bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic field dependent Hall data analysis of electron transport in modulation-doped AlGaN/GaN heterostructures
Authors: Dziuba Z. , Antoszewski J. , Dell J. M. , Faraone L. , Kozodoy P. , Keller S. , Keller B. , DenBaars S. P. , Mishra U. K.
Published: July 2002

Experimental magnetic field dependent Hall and resistivity data is presented for two modulation-doped AlGaN/GaN heterostructures in the temperature range from 6 to 300 K and for a magnetic field up to 12 T. The mobility and concentration of electrons within the two-dimensional electron gas (2DEG) at the AlGaN/GaN interface and within the underlying GaN layer are readily separated and characterized using quantitative mobility spectrum analysis. The observed transport parameters of the 2DEG are explained using the classical band theory for a degenerate electron gas. Analysis of the temperature dependencies of mobility and electron concentration in the GaN layer and 2DEG indicates that electron transport in the GaN layer is dominated by carriers in the conduction band for the case of low-doping (<1017 cm−3), and by conduction via an impurity band for highly doped material (>1018 cm−3). The simultaneous analysis of the multilayer AlGaN/GaN structure applied in this work renders the results applicable directly to modulation-doped field-effect transistors based on similar structures.
The importance of Grüneisen parameters in developing interatomic potentials
Authors: Porter Lisa J. , Justo Joao F. , Yip Sidney
Published: July 2002

We show that three representative interatomic potential functions for Si fail to describe properly the Grüneisen parameters γn, although one model is able to give a satisfactory account of the thermal expansion coefficient, α, by virtue of a fortuitous cancellation of errors in γn for the acoustic and optic modes. This suggests that in developing robust interatomic potentials intended to predict the thermomechanical behavior of crystals, the database used in parameter adjustment should include the fundamental quantities, γn, rather than the composite quantity α.
Ion mass and temperature dependence of damage production in ion implanted InP
Authors: Wendler E. , Opfermann T. , Gaiduk P. I.
Published: July 2002

Ion beam induced damaging and amorphization of crystalline InP is investigated. 100 keV B+, 300 keV Si+, 200 keV Ar+ and 600 keV Se+ ions are implanted into 〈100〉 InP at temperatures ranging from 80 K to 420 K. The implanted layers are analyzed using Rutherford backscattering spectrometry in channeling configuration, cross section transmission electron microscopy and optical spectroscopy in the sub-gap frequency region. The temperature dependence of damage production can be represented assuming a thermally stimulated defect diffusion within the primary collision cascades, resulting in a shrinkage of the remaining defect clusters. At a critical temperature T∞ these clusters dissolve completely and only point defect complexes nucleate. Then, amorphization occurs only at very large ion fluences (≈1016cm−2) and the process seems to be influenced by the high amount of implanted ions. A defect band forms around the projected range of the implanted ions, which may act as a diffusion barrier for point defects. The range of T∞ from ≈350 K for B+ and ≈420 K for Se+ ions corresponds to the annealing stage II of defects in InP, which can be related to the mobility of phosphorous interstitials. This indicates that phosphorous interstitials play an important role during ion irradiation of InP.
1/f
noise in anisotropic and giant magnetoresistive elements
Authors: van de Veerdonk R. J. M. , Beliën P. J. L. , Schep K. M. , Kools J. C. S. , de Nooijer M. C. , Gijs M. A. M. , Coehoorn R. , de Jonge W. J. M.
Published: July 2002

Microfabricated magnetoresistive elements based on either the anisotropic or the giant magnetoresistance effect were tested for their frequency dependent resistance noise behavior at room temperature in a dc magnetic field, using a dc sense current. Thermal resistance noise was the dominant noise source above about 10 kHz. At low frequencies the resistance noise was found to be dominated by a 1/f contribution that depends on the applied magnetic field. The 1/f noise is relatively low and field independent when the element is in a saturated state and contains a relatively large and field dependent excess contribution when the magnetic field is in the sensitive field range of the element. The 1/f noise level observed in saturation is comparable to the 1/f noise level found in nonmagnetic metals; the excess noise has a magnetic origin. The variation of the excess noise level with the applied dc magnetic field can be explained qualitatively using a simple model based on thermal excitations of the magnetization direction.
Domain pattern formation in epitaxial rhombohedral ferroelectric films. II. Interfacial defects and energetics
Authors: Romanov A. E. , Lefevre M. J. , Speck J. S. , Pompe W. , Streiffer S. K. , Foster C. M.
Published: July 2002

The coherency defect technique is developed for the domain pattern energetics in rhombohedral (001) epitaxial ferroelectric films. The coherency defects that are necessary to maintain the epitaxy during the ferroelectric phase transition are considered to be the only sources of elastic strains and stresses (and, correspondingly energy) in the film/substrate system. The coherency defects include: (i) a uniform distribution of edge dislocations which are responsible for the in-plane tension or compression and have Burgers vectors parallel to the interface; and two kinds of mesoscale defects: (ii) Somigliana screw dislocations which are responsible for in-plane shear; and (iii) wedge disclinations which are responsible for the out of plane rotations in neighboring domains. Using this approach, analytical expressions were found for the elastic energy in the film/substrate system for both the {101}-ri/rj and the {100}-ri/rj domain patterns. These two configurations differ by the orientation of domain walls, coherency defect content, and also the morphology of the free surface (flat versus puckered surfaces). Calculations are performed for screened mesoscale coherency defect configurations that represent a single embedded domain pattern and multidomain patterns. The following mesoscale defect configurations are used for these calculations: Somigliana dislocation dipoles, wedge disclination dipoles, Somigliana dislocation quadrupoles, and disclination quadrupoles. It is predicted that there is no critical thickness for domain pattern formation in rhombohedral ferroelectric epitaxial films. Agreement is shown between experimentally observed domain widths and theoretically predicted values.
Deep states in nitrogen-doped p-ZnSe
Authors: Funato Mitsuru , Fujita Shizuo , Fujita Shigeo
Published: July 2002

