bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of electromigration-induced edge drift in Al–Cu thin-film interconnects
Authors: Kim Choong-Un , Morris J. W. , Lee Hyuck-Mo
Published: July 2002

This article employs a one-dimensional diffusion model to study the phenomenon of electromigration-induced edge drift in a finite, Al–Cu thin-film conductor. Edge drift is caused by the accumulation of vacancies at the negative (upstream) terminal of the conductor as Al diffuses with the electrical current. When the Cu content exceeds its solubility limit, grain boundaries are decorated with Al2Cu precipitates, which must be dissolved before significant Al diffusion occurs. Assuming one-dimensional flow in a homogeneous, polygranular film, we compute the rate of growth of the precipitate-free zone at the upstream terminal, and estimate the incubation time for the onset of edge drift. The results predict an incubation time that increases with the grain size and the initial Cu content, and decreases with the square of the current density. The incubation time is inversely proportional to the “electromigration diffusivity”, DE=DBCuδZCu*, the product of the grain boundary diffusivity of Cu, the effective grain boundary thickness, and the effective valence of the Cu ion. The results are used to compare a number of prior experimental studies, which are shown (with one exception) to produce consistent values for DE. An analysis of the experimental results suggests that edge drift begins almost as soon as the precipitate-free zone length exceeds the “Blech length” for the line, suggesting that the presence of Al2Cu precipitates in the grain boundaries is essential to retard Al electromigration.
Study of solid surfaces by metastable electron emission microscopy: Energy-filtered images and local electron spectra at the outermost surface layer of silicon oxide on Si(100)
Authors: Yamamoto Susumu , Masuda Shigeru , Yasufuku Hideyuki , Ueno Nobuo , Harada Yoshiya , Ichinokawa Takeo , Kato Makoto , Sakai Yuji
Published: July 2002

We have observed images and local electron spectra of an oxide pattern on Si(100) using metastable electron emission microscopy (MEEM) recently developed at our laboratory. Low-energy electron microscopy (LEEM) was also used. For both MEEM and LEEM, the energy-filtered images were obtained for the first time. It was shown that MEEM gives the information on the outermost surface layer selectively, while LEEM provides averaged information on several surface layers. The intensity of the band in the local electron spectrum of MEEM can be related to the distribution of the relevant orbitals exposed outside the surface, with which metastable atoms interact effectively. Thus, using energy-filtered MEEM, we can observe the map reflecting the distribution of individual orbitals at the outermost surface layer.
Comparison of hydrogen atom density measurements in three types of discharges using H2–N2 gas mixtures
Authors: Ganguly B. N. , Bletzinger P.
Published: July 2002

Two-photon laser induced fluorescence measurement of H atom density in %H2–%N2 gas mixture discharges has shown that the flux of H atom remains nearly constant over a wide range of gas compositions and pressures in three types of discharge devices. This unique feature is attributable to an efficient multiquantum N2 vibrational energy transfer to the dissociation of H2. This result shows that an appropriate choice of molecular gas mixtures can be used to extend the discharge operating conditions and device size scaling without compromising atom flux production.
Faceted GaInAs/InP nanostructures grown by selective area chemical beam epitaxy
Authors: Finnie P. , Charbonneau S. , Buchanan M. , Lacelle C. , Fraser J. , Roth A. P.
Published: July 2002

InP was grown by chemical beam epitaxy in narrow windows of widths varying between 20 and 2 μm, oriented along the [011] or [01̄1] directions opened in a SiO2 mask on an (001) InP substrate. Several facets appear along the sidewalls and on the edge of the mesas owing to different growth rates on different crystallographic planes. These can be understood as consequences of the migration of group III species from one crystallographic plane to another. We have studied the formation of such facets and their effects on the growth of GaInAs/InP structures of various thicknesses. The samples were studied using a field emission scanning electron microscope (SEM) and low temperature photoluminescence (PL). SEM micrographs show that for lines oriented along the [011] direction the dominant InP sidewall facets are (111)B planes on which GaInAs does not grow as long as Ga and In species can migrate towards (001). For the orthogonal direction, however, the lateral growth rate of the InP sidewalls is large and the faceting of the mesas is more complicated. The PL spectra of GaInAs quantum wells grown on such mesas exhibit several peaks whose energy depends on the initial width of the mask. They can be interpreted in terms of crystallographic plane dependent migration and desorption rates of Ga and In species. The (111)B facets of [011] directed mesas were used to produce inverted V-shaped mesa wire structures.
Hydrogen-oxygen-vacancy complexes in Czochralski-grown silicon crystal
Authors: Hatakeyama H. , Suezawa M.
Published: July 2002

We studied the process of generation and some properties of shallow donors generated by annealing a hydrogen-doped Czochralski-grown silicon crystal after electron irradiation. Hydrogen was doped by annealing specimens at 1200 °C in hydrogen atmosphere followed by quenching. Specimens were irradiated with 3 MeV electrons. Three kinds of shallow donors, termed D1–D3, were generated and annihilated due to isochronal annealing between 300 and 650 °C as shown by the measurement of optical absorption at 6 K. The generation process of D1 was well described by the first order reaction. The activation energy for the generation of D1 was about 1.8 eV. This agreed well with the migration energy of a vacancy-oxygen pair (VO pair). Hence, it is proposed that D1 is a complex of one VO pair and a hydrogen aggregate. Neutral D1 is in a metastable state. The optical threshold energy for the excitation of the neutral charge state of D1 was about 0.36 eV. The energy barrier from the neutral to the stable state of D1 was estimated to be about 0.13 eV.
Use of dipole doping to suppress switching in indium phosphide double heterojunction bipolar transistors
Authors: McAlister S. P. , McKinnon W. R. , Driad R. , Renaud A. P.
Published: July 2002

