bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of the detonation products of lead azide. II. Formation of charged particles
Authors: Heflinger D. , Bar I. , Ben-Porat T. , Erez G. , Rosenwaks S.
Published: July 2002

The formation and temporal behavior of charged particles ensuing from the detonation of lead azide (LA) is studied. An ion probe tailored for measurements in the hostile environment produced following the detonation is described. The positive ions (most probably singly ionized lead atoms) and the electrons, which are simultaneously collected on separate electrodes, are embedded in the outer half of the expanding product cloud resulting from the detonation. They show similar temporal behavior and their maximum velocity is ∼4.5 km/s. The density of each of them at a distance of 2.5 cm from the LA sample is 4.3×1011–1.3×1012 cm−3 for detonation of 5–15 mg of LA, respectively. From the hydrodynamics of the expanding cloud and the density of the electrons, their temperature is estimated to be in the range 2600–3000 K. The results of the measurements are discussed in view of the mechanisms believed to govern the expansion of the product cloud following the detonation.
(GaAs)m(AlAs)n short-period superlattice quantum-well lasers
Authors: Chand Naresh , Dutta N. K. , Lopata J. , Hull R. , Geva M.
Published: July 2002

We report on the fabrication and performance characteristics of (GaAs)3/(AlAs)1 short-period superlattice (SPS) quantum-well lasers emitting at 737 nm. The SPSs consists of eight periods of 3 and 1 monolayers of GaAs and AlAs, respectively. The (GaAs)m/(AlAs)n SPSs have many advantages over their equivalent AlGaAs alloy counterparts. The broad-area threshold current density, Jth, for 500-μm-long lasers is 510 A cm−2. The 500-μm-long ridge waveguide lasers have a threshold current of 48 mA with a characteristic temperature of 68 K in the temperature range 19–60 °C. The external differential quantum efficiency near threshold is 0.58 mW/mA/facet. The devices lase in a single mode with spectral width within the resolution limit of the spectrometer.
Magnetic imaging of surfaces on the atomic scale by spin-polarized scanning tunneling microscopy (invited) (abstract)
Authors: Wiesendanger R.
Published: July 2002

Based on the direct observation of vacuum tunneling of spin-polarized electrons by means of a scanning tunneling microscope (STM) with a ferromagnetic CrO2 tip, it has been possible to probe the alternating magnetization of different terraces of a Cr(001) surface on a nanometer scale.1,2 More recently, we have achieved even atomic resolution in magnetic-sensitive STM studies of Fe3O4 (001) surfaces by using atomically sharp, in situ prepared Fe probe tips.3,4 An atomic-scale contrast between the different magnetic ions Fe(2+) and Fe(3+) in magnetite has been detected with these Fe tips which is not present by using nonmagnetic W tips.5 The experimental results based on magnetic-sensitive STM and local tunneling spectroscopy indicate the presence of a Wigner glass state at the Fe3O4 (001) surface at room temperature. The perspectives of atomic-resolution magnetic-sensitive STM studies for the characterization of magnetic surfaces, ultra-thin films, and multilayers in general will be discussed.
Spin-polarized electron tunneling from a ferromagnet into GaAs(110) (invited) (abstract)
Authors: Alvarado S. F.
Published: July 2002

A spin-polarized scanning tunneling microscope technique is presented. It is based on the detection on spin-polarization effects for tunneling between a ferromagnet and a semiconductor, the latter acting as a miniature spin detector. The spin detection is made by means of measuring the circular polarization of the radiation emitted from the semiconductor upon the radiative recombination of the minority carriers injected from the magnetic material, through the tunneling junction, into the semiconductor. Spin-polarized STM measurements were performed using a Ni tip and GaAs(110). We demonstrate the following effects on the spin polarization: (1) Sign change upon reversal of the magnetization of the tip. (2) Spectroscopy: polarization versus energy of the tunneling electrons. (3) Magnitude changes dependent on surface cleanliness of the tip apex. The data indicate that the polarization of the electrons tunneling from the Fermi level of Ni can have a polarization close to 50% in magnitude. This high value and the sign of the polarization indicate that 3d levels might be the dominant tunneling contribution. This has been cross-checked by experiments on Fe tips. We note that this technique intrinsically has nanometer lateral resolution determined by the diameter of the tunneling current filament. Imaging of magnetic domains would be realized by tunneling from a magnetic surface into a tip acting as the detector. The present technique is a fully new spin-polarized STM approach in that it involves tunneling from a magnetic into a nonmagnetic material. Thus, effects like magnetic forces and magnetostriction should be excluded from the interpretation of the results and from potential spin-polarized STM imaging. This might not be the case for tunneling between two magnetic materials.
Experimental testing of point-charge model of magnetic force scanning tunneling microscopy
Authors: Mayergoyz I. D. , Adly A. A. , Gomez R. D. , Burke E. R.
Published: July 2002

A magnetic force scanning tunneling microscopy technique for imaging of magnetization patterns is briefly described and a ‘‘point-charge model’’ for this technique is stated. This model is experimentally tested by comparing the measured images of some deliberately recorded magnetization patterns with the images theoretically computed for the same patterns on the basis of the point-charge model. The good agreement between experimentally measured and theoretically computed images is found. This agreement suggests the validity of the point-charge model.
Antiferromagnetic phase diagram of YBa2(Cu1−xFex)3Oy
Authors: Mirebeau I. , Bellouard C. , Hennion M. , Caignaert V. , Suard E.
Published: July 2002

