bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic state and thermodynamical properties of inhomogeneous epitaxial layers: Application to immiscible III-V alloys
Authors: Glas Frank
Published: July 2002

The elastic strain and stress fields and the elastic energy of the system composed of a crystalline epitaxial layer of finite thickness coherently grown on a bulk substrate are calculated, when the intrinsic stress-free lattice parameter of the layer is modulated along directions parallel to the substrate surface. When the modulation has components with spatial periods of the same order as the thickness of the layer, the elastic energy is considerably reduced with respect to the same modulation occurring in a bulk sample. There exists an optimal period of elementary sinusoidal modulation, proportional to the layer thickness. Consequently, for immiscible alloys where changes of composition induce changes of intrinsic lattice parameter, the critical temperature (below which they become thermodynamically unstable with respect to composition modulations) is much higher (and the domain of instability larger) if the material is in the epitaxial layer form than in the bulk form. This extends Cahn’s theory of spinodal decomposition to epitaxial layers. It is also pointed out that if a modulation has started to occur in a growing layer, the elastic deformation induced near the free surface should have important consequences on the subsequent growth of this layer. These results are applied to III-V semiconductors immiscible alloys, such as InxGa1−xAsyP1−y, where such composition modulations are known to exist. New values of the critical temperatures for these alloys are calculated and compared with lower former estimates. The mode of development of these modulations is discussed in light of previous experimental results and these new calculations.
On the pinning of domain walls in low magnetization materials
Authors: Wörner C. H. , Valdés J. E.
Published: July 2002

The proposal of an interaction potential between 180° Bloch walls and nonmagnetic obstacles, precipitates or pores, is presented. Use is made of the analogy between this phenomenon and the pinning of grain boundaries in polycrystalline materials. The model takes into account the increase of energy due to the flexibility of the boundary and it is applicable to low magnetization materials. Using this approach it is possible to calculate the coercive force and to compare it with early findings. The numerical values obtained are in agreement with the values we find in the literature with the advantage that the proposed potential is expressed in a simple analytical form.
Hysteresis of submicron permalloy particulate arrays
Authors: Smyth J. F. , Schultz S. , Kern D. , Schmid H. , Yee Dennis
Published: July 2002

We have prepared controlled arrays of submicron permalloy particles as part of a program to determine the magnetic properties of verifiable single-domain particles (SDP) and the effects of interactions between SDPs. We have measured the hysteresis loops of these arrays as a function of size, aspect ratio, and spacing. The arrays are fabricated utilizing electron beam lithography and liftoff techniques. Each array consists of approximately 106 identical particles with identical spacing between the particles. Particle size and spacing [center-to-center distance (ccd)], vary from 5 μm×15 μm (width by length) with ccd of 25 μm to 0.07 μm×0.1 μm with ccd of 0.25 μm. The thickness of the films varied from 0.072 to 0.049 μm. Hysteresis loops are obtained utilizing a SQUID magnetometer. For well-separated particles we observe an order of magnitude increase in Hc with decreasing particle size, and then a subsequent decrease in Hc as the ccd is reduced. However, even for the smallest particles, the ratio of the remanence to saturation magnetization indicates that they are not single domain, and the loop shape is not indicative of Stoner–Wohlfarth behavior.
Structural properties of the ZnSe/GaAs system grown by molecular-beam epitaxy
Authors: Petruzzello J. , Greenberg B. L. , Cammack D. A. , Dalby R.
Published: July 2002

We report the results of an investigation of the structural properties and relaxation of misfit stress with transmission electron microscopy and x-ray diffraction techniques. Epitaxial films of ZnSe were grown on GaAs by molecular-beam epitaxy of thicknesses ranging from 0.05 to 4.9 μm. The films contain stacking fault defects up to thicknesses of about 150 nm. Above 150 nm perfect misfit dislocations are generated from surface sources and the stacking fault defects to accommodate the lattice mismatch. The majority of dislocations observed are of the 60° type with Lomer edge type dislocations observed in a much lower concentration. The density of misfit dislocations increases with increasing epilayer thickness. Above about 1 μm the films exhibit biaxial tension which we believe is due to thermal expansion differences of ZnSe and GaAs. Good agreement is observed between microscopic and diffraction measurements of the relaxation phenomena.
Cross-sectional high-resolution electron microscopy investigation of argon-ion implantation-induced amorphization of silicon
Authors: Claverie A. , Vieu C. , Fauré J. , Beauvillain J.
Published: July 2002

Cross-sectional high-resolution transmission electron microscopy and related diffraction techniques are applied to the characterization of argon implantation-induced amorphization of silicon at room temperature. Damage calculations have been performed to provide a theoretical support for the cross-sectional transmission electron microscopy observations. It is shown that the amorphous-crystalline interfacial roughness is strongly dependent on ion dose and hence on its depth location. The a-c transition region was found to have sharply defined boundaries and sometimes exhibits defects such as dislocations and stacking-fault nuclei. Combining the experimental measurement of the extension of the a layer for increasing dose, with concepts arising from the ‘‘critical damage energy density’’ model leads to a value of about 10 eV/atom for the c→a transformation. It is suggested that temperature effects are responsible for the observation that higher damage energy densities are apparently needed to produce a first continuous a layer than to extend this layer to greater depth.
A new model for the diffusion of arsenic in polycrystalline silicon
Authors: O’Neill A. G. , Hill C. , King J. , Please C.
Published: July 2002

