bluuebunny/crossref_metadata_2025 huggingface dataset:
Angular spectrum of visible resonant cavity light-emitting diodes
Authors: Stavrinou P. N. , Whitehead M. , Parry G. , Button C. C.
Published: July 2002

We present spectrally resolved angular radiation patterns from visible resonant cavity light-emitting diodes. Recording the pattern for a fixed wavelength clearly demonstrates how the overall angular emission is strongly influenced by the relative spectral alignment of the cavity resonance and the underlying quantum well emission, i.e., the detuning. Combined with measurements of total optical power, taken over a range of solid angles, the results highlight the importance of accounting for the collection optics of a proposed application when an optimum design is considered.
Optical properties of MnAl2S4 and MnAl2Se4 single crystals
Authors: Park Tae-Young , Jin Moon-Seog , Choe Sung-Hyu , Goh Jae-Mo , Kim Wha-Tek
Published: July 2002

MnAl 2 S 4 and MnAl2Se4 single crystals were grown by the chemical transport reaction method. Optical energy gaps of the MnAl2S4 and MnAl2Se4 single crystals were 3.75 and 3.21 eV, respectively, at 300 K. Emission peaks due to donor-acceptor pair recombinations were observed at 450 and 603 nm in the MnAl2S4 single crystal and at 488 and 655 nm in the MnAl2Se4 single crystal. Optical absorption peaks and emission peaks described as appearing due to Mn2+ ion sited in Td symmetry were observed at 414, 450, 482, and 527 nm in the MnAl2S4 single crystal and at 416, 455, 488, and 532 nm in the MnAl2Se4 single crystal.
Comment on “Dynamic analysis of the response of lateral piezoresistance gauges in shocked ceramics” [J. Appl. Phys. 82, 2845 (1997)] and “Determination of lateral stresses in shocked solids: Simplified analysis of piezoresistance gauge data” [J. Appl. Phys. 83, 747 (1998)]
Authors: Rosenberg Z.
Published: July 2002

The issue of lateral stress measurements, with piezoresistance gauges, is discussed by Feng et al. [J. Appl. Phys. 82, 2845 (1997)] and Feng and Gupta [J. Appl. Phys. 83, 747 (1998)] using 2D numerical simulations. The purpose of this Comment is to highlight the difficulties with these simulations and to emphasize some of the results obtained in these articles, which strongly support our analytical approach to the subject.
Investigation of the temperature dependence in Black’s equation using microscopic electromigration modeling
Authors: Tammaro M.
Published: July 2002

The long-standing speculation regarding an additional temperature dependence in Black’s equation is addressed. Under the premise that the activation energy measured using Black’s equation, Ea, should be equal to the energy barrier for atomic hopping, we provide evidence for an additional T2 dependence of the failure time. Microscopic modeling provides an explicit connection between the macroscopic model behavior and the microscopic parameters of the model. These results may provide a reliable method for measuring activation energies for self-diffusion in pure metals.
A study on the oxidation kinetics of silicon in inductively coupled oxygen plasma
Authors: Choi Yong Woo , Ahn Byung Tae
Published: July 2002

The oxidation kinetics of silicon in inductively coupled oxygen plasma (ICP) was studied at temperatures ranging from 350 to 450 °C. The oxide growth kinetics was described by a linear-parabolic growth law, with a rapid initial growth and a negative linear-rate constant. Under oxygen pressure of 10 mTorr, the initial oxide growth at 350 °C (thickness below 25 nm) was faster than at 400 °C. An analysis of transverse-optical mode frequencies and etch rates indicated that the density of the surface oxide was lower than that of the bulk oxide. The oxidation kinetics was explained qualitatively by assuming that the ICP oxide consisted of a surface layer with a larger diffusion coefficient and a bulk layer with a smaller diffusion coefficient. On the other hand, the ICP oxidation of silicon with a thin chemical oxide showed a positive linear-rate constant and no surface layer effect, supporting the fact that the oxide grown by the ICP oxidation has a low-density surface layer with a larger diffusion coefficient.
Experimental investigation of deviations from local thermodynamic equilibrium in high-pressure mercury discharges
Authors: Elloumi Hatem , Kindel Eckhard , Schimke Conrad , Zissis Georges
Published: July 2002

The determination of the radial profile of the ground state density without assuming local thermodynamic equilibrium (LTE) conditions in around atmospheric pressure (0.01 MPa<p<0.3 MPa) discharges used as light sources is a worthy investigation subject. This work deals with the high-pressure mercury discharge which could be considered as a “test case.” Particularly useful for the diagnostics of these plasmas is the self-reversed resonance mercury line 253.7 nm. In this article, two independent experimental methods were used: emission spectroscopy, called the “ΔλR method,” and interference shift measurements “hook method.” Using the Hg-253.7 nm resonance line, both experimental methods indicate similar deviations from LTE in particular for the lower pressure discharges (p<0.04 MPa). In those cases, the experimental errors for both methods are significantly lower than the detected deviations. Furthermore, the measured deviations are in good agreement with predicted values from a two-temperature, two-dimensional fluid model developed elsewhere.
Effects of stress relaxation of epitaxial SrRuO3 thin film on microstructures
Authors: Pan X. Q. , Jiang J. C. , Tian W. , Gan Q. , Rao R. A. , Eom C. B.
Published: July 2002

