bluuebunny/crossref_metadata_2025 huggingface dataset:
Spontaneous appearance of high index facets during the evolution of step bunching on vicinal GaAs(001)
Authors: Hata K. , Ikoma T. , Hirakawa K. , Okano T. , Kawazu A. , Ueda T. , Akiyama M.
Published: July 2002

The evolution of step bunching on vicinal GaAs(001) annealed at different temperatures in AsH3 and H2 ambient was studied by scanning tunneling microscopy. The results provided aspects of the evolution of step bunching from the initial to the final stage. We observed a appearance of additional step bunchings which have a peculiar azimuth of 〈210〉, 〈21̄0〉, 〈310〉, and 〈31̄0〉.
X-ray-diffraction analysis of complex structures in ZnCdSe/ZnSe strained-layer superlattices
Authors: Nakashima Kiichi , Kawaguchi Yoshihiro
Published: July 2002

The procedure for analyzing the discrimination of fundamental peaks in x-ray-diffraction measurements of strained-layer superlattice (SLS) structures is generalized and refined for application to more complex structures, including both lattice-relaxed and coherently strained layers. Two criteria are proposed for investigating lattice relaxation and coherent deformation of relevant layers. A ZnCdSe/ZnSe SLS grown on a GaAs substrate with a ZnSe buffer layer is structurally analyzed by the generalized procedure with the two criteria, demonstrating its effectiveness in analyzing such complex structures. It is revealed from this analysis that the ZnSe buffer layer is almost totally lattice relaxed relative to the GaAs substrate and that the ZnCdSe/ZnSe SLS layer is coherently strained relative to the relaxed ZnSe buffer layer. Quantitative analysis with a kinematical step model also confirms the above results in terms of peak position and peak intensity profile. The full width at half-maximum of the SLS peaks in the experimental profiles, however, is much broader than that of the calculated profiles. Based on this difference, some consideration of the peak broadening mechanism is offered from the viewpoint of broadening symmetry, revealing that this analysis works as a simple checking method on the peak broadening mechanism.
Addendum to ‘‘Determination of the crystallographic orientation of cubic media to high resolution using optical harmonic generation’’ [J. Appl. Phys. 74, 6072 (1993)]
Authors: Bottomley D. J. , Lüpke G. , Mihaychuk J. G. , van Driel H. M.
Published: July 2002

Calculations of the optical harmonic response of cubic centrosymmetric and noncentrosymmetric media with arbitrary facial orientation are extended to the general case of s- or p-polarized fun- damental beam radiation and s- or p-polarized harmonic beam radiation.
Dielectric surface preflashover processes in vacuum
Authors: Li C. R. , Sudarshan T. S.
Published: July 2002

Surface preflashover phenomena across alumina ceramics in vacuum are presented. The localized plasmas are identified by the spatially resolved images that appear in a pulse surface preflashover phase using an intensified charge-coupled-device camera, along with coordinated, time-resolved preflashover current and luminosity. It was observed that significant preflashover activity associated with the localized plasmas on the surface appears after a flashover event and disappears after a few pulse voltage applications. Large bursts of current pulses (several hundred milliamperes) with no counterpart in the luminosity signal were also observed during preflashover. These phenomena cannot be explained satisfactorily by electron stimulated gas desorption. The effect of adsorbed gases and surface polishing on preflashover are presented and discussed. The experimental results indicate that preflashover essentially depends on the surface state (physical and chemical) of the solid insulator. The preflashover conduction associated with the localized plasmas is attributed to the detrapping and impact-ionization processes associated with defect centers. The results reported support the surface flashover model based on trapping/detrapping and impact ionization processes.
Formation of nitrogen-oxygen donors in N-doped Czochralski-silicon crystal
Authors: Chen C. S. , Li C. F. , Ye H. J. , Shen S. C. , Yang D. R.
Published: July 2002

The behavior of nitrogen and the formation of nitrogen-oxygen donors in nitrogen-doped Czochralski-silicon crystals (Cz-Si) were studied by the electrical and infrared-absorption measurements in samples annealed in different conditions. Experiments showed that nitrogen-oxygen shallow donors are formed during the nitrogen-doped Cz-Si crystal growth. Nitrogen-oxygen thermal donors are generated in temperature range of 300–550 °C, and the behavior of these thermal donors resembles that of the thermal donors in Cz-Si crystals [P. Wagner, C. Holm, and E. Stirtl, Festkoperprobleme 24, 191 (1984)], but they are monovalent donors with the same levels as those of neutral thermal donors; no new electrically active center is generated in the temperature range of 600–900 °C.
Enhancement of electron transfer and negative differential resistance in GaAs-based real-space transfer devices by using strained InGaAs channel layers
Authors: Lai Jiun-Tsuen , Lee Joseph Ya-min
Published: July 2002

Real-space transfer devices using strained InxGa1−xAs (x=0.15, 0.25) channel and unstrained GaAs channel are fabricated. The electron transfer and negative differential resistance of InGaAs channel devices are enhanced. The InGaAs channel devices with a larger indium mole fraction show lower leakage currents and can be operated at a higher collector voltage. For operation in the negative resistance field-effect transistor mode, the drain current peak-to-valley ratios of strained InGaAs channel devices are larger than that of unstrained GaAs channel devices. The peak-to-valley ratio can be increased more than two orders of magnitude by using strained InGaAs channel layers.
Misfit dislocation generation mechanisms in InGaAs/GaAs heterostructures
Authors: Kui J. , Jesser W. A. , Jones S. H.
Published: July 2002