Experimental data are presented for the diffusion of arsenic in polycrystalline silicon (polysilicon) during a 15-min anneal at 900 °C. A new model is used to describe the data, which takes account of the different dopant diffusion rates in grains and in grain boundaries. The model is divided into early and late stages and, in particular, the early stage model identifies grain growth as the major mechanism by which arsenic transfers from the grains to the grain boundaries. In the late stage model, grain growth can be ignored and analytic solutions for the arsenic concentration are derived.
Fundamental oscillations up to 200 GHz in resonant tunneling diodes and new estimates of their maximum oscillation frequency from stationary-state tunneling theory
Authors: Brown E. R. , Goodhue W. D. , Sollner T. C. L. G.
Published: July 2002

Fundamental oscillations have been measured up to 200 GHz in resonant-tunneling diodes at room temperature. Oscillations in the range 102–112 GHz were achieved with diodes mounted in a WR-6 waveguide resonator, and the peak output power in this range was approximately 5 μW. The same diodes oscillated between 192 and 201 GHz and generated about 0.2 μW when mounted in a WR-3 resonator. The estimated maximum oscillation frequency ( fmax) for these devices is 244 GHz, assuming the average drift velocity across the depletion layer to be 4×107 cm s−1. This estimate has been obtained from a new phenomenological theory of the negative differential conductance which accounts for the frequency-dependent spreading resistance and transit-time delay. The theory is also used to show that diodes having fmax exceeding 600 GHz are feasible simply by modifying the doping profile in the regions on either side of the double-barrier structure.
Theory for the plasma anodization of silicon under constant voltage and constant current conditions
Authors: Taylor S. , Eccleston W. , Barlow K. J.
Published: July 2002

A theoretical model is presented for the anodization of silicon in an oxygen plasma. The model is based on the space-charge-limited drift of negative oxygen ions through the growing oxide film to react at the silicon-silicon dioxide interface. In the initial linear growth region the supply of ions from the plasma determines the growth, whilst for longer periods the drift of ions through the oxide gives rise to parabolic growth kinetics. The theory is consistent with the lack of orientation dependence observed for the plasma oxidation of silicon. It is applicable over the whole of the growth kinetics, for anodization in both constant voltage and constant current modes. The dependence of current and voltage with time during anodization may also be predicted. The theory is shown to be applicable to oxide growth occurring in both an inductively coupled, rf-stimulated plasma and a microwave-sustained oxygen plasma over a range of experimental conditions.
Preparation and properties of III-V nitride thin films
Authors: Kubota K. , Kobayashi Y. , Fujimoto K.
Published: July 2002

Thin films of III-V nitride semiconductors (AlN, GaN, InN), mixed-crystalline films (AlxIn1−xN), and multilayered films (GaN/InN)n were grown by rf magnetron sputtering at low substrate temperatures below 500 °C. These films were characterized by x-ray diffraction, Raman scattering, optical absorption, and electrical measurements; it was proved that they have high crystal quality comparable to the previously reported data obtained by other growth methods. Dependence of the band-gap energy of AlxIn1−xN on composition x was determined. Multilayered films of (GaN/InN) were prepared for the first time; these films showed the characteristic diffusion property to make an ordered-alloy superlattice.
Strain relaxation of Ge films grown on a Si(001)-2×1 surface by molecular beam epitaxy
Authors: Hida Y. , Tamagawa T. , Ueba H. , Tatsuyama C.
Published: July 2002

The crystalline quality of Ge films grown on a Si(100)-2×1 substrate by molecular beam epitaxy has been studied by means of in situ reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering spectroscopy. The Ge layers of thickness up to 4000 Å were deposited at 400 °C with and without thin (200 Å) Ge buffer layers predeposited at 200 °C. It was found that the buffer layers were effective in improving the quality of the thin Ge overlayers, while they had no noticeable effect on the thick layers whose x-ray and Raman spectra were comparable to that of bulk Ge. It was also found that, even after the Ge films became relaxed with a generation of dislocations, they were not free from strain due to the lattice mismatch. This finding is supported by the quantitative estimate of the Raman frequency shift in terms of the uniaxial strain model.
Magnetic-state-dependent interatomic potential for iron (abstract)
Authors: Harrison Ralph J. , Krasko Genrich L.
Published: July 2002