We report the effect of lattice stress relaxation on the microstructures of epitaxial thin films by domain structure studies of epitaxial SrRuO3 thin films grown on vicinal (001) SrTiO3 substrates. X-ray diffraction analysis revealed that the as-grown films are single domain and have a strained lattice due to the lattice mismatch with the substrate. In contrast, plan-view transmission electron microscopy (TEM) images obtained from the same films showed the coexistence of domains with three different crystallographic orientations. The discrepancy is attributed to the lattice stress relaxation occurring on the TEM specimens as the substrate material is eliminated by ion milling or etching, resulting in the formation of elastic domains with different crystallographic orientations. These studies directly reveal a crucial effect of the lattice strain relaxation on the microstructures and properties of epitaxial thin films when the substrate material is removed.
Electroabsorption studies on quasi-three-dimensional and quasi-two-dimensional excitons in (Zn,Cd)Se/Zn(S,Se) structures
Authors: Lilienkamp T. , Michler P. , Ebeling W. , Gutowski J. , Behringer M. , Fehrer M. , Hommel D.
Published: July 2002

The electro-optical properties of (Zn,Cd)Se/Zn(S,Se) quantum well structures grown on GaAs substrates have been studied with differential electroabsorption spectroscopy at room temperature and compared to model calculations. (Zn,Cd)Se wells of 20, 10, and 4×5 nm are investigated, corresponding to well widths of four, two, and one times the exciton Bohr radius in this material system, respectively. We observe the quantum confined Stark effect for the 4×5 nm sample and find a Stark shift of 18 meV in the heavy-hole exciton peak for an electric field change from 82 to 175 kV/cm. In contrast, the 10 nm sample shows a rather weak and more Franz–Keldysh-like signal. We show that the 20-nm-thick quantum-well sample behaves like bulk material, i.e., the electro-absorption signal is well described by Franz–Keldysh oscillations.
Valence band discontinuity, surface reconstruction, and chemistry of (0001), (0001̄), and (11̄00) 2H–AlN/6H–SiC interfaces
Authors: King S. W. , Davis R. F. , Ronning C. , Benjamin M. C. , Nemanich R. J.
Published: July 2002

A detailed examination of the valence band discontinuity (ΔEv) formed at the (0001), (0001̄), and (11̄00) interfaces between 2H–AlN and 6H–SiC has been conducted using x-ray and UV photoelectron spectroscopies. The ΔEv was observed to range from 0.6–2.0 eV depending on the growth direction (i.e., AlN on SiC vs SiC on AlN), as well as the crystallographic orientation, cut of the SiC substrate (i.e., on versus off axis), and SiC surface reconstruction and stoichiometry. A ΔEv of 1.4–1.5 eV was observed for AlN grown on (3×3) (0001)Si6H–SiC on-axis substrates; a ΔEv of 0.9–1.0 eV was observed for off-axis substrates with the same surface reconstruction. The values of ΔEv for AlN grown on (√3×√3)R30°(0001) 6H–SiC on-and-off-axis substrates were 1.1–1.2 eV. A larger valence band discontinuity of 1.9–2.0 eV was determined for 3C–SiC grown on (0001) 2H–AlN. Smaller values of ΔEv of 0.6–0.7 and 0.8–0.9 eV were observed for AlN grown on on-axis (0001̄)C and (11̄00)6H–SiC substrates, respectively.
Near infrared avalanche photodiodes with bulk Al0.04Ga0.96Sb and GaSb/AlSb superlattice gain layers
Authors: Cheng X-C. , McGill T. C.
Published: July 2002

We demonstrate the use of bulk Al0.04Ga0.96Sb and GaSb/AlSb superlattice as the gain material in a separate absorption/multiplication avalanche photodiode with sensitivity up to 1.74 μm. Both gain schemes were implemented in a molecular-beam epitaxy grown structure with a selectively doped InAs/AlSb superlattice as the n-type layer. Hole impact ionization enhancement was observed in Al0.04Ga0.96Sb by using a two wavelength injection scheme. The superlattice gain layer device exhibited multiplication factors in excess of 300, and surface limited dark current at a level comparable to InGaAs/InAlAs devices of similar design. The superlattice gain layer was found to be more promising than its bulk counterpart due to its inherent lower dark current.
The effect of threading dislocations, Mg doping, and etching on the spectral responsivity in GaN-based ultraviolet detectors
Authors: Torvik J. T. , Pankove J. I. , Nakamura S. , Grzegory I. , Porowski S.
Published: July 2002

Presently, GaN-based photodiodes grown on sapphire substrates exhibit broad spectral cutoffs, poor visible rejection, and low quantum efficiencies when illuminated through a p-type contact/window layer. We, therefore, investigate the effect of threading dislocations, Mg doping, and etching on the photoconductivity spectra in GaN. Highly resistive bulk GaN:Mg nearly free of threading dislocations has more than 1000 times better visible rejection ratio compared to highly dislocated and comparably doped p-GaN thin films grown on sapphire substrates. However, the heavy Mg compensation in both the bulk and the epitaxial film causes a broad spectral cutoff. Unintentionally doped semi-insulating GaN with low dislocation density has a similar rejection ratio compared to bulk GaN:Mg, but much sharper spectral cutoff due to the absence of intentional doping. Furthermore, postgrowth processing steps such as etching and polishing significantly increase the surface recombination compared to the as-grown surface.
Reaction of Si (100) with silane–methane low-power plasma: SiC buffer-layer formation
Authors: Bittencourt C.
Published: July 2002

