bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of microstructures in hydrogenated silicon films prepared by diluted-hydrogen and hydrogen-atom-treatment methods
Authors: Hsu Kuo-Chiang , Chang Hua , Hwang Huey-Liang
Published: July 2002

Nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, and Raman studies on microstructures of hydrogenated silicon films that were fabricated by diluted-hydrogen and hydrogen-atom-treatment methods are presented. The diluted-hydrogen samples tend to show a very sharp line shape in the NMR spectra at substrate temperatures higher than 300 °C, and the addition of atomic hydrogen treatment can produce the same NMR spectra at a lower temperature of about 250 °C. The Raman scattering spectra show that the atomic hydrogen treatment creates the microcrystalline phase while the diluted-hydrogen method produces amorphous phase plus a small quantity of microcrystalline phase. The infrared-absorption spectra also indicate an increase of SiH2 bonding configuration and a hydrogen content reduction when atomic hydrogen treatment is employed. The relation between the origin of the sharp line shape in the NMR spectra and the formation of the microcrystalline phase is also discussed. Together with increase of dark conductivity and reduction of the photo-to-dark conductivity ratio, these samples indicate that with appropriate hydrogen incorporation during deposition, and with plasma hydrogen treatment, these films should possess a much more compact structure. These results suggest that the degree of crystallinity of hydrogenated silicon films can be systematically adjusted. A qualitative model based on our experimental data is presented to illustrate the formation procedures of microcrystalline-phase hydrogenated silicon under the influence of plasma hydrogen and hydrogen dilution.
Reaction of iron and silicon during ion implantation
Authors: Crecelius G. , Radermacher K. , Dieker Ch.
Published: July 2002

Using energy-loss spectroscopy, energy dispersive x-ray analysis, electron diffraction, and He+-ion channeling the reaction of Fe during implantation into Si(111) has been investigated at various target temperatures and implantation doses. In samples implanted at 275 °C with 2.8×1017 Fe+ cm−2 a continuous α-FeSi2 layer accompanied by α-phase precipitates is formed. At 450 °C Fe agglomerates mostly in α-phase precipitates with only a few being β-FeSi2. At 350 °C 1×1017 Fe+ cm−2 produce precipitates electronically close to FeSi2 but crystallographically poorly defined. At 4×1017 Fe+ cm−2 a β-FeSi2 layer is formed at the surface and a 20-nm-thick α-FeSi2 one followed by α-FeSi2 precipitates deeper in the volume. Channeling reveals a minimum yield decreasing with dose indicating improved α-phase crystal quality. A sharp increase at 3.3×1017 cm−2 indicates an α–β phase transition. FeSi has not been detected. Precipitates of well defined silicide phases are formed already during implantation. Dose and temperature have a profound influence on the phase formed.
Impact ionization coefficients in strained InGaAs/InAlAs multiquantum wells
Authors: Gutierrez-Aitken A. L. , Bhattacharya P. K.
Published: July 2002

We have measured electron and hole impact ionization coefficients in biaxially strained InxGa1−xAs/InyAl1−yAs (0.44≤x≤0.62, 0.44≤y≤0.62) multiquantum wells for the first time. It is seen that β/α is enhanced due to strain-induced changes in band gap, band offsets, and bandstructure for tensile strain in the well and compressive strain in the barrier. The results have been interpreted by considering band-to-band impact ionization and band-edge discontinuity impact ionization processes.
Effect of processing geometry on YBa2Cu3O7−x plasma emission during superconducting thin film growth by pulsed laser evaporation technique
Authors: Pramanick S. , Narayan J.
Published: July 2002