Neutron-diffraction measurements have been performed in the YBa2(Cu1−xFex)3Oy system with 0.01≤x≤0.12 and y≂6.2. At low iron content, two magnetic transitions are observed with decreasing temperature and a noncollinear phase (AF1+AF2) at low temperature. This results from the occurrence of a very small magnetization μCu1 on the oxygen-deficient layers. Around x=0.03, the AF1 phase disappears, μCu1 remaining very small. At higher iron content, in the AF2 phase an increase of μCu1 with increasing x is observed, together with a spin-glass-like freezing of the iron spins. The results suggest that charge-transfer effects play a crucial role in the change of magnetic phases at low x values, whereas the magnetic effects of iron substitution dominate at higher concentrations.
Effects of grain size dispersion on noise in longitudinal thin film media
Authors: Min Tai , Zhu Jian-Gang
Published: July 2002

The effect of grain size dispersion on medium noise in longitudinal thin film recording media is studied by micromagnetic modeling. A previous micromagnetic model has been extended for modeling films with grain size variation. It is found that for the typical thin film media, moderate grain size dispersion in the film does not affect the media noise at low recording densities. At high recording densities, when adjacent transition boundaries percolate, the film with grain size dispersion exhibits a less degree of superlinear noise enhancement than the film with identical size grains. It is concluded that for films with medium noise dominated by magnetic interactions, moderate grain size dispersion slightly suppresses the collective magnetization processes at high recording densities, thereby, reducing medium noise.
Magnetic susceptibility study of the effect of diamagnetic doping on the magnetic ordering in the S=1/2 antiferromagnetic Cu1−x(Mg,Zn)x(HCO)2 4H2O (abstract)
Authors: Visser D. , Clarke S. J. , Harrison A.
Published: July 2002

The two-dimensional S=1/2 Heisenberg antiferromagnet copper(II) formate tetrahydrate has been doped with diamagnetic impurities, Mg2+, Zn2+, up to levels of 15%. The decrease of TN as function of the doping concentration has been compared to that of La2CuO4 and other 2D magnets. A discussion of recent ESR results and single crystal magnetic susceptibility data in view of the interaction between the magnetic ions is presented.
Synthesis, crystal structure, and magnetic properties, of a Cu(II) solid state compound derived from the 3,4,9,10-perylenetetracarboxylate anion and diethylenetriammine (abstract)
Authors: Valdes-Martinez Jesus , Cervantes-Lee Francisco , ter Haar Leonard W.
Published: July 2002

Several strategies have been used to develop novel magnetic materials. One of these is the simultaneous use of at least two different ligands, one an oxygen donor and the other a nitrogen donor. With this approach, using the optically interesting oxygen donor, 3,4,9,10-perylenetetracarboxylate anion (petca), and the tridentate nitrogen donor ligand, diethylenetriamine (dien), we recently reported1 the synthesis of the solid state Cu(II) compound, [Cu5(dien)5(petca)2H2O4] (ClO4)2⋅2H2O (1). This compound contains three different copper species, a chain [Cu2 (dien)2(petca)n, a dimer [Cu2(dien)2(petca)(H2O)2] and a monomer [Cu(dien)(H2O)2] and several exchange pathways. We wish to report, that the dimeric units in 1 have been isolated. The structure of the compound [Cu2(dien)2(petca)H2O)2]⋅13H2O (2), has been determined by an x-ray single-crystal diffraction study. Two different dimers are present in the unit cell: isolated dimer units, surrounded by water molecules; and dimers connected through hydrogen bonds in a way that resembles the chains in 1. In both cases, the Cu(II) ion is coordinated to one oxygen from the petca anion, the three nitrogen atoms of the dien, and a water molecule, in a nearly square planar geometry. The magnetic properties of this compound relate to these structural features.
Anisotropic magnetism in hybridizing uranium systems
Authors: Kioussis N. , Yu H. J. , Cooper B. R. , Sheng Q. G. , Wills J. M.
Published: July 2002

The isostructural uranium monopnictides and monochalcogenides have become prototype systems in actinide research with respect to their unusual magnetic properties. We have investigated the origins in the electronic structure of the variation in magnetic behavior as the degree of 5f-electron localization changes from localized to itinerant on going up the pnictogen or chalcogen column, thus decreasing the U-U separation. We have applied a synthesis of: (1) A phenomenological theory of orbitally driven magnetic ordering which includes both the hybridization-induced and the RKKY exchange interactions on an equal footing, and (2) Ab initio electronic structure calculations, based on the linear-muffin-tin-orbital method, allowing a first-principles evaluation of the parameters entering the model Hamiltonian. We have investigated systematically characteristic trends and changes of the 5f-state resonance width, the hybridization potential, and the hybridization-induced and RKKY exchange interactions with chemical environment, on going down the pnictogen or chalcogen column and on going from the weakly hybridizing pnictides to the more strongly hybridizing chalcogenides.
The computation of magnetic fields of permanent magnet cylinders used in the electrophotographic process
Authors: Mayergoyz I. D. , Furlani E. P. , Reznik S.
Published: July 2002

General analytic expressions for the magnetic field distributions created by permanent magnet cylinders are derived in this paper. These expressions take into account a finite slope of the B-H loop in the second quadrant, and they are valid for arbitrary magnetization patterns within the cylinders. They can be used for parametric studies of field strength and can also be used to test new and existing numerical techniques for computing the field due to permanent magnet materials. The theoretical discussion is illustrated by some sample calculations.
Critical scattering from erbium
Authors: Lin H. , Collins M. F. , Holden T. M.
Published: July 2002

