bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen-surface reactions during the growth of hydrogenated amorphous silicon by reactive magnetron sputtering: A real time kinetic study by in situ infrared absorption
Authors: Katiyar M. , Yang Y. H. , Abelson J. R.
Published: July 2002

This article experimentally identifies the hydrogen incorporation and release processes which control the final hydrogen content of hydrogenated amorphous silicon films (a-Si:H). We deposit films using reactive magnetron sputtering of a silicon target in an Ar and H2 atmosphere. Hydrogen incorporation or loss is measured using real time infrared reflectance spectroscopy. An optical cavity substrate increases the sensitivity, allowing us to observe Si–H bonding in layers ≥5 Å thick via the stretching mode absorption (1800–2300 cm−1). We observe a narrow component at ∼2100 cm−1 corresponding to all SiHx bonds on the physical surface; the line width allows us to distinguish this contribution from the broader bulk modes. Various combinations of growth flux (isotope labeling, hydrogen partial pressure between 0.1 and 2.0 mTorr) and substrate material (on SiO2, a-Si, or a-Si:D) at substrate temperatures between 120 and 350 °C are used to distinguish various mechanisms. From the deposition of a-Si:H films on SiO2, we quantify the H surface coverage at the end of the nucleation stage (10 Å of growth) to be 1.2±0.3×1015 cm−2, essentially independent of growth conditions. From the evolution of Si–H bonding during the initial growth (≤25 Å), we infer a reduction of the surface area (smoothening) during nucleation and coalescence. During a-Si:H growth on unhydrogenated a-Si, we observe H implantation up to a depth of 40 Å, and derive the total flux of arriving H as a function of hydrogen partial pressure. For the exposure of a-Si:H to atomic deuterium and of a-Si:D to atomic H, we observe a loss/gain of surface H due to abstraction or exchange reactions. When a-Si:D or a-Si films are deposited on a-Si:H films, we observe H loss from the bulk of the film due to fast particle bombardment at the growing surface. Based on the experimental evidence, we discuss the dependence of various H incorporation and release processes on the incident H flux, substrate temperature, and deposition rate.
Heteroepitaxial relationships for CrSi2 thin films on Si(111)
Authors: Long Robert G. , Becker James P. , Mahan John E. , Vantomme André , Nicolet Marc-A.
Published: July 2002

Vacuum evaporation techniques were applied to the epitaxial growth of CrSi2 on Si(111) substrates. There are two CrSi2 matching faces which offer good lattice matchings, and which are observed experimentally: the (001) and the (111). These are present together in films grown by reactive deposition at temperatures from 450 to 1000 °C, with the latter matching face becoming more dominant as the growth temperature is raised. During an anneal at 1100 °C, however, the regions of the (111) matching face disappear in films ≥∼84 Å thick. Moderately good epitaxial alignment is obtained with the other matching face operative. Films ≤∼30 Å thick yield an opposite result: they adopt exclusively the (111) matching face as a result of this anneal. For both heteroepitaxial relationships, a strong islanding tendency is manifested during growth (unless the CrSi2 layer is more than a few thousands of angströms thick), which is accentuated by such a post-growth anneal. The population of CrSi2 islands exhibits a gradual strain relaxation with increasing average island size.
Scanning tunneling microscope investigation of the growth morphology of titanium silicide on Si(111) substrates
Authors: Stephenson A. W. , Welland M. E.
Published: July 2002

A scanning tunneling microscope in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(111) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Atomic resolution images of a titanium silicide crystallite facet, formed at 850 °C, have been identified as a 2×2 silicon termination of a C54-TiSi2(010) surface. Possible epitaxial silicide/silicon relationships are provided. Theoretical consideration has been given to the interatomic bonding in the C54-TiSi2 lattice and the dangling bond density of ideally terminated silicide planes has been calculated. The highly reconstructed atomically flat surface of a large crystallite, formed at 1200 °C, has been assigned as a C54-TiSi2(311) plane giving the epitaxial relation C54-TiSi2(311)∥Si(111). The presence of pairs and linear chains of defects, with common orientations, is attributed to the decomposition of a diatomic gas on the facet, producing sites of preferential adsorption on the silicide surface.
Theory of EL2 and EL5 formation in melt-grown GaAs:Si
Authors: Morrow Richard A.
Published: July 2002

Reactions conjectured to occur during the cooldown of GaAs grown from the melt are presented. These are used to fit existing data on the dependence of various concentrations (carrier, EL2, and EL5) on melt composition in crystals grown from a Ga-rich melt doped with silicon. Acceptable fits are based on the following model assumptions: (1) EL2 is AsGaVGa, (2) EL5 is the acceptor complex SiGaVGa, and (3) freeze-out of the reaction VGa+AsGaVAs=AsGaVAsVGa during cooldown is responsible for a large VGa concentration and a concomitant restricted EL2 concentration in the crystal.
Formation of carbon nitride films with high N/C ratio by high-pressure radio frequency magnetron sputtering
Authors: Okada T. , Yamada S. , Takeuchi Y. , Wada T
Published: July 2002

