bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis and modeling of the base currents of Si/Si1−xGex heterojunction bipolar transistors fabricated in high and low oxygen content material
Authors: Shafi Z. A. , Ashburn P. , Post I. R. C. , Robbins D. J. , Leong W. Y. , Gibbings C. J. , Nigrin S.
Published: July 2002

A comparison is made of the dc electrical characteristics of Si/Si1−xGex heterojunction bipolar transistors (HBTs) fabricated on high oxygen content material grown using molecular beam epitaxy and low oxygen content material grown using chemical vapor deposition. The base currents of Si/Si0.85Ge0.15 HBTs with a high oxygen content are significantly higher than those of comparable devices with a low oxygen content and the base currents of both are higher than those of silicon homojunction devices. In addition the base current in the low oxygen content devices increases significantly when the germanium concentration is increased from 15% to 23%. The roles of the lifetime in the base and the out-diffusion of the boron from the Si1−xGex base are investigated using a two-dimensional drift-diffusion device simulator. It is shown that the increased base currents in the HBTs are caused by recombination in the neutral base, and that the lifetime in the Si1−xGex is an important parameter in determining the base current. Modelling of the measured base currents indicates that the lifetime in the high oxygen content Si0.85Ge0.15 is reduced by a factor of approximately 15 compared with silicon, but in the low oxygen content Si0.77Ge0.23 by a smaller factor of approximately 4. Boron out-diffusion from the base is present in both the high and low oxygen content HBTs, but it appears to be significantly less in the former. A high oxygen content in Si1−xGex HBTs therefore has the disadvantage of decreasing the lifetime, but the advantage of decreasing the boron out-diffusion from the base.
Extinction spectra of high conductivity fibrous aerosols
Authors: Jelinek A. V. , Bruce C. W.
Published: July 2002

Straight, well separated fibers were arrayed in planes normal to the propagation (k) vector for extinction measurements over a frequency range of 20 to 160 GHz. The fiber half-length (h) was chosen so that the key parameter kh varies from about 0.9 to about 7. All fibers were oriented parallel with the incident E-field. Initial studies pertained to determination of effects of attachment to the substrate material and separation distances between fibers. Fiber composition is graphite coated with nickel to a thickness that should appear to the incident waves to be solid nickel. The resulting spectra exhibit the expected strong scattering resonances and, more importantly, agree in form and magnitude with a recently developed theory in the vicinities of the anti-resonances. The latter provide a strong test of the assumed form of the current function induced in the fiber. Graphical comparisons of the computed and measured spectra show that good agreement is obtained.
Importance of poly(N-vinylcarbazole) dopant to poly(3-octylthiophene) electroluminescence
Authors: Wang Guangming , Yuan Chunwei , Wu Haiming , Wei Yu
Published: July 2002

A new method is presented of enhancing the electroluminescence of short alkyl side chain poly(3-alkylthiophene) (P3AT) by adding poly(N-vinylcarbazole) (PVK) as a dopant in the emitting layer. The organic electroluminescent (EL) diode with one layer of poly(3-octylthiophene) (P3OT) and P3OT doped with various amounts of PVK (P3OT/PVK) as the EL-emitting layer sandwiched between indium/tin oxide and aluminum electrodes has been fabricated by spin-coating technique. The P3OT/PVK EL diode starts to emit red-orange light with an optimum molar ratio of P3OT to PVK at 5 V. Evidence suggests that the PVK dopant plays an important role in the P3OT electroluminescence. Such a molar ratio of P3OT to PVK is found to exist and acting mechanism of the dopant is investigated for enhancing the P3OT electroluminescence.
Silicon incorporation in GaAs: From delta-doping to monolayer insertion
Authors: Wagner J. , Newman R. C. , Roberts C.
Published: July 2002

Raman spectroscopy was used to study the incorporation of Si into doping layers in GaAs, grown by molecular beam epitaxy at a temperature of 400 °C, for Si concentrations ranging from the δ-doping level to a ML coverage. The strength of the scattering by local vibrational modes of substitutional Si was almost constant for Si areal concentration [Si]A in the range 5×1012<[Si]A<5×1013 cm−2 but then decreased, dropping below the detection limit for [Si]A≳3×1014 cm−2. At these concentrations a new vibrational band emerged at a frequency close to 470 cm−1 and developed into the optic zone center phonon of a coherently strained epitaxial layer of Si embedded in GaAs when a coverage of ≊1.5 ML (9.3×1014 cm−2) was reached. These findings strongly indicate that the observed saturation and the eventual decrease of the concentration of substitutional silicon is caused by an increasing incorporation of deposited Si into two-dimensional islands of covalently bonded Si.
Theoretical investigation of fabrication-related disorder on the properties of photonic crystals
Authors: Fan Shanhui , Villeneuve Pierre R. , Joannopoulos J. D.
Published: July 2002

How various deviations in perfect photonic crystals, which may arise during fabrication, can affect the size of photonic band gaps is investigated theoretically. The emphasis is on determining the effects of misalignment of basic structural elements and overall surface roughness, because of their general fabrication relevance. As an example, calculations on a newly proposed three-dimensional photonic crystal are performed. It is shown that the size of the gap is tolerant to significant amounts of deviation from the perfect structure.
Anomalous charge-transfer in La2−βSrβCuO4 and in Nd2−zCezCuO4
Authors: Blackstead Howard A. , Dow John D.
Published: July 2002

