bluuebunny/crossref_metadata_2025 huggingface dataset:
Mass density and hydrogen content of diamondlike carbon as related to the preparation by plasma-enhanced chemical vapor deposition
Authors: Vigild M. E. , Findeisen E. , Feidenhans’l R. , Barholm-Hansen C. , Bentzon M. D. , Bindslev Hansen J.
Published: July 2002

Combined neutron and x-ray reflectometry was used to determine mass density and hydrogen content for thin (less than 3000 Å) diamondlike hydrocarbon films on polished Si (100) substrates. The introduction of ‘‘twin’’ neutron reflectometry improved the accuracy of the measurements. Mass densities in the range of 1.60–1.84 g/cm3 and hydrogen contents of 31–36 at. % were determined together with the nucleus densities of hydrogen and carbon. A series of diamondlike carbon films prepared by plasma-enhanced chemical vapor deposition was investigated to analyze the effect of depositing films at different flow rates of methane precursor gas. The mass density was found to increase with flow in the range of 1–8 standard cm3/min (sccm). For higher flows the mass density was constant. The hydrogen content did not depend on the rate of flow in the interval of 2–30 sccm. Finally, results of the mass density measured in a simpler way, using a scanning electron microscope and a profilometer, were compared to the results obtained by neutron and x-ray reflectometry.
Quarternary giant magnetoresistance random access memory
Authors: Wang Zhigang , Nakamura Yoshihisa
Published: July 2002

We designed a quarternary memory using weakly coupled giant magnetoresistance (GMR) multilayers based on the fact that there are four stable states when the applied field is zero. Compared with conventional binary memory, the major advantage of the quarternary GMR memory is that we can simply double its capacity. This design’s feasibility has been proved by experiments.
Negative remanence in magnetic nanostructures
Authors: Yan X. , Xu Y.
Published: July 2002

In nearly percolating magnetic granular (Ni,Fe)-SiO2 films, we observed negative remanence, or a negative remanent magnetization, upon removing the positively applied magnetic field to zero. We propose that there exist both superparamagnetic and ferromagnetic components due to different connectivity of magnetic nanometer scaled particles, as supported by the dark field transmission electron microscopy images. The magnetostatic interaction between them favors an opposite alignment. Moreover, the two-component hypothesis explains satisfactorily the dependencies of the negative remanence on temperature and magnetic volume fraction.
Magnetization reversal processes of NiFe elements exchange coupled by NiO antiferromagnetic films
Authors: Ding Juren , Zhu Jian-Gang
Published: July 2002

The magnetic domain structures and their switching processes of small peamalloy (NiFe) elements exchange coupled by antiferromagnetic (NiO) thin films were studied using magnetic force microscopy. A uniform external magnetic field generated by an electromagnet was applied to the magnetic force microscopy so that the domain switching processes could be observed in situ. The effects of NiFe film thickness, element size, and aspect ratio on domain behavior were investigated. It was found that the unidirectional exchange coupling field of small elements was much greater than that measured on the sheet bilayer films. The results of this study showed little indication of the existence of domain structure in NiO antiferromagnetic film.
Spin-dependent interface transmission and reflection in magnetic multilayers (invited)
Authors: Stiles M. D.
Published: July 2002

First-principles calculations of transmission and reflection from Ag/Fe, Au/Fe, Cu/Co, and Cu/Ni interfaces show very strong spin dependence that differs significantly from expectations based on free electron approximations. The results can be used to understand both the giant magnetoresistance and the oscillatory exchange coupling observed in magnetic multilayers of these materials. The spin dependence of the reflection probabilities is strong enough to give a large giant magnetoresistance even if there is no spin-dependent defect scattering. The calculated reflection amplitudes determine the strength of the oscillatory exchange coupling.
Two magnon ferromagnetic resonance linewidths in uniaxial and planar single crystal hexagonal ferrites (abstract)
Authors: Franklin D. R. , Hurben M. J. , Patton C. E.
Published: July 2002

Hexagonal ferrites are ideally suited for millimeter-wave applications but have seen limited use because of large losses. High field effective linewidth and ferromagnetic resonance (FMR) linewidth measurements for single crystal barium ferrite indicate that these losses may be due to an inhomogeneity related two magnon scattering contribution to the relaxation. Calculations of the linewidth as a function of the external magnetic field orientation in films and thin disks give characteristic dependencies if two magnon scattering relaxation is present. The objective of this work was to measure the angle dependence of the FMR linewidth in single crystal Zn–Y and Ba–M hexagonal ferrite disks and compare the results to the theoretical predictions. The materials were flux grown Mn doped Ba–M and Zn–Y single crystals. The Ba–M and Zn–Y disks were 1.0 mm in diameter and 0.13 and 0.4 mm thick, respectively. Measurements were made at 53 GHz for the Ba–M and 9.5 GHz for the Zn2–Y. The FMR was measured by a shorted waveguide technique as a function of the field angle relative to the out-of-plane direction. The linewidth for the Ba–M increased from minimum value of 35 Oe at 0°, to a maximum of 330 Oe at 68°, followed by a small decrease out to the maximum angle of 75° for which FMR could be observed. For the Zn2–Y, the linewidth increased from 95 Oe at 0° to 420 Oe at 5°, followed by a decrease to 37 Oe at 90°. The two-magnon model gives large peaks in the linewidth as a function of angle, the sizes and positions of which depend upon the anisotropy and the size of the scattering inhomogeneities. The experimental results for both materials are in good agreement with model two-magnon calculations based on an inhomogeneity size on the order of 0.5 μm.
Oxidation mechanism of the ammonium-fluoride-treated Si(100) surface
Authors: Kluth G. Jonathan , Maboudian Roya
Published: July 2002

