bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser ablation of Pb(Ti0.48Zr0.52)O3 target: Characterization and evolution of charged species
Authors: Amoruso S. , Berardi V. , Dente A. , Spinelli N. , Armenante M. , Velotta R. , Fuso F. , Allegrini M. , Arimondo E.
Published: July 2002

Time-of-flight mass spectrometry and covariance mapping technique were used to investigate the formation of charged species in the laser ablation of a Pb(Ti0.48Zr0.52)O3 target. Mass spectra were taken both in high vacuum and in an oxygen environment in order to clarify the effect of oxygen on the plume composition. The analysis of charged species in the plume has been carried out as a function of the distance between the target and spectrometer axis. The relative abundance of atomic, monoxide, and heavy ions has been measured and discussed. The velocities of atomic ions in the laser plume have been evaluated. Pb transport from target to substrate has been found to be mainly due to Pb-containing heavy species, whose abundance is greater in an oxygen environment with respect to a high vacuum condition. Ionic yield and covariance mapping analysis indicate that oxidation of ablated material starts at a distance very close to the plume range. The data show that, close to the deposition distance, a significant fraction of the plume is composed of heavy species containing Pb and oxides. These are appropriate precursors for the stoichiometric growth of ferroelectric films. The oxides are important carriers of oxygen atoms, and the Pb-containing heavy species preserve the metal stoichiometry with respect to Pb, which is the most volatile element in the composition.
Measurement of the mean free path of dislocation glide in the InGaAs/GaAs materials system
Authors: Matragrano M. J. , Ast D. G. , Watson G. P. , Shealy J. R.
Published: July 2002

A method for measuring the mean free path of laterally gliding threading dislocations, based on the growth of lattice mismatched epitaxial layers on patterned substrates has been presented. The method requires no post growth annealing and permits an accurate measurement of the glide process as it occurs during epitaxial growth. This method has been applied to an InGaAs/GaAs heterostructure with a lattice mismatch of 0.5%. Mean free paths of 960 μm for α type dislocations and 1000 μm for β type dislocations were measured. It was concluded that the interaction of threading dislocations with clusters of misfit dislocations and/or edge type dislocations was the primary mechanism of glide length attenuation.
Microstructural characterization of Fe80B20 eutectic spherulites by small-angle neutron scattering and transmission electron microscopy
Authors: Deriu A. , Malizia F. , Ronconi F. , Vittori-Antisari M. , King S. M.
Published: July 2002

The evolution of the microstructure during the crystallization of amorphous Fe80B20 alloys has been investigated by small-angle neutron scattering (SANS) and transmission electron microscopy (TEM). Samples with an increasing degree of crystallization were obtained by performing different isothermal heat treatments on melt spun amorphous ribbons. The TEM measurements indicate that the crystallization occurs by nucleation and growth of Fe3B spherulites which include small finely divided α-Fe acicular crystallites. On the basis of the TEM results a model for the quantitative analysis of the SANS data has been formulated. The results show that, due to the high growth velocity of the nucleated particles, only their number increases with increasing annealing time, while their size and internal structure is not dependent on the degree of crystallization.
Effects of wavelength on boron nitride ablation by a transversely excited atmospheric CO2 laser
Authors: Sumiyoshi Tetsumi , Tomita Hiroyuki , Takahashi Akihisa , Obara Minoru , Ishii Katsunori
Published: July 2002

Laser ablation processing for a reaction sintered cubic boron nitride (cBN) by a transversely excited atmospheric CO2 laser was found to be strongly affected by the laser wavelength. The ablation threshold of the cBN was 3 J/cm2 at 9.3 μm for the 9R(10) branch but 18 J/cm2 at 10.8 μm for the 10P(38) branch since the 9.3 μm laser pulse is resonantly absorbed by the transverse optical phonons. Although the reflectance of the cBN surface is very high, it was efficiently ablated by the 9.3 μm laser irradiation. Hot pressed hexagonal BN samples were also ablated by a 9 μm band laser as well as a 10 μm laser, however, a boron rich layer was left on the ablated surface.
Nonlinear exposure dependence of the holographic recording and relaxational structural changes in amorphous As2S3 films
Authors: Ozols A. , Salminen O. , Riihola P. , Mönkkönen P.
Published: July 2002

The exposure dependence of holographic recording has been experimentally studied in nonannealed amorphous As2S3 films for small exposures ≤1 J/cm2. The square root of the diffraction efficiency is found to depend nonlinearly on exposure. This nonlinearity increases when the film thickness is increased and the holographic grating period is decreased. No exposure threshold has been found down to exposures of 0.025–0.17 J/cm2, depending on period. Different reasons for the observed nonlinearity are discussed. Nonlinearity is explained in terms of a phenomenological model based on the simultaneous action of the photostructural and relaxational structural changes, and this model has previously been used to explain the existence of relaxational self-enhancement and the intensity threshold for holographic recording in amporphous As2S3 films. The model itself is developed further.
Crystallographic and magnetic properties of NdFe10.7TiM0.3(M=B, Ti)
Authors: Kim Chul Sung , Lee Young Jong , Lee Seung Wha , Kim Y. B. , Kim C. S.
Published: July 2002

