bluuebunny/crossref_metadata_2025 huggingface dataset:
Sm
2
(Co,Fe,Cu,Zr)
17
magnets for use at temperature ⩾400 °C
Authors: Chen C. H. , Walmer M. S. , Walmer M. H. , Liu S. , Kuhl E. , Simon G.
Published: July 2002

Sintered Sm2(Co,Fe,Cu,Zr)17 magnets with improved high temperature performance have been obtained by reducing the iron content in the magnet alloys. A record intrinsic coercive force, HCI, of 8.3 kOe at 400 °C was obtained when the iron content was decreased to 7 wt%. At 400–600 °C, 2:17 magnets with low iron content have demonstrated lower irreversible loss of magnetic flux, higher maximum energy product, and lower temperature coefficient of HCI. A temperature coefficient of HCI=−0.12%/°C (20–400 °C) was obtained for the low Fe magnets, compared to −0.23%/°C for commercial 2:17. Reducing iron content increases both Curie temperature and anisotropy field. Therefore, it is anticipated that new 2:17 magnet materials capable of operating at 400 °C or higher temperatures can be developed by reducing or eliminating the iron content and making other adjustments in composition and heat treatment.
Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis
Authors: Studenikin S. A. , Golego Nickolay , Cocivera Michael
Published: July 2002

Photoluminescent, undoped ZnO films have been fabricated using spray pyrolysis of zinc nitrate solution. The luminescent films had a polycrystalline hexagonal wurtzite type structure with no preferred orientation. Photoluminescence intensity was critically dependent on substrate temperature during spray pyrolysis and on post-annealing temperature. Green, photoluminescent films possessed a porous structure while orange films possessed a close packed granular morphology. Green luminescence appears to be due to oxygen vacancies in a layer just below the crystallite surface.
Influence of wavelength on the Raman line shape in porous silicon
Authors: Agulló-Rueda F. , Moreno J. D. , Montoya E. , Guerrero-Lemus R. , Martı́nez-Duart J. M.
Published: July 2002

The line shape of the one phonon Raman peak has been used extensively in the literature to estimate the crystallite size in porous silicon. However it has been shown that the line shape obtained on top surface experiments depends on the excitation wavelength. Because the porosity depends on depth, previous results are masked by the change in penetration depth. In this communication we report depth-resolved micro-Raman spectra at 514.5 and 632.8 nm. The spectra were measured at different points along a cross section of porous silicon films. We show that even when the same layer and, therefore, the same porosity is probed the Raman peak is broader at shorter wavelengths. To explain the results we suggest a contribution of indirect gaps to the resonant Raman cross section induced by quantum confinement.
Band-gap changes and band offsets for ternary Si1−x−yGexCy alloys on Si(001)
Authors: Osten H. Jörg
Published: July 2002

An estimation for the band offsets and the fundamental band gap will be presented for Si1−x−yGexCy alloys tensile or compressive strained on Si(001). This estimation considers both the band lineup at the interface of two different materials as well as the strain effects. Unknown material parameters have been adjusted to obtain the best agreement with experimental results for tensile strained Si1−yCy layers. The obtained results agree very well with the first experimental data for the effect of C on band-structure properties in Si1−x−yGexCy. For a completely strain-compensated (cubic) Si1−x−yGexCy layer, we predict significant “Ge effects” (smaller gap than Si, valence-band offset to Si) with values depending on the Ge content.
Nonlinear behavior of thin film SrTiO3 capacitors at microwave frequencies
Authors: Kozyrev A. B. , Samoilova T. B. , Golovkov A. A. , Hollmann E. K. , Kalinikos D. A. , Loginov V. E. , Prudan A. M. , Soldatenkov O. I. , Galt D. , Mueller C. H. , Rivkin T. V. , Koepf G. A.
Published: July 2002

The voltage-dependent dielectric constant (ε) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capacitor, the level of the third order IMD product was 21 dB below the fundamental signal level. Application of a 107 V/m dc electric field bias across the capacitor suppressed the third order IMD by an additional 10 dB. The nonlinear properties of thin film STO capacitors as a function of microwave voltage were determined by comparing the experimental and theoretical dependencies of the IMD products.
Effects of disorder on the blockade voltage of two-dimensional quantum dot arrays
Authors: Müller Heinz-Olaf , Katayama Kozo , Mizuta Hiroshi
Published: July 2002

The influence of both geometric and offset charge disorder of two-dimensional quantum dot arrays (also known as network tunnel junctions) on their Coulomb blockade voltage Vb is studied using extensive Monte–Carlo simulations. A general increase of Vb with increasing disorder is confirmed, but an exception to the rule is found for intermediate degrees of offset charge disorder. Detailed studies of the Vb distribution reveal a stability of its minimal value against geometric disorder, whereas this figure is considerably increased for high offset charge disorder. Implications of our results for single electron device design are discussed.
Second harmonic generation capabilities of ion implanted LiNbO3 waveguides
Authors: Rams J. , Olivares J. , Chandler P. J. , Townsend P. D.
Published: July 2002