High-resolution electron energy loss (HREEL) spectroscopy, Auger electron spectroscopy, and low-energy electron diffraction have been employed to examine the initial oxidation stage of ammonium-fluoride-treated Si(100) surfaces exposed to air. The NH4F treatment results in a hydrogen-terminated surface, as shown by the presence of the Si–H stretch (2100 cm−1), SiH2 scissor (910 cm−1), and SiH bend (650 cm−1) in the HREEL spectra. Initial oxidation on this surface occurs through oxygen insertion in the silicon backbonds, as indicated by the presence of the asymmetric bridge-bonded oxygen stretch in the region between 1060 and 1160 cm−1. Oxygen is observed in both surface and bulk bridge-bonding sites for even the shortest air exposures, suggesting that initial oxygen uptake is not taking place in a layer-by-layer fashion. Auger electron spectroscopy shows a slow uptake of oxygen over the first few days of air exposure, followed by more rapid oxidation. Over the first two weeks of air exposure, the Si–H stretch at 2100 cm−1 gradually disappears, accompanied by the appearance of oxygen backbonded Si–H peaks around 2250 cm−1. This behavior indicates that, despite the increase in oxygen uptake, the hydrogen termination remains intact, and further confirms that oxygen uptake occurs through insertion in the silicon backbonds. Water is found to play a significant role in the initial oxidation.
Influence of nonstoichiometry on the Urbach’s tails of absorption spectra for CuInSe2 single crystals
Authors: Shioda T. , Chichibu S. , Irie T. , Nakanishi H. , Kariya T.
Published: July 2002

Optical absorption spectra of CuInSe2 single crystals were measured for the samples with −0.150≤x≤0.053, where x represents a degree of nonstoichiometry in the formula Cu1−xIn1+xSe2. The Urbach’s tail was observed for all samples between 90 K and room temperature. The Urbach’s energy, which represents an arbitrary intensity of exciton–phonon interaction, was almost constant for the Cu-rich samples (x<0), while it increased with increasing In composition for the In-rich ones (x≳0). Such an increase of the Urbach’s energy was explained to be due to enhanced electronic distortion caused by the compositional deviation from stoichiometry in terms of simultaneous influence of electron–phonon interaction and structural disorder.
A low-energy electron diffraction investigation of the surface deformation induced by misfit dislocations in thin MgO films grown on Fe(001)
Authors: Dynna M. , Vassent J. L. , Marty A. , Gilles B.
Published: July 2002

The elastic deformation of the surface of thin MgO films grown on Fe(001) induced by the presence of misfit dislocations at the Fe/MgO interface has been put into evidence using low-energy electron diffraction. Satellite spots surrounding the fundamental Bragg reflections appear once the critical thickness is exceeded and undergo a shift in position as thickness of MgO increases, gradually blending in with the fundamental reflections. The evolution of the position of the spots is in good agreement with kinematical diffraction calculations in which the surface deformation is determined from the displacement field of a single misfit dislocation using both isotropic and anisotropic elasticity theory.
The nonlinear, complex, induced-piezoelectric response of Pb(Mg1/3Nb2/3)O3–PbTiO3 relaxors
Authors: Li Jie-Fang , Viehland Dwight
Published: July 2002

The amplitude-dependent complex piezoelectric response of Pb(Mg1/3Nb2/3)O3–PbTiO3 relaxors has been investigated using a single-beam interferometric technique as a function of ac electrical drive, superimposed dc electrical bias, and measurement frequency. Large-induced longitudinal piezoelectric coefficients (d33) were observed over a wide dc bias range below polarization saturation. On approaching polarization saturation, the value of d33 decreased, approaching values typical of poled piezoelectric ceramics. Investigations as a function of ac drive revealed the presence of strong nonlinearities, where the value of d33 increased with increasing drive. The nonlinear contribution was found to be on the order of 50% of the total response. In addition, large unexpected piezoelectric loss factors tan δ(piezo) were observed in materials which were otherwise characterized by anhysteresis in the polarization electric field behavior. The value of tan δ(piezo) was found to be in the range between 0.10 and 0.20, depending on various measurement conditions. With either increasing dc bias or ac drive, the value of tan δ(piezo) was found to decrease significantly.
Creation of submicron orientational structures in thin liquid crystal polymer layers
Authors: Rüetschi M. , Fünfschilling J. , Güntherodt H.-J.
Published: July 2002

Submicron orientational patterns were created in thin liquid crystal polymer films. A rubbed polyimide layer was locally modified by hatching it with the small tip of a simplified atomic force microscope. A nematic liquid crystal mixture, that can be spin-coated and polymerized at room temperature was used to transfer the lithography from the polyimide into the liquid crystal polymer. Gratings as small as 240 nm periods were fabricated. The domain boundaries, which are nothing else than twist disclination lines attached to the substrate surface, influence the topography on a distance much larger than the film thickness. This topographic effect, which consists of a groove and a wall parallel to the boundary, was investigated as a function of the scratching angle, the film thickness and the size of the structure.
Novel method for minority-carrier mobility measurement using photoconductance decay with chemically passivated and plasma damaged surfaces
Authors: Stephens A. W. , Green M. A.
Published: July 2002

