
Published: July 2002

We report high magnetic field magnetization measurements of the ferromagnetic compounds Sm2Fe17, Sm2Fe17N3, Sm2Fe17D5, and Pr2Fe17, Pr2Fe17N3 performed up to 200 kOe in continuous field. We present isothermal magnetization curves measured at 4.2 and 300 K on oriented powder samples. The M(H) curves show that the axial anisotropy fields obtained for the nitride are larger than those previously reported. The results are discussed in the light of neutron powder diffraction data. The magnetic anisotropy constants K1 and K2 have been determined, taking into account the angular distribution of the grain axis.
Effect of ion beam assistance on the microstructure of nonhydrogenated amorphous carbon
Authors: Rossi François , André Bernard , van Veen A. , Mijnarends P. E. , Schut H. , Delplancke Marie Paule , Gissler W. , Haupt J. , Lucazeau G. , Abello L.
Published: July 2002

Nonhydrogenated diamondlike carbon films have been prepared by dual ion beam sputtering and ion-beam-assisted magnetron. The assistance parameters—ion energy, ion mass, ion flux/atom flux—have been systematically varied, and the films have been characterized by Rutherford backscattering spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, positron annihilation spectroscopy, Raman spectroscopy, and nanoindentation. It was found that the density and the degree of disorder of the films go through a maximum with ion energy, and the void concentration goes through a minimum. Microstructure analysis shows that the films are mostly sp2 bonded, with a maximum of about 16% concentration of sp3 bonding from the largest values of density. The evolution of density with ion flux and energy is consistent with a combined effect of atomic displacements in the film leading to densification, and damage buildup leading to progressive graphitization as the energy is increased. The large hardness/elastic modulus ratios obtained should lead to excellent friction properties.
Self-consistent calculations in silicon carbide inversion layers
Authors: Ouisse T.
Published: July 2002

Schrödinger’s and Poisson’s equations have been self-consistently solved in the Hartree approximation in order to study the behavior of electron inversion layers in the cubic form of silicon carbide. The major crystalline orientations have been investigated in a large temperature range. Owing to the specific ratio between the longitudinal and transverse effective masses, it is shown that quantization as well as the subband occupation may differ from the case of silicon in a noticeable way, depending on temperature or electron density: Even for rather low electron concentrations, several subbands could be occupied at very low temperature and for (100) orientation. In addition, the average penetration depth of the inversion layer is always lower than that of silicon at room temperature, which could result in increased surface roughness scattering.
Stability of rhenium thin films on single crystal (001) β-SiC
Authors: Chen J. S. , Kolawa E. , Nicolet M.-A. , Baud L. , Jaussaud C. , Madar R. , Bernard C.
Published: July 2002

Thermal reactions of Re thin films in contact with single crystalline (001) β-SiC at temperatures between 700 and 1100 °C for 30 min are investigated by MeV He++ backscattering spectrometry, x-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy (plan-view and cross-sectional). No reaction between Re and SiC is observed for any annealing conditions. The average grain size of the as-deposited Re film is 220 nm and increases to 280 nm after annealing at 1100 °C for 30 min. A strong {0001}Re fiber texture is also observed after annealing. The chemical stability of Re thin films on SiC is consistent with the earlier study of solid-phase stability in the ternary Re-Si-C system which shows that Re and its silicides have tie lines with SiC at 1600 °C. It also coincides with calculations of the free energy of reaction from assessed thermodynamic data for rhenium silicides and SiC. The implications of this Re stability with SiC for applications of Re as a metal for electrical contact to SiC-based devices are discussed.
Rapid photochemical deposition of silicon dioxide films using an excimer lamp
Authors: Bergonzo Philippe , Boyd Ian W.
Published: July 2002

We have exploited the excimer light generation principle to generate large photon fluxes over a narrow band of very short wavelengths around 172 nm. By irradiating gas mixtures of silane and oxygen with this light, we have succeeded in directly photodepositing silicon dioxide films. Very high deposition rates (500 Å/min) have been obtained for substrate temperatures as low as 300 °C. The deposited films have been characterized using ellipsometry and Fourier transform infrared spectroscopy. The influence of the deposition parameters on the film properties and their optimization are discussed. In particular, we describe the minimization of hydrogen incorporation in the films, rendering this new technique promising for applications in optical and electronic thin film processing.
Nucleation of Al-Cu-Fe alloy in a drop tube
Authors: Zhang F. X. , Yu R. C. , Wang W. K.
Published: July 2002