An experimental investigation of misfit dislocation generation mechanisms at an InGaAs/GaAs heterointerface is reported. InGaAs epitaxial layers were grown by low-pressure organometallic vapor-phase epitaxy on patterned and unpatterned GaAs substrates having etch-pit densities (EPD) of 200, 1400, and 10 000 cm−2. After epitaxial growth, the samples were annealed at temperatures between 650 and 750 °C, and analyzed by optical and transmission electron microscopy. For the range of substrate EPD studied, it was found that the substrate EPD controls the onset of misfit dislocation generation for low-temperature epitaxy (<600 °C) on unpatterned substrates. When epilayers were annealed at 750 °C, the density of misfit dislocations was independent of the substrate EPD. These studies also show that the dominant misfit dislocation generation mechanism for films grown on patterned substrates is nucleation at the growth-mesa edge. The density of preexisting threading dislocations has little influence on misfit dislocation generation for films selectively deposited within 100×100 μm2 growth windows. For selective heteroepitaxy, misfit dislocation generation strongly depends on the crystallographic orientation of the growth-mesa edge.
Departures from the principle of superposition in silicon solar cells
Authors: Robinson S. J. , Aberle A. G. , Green M. A.
Published: July 2002

The principle of superposition forms the theoretical basis on which the comparison of illuminated and dark current-voltage (I-V) characteristics of solar cells depends. Two cases predicted from computer simulations where the superposition principle does not apply in silicon p-n junction solar cells are reported. These predictions are confirmed experimentally with measurements taken on existing high-efficiency devices, and cannot be accurately described by previous explanations for departures from this principle. The first case, which is the more important in terms of operation under 1 sun illumination, occurs in cells where recombination via defect levels (Shockley–Read–Hall recombination) with unequal electron and hole capture rates dominates the I-V characteristics. The second case is evident at small forward voltages in almost all silicon solar cells. It is shown that the former is due to a saturation in the recombination rate, while the latter is the result of a bias-dependent modification of the carrier concentrations across the p-n junction depletion region which is different under illumination from that in the dark.
Magnetic properties of cubic R
xY1−xAl2 (R=Dy, Tb) intermetallic random anisotropy magnets (invited)
Authors: del Moral A. , Arnaudas J. I. , de la Fuente C. , Ciria M. , Joven E. , Gehring P. M.
Published: July 2002

A short review is made of the key magnetic properties of dilute cubic RxY1−xAl2 (R=Tb, Dy) intermetallics, in order to show their main magnetic features. Dilution by Y introduces a weak random magnetic anisotropy (RMA). The rich magnetic phase diagram is described, including paramagnetic (P), spin glass (SG), correlated spin glass (CSG), random ferromagnetic (RFM), and ferromagnetic (F) phases, with a triple point and two multicritical ones. The paper deals with the induced macroscopic magnetic anisotropy cooling in a magnetic field below TSG or TCSG transition temperatures, which can be either unidirectional or uniaxial or both. High-field (3 T) magnetostriction in Tb series shows, for x=0.48, 0.59, 0.87, a decrease of the Callen α exponent (λt∼mα, m=reduced magnetization) below 3. The Sompolinsky irreversibility parameter Δ has been determined for the Tb series in the SG regime, and a replica model is presented to explain the Δ(T) dependence. The character of the P→SG or P→CSG transitions is addressed, through the scaling of nonlinear susceptibility (SG regime) or a ferromagnetic-like scaling of magnetization, respectively. From quasielastic neutron scattering around Q=[1,1,1] we determine the temperature dependence of the magnetic correlation length in Dy0.8Y0.2Al2, which peaks, but does not diverge at Tc. A magnon excitation at 3.5 meV is reported for x=0.8.
A normal incidence two-color p-type compressive strained-layer In0.4Ga0.6As/GaAs quantum well infrared photodetector for 3–5 μm and 8–12 μm detection
Authors: Wang Y. H. , Chu J. , Li Sheng S. , Ho Pin
Published: July 2002

A normal incidence p-type compressive strained-layer In0.4Ga0.6As/GaAs quantum well infrared photodetector (PSL-QWIP) grown on (100) semi-insulting GaAs substrate with molecular beam epitaxy technique for 3–5 μm mid-wavelength infrared and 8–14 μm long-wavelength infrared detection was demonstrated for the first time. This PSL-QWIP shows a broadband double-peak response between 8 and 9 μm wavelength by utilizing the resonant transport coupling mechanism between the heavy-hole type-I states and the light-hole type-II states. Using the compressive strain in the InGaAs quantum well, normal incident absorption was greatly enhanced by reducing the heavy-hole effective mass and increasing density of states of off-zone center. Maximum responsivities of 93 mA/W and 30 mA/W were obtained at peak wavelengths of λp1=8.9 μm and λp3=5.5 μm, respectively, with Vb=1.6 V. Detectivity at λp1=8.9 μm was found to be 4.0×109 cm√Hz/W at Vb=0.3 V and T=75 K.
Atomic diffusion mechanism and diffusivity of nitrogen into Sm2Fe17
Authors: Christodoulou Chris N. , Komada Norikazu
Published: July 2002