NdFe10.7TiM0.3(M=B, Ti) has been studied with x-ray diffraction, Mössbauer spectroscopy, and a vibrating sample magnetometer. The alloys were prepared by arc-melting under an argon atmosphere. The NdFe10.7TiB0.3 exhibits a pure single phase, whereas the NdFe10.7Ti1.3 contains some α-Fe, from x-ray and Mössbauer measurements. The NdFe10.7TiB0.3 has the ThMn12-type tetragonal structure with a0=8.587 Å and c0=4.788 Å. The Curie temperature (TC) is 570 K from Mössbauer spectroscopy performed at various temperatures ranging from 13 to 770 K. Each spectrum below TC was fitted with five subspectra of Fe sites in the structure (8i1, 8i2, 8j1, 8j2, and 8f ). The area fraction of the subspectra at room temperature are 16.4%, 8.2%, 14.8%, 21.3%, and 39.3%, respectively. Magnetic hyperfine fields for the Fe sites decrease on the order of Hhf(8i)≳Hhf(8j)≳Hhf(8f ). The average hyperfine field Hhf(T) of the NdFe10.7TiB0.3 shows a temperature dependence of [Hhf(T)−Hhf(O)]/Hhf(O)=−0.39(T/TC)3/2 & −0.17(T/TC)5/2 for T/TC<0.7, indicative of spin-wave excitation. Annealing the alloy at around TC for 60 min resulted in a two phase microstructure consisting of a ThMn12-type structure and α-Fe.
Phonon density of states of nanocrystalline Fe prepared by high-energy ball milling
Authors: Fultz B. , Robertson J. L. , Stephens T. A. , Nagel L. J. , Spooner S.
Published: July 2002

We performed inelastic neutron scattering measurements on nanophase Fe powders prepared by high-energy ball milling. Neutron energy loss spectra were measured for two states of the material: (1) as milled, when the material had a characteristic nanocrystallite size of 12 nm; and (2) annealed, when the material had a characteristic crystallite size of 28 nm. The longitudinal peak in the phonon density of states (DOS) of the nanophase Fe was broadened, compared to that of the annealed material. We attribute this broadening to short phonon lifetimes in nanocrystals. The nanophase material also showed an enhanced density of states at low energies below 15 meV, which may indicate the presence of intercrystallite vibrations. These differences in phonon DOS should have only a small effect on the difference in vibrational entropy of nanocrystalline and larger-grained Fe.
Homoepitaxial growth of GaN using molecular beam epitaxy
Authors: Gassmann A. , Suski T. , Newman N. , Kisielowski C. , Jones E. , Weber E. R. , Liliental-Weber Z. , Rubin M. D. , Helava H. I. , Grzegory I. , Bockowski M. , Jun J. , Porowski S.
Published: July 2002

In this article, experimental results are presented for the homoepitaxial deposition of a GaN overlayer onto a bulk single-crystal GaN substrate using molecular beam epitaxy. Transmission electron microscopy shows a superior structural quality of the deposited GaN overlayer when compared to heteroepitaxially grown layers. Photoluminescence shows narrow excitonic emission (3.467 eV) and the very weak yellow luminescence, whereas the bulk substrate luminescence is dominated by this deep level emission. These results show that homoepitaxy of GaN can be used to establish benchmark values for the optoelectronic properties of GaN thin films.
Structural and optical characterization of ZnSe single crystals grown by solid-phase recrystallization
Authors: Tournié E. , Morhain C. , Neu G. , Laügt M. , Ongaretto C. , Faurie J.-P. , Triboulet R. , Ndap J. O.
Published: July 2002

We investigate by high-resolution x-ray diffraction (HRXRD), temperature-dependent photoluminescence (PL) and reflectivity spectroscopies, and low-temperature selective-photoluminescence spectroscopy ZnSe single crystals grown by solid-phase recrystallization. HRXRD reveals the high structural perfection of the samples which exhibit rocking-curve linewidths in the 15–20 arcsec range. The low-temperature PL spectra are dominated by the so-called Ideep1 excitonic line, a neutral-acceptor bound-exciton line I1, the free-exciton emission FX, and the n=2 excited state of FX. We identify the main residual impurities to be Li acceptors. Donor–acceptor pair bands are very hardly detected at low temperature which indicates a low donor content. A major characteristics of these samples is the quasi-absence of any Cu-related deep emission which generally plagues the PL spectra of bulk ZnSe. Consequently, Ideep1 is ascribed to Zn-vacancy–donor complexes. Finally, from the temperature dependence of the PL emission and reflectivity, the band-gap energy of bulk ZnSe is found to linearly shrink with the temperature above 80 K at a rate of −4.3×10−4 eV K−1. The room-temperature gap is estimated to 2720±2 meV. Our results indicate that solid-phase recrystallization produces ZnSe samples with the highest structural quality and purity achievable at present time.
Ultrafast photoexcitation and coherent dynamics of electrons in triple tunnel-coupled quantum wells
Authors: Raichev O. E. , Vasko F. T. , Hernández-Cabrera A. , Aceituno P.
Published: July 2002

Coherent dynamics of electrons in the triple tunnel-coupled quantum wells after an ultrafast optical excitation is studied. In a collisionless approximation, we consider temporal evolution of the electron distribution for different parameters of structure and pulse duration. Due to mixing of the tunneling frequencies under the three-level tunneling resonance condition, transient evolution of the dipole moment demonstrates different kinds of behavior: harmonic oscillations, beats, and nonperiodic oscillations.
Hot-phonon effects on electron transport in quantum wires
Authors: Mickevičius R. , Mitin V. , Paulavičius G. , Kochelap V. , Stroscio M. A. , Iafrate G. J.
Published: July 2002