Previous empirical or semiempirical potentials for use in atomic computer simulation in iron and its alloys1,2 have not explicitly taken account of the magnetism per se, but because the magnetic effects have contributed to the mechanical and thermal properties used in choosing the parameters of the potential, it would not be accurate to say that these potentials ignore the magnetic effects completely. However, for cases where the magnetic properties significantly change about defects in crystal, these potentials may be inadequate to simulate their structures or energies. We try to remedy this situation by a modification of the embedded-atom method (EAM) for obtaining interatomic potentials, in which the EAM potential for nonmagnetic iron is first obtained, and then extra terms in the potential are introduced to match the magnetic interactions. These interactions may be obtained by comparison of the properties of both nonmagnetic and magnetic iron as obtained, say, by doing both a non-spin-polarized and a spin-polarized computation for bcc iron. We have actually used the Stoner method in conjunction with a linear muffin-tin orbitals computation of nonmagnetic iron3 to calculate the magnetic contribution to energy, since the Stoner parameter is adjusted to fit experimental data and, therefore, the method is perhaps more suitable for determining an empirical potential than are the currently available a priori theoretical methods. We then use the experimental values of the cohesive energy and elastic constants, but with the magnetic contributions subtracted out, to fit the nonmagnetic iron EAM potential. Additional terms in the potential are then added to reproduce the magnetic effects.
Localization of the electrical activity of structural defects in polycrystalline silicon
Authors: Cabanel C. , Laval J. Y.
Published: July 2002

In order to improve the capabilities of the electron-beam-induced current method, a technique based on scanning transmission electron-beam-induced current has been developed. It is shown that it enables the direct correlation of structural defects with their electrical activity. It implies the fabrication of ultrathin Schottky diodes (thickness ≤600 nm). From an approximate theoretical model it was inferred that the spatial resolution reaches about 200 nm in our experimental conditions. Experimental data are obtained on electron-grade and on upgrade metallurgical grade polycrystalline silicon, grown by the heat exchange method on which the behavior of carbon at grain boundaries and defects has been studied by transmission electron microscopy, high-resolution electron microscopy, and electron energy loss. There is a good agreement between the experimental data on electrical activity and the calculated approximation. The present method shows that the electrical activity is mainly related to the presence of impurities. Carbon seems to trap recombining impurities and specifically oxygen. Isolated dislocations are always electrically active whereas ‘‘clean’’ twin boundaries are not active. The activity at boundaries is always localized on extrinsic dislocations or on precipitates. Asymmetric profiles are also observed on boundaries and stacking faults. This was related to the existence of segregated zones lying on one side of these defects which are likely to act as diffusion barriers.
Crystal structure and magnetic properties of Mn-Al-Ni films prepared by sputtering
Authors: Morisako A. , Kohshiro N. , Matsumoto M. , Naoe M.
Published: July 2002

Mn60−xAl40Nix ternary alloy films were prepared by dc magnetron sputtering, and their crystallographic characteristics and magnetic properties have been investigated. The τ and κ phases were synthesized at a Ni content of below 7 at. % and above 10 at. %, respectively, and the mixture of both phases were synthesized at a Ni content of about 7.9 at. %. The lattice constant a of tetragonal structure in the films increased from 3.98 to 4.12 Å, and c decreased from 3.48 to 3.21 Å with the increase of Ni content. The Ms for the τ phase increased from 120 to 240 emu/cc by a few percent Ni substitution. Mn-Al-Ni films with single τ phase showed anisotropic characteristics in the magnetization curve due to the preferential orientation of the c axis for tetragonal crystallite.
In0.53Ga0.47As/InP heterojunctions with low interface defect densities
Authors: Lee C. D. , Forrest S. R.
Published: July 2002

Interface charge densities and conduction-band offset energies for liquid-phase epitaxially grown n-N isotype In0.53Ga0.47As/InP heterojunctions have been measured using capacitance-voltage methods. Extremely low interface charge densities have been obtained in some samples, and they are found to be independent of both the measurement temperature and the magnitude of lattice mismatch. Our samples show a clear peak and notch in the apparent free-carrier concentration profile at temperatures as low as 83 K. This is in contrast to results reported previously where the notch, due to the carrier depletion at the heterojunction, was observed to vanish at low temperature. An electron trap has been identified in one of the samples. The trap is uniformly distributed within the bulk of the In0.53Ga0.47As layer at a density of 5×1014 cm−3. In spite of the presence of this relatively low density defect, the heterojunctions grown for this study apparently have considerably lower interface defect densities than observed by others.
Indium-tin oxide thin films prepared by chemical vapor deposition
Authors: Maruyama Toshiro , Fukui Kunihiro
Published: July 2002

Transparent conductive indium-tin oxide thin films were prepared by an atmospheric-pressure chemical vapor deposition method. The source materials were indium acetylacetonate and tin (II) acetylacetonate which are nontoxic and easy to handle. The polycrystalline films were obtained at a reaction temperature in the range of 350–500 °C. For the 215-nm-thick film deposited at 450 °C, the resistivity was 1.8 × 10−4 Ω cm, and the transmittance was more than 90% in the wave number range of above 400 nm. The atomic ratio Sn/In of the film was 0.031. The effects of tin doping on the structure and characteristics of the film were discussed on the basis of the carrier concentration and mobility which were measured by using the Hall effect.
New model for the characterization of bulk traps by current deep level transient spectroscopy in metal-oxide-semiconductor transistors
Authors: Bauza D. , Ghibaudo G.
Published: July 2002