An atomic diffusion mechanism (voidal diffusion) is proposed where nitrogen atoms migrate inside the Sm2Fe17 lattice by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site and subsequently into a new 9(e) site. For the first time, the anisotropic nature of diffusion and growth kinetics together with direct observation of the diffusion fields by Bitter domain patterns have been taken into account and employed for nitrogen diffusivity measurements. The planar and axial preexponential factors were found to be DOX=0.72×10−6 m2 s−1 and DOZ=2.26×10−6 m2 s−1, respectively. The activation enthalpy, ΔH, for diffusion was found to be 143 kJ/mol.
Analysis of the thickness dependence of critical current density for electrophoretically deposited YBa2Cu3O7−x films
Authors: Bhattacharya Dipten , Choudhury P. , Roy S. N. , Maiti H. S.
Published: July 2002

An attempt has been made to explain the experimental observations regarding variation of critical current density with the film thickness between 3 and 65 μm in electrophoretically deposited YBa2Cu3O7−x films on silver (Ag) substrates. It is not possible to explain the phenomenon using the commonly believed concept of self-field degradation of critical current density of weak links between the grains. On the other hand, an increased grain-boundary resistance due to a lesser degree of silver penetration from the substrate with increasing film thickness is proposed to be the cause of the observed degradation of critical current density. The experimental data agree quite well with those theoretically calculated.
Existence of a Pb1-like defect center in porous silicon
Authors: Xiao Y. , McMahon T. J. , Pankove J. I. , Tsuo Y. S.
Published: July 2002

We performed a detailed study of electron spin resonance (ESR) spectra of porous silicon (PS) samples at different stages of treatment and with different porosities. In addition to the commonly observed Pb0-like dangling bond, results of curve fitting to our ESR spectra show that a Pb1-like center, similar to the Pb1 center observed at the (100) crystalline-Si/SiO2 interface, appears in the PS nanostructure. The ratio of the number of Pb1-like centers to that of Pb0-like centers is related to the PS porosity. Remote hydrogen plasma processing of the annealed PS does not change the ratio significantly, although the total numbers of Pb0-like and Pb1-like centers are reduced and photoluminescence efficiency is improved.
Hysteresis in the switching of hot electrons in InP/InGaAs double-heterojunction bipolar transistors
Authors: McAlister S. P. , McKinnon W. R. , Abid Z. , Guzzo E. E.
Published: July 2002

The switching behavior of a composite-collector InP/InGaAs heterojunction bipolar transistor is found to be hysteretic at temperatures below 200 K. This arises from the underlying S-shaped negative differential conductivity associated with the hot-electron transport of electrons across the heterojunction barrier in the collector structure of such transistors.
Cumulative interface roughness and magnetization in antiferromagnetically coupled NiCo/Cu multilayers
Authors: Meng X. , Bian X. , Abdouche R. , Muir W. B. , Ström-Olsen J. O. , Altounian Z. , Sutton M.
Published: July 2002

Cumulative interface roughness and its influence on the magnetization process in antiferromagnetically coupled (Ni80Co20/Cu)×N multilayers is studied. In these multilayers, Cu and Ni80Co20 thicknesses are fixed at 20 and 15 Å, respectively, in order to obtain the antiferromagnetic coupling at the second oscillation peak of giant magnetoresistance (GMR) versus Cu thickness. Low-angle x-ray reflectivity measurements show that cumulative interface roughness increases with increasing bilayer number N. In-plane magnetization hysteresis measured with both SQUID and surface magneto-optic Kerr effect (SMOKE) magnetometers are compared. When the cumulative interface roughness is significant, SMOKE hysteresis loops, which are sensitive to the top 5 or 6 magnetic layers, display a nonlinear plateau region at small fields. Comparison of low-angle x-ray, and SMOKE results show that interfaces of relatively high quality in top layers only exist for sputtered multilayer with N<10.
Dispersion curves and transmission spectra of a two-dimensional photonic band-gap crystal: Theory and experiment
Authors: Qiu Y. , Leung K. M. , Carin L. , Kralj D.
Published: July 2002

An on-shell method that combines plane-wave and finite-difference techniques for the calculation of dispersion curves and transmission spectra for electromagnetic fields in photonic band-gap crystals is presented. The overall problem is decomposed into a field problem of determining the plane-wave scattering from an individual crystalline layer and a conventional one-dimensional network problem of combining this scattering to obtain the band structure of the entire crystal or the scattering properties of a crystal with a finite thickness. Results of the calculation are compared with experimental data measured using ultrawideband microwave pulses for a two-dimensional photonic band-gap crystal.
Effects of variations in layer thicknesses on the reflectivity spectra of semiconductor Bragg mirrors
Authors: Murtaza S. S. , Campbell J. C.
Published: July 2002

We report the results from an investigation of the effects of variations in layer thicknesses during the growth of semiconductor Bragg mirrors on their reflectivity spectra. Different types of variations are investigated and an effort is made to point out the sources of such errors in a typical growth process. It is expected that this study will shed light on the critical control parameters for very highly reflecting mirrors and help the interpretation of the measured spectra from a finished growth run and the identification of the possible sources of deviations from the ideal structure.
Effect of thermal annealing on Te precipitates in CdTe wafers studied by Raman scattering and cathodoluminescence
Authors: Sochinskii N. V. , Serrano M. D. , Diéguez E. , Agulló-Rueda F. , Pal U. , Piqueras J. , Fernández P.
Published: July 2002