Hot (nonequilibrium) phonon effects on electron transport in rectangular GaAs/AlAs quantum wires have been investigated by a self-consistent Monte Carlo simulation. Confinement and localization of optical phonons have been taken into account. We have demonstrated that at room temperature hot optical phonons lead to a significant increase in electron drift velocity. This hot-phonon drag effect is due to the strongly asymmetric nonequilibrium phonon distribution. As a result, phonon absorption for forward transitions (electron gains momentum along electric field) is enhanced, whereas absorption for backward transitions (electron gains momentum against electric field) is suppressed. At low temperatures diffusive heating of electrons by hot phonons dominates over hot-phonon drag and the electron drift velocity decreases.
Thermally stable PdIn ohmic contacts to n-GaAs via exchange mechanism
Authors: Chen D. Y. , Chang Y. A. , Swenson D.
Published: July 2002

Thermally stable, low-resistance ohmic contacts to GaAs (Si-doped, n=1.6–1.8×1018 cm−3) were formed using PdIn metallization sputter-deposited from an alloy target. Average specific contact resistances (ρc) in the 10−6 Ω cm2 range were reached upon annealing at 600 °C or higher. Contacts annealed under the optimum condition of 850 °C for 15 s exhibited an average ρc of 2.5×10−6 Ω cm2. The 100 h of thermal aging at 400 or 500 °C increased their average ρc to 3.0×10−6 and 1.0×10−5 Ω cm2, respectively. The ohmic behavior of the annealed contacts was ascribed to the exchange of In and Ga atoms between the metallization and the semiconductor and the concomitant formation of InxGa1−xAs, whose presence at the contact interface was confirmed using cross-sectional transmission electron microscopy.
Parametric and modulation instabilities of magnetostatic surface spin waves in ferromagnetic films
Authors: Jun Su , Nikitov S. A. , Marcelli R. , De Gasperis P.
Published: July 2002

First-order parametric instability, modulation instability and second-order instability processes for magnetostatic surface waves propagating in a pure yttrium iron garnet film are investigated in accordance with the relationship between the magnetostatic wave frequency and the internal magnetic field. The transitions from the second-order parametric instability, via the modulation instability, to the first-order parametric instability (when the magnetic field is 720 Oe and the carrier frequency varies from 3.87 to 4.39 GHz), and the transitions from the first-order parametric instability to the second-order parametric instability via the modulation instability (when the carrier frequency is 3.5 GHz and the magnetic field varies from 427 to 647 Oe) are experimentally observed and explained. Dispersion characteristics of the magnetostatic surface waves are modified by introduction of a metallic ground plane in order that the Lighthill criterion, as one of the prerequisites for onset of the modulation instability, is fulfilled in a limited frequency range. Characteristics of magnetostatic surface wave modulation instability are studied by solving the nonlinear Schrödinger equation. Dependencies of the modulation frequency induced by the modulation instability on the carrier frequency and the magnetic field are found to be consistent with experimental data.
Properties of low-pressure chemical vapor epitaxial GaN films grown using hydrazoic acid (HN3)
Authors: Chtchekine D. G. , Fu L. P. , Gilliland G. D. , Chen Y. , Ralph S. E. , Bajaj K. K. , Bu Y. , Lin M. C. , Bacalzo F. T. , Stock S. R.
Published: July 2002

We have grown high-quality GaN films on sapphire using a new nitrogen precursor, hydrazoic acid (HN3). Films were grown at 600 °C on (0001) sapphire substrates in a low-pressure chemical-vapor-deposition system using triethylgallium and hydrazoic acid as precursors. Subsequently, we have conducted a complete study of the surface, structural, electrical, and optical properties of these GaN films, and our early results are very encouraging. All films were of wurtzite crystal structure, slightly polycrystalline, and n type at about 2×1017 cm−3. We find the films to be efficient light emitters in the near-band edge region of the spectrum. Analysis of the emission energies and kinetics suggests that the midgap emission results from a superimposed deep-donor-to-shallow-acceptor emission and a deep-donor-to-valence-band emission, where the deep donor consists of a distribution of energy levels, thereby yielding a broad emission band.
Optimization of the intersubband second-order susceptibility in graded ternary alloy quantum wells by combined supersymmetric and coordinate transform methods
Authors: Milanović V. , Ikonić Z.
Published: July 2002

The authors report on a systematic procedure for optimizing the second order susceptibility in semiconductor quantum wells (QWs) based on ternary alloys. The procedure relies on combining the supersymmetric and the coordinate transform methods. Optimized design of AlxGa1−xAs based QWs gives nonlinearity somewhat larger than is presently achieved in this system.
Preparation of Sm2Fe17−xGaxNy/Cy magnets by a hydrogenation-disproportionation-desorption-recombination process
Authors: Kubis M. , Cao L. , Handstein A. , Gebel B. , Müller K.-H. , Schultz L.
Published: July 2002

A hydrogenation-disproportionation-desorption-recombination process (HDDR) was applied to Sm2Fe17−xGax (x=0.5, 1, and 2). The process was studied by means of temperature-pressure analysis, x-ray diffraction and scanning electron microscopy. It was shown that Ga not only stabilizes the interstitially modified compounds Sm2Fe17−xGaxCy (0<y<3) but it also stabilizes the parent compound Sm2Fe17−xGax against the disproportionation by hydrogen. Therefore, only for x=0.5 can a nearly complete HDDR be performed. The HDDR-treated and subsequently nitrogenated or carburized samples show coercivities μ0JHC up to 3.1 T and 2.5 T, respectively. Hot compaction increases the density of the Sm2Fe16.5Ga0.5Cy powder; however, it leads to a loss of coercivity due to decomposition into α-iron and samarium carbides.
Quadratic power dependence of the magneto-optic coupling coefficient in the magnetostatic wave–optical interaction (abstract)
Authors: Prabhakar A. , Stancil D. D.
Published: July 2002