The formation of a SiC buffer layer on Si (100) at substrate temperature as low as 950 °C using radicals of methane molecules obtained in a low-power-density glow-discharge plasma, is presented. The x-ray photoemission spectroscopy and low-energy-yield spectroscopy performed in the constant final-state mode suggest that the layers obtained were stoichiometric. To understand the mechanism of heteroepitaxial silicon carbide growth, the early stage of SiC nucleation was observed by atomic force microscopy and reflection high-energy electron diffraction. The results reveal that three-dimensional epitaxial crystallites nucleate at the earliest growth stage followed by a further Volmer–Weber growth.
Formation of SiC-surface layer by ion implantation
Authors: Theodossiu E. , Baumann H. , Bethge K.
Published: July 2002

A homogeneous SiC-surface layer is formed by implantation of 40 keV 13C carbon ions into single-crystalline silicon 〈100〉 with a fluence of 3.8×1017 ions/cm2 and subsequent electron beam rapid thermal annealing (EB-RTA). The carbon-depth distributions were analyzed with the resonant nuclear reaction 13C(p,γ)14N. In contrast to furnace annealing, EB-RTA (1150 °C for 15 min) leads to a carbon redistribution resulting in the formation of a homogeneous SiC-surface layer of about 50 nm in thickness. The carbon redistribution was investigated on silicon samples with an oxygen-depth marker using Rutherford backscattering spectroscopy. SiC bonds were detected by Fourier transform infrared spectroscopy measurements.
Twist elastic constants of liquid crystals using in-plane electrode structure
Authors: Ikeda K. , Okada H. , Onnagawa H. , Sugimori S.
Published: July 2002

A new technique for measuring the twist elastic constant (K22) using an in-plane electrode structure was investigated. The variation in the temperature dependence of K22 with respect to the molecular structure of fluorinated phenylbicyclohexane (PBC) compounds was investigated. The experimental cell has in-plane electrodes between which the liquid crystal molecules are aligned to an offset angle of azimuthal direction. Using K22 as a fitting parameter, the transmittance–voltage curves for the calculated results are fitted to those for the experimental results. In addition, the investigation of the temperature dependence of K22 revealed that the PBC compound having a para-cyano substituent at the benzene ring exhibits larger splay elastic constant and larger temperature dependence. Finally, molecular association is discussed based on the experimental results.
Simulation of material properties of amorphous carbon nitride with different nitrogen concentrations
Authors: Lu Y. F. , He Z. F. , Ren Z. M.
Published: July 2002

A theoretical model is proposed to simulate the structures of amorphous carbon nitride. An understanding of amorphous carbon nitride structures and properties is helpful for the synthesis and applications of this novel material. The influence of the nitrogen concentration on the cohesive energy, bulk modulus, and vibration density spectra was studied. An expression for the total cohesive energy of an atom group was established. The material structure can be simulated by minimizing the total energy of the atoms in the group. From the simulated structure of the material, many properties can be predicted. The results show that either high or low nitrogen concentrations in the carbon nitride led to a high bulk modulus. The vibrational spectra of carbon and nitrogen atoms depend on the nitrogen concentration. This theoretical model may help to identify an appropriate process to grow hard films with expected properties.
Magnetotransport in chemically derived La–Ca–Mn–O thin films
Authors: Moshnyaga V. , Khoroshun I. , Weidinger A. , Klimm S. , Zeitler M. , Tidecks R. , Horn S. , Samwer K.
Published: July 2002

La 2/3 Ca 1/3 MnO 3 thin films were prepared by a metal organic aerosol deposition technique and characterized with respect to their structure and magnetotransport properties. The results demonstrate that the metal–insulator transition temperature TMI can be varied in the range of TMI∼150–250 K, thereby increasing the maximal magnetoresistance MR=ΔR/R(5T) from 200% to 1400%, and yielding a large MR over a wide temperature range for a given sample. All of the above properties depend sensitively on the microstructure of the films.
Recording process and reproducing feasibility of transverse magnetic recording
Authors: Atarashi E. , Shiiki K.
Published: July 2002

The process and reproducibility of transverse magnetic recording are studied by micromagnetic simulation using the Landau–Lifshitz–Gilbert equation and compared with longitudinal recording. The magnetization process of the recording medium by a conventional thin-film head is simulated for various anisotropic fields of media. It is confirmed that the transverse bit can be recorded by using a conventional head and medium at high-bit density. The magnetic-field component perpendicular to the medium is very important for magnetization reversal. In transverse recording, there is a disadvantage in that the field perpendicular to the medium along the bit direction is very small. The transverse recording bits can be detected by a magnetoresistive element with two sensing regions at each end that have opposing biases of different signs. As the track width becomes narrow, the output for transverse bits becomes large relative to that for longitudinal bits.
Effect of wire length on Coulomb blockade in ultrathin wires of recrystallized hydrogenated amorphous silicon
Authors: Ng V. , Ahmed H. , Shimada T.
Published: July 2002

Electron transport in nanowires of hydrogenated amorphous silicon recrystallized by electron beam annealing has been studied. Wire length dependent electrical characteristics have been measured at low temperatures. Evidence for single electron effects with I–V characteristics has been presented. Transmission electron microscopy and photoluminescence data are used to support the electrical results. The Coulomb blockade effect is modeled as a combination of hopping conduction between a limited number of trapping sites in amorphous regions or at grain boundaries with additional Coulomb blockade effects as well as a confinement due to potential fluctuations arising from dopant distribution.
Production of amorphous zones in GaAs by the direct impact of energetic heavy ions
Authors: Bench M. W. , Robertson I. M. , Kirk M. A. , Jenčič I.
Published: July 2002