A combination of Raman scattering and cathodoluminescence techniques has been used to study the spatial distribution of Te precipitates in the volume of CdTe wafers. Starting with the as-grown crystals with random distribution of precipitates over the whole volume, improvement at different stages of thermal annealing is demonstrated. As-grown p-CdTe wafers were annealed at 500–600  °C either in Ga melt or in Cd vapor for 2 or 22 h. The kinetics of dissolution of Te precipitates was found to be similar for both the Ga melt and Cd vapor annealing processes. Short-time annealing causes the disappearance of small Te precipitates, while the larger ones, 5–10 μm in size which decorate the extended structural defects, still remain. After a long-time annealing, the complete disappearance of Te precipitates occurs in the wafers volume. Interestingly, it was observed that the disappearance of Te precipitates during annealing starts in the central part of the bulk wafer and is followed by a precipitate gettering at the wafer surface. This implies that it is possible to obtain precipitate-free CdTe wafers by postgrowth annealing.
Surface defect production on Ge(001) during low-energy ion bombardment
Authors: Floro J. A. , Kellerman B. K. , Chason E. , Picraux S. T. , Brice D. K. , Horn K. M.
Published: July 2002

Low-energy ion bombardment of the Ge(001)-(2×1) surface produces surface point defects, which are detected and quantified using in situ reflection high-energy electron diffraction. Surface defect production rates are determined for a range of ion energies and ion masses. At low substrate temperatures (T≊−100 °C), copious production of surface defects is observed, with defect yields as high as 20–30 defects per ion for 500 eV Ar and Xe bombardment. The observed He surface defect yields exceed the surface yield predicted by binary collision simulations, indicating that defects created in the subsurface region migrate to the surface for these conditions. The observed surface defect yield is reduced at elevated substrate temperatures. Based on a simple model this reduction is attributed to surface recombination of point defects created within the same cascade. A constant surface defect yield is reached at temperatures greater than 100 °C which is consistent with the net defect production due to the vacancies left by sputtering. However, even at elevated temperatures, significantly larger populations of mobile point defects than can be accounted for by sputtering may reside transiently on the surface, which can modify the overall surface morphology.
Simulation of nucleation and emission of dislocations by molecular-dynamics method
Authors: Zhang Y. W. , Wang T. C. , Tang Q. H.
Published: July 2002

The nucleation and emission of dislocations from the crack tip under mode II loading are analyzed by the molecular-dynamics method in which the Finnis–Sinclair potential has been used. A suitable atom lattice configuration is employed to allow one to fully analyze the nucleation, emission, dissociation, and pileup of the dislocations. The calculated results show that although the pure mode II loading is applied, the crack tip generally exhibits a combined mode. The stress distributions before the dislocation emission are in agreement with the elasticity solution, but are not after the emission. The critical stress intensity factor corresponding to the dislocation nucleation KIIe is dependent on the loading rate K̇II. The separations of a pair of partial dislocations and the full dislocations are also dependent on the loading rate. When the first partial dislocation is blocked, a pileup of dislocations can be set up. It is also found that the dislocation can move at subsonic wave speed (less than the shear wave speed) or at transonic speed (greater than the shear wave speed but less than the longitudinal wave speed) depending on the loading rate, but at the longitudinal wave speed which just corresponds to K̇II=1.15 MPa √m/ps for copper, the atom lattice breaks down.
Magnetic properties of layered CuO in CuO/Al2O3 and CuO/MgO multilayers
Authors: Sohma M. , Kawaguchi K. , Fujii Y.
Published: July 2002

Mössbauer spectra of 57Fe-doped polycrystalline CuO/Al2O3 multilayers (CAOs) show magnetic ordering at liquid helium temperature. The Néel temperature of a CAO with thin CuO layers is lower than that of bulk CuO judged from the temperature dependence of the Mössbauer spectra. A paramagnetic component was observed in all magnetic susceptibility measurements of the multilayers. The component also exists in both polycrystalline and epitaxial CuO/MgO multilayers. We have found that the component was localized at interfaces and derived from the spin fluctuation effect.
Photoluminescence spectroscopy of growth-interrupted GaAs/AlAs single quantum wells subjected to hydrogenation
Authors: Yu Haiping , Mookherjee Paul B. , Murray Ray , Yoshinaga Atshushi
Published: July 2002

Photoluminescence excitation and emission spectroscopy (PLE and PL) have been used to investigate growth interrupted GaAs/AlAs single quantum wells (SQW). Evidence that growth interruption leads to smooth interfaces which vary in height by integer monolayer steps is provided by changes in the PLE and PL spectra from a broad single band for noninterrupted SQW to narrow multiple peak features for growth interrupted samples. However, growth interruption results in a dramatic reduction in the intensity of the photoluminescence signal above 30 K. Hydrogenation treatment has been found to recover the photoluminescence intensity in the growth-interrupted samples which we attribute to passivation of nonradiative centers incorporated during the growth interruption process. Similar, though less dramatic, effects have been observed for noninterrupted samples. Time-resolved photoluminescence spectroscopy and decay time measurements show that hydrogen passivates carbon acceptors also incorporated during interruption, and in addition, demonstrate the migration process of excitons from the narrower to the wider well widths in these SQW. This latter exciton kinetic information fully supports the view that growth interruption results in large lateral islands with monolayer fluctuations in well width.
Substrate selective deposition and etching of silicon thin films
Authors: Westlake W. , Heintze M.
Published: July 2002