The transverse magnetostatic wave (MSW)–optical interaction was first observed at relatively low microwave power levels.1 Recent investigations show that a change in the demagnetizing field due to an increase in the dynamic magnetization causes a shift in the microwave passband2 and consequently a shift in the MSW–optical interaction spectrum.3 We now focus our efforts on determining the effects of high microwave power on the magneto-optic coupling coefficient (κ). A microstrip T transducer excited forward volume MSWs in a bismuth–lutetium–iron–garnet film. An optical beam is edge coupled into the film and the transverse MSW–optical interaction is observed at high microwave power levels. Using the coupled mode equations, we show that the amplitude of the diffracted beam is a product of κ and the phase mismatch between the optical guided modes and the MSWs. Taylor series expansions for κ and the MSW wave number (β) are used to obtain an analytic expression for the MSW–optical interaction. The dependence on β causes a shift in the interaction passband with increasing microwave power. After correcting for this nonlinear shift, the residual dependence of the output optical intensity on input microwave power is attributed to a quadratic power dependence in κ. The coefficient of the quadratic term was observed to be ∼1% of the coefficient of the linear term and its effect at high power is comparable to that of the nonlinear frequency shift.
Current crowding effects in a CdS/LaS cold cathode
Authors: Mumford P. D. , Cahay M.
Published: July 2002

We analyze the importance of current crowding in a new cold cathode emitter that consists of a thin wide band-gap semiconductor material sandwiched between a metallic contact and a low work function semimetallic thin film. Potential material candidates are suggested to achieve low-voltage (<10 V), room-temperature cold cathode operation with emission currents of several tens of A/cm2. We calculate the lateral potential drop that occurs across the emission window of cold cathodes with rectangular geometry and describe its effects on the emitted current density profile. The power dissipation in the cold cathode is calculated as a function of a dimensionless parameter characterizing the importance of current crowding. We determine the range of dc bias over which cold cathodes of different width must be operated to minimize current crowding effects.
On the fabrication of stacked monolayers of Fe/Cu on Cu(100) by pulsed laser deposition
Authors: Sundar Manoharan S. , Shen J. , Jenniches H. , Klaua M. , Kirschner J.
Published: July 2002

We report on the artificial fabrication of stacked monolayers of Fe/Cu on Cu(100) by pulsed laser deposition at 300 K employing reflection high energy electron diffraction (RHEED). In contrast to the thermally deposited films, an improved two dimensional growth of the Fe and Cu layers has been achieved by virtue of the dynamics of the pulsed laser deposition technique. The observed RHEED oscillations for each Fe and Cu layer and their growth mode as studied by scanning tunnelling microscopy indicate layer-by-layer growth. The magnetic properties studied by magneto-optical Kerr effect measurements show that the easy axis of magnetization is in plane with no specific anisotropy observed in the plane. Ferromagnetic response was absent for thicknesses less than 2 ML in total within the measured temperature range down to 100 K. The Curie temperature increases from 225 K for a trilayer Fe/Cu/Fe to 400 K for a total layer thickness of 10 ML.
Determination of azimuthal anchoring energy in grating-aligned twisted nematic liquid-crystal layers
Authors: Wood E. L. , Bradberry G. W. , Cann P. S. , Sambles J. R.
Published: July 2002

In this work sinusoidal diffraction gratings with a range of pitches and amplitudes are used to align nematic liquid-crystal layers in a twisted homogeneous configuration. The grating profiles are accurately characterized using optical surface plasmon polariton spectroscopy, which then allows a calculation of the anchoring energy as predicted by the simple Berreman expression. The experimental Rapini–Papoular anchoring energy is also obtained by a measurement of the director twist away from the alignment direction (easy axis) at room temperature. A linear relationship is found between the two anchoring energies, except when it falls below 4×10−7 J m−2. Noticeably, the correlation between the two theories is not unity, if room-temperature elastic constants are used in the calculation. This apparent inconsistency is explained if the effect of surface memory on the system is considered. Indeed, if elastic constants, corresponding to a higher temperature at which surface memory effects are absent, are used in the Berreman expression, good agreement between the predicted and experimentally measured energies is found.
X-ray diffraction pole figure measurements of diamond films grown on platinum (111)
Authors: Tachibana Takeshi , Yokota Yoshihiro , Kobashi Koji , Shintani Yoshihiro
Published: July 2002

Diamond films of various thicknesses (1.5–12 μm) deposited on Pt (111) by microwave enhanced chemical vapor deposition (CVD) were studied by scanning electron microscopy (SEM) observation and x-ray diffraction pole figure measurements. The dependence of the surface morphology and structure of the films on the film thickness were investigated. It was observed by SEM that the coalescence between neighboring (111) diamond surface became more significant as the film thickness increased. It was revealed by the x-ray diffraction measurements that the 〈111〉 texture of the diamond films improved with increasing film thickness, whereas azimuthal alignment of the crystals was roughly independent of the film thickness. It was also found that the deposited films contained epitaxial diamond crystals which were azimuthally rotated by 60° about the [111] axis of the samples. With increasing film thickness, the ratio of the 60°-rotated crystals to nonrotated crystals increased, which is primarily attributed to twin formation during growth of the CVD diamond on its (111) surface.
Atomic level stress and light emission of Ce activated SrS thin films
Authors: Warren W. L. , Vanheusden K. , Tallant D. R. , Seager C. H. , Sun S.-S. , Evans D. R. , Dennis W. M. , Soininen Erkki , Bullington J. A.
Published: July 2002