The hypothesis of charge-transfer to the cuprate-planes, which lies at the foundation for most theories on high-temperature superconductivity, is re-examined in light of recent neutron-diffraction data for La2−βSrβCuO4 and Nd2−zCezCuO4. The data indicate that the cuprate-planes do not acquire holes as the materials become superconducting, and that the superconducting p-type regions of the unit cells are in the noncuprate-plane charge-reservoir layers instead.
Multivacancy clusters in neutron-irradiated silicon
Authors: Xu Yuesheng , Liu Caichi , Li Yangxian , Wang Hongmei
Published: July 2002

The characteristics and annealing behavior of vacancy clusters in neutron transmutation doped silicon (NTDSi) were investigated by means of Fourier transform infrared spectroscopy (FTIR) and photoinduced transient spectroscopy. FTIR results showed that three IR absorption peaks at 707, 742, and 776 cm−1 occurred during annealing NTDSi samples at temperatures ranging from 350 to 600 °C. By comparing the present results with those reported earlier, the defect center (we named it the P0 center) which gives rise to these IR absorption bands, was assumed to be a multivacancy cluster consisting of at least six vacancies. Photoinduced transient spectroscopy results showed that a P0 center introduced two acceptor levels at Ec−0.427 eV and Ec−0.524 eV inside the energy gap.
Magnetooptical Kerr effect in Fe-SiO2 granular films
Authors: Jiang Z. S. , Jin G. J. , Ji J. T. , Sang H. , Du Y. W. , Zhou S. M. , Wang Y. D. , Chen L. Y.
Published: July 2002

Polar Kerr rotation spectra of Fe-SiO2 granular films were measured at room temperature in the wavelength region between 400 nm and 850 nm. A maximum Kerr rotation angle (θk) of 18 min was obtained when a magnetic field of 10 kOe was applied perpendicular to the film plane. We have measured θk as a function of Fe volume fraction (fv) of the films. It was found that θk increases with increasing fv and peaks at fv≊0.4. We also found that θk decreases as the Fe particle size increases. Our results indicate that the density of Fe particles and the interfaces between Fe particles and SiO2 matrix may play an important role in the Kerr rotation of granular films.
Strain-induced band-gap modulation in GaAs/AlGaAs quantum-well structure using thin-film stressors
Authors: Deng F. , Liu Q. Z. , Yu L. S. , Guan Z. F. , Lau S. S. , Redwing J. M. , Geisz J. , Kuech T. F.
Published: July 2002

The effect of strain-induced band-gap modulation has been studied in a GaAs/AlGaAs multiple-quantum-well structure with the wells located at various depths in the structure. The energy change in the quantum wells was calculated based on simple elasticity theory and measured using photoluminescence on the structure where a thin-film stressor array was deposited. Metallic thin-film stressors were made by conventional thin-film deposition techniques followed by photolithography. It was found that the elasticity theory describes the energy changes reasonably well in comparison with the experimental results. For stressor layers that react with the heterojunction structure, the situation was more complex and requires more detailed analysis. Based on the calculated and experimental results it appears possible to fabricate quantum wire with lateral dimensions of less than 100 nm using thin-film technology and e-beam lithography.
Shock response of polycrystalline silicon carbide undergoing inelastic deformation
Authors: Feng R. , Raiser G. F. , Gupta Y. M.
Published: July 2002

Longitudinal stress profiles have been measured in polycrystalline silicon carbide (SiC) shocked to peak stresses from 7.3 to 23 GPa. Dispersive wave fronts, consistent with the expected inelastic response, were observed beyond the previously reported Hugoniot elastic limit (HEL) of 11.7 GPa. Detailed numerical analyses were carried out to interpret the observed inelastic response using both a strain-hardening, plasticity model and a pressure-dependent strength, stress relaxation model. Both models show good agreement with the data; the latter provides a better fit to the transient features in the measurements suggesting rate dependence in the material response. The computed Hugoniot curve matches all of the peak state data for two different types of SiC that display more than 20 % variation in HEL. This suggests that the measured HEL for SiC is not a proper indicator of the material strength in the shocked state. The results also show that the longitudinal data and analyses are insufficient to resolve issues related to material strength and mechanisms governing inelastic deformation in shocked SiC. The need for a more complete characterization of the shock response of a high-strength brittle material is discussed.
Fracture testing of silicon microcantilever beams
Authors: Wilson C. J. , Ormeggi A. , Narbutovskih M.
Published: July 2002

Silicon microcantilever beams are fractured and characterized. The specially designed beams, etched into two wafers, are loaded to fracture in bending using a unique measurement system. A finite element model of the beams is created, and ABAQUS is used to calculate the displacements and stresses produced by an applied load force. A special testing scheme is devised to obtain certain model parameters: E〈110〉, the Young’s modulus along the length of the beam and Lforce, the position of the applied force. With these parameters defined, the model is well correlated with that of the experimental data. The fracture stress (strength) of the beam is obtained from the stress produced in the model at the fracture location by a load equivalent to the experimental fracture force. This fracture stress can be used as a design parameter for silicon micromechanical structures. Numerous beams are fractured from both the front and back sides of the wafer, and statistical fracture strength results are compiled for each of these cases. The fracture strength of the front side, 3.3 GPa (average), is significantly greater than that of the backside, 1.0 GPa (average). This dissimilarity is attributed to the differences in the surface roughness of these sides.
In situ observation of the periodical domain structure formation in KTiOPO4
Authors: Buritskii K. S. , Dianov E. M. , Maslov V. A. , Chernykh V. A. , Shcherbakov E. A.
Published: July 2002