The damage produced in GaAs by implantation with low energy heavy ions has been studied as a function of ion mass and implantation temperature (30 and 300 K). The experiments were performed in situ in the microscope-accelerator facility at Argonne National Laboratory. In samples implanted and examined at 30 K, spatially isolated amorphous regions were produced by the direct impact of 50 keV Ar, Kr, and Xe ions. The probability that the impact of an individual ion formed an amorphous zone increased as the ion mass increased from Ar to Kr but not from Kr to Xe. The average dimension of the amorphous zones also increased with ion mass, being greater for the Xe than for the Kr ion implantation. On warming to room temperature, the amorphous zones decreased in size and density as the sample temperature was increased above 200 K. In samples implanted and examined at 300 K, the probability of forming an amorphous zone by direct impact increased as the ion mass increased from Kr to Xe, although the probability was always less than at 30 K. The density of amorphous zones produced at 300 K was similar to that remaining in a sample implanted at 30 K and then warmed to room temperature. With time at 300 K the amorphous zones decreased in size and eventually crystallized completely, leaving no trace of their prior existence.
Stranski-Krastanov growth of Al on Cr layers during molecular-beam epitaxial growth and its influence on the structure of epitaxial (001) Al/Cr superlattices
Authors: Kingetsu Toshiki , Kamada Yasuhiro , Yamamoto Masahiko
Published: July 2002

We studied growth behavior and structures of (001) Al/Cr superlattices grown by molecular-beam epitaxy, using in situ reflection high-energy electron diffraction (RHEED) during growth and ex situ x-ray diffraction (XRD). The (001) epitaxial growth proceeded with an orientational relationship of Al(001)[010]‖Cr(001)[110]. The (001) Al layers grew on Cr layers in the Stranski-Krastanov mode. The maximum thickness of the layer growth at the early stage was two monolayers (ML), and three-dimensional islands emerged upon further growth. Besides island growth during deposition, postgrowth evolution of the Al islands after completion of deposition was clearly observed from time-resolved measurements of the Bragg spot intensity of Al in RHEED. The strain relief of Al upon formation of islands was detected. The Al island evolution was found to depend on the growth temperature. The activation process of the island formation is discussed. The interfaces in the superlattices with 2-ML-thick Al layers were very flat. The (001) interplanar spacing of the 2-ML-thick Al layers contracted as compared with that of bulk Al, and was as small as 0.158 nm. The contraction was found to result in the emergence of a satellite peak in a middle-angle range XRD profile using model calculations. The interfaces in superlattices with Al layers thicker than 2 ML were rough because of the island growth of Al. The XRD-determined period of a nominal Al(x ML)/Cr(y ML) superlattice was (0.158×2+0.144y) nm or slightly larger, being much smaller than (0.202x+0.144y) nm expected from the interplanar spacings of bulk Al and Cr, where 0.202 and 0.144 nm are interplanar spacings of (001) bulk Al and Cr, respectively.
Structural and electrical properties of polycrystalline silicon produced by low-temperature Ni silicide mediated crystallization of the amorphous phase
Authors: Yoon Soo Young , Park Seong Jin , Kim Kyung Ho , Jang Jin , Kim Chae Ok
Published: July 2002

Amorphous silicon (a-Si) film was crystallized at 500 °C, using a thin Ni layer. The crystallization proceeds by the migration of NiSi2 precipitates through the a-Si network. The crystallization kinetics change with the density of NiSi2 precipitates on the a-Si. The high density of NiSi2 precipitates, formed at a thick Ni layer, leads to vertical migration of the NiSi2 precipitates. With decreasing Ni thickness on a-Si, the spacing between NiSi2 precipitates increases and leads to lateral growth of crystallites. The room temperature conductivity is around 10−5 S/cm, and the activation energy is around 0.49 eV when the average Ni thickness is 24 Å or less. The sheet resistance of the ion-doped, poly-Si film decreases with the average Ni thickness because the crystalline quality improves as the Ni thickness is decreased.
Comparison of the annealing characteristics of resistivity and vacancy defects for implant isolated n-type GaAs
Authors: Knights A. P. , Ruffell S. , Simpson P. J.
Published: July 2002

Gallium arsenide layers, Si-doped at concentrations of 2×1019, 1×1019, and 5×1018 cm−3, grown on SI substrates were implanted using multiple-energy regimes, with O+, He+, and H+, respectively, to produce resistive structures. Sample resistivity was measured following annealing in the temperature range 400–800 °C. Maximum resistivity values were achieved after annealing at 600 °C for the O+ and He+ and 500 °C for the H+ implanted sample. Equivalently implanted and annealed semi-insulating GaAs samples were analyzed using positron annihilation spectroscopy in the gamma-ray Doppler-broadening mode, a technique which is predominantly sensitive to negatively charged, or neutral, vacancy-type defects. The annealing behavior of the resistivity is in good agreement with previous reports. Vacancy defects to which the positron is sensitive are found to be removed from all semi-insulating samples at a temperature which is 100 °C below that at which maximum resistivity is achieved. Therefore, such vacancy types can be eliminated as the defect responsible for optimum electrical isolation of GaAs following implantation, and the source of vacancies necessary to annihilate such defects.
Investigation of magnetization reversal in Sm–Fe–Cu(Zr)–Ga–C nanocomposite magnets
Authors: Zhang Shao-ying , Zhang Hong-wei , Shen Bao-gen , Zhao Tong , de Boer F. R. , Buschow K. H. J.
Published: July 2002