The undercooling and nucleation of an Al65Cu20Fe15 alloy have been investigated with a 1.2-m-long drop tube. Melted droplets solidified either in vacuum or in helium. Various size samples in the forms of a globule, a fine fiber, and an irregular splat were collected at the bottom of the tube. X-ray phase identification results indicated that the CsCl-type cubic phase with a lattice parameter of 2.94 Å and Al-Cu-Fe icosahedral quasicrystalline phase coexist in most specimens. However, the amount of each phase differs in various size samples due to the different cooling rates and undercoolings. It can be concluded that deep undercooling is beneficial to the formation of the Al-Cu-Fe icosahedral phase. In addition, in some rapidly solidified fine fibers (<105 K/S), the Al-Cu-Fe ternary amorphous phase was also observed by high resolution transmission electron microscopy.
Magnetic properties of R2Fe17−xGax compounds (R=Y, Ho)
Authors: Wang J. L. , Zhao R. W. , Tang N. , Li W. Z. , Gao Y. H. , Yang F. M. , de Boer F. R.
Published: July 2002

Structural and magnetic properties of the R2Fe17−xGax compounds (R=Y, Ho, 0≤x≤6) have been investigated. All investigated Ho2Fe17−xGax compounds with x≳2.5 crystallize in the Th2Zn17-type structure and the others in the Th2Ni17-type structure. The substitution of Ga for Fe leads to an increase in lattice constants a, c, and unit-cell volume, a decrease of the average Fe moment, and a maximum of the Curie temperature as a function of the Ga concentration at x=3, for both Y2Fe17−xGax and Ho2Fe17−xGax compounds. The high magnetic field leads to a spin phase transition from the ferromagnetic to the canted phase. The critical field Bc of the transition decreases with increasing Ga concentration. The exchange interaction constants JTT between Fe–Fe spins and JRT between R–T spins have been derived from a mean field analysis of Curie temperature. It has been found that JTT increases at first, going through a maximum at x=3, then decreases with increasing x, whereas JRT is almost independent of the Ga content, which is consistent with the result obtained from a mean field analysis of the high field magnetization curves of the Ho2Fe17−xGax.
Kramers’s rate theory, broken symmetries, and magnetization reversal (invited)
Authors: Braun Hans-Benjamin
Published: July 2002

The theory of thermally induced magnetization reversal in small particles is reviewed. The conventional Néel–Brown theory for uniform magnetization reversal and its derivation from Kramers’s rate theory are first discussed. For sufficiently elongated particles, however, a nonuniform energy barrier (‘‘nucleus’’) has lower energy than the uniform barrier and thus yields a lower coercivity. This coercivity reduction is shown to occur also for vanishing hard-axis anisotropy when the nucleus breaks the rotational symmetry around the easy axis. The prefactor of the Arrhenius factor is calculated for uniform and nonuniform barriers.
Magnetoresistance and magnetic properties of NiFe/oxide/Co junctions prepared by magnetron sputtering
Authors: Plaskett T. S. , Freitas P. P. , Barradas N. P. , da Silva M. F. , Soares J. C.
Published: July 2002

NiFe/oxide/Co junctions were fabricated by magnetron sputtering for studies of polarized electron transport across the insulating barrier. Al2O3, Al-Al2O3, and MgO insulating barriers were prepared with junction resistances from 0.5 to 116 Ω. The I-V characteristics at room temperature are linear. For low barrier resistance, the magnetoresistance of the structure is dominated by the anisotropic magnetoresistance of the ferromagnetic electrodes. For the higher barrier resistances, a different magnetoresistance effect is observed, which is tentatively related to tunneling or spin-valve effects across the insulating junction.
Temperature-time duality and deep level spectroscopies
Authors: Agarwal Sandeep , Mohapatra Y. N. , Singh Vijay A.
Published: July 2002