A method for measuring minority-carrier mobility using microwave-detected photoconductance decay without requiring bulk lifetime, estimates is presented. Three different measurements on a single sample yield values for surface recombination velocity, bulk lifetime, and diffusivity. For each measurement the surface conditions of the sample are changed, allowing extraction of different parameters. The usefulness of 0.08 molar ethanol/iodine solution as a means of achieving such good surface passivation is demonstrated. The following procedure was used to achieve high surface recombination. A CF4 plasma surface etch was shown to achieve the same level of surface damage as mechanical abrasion. The advantage of the new method is that it completely eliminates the chance of breaking samples during the abrasion process, which is of particular advantage for thin samples. The new experimental method for minority-carrier mobility measurement is evaluated using carrier lifetime measurements made on a commercially available Leo Giken ‘‘Wafer-τ’’ lifetime tester.
Characterization of traps in GaAs/W Schottky diodes by optical and electrical deep-level transient spectroscopy methods
Authors: Takanashi Yoshifumi
Published: July 2002

Both the electrical and optical deep-level transient spectroscopy (DLTS) measurement methods were used to investigate the deep traps in GaAs that originate from the impurities included in the Schottky metal on GaAs/W Schottky diodes. Two distinct electron traps (E1 and E2, whose activation energies EA were, respectively, 0.65 and 0.58 eV) were detected by electrical DLTS in samples annealed at 650 °C. The variation of EA and trap density with changes in the Schottky barrier height indicates that the trap E1 can be identified with the EL2 usually observed in the GaAs grown by metalorganic chemical-vapor deposition. In addition, the trap E2 is found to be due to impurities that diffused from the W film into the GaAs layer during annealing. Four hole traps with activation energies of 0.76, 0.55, 0.43, and 0.30 eV were detected by optical DLTS in samples annealed at 650 °C. Comparison of the EA for each hole trap with data previously reported indicates that the first three traps are due to the transition metals Cr, Fe, and Cu, and that the last one is due to a native defect. These metals are confirmed from secondary-ion-mass spectroscopy analysis to be included in the as-deposited W film. Furthermore, these traps are also observed in GaAs permeable base transistors with the buried W gate.
Real time spectroellipsometry for optimization of diamond film growth by microwave plasma-enhanced chemical vapor deposition from CO/H2 mixtures
Authors: Lee Joungchel , Hong Byungyou , Messier R. , Collins R. W.
Published: July 2002

Real time spectroellipsometry has been applied to determine the deposition rate and thickness evolution of the nondiamond (sp2-bonded) carbon volume fraction in very thin (<1000 Å), but fully coalesced, nanocrystalline diamond films prepared on Si substrates by microwave plasma-enhanced chemical vapor deposition from gas mixtures of CO and H2. At a substrate temperature of ∼800 °C, high quality diamond films can be obtained over two orders of magnitude in the CO/H2 gas flow ratio, from 0.04, the lowest value explored, to ∼5. A well-defined minimum in the sp2 C volume fraction (0.03 in a 600 Å film) is observed for a CO/H2 ratio of 0.2, corresponding to the C–H–O diamond-growth phase-diagram coordinate XH/Σ=[H]/{[H]+[C]} of 0.9. Under these conditions, the deposition rate increases with increasing temperature over the range of ∼400–800 °C with an activation energy of 8 kcal/mol, behavior identical to that observed for diamond film growth from a CH4/H2 ratio of 0.01. This observation shows that the dominant film precursors in the diamond growth process from CO/H2=0.2 are hydrocarbons whose flux at the growing film surface is controlled through the reaction of excited CO with H or H2 in the plasma. A broad subsidiary minimum in the sp2 C content is observed, centered near a CO/H2 ratio of 2, corresponding to an XH/Σ value of ∼0.5. Under these gas flow conditions, the deposition rate is a complicated function of temperature, exhibiting a peak near 550 °C. This peak shifts to lower temperature with further increases in the CO/H2 ratio above 2, suggesting a nonhydrocarbon precursor and a different growth mechanism for diamond prepared at high CO/H2 ratio and low temperature.
Calculation of electron and hole impact ionization coefficients in SiGe alloys
Authors: Yeom K. , Hinckley J. M. , Singh J.
Published: July 2002

Silicon–germanium alloys offer a system where the ratio of the electron impact ionization coefficient (α) and hole impact ionization coefficient (β) varies from a value larger than unity (in high silicon content alloys), to a value smaller than unity (in high germanium content alloys). We report results for α and β for this alloy system. The electron results are based on a multivalley nonparabolic band structure. The hole results are based on a six-band k⋅p model for low energies coupled to an eight-band model for high energies. We find that for the alloy Si0.4Ge0.6, α∼β. Alloy scattering is found to play an important role in determining the impact ionization coefficient. For compositions around Si0.5Ge0.5, the strong alloy scattering is found to suppress the impact ionization coefficient.
Effects of fluorine addition on the structure and optical properties of SiO2 films formed by plasma-enhanced chemical vapor deposition
Authors: Ishii K. , Takami A. , Ohki Y.
Published: July 2002

In order to elucidate the effects of fluorine addition on the structure and optical properties of SiO2 films formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane, photoluminescence and optical absorption characteristics were studied using synchrotron radiation as an excitation photon source. Luminescence appears at 4.4 eV in both fluorine-doped and nondoped films when excited by 7.6 eV photons, indicating that the oxygen vacancies exist in both films. The decay profile of the 4.4 eV luminescence deviates from a single-exponential curve, indicating that the microscopic structure is not uniform in the sample. This deviation becomes smaller by the fluorine addition. Infrared absorption spectra indicate that the width of the distribution of bond angle ∠SiOSi becomes smaller with an increase in the fluorine content. From these results, it is considered that the distribution of the bond angle decreases by the addition of fluorine. The optical gap energy was found to increase with the increase in the fluorine content. Presumably, this is also due to the homogeneous structure with a uniform bond angle caused by the fluorine addition.
Role of the buffer layers in determining the antiferromagnetic coupling and magnetoresistance of NiFeCo/Cu superlattices
Authors: Diao Z. T. , Goto S. , Meguro K. , Tsunashima S. , Jimbo M.
Published: July 2002