Data are reported on the depth profile of the nonlinear optical capabilities (d33) of He+ ion-implanted LiNbO3 waveguides. Reflected second harmonic generated light has been measured for guides formed by implant doses ranging from 5×1014 to 2×1016 ions/cm2. Directly after the irradiation the nonlinearity in the waveguide region is reduced to ∼75% that of virgin LiNbO3 for doses higher than 3×1015 ions/cm2. At the optical barrier the nonlinearity becomes zero for doses above 1×1016 ions/cm2. Annealing at 200 °C completely recovers the nonlinearity of the waveguiding layer, as well as reducing absorption losses within the guide. For the highly disordered material in the barrier, the harmonic signal shows that only partial recovery of the structure occurs, even by annealing at higher temperatures. This difference in response allows the production of low loss waveguides with high nonlinearity.
Outdiffusion of the excess carbon in SiC films into Si substrate during film growth
Authors: Sun Yong , Miyasato Tatsuro , Sonoda Nobuo
Published: July 2002

The excess of C atoms diffused into the (111) Si substrate during the growth of the cubic SiC films are detected by Auger electron spectroscopy, infrared absorption, and x-ray photoelectron spectroscopy. The diffusion coefficient of the C atoms into the Si substrate at 820 °C is 8.4×10−15 cm2 s−1, which is close to the value of the diffusion of the C atoms into Si crystal using solid source. The C atoms mainly occupy substitutional sites in the Si substrate when the substrate temperature is below 750 °C, and both substitutional and nonsubstitutional sites when it is above 820 °C.
Transient enhanced diffusion and defect microstructure in high dose, low energy As+ implanted Si
Authors: Krishnamoorthy V. , Moller K. , Jones K. S. , Venables D. , Jackson J. , Rubin L.
Published: July 2002

(001) CZ silicon wafers were implanted with As+ at 100 keV to a dose of 1×1015/cm2 in order to produce a continuous amorphous layer to a depth of about 120 nm. Furthermore, the implant condition was such that the peak arsenic concentration was below the arsenic clustering threshold. Subsequently, a second As+ or Ge+ implant was performed at 30 keV to doses of 2×1015/cm2, 5×1015/cm2 and 1×1016/cm2, respectively, into the as-implanted samples. All of the samples were annealed at 800 °C for 1 h. The second implant was designed to be contained within the amorphous region created by the initial implant. The second As+ implant was also designed to provide the additional arsenic needed to exceed the critical concentration for clustering at the projected range. Of the three samples with the dual As+ implant the clustering threshold was exceeded for the two lower doses while the SiAs precipitation threshold was exceeded at the highest dose. In the case of the dual As+/Ge+ implants the clustering and precipitation thresholds were not reached. Since arsenic and germanium are similar in mass the extent of damage created by these implants would be comparable. The implanted and annealed specimens were analyzed using secondary ion mass spectroscopy and transmission electron microscopy. The difference in the defect evolution and the transient-enhanced diffusion of arsenic beyond the end-of-range region between the As+ and Ge+ implanted and annealed samples was used to isolate the effects of arsenic clustering and precipitation. The results showed that point defects induced during clustering and/or precipitation did not contribute to the enhanced diffusion of arsenic although these defects did coalesce to form extended defects at the projected range. However, damage beyond the end-of-range region did cause enhanced diffusion of arsenic.
Growth of poly- and single-crystal ScN on MgO(001): Role of low-energy N2+ irradiation in determining texture, microstructure evolution, and mechanical properties
Authors: Gall D. , Petrov I. , Hellgren N. , Hultman L. , Sundgren J. E. , Greene J. E.
Published: July 2002

ScN layers, 345 nm thick, were grown on MgO(001) substrates at 750 °C by ultrahigh-vacuum reactive magnetron sputter deposition in pure N2 discharges at 5 mTorr. The N2+ to Sc ratio incident at the substrate and growing film was maintained constant at 14, while the ion energy EN2+ was varied from 13 to 50 eV. All films were stoichiometric with N/Sc ratios of 1.00±0.02. However, microstructural and surface morphological evolution were found to depend strongly on EN2+. The nucleation and initial growth stages of ScN films deposited with EN2+=13 eV are dominated by the formation of 111- and 002-oriented islands, but preferred orientation rapidly evolves toward a purely 111 texture by a film thickness of ≃50 nm as 002 grains grow out of existence in a kinetically limited competitive growth mode. In distinct contrast, films deposited with EN2+=20 eV grow in a cube-on-cube epitaxial relationship with the substrate and exhibit no indication of 111-oriented grains, even in the earliest stages. Increasing EN2+ to 50 eV still results in epitaxial layers, but with high in-plane compressive stress and the presence of N2 gas bubbles. All epitaxial layers contain rectangular nanopipes ≃1 nm wide and aligned along the growth direction. The nanopipes result from atomic shadowing near the bottom of a periodic array of surface cusps which form along orthogonal 〈100〉 directions due to kinetic roughening during growth. The hardness H and elastic modulus E of the epitaxial ScN(001) layer grown with EN2+=20 eV are 21.1±1.1 and 356±18 GPa, respectively. H and E increase (decrease) with increasing (decreasing) EN2+.
The reduction and enhancement of spontaneous ordering in (InP)2/(GaP)2 quantum wells grown by solid source molecular beam epitaxy
Authors: Chou S.-T. , Cheng Y. C. , Tai K. , Huang K. F. , Lin W. J. , Lan W. H.
Published: July 2002

