bluuebunny/crossref_metadata_2025 huggingface dataset:
In-situ x-ray diffraction study of the impurity effects on the cubic-to-tetragonal phase transformation in PbTiO3
Authors: Takesue Naohisa , Chen Haydn
Published: July 2002

The impurity effects on the phase transformation in a single crystal of PbTiO3 have been studied by in-situ x-ray diffraction technique. Two single crystals were employed for this study with one containing a noticeable amount of impurities. For the pure sample, a hysteresis of the structural parameters such as lattice constants with respect to temperature for the cubic-to-tetragonal phase transformation is found. Such a hysteresis loop is typical of the martensitic phase transformation. Existence of the athermal regions has been reported for the first time. The athermal regions were found to be asymmetric between heating and cooling cycles. Furthermore, the lattice parameters in both athermal regions show different behavior. These are caused by microstrain effects during growth of the product phase. In fact, within the athermal regions both the cubic and tetragonal phases are highly strained due to coherency strain. The sample with impurities has shown the existence of embryonic tetragonal fluctuations at temperatures far above Tc. The presence of impurities lowers Tc, and results in a wider hysteresis. The athermal regions are narrower for the sample with impurities, suggesting preferred heterogeneous nucleation at impurity defects.
Voltage tunable multiple quantum well distributed feedback filter with an electron beam written Schottky grating
Authors: Zia O. , Bhattacharya P. K. , Singh J. , Brock T.
Published: July 2002

A novel optoelectronic filter with voltage-tunable transfer characteristics has been developed and implemented in a multiquantum well waveguide device. By virtue of the quantum-confined Stark effect, the refractive index in quantum wells at the periphery of a guiding region can be given a periodicity in the guiding direction by application of a bias on an electron-beam patterned Schottky grating atop the guide. If the period of the Schottky grating and associated index profile satisfies the Bragg condition, as in a resonant distributed feedback structure, band-reject filtering results. Altering the bias on the Schottky grating changes the refractive index in the wells, thereby providing tunability of the wavelength at which Bragg diffraction occurs.
Measurement of residual strain in InGaAs buffer layers
Authors: Maigné P. , Baribeau J.-M.
Published: July 2002

InxGa1−xAs layers have been grown with different indium compositions and thicknesses above the critical layer thickness and the extent of strain relaxation has been measured using high-resolution x-ray diffraction. Our results show that, in thick layers, the residual strain is dependent upon the lattice mismatch. In a range from 30 times up to 300 times the critical layer thickness, the residual strain represents about 15% of the lattice mismatch, regardless of the In composition. Comparison with published experimental data shows that the magnitude of the residual strain is difficult to predict and depends upon parameters which are yet to be identified. Our data also shows a different behavior for a sample with thickness 450 times the critical layer thickness where strain relaxation is almost complete. One possible explanation is a relaxation process which takes place during the growth of the structure, leading to a time dependent lattice mismatch.
Nonlinear optical absorption in separate confinement multi-quantum-well structures due to spatial band bending
Authors: Väterlein C. , Fuchs G. , Hangleiter A. , Härle V. , Scholz F.
Published: July 2002

We report on nonlinear optical absorption in InGaAs/InGaAsP separate confinement multiple-quantum-well (SCMQW) structures based on charge-carrier induced band bending. Carrier induced localization and related changes in absorption strength due to spatial separation of electrons and holes are utilized to design SCMQW structures with a new type of optical nonlinearity. Experimentally, we observe strong changes in optical absorption with increasing excitation power. Compared to exciton bleaching, the new nonlinearity dominates at low carrier densities.
Ion beam induced lateral diffusion in Ni/Si couples at room temperature
Authors: Desimoni J. , Hagège S. , Traverse A.
Published: July 2002

Diffusion induced by 4 MeV Au++ irradiation at room temperature, normal and parallel to the substrate surface, has been studied on samples formed by a Ni film (the Ni island) which partially covers a Si film. Normal diffusion, monitored by Rutherford backscattering spectroscopy, is in agreement with previous results obtained on Ni films deposited on Si wafers. A noticeable spread of the edge of the Ni island along the lateral direction, that increases with irradiation fluence, was observed by transmission electron microscopy. When diffusion is thermally induced, the lateral diffusion length was found to be of the same order of magnitude as the normal one. However, under ion irradiation, the lateral diffusion length is two orders of magnitude larger than the normal diffusion. Electron diffraction patterns reveal the existence of mainly Ni2Si and Ni3Si2 in the layer formed along the lateral direction.
Interface structure of CdSe/ZnSe epilayers
Authors: Wu Xue-Hua , Peng Zhong-Ling , Yuan Shi-Xing , Li Fang-Hua
Published: July 2002

Interface microstructure and strain relaxation of atomic-layer-epitaxy-grown CdSe on ZnSe/GaAs〈001〉 was investigated by using transmission electron microscopy and high-resolution transmission electron microscopy techniques. The CdSe epilayer is characterized with fluctuated misorientations along the interface, irregularly distributed stacking faults (SFs), and an array of misfit dislocations (MDs) composed of 60°, Lomer, and partial types. It is found that the fluctuated misorientation is related to the proportion of different 60° MDs and SFs, and this causes the local strain difference. The analysis of MDs is facilitated by using a digital image processing method and allows a clear image of a Lomer MD formed by interaction of two 60° MDs as well as the value of remaining local strains by measuring the density of MDs.
Synchrotron-radiation photochemical-vapor deposition of amorphous carbon
Authors: Endo A. , Takami S. , Osawa T. , Honma I. , Komiyama H.
Published: July 2002