New models for the characterization of volume traps in enhancement-mode field-effect transistors by current deep level transient spectroscopy are proposed. First, a simple model that is based on the use of the experimentally measured transconductance of the device is studied. Consequently it includes, in principle, the dependence of the mobility with gate bias and therefore allows us to avoid any approximation in the gate voltage dependence of the mobility. Different improvements of this first model are then proposed, compared, and discussed with respect to previous works. Finally a more complete model that requires less approximations is derived. These different approaches are finally used for the characterization of a deep trap in a bulk enhancement-mode field-effect transistor.
Impurity enhancement of the 1.54-μm Er3+ luminescence in silicon
Authors: Michel J. , Benton J. L. , Ferrante R. F. , Jacobson D. C. , Eaglesham D. J. , Fitzgerald E. A. , Xie Y.-H. , Poate J. M. , Kimerling L. C.
Published: July 2002

The effect of impurity coimplantation in MeV erbium-implanted silicon is studied. A significant increase in the intensity of the 1.54-μm Er3+ emission was observed for different coimplants. This study shows that the Er3+ emission is observed if erbium can form an impurity complex in silicon. The influence of these impurities on the Er3+ photoluminescence spectrum is demonstrated. Furthermore we show the first room-temperature photoluminescence spectrum of erbium in crystalline silicon.
Heterointerfaces in quantum wells and epitaxial growth processes: Evaluation by luminescence techniques
Authors: Herman M. A. , Bimberg D. , Christen J.
Published: July 2002

This paper presents a review devoted to the problem of how optical and structural properties of quantum-well heterostructures (QWH) can be correlated in detail, and how these properties may be connected with the parameters of the epitaxial growth process. It demonstrates how luminescence techniques, mainly photoluminescence (PL) and cathodoluminescence imaging (CLI), may be used for evaluation of the structural disorder on the atomic scale, which occurs at the growth surfaces creating the interfaces of the QWH. The physics of the excitonic luminescence in QWH (theory and experiment) is presented in detail in the first part of the review. This is followed by a comprehensive discussion of experimental aspects (hardware and software) of the luminescence techniques, as applied for studying QWH grown by molecular-beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). The specific features of both the epitaxial growth techniques, when used for growing QWH are presented in the next part of this review. Finally, the possibilities of application of PL and CLI to studies on growth of QWH by MBE and MOVPE are demonstrated on a couple of selected examples. The review concludes with a short discussion on possible interpretation mistakes which may occur when one applies the CLI to studies of interfaces in QWH without taking into account the basic parameters of the excitonic luminescence lines creating the CL images of the relevant interfaces.
Anisotropic Nd–Fe–B thin-film magnets for milli-size motor
Authors: Yamashita S. , Yamasaki J. , Ikeda M. , Iwabuchi N.
Published: July 2002

Efforts were made to obtain anisotropic thin-film magnets at low substrate temperature. This is an important criterion for practical applications such as to build motors. The influence of substrate materials as well as film thickness on the c-axis orientation were studied. It has been shown that thin-film magnets with the easy axis of magnetization normal to the film plane could be deposited at a substrate temperature of around 450 °C by choosing the composition near the line from Nd13Fe76B11 to Nd13Fe70B17 in the ternary phase diagram. It was found that the anisotropic film magnets could be also deposited on the metallic substrate. The c-axis orientation tended to be isotropic with an increase in film thickness. The obtained results were used to fabricate a milli-size motor by depositing 20-μm-thick Nd–Fe–B films on a silicon steel disk substrate of 5-mm diam. The milli-size motor exhibited a torque of 0.8 g mm at a rotational speed of 3000 rpm.
Spectroscopic characterization and roughness-spectrum of interfaces in III-V multiple-quantum-well structures
Authors: Ils P. , Kraus J. , Schaack G. , Weimann G. , Schlapp W.
Published: July 2002

Resonance Raman spectroscopy and resonant excitation of luminescence have been applied to a p-doped GaAs-(Al,Ga)As multiple-quantum-well structure to determine the effects of well thickness variations and of interface roughness from observed shifts of hole-intersubband transitions with the exciting laser frequency. Energy and intensity variations of luminescence transitions have also been studied, again depending on laser energy. A fitting procedure is described that provides, e.g., data on the distribution of the well widths, i.e., on the interface quality. From these data along with other data available in the literature, a few semiquantitative but representative values of the power spectrum S(k) of the interface roughness at different spatial frequencies k have been determined. These values are averages over the frequency intervals of specific sensitivity of the different methods, which overlap only partially but cover almost the whole range of spatial frequencies k (D−1 ≤ k ≤ a−10), where D is the sample size and a0 the thickness of a monolayer. It appears that the power spectra derived are ‘‘red’’ (dS/dk<0) or, for some structures, possibly ‘‘white’’ (dS/dk=0).
The molecular beam epitaxial growth of GaAs/GaAs(111)B: doping and growth temperature studies
Authors: Woolf D. A. , Sobiesierski Z. , Westwood D. I. , Williams R. H.
Published: July 2002