We have studied methods for reducing and enhancing the intensity of the spontaneous ordering in the growth of (InP)2/(GaP)2 short-period superlattice (SPS) quantum wells using solid source molecular beam epitaxy. The intensity of the ordering effect can be greatly reduced by using substrates with larger tilt angles or greater height and density of steps on the surface. We believe that the competition between bonding preference and the surface migration ability of group III adatoms plays an important role in this growth mechanism. By using a long growth interruption, we have also demonstrated that the intensity of the spontaneous ordering can be enhanced which gives us the capability to vary the emission energy of (InP)2/(GaP)2 SPS quantum wells and its dependence on temperature.
Electrochromism of sputter deposited Ni–Cr oxide films
Authors: Azens A. , Granqvist C. G.
Published: July 2002

Electrochromic thin films of Ni oxide and Ni–Cr oxide were made by reactive dc magnetron sputtering from magnetic Ni as well as nonmagnetic Ni–Cr alloy targets, respectively. The physical and electrochemical properties of the two materials were similar, indicating that anodically coloring electrochromic films can be made by a standard large area deposition technique.
Computational model of Q-switch Nd:YAlO3 dual-wavelength laser
Authors: Su H. , Shen H. Y. , Lin W. X. , Zeng R. R. , Huang C. H. , Zhang G.
Published: July 2002

A rate equation model with spatial hole-burning effect is built to calculate the Q-switch Nd:YAlO3(Nd:YAP) simultaneous dual wavelength laser. We find that 1341.4 and 1079.5 nm lasers can be simultaneously generated in a linear cavity by controlling the output coupling rates of the two coherent radiations. In contrast, a Y cavity and devices to control relative Q-switch delay were necessary for the dual wavelength Nd:YAG laser in previous work. Also with this model, some optimum parameters’ relationships to pump energy are investigated.
Exact expression for the impedance of a rectangular strip and second-order effects in Bleustein–Gulyaev wave scattering
Authors: Biryukov S. V. , Polevoi V. G. , Weihnacht M.
Published: July 2002

A novel exact expression for the impedance of a rectangular strip of arbitrary width and height, in the case of horizontally polarized waves, is derived. The impedance exactly describes the relation between the displacement and stress distributions on one side of the strip if its other sides are free from stresses. The expression found is for the main part in summed form, and it enables to calculate the impedance in a rigorous manner, including when various limiting cases of strip sizes are considered and all harmonics of the field inside the strip are taken into account. The impedance representation obtained is applied to the problem of Bleustein–Gulyaev wave scattering on a single rectangular strip of small height. The exact asymptotic expressions for the complex amplitudes of the reflection and transmission coefficients are calculated for this problem, taking into account second-order effects with respect to a small parameter such as the ratio between the strip height and the wavelength. The bulk wave contribution to these effects fits naturally into the method of calculation. The primary aim of this investigation is to illustrate the character and origin of the various contributions to the second-order effects. The calculated results may be useful to tackle the same problem in the considerably more important and complicated case of Rayleigh-wave-like scattering, which so far remains to be solved in a rigorous manners. Second-order effects considerably shift the resonance frequencies in surface acoustic wave devices and a correct model explaining this shift is necessary, particularly for experimental investigations. It is shown that the second-order effects are not purely quadratic with respect to the small parameter considered, rather, the main contribution contains as a multiplier, the logarithm of this parameter. This contribution is related to static fields in the strip and may be calculated using a simpler expression for the strip impedance. The strip width dependence of the second-order effects is also investigated. The strongly piezoelectric crystal LiIO3 and an aluminum strip are chosen to illustrate the results.
Thermal treatment effect of the GaN buffer layer on the photoluminescence characteristics of the GaN epilayer
Authors: An H. Y. , Cha O. H. , Kim J. H. , Yang G. M. , Lim K. Y. , Suh E. -K. , Lee H. J.
Published: July 2002

Photoluminescence properties of undoped wurtzite GaN epilayers grown on sapphire substrates with different buffer layer treatment conditions in metalorganic chemical vapor deposition (MOCVD) growth have been studied as a function of temperature. At low temperatures, very well resolved spectral features associated with the GaN band structure were observed. From the photoluminescence (PL) data for free excitons, an accurate value of the A exciton binding energy was found. The localization energies of the excitons bound to neutral acceptor are found to agree with Haynes’ rule with the proportionality factor close to 0.1. The longitudinal optical (LO) phonon assisted photoluminescence associated with both the bound and free excitons has been observed. The characteristics of free excitons and their LO phonon replica have been studied in detail with the temperature variation and related to the point defects. The behavior of the peak energy and the full width at half maximum of the exciton band as a function of temperature reveals the change of the dominant recombination mechanism of the exciton with the temperature. Besides, all samples show peaks in the energy range of 3.15–3.19 eV, whose intensities vary with the annealing time of the buffer layer. We found that these peaks are related to the Zn impurity, which is unintentionally incorporated due to the memory effect in the MOCVD system. Intensities of Zn impurity related recombination and yellow band luminescence are also examined as functions of the temperature and annealing time of the buffer layer. Based on these temperature dependent PL results, the optimum growth conditions have been suggested.
Distributed exchange interactions and temperature dependent magnetization in amorphous Fe88−xCoxZr7B4Cu1 alloys
Authors: Gallagher K. A. , Willard M. A. , Zabenkin V. N. , Laughlin D. E. , McHenry M. E.
Published: July 2002

