bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical investigation of electrical characteristics of AlGaN/GaN modulation doped field-effect transistors
Authors: Stengel Franck , Mohammad S. Noor , Morkoç Hadis
Published: July 2002

A theoretical investigation of the electrical characteristics of GaN/AlxGa1−xN (x is the Al mole fraction in AlGaN) modulation doped field-effect transistors (MODFETs) is carried out. Using a self-consistent solution of Schrödinger’s equation and Poisson’s equation, relations between the concentration of two-dimensional electron gas (2DEG), the Fermi level in GaN, and the average distance of the electrons from the heterointerface are calculated. A relation between the gate bias and the 2DEG concentration is obtained for a flat quasi Fermi level in AlxGa1−xN. Based on the relation between the 2DEG concentration and the applied gate bias, a model for the drain current and the transconductance of the device is developed. The effects of the n-AlxGa1−xN layer thickness, the spacer i-AlxGa1−xN layer thickness, the n-AlxGa1−xN doping level, the aluminum mole fraction x, and the channel length L on the electrical characteristics of MODFETs are presented. Theoretical results are compared with the recent experimental data, which show striking agreement. The 2DEG concentration is found to be as high as 1013 cm−2, and transconductance as high as 1000 mS/mm. Finally, the effect of the difference in the properties of AlN and GaN, and a device structure that would take advantage of the bending of the quasi Fermi level for electrons in n-AlxGa1−xN, are discussed.
An infrared absorption investigation of hydrogen, deuterium, and nitrogen in ZnSe grown by molecular beam epitaxy
Authors: Yu Zhonghai , Buczkowski S. L. , Hirsch L. S. , Myers T. H.
Published: July 2002

Surprising concentrations of hydrogen and deuterium, as high as 5×1020 cm−3, were incorporated into nitrogen-doped ZnSe grown on GaAs by molecular beam epitaxy. Infrared absorption bands due to local vibration modes were observed at 3193 and 783 cm−1 for ZnSe:N,H samples, and at 2368 cm−1 for ZnSe:N,D samples using Fourier transform infrared spectroscopy. The isotopic shift in the absorption band agrees with predictions of a simple harmonic oscillator approximation for N–H bonding. The variation of the absorption band associated with substitutional nitrogen with nitrogen concentration indicates that not all nitrogen is substitutional, and also exhibited significant changes related to hydrogen incorporation.
Recombination-enhanced Fe atom jump between the first and the second neighbor site of Fe–acceptor pair in Si
Authors: Sakauchi S. , Suezawa M. , Sumino K. , Nakashima H.
Published: July 2002

We studied the recombination-enhanced Fe atom jump between the first (1st) and second (2nd) neighbor sites of Fe–Al and Fe–B pairs in Si. We first annealed specimens at 80 °C to generate Fe–acceptor pairs after doping of Fe. Concentrations of the 1st and 2nd neighbor Fe–acceptor pairs were determined by electron-spin-resonance (ESR) measurement after annealing at around 150 K under optical excitation. The concentration of the 1st neighbor pair was decreased and that of the 2nd neighbor pair was increased by the above annealing. Activation energy for the above changes was about 0.11 eV in the case of the Fe–Al pair. This is much smaller than that (0.8 eV) of thermal annealing alone. In the case of the Fe–B pair, the ESR signal of the 2nd neighbor pair could be detected due to annealing at around 160 K under optical excitation.
Space-charge-perturbed electrophoresis in nonpolar colloidal dispersions
Authors: Chen I. , Mort J. , Machonkin M. A. , Larson J. R. , Bonsignore F.
Published: July 2002

This paper reports the observation of bipolar, space-charge-perturbed transport in colloidal dispersions using an experimental technique that time-resolves electrophoresis in nonpolar colloidal systems. Unlike existing methods for studying electrophoresis, this technique can be applied to dispersions of diverse types and concentrations over a wide range of electric fields, including the space-charge-perturbed conditions often encountered in practical applications. The phenomenon is investigated as a special case of dielectric relaxation in a leaky capacitor connected in series to a perfect one. Using the first principle charge transport theory, such dielectric relaxation, occurring under the non-Ohmic supply and space-charge-perturbed transport conditions, is shown to differ from that expected by the conventional equivalent-circuit treatment. The combined theoretical analysis and the experimental technique provides a means for independently determining the densities and mobilities of charged species in such systems. Using a liquid developer for electrography as a prototype system, results are presented that illustrate the power of the technique as a new tool to provide new insights into the generic transport and generation mechanisms of charged species in colloidal systems.
High power gain for stimulated Raman amplification in CuAlS2
Authors: Bairamov B. H. , Aydinli A. , Bodnar’ I. V. , Rud’ Yu. V. , Nogoduyko V. K. , Toporov V. V.
Published: July 2002