Relaxation of deep levels in semiconductors is studied through capacitance transients. We explore the temperature-time duality relationship which is inherent in such thermal relaxation processes. Using duality considerations we show the existence of four distinct spectroscopies. We demonstrate that the techniques for spectroscopic evaluation of capacitance transients are based on differential operators and provide a novel interpretation to spectroscopy. We extend this approach to higher order spectroscopy. Two families of higher order spectroscopy are analyzed using the formalism of temperature-time duality and differential operators. From duality considerations we have suggested a novel deep level spectroscopy as well as various improvements in line shapes and spectroscopic quality of existing techniques.
Photoluminescence investigation of GaN films grown by metalorganic chemical vapor deposition on (100) GaAs
Authors: Hong C. H. , Pavlidis D. , Brown S. W. , Rand S. C.
Published: July 2002

GaN films were grown on (100) GaAs substrates by metalorganic chemical vapor deposition and were found to be of (200) cubic or (111) cubic/(0002) hexagonal phase. Their photoluminescence characteristics remained invariant with material phase. We report assignment of band-edge photoluminescence near 3.36 eV and 3.15–3.31 eV in apparently cubic GaN to intrinsic/bound excitons and phonon-assisted, donor-acceptor pair recombination respectively, on the basis of observed temperature and intensity dependences. A free exciton energy of 3.375 eV is deduced at 6.5 K.
Carrier trapping in ultrafast metal-semiconductor-metal photodetectors on InGaAs/GaAs-on-GaAs superlattices
Authors: Hugi J. , Haddab Y. , Sachot R. , Ilegems M.
Published: July 2002

High-resolution time-resolved measurements and deep level transient spectroscopy (DLTS) measurements on InxGa1−xAs/GaAs-on-GaAs superlattices grown by molecular-beam epitaxy with InAs content x from 0% to 63% are presented. The pulse response of metal-semiconductor-metal photodetectors (MSMPDs) on these layers varies from transit-time-limited responses to ultrafast lifetime-limited responses with electron and hole lifetimes of 3 and 15 ps. DLTS measurements on the superlattices and a two-dimensional self-consistent numerical simulation of the MSMPDs pulse response indicate a strong influence of the oxygen related electron trap EL3 on the electron lifetimes. The dark currents range between 500 pA and 700 nA at 5 V for 20×20 μm2 devices and the spectral response shows cutoff wavelengths up to 1550 nm.
Trap level spectroscopy of undoped and Ga-doped Bi12GeO20 using thermally stimulated conductivity
Authors: Bloom D. , McKeever S. W. S.
Published: July 2002

The analysis of thermally stimulated conductivity (TSC) measurements performed on ‘‘pure’’ Bi12GeO20 (BGO) and BGO doped with gallium is presented. The TSC data show many overlapping TSC peaks in the temperature range of interest (80–300 K) which arise from a complex array of trapping states. The TSC signals from both pure BGO and Ga-doped BGO are similar, consisting of a series of large peaks below 160 K and many smaller overlapping peaks between 180 and 300 K. The analysis shows that the large peaks below 160 K in undoped BGO arise from two trapping centers, each characterized by a distribution of activation energies centered at ∼0.24 and ∼0.29 eV, with distribution widths of ∼0.065 eV. In the Ga-doped BGO sample the large peak seen below 180 K arises from a single trapping center at ∼0.29 eV with a distribution width of ∼0.085 eV. Activation energies, frequency factors, and concentrations of trapping states have been determined.
Perturbation of the substrate temperature by the impingement of laser ablated particles
Authors: Xu Xianfan
Published: July 2002

During the process of pulsed laser deposition of thin film materials, the impingement of the laser ablated particles onto the substrate increases the substrate temperature. In this work, the energy exchange between the ablated particles and the substrate is analyzed. The substrate temperature rise is studied via a transient two dimensional heat transfer model. In particular, the temperature evolution of silicon and quartz substrates induced by the deposition of carbon and copper films are examined. The effects of experimental conditions on the maximum substrate temperature rise are discussed.
An electrostatic model of split-gate quantum wires
Authors: Sun Yinlong , Kirczenow George , Sachrajda Andrew S. , Feng Yan
Published: July 2002