A comparative study has been performed concerning the microstructure and the magnetization reversal behavior of nearly single-phase Sm2Fe14.5Cu0.5Ga2C2 and two-phase exchange-coupled Sm2Fe14.5Cu0.5Ga2C2/α-Fe and Sm2Fe14.25Zr0.25Cu0.5Ga2C2/α-Fe synthesized by melt spinning and annealing. A few percent of Zr substitution proved to be effective in refining the microstructure and enhancing exchange-spring behavior after melt spinning and annealing. All samples show remanence enhancement, the value of Mr/Ms being the largest for the sample containing Zr. Measurements of the reversible susceptibility show a large reversible magnetization in the two-phase materials. For the Zr-containing sample, a more detailed analysis of the temperature dependence of the exchange-spring behavior was made.
Impact of microstructure on the electrical stress induced effects of pulsed laser ablated (Ba, Sr)TiO3 thin films
Authors: Saha S. , Krupanidhi S. B.
Published: July 2002

Electrical stress induced degradation effects in thin films of (Ba, Sr)TiO3 were studied. A comparison of the processing conditions of the films was made to examine its subsequent impact on the electrical stress induced degradation in this regard. Films from two different processing approaches were taken up for this study, which had different microstructures and grain sizes originating from their difference in annealing/thermal conditions. A continuous electron injection under low field conditions was found to cause a change in the capacitance and the leakage current characteristics of the thin films. It is generally observed that the process of electron injection causes an accumulation of charge at the trap sites, thereby changing the local field near the vicinity of the trapped charge. This leads to the change in the electrical properties, such as the capacitance of metal–insulator–metal capacitors as observed. Capacitance–voltage (C–V) measurements were performed to estimate the electronic capture cross section and the neutral trap density from the voltage shifts induced in the C–V curves. The trapping efficiency, capture cross section, and trap densities were obtained as functions of the injected charge fluence. The obtained values showed that in situ crystallized films exhibited better electrical response under continuous electrical stress than those which were ex situ crystallized. However, time-dependent dielectric breakdown studies (long-term response) on the two types of films indicated that ex situ crystallized films are more resistant toward breakdown than their in situ crystallized counterparts. The observations showed that the microstructure played an important role in the degradation properties. The electrical breakdown in both cases is believed to originate from different parts of the film. In the case of the ex situ crystallized films the breakdown takes place at the grain boundaries, while in the in situ case it appears to originate at the electrode/film interfaces.
Low-energy spin-wave excitations in the bilayer manganite La1.2Sr1.8Mn2O7
Authors: Rosenkranz S. , Osborn R. , Mitchell J. F. , Vasiliu-Doloc L. , Lynn J. W. , Sinha S. K.
Published: July 2002

Inelastic neutron scattering experiments were performed on a single crystal of the bilayer manganite La1.2Sr1.8Mn2O7. Low-energy spin-wave excitations were observed along the c direction with a maximum energy of ∼ 0.5 meV at the zone boundary. The dispersion of these acoustic spin wave modes is modeled by a nearest-neighbor Heisenberg model with an interbilayer exchange interaction between neighboring spins in different bilayers of 0.048(1) meV and an anisotropy gap of Δ = 0.077(3) meV. These results confirm the two-dimensional nature of the spin correlations in the bilayer manganites, with a ratio of the in-plane to interbilayer interaction of ∼ 200. The temperature dependence of the energies and intensities of the spin wave excitations are in agreement with our earlier conclusion that the ferromagnetic transition is second order.
Field independent rotational hysteresis loss on exchange coupled polycrystalline Ni–Fe/Mn–Ir bilayers
Authors: Tsunoda Masakiyo , Takahashi Migaku
Published: July 2002

The behaviors of the magnetic torque curve and the rotational hysteresis loss of the polycrystalline ferromagnetic (F)/antiferromagnetic (AF) bilayer whose AF layer is thinner than the critical thickness are discussed as a function of the applied field. The critical thickness of the AF layer is the threshold beyond which the unidirectional anisotropy appears. At the high field, we found some differences between the calculated torque curves based on the simple model proposed by Meiklejohn and the measured ones for the Ni–Fe/Mn–Ir bilayer: (1) the lack of the sin 2θ component and (2) the constant rotational hysteresis loss, in the experimental results. These differences are explained well by the model in which the two-dimensionally random distribution of the magnetic anisotropy axes of the AF grains is taken into account. We conclude the distribution of the anisotropy axes of the AF grains is an indispensable factor to understand the unidirectional anisotropy of the polycrystalline F/AF bilayers.
Magnetic and thermal relaxation in (NiFe/CoFe)/PtMn and NiFe/NiMn bi-layers for spin valve heads
Authors: Han Dehua , Gao Zheng , Mao Sining , Ding Juren
Published: July 2002