High-energy synchrotron-radiation (SR) light was applied to the photochemical-vapor deposition of amorphous carbon films. The source gases of CH4 and H2 have been directly photodissociated by the ultraviolet SR light to deposit carbon films on the heated Si substrate. The film structure was characterized by field-emission scanning electron microscopy, Raman scattering, and infrared absorption, which indicate the film to be a rigid amorphous structure comprised of mostly sp3-bonded carbon. The advantage as well as the limitation of the synchrotron-radiation process to the synthesis of functional materials is discussed.
Hole–phonon scattering rates in gallium arsenide
Authors: Scholz Reinhard
Published: July 2002

Starting from an 8*8 k⋅p band structure, phonon scattering rates between hole subbands can be calculated with realistic electronic wave functions. Pronounced differences to published light hole and heavy hole scattering rates are found for GaAs, partially due to the density of states of the nonparabolic light hole band and to different overlap between the wave functions. Results are presented for some quantities of interest for transport calculations, like the average velocity after polar LO–phonon scattering. The scattering rates of the split–off holes are calculated within the same formalism. It is shown that their lifetime is mainly limited by optical phonon deformation potential scattering towards the heavy and light hole bands. All band parameters and phonon occupations correspond to room temperature.
SiC/SiO2 micropatterning by ultraviolet irradiation and heat treatment of a poly(phenylsilyne) film
Authors: Watanabe Akira , Komatsubara Tadaharu , Ito Osamu , Matsuda Minoru
Published: July 2002

High-resolution micropatterning of SiC/SiO2 was achieved by combining the photochemical and thermal properties of poly(phenylsilyne) having Si-network structure. The laser flash photolysis of poly(phenylsilyne) film showed the formation of silyl radical as an intermediate during photodegradation. The XPS spectrum of the Si—Si chain changed into that of the siloxane chain by photo-oxidation in air. FT-IR spectra showed the formation of SiC and SiO2 by the heat treatment of unirradiated and irradiated poly(phenylsilyne) films, respectively. SiC/SiO2 micropatterning was obtained by the heat treatment of poly(phenylsilyne) film after ultraviolet irradiation with a photomask.
Deformation of adaptive heterophase materials
Authors: Roytburd Alexander L. , Slutsker Julia
Published: July 2002

A crystal which can be in two possible phase states is considered. During tensile extension the crystal is deformed elastically. After a certain amount of elastic strain a phase transformation begins. For each fixed level of strain an equilibrium mesostructure is established, which corresponds to a minimum in the free energy of the crystal. The equilibrium mesostructure consists of plane, parallel layers of a product phase separated by layers of an initial phase. The product phase itself consists of two or more different domains (twins) forming plane, parallel alternations. The volume fractions of the phases and of different twin components in the product phase are functions of strain and temperature. Above a critical temperature the product phase is a single domain (untwinned). The stress-strain curve which reflects the evolution of the equilibrium mesostructure is calculated. For deformation under a strain control the calculated equilibrium stress-strain curve has a section with negative slope that corresponds to a negative Young’s modulus. If deformation proceeds under stress control, hysteretic stress-strain curves on loading and unloading will result from a section with negative slope.
Optoelectronic properties of Pb5CrO8 thin films for application to contact-type line-image sensor
Authors: Toda Kohji , Watanabe Shingo
Published: July 2002

Optical and optoelectronic properties of Pb5CrO8 thin films prepared by an electron-beam evaporation deposition technique are described. The relation between spectral response of Pb5CrO8 thin-film lateral photodetectors and annealing temperature for developing the crystallization of the films is examined. An obtained spectral response for adequate annealing temperature covers an entire visible region. A large ratio of photocurrent and dark current and a fast response characterize the thin-film photodetector. An application to a color contact-type line-image sensor is also discussed to demonstrate the optoelectronic ability of the lateral Pb5CrO8 thin film.
Theoretical consideration of equilibrium dissociation geometries of 60° misfit dislocations in single semiconductor heterostructures
Authors: Zou J. , Cockayne D. J. H.
Published: July 2002

Equilibrium geometries of 60° dissociated misfit dislocations are considered theoretically using elasticity theory. The prediction of equilibrium dissociation width and the position of each partial with respect to the interface is given. The experimental results are in excellent agreement with the theory.
Atomic and magnetic ordering in CoPt alloy
Authors: Gomonaj Elena V. , Klugmann Eugeniusz
Published: July 2002

Magnetic susceptibility behavior of the disordered equiatomic CoPt alloy is studied during annealing up to the Curie point. A shift of the Curie point from 800 to 780 K during the ordering process and a number of other peculiarities at 470, 630, and 710 K are observed. To explain these peculiarities, the temperature dependence of the static magnetic susceptibility is calculated in the framework of phenomenological theory with a given structural order parameter. It is shown that the first maximum at 470 K is due to the rotation of the sample magnetization from the [111] to the [001] axis; its position depends upon the annealing rate and it disappears in the course of successive annealing runs. Two other maxima at 630 and 710 K can be attributed to the mutual reorientation of the Co and Pt magnetizations assuming that the Co-Pt exchange interaction depends strongly on temperature. The behavior of magnetic susceptibility in the paramagnetic region is shown to follow the Curie-Weiss law with small corrections arising in the structurally ordered state. The corresponding Curie temperature is found to depend on the structural order parameter. Theoretical results are compared with the experimental ones. Good agreement is achieved in the wide temperature interval below the Curie point.
Nb lift-off procedure for micropatterning Bi2Sr2Ca1Cu2O8+x thin films
Authors: Attanasio C. , Coccorese C. , Maritato L. , Prischepa S. L. , Salvato M.
Published: July 2002