The strain-induced periodical domain inversion in KTiOPO4 (KTP) has been demonstrated in situ by means of the nonlinear light diffraction. The temperature of 730 °C was found to be the starting point of the opposite sign domains growth under the heating of −Z KTP substrate (rate 70 °C/s) with the periodical SiO2 mask. The depth of the fabricated domain-reversed structure of the 5.5 μm period was measured to be 4.5 μm.
Electrical breakdown induced by silicon nitride roughness in thin oxide–nitride–oxide films
Authors: Reisinger H. , Spitzer A.
Published: July 2002

The time-dependent dielectric breakdown (TDDB) characteristics of thin oxide–nitride–oxide (ONO) films containing 4 nm of Si3N4 were found to be exclusively determined by statistical thickness fluctuations of the Si3N4 layer. A two-parameter statistical model describes this roughness and explains experimental TDDB data. A simple growth model combined with the statistical model reduces the number of parameters to one. It is consistent with TDDB data and is in quantitative agreement with transmission electron microscopy and x-ray photoelectron spectroscopy data and is able to explain the origin of the roughness. On an ONO with a 4 nm Si3N4 layer and an area of 8 cm2—corresponding to the total storage area of a 256 Mbit dynamic random access memory—the thinnest point in the Si3N4 can be expected to be below 10 Å. So eliminating the Si3N4 roughness would bring a drastic improvement in reliability.
Diameter-dependent optical losses in pillar microcavities
Authors: Tezuka Tsutomu , Nunoue Shinya
Published: July 2002

Optical losses in pillar microcavities have been measured directly by means of microreflectance spectroscopy. It was shown that optical losses of the cavities larger than about 3 μm were constant irrespective of the diameters. The lasing thresholds were also measured, and they were proportional to the cavity volume. This result is consistent with the measured constant optical losses. A drastic increase in optical losses has been observed when reducing the cavity diameters below 2 μm. The scattering losses due to sidewall roughness were considered mainly responsible for the optical losses.
Energy gap of nanoscale Si rods
Authors: Yorikawa H. , Uchida H. , Muramatsu S.
Published: July 2002

The electronic structure of silicon rods has been studied by means of the tight-binding recursion method to investigate the dependence of the energy gap (Eg) on a rod length and the direction of the rod axis. An empirical expression for Eg is derived from numerical results for the rods in 〈100〉, 〈110〉, and 〈111〉 directions. This expression is applicable to the energy gaps of wires and crystallites, which can be regarded as limiting cases of rods.
Bolometric and nonbolometric infrared photoresponses in ultrathin superconducting NbN films
Authors: Johnson M. W. , Herr A. M. , Kadin A. M.
Published: July 2002

The photoresponse of 10 nm thick superconducting NbN meander lines is measured using an amplitude modulated infrared semiconductor laser operating at a wavelength of 1300 nm. The response time of the film is found to be less than 1 ns with a measured responsivity of up to 1500 V/W of absorbed power at 100 kHz. Thermal properties of the film are extracted from current–voltage characteristics using a self-heating hot spot model. At temperatures well below the superconducting transition, the magnitude of the photoresponse is found to be an order of magnitude too large to be purely bolometric, even when electron heating and effects due to intergranular weak links are taken into account. The photoresponse is seen to be bolometric near Tc. Other contributions to photoresponse are discussed, including kinetic inductance and the photofluxonic effect.
Magnetization processes and optimal performance of magnetostrictive piezoelectric sensors
Authors: Prieto J. L. , Aroca C. , Sánchez M. C. , López E. , Sánchez P.
Published: July 2002

We discuss the magnetization processes in sensors based on the superposition of a magneto- restrictive amorphous ribbon over a piezoelectric plate. These devices show an unusual behavior of the null condition in the response curves, which we have related theoretically and experimentally to the coercive force of the magnetic sample and to changes in the magnetic domain pattern for frequencies near the longitudinal piezoelectric resonance frequency.
Exchange coupling of sputter deposited NiCo–O/NiFe thin films
Authors: Tan Minshen , Tong Hua-Ching , Tan Swie-In , Rottmayer Robert
Published: July 2002

NiCo–O serving as the exchange layer in a spin valve structure has the advantage of being corrosion resistant and unable to shunt sensing current in the read element. In this work, NiCo–O/NiFe thin films are studied. The films are deposited on Si wafers using rf sputtering. The antiferromagnetic NiCo–O films were reactively sputtered from alloy targets; one has a 55 at. % of Co and the other 60 at. % of Co. The chamber atmosphere is an Ar/O2 mixture containing ∼10% O2. For films with optimal structures, the exchange strength is around 40–45 Oe. The coercivity of the pinned layer is between 18–26 Oe. The exchange strength and coercivity as a function of NiCo–O thickness (with NiFe thickness fixed at 200 Å) and NiFe thickness (with NiCo–O fixed at 250 Å) are examined. The exchange field is found to increase with the antiferromagnet thickness initially and then flatten out or drop slightly when the antiferromagnet thickness is beyond 200–250 Å. Blocking temperature of the bilayer films is also measured, and is found to increase with NiCo–O thickness. NiCo–O with lower Co content shows slightly higher blocking temperature.
Epitaxial ferromagnetic MnAs thin films grown on Si (001): The effect of substrate annealing
Authors: Akeura K. , Tanaka M. , Nishinaga T. , De Boeck J.
Published: July 2002