A series of investigations are presented which address various aspects of the growth, by molecular beam epitaxy, of n-type (Si doped) on-axis GaAs/GaAs(111)B. In situ characterization by reflection high-energy electron diffraction has identified four surface phases on the static (zero growth rate) surface, and three reconstructions which occur, depending upon the substrate temperature, during growth. The n-type doping properties of GaAs/GaAs(111)B epilayers have been compared with n-GaAs/GaAs(100) structures. Hall effect and low-temperature photoluminescence measurements have demonstrated that it is possible to dope GaAs/GaAs(111)B with Si in the 6×1014 to 1018 cm−3 range. A variable growth temperature study is also presented which examines the surface structural, electrical, optical, and surface morphological properties of n-GaAs/GaAs(111)B grown in the 400 to 650 °C temperature range. The onset of electrical conduction, and optically active material, was found to be directly related to changes in the dynamic surface structure. The variable growth temperature study also revealed a temperature regime within which it was possible to significantly improve the surface morphology of on-axis GaAs/GaAs(111)B structures whilst retaining good electrical and optical properties.
Characterization of microporous Si by flow calorimetry: Comparison with a hydrophobic SiO2 molecular sieve
Authors: Canham L. T. , Groszek A. J.
Published: July 2002

Flow microcalorimetry has been used to study microporous silicon produced by electrochemical corrosion of bulk p-type silicon wafers in highly concentrated (50 wt %) aqueous hydrofluoric acid. Calorimetry data on pore size and hydrophobicity of freshly etched crystalline silicon structures are compared with similar measurements on silicalite, a well-studied microporous form of crystalline silicon dioxide. Silicalite has a tetrahedral SiO2 framework with interconnected ‘‘ultramicropores’’ that only readily admit molecules of less than 6 Å diameter. Its measured heat of immersion in n-heptane (kinetic diameter 4.3 Å) consequently far exceeds that in iso-octane (kinetic diameter 6.2 Å) and it preferentially adsorbs the normal alkane from the branched alkane. In direct contrast the microporous Si layers studied exhibited comparable heats of immersion for n-heptane and iso-octane, and did not show any preferential adsorption of the narrower molecule. In addition, the microporous Si layers studied exhibited appreciable heats of immersion in 1, 3, 5 tri-isopropylbenzene (kinetic diameter 8.5 Å). The majority of their pore volume is thus constrained to the ‘‘supermicropore’’ size regime of 10–20 Å width. Both silicalite and freshly etched microporous Si are shown, however, to be highly hydrophobic and organophilic materials. Their exothermic heats of immersion in n-heptane far exceed those in water and both materials preferentially interact with the polar alcohol (n-butanol) more strongly from water than from n-heptane.
Characterization of praseodymium-doped InGaP epilayer grown by liquid phase epitaxy
Authors: Lai M. Z. , Chang L. B.
Published: July 2002

Praseodymium (Pr) is added to the InGaP growth melt during liquid phase epitaxy (LPE). The epilayers are grown, by using a supercooling method, on (100) Cr-doped semi-insulating GaAs substrates at a growth temperature of 790 °C. An examination of the structural properties of the InGaP-grown layers reveals that its lattice constant increases slightly with increasing Pr concentration in the growth melts. Similarly, an examination of the electrical properties reveals that, depending on the amount of Pr in the growth melt, n-type InGaP epilayers with room-temperature electron concentrations in the range of 3.4×1016 cm−3 to 5.2×1015 cm−3 and electron mobilities from 730 to 1310 cm2/V s can be prepared. The photoluminescence spectral results show that by increasing the amount of Pr in the growth melt, smaller full-width at half-maximum values and better band-edge emission intensities result. The experimental results support the fact that Pr exhibits a gettering effect in the InGaP LPE process, as no characteristic Pr intra-4f-shell transitions are observed in the luminescence spectra of the InGaP-grown layers.
Effects of elevated temperature hydrogen exposure on short-wavelength optical losses and defect concentrations in germanosilicate optical fibers
Authors: Atkins R. M. , Lemaire P. J.
Published: July 2002

The relation between short-wavelength optical loss increases and defect concentrations has been examined for a single-mode germanosilicate fibers exposed to H2 at 150–250 °C. The hydrogen-induced loss increases under these conditions appear to be due to the reaction of H2 at Ge–E′ centers, resulting in the formation of —OH and ‘‘GeH’’ defects, the latter of which is responsible for a 650-nm fluorescence. ‘‘GeO’’ defect levels, characterized by fluorescence and optical loss measurements, remain unchanged in these experiments. This indicates that ‘‘GeO’’ defects are not part of the reaction mechanism that lead to short-wavelength loss increase at these temperatures, although they may cause other types of short-wavelength loss increase at higher temperatures.
Resonant tunneling current calculations using the transmission matrix method
Authors: Zebda Y. , Kan’an A. M.
Published: July 2002

A solution of resonant tunneling problem in multilayered heterostructures is presented based on the quantum mechanical wave impedance concept. The transmission matrix is found using open and short circuit tests. By using the transmission matrix approach the transmissivity of the structure is determined as a function of the incident electron energy. The J-V curves are obtained by using this approach and the results compare to previous models with good agreement. The method was then applied to a new quantum well three barrier structure, where the inner barrier height is variable. It is found that increasing the inner barrier height causes the resonance energies to align, meaning that coupling between the wells increases. By using this method, the analysis and tunneling current calculations of any structure can be easily figured and new tunneling quantum well devices can be designed.
C-V and I-V characteristics of quantum well varactors
Authors: Sun J. P. , Mains R. K. , Chen W. L. , East J. R. , Haddad G. I.
Published: July 2002