We find that the Ce3+ ion in polycrystalline sputtered SrS:Ce thin films resides in a distorted octahedral environment, as opposed to the cubic host environment. Using electron paramagnetic resonance and x-ray diffraction analysis, we show that the degree of axial distortion is related to the preferential growth direction of the SrS films. To first order, the blue-emission properties (emission wavelength and decay times) of the SrS:Ce films do not appear to be affected by the amount of distortion in the local Ce3+ environment.
Steady-state and transient current transport in undoped polycrystalline diamond films
Authors: Jauhiainen Anders , Bengtsson Stefan , Engström Olof
Published: July 2002

We have investigated steady-state and transient electrical properties of undoped polycrystalline diamond thin films deposited using hot filament chemical vapor deposition on (100)-oriented n-type and p-type silicon substrates. The capacitance-voltage characteristics are strongly influenced by slow traps located close to the interface between the diamond layer and the silicon substrate. When interpreting data one has to consider that the traps are not in thermal equilibrium during measurements. The steady-state current through the diamond film has the same behavior for films deposited on both n-Si and p-Si. Its temperature and field dependency can be interpreted in terms of Poole–Frenkel transport involving ionized sites with overlapping potentials in the diamond film. Electrically excited current transients decay with time according to a power law. The kinetics depend only weakly on temperature. Further, the transients contain very long time scales and show much similarity to earlier reported optically excited ones. The temperature and voltage dependency of the transient current magnitude are similar to the ones of the steady-state current for a nonzero field across the diamond layer during the transient. It is possible to qualitatively account for the steady-state and transient transport within the framework of the same basic model assuming that the traps involved in the transport have a certain spatial and energy distribution. From an application point of view the leakage currents in the diamond film are of acceptable magnitude for many diamond based silicon-on-insulator applications intended for operation at moderate temperatures and voltages. Finally, the films also show promising behavior with respect to material reliability; from the electrical measurements no sign of degradation of the diamond films due to long term current stress can be seen.
Performance of heterojunction p+ microcrystalline silicon n crystalline silicon solar cells
Authors: van Cleef M. W. M. , Rath J. K. , Rubinelli F. A. , van der Werf C. H. M. , Schropp R. E. I. , van der Weg W. F.
Published: July 2002

We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition (PECVD) on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20–30 nm p+ μc-Si:H layers with a considerably large crystalline volume fraction (∼22%) and good window properties were deposited on crystalline silicon under moderate PECVD conditions. The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer layer used. A large improvement of open circuit voltage is observed in these solar cells when a thin 2–3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature (∼100 °C) is inserted between the microcrystalline and crystalline silicon [complete solar cell configuration: Al/(n)c-Si/buffer/p+μc-Si:H/ITO/Ag)]. Detailed modeling studies showed that the wide band-gap a-Si:H buffer layer is able to prevent electron backdiffusion into the p+ μc-Si:H layer due to the discontinuity in the conduction band at the amorphous-crystalline silicon interface, thereby reducing the high recombination losses in the microcrystalline layer. At the same time, the discontinuity in the valence band is not limiting the hole exit to the front contact and does not deteriorate the solar cell performance. The defect density inside the crystalline silicon close to the amorphous-crystalline interface has a strong effect on the operation of the cell. An extra atomic hydrogen passivation treatment prior to buffer layer deposition, in order to reduce the number of these defects, did further enhance the values of Voc and fill factor, resulting in an efficiency of 12.2% for a cell without a back surface field and texturization.
Growth and characterization of silicon thin films employing supersonic jets of SiH4 on polysilicon and Si(100)
Authors: Mullins C. B. , Pacheco K. A. , Banerjee S.
Published: July 2002

Supersonic jets of silane were employed to deposit silicon on both polysilicon and Si(100) at substrate temperatures ranging from 500 to 650 °C. The growth rate and film uniformity were studied as a function of silane kinetic energy. Increasing the SiH4 precursor kinetic energy from 0.4 eV (10% SiH4 in He mixture) to 1 eV (1% SiH4 in H2 mixture) results in as much as an order of magnitude increase in reaction probability. The advantage of using high kinetic energy precursors to enhance deposition is reflected in the centerline growth rates obtained employing supersonic jets of SiH4. At higher substrate temperatures, the high kinetic energy SiH4 jet has a higher growth rate than the low kinetic energy SiH4 jet, although, the flux of the high energy jet at the centerline is a factor of 8 less than the flux of the low energy jet. The silane flux distribution from the supersonic jet is dependent on the gas mixture; a flux distribution of cos55 θ results from the 1% silane in hydrogen jet (1 eV) compared to a cos28 θ distribution from the 10% silane in helium jet (0.4 eV).
Derivation of an analytical model to calculate junction depth in HgCdTe photodiodes
Authors: Holander-Gleixner S. , Robinson H. G. , Helms C. R.
Published: July 2002