The temperature dependence of the magnetization for Fe88Zr7B4Cu1 amorphous alloy has been measured. M(T) has been fit using a Handrich–Kobe model with a modified Brillouin function with an additional exchange fluctuation term. Here for the first time, an asymmetrical distribution of the exchange interactions is proposed based on empirical knowledge of the Bethe–Slater curve. A two-parameter exchange fluctuation is shown to give significantly better fits to M(T) for these amorphous alloys.
Optical study of continuous thin films and island films of Ir: Interband absorption and localization of conduction electrons of Ir
Authors: Anno E.
Published: July 2002

For continuous thin Ir films and Ir island films consisting of Ir particles larger than about 25 Å in diameter, optical absorption has been measured in the photon energy range of 0.5–6.5 eV. Interband absorption does not exist at about 1.1 eV. Absorption at about 3.1 eV is identified as interband absorption because its particle-size dependent change is similar to that of interband absorption of Cu particles. Interband absorption at about 4.4 eV is suggested to arise from transitions between bands 2 and 5 near L in the direction Q. Absorption due to plasma oscillations of conduction electrons is absent in the island films, showing that the conduction electrons are localized by a strong correlation interaction between the conduction electrons having strong d character. Strong d character based on large hybridization of s and d bands is responsible for the strong correlation interaction.
Hole antiresonances above quantum wells
Authors: Ekbote S. , Cahay M. , Roenker K.
Published: July 2002

We use the 6×6 Luttinger–Kohn Hamiltonian to study the effects of the spin-orbit split-off band on the properties of holes tunneling across quantum wells. The transmission coefficients of heavy and light holes decrease sharply (antiresonances) at energies coinciding with the formation of boundstates in two effective potential wells for the light and SO bands. The origin of these effective potential wells is due to the difference between the threshold energies for free propagation in the quantum well and cladding layers for holes in the light and SO bands. The dwell times of holes increase sharply at the antiresonance energies and are found to reach a maximum at some values of the transverse wave vector for holes incident with a fixed energy. The probability of heavy- to light-hole conversion (and vice versa) is found to increase at the antiresonance energies. These results are compared with the predictions of a one-band effective-mass description of hole transport to stress the importance of the coupling between the hole subbands on the tunneling properties of holes across quantum wells.
Electromechanical responses of antiferroelectric and ferrielectric liquid crystals
Authors: Jákli A.
Published: July 2002

We measured the electromechanical responses in the antiferroelectric and ferrielectric phases of the prototype antiferroelectric material (R)-MHPOBC. Our observations confirm recent results, which indicated that SmCβ* is ferrielectric, or at least not normal ferroelectric. In addition, we found that the layers become permanently tilted after dc field treatments. The mechanical responses are practically zero normal to the plates due to mechanical compensation of the electroclinic effect. This fact may have an importance in display applications, since a vertical vibration usually disturbs electro-optical performances.
Crystallization of amorphous carbon thin films in the presence of magnetic media
Authors: Ramírez A. G. , Itoh T. , Sinclair R.
Published: July 2002

Amorphous carbon thin films, which are often used as protective coatings for magnetic hard disks, were deposited in a carbon/cobalt alloy/carbon trilayers (20/10/20 nm) and subjected to thermal annealing and cooling. The associated microstructural changes were analyzed by in situ transmission electron microscopy (TEM). TEM micrographs show that the amorphous carbon in contact with the magnetic media increases in graphitic content at annealing temperatures near 400 °C. It fully crystallizes between 500 and 600 °C. The microstructural changes at these temperatures suggest that the metals of the magnetic layers mediate graphitization, similar to the behavior of other eutectic metal metalloid systems (e.g., Al–Si, Ag–Ge). Calorimetric and magnetic measurements are consistent with a graphitization mechanism that includes a diffusional process. This article presents the experiments and proposes a graphitization mechanism.
Addendum: “Stress measurements in silicon devices through Raman spectroscopy: Bridging the gap between theory and experiment” [J. Appl. Phys. 79, 7148 (1996)]
Authors: De Wolf I. , Anastassakis E.
Published: July 2002

Micro-Raman spectroscopy is often applied to measure local mechanical stress in silicon microelectronics devices. A procedure was proposed [De Wolf et al., J. Appl. Phys. 79, 7148 (1996)] for deriving quantitative information about the stress from the Raman spectra. The calculations were shown to be less tedious when performed in the reference system of the sample, as suggested by Anastassakis [Light Scattering in Semiconductor Structures and Superlattices, (Plenum, New York, 1991), p. 173]. However, there are a few errors in some equations in De Wolf et al. We give here the correct equations, and discuss the consequences of the errors. An alternative convenient approach is recommended.
Refractive index changes of Pd-coated magnesium lanthanide switchable mirrors upon hydrogen insertion
Authors: von Rottkay K. , Rubin M. , Duine P. A.
Published: July 2002