In this study investigations into silicon plasma deposition and etching using both a very high-frequency plasma in dilute SiH4/H2 near partial chemical equilibrium and the pulsed silane flow method are presented. Under the conditions used, simultaneous growth of microcrystalline silicon (μc-Si:H) and etching of amorphous silicon (a-Si:H) is observed. The results obtained support a μc-Si:H growth model in which the net growth rate is regarded as the difference of a silicon deposition and an etch rate. By patterning amorphous silicon using laser crystallization it was possible to produce uniform and well resolved submicrometer microcrystalline silicon features.
Solid-phase epitaxial regrowth and dopant activation of P-implanted metastable pseudomorphic Ge0.12Si0.88 on Si(100)
Authors: Lie D. Y. C. , Theodore N. D. , Song J. H. , Nicolet M.-A.
Published: July 2002

Several 265-nm-thick metastable pseudomorphic Ge0.12Si0.88 films grown on a Si(100) substrate by molecular-beam epitaxy were implanted at room temperature with 100 keV phosphorus ions to a dose of 1.5×1015/cm2. The implantation amorphizes the top portion (∼190 nm) of the GeSi layer and leaves the rest of the film single crystalline. Implanted and nonimplanted samples were subsequently annealed simultaneously in vacuum for 30 min from 400 to 800 °C. The implanted samples undergo layer-by-layer solid-phase-epitaxial regrowth during annealing at or above 500 °C. The regrown GeSi layer is relaxed with a high density of threading dislocations (∼1010–1011/cm2). The nonamorphized portion of the layer remains fully strained when annealed between 400 and 600 °C. At or above 700 °C misfit dislocations are observed at the Si/Ge0.12Si0.88 interface. After 800 °C annealing the strain in the whole epilayer is fully relaxed. The strain relaxation is facilitated by the implantation. The presence of phosphorus in GeSi raises its regrowth velocity by about an order of magnitude over that of Ge0.12Si0.88 amorphized by irradiation of Si. The implanted phosphorus reaches ∼100% activation after the completion of solid-phase-epitaxial regrowth. The room-temperature sheet electron mobility in GeSi is ∼20% below that of a Si sample implanted and annealed under the same conditions. It is concluded that metastable Ge0.12Si0.88 on Si(100) amorphized at room temperature by P implantation and recrystallized by solid-phase epitaxy can- not recover its crystalline perfection and its pseudomorphic strain upon steady-state furnace annealing.
Desorption of ultraviolet-ozone oxides from InP under phosphorus and arsenic overpressures
Authors: Hofstra P. G. , Thompson D. A. , Robinson B. J. , Besland M. P. , Gendry M. , Regreny P. , Hollinger G.
Published: July 2002

The thermal desorption of ultraviolet-ozone oxide on InP substrates prepared for molecular-beam epitaxy has been performed with overpressures of P2, As2, and As4. Surface analysis using reflection high-energy electron diffraction, Auger electron spectroscopy, and x-ray photoelectron spectroscopy and thermodynamic calculations indicate that thermal desorption proceeds via a reaction between the oxide and atomic phosphorus from the substrate to produce volatile phosphorus oxides such as P2O3. The overpressure species serves to stabilize the substrate against surface dissociation once the oxide is removed. In the case of an arsenic overpressure the desorption of the final monolayer of oxide is slowed, relative to the case of phosphorus overpressure, due to the formation of InAs.
Nucleation of dislocation loops in strained epitaxial layers
Authors: Jain Uma , Jain S. C. , Harker A. H. , Bullough R.
Published: July 2002

The combined effect of the misfit strain and the strain caused by a neighboring defect on the activation energy of nucleation of dislocation loops is calculated. Defects of different sizes and shapes and located at different distances from the loop are considered. At very low mismatches (<0.5%) and with very small defects, the activation energy is not sufficiently reduced and large layer thicknesses are required for nucleation. At mismatches of 1% or more, and with defect sizes of 1.5 nm or larger, heterogeneous nucleation at growth temperatures becomes possible. These defects are more efficient in reducing the energy when they are at the center of the loop. Though impurities located within the core of the dislocations can reduce the core parameter substantially and therefore reduce the activation energy, in practice this is unlikely to occur. Very large defects such as SiO2 and SiC precipitates reduce the activation energy of nucleation over large distances thereby inducing the nucleation of several loops which results in very rapid relaxation of strain. In highly mismatched layers (4%–8%) homogeneous nucleation occurs at about 400–500 °C. Why the periodic arrangement of misfit dislocations is observed only in the highly mismatched layers is explained.
Polarization dependence of the absorption coefficient for an array of strained quantum wires
Authors: Vurgaftman Igor , Singh Jasprit
Published: July 2002

The polarization dependence of the absorption coefficient for compressively and tensilely strained quantum wires is investigated as a function of strain, the lateral wire width, and the ratio between the wire and barrier widths. The results are markedly different from the case of an unstrained quantum wire and exhibit a significant variation with the sign and amount of built-in strain. In the case of compressive strain, absorption of light polarized in the growth direction is considerably suppressed near the band edge, while the ratio between the absorption coefficients for light polarized parallel and perpendicular to the wire is determined by the relative prominence of strain and quantum confinement effects. A discussion of how these results can be applied to characterization of the achieved quantum confinement by polarization-resolved photoluminescence spectroscopy is given.
Particle size dependence of the giant magnetoresistance in granular Ag-Co0.4Ni0.6 grown epitaxially on MgO{100}
Authors: Kubinski D. J. , Holloway H. , Hangas J.
Published: July 2002