A comparative study on deep levels in p-ZnSe grown by molecular beam epitaxy (MBE), metalorganic MBE (MOMBE), and metalorganic vapor phase epitaxy (MOVPE) has been carried out to elucidate the nature of deep levels. A hole trap level which is located at 0.38 eV above the valence band is inevitably introduced into p-ZnSe by nitrogen doping. The formation of other hole traps, which are associated with native defects due to nonstoichiometry of ZnSe, depends on growth methods and conditions. Electron traps were also observed in MOMBE and MOVPE-grown samples. The absence of the electron trap levels in MBE samples indicates that the traps originate from insufficient decomposition of metalorganic source precursors. These findings illustrate the necessity of forming p-ZnSe (1) under stoichiometric conditions, (2) with novel growth/process techniques to suppress evaporation of Zn and Se, and, (3) with well-decomposed metalorganic sources, in order to achieve p-type doping with a low degree of compensation by hole and electron traps.
Vertical metrology using scanning-probe microscopes: Imaging distortions and measurement repeatability
Authors: Edwards Hal , McGlothlin Rudye , U Elisa
Published: July 2002

We report a study of the repeatability as limited by instrumental imaging distortions in scanning-probe microscope (SPM) measurements of the heights of nominal 44 and 88 nm steps in calibration artifacts. By imaging the same series of locations on different days, we are able to distinguish sample variations from variations originating in the imaging process. In particular, the value and repeatability of the measured step heights are found to depend upon the algorithm used to infer the step height from the SPM image. The three general approaches tested are: a manual single-point method, which represents the most commonly used practice in the SPM community; a histogram-based method, which is available in commercially available SPM image-analysis software; and the polynomial step-function fit (PSFF), which explicitly removes image bow and sample tilt. Factors related to variations in the sample measured such as image size and location on the sample cause up to 10% variations in the step-height measurements. Methods of statistical analysis of variance are used to separate sample variations from instrumental and algorithmic variations. Once image bow and sample tilt have been corrected properly using PSFF, SPM instrument-related variations are under 1%. Without PSFF, the step height exhibits a systematic dependence upon sample tilt and image bow which distorts the measured step height. In our study, these distortions occur at a level of up to 2% of the step height. However, these step-height distortions can be far more extreme in some cases. We show an example of a step-height measurement in which the image bow is so extreme that the step can barely be identified without first correcting the image bow. We also show that, for small images of a rough sample, tip wear can contribute a significant systematic error in the step height measurement, of up to 1% per image in this case, which can lead to large cumulative errors if the tip is not changed often enough. Thus, understanding SPM image distortions and their effect on step-height measurement repeatability is crucial to SPM metrology.
Monte Carlo study of electron initiated impact ionization in bulk zincblende and wurtzite phase ZnS
Authors: Bellotti E. , Brennan K. F. , Wang R. , Ruden P. P.
Published: July 2002

This paper presents a theoretical study of the high field electronic transport properties of the cubic and hexagonal phases of zinc sulfide (ZnS) using an ensemble Monte Carlo method. Essential features of the model are the inclusion of realistic energy band structures calculated from a local pseudopotential method and numerically calculated impact ionization transition rates. The polar optical phonon scattering rate has also been computed numerically from the band structure. The relevant transport quantities have been computed for field values between 100 kV/cm and 2 MV/cm. On the basis of these calculations it is predicted that the electron distribution is cooler and the average energy lower in the wurtzite phase than in the zincblende phase over the entire field range examined. The difference in average energy between the two phases becomes pronounced for field magnitudes above 1 MV/cm while it is smaller in the field range between 700 kV/cm and 1 MV/cm. As a result, the ionization coefficients are expected to be higher in the zincblende phase than in the wurtzite phase. This can be attributed to differences in the density of states between the two polytypes. The quantum yield has also been computed. It is found that even though the threshold for impact ionization is relatively hard in both polytypes, the threshold for the wurtzite phase is harder than the threshold for the zincblende phase.
Inelastic electron tunneling into p-Bi2Te3 crystals
Authors: Nagao Jiro , Hatta Eiji , Mukasa Kōichi
Published: July 2002

Inelastic electron tunneling experiments have been performed in p-Bi2Te3–Al oxide-Al junctions. Strong peaks due to electron-acoustic phonon coupling are observed in the second derivative tunnel spectrum. These phonon structures are closely related to the effective Debye temperature. This gives direct evidence that the electron-acoustic phonon scattering plays a dominant role in p-Bi2Te3.
Material and n-p junction properties of N-, P-, and N/P-implanted SiC
Authors: Gardner Jason A. , Edwards Andrew , Rao Mulpuri V. , Papanicolaou N. , Kelner G. , Holland O. W. , Capano M. A. , Ghezzo M. , Kretchmer J.
Published: July 2002