Neutron scattering techniques have been used to measure the nature of the phase transition in erbium at TN=87 K from paramagnetism to an incommensurate longitudinally ordered phase with q=0.283 along c at TN. A high-purity crystal showed normal critical scattering in the paramagnetic state with a different correlation length in the basal plane and along c. It is found that γ=0.96±0.06 and both correlation lengths diverge with the same critical exponent ν=0.49±0.02 for a range of reduced temperature from 0.0015 to 0.07. In the ordered state, power-law behavior was observed with β=0.48±0.02 for reduced temperatures between 0.005 and 0.04. Below TN the intensity of the third harmonic varied as the cube of the reduced temperature. All these observations correspond to mean-field behavior.
Polarization analysis of the magnetic excitations in Invar and non-Invar amorphous alloys
Authors: Lynn J. W. , Rosov N. , Fish G.
Published: July 2002

Conventional spin wave theory works remarkably well in describing the spin dynamics of both Invar and non-Invar isotropic ferromagnets, with the important exception that for Invar systems the magnetization decreases much more rapidly with temperature than can be explained based on the measured spin wave dispersion relations. We have been carrying out triple-axis polarized inelastic neutron scattering experiments on the amorphous ferromagnets Fe86B14 (Invar system) and Fe40Ni40P14B6 (METGLAS■ 2826) in order to separate the longitudinal magnetic fluctuations from the transverse (spin wave) excitations, and thereby determine if the presence of longitudinal excitations might resolve this discrepancy. The present measurements exhibit longitudinal excitations below Tc, but in both materials. Possible interpretations of the results are discussed.
Magnetic interactions in homometallic chains M(4-cyanopyridine)2Cl2, M=Cu,Ni,Co
Authors: Zhang Wanru , Landee Christopher P. , Turnbull Mark M. , Willett Roger D.
Published: July 2002

The title compounds have been synthesized for the first time. The crystal structure of the copper compound has been determined to consist of chloride bibridged chains of copper ions with the 4-cyanopyridine (4-CNPy) ligands coordinated to the metal ions through the pyridine nitrogens in the axial sites. Chemical and powder x-ray analysis indicate that the compounds form an isostructural series. The powder susceptibilities have been measured between 2 and 300 K. Effects of either antiferromagnetic exchange (Cu) or ferromagnetic exchange (Ni,Co) within the chains have been seen. The copper data has been fit to the model of a S=1/2 Heisenberg antiferromagnetic linear chain with exchange constant J/k=−13.7(2) K. The nickel data has been fit to the model of a S=1 ferromagnetic Heisenberg linear chain with exchange constant J/k=+4.8(2) K. The cobalt data has been fit to the model of a S=1/2 ferromagnetic Ising linear chain with exchange constant J/k=+6.6(2) K. The Ni and Co compounds order antiferromagnetically at 7.2(2) and 2.1(2) K, respectively. Magnetic-field-induced phase transitions have been observed in the Ni and Co compounds. The critical fields extrapolated to T=0 are Hc=4.2(2) kOe and Hc=1.4(2) kOe for the Ni and Co compounds, respectively.
Magnetic properties of the Tb4+ ion in Li2TbF6
Authors: Guillot M. , El-Ghozzi M. , Avignant D. , Ferey G.
Published: July 2002

Both oxygen and fluorine have a partiality towards the stabilization of high oxidation states of rare earths such as Ce, Pr, and Tb. During the study of the MF-M′F4 (M=alkaline metal; M′ rare ion) the compound Li2TbF6 was found to constitute the only representative obtained under ambient pressure of a structural type, namely α-Li2ZrF6, known until now only through the high pressure form. Magnetic measurements were carried out over the 1.4–300 K temperature range in continuous magnetic fields up to 20 T produced by a Bitter magnet. At low temperature and in H higher than 15 T, the magnetization M was observed to be field independent and very close to the calculated Tb4+ free ion value, i.e., 7μB; on the other hand, the data do not present any departure from the Brillouin function of the pure 8S7/2 state. The reciprocal magnetic susceptibility obeys a Curie–Weiss law leading to an effective moment of 7.86μB. A very unusual behavior of the terbium ion which presents a 4+ oxidation state is concluded: to the best of our knowledge, it is the first observation of this form. The so-found spherical 4f7 electronic configuration is the most compatible with the high coordination number of eight. Finally, the discussion within the series Li2MIVF6 compounds to understand the influence of both electronic configuration and size effects is presented.
Characterization and growth of magnetotactic bacteria: Implications of clean up of environmental pollution
Authors: Bahaj A. S. , James P. A. B. , Ellwood D. C. , Watson J. H. P.
Published: July 2002

Magnetotactic bacteria possess a magnetic moment due the presence of membrane bounded crystals of magnetite, (Fe3O4) called magnetosomes within their structure. Through manipulation in an applied magnetic field it is possible to determine the size, speed, and magnetic moment of individual bacteria, and hence an average for a culture. Variations in these characteristics with growth have been measured, indicating the suitability of this particular magnetic spirillum for metal loading. A correlation between the increase in magnetic moment and iron uptake of this bacterium leads us to propose a new mechanism for the biomineralization of magnetite. This paper describes the characterization of a magnetotactic spirillum and the implications these results have for its use in the clean up of environmental pollution.
Modeling growth of Si1−xGex epitaxial films from disilane and germane
Authors: Malik Rajeev , Gulari Erdogan , Li Shin Hwa , Bhattacharya Pallab K.
Published: July 2002