In situ processing of YBa2Cu3O7−x (YBCO) superconducting thin films by pulsed laser evaporation is a technique critically dependent on the processing conditions (substrate temperature, oxygen partial pressure, etc.), and on the deposition setup, including the oxygen nozzle geometry required for incorporation of oxygen. It has been found necessary to have the oxygen nozzle directed toward the substrate during deposition for growth of high-quality superconducting thin films without further annealing [R. Singh et al., Appl. Phys. Lett. 55, 2351 (1989)]. We have taken optical emission spectra of YBCO plasma during the following deposition conditions: (1) In vacuum, (2) in 200 mTorr oxygen ambient with the oxygen nozzle directed toward the substrate, (3) in 200 mTorr oxygen ambient with oxygen nozzle directed toward the target, and (4) with the +300 V biasing ring interposing the target and the substrate. Our results show that enhanced formation of YO and CuO might lead to the growth of thin film at a lower substrate temperature under the biasing condition. A strong dependence of the Cu i (324.75 nm) neutral peak intensity and of the appearance of the molecular peaks such as YO on different deposition geometry is seen. These results provide important insight into the characteristics of plasma transport and deposition by the pulsed laser evaporation technique.
Range of chaotic motion of a domain wall in a periodic drive field
Authors: Kosinski R. A.
Published: July 2002

The motion of a domain wall in a periodic drive field H0 sin ωt is studied with Slonczewski’s equations of motion using the fully implicit finite difference scheme. The motion is periodic, quasiperiodic or chaotic, depending on the values of the frequency ω, damping constant α, and magnetic filed strength H0, while the other parameters are held constant. For a given magnitude of the magnetic field, there is a narrow frequency range in which the periodic motion extends to low values of the damping constant. This may be due to spin-wave-like excitations appearing in the sample. For low frequencies the border between the chaotic and periodic motion corresponds to the Walker limit of stationary motion yielding a value of the damping constant according to α=H0/2πM, where M is the magnetization.
Effect of thermal annealing on the Raman spectrum of Si1−xGex grown on Si
Authors: Nakano Noboru , Marville Louis , Jang Syun-Ming , Liao Kenneth , Tsai Curtis , Tsai Julie , Kim Hyoun-Woo , Reif Rafael
Published: July 2002

The effect of thermal annealing on the Raman spectrum of Si1−xGex grown on Si was investigated in the temperature range of 800–1100 °C on three samples having Ge contents x of 0.2 and thicknesses of 0.08, 0.16, and 0.40 μm. For annealing below 900 °C, the stress relaxation played an important role on the Raman shift. The degree of the stress relaxation in a dislocation free sample was smaller than that in a sample with partially relaxed stress. Stress was not completely relaxed in any of our samples after annealing. For annealing above 900 °C, diffusion strongly affected the Raman shift and the spectral lineshape. The critical thickness was close to 0.08 μm.
The structural and electrical properties of low-resistance Ni contacts to InP
Authors: Fatemi Navid S. , Weizer Victor G.
Published: July 2002

We have investigated the electrical and metallurgical behavior of the Ni-InP contact system. Specific contact resistivity (Rc) values in the low 10−7 Ω cm2 range are achieved with Ni-only contacts on n-InP (Si: 1.7×1018 cm−3) by sintering at 400 °C for several minutes. The post-sinter contact metallization consists of three layers, arranged in the sequence: InP/Ni3P/Ni2P/In. Extended sintering (40 min) at 400 °C brings about a rise in Rc to the 10−4 Ω cm2 range. After extended sintering, the contact metallization is found to consist of only two layers, arranged in the sequence: InP/Ni2P/In. Based on the correlation between low Rc and the presence of Ni3P at the metal-InP interface, it is suggested that the presence of Ni3P is the cause of the low Rc values. We show that the sintering schedule used to achieve low values of Rc is accompanied by substantial metal-InP interdiffusion that results in severe device degradation. We show, finally, that it is possible to achieve low values of Rc without incurring these device-destroying metallurgical interactions by introducing a thin layer of Ni3P between the InP and the current carrying metallization.
Epitaxial ZnS films grown on GaAs (001) and (111) by pulsed-laser ablation
Authors: McCamy J. W. , Lowndes Douglas H. , Budai J. D. , Zuhr R. A. , Zhang Xiao
Published: July 2002