Elevated temperature (ET) multiple energy N, P, and N/P implantations were performed into p-type 6H-SiC epitaxial layers. For comparison, room temperature (RT) N and P implantations were also performed. In the N/P coimplanted material a sheet resistance of 2.1×102 Ω/□ was measured, which is lower compared to the values measured in N or P implanted material of the same net donor dose. The RT P implantation resulted in heavy lattice damage and consequently low P electrical activation, even after 1600 °C annealing. After annealing the Rutherford backscattering yield either coincided or came close to the virgin level for ET implantations and RT N implantation, whereas for RT P implantation the yield was high, indicating the presence of high residual damage. Vertical n-p junction diodes were made by selective area ET N, P, and N/P implantations and RT N and P implantations using a 2.5 μm thick SiO2 layer as an implant mask. The diodes were characterized by capacitance–voltage and variable temperature current–voltage measurements.
InGaAs/GaAs quantum nanostructure fabrication on GaAs (111)A vicinal substrates by atomic layer epitaxy
Authors: Lee Jeong-Sik , Isshiki Hideo , Sugano Takuo , Aoyagi Yoshinobu
Published: July 2002

InGaAs/GaAs quantum wire (QWR) and dots were fabricated on GaAs (111)A vicinal substrates by the atomic layer epitaxy (ALE) technique. In0.25Ga0.75As QWR structures were formed on metal–organic vapor phase epitaxy grown homogeneous multiatomic height steps on GaAs (111)A vicinal substrates misoriented toward the [112̄] direction. In photoluminescence measurement only one spectrum, which shows strong polarization dependence, was observed. The result and its narrow full width at half maximum of 10 meV imply that the elimination of the wetting layer and excellent size uniformity are realized by the ALE technique. Spontaneous alignment of InGaAs quantum dots was also achieved by the ALE method. Boxlike shaped dot arrays of which height is restricted by the step height indicate that the ALE growth on the (111)A vicinal surface has an effect on not only the arrangement but also on the size and shape control of quantum nanostructures.
The effect of particle size and consolidating pressure on the magnetic properties of Sm(Co, Fe, Cu, Zr)z bonded magnets
Authors: Ma B. M. , Gong W. , Bounds C. O. , Chen C. H. , Walmer M. S.
Published: July 2002

The effect of particle size on the Br, BHmax, and specific density of Sm(Co0.62Fe0.29Cu0.07Zr0.02)8.0 and Sm(Co0.70Fe0.21Cu0.07Zr0.02)8.0 bonded magnets has been systematically investigated. It was found that magnets made of powders with a top size less than 53 μm have a low specific density and, hence, low Br and BHmax values. Magnets made of powders of about 250 μm top size, with a composition of Sm(Co0.62Fe0.29Cu0.07Zr0.02)8.0, yield a specific density of 7.0 gm/cc (84% of theoretical density), a Br of more than 8.1 kG, and a BHmax of about 15 MGOe. A slight decrease in Br and BHmax was noticed when the particle top size was increased beyond 710 μm. This decrease in Br and BHmax may be attributed to the imperfect grain alignment arising from the fact that each particle may consist of multigrain grains with a random orientation. Because of the slightly lower Fe content, magnets with a composition of Sm(Co0.70Fe0.21Cu0.07Zr0.02)8.0 exhibit slightly lower Br and BHmax values when compared to those of Sm(Co0.62Fe0.29Cu0.07Zr0.02)8.0. However, the relationships of Br and BHmax to particle size are nearly identical for both compositions. When the powder top size was fixed at about 250 μm, a consolidating pressure of 8 T/cm2 appears to be sufficient to obtain magnets with a specific density of 6.9 gm/cc and with a reasonable grain alignment. For Sm(Co0.62Fe0.29Cu0.07Zr0.02)8.0 bonded magnets, a Br of 8.5 kG, Hc of 8.5 kOe, Hci of 23.0 kOe, BHmax of 16.7 MGOe, and Hk of 8.0 kOe were obtained.
A variational approach to exchange energy calculations in micromagnetics
Authors: Donahue M. J.
Published: July 2002

This article presents a magnetization interpolation method for micromagnetic exchange energy calculations using a variational procedure to relax spins on a supplemental (refined) lattice. The approximations implicit in standard micromagnetic discretization schemes fail when angles between neighboring spins in the model become large, but the proposed approach effectively reduces the angle between neighboring spins, alleviating many of the associated problems. Moreover, this method does not introduce excessive discretization-induced vortex pinning observed with some large angle exchange energy formulations. This article includes details on proper post-interpolation exchange torque calculation, bounds on nearest-neighbor angles for interpolated lattices, a simple model predicting discretization-induced Néel wall collapse, and an example of a collapsed (1 cell wide) domain wall that can be restored by the proposed technique.
Magnetic and transport properties of radiation damaged La0.7Ca0.3MnO3.0 thin films
Authors: Browning V. M. , Stroud R. M. , Fuller-Mora W. W. , Byers J. M. , Osofsky M. S. , Knies D. L. , Grabowski K. S. , Koller D. , Kim J. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