The optical effect upon insertion of hydrogen into Pd-coated magnesium lanthanide switchable mirrors is investigated in terms of the changes of their complex refractive indices. A significant change in the optical constants of LnMg layers is seen between the as-deposited state and the dehydrided state after one cycle. Furthermore, the optical effect of switching the Pd cap layer to a PdH cap layer was determined. It is shown that the Pd layer mainly limits the visible transmittance of the hydrided stack to about 35%–40%. Whereas the extinction coefficient of dehydrided LnMg layers at 550 nm is between 2.2 and 3.1, it is as low as 10−4 in the transparent state. This is of great promise to applications requiring large optical contrast (e.g., optical switches).
Solid phase epitaxial regrowth of n-GaAs with Ti–Ge–Ni metallization for ohmic contacts
Authors: Kim T.-J. , Krishnamoothy V. , Puga-Lambers M. , Holloway P. H.
Published: July 2002

Thin films of Ni, Ge, and Ti were evaporated onto n-GaAs to investigate the correlation between solid-phase interfacial reactions and development of ohmic contacts. Initially 65 nm Ni films were electron beam deposited onto the GaAs substrate and vacuum annealed in situ at 300 °C to produce a ∼130-nm-thick Ni2.4GaAs phase. After Ni2.4GaAs was produced, 30 nm of Ge and 25 nm Ti films were deposited and vacuum annealed at 500 °C. This resulted in solid phase epitaxial regrowth of ∼30 nm of GaAs from decomposition of Ni2.4GaAs first into NiAs and NixGa with subsequent decomposition of these binary phases into NiTiy and GaAs. Similar structures without the in situ annealing (i.e., sequentially deposited Ni, Ge, and Ti, all annealed simultaneously at 500 °C) were studied for comparison. Only the in situ annealed structure showed ohmic behavior after annealing at 500 °C for 5 min, and only when Ge was incorporated into regrown GaAs.
Fowler–Nordheim current injection and write/erase characteristics of metal–oxide–nitride–oxide–Si structure grown with helicon-wave excited plasma processing
Authors: Kimura Shinjiro , Ikoma Hideaki
Published: July 2002

Metal–oxide nitride–oxide–Si (MONOS) structure was fabricated using the oxidation-nitridation series with helicon-wave O2–Ar and N2–Ar plasmas, respectively. The detrimental effect of Ar ion etching was minimized during the fabrication process by controlling the plasma–sheath width. The top oxide was very thin (∼1–2 nm) as compared with nitride (∼12–13 nm) and bottom oxide (∼7–8 nm). Fowler–Nordheim tunneling electron injection was performed in this MONOS diode for both dc and pulsed stress voltages with the electrical polarity being changed. For the positive stress voltage, the shift of the threshold voltage Vth was negative and larger for the smaller stress voltage. It was higher for the pulsed stress than for the dc one. On the other hand, Vth shift is positive and smaller for the pulsed stress than for the dc one for the negative stress bias. These findings can almost be explained by the avalanche breakdown model together with the effect of the total amount of the injected carries. Terman analysis indicated that the interface state density did not increase after both positive and negative stresses, which was probably due to film structure and the presence of a small amount of Si oxynitride (or Si–N–O bonds) at the insulator/Si interface. Write/erase characteristics were also briefly discussed.
Ferroelectric phase transitions and three-dimensional phase diagrams of a Pb(Zr, Ti)O3 system under a hydrostatic pressure
Authors: Oh S. Hoon , Jang Hyun M.
Published: July 2002

Effects of hydrostatic pressure on various ferroelectric phase transitions in Pb(Zr, Ti)O3 (PZT) have been investigated using the thermodynamic formalism based on the Landau–Devonshire phenomenological theory. For these purposes, the rotostrictive coefficient related to the coupling between the tilting of oxygen octahedron and the stress was evaluated first. We have then simulated three-dimensional phase diagrams of the PZT system using composition, stress and temperature as the three independent thermodynamic variables. It has been shown that, with increasing tensile hydrostatic pressure, the para–ferro transition temperature increases but the transition temperature between the two ferroelectric rhombohedral phases (FR(HT)–FR(LT)) decreases, and the morphotropic phase boundary moves slightly toward the tetragonal-phase field. Contrary to these, opposite tendencies are predicted under a compressive stress. The thermodynamic computation of ferroelectric properties in the vicinity of the FR(HT)–FR(LT) phase transition further suggests that a first-order phase transition gradually becomes a continuous second-order transition with increasing tensile stress.
Epitaxial growth of stoichiometric (100) GaAs at 75 °C
Authors: Taylor Patrick J. , Jesser W. A. , Martinka M. , Dinan J. H.
Published: July 2002