Carbon nitride films were synthesized on Si(100) substrate using relatively high-pressure radio-frequency magnetron sputtering of a graphite target in a nitrogen ambient. The influences of the substrate temperature and pressure on deposition rate and the film composition ratio N/C and C–N bonding were investigated using x-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). The deposition rate had a maximum in the investigated pressure region and reduced as the substrate temperature increased. XPS spectra showed that the deposited films were very large N/C value up to 1.20, and unchanged after thermal treatment up to about 900 K, the latter confirmed by FTIR.
Platinum gettering in silicon by silicon phosphide precipitates
Authors: Correia A. , Pichaud B. , Lhorte A. , Quoirin J. B.
Published: July 2002

The gettering efficiency process in platinum doped fast rectifiers was studied by transmission electron microscopy. We have observed orthorhombic PtSi-(monocrystalline) precipitates in n+ gettering region with the following epitaxial relationship: (011)PtSi//{110}Si with [010]PtSi//〈112〉Si. This precipitation can be related to the presence of orthorhombic SiP particles induced by highly phosphorus doping, without any dislocations around them.
Analysis of post-transit photocurrents and electroluminescence spectra from a-Si:H solar cells
Authors: Yan Baojie , Han Daxing , Adriaenssens Guy J.
Published: July 2002

The post-transit photocurrent analysis has been applied for the characterization of hydrogenated amorphous silicon solar cells. The results show a density-of-states feature, about 0.6–0.7 eV below the conduction band edge, which could be ascribed to the D−/0 transition. Comparison with the electroluminescence spectra for different samples further confirms that the post-transit photocurrent analysis is a useful method for the evaluation of the profile of density of states in amorphous silicon solar cells. However, a more detailed analysis demonstrates that the exact magnitude of the density of states will be difficult to obtain from just the post-transit photocurrent analysis under all circumstances, since the total number of states itself is involved in the occupation factor. It may explain the crossing of density-of-state profiles between the annealed and light-soaked states. Light soaking creates new defects at lower levels than the normal D−/0 transitions of native defects.
Magnetostatic effect on magnetic flux penetration in superconducting Nb film covered with a micron–size magnetic particle array
Authors: Nozaki Yukio , Otani Yoshichika , Runge Katharina , Miyajima Hideki , Pannetier Bernard , Nozières Jean Pierre
Published: July 2002

Magnetostatic effect of the micron-size ferromagnetic particle array on a two-dimensional superconductor was investigated. In the bare Nb film, a sharp drop in magnetization due to the flux rearrangement was observed, whereas this behavior was not observed for the Nb film with the ferromagnetic particles arranged on top. The difference in the magnetization curves may be attributed to the regulated flux penetration process by the ferromagnetic particle array.
Magnetic force microscopy of domain wall fine structures in iron films
Authors: Schneider M. , Müller-Pfeiffer St. , Zinn W.
Published: July 2002

This work investigated the micromagnetic structure of single crystal iron films by means of magnetic force microscopy (MFM) and Kerr microscopy. The investigated samples were epitaxially grown Fe films on a GaAs substrate. The film thickness varied between 30 and 500 nm. With the Kerr microscope it was possible to localize efficiently the domain walls in iron films. These walls were subsequently imaged by MFM with a high resolution. The MFM was used to observe the fine structure of a Bloch wall. Experimental results were compared with the calculated MFM response that was based on a two-dimensional, micromagnetic model describing the magnetic structure of an asymmetric Bloch wall. Both theory and experiment demonstrated the influence of the stray field of the tip on the wall structure. We could observe symmetric Néel walls and cross tie walls in a 30 nm iron film after reducing the sensitivity of the MFM tip. In order to achieve this state the tip was demagnetized by an external field.
Giant magnetoresistance in evaporated Ni-Fe/Cu and Ni-Fe-Co/Cu multilayers
Authors: Zeltser Alexander M. , Smith Neil
Published: July 2002

The magnetic and transport properties of electron-beam evaporated (Ni83Fe17/Cu)10 and (Ni66Fe16Co18/Cu)10 multilayers were studied as a function of the Cu spacer, magnetic layer and Ta buffer layer thicknesses, as well as annealing conditions. All multilayers exhibited very small giant magnetoresistance (GMR) effect (<0.3%) in the as-deposited state, however, after magnetic post-annealing at 300–325 °C, GMR increased up to 4.5%–7%, depending on the multilayer type. In contrast to sputtered Ni-Fe-(Co)/Cu multilayers, GMR showed no oscillatory behavior as a function of Cu thickness. Similar to that reported in sputtered ‘‘discontinuous’’ Ni-Fe/Ag multilayers, it is believed that Cu diffusion along the Ni-Fe-(Co) grain boundaries creates intra-layer magnetic discontinuities in Ni-Fe-(Co) layers which promote inter-layer antiferromagnetic coupling. The evaporated Ni-Fe/Cu multilayers exhibited very low remanence, exceptionally low hysteresis, and quite uniform GMR properties through the thickness of the multilayer. All of these makes them potentially attractive for application to future magnetoresistive reproduce heads for very high areal density magnetic storage systems.
Analysis of the experimental distribution of relaxation times around the liquid–glass transition of poly(vinylidene fluoride)
Authors: Teyssèdre G. , Demont P. , Lacabanne C.
Published: July 2002