We present a theoretical model of split-gate quantum wires that are fabricated from GaAs-AlGaAs heterostructures. The model is built on the physical properties of donors and of semiconductor surfaces, and considerations of equilibrium in such systems. Based on the features of this model, we have studied different ionization regimes of quantum wires, provided a method to evaluate the shallow donor density, and calculated the depletion and pinchoff voltages of quantum wires both before and after illumination. A real split-gate quantum wire has been taken as an example for the calculations, and the results calculated for it agree well with experimental measurements. This paper provides an analytic approach for obtaining much useful information about quantum wires, as well as a general theoretical tool for other gated nanostructure systems.
Rotational hysteresis loss study on exchange coupled Ni81Fe19/NiO films
Authors: Soeya Susumu , Nakamura Shin , Imagawa Takao , Narishige Shinji
Published: July 2002

Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19 film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19 film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature for tNiO<50 nm (tNiO:NiO film thickness) seems to originate from a decrease of antiferromagnetic NiO anisotropy with decreasing tNiO.
Analysis of the offset voltage of InGaP/GaAs single-, double-, and composite double-heterojunction bipolar transistors
Authors: Fresina Michael T. , Hartmann Quesnell J. , Ahmari David A. , Gardner Nathan F. , Stillman Gregory E.
Published: July 2002

The common-emitter offset voltages of In0.49Ga0.51P/GaAs single-, double-, and composite double-heterojunction bipolar transistors have been investigated. The physical mechanisms behind variations in the offset voltage between these devices and versus base current for each device have been examined. We have found that the offset voltages of the composite collector devices decrease and remain more constant at low base currents as the composite spacer thickness decreases due to reduced space-charge recombination in the base-collector junction. An InGaP/GaAs composite-collector device with a thin spacer has an offset voltage that is almost as low and uniform as a double-heterojunction bipolar transistor (DHBT) with comparable breakdown voltage but without the poor common-emitter current characteristics of the DHBT.
X-ray study of elastic and plastic strains in Na+-implanted (001) monocrystalline MgO
Authors: Brenier R. , Beranger M. , Canut B. , Gea L. , Ramos S. M. M. , Thevenard P. , Badawi K. F.
Published: July 2002

The morphological, plastic, and elastic strains in Na+-implanted MgO have been determined by measuring the swelling of the surface with a profilometer and in-plane lattice parameters by x-ray diffraction at glancing incidence. The high value of the stress (∼9 GPa) is related to a high dislocation density with very low mobility leading to weak plastic flow (∼0.2%).
Efficient dry etching of Si with vacuum ultraviolet light and XeF2 in a buffer gas
Authors: Li B. , Streller U. , Krause H.-P. , Twesten I. , Schwentner N.
Published: July 2002

Replicas of a mask are etched in Si wafers with a micrometer lateral resolution and typical depths of 200 nm by irradiation with filtered synchrotron radiation using cutoff wavelengths of 105, 122, and 150 nm. An excellent selectivity and anisotropy is obtained by suppressing the spontaneous etching of the XeF2 etch gas (typical 10−2 mbar) with O2 or Ar buffer gas (typical 1 mbar). The efficiency of etching increases by more than two orders of magnitude by reducing the wavelength from longer than 150 nm to the spectral range of 105–122 nm. The number of removed Si atoms per incident photon reaches a value above unity for the short wavelengths. This very high quantum efficiency, which exceeds that in the visible spectral range by more than four orders of magnitude, is attributed to selective electronic excitation of a thin fluorosilyl layer on top of the Si wafer. The low probability of absorption in this layer implies a reaction efficiency far above unity.
Fractal statistics of partial discharges with polymeric samples
Authors: Ficker T.
Published: July 2002

The statistics of microdischarges crossing the air gap between conductors and insulating polymers have been measured. Long tails of pulse-amplitude and pulse-time-separation distributions have been observed and their nature examined. The long tails of time distributions followed quasiexponential behavior while the long tails of amplitude distributions satisfied a fractal power law. The transformation relations between both sets of statistics have been determined and their consequences discussed. The clearly resolved fractal long tails of amplitude distributions have not been observed so far and represent a new result in the statistical studies of microdischarges. Also the determined transformation relations between time and amplitude statistics bring a new insight into the studied phenomenon.
Growth and topography study of MnBi films
Authors: Rüdiger U. , Berndt H. , Schirmeisen A. , Fumagalli P. , Güntherodt G.
Published: July 2002