The antiferromagnetic (AF) coupling and magnetoresistance in magnetron-sputtered polycrystalline NiFeCo/Cu superlattices grown on Fe, Cr, or Zr buffer layers have been studied. The results show that the buffer layers significantly affect AF coupling and magnetoresistance through modulating the growth of specifically oriented crystalline multilayered structures. Two types of buffer layer effects on AF coupling and magnetoresistance are considered: modifying interfacial roughness and producing layer-thickness fluctuations. Interfacial roughness is formulated in the first Born approximation using data obtained from specular and off-specular (diffuse) x-ray scattering measurements. For the roughness at interfaces, correlated interfacial roughness is found to predominate and estimated to be as large as 5–7 Å with a moderately large lateral correlation length scale ξ that ranges from 120 to 200 Å. Upon approximately the same AF coupling, for Fe or Cr buffer layered superlattices characteristic of well-defined, comparatively flat layer structures throughout the entire specimen, magnetoresistance turns out to increase as the correlated interfacial roughness increases. For the superlattices grown on Zr buffer layers in which the presence of columnar structures is confirmed, a constant, weak AF coupling and moderate magnetoresistance concur and are both almost independent of buffer layer thickness. These features are explained in terms of the averaging of AF coupling and magnetoresistance through parallel layer-thickness fluctuation structures approximating the columnar structures.
Monte Carlo calculation of the electron capture time in single quantum wells
Authors: Abou-Khalil Michel , Goano Michele , Reid Benoit , Champagne Alain , Maciejko Roman
Published: July 2002

The electron capture time in single quantum wells is calculated by considering capture and escape as scattering events in Monte Carlo simulation. The calculation is performed for an AlGaAs/GaAs quantum well as a function of the well width at 300 K. The overall capture time of carriers is found to be controlled by the transition from the free state to the uppermost confined levels. Subsequent interband transitions cause rapid decay into lower levels.
Calculating orientation distributions in magnetic tapes
Authors: Harrell J. W. , Yu Y. , Ye Y. , Parakka J. P. , Nikles D. E. , Zolla H. G.
Published: July 2002

Two new methods are presented for determining the orientation distribution of magnetic tapes from measurements of the angular dependence of remanence. A closed-form expression is presented which allows a two-dimensional distribution function to be directly calculated from the remanence measurements. We have also adapted the method of maximum entropy to numerically calculate the distribution function. This method selects a distribution function which maximizes the entropy subject to the constraint that the fit cannot be better than the assumed error in the measurements. Results are presented which show very good agreement between the two methods.
Nuclear magnetic resonance study of ion ordering and ion shifts in relaxor ferroelectrics
Authors: Glinchuk M. D. , Laguta V. V. , Bykov I. P. , Nokhrin S. , Bovtun V. P. , Leschenko M. A. , Rosa J. , Jastrabı́k L.
Published: July 2002

Nuclear magnetic resonance (NMR) spectra of Nb93, Sc45, and Pb207, as well as dielectric permittivity temperature dependence in a wide frequency range, were investigated in the solid solution of ferroelectric relaxors (x)PbMg1/3Nb2/3O3—(1−x)PbSc1/2Nb1/2O3 (0⩽x⩽1). The peculiar behavior of the observed quantities and parameters, e.g., disappearance of dielectric dispersion for x∼0.5, is revealed. The analysis of the dependence of Sc45 NMR spectra on x has shown that ordering of the regions containing Sc ions takes place for x>0.45. The investigation of the temperature dependence of Nb93 and Sc45 NMR linewidths in PbSc1/2Nb1/2O3 had shown that at T<Tmax Nb and Sc ions are shifted from their equilibrium lattice sites (Tmax corresponds to dielectric permittivity maximum). Latter shifts contribute to the temperature dependence of lattice polarization P, whereas the jump of P at T=Tmax is determined by shifts of Pb ions. The peculiarities of NMR line shape in the investigated materials gave evidence of randomness in the directions and values of Nb, Sc, and Pb dipole moments.
Phonon modes of ZnS1−xTex alloys epitaxially grown on (100) GaAs substrates
Authors: Jin C. X. , Ling Z. , Wang D. H. , Huang D. M. , Hou X. Y. , Wang Xun
Published: July 2002

ZnS 1−x Te x (0<x<1) alloys grown on (100) GaAs substrates by molecular beam epitaxy are investigated using the x-ray diffraction and Raman scattering. The frequencies of long wavelength ZnTe-like and ZnS-like longitudinal optical phonons determined from Raman scattering show linear variation with the composition x. The frequency of the zone-center optical phonons as a function of x of the ZnS1−xTex mixed crystal shows a typical two-mode behavior, which is in good agreement with the theoretical results from a modified random-element isodisplacement model.
Optical spectroscopy investigations of half metallic ferromagnetic Heusler alloy thin films: PtMnSb, NiMnSb, and CuMnSb
Authors: Bobo J. F. , Johnson P. R. , Kautzky M. , Mancoff F. B. , Tuncel E. , White R. L. , Clemens B. M.
Published: July 2002

