bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and grain boundary defects of glow-discharge polycrystalline silicon films deposited using disilane
Authors: Hasegawa S. , Fujimoto E. , Inokuma T. , Kurata Y.
Published: July 2002

Undoped polycrystalline Si (poly-Si) films were deposited as a function of the rf power (0–30 W) and deposition temperature Td (600–750 °C) using disilane by a plasma-enhanced chemical vapor deposition (PECVD) method. For comparison, poly-Si films were also deposited using monosilane. The preferential orientation to a random, (100), or (110) texture was able to be selected by changing some of these deposition conditions. It was suggested that the change in the texture is caused by a change in the surface-diffusion coefficient of SiHn adsorbates and by effects of an ion bombardment, rather than by a change in the deposition rate. For PECVD poly-Si films, both the x-ray relative intensity and the crystallinity rapidly decreased with decreasing the film thickness thinner than 0.4 μm. The value of g in the electron spin resonance spectra and that of stress strongly depended on the rf power and the thickness. Further, when a film was annealed in H2 plasma, the value of g shifted from 2.0054 to 2.0043. These results are examined in terms of formation of different types of grain boundary which can be associated with a lattice deformation around the dangling bonds.
Laser-induced structural modifications of glassy carbon surfaces
Authors: Vitali G. , Rossi M. , Terranova M. L. , Sessa V.
Published: July 2002

Structural modifications induced by pulsed laser irradiations in the surface layers of glassy carbon have been monitored by reflection high energy electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy. The glassy carbon samples were irradiated by 30 superimposed laser pulses (λ́=6.943 nm). The energy density (100–500 mJ/cm2 per pulse) delivered to the material and the repetition rate of the laser (0.05 Hz) have been chosen so that the temperature increase of the irradiated surface layers was below the melting point of the glassy carbon. The combined use of the analysis techniques indicated that the beginning of the solid state processes, leading to microstructural modifications of the surface layers, occurs at energy density of 300 mJ/cm2. An increase of the average crystalline size of graphitic clusters occurs upon radiation performed at fluences of 300 and 400 mJ/cm2, whereas at higher energy density the material undergoes complete amorphization. The analysis of chemical state and microstructure of the irradiated samples clearly demonstrates that graphitization or, conversely amorphization of glassy carbon surface layers can be achieved by a proper choice of the laser irradiation conditions.
Flow-field properties under deposition of films from low-density jets
Authors: Vasenkov A. V. , Belikov A. E. , Sharafutdinov R. G. , Kuznetsov O. V.
Published: July 2002

Statistical methods in the direct simulation Monte Carlo procedure and the electron-beam fluorescence technique were applied to investigate the distribution of density and temperature in a monatomic jet. Initial conditions of investigated flows corresponded to ones under deposition of films from low-density jets. In calculations, the variable hard-sphere model for molecular interaction potential and the diffuse reflection model with complete thermal accommodation were used. Distribution of density of Ar atoms in the jet and in the vicinity of the substrate were measured by the spontaneous radiation of Ar i 5912 Å (transition 4p[1/2]←4d[3/2]0,1←1′) and Ar ii 4610 Å (4s′2D←4p′2F0,5/2←7/2) induced by the electrons. The temperature was evaluated with the rotational temperature of N2, a small portion (5%) of which was added into Ar. The quantities of rotational temperature were found by measuring the intensity of the rotational lines of the first negative system N+2 band. The experimental data provide information about profiles of density along and across the center line of a jet over a range of 1×10−3≤Kn*≤4×10−2. On the whole, the computed profiles of density are in good agreement with the measurements. The calculated properties in the vicinity of the substrate allowed the performance of a detailed analysis of the gas-surface interaction process. A good comparison of the computed profile of the flux of molecules, incident on the substrate, and data on the film thickness of a-Si:H were obtained.
Analysis of contact degradation at the CdTe-electrode interface in thin film CdTe-CdS solar cells
Authors: Singh V. P. , Erickson O. M. , Chao J. H.
Published: July 2002

Atmospheric degradation of unencapsulated CdS-CdTe solar cells was modeled by assuming that the initial CdTe/electrode interface becomes a metal/insulator/semiconductor junction as oxygen diffuses through the metal to form a CdTeO3 insulating layer. The doping concentration, barrier height, and oxide thickness were varied and the numerical calculations were compared with the experimental data on cells aged in a humidity chamber. The degradation was most severe when the cells were exposed to high humidity and high temperature. This degradation could be modeled in terms of the growth of the insulating CdTeO3 layer from 20 Å to 80 Å.
GaAs to InP wafer fusion
Authors: Ram R. J. , Dudley J. J. , Bowers J. E. , Yang L. , Carey K. , Rosner S. J. , Nauka K.
Published: July 2002

This paper presents an analysis of the various properties of the fused interface between GaAs and InP. Interface dislocations are characterized by transmission electron microscopy. Bipolar electrical properties are studied by electron beam induced current measurements and by electrical measurements of fused diode and laser structures. Absorptive optical losses at the interface are estimated from measurements on fused Fabry–Perot resonators and optical scattering losses from interface roughness are estimated by atomic force microscopy. Finally a preliminary mechanical analysis of fracture patterns of fused mesas is presented. The results from our analysis are used to develop guidelines for the fabrication of fused optoelectronic devices.
Antisite defects created in neutron irradiated GaP crystals
Authors: Palczewska M. , Jasinski J. , Korona K. , Kaminska M. , Bourret E. D. , Elliot A. G.
Published: July 2002