A theoretical model for quantum well varactors is presented. The model is used to calculate the device C-V and I-V characteristics and very good agreement has been found between the calculated and measured results. Based on the model, a triple barrier double well varactor has been designed and fabricated. A very high capacitance ratio within a very small bias range is achieved, as designed. Details of the design calculations and experimental results are presented.
Characterization of deep levels introduced by alpha radiation in n-type silicon
Authors: Asghar M. , Zafar Iqbal M. , Zafar N.
Published: July 2002

A detailed deep-level transient spectroscopy study of the characteristics of deep-level defects introduced by 5.48 MeV alpha particles in low-doped n-Si is reported. The deep-level characteristics studied include emission rate signatures, activation energies, capture cross sections and their temperature dependence, and defect concentrations and their spatial profiles. At least five deep levels in the upper-half band gap and two levels in the lower-half gap have been observed as a result of irradiation and characterized in detail. A systematic study of their generation rates up to a dose of about 3×1010 alpha particles/cm2 has been performed providing insights into the dose dependence of their formation mechanisms. Interesting room temperature transformation phenomena have been observed in our deep-level spectra during room temperature storage of the irradiated samples. Extensive isochronal thermal annealing measurements have been carried out to obtain data on the anneal-out characteristics of the radiation-induced deep levels and to identify these with the known defects wherever possible. A number of new annealed-in levels have been observed during this investigation. A detailed comparison with the published results is presented.
Radiation-induced mobility degradation in p-channel double-diffused metal-oxide-semiconductor power transistors at 300 and 77 K
Authors: Zupac D. , Galloway K. F. , Schrimpf R. D. , Augier P.
Published: July 2002

The effects of radiation-induced interface-trapped charge and oxide-trapped charge on the inversion-layer hole mobility in p-channel double-diffused metal-oxide-semiconductor power transistors at 300 and 77 K are investigated. The mobility degradation is more pronounced at 77 K than at 300 K, due to increased importance of Coulomb scattering from trapped charge when phonon scattering is significantly reduced. The mobility degradation is primarily due to interface-trapped charge, but the effects of oxide-trapped charge must be taken into account in order to properly describe the mobility behavior, particularly at cryogenic temperatures.
Electron transport in N,N′-bis(2-phenethyl)-perylene-3,4: 9,10-bis(dicarboximide)
Authors: Magin Edward H. , Borsenberger Paul M.
Published: July 2002

By time-of-flight techniques, electron mobilities have been measured in vapor deposited films of the title compound. The results were compared to predictions of the disorder formalism, due to Bässler and co-workers, and models based on polaron formation. The results lead to the conclusion that fluctuations in hopping site energies are the major contribution to the activation energy. For consistency between experiment and predictions of the formalism, however, an additional source of activation is required. The source of this activation is believed due to either polaron formation or trapping. The width of the hopping site manifold, σ, is determined as 0.080 eV and the positional disorder parameter, Σ, as 1.0. The interpretation of these results by a model in which transport occurs only by polaron displacement leads to inconsistencies with both the temperature and field dependencies of the mobility.
Structure induced magnetic anisotropy behavior in Co/GaAs(001) films
Authors: Blundell S. J. , Gester M. , Bland J. A. C. , Daboo C. , Gu E. , Baird M. J. , Ives A. J. R.
Published: July 2002

Epitaxial Co has been grown on GaAs(001) and studied by both low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED), and by the magneto-optic Kerr effect (MOKE) and polarized neutron reflection (PNR). Three samples were fabricated using different growth procedures: (1) ‘‘interrupted’’ growth (including an anneal); (2) and (3) continuous growth of similar thicknesses. For sample 1, RHEED patterns indicate an initial growth in the bcc phase followed by a relaxation into a distorted single phase at completion of growth, whereas samples 2 and 3 showed a multicrystalline structure after growth. LEED patterns were used to check the existence of the 2×4 reconstruction patterns before growth, but no LEED patterns could be obtained after more than 2 Å Co was deposited, in contrast to the RHEED patterns which remained visible throughout the growth. Structural analysis of the completed films indicates the formation of a ∼10 Å CoO layer on the Co/air interface, and gives thicknesses for magnetic material of (1) 30 Å and (2) 80 Å. Sample 1 showed a dominant fourfold magnetic anisotropy with the easy axis parallel to the (100) direction and with a strength 2K4/M of ∼0.5 kOe, smaller in magnitude than that reported for bcc films on GaAs(110) but along the same axis [G. A. Prinz et al., J. Appl. Phys. 57, 3672 (1985)]. However, samples 2 and 3 showed only a large uniaxial anisotropy along the (110) direction of strength 2K1/M of ∼1.5 kOe and ∼2.5 kOe, respectively, similar in magnitude to those previously observed [G. A. Prinz et al., J. Appl. Phys. 57, 3676 (1985)]. We attribute the origin of the contrasting magnetic anisotropy behavior observed to the differences in final structure.
Low temperature properties of soft magnetic materials: Magnetic viscosity and 1/f thermal noise
Authors: Durin G. , Falferi P. , Cerdonio M. , Prodi G. A. , Vitale S.
Published: July 2002