We have studied two different types of epitaxial ferromagnetic MnAs thin films on Si (001) substrates grown by molecular beam epitaxy. When the Si substrates were annealed at a relatively high temperature (∼900 °C) and then MnAs was grown, we obtained epitaxial MnAs films with twofold crystal symmetry (type I). In contrast, when the thermal cleaning of the Si substrate was done at a lower temperature (∼600 °C), epitaxial MnAs thin films had fourfold crystal symmetry (type II). The growth plane in both types of MnAs thin films was the (1̄101) of the hexagonal MnAs. The type I MnAs films are single domain with strong magnetic anisotropy, whereas the type II MnAs films are double domain with the lack of strong magnetic anisotropy.
Initial stage of cubic boron nitride film growth from vapor phase
Authors: Ichiki Takanori , Amagi Satoshi , Yoshida Toyonobu
Published: July 2002

Low pressure inductively coupled plasma enhanced chemical vapor deposition (ICP–CVD) made it possible to deposit stoichiometric boron nitride (BN) films with thickness gradients in lateral direction, which clearly demonstrate the progresses of the cubic BN (c-BN) growth. Under optimal deposition conditions, single-phase c-BN grows on the initially deposited sp2-bonded BN layer, which had a critical sp2-BN-thickness of around 30–100 nm, depending on the substrate potential. It was also found that there exists a very close correlation between the c-BN formation and Ar incorporation, which was lower than the detection limit in the sp2-bonded BN layer, while approximately 0.4 at. % Ar was detected in the c-BN layer. Furthermore, drastic change of the strained state in the film and the surface morphology upon the transition of the growth phase from sp2-bonded BN to c-BN was confirmed. As a whole, the comprehensive characterization of the initial progress of the c-BN formation suggested that a highly compressed state in the atomistic local scale, not in the macro scale, should be essential for the c-BN formation.
Effect of annealing temperature on optical and structural properties of ion-beam-synthesized semiconducting FeSi2 layers
Authors: Yang Z. , Homewood K. P.
Published: July 2002

Both as-implanted and annealed ion-beam-synthesized semiconducting FeSi2 layers on Si(001) substrates have been investigated by optical transmission measurements and transmission electron microscopy. The effect of annealing temperature on the minimum direct band gap and the Urbach tail width was found to be associated with the growth of β grains, the formation of a well-defined polycrystalline layer, and the removal of implantation-induced damage during the subsequent thermal annealing. The direct band gap and the Urbach tail width in the sample first annealed at a temperature to produce a metallic α phase, followed by a prolonged annealing at a temperature to produce a β phase, were also studied.
Study of magnetic reorientation phenomenon and magnetic properties of Pd/(Pt/Co/Pt) multilayers
Authors: Xiao Ying , Xu Jun-Hao , Rao K. V.
Published: July 2002

Pd/(Pt/Co/Pt) modulated multilayer films have been prepared at substrate temperatures ranging from room temperature to 350 °C by e-beam evaporation using various buffer layers. All these films exhibit perpendicular magnetic anisotropy with square polar Kerr hysteresis loops at room temperature. However, we observed a magnetic reorientation from a perpendicular anisotropy to an in-plane anisotropy at temperatures close to the Curie temperature TC. The reorientation temperature TR is found to depend strongly on the nonmagnetic Pd-spacer thickness in these Pd/(Pt/Co/Pt) multilayers. Over the room temperature to TR range, we observe a linear temperature dependence of magnetic coercivity. AFM and STM characterization of the role of buffer layers on the microstructure, the dependence of coercivity and squareness of the Kerr hysteresis loops on the buffer layers of Au, Pt, and Al are also presented.
Dielectric tensor characterization of Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films
Authors: Celinski Z. , Yan Zheng
Published: July 2002

We have grown Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 alloy films on glass substrates under UHV conditions. These films exhibit good magneto-optical (MO) properties. Unfortunately, the measurements of Kerr rotation and ellipticity cannot by themselves provide reliable evaluation of the MO properties of a layer capped with a dielectric SiO film. The purpose of the present work was to determine the dielectric tensor in the MnBi1−xSbx films within a multilayer stack, using a combination of ellipsometric, reflection/transmission, and polar MO Kerr effect measurements in the wavelength range of 360–860 nm. We have evaluated the Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films based on the intrinsic MO figure of merit (FOM) defined by FOM=‖εxy‖/(2 Im εxx), where εxy and εxx are the diagonal and off-diagonal elements of the dielectric tensor of the MO material. For short wavelengths (360–550 nm) the measured FOM in the Mn0.53Bi0.47 and Mn0.52Bi0.44Sb0.04 films is significantly larger (≥factor of 2) than that commonly observed in TbFeCo films (∼0.01).
Variations in auto-oscillation frequency at the main resonance in rectangular yttrium–iron–garnet films
Authors: Prabhakar A. , Stancil D. D.
Published: July 2002