This article compares the nature of phosphorus antisite defects in as-grown and neutron irradiated GaP crystals. Electron spin resonance studies indicate that these defects in both kinds of crystals have identical close neighbors consisting of four phosphorus atoms. Neutron irradiation of GaP introduced an additional defect (called WA1), which is linked to a gallium antisite. Characteristic absorption bands and conductivity of neutron irradiated GaP crystals are discussed as well.
Structural characterization of lattice matched AlxIn1−xAs/InP and GayIn1−yAs/InP heterostructures by transmission electron microscopy and high-resolution x-ray diffraction
Authors: Carlino E. , Giannini C. , Tapfer L. , Catalano M. , Tournié E. , Zhang Y. H. , Ploog K. H.
Published: July 2002

In this work we report on transmission electron microscopy and high-resolution x-ray diffractometry studies of lattice matched AlxIn1−xAs/InP and GayIn1−yAs/InP epilayers grown by molecular beam epitaxy on InP(100) substrates. High-resolution and diffraction contrast electron microscopy measurements show the presence of different contrast zones in the epilayers. The analysis of high-resolution x-ray diffraction measurements and computer simulations ascribe these zones to the presence of a compositional gradient in the epilayers. A comparison among investigated samples grown under slightly different growth conditions combined with an analysis of the crystal defects is presented. Growth-induced small variations in the chemical composition of the epilayer can produce differences in the structural quality of the epitaxial layer. Finally, a few monolayers thick and highly strained film of InAsP, is observed in all investigated samples at the substrate/epilayer interface. The formation of this interface layer is explained by the exchange of As and P during exposure of the InP surface to As4 before the growth.
Investigation of reactive-ion-etch-induced damage of InP/InGaAs multiple quantum wells by photoluminescence
Authors: Steffensen O. M. , Birkedal D. , Hanberg J. , Albrektsen O. , Pang S. W.
Published: July 2002

The effects of CH4/H2 reactive ion etching (RIE) on the optical properties of an InP/InGaAs multiple-quantum-well structure have been investigated by low-temperature photoluminescence (PL). The structure consisted of eight InGaAs quantum wells, lattice matched to InP, with nominal thicknesses of 0.5, 1, 2, 3, 5, 10, 20, and 70 monolayers, respectively, on top of a 200-nm-thick layer of InGaAs for calibration. The design of this structure allowed etch-induced damage depth to be obtained from the PL spectra due to the different confinement energies of the quantum wells. The samples showed no significant decrease of luminescence intensity after RIE. However, the observed shift and broadening of the PL peaks from the quantum wells indicate that intermixing of well and barrier material increased with etch time.
Prediction of solid-state amorphizing reaction using effective driving force
Authors: Kwak J. S. , Chi E. J. , Choi J. D. , Park S. W. , Baik H. K. , So M. G. , Lee S. M.
Published: July 2002

It is proposed that the nucleation and growth of the amorphous phase through the solid-state amorphizing reaction in thin-film diffusion couples can be predicted by using the concept of effective driving force. The effective driving force consists of two factors: (i) the thermodynamic driving force given by maximum free-energy difference between the physical mixture of binary elements and the amorphous phase (ΔGmax), and (ii) the kinetic factor given by a ratio of the effective radius of the interstitial site in the host matrix to the atomic radius of the diffusing species (Rm/d). From the comparison of reported experimental results, it is shown that the criterion of effective driving force holds well for predicting the nucleation of the amorphous phase in metal/silicon systems as well as that of metal/metal systems. In addition, the concept of effective driving force holds well for predicting the growth tendency of the amorphous phase in metal/silicon systems.
Deep-level impurities in edge-defined film-fed-growth silicon
Authors: Park S. H. , Schroder D. K.
Published: July 2002

Edge-defined film-fed-growth (EFG) Si is investigated using deep-level transient spectroscopy and surface photovoltage. An impurity energy level of CrB was found at 0.27 eV above the valence band in EFG Si contaminated with Cr. The Cr diffusion coefficient in EFG Si was obtained as 2×10−17 cm2/s at room temperature using association and dissociation of CrB pairs after a 210 °C dissociation anneal. Most of the deep-level transient spectroscopy (DLTS) spectra are not analyzable with conventional methods due to abnormally broad peaks. DLTS spectra of as-grown EFG Si are modeled using a Gaussian distribution of impurity energy states. The simulated DLTS peaks agree well with measured data explaining the origin of the deep-level impurities of EFG Si.
Electron density modulation effect in a quantum-well infrared phototransistor
Authors: Ryzhii V. , Ershov M.
Published: July 2002

Effect of the modulation of the electron density in the quantum well (QW) of intersubband single quantum-well infrared phototransistor (QWIPT) on its performance is considered theoretically. We show that the sheet electron concentration can significantly differ from the sheet concentration of the donors in the QW. The sheet electron concentration can increase with applied bias, which leads to an increase of the dark current, photocurrent, responsivity and detectivity of the QWIPT. The effect of the electron tunneling from the QW is also discussed.
Two-dimensional fluid simulation of expanding plasma sheaths
Authors: Hong MunPyo , Emmert G. A.
Published: July 2002