Pulsed KrF (248 nm) laser ablation of a polycrystalline ZnS target has been used to grow very smooth and carbon-free, epitaxial ZnS thin films on GaAs (001) and (111). Films were grown at temperatures of 150–450 °C, using a rotating substrate heater and deposition geometry that produces highly uniform film thickness, without nucleation or surface-roughening problems. X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) show that the ZnS films are fully epitaxial (in-plane aligned). Films grown at the optimum temperature of 325 °C have x-ray rocking curve widths that are indistinguishable from molecular-beam-epitaxy-grown ZnS/GaAs films of the same thickness. Rutherford backscattering spectrometry and HRTEM show that in films ∼275 nm thick, the ∼150 nm nearest the GaAs-ZnS interface is highly faulted, due to the ∼4.1% lattice mismatch and/or the low ZnS stacking fault energy, but the upper ∼125 nm is much less defective. The anisotropy of the ZnS epitaxial growth rate between the GaAs (001) and GaAs (111) surfaces was found to be slightly temperature dependent.
The theory of the photoabsorption in n-type Si-SiGe quantum well infrared photodetectors
Authors: Shadrin V. D. , Coon V. T. , Serzhenko F. L.
Published: July 2002

The theory of multiple quantum well n-type Si-SiGe infrared detectors is presented. The coefficient of photoabsorption, quantum efficiency, and responsivity of Si-SiGe quantum well (QW) detectors are calculated taking in consideration the effects of depolarization and electron-electron exchange interaction. We show that the Si-SiGe quantum well detectors possess lower dark current, lower tunneling rates, and better photoabsorption characteristics including responsivity when compared to GaAs-AlGaAs photodetectors. These imply that Si-SiGe QW detector performance characteristics are superior to GaAs-AlGaAs QW infrared photodetectors aside from its advantage of compatibility to Si-readout circuity.
Hydrogenated amorphous silicon films prepared at high substrate temperature: Properties and light induced degradation
Authors: Banerjee Ratnabali , Ghosh Sukriti , Chattopadhyay S. , Bandyopadhyay A. K. , Chaudhuri P. , Batabyal A. K. , Barua A. K.
Published: July 2002

Of the different deposition parameters, the substrate temperature Ts has a profound effect on the microstructure and optoelectronic properties of hydrogenated amorphous silicon (a-Si:H). A detailed study was done to evaluate a-Si:H materials deposited at high substrate temperatures (≥325 °C). Their characteristics and nature of light induced degradation were compared to a-Si:H deposited at 200 °C. Electrical properties were studied with coplanar electrode structure as well as on Schottky barrier devices. Absorption measurements in the visible and infrared regions and spin-density measurements were carried out. For high Ts (≥325 °C) the presence of acceptorlike defects are indicated in addition to the neutral dangling bonds. Annealing recovery from the light soaked state is slower as compared to a film deposited at 200 °C. The results have been discussed in connection with the role of hydrogen motion in the annealing of light induced defects.
The effect of anisotropy on the electrical properties of compacted semiconductor (Bi2Te3)25-(Sb2Te3)75 powders
Authors: Joraide A. A.
Published: July 2002

The effect of anisotropy on the electrical resistivity, and the Seebeck coefficient have been studied for a compressed powder alloy of (Bi2Te3)25-(Sb2Te3)75 doped with 0.9 wt % of Se. The results from these measurements support the theoretical model, which is proposed to explain the anisotropy of semiconductor materials having uniaxial symmetry.
Molecular-beam-epitaxial-grown n-GaAs1−xSbx/N-GaAs heterostructures characterized by admittance spectroscopy
Authors: Zhao J. H. , Lu Z.
Published: July 2002

The admittance spectroscopy technique has been applied to characterize deep traps in molecular-beam-epitaxial-grown n-GaAs1−xSbx/N-GaAs heterostructures. A single dominant defect has been identified in each of the three samples containing different Sb content. It was found that the substrate misorientation appears to affect the formation of the dominant defect, resulting in different defects in samples grown under the same conditions. It shows a good agreement when comparing the admittance spectroscopy results with those of the previously reported work of deep-level transient spectroscopy.
Influence of polysilicon gate formation conditions on thin gate oxide (4–6 nm) dielectric and charging properties
Authors: Itsumi Manabu , Shiono Noboru , Shimaya Masakazu
Published: July 2002