Measurements of structure and magnetoresistance in Ag-Co0.4Ni0.6 granular alloy films with 10–23.5 vol % Co0.4Ni0.6 grown epitaxially on MgO{100} are reported. These results elucidate the effects of varying both the concentration and the size of the Co0.4Ni0.6 precipitates in the Ag matrix. It is shown that the magnetic-field-dependent contribution to the resistivity, Δρmag, is consistent with the predictions of a two-current model in which spin-dependent scattering is dominated by the interfaces between the precipitates and the matrix. Specifically we observe: (a) A regime where Δρmag is linear in c/D, with c the volume fraction of precipitates with diameter D. (b) Extrapolation of these linear dependences to Δρmag=0 gives values of 1/D that are proportional to 1/c. (c) With D≳100 Å there is a reduced dependence on particle size that is consistent with transition to a quadratic dependence. In contrast, at the lower concentrations the behavior of the maximum resistivity, ρmax, is not well described by the two-current model that suffices for Δρmag. This suggests that refinement of the model may be needed for granular films that have relatively large and widely spaced ferromagnetic precipitates.
Measurement of tortuosity of anisotropic acoustic materials
Authors: Melon Manuel , Lafarge Denis , Castagnède Bernard , Brown Niven
Published: July 2002

Tortuosity, i.e., the high-frequency limit of the squared propagation index, has been evaluated with narrow-band piezoelectric transducers on reticulated plastic foams. The basic procedure was recently reported by J. F. Allard, B. Castagnède, M. Henry and W. Lauriks [Rev. Sci. Instrum. 65, 754 (1994)]. Further experimental work related to the anisotropic nature of these materials has been done by probing the propagation index versus angle. A simple numerical routine has been implemented in order to recover the propagation index along principal directions. These predicted values compare well with measurements taken directly along the principal geometric axes of the samples. Slight angular deviations of the principal axes themselves have been observed. This is the very first account of the anisotropy of tortuosity measured by ultrasonic methods in air-saturated porous materials.
Spectroscopic ellipsometry studies of very thin thermally grown SiO2 films: Influence of oxidation procedure on oxide quality and stress
Authors: Boultadakis S. , Logothetidis S. , Papadopoulos A. , Vouroutzis N. , Zorba Ph. , Girginoudi D. , Thanailakis A.
Published: July 2002

The qualitative and structural modifications of very thin SiO2 films with thicknesses between 80 and 500 Å, caused by the oxidation procedure, were studied with spectroscopic ellipsometry (SE). Analysis of the experimental dielectric function obtained by SE provides the structural characteristics and the thickness of the oxide in fairly good agreement with electron microscopy results in cross-sectional geometry. The calculated voids volume fraction was found to drop below 3% (2%) for oxides thicker than 250 (400) Å grown at 900 (1000) °C. The densification of thermal oxides grown at low oxidation temperature predicted by Fourier transform infrared is discussed and compared to the SE results, whereas a relation between the final oxidation time and the viscoelastic relaxation time was found. Furthermore, it is shown that in situ SE can be used to monitor the process of oxide removal with very low-energy Ar+ ions and to control the conditions in order to avoid oxide and Si substrate damage. It is found that Ar+ ions with energy of about 10 eV are required to avoid oxide damage and Si substrate amorphization at a depth below 6 Å. The latter finding is also corroborated by atomic force microscopy images obtained from Si substrates after oxide etching. Finally, the influence of oxide thickness and the oxidation procedure on the E1 structure of c-Si was studied and from the results the stress applied by the oxide on the Si substrate was determined. From this study it was found that the stress depends strongly on the oxidation duration as well as on the oxidation procedure and the type of Si substrate and is minimized at an oxidation temperature 900 °C for oxidation duration longer than 70 min.
Passive Q switching and mode-locking of Er:glass lasers using VO2 mirrors
Authors: Pollack S. A. , Chang D. B. , Chudnovky F. A. , Khakhaev I. A.
Published: July 2002

Passive Q switching of an Er:glass laser with the pulse width varying between 14 and 80 ns has been demonstrated, using three resonator vanadium-dioxide-coated (VO2) mirror samples with temperature-dependent reflectivity and differing in the reflectivity contrast. The reflectivity changes because of a phase transition from a semiconductor to a metallic state. Broad band operating characteristics of VO2 mirrors provide Q switching over a wide range of wavelengths. In addition, mode-locked pulses with much shorter time scales have been observed, due to exciton formation and recombination. A simple criterion is derived for the allowable ambient temperatures at which the Q switching operates effectively. A simple relation has also been found relating the duration of the Q-switched pulse to the contrast in reflectivities of the two mirror phases.
Evidence for inhomogeneous growth rates in partially relaxed InGaAs/InP heterostructures
Authors: Gendry M. , Porte L. , Hollinger G. , Loubet J. L. , Miossi C. , Pitaval M.
Published: July 2002