The transient sheath expansion around square and cross-shaped targets is simulated numerically with a two-dimensional fluid model. The angular distribution of the ions impinging on the target surface and the nonuniformity of the incident ion dose are calculated. The incident ion dose peaks near, but not at, the convex corner and has a minimum at the concave corner. The dip of the dose profile at the convex corner is shown to be caused by the product of a decreasing normal velocity profile and an increasing ion density profile along the target surface from the center to the corner.
Formation of amorphous interlayers by solid-state diffusion in Ti thin films on epitaxial Si–Ge layers on silicon and germanium
Authors: Lai J. B. , Liu C. S. , Chen L. J. , Cheng J. Y.
Published: July 2002

The formation of amorphous interlayers (a-interlayers) by solid-state diffusion in ultrahigh-vacuum-deposited polycrystalline Ti thin film on germanium and epitaxial Si1−xGex (x=0.3, 0.4, and 0.7) alloys grown on (001)Si has been investigated by transmission electron microscopy and Auger electron spectroscopy. Amorphous interlayers, less than 2 nm in thickness, were observed to form in all as-deposited samples. The growth of the a-interlayers was found to vary nonmonotonically with the composition of Si–Ge alloys in annealed samples. On the other hand, the formation temperature of crystalline phase was found to decrease with the Ge content. The results are compared with those of the Ti/Si system.
Two-dimensional simulation study of field-effect operation in undoped poly-Si thin-film transistors
Authors: Kong Hyang-Shik , Lee Choochon
Published: July 2002

We propose a simulation model to explain two-dimensional field-effect operation of undoped poly-Si thin-film transistors (TFTs) under small drain voltages. Our model includes both thermionic-emission and drift-diffusion conduction processes. We calculated grain-boundary potential barriers, channel currents, and various device parameters depending on grain size and defect density. In order to validate our model, we compared calculated currents with experimental data for two types of poly-Si TFTs. We could obtain good current fits simultaneously in both subthreshold and linear regions by adopting proper densities of states in the poly-Si channels. We could also explain well the temperature-dependent current changes and the current activation energy versus the gate voltage. Finally, we succeeded in modeling the drain current under small drain voltages by using the combined transport process in the two-dimensional grain-boundary structure.
Excimer laser processing of indium-tin-oxide films: An optical investigation
Authors: Szörényi T. , Laude L.D. , Bertóti I. , Kántor Z. , Geretovszky Zs.
Published: July 2002

dc sputtered indium-tin-oxide films have been excimer laser irradiated at subablation threshold fluences (<510 mJ/cm2). Optical characterization of irradiated products has been performed aiming at resolving the finer structure appearing in the IR–visible absorption spectra, as a function of laser fluence, and assigning such features to specific electronic defects which are produced upon irradiation. Four individual Gaussian-like contributions to absorption spectra are identified at 0.7, 1.0, 1.6, and 2.6 eV, the intensity of which is observed to vary with fluence. Being absent in the original films and emerging in optical spectra at fluences exceeding 300 mJ/cm2, the 2.6 eV contribution is most characteristic to excimer laser processing and is responsible for the darkening of the film. Thermal model calculations reveal that such defects are produced only upon melting and fast resolidification of the film. The evolution of the chemistry actually taking place in the film upon irradiation is followed by x-ray photoelectron spectroscopic analysis. A chemical approach to the production of such defects is proposed in which oxygen displacement in the atomic matrix leads to the formation of neutral ternary complexes of the type SnIn2O4.
Time-resolved energy distribution of F− from pulsed radio frequency discharges
Authors: Smith Brian A. , Overzet Lawrence J.
Published: July 2002

Negative ions extracted from a pulsed rf discharge can have a complex energy dependence as a function of time. This dependence could influence semiconductor processing in pulsed discharges, particularly if precise control of the ion energy is important. In this article, the time-dependent ion energy distributions of F− from pulsed discharges through CF4 are presented for the first time using a mass spectrometer equipped with a Bessel Box energy analyzer. The time-dependent changes in the ion energy distributions are produced by the manner in which the plasma disintegrates. Negative ions reaching the processing surface very early in the afterglow have lower energies because they experience less acceleration by the dc bias potential before entering the mass spectrometer. These ions appear in a very intense, short duration pulse, suggesting that they are formed in the after glow near the entrance to the mass spectrometer. Ions reaching the surface later in the afterglow have higher energies because they travel further through the dc bias potential. These ions appear with less intensity over a longer period of time.
Structural and optical properties of amorphous silicon oxynitride
Authors: Yeh Jiun-lin , Lee Si-Chen
Published: July 2002

Hydrogenated amorphous silicon oxynitride (a-SiOxNy:H) films have been prepared by glow discharge of silane (SiH4, 50% in H2), nitric oxide (N2O) and hydrogen (H2) at a substrate temperature of 250 °C. The properties of these films have been investigated using infrared absorption, ellipsometry, optical absorption, and photoluminescence, etc. Various molecular bonds were identified by the infrared absorption measurement. For example, the 2185, 975, 935, and 700 cm−1 peaks are associated with the stretching, bending, wagging, and rocking modes of the Si–H2 bond back bonded to two oxygen atoms, respectively. It was found that the water molecules H2O appear as the gas phase composition of N2O becomes larger than 0.965; they are trapped in the film by their hydrogen bonds. It is found that the low frequency (≤1 MHz) refraction indexes of the deposited films are 0.2 higher than the high frequency (632.8 nm) values. Two peaks are observed in the photoluminescence spectra of a-SiOxNy:H when the gas phase composition is smaller than 0.8; this indicates that the a-SiOxNy:H films are inhomogeneous.
Piezoreflectance and photoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures
Authors: Lin D. Y. , Lin F. C. , Huang Y. S. , Qiang H. , Pollak Fred H. , Mathine D. L. , Maracas G. N.
Published: July 2002