A Langmuir–Hinshelwood-type kinetic model is developed for modeling growth of silicon–germanium alloys from disilane and germane on Si substrates. Gas source molecular beam epitaxy was employed to grow Si1−xGex films at various germanium fractions, x, in the alloy and at different temperatures. The model correctly predicts experimentally observed and previously reported behavior; a monotonic decrease with germanium fraction at higher substrate temperatures (700 °C) and a maximum in the growth rate for lower temperatures (550 °C and 610 °C).
Film thickness dependence of microwave surface resistance for YBa2Cu3O7 thin films
Authors: Mogro-Campero A. , Turner L. G. , Kadin A. M. , Mallory D. S.
Published: July 2002

Microwave surface resistance close to 10 GHz has been measured as a function of temperature for epitaxial thin films of YBa2Cu3O7 (YBCO) on LaAlO3 in the film thickness range of 0.2–0.8 μm. The films were made by a reduced-temperature post-anneal technique. The surface resistance (Rs) scaled to 10 GHz decreases with increasing film thickness as is expected due to the finite film thickness with respect to the magnetic penetration depth. Below about 70 K there is an increase in Rs for the thickest films, attributed to a change in microstructure from c axis normal to the substrate plane, to c axis in the plane of the substrate; the resulting minimum in Rs occurs at a film thickness of 0.6 μm. The critical current density (Jc) at 77 K is highest for the thinner films, so that films with the highest Jc do not have the lowest measured Rs. These results suggest that the optimum YBCO film thickness for microwave devices patterned from these films may be about 0.6 μm, depending on operating temperature.
Incoherent rotations in thin films with in-plane anisotropy
Authors: Nowak J.
Published: July 2002

The hard axis magnetization process in thin ferromagnetic films has been investigated by Lorentz electron microscopy. At the onset of an external magnetic field the local magnetization vectors of the individual domains rotate incoherently. To describe these rotations an internal field is introduced. The magnitude of this field and its evolution during the magnetization process are calculated in terms of experimentally measured directions of local magnetization. The internal field increases as the domains appear, attaining two extreme values and then gradually vanishes close to saturation. Its maximum value depends on the width of stripe domains arising when the film is magnetized along the hard axis. The internal field is the main factor determining the shape of the hysteresis loop.
A spectral flux method for solving the Boltzmann equation
Authors: Alam Muhammad A. , Stettler Mark A. , Lundstrom M. S.
Published: July 2002

A spectral method for solving the Boltzmann equation by the scattering matrix approach is presented. The algorithm discussed can be used to simulate both bulk and device properties with arbitrary field profiles. Although the primary goal is to reduce the data storage problem of the scattering matrix approach, many of the concepts and mathematical properties developed may be useful for other traditional spectral methods as well.
Dislocation bends in piezoelectric materials
Authors: Shintani K.
Published: July 2002

It is shown that J. Lothe’s theorem [Philos. Mag. 15, 353 (1967)] is also valid for piezoelectric materials by replacing the energy factor. Its proof follows L. M. Brown’s procedure [Philos. Mag. 15, 363 (1967)]. The forces on dislocation bends in lithium niobate are numerically calculated.
Investigations of semiconductor superlattices by depth-sensitive x-ray methods
Authors: Rhan H. , Pietsch U. , Rugel S. , Metzger H. , Peisl J.
Published: July 2002

Focusing on the structure determination of a GaInAs/InP superlattice (SL), the potential of grazing incidence diffraction (GID) to resolve structure parameters on a microscopic scale is compared to x-ray reflectivity and conventional x-ray diffraction (XRD) measurements. Usually, information on the density profile perpendicular to the surface is obtained by x-ray reflectivity and on lattice mismatch by XRD. Since the penetration depth of x rays is much larger than the total thickness of the SL these methods measure parameters averaged over the whole structure. Furthermore, the depth sensitivity of both methods is small in the case of extremly high thickness ratios of the sublayers within the SL period. These disadvantages can be overcome using GID geometry for which the information depth can be reduced by keeping the angle of incidence αi and exit αf of the x-ray beam with respect to the surface in the range close to the critical angle of total external reflection αC. This enables a depth-selective structure determination. As an example measurements and computer simulations corresponding to the three different methods of a lattice-matched (GaIn)As/InP SL are presented. The periodic length of the present SL and the thickness of the top layer was determined by all three methods to monolayer accuracy.
L-H interface improvement for ultra-high-efficiency Si solar cells
Authors: Kuznicki Z. T.
Published: July 2002

Several phenomena normally considered separately are taken into account simultaneously to model a new solar-cell structure. A planar substructure inserted in the cell emitter is provided at its faces with two L-H interfaces which modify recombination mechanisms inside the accumulation layers. The theoretical modelization and one-dimensional numerical simulation show its yield to be 35%–40%. In practice, fabrication would necessitate uniting recent technological advances in the same fine substructure to ensure: (i) absorption of low-energy photons; (ii) optical confinement of the infrared light; and (iii) formation of the L-H interfaces at its edges.
Segregation and interdiffusion of In atoms in GaAs/InAs/GaAs heterostructures
Authors: Kawai T. , Yonezu H. , Ogasawara Y. , Saito D. , Pak K.
Published: July 2002