High power microwave experiments conducted on a rectangular yttrium-iron-garnet (YIG) film indicate a parabolic dependence of the square of the frequency of auto-oscillations on microwave signal amplitudes, an observation that is qualitatively consistent with prior theoretical predictions. Forward volume magnetostatic waves were excited using a microstrip transducer kept in contact with a YIG film placed in a constant external magnetic field (Hdc=3.48 kG). Variations in the input microwave power (10 dBm<P<25 dBm) and frequency (5.1 GHz<f<5.7 GHz) were used to locate and study auto-oscillations close to the Suhl instability at the main resonance. A novel method of viewing changes in the microwave passband using density plots enables us to study variations in the dipole gaps in the passband near the Suhl instability. A broadening of dipole gaps into fingerlike regions of weak transmission marks the onset of auto-oscillations beyond the instability threshold. These regions are associated with a variety of spin-wave dynamics ranging from simple auto-oscillations to auto-oscillations with period doubling and in some cases an abrupt transition to a turbulent wide-band power spectrum. The parabolic dependence of the auto-oscillation frequency persists despite a period-doubling bifurcation.
Optical limiting characteristics and mechanism of silver bromide nanosols
Authors: Sahyun M. R. V. , Hill Susan E. , Serpone N. , Danesh Reza , Sharma Devendra K.
Published: July 2002

Optical limiting behavior has been observed in nanosols comprising ∼60 Å particles of silver bromide. Switching times, τ, are consistently in the ns regime, and values of I1/2;, the pulse laser exposure which elicits a 50% decrease in transmittance of the medium, can be less than 20 mJ/cm2. Nonlinearity of response, I1/2, and response time all increase with decreasing pAg [Eq. (2)] of the nanosols. The materials can be cycled apparently indefinitely, provided they are subjected only to very short (sub-ns) laser pulses. Optical switching can be spectrally sensitized; comparable response parameters are obtained under these conditions. The nonlinear character of the response is, however, very different: a true response threshold, I0=∼12 mJ/cm2, is observed. Analysis of the optical limiting mechanism suggests that iodide doping, which is adventitious in the present case, is critical to the observed photophysical behavior of the nanosols.
Polyvinylidene fluoride films as infrared to visible converter material
Authors: Beaulieu R. , Lessard R. A. , Chin S. L.
Published: July 2002

Polyvinylidene fluoride (PVDF) films suitable for infrared to visible image conversion at the wavelength of 10.6 μm are investigated. Birefringent PVDF films are simultaneously exposed to infrared and visible radiations and the variation of the transmitted intensity at 633 nm is measured as a function of the incident infrared intensity. The possibility of using PVDF films as infrared to visible image converter material is demonstrated.
Oxygen-related vibrational modes produced in Czochralski silicon by hydrogen plasma exposure
Authors: Stein H. J. , Medernach J. W.
Published: July 2002

Plasma hydrogenation of Czochralski Si has been performed to investigate the introduction of Si–O stretch modes and their correlation with thermal donor formation. Plasma hydrogenation at 275 °C introduces a well-resolved vibrational absorption band at 1005 cm−1, while absorption due to electronic excitations for thermal donors remains weak. We attribute this band to a Si–O precursor center for thermal donor formation, and suggest it is the oxygen dimer center discussed in other studies of oxygen in Si. Vibrational modes introduced at 990 and 1000 cm−1 during post-hydrogenation furnace annealing at 400 °C correlate with thermal donors TD2 and TD3, respectively. Stretch frequencies for Si–O in thermal donor centers are compared to those for oxygen aggregates in oxygen-implanted and electron-irradiated Si.
Mean-field theory of electromigration-induced void drift and coalescence in metal thin films
Authors: Boularot Hervé , Bradley R. Mark
Published: July 2002

We introduce a mean-field theory for the drift and coalescence of voids in a thin, passivated, single-crystal metal film carrying an electrical current. In our model, each circular void drifts along the local electric field with a speed inversely proportional to its radius, and current crowding is taken into account. Our approach leads to a mean-field equation of motion which we integrate numerically. We find that the number of voids of area a per unit film area, n(a,t), follows the scaling form n(a,t)=t−2(a/t) for large times t. As a result, the mean void area grows linearly in time in the long-time regime.
Homogeneity assessment of YBa2Cu3O7−x single crystals by cathodoluminescence microscopy
Authors: Díaz-Guerra C. , Piqueras J.
Published: July 2002

Cathodoluminescence (CL) of YBa2Cu3O7−x single crystals has been investigated in the scanning electron microscope. Some regions of the crystals, generally related to steps or other growth features, show enhanced CL intensity. Spectra show the presence of two emission bands centered at about 2.85 and 2.3–2.4 eV, respectively. In the regions of higher CL intensity, the 2.3 eV band, which has been attributed to defects in the oxygen sublattice, shows a higher relative intensity.
Electric-field controlled color effect in cholesteric liquid crystals and polymer-dispersed cholesteric liquid crystals
Authors: Chilaya Gouram , Hauck Gerd , Koswig Hans Dieter , Sikharulidze David
Published: July 2002

In cholesteric liquid crystals with positive dielectric anisotropy an electric-field induced texture with unusual properties was observed. This texture appears after the reduction of the electric field during the transition from the homeotropic nematic state into the planar cholesteric one. The angular and the electric-field dependence of the spectral characteristics indicate that this state possesses diffractive grating properties. A voltage controlled color shift and a laser beam deflection are possible. An electric-field controlled cholesteric diffractive grating texture was also observed in polymer-dispersed films. In this case the electric-field induced transition into the homeotropic state is not necessary.
Optimized SiO2/InP structures prepared by electron cyclotron resonance plasma
Authors: Besland M. P. , Jourba S. , Lambrinos M. , Louis P. , Viktorovitch P. , Hollinger G.
Published: July 2002