The magnetic and thermal relaxation of exchange coupling, Hex in (NiFe/CoFe)/PtMn and NiFe/NiMn bi-layers has been studied as a function of temperature between 25 °C to 250 °C and magnetic field between 0 to 1000 Oe applied antiparallel to the original exchange coupling direction. The blocking temperature was about 380 °C for (NiFe/CoFe)/PtMn, and 410 °C for NiFe/NiMn. It was found that the Hex of the bi-layers decreased gradually with increasing time, while the coercivities remained almost a constant. The Hex, measured at a temperature of 250 °C and a reversal field of 1000 Oe, decreased from 205 Oe to 0 and then changed its sign. The relaxation time, defined as the time required for Hex to reach zero, is about 3.1 h and 8.7 h for (NiFe/CoFe)/PtMn and NiFe/NiMn, respectively. The relaxation process for the bi-layers could be understood as the superposition of two exponential decays with different rates associated with a distribution of energy barriers. Spin valve heads using NiFe/NiMn bi-layers could be expected more stable, than those using (NiFe/CoFe)/PtMn bi-layers at an elevated operating temperature.
Anisotropy determination in epitaxial Sm–Co/Fe exchange springs
Authors: Pechan Michael J. , Teng Nienchtze , Stewart Jason-Dennis , Hilt J. Zachary , Fullerton Eric E. , Jiang J. S. , Sowers C. H. , Bader S. D.
Published: July 2002

We report in-plane anisotropy in epitaxial Sm–Co(x)/Fe(y) bilayers as determined by ferromagnetic resonance (FMR). Four samples, (x,y)=(35,30) and (20, 20) nm each on MgO (110) and (100) substrates, have been prepared via magnetron sputtering. The two substrate orientations result in twofold and fourfold Sm–Co symmetry respectively, with the Sm–Co c-axis in-plane. Magnetization curves indicate elastic exchange spring Fe behavior in reversing fields up to the Sm–Co switching fields (6 and 8 kG at room temperature in the (35, 30) and (20, 20) nm films, respectively). 35 GHz in-plane FMR measurements were made in order to map the crystalline anisotropy of the Fe layer as well as the induced anisotropy from the exchange coupling to the Sm–Co layer. The twofold Sm–Co samples exhibit a clear superposition of the near fourfold Fe crystal field anisotropy (530 Oe) and the unidirectional exchange-bias anisotropy (≈650 Oe) arising from the Fe/Sm–Co interface. The crystalline Fe anisotropy in the fourfold Sm–Co samples is less well defined, presumably due to poorer epitaxy of the Fe layer for this orientation.
Role of ferric ions in the magnetic interactions of substituted iron molybdate
Authors: Ghose J. , Roy A.
Published: July 2002

The magnetic interactions in titanium and zinc substituted Fe2MoO4 has been analyzed with respect to Fe2+ and Fe3+ ion concentrations on the two sites of the spinel lattice. The concentrations of Fe2+ and Fe3+ ions on the two sites of the spinel lattice were determined from the Mössbauer spectra of the samples recorded in their paramagnetic state. Substitution of molybdenum by titanium decreases the Fe3+ ions on both the sites of the ferrite. The decrease in Fe3+ ion concentration leads to a decrease in the A–O–B and B–B interactions and thus decreases the ferrimagnetic Curie temperature (TC). Substitution of Fe2+ on the tetrahedral site (A) of Fe2MoO4 by Zn2+ leads to an increase in Fe3+ concentration on the tetrahedral site and thus the A–A and A–O–B interactions become strong which explains the absence of compensation temperature. Also, the increase in Fe3+ content on the A site of the zinc substituted samples initially leads to a relatively small decrease in TC, compared to the titanium substituted samples.
Damage accumulation in Si crystal during ion implantation at elevated temperatures: Evidence of chemical effects
Authors: de Souza J. P. , Suprun-Belevich Yu. , Boudinov H. , Cima C. A.
Published: July 2002

Damage in Si induced by irradiation with various light/medium mass ions at elevated temperatures and high doses (⩾3×1016 cm−2) was studied using Rutherford backscattering spectroscopy, cross-section transmission electron microscopy, and high resolution x-ray diffraction. The results obtained have shown that there is a marked variation in the damage accumulation for different ion species. For O+ and N+ ions a distinct layer with a low level of damage presenting negative strain is formed at the surface. It has been found that the magnitude of the strain does not correlate with the energy deposited in the collision cascades. In the cases of Ne+ and Mg+ implantation, a low damage accumulation occurs near the surface but no negative strain is formed. In contrast to the N+ and O+ cases, with the increase of the Ne+ or Mg+ dose (>1×1017 cm−2) the damage profile stretches almost to the crystal surface. It is proposed that in addition to the mechanism of spatial separation of Frenkel pairs taking place in the collision cascades, the ability of the implanted ions to form precipitates and complexes with Si atoms noticeably influences the damage formation during implantation at elevated temperatures.
Hydrostatic pressure and thermal loading induced optical effects in double-coated optical fibers
Authors: Chang Win-Jin , Lee Haw-Long , Yang Yu-Ching
Published: July 2002

The stresses, microbending losses, and refractive index changes induced simultaneously by hydrostatic pressure and thermal loading in double-coated optical fibers are analyzed. The lateral pressure and normal stresses in the optical fiber, primary coating, and secondary coating are derived. Also presented are the microbending losses and refractive-index changes in the glass fiber. The normal stresses are affected by the hydrostatic pressure, temperature drop, material properties of the primary and secondary coatings, and their thickness. To minimize the microbending losses, the polymeric coatings should be suitably selected, and it is found that the thickness, Young’s modulus, and Poisson’s ratio of the secondary coating should be increased, but the Young’s modulus and Poisson’s ratio of the primary coating should be decreased. Similarly, in order to reduce the changes of refractive index in the glass fiber, the thickness, Young’s modulus, and Poisson’s ratio of the secondary coating should be increased, and on the contrary the Young’s modulus and Poisson’s ratio of the primary coating should be decreased.
Monte Carlo simulation of electron transport in carbon tetrafluoride discharge plasma
Authors: Vasenkov A. V.
Published: July 2002