The spontaneous Raman spectra of the chalcopyrite structure crystal CuAlS2, which is promising for nonlinear optical applications, has been investigated at 8 and 300 K. The main aim of this study is to compare the absolute spontaneous Raman scattering efficiency in CuAlS2 crystals with that of their isomorphous analog, zinc-blende structure GaP crystals, known as one of the most efficient materials for Raman amplification. Observation of a high value of absolute scattering efficiency S/L dΩ (where S is the fraction of incident power that scatters into the solid angle d Ω and L is the optical path length with S/L dΩ=9.5×10−5 cm−1 sr−1), together with relatively narrow linewidth (Γ=5.1 cm−1, full width at half maximum at room temperature and Γ=1.5 cm−1 at 8 K for the strongest Γ1 phonon mode of CuAlS2 at 314 cm−1) indicate that CuAlS2 has the highest value of the stimulated Raman gain coefficient gs/I where I is the incident laser power density. The calculated value of this gain is gs/I=2.1×10−6 cm−1/W at 300 K and 5.0×10−6 cm/W, at 8 K for 514.5 nm laser excitation, and is larger than those for the appropriate vibrational modes of various materials (including GaP, LiNbO3, Ba2NbO5O15, CS2, and H2) investigated so far. The calculations show that cw Raman oscillator operation in CuAlS2 is feasible with low power threshold of pump laser.
Deposition of high-quality NiMnSb magnetic thin films at moderate temperatures
Authors: Caballero J. A. , Park Y. D. , Cabbibo A. , Childress J. R. , Petroff F. , Morel R.
Published: July 2002

Thin films of the ferromagnetic Heusler alloy NiMnSb, of interest for magnetic multilayer devices because of their predicted half-metallic (i.e., 100% spin-polarized) transport properties, have been successfully deposited by rf magnetron sputtering from a single composite target. A novel combination of low argon gas pressure, low deposition rates, and moderate substrate temperatures (250–350 °C) are shown to result in high-quality, low-roughness polycrystalline films of the C1b-type crystal structure, with thicknesses as low as 100 Å, without the need for any post-deposition annealing. The structural properties of these films, determined by x-ray diffraction and atomic force microscopy are presented as a function of deposition conditions. The magnetic properties and resistivity are consistent with bulk MiMnSb, which suggests that they will be effective as spin-polarized conducting layers in multilayer thin-film structures.
Space-charge limited conduction processes in polybenzo[c]thiophene films
Authors: Musa I. , Higgins S. J. , Eccleston W.
Published: July 2002

We present an observation of extensive space-charge limited conduction in polybenzo[c]thiophene. Strong evidence for the presence of three discrete trapping levels is shown with a possibility of a fourth trap level. The density of traps for the first three levels is found to be 5×1016 cm−3, 1×1017 cm−3, and 2×1017 cm−3, respectively. A lower limit for the free-carrier mobility was estimated to be ∼3×10−5 cm2 V−1 s−1.
Rotational twins in heteroepitaxial CuInSe2 layers on Si(111)
Authors: Krejci M. , Tiwari A. N. , Zogg H. , Schwander P. , Heinrich H. , Kostorz G.
Published: July 2002

The microstructure of (112) oriented CuInSe2  heteroepitaxial layers grown with molecular beam epitaxy on (111) oriented Si wafers was investigated by transmission electron microscopy. Experimental and calculated diffraction patterns of different zone axes were compared. Extra spots are caused by rotational twins on (112) growth planes. Six twin variants rotated by ±120°, ±60° and 180° about the [221] axis were identified in the layers. The tetragonal chalcopyrite structure of CuInSe2, the crystal symmetry of the substrate and variations of the growth conditions during the growth are responsible for the formation of these rotational twins. Coherent twin boundaries as well as partly coherent boundaries of twin variants rotated by ±60° and 180° were imaged by high resolution transmission electron microscopy. The boundaries can be formed by inserting partial dislocations with Burgers vectors a/6〈111̄〉 into the CuInSe2  structure. This suggests that the annealing of the samples induces the annihilation of these partial dislocations and consequently reduces the density of twins in the CuInSe2  layers.
Major loop reconstruction from switching of individual particles
Authors: Zheng Guobao , Pardavi-Horvath Martha , Vertesy Gabor
Published: July 2002

Major hysteresis loops of groups of isolated 60 μm square garnet particles of a regular two-dimensional array, have been measured magnetooptically. Individual loops for each particle were measured, and the statistics of the distribution of coercivities and interaction fields was determined. It is shown that from the measured coercivity distribution and calculated magnetostatic interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the major loop of an assembly of 5×5 particles were calculated numerically for two cases: first, when calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the major loop of the same 25 particles, embedded into a 9×9 square, was reconstructed taking into account the interactions among all 81 particles. The numerically simulated major hysteresis loops agree very well with the measured loops, demonstrating the reliability of numerical modeling.
Local magnetization and hyperfine field systematics of s−p and noble impurities in Gd and Ni hosts
Authors: de Oliveira A. L. , Tovar Costa M. V. , de Oliveira N. A. , Troper A.
Published: July 2002

Magnetic hyperfine data of s−p and noble impurities diluted in ferromagnetic Gd and Ni hosts are described within a simple model which is an extension on that one of Daniel and Friedel. We also include in the present model the effect of next-neighbor perturbation, due to the translational invariance break introduced by the impurity. Performing a self-consistent calculation of the local magnetic moments at the impurity site, one obtains the conduction electron polarization (CEP) hyperfine field. It is found that the model can explain the different hyperfine field trends observed in Gd and Ni hosts, e.g., a remaining negative value along the s−p series in the Gd case and a change of sign behavior in the Ni case. Period effects observed in noble impurities are also discussed and the theoretical calculations are in good agreement with available experimental data.
Two-dimensional arrays of magnetic stripes and dots patterned by laser ablation
Authors: Le Gall H. , Gouzerh J. , Youssef J. Ben , Koshkina O. , Guitonni J. , Desvignes J. M. , Fessant A. , Spenato D. , Gieraltowski J. , Loaëc J.
Published: July 2002