Magnetization and resistivity measurements are reported for a series of radiation damaged La0.7Ca0.3MnO3 pulsed laser deposited thin films. When plotted as a function of activation energy, trends in the electrical transport properties are similar to those exhibited in the magnetic properties. A sharp drop in both Tc and Tp in samples with activation energies greater than ∼110 meV suggests a “decoupling” of the magnetic and transport properties. The results suggest the magnetic order is no longer sufficient to delocalize the system of the extra disorder induced by the radiation damage.
Structure and soft magnetic properties of sputter deposited MnZn-ferrite films
Authors: Gillies M. F. , Coehoorn R. , van Zon J. B. A. , Alders D.
Published: July 2002

In this paper we report the soft magnetic properties of thin films of sputtered MnZn ferrite deposited on thermally oxidized Si substrates. A high deposition temperature, 600 °C, together with the addition of water vapor to the sputtering gas was found to improve the initial ac permeability, μ. The highest value obtained was approximately 30. For MnZn-ferrite films with much larger grain sizes, as obtained by deposition on a polycrystalline Zn-ferrite substrate, a μ of 100 was obtained. The results are discussed in terms of the so-called nonmagnetic grain boundary model.
Atomistic analysis of the vacancy mechanism of impurity diffusion in silicon
Authors: List S. , Ryssel H.
Published: July 2002

The complete set of the four macroscopic transport coefficients describing the coupled diffusion of impurity atoms and vacancies in silicon is calculated from the atomistic mechanism by accurately taking into account the effects of the microscopic forces between dopants and vacancies. The aim of these simulations is to come to a decision concerning the validity of models like the pair diffusion model [e.g., M. Yoshida, J. Appl. Phys. 48, 2169 (1977); R. B. Fair and J. C. C. Tsai, J. Electrochem. Soc. 124, 1107 (1977); F. F. Morehead and R. F. Lever, Appl. Phys. Lett. 48, 151 (1986); B. J. Mulvaney and W. B. Richardson, Appl. Phys. Lett. 51, 1439 (1987)] or the “non-Fickian diffusion” model [M. Kurata, Y. Morikawa, K. Nagami, and H. Kuroda, Jpn. J. Appl. Phys. 12, 472 (1973); Y. Morikawa, K. Yamamoto, and K. Nagami, Appl. Phys. Lett. 36, 997 (1980); V. V. Kozlovski, V. N. Lomasov, and L. S. Vlasenko, Radiat. Eff. 106, 37 (1988); O. V. Aleksandrov, V. V. Kozlovski, V. V. Popov, and B. E. Samorukov, Phys. Status Solidi 110, K61 (1988), K. Maser, Exp. Tech. Phys. (Berlin) 34, 213 (1986), K. Maser, Ann. Phys. (Leipzig) 45, 81 (1988), K. Maser, Exp. Tech. Phys. (Berlin) 39, 169 (1991)] that make contradicting predictions for very fundamental properties like the relative direction of the fluxes of dopants and vacancies driven by a vacancy gradient and for the relation α=Td0/Dd0 between two of the four transport coefficients. Simulation results are shown for a variety of assumed interaction potentials that establish a functional dependence between α and measurable quantities, like the factor Dd/Dtracer of enhancement of dopant diffusivity over tracer diffusion, that holds for an arbitrary interaction. The comparison with experimental values for Dd/Dtracer leads to confirmation of the pair diffusion model for boron and phosphorous. For arsenic and antimony, the large scatter of the experimental data prohibits an equally definite conclusion, but at least a qualitative confirmation of pair diffusion theory (i.e., α>0 which means that dopant and vacancy fluxes have the same direction if caused by a vacancy gradient) is possible.
Frequency-resolved microwave reflection photoconductance
Authors: Romanowski A. , Buczkowski A. , Karoui A. , Rozgonyi G. A.
Published: July 2002

The effect of the carrier recombination process in silicon on the microwave reflection coefficient is analyzed in the frequency domain. The process is described using a two level recombination/trapping model. Carrier recombination kinetics are characterized by four parameters, two of which are related to the recombination and the other to the trapping processes. These parameters are evaluated for Czochralski silicon wafers based on Nyquist plots. In the evaluation procedure, a nonlinear simplex method is used for fitting the experimental data to the model.
Partitioning of Ga and Co atoms in a Fe3B/Nd2Fe14B nanocomposite magnet
Authors: Ping D. H. , Hono K. , Hirosawa S.
Published: July 2002

Partitioning of Co and Ga atoms in nanocrystalline Nd4.5Fe73B18.5Co3Ga1 at various stages of crystallization has been studied by atom probe field ion microscopy. In the early stage of crystallization, Co and Ga atoms are rejected from primary particles of the soft magnetic Fe3B phase and partitioned to the amorphous matrix phase. These atoms are enriched at the Fe3B/amorphous interfaces in this stage. Three crystalline phases (Fe3B, Nd2Fe23B3, and Nd2Fe14B) are present in the fully crystallized state, and Ga partitions preferentially only to the Nd2Fe14B phase. In a fully crystallized Fe3B/Nd2Fe14B nanocomposite with optimum magnetic hardness, Co and Ga atoms are partitioned to the Nd2Fe14B phase, and evidence for a slight enrichment of Ga atoms at the Nd2Fe14B/Fe3B interface has been found. Based on these results, the role of Ga and Co additions for improved hard magnetic properties is discussed.
A study of the Franz–Keldysh oscillations in electromodulation reflectance of Si-delta-doped GaAs by a fast Fourier transformation
Authors: Chang W.-H. , Hsu T. M. , Lee W. C. , Chuang R. S.
Published: July 2002