We deposited (Mn/Bi)x bilayers (x=1,2,3) on clean glass substrates at room temperature and at 200 °C without using a protective SiOx layer in order to investigate the interaction between preparation conditions, homogeneity, granularity, and topography. During annealing at temperatures between 280 and 320 °C, no protective SiOx layer on top of the (Mn/Bi)x bilayers influences the formation of MnBi crystallites, i.e., the granularity. The coercive field of the resulting MnBi films is enhanced reaching values of up to 1.25 T. The large coercive fields indicate a single-domain MnBi crystallite size of only 0.2–0.3 μm, which is favorable for a possible application as a magneto-optic storage material.
Degradation of inversion layer electron mobility due to interface traps in metal-oxide-semiconductor transistors
Authors: Matsuoka Toshimasa , Taguchi Shigenari , Khosru Quazi Deen Mohd , Taniguchi Kenji , Hamaguchi Chihiro
Published: July 2002

Degradation of inversion layer electron mobility during Fowler–Nordheim electron injection has been investigated using n-channel metal-oxide-semiconductor transistors. The change of the reciprocal effective mobility, Δ(1/μEFF), has been found to be linearly related to the generated interface trap density, ΔNit, at a given effective electric field normal to the Si/SiO2 interface. The effect of trapped charges in the oxide on the mobility degradation is rather insignificant, which is attributed to the location of trapped charges from the Si/SiO2 interface. The dependence of mobility degradation on inversion layer electron density has also been explained using a transport theory based on two-dimensional electron gas.
Crystallographic characterization of tetragonal (Pb,La)TiO3 epitaxial thin films grown by pulsed laser deposition
Authors: Kang Young Min , Ku Ja Kang , Baik Sunggi
Published: July 2002

Crystallographic properties such as lattice constants, degree of c-axis orientation, and c/a ratio of tetragonal Pb1−xLaxTiO3 (PLT, x=0–0.28) epitaxial thin films grown by pulsed laser deposition on single-crystal substrates such as MgO(001) and SrTiO3(001) were evaluated. General x-ray-diffraction techniques—θ-2θ scan and Φ scan—were used to confirm the epitaxial relations between films and substrates. The epitaxial relations were PLT(001) or (100)//substrate (001) and PLT[100] or [001]//substrate[100]. Then, using the {303} asymmetric rocking curve technique, more quantitative crystallographic informations of PLT films could be obtained. The c/a ratio and lattice constant along the a axis of c-axis-oriented PLT tetragonal unit cell were calculated from the peak location of {303} rocking curve, which is slightly different from that of the powder or bulk PLT. The existence of a-axis-oriented domains was also verified in PLT films grown on SrTiO3 substrate by {303} rocking curve. The origin of the observed ferroelectric domain structures is discussed based on transformation and thermal strains involved in the film preparation.
Electrostatic problem of a point charge in the presence of a semi-infinite semiconductor
Authors: Donolato C.
Published: July 2002

The space-charge region that is induced in a semiconductor half-space by an external point charge is analyzed. The mathematical model gives rise to a free-boundary problem, since the edge of the space-charge region [a surface z=z(d) in cylindrical coordinates] is unknown. From the formal solution of Poisson’s equation, an integral equation for z(d) is derived and approximately solved. The solution describes the depletion region in dependence of the magnitude of the point charge, the charge-semiconductor distance, and the doping and dielectric constant of the semiconductor. Approximate expressions are derived for the force acting on the point charge and are used to estimate the electrostatic force between the tip and a silicon sample in a scanning force microscope. The limitations of the model are discussed and its generalization to the case of a semiconductor coated by a dielectric film is outlined.
Nonlinear effect of self-induced electric field on diffusion-induced stresses
Authors: Lin Senpuu , Hwang Chi-Chuan
Published: July 2002