The dielectric relaxation mode associated with the liquid–glass transition of a semicrystalline polymer, polyvinylidene fluoride, has been analyzed by the technique of fractional polarizations. A discrete distribution of relaxation times following an Arrhenius law has been obtained experimentally. The activation energy of the ‘‘single’’ relaxation processes are distributed according to a Gaussian function. Moreover, these relaxation times follow a compensation law. These results allowed us to introduce a continuous distribution of relaxation times whose parameters are temperature dependent. The validity of the method was verified for the thermally stimulated current spectra. Use is made of this distribution to compute the temperature and frequency dependence of the complex dielectric permittivity.
Mass-resolved ion energy measurements at both electrodes of a 13.56 MHz plasma in CF4
Authors: Snijkers R. J. M. M. , van Sambeek M. J. M. , Hoppenbrouwers M. B. , Kroesen G. M. W. , de Hoog F. J.
Published: July 2002

The ion energy distributions (IEDs) at the electrodes in a capacitively coupled 13.56 MHz plasma in CF4 have been measured mass resolved with a Balzers quadrupole in combination with a home-built energy analyzer. Mass-resolved determination offers the possibility to compare the IED of different ions achieved in the same sheath. The IEDs have been determined at both the largest and the smallest electrode. Apart from the IEDs of the CF4 species, the IEDs of ionic species in plasmas in argon and nitrogen also were determined. Apart from the CF4 ionic species CF+3, CF+2, CF+, and F+, CHF+2 ions also are present in the CF4 plasma due to residual water in the reactor. Because the CHF+2 ions are not produced in the sheath and because we do not detect elastically scattered ions, the IEDs of these ions show the typical bimodal distribution for rf plasmas which corresponds to an IED of ions which have not collided in the sheath. From these IEDs we can obtain the sheath characteristics, such as the averaged sheath potential. From the IEDs of CF+n ions one can conclude that, in the sheath of the CF4 plasma, a large number of chemical reactions takes place between the CF+n ions and the neutrals.
Electron transverse energy distribution in GaAs negative electron affinity cathodes: Calculations compared to experiments
Authors: Vergara G. , Herrera-Gómez A. , Spicer W. E.
Published: July 2002

The transverse momentum of photoelectrons released from a negative electron affinity GaAs cathode is small compared with other thermonic or field emission electron sources. A low photoelectron transverse momentum in vacuum promises highly focused, low-energy beams useful for numerous applications. A simplified theory for electron emission from GaAs predicts much lower electron transverse momentum than those previously measured experimentally. To address this theory–experiment mismatch, Monte Carlo based calculations were compared with experimental data. We checked the possibility of there being electron scattering in the Cs,O layer; however, none of the scattering checked (isotropically distributed, cosine distributed, and Rutherford scattering) properly fit the experimental results. The assumption of conservation of the parallel component of the crystal momentum k during the emission is mainly believed to be responsible for the calculation-theory disagreement. The best simulation-experiment fit was obtained through a relaxation of the conditions imposed on the transverse momentum inside and outside of the semiconductor when an ideal interface is considered. We obtained the best results by assuming that the effective mass of the electron inside GaAs is equal to the effective mass of the electron in vacuum. Two independent experiments confirmed that in both cases, this same-mass approximation gives the best fit. The physical meaning of this is not clear, but it seems to be related to the amorphous nature of the Cs,O layer. We conclude that the way to get lower electron transverse energy spread cathodes is to study alternative activation methods and new materials with smaller effective electron masses.
Particle agglomeration study in rf silane plasmas: In situ study by polarization-sensitive laser light scattering
Authors: Courteille C. , Hollenstein Ch. , Dorier J.-L. , Gay P. , Schwarzenbach W. , Howling A. A. , Bertran E. , Viera G. , Martins R. , Macarico A.
Published: July 2002

To determine self-consistently the time evolution of particle size and their number density in situ multi-angle polarization-sensitive laser light scattering was used. Cross-polarization intensities (incident and scattered light intensities with opposite polarization) measured at 135° and ex situ transmission electronic microscopy analysis demonstrate the existence of nonspherical agglomerates during the early phase of agglomeration. Later in the particle time development both techniques reveal spherical particles again. The presence of strong cross-polarization intensities is accompanied by low-frequency instabilities detected on the scattered light intensities and plasma emission. It is found that the particle radius and particle number density during the agglomeration phase can be well described by the Brownian free molecule coagulation model. Application of this neutral particle coagulation model is justified by calculation of the particle charge whereby it is shown that particles of a few tens of nanometer can be considered as neutral under our experimental conditions. The measured particle dispersion can be well described by a Brownian free molecule coagulation model including a log-normal particle size distribution.
A model noise temperature for nonlinear transport in semiconductors
Authors: Varani L. , Houlet P. , Vaissière J. C. , Nougier J. P. , Starikov E. , Gruzhinskis V. , Shiktorov P. , Reggiani L. , Hlou L.
Published: July 2002

We present an analytical modeling of the noise temperature associated with velocity fluctuations obtained in the framework of the linear-response theory around a steady state. The expressions are rigorously related to an eigenvalue expansion of the response matrix and are applicable to ohmic as well as to nonohmic (hot-carrier) conditions. Theory requires as input parameters the reciprocal carrier effective mass, the drift velocity, the carrier energy, the variance of velocity fluctuations, and the covariance of velocity-energy fluctuations as functions of the electric field in stationary and homogeneous conditions. The analytical results obtained for the case of holes in Si and electrons in GaAs at T=300 K are validated by comparison with experiments.
Optical properties of nanoscale, one-dimensional silicon grating structures
Authors: Zaidi Saleem H. , Chu An-Shyang , Brueck S. R. J.
Published: July 2002