We have studied the piezoreflectance (PzR) spectra at 300 and 80 K related to the intersubband transitions from two different (001) GaAs/AlGaAs structures, an asymmetric triangular quantum well and a rectangular quantum well, fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. A comparison of the relative intensity of heavy- and light-hole related features in the PzR spectra and those in the photoreflectance emphasizes the contribution of the strain dependence of the energies of the confined states which allows us to identify the features associated with the heavy- and light-hole valence bands unambiguously. Comparison of the observed intersubband transitions with the envelope function calculations provides a self-consistent verification that the DACG method generated the desired potential profiles. Furthermore, the temperature dependence of both the energy position and broadening parameter of the fundamental conduction to heavy-hole (11H) and light-hole (11L) excitonic features are investigated in the range of 20–300 K. The anomalous behavior of the temperature dependence of the linewidth of 11H(L) excitonic features of the samples are discussed.
Precipitate-hardened aluminum alloys formed using pulsed laser deposition
Authors: Knapp J. A. , Follstaedt D. M. , Myers S. M.
Published: July 2002

Very high-strength alloys of Al have been formed by pulsed laser deposition (PLD) from Al and Al2O3 targets. The deposition is alternated between the two targets, with the ratio of pulses per target controlling the O content, and the amount of each element deposited in a cycle kept low enough that the final material is uniform in composition and has a highly refined microstructure. A fine, uniform dispersion of ∼1-nm-diam γ-Al2O3 precipitates is formed in fcc Al, with an overall composition of up to 33 at. % oxygen. This microstructure is very similar to one that we previously synthesized by using ion implantation of O into Al, where nanoindentation testing revealed yield stresses of ∼1 GPa or more. Nanoindentation testing of these new PLD materials shows yield stresses as high as 5.1 GPa, greatly exceeding the strengths of aerospace Al alloys and even high-strength steels. The key to the properties of these materials is the dispersion of small, hard precipitates spaced only a few Burgers vectors apart; dislocations are apparently unable to cut through and must bow around them. While the nature of ion implantation limits both its cost effectiveness and the thickness of the layers that can be formed, PLD is a completely new approach with major advantages over implantation, including greater control of the composition versus depth, deposition on arbitrary substrates to any thickness, and scalability of the process to larger areas.
An analytical model of metal cutting with a laser beam
Authors: Kaplan A. F. H.
Published: July 2002

The process of cutting metals with a laser beam is described by a mathematical model comprising a set of balance equations for mass, momentum and energy. The molten film at the cutting front is defined as the control volume. The calculation yields average values for the characteristic macroscopic state of the cutting front, such as temperature, melt film velocity and melt film thickness. All relevant parameters of the laser beam and of the process gas flow are considered in the model. Numerical evaluations of the analytical model provide explanations of the process behavior both for inert and for oxygen gas cutting. The narrowing of the kerf width with increasing speed is explained by heat conduction. Furthermore, the model shows that the evaporation temperature is reached at high velocities for thin metal sheets. Using a higher gas pressure increases the diffusion limited oxidation rate and provides higher cutting velocities, while the fraction of oxidized metal decreases substantially with increasing speed.
Oscillatory interlayer exchange coupling in Ni81Fe19/Cu/Ni81Fe19/Fe50Mn50 spin valves
Authors: Leal J. L. , Kryder M. H.
Published: July 2002

An oscillatory behavior is observed in the indirect exchange coupling between the two ferromagnetic layers in sputtered 〈111〉 and 〈100〉 Ni81Fe19/Cu tCu/Ni81Fe19/Fe50Mn50 spin valves. The periods of oscillations are 13.5 and 12.5 Å for the 〈111〉 and 〈100〉 growth directions, respectively. The coupling changes from strongly ferromagnetic, at tCu=12 Å, to antiferromagnetic at tCu=9.5 Å. At the transition region where the bilinear coupling changes sign (tCu≊10 Å) biquadratic coupling is observed. The amplitude of the giant magnetoresistive effect reaches its maximum value for tCu=12 Å, and values in excess of 4% are still observed for tCu=10 Å. Interface sharpness is shown to be a key parameter for the observation of antiferromagnetic coupling, as well as a large magnetoresistive effect, at very thin copper layers.
Carrier diffusion in porous silicon studied by transient laser-induced grating spectroscopy
Authors: Tomasiunas R. , Pelant I. , Kočka J. , Knápek P. , Lévy R. , Gilliot P. , Grun J. B. , Hönerlage B.
Published: July 2002