This article investigates the nonlinear effect of the self-induced electric field on the diffusion-induced stresses in a thin plate. We first approximate the nonlinear concentration-dependent diffusivity as a series of third-degree polynomials by the least-squares curve-fitting techniques and then calculate the distributions of concentration by the Galerkin method; afterwards and using these results, the diffusion-induced stresses inside the slab are determined. It is found that the differences from the linear case with a constant diffusivity are apparent. Furthermore, the nonlinear effect is modest for low-surface-concentration diffusion but extremely pronounced for high-surface-concentration diffusion.
Coprecipitation of oxygen and carbon in Czochralski silicon: A growth kinetic approach
Authors: Huh J.-Y. , Gösele U. , Tan T. Y.
Published: July 2002

Oxygen (O) and carbon (C) coprecipitation in Czochralski Si is studied in terms of a diffusion-limited growth model. The interfacial energy increase upon C incorporation into oxide precipitates as well as the changes of O and C concentrations in the Si matrix with annealing time have been taken into account. A comparison of the model predictions with available experimental data has led to the following conclusions: (i) Regardless of the C content in the crystal, it is necessary to introduce sinks for the precipitation-induced excess Si self-interstitials (I) in the matrix for high annealing temperatures. (ii) At annealing temperatures below about 1000 °C, the enhancement effect of C on O precipitation results primarily from an increase in the precipitate density. (iii) The transition in the C precipitation behavior observed in C-rich Si crystals at annealing temperatures around 800 °C is related to a change in the availability of effective I sinks in the Si matrix at these temperatures. (iv) An enhancement of C diffusivity in the presence of excess I plays an important role in increasing the precipitate growth rate, particularly at low temperatures for which no efficient I sinks are available in the Si matrix.
Effect of internal stresses and microstructure of sputtered TiN films on solid-phase reactions with Al-Si-Cu alloy films
Authors: Yamauchi T. , Yamaoka T. , Yashiro K. , Sobue S.
Published: July 2002

Solid-phase reactions at the interface between Al-Si-Cu and reactively sputtered TiN thin films have been investigated by cross-sectional transmission electron microscopy, Auger electron spectroscopy, and x-ray diffraction. In the case in which the internal stress in the TiN thin film is extremely compressive at 209 MPa, a very thin amorphous Al-Ti-Si ternary compound layer (a-Al-Ti-Si) containing microcrystallites, about 4 nm thick, is found to form at the Al-Si-Cu/TiN interface by annealing at the temperature of 450 °C for 30 min. On the other hand, in the case of a minimally compressive stress of 21 MPa, it is in a marked contrast to form a polycrystalline TiAl3 layer (c-TiAl3) on the amorphous intermediate layer ununiformly. Behavior of the internal stress in the latter TiN film as a function of heating and cooling temperature shows nonlinear characteristics, indicating that a rearrangement of the TiN film actively occurs even at low temperatures below 300 °C. Monte Carlo simulations of internal microstructures based on a ballistic aggregation model suggest to us that a short migration length corresponding to the condition of low internal stress brings about numerous vacancies and disordered regions in the TiN films. It is considered that the rearrangement of the TiN films with a diffusion of Ti atoms governs the solid-phase reactions at the Al-Si-Cu/TiN interfaces and that the formation of the bilayer of c-TiAl3/a-Al-Ti-Si originates in phase separation of the resultant Al-Ti mixing layer.
Photoconductivity of InSb/GaAs heterostructures at low temperature
Authors: Beaulieu Y. , Webb J. B. , Brebner J. L.
Published: July 2002

We have studied the low-temperature coplanar photoconductivity under pulsed and continuous illumination of n-InSb/GaAs heterostructures grown by metalorganic magnetron sputtering. The large lattice mismatch and the large difference in band gaps give rise respectively to a highly defective region at the interface and a high built-in field. The effect of these factors on the photoconductivity is then a function of the InSb film thickness. For photon energies above the InSb band gap, up to about 0.4 eV, we observe positive photoconductivity due to an increase in the electron density coupled with trapping of the excess minority carriers. As the film thickness is decreased from 2 to 0.2 μm, the photoconductivity gain per absorbed photon decreases and the decay time increases, indicating the presence of trapping or recombination centers near the interface. Illumination with photons of energy greater than 0.4 eV leads to the coexistence of a positive photoconductivity and a very slow transient negative photoconductivity which gives rise to high gains at low modulation frequencies. The negative photoconductivity increases with increasing photon energy and decreasing film thickness. Illumination at 0.9 eV from both sides of the heterostructure shows that the effect is related to diffusion and trapping of hot electrons at the interface. The onset of the saturation of the effect indicates a concentration of electron traps of about 1014 cm−2 at the interface, a value similar to the concentration of edge-type dislocations that predominantly accommodate the lattice mismatch in this heterostructure.
Helix deformation and bistable switching of ferroelectric liquid crystals
Authors: Jákli A. , Markscheffel S. , Saupe A.
Published: July 2002