In this article we show the effect on the dc and rf transport characteristics of dipole doping at the collector heterojunction in an InP/InGaAs double heterojunction bipolar transistor. We show how the switching characteristics of devices with abrupt undoped InGaAs/InP collector heterojunctions can almost be eliminated by using the dipole doping at that interface and how the high-frequency performance is also improved.
Photoluminescence study of the dynamical properties of GaAs sawtooth superlattices
Authors: Johnston M. B. , Gal M. , Li G. , Jagadish C.
Published: July 2002

The dynamic properties of a sawtooth superlattice (δ-doped nipi) were examined by photoluminescence (PL) spectroscopic techniques. The structure was grown on a semi-insulating GaAs substrate by metalorganic vapor phase epitaxy, using C and Si δ-doping. The excitation intensity dependence of the sample’s PL was measured over six decades which produced a shift of 200 meV in the peak of the PL photon energy. The dynamic properties of the sawtooth superlattice were probed using time resolved PL and carrier lifetime measurements. Time resolved PL was measured over 6 orders of magnitude in delay time. The luminescence wavelength from the sawtooth superlattice sample was found to shift to low energies over time after pulsed excitation, indicating the temporal evolution of the band edges. A new and sensitive technique for measuring radiative recombination lifetimes at low excitation intensities was developed. Therefore δ-doped sawtooth superlattices are shown to have a tunable band gap as well as an intensity tunable carrier lifetime.
AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactors
Authors: Fu Y. , Stake J. , Dillner L. , Willander M. , Kollberg E. L.
Published: July 2002

By the Schrödinger and Poisson equations, we have theoretically investigated AlGaAs/GaAs and InAlAs/InGaAs single barrier varactors. The energy band structure, carrier distribution, and conduction current are fully exploited for varactor design. We have explained the experimental current–voltage and capacitance–voltage measurements very well. A simple analytical model for energy band structure is derived based on the Schrödinger and Poisson equation calculation. It is found that a barrier structure of 3 nm Al0.3Ga0.7As/3 nm AlAs/3 nm Al0.3Ga0.7As for an Al0.3Ga0.7As/GaAs varactor and a barrier structure of 8 nm In0.52Al0.48As/3 nm AlAs/8 nm In0.52Al0.48As for In0.52Al0.48As/In0.47GaAs are optimal for minimal conduction currents.
Silicon-based interminiband infrared laser
Authors: Friedman Lionel , Soref Richard A. , Sun Gregory
Published: July 2002

Simulations of a room-temperature p-i-p coherently strained Si0.5Ge0.5/Si superlattice quantum-parallel laser diode have been made. Calculations have been made of the local-in-k-space population inversion between the nonparabolic heavy-hole valence minibands, HH2 and HH1. Lasing is at 5.4 μm and the optical dipole matrix element is 3.7 A. Analysis of radiative-and-phonon scattering between the “mixed” bands indicates a lifetime difference between the upper and lower states of 2.4 ps. At an injected current density of 5000 A/cm2, a laser gain of 134 cm−1 is calculated.
On the amorphization process in Al0.6Ga0.4As/GaAs heterostructures
Authors: Turkot B. A. , Robertson I. M. , Rehn L. E. , Baldo P. M. , Forbes D. V. , Coleman J. J.
Published: July 2002

The amorphization mechanism operative in Al0.6Ga0.4As/GaAs structures has been studied by using ion implantation at 77 K with different ion masses (Kr and Ar) and energies (1–3 MeV). The damage produced by these implantations was examined by using a combination of ion channeling and cross-sectional transmission electron microscopy. For all implantation conditions, the damage accumulation produced by implantation at 77 K increases with depth into the Al0.6Ga0.4As layer. Neither point defects nor energetically dense cascade events by themselves were found to be sufficient to drive Al0.6Ga0.4As amorphous at 77 K. Amorphization occurred only when a high density of energetically dense cascade events were superimposed on a high total defect population. These conditions were satisfied deep in the layer and only for the 1.5 MeV Kr ion implantation. With the exception of the amorphous material, the damage structure produced at 77 K recovered on warming to room temperature. The recovered structure consisted of dislocation loops or planar defects.
Tantalum-microcrystalline CeO2 diffusion barrier for copper metallization
Authors: Yoon Dong-Soo , Baik Hong Koo , Lee Sung-Man
Published: July 2002