An enhanced analytical model is derived to calculate the junction depth and Hg interstitial profile during n-on-p junction formation in vacancy-doped HgCdTe. The enhanced model expands on a simpler model by accounting for the Hg interstitials in the p-type, vacancy-rich region. The model calculates junction depth during both the initial, reaction-limited regime of junction formation and the diffusion-limited regime. It also calculates junction depth under conditions when the abrupt junction approximation of the simpler model fails. The enhanced model can be used to determine the limits of the annealing conditions and times for which the junction depth calculated analytically is valid. The decay length of interstitials into the p-type region estimated analytically places an upper bound on the grid spacing needed to accurately resolve the junction in a numerical simulation.
Influence of a structural disorder in the Co0.62Al0.38 alloy film on the magneto-optical, magnetic, and optical properties
Authors: Kudryavtsev Yu. V. , Lee Y. P. , Kim K. W.
Published: July 2002

The influence of the order-disorder structural transition on the magneto-optical, magnetic, and optical properties of Co-rich β-phase Co-Al alloy films has been investigated. The disordered state in these alloy films was obtained by means of vapor quenching deposition onto glass substrates cooled by liquid nitrogen. The experimental studies of the magneto-optical and optical properties of the ordered and disordered Co-Al alloy films have been carried out in 0.5–5.0 eV energy range at room temperature. The influence of the order-disorder structural transition on the magneto-optical, magnetic, and optical properties was discussed by using the results of the first-principles calculations of the electronic structure of the CoAl compound and the structural defect approach.
Frequency and temperature dependence of complex strontium titanate electrorheological fluids under an alternating electric field
Authors: Ma Yong , Zhang Yuling , Lu Kunquan
Published: July 2002

The electrorheological (ER) behavior of suspensions of water-free complex strontium titanate particles, synthesized by means of a modified sol-gel technique, in silicone oil have been investigated under an ac electric field. The frequency and temperature dependence of the shear stress show that the shear stress decreases monotonically with frequency and reaches a maximum value when the temperature is around 70 °C. The ER behavior has been explained on the basis of dielectric measurements.
Interfacial reaction and defect microstructure of epitaxial MnSb/Si(111) grown by hot-wall epitaxy
Authors: Tatsuoka H. , Isaji K. , Sugiura K. , Kuwabara H. , Brown P. D. , Xin Y. , Humphreys C. J.
Published: July 2002

The microstructure of epitaxial MnSb grown on (111) oriented Si substrates by hot-wall epitaxy has been assessed using the combined techniques of x-ray diffraction, and conventional and high resolution transmission electron microscopy (HRTEM). The growth temperature and layer thickness dependence of epitaxial MnSb/Si has been investigated. Optimum MnSb(0001) layers were grown under conditions of low substrate temperature of 300 °C with a Sb/Mn flux ratio of 10. HRTEM imaging revealed the existence of a thin interfacial layer of MnSi rotated 30° with respect to the Si substrate to reduce the lattice mismatch. The predominant epitaxial relationship is given by (0001),[1̄21̄0]MnSb∥(111),[12̄1]MnSi∥(111),[01̄1]Si. Only evidence for the presence of MnSi was found with no evidence for any other Mn–Si phases. The formation of MnSi is enhanced at an elevated substrate temperature of 350 °C, while the quality of the MnSb overlayer is correspondingly degraded leading to polycrystalline growth.
Thermal conductivity of κ-Al2O3 and α-Al2O3 wear-resistant coatings
Authors: Cahill David G. , Lee S.-M. , Selinder Torbjörn I.
Published: July 2002

The thermal conductivities of α-Al2O3 and κ-Al2O3 wear-resistant coatings are measured using the 3ω method in the temperature range 80<T<600 K. The coatings are 13 μm thick and deposited by chemical vapor deposition on substrates of Co-cemented WC. The α-Al2O3 coating has a thermal conductivity comparable to sapphire at T>300 K. The relatively small thermal conductivity of κ-Al2O3, a factor of ∼3 smaller than α-Al2O3, suggests that this metastable phase of alumina can be applied as an effective thermal barrier for cutting tools.
Investigation of granular films composed of interdispersed La1/3Ca2/3MnO3 particles and metallic Au particles
Authors: Rubinstein Mark , Broussard P. R. , Allen L. H. , Hathaway Kristl B. , Miller Michael M. , Sun Jonathan Z.
Published: July 2002

We have carried out a study of composite, granular films made by depositing LCMO (a colossal magnetoresistive perovskite) and pure, metallic Au onto MgO and LaAlO3 substrates. Elevated temperature caused the Au to segregate into small, micron-sized granules. The films were studied by atomic force microscopy, magnetoresistivity, superconducting quantum interference device magnetization measurements, and magnetically modulated microwave absorption.
Alternating current susceptibility study of Dy2Fe17−xGax compounds
Authors: Skumryev V. , Rao Xiao-lei , Coey J. M. D. , Sheludko N.
Published: July 2002

The effect of Ga substitution for Fe in Dy2Fe17−xGax on the low field ac susceptibility is reported. The easy magnetization direction is along the c axis for x>6.3 while for x<6 it is within the basal plane. With decreasing temperature a spin reorientation from an easy axis to an easy cone takes place when 6⩽x⩽6.3. Two other anomalies have been found in the susceptibility of easy plane materials when the field is applied in the plane: (i) at T near room temperature, both χ′ and χ″ depend on the frequency and χ″ shows a maximum which shifts to higher temperatures with increasing frequency; (ii) at low temperatures an increase in χ′ is accompanied by time varying effects. While the latter is related to the domain structure reconstruction due to a change of the in-plane anisotropy, the former is caused by thermally activated domain wall pinning and depinning. The Ga substitution strongly affects the low temperature anomaly which disappears when x>2, whereas it has little effect on the high temperature one.
Structural stability, magnetism, and surface magneto-optic Kerr effect spectra of MnAg(001) surface alloys
Authors: Hong Soon C. , Kim Miyoung , Freeman A. J.
Published: July 2002