Fabrication of two-dimensional (2D) arrays of magnetic stripes, bars, or dots is presented from a very simple, fast, and low-cost technique of laser ablation. This process is alternative to the usual but heavier photolithography method. Ablated tracks have been produced with partial or total etching down to the magnetic layer substrate and width from 3 to 100 μm depending on the laser defocusing and power and the velocity of the sample induced by a motorized X–Y translation stage. Different patterns corresponding to magnetic stripes, bars, and square 2D arrays with different shape factors and sizes decreasing from millimeter to micron scale have been realized. Strong changes in both static and dynamical magnetic properties of the films are induced by the laser patterning as discussed from the magnetization processes of domain walls at low frequency and the complex permeability in the microwave range.
Raman-scattering criteria for characterization of anneal-restored zinc blende single crystals: Application to Si+-implanted InP
Authors: Artús L. , Cuscó R. , Ibáñez J. , Martin J. M. , González-Díaz G.
Published: July 2002

We have studied the lattice recovery by rapid thermal annealing of Si+-implanted InP using Raman spectroscopy. The crystallinity recovery for different annealing temperatures of samples totally amorphized by the implantation can be monitored by means of their Raman spectra. However, free-charge coupling with the LO mode and possible misorientation of the recrystallized material may alter substantially the first-order Raman spectrum, making it unreliable for a good characterization of the lattice recovery. The study of second-order Raman spectrum overcomes the problems present in the analysis of first-order Raman spectrum and provides suitable criteria to assess the recrystallization of the implanted and annealed samples. After rapid thermal annealing at 875 °C for 10 s, the intensity of the second-order peaks approaches 70% of its value in virgin InP, and third-order Raman peaks are also clearly detected, evidencing the good lattice recovery achieved.
Acceleration–deceleration process of thin foils confined in water and submitted to laser driven shocks
Authors: Romain J. P. , Auroux E.
Published: July 2002

An experimental, numerical, and analytical study of the acceleration and deceleration process of thin metallic foils immersed in water and submitted to laser driven shocks is presented. Aluminum and copper foils of 20 to 120 μm thickness, confined on both sides by water, have been irradiated at 1.06 μm wavelength by laser pulses of ∼20 ns duration, ∼17 J energy, and ∼4 GW/cm2 incident intensity. Time resolved velocity measurements have been made, using an electromagnetic velocity gauge. The recorded velocity profiles reveal an acceleration–deceleration process, with a peak velocity up to 650 m/s. Predicted profiles from numerical simulations reproduce all experimental features, such as wave reverberations, rate of increase and decrease of velocity, peak velocity, effects of nature, and thickness of the foils. A shock pressure of about 2.5 GPa is inferred from the velocity measurements. Experimental points on the evolution of plasma pressure are derived from the measurements of peak velocities. An analytical description of the acceleration–deceleration process, involving multiple shock and release waves reflecting on both sides of the foils, is presented. The space–time diagrams of waves propagation and the successive pressure–particle velocity states are determined, from which theoretical velocity profiles are constructed. All characteristics of experimental records and numerical simulations are well reproduced. The role of foil nature and thickness, in relation with the shock impedance of the materials, appears explicitly.
Comparison of PdGeTiPt and NiGeAu ohmic contacts to n-GaAs and PdGeTiPt and TiPd contacts to p+-GaAs
Authors: Jones K. A. , Cole M. W. , Han W. Y. , Eckart D. W. , Hilton K. P. , Crouch M. A. , Hughes B. H.
Published: July 2002

NiGeAu and PdGeTiPt ohmic contacts to n-GaAs and TiPd and PdGeTiPt ohmic contacts to p+-GaAs are examined by comparing their contact resistances, chemical intermixing as determined by Auger electron microscopy, interface structure as determined by transmission electron microscopy, and surface roughness as determined by surface profiling all measured as a function of annealing time and temperature. The n-PdGeTiPt contact annealed for short times, ⩽15 s, and at low temperatures, ⩽395 °C, was superior to the NiGeAu contact because it had a comparable contact resistance, less interface mixing, better lateral homogeneity, and a smoother surface. However, its contact resistance increased substantially with the annealing time and temperature, whereas the NiGeAu contact was relatively unaffected. For all annealing times and temperatures except the one at 550 °C, the TiPd contact to p+ GaAs was superior as it had a lower contact resistance and a comparable amount of interface intermixing, lateral homogeneity, and surface roughness. However, it had a complete chemical breakdown at 550 °C, whereas the PdGeTiPt contact resistance remained relatively stable.
Radical density measurements in an oxyacetylene torch diamond growth flame
Authors: Welter M. D. , Menningen K. L.
Published: July 2002

The column densities of several molecular radicals are measured in an oxyacetylene torch flame during the growth of diamond films. The column densities of CH, C2, CN, and OH radicals are determined by highly sensitive absorption spectroscopy. The radical densities are measured as a function of position in the flame for different fuel/oxidant ratios. The growth of diamond is confirmed by scanning electron microscopy and Raman spectroscopy. Of the four radicals measured, the C2 and CH densities are the most correlated with diamond growth. The measured densities are compared with numerical flame models.
Determination of nonlinear absorption mechanisms using a single pulse width laser
Authors: Li Feng , Song Yinglin , Yang Kun , Liu Shutian , Li Chunfei , Wu Yiqun , Zuo Xia , Yu Congxuan , Zhu Peiwang
Published: July 2002