The segregation and interdiffusion of In atoms in the GaAs/InAs/GaAs heterostructures were investigated by secondary-ion mass spectroscopy. When the 1-ML-thick InAs layer was grown in a layer-by-layer growth mode with no dislocations, the segregation of In atoms became marked with the increase of the growth temperature. However, the segregation was observed even at a relatively low growth temperature of 400 °C in molecular beam epitaxy. It was found that the segregation was markedly enhanced by dislocations near the heterointerface when thick InAs layers were grown in a three-dimensional island growth mode. The interdiffusion of In atoms toward the growth direction occurred after thermal annealing, which could be assisted by vacancies propagating from the film surface into the epilayer. It became apparent that the interdiffusion was effectively suppressed by a thin AlAs layer inserted in the GaAs cap layer.
Effect of H on Si molecular-beam epitaxy
Authors: Eaglesham D. J. , Unterwald F. C. , Luftman H. , Adams D. P. , Yalisove S. M.
Published: July 2002

In Si crystal growth by molecular-beam epitaxy (MBE) at low temperatures there is known to be an epitaxial thickness: an initially crystalline regime before the deposited film becomes amorphous. The predominant impurity in MBE is hydrogen, but the role of background H in low-temperature MBE has not previously been assessed. Here the effect of deliberate dosing of the Si surface with atomic H during low-T growth is studied. The epitaxial thickness is shown to be sensitive to very small additional H fluxes (≊10−9 Torr, i.e., an increase in H only marginally above ambient). With further increases in dose rate, the epitaxial thickness decreases as hepi=h0−k(ln PH). Using secondary-ion-mass spectrometry data on the segregated H at the interface, we argue that breakdown in epitaxy is not caused directly by the surface concentration of adsorbed impurities. It is deduced that very small concentrations of H may influence the Si surface diffusion rate. The possible effect of background H adsorption on previous experiments on Si steps and surface diffusion is discussed.
Interaction mechanisms of near-surface quantum wells with oxidized and H-passivated AlGaAs surfaces
Authors: Emiliani V. , Bonanni B. , Presilla C. , Capizzi M. , Frova A. , Chang Ying-Lan , Tan I.-Hsing , Merz J. L. , Colocci M. , Gurioli M.
Published: July 2002

The tunneling mechanism of electrons and holes to surface states from near-surface Al0.3Ga0.7As/GaAs quantum wells has been investigated by steady-state and time-resolved photoluminescence spectroscopy, near liquid-helium temperature, of the excitonic e1-hh1 transition in the well. The ensemble of the data, taken over a wide range of optical excitation levels, for various values of the tunneling-barrier thickness, and before and after passivation of the surface by hydrogen, allows a description both of the details of the tunneling mechanism and of the character and behavior of relevant surface states. The main results are summarized as follows: (i) steady-state tunneling is ambipolar, namely, separate for electrons and holes, rather than excitonic; (ii) Spicer’s advanced unified defect model for an oxidized GaAs surface, antisite-As donors as dominating surface traps, provides an appropriate description of the state distribution at the interface between AlGaAs and its oxide; (iii) hole accumulation in surface states, resulting from the nominally different unipolar tunneling probability for the two carriers (and increasing with excitation level), generates a dipole electric field across the tunneling barrier, extending into the well; (iv) hydrogenation efficiently passivates electron trapping in surface states, but not hole tunneling and the consequent generation of a surface field by illumination; (v) the experimental findings agree with a model for ambipolar tunneling based on a self-consistent quantum-mechanical approach.
Effect of threading dislocations on mobility in selectively doped heterostructures grown on Si substrates
Authors: Ohori T. , Ohkubo S. , Kasai K. , Komeno J.
Published: July 2002

We studied the effect of threading dislocation scattering on the mobility of a two-dimensional electron gas. To verify our theory, we grew Si-doped AlGaAs/GaAs selectively doped heterostructures with different dislocation densities by changing the number of thermal annealing cycles. The theory agreed well with our experimental results. Previous work on high electron mobility transistors (HEMTs) fabricated on Si indicated that the device characteristics are insensitive to the dislocation density. Our theory states that the room temperature mobility reduction by dislocations with a density below 108 cm−3 does not affect HEMT device performance, which is consistent with empirically known results.
Formation of ohmic contacts to p-type diamond using carbide forming metals
Authors: Nakanishi Jiro , Otsuki A. , Oku T. , Ishiwata O. , Murakami Masanori
Published: July 2002

The measurement of the specific contact resistance, ρC, and microstructural analysis at the metal/diamond interface were carried out for diamond with various acceptor concentrations, NA, in order to understand the carrier transport mechanism at the metal/diamond interface. The ρC measurements were carried out for polycrystalline boron-doped semiconducting diamonds which were prepared by the microwave plasma chemical vapor deposition. The acceptor concentrations, estimated by the boron concentrations measured by secondary ion mass spectroscopy, ranged from 3×1018 to 3×1020 cm−3. Ti and Mo films, which form carbides with diamond, were deposited on the diamonds using the electron-beam evaporation technique. The ρC values were measured by the circular transmission line method before and after annealing at temperatures in the range of 400–600 °C. The dependence of the ρC values on the acceptor concentrations suggested that the dominant transport mechanism was the field-emission for the diamond with NA around 1020 cm−3 and the thermionic-field-emission for the diamond with NA from 3×1018 cm−3 to 4×1019 cm−3. The ρC values of the Ti contacts were observed to decrease upon annealing, whereas those of the Mo contacts decreased gradually with increasing annealing temperature. However, the ρC values of both the Ti and Mo contacts reached at the same value of ∼1×10−6 Ω cm2 after annealing at 600 °C for the diamonds with NA higher than 1020 cm−3. Note that the ρC values of the Mo contact were extremely stable at high temperatures: the ρC values did not deteriorate after annealing at 600 °C for more than 3 h. The thermally stable molybdenum carbide (α-Mo2C) and amorphous layers were observed at the Mo/diamond interface after annealing at 600 °C by cross-sectional transmission electron microscopy and x-ray diffraction.
Efficient frequency doubling and locking of diode laser light in periodically segmented KTiOPO4 waveguide
Authors: Eger D. , Oron M. , Katz M. , Zussman A.
Published: July 2002