Stoichiometric single crystal GaAs is grown at 75 °C by a migration-enhanced-epitaxy (MEE) technique. The stoichiometry of the GaAs films can be controlled by adjusting the arsenic monolayer deposition time. For Ga-rich compositions, epitaxy breaks down and polycrystalline GaAs results. For stoichiometric GaAs, and As-rich compositions, epitaxy proceeds and specular stoichiometric films can be grown to significant thicknesses. Reflection high-energy electron diffraction patterns indicated some surface roughness that increased for compositions closer to those which were Ga rich. This work suggests that low-temperature growth by MEE results in limited-thickness-epitaxy (LTE) behavior for GaAs that is very different from low-temperature growth by conventional molecular beam epitaxy. The results of this work indicate the importance of stoichiometry, surface roughening, and surface diffusion phenomena in determining regimes where epitaxy occurs at low temperature. A maximum LTE epitaxial thickness of 300 nm was obtained at 75 °C.
Magnetic hysteresis in granular CuCo alloys
Authors: Allia P. , Coisson M. , Tiberto P. , Vinai F. , Knobel M.
Published: July 2002

Room-temperature hysteresis loops of granular Cu100−xCox alloys (5⩽x⩽15) obtained by planar flow casting in air and submitted to proper annealing treatments have been measured up to a field of 10 kOe by means of a vibrating sample magnetometer. In major loops (|Hvert|=10 kOe), the reduced remanence-to-saturation ratio mr=Mr/Ms and the coercivity Hc measured on all studied materials appear to be related by an almost linear law of the type mr≈1/3 (μHc/kT), μ being the average magnetic moment on Co particles. A similar relation is also observed on minor symmetrical loops (100 Oe⩽|Hvert|⩽9 kOe). The observed results are accounted for by a model which considers the hysteresis as originating by magnetic interactions among nearly superparamagnetic Co particles.
Enhancement of magnetic anisotropy of nanocrystalline FeTaN films by Ti underlayers and magnetic field annealing
Authors: Shin Dong-Hoon , Kim Choong-Sik , Ahn Dong-Hoon , Nam Seoung-Eui , Kim Hyoung-June
Published: July 2002

Evolution of magnetic anisotropy in a FeTaN film was investigated as a function of annealing temperature, induction of magnetic field, and the addition of a Ti underlayer. The Fe78.8Ta8.5N12.7 films studied in this work are deposited as amorphous, and their soft magnetic properties evolve through the crystallization during postdeposition annealing. The films show a strong isotropy at all annealing temperatures with and without magnetic field induction. Employment of a Ti underlayer greatly enhances the uniaxial anisotropy when combined with a magnetic field annealing. The enhanced anisotropy leads to a significant improvement of high frequency response through an increased ferromagnetic resonance frequency.
Electronic and structural properties of the pentanary alloy GaxIn1−xPySbzAs1−y−z
Authors: Shim Kyurhee , Rabitz Herschel
Published: July 2002

The universal tight binding (UTB) model based on a modified pseudocell (MPC) is used to obtain the electronic band structure, band gaps, and the density of states (DOS) for the pentanary alloy GaxIn1−xPySbzAs1−y−z of dimension N=3 (i.e., x, y, and z). The composition dependent local and alloy bond lengths of the alloy are formulated within the MPC, and the lattice matching conditions for several substrates are found by optimizing the bond length of the substrate to the alloy bond length. The principal band gaps [E(Γ),E(L),E(X)] of the alloy are calculated lattice matched to GaAs, InP, InAs, and GaSb. The main contribution of each component upon the DOS is investigated through observation of variations with composition. In addition, the band gap calculations are performed for the ternary (N=1; GaxIn1−xP, GaPyAs1−y, and GaxIn1−xAs) and quaternary (N=2; GaxIn1−xPyAs1−y) alloys which are limiting composition cases for the pentanary alloy GaxIn1−xPySbzAs1−y−z. The latter reduced results are in good agreement with available experimental data, thereby validating the UTB model. The UTB model may also be extended to treat higher dimensional material systems (N⩾4).
Modeling photocurrent action spectra of photovoltaic devices based on organic thin films
Authors: Pettersson Leif A. A. , Roman Lucimara S. , Inganäs Olle
Published: July 2002

We have modeled experimental short-circuit photocurrent action spectra of poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and C60, respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in order to study the influence of the geometrical structure with respect to the efficiency of the thin film devices. In this way the photocurrent was optimized.
Temperature dependent field effect in organic-based thin-film transistor and its spectroscopic character
Authors: Schauer F.
Published: July 2002

The thin field-effect transistor based on amorphous semiconductors, both inorganic or organic, is modeled using an iterative scheme. The results of modeling gave insight into the temperature dependencies of this device. Further, we present the differential procedure, based on the first and higher derivatives of the data, to derive from the field-effect characteristics the basic parameters of the active semiconductor film, i.e., the density of states and field-effect mobility.
Magneto-impedance effects in multilayered permalloy materials
Authors: Sommer R. L. , Gündel A. , Chien C. L.
Published: July 2002

The magneto-impedance (MI) effect has been studied in permalloy/Ag multilayers with different Ag thickness and number of bilayer. By increasing the Ag thickness from 7 to 25 Å, the magnitude of the MI is greatly increased and the shape of the MI versus H curves evolves from a single-peak centered at H=0 to a double-peak structure. This is attributed to the different anisotropy axes induced by strain in the multilayers.
Defect clusters and thermoluminescence in LiF crystals
Authors: Davidson A. T. , Kozakiewicz A. G. , Wilkinson D. J. , Comins J. D.
Published: July 2002