We report laser-induced transient grating experiments performed at room temperature on self-supporting p-type porous silicon films with different porosities. With this technique the diffusion of the photocarriers can be studied with a time resolution of some tens of picoseconds. The gratings were created by two interfering pulses of the second-harmonic radiation from an Nd3+:yttrium-aluminum-garnet YAG laser (2.3 eV) and their decay in time was studied by a time-delayed pulse of the fundamental YAG laser frequency (1.15 eV). The observed grating decay time is very fast (hundreds of picoseconds) and shortens with decreasing porosity. Diffusion constants D=45, 24, and 5 cm2 s−1 have been found for the porosities of 64%, 68%, and 73%, respectively. To explain these high values of D we consider a simple kinetic model which takes into account two different types of carriers, delocalized and trapped ones.
Improvement of soft magnetism of Fe90Co10 sputtered films by addition of N and Ta
Authors: Nakagawa Shigeki , Tanaka Satoshi , Suemitsu Katsumi , Naoe Masahiko
Published: July 2002

The addition of Ta and N into sputtered Fe90Co10 alloy films was performed to attain soft magnetism with large saturation magnetization 4πMs. Addition of Ta below 8 at. % to Fe90Co10 films improved their crystallinity and the lattice constant of Fe crystallites was expanded. These films revealed relatively large 4πMs of about 21 kG. The deposition of Fe–Co and Fe–Co–Ta films at N2 partial pressure PN2 below 0.1 m Torr was so effective as to cause fine granulation in the film. Fe84.6Co8.4Ta7:N films deposited at PN2 of about 0.2 mTorr and postannealed at 150 °C revealed 4πMs and relative permeability μr of about 16 kG and 1500, respectively.
Relaxation process for second-order nonlinear optical properties of poled diazo-dye-substituted polymers
Authors: Shuto Yoshito , Amano Michiyuki
Published: July 2002

This paper describes the transient properties of the second-order nonlinear optical processes of a diazo-dye-substituted poled polymer. The temperature dependences of bulk susceptibilities and Pockels coefficients are measured below the glass transition temperature to estimate relaxation times. The activation energy of 43 kcal/mol determined from the Arrhenius plots of the relaxation times is assigned to that of the α relaxation of the diazo-dye-substituted polymer. The relaxation of the induced alignment of diazo-dye side groups is restricted because of both the large dye molecular volume of greater than 500 Å3 and the very small cumulative distribution of local free volumes which are greater than the dye molecular volume in the polymer.
Direct measurements of strain in shock-loaded glass specimens
Authors: Rosenberg Z. , Bourne N. K. , Millett J. C. F.
Published: July 2002

Direct measurements of the uniaxial strain behind wave fronts in shock-loaded glass specimens reveal several new aspects of their dynamic response. The measurements have been achieved by embedding longitudinal strain gauges in the shocked specimen in such a way that gauge length is along the shock propagation direction. Following the resistance changes of the gauges, we were able to find differences in the loading characteristics below and above the Hugoniot elastic limit of the glass, measure residual strains when shocked above this limit, and find interesting aspects of the strain histories behind the failure wave fronts.
Spin-dependent scattering in the nonmagnetic layers of annealed Co/Cu multilayers
Authors: Laidler H. , Hickey B. J.
Published: July 2002

Annealing a multilayer is expected to promote diffusion at the interfaces. We have found that the interface roughness and giant magnetoresistance of Co/Cu (111) multilayers, grown by molecular-beam epitaxy on sapphire substrates, remain unchanged during anneals of up to 320 °C despite the zero-field resistivity having doubled. We explain this large increase in resistivity as due to the formation of an alloy between the top Cu layer of the multilayer and the Au protective cap. We propose that scattering in the AuCu alloy is spin dependent. We discuss this idea in the light of mean free path effects and the density of states of the entire sample.
Femtosecond near-field spin microscopy in digital magnetic heterostructures (invited)
Authors: Levy J. , Nikitin V. , Kikkawa J. M. , Awschalom D. D. , Samarth N.
Published: July 2002

We have designed a low-temperature femtosecond-resolved near-field scanning optical microscope to study spatiotemporal excitonic spin behavior in magnetic semiconductor heterostructures. Local disorder introduced by focused-ion-beam implantation reduces the otherwise large Zeeman splittings in modest magnetic fields, creating a planar spin-dependent energy landscape for diffusing carriers. Near-field polarization-resolved static and femtosecond measurements map out excitonic spin behavior with ∼125 nm spatial resolution, revealing spin-dependent diffusion. We demonstrate the applicability of two distinct time-resolved techniques in the near field, and discuss limitations on the measurement of polarized luminescence from semiconductors in the near field.
Structural origin of magnetic biased field in NiMn/NiFe exchange coupled films
Authors: Wong Bunsen Y. , Mitsumata Chiharu , Prakash Shiva , Laughlin David E. , Kobayashi Toshio
Published: July 2002

The factors which determine the strength of the magnetic biased field in NiMn/NiFe exchange biased system have been examined. It was found that a strong NiFe (111) texture and columnar growth are essential to obtaining a high exchange field as they facilitate epitaxy between the NiFe and NiMn layers. The application of an amorphous Ta nucleation layer and substrate biasing during NiFe deposition have been found to promote these structures. Fcc NiMn replicates the (111) texture through epitaxial growth on NiFe according to a cube on cube orientation relationship. Upon annealing, atomic ordering takes place in the NiMn layer and gives rise to an exchange biased field. However, a fully L10 ordered structure cannot be achieved at the NiMn/NiFe interface due to the stabilizing effect of fcc NiFe. Away from the interface, the transformation proceeds and the ordered tetragonal unit cell creates a lattice strain within the NiMn film. This strain is relieved by twinning within the NiMn grains of the film. The strength of the exchange field has been found to be closely associated with the density of twins and their proximity to the NiMn/NiFe interface.
Characterization of defects in self-ion implanted Si using positron annihilation spectroscopy and Rutherford backscattering spectroscopy
Authors: Fujinami M. , Tsuge A. , Tanaka K.
Published: July 2002