The influence of poly-Si gate formation conditions on the electrical properties and reliability of very thin thermal SiO2 films is described. Phosphorus-doped (P) poly-Si, phosphorus ion (P+) implanted poly-Si, and arsenic ion (As+) implanted poly-Si are used as metal-oxide-semiconductor capacitor gates for comparison. Long-term bias temperature aging tests show that As+ poly-Si is the most stable of the three gates from the standpoint of minimum flatband voltage shift. It is shown that electron trapping probability in the oxides is extremely low compared with that of hole trapping. This result is closely related to a mechanism in which electron tunneling distance in the oxides is 3 nm and electrons trapped in oxides thinner than 6 nm detrap easily to the oxide exteriors. Furthermore, it is shown that long-term avalanche injection of electrons induces electron trapping for 6-nm-thick oxides which are free from electron trapping under ordinary stressing conditions. The capture cross section of this electron trapping is estimated to be about 6×10−22 cm−2, three or four orders of magnitude lower than the values already reported. It is thought that the energy level of this trap center is deeper than 3.1 eV below the SiO2 conduction band edge.
On the effect of processing parameters in the chemical-vapor deposition of YBa2Cu3O7−δ thin films on polycrystalline silver
Authors: Chen L. , Piazza T. W. , Schmidt B. E. , Kelsey J. E. , Kaloyeros A. E. , Hazelton D. W. , Walker M. S. , Luo L. , Dye R. C. , Maggiore C. J. , Wilkins D. J. , Knorr D. B.
Published: July 2002

Results are reported from an investigation of the effects of selected processing parameters on the morphology and properties of YBa2Cu3O7−δ (YBCO) superconducting thin films grown directly on polycrystalline silver substrates by chemical-vapor deposition (CVD). These results were achieved through a set of experimental studies which examined: (i) recrystallization mechanisms of polycrystalline silver and their effect on the deposition of YBCO thin films; and (ii) CVD processing conditions leading to the growth of high-quality YBCO films. The samples were analyzed using dynamic impedance, four-point resistivity probe, x-ray diffraction, Rutherford backscattering, and scanning electron microscopy. These studies showed that substrate temperature played a critical role not only in the formation of YBCO films, but also in the recrystallization of silver substrates, which in turn greatly influenced film growth. The studies also led to the identification of a two-stage processing scheme for the growth of YBCO films on silver. The first processing stage consisted of a substrate conditioning cycle which involved a 10 min ramping from room temperature to deposition temperature where the substrates were held for an additional 10 min in a flow of 70 sccm O2 at a reactor working pressure of 2 Torr. The second processing stage involved actual film deposition at 760–800 °C for 3–10 min (depending on desired film thickness) in a mixed flow of 70 sccm O2 and 210 sccm N2O at a reactor working pressure of 4 Torr. Samples thus produced were highly oriented along the c axis perpendicular to the substrate with a zero resistance transition temperature of 87 K and a critical current density of 2×104 A/cm2 (77 K, B=0). The films had a thickness of 200–700 nm depending on the length of the growth cycle, which corresponded to the growth rates in the range 65–130 nm/min. A growth mechanism for YBCO on polycrystalline silver, which emphasized the role of silver recrystallization, was consequently proposed and discussed.
Modeling of molecular-beam epitaxy and metalorganic vapor-phase epitaxy on nonplanar surfaces
Authors: Ohtsuka Mitsuru , Suzuki Akira
Published: July 2002

A model allowing a consistent description of growth evolution on nonplanar surface by molecular-beam epitaxy and metalorganic vapor-phase epitaxy is presented. It is shown that the elemental kinetic processes (surface migration, incorporation, and desorption) are reduced to an equation of continuity for an atomic flow on a growing surface. In metalorganic vapor-phase epitaxy this continuity equation is coupled with the diffusion equation in vapor phase, permitting to calculate self-consistently the density distribution of reactant species in vapor and of the adatoms in the surface layer. In the model, anisotropic properties of growth are introduced by taking into account the surface diffusion around the atomic steps. The formulation of the model is based on the discrete form of the continuity equation (rather than the differential form) and therefore allows description of the growth behavior including the development of sharp facet edges. The method of simulation for nonplanar growth is illustrated in detail.
Measurement of effects of ripple on the permeability of thin film head materials (abstract)
Authors: Jacobs I. S. , Thompson J. L. , Tkaczyk J. E. , Kryder M. H.
Published: July 2002