We studied electro-optical properties of 5–13-μm-thick films of a short pitch (0.43 μm) chiral smectic C liquid crystal in bookshelf structure. The anchoring at the silane treated surfaces was relatively weak so that the relaxed bookshelf structure was helical also at the surfaces. Low fields cause only a deformation of the helix. Larger fields unwind the helix and give a quasibistable switching since the helix recovers at zero fields only slowly via a nucleation process. There is an intermediate field range where the sample is in a mixed state: In some areas the helical structure remains stable, while in other areas switching occurs between two unwound states. In the intermediate field range bistable switching with a gray scale is possible.
Nuclear magnetic resonance of 7Li in LiB3O5 crystal and glass
Authors: Matsuo Toru , Shibasaki Masayuki , Saito Norio , Katsumata Tooru
Published: July 2002

The spin lattice relaxation rates and the absorption spectra of 7Li in both LiB3O5 crystal and glass samples were measured from room temperature up to 800 K. At low temperatures the dominant contribution to the relaxation rates was due to the spin–phonon interaction in both samples. For the crystal the relaxation caused by the diffusion of Li ions became significant above 580 K, and the narrowing of the absorption line started at about 540 K. For the glass the narrowing of the line started at room temperature and the effect of the diffusion of Li ions on the relaxation rate appeared above 400 K. From these results the correlation time τc of the fluctuating field at Li nuclei was obtained. Each value of the activation energy of τc deduced from the relaxation rate and the narrowing of the line, respectively, agreed well.
Phase properties of microwave magnetic envelope dark solitons in yttrium iron garnet thin films (abstract)
Authors: Nash J. M. , Kabos P. , Patton C. E. , Staudinger R.
Published: July 2002

Although microwave magnetic envelope (MME) solitons in thin yttrium iron garnet (YIG) films have been studied extensively, only recently have dark MME solitons been observed. For a single dark soliton, one has a carrier phase change of 180°. Reference 1 indicated such a change. However, the overall phase change for a given dark pulse experiment should be 0°. The objective of this work was to examine phase profiles in detail for the dark soliton experiment. The results show that both of the above expectations are, in fact, satisfied. The 5 GHz measurements were done with a magnetostatic wave delay line structure with a long and narrow 7.2 μm thick single crystal YIG film oriented in the surface wave configuration. A GaAs microwave switch produced 300 ns long bright pulses separated by a dark pulse with a width in the 15 ns range. The input power was 0.002–1.0 W. The output signal showed two main dark pulse features, independent of power level. At low powers, the changes in phase across both features were small. With increasing power, the phase change across the first feature showed a rapid increase while the phase change for the second feature remained small. The phase change for the first feature saturated at a large value, on the order of 200°, relative to the phase change at low power.

The phase change over the entire profile was always 0°. These results show: (1) There are significant phase changes at both low and high powers in the dark pulse experiment. (2) The phase change for a dark soliton must take into account the background phase change at lower powers. (3) Only the main dark pulse feature in the output profile exhibits single dark soliton characteristics through a clearly resolved phase saturation at 180°. (4) These thresholds are on the order of 10 mW for typical strip line transducer structures, compared to 100 mW or so for bright solitons.
Stress and interface morphology contributions in the crystallization kinetics of a GexSi1−x thin layer on (100) Si
Authors: Corni F. , Frabboni S. , Tonini R. , Ottaviani G. , Queirolo G.
Published: July 2002