We developed an in situ micropatterning process for Bi2Sr2Ca1Cu2O8+x thin films using the large volumetric increase of refractory metals such as Nb, V, and Ta, during the hard regime of oxidation. After the lift-off procedure, our films showed nonreduced critical currents and temperatures. This technique is particularly suitable for multilayer in situ lithographic process.
Laser ionization time-of-flight mass spectrometric study on laser ablation of a graphite-like material of (BC2N)ncomposition
Authors: Kokai F. , Taniwaki M. , Koga Y. , Kakudate Y. , Fujiwara S. , Fukuda K. , Kawaguchi M.
Published: July 2002

Laser ionization time-of-flight mass spectrometry has been used to probe Nd:YAG laser ablation products from a new graphite-like material of (BC2N)n composition. Neutral species mainly composed of one, two, and three atoms were detected. A center-of-mass velocity of 1.2×105 cm/s was obtained for the expansion of the products. An influence of collision-induced reactions is suggested for the product generation.
Structural and electro-optical investigation of a vapor-deposited chromophore-polymer thin film
Authors: Wu P. K. , Yang G.-R. , Ma X.-F. , Cococziela A. , Lu T.-M.
Published: July 2002

Nonlinear optical films were vapor deposited onto Si(100) and indium-tin-oxide-coated glass. These films are guest-host-type polymer and showed electrooptical (EO) properties after poling. The host polymer is Teflon AF 1600 (AF) and the guest is dimethylaminonitrostilbene (DANS). Deposition is done by coevaporation. EO effects were found in films containing 5–25 vol % DANS. The EO coefficient r33, is found to be a function of composition, poling temperature, and cooling rate during poling. The highest EO coefficient obtained is 2.4 pm/V from a film with 10 vol % DANS and poled at a temperature of 130 °C. Very little or no EO effects were found for films with ≳25 vol % DANS. This is found to be a result of phase separation and subsequent crystallization of DANS. A decrease of EO effect at higher poling temperature is possibly a result of thermal disorder which was ‘‘frozen’’ during cooling. DANS was also found to react with Teflon AF 1600 at a higher DANS concentration.
Non-ohmic behavior of the high-field magnetoresistances in AlGaAs/GaAs heterostructures
Authors: Allerman A. A. , Xu W. , Hauser N. , Jagadish C.
Published: July 2002

In this communication are reported the measurements and analyses of the dependence of the high-field magnetoresistivities (ρxx and ρxy) on the current density in AlGaAs/GaAs heterojunctions. The strong current dependence of spin-up and spin-down levels can be observed both for ρxx and for ρxy. It was found that at low temperatures and in strong magnetic fields a quantity R=B(dρxy/dB), with B the magnetic field, exhibits similar line shape and non-ohmic behavior to those of ρxx, which dramatically confirms the unresolved proportionality ρxx∼R. In high magnetic fields R shows a stronger oscillation structure, especially for spin levels, than that observed in ρxx.
Cathodoluminescence and photoluminescence from chemical-vapor-deposited diamond
Authors: Yang Xinxing , Barnes A. V. , Albert M. M. , Albridge R. G. , McKinley J. T. , Tolk N. H. , Davidson J. L.
Published: July 2002

Cathodoluminescence spectra from microwave-plasma-assisted chemical-vapor-deposited (CVD) diamond have been studied as a function of temperature at low electron energies. This investigation shows a pronounced difference between the luminescence spectra from CVD diamond films as grown and CVD diamond films annealed by rapid thermal processing at 1000 °C for one minute in an argon atmosphere. The relative intensities of the dominant features at 443 nm (2.8 eV) and 510 nm (2.4 eV) are very different for unannealed and annealed samples. We report measurements of the temperature dependence of these luminescence bands.
Heat and mass transfer in semiconductor melts during single-crystal growth processes
Authors: Kakimoto Koichi
Published: July 2002

The quality of large semiconductor crystals grown from melts is significantly affected by the heat and mass transfer in the melts. The current understanding of the phenomena, especially melt convection, is reviewed starting from the results of visualization using model fluids or silicon melt, and continuing to the detailed numerical calculations needed for quantitative modeling of processing with solidification. The characteristics of silicon flows are also reviewed by focusing on the Coriolis force in the rotating melt. Descriptions of flow instabilities are included that show the level of understanding of melt convection with a low Prandtl number. Based on hydrodynamics, the origin of the silicon flow structure is reviewed, and it is discussed whether silicon flow is completely turbulent or has an ordered structure. The phase transition from axisymmetric to nonaxisymmetric flow is discussed using different geometries. Additionally, surface-tension-driven flow is reviewed for Czochralski crystal growth systems.
Photoconductive and photovoltaic response of high-dark-resistivity 6H-SiC devices
Authors: Cho Pak S. , Goldhar Julius , Lee Chi H. , Saddow Stephen E. , Neudeck Philip
Published: July 2002