The line shapes of electromodulation spectra exhibit extended Franz–Keldysh oscillations and interference beats in the presence of a uniform built-in electric field. For this work, we used photoreflectance and contactless electroreflectance to study the Franz–Keldysh oscillations in Si-delta-doped GaAs. The fast Fourier transformation taken to the photoreflectance and contactless electroreflectance spectra, produced more complicated results than were observed in previous studies, when the effect of the modulation field is nonnegligible. This indicates that the interference beats are not only due to different effective heavy-hole and light-hole mass but also to the modulation field. We propose that the Franz–Keldysh oscillations generally contain four frequencies, which correspond to the heavy-hole and light-hole splitting. A comparison between the experiments and some numerical simulations attest to the validity of our proposal. This line shape analysis could then be applied to estimate the strength of a modulation field in contactless electroreflectance and photoreflectance.
Effect of substrate temperature on Si compensation in δ-doped InSb and AlxIn1−xSb grown by molecular beam epitaxy
Authors: Liu W. K. , Goldammer K. J. , Santos M. B.
Published: July 2002

Dopant compensation was studied for Si δ-doped InSb samples grown on GaAs (001) substrates. Hall-effect measurements indicate a sharp decline in electron density with increased substrate temperature when doping and cap-layer growth occur on the pseudo-(1×3) surface reconstruction, while little temperature dependence is observed for doping and growth on the c(4×4) surface reconstruction. Hall-effect measurements on samples grown with the substrate temperature differing between the dopant and cap layers rule out simple diffusion and desorption of Si atoms, and, along with secondary-ion mass spectrometry measurements, suggest that the temperature dependence of the carrier density results from compensation occurring primarily during growth of the cap layer. Similar behavior was observed in AlxIn1−xSb samples δ-doped with Si.
Dependence of gate control on the aspect ratio in metal/metal-oxide/metal tunnel transistors
Authors: Buot F. A. , Rendell R. W. , Snow E. S. , Campbell P. M. , Park D. , Marrian C. R. K. , Magno R.
Published: July 2002

The design criteria for large transconductance/high output impedance or high-gain operation of metal-oxide tunneling transistors is given. The dependence of the gate control on the aspect ratio of thickness to width of the tunneling oxide is investigated by computer simulation. This device structure can only operate similar to conventional semiconductor transistors for aspect ratio considerably less than one. It ceases to function as a transistor for larger aspect ratio due to insufficient penetration of the gate control field into the tunneling oxide. To demonstrate this, the current–voltage characteristics are computed for aspect ratios equal to 7/30, 1, 21/10, and the different tunneling-current behaviors compared with our experimental results on Ti/TiOx/Ti and Nb/NbOx/Nb tunnel transistors.
Modeling the conductivity of highly consolidated, bi-connected porous rocks
Authors: Chinh Pham Duc
Published: July 2002

The realistic interconnected pore space geometry of certain fluid-saturated rocks is modeled as a suspension of open spherical water pockets in the remaining rock matrix, intersected by a random network of thin cylindrical tubes. The pockets, connected to the tubes, contribute much to the porosity, while the tubes dominate the electrical conductivity of the rocks. An unsymmetrical effective medium approximation scheme is used to derive the conductivity equations for the model, which are consistent with Archie’s empirical law for consolidated rocks.
Ultrafast coherent oscillations of electron wave packet in a step-doped stepped quantum well structure
Authors: Mohaidat Jihad M.
Published: July 2002

The temporal development of an electron wave packet initially photoexcited in the region with smaller band gap of a stepped quantum well is investigated numerically by directly solving the time-dependent Schrödinger equation. It is shown that, if the photoexcitation region is about 55% to 65% of the total quantum well width, then coherent ultrafast oscillations will occur between the photoexcitation region and the rest of the well region. The period of these ultrafast oscillations increases exponentially with the quantum well width. In addition, the oscillation period ranges between 15 fs for a 50 Å quantum well width and 700 fs for a 500 Å quantum well width. The above coherent oscillations are also found to be insensitive to variations in the quantum well depth.
Excitonic states of weakly confining quantum wires
Authors: Jaziri S. , Ferreira R.
Published: July 2002

We consider theoretically the excitonic levels of weakly one-dimensional quantum wires in the regime where the center of mass of the exciton is confined by the wire potentials for the electron and hole. A general frame based on the effective-mass approximation is presented and the case of harmonic profiles for the electron and hole confining potentials is considered in detail. The breaking of translation symmetry by the wire potentials introduces a series of transitions between the 1S and 2S excitons of the underlying isolated well, which lead to an intrinsic high-energy tail for the wire absorption. We evaluate also the exciton scattering by longitudinal acoustical phonons and by alloy disorder in Ga1−xInxAs-based systems.
Morphology and magnetic analysis of MnSb films grown by hot-wall epitaxy
Authors: Low B. L. , Ong C. K. , Han G. C. , Gong H. , Liew T. Y. F. , Tatsuoka H. , Kuwabara H. , Yang Z.
Published: July 2002