A tantalum diffusion barrier incorporating microcrystalline CeO2 is proposed for Cu metallization and investigated using Auger electron spectroscopy, x-ray diffraction, optical microscopy, transmission electron microscopy, and sheet resistance measurements. The Cu/Ta+CeO2/Si contact system retained its structure up to 800 °C without an increase in resistivity. The cerium dioxide (CeO2) was stuffed along the grain boundaries during the deposition of Ta layer. Because of its heavier atomic weight than O2- or N2-stuffed elements, it inhibited an interdiffusion of Cu and Si through grain boundaries which can act as fast diffusion paths. It also resulted in preventing the outdiffusion of Ta into the overlayer Cu as well as suppressing the formation of Ta silicide up to 800 °C. It appears that the barrier properties of the tantalum incorporating microcrystalline CeO2 are superior to polycrystalline transition metal barriers, polycrystalline nitride barriers, ternary amorphous compound barriers, and N2- and O2-stuffed barriers.
Role of 0.66 eV dominant trap in annealed low-temperature grown molecular beam epitaxial GaAs
Authors: Chen N. C. , Wang P. Y. , Chen J. F.
Published: July 2002

The sample considered herein is an annealed low-temperature (LT) molecular beam epitaxially grown GaAs of n-LT-i-p structure with the LT layer grown at 300 °C. Characteristics involving the dominant trap level located at about 0.66 eV below the conduction band are obtained by analyzing the data of the admittance spectroscopy, capacitance–voltage, current–voltage, and frequency-dependent conductance experiments. This trap pins the fermi level of the LT layer and makes the LT layer semi-insulating. In this structure, the level interacts with both the conduction band and the valence band with a hole emission time constant characterized by an activation energy of 0.77 eV and a cross section of 1.1×10−13 cm2. This level is also an effective generation-recombination center when the temperature exceeds 300 K.
Transfer impedance calculations of electronic noise in two-terminal semiconductor structures
Authors: Starikov E. , Shiktorov P. , Gružinskis V. , Varani L. , Vaissiere J. C. , Nougier J. P. , González T. , Mateos J. , Pardo D. , Reggiani L.
Published: July 2002

The time-domain formulation of the transfer-impedance method is developed to calculate the impedance field of two-terminal semiconductor structures. The voltage noise spectrum associated with velocity fluctuations is then calculated for overmicron and submicron n+nn+ GaAs diodes in the framework of a closed hydrodynamic approach based on the velocity and energy conservation equations. Transit-time effects are found to influence substantially the noise spectrum in a wide frequency range above 10 GHz. The good agreement found with Monte Carlo simulations validates the proposed theoretical approach.
Concentration of neutral oxygen vacancies in buried oxide formed by implantation of oxygen
Authors: Seol Kwang Soo , Futami Tsuyoshi , Ohki Yoshimichi
Published: July 2002

Using synchrotron radiation as a photon source, photoluminescence spectra were obtained for buried oxide formed by implantation of oxygen. From the spectra, the oxide was known to have relaxed and unrelaxed neutral oxygen vacancies with respective concentrations of 1.4×1020 and 1.0×1017 cm−3.
Optimized performance of a powerful hollow-cathode rf oscillator
Authors: Felsteiner J. , Ish-Shalom S. , Slutsker Ya. Z.
Published: July 2002

We have studied the power, efficiency, and frequency ranges achievable for a hollow-cathode based rf oscillator. We have found that the rf component of the discharge current can be much larger (up to ∼4.5 times) than its dc component. In the achievable frequency range of 15–120 MHz, this very simple cold cathode device produces up to 40 kW of rf power with efficiency of up to 25%. The internal resistance of this device always remained within a convenient range of 20–40 Ω. The rf pulse duration could be prolonged up to tens of μs. The physical processes governing the oscillator parameters are discussed.
Infrared photoresponse of (Mn,Sb) doped-Pb(Zr,Ti)O3/YBa2Cu3O7 heterostructure detectors
Authors: Wu N. J. , Chen Y. S. , Fan J. Y. , Ignatiev A.
Published: July 2002

Mn- and Sb-doped PbZrTiO3 (PMSZT) thin-film pyroelectric infrared (IR) detectors have been fabricated on c-oriented YB2Cu3O7−x (YBCO) thin films. The thin-film heterostructure detectors were prepared by pulsed laser deposition on LaAlO3 (100) substrates. The YBCO layer in the PMSZT/YBO heterostructure was used as an atomic template for the epitaxial growth of the PMSZT thin films, as well as a nonsuperconducting electrode and IR reflector in the detector. Mn and Sb doping was used to tune the Pb(Zr,Ti)O3 transition temperature to a lower value (Curie temperature of ∼170 °C), and hence, to modify the pyroelectric and dielectric properties of the pyroelectric material. The PMSZT IR detectors were examined as to their photovoltage and photocurrent in response to IR source temperature, detector temperature, and chopper frequency. The experimental results show that the detector has relatively high IR sensitivity with a detectivity D* of ∼4×108 cm Hz1/2/W, and is capable of operating from room temperature to ∼170 °C.
The wavelength-dependent transfer function for optically controlled semiconductor lasers
Authors: Parker M. A. , Shire D. B.
Published: July 2002

The optically controlled laser, consisting of a main semiconductor laser with an associated control beam, is important for integrated high-speed optical signal processing and for optical communications. The optically controlled laser rate equations, the gain, and the wavelength-dependent transfer function are derived from the Liouville equation for the density matrix describing a system with multiple perturbing optical fields. The results explain the primary effects of coherent and noncoherent control signals upon the output of the main laser (illustrated with experimental data). Predictions are obtained for optical gain, wavelength conversion, and FM-to-AM demodulation.
Particulate distributions in Pt films prepared by pulsed laser deposition
Authors: Leuchtner R. E. , Horwitz J. S. , Chrisey D. B.
Published: July 2002