The behavior of vacancy-type defects and displaced Si atoms in Si(100) caused by self-ion implantation has been investigated by variable-energy positron annihilation spectroscopy and Rutherford backscattering spectroscopy/channeling. It is found that the recovery process of the defects strongly depends on the morphology of the implanted region. The divacancies produced by an implantation of 2×1014Si+⋅cm−2, which is less than the critical value required for amorphization, aggregate into large vacancy clusters by annealing at 300 °C. These vacancy clusters diffuse towards the surface at temperatures above 600 °C and anneal out at around 800 °C. The specimen implanted with 2×1015Si+⋅cm−2, in which a complete amorphization takes place in the damaged region, shows a different annealing characteristic. In the first stage (∼600 °C), the amorphous zone is transformed into crystalline material by solid phase epitaxial growth, although large vacancy clusters still remain. These agglomerate clusters continue to grow in a second annealing stage which takes place at around 700 °C. Annealing at 900 °C is required to eliminate these vacancy-type defects.
Moment expansion approach to calculate impact ionization rate in submicron silicon devices
Authors: Sonoda Ken-ichiro , Yamaji Mitsuru , Taniguchi Kenji , Hamaguchi Chihiro , Dunham Scott T.
Published: July 2002

A method to calculate the impact ionization rate in submicron silicon devices is developed using both an average energy and an average square energy of electrons. The method consists of an impact ionization model formulated with the average energy and conservation equations for the average square energy in the framework of an energy transport model. Parameters for the transport equations are extracted in such a way that calculated moments based on these equations match Monte Carlo simulation results. The impact ionization generation rate in an n+nn+ structure calculated with this method agrees well with the results obtained from Monte Carlo simulation. The new method is also applied to a submicron n-MOSFET. The calculated distribution of the generation rate is found to be quite different from the results based on a conventional method.
Hot carrier transport in multibarrier heterostructure diode
Authors: Reklaitis A.
Published: July 2002

Current switching and instability in a multibarrier heterostructure GaAs/Al0.45Ga0.55As diode is studied by Monte Carlo particle technique. It is found that interband impact ionization plays an important part in the phenomena considered. Current switching and instability originate from electron tunneling across the heterointerface nearest to the cathode with the subsequent impact ionization. Tunneling across the remaining heterointerfaces is of no importance in the diode operation under an instability regime.
Operation of bistable phase-locked single-electron tunneling logic elements
Authors: Ohshima T. , Kiehl R. A.
Published: July 2002

The operation of bistable phase-locked single-electron tunneling logic elements is analyzed using a deterministic model of single-electron tunneling. The elements consist of capacitively coupled junctions pumped by an ac signal at twice the tunneling frequency and activated by clocking the dc bias. Logic states are defined by the tunneling phase with respect to the ac pump. The bistable operation is examined over ranges of input phase, clock phase, capacitive coupling factor, dc bias, and ac pump amplitude. Useful operating ranges are determined for stable locking and control of the logic state by input signals, and the signal transfer between coupled stages is examined. It is shown that operation is possible with reasonable input phase margins over wide ranges in dc bias and pump amplitude and for parameter ranges compatible with extremely low power-delay products. Circuit architectures for exploiting the attractive power-delay performance and extremely high gate density possible with this approach are discussed.
Charge trapping and current–voltage bistability in InGaAs quantum wires
Authors: Cingolani R. , Rinaldi R. , De Vittorio M. , Vasanelli L. , Cola A. , Marti U. , Reinhart F. K.
Published: July 2002

We report a clear evidence of bistability in the current–voltage characteristics of p-i-n heterostructures containing InGaAs V-shaped quantum wires. The observed phenomenon is explained in the framework of a single carrier transport model in which the quantum wires act like traps for the vertical current. The charge trapping phenomenon is indeed demonstrated by temperature-dependent photocurrent experiments.
Growth, microstructure, and strain relaxation in low-temperature epitaxial Si1−xGex alloys deposited on Si(001) from hyperthermal beams
Authors: Lee N.-E. , Matsuoka M. , Sardela M. R. , Tian F. , Greene J. E.
Published: July 2002