MnSb epitaxial layers were grown on GaAs (100) substrates by hot-wall epitaxy and the structural, morphology, and magnetic properties of the layers were examined. In this study, three films of different nominal film thicknesses were fabricated. All the films are found to be well-interconnected with low intergrain resistance values being measured. It is observed that the thinnest film has grains elongated along GaAs [011] and the grains in the other two thicker films exhibit two orthogonal domains. The above microstructures of the films have a marked impact on the easy magnetization axis. The easy magnetization axis of the thinnest film is found to be along that of the elongation direction of the grains and that for the thickest film is along GaAs [011̄] (along the [12̄.0] axis of MnSb). The largest measured values for coercivity, remanent magnetization, and squareness ratio of the three films are found in the thinnest film, and the thickest film has the highest saturation magnetization. Finally, the angular variation of coercivity curves indicate that the magnetization reversal mechanism of the three films is an incoherent rotation type, i.e., non-Stoner–Wohlfarth type.
Study on ferroelectric domains in BaTiO3 crystalline films and bulk crystals by atomic force and scanning electron microscopies
Authors: Tsunekawa S. , Fukuda T. , Ozaki T. , Yoneda Y. , Okabe T. , Terauchi H.
Published: July 2002

We have observed cylindrical 180° domains and 90° multi-domains in crystalline films by an atomic force microscope and in bulk crystals by a scanning electron microscope. The size distribution of the 180° domain end is determined graphically by a computer analysis. A discussion is given on why a thickness dependence of the end size coincides with the theoretical relation proportional to the square root of the thickness over a wide range from 4 nm to 1.4 mm and why the 90° domain is found only in a film of a particular thickness.
Coexistence of magnetism and superconductivity in epitaxial GdN/W/NbN/W (100) and related NbN/W (100) multilayers
Authors: Osgood R. M. , Pearson J. E. , Sowers C. H. , Bader S. D.
Published: July 2002

The magnetic and superconducting properties of epitaxial, (100)-oriented GdN/W/NbN/W multilayers have been characterized. We find that the interfacial layer of W permits the GdN/NbN layers to be grown smoothly. We report large flux trapping effects in the magnetic hysteresis loops measured below the superconducting transition temperature (Tc) of the multilayer, indicating the coexistence of magnetism and superconductivity down to layer thicknesses of 40 and 22 Å for the NbN and GdN, respectively. Nonmagnetic NbN/W multilayers were also studied; in these multilayers, Tc is enhanced considerably above that of a single NbN layer due to coupling of the superconducting layers across the W spacers. This behavior is fitted to a model which explicitly includes interlayer coupling.
Magnetization of a trilayer structure with alternate in-plane and out-of-plane anisotropies
Authors: Yorozu Takehiko , Hu Xiao
Published: July 2002

Both analytical and numerical calculations are carried out in order to determine the equilibrium magnetization configuration in trilayer magnetic materials having an in-plane anisotropy defect sandwiched in between perpendicular anisotropy layers. By varying the applied fields, the configuration follows the state where the sum of anisotropy, exchange, and Zeeman energy becomes minimum. The magnetization reversal occurs abruptly at increased field which either corresponds to nucleation or pinning coercivity. It is also shown that these coercivity values depend on the thickness of the in-plane layer. An explicit analytical solution can be derived for the extreme case in the upper and lower bounds of in-plane defect layer thickness, i.e., in the case for null and infinitive in-plane layer thickness. The results are compared with the previous data reported for systems having a perpendicular anisotropy defect layer.
Cumulative second-harmonic generation accompanying nonlinear shear horizontal mode propagation in a solid plate
Authors: Mingxi Deng
Published: July 2002

In this article cumulative second-harmonic generation accompanying nonlinear shear horizontal (SH) mode propagation in a solid plate has been studied. Generally, the amplitude of the driven second harmonic arising from the self-interaction of the shear wave is independent of propagation distance. However, for SH mode propagation consisting of two shear waves, the cross interaction between two shear waves may cause the cumulative effect of the driven second harmonic once the phase velocity of SH mode propagation equals the longitudinal velocity of the plate material. In practical issues, one is especially interested in the case in which the second-harmonic amplitude is dependent of propagation distance. With appropriate boundary and initial conditions of excitation, the process of cumulative second-harmonic generation is clearly shown, and the analytical expression of the cumulative second harmonic is determined. Although the plane second harmonic without the cumulative effect cannot be determined in the present analysis, the cumulative second-harmonic generation accompanying nonlinear SH mode propagation is of substantial significance due to its obvious effect.
Electrical characterization of InP/GaInP quantum dots by space charge spectroscopy
Authors: Anand S. , Carlsson N. , Pistol M.-E. , Samuelson L. , Seifert W.
Published: July 2002

An investigation of coherently grown InP quantum dots embedded in Ga0.5In0.5P by conventional space charge spectroscopy methods is reported. Deep level transient spectroscopy (DLTS) is used to obtain quantitative information on the electron emission from the dots. The applied field is found to significantly enhance the electron emission rates as seen by shifts in the peaks towards lower temperatures with increased field. Taking the field induced barrier lowering into account, the emission energy for the one electron ground state of the dot is determined as 240±10 meV. The correlation between the measured signal and the observed electron accumulation in capacitance–voltage measurements is clearly demonstrated. Further, studies of the electron emission when the average electron population in the dots was varied show that the emission energies are modified by the coulomb charging energy. Admittance measurements as a function of temperature, bias and frequency were also performed, and the results are qualitatively explained in terms of response from the dots. These observations are consistent with the effect of the signal frequency on the measured carrier concentration profile. To complete the picture and in order to distinguish the DLTS signature of the dots from that of the deep levels in GaInP, electron traps in the barrier material were also characterized. Two main electron traps, one with an activation energy of about 950 meV and the other having an activation energy of 450 meV, were present in all the samples.
Band structures and optical properties of Ga1−xInxAs quantum wires grown by strain-induced lateral ordering
Authors: Li Liang-Xin , Chang Yia-Chung
Published: July 2002