We report a detailed study of nanostructure fabrication and optical characterization of sub-μm-period, one-dimensional, Si grating structures. Nanoscale wall width structures were fabricated by combining laser interferometric lithography with anisotropic wet-chemical etching (KOH) and thermal oxidation. Structure wall widths were characterized by Raman scattering (RS) and scanning electron microscopy. Salient features of the RS measurements as a function of wall widths from ∼100 to 10 nm were: (a) large cross-section enhancements, ∼100×, for linewidths ∼50 nm; (b) asymmetric line shapes with tails extending to smaller Raman shifts for linewidths ∼20 nm; and (c) splitting of the bulk Raman mode, again to lower Raman shifts, for linewidths ∼10 nm. For room temperature photoluminescence (PL) measurements, the grating structures were excited at 257 nm. PL measurements are reported for oxidized and unoxidized grating structures with peaks varying between 380 and 700 nm. PL was only observed for Si structures with dimensions less than about 10 nm. PL intensities and spectral line shapes varied significantly as a result of surface modification treatments such as high temperature anneal in a N2 atmosphere, immersion in boiling H2O, and long-term exposure to ambient air. The measurements indicate a strong correlation of the visible PL with crystal size (∼5–10 nm); however, it remains unclear if the mechanism responsible is quantum confinement, passivation of the surface by Si:Hx complexes, or optically active surface states.
Temperature dependence of magnetoresistance in spin valves with different thicknesses of NiO
Authors: Lai Chih-Huang , Regan Thomas J. , White Robert L. , Anthony Thomas C.
Published: July 2002

Spin valves with structure X Å NiO/60 Å NiFe/30 Å Cu/80 Å NiFe/50 Å Ta, with X from 250 to 1450, were fabricated by dc magnetron sputtering, and their magnetoresistances and NiO/NiFe exchange fields were measured over the temperature range from 20 to 200 °C. ΔR/R decreased almost linearly with increasing temperature from 20 °C up to a transition temperature, and then decreased more steeply to zero at the blocking temperature. A broad distribution of NiO/NiFe exchange paths was observed peaking around 70 °C. The ΔR/R transition temperatures and the exchange path distributions were dependent on NiO thickness. Part of the reduction of ΔR/R at high temperatures may be attributed to fluctuations in NiO spin orientation which cause imperfect antiparallel alignment of the magnetizations of the NiFe layers.
Transmission electron microscopy on {113} rodlike defects and {111} dislocation loops in silicon-implanted silicon
Authors: Pan G. Z. , Tu K. N.
Published: July 2002

Plan-view and cross-sectional transmission electron microscopy have been used to study the microstructural evolution of {113} rodlike defects as well as their correlation with the formation of {111} dislocation loops in silicon implanted with 50 keV 3.6×1014 Si+/cm2 8.0 mA and rapid thermally annealed at temperatures ranging from 500 °C to 1100 °C for various times. It was determined that the size distribution of {113} defects is a log-normal Gaussian probability function. The nucleation, growth and dissolution of both {113} defects and {111} dislocation loops were observed. The nuclei of {113} defects form first and are circular interstitial clusters. The nucleation is homogeneous in the matrix at the crystalline side of the amorphous/crystalline interface and occurs at a temperature as low as 600 °C for 1 s. These circular nuclei grow into rods along the 〈110〉 direction in a {113} habit plane. It was found that the growth of {113} defects inclined to wafer surface is more preferable than that parallel to the surface. Similarly their dissolution is also faster. The dissolution strongly depends on annealing temperature. Most of them can last for more than 240 s at 800 °C but only 15 s at 900 °C. It was found that the formation of {111} faulted Frank dislocation loops is an independent event of nucleation and growth. This nucleation of {111} Frank dislocation loops does not occur until the {113} defects begin to grow along a 〈110〉 direction and/or dissolve into the matrix. This means that there is a period when {113} defects release interstitial point defects before the {111} dislocation loops nucleate from the matrix. The growth of {111} dislocation loops, together with {113} defects, is a nonconservative Ostwald ripening process. The {113} defects were found to disappear completely at 900 °C for 120 s, but the {111} dislocation loops disappear at 1100 °C for 60 s.
Dynamics of collisionless rf plasma sheaths
Authors: Miller Paul A. , Riley Merle E.
Published: July 2002

The behavior of rf plasma sheaths has been the subject of much scientific study and also is technologically important for plasma etching and deposition in the manufacture of integrated circuits. This paper presents a semianalytic model of rf sheaths and describes an experiment that tested the model. An approximation to the first integral of the Poisson equation allows solving for the response of plasma sheaths to an imposed rf bias voltage. This approximation enables the plasma sheaths to be included within an electrical model of the plasma and external rf circuit components, and affords a prediction of the ion energy distributions impacting the electrodes, which are in contact with the plasma. The model is a significant advance beyond previous sheath models because it has no restriction on the ratio of the rf period to the ion transit time across the sheath. The model is applicable to those high-density, low-pressure plasmas in which the Debye length is a small fraction of the ion mean-free path, which itself is a small fraction of the plasma dimension. The experimental test of the model was conducted by comparing the predicted and measured rf potential, current, and power at the sheath adjacent to a capacitively coupled, rf-biased electrode in a plasma reactor with argon discharges sustained by an inductively coupled plasma source. The comparisons included both linear and nonlinear components of the rf electrical parameters. Results of the experiment were in substantial agreement with model predictions.
Raman microstructural analysis of silicon-on-insulator formed by high dose oxygen ion implantation: As-implanted structures
Authors: Macı́a J. , Martı́n E. , Pérez-Rodrı́guez A. , Jiménez J. , Morante J. R. , Aspar B. , Margail J.
Published: July 2002