A simple optical setup that uses a single pulse width laser is proposed to identify different kinds of nonlinear optical absorption mechanisms. It can easily distinguish reverse saturable absorption from two photon absorption, and direct three photon absorption from two photon absorption followed by one photon absorption from the excited level. Both theoretical and experimental results are reported.
Reproducible growth of highly crystalline YBa2Cu3O7 thin films on SrTiO3 by scanning pulsed laser deposition
Authors: Wong André , Liang Ruixing , Gardner Michael , Hardy Walter N.
Published: July 2002

YBa 2 Cu 3 O 7 (YBCO) thin films were grown on SrTiO3 substrates using a pulsed laser deposition system. The laser beam was scanned over the target in such a way to obtain a reproducible method of composition control at the substrate. In total, 17 films were grown under optimal conditions; they exhibited mean transition temperatures of 89.7±0.4 K and mean (005) rocking curve widths of 0.07°±0.02°. The narrowest (005) rocking curve width was measured to be 0.037°. Both the rocking curve line shapes and the dependence of transition temperature on rocking curve width suggest the films have single-crystal-like structure.
Comparative study of the structure and local magnetic order in bulk and ultrafine particles of Fe–Mn (32%–35%) and Fe3Pt
Authors: Baldokhin Yu. V. , Kolotyrkin P. Ya. , Petrov Yu. I. , Shafranovsky E. A.
Published: July 2002

A study of Fe–Mn (32% and 34.65%) and Fe3Pt Invar alloys during the transition from bulk to ultrafine (3–20 nm) particles is done at room temperature by combined x-ray diffraction and Mössbauer spectroscopy techniques. The particles obtained by evaporation of foil or filings of bulk material in an Ar atmosphere at pressures from 0.16 to 50 Torr were rapidly quenched during their production. Mössbauer measurements confirmed the availability of two spin states in both Fe–Mn and Fe3Pt fcc particles like it has been observed in previous studies of fine Fe and Fe–Ni (30%–35%) particles. Various heat treatments of the Fe3Pt foil with the initial bcc structure made it possible to obtain its fcc modification in either the entirely ordered or the entirely disordered state. It was shown that the disordered fcc structure in a bulk sample could exist in two spin states (ferromagnetic and paramagnetic) whereas the ordered structure was only in a ferromagnetic state.
Microcracks, spall and fracture in glass: A study using short pulsed laser shock waves
Authors: Li Xin-Zen , Nakano M. , Yamauchi Y. , Kishida K. , Tanaka K. A.
Published: July 2002

This work reports our meso mechanical study of microcrack behavior, especially the process leading from microcracking to macro failure. Using laser loading with a duration on the order of nanoseconds, spallation in a cylindrical geometry was achieved in soda-lime glass at the microcrack evolution stage. Laser induced shock waves were used to conduct crack initiation experiments for the first time. The specimens were examined after experiments and were compared with those loaded by conventional static and dynamic methods. A meso scale failure model of nucleation, growth, and percolation (NGP) is suggested based on the experiments. The NGP model is characterized by a randomly generated microcrack field and by simultaneous percolation statistics in time iteration. In most existing damage models, the statistical average of microcracks over a finite space is required in order to evaluate the damage variables. This procedure is not necessary in the NGP model. We show that the connection of microcracks can be very complicated, possibly a self-organized process.
Theoretical study of plasma expansion in a magnetic field in a disk anode vacuum arc
Authors: Beilis I. I. , Keidar M. , Boxman R. L. , Goldsmith S.
Published: July 2002

The low-density plasma flow in an axial magnetic field to a disk-shaped anode in a vacuum arc was studied theoretically using a two-dimensional model. The plasma expansion was modeled using the sourceless steady-state hydrodynamic equations, where the free boundary of the plasma was determined by a self-consistent solution of the gas-dynamic and electrical current equations. The anode was modeled as a current and plasma collector, which does not influence the plasma flow field. Magnetic forces from both the azimuthal self-magnetic field, and the imposed axial magnetic field were taken into account. It was found that the self-magnetic field does not substantially influence either the plasma jet shape, density, velocity, or the current density distribution for arc currents I⩽200 A. On the other hand, the plasma jet angle (α0) at the starting plane and the radial plasma density gradient force in the expansion region do have a strong influence on the plasma and current flow. The mass and current flow in a 500 A arc are compressed in the near axis region, leading to an increase in the plasma and axial current density by a factor of 1.5 at a distance of about two plasma jet radii from the starting plane. The calculated arc current–voltage characteristics agree qualitatively with experiments on arc behavior in an axial magnetic field.
Local noise analysis of a Schottky contact: Combined thermionic-emission–diffusion theory
Authors: Gomila G. , Bulashenko O. M. , Rubı́ J. M.
Published: July 2002

A theoretical model for the noise properties of Schottky barrier diodes in the framework of the thermionic-emission–diffusion theory is presented. The theory incorporates both the noise induced by the diffusion of carriers through the semiconductor and the noise induced by the thermionic emission of carriers across the metal–semiconductor interface. Closed analytical formulas are derived for the junction resistance, series resistance, and contributions to the net noise localized in different space regions of the diode, all valid in the whole range of applied biases. An additional contribution to the voltage-noise spectral density is identified, whose origin may be traced back to the cross correlation between the voltage-noise sources associated with the junction resistance and those for the series resistance. It is argued that an inclusion of the cross-correlation term as a new element in the existing equivalent circuit models of Schottky diodes could explain the discrepancies between these models and experimental measurements or Monte Carlo simulations.
Design and modeling of precision solid liner experiments on Pegasus
Authors: Bowers R. L. , Brownell J. H. , Lee H. , McLenithan K. D. , Scannapieco A. J. , Shanahan W. R.
Published: July 2002