Epitaxial Si1−xGex(001) alloy films, with 0.15≤x≤0.30, were grown on Si(001) at temperatures Ts ranging from 300 to 550 °C using hyperthermal Si (average energy 〈ESi〉≂18 eV) and Ge (〈EGe〉≂15 eV) beams. The deposition rate was 0.1 nm s−1 and film thicknesses ranged from 30 nm to 0.8 μm. The energetic Si and Ge beams are generated by bombarding Si and Ge targets with 1 keV Kr+ ions from double-grid, multiaperture, broad ion-beam sources in a system geometry established based upon TRIM simulations of energy-dependent angular distributions of sputtered and backscattered particles. A combination of high-resolution plan-view and cross-sectional transmission electron microscopy, high-resolution x-ray diffraction, Rutherford backscattering spectroscopy, channeling, and axial angular-yield profiles demonstrated that the films are of extremely high crystalline quality. Critical layer thicknesses hc for strain relaxation in these alloys were found to increase rapidly with decreasing growth temperature. For Si0.70Ge0.30, hc ranged from 35 nm at Ts=550 °C to 650 nm at 350 °C compared to an equilibrium value of ≂8 nm. At even lower growth temperatures, hc becomes larger than critical epitaxial layer thicknesses, ≳1 μm at 300 °C. In addition, atomic force microscopy studies showed that strain-induced roughening, which occurs at elevated growth temperatures, is strongly suppressed at Ts between 300 and 400 °C with no indication of kinetic roughening.
Comparative study on dry oxidation of heteroepitaxial Si1−xGex and Si1−x−yGexCy on Si(100)+
Authors: Xiang J. , Herbots N. , Jacobsson H. , Ye P. , Hearne S. , Whaley S.
Published: July 2002

The goal of this study is to investigate the effect of carbon incorporation upon thermal oxidation of Si1−xGex alloys and its role on strain compensation in Si1−xGex alloys. Si1−xGex and Si1−x−yGexCy alloys on Si(100) are grown by combined ion and molecular beam deposition and are then oxidized at 1000 °C in a dry oxygen ambient for two h. The thickness and the composition of all samples before and after oxidation are measured by Rutherford backscattering spectrometry (RBS) combined with ion channeling at 2.0 MeV and carbon nuclear resonance analysis at 4.3 MeV using 4He++ ions. In agreement with previously reported results of dry oxidation on Si1−xGex thin films, 2.0 MeV RBS analysis shows that a layer of SiO2 is formed on the top surface of both Si1−xGex and Si1−x−yGexCy thin films, while Ge segregates towards the top surface and at the SiO2/Si1−xGex and SiO2/Si1−x−yGexCy interfaces. However, it is observed for the first time that dry oxidation rates of Si1−xGex thin films decrease with increasing Ge fraction x for x≳0.20 and with increasing minimum yield. Ion channeling analysis and strain measurements indicate that the incorporation of C rather than the amount of C itself affects the dry oxidation mechanism because of its strong influence on film strain and crystalline quality. These results are discussed in conjunction with observations by secondary ion mass spectrometry, high resolution transmission electron microscopy, Fourier transform infrared spectrometry, and tapping mode atomic force microscopy.
Charge transport in silicon carbide: Atomic and microscopic effects
Authors: Schroeder A. , Pelster R. , Grunow V. , Lennartz W. , Nimtz G. , Friederich K.
Published: July 2002

It is shown that charge transport in SiC ceramics includes atomic mechanisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature-dependent dc and ac measurements. The complex dielectric function (DF) of boron-doped SiC ceramics with various additives has been measured at frequencies from 5 Hz to 2 GHz and at temperatures between 100 and 330 K. In addition, the dc conductivity was measured between 40 and 220 K. A transport mechanism on an atomic scale determines the temperature dependence of the dc conductivity. At low temperatures 3D variable range hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius-like carrier activation becomes dominant. The ac behavior depends on the dc conductivity, but it reflects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 104. Two polarization processes have been identified. The low-frequency process is related to a conduction current relaxation, i.e. to a partial interfacial polarization in conducting paths. The Barton-Nakajima-Namika relation holds, relating dc conductivity, relaxation time, and relaxator strength. On the other hand, the high-frequency process is attributed to Maxwell-Wagner-Sillars interfacial polarization in crystalline SiC grains with a size of several μm.
Recombination and metastability in amorphous silicon p-i-n solar cells made with and without hydrogen dilution studied by electroluminescence
Authors: Han Daxing , Wang Keda , Yang Liyou
Published: July 2002

Electroluminescence (EL) energy spectra and their total intensity as a function of temperature and generation rate of a-Si:H p-i-n solar cells made with and without hydrogen dilution were measured. The slope of the EL efficiency versus T curve was 29 K, and the defect luminescence band was dominated by a single peak at ∼0.9 eV for the cell made with H dilution; while a slope of 37 K and a dual-peak defect band at ∼0.9 and ∼0.75 eV were observed for the cell made without H dilution. We explain that the energy distribution of the localized states, including both the tail and deep states, has been modified by hydrogen dilution. The photodegradation kinetics of the defect band suggests that the two distinctive defect states in the EL spectra are actually those identified independently as ‘‘fast’’ and ‘‘slow’’ defects, respectively, from photodegradation studies of solar cells prepared under the same conditions. Our results indicate that not only the total density but also the energy distribution of the defects are crucial in determining the photoelectronic properties including the stability of amorphous silicon-based materials. We conclude that EL is a more sensitive tool to study recombination and localized states in a-Si:H p-i-n solar cells compared to photoluminescence.
MeV P ion implantation damage and rapid thermal annealing effects in Fe-doped InP using Raman scattering
Authors: Shi Bo-Rong , Cue Nelson , Xu Tian-Bing , Au Standey
Published: July 2002