A microstructural analysis of silicon-on-insulator samples obtained by high dose oxygen ion implantation was performed by Raman scattering. The samples analyzed were obtained under different conditions thus leading to different concentrations of defects in the top Si layer. The samples were implanted with the surface covered with SiO2 capping layers of different thicknesses. The spectra measured from the as-implanted samples were fitted to a correlation length model taking into account the possible presence of stress effects in the spectra. This allowed quantification of both disorder effects, which are determined by structural defects, and residual stress in the top Si layer before annealing. These data were correlated to the density of dislocations remaining in the layer after annealing. The analysis performed corroborates the existence of two mechanisms that generate defects in the top Si layer that are related to surface conditions during implantation and the proximity of the top Si/buried oxide layer interface to the surface before annealing.
Growth kinetics of amorphous interlayers and formation of crystalline silicide phases in ultrahigh vacuum deposited polycrystalline Er and Tb thin films on (001)Si
Authors: Luo C. H. , Chen L. J.
Published: July 2002

The growth kinetics of amorphous interlayer (a-interlayer) and formation of crystalline silicide phases in ultrahigh vacuum deposited polycrystalline Er and Tb thin films on (001)Si have been investigated by cross-section transmission electron microscopy. The growth of the amorphous interlayer in both Er/Si and Tb/Si systems was observed to exhibit similar behaviors. The growth was found to follow a linear growth law initially in samples annealed at 190–240 °C. The activation energy of the linear growth and maximum thickness of the a-interlayer were measured to be 0.5 eV, 15.5 nm, and 0.35 eV, 16 nm in Er/Si and Tb/Si systems, respectively. Crystalline silicides (ErSi2−x or TbSi2−x) were found to form at the amorphous interlayer/Si interfaces in samples after prolonged and/or high-temperature annealing. Simultaneous growth of the a-interlayer and crystalline phase was observed and the growth rate of a-interlayer was faster than the growth of epitaxial ErSi2−x and TbSi2−x phases in samples annealed at 270–300 °C in Er/Si and Tb/Si systems, respectively. The competitive growth can be understood from energetic consideration. A high density of recessed amorphous regions were found to form between isolated epitaxial silicide regions which led to uneven silicide/Si interfaces and eventually pinholes in the silicide films at high temperatures. The formation mechanism of rough silicide/Si interface is discussed.
Real-space field of surface sources and the problem of fast leaky wave generation in a piezoelectric half-space
Authors: Biryukov S. V. , Weihnacht M.
Published: July 2002

The total solution of the boundary problem for a piezoelectric half-space in the case of oscillating electric and mechanical surface sources is presented as a function of coordinates in terms of a Fourier integral over the real axis of acoustic wave slowness along the surface. In order to investigate the possible leaky wave contributions to the total field, the surface impedance matrix and the Fourier transform of the Green function are uniquely defined for complex values of slowness, using the procedure of analytic continuation from the real axis of slowness and appropriate energy relations. A simple expression for the calculation of plane partial wave group velocities is also proposed. This approach allows the problem to be automatically confined to leaky waves that can be generated by surface sources. The main calculations refer to the new fast leaky waves, which have high phase velocities not far from the velocities of the fastest bulk wave in piezoelectric crystals, and are thus potentially attractive for high frequency surface acoustic wave devices. In the case of lithium tetraborate with Euler angles (0°, 47.3°, 90°), having assumed a simple distribution of surface sources, the total field is exactly calculated in the vicinity of the sources both at the surface and in the bulk region. The asymptotic field far from the sources is also estimated: it is shown that fast leaky waves, ordinary surface acoustic waves, and so-called limiting bulk waves are present, and all contributions in relation to the distance from the sources are compared. It is shown that the fast leaky waves may have different ratios between the magnitudes of components, when propagating in opposite directions. Both free and metallized crystal surfaces are considered. In contrast to the free surface case, it is found that in the case of a metallized surface the new leaky waves give a good approximation of the total field near the sources. This different behavior should be taken into account in practical applications. A comparison with the well-known case of lithium niobate (0°, −49°, 0°), in which the slow leaky waves can be generated, is made. Previously published numerical and experimental results are discussed. Some experimental results are reinterpreted and explained by applying the present approach to a model of the experimental situation considered.
Optimal control of ultrasoft cantilevers for force microscopy
Authors: Bruland K. J. , Garbini J. L. , Dougherty W. M. , Sidles J. A.
Published: July 2002