The stability of a MnAg surface alloy was investigated employing the total energy and atomic force full-potential linearized augmented plane wave method based on the local density approximation for: (i) a clean Ag(001), (ii) 1 ML Mn overlayer [1 Mn/Ag(001)], (iii) 1 and 2 ML MnAg alloys as overlayers on Ag(001) [1(MnAg)/Ag(001) and 2(MnAg)/Ag(001)], and (iv) 1 ML Mn diffused into Ag(001) substrate [Ag/1 Mn/Ag(001)]. Results obtained show that 2(MnAg)/Ag(001) is much more stable than 1 Mn/Ag(001) (by a large energy difference of 150 meV), whereas 1(MnAg)/Ag(001) is marginally more stable (by a slight energy difference of 8 meV) compared to a separate phase of Ag and Mn atoms. Surface Mn and subsurface Mn atoms in 2(MnAg)/Ag(001) were found to be coupled antiferromagnetically with magnetic moments of 3.96 and −3.55 μB, respectively. The surface corrugation (Δz=0.05 a.u.) of 2(MnAg)/Ag(001) was found to be much smaller than that (Δz=0.5 a.u.) of another magnetically stabilized surface alloy system, MnCu/Cu(001). No significant magneto-optical effects for antiferromagnetic Ag/Mn/Ag(001) were found due to the small off-diagonal elements of the optical conductivity.
Unusual properties of the molecular nanomagnet Mn12ac
Authors: Novak M. A. , Gomes A. M. , Rapp R. E.
Published: July 2002

The magnetic properties of the Mn12ac molecular cluster were studied by complex susceptibility and specific heat measurements. Extensive ac susceptibility measurements done under a magnetic field on good quality single crystals and zero field powder sample specific heat results indicate that the usual uniaxial Hamiltonian, as well as the recently proposed fourth order corrections, are not enough to account for all the observations. Very low temperature specific heat data indicate the presence of zero field splitting of the ground state which is much larger than expected. Finally, specific heat measurements done under a magnetic field permitted us to observe and measure the lattice spin relaxation by calorimetric methods with time constants that are in agreement with the magnetic relaxation measurements.
Photovoltaic measurement of the built-in potential in organic light emitting diodes and photodiodes
Authors: Malliaras G. G. , Salem J. R. , Brock P. J. , Scott J. C.
Published: July 2002

We measure the voltage at which the current under illumination in poly[2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene] based light emitting diodes is equal to the dark current. At low temperatures, this voltage, which we term the “compensation” voltage, is found to be equal to the built-in potential, as measured with electroabsorption on the same diode. Diffusion of thermally injected charges at room temperature, however, shifts the compensation voltage to lower values. A model explaining this behavior is developed and its implications for the operation of organic light emitting diodes and photovoltaic cells are briefly discussed.
Deriving of single intensive picosecond optical pulses from a high-power gain-switched laser diode by spectral filtering
Authors: Vainshtein S. N. , Simin G. S. , Kostamovaara J. T.
Published: July 2002

Single 25 ps/16 W optical pulses were achieved by spectral filtering from a multiheterostructure gain-switched laser diode with its quasisteady-state modes suppressed by a factor of 103 as compared with the peak power. A significant transient spectrum broadening makes this possible provided that a very high dI/dt rate of the pumping current pulse is used. A simple numerical model is suggested which describes adequately both the spectral and transient features of the observed phenomenon. It follows from the model that single picosecond optical pulses can be obtained from any type of high power semiconductor laser.
Photoconductivity in self-organized InAs quantum dots
Authors: Fan J. C. , Lin Y. J. , Chen Y. F. , Chen M. C. , Lin H. H.
Published: July 2002

Photoconductivity in self-organized InAs/GaAs quantum dots is reported. It is found that the photoconductivity ratio between the InAs dots and GaAs matrix increases with increasing temperature. We point out that the photoconductivity of InAs dots can be attributed to the thermal activation of photocarriers into the GaAs matrix, where the conduction takes place. We also found that self-organized InAs dots exhibit the effect of persistent photoconductivity (PPC). The PPC effect is interpreted in terms of the spatial separation of photocreated electrons and holes. After photoexcitation, the hole remains in the dot, and the electron is thermally activated into the GaAs barrier. The return of the electron is prevented from the existence of band bending at the InAs dot and GaAs interface.
Theoretical investigation of electroluminescence and current–voltage characteristics in p-i-n resonant tunneling light emitting diode
Authors: Fu Y. , Willander M.
Published: July 2002

Multiple-line electroluminescence (EL) spectrum and current bistability in the current–voltage (I–V) relationship have been reported in a p-i-n resonant tunneling light emitting diode based on an AlGaAs/GaAs heterostructure. In this work we report our theoretical investigation of the energy band structure of this diode under different forward bias Vex. It is concluded that the occupation of excited hole subbands and the interactions between light and heavy holes are the most possible reasons to account the observed the multiple-line EL spectrum. The current bistability in I–V relationship is attributed to the charge accumulation in the central i region when carriers begin to tunnel directly through the diode at Vex=Vi, where Vi is the built-in potential in the p-i-n structure.
Wide optical bandwidth asymmetric Fabry–Pérot reflection modulator using the quantum confined Stark effect
Authors: Xu M. G. , Fisher T. A. , Dell J. M. , Clark A.
Published: July 2002