The thermoluminescence of pure and doped LiF crystals has been measured following irradiation at ambient temperature with gamma and x rays. Factors investigated include the effects of dopants (Mg, Mg/Ti, and Fe) and of dose. Results are compared with optical absorption and photoluminescence measurements made previously on the same crystals. The role of F, F3+, Z2, and colloids in the emission process is discussed. Some new effects are reported including step-like features in the glow curves at large doses. These are explained in terms of the clustering of interstitial defects formed during radiolysis.
Preparation of amorphous CNx thin films by pulsed laser deposition using a radio frequency radical beam source
Authors: Aoi Yoshifumi , Ono Kojiro , Kamijo Eiji
Published: July 2002

Amorphous CNx thin films were deposited by pulsed laser deposition (PLD) combined with a nitrogen rf radical beam source which supplies active nitrogen species to the growing film surface. The dominant active nitrogen species are excited N2 molecules and nitrogen atoms. The deposited films were characterized by scanning electron microscope, x-ray photoelectron spectroscopy (XPS), Raman scattering, and Fourier transform infrared (FTIR) spectroscopy. Nitrogen content of the deposited films increased with increasing rf input power and N2 pressure in the PLD chamber. The N/C ratio 0.23 was obtained at 400 W of rf input power and 1.3 Pa. XPS N 1s spectra shows the existence of N–sp2C and N–sp3C bonds in the deposited films. The fraction of the N–sp3C increased with increasing of N2 pressure in the PLD chamber during the operation of radical beam source. FTIR and Raman spectra of the deposited films indicated that N≡C bonds in the films were few as compared to the other carbon and nitrogen bonds.
Electrohydrodynamic flow associated with unipolar charge current due to corona discharge from a wire enclosed in a rectangular shield
Authors: Feng James Q.
Published: July 2002

Unipolar charge current can be generated through corona discharge from a thin wire enclosed in a shield electrode. Except for an ionization sheath adjacent to the coronating wire surface, most parts of the region in the enclosing shield contain drifting ions of a single polarity in response to the electric field. Momentum transfer as a consequence of collisions between drifting ions and electrically neutral air molecules gives rise to the electrohydrodynamic flow known as “corona wind.” Although primarily driven by the Coulomb force due to unipolar charge in the electric field, the electrohydrodynamic flow cannot simply follow the direction of electric field lines because of the confinement of the solid walls of the shield. Therefore, the structure of the electrohydrodynamic flow can vary significantly depending on the system configuration. In the present work, the electrohydrodynamic flow in a rectangular shield is studied by solving the nonlinearly coupled governing equations via the Galerkin finite-element method. A highly symmetric system with the wire positioned at the center of a square shield is shown to contain eight equal-sized, two-dimensional recirculation vortices. The number of recirculation vortices tends to be reduced by a slight asymmetry in the system. The flow structure of two major counter-rotating recirculation vortices is found to be most common in systems where the wire is positioned off the center of the rectangular shield in a two-dimensional domain. The results reported here may be brought to bear upon the “corona wind” effect in charging devices such as corotrons and scorotrons used in electrophotographic printing processes.
GaN: Processing, defects, and devices
Authors: Pearton S. J. , Zolper J. C. , Shul R. J. , Ren F.
Published: July 2002

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.
Second-harmonic generation in reactive-ion etched N′ ethyl N-ethanol-4-(nitrophenylazo)phenylamino polymer waveguides at telecommunication wavelengths
Authors: Cho Wook-Rae , Ricci Vincent , Pliška Tomáš , Canva Michael , Stegeman George I.
Published: July 2002

Second-harmonic generation using modal dispersion phase matching has been demonstrated in reactive-ion etched waveguides based on the organic nonlinear polymer poly (methyl methacrylate)-Disperse Red 1 [N′ ethyl N-ethanol-4-(nitrophenylazo)phenylamino]. The measured propagation losses were 6 dB/cm at the fundamental and 10 dB/cm at the second-harmonic wavelength, dramatically less than we obtained previously for the harmonic in photobleached waveguides. A figure of merit of η=0.1%/W for second-harmonic generation was obtained with a modest nonlinearity of 3 pm/V at 1607 nm in a 2 mm long waveguide.
Coherent, wavelength-tunable ultrasonic acoustic modes stimulated in optical fiber, microcapillaries, and planar microfluidic networks using crossed-picosecond laser pulses
Authors: Rogers John A.
Published: July 2002

This article describes the use of optical methods based on picosecond transient gratings to stimulate and detect ultrasonic acoustic modes in several important structures with dimensions less than ∼200 μm: single-mode optical fiber, cylindrical microcapillary tubes, and planar microfluidic networks. The measurements illuminate the characteristics and dispersion of acoustic modes—Rayleigh and Lamb-like modes and Scholte–Stoneley waves—in three-dimensional microsystems with feature sizes that are comparable to the modal wavelengths. The results demonstrate, for example, the ability to measure, rapidly and nondestructively, the mechanical characteristics of on-fiber metal and polymer coatings. They also illustrate real-time monitoring of the elastic and loss moduli, and thermal diffusivities of nanoliter volumes of material contained in planar microfluidic channels during the course of photochemical curing reactions. The techniques are potentially useful for applications ranging from characterization of high-frequency acoustic modes in optical fiber that may be relevant to new types of in-fiber acousto-optic filters and modulators, to detection in microfluidic total analysis systems.
Effects of resonant pumping on the temporal and spatial evolution of a laser produced lithium plasma
Authors: Bowe Paul , Conway Jim , Dunne Padraig , McCormack Thomas , O’Sullivan Gerry
Published: July 2002