The goals of optimal control in force microscopy are: (1) to obtain favorable cantilever dynamic properties and (2) to control the cantilever to a desired amplitude, while (3) exerting as little control force as possible, and (4) preserving the force signal-to-noise ratio of the uncontrolled cantilever. This article describes the experimental implementation of an optimal controller that achieves these goals. The application of this controller to an ultrasoft cantilever with spring constant of 110 μN/m at 10 K reduced the resonant quality from 15 000 to 220, reduced the Brownian amplitude from 11.2 Å to 1.4 Å, used less than 7×10−17 N of control effort, left the force sensitivity unaltered at 9.8×10−18 N/ Hz, and demonstrated feedback control can force cantilever motion to track a reference input.
Micromagnetic localization
Authors: Skomski Ralph
Published: July 2002

The localization of nucleation modes in inhomogeneous ferromagnets and its influence on the coercivity are investigated. From the formal analogy between quantum mechanics and micromagnetics follows that anisotropy inhomogeneities may cause localization. The nucleation modes of one-dimensional arrays, such as multilayers composed of hard and soft magnetic materials, are localized even if the superlattice exhibits a nearly ideal periodicity. Gaussian distributions of the layer thicknesses lead to Urbach tails and very low coercivities, but a maximum thickness lm of the soft layers suppresses the Urbach tails. The related problem of magnetic viscosity leads to a supersymmetric Fokker–Planck description where the time dependence of the magnetization is given by the ground-state mode of a fermionic potential.
Magnetic properties of Nd8Fe77Co5B6CuNb3 melt-spun ribbons
Authors: Chiriac H. , Marinescu M.
Published: July 2002

Nd 8 Fe 77 Co 5 B 6 CuNb 3 amorphous ribbons were obtained by the melt spinning method using a large range of the wheel circumferential speed (18–40 m/s). For 10 min time of heat treatment at 700 °C the following values of the magnetic characteristics were achieved: Hc=3.95 kOe, Br=11.25 kG, and (BH)max=15 MGOe. The 13% increase of the remanent ratio Mr/M1.5 T by applying a magnetic field during annealing suggests an improvement of the exchange coupling effect. The Curie temperature of both hard and soft magnetic component phases increases with about 50 °C due to the Co content but it is not affected by the magnetic field annealing.
High-resolution diffraction using Larmor precession and multiple narrow beams defined by multislits
Authors: Rekveldt M. Th.
Published: July 2002

A new method is introduced to measure lattice constants by diffraction in Bragg scattering with very high accuracy. Use is made of Larmor precession of a polarized neutron beam in a magnetic field. The number of precessions is directly related to the wavelength of the neutron beam and the phase change in one precession period can be identified as a slightly changed wavelength, which itself is indicative of a change in the lattice parameter. Using multislits as diaphragms in the incident and in the diffracted beams, one selects angular regions of the neutron beam in the scattering plane with phase angles in Larmor precession that differ only by multiples of 2π at a certain magnetic field in the Larmor coil. Doing this, a very high resolution in lattice parameter can be obtained with an intensity, which is a multiple of the intensity in conventional diffraction methods at this high angular resolution. Use of the proposed method in materials science applications such as stresses in materials and in crystal size determination in materials is discussed.
Properties of interface states at Ta2O5/n-Si interfaces
Authors: Zhang S. K. , Fu Z. W. , Ke L. , Lu F. , Qin Q. Z. , Wang Xun
Published: July 2002

Properties of interface states at Ta2O5/n-Si interfaces are studied by capacitance–voltage and deep-level transient spectroscopy (DLTS) measurements. The results show that the “slow” states at Ta2O5/n-Si interfaces are not significantly affected by annealing, and their density is about 1.4–1.9×1011 cm−2. The energy distribution of the “fast” interfacial states in the band gap of silicon is obtained, and some high concentration interfacial states are found. DLTS spectra reveal that the interfacial defects at Ta2O5/n-Si interfaces are all donor type. The high concentration interfacial defects are located at about 0.40 eV below the conduction-band minimum of silicon. Their density decreases with increasing annealing temperature. The origin is suggested to be related to the diffusion of tantalum into the Si substrate.
Far infrared spectroscopy with high resolution cyclotron resonance filters
Authors: Skierbiszewski C. , Knap W. , Dur D. , Ivchenko E. L. , Huant S. , Etienne B.
Published: July 2002

A new type of far infrared spectroscopy based on a cyclotron resonance notch filter is demonstrated. The resonant absorption energy of such a filter is tuned by an external magnetic field. GaAs/AlGaAs heterostructures with high mobility two-dimensional electron gas are used to obtain an optimal cyclotron resonance filter. With such a filter, we can analyze far infrared radiation in the range 40–150 cm−1 with a resolution of up to 2 cm−1. A procedure for the determination of the spectral characteristics of an unknown source from the detector signal is presented. We show that the cyclotron-resonance-notch-filter based spectrometer provides ultralow background radiation conditions for measurements. We also demonstrate the performance of the spectrometer by an analysis of electrostimulated far infrared emission of InSb, GaAs bulk crystals as well as of selectively doped quantum wells.
In situ infrared and visible-light ellipsometric investigations of boron nitride thin films at elevated temperatures
Authors: Franke E. , Schubert M. , Hecht J.-D. , Neumann H. , Tiwald T. E. , Thompson D. W. , Yao H. , Woollam J. A. , Hahn J.
Published: July 2002