Pulsed power driven solid liners may be used for a variety of physics experiments involving materials at high stresses. These include shock formation and propagation, material strain-rate effects, material melt, instability growth, and ejecta from shocked surfaces. We describe the design and performance of a cylindrical solid liner that can attain velocities in the several mm/μs regime, and that can be used to drive high-stress experiments. An approximate theoretical analysis of solid liner implosions is used to establish the basic parameters (mass, materials, and initial radius) of the driver. We then present one-dimensional and two-dimensional simulations of magnetically driven, liner implosions which include resistive heating and elastic–plastic behavior. The two-dimensional models are used to study the effects of electrode glide planes on the liner’s performance, to examine sources of perturbations of the liner, and to assess possible effects of instability growth during the implosion. Finally, simulations are compared with experimental data to show that the solid liner performed as predicted computationally. Experimental data indicate that the liner imploded from an initial radius of 2.4 cm to a target radius of 1.5 cm, and that it was concentric and cylindrical to better than the experimental resolution (60 μm) at the target. The results demonstrate that a precision solid liner can be produced for high-stress, pulsed power applications experiments.
Quasidrift limited field-effect current in polycrystalline silicon: Wide grain boundary approximation
Authors: Eccleston W.
Published: July 2002

The current flowing along polycrystalline silicon surfaces, as a function of the field applied normal to the surface, is developed for grains of finite size with finite grain boundaries which may contain amorphous material. The work is relevant to the operation of thin film transistors and expressions are developed for the dependence of drift channel current and field effect mobility on gate and drain voltage and, particularly, grain size. Comparison is made with real device characteristics. The results are consistent with a trapping level at the conduction band edge. In electrical terms, such a condition is indistinguishable from no trapping but with a proportionately lower mobility. The assumption of finite grain boundaries, with effective concentrations of traps, is demonstrated to provide potentially very useful analytical results that further the understanding of field effect in all polycrystalline materials.
Temperature measurements in low-pressure, diamond-forming, premixed flames
Authors: Kim J. S. , Cappelli M. A.
Published: July 2002

Measurements of temperature in low-pressure, diamond-forming, premixed flames are described. Optical emission spectroscopy was used to determine the rotational temperature of CH radicals in the flame by analyzing the spectra of the R-branch of the A 2Δ→X 2Π(0,0) CH electronic transition near 430 nm. Emission spectroscopic measurements were taken in stagnation-point flames of three different fuel types; CH4–O2, C2H4–O2, and C2H2–O2 flames, all of which have been previously used in the synthesis of diamond films. The emission-based rotational temperatures are found to be in good agreement with temperature measurements from thermocouples for the CH4–O2 flame, when a correction is made to the thermocouple measurements for radiation losses. The measurements show that peak flame temperatures are higher than the adiabatic flame temperatures for both C2H4–O2, and C2H2–O2 flames; a result that is also predicted by a numerical simulation of the stagnation-point strained flame that accounts for detailed finite-rate gas-phase chemistry and heterogeneous surface reactions.
Threshold current of quantum-disk and quantum-wire gain-coupled distributed feedback lasers
Authors: Susa Nobuhiko
Published: July 2002

This numerical study shows that an effective way to reduce the threshold current density (Jth) in the quantum well (QW) and bulk types of gain-coupled distributed feedback (DFB) lasers is to reduce the active volume. Although the threshold current per unit cavity width (Ith) is smaller for the quantum-disk type than it is for the quantum-wire or QW type of gain-coupled DFB laser, both the optimum number of vertically stacked quantum structures (Nst) for minimizing Ith when cavity length (L) is fixed and the optimum L for minimizing Ith when Nst is fixed are the largest for the quantum-disk type. When L=500 μm, for example, the optimum Nst is 7 for the quantum-disk type, 5 for the quantum-wire type and 3 for the QW type. Threshold current density (Jth) is almost independent of facet reflectivity (r), and this implies that all these lasers oscillate at the Bragg mode. The Jth of the bulk laser, however, is influenced most by r because this type has the smallest differential gain.
Simulations of reactive sputtering with constant voltage power supply
Authors: Zhu Shenglong , Wang Fuhui , Wu Weitao
Published: July 2002

Reactive sputtering in an Al–O2/Ar system equipped with a dc power supply in constant voltage mode was simulated using a kinetics model in this article. This work showed that reactive sputtering under reactive gas flow rate control and with a constant voltage power was much different from that with a constant current power both in time-dependent and parameter-dependent behaviors. Numerical analysis predicted the continuous, N-shaped V–I characteristics curves, which agreed with the experimental curves reported by other researchers in a system which was equipped with a dc constant current power but operated under voltage control [R. McMahon et al., J. Vac. Sci. Technol. 20 376 (1982); K. Steenbeck et al., Thin Solid Films 92, 371 (1982)] and in a system with a modulated power supply [H. Ohsaki et al., Thin Solid Films 281-282, 213 (1996)]. The continuous parameter-dependent functions indicated that there is no hysteresis effect in constant voltage reactive sputtering, which is a well-known feature of dc constant current magnetron sputtering of an Al–O2/Ar system. According to the simulated kinetics curves using the same simulation parameters, the initial values of oxide coverage on the target surface had no influences on the final parameters at steady state in a constant voltage system, whereas different initial coverage may lead to different steady states (hysteresis behaviors) in a constant current system. This article showed that the films of any reactive gas contents, i.e., the M1−θ−(MOx)θ films (M=metal, MOx=compound of metal) with full range of 0⩽θ⩽1, could be produced in constant voltage reactive sputtering which is a system simpler than those under partial pressure control.
Formation of amorphous aluminum oxide and gallium oxide on InP substrates by water vapor oxidation
Authors: Chou L. J. , Hsieh K. C. , Wohlert D. E. , Cheng K. Y. , Finnegan N.
Published: July 2002