A quantitative statistical analysis of micron-sized particulates on the surface of oriented platinum thin films prepared on 〈100〉 MgO at 450 °C was made. The particulates, resulting from nanosecond laser ablation, were evaluated as a function of laser fluence (0.75–3.00 J/cm2) to determine the average diameter, spatial distribution relative to the center of the ablation plume, and areal density. The latter quantity is defined as the number of particulates per cm2 of film surface per nanometer of film thickness. The particulate shape was generally spherical with a diameter range of ∼0.05–1.0 μm. For a given laser fluence, the areal density was roughly constant from the plume center outwards suggesting that particulate reduction by growing films in off-axis regions may be problematic. Increasing the laser fluence from 0.75 to 3.00 J/cm2 however, resulted in a two order of magnitude reduction in the areal density, although an increase in the average particulate diameter from 0.26 to 0.47 μm occurred. This result implies that, at least with metals, increasing laser fluence is a pragmatic approach to particulate reduction in laser deposited thin films.
The origin of Ga2O3 passivation for reconstructed GaAs(001) surfaces
Authors: Guo-Ping Jiang , Ruda Harry E.
Published: July 2002

Ab initio molecular-orbital calculations are used to study the origin of the Ga2O3 passivation mechanism for GaAs(100) reconstructed surfaces. Two cluster models are used to simulate the main features of reconstructed and oxygen chemisorbed GaAs(100) surfaces. The simulation results show that the reduction in the density of surface states located within the bulk energy gap derives from the initial near-bridge-bonded O atoms. The calculated electronic energy spectra reveal that the surface-state energy gap lies completely outside of the bulk energy gap in distinct contrast to the case for S passivation. At the optimized geometry, each surface Ga atom (situated beneath the adsorbed O) is distorted by 0.40 Å from its ideal position, resulting in a strained surface. O atoms are almost buried in the GaAs(100) surface; each is located 0.30 and 0.25 Å above the reconstructed GaAs(100) surface, respectively. The O–Ga bond length is 1.63 Å and the Ga–O–Ga bond angle is 157.4°. Each O atom deviates from the bridge position by 0.11 and 0.19 Å from the vertical position, respectively. This causes further deposition to result in the formation of an amorphous oxide film, which provides an effective protection layer against further oxidation of the near-bridge-site oxidized GaAs surface. The calculated electronic structure and local density of states also reflect a large charge accumulation near the adsorbed O atoms.
The origin and interpretation of fine scale magnetic contrast in magnetic force microscopy: A study using single-crystal NdFeB and a range of magnetic force microscopy tips
Authors: Al-Khafaji M. , Rainforth W. M. , Gibbs M. R. J. , Bishop J. E. L. , Davies H. A.
Published: July 2002

We have used an oriented single crystal of Nd2Fe14B1 as a test sample to explore contrast and resolution as a function of magnetic force microscopy tip coating, tip scan height, and sample history. We find that resolution is independent of tip type; that contrast is greater at low scan heights and for higher moment tips; and that the fine length scale structure of order 25 nm can be resolved. We further show how the surface structure is a function of state (demagnetized or remanent), demonstrating the ability of well-characterized studies to give high-resolution information on carefully prepared samples.
Off-axis electron holographic mapping of magnetic domains in Nd2Fe14B
Authors: McCartney M. R. , Zhu Yimei
Published: July 2002

Off-axis electron holography employing a field-free Lorentz lens has been used to provide direct imaging of magnetic induction in Nd2Fe14B with nanometer-scale resolution and high signal-to-noise. Using this technique, reconstructed phase images have been used to measure domain wall widths for 90° and 180° walls in a sintered sample were measured to be no greater than 7 and 9 nm, respectively. Induction maps show an unusual array of 90° domains in a hot-press sample, with singularities at the intersections of the domains. In situ thermal annealing of the hot-pressed sample resulted in magnetization rotation accompanied by domain wall movement and evidence for pinning at a structural defect. Heating of the sample above 400 °C resulted in the accumulation of small particles in thin regions and decrease in magnetic contrast.
Hysteresis and self-sustained oscillations in space charge limited currents
Authors: Gartstein Yu. N. , Ramesh P. S.
Published: July 2002

A problem of charged particle flow between parallel plate electrodes with arbitrary injection velocities is revisited. A steady-state analysis is advanced by the kinetic simulations which show that the system under consideration can exhibit features characteristic of nonlinear dynamical systems. In particular, a hysteresis between supply limited and space charge limited currents is demonstrated as well as the existence of self-sustained oscillations in the space charge limited regime. These phenomena are most sharply pronounced for narrow distributions of the initial injection energies.
Formation of carbon nanotubes and their filling with metallic fibers on ion-emitting field anodes
Authors: Okuyama F. , Hayashi T. , Fujimoto Y.
Published: July 2002

Detailed descriptions are given of the formation of carbon nanotubes and their filling with metallic fibers developing on field anodes in the so-called “anode activation” process. Unlike the multishell carbon nanotubes produced in arc plasmas, these carbon nanotubes are not perfectly graphitized. The metallic fibers grown inside the nanotubes contain chromium as their major component, but their structure does not conform to any of the known compounds of Cr nor to elemental Cr. The growth morphology and lateral dimension of metallic fibers are governed by those of their host nanotubes, thus falling into the category of “template-mediated” crystal growth. Evidence is also presented that the metallic fibers epitaxially promote the graphitization of the carbon nanotubes.
Analysis of the Fiske modes in a short Josephson junction with nonuniform critical current density
Authors: Gambardella Umberto , Celentano Giuseppe , Boffa Vincenzo , Pace Sandro
Published: July 2002