A study of the solid phase epitaxial growth from amorphous phase of a strained GexSi1−x thin layer made by 74Ge+ ion implantation of (100) Si is presented. Ge peak concentration is 6.4 at. %. Principally employed techniques are time-resolved reflectivity for crystallization rate measurements and cross sectional transmission electron microscopy (TEM) for interface morphology imaging. The kinetics is heavily affected by the mismatch stress induced by the Ge; the experimental data cannot however be explained considering only the average stress along the amorphous-crystal interface and another mechanism is required. Cross sectional TEM observations of partially crystallized samples reveal the tendency of the interface to roughen with an evolution reflected in the crystallization activation energy. Such roughness is assumed to affect the kinetics by locally enhancing the interface stress.
Effects of magnetostatic interaction between two single-domain cobalt bars on crystal anisotropy and switching field (abstract)
Authors: Chou Stephen Y. , Kong Linshu
Published: July 2002

Understanding the magnetostatic field effects of a single-domain particle on its neighbors during thin film deposition and magnetic recording is essential to the development of magnetic recording media. Here, we present a unique study of such effects using nanofabrication technology and magnetic force microscopy (MFM). We fabricated pairs of single-domain cobalt bars. Each bar is 35 nm thick, 50 nm wide, and 1 μm long. The twin bars were oriented side-by-side on a silicon substrate with a spacing that varied from 50 to 1000 nm. In fabrication, a resist template was first created on a silicon substrate, followed by e-beam evaporation of cobalt into the bar-shaped openings of the template as well as on the top of the template. Then, the resist was dissolved making the cobalt on top of the template lift off. MFM showed that as-fabricated bars are single magnetic domain. We polarized the magnetic moment of the twin single-domain bars in the same direction and measured the magnetic field needed for the following three cases: (a) switching the magnetization direction of only one of the twin bars, Ha; (b) switching both bars, Hb; and (c) switching one bar after the other bar was physically removed by a nanotechnique, Hc.

Two important facts can be seen. First, for a given bar spacing, Ha<Hc<Hb. The difference is due to the demagnetization field of the neighbor which is on the order of 150 Oe depending upon the bar spacing. Second, all three switching fields decrease significantly as the bar spacing becomes larger. For example, Hc is 2050 Oe for a 300 nm bar spacing and 1700 Oe for 1000 nm spacing. The spacing dependence of Hc clearly indicates that the intrinsic switching field of a bar has been significantly affected, during the cobalt deposition, by the demagnetization field of its neighbors. One explanation is that as soon as the first few atomic layers of cobalt were deposited into a nanoscale bar shaped opening in the resist template, a single domain is formed immediately creating a magnetostatic field. This field forces the c axis of the cobalt that was later deposited into the neighboring bar to align with the field, therefore enhancing the crystal anisotropy and intrinsic coercivity of the neighboring bar. This study implies that a small topology variation on a magnetic disk substrate may cause a large local coercivity variation due to the enhancement of crystal anisotropy and coercivity induced during the deposition by the demagnetization field of neighboring single-domain particles.
Giant magnetoresistance and high sensitivity in annealed NiFeCo/Ag multilayers
Authors: Dykes J. W. , Kim Y. K. , Tsoukatos A. , Gupta S. , Sanders S. C.
Published: July 2002

We report giant magnetoresistance in annealed NiFeCo (2 nm)/Ag (3.3 nm) multilayers. For all samples, post-deposition annealing was necessary for the observation of GMR analogous to the NiFe/Ag discontinuous multilayer system. The GMR ratio and field sensitivity varied widely as a function of annealing from temperatures between 300 and 400 °C. A high MR ratio of 6.3% and field sensitivity of 11.3%/(kA/m) (0.9%/Oe) were observed for a seven bilayer sample annealed to 350 °C. In general, NiFeCo/Ag multilayers produced greater MR responses and field sensitivities than comparably annealed NiFe/Ag multilayers. NiFeCo/Ag multilayers, however, display more hysteresis than NiFe/Ag multilayers and lack a zero-crossing saturation magnetostriction versus annealing temperature. Multilayer deposition in a magnetic field severely degrades MR performance since it prevents the magnetostatic antiparallel alignment between magnetic grains which occurs in the absence of an applied magnetic field following sample annealing.