Simultaneous frequency doubling and Bragg reflection locking of the light of a GaAlAs diode laser was obtained by two separate sections of a periodically segmented waveguide of KTiOPO4. A blue light power of 3.6 mW at 429 nm was generated. The blue light was emitted in a circular beam with a low divergence angle of 5.7°.
Temperature dependence of the transverse lasing mode in vertical-cavity lasers
Authors: Deng H. , Lin C. C. , Huffaker D. L. , Deng Q. , Deppe D. G. , Rogers T. J.
Published: July 2002

Experimental data are presented along with calculations which describe the near-threshold transverse modes in AlAs/GaAs/InGaAs vertical-cavity surface-emitting lasers with two different Bragg reflector systems. The Bragg reflector systems are composed of either AlAs/GaAs quarter-wave stacks or a combination of AlAs/GaAs and ZnSe/CaF. The temperature dependence of the lasing mode is studied for three different structures, and it is shown that higher-order lasing modes are favored with strong cavity detuning when the cavity resonance exists at a wavelength greater than the bulk emission peak. Important aspects of the lasing behavior are described by calculating the spontaneous mode which occurs at the lasing wavelength.
Mechanical fatigue in thin films induced by piezoelectric strains as a cause of ferroelectric fatigue
Authors: Khachaturyan K.
Published: July 2002

It is proposed that the ferroelectric fatigue is caused by alternating mechanical stresses induced piezoelectrically. It is shown that the mechanical stresses that develop upon switching are sufficient to cause significant mechanical fatigue after the similar number of cycles after which the ferroelectric fatigue is typically observed. It is proposed that the degradation of the ferroelectric properties is a direct consequence of the mechanical fatigue. It is attempted to explain some of the reported correlations between ferroelectric fatigue and microstructure, temperature, test conditions, and electrode material. It is proposed that electromigration and chemomechanical effect are essential in the mechanism of the effect of electrode material on ferroelectric fatigue.
Intrinsic origin and composition dependence of deep-level defects at the inverted GaAs/AlxGa1−xAs interface grown by molecular-beam epitaxy
Authors: Krispin P. , Hey R. , Kostial H.
Published: July 2002

Electronic states in n-type GaAs/AlxGa1−xAs heterojunctions are studied by deep-level transient spectroscopy in the Al mole fraction range from 0.25 to 1.00. A series of four well-defined deep levels is characteristic of the inverted GaAs/AlxGa1−xAs interface (GaAs grown on AlxGa1−xAs) grown by conventional molecular-beam epitaxy under arsenic-stable conditions without growth interruption. It is shown that the series of four levels originates from intrinsic defects which are associated with arsenic vacancies and antisites. In particular, two charge states of the isolated arsenic vacancy VAs can be identified. Except for the sheet very close to the inverted heterointerface the n-type AlxGa1−xAs layers are found to be almost free of intrinsic deep levels in the entire composition range. For the investigated samples, the compositional dependence of the level energies shows that the arsenic vacancy is predominantly surrounded by Al atoms. From the compositional dependence of the level concentrations it follows that the arsenic vacancies at the interface are formed above a composition threshold of 0.25. The distinct distribution of intrinsic defects at the inverted GaAs/AlxGa1−xAs interface can be explained by a stable defect configuration near the AlxGa1−xAs surface during growth.
Generation and relaxation phenomena of positive charge and interface trap in a metal-oxide-semiconductor structure
Authors: Khosru Quazi Deen Mohd , Yasuda Naoki , Taniguchi Kenji , Hamaguchi Chihiro
Published: July 2002

Positive charge and interface trap generation during substrate hot-hole injection into the insulating layer of a metal-oxide-semiconductor (MOS) structure is investigated using p-channel MOS transistors with very thin oxide thicknesses in the range from 4.6 to 15 nm. Positive charge fluence in the oxide due to hole trapping is found to saturate for an injected hole density over 1016 cm−2. A model based on first-order trapping kinetics is presented to describe hole trapping phenomena including the dispersive nature of capture cross-section. A universal relationship between hole trapping and interface trap generation is observed experimentally, which implies direct involvement of holes in the formation of interface traps at the Si/SiO2 interface. A generation mechanism related with hole trapping in the oxide is discussed. Postinjection relaxation of the trapped holes showed strong dependence on both oxide field magnitude and polarity. Detrapping under positive gate voltage is consistent with the concept of direct tunneling discharge, while negative gate voltage relaxation occurs due to electron injection by Fowler–Nordheim mechanism. A new phenomena of time dependent annealing of interface traps has been observed after the end of hole injection, which contradicts the reports of time delayed interface trap generation.
Strong Stark effect of the intersubband transitions in the three coupled quantum wells: Application to voltage-tunable midinfrared photodetectors
Authors: Huang Yimin , Lien Chenhsin
Published: July 2002