The surface morphology and the dislocation structure of thin and thick strained InxGa1−xAs heterostructures grown on (001) InP substrates have been investigated using atomic force microscopy and transmission electron microscopy. Long linear ridges, correlated to underlying dislocations, are detected in thin partially relaxed In0.82Ga0.18As layers. The ridges (with increased height along the [110] direction comparing to along the [1-10] direction) are associated with strong inhomogeneous local growth rates induced by elastic strain relief and controlled by kinetic factors. The crosshatched surface morphology in thick In0.65Ga0.35As layers is explained using the same mechanism with multiplication and interaction of dislocations and ridges during growth. In contrast, the development of isolated ‘‘V-shaped’’ grooves parallel to [1-10] in lattice-matched capping layers grown on partially relaxed In0.82Ga0.18As quantum wells is explained by the generation of surface corrugations in tensile strained regions appearing just above the elongated relaxed region of In0.82Ga0.18As.
Pre-breakdown currents in water and aqueous solutions and their influence on pulsed dielectric breakdown
Authors: Jones H. M. , Kunhardt E. E.
Published: July 2002

The current response of water and sodium chloride solutions subjected to high amplitude electric fields of sub-microsecond duration has been investigated. The prebreakdown current-voltage relationship for these liquids is found to be nearly linear for the fields considered and therefore described by a resistance, R. For constant concentration, R does not extrapolate to zero as the gap width, d, goes to zero, suggesting the presence of a high resistivity sheath near the electrodes. Estimates for the sheath parameters (electric field in the sheath, Es, width, dl, formation time, tl, and ion density, ns) are obtained from these measurements. The contribution of electronic and ionic currents to the breakdown probability has been assessed from the power input to the liquid during the pre-breakdown phase. For applied fields with sub-microsecond duration, the input power is primarily due to electron field emission currents whose magnitude is a function of the field in the ionic sheath. For the parameters investigated, ionic currents are found to play a secondary role in these time scales.
Low temperature crystallization of sputtered carbon films
Authors: Yañez-Limón J. M. , Ruiz F. , González-Hernández J. , Chao B. S. , Ovshinsky S. R.
Published: July 2002

The crystallization of amorphous carbon films, under inert atmospheres, occurs at annealing temperatures above 800 °C. In this work we have found that when the annealing of carbon films is performed under atmospheric conditions, crystallization occurs at temperatures as low as 200 °C. The catalytic effect of oxygen in the crystallization process is understood in terms of the generation of a porous structure in the carbon film due to the vaporization of carbon oxides.
Cross-sectional photoluminescence and its application to buried-layer semiconductor structures
Authors: Schaafsma D. T. , Christensen D. H.
Published: July 2002

We present an overview of a cross-sectional scanning microphotoluminescence technique that is used to examine various buried-layer semiconductor structures for which traditional surface-normal techniques cannot yield sufficient information or must be coupled with time-consuming and painstaking processes such as wet etching. This technique has a wide range of applications; two—defect-driven interdiffusion in quantum wells and the modification of spontaneous emission from quantum wells in vertical-cavity surface-emitting lasers (VCSELs)—are discussed here. The data obtained using this method can be used to distinguish emission spectra from quantum wells as little as one micrometer apart in depth and a few nanometers different in wavelength. The comparison of normal incidence with cross-sectional data from VCSELs can be used to more effectively optimize the match between cavity resonance and quantum well emission in high-Q devices.
Scattering by immersed two-dimensional elastic galois gratings
Authors: Costa Bernadette , Folacci Antoine , Gabrielli Paul
Published: July 2002

Diffraction of ultrasonic waves by immersed elastic bigratings constructed from the Galois fields GF(23) along one direction and GF(32) along the orthogonal one is studied. Such bigratings permit a significant attenuation of the specular echo in a wide frequency domain. This attenuation is experimentally observed and agrees with the theoretical one predicted by a model which takes into account the elasticity of the structure, the boundary conditions at the fluid–solid interface, and the periodicities of the gratings. Furthermore, numerous sharp minima in the spectrum of the reflection coefficient are experimentally observed and theoretically predicted; they correspond to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) and they contribute significantly to the attenuation of the specular echo.
In-plane orientation of c-axis-oriented Bi2Sr2Can−1CunO2n+4+δ thin films on Nd:YAlO3 substrates
Authors: Tsukada I. , Uchinokura K.
Published: July 2002

In-plane orientation of thin-film Bi2Sr2Can−1CunO2n+4+δ superconductors on Nd:YAlO3 is investigated in detail. Thin films of c-axis-oriented Bi2Sr2Can−1CunO2n+4+δ are grown in situ by molecular beam epitaxy with pure ozone as an oxidant. The in-plane orientation of the films was observed by reflection high-energy electron diffraction. A characteristic epitaxial relationship was found between Bi2Sr2Can−1CunO2n+4+δ (001) and Nd:YAlO3 (001), and as a result, Bi2Sr2Can−1CunO2n+4+δ was grown without the a-b twinning structure, maintaining the relation Bi2Sr2Can−1CunO2n+4+δ [010] ∥Nd:YAlO3[100]. This relationship is found to be independent of the phases (n) of the films. Moreover, it is affected by the steps running parallel to Nd:YAlO3 [100]. With increase of the step density by changing the misorientation angle from (001) toward [010], the in-plane orientation of Bi2Sr2Can−1CunO2n+4+δ is rotated by 90° around its c-axis direction. The epitaxial relationship is discussed with respect to the anisotropic growth rate of Bi2Sr2Can−1CunO2n+4+δ films.
Photoluminescence of nanocrystallites embedded in hydrogenated amorphous silicon films
Authors: Liu Xiang-Na , Tong Song , Wang Lu-Chun , Chen Guang-Xu , Bao Xi-Mao
Published: July 2002