A fabrication procedure for high quality SiO2/InP structures has been developed using electron cyclotron resonance plasmas. The oxidation of InP in an oxygen plasma was investigated by x-ray photoelectron spectroscopy (XPS) to prepare an appropriate interfacial layer made of a single phase native InP oxide. Silicon dioxide films were prepared on InP (100), using SiH4 and oxygen plasma. We studied the effect of plasma parameters (substrate temperature, flow rates, flow rate ratio, chamber pressure, and annealing conditions) on the structural and electrical properties of the films. The SiO2 films and the SiO2/InP interface were characterized in situ by XPS and ex situ by spectroellipsometry. Electrical C–V and I–V measurements were performed on metal-insulator-semiconductor structures. The films are stoichiometric for deposition temperatures ranging from 150 to 300 °C, with bulk properties very close to those of thermally grown silicon dioxide. Optimized SiO2/InP structures, with an interface state density of 1011 cm−2 eV−1 and a long term drift that is lower than 0.2 eV after 103 s of stress under 4 V in accumulation, are obtained for a deposition temperature of 200 °C and with a 10 Å thick single phase native oxide prepared in indirect plasma conditions. It was found that the electrical interfacial properties of the SiO2/InP structures are directly correlated to their interfacial chemistry.
Characterization of stoichiometric surface and buried SiN films fabricated by ion implantation using extended x-ray absorption fine structure
Authors: Paloura E. C. , Lioutas C. , Markwitz A.
Published: July 2002

The microstructural properties of stoichiometric surface and buried Si3N4 films, fabricated with 15N ion implantation into Si wafers, are studied using the extended x-ray absorption fine structure (EXAFS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopies. Complementary information about the film composition and structure is provided by nuclear reaction analysis (NRA) and cross-section transmission electron microscopy (XTEM). The films have been characterized in the as-implanted state and after annealing in the temperature range 1100–1200 °C. For all the examined films, the N/Si ratio at the peak of the nitrogen profile, as measured by NRA is 1.33, a value that corresponds to stoichiometric nitrides. However, small compositional deviations towards a N-rich composition are detected by EXAFS in the surface nitrides. The excess nitrogen is also detectable in the NEXAFS spectra, where it introduces a characteristic resonance line superimposed to the absorption edge. Finally, XTEM observations confirm the formation of the nitride layers and reveal different degrees of damage at the Si3N4/Si interface for the low and high energy implantations, respectively.
Photoluminescence observations in band-gap tailored InGaAlAs epilayers lattice matched to InP substrate
Authors: Chua S. J. , Ramam A.
Published: July 2002

Epitaxial layers of In1−x−yGaxAlyAs with band-gap energies varying in the range 0.78 to 1.42 eV and Al mole fraction varying from 0.05 to 0.43, are grown lattice matched to InP substrate by molecular-beam epitaxy. The lattice mismatch of the independently grown epilayers is less than 0.03%. Photoluminescence in the temperature range of 4–90 K showed a characteristic ‘‘dip behavior’’ in the plot of emission energy versus temperature, which is observed for the first time to disappear in layers with Al mole fraction less than 0.22. This anomaly is postulated to originate from the onset of multiple competing transitions and exciton localization.
Complex effective permittivity of a lossy composite material
Authors: Sareni B. , Krähenbühl L. , Beroual A. , Brosseau C.
Published: July 2002

In recent work, boundary integral equations and finite elements were used to study the (real) effective permittivity for two-component dense composite materials in the quasistatic limit. In the present work, this approach is extended to investigate in detail the role of losses. We consider the special but important case of the axisymmetric configuration consisting of infinite circular cylinders (assumed to be parallel to the z axis and of permittivity ε1) organized into a crystalline arrangement (simple square lattice) within a homogeneous background medium of permittivity ε2=1. The intersections of the cylinders with the x–y plane form a periodic two-dimensional structure. We carried out simulations taking ε1=3−0.03i or ε1=30−0.3i and ε2=1. The concentration dependence of the loss tangent of the composite material can be fitted very well, at low and intermediate concentrations of inhomogeneities, with a power law. In the case at hand, it is found that the exponent parameter depends significantly on the ratio of the real part of the permittivity of the components. We argue, moreover, that the numerical results discussed here are consistent with the Bergman and Milton theory [D. J. Bergman, Phys. Rep. 43, 377 (1978) and G. W. Milton, J. Appl. Phys. 52, 5286 (1981)].
Amphoteric behavior and precipitation of Ge dopants in InP
Authors: Yu Kin Man , Moll A. J. , Walukiewicz W.
Published: July 2002

We have directly correlated the electrical behavior, the impurity lattice site location, ion damage, and the local bonding environments of Ge-dopant ions implanted into InP. We have found that after rapid thermal annealing the free electron concentration in the samples implanted at room temperature (RT) are always higher than those implanted at liquid nitrogen temperature (LNT). Although the macroscopic structure seems to be insensitive to the implantation temperature, significantly more local disorder is created in the LNT implanted amorphous layers. Moreover, the amphoteric bonding structure of the Ge atoms is found to be well established already in the as-implanted amorphous InP. After high temperature annealing (≳800 °C), the Ge atoms rearrange locally with more of the Ge substituting the In site than the P site resulting in n-type conductivity. The solid solubility of Ge in the InP is measured to be ∼1.4–1.6×1020/cm3 while the free electron concentration is estimated to saturate at ∼3.4×1019/cm3. The relatively low electron concentration can be explained by Ge precipitation and the compensation of GeIn donors by GeP acceptors in the RT implanted case. The further reduction in electron concentration in the LNT implanted samples is believed to be related to the high residual damage found in these samples. The high solubility of Ge in InP can be attributed to the availability of two possible sublattice sites for the dopant and the compensation of the local strains due to the amphoteric substitution of the Ge. The concentration ratio of the GeIn to GeP determined in the heavily implanted material has been used to estimate the difference in the formation energy of Ge substituting those two different sites.
Epitaxial growth of C60 crystals vapor-deposited on a KI (001) surface
Authors: Yanagi Hisao , Doumi Shinjiro , Sasaki Takashi , Tada Hirokazu
Published: July 2002