The optoelectronic properties of high-resistivity p-type hexagonal silicon carbide (6H-SiC) have been investigated using lateral photoconductive switches. Both photovoltaic and photoconductive effects are reported, measured at 337 nm, which is above the 6H-SiC absorption edge. These photoconductive switches have been fabricated with dark resistances of up to 1 MΩ; photoconductive switching efficiencies of more than 80% have been achieved. In addition, these devices displayed a high-speed photovoltaic response to nanosecond laser excitations in the ultraviolet spectral region; in particular, the observed photovoltaic response pulse width can be shorter than the exciting laser pulse width. This subnanosecond photovoltaic response has been modeled and good qualitative agreement with experiment has been obtained.
Photocapacitance investigation of the two photoquenching levels in n-type GaAs crystals with excess arsenic atoms
Authors: Nishizawa Jun-ichi , Oyama Yutaka , Dezaki Kazushi
Published: July 2002

The photocapacitance method is applied to the n-type GaAs crystal in which the excess arsenic atoms are introduced by the 67 h annealing at 900 °C under extremely high arsenic vapor pressure of 1950 Torr. The ion density photocapacitance spectra and the temperature dependencies of the photoinduced ion density measurements reveal two photoquenching levels with different level positions and thermal recovery temperatures, respectively.
Study of surface charges on dielectric electrodes in a radio-frequency glow discharge
Authors: Kakuta S. , Kamata T. , Makabe T. , Kobayashi S. , Terai K. , Tamagawa T.
Published: July 2002

A radio-frequency glow discharge between dielectric electrodes is observed by spatiotemporal optical emission spectroscopy. No emission from an excited Ar ion is observed in the dielectric electrode discharge, but it is observed in a discharge with metallic electrodes under the same external conditions. On the other hand, marked excitation of Ar(3p5) on the driven dielectric is observed. The maximum of the spatiotemporal net excitation rate profile is larger and the amplitude of the current is smaller compared with the discharge sustained by metallic electrodes. These differences are attributed to the effect of surface charge on the dielectric. The dielectric immersed in the discharge is exposed to the ion and electron fluxes, and is charged negatively to balance both fluxes. The accumulated charge on the dielectric is estimated by a capacitor in series with the grounded electrode. As a result, the surface charge affects the field just in front of the dielectric and enhances excitation and ionization.
Substrate bias effect on the random telegraph signal parameters in submicrometer silicon p–metal–oxide–semiconductor transistors
Authors: Simoen E. , Claeys C.
Published: July 2002

The effects of the substrate bias VBS on the parameters of random telegraph signals (RTSs) in submicrometer silicon p-channel metal–oxide–semiconductor transistors are analyzed. The fractional RTS amplitude increases slightly for more positive VBS, suggesting a close connection with the capture cross section of the corresponding near-interface oxide trap. A strong exponential dependence of the capture time constant on the drain current ID is observed, which can be explained by the transverse-field dependence of the hole capture cross section. The emission time constant on the other shows only a weak dependence on VBS, or ID. Finally, the corresponding low-frequency noise peaks at constant frequency f are studied in detail; excellent agreement is observed between the theoretical Lorentzian spectrum and the experimental peaks.
Monte Carlo simulation of bulk hole transport in AlxGa1−xAs, In1−xAlxAs, and GaAsxSb1−x
Authors: Martinez Marino J. , Look David C. , Sizelove John R. , Schuermeyer Fritz L.
Published: July 2002

We report a Monte Carlo study of hole transport in AlxGa1−xAs, In1−xAlxAs, and GaAsxSb1−x. The effects of alloy scattering are significant in all three cases, but mobilities are still high enough to be advantageous in particular device applications. We separately calculate the Hall r factors by a Boltzmann transport method and show that these factors are vitally important when attempting to compare Monte Carlo drift mobilities with experimental Hall data.
Dielectric mixture model for a hollow-ceramic-sphere composite
Authors: Liu Jay G. , Wilcox David L.
Published: July 2002

A layered sphere Bruggeman effective-medium model was developed to describe the dielectric constant of a porous ceramic composite having potential for use as a very low dielectric constant inorganic material for microelectronic packaging. The uniqueness of this model is that it incorporates a hollow sphere geometry and thus fits closely the microstructure of the dielectric composite in a broad range of compositions. Samples of the composite were prepared by adding hollow alumina-silica alloy ceramic spheres to a cordierite matrix. Their dielectric properties were characterized as a function of volume fraction of hollow spheres. The measured results were in good agreement with the model prediction.
Atomic oxygen effect on the in situ growth of stoichiometric YBa2Cu3O7−δ epitaxial films by facing targets 90° off-axis radiofrequency magnetron sputtering
Authors: Oya Gin-ichiro , Diao Chien Chen , Imai Syozo , Uzawa Takaaki , Sawada Yasuji , Sugai Tokuko , Nakajima Kensuke , Yamashita Tsutomu
Published: July 2002