We have fabricated light-emitting nanocrystallites embedded in an a-Si:H matrix using a conventional plasma-enhanced chemical-vapor-deposition system. It was found that the photoluminescence properties are directly related to the deposition parameters. The quantum size effect model is proposed to explain the photoluminescence. Two structural prerequisites are proposed for this kind of films to exhibit effective light emission: One is an upper limit for mean crystallite size of about 3.4 nm, the other is an upper limit for crystallinity of about 30%.
Thermal index guiding in gain-guided blue-green II–VI semiconductor lasers
Authors: Buijs Maarten , Tijburg Rudy , Haberern Kevin , Gaines Jim
Published: July 2002

We have studied the lateral waveguiding properties of gain-guided and index-guided II–VI lasers under pulsed conditions and their influence on threshold current density Jth and differential quantum efficiency η. Thermal index guiding was found to reduce the astigmatism of the gain-guided devices. The thermally induced lateral field confinement leads to a lowering of Jth and an increase of η with pulse width with a maximum after a few μs because of maximum overlap of the near field with the gain profile. For the index-guided devices the lateral waveguiding is fully determined by the built-in refractive index profile and no dependencies on pulse width are observed.
Kinematic versus dynamic approaches of x-ray diffraction simulation. Application to the characterization of InGaAs/InGaAlAs multiple quantum wells
Authors: Idiart-Alhor E. , Marzin J. Y. , Quillec M. , Le Roux G. , Patriarche G.
Published: July 2002

A comparison of kinematic and dynamic approaches of x-ray diffraction is made for multiple InGaAs/InGaAlAs quantum wells (MQW), around 002 and 004 reflections, as a function of strain and thickness. A domain of validity of the kinematic approximation is then defined. It turns out that the kinematic approximation is sufficient in the case of typical structures for MQW lasers or modulators. An automatic procedure including kinematic treatment of double-diffraction X and photoluminescence data is proposed, which allows us to determine precisely the compositions, thicknesses, and even the band offsets of MQW structures. This procedure is applied to a 20 period sample. The results are confirmed by transmission electron microscopy measurement.
Silicon contamination of diamond films deposited on silicon substrates in fused silica based reactors
Authors: Borges C. F. M. , Schelz S. , St.-Onge L. , Moisan M. , Martinu L.
Published: July 2002

Deposition of thin diamond films on silicon (Si) substrates and in a reactor with fused silica walls can lead to the incorporation of Si impurities. In the present work, impurities in the bulk of the films were analyzed quantitatively using complementary diagnostic techniques (elastic recoil detection, electron microprobe analysis and secondary ion mass spectrometry), while surface analysis was achieved with x-ray photoelectron spectroscopy. The Si contamination level in the bulk reaches up to 0.16 at. %. We show that the presence of Si impurities correlates with the fluorescence background that accompanies the 1332 cm−1 diamond peak in the Raman spectra. Experiments were performed to distinguish between the Si originating from the wall and from the Si substrate. The effect of O2 added to the process gases is also investigated. The diamond films were prepared in a recently developed plasma reactor using a novel configuration of surface-wave-sustained discharge: the reactor operation is akin to that of the well-known plasma-ball systems.
High-rate deposition of a-Si:H films using a flow plasma–chemical method with electron beam activation
Authors: Sharafutdinov R. G. , Skrynnikov A. V. , Parakhnevich A. V. , Ayupov B. M. , Badalian A. M. , Polyakov O. V. , Baklanov M. R. , Mogilnikov K. P. , Biryukov S. A.
Published: July 2002

A high-rate a-Si:H film deposition using the new plasma–chemical gas jet method with electron beam activation has been developed. The films were characterized by ellipsometry, IR-spectrometry, and electrophysical measurements. Comparison of the film characteristics with those obtained by Hichikawa et al., J. Appl. Phys. 73, 4227 (1993), in the usual planar technology has demonstrated satisfactory film quality with a growth rate of films an order of magnitude greater than the rates in a planar reactor.
Quantitative analysis of electron-beam-induced current profiles across p–n junctions in GaAs/Al0.4Ga0.6As heterostructures
Authors: Bonard Jean-Marc , Ganière Jean-Daniel
Published: July 2002

We propose a method for the quantitative evaluation of electron-beam-induced current profiles measured in normal-collector geometry across p–n junctions in GaAs/Al0.4Ga0.6As heterostructures. We obtain a theoretical expression, valid for all distances from the junction, and which takes into account the spatial extension of the electron–hole pairs generation. From the comparison of calculated with experimental profiles, we evaluate the minority carrier diffusion length and to some extent the surface recombination velocity.
Heterojunctions and devices of colloidal semiconductor films and quantum dots
Authors: Könenkamp R. , Hoyer P. , Wahi A.
Published: July 2002

We present an electrical characterization of thin film semiconductor devices based on porous nanocrystalline TiO2 anatase films. It is shown that the TiO2 films can be doped, and that injecting or blocking electrical contacts can be established by metals, semiconductors, and oxides. This opens the way for the preparation of a number of all-solid-state devices. We present results on a Schottky barrier, a two-layer heterojunction on amorphous silicon, and a distributed heterojunction involving PbS quantum dots adsorbed on the internal surface of the TiO2 film.