We have studied the optical properties of cosputtered XMnSb (X=Cu,Ni,Pt) thin films using light transmission, reflection, and spectroscopic ellipsometry. We observed a feature in the transmission spectra suggestive of a band gap of energy ∼1.5–1.7 eV for PtMnSb, ∼1.1 eV for NiMnSb and ∼0.8 eV for CuMnSb. This behavior is consistent with predictions of the half metallic ferromagnetic state in PtMnSb and NiMnSb. The values for PtMnSb and NiMnSb are larger than the ones previously reported by other authors on bulk polished samples. We interpret this discrepancy as due to surface contamination of the bulk samples or nonstoichiometry in our films.
Femtosecond degenerate four-wave mixing in 500 μm undoped GaAs and 350 μm undoped InP far below band gap
Authors: Yu Sungkyu , Kim Dongho , Kim D. S. , Lee Y. H. , Cho Y. H. , Choe B. D. , Lee Jong Hyun
Published: July 2002

Rather strong third- and fifth-order four-wave mixing (FWM) signals from a 500 μm thick undoped GaAs and a 350 μm thick undoped InP in detuning ranges of 80–170 meV below the band gap are observed. The spectrally resolved (SR) FWM signals for both samples far below the band gap are blue-shifted significantly with respect to the spectrum of the excitation laser pulse for negative time delays, whereas they are red-shifted for positive time delays. The temporal and spectral shapes of the FWM signals for both samples far below the band gap are nearly consistent with those of the excitation laser pulse but are remarkably different from the excitonic properties. The shifts of SR-FWM signals far below the band gap are interpreted as due to the instantaneously created virtual states by the temporal overlap of the excitation laser pulses.
Angle dependence of the ferromagnetic resonance linewidth in easy-axis and easy-plane single crystal hexagonal ferrite disks
Authors: Hurben M. J. , Franklin D. R. , Patton C. E.
Published: July 2002

Ferromagnetic resonance measurements were made as a function of the static external field angle for c-plane disks of single crystal flux grown manganese substituted barium M-type (Ba-M) and zinc Y-type (Zn-Y) hexagonal ferrites at 50 and 8.8 GHz, respectively. A shorted waveguide technique was used. Analysis of the FMR field versus angle results confirmed the operational assumption of a uniform mode response. For the easy-axis Ba-M disk, the linewidth was 69 Oe when the external field and magnetization vectors were perpendicular to the disk. The linewidth increased to a maximum measured value of 472 Oe when the magnetization was directed 45.4° from the sample normal. For the easy-plane Zn-Y disk, the linewidth had a minimum value of 18 Oe when the field and magnetization vectors were in-plane. The maximum linewidth was 391 Oe when the magnetization was directed 25.2° from the disk normal. The linewidths were larger than predicted for reasonable values of the Landau–Lifshitz damping and showed angle dependences which indicated nonintrinsic contributions to the loss. A modified two magnon scattering calculation based on the model of Sparks, Loudon, and Kittel was used to investigate these linewidth differences. The calculation included anisotropy modifications to the spin wave band for each material. The angle dependences of the excess linewidths show qualitative agreement with the two magnon predictions, with inhomogeneity sizes on the order of 1 and 0.25 μm and volume fractions of 0.01 and 0.005 for the Ba-M and Zn-Y disks, respectively.
Self-consistent simulation of a planar electron-cyclotron-wave-resonance discharge
Authors: Krimke Ralf , Urbassek Herbert M.
Published: July 2002

A discharge heated inductively by resonant absorption of electron cyclotron waves discharge is modeled in a planar geometry. The simulation algorithm is based on a kinetic particle-in-cell (PIC/MC) simulation of the plasma properties; the electromagnetic field is calculated macroscopically using the Appleton–Hartree theory for the dielectric tensor. The results are checked against a simplified analytical theory and experimental data by B. Pfeiffer [J. Appl. Phys. 37, 1624,1628 (1966)] for a 15 mTorr argon discharge. As a result, we show that an inhomogeneous density profile in the discharge strongly affects the electromagnetic fields in the plasma. Power deposition is calculated both in and outside of the resonance.
Uniformity and physical properties of semi-insulating Fe-doped InP after wafer or ingot annealing
Authors: Avella M. , Jiménez J. , Alvarez A. , Fornari R. , Gilioli E. , Sentiri A.
Published: July 2002

Semi-insulating Fe-doped InP was annealed under different conditions and investigated by Hall effect, extrinsic photocurrent mapping, chemical etching, and optical microscopy. The resistivity is increased for any treatment, particularly in wafer-annealed InP. This result is probably due to strong losses of shallow donors. Remarkable differences exist between the structural properties of the wafer and ingot annealed material; wafer annealing produces a quick elimination of growth striations and decoration microdefects while ingot-annealed InP still retain these microdefects, both along dislocations and in the matrix. The photocurrent maps indicate that the thermal treatments normally improve the doping uniformity (especially the short-range fluctuations). The best uniformity is achieved for the long annealing time (⩾50 h), while a slow cooling rate improves the mobility.
Si/Δ layer in GaAs(001): Its effect on the crystal structure and roughness of the GaAs cap layer
Authors: Lucas Norene , Zabel Hartmut , Morkoç Hadis
Published: July 2002