The Monte Carlo technique was used to investigate electron transport in the carbon tetrafluoride discharge plasma. A set of total elastic and inelastic cross sections was assembled on the basis of the critical survey of Christophorou, Olthoff, and Rao [J. Phys. Chem. Ref. Data 25, 1341 (1996)]. Particular attention was given to the derivation of the total and angular elastic cross sections at energies close to the Ramsauer minimum. The experimental angular elastic cross sections were fitted to analytic functions suitable for implementation in the Monte Carlo calculations. Angular inelastic cross sections were analytically represented using a simple Born approximation. Superelastic collisions were included in the calculations in an effort to account properly for the behavior of electrons at low energies. The transport coefficients obtained with the direct simulation agreed with the measurements for the electric field to gas density ratios (E/N) in the range 0.01⩽E/N⩽300 Td. Agreement between calculated and measured reaction-rate coefficients was obtained for E/N below 200 Td. The effect of both elastic and inelastic anisotropic scattering on electron transport in carbon tetrafluoride was studied in detail. This system is typical of molecular systems with a Ramsauer minimum in the elastic cross section.
Time-resolved measurements of ion energy distributions in dual-mode pulsed-microwave/radio frequency plasma
Authors: Zabeida O. , Hallil A. , Wertheimer M. R. , Martinu L.
Published: July 2002

In the present work we systematically study the ion energy distribution functions (IEDFs) in argon discharges produced by a combination of pulsed (1–2 kHz) microwave (MW) and continuous wave (cw) radio frequency (rf) excitations. We show that the IEDFs for the pulsed MW discharges are structured, with individual features originating from different periods of the pulse. In the dual-mode MW/rf discharge, significant modulation of the self-bias voltage, Vb, during the MW pulse cycle is observed, which we attribute to changes in the overall plasma impedance: We demonstrate that in the pulsed-MW/cw-rf mode the impedance is highly resistive when the MW signal is on, while it is predominantly capacitive during the period between individual pulses. Using the measured time evolution of Vb in combination with time-resolved measurements of individual ion species, IEDFs at the rf-powered electrode at each instant of the MW pulse have been obtained. This approach is then used to reconstruct the total IEDF in pulsed-MW/cw-rf plasma in order to determine the total energy delivered by the impacting ions.
Raman investigation of CdxZn1−xSe/ZnSe quantum wires: Strain relaxation and excitation profile
Authors: Schreder B. , Materny A. , Kiefer W. , Kümmell T. , Bacher G. , Forchel A. , Landwehr G.
Published: July 2002

CdZnSe/ZnSe quantum wires with two different Cd contents and different wire sizes were investigated by resonance Raman spectroscopy. Depending on the Cd content, the longitudinal optical (LO) phonon signal of the wires shows an asymmetry or a clear separation into two peaks. This separation is caused by an inhomogeneous strain parallel to the layer plane due to the strain relaxation at the wire edges. Thus, the observed signals can be attributed to LO phonons excited in the center and in the edge of the structures. Furthermore, high outgoing resonances for the LO overtones (2LO and 3LO) can be observed. For two wire sizes [(a): length L=100 μm, width W=18 nm; (b): L=49 nm, W=27 nm] excitation profiles of the LO fundamental and of the first overtone of Cd0.2Zn0.8Se quantum wires have been recorded. The excitation profiles clearly show structures attributed to the incoming and outgoing resonances. The intensity ratio 2LO/1LO, which is also a good measure for the electron–phonon coupling in a system, was determined for various wire sizes. The results point to a decrease of the electron–phonon coupling for decreasing wire length (transition from wire to dot).
Reduced master equation analysis of multiple–tunnel junction single-electron memory device
Authors: Jalil M. B. A. , Wagner M.
Published: July 2002

We employ a master equation (ME) approach in the charge transport analysis across a uniform multiple–tunnel junction (MTJ) memory trap, using a much-reduced state list derived from circuit symmetry, and previous assumptions by Jensen and Martinis. This enables all significant single tunneling and higher-order cotunneling sequences to be accounted for, while avoiding the computational cost of the full ME method. The reduced ME method is conceptually simpler and yields greater accuracy, compared with previous approximations based on tunneling probabilities. For an MTJ trap with zero stray capacitance C0, the results obtained are found to agree very closely with the full ME results up to a temperature of T≈3T0/10, where T0=e2/kBC, whereas previous methods break down at T≈T0/10. Furthermore, unlike the earlier methods, the reduced ME approach can be applied to the realistic but less symmetric case of a trap with finite C0, and remains valid up to the trap’s maximum operating temperature of T≈T0/100. Finally, our reduced ME results are in close agreement with available experimental data at T<T0/200.
Light-induced charge-transport properties of photorefractive barium–calcium–titanate crystals doped with iron
Authors: Veenhuis H. , Börger T. , Buse K. , Kuper C. , Hesse H. , Krätzig E.
Published: July 2002