This article analyzes the magnetic field dependence of the Fiske steps in a small Josephson junction having a peaked critical current density distribution along the direction orthogonal to the field. The maximum current of the resonant states is computed in the case of a periodic and a nonperiodic distribution of the peaks in the current density along the junction. It is observed that the maximum amplitude of the Fiske steps and the magnetic field at which they occur are affected by the nonperiodic peak distribution. These results are discussed in connection with parallel networks of junctions coupled by negligible inductances.
Diffusion studies of Ra and Pb in GaAs by the alpha-particle energy loss method
Authors: Adamcyk M. , Beaudoin M. , Kelson I. , Levy Y. , Tiedje T.
Published: July 2002

The temperature dependence of the diffusion of lead in GaAs is determined by measuring the modification to the energy spectrum of emitted alpha particles from the decay chain of implanted Pb212 atoms. Diffusion rates are measured for temperatures up to 900 °C. Higher rates are observed for the diffusion in silicon-doped GaAs than in semi-insulating GaAs. An upper limit for the diffusion of radium in GaAs is similarly obtained from the decay of the Ra224 isotope. Implications for the use of implanted alpha sources for thickness monitoring during epitaxial film growth by the alpha-particle energy loss method are discussed.
Incorporation of rare earths into II–VI compounds during molecular beam epitaxial growth: Extended x-ray absorption fine structure study of Sm-doped ZnTe
Authors: Wruck D. , Boyn R. , Parthier L. , Henneberger F. , Röhrich J.
Published: July 2002

We present extended x-ray absorption fine structure measurements performed on ZnTe:Sm films, at the Sm L3 edge. The films (thickness several hundred nanometers) were grown by molecular beam epitaxy on GaAs substrates and were doped during growth, achieving Sm concentrations of the order 1021–1022 cm−3. Analysis of the experimental data by standard methods shows that the majority of the Sm atoms are located on sites with octahedral Te coordination, with a Sm–Te bond length about 3.1 Å. These sites are thought to form during growth from zinc-blende-type interstitial sites due to the high chemical affinity between the rare earth and the chalcogen atoms. This result agrees with suggestions made by us in earlier work, based on optical and structural studies. A small part of the Sm atoms (with a fraction depending on the total Sm concentration) is found to be on sites with nearest-neighbor oxygen atoms. The results indicate two subspecies of the latter type, the mean Sm–O bond lengths lying between about 2.3 and 2.5 Å. The Sm–O sites are restricted to a layer of about 50 nm thickness at the outer boundary of the doped region. This is inferred from the depth distribution of O atoms determined by elastic recoil detection analysis.
Adsorption and desorption of S on and off Si(001) studied by ab initio density functional theory
Authors: Çakmak M. , Srivastava G. P.
Published: July 2002

We present detailed ab initio density functional calculations of equilibrium atomic geometry, electronic states, and chemical bonding for the adsorption of elemental S on Si(001). Following recently reported room temperature low-energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy, and work function measurements by Papageorgopoulos et al. [Phys. Rev. B 55, 4435 (1997)], three different adsorption models have been studied: hemisulfide (2×1) structure, monosulfide (1×1) structure, and disulfide (1×1) structure. For hemisulfide and monosulfide structures, the calculated location of S above the Si(001) surface is in excellent agreement with the experiment. An analysis of surface free energy suggests that, in the allowed range of S chemical potential, the monosulfide structure is more stable than the hemisulfide and disulfide structures. A signature of desorption of the SiS unit is obtained from the study of the disulfide structure.
Aging tests of InP-based laser diodes heteroepitaxially grown on Si substrates
Authors: Sasaki T. , Mori H. , Tachikawa M. , Yamada T.
Published: July 2002

Aging tests were performed for InP-based laser diodes heteroepitaxially grown on Si substrates. To define the degradation mode, we studied the change in the device characteristics and electroluminescence (EL) images during the aging tests. The degradation proceeds in a mode characterized by a gradual increase in the threshold current without a change in the current–voltage characteristics. During the aging, dark spot defects (DSDs) appear in the EL images, where no defects were observed initially. However, after a certain number of DSDs are generated, no more degradation occurs in the current–light-output characteristics and EL images. The correlation of the saturation behavior between the threshold current and DSD number is quite good, and strongly suggests that the device degradation is dominated by the development of nonradiative defects in the active layer. Judging from the saturation density of DSDs, we can assume that the origin of the nonradiative defects is closely related to the dislocation that remains in the heteroepitaxial layers.
Stimulated emission in GaN thin films in the temperature range of 300–700 K
Authors: Bidnyk S. , Little B. D. , Schmidt T. J. , Cho Y. H. , Krasinski J. , Song J. J. , Goldenberg B. , Yang W. , Perry W. G. , Bremser M. D. , Davis R. F.
Published: July 2002