We have investigated structural properties of GaAs grown on top of a pseudomorphic Si interlayer deposited on a GaAs substrate by x-ray reflectivity and Bragg scattering. The surface and interface roughness of the 500-Å thick GaAs overlayer depends critically on the thickness of the Si interlayer. The surface roughness increases from roughly 10 to 40 Å when the Si thicknesses increases from 3 to 5 Å, whereas the interface roughness remains roughly constant for Si thicknesses larger than 9 Å. The Laue oscillation of the GaAs(004) Bragg reflection together with the asymmetry of the intensity allow a precise determination of the pseudomorphic Si interlayer.
Intense field effects on hydrogen impurities in quantum dots
Authors: Fanyao Qu , Fonseca A. L. A. , Nunes O. A. C.
Published: July 2002

Calculations of the binding energy of an on-center donor hydrogenic impurity in a quasizero-dimensional quantum-well system [quantum dot (QD)] placed in an intense, high-frequency laser field are presented. A nonperturbative theory and a variational approach are used as the framework for this calculation. The effect of the intense laser field is to “dress” the impurity potential making it dependent upon the laser field amplitude. A rapid decrease of the binding energy, for different values of the QD radius and for both infinite and finite potential barriers, with increasing field intensity is predicted. An application is made for a spherical QD made of GaAs/Ga1−xAlxAs heterostructures.
Surface energy model for the thickness dependence of the lateral oxidation of AlAs
Authors: Naone R. L. , Coldren L. A.
Published: July 2002

The lateral oxidation rate of AlAs layers decreases dramatically for layers thinner than about 500 Å, because the activation energies for the rate constant of the reaction at the oxidation front increases by an amount inversely proportional to the layer thickness. We derive a model for the thickness dependence of the lateral oxidation rate of AlAs based on the surface energy of the curvature observed at the oxide tip. From the model, we show that the linear oxidation rate has an exp(−θ0/θ) dependence on the AlAs layer thickness θ, and we can predict the slowing of oxidation when the AlAs layer is cladded with AlGaAs barriers. Also, we estimate the surface energy of the AlAs/oxide interface to be 50 eV/nm2.
Optical quenching of photoconductivity in GaN photoconductors
Authors: Huang Z. C. , Mott D. B. , Shu P. K. , Zhang R. , Chen J. C. , Wickenden D. K.
Published: July 2002

The observation of optical quenching of photoconductivity in GaN photoconductors at room temperature is reported on. Three prominent quenching bands were found at Ev+1.44, 1.58, and 2.20 eV, respectively. These levels are related to three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He–Ne laser shining on the photoconductor.
Agglomeration of TiSi2 thin film on (100) Si substrates
Authors: Rha J. J. , Park J. K.
Published: July 2002

We have investigated the detailed process of agglomeration of TiSi2 thin film on (100) Si substrates as a model system for our recent geometrical model of agglomeration based on the spheroidization at both the surface and film/substrate interface. Agglomeration occurs by a nucleation of holes at grain-boundary vertices as a result of spheroidization at both interfaces and by their subsequent growth along grain boundaries in accordance with the prediction of our model. The critical condition of the ratio of the grain size to film thickness is predicted and confirmed to be between 5 and 6 depending on the magnitude of free-energy barrier.
Mechanical vibration of freely suspended ferroelectric liquid-crystal film excited by sound and electric field
Authors: Uto Sadahito , Tazoh Eisuke , Ozaki Masanori , Yoshino Katsumi
Published: July 2002

A study of the mechanical vibrations of a freely suspended (FS) ferroelectric liquid-crystal film has been carried out. Upon excitations by sound irradiation and also by electric-field application, the mechanical vibration of the FS film of the ferroelectric liquid crystal is effectively excited. In the frequency dependence, resonance vibrations are observed for both excitations and the resonance frequencies, and light reflection patterns are found to be different for both excitations, suggesting the different oscillating modes of the FS film for both excitations. In electric-field excitation, the vibration mode, which is consistent with the molecular model of the origin of the vibration due to the reorientation of Ps by Ps⋅E torque is found. In addition, the application of the FS film of the ferroelectric liquid crystal as sensitive acoustic sensors is proposed.
Carrier lifetime and exciton saturation in a strain-balanced InGaAs/InAsP multiple quantum well
Authors: Mottahedeh R. , Prescott D. , Haywood S. K. , Pattison D. A. , Kean P. N. , Bennion I. , Hopkinson M. , Pate M. , Hart L.
Published: July 2002

The bleaching of the n=1 heavy-hole and light-hole exciton absorption has been studied at room temperature and zero bias in a strain-balanced InGaAs/InAsP multiple quantum well. Pump-probe spectroscopy was used to measure the decay of the light-hole absorption saturation, giving a hole lifetime of only 280 ps. As only 16 meV separates the light- and heavy-hole bands, the short escape time can be explained by thermalization between these bands followed by thermionic emission over the heavy-hole barrier. The saturation density was estimated to be 1×1016 cm−3; this is much lower than expected for tensile-strained wells where both heavy and light holes have large in-plane masses.
Activation of blue emission from oxidized porous silicon by annealing in water vapor
Authors: Koyama Hideki , Matsushita Yuka , Koshida Nobuyoshi
Published: July 2002

It is reported that annealing of oxidized porous silicon in water vapor at about 250 °C drastically enhances its blue emission. Infrared absorption studies indicate that this annealing increases all the absorption peaks related to O–H vibrations, except for that of free silanol which disappears completely. No traces of carbon-related signals are observed. The latter result makes a clear objection to blue-band luminescence models based on adsorbed carbon compounds. Although the possibility of silanol-related origins cannot be ruled out, we suggest that some passivation effect of adsorbed hydroxyls on Si nanostructures surrounded by SiO2 is also likely.
Transient response of the radio frequency inductively coupled plasma to a sudden change in power
Authors: Mostaghimi J. , Paul K. C. , Sakuta T.
Published: July 2002