We study the magnetic properties of soft ferromagnetic cores in the low field linear regime at low temperature (T≤4.2 K) in view of their use in connection with superconducting quantum interference device magnetometers. We have tested several amorphous and polycrystalline alloys in the form of strip-wound toroidal cores. Both high and near zero magnetostrictive alloys are studied. All samples are found out to be the source of thermal magnetic noise of 1/f spectral density in agreement with the fluctuation dissipation theorem. This type of thermal noise comes from a frequency independent imaginary permeability. This feature is equivalently described in terms of the magnetic viscosity effect or by considering the constant (residual) term in the classical Legg’s equation of magnetic losses at low fields. Measurements on a few Co-based amorphous alloys show that the magnetic viscosity is independent of temperature. Explanations in terms of thermal activation processes and quantum tunneling are discussed.
Magnitude, origin, and evolution of piezoelectric optical nonlinearities in strained [111]B InGaAs/GaAs quantum wells
Authors: Cartwright A. N. , McCallum D. S. , Boggess Thomas F. , Smirl Arthur L. , Moise T. S. , Guido L. J. , Barker R. C. , Wherrett B. S.
Published: July 2002

The magnitudes of excitonic nonlinearities were compared at 12 K in InGaAs/GaAs multiple quantum well structures with growth directions oriented along the [100] and [111] crystal axes by measuring both the steady-state and time-resolved differential transmission spectra. As expected, the spectra for the [100] sample are indicative of excitonic bleaching at all times and for all excitation levels, and a carrier recombination time of 0.8 ns and a nonlinear cross section (change in absorption coefficient per carrier pair) of ∼8×10−14 cm2 are extracted for the [100] sample. By comparison, for low excitation levels, the spectra for the [111] sample are consistent with a blueshift of the exciton, indicating a screening of the strain-induced piezoelectric field. At higher excitation levels, the spectra are dominated by excitonic bleaching. Under identical 1 ps pulsed excitation conditions, the magnitudes of the changes in the absorption coefficient caused by screening in the [111] sample are comparable to those measured for bleaching in the [100] sample. By contrast, the steady-state changes in the absorption coefficient caused by screening in the [111] sample are an order of magnitude larger than the changes caused by bleaching in the [100] sample. It was demonstrated that the larger steady-state response for the [111] sample is caused by carrier accumulation over the longer (density-dependent) lifetime for that sample and that it is not the result of a larger nonlinear cross section. The slow, nonexponential, density-dependent recombination rates measured for the [111] sample are consistent with carrier escape and drift to screen the entire multiple quantum well structure and are not consistent with screening within the individual quantum wells.
Ensemble Monte Carlo with Poisson solver for the study of current fluctuations in homogeneous GaAs structures
Authors: González T. , Pardo D.
Published: July 2002

An analysis of current fluctuations in homogeneous N-type GaAs structures of different lengths and doping concentrations is presented. The study is performed coupling self-consistently a Poisson solver to a many-particle Monte Carlo simulation. The autocorrelation functions and the spectral density of current fluctuations are calculated, dividing them into different contributions that allow one to analyze several sources of noise. A qualitative analysis of shot noise is made. Different behaviors of current fluctuations are found among the different structures; these are interpreted in terms of the microscopic processes occurring inside them. For high values of impurity concentration (1017 cm−3), the coupling between fluctuations in the electric field and in carrier velocity is found to be an important source of noise.
Interaction of electrons with coupled plasmon phonons in GaAs
Authors: Peschke Christoph
Published: July 2002

A model is derived for the interaction of electrons with coupled plasmon-phonon modes in GaAs. The Hamilton operator of the plasmon-phonon system is diagonalized by a linear combination of plasmon and phonon states. The minimum wavelengths of the modes are obtained by an analysis using the dielectric function. The scattering rates for scattering of the electrons by the coupled modes are computed for electron concentrations ranging from 1015 to 1019 cm−3 at T=300 K. At electron concentrations of about n≳7×1017 cm−3, scattering of electrons by coupled modes is an important energy relaxation process.
Inverse parabolic quantum well and its quantum-confined Stark effect
Authors: Chen W. Q. , Wang S. M. , Andersson T. G. , Thordson J.
Published: July 2002

An inverse parabolic quantum well was successfully grown by molecular-beam epitaxy using a digital compositional grading superlattice composed of Al0.36Ga0.64As/GaAs. The photoluminescence and photocurrent measurements for this structure gave a good agreement between experimental and theoretical results. Large Stark shift and amplitude reduction of 1e-1hh exciton resonance under applied electric field were found in the photoluminescence spectra, which are substantially larger than the conventional square quantum well. These properties benefited from the concept of local-to-global state transitions.
Experimental investigation and modeling of the role of extended defects during thermal oxidation
Authors: Huang R. Y. S. , Dutton R. W.
Published: July 2002

A special test structure consisting of a box-shaped boron profile capped by a lightly doped arsenic layer has been used to determine that extended defects absorb some of the interstitials injected during a wet thermal oxidation. Reduced oxidation-enhanced diffusion of the boron layer is observed for samples containing the extended defects. Secondary ion mass spectrometry measurements are combined with transmission electron microscopy measurements to calculate the DIC*I product which is found to be in good agreement with values previously obtained from gold diffusion experiments. In addition, a lower bound on the ratio of the net number of silicon atoms injected during the oxidation to the number of silicon atoms consumed is calculated. A one-dimensional model for the growth of the extended defects has been implemented into suprem-iv. Simulations with the new model agree with experimental data. The growth of the extended defects is also shown to be a reaction-limited process.
Understanding and electrochemical control of YBa2Cu3O7−x thin film epitaxy on yttrium stabilized zirconia
Authors: MacManus-Driscoll J. , Geballe T. H. , Bravman J. C.
Published: July 2002