We report the results of an experimental study on stimulated and spontaneous emission from high-quality single-crystal GaN films grown on 6H-SiC and (0001) sapphire substrates in the temperature range of 300–700 K. We observed edge-emitted stimulated emission (SE) at temperatures as high as 700 K for samples grown on both SiC and sapphire substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with temperature and empirical expressions for the energy positions are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanism in this temperature range. The temperature sensitivity of the SE threshold for different samples was studied and the characteristic temperature was found to be 173 K in the temperature range of 300–700 K for one of the samples studied. We suggest that the unique properties of SE in GaN thin films at high temperatures could potentially be utilized in optoelectronic devices.
A small angle neutron scattering and Mössbauer spectrometry study of magnetic structures in nanocrystalline Ni3Fe
Authors: Frase H. N. , Fultz B. , Spooner S. , Robertson J. L.
Published: July 2002

Results are reported from small angle neutron scattering and Mössbauer spectrometry measurements on nanocrystalline Ni3Fe. The nanocrystalline materials were prepared by mechanical attrition and studied in the as-milled state, after annealing at 265 °C to relieve internal stress, and after annealing 600 °C to prepare a control sample comprising large crystals. The small angle neutron scattering (SANS) measurements were performed for a range of applied magnetic fields. Small differences were found in how the different samples reached magnetic saturation. From the SANS data obtained at magnetic saturation, we found little difference in the nuclear scattering of the as-milled material and the material annealed at 265 °C. Reductions in nuclear scattering and magnetic scattering were observed for the control sample, and this was interpreted as grain growth. The material annealed at 265 °C also showed a reduction in magnetic SANS compared to the as-milled material. This was interpreted as an increase in magnetic moments of atoms at the grain boundaries after a low temperature annealing. Both Mössbauer spectroscopy and small angle neutron scattering showed an increase in the grain boundary magnetic moments after the 265 °C annealing (0.2 and 0.4μB/atom, respectively), even though there was little change in the grain boundary atomic density.
Non-steady-state photoelectromotive force in reduced lithium niobate crystals
Authors: Veenhuis H. , Buse K. , Krätzig E. , Korneev N. , Mayorga D.
Published: July 2002

Non-steady-state photocurrents are generated in reduced iron-doped lithium niobate crystals. Dependences on spatial frequency and on modulation amplitude are well described by the existing theory and a Debye screening length of 19 nm is determined. The non-steady-state photocurrents strongly decrease for frequencies above 2 Hz. The technique might be useful for material characterization and for convenient active stabilization of holographic setups against mechanical vibrations.
Measurement of the Al mole fraction of bulk AlGaN and AlGaN/GaN heterostructure by photoconductance method
Authors: Yu L. S. , Qiao D. , Lau S. S. , Redwing J. M. , Lin J. Y. , Jiang H. X.
Published: July 2002

A photoconductance method was used to determine the band-gap energy and, therefore, the Al mole fraction of bulk AlxGa1−xN and AlxGa1−xN/GaN heterostructures. The results are compared with those obtained by a more elaborate photoluminescence method.
Quantum wires under photoexcitation
Authors: Ruda H. , Shik A.
Published: July 2002

We present a theoretical analysis of the influence of external illumination on the main quantum wire parameters, namely, the linear electron density v and width of the electron channel a. The analysis is performed for two different types of quantum wires: those fabricated by etching and those formed electrostatically using a split Schottky-gate structure. Both impurity and interband optical excitation are considered. Changes in the wire parameters are shown to depend on the width of the wire structure, the light frequency and the electrical parameters of the gate circuit. Typically, these changes are more pronounced in etched rather than gated structures and can be used for tuning the wire parameters over a wide region.
Al concentration control of epitaxial AlGaN alloys and interface control of GaN/AlGaN quantum well structures
Authors: Choi Sung Chul , Kim Jong-Hee , Choi Ji Youn , Lee Kang Jae , Lim Kee Young , Yang Gye Mo
Published: July 2002

Al x Ga 1−x N alloys were grown by metalorganic chemical vapor deposition on c-plane sapphire substrates under various growth conditions. Both the Al concentration and the growth rate of AlGaN are strongly affected by gas-phase parasitic reaction between ammonia and group-III sources. As the ammonia flow rate increases with fixed flow rate of group-III sources, both the Al concentration in the solid and the growth rate of AlGaN are decreased. It was also found that the Al concentration in the solid saturated as increasing the Al gas composition increased by increasing the flow rate of the Al source precursor. In addition, the influence of growth interruption when group-III sources are temporarily shut off on the optical quality of GaN/AlGaN single quantum well (QW) structures was studied to realize high-quality QWs. Photoluminescence measurements revealed that the emission peak is blueshifted as the interruption time increases and the emission intensity is maximized at a several-second short interruption time, indicating the GaN QW layer is evaporated during interruption at a high growth temperature.
Many-spin effects and tunneling properties of magnetic molecules
Authors: Al-Saqer M. , Dobrovitski V. V. , Harmon B. N. , Katsnelson M. I.
Published: July 2002

Spin tunneling in molecular magnets has attracted much attention, however theoretical considerations of this phenomenon up to now have not taken into account the many-spin nature of molecular magnets. We present, to our knowledge, the first successful attempt of a realistic calculation of tunneling splittings for Mn12 molecules, thus achieving a quantitatively accurate many-spin description of a real molecular magnet in the energy interval ranging from about 100 K down to 10−12 K. Comparison with the results of the standard single-spin model shows that many-spin effects affect the tunneling splittings considerably. The values of ground state splitting given by single-spin and many-spin models differ from each other by a factor of 5.
Effect of particle size on the magnetization process in lithographic arrays of Nd2Fe14B
Authors: Kubota Hitoshi , Ikari Takayuki , Ando Yasuo , Kato Hiroaki , Miyazaki Terunobu
Published: July 2002