The spatial and temporal distribution of neutral and singly ionized lithium atoms has been investigated when slab targets are irradiated by ∼1 μs pulses of both resonant and nonresonant radiation from a flashlamp pumped dye laser. The ion population is enhanced when the laser is tuned to resonance with the 2s–2p line of neutral lithium and there is evidence for efficient ion production at distances up to 7.5 mm from the target surface. In particular, the rate of ionization is increased and the maximum ion population is attained between 120 and 200 ns earlier in the resonant case depending on distance above the target surface. However, the on-resonance ion population close to the surface is found to decrease significantly some 200 ns into the laser pulse. Ion and neutral drift velocities were measured by direct observation of the surface of the expanding plasma plume at a laser flux close to 8×108 W cm−2 and were consistent with the value predicted by a dynamic source model for accelerated plasma expansion, as was the value for the electron temperature derived from line intensity ratios. Power density thresholds for ion production have been studied and in particular, the threshold for Li2+ production is established.
Single-electron tunneling through Si nanocrystals dispersed in phosphosilicate glass thin films
Authors: Inoue Yoku , Tanaka Atsushi , Fujii Minoru , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

Electrical transport properties of extremely thin phosphosilicate glass (PSG) films containing Si nanocrystals (nc-Si) a few nanometers in diameter were studied. Samples were prepared by cosputtering Si and PSG targets, and post annealing. Periodic Coulomb staircases were clearly observed in the dc current–voltage (I–V) characteristics along the vertical direction of films. Although the step structure was broadened with increasing the temperature, it remained up to 200 K. The I–V curve could be well fitted by Monte Carlo simulation with a simple double-barrier structure model. Advantages of using PSG instead of SiO2 as surrounding matrices of nc-Si to observe single-electron tunneling effects are discussed.
Space charge distribution of organic-molecular-beam-deposited titanylphthalocyanine films on metal electrodes
Authors: Majima Yutaka , Yamagata Kazuhiro , Iwamoto Mitsumasa
Published: July 2002

Space charge distributions of organic-molecular-beam-deposited titanylphthalocyanine (TiOPc) films on metal electrodes were investigated by measuring surface potentials at various deposited thickness of TiOPc. The island growth of TiOPc on Au, Cr, and Al electrodes was observed by atomic force microscopy (AFM) image. Mean thickness of TiOPc films was evaluated by combining the results of absorbance and frequency-height dependence of the AFM image. Surface potentials across TiOPc film due to the space charges at the metal-TiOPc interface were discussed by taking into account the surface roughness. It was found that the surface potential-film thickness relationship of TiOPc films is similar to that of Langmuir–Blodgett films. The space charge distributions in TiOPc films on Au, Cr, and Al electrodes were determined from the thickness dependence of the surface potentials. It is shown that the space charges are excessive electrons transferred from metals, and 90% of them concentrate on the metal-TiOPc interface at a range of 10 nm.
Magnetic properties of DyCo10Mo2−xSix compounds
Authors: Zhang C. , Mohanty R. C. , Raman A. , Grenier C. G. , Ferrell R. E. , Shaheen S. A.
Published: July 2002

DyCo 10 Mo 2−x Si x compounds (x=0, 0.5, 1.0, 1.5, and 2.0) with the ThMn12 structure were prepared by means of arc melting and subsequent vacuum annealing. Their magnetic properties were investigated by x-ray diffraction on fixed aligned powder samples and by magnetization measurements. The lattice constants decrease linearly with increasing value of x. The easy-magnetization direction (EMD) of all samples is parallel to the c axis at room temperature except for DyCo10Si2 whose EMD is within the basal plane. Deviations of the spontaneous moment up from a linear x dependence are observed. The Curie temperature attains a maximum value at x=1.5. There is a spin reorientation transition from the easy axis to the easy cone upon cooling for compounds with x=0.5, 1, and 1.5.
Photorefractive properties of Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystals with different doping levels and different dimensions
Authors: Shen Xiaonong , Zhao Jianhua , Lu Xinliang , Jiang Quanzhong , Zhang Jingwen , Xia Hairui , Song Lihuan , Zhang Shujun , Han Jianru , Chen Huanchu
Published: July 2002

In this article, we studied the photorefractive properties of Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 (Cu:KNSBN) crystals with different doping levels. The properties include two-wave mixing and self-pumped phase conjugation, absorption properties, and the growth conditions as well. We also studied the effect of the dimensions on the photorefractive properties. The mechanism of Cu-doped KNSBN and the influence of the different sites occupied by Cu+ on photorefractive properties are also discussed. All the experimental results showed that doping levels have much effect on the photorefractive properties. The crystal with a higher doping level has a higher effective charge carrier density, a larger coupling coefficient, and a faster response rate, which is suitable for fast optical information processing. Dimension has much influence on the photorefractive properties. The thinner crystal with the same doping level has a larger coupling gain coefficient, and a shorter response time.