Amorphous aluminum oxide layers have been successfully produced by oxidizing amorphous (Al, As) compounds using water vapor at temperatures as low as 300 °C. The amorphous (Al, As) was deposited on InP substrates by molecular beam epitaxy at low temperatures (∼100 °C), and was found to have 50% more As than crystalline AlAs. Auger electron spectroscopy depth profiles indicate a complete depletion of As in the amorphous aluminum oxide layer. However, complete removal of As in the amorphous (Ga, As) layer requires a minimum oxidation temperature higher than 300 °C. This method has extended the use of native oxides to lattice-mismatched heterostructures.
Shock jump equations for unsteady wave fronts of finite rise time
Authors: Sano Yukio , Miyamoto Isamu
Published: July 2002

First, generalized Rankine–Hugoniot equations for unsteady wave fronts of finite width are derived. It is clarified from these jump equations for particle velocity, stress, and specific internal energy that shock jump conditions involve the effects of rise time of the front or the change in its velocity with time, in addition to that of strain rate and acceleration, and that the treatment in the previous study [Sano, J. Appl. Phys. 82, 5382 (1997)] where the rise time is reduced to an infinitesimal eliminates the terms of this change in the jump equations. Furthermore, the jump equations of the general form derived here support this elimination. Next, the influence of the rise time on the three jumps at the impacted surface of a lithium fluoride single crystal is calculated based on its experimental data and shown to be negligibly small. However, its influence, calculated for sandstone in a similar manner, is great. Finally, a parametric investigation of the influence is carried out for specific strain waves.
Electrical characterization of a He ion implantation-induced deep level existing in p+n InP junctions
Authors: Quintanilla L. , Pinacho R. , Enrı́quez L. , Peláez R. , Dueñas S. , Castán E. , Bailón L. , Barbolla J.
Published: July 2002

The electrical characterization of a He ion implantation-induced deep level existing in fully implanted p+n InP junctions isolated by He bombardment has been carried out in this work. A discrete deep level located at 0.19 eV below the conduction band was detected by deep level transient spectroscopy (DLTS). Several emission characteristics of this trap were derived by the correlation between DLTS and capacitance–voltage transient technique.
Effects of helicon-wave excited N2 plasma treatment on Fermi-level pinning in GaAs
Authors: Hara Akio , Kasahara Fumio , Wada Satoshi , Ikoma Hideaki
Published: July 2002

GaAs(100) substrate was treated in a helicon-wave excited N2 plasma for short time (5–15 min). Current density–voltage (J–V) characteristics were measured for the Schottky or tunnel metal–insulator–semiconductor diode. The Schottky barrier height obtained from Richardson plot was about 0.3–0.4 eV for the plasma-treated samples independent of the plasma exposure time, while it was about 0.7 eV for the untreated one. The ideality factor and the reverse (leakage) current were much higher for the plasma-treated GaAs than for the untreated ones. The bulk carrier density showed a small decrease near the GaAs surface only for the plasma-treated samples. A very high density of the interface states was observed at EC=0.3–0.4 eV from the analysis of J–V characteristics based on the Horváth’s theory [J. Appl. Phys. 63, 976 (1988)]. These experimental results indicated that the high density of the interface states were generated at the energy of EC=0.3–0.4 eV, probably due to plasma-induced damage, and the surface Fermi level was strongly pinned at this energy position, though the midgap pinning was removed or partially removed due to the plasma treatment. Some possible reasons of this removal of the midgap pinning are also discussed.
Extra-low-frequency noise in giant magnetoresistance recording heads
Authors: Xiao Min , Klaassen Klaas B. , van Peppen Jack C. L. , Kryder Mark H.
Published: July 2002

Extra-low-frequency (ELF) noise was studied in flyable, shielded giant magnetoresistance heads using a novel, ultralow-noise amplification system. The noise power spectral density had a 1/f character over wide frequency and sense current ranges. The noise was mainly magnetic in origin. It, however, did not follow Hooge’s formula. Random telegraph noise (RTN) was observed after moderate current stressing. The characteristic dwell times of the two-level RTN switching changed with applied field, sense current, and temperature. We consider the ELF noise as a sensitive diagnostic tool, since deviations from the 1/f dependence signal the existence of undesirable magnetic states.
Magnetostatic modes in quasiperiodic Fibonacci magnetic superlattices
Authors: Anselmo D. H. A. L. , Cottam M. G. , Albuquerque E. L.
Published: July 2002

The magnetostatic modes are studied in multilayer structures that exhibit deterministic disorders. Some models that have attracted particular attention are the quasiperiodic magnetic multilayers that obey a substitutional sequence of the Fibonacci type. The spin wave spectra are evaluated in the geometry where the magnetization is perpendicular to the surfaces of the layers of the superlattice by using a transfer-matrix approach. Numerical results are presented for the ferromagnets EuS and Fe and for the antiferromagnet MnF2.
Effect of thin V insertion layer into Ta film on the performance of Ta diffusion barrier in Cu metallization
Authors: Kwak Joon Seop , Baik Hong Koo , Kim Jong-Hoon , Lee Sung-Man , Ryu Hyuk Ju , Je Jung Ho
Published: July 2002