Controlled arrays of Nd2Fe14B were fabricated via sputtering and lithography. The arrays were fabricated via optical and electron-beam lithography combined with dry etching technique. Designed particle size varies from 10 μm×10 μm to 0.5 μm×0.5 μm. We found an increase in the coercive field with decreasing particle size. Results of the micromagnetic calculations suggest that the coercive-field increase in smaller-sized particles is originated from the effective decrease in the exchange-coupling energy owing to the relative increase in the fraction of surface grain.
The role of amorphous Ni50P50 precoating layer in CoCrPtTa thin film media
Authors: Lee Yueh Hua , Wang Jian Ping , Lu Li
Published: July 2002

The role of amorphous Ni50P50 as a precoating layer in CoCrPtTa/Cr/glass substrate thin film media has been studied. It was found that the amorphous NiP layer has a strong effect on grain size reduction. Both in-plane transmission electron spectroscopy (TEM) images and atomic force microscopy images reveal a large reduction of the average grain size of the Cr underlayer deposited on the precoating Ni50P50 layer compared to the one on a glass substrate. The TEM images also show that Ni50P50 film has isolated unique granular features approximately 10 nm in diameter, consisting of grains with uniform size that could be inherited by the subsequent layers. Samples with NiP precoating layer on the glass substrate show a slightly higher surface roughness that may be attributed to the smaller Co grains induced. Magnetic force microscopy images at remanent state also indicate a much smoother micromagnetic structure for media with a NiP precoating layer.
Enhancement of spin correlations in ZnTe/MnTe superlattices by Cl doping
Authors: Stumpe L. E. , Rhyne J. J. , Kaiser H. , Lee S. , Bindley U. , Furdyna J. K.
Published: July 2002

The effect of adding Cl impurity on the long-range antiferromagnetic coupling in [ZnTe|MnTe] magnetic semiconductor superlattices has been investigated by neutron diffraction. Superlattices consisting of five atomic planes of ZnTe alternated with ten atomic planes of MnTe[(ZnTe)5|(MnTe)10] have been prepared by molecular-beam epitaxy both with and without Cl added to the MnTe and/or ZnTe layers during deposition. This was motivated by the fact that Cl can form either shallow or deep donor levels in II–VI semiconductors, and that such electronic states can serve to modify magnetic interlayer exchange. Unchlorinated samples showed magnetic correlation lengths of around 225 Å (≈5 bilayers) at low temperatures, and the addition of Cl increased the correlation length at 15 K by nearly a factor of 2 to approximately 450 Å.
High-resolution study of magnetization and susceptibility by spin-polarized scanning tunneling microscopy
Authors: Wulfhekel W. , Ding H. F. , Kirschner J.
Published: July 2002

We present static measurements of the domain structure and dynamic results on domain wall motion and local susceptibility obtained by spin-polarized scanning tunneling microscopy. The topography and the magnetic structure of the sample are recorded simultaneously with down to 10 nm resolution. With this technique, domain wall movement on Co(0001) is studied in situ. In some cases, the magnetization of the sample is locally influenced by the stray field of the tip. Measuring higher harmonics in the tunneling current allows one to quantify this influence and measure magnetic susceptibilities on similar scales.
Rapid thermal processing of sendust for magnetic recording applications
Authors: Macken Declan , Scullion Paul , Duddy Kevin
Published: July 2002

This article describes a method of thermally processing sputter deposited 2-μm-thick sendust films to produce the high permeability form of this alloy. The method described utilizes rapid thermal processing to induce the phase change in the sendust from the disordered α type to the DO3 ordered structure as confirmed by Mössbauer spectroscopy. Optimization of the treatment resulted in films with a coercivity of less than 0.4 Oe. At a treatment temperature of 500 °C, a soak time of 120 s was required to achieve optimum coercivity and at a temperature of 600 °C a time of 15 s was required. Treatments at the lower end of the temperature range with longer process times gave a greater degree of reproducibility in the coercivity results. This thermal treatment gives rise to several performance advantages of the material relative to untreated sendust, including reduced “out-of-plane” magnetic component, improved high frequency permeability, and surface passivation. In addition, the processing times associated with rapid thermal processing are considerably less than for conventional annealing, making this an attractive production solution for magnetoresistive shielding applications.
Micromagnetic simulation of an ultrasmall single-pole perpendicular write head
Authors: Abelmann Leon , Khizroev Sakhrat K. , Litvinov Dmitri , Zhu Jian-Gang , Bain James A. , Kryder Mark H. , Ramstöck Klaus , Lodder Cock
Published: July 2002

In single-pole heads with a pole tip cross section trimmed down to 100×200 nm by focused ion beam, the remanence of the head becomes nonzero if the pole length is too high. Large-scale three-dimensional micromagnetic simulations are performed to understand this effect, and to calculate the critical length of the pole as a function of the pole tip cross section.
Polarization properties of partially coherent Gaussian Schell-model electromagnetic beams
Authors: Seshadri S. R.
Published: July 2002