Epitaxial growth of C60 was found in vapor-deposited crystals on the KI (001) cleavage surface. C60 molecules deposited on the KI surface, which was preheated at 400 °C, then kept at 195 °C during deposition, crystallized in the face-centered cubic form with two types of morphologies. Truncated pyramidal crystals epitaxially grew along the KI 〈110〉 directions, taking its C60 (001) face parallel to the KI (001) surface. This epitaxial nucleation occurred at a corner of the crossing steps along the KI 〈100〉 directions which were caused by thermal etching of the KI surface. Molecular mechanics and dynamics calculations revealed that a three-dimensional (2×2) commensurate lattice matching between the C60 {100} planes and the KI {100} faces at the step corner attributed to the epitaxial nucleation of the pyramidal crystal, in spite of the lower stability of the monolayer interaction of the C60 (001) face with the KI (001) surface. On the other hand, platelike crystals grew, with the C60 (111) face parallel to the KI (001) surface. A stable monolayer nucleation of the close-packed C60 (111) face made this two-dimensional growth of the platelike crystals possible to occur on the (001) terrace of the etched KI surface.
Radiation damage induced by MeV alpha particles in polycrystalline diamond films
Authors: Gonon P. , Prawer S. , Nugent K. W. , Jamieson D. N.
Published: July 2002

Radiation damage induced by 2 MeV alpha particles in polycrystalline diamond films has been studied as a function of the irradiation dose D (1012≤D≤1017 cm−2). The films were characterized using Raman/photoluminescence spectroscopy and I(V,T) measurements. The results can be summarized as follows. In undoped samples the H3 luminescent center (N–V–N) is observed for D≥1014 cm−2. The Raman diamond peak is broadened and shifted to lower frequencies for D≳1015 cm−2. No new graphitic component is detected after irradiation. On the contrary graphitic sp2 defects are annealed by irradiation. For D=3×1016 cm−2 new Raman defect peaks are detected at 1496 and 1635 cm−1. I(V,T) characteristics remain unaffected for D≤1016 cm−2. An increase in the conductivity is observed for D=3×1016 cm−2. At this dose we observe an activation energy of 0.4 eV and thermally stimulated currents related to defect levels at about 0.3 eV. A boron-doped sample (100 Ω cm) has been irradiated at 1017 cm−2 for comparison. After irradiation the conductivity of this sample is reduced and the activation energy of the conductivity is also reduced. Less damage is detected by Raman spectroscopy in the B-doped material.
Two-dimensional spectral distribution of photoluminescence from porous silicon
Authors: Matsuda T. , Tanino K. , Shinbo A. , Ishii H. , Ikeshita T. , Ohzone T.
Published: July 2002

The two-dimensional spectral distribution of photoluminescence (PL) was analyzed for p-type and n-type porous silicon (PS) prepared under different illumination conditions. In the p-type PS fabricated under strong illumination, the peak position of PL distribution in the depth direction moves inside the PS as the wavelength becomes longer. On the other hand, the PS prepared in the dark has an intensity peak of longer wavelength near surface. In the n-type PS formed under illumination, PL intensity has a peak near the PS surface and the peak position of the PL intensity does not move much. The difference between p-type and n-type PS suggests that the penetration depth of the light becomes greater as the PS layer grows and thus the effect of illumination may extend far inside p-type PS.
Thermal degradation of ZnO/InP interfaces: Heteroepitaxial growth of precipitated indium on InP{111} planes
Authors: Kečkéš J. , Ortner B. , Červeň I. , Jakabovič J. , Kováč J.
Published: July 2002

X-ray diffraction has been used to characterize the heteroepitaxial growth of indium formed at the interfaces between ZnO thin film and InP monocrystalline substrates. The In formation was induced by a thermal degradation of InP during annealing in the range of 400–700 °C for 3 min. The results prove that the evolution of the degradation is controlled by the decomposition of close-packed InP{111} planes, while the polar character of these planes plays a very important role. Moreover, for all four employed orientations of InP substrates [namely (111)A, (111)B, (110) and (100)], In is found to grow (101) on InP{111} planes. On an InP{111} interface plane, In crystallites can occur in six possible orientations characterized by a condition In〈100〉∥InP〈110〉. To estimate a mismatch of the heteroepitaxy, a geometrical model of the atomic arrangement at In(101)-InP(111) interface is given.
Spin-polarized tunneling between ferromagnetic films
Authors: Gu R. Y. , Xing D. Y. , Dong Jinming
Published: July 2002

Using a tunneling Hamiltonian including spin-flip effect, we derive general expressions for spin-polarized tunneling conductances as well as magnetoresistance (MR) in ferromagnetic/ insulator/ferromagnetic junctions. The spin-dependent density of states due to exchange splitting is responsible for the large negative MR, and the spin-flip tunneling is found to diminish the magnitude of MR. Qualitative agreement is obtained between our theoretical results and recent experimental data.
Dispersion and dipolar orientational effects on the linear electroabsorption and electro-optic responses in a model guest/host nonlinear optical system
Authors: Goodson T. , Wang C. H.
Published: July 2002