The quantum-confined Stark effect in the AlInAs/GaInAs three-coupled-quantum-well (TCQW) structures is studied theoretically. The basic AlInAs/GaInAs TCQW structures are composed of three GaInAs quantum wells separated by two narrow AlInAs barriers. Results indicated that the three-depth TCQW structure exhibits a very large and near-linear voltage-control Stark shift for the energy of the 1→4 intersubband transition (ΔE41). The amount of the Stark shift of calculated intersubband energy separation ΔE41 is about 175 meV as the applied electric field varied from −90 to 90 kV/cm. This large Stark shift can be exploited for fabricating a voltage-tunable midinfrared photodetector operating in λ=3–5 μm atmospheric window region. By employing the enhanced Stark shift of the 1→4 intersubband transition in the three-depth TCQW structure, a highly sensitive voltage-tunable midinfrared photodetector is proposed. The operation of this device is based on the infrared absorption by electrons in the ground-state subband E1 transited to the third-excited-state subband E4 of the TCQW. Since the infrared radiation is absorbed via the intersubband resonance absorption (ℏω=E4−E1), the detected infrared wavelength can be tuned by the ΔE41 which can be adjusted by an applied electric field. The tunability of this three-depth TCQW structure has been studied theoretically. Based on the theoretical calculations, a tuning range from 3 to 5 μm is predicted for the three-depth TCQW structure by varying the applied electric field in the 90 to −80 kV/cm range.
Photoluminescence measurements on phosphorus implanted silicon: Annealing kinetics of defects
Authors: Othonos Andreas , Christofides Constantinos
Published: July 2002

Room-temperature photoluminescence measurements are performed in order to study the effect of thermal annealing on phosphorus implanted silicon wafers. Measurements are carried out at near band gap excitation with a Nd:YAG laser operating at 1.06 μm. Photoluminescence measurements are also carried out with 0.488 μm laser excitation. It was found that implantation conditions (dose and energy) and annealing temperature strongly influence the intensity of the photoluminescence signal. Contribution from the bulk silicon and the effects from the ion implantation to the photoluminescence signal are discussed.
Structure of 60° dislocations at the GaAs/Si interface
Authors: Vilà A. , Cornet A. , Morante J. R. , Ruterana P. , Loubradou M. , Bonnet R.
Published: July 2002

High-resolution electron microscopy technique has been applied to a detailed study of the 60° dislocations at the atomic layer molecular-beam-epitaxial GaAs/Si interface. Their deformation fields strongly interact with neighbor dislocations inducing irregular spacing between the cores and possible dissociations. Biatomic silicon steps were observed at the interface, but never inside 60° dislocation cores. Computer image simulation and elasticity calculations of the atomic displacement field have been used in order to determine the structure of the 60° dislocation; however, due to the Eshelby effect and to interaction with some neighbor dislocations, in many cases no theoretical model could explain some observations.
Ion energy effects in AlN thin films grown on Si(111)
Authors: Meng W. J. , Doll G. L.
Published: July 2002

We report growth of epitaxial AlN thin films on Si(111) in a N2 glow discharge using a modified direct current magnetron sputter source in ultrahigh vacuum. Using such a device, we show that the plasma potential of the glow discharge can be varied over a wide range without significant changes in film growth rate and ion density. We show further that changing the plasma potential significantly and systematically affects the lattice strain and the mosaic structure of the AlN films. We show significant straining of the AlN lattice and suggest the occurrence of AlN lattice damage when N+2 ions acquire energies in excess of +50 eV.
Theory of non-Heisenberg exchange: Results for localized and itinerant magnets
Authors: Mryasov O. N. , Freeman A. J. , Liechtenstein A. I.
Published: July 2002

A general formulation of intersite (layer) exchange coupling in bulk (multilayer) materials is proposed based on the treatment of the spin (layer) rotation as a perturbation in terms of the force theorem and multiple scattering theory. The expansion of the intersite (layer) exchange interaction energy gives expressions for the bilinear and biquadratic exchange. For metals, the approach is illustrated by linear muffin-tin orbital calculations of exchange coupling constants in the ferromagnetic 3D-metals and the fcc phase of bulk Fe. Long range oscillations of strongly volume dependent exchange coupling in fcc Fe appears to be the origin of the spin-density-wave instability in this metastable phase. The correctness of expressions for the limit of localized magnets is demonstrated by calculations for the antiferromagnetic insulator NiO. In contrast with other theories, this method can be used for both nonmagnetic and magnetic spacers in metallic multilayers, as is illustrated by calculations of the interfacial and interlayer exchange in Fe/Mn and Co/Mn.
Characterization of GaAs/InGaAs quantum wells using photocurrent spectroscopy
Authors: Barnes J. , Nelson J. , Barnham K. W. J. , Roberts J. S. , Pate M. A. , Grey R. , Dosanjh S. S. , Mazzer M. , Ghiraldo F.
Published: July 2002

We report on characterization studies of high quality metal-organic vapor phase epitaxy and molecular beam epitaxy grown GaAs/InGaAs quantum wells, set within p-i-n diodes, to determine the well widths, indium mole fractions, and conduction band offset. We present photocurrent spectra containing a larger number of transitions than revealed in photoluminescence or photoluminescence excitation experiments. The energies of these transitions have been modeled using a theoretical characterization tool known as ‘‘contouring,’’ which is used in this strained system for the first time. This has enabled determination of the conduction band offset in GaAs/InGaAs quantum wells, to a value between 0.62 and 0.64, for a range of indium fractions between 0.155 and 0.23. As a final, additional check on our results, we compare the field dependence of the e1-hh1 exciton transition energy with our theoretical calculations and find good agreement.
Field-ion scanning tunneling microscopy study of the atomic structure of 6H–SiC(0001) surfaces cleaned by in situ Si molecular beam etching
Authors: Li L. , Hasegawa Y. , Sakurai T. , Tsong I. S. T.
Published: July 2002