Films of c-axis YBa2Cu3O7−x were grown on yttrium stabilized zirconia (YSZ) substrates by rf magnetron, off-axis sputtering. Due to the large lattice mismatch in this system, three epitaxial orientations may be stabilized, resulting in a large proportion of high angle grain boundaries and, hence, to a reduction in critical currents. A broad study of the effects on epitaxy of various deposition parameters, such as temperature, oxygen partial pressure, and total pressure has been investigated. In addition, film/substrate strain effects have been studied by depositing films of different compositions, i.e., EuBa2Cu3O7−x and (Ca,Y)Ba2Cu3O7−x, onto YSZ. The influence of both kinetic and thermodynamic parameters on the formation of different orientations has been considered and an understanding of how to control them has been reached. YSZ was used not only as a substrate for YBa2Cu3O7−x film growth but also as an ionic conductor to control oxygen activity at the film/substrate interface during deposition. By using this electrochemical technique it has been possible to form uniepitaxial films with good transport properties, under conditions which would otherwise have yielded films of mixed epitaxy and thus degraded superconducting properties.
Raman scattering study of residual strain in GaAs/InP heterostructures
Authors: Attolini G. , Francesio L. , Franzosi P. , Pelosi C. , Gennari S. , Lottici P. P.
Published: July 2002

A Raman spectroscopy study on highly mismatched GaAs layers with thickness ranging from 15 nm to 6.6 μm and grown by metal-organic vapor-phase epitaxy on InP (001) substrates, is reported. Both LO and TO GaAs phonons have been observed in backscattering and Brewster geometries. In the thinnest samples large frequency red shifts with respect to the bulk are measured indicating large residual tensile strains. The Raman measurements agree with x-ray-diffraction measurements and confirm that layers thinner than 30 nm exhibit a 3D growth mechanism as suggested by transmission electron microscopy investigations.
Formation of interface layers in GaxIn1−xAs/InP heterostructures: A re-evaluation using ultrathin quantum wells as a probe
Authors: Seifert W. , Hessman D. , Liu X. , Samuelson L.
Published: July 2002

We have studied the metalorganic vapor phase epitaxy growth of ultrathin GaInAs/InP and GaAs/InP quantum well (QW) structures using photoluminescence (PL) spectra as a probe for interface effects. In parallel we have also studied InAsxP1−x ‘‘interface QWs’’ formed by simply exposing InP to AsH3. We see a correlation between QW properties (PL peak position, effective thickness, PL half-width) and the surface phase during the growth of the QW material. For GaAs QWs grown under conditions where typically the As-excess c(4×4)/d(4×4)- or (1×2)-like (with As double layers) surface reconstructions, we find a strong red-shift of the PL peak positions. The red-shift becomes smaller the closer the growth conditions come toward the border to the (2×4) reconstruction (with only one As-termination layer). We thus conclude that the surface itself is one source for As carryover. For GaInAs QWs a boundary between an As-excess/no As-excess surface reconstruction seems to exist at higher AsH3/lower T values. Near to this border GaInAs QWs can be deposited which show PL-half-widths between 7 and 11 meV even for the range of 1–5 ML nominal thickness. The P/As replacement reaction at the lower interface is for short AsH3 interaction times (≤1 s) restricted to less than 1 ML and contributes a relatively constant amount to the effective thickness of the QW. Similarly, we show that InAsxP1−x interface QWs formed by short time interaction of InP and AsH3, originate less from a reaction into the depth of the InP, but more from a consumption of the As which is adsorbed onto the InP surface after the AsH3 treatment.
Flattened diamond crystals synthesized by microwave plasma chemical vapor deposition in a CO-H2 system
Authors: Hirabayashi Keiji , Matsumoto Seiichiro
Published: July 2002

Flattened diamond crystals have been synthesized by microwave plasma chemical vapor deposition method in a CO-H2 system. The flattened diamond crystals are formed by a re-entrant corner effect, which resulted from formation of the twin boundaries. The flattened diamond crystals grow with {111} oriented surface, but do not grow epitaxially on the Si {111} substrate, probably because the source gas includes oxygen-containing gas (CO gas) and the substrate temperature is comparatively low.
Noise and impedance of n+-n-n+ InP microwave generators
Authors: Gruzinskis V. , Mitin V. , Starikov E. , Shiktorov P.
Published: July 2002

The noise power spectral density Pn in a submicron InP diode loaded by resistor R is calculated using the Monte Carlo particle technique. It is shown that at biases above the generation threshold the Pn has a peak at the frequency fpeak which corresponds to the highest generation frequency at the given R. The excess noise is the shot noise of electrons accumulated into the layers. Measuring the frequency of the peak fpeak as the function of R one can obtain the negative values of the real part of impedance at the high-frequency side of the negative impedance spectrum.