Band structures and optical matrix elements of strained multiple quantum wires (QWRs) are investigated theoretically via the effective bond-orbital model, which takes into account the effects of valence band anisotropy and the band mixing. In particular, the Ga1−xInxAs QWRs grown by strain-induced lateral ordering (SILO) are considered. Recently, long wavelength Ga1−xInxAs QWR lasers have been fabricated via a single step molecular beam epitaxy technique which uses the SILO process [S. T. Chou, K. Y. Cheng, L. J. Chow, and K. C. Hsieh, Appl. Phys. Lett. 17, 2220 (1995); J. Appl. Phys. 78, 6270 (1995); J. Vac. Sci Technol. B 13, 650 (1995); K. Y. Cheng, K. C. Hsien, and J. N. Baillargeon, Appl. Phys. Lett. 60, 2892 (1992)]. Low threshold current and high optical anisotropy have been achieved. Multiaxial strains [combinations of biaxial strains in the (001) and (110) planes] for QWRs are considered. Our calculated anisotropy in optical matrix elements (for light polarized parallel versus perpendicular to the QWRs’ axis) is in good agreement with experiment. We also find that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.
Characterization of InGaAsP/InP p-i-n solar cell structures using modulation spectroscopy and secondary ion mass spectrometry
Authors: Jaeger A. , Sun W. D. , Pollak Fred H. , Reynolds C. L. , Geva M. , Stampone D. V. , Focht M. W. , Raisky O. Y. , Wang W. B. , Alfano R. R.
Published: July 2002

We have investigated two In1−xGaxAsyP1−y/InP p-i-n multiple quantum well (MQW) solar cell structures (with and without gold contacts) with intrinsic (i)-InP spacers and two InP p-i-n control samples (with and without gold contacts) using electroreflectance (ER), piezoreflectance (PZR) and secondary ion mass spectrometry (SIMS). From a comparison of the rich ER/PZR spectra from the MQW regions (including the quantum confined Stark effect produced by an external bias) with an envelope function calculation (strain and electric field), we have been able to completely characterize the composition (x,y), strain and well width of the MQWs. From the electric fields originating in the i-InP region of the two InP p-i-n control samples, as determined from the observed Franz–Keldysh oscillations (FKOs), we have evaluated the amount of p-dopant interdiffusion, in agreement with the SIMS data. The FKOs from the i-InP region of the MQW samples reveal an unusual effect, i.e., a modulation-doping field (superimposed on the p-i-n field) due to a transfer of charge from the n-InP layer to the nearest InGaAsP quantum well.
Quantum transport in an atom-sized bridge between a metal surface and a tip of a scanning tunneling microscope at finite temperatures
Authors: Kawahito Y. , Kasai H. , Nakanishi H. , Okiji A.
Published: July 2002

Electron transport in an atom-sized bridge between a metal surface and a tip of a scanning tunneling microscope is investigated with the aid of the Hubbard Hamiltonian laying stress on the effects of the intrasite Coulomb interaction. The second-order self-energy with respect to the Coulomb interaction in a single-site approximation is taken into account in calculating the conductance of the bridge. On the basis of the calculation, we closely examine the effects of the Coulomb interaction on the conductance as a function of the stretching length of the bridge. We show that the conductance value at the plateau deviates from multiples of the universal value of 2e2/h, and that the deviation increases as the temperature is decreased. This temperature dependence increases with the strength of the Coulomb interaction. In addition, when the subband edge lies energetically near the Fermi energy, a valley appears between the plateaus. When the Coulomb interaction is weak at a high temperature, or when the Coulomb interaction is strong at a low temperature, the valley becomes a quasiplateau.
Microwave dielectric properties and characteristics of polar lattice vibrations for Ba(Mg1/3Ta2/3)O3–A(Mg1/2W1/2)O3 (A=Ba, Sr, and Ca) ceramics
Authors: Furuya Mitsuru
Published: July 2002

The microwave dielectric properties and the polar lattice vibrations were investigated for complex perovskite-structured Ba(Mg1/3Ta2/3)O3–A(Mg1/2W1/2)O3 (A=Ba, Sr, and Ca) ceramics. With 0.5 mol % Ba(Mg1/2W1/2)O3 content, the dielectric quality factor, Qd, reached the maximum of 40 000 at 10 GHz. In order to better understand the dielectric loss mechanism, the characteristics of the polar lattice vibrations were analyzed by using infrared reflectivity measurements. The results suggest that the intrinsic dielectric loss mechanism for Ba(Mg1/3Ta2/3)O3–A(Mg1/2W1/2)O3 can be attributed to the increase in the crystal potential anharmonicity caused by the A-site cation substitutions and to the phonon degeneracy or coupling due to the incorporation of Mg and W into the B-site sublattice of Ba(Mg1/3Ta2/3)O3. For Ba(Mg1/3Ta2/3)O3–Ba(Mg1/2W1/2)O3 and Ba(Mg1/3Ta2/3)O3–Sr(Mg1/2W1/2)O3 the complex dielectric constants calculated from the reflectivity spectra were consistent with those measured at 10 GHz. However, for Ba(Mg1/3Ta2/3)O3–Ca(Mg1/2W1/2)O3, the difference between the calculated and measured Q values became larger with increasing Ca(Mg1/2W1/2)O3 content. It is supposed that these behaviors are partly due to the Debye-like losses caused by the defect dipoles formed by the incorporation of Ca into the B-site sublattice.
Domain structure in chemically ordered InxGa1−xN alloys grown by molecular beam epitaxy
Authors: Doppalapudi D. , Basu S. N. , Moustakas T. D.
Published: July 2002