(110)- and (103)-oriented almost stoichiometric YBa2Cu3O7−δ (YBCO) films have been grown epitaxially on hot SrTiO3 (110) substrates using a 90° off-axis rf magnetron sputtering technique, for fabrication of vertical sandwich-type YBCO/insulator/YBCO or YBCO/normal metal/YBCO Josephson junctions utilizing the high-quality YBCO films. The YBCO epitaxial films with high transition temperatures Tc of ∼90 K have been deposited in situ only under the conditions of substrate temperatures Ts of ∼650–∼700 °C and oxygen partial pressure PO2 of ∼5×10−3–∼10×10−3 Torr, which are in close proximity to the critical stability/decomposition line for YBa2Cu3O6 in the ordinary Y–Ba–Cu–O phase diagram. Using a quadrupole mass spectrometer, a high density of atomic oxygen has directly been observed to be efficiently produced in the sputter glow discharge under the above optimum conditions of PO2. This atomic oxygen has played a key role in promoting the formation of the perovskite structure and the epitaxial growth of the YBCO films. Furthermore, Shapiro steps have successfully been observed for a Nb–YBCO point-contact junction, which is made by pressing a Nb needle on a surface-etched YBCO epitaxial film, under 525.4 GHz submillimeter-wave irradiation.
Correlation between electronic and atomic structures in Ag-Ni multilayers
Authors: Jonnard P. , Vergand F. , Bonnelle C. , Badawi K. F.
Published: July 2002

Ag3Ni3 and Ag4Ni4 multilayers have been analyzed by electron induced x-ray emission spectroscopy. Changes observed on the density of valence states in the Ni layers, with respect to the bulk, are correlated with the variation of the lattice parameter deduced from x-ray diffraction experiments. In the Ag layers no change in the density of valence states is observed and the lattice parameter is almost unchanged. These behaviors are explained by an electronic effect due to the low dimensionality of the multilayers.
Zener tunneling condition and the hysteresis of trapped charge in an AlGaSb barrier in AlGaSb/InAs/AlGaSb double-barrier structures
Authors: Buot F. A. , Rajagopal A. K.
Published: July 2002

The essential role played by the physics of Zener tunneling of electrons from the barrier to the drain in finding the solutions to the field, energy-balance, and quantum transport equations in AlGaSb/InAs/AlGaSb double-barrier structures is pointed out. It is shown that the self-consistent physical consideration on these equations is crucial in obtaining interesting and realistic novel hysteresis phenomena of the trapped hole charge in an AlGaSb barrier. There exist solutions to the above equations which do not incorporate the physics of Zener tunneling, which are therefore physically untenable.
Electrical properties of p-type InGaAsP and InGaAs irradiated with He+ and N+
Authors: Sargunas V. , Thompson D. A. , Simmons J. G.
Published: July 2002

He+ and N+ ion irradiation of epitaxial p-type In0.76Ga0.24As0.58P0.42 and In0.53Ga0.47As was performed at 300 K to obtain high-resistivity regions. In both the ternary and quaternary samples the resistivity first increases with ion dose. A maximum is reached at a critical dose depending on the ion species and initial doping concentration. Above this dose the conductivity converts to n type and the resistivity steadily decreases to ∼102 Ω cm in InGaAsP and ∼2 Ω cm in InGaAs. After thermal annealing the type converted samples revert to p type. However, for ion doses ≥1013 cm−2 the high resistivities remain stable up to 700 K. The results suggest that simple point defects, rather than complexes are responsible for the changes in the electrical properties of the samples.
Character and distribution of vacancies in Czochralski-grown silicon ingots
Authors: Dannefaer S. , Bretagnon T.
Published: July 2002

Positron lifetime investigations of vacancy distributions in ingots of silicon have shown that vacancies are retained after growth at nearly constant concentrations close to 3×1016 cm−3. The vacancies are generally monovacancies and are suggested to be trapped by oxygen clusters. Trapped divacancies can also be formed but they are unstable upon heat treatment at 1000 °C for 16 h. This observation is invoked to explain anomalous oxygen precipitation. This heat treatment has little effect on the distributions of monovacancies in the ingots investigated, so the complexes between vacancies and oxygen clusters are suggested to be formed at temperatures above 1000 °C during the growth.
Interfacial reactions in the SiO2/Ru and SiO2/Ru/Al-Si structures
Authors: Wang Shi-Qing , Hong Stella , White Allen , Hoener Carolyn , Mayer J. W.
Published: July 2002

The interfacial reactions in the structures of Si/SiO2 (500 nm)/Ru (65 nm) (up to 950 °C) and Si/SiO2 (500 nm)/Ru (72 nm)/Al-0.8 wt % Si (630 nm) (up to 700 °C), were studied by Rutherford backscattering spectrometry, Auger electron spectroscopy, x-ray diffractometry, selected-area electron diffraction, energy-dispersive x-ray spectroscopy, scanning electron microscopy, and transmission electron microscopy. A thin film of Ru was stable on a SiO2 substrate up to 30 min in a vacuum anneal at 950 °C. In vacuum, a fast interfacial reaction between the Ru thin film and the Al-Si overlayer started between 550 and 575 °C, resulting in the formation of a Si/SiO2/ RuAl2/(Ru4Al13)Al-Si layer sequence. Similar to the reactions between Pt group metals and Al, the Ru/Al reaction is nonuniform. It is controlled by a nucleation mechanism and is accompanied by void formation.
Spatial variations of radio frequency plasma density caused by a step-change of secondary electron emission coefficient
Authors: Xu Weilu , Sides Paul J.
Published: July 2002

A two-dimensional analytical model for the effect of a nonuniform secondary electron emission coefficient (γ) on the powered electrode of a radio frequency plasma was developed. The nonuniformity was a step change in the value of γ. The ambipolar diffusion equation, including ionization by thermal and secondary electrons, took the form of a Helmholtz equation for which the solution by separation of variables was a series of cosine functions. The discontinuity of γ on the electrode engendered substantial nonuniformity of plasma density. For example, the nonuniformity at the sheath edge of an Ar plasma (50 mTorr, 200 V) was 40% for a substrate having a γ of 0.027 and resting on a platen having a γ of 1.00.
Nucleation of the Al4Mn alloy during containerless solidification in a drop tube
Authors: Yu R. C. , Zhang F. X. , Zhang J. , Wang W. K.
Published: July 2002