Field ion-scanning tunneling microscopy has been used to study 6H–SiC(0001) surfaces with Si adlayers on the Si-terminated surface, formed by in situ Si molecular beam etching at 950 °C. The as-cleaned surface showed a (∛×∛) reconstruction. The (2×3), (2∛×6√6), and (3×3) phases were formed by evaporating Si on this clean 6H–SiC(0001)-(∛×∛) surface. A Si(111) film 6 monolayers thick was also epitaxially grown on the 6H–SiC(0001)-(∛×∛) surface at 800 °C, and the surface exhibited the Si(111)–(7×7) reconstruction. A surface vacancy model for the (∛×∛) reconstruction is proposed, and a possible application of utilizing the various surface reconstructions to control the growth of different polytypes of SiC on the 6H–SiC substrate is discussed.
Facet and bulk heating of GaAs/AlGaAs high-power laser arrays studied in spatially resolved emission and micro-Raman experiments
Authors: Puchert R. , Bärwolff A. , Menzel U. , Lau A. , Voss M. , Elsaesser T.
Published: July 2002

Temperature profiles in the bulk and at the front facet of a 20 emitter GaAs/AlGaAs double-quantum-well laser array are studied by spatially resolved luminescence and micro-Raman spectroscopy. For optical output powers of about 1 W, the facet temperature of the individual emitters differs by up to 90 K. In contrast, the temperature distribution inside the resonator is highly uniform with temperature differences of less than 2 K. The facet temperature distribution correlates with the near-field intensity pattern of the laser array. Reabsorption of laser emission close to the facet and subsequent surface recombination of the photogenerated carriers represent the main heating mechanism.
Quasiparticle relaxation rates in Nb/AlOx/Nb tunnel junctions due to Abrikosov vortices
Authors: le Grand J. B. , Martin J. , Huebener R. P. , Hamster A. W. , Brons G. C. S. , Flokstra J.
Published: July 2002

Superconducting tunnel junctions are promising detector elements in next generation x-ray, UV, and optical detectors. Quasiparticles which approach the neighborhood of the normal core may scatter and emit a phonon, thus being trapped in the Abrikosov vortex (AV). In this way the existence of AVs reduces the lifetime of quasiparticles. In this experiment excess quasiparticles in a Nb/AlOx/Nb junction are created by means of a low temperature scanning electron microscope. The trapping rate of quasiparticles into AVs is determined by measuring the quasiparticle lifetime as a function of the AV concentration. This results in an effective trapping radius of the AV of 14.8±1.4 nm, which is slightly less than theoretical expectations.
Faraday effect of sodium borate glasses containing divalent europium ions
Authors: Tanaka Katsuhisa , Fujita Koji , Soga Naohiro , Qiu Jianrong , Hirao Kazuyuki
Published: July 2002

Faraday effect measurements have been carried out at room temperature for some sodium borate glasses containing Eu2+ ions prepared under a reducing atmosphere. The wavelength dependence of the Verdet constant is analyzed in terms of a theory derived by Van Vleck and Hebb to obtain effective transition wavelength of the 4f7→4f65d transition which causes the Faraday effect of Eu2+ in the visible range. The effective transition wavelength increases with an increase in the optical basicity of glass. This phenomenon is explained in terms of the crystal field theory; because the splitting of 5d levels is larger in the glass with larger optical basicity, the energy required for the 4f7→4f65d transition decreases as the optical basicity of glass increases. The magnitude of Verdet constant increases with an increase in the effective transition wavelength when the concentration of EuO remains constant.
Er in molecular beam epitaxy grown GaAs/AlGaAs structures
Authors: Gusev O. B. , Prineas J. P. , Lindmark E. K. , Bresler M. S. , Khitrova G. , Gibbs H. M. , Yassievich I. N. , Zakharchenya B. P. , Masterov V. F.
Published: July 2002

Er-doped GaAs/AlGaAs structures were grown by the molecular beam epitaxy technique with concentrations of Er in the range 1017–2×1019 cm−3. Photoluminescence (PL) of Er3+ ions and Er-induced defects was studied at liquid helium and higher temperatures. A strong diffusion of erbium and interdiffusion of gallium and aluminum ions are observed [at the boundary of GaAs/AlGaAs quantum wells (QWs)] which leads at high erbium concentrations to degradation of the QW’s and macroscopic (average) leveling of erbium and aluminum concentrations over the whole semiconductor structure. From high-resolution PL spectra the existence of three types of Er centers is deduced, which differ by positions of fine structure lines, PL lifetimes, and temperature dependence. The results indicate that these centers are accompanied by the appearance of three types of carrier traps with binding energies of 20, 50, and about 400 meV, respectively. The experiments show evidence that carriers captured into these traps control the Auger excitation of Er ions assisted by multiphonon emission of local phonons. Temperature quenching of erbium PL is controlled by depopulation of defect states in the case of Auger excitation via the most shallow hole trap (20 meV) and by competition of multiphonon nonradiative capture with the Auger transitions in the case of the deepest defect (400 meV). De-excitation processes of excited f electrons accompanied by generation of electron-hole pairs were also considered. Such processes are especially important in the case of resonance excitation. Besides erbium PL at 1.54 μm, PL of erbium ions was observed from upper excited states at 0.82 and 0.98 μm, which demonstrates the possibility of realizing a three-level scheme of light emission.