The polarization properties of highly directional electromagnetic beams generated by planar sources of different states of spatial coherence are investigated. Cross-spectral density matrix based on modifications of Gaussian Schell-model beams is used. Azimuthally symmetric beams and dipolar beams are considered. The electromagnetic fields are assumed to be completely polarized in the source plane. The variations of the degree of polarization in the transverse directions and in the axial direction are examined. The dependence of these variations on the degree of coherence of the source distribution is discussed. The depolarization effects of the fluctuations in the source distribution are found to be less severe for the dipolar beam as compared to that for the azimuthally symmetric beam.
Glass-forming range of the Ni–Mo system derived from molecular dynamics simulation and generalized Lindemann criterion
Authors: Zhang Q. , Li Z. C. , Lin C. , Liu B. X. , Ma E.
Published: July 2002

By using molecular dynamics simulation with an n-body Ni–Mo potential, the relative stability of the Ni- and Mo-based solid solutions versus their amorphous counterparts is studied as a function of solute concentrations at 300 K. It is observed that the supersaturated terminal solid solutions have the maximum or critical solute concentrations of 21 at. % of Mo in Ni and 25 at. % of Ni in Mo, respectively, beyond which the solid solutions transform into an amorphous phase. As such, the glass-forming range of the Ni–Mo system is derived to be 21–75 at. % of Mo. The computed critical solute concentrations are compared with those predicted by the generalized Lindemann melting criterion as well as with those revealed by ion-beam mixing/solid-state interdiffusion reaction experiments of Ni–Mo multilayered films.
Optical properties of titanium containing amorphous hydrogenated carbon films (a-C:H/Ti)
Authors: Schüler Andreas , Ellenberger Christoph , Oelhafen Peter , Haug Christian , Brenn Rüdiger
Published: July 2002

Titanium containing amorphous hydrogenated carbon films have been deposited by reactive magnetron sputtering. Real time laser reflectometry and ex situ spectrophotometry have served as a means for the determination of the optical constants index of refraction n and extinction coefficient k. The experimental results are being compared to effective medium theories, modeling the optical constants of the composite material from the ones of amorphous hydrogenated carbon and TiC. We have used the effective medium theories to estimate the bulk stoichiometries which agree with Rutherford backscattering experiments but deviate from the surface stoichiometries determined by in situ photoelectron spectroscopy. These deviations can be explained by surface diffusion and three dimensional cluster growth.
Surface and interface shear horizontal acoustic waves in piezoelectric superlattices
Authors: Bousfia A. , El Boudouti E. H. , Bria D. , Nougaoui A. , Djafari-Rouhani B. , Velasco V. R.
Published: July 2002

The propagation of acoustic waves of shear horizontal polarization in infinite and semi-infinite superlattices made of two piezoelectric media is studied within a Green’s function method. Localized modes induced by a free surface of the superlattice or a superlattice/substrate interface are investigated theoretically. These modes appear as well-defined peaks of the total density of states inside the minigaps of the superlattice. The spatial localization of the different modes is studied by means of the local density of states. The surface of the superlattice and the superlattice/substrate interface are considered to be either metallized or nonmetallized. We show the possibility of the existence of interface modes, which are without analogue in the case of the interface between two homogeneous media (the so-called Maerfeld–Tournois modes). We also generalize to piezoelectric superlattices a rule about the existence and number of surface states, namely when one considers two semi-infinite superlattices together obtained by the cleavage of an infinite superlattice, one always has as many localized surface modes as minigaps, for any value of the wave vector k∥ (parallel to the interfaces). Specific applications of these results are given for CdS–ZnO superlattices with a free surface or in contact with a BeO substrate.
New substitutions in hexagonal ferrites to reduce anisotropy without using Co
Authors: Lubitz Peter
Published: July 2002

Hexagonal ferrites that have a previously unexplored class of substitutions have been made and evaluated by observing their microwave absorption spectra as a function of temperature. The temperature dependences of the uniaxial anisotropy and the magnetic moment were determined. Substitutions were investigated in hexagonal ferrites normally having a small divalent ion in their composition, e.g., Zn–Z(Zn2Ba3Fe24O41), Ni–W, etc. Substitution of a further Zn and Ti for Fe reduced the uniaxial anisotropy field, Ha, of Zn–Z rapidly from ∼5 kOe to a few kilo-oersteds, while replacing only about one of the 24 Fe atoms in the formula unit. This rapid reduction in anisotropy along with moderate reductions in TC and moment at 300 K, imply that these substitutions preferentially fill the high anisotropy, up-spin, trigonal bipyramidal sites. The anisotropy field is only weakly temperature dependent near 300 K. This contrasts strongly with the case of Co substitutions often used to obtain Ha in this range. Complications caused by rapid magnetic relaxation and magnetostriction associated with divalent Co are also avoided.
Formation and decomposition of hydrogen-related electron traps at hydrogenated Pd/GaAs (n-type) Schottky interfaces
Authors: Nie H.-Y.
Published: July 2002

We demonstrate a capability for exploring the behavior of hydrogen at a Pd/GaAs (n-type) Schottky interface containing a native oxide. By applying a large forward current to such a hydrogenated interface, a remarkable reduction of the hydrogenation effect was observed. Formation of hydrogen-related electron traps near the interface could be responsible for this reduction of hydrogenation effect. Moreover, evidence was observed for the decomposition of those electron traps when hydrogen was forced to diffuse out from such interfaces.