The undercooling and nucleation of Al4Mn alloy have been investigated by containerless solidification in a 1.2 m long drop tube, where molten droplets of the alloy solidified in the vacuum. Droplets in various sizes were collected at the bottom of the tube. It is found by x-ray and electron diffractions that besides the orthorhombic Al6Mn and β-Mn phases, two approximants of the decagonal quasicrystal, one approximant of the icosahedral quasicrystal and decagonal domains can be observed in the as-solidified droplets. However, due to the different cooling rate or undercooling in the samples of different sizes, the phases mentioned above will appear in different amounts. The relationship between the phase composition and the sample size is discussed on the basis of the classical nucleation and growth theories.
Theoretical performance limits of 2.1–4.1 μm InAs/InGaSb, HgCdTe, and InGaAsSb lasers
Authors: Flatté M. E. , Grein C. H. , Ehrenreich H. , Miles R. H. , Cruz H.
Published: July 2002

Ideal threshold current densities of 2.1–4.1 μm IR lasers are calculated for active layers composed of InAs/InGaSb superlattices, InGaAsSb quantum well quaternaries, InAsSb bulk ternaries, and HgCdTe superlattices. The fully K-dependent band structure and momentum matrix elements, obtained from a superlattice K⋅p calculation, are used to calculate the limiting Auger and radiative recombination rates and the threshold current density. InGaAsSb quantum wells and InAs/InGaSb superlattices are found to be more promising laser candidates than HgCdTe superlattices and InAsSb bulk ternaries. The calculated threshold current densities of InAs/InGaSb superlattices are similar to those of InGaAsSb active layers at 2.1 μm, but are significantly lower at longer wavelengths. Comparison with experiment indicates that the threshold current densities of InGaAsSb-based devices are about three times greater than those calculated for 25 cm−1 gain. The threshold current densities of present InAs/InGaSb superlattices are about 100 times above their theoretical limit.
Pr3+ luminescence in GaAs and AlxGa1−xAs implanted with Pr
Authors: Thee Paul L. , Yeo Yung Kee , Hengehold Robert L. , Pomrenke Gernot S.
Published: July 2002

Praseodymium (Pr) emissions were investigated for Pr-implanted GaAs and AlxGa1−xAs as a function of Al mole fraction, sample temperature, and anneal temperature using photoluminescence (PL) spectroscopy. Two groups of major PL peaks have been observed near 1.6 and 1.3 μm, which can be attributed to the crystal-field-split spin-orbit level transitions of 3F3→3H4 and 1G4→3H5 of Pr3+ (4f2), respectively. The PL intensity of Pr3+ varies dramatically with the Al mole fraction in Pr-implanted AlxGa1−xAs. For GaAs, the PL peak intensity near 1.3 μm is strong and the peak intensity near 1.6 μm is weak, whereas the PL peak intensity near 1.6 μm is much stronger than that of 1.3 μm for AlxGa1−xAs. Furthermore, Al0.15Ga0.85As shows particularly strong luminescence peaks near 1.6 μm. PL emissions of Pr were detected for sample temperatures as high as 100 K, and several hot lines were identified. The optimal anneal temperatures for the Pr-implanted GaAs and AlxGa1−xAs are between 725 and 775 °C. Dual implantations of Pr and Er into Al0.15Ga0.85As have also been made, but the PL results showed that the emission intensities of both ions did not increase compared to those of identically prepared singly implanted samples. Possible excitation processes are explored in light of these results.
Photoconductivity and n- to p-type transition in silicon planar-doped GaAs structures with a doped cap layer
Authors: de Oliveira A. G. , Ribeiro G. M. , Soares D. A. W. , Corrêa F. J. A. , da Silva M. I. N. , Chacham H.
Published: July 2002

Photoconductivity, photo-Hall free carrier concentrations and mobilities were measured on molecular beam epitaxy-grown silicon planar-doped GaAs samples, with silicon concentrations between 1.4×1012 and 8.8×1013 cm−2, as functions of temperature. In all samples, the planar-doped region is placed 0.2 μm below a n+-doped cap layer at the surface. We perform a systematic investigation of the persistent as well as nonpersistent photoconductivity effects which are present in all samples. A phenomenological analysis shows the presence of two distinct conduction channels, one with n characteristics and the other with p characteristics. These channels can present either bulk or two-dimensional characteristics, depending on the light intensity and on the temperature. A model based on spatial separation between electrons and holes is proposed to account for the persistence of the photoconductivity effect.
Determination of the hydrogen content in an evaporated Pd film by neutron interferometry
Authors: Tasaki Seiji , Kawai Takeshi , Ebisawa Toru
Published: July 2002

We demonstrate the feasibility of a microneutron interferometer (MINI) to determine the hydrogen content in the evaporated palladium film. MINI consists of a thin palladium film sandwiched by two semitransparent multilayer mirrors of Ni/Ti. The atomic ratio H/Pd as obtained from the shift of the interference fringes, under hydrogen pressures of 1 and 2 atm, was found to be 0.43 and 0.55, respectively. The accuracy of the results is 5.5%. MINI realized a better determination of the atomic ratio H/Pd than that in the case of a single Pd film by obtaining high neutron reflectivity in a relatively high Q range.
Modeling the diffusion of implanted Be in GaAs
Authors: Hu J. C. , Deal M. D. , Plummer J. D.
Published: July 2002