A two-dimensional, axisymmetric model was developed to study the response of a radio-frequency inductively coupled plasma to a sudden change in its active power. The time-dependent equations for the conservation of mass, momentum, and energy, along with Maxwell’s equations were solved numerically. Results were obtained for a pressure range of 200–760 Torr, a frequency range of 1–3 MHz; torch diameters between 40 and 75 mm; and, argon/hydrogen flow rates of 40–75 slpm. Initially, the plasma was assumed to be under steady-state condition at 20 kW. The plasma power was then reduced to 10 kW for 35 ms and, the response of the plasma fields and the coil current were predicted numerically. When power was reduced, the coil current reduced significantly in 2 ms. It then increased to a maximum before smoothly decreasing to its new steady-state value. The response of the plasma depended, to different degrees, on all the parameters considered here. Depending on the position within the torch, it could vary from 2 ms to several tens of millisecond. The plasma response was fastest within the skin-depth region where power was dissipated. The response time was most strongly affected by the changes in discharge pressure; and was least affected by the induction frequency. The response time increased with pressure and/or torch diameter, and decreased with frequency and/or flow rate. We also found that, depending on the magnitude of induction frequency, an increase in plasma power may cause an oscillatory plasma behavior.
Coupled charge trapping dynamics in thin SiO2 gate oxide under Fowler–Nordheim stress at low electron fluence
Authors: Samanta Piyas , Sarkar C. K.
Published: July 2002

Degradation of thermally grown silicon dioxide (SiO2) in metal-oxide-semiconductor capacitors due to bulk trapped positive charges has been theoretically investigated under Fowler–Nordheim (FN) stress at a low electron fluence Qinj (⩽0.01 C/cm2) from the accumulated layer of (100) n-Si. n+-polySi gate capacitors with 22-, 27-, and 33-nm-thick SiO2 dielectrics were studied under FN stress over a wide range of cathode electric fields (6–12.5 MV/cm). The present numerical simulation is based on the tunneling electron initiated trap-to-band ionization and band-to-band ionization as the possible coupled mechanisms for the generation of positive charges and holes, respectively. Our theoretical analysis is carried out with a Gaussian and a uniform distribution of trapped holes in the oxide in addition to the widely used constant value (5 nm from the Si–SiO2 interface) of the centroid of the trapped holes. Utilizing the available experimental data, the estimated values of various trapping parameters obtained from the present coupled dynamics differ by ∼35%–40% from those obtained by the traditional analysis based on either one of the above two mechanisms. The results obtained from the present model are in good agreement with the experimental data of FN voltage shift as a function of electron fluence Qinj reported by Fazan et al. [Solid State Electron. 30, 829 (1987)]. By recourse to estimation of the trapping parameters from the experimental data under constant current stress, a comparative study of degradation under various modes of FN stressing (constant current and constant gate voltage) is also presented. The present simulation predicts the occurrence of plateau level consistent with earlier experimental observations in various measurables (FN voltage shift and FN tunneling current transient, under constant current and field stress, respectively) at a Qinj around 2×10−3 C/cm2 independent of oxide thickness and mode of stressing.
Infrared luminescence decay and guided spectroscopy of Er3+ doped CaF2 molecular beam epitaxy layers
Authors: Marco de Lucas M. C. , Daran E. , Jacquier B. , Garapon C. , Mugnier J. , Pernas P. , Muñoz–Yague A.
Published: July 2002

The decay of infrared luminescence in Er3+-doped CaF2 layers grown by molecular beam epitaxy (MBE) on CaF2 substrates has been studied as a function of the Er3+ concentration in the 0.05–35 mol % range. Excitations at 800 nm, 980 nm, and 1.48 μm were used to obtain the decay of the I13/24→4I15/2 luminescence at 1.54 μm. The results have been compared with those reported for the Er3+ doped CaF2 bulk and correlated with the low temperature luminescence study of these layers in a back-scattering configuration. The I13/24 level lifetime was found to be almost constant (∼11 ms) for Er3+ concentrations up to 5 mol %. At higher concentrations, the lifetime decreases due to energy transfer, but remains longer than the lifetime measured in the bulk because of the different formation of aggregated centers induced by the MBE growth conditions. The strong increase of the luminescence intensity observed from 10 to 35 mol % Er3+ concentrations despite the decrease of the lifetime is discussed. Finally, end-fire coupling of a diode laser at 977 nm and propagation through 15 mm long waveguides have been observed for moderate Er3+ concentrations (6 mol %). Guided and side IR luminescence spectra have been obtained and contrasted.
Observation of spin-dependent transport and large magnetoresistance in La0.7Sr0.3MnO3/SrTiO3/La0.7Sr0.3MnO3 ramp-edge junctions
Authors: Kwon C. , Jia Q. X. , Fan Y. , Hundley M. F. , Reagor D. W.
Published: July 2002

We have fabricated ferromagnet-insulator-ferromagnet junctions using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. Pulsed laser deposition was used to deposit the multilayer thin films and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance is dependent on the relative orientation of the magnetization in the electrodes. A junction magnetoresistance (JMR) as large as 30% is observed at low temperatures and low fields. In addition, we have found that JMR is reduced at high temperatures (T>100 K) and decreases monotonically with increasing field at high fields (0.5 T<H<1 T). Possible causes for these are also discussed.
Magnetic anisotropy of doped manganite thin films and crystals
Authors: Suzuki Y. , Hwang H. Y. , Cheong S-W. , Siegrist T. , van Dover R. B. , Asamitsu A. , Tokura Y.
Published: July 2002