Observation of long range atomic ordering in InGaN films grown by molecular beam epitaxy on A-plane sapphire is reported, based on x-ray diffraction and transmission electron microscopy studies. The InGaN films have a domain structure, with alternating domains of ordered and disordered phases, close to the film/substrate interface. Closer to the film surface, disordered domains are no longer observed. The degree of ordering was found to increase with growth rate (at the low growth rates used in these materials), which is consistent with ordering being a surface phenomenon.
In-plane strain distribution in free-standing GaAs/InGaAs/GaAs single quantum well surface nanostructures on GaAs[001]
Authors: Ulyanenkov A. , Baumbach T. , Darowski N. , Pietsch U. , Wang K. H. , Forchel A. , Wiebach T.
Published: July 2002

The vertical variation of in-plane strain induced by an In0.1Ga0.9As single quantum well (SQW) embedded in a free-standing wire structure on GaAs[001] has been investigated by depth resolved x-ray grazing incidence diffraction. If the wires are oriented along the [110] direction both the shape and strain influence on the x-ray intensity distribution can be separated by running transverse or longitudinal scans across the grating truncation rods (GTRs) close to the (2̄20) and (2̄2̄0) in-plane Bragg reflection, respectively. The GTRs themselves are modulated due to the vertical layering of the wires. The vertical strain variation in the vicinity of SQW is particularly inspected at the weak (200) Bragg reflection which is most sensitive to the scattering density difference between the SQW and GaAs. The theoretical analysis is based on the distorted wave Born approximation for grazing incidence geometry. The structural parameters of the surface nanostructure were determined with high accuracy by fitting of the complete set of experimental GTRs simultaneously. In agreement with finite-element calculations we find a maximum in-plane lattice displacement within the SQW of (Δa‖/a≈3.5×10−4) with respect to the substrate. It induces dilative in-plane strain in the GaAs confinement layers decreasing towards the upper free surface and the bulk, respectively. The evaluated in-plane strain within the SQW is used for estimating the strain induced redshift of the photoluminescence wavelength of the respective optical device.
Magnetoresistance in modulated width Ni80Fe20 wires
Authors: Yao C. C. , Hasko D. G. , Xu Y. B. , Lee W. Y. , Bland J. A. C.
Published: July 2002

The magnetization reversal processes and magnetoresistance behavior in micron-sized Ni80Fe20 wires with periodically modulated width have been studied. The wires were fabricated by electron beam lithography and a lift-off process. A combination of the magneto-optical Kerr effect and magnetoresistance measurements shows that the lateral shape of the wires greatly influences the magnetic and transport properties. For the field applied along the wire axis, the hysteresis loops are strongly influenced by the wire shapes. In contrast to the fixed width wires, the modulated width wires show an additional transverse magnetoresistance, which has been attributed to the shape-dependent demagnetizing fields and the inhomogeneous current density. The resistance of the modulated width wires is dominated by the contribution due to the narrow part of the wires; however, the inhomogeneous current density in the wide part of the wires contributes a significant transverse magnetoresistance.
Thermoelastic strain and plastic yielding in aluminum nitride on sapphire
Authors: Dobrynin A. V.
Published: July 2002

The stress state of AlN/Al2O3 heteroepitaxial structures was probed by curvature measurements. The structures were found to be concave at the deposition temperature and to become convex or remain concave—depending on film thickness—on cooling to room temperature. The conclusion is drawn that film deposition is accompanied by plastic deformation. The upper layers of the film may be unstressed. Cooling from the deposition temperature gives rise to elastic strains of opposite sign. An interplay between various contributions determines the magnitude and sign of structure curvature. The calculated variation of strain with film thickness differs from that predicted by Stoney’s equation and is at variance with the present experimental data.
Low temperature transport of Al into and through copper starting with Cu/Al/SiO2 bilayers
Authors: Lanford W. A. , Bedell S. , Isberg P. , Hjorvarsson B. , Lakshmanan S. K. , Gill W. N.
Published: July 2002

The transport of Al into Cu is studied by annealing Cu/Al/SiO2 bilayers. In order to minimize the effects of contaminants, samples were prepared and annealed in all-metal ultrahigh vacuum systems. In situ resistivity was used to continuously monitor the transport of Al into the bulk of the copper films. It is observed: that Al begins to move into the copper at low temperature (100–150 °C); that during an isothermal anneal, there is an initial rapid increase in Al concentration in the copper followed by a very slow increase; the equivalent of about 45 A of Al remains bound at the SiO2 interface or at the surface of the copper and is not free to dissolve in the copper. These data are not consistent with simple one-dimensional bulk diffusion but are in good agreement with a model of three-dimensional copper grains in which there is rapid transport to saturate the grain boundaries followed by diffusion of Al from the grain boundaries into the copper grains. One important observation is that because of the role of grain boundaries, it is possible to transport Al through copper films without a significant increase in the resistivity of the copper films. The potential application of this system for microelectronic interconnects in modern microelectronics is discussed.