The damage in Fe-doped InP induced by 1.0 MeV P ion implantation with doses ranging from 5×1013 to 2×1015 cm−2 and effects of rapid thermal annealing (RTA) in the range of 700-1050 °C were investigated by means of Raman scattering. The shift and asymmetrical broadening of the longitudinal optical phonon peak (LO) and the appearance of a transverse optical mode (TO) show that the Raman scattering is very sensitive to implantation damage. For doses larger than 5×1014 cm−2, the TO and LO peaks were markedly broadened, even merged into a single peak, indicating an amorphous structure in the near surface region. Much of the primary damage can be annealed out after RTA at 800 °C for all implantation doses. For RTA below 900 °C, the residual damage decreased with increasing annealing temperature for the low dose case of 1×1014 cm−2, but increased for the high dose case of 2×1015 cm−2. Only when the annealing temperature is over 900 °C, the residual defects of the high dose case drastically decrease, and nearly full recovery is obtained when the annealing temperature is raised to 1000–1050 °C.
Giant microwave magneto-impedance in a single crystal of La0.7Sr0.3MnO3: The effect of ferromagnetic antiresonance
Authors: Lofland S. E. , Bhagat S. M. , Tyagi S. D. , Mukovskii Y. M. , Karabashev S. G. , Balbashov A. M.
Published: July 2002

We show that at about 340 K a large (≂50%) change in the microwave absorption of a single crystal of La0.7Sr0.3MnO3 is caused by the application of a field of just a few tens of mT. This large magneto-impedance occurs at 10 GHz but is absent at 36 GHz, and we demonstrate that these results follow from the frequency and field dependence of the dynamic permeability of a ferromagnetic conductor.
Growth of c-axis oriented gallium nitride thin films on an amorphous substrate by the liquid-target pulsed laser deposition technique
Authors: Xiao R. F. , Liao H. B. , Cue N. , Sun X. W. , Kwok H. S.
Published: July 2002

Gallium nitride (GaN) thin films with a wurtzite structure were grown on fused silica (FS) substrates by pulsed laser ablation of a liquid gallium target in the presence of ammonia gas. X-ray diffraction measurement shows a single c-axis orientation for the GaN film grown with a thin (<1000 Å) zinc oxide (ZnO) film as an alignment layer. There is a great improvement in the surface morphology as well as optical transmission for the GaN film grown on the ZnO buffered FS substrate. The energy band gap obtained from the absorption spectrum is about 3.45 eV.
Majority and minority electron and hole mobilities in heavily doped gallium aluminum arsenide
Authors: Bennett Herbert S.
Published: July 2002

The majority electron and minority hole mobilities were calculated in Ga1−yAlyAs for donor densities between 1016 and 1019 cm−3. Similarly, the majority hole and minority electron mobilities were calculated for acceptor densities between 1016 and 1020 cm−3. The mole fraction of AlAs, y, varies between 0.0 and 0.3 in these calculations. All the important scattering mechanisms have been included. The ionized impurity and carrier–carrier scattering processes were treated with a quantum-mechanical, phase-shift analysis. These calculations are the first to use a phase-shift analysis for minority carriers scattering from majority carriers in ternary compounds such as Ga1−yAlyAs. The results are in good agreement with experiment for majority mobilities and predict that at high dopant densities minority mobilities should increase with increasing dopant density for a short range of densities. This effect occurs because of the reduction of plasmon scattering and the removal of carriers from carrier–carrier scattering due to the Pauli exclusion principle. These calculations do not treat the density-of-state modifications due to heavy doping, which should have only a small effect on the mobility at room temperature. The results are important for device modeling because of the need to have physically reasonable values for minority mobilities when simulating the electrical behavior of heterojunction bipolar transistors.
Conductance measurements on P
b centers at the (111) Si:SiO2 interface
Authors: Uren M. J. , Stathis J. H. , Cartier E.
Published: July 2002

The electrical properties of the Pb center have been measured using the conductance technique over the temperature range 130–290 K. A high concentration of Pb centers was created by vacuum annealing of 28-nm-thick thermal oxides on (111) silicon surfaces. Fitting the conductance data allowed the contribution of the (0/−) Pb level to be separated from the U-shaped background states. The (0/−) peak in the density of states was found to be asymmetrical with a broad shoulder on the conduction band side. The Pb levels were found to show a capture cross section which fell toward the band edges and which could be fitted by assuming an activated cross section with an activation energy which increased toward the band edges. By contrast, the background states showed a cross section which was temperature and band bending independent.
The use of carbides as electrode material in a pseudospark switch
Authors: Görtler A. , Schwandner A. , Frank K. , Christiansen J.
Published: July 2002

In the following, we report tests of a semiconductor as electrode material in a pseudospark switch, which is a low pressure gas discharge switch. Polycrystalline SiC or BC disks with central apertures were embedded in molybdenum outer electrodes in a typical single stage pseudospark geometry. The maximum voltage applied was 15 kV. Tests were done with peak currents from 50 up to 500 kA at pulse lengths of a few microseconds. We found no difference in the voltage breakdown or the rise in current when compared to conventional metal electrodes such as molybdenum or tungsten. Fast shutter photographs showed that the discharge burns in an intense column at the center and is distributed very homogeneously over the SiC surfaces. Neither cathode nor anode spots were observed. Small sparks occurred at the metal–carbide interface after current zero, which led to a reignition in the second current half-wave. An estimation after the tests showed an erosion rate of about 5 μg/C approximately one order of magnitude lower than in metals. We assume that the current is transported in a thin surface layer. Ion bombardment in the pseudospark discharge heats this layer to more than 2000 K, lowering the specific resistance. The negative temperature–resistance characteristic means that cathode spots are avoided.