In situ infrared (IR) spectroscopy and visible-light (VIS) spectroscopic ellipsometry over the spectral range from 700 to 2000 cm−1 and 1.5–3.5 eV, respectively, were used to investigate the optical behavior of boron nitride (BN) thin films at temperatures from room temperature (RT) to 600 °C. The polycrystalline hexagonal (h) and mixed-phase h- and cubic (c)-BN thin films were deposited by magnetron sputtering on [001] silicon. We observe a reversible moisture incorporation process in as-grown h-BN samples. When stored in normal ambient, the h-BN thin films absorb water into thin-film micropores. When annealed in ultrahigh vacuum or a dry nitrogen atmosphere, the samples expel moisture but retain their microstructure. This is observable by reduction of the thin-film refractive indices in accordance with changes in the IR lattice resonance behavior. The optical properties of high c-BN content thin films remain unchanged during annealing. And both intrinsic h- and c-BN thin-film VIS refractive indices are nearly temperature independent, at least up to 600 °C. Therefore, RT BN optical constants can be used for feedback loop control in in situ thin-film growth at temperatures up to 600 °C.
An ab initio molecular-orbital study on hydrogen-abstraction reactions at the growing surface of hydrogenated amorphous silicon
Authors: Nakajima Kenji , Miyazaki Kaori , Koinuma Hideomi , Sato Kota
Published: July 2002

Energy profiles have been evaluated by an ab initio molecular-orbital method for hydrogen-abstraction reactions from surface model compounds of growing hydrogenated amorphous silicon (a-Si:H) by a SiH3 radical, a presumed main precursor to a-Si:H, as well as by a hydrogen radical which should coexist in the silane plasma chemical vapor deposition. The activation energies calculated for these two reactions decrease as the cluster size of the film surface model SinH2n+2 increases from n=1 to n=4 to converge for n⩾4. This trend is in parallel with the variation of atomic charge delocalization. Both activation energies (0.22 and 0.28 eV, respectively) for the largest model, Si7H16, were low enough to induce the hydrogen abstractions from the surface to form dangling bonds, which spontaneously react with SiH3 radicals to form Si–Si bond. From thus produced H3Si–Si≡surface, hydrogen can be eliminated with SiH3 (or H) to reproduce a dangling bond. The initial step of the a-Si:H film growth is deduced by the calculation to proceed through sequential reactions of spontaneous addition of SiH3 to the dangling bonds, and the hydrogen abstraction to reproduce dangling bonds.
a-
SiGe:H
based solar cells with graded absorption layer
Authors: Zimmer J. , Stiebig H. , Wagner H.
Published: July 2002

An experimental and numerical study of a-SiGe:H based solar cells with a band gap graded i layer in the shape of a “V” is presented. The variation of the location of the band gap minimum has a strong influence on solar cell performance. Under air mass (AM) 1.5 illumination the cells show a strong increase in open circuit voltage and a distinct decrease in the fill factor when shifting the band gap minimum from the front to the rear part of the i layer. Comparisons of experimental and simulated data of the dark I/V behavior, the I/V curves under illumination and the quantum efficiency allow insights into the transport and recombination behavior within the solar cell. The simulations reveal that the position as well as the charge state of the defects and, under illumination additionally the recharging behavior of the defect states, determine the device characteristics.
A theory for the nonlinear response of high power p-i-n photodetectors
Authors: Herbert D. C. , Chidley E. , Allenson M. , Wight D.
Published: July 2002

A new physics based model is developed for the small signal and large signal high current response of p-i-n photodetectors. The numerical treatment is not constrained by the numerical stability requirements of conventional treatments of the time-dependent drift-diffusion equations. The results confirm the finding of other authors in predicting the formation of an extended low field region at high currents, with consequent collapse of the frequency response. Comparison with available experimental data for GaAs based photodiodes suggests that 2D effects are important in the high current limit, but can be allowed for within the 1D model by reducing the calculated particle velocity in the low field region.
Effect of growth temperature and substrate materials on epitaxial growth of coronene
Authors: Cho K. A. , Shimada T. , Sakurai M. , Koma A.
Published: July 2002

Epitaxial growth of coronene (C24H12) was studied on substrates having no dangling bonds, such as hydrogen-terminated Si(111), MoS2, MoTe2, and muscovite by using reflection high-energy electron diffraction. It was found that the sticking coefficient and the maximum thickness of the epitaxial films are strongly dependent on the substrate materials as well as on the growth temperature. The effectiveness of molecular mechanics calculation was examined by comparing the calculated stabilization energies with the experimental results. Importance of the electronic structure of substrates has been revealed and its effect was estimated quantitatively using the Lifshitz theory. The tendency that the interfacial dispersion force is stronger for smaller band gap materials has been shown from theoretical consideration.
Dependence of lateral oxidation rate on thickness of AlAs layer of interest as a current aperture in vertical-cavity surface-emitting laser structures
Authors: Koley B. , Dagenais M. , Jin R. , Simonis G. , Pham J. , McLane G. , Johnson F. , Whaley R.
Published: July 2002

The dependence of the wet oxidation process on the AlAs layer thickness used in selectively oxidized vertical-cavity surface-emitting-laser structures is studied in detail. A theoretical model based on a diffusion-reaction process is proposed. A rapid reduction in the oxidation rate is predicted with a reduction in the layer thickness of the ultrathin AlAs layer. The theoretical predictions are verified through experiments.
Beyond the solid on solid model: An atomic dislocation formation mechanism
Authors: Dalla Torre J. , Rouhani M. Djafari , Malek R. , Estève D. , Landa G.
Published: July 2002