Nominally pure and iron doped, as-grown, and thermally annealed photorefractive barium–calcium–titanate crystals of the congruently melting composition Ba0.23Ca0.77TiO3 (BCT) are investigated by holographic and conventional electrical techniques. Refractive-index changes, two-beam-coupling gains, photoconductivities, dark conductivities, and bulk-photovoltaic current densities are measured. As-grown and oxidized crystals are hole conductive and at usual illumination conditions (light wavelength 514.5 nm, light intensity between 0.1 and 1 W/cm2) all measured properties are excellently described by an one-center charge-transport model. The effective electrooptic coefficient r333 is only about 30 pm/V and thus much smaller than the value obtained from interferometric measurements. Two-beam-coupling gains as high as 7 cm−1 are achieved. Doping with iron increases considerably the effective trap density, and bulk-photovoltaic fields of the order of some kilovolts per centimeter are observed in iron-doped crystals. Typical response times of iron-doped, as-grown, or oxidized crystals are about 0.5 s at 1 W/cm2. Reduction yields electron-conductive BCT. The dark storage time increases from 6 min in the as-grown state to 3 h upon a slight reduction treatment, but decreases for strongly reduced samples. The investigation reveals that BCT will become a very promising alternative to barium–titanate crystals (BaTiO3) for many applications.
Relation between the suppression of the generation of stacking faults and the mechanism of silicon oxidation during annealing under argon containing oxygen
Authors: Suzuki Toshiharu
Published: July 2002

The effect of the oxygen partial pressure during annealing under argon on the generation and growth of defects that adversely affect the yield and reliability of thermally grown oxide was investigated by precisely and widely controlling the oxygen partial pressure. Below the critical oxygen partial pressure of 6×10−3 atm, the generation of oxidation-induced stacking faults (OSFs) during a subsequent oxidation step was effectively suppressed from the density of about 103 cm−2 to less than 20 cm−2. Annealing for 10 min under an appropriate oxygen partial pressure was shown to be sufficient for annihilation of the origin of OSF and this effect continued even during a subsequent oxidation in dry oxygen. Mechanism by which the generation of OSFs is suppressed was discussed considering the oxidation mechanism under the reduced oxygen partial pressures. It is suggested that electric field that builds across the oxide layer when the oxygen partial pressure is below the critical value causes silicon interstitials and impurities such as metals to drift from the substrate to the outer surface of the oxide. During the annealing origins of OSFs such as Si–O clusters and metallic impurities are eliminated and mechanical damage is also annealed out before climbing to be the nuclei of OSFs.
Sensitive characterization of phase and amplitude semiconductor nonlinearities for broadband 20 fs excitation
Authors: Kunde J. , Arlt S. , Gallmann L. , Morier-Genoud F. , Siegner U. , Keller U.
Published: July 2002

We present an experimental technique that allows for the detection of pump-induced transmission and phase changes with high sensitivity and ultrafast temporal resolution over an arbitrarily wide time window. This is achieved combining spectral interferometry with high-frequency-chopping differential transmission measurements. With this setup, exciton and continuum nonlinearities in a semiconductor are studied for broadband excitation. We find that the pump-induced phase changes at the exciton and in the continuum decay on distinctly different time scales, indicating different microscopic origins.
Splitting of the heavy and light hole bands due to the indium-induced strain in three inch indium-alloyed semi-insulating GaAs substrates
Authors: Kim J. S. , Yu P. W.
Published: July 2002

Photoreflectance (PR) spectroscopy, x-ray diffraction (XRD) method, and secondary ion mass spectroscopy (SIMS) were used to observe the radial distribution of the band-edge transition and modification of the band structure due to the variation of indium in 3 in. indium-alloyed semi-insulating GaAs (InxGa1−xAs) grown by the liquid-encapsulated Czochralski method. The data from room temperature PR measurements showed the variation of the transition energy with positions indicating the radial distribution of the indium content across the wafers; indium content being higher around the edge region than the central area. The splitting of the degenerate valence band around the edge region of the wafers was also shown in PR data due to the different indium content in adjacent regions where indium content varies rapidly. The XRD measurements showed the drastic change in the distribution of lattice constant on where the splitting of the heavy and light holes happened and the SIMS analysis was adopted to confirm the distribution of indium content across the wafer. The possible model was proposed from the experimental data.
Dynamics of intramolecular vibrational-energy redistribution (IVR). IV. Excess energy dependence, t-stilbene
Authors: Felker P. M. , Lambert W. R. , Zewail Ahmed H.
Published: July 2002

The results of picosecond time-resolved measurements of intramolecular vibrational-energy redistribution (IVR) in jet-cooled t-stilbene are presented. The results show that the changes in the nature of IVR that were found to occur with increasing energy in anthracene also occur in t-stilbene. In particular, at intermediate energies a number of different excitations give rise to phase-shifted quantum beats in fluorescence decays, indicating restricted IVR in the molecule. At higher energies decay behavior characteristic of dissipative IVR is observed. These results are compared with those of anthracene and are discussed in terms of molecular symmetry and vibrational density of states. The results suggest the generality of the conclusions about large molecule IVR that have been stated in the other papers of this series.
Theory of vibrational coherence and population decay in isotopically mixed crystal
Authors: Velsko S. , Hochstrasser R. M.
Published: July 2002

A simple theoretical description of vibrational coherence decay in isotopically mixed molecular crystals at low temperatures is presented. Dephasing due to impurity scattering is shown to be a slower than exponential decay process for optically excited vibrons lying near band singularities, by analogy with the theory of coherence loss in weakly disordered crystals [I. I. Abram and R. M. Hochstrasser, J. Chem. Phys. 72, 3617 (1970)]. When the nature of the initially excited state is properly taken into account, the rate constant for population decay displays an unexpected interplay between trapping and self-relaxation which can, in principle, lead to a nonmonotonic dependence of the relaxation rate with impurity concentration. Our results are related to recent time resolved CARS measurements in low temperature molecular crystals.