The diffusion of implanted Be in liquid-encapsulated Czochralski GaAs samples is modeled using SUPREM-IV.GS, a simulator for GaAs and Si processing technology. The ‘‘plus one’’ approach for defect generation after implantation, as well as an assumption of local Ga interstitial sinks, are used to successfully simulate the high Be diffusivity, the uphill diffusion and the time-dependent Be diffusivity. The fast diffusion of implanted Be can be simulated using the same intrinsic Be diffusivity as that used in the simulation of the slow diffusion of molecular beam epitaxy grown-in Be. The roles of extended defects and nonequilibrium Ga point defects in the implanted Be anomalous diffusion behavior are taken into account.
Molecular-dynamics study of the synthesis and characterization of a fully dense, three-dimensional nanocrystalline material
Authors: Phillpot S. R. , Wolf D. , Gleiter H.
Published: July 2002

A molecular-dynamics simulation method is described that permits space-filling, fully dense three-dimensional nanocrystalline materials to be grown by crystallization from the melt. The method is illustrated by computer synthesis of an eight-grain polycrystal of Cu with a grain size of 43 Å. At the beginning of the simulation, eight small pre-oriented single-crystal seeds are embedded in the melt which is subsequently cooled below the melting point to enable crystal growth under an applied external pressure. The fully relaxed nanocrystalline material contains large perfect-crystal regions separated by well-defined grain boundaries, most of which have approximately the same width and energy. In spite of the rather small number of grains in the simulation cell, the thermal expansion of the material is practically isotropic, and almost identical to that of the perfect crystal. The elastic moduli are also almost isotropic and are somewhat lower than in the coarse-grained material. The material exhibits a low-temperature anomaly in the specific heat.
Paramagnetic defects induced by mechanical stress in calcium sulfide phosphor
Authors: Caurant D. , Gourier D. , Demoncy N. , Ronot I. , Pham-Thi M.
Published: July 2002

The effect of uniaxial pressing and grinding on pure and europium-doped CaS powders prepared by the alkaline polysulfide flux method is studied by electron spin resonance (ESR). F+ centers are generated in the bulk of CaS particles and their ESR spectra can be easily separated from those of other paramagnetic species by using a 90° out-of-phase detection. This is possible because of the very long transverse relaxation time T2 of F+ centers. It is thought that the primary defects induced by mechanical stress are bulk F centers, which are next partially converted into F+ centers by reduction of metal ion impurities (Eu3+,Cr3+,Cu2+) and native hole centers. Heat treatments in air of mechanically stressed powders show that F+ centers firstly diffuse to the particle surface forming low-symmetry F+s centers for temperatures ranging from 300 to 700 K. Atmospheric oxygen then transforms F+s centers into ESR silent O− ions except if they are stabilized by Na+ impurities originating from the flux. For temperatures higher than 700 K, these centers are then converted into paramagnetic SO−2 centers. The reason for the strong decrease of Eu2+ emission intensity (λ=645 nm) following mechanical stress is discussed. The study of the fine structure of Mn2+ natural impurity shows that local order is strongly disturbed in CaS particles by mechanical stress.
Determination of free carrier concentration in n-GaInP alloy by Raman scattering
Authors: Sinha K. , Mascarenhas A. , Kurtz Sarah R. , Olson J. M.
Published: July 2002

We present results of Raman scattering from coupled phonon-plasmon modes in Se-doped n-Ga0.52In048P alloy. Due to the small energy separation between the Γ- and the L-point conduction-band minima for this alloy composition, a significant fraction of the free carriers at room temperature are present in the L-conduction-band valley, giving rise to a multicomponent plasma. The carrier concentrations extracted from the Raman spectra for the different epilayers are in good agreement with the free electron concentrations determined by capacitance-voltage measurements. We employ the light scattering technique to extract the carrier concentration in the n-type emitter layer of a GaInP-based solar cell.
Possible evidence for quantum-size effects in self-assembled ultrathin films containing conjugated copolymers
Authors: Hong H. , Tarabia M. , Chayet H. , Davidov D. , Faraggi E. Z. , Avny Y. , Neumann R. , Kirstein S.
Published: July 2002

We present photoluminescence (PL), UV absorption, electroluminescence and x-ray reflectivity studies of self-assembled multilayer films containing alternate layers of conjugated copolymers, and nonconjugated insulating polymers. We show that the PL emission properties of these organic quantum wells can be ‘‘tuned’’ by a proper choice of the conjugated copolymer and the thickness of the insulating layers. Particularly, some of the self-assembled ultrathin films containing thin (∼7 Å) insulating polymeric layers exhibit a blue shift upon decreasing the thickness of the assembly. The PL shift is roughly proportional to 1/d2 where d is the thickness of the assembly, as expected for confined photogenerated electron-hole pair in an infinite square potential well. In contrast, the PL emission of similar assemblies but containing thick (∼40 Å) insulating layers is independent of the assembly thickness and exhibit emission in the blue. This may suggest a strong spatial confinement. Light emitting diodes based on self-assembled multilayer films with improved efficiency and stability and with threshold voltage as low as 2.6 V could be fabricated.
Photorefractive composite materials with bi-functional charge transporting second-order nonlinear optical chromophores
Authors: Zhang Yue , Ghosal Saswati , Casstevens Martin K. , Burzynski Ryszard
Published: July 2002