In order to increase the failure temperature of Ta diffusion barrier for Cu, we investigated the effect of a thin V insertion layer (100 Å) into Ta film with/without ion bombardment on Ta diffusion barrier performance in Cu metallization. When the Ta/V/Ta diffusion barrier was deposited without concurrent ion bombardment, the insertion of the thin V layer into Ta film was not effective to improve the barrier performance of Ta film, because of the thermal instability of the Ta/V/Ta multilayer caused by the reaction between the Ta/V/Ta films and Si substrate. Meanwhile, when the Ta/V/Ta diffusion barrier was deposited with ion bombardment, the insertion of the thin V layer into Ta film improved barrier properties significantly. This was attributed not only to the densification of grain boundaries in Ta/V/Ta films, but also to the formation of two thermally stable sharp interfaces between Ta and V by ion bombardment, resulting in the reduction of the fast diffusion of Cu through Ta/V/Ta films.
Reduction of density of subgrain boundaries and misfit dislocations in epitaxial (001) SrTiO3 thin films: Effect on dielectric tunability
Authors: Ryen L. , Wang X. , Petrov P. , Carlsson E. , Helmersson U. , Olsson E.
Published: July 2002

The effect of a YBa2Cu3O7−x buffer layer on the quality of rf magnetron sputtered epitaxial (001) SrTiO3 thin films on a LaAlO3 substrate has been investigated using high resolution transmission electron microscopy. Magnetron sputtered SrTiO3 films generally exhibit a columnar subgrain morphology. By using a 75 nm thick YBa2Cu3O7-x buffer layer the subgrain boundary area was reduced considerably compared to single layer films. Subgrain widths around 130 nm were observed, which corresponded to an 80% reduction of the subgrain boundary area. The density of misfit dislocations was also reduced by 80%. By using a YBa2Cu3O7−x buffer layer, the dielectric tunability increased 30%, to a value of 1.43 (at U=16.5 V/μm, 2 MHz, 77 K). The influence of interfacial strain and misfit dislocations on the subgrain structure, and corresponding effect on the dielectric constant εr, is elucidated.
Effect of an epitaxial CoSi2 layer on diffusion of B and Sb in Si during annealing and oxidation
Authors: Tyagi A. K. , Kappius L. , Breuer U. , Bay H. L. , Becker J. S. , Mantl S. , Dietze H. J.
Published: July 2002

The diffusion of B and Sb in Si, with and without a 20-nm-thick epitaxial CoSi2 layer on top, is investigated, during annealing and oxidation, using doping superlattices (DSLs). CoSi2 layers were grown on Si by molecular beam allotaxy. DSLs were grown by molecular beam epitaxy. They consisted of six spikes with peak concentrations of 1018 cm−3(B) and about 1019 cm−3(Sb) with peak centers spaced 100 nm apart. The shallowest spike was capped with 100 nm of Si followed by 20 nm of CoSi2. Annealing in pure N2 and oxidation in pure O2 were performed at temperatures ranging from 800 to 1200 °C. Concentration depth profiles were measured by secondary ion mass spectrometry. The results showed that the dopant diffusion in Si was markedly different with or without a CoSi2 layer. For specimens without CoSi2 layer, we observed oxidation enhanced diffusion of B and oxidation retarded diffusion of Sb in accordance with the literature. However, the effect of CoSi2 layer was a strong retardation of B diffusion and an enhancement of Sb diffusion. The B diffusivity was retarded by a factor of 2–10 as compared to the thermal diffusivity and by a factor of 20–100 as compared to the corresponding diffusivity for oxidation of Si without a CoSi2 layer. Sb diffusivity was enhanced by a factor of 2 with respect to thermal diffusivity and by about a factor of 5 as compared to the case without a CoSi2 layer.
Effects of the conduction-band nonparabolicity on the gain in bulk InAsSb semiconductor lasers
Authors: Paskov P. P.
Published: July 2002

A simple model for the optical gain in bulk semiconductors with a nonparabolic band structure is developed. The model is applied to the calculation of gain spectrum and gain-carrier-density dependence in InAsxSb1−x. A comparison with the conventional parabolic band model is also performed. The obtained results show the necessity to include the conduction-band nonparabolicity in modeling of lasers based on narrow-gap semiconductor materials.
Electron energy loss spectroscopy studies of the amorphous to crystalline transition in FeF3
Authors: Saifullah M. S. M. , Botton G. A. , Boothroyd C. B. , Humphreys C. J.
Published: July 2002

Electron beam-induced crystallization studies in amorphous FeF3 films using electron energy loss spectroscopy (EELS) are discussed in this letter. Time-resolved EELS studies show that the coordination polyhedra in amorphous FeF3 (a-FeF3) are randomly arranged FeF6 octahedra. They arrange themselves to give long range order during crystallization to FeF2 and FeF3 under the electron beam. Changes in the d-band occupancy by one electron as well as the sensitivity of the ratio of the Fe L3 and L2 edges to the electronic configuration of the iron ion are clearly seen during the crystallization process.
Fabrication and characteristics of three-dimensionally buried porous silicon optical waveguides
Authors: Takahashi Morio , Koshida Nobuyoshi
Published: July 2002