Magnetic anisotropy of La0.7Sr0.3MnO3 thin films and single crystals has been measured using vibrating sample and torque magnetometry. The magnetic anisotropy of thin films is dominated by strain anisotropy due to the lattice mismatch between the film and SrTiO3 substrate. The two and fourfold symmetries in the (110) and (001) films, respectively, can only be explained by stress anisotropy contributions (Kstress∼8.4×104 ergs/cm3). Magnetic anisotropy of La0.7Sr0.3MnO3 single crystals, subject to no external stress, reveals a uniaxial anisotropy in the (100) plane of K1∼1.8×104 ergs/cm3 that can be attributed to magnetocrystalline effects.
New applications of nanocrystalline Fe(Co–Fe)–Hf–O magnetic films to micromagnetic devices
Authors: Sato T. , Miura Y. , Matsumura S. , Yamasawa K. , Morita S. , Sasaki Y. , Hatanai T. , Makino A.
Published: July 2002

New applications of high resistivity magnetic films, such as Fe–Hf–O and Co–Fe–Hf–O as thin film inductors and flux gate magnetic sensors, were investigated. These soft magnetic films with nanocrystalline structure and high resistivity were deposited on glass substrate by using the oxygen reactive sputtering method. Planar sandwich thin film inductors developed by using Fe–Hf–O or Co–Fe–Hf–O films had a higher quality factor than that of an amorphous Co–Ta–Hf film inductor. The dc–dc converter using the Fe(Co–Fe)–Hf–O film inductor had higher conversion efficiency. These high resistivity magnetic films were also applied to a flux gate magnetic sensor operated at high frequencies over 1 MHz. Both high frequency excitation and high permeability (owing to the low eddy current loss) enabled us to increase the sensitivity, e.g., 39 mV/Oe/turn at 5 MHz when using a 3 μm thick Fe–Hf–O film (12×6 mm2 area).
Impedance magnetic resonance imaging: A method for imaging of impedance distributions based on magnetic resonance imaging
Authors: Ueno S. , Iriguchi N.
Published: July 2002

Magnetic resonance imaging (MRI) techniques have become important tools in medicine and biology. Conventional MRI, however, produces no information about the electrical properties of the body. This article proposes a new and noninvasive method for imaging electrical properties such as conductivity and impedance based on MRI techniques. The basic idea is to use the shielding effects of induced eddy currents in the body on spin precession. Two types of methods are introduced; (i) a large flip angle method, and (ii) a third coil method. The large flip angle method enhances the shielding effects of conducting tissues at the given Larmor frequency. The third coil method detects the shielding effects of conducting tissues at an arbitrary frequency. Both phantom and animal experiments have been carried out to verify this concept using a MRI system of 7.05 T with a bore size of 183 mm in diameter.
A new paradigm for two-dimensional spin liquids
Authors: Singh R. R. P. , Starykh O. A. , Freitas P. J.
Published: July 2002

Motivated by the geometry of the materials Na2Ti2As2O and Na2Ti2Sb2O, we study a square-lattice Heisenberg antiferromagnet, with spins located at the bond centers. The largest exchange constant J couples neighboring spins in a given row or column. This leads to a mesh of isolated spin chains running along the X and Y axes. A weaker exchange constant J′ couples the nearest-neighbor spins on the lattice. Classically, J′ fails to fix the relative spin orientation for different chains and hence the ground state is highly degenerate. Quantum order by disorder effect is studied by spin-wave theory and numerical methods. It is shown that a four-sublattice order is favored by quantum fluctuations. However, several arguments are presented that suggest that the ground state of the system remains disordered, thus providing us with a paradigm for a two-dimensional spin liquid.
The Meyer–Neldel rule in the conductivity of polycrystalline semiconducting FeSi2 films
Authors: Tassis D. H. , Dimitriadis C. A. , Valassiades O.
Published: July 2002

The electrical transport properties of polycrystalline semiconducting β-FeSi2 films have been evaluated by conductivity (σ) measurements over the temperature range 50–300 K. At low temperatures (T<200 K), a variable range hopping conduction was observed, from which the number of states near the Fermi level and the degree of disorder in the material were obtained. At moderate temperatures (200–300 K), the ln σ vs 103/T curves show anomalous features such as kinks or continuous bending. In this temperature range, the conductivity data satisfy the Meyer–Neldel rule, (MNR), which is of fundamental importance for the transport properties of the β-FeSi2. The results show that the MNR parameters are related with the degree of disorder in the material.
Limitations on distinguishing between representations of relaxation data over narrow frequency ranges
Authors: Snyder Chad R. , Mopsik Frederick I.
Published: July 2002

In this article, we examine the ability to distinguish between relaxation functions with data over a limited range of frequency. It is demonstrated that over these limited frequency ranges under a variety of conditions, the Cole–Cole equation can be used to fit data generated by the Havriliak–Negami equation. These results show that discerning between several very different broad relaxation functions fit to data obtained over narrow time or frequency ranges is nearly impossible within experimental accuracy. Therefore, the uniqueness of the fit parameters, and hence the ability to verify model predictions, is brought into question. Furthermore, as this conclusion is drawn from comparison of exact functions that experience no dispersion overlaps or instrumental systematic errors that can mask exact fits, the true situation with experimental data is even worse. The same conclusion can be applied to time domain data.