Two bi-functional chromophores have been developed which possess second-order nonlinear optical and charge transporting properties. The second-order nonlinearity arises from conjugated structures terminated by donor and acceptor groups and the charge transporting property results from a triphenylamine moiety. The bi-functionality of the chromophores is confirmed by the large electro-optic coefficients and high photoconductivities of composites containing these chromophores. These chromophores have been incorporated into an inert polymer to form photorefractive composite materials when doped with a photocharge generation sensitizer. The electro-optic coefficients and photoconductivity of these composites have been measured as a function of the applied electric field as well as of the chromophore concentration. The dependence of the four-wave mixing diffraction efficiency, holographic grating writing rate, and two-beam coupling gain on the applied electric field and on the chromophore concentration have been evaluated. A quantitative relationship between the chromophore concentration and a number of photorefractive parameters including electro-optic coefficient, photoconductivity, four-wave mixing diffraction efficiency, and grating writing rate has been established.
Strain-induced electron and hole lateral confinement in semiconductor heterostructures with curved heterointerfaces
Authors: De Caro Liberato , Tapfer Leander
Published: July 2002

In this work we theoretically investigate the effect of the strain on the conduction and valence bands of quantum wells of two-dimensional (2D) strained semiconductor heterostructures with curved and parallel heterointerface profiles. InxGa1−xAs/GaAs heterostructures with hyperbolic heterointerface profiles are considered. The discussed structures are geometrically similar to V-shaped quantum wires, with the only difference that the well thickness is constant (absence of the crescent shape). The variation of the crystallographic orientation along the curved heterointerface profiles leads to relevant nonuniform strain fields. The nonuniform lattice deformations induce: (i) large band-gap modulations up to several tens of meV; (ii) piezoelectric effects; and (iii) lateral electron and hole potential modulations of several tens of meV. These potential modulations may allow 1D electron and hole confinement in thin InxGa1−xAs curve quantum wells (few nm thick), and should be taken into account in the calculation of the lateral confinement potentials obtained by geometrical constraints in strained V-groove-shaped quantum wires.
Growth of Er-doped silicon using metalorganics by plasma-enhanced chemical vapor deposition
Authors: Andry P. S. , Varhue W. J. , Ladipo F. , Ahmed K. , Adams E. , Lavoie M. , Klein P. B. , Hengehold R. , Hunter J.
Published: July 2002

Epitaxial growth of Er-doped silicon films has been performed by plasma-enhanced chemical vapor deposition at low temperature (430 °C) using an electron cyclotron resonance source. The goal was to incorporate an optically active center, erbium surrounded by nitrogen, through the use of the metalorganic compound tris (bis trimethyl silyl amido) erbium. Films were analyzed by Rutherford backscattering spectrometry, secondary ion mass spectroscopy, and high resolution x-ray diffraction. The characteristic 1.54 μm emission was observed by photoluminescence spectroscopy. Previous attempts to incorporate the complex (ErO6) using tris (2,2,6,6-tetramethyl- 3,5-heptanedionato) erbium (III) indicated that excessive carbon contamination lowered epitaxial quality and reduced photoluminescent intensity. In this study, chemical analysis of the films also revealed a large carbon concentration, however, the effect on epitaxial quality was much less destructive. A factorial design experiment was performed whose analysis identified the key processing parameters leading to high quality luminescent films. Hydrogen was found to be a major cause of crystal quality degradation in our metalorganic plasma-enhanced process.
Coupled LO–plasmon modes in semi-insulating GaAs of ZnSe/GaAs heterojunctions
Authors: Pagès O. , Renucci M. A. , Briot O. , Aulombard R. L.
Published: July 2002

Raman spectroscopy is used to investigate strong band bending at the interface in semi-insulating substrates of ZnSe/GaAs heterostructures grown at high epitaxy rates. Direct evidence is given of the enhancement of polar modes strength, on the substrate side, by the electric field of the space-charge zone associated with Fermi-level pinning. The latter is qualitatively analyzed by following band flattening under illumination through the evolution of interfacial coupled LO-phonon–plasmon modes. Corresponding Raman line shapes are discussed within the phenomenological approach of D. H. Hon and W. L. Faust [Appl. Phys. 1, 241 (1973)].
Study of the orientational relaxation dynamics in a nonlinear optical copolymer by means of a pole and probe technique
Authors: Michelotti F. , Toussaere E. , Levenson R. , Liang J. , Zyss J.
Published: July 2002

We report measurements on the isothermal relaxation dynamics of the electro-optic coefficient of electric field poled polymeric films. The studied copolymer is a side-chain Disperse Red 1/poly–methyl–metacrylate combination. The isothermal relaxation is shown to follow a Kohlrausch–Williams–Watts stretched exponential dependence, whose parameters are strongly dependent on temperature. The average decay constant is shown to follow an Arrhenius temperature dependence below the glass transition Tg, while it is described by a Vogel–Fulcher–Tamann–Hesse above Tg. The values of the decay constant confirm a much better stability of the polymer orientation with respect to guest-host systems. The relaxation is shown to be due to a broad distribution of relaxation rates, the width of which can be qualitatively described by an empirical relation, which has already been used for guest-host systems. The measurement technique permits monitoring relaxation in a standard dielectric relaxation configuration and performing unconventional poling of the samples.