A fabrication technology of three-dimensionally buried porous silicon (PS) optical waveguide with an extremely high refractive index contrast is presented, including its fundamental properties. The origins of attenuation losses are investigated by experimental and theoretical analyses in terms of microscopic observations, edge emission measurements, polarization mode determination of guided wave, and evaluation of bending loss. The results of these studies indicate that attenuation losses in PS waveguides are due to self-absorption by residual silicon, structural and optical inhomogeneities in the core region, and roughness at interfaces between the core and the cladding layers. Some possible ways for reducing these attenuation losses are discussed. It is also demonstrated that a buried bent PS waveguide with an extremely small curvature of 250 μm can be fabricated by simple planar processing, and that a visible optical wave propagates along it owing to a significantly high refractive index contrast between the core and the cladding layers.
Spatially resolved near-field photoluminescence spectroscopy of II–VI quantum wells: The role of localized excitons
Authors: Cingolani R. , Bastard G. , Labardi M. , Fuso F. , Allegrini M. , Sorba L. , Vanzetti L. , Franciosi A.
Published: July 2002

Excitons localized at compositional potential fluctuations are individually resolved by spatially resolved near-field luminescence in ZnCdSe/ZnSe quantum wells at room temperature. Localization is found to occur on a scale length of about 100 nm, in Cd-rich clusters in which the local Cd content varies by approximately 1% with respect to the nominal composition, and in steps of about 0.3%. A microscopic modeling of the localized exciton states is presented, which describes quite well the observed near-field spectra.
Spatial and spectroscopic profiles of the Kondo resonance for magnetic atoms on metal surfaces
Authors: Kawasaka Taihei , Kasai Hideaki , Diño Wilson Agerico , Okiji Ayao
Published: July 2002

Manifestations of the Kondo effect on an atomic length scale on and around magnetic atoms adsorbed on a nonmagnetic metal surface differ depending on which spectroscopic mode the scanning tunneling microscope is operated. In a previous report [T. Kawasaka, H. Kasai, and A. Okiji, Phys. Lett. A 250, 403 (1998)], we suggested the possibility that the Kondo effect can be directly observed as protrusions (peaks) in the spatial distribution of the tunneling current. By comparing the spatial distributions of the conduction electron and localized electron wave function, we show that the protrusions (peaks) observed in the spatial distribution of the tunneling current correspond to the resonances observed in the differential conductance spectra. Furthermore, we investigate the temperature dependence of the spatial distribution of the tunneling current and of the differential conductance to study the electronic properties of various metal–adatom systems.
Early stages of mechanical crystallization of amorphous FeZrBCu soft magnetic material
Authors: Friedrich J. , Herr U. , Samwer K.
Published: July 2002

Nanocrystalline Fe-base soft magnetic materials with high saturation magnetization have been obtained by crystallization of FeZrBCu amorphous precursors. In contrast to the conventional thermal crystallization, mechanical crystallization provides an alternative route for the generation of nanocrystalline materials using a high energy ball mill. Special emphasis is put here on the early stages of the transformation. X-ray diffraction, Mössbauer spectroscopy, and magnetization measurements are used to characterize the material. We find a continuous increase of the saturation magnetization accompanied by an increase of the crystalline fraction. The coercivity shows a different behavior with a large increase after very short milling within the amorphous state and a decrease for long milling duration. The influence of the deformation process on the magnetic anisotropy and the coercivity is discussed in terms of stresses and shear band formation. The results are compared with the phase formation and magnetic properties of thermally crystallized FeZrBCu samples and mechanical crystallization studies of other materials.
Dynamic conductance of Ni80Fe20/AlOx/Ni80Fe20 tunnel junctions
Authors: Nowak Janusz , Song Dian , Murdock Ed
Published: July 2002

The shape of the dynamic conductance versus voltage of NiFe/AlOx/NiFe tunnel junctions is correlated with the intensity and duration of oxidation. A shift of the conductance minimum towards positive voltage (up to 100 mV) indicates that the Al layer is only partially oxidized. In contrast, a shift of the conductance minimum towards negative voltage indicates oxidation of the bottom electrode and/or damage to the top surface of the barrier. Annealing of the junctions makes the tunnel barrier effectively thinner and taller, as implied by a fit to the parabolic conductance. These qualitative trends are observed for Al layers ranging from 10 to 35 Å in thickness. In conjunction with these effects, we observe the highest magnetoresistance (up to 26.6%) and the best bias dependence of the magnetoresistance when the conductance is symmetric.
Capture cross sections of defect states at the Si/SiO2 interface
Authors: Albohn J. , Füssel W. , Sinh N. D. , Kliefoth K. , Fuhs W.
Published: July 2002

Modulation capacitance voltage spectroscopy is applied to study the interaction between interface defect states and the conduction band of thermally oxidized n-type silicon wafers, which were prepared using a broad spectrum of preparation conditions. The modulation frequency response of metal oxide semiconductor samples is measured in depletion and accumulation as a function of temperature and of the position of the Fermi level at the interface. The data reveal two different sets of states, the capture cross sections of which differ by as much as 1–2 orders of magnitude. We assign these states to two types of defects which originate from silicon dangling bonds with three backbonded silicon atoms (Pb center) and silicon dangling bonds where one backbond is substituted by oxygen (PL center). Both capture cross sections decrease monotonically with decreasing energetic separation from the conduction band.