We investigate the growth of mismatched thin films by a kinetic Monte Carlo computer simulation. The strain is introduced through an elastic energy term based on a valence force field approximation and stress is relaxed along “atomic chains” at each step of the simulation. The calculations use a set of elementary atomic processes including, besides well-known standard processes, the collective incorporation of atoms. This leads us to introduce a new “hanging” position with only one bond created toward the substrate contrary to solid on solid models. This position plays a role of defects initiation, and thus an atomic dislocation nucleation mechanism is described. Finally, we present the influence of a step in the dislocations creation.
A numerical study of operational characteristics of organic light-emitting diodes
Authors: Kawabe Y. , Morrell M. M. , Jabbour G. E. , Shaheen S. E. , Kippelen B. , Peyghambarian N.
Published: July 2002

We propose a simple model for the device characteristics of organic single layer molecular or polymeric light-emitting diodes. The model is based on Poisson’s equation and the conservation law of charges. A bimolecular recombination process is incorporated phenomenologically, and boundary conditions are given by carrier injection functions. Equations for the electric field and carrier concentrations are formulated for single-carrier and double-carrier injection cases. The equations are solved for different parameters including carrier mobility and energy barrier height at the electrodes. Current–voltage characteristics, relative quantum efficiency, and emission distribution are obtained. The results show that (1) at least one barrier height should be low for device operation at low voltage, (2) high mobility is essential for devices with high brightness, (3) low electron mobility confines the emission region near the cathode and should be avoided to prevent electrode quenching. We also discuss the effects from persistent charged traps. The calculation was compared with experimental results obtained by single layer conjugated polymer and dye-doped polymer devices. Fairly good agreement between experiments and calculations on the current–voltage characteristics and relative quantum efficiency were obtained using reasonable physical parameters.
Effect of resistivity and current density on photoluminescence in porous silicon produced at low HF concentration
Authors: Gaburro Zeno , You Hoydoo , Babić Davorin
Published: July 2002

An experimental study of the intensity of photoluminescence (PL) of porous silicon (PSi) prepared from an anodic dissolution of Si at low HF concentration (12.5%) of p-type (100) 0.01, 1 and 10 Ω cm substrates as a function of substrate resistivity and etching current density has been performed. Based on the experimental results a photoluminescence efficiency diagram is proposed. Etching of p+-type silicon samples without light illumination produces PSi layers whose PL spectra show interference fringes. Comparison of the fringes in PL and in light reflectivity demonstrates unambiguously that they originate from the interference of the light reflected at the PSi/bulk Si interface and depend on the thickness of the PSi layer. The intensity and frequency of the interference fringes are found to be strongly dependent on the anodization current. Implications of PSi layer fabrication at low HF concentration are discussed.
Use of rotational transverse magnetometry to measure anisotropy energy
Authors: Li Ning , Lairson Bruce M.
Published: July 2002

Rotational transverse magnetometry (RTM) is used to measure the anisotropy field of magnetic thin films. Magnetization reversal during rotation of a two-dimensional isotropic sample in an applied field is discussed. The relation between the transverse magnetization My and the applied field H is solved numerically. An excellent approximation for the transverse magnetization is My/Ms=A(1−H/Hk)2.5, where A=1.1434, and Hk is the anisotropy field. For curve fitting to experimental data, both A and Hk were used as fitting parameters. Comparison between a constructed torque hysteresis method and this vibrating sample magnetometer (VSM) RTM method is made theoretically and experimentally. Both results show that the VSM RTM will give a good extrapolation of the anisotropy field, whereas the torque measurement will slightly overestimate it. Anisotropy energy is calculated based on the anisotropy field and saturation magnetization for magnetic thin films.
Magnetic correlations in nanostructured metals and an extended random-anisotropy model
Authors: Löffler J. F. , Braun H. B. , Wagner W.
Published: July 2002

Small-angle neutron scattering experiments on nanostructured Fe reveal spatial magnetic correlations across the interfaces which depend strongly on grain size, with a minimum at around 35 nm. This grain size is comparable with Lcrit=πδ, with δ the bulk domain-wall width. We present an extended random-anisotropy model to explain the magnetic properties in the low grain size regime and explain the crossover of the magnetic properties for grain sizes comparable with Lcrit.
Statistical modeling of magnetic and magnetostrictive properties due to domain wall motion
Authors: Bergqvist A. , Lundgren A. , Stillesjö F. , Engdahl G.
Published: July 2002

Statistical mechanics is employed to derive magnetic and magnetostrictive bulk properties of soft magnetic materials dominated by domain wall motion. The material is divided into an ensemble of volumes with uniform magnetization, each one much smaller than a typical domain but much larger than one atom. The energy for such a volume element is assumed to consist of an internal energy: the interaction with the external magnetic field and a mean field interaction with the surrounding medium. The volume fractions of discrete mesoscopic magnetizations in the medium are derived from Boltzmann statistics. The bulk magnetization is the weighted average of the mesoscopic values. Magnetostriction is treated in a similar way. Calculations are compared with two-dimensional measurements on grain-oriented silicon–iron.