A normally on, high-performance quantum confined Stark effect asymmetric Fabry–Pérot reflection modulator with enhanced optical bandwidth is reported. The wide optical bandwidth is achieved by utilizing the variation in refractive index in the vicinity of the heavy-hole exciton. The nominal operating wavelength is set in the region where the on-state refractive index starts to increase and allows the Fabry–Pérot resonance condition to be maintained over a wide wavelength range. An optical bandwidth of 5 nm is achieved for an operating voltage of 7 V, insertion loss <2.5 dB, reflectance change >55% and contrast ratio >15 dB, and 7 nm if the contrast ratio is relaxed to >10 dB. These values are twice as wide as previously reported for a quantum confined Stark effect modulator structure, and correspond to an operating temperature range of 25 °C compared to 10 °C for conventional structures.
Pyrene fluorescence dynamics within a polymer microspherical cavity
Authors: Fujiwara Hideki , Sasaki Keiji , Masuhara Hiroshi
Published: July 2002

Drastic changes of fluorescence dynamics in pyrene-doped poly(methylmethacrylate) latex microspheres were observed with a time-resolved microspectroscopy system. Fluorescence decay curves of pyrene monomer emission from the microspheres consist of fast and slow decay components. Rate constants of the slow components are almost the same for all particles, which correspond to the value in free space. The fast components increase in the relative intensity and in the decay rate with decreasing particle size, which can be ascribed to cavity-enhanced spontaneous emission. The rate constant for a 2.4 μm sphere is increased by a factor of ∼17 compared to the bulk value. The results are discussed on the basis of the cavity quantum electrodynamic theory.
Formation of magnetic nanocrystals in a glass ceramic studied by small-angle scattering
Authors: Lembke U. , Hoell A. , Kranold R. , Müller R. , Schüppel W. , Goerigk G. , Gilles R. , Wiedenmann A.
Published: July 2002

The formation of nanosized crystallites of magnetite, Fe3O4, by heat treatment of a glass containing iron oxide was investigated. The magnetic properties of the glass ceramic manufactured strongly depend on the heat treatment conditions. The evolution of size distribution and volume fraction of the nanocrystallites formed was studied by small-angle x-ray scattering (SAXS). The size distribution of the nanocrystalline phase turned out to show bimodal shape. The possibility of magnetic contrast variation offered by small-angle neutron scattering (SANS) was utilized in order to distinguish the small-angle scattering of magnetite from the scattering contributions of nonmagnetic iron containing crystallites that can additionally be formed during the heat treatment. The results obtained reveal that both size grades of particles observed in the size distribution are superparamagnetic consisting of magnetite. The evolution of the volume fraction of magnetite in dependence on the heat treatment was found to be correlated with the magnitude of the specific saturation magnetization of the glass ceramic. The volume size distributions derived from magnetic SANS revealed peaks at smaller radii in relation to those from nuclear SANS and SAXS data. Therefore, the existence of a nonmagnetic surface layer is suggested that surrounds the magnetically active core of the magnetite nanocrystals.
An approximate model for wave propagation in piezoelectric materials. I. Laminated composites
Authors: Nayfeh Adnan H. , Faidi Waseem , Abdelrahman Wael
Published: July 2002

A continuum mixture theory with microstructure is developed for guided wave propagation in bilaminated periodic composites of piezoelectric materials. The theory leads to the simple governing coupled equations for the actual composites which retain the integrity of the propagation process in each constituent but allow them to coexist under analytically derived interaction parameters. As a consequence of the analysis, effective mixture properties of the composite are obtained in the zero-frequency (static) limit. The accuracy of the approximation is demonstrated by direct comparison with the exact solution for the propagation of harmonic waves in the system.
Maximum operating power of 1.3 μm strained layer multiple quantum well InGaAsP lasers
Authors: Elenkrig B. B. , Smetona S. , Simmons J. G. , Makino T. , Evans J. D.
Published: July 2002

The maximum operating optical powers of ridge-waveguide 1.3 μm strained layer multiple quantum well lasers with various barrier heights and cavity lengths have been investigated over a wide temperature range. The linear reduction of output power with temperature was confirmed for all lasers under investigation. An empirical expression has been derived which shows that at a given temperature the laser output power is directly proportional to the laser’s characteristic temperature T0, and inversely proportional to its threshold current Ith.
Possible order–disorder phase diagrams in (Ax′A1−x″)BO3 and A(Bx′B1−x″)O3 complex perovskites
Authors: Gu Bing-Lin , Gui Hong , Liu Zhi-Rong , Zhang Xiao-Wen
Published: July 2002

The structural phase diagrams of complex perovskites have been studied within the mean-field approximation. Various phase diagrams with possible topological structures of the 1/2{111}-, 1/2{110}-, and 1/2{100}-type ordered structures are obtained. The 1/2{111}- and 1/2{100}-type ordered structures have similar phase diagrams and their ordering transitions are both of the second kind, while the 1/2{110}-type ordered structure has a more complex phase diagram and its ordering transition is of the first kind. In the two-phase region of the phase diagram, the ordered phase’s composition deviates from that of the chemical formula towards the completely ordering composition which is 0.5 for the 1/2{111}-, 1/2{100}-type ordered structures and 0.25, 0.50, 0.75 for the 1/2{110}-type ordered structures.