Linear electroabsorption (LEA) and linear electro-optic (LEO) measurements are demonstrated using a model guest/host system consisting of disperse red 1 (DR1) doped in poly(methyl)methacrylate (PMMA). The LEA response is measured over a wavelength range of 300–700 nm. Electro-optic measurements of the real and imaginary parts of the electric field-induced Pockels coefficient are carried out at wavelengths near and far from the resonant absorption. A shift in the absorption maximum and change in the band shape of the LEA spectrum are related to the linear Stark effect and dipolar orientation. Expressions for the real and imaginary parts of the Pockels coefficient derived from the two experiments are provided. Induced dipolar order as a result of the contact poling process is investigated by the LEA measurement. Information concerning the relaxation of the induced dipolar order, investigated by the LEA measurement, is compared to the relaxation results obtained by using the second-harmonic generation technique.
Adhesion enhancement of ion beam mixed Cu/Al/polyimide
Authors: Chang G. S. , Jung S. M. , Lee Y. S. , Choi I. S. , Whang C. N. , Woo J. J. , Lee Y. P.
Published: July 2002

Cu (400 Å)/polyimide was mixed with 80 keV Ar+ and N2+ from 1.0×1015 to 2.0×1016 ions/cm2. The same processes were repeated for the Cu (400 Å)/Al (50 Å)/polyimide system which has Al as a buffer layer. The quantitative adhesion strength was measured by a standard scratch test. X-ray photoelectron spectroscopy was employed to investigate the change in the chemical bonds of the ion beam mixed polyimide substrate and the intermediate effects for the adhesion enhancement in Cu/Al/polyimide. Two distinct tendencies are observed in the adhesion strength: Cu/Al/polyimide is more adhesive than Cu/polyimide after ion beam mixing, and N2+ ions are more effective in the adhesion enhancement than Ar+. The formation of an interlayer compound of CuAl2O4 accounts for the former, while the latter is understood by the fact that N2+ ions produce more pyridinelike moiety, amide group and tertiary amine moiety which are known as adhesion promoters.
The current–voltage characteristics of the resistive direct current superconducting quantum interference device
Authors: Greenberg Y. S. , Krivoy G. S. , Koch H.
Published: July 2002

Partly resistive superconducting quantum interference devices (RSQUIDs) are used in noise thermometry in the liquid helium temperature range. Here we analyze one type of RSQUID that has not been thoroughly investigated until now: the direct current biased with two Josephson junctions and a resistive part connected in series with the junctions and closed by a superconducting ring. The resistive shunted model of the Josephson junctions is used for the analysis. Two cases of the RSQUID ring inductance were analyzed: (i) negligibly small and (ii) small but finite inductance. The expressions for the low frequency output signal, the current–voltage (I–V) characteristics, and the current flowing through the resistive part are obtained. In the dependence of the output frequency versus the dc current passed through the resistive part often an offset is observed experimentally. This phenomenon can be explained as an influence of an asymmetry in the critical currents of the junctions.
Photoemission through thin dielectric coating films
Authors: Buzulutskov A. , Breskin A. , Chechik R.
Published: July 2002

Photoemission through thin coating films was studied in the scope of protection of sensitive photocathodes. The transmission of low energy (∼1 eV) electrons was measured for a large number of dielectric films (LiF, NaF, CsF, NaI, MgF2, BaF2, SiO, SiO2, Al2O3, n-C36H74), evaporated in vacuum on CsI and CuI photocathodes. Some films like CsF, NaI and n-C36H74 were found to have a fairly large electron attenuation length, varying from about 20 to 100 Å at a maximum initial electron energy of 1 eV. A thin CsF layer deposited on top of CuI and Al photocathodes was found to significantly increase their quantum yield. An enhancement of the photoyield following exposure to water vapour was observed for alkali fluoride-coated photocathodes. We interpret this effect as a decrease of the electron affinity by about 0.3–0.4 eV, induced by adsorption of polarized H2O dipoles.
Si3N4/Si/In0.05Ga0.95As/n–GaAs metal–insulator–semiconductor devices
Authors: Park Dae-Gyu , Li Ding , Tao Meng , Fan Zhifang , Botchkarev Andrei E. , Mohammad S. Noor , Morkoç Hadis
Published: July 2002

We report a novel metal–insulator–semiconductor (MIS) structure exhibiting a pseudomorphic In0.05Ga0.95As layer on GaAs with interface state densities in the low 1011 eV−1 cm−2. The structure was grown by a combination of molecular beam epitaxy and chemical vapor deposition methods. The hysteresis and frequency dispersion of the MIS capacitor were lower than 100 mV, some of them as low as 30 mV under a field swing of about ±1.3 MV/cm. The 150-Å-thick In0.05Ga0.95As channel between Si and GaAs is found to bring about a change in the minority carrier recombination behavior of the GaAs channel, in the same way as done by In0.53Ga0.47As channel MIS structures. Self-aligned gate depletion mode In0.05Ga0.95As metal–insulator–semiconductor field-effect transistors having 3 μm gate lengths exhibited field-effect bulk mobility of 1400 cm2/V s and transconductances of about 170 mS/mm.