A new low-field resonance technique for determining the initial hard-axis permeability (μ′) of materials used in thin film and magnetoresistive heads has been developed. The initial μ′ is found to be less than 4πM/Hk due to the internal effective magnetostatic field caused by magnetization ripple, in agreement with the theory and transverse biased initial susceptibility measurements of Hoffmann and others.1 A proper consideration of the in-plane susceptibility for real films includes the Stoner–Wohlfarth anisotropy field (Hk)−1 dependence, modified by the factors due to ripple (B) and skew (E) as defined by Hoffmann. The latter produce internal fields that shift the Kittel resonance versus field plots (fr2 vs H) leftward from their ideal symmetric positions for angles of 0° and 90° between bias field and preferred axis. When resonance data from both orientations are computer-fit simultaneously, we determine the parameters B and E as well as Hk and Ms. The coefficients are believed to have more consistency than those obtained using Hk found from hard-axis magnetization curves. The reduction of the hard-axis μ′ is by the factor (1+B+E)−1. The high frequency properties (300 kHz–3 GHz) were measured using a test jig consisting of a shorted-shield strip-line section with a one-turn inductively coupled loop containing the sample and located at the shorted end. Bias fields from zero to 100 Oe were employed. We measure fR reproducibly with an uncertainty of less than 1%. This analysis determines the parameters of interest from measurements under dynamic conditions. Results include Permalloy and amorphous TM-metalloid films. When annealed to reduce Hk, their B parameters increase in accord with theory, assuming invariant structure constants, S. Beyond head applications, the method relates to thin film microwave devices operating near fr in zero field.2
Positron annihilation study of microvoids in centrifugally atomized 304 stainless steel
Authors: Kim J. Y. , Byrne J. G.
Published: July 2002

Positron trapping in microvoids was studied by positron-lifetime and positron Doppler line-shape measurements of centrifugally atomized 304 stainless-steel powder, which was hot-isostatically-press consolidated. This material contained a concentration of several times 1023/m3 of 1.5-nm-diam microvoids. Positron annihilation was strongly influenced by the microvoids in that a very long lifetime component τ3 of about 600 ps resulted. The intensity of the τ3 component decreased with decreasing number density of 1.5 nm microvoids. The Doppler peak shape was found to be much more strongly influenced by microvoids than by any other defects such as precipitates or grain boundaries. In particular microvoids produced significant narrowing of the Doppler distribution shape.
Growth and characterization of GaSe and GaAs/GaSe on As-passivated Si(111) substrates
Authors: Palmer J. E. , Saitoh T. , Yodo T. , Tamura M.
Published: July 2002

We have grown and characterized epitaxial layered structure GaSe on As-passivated Si(111) and GaAs on GaSe on As-passivated Si(111) for the ultimate purpose of using the layered structure GaSe as a lattice mismatch/thermal expansion mismatch buffer layer in the GaAs on Si system. Films were grown on nominally (111) oriented Si substrates by molecular beam epitaxy and characterized by in situ reflection high energy electron diffraction, as well as by ex situ scanning electron microscopy and both plan-view and cross-sectional TEM (transmission electron microscopy). In this study, GaSe was grown epitaxially on As-passivated Si(111) substrates at 500 °C with Se/Ga BEP (beam equivalent pressure) ratios of ∼10 and ∼20. Small droplets were observed on the surface after GaSe growth. These are thought to be droplets of unreacted Ga. The density and size of the droplets decreases with the increasing Se/Ga BEP ratio. When the GaSe surface is exposed to As, the droplets become GaAs islands. Subsequent GaAs growth was carried out at 400 and 500 °C, giving the following results for 300-Å-thick films: as grown GaAs films were highly twinned, and some polycrystalline GaAs was present in the film grown at 400 °C. In situ annealing at 650 °C for 10 min reduced the density of twins in both cases. In plan-view TEM, Moiré fringes from both GaAs and GaSe are observed and show conclusively that the GaAs grew epitaxially on the GaSe without contacting the Si substrate. Cross-sectional TEM shows the interface between the Si and GaSe is not smooth on the atomic scale. In spite of this, the GaSe becomes smooth with about 2 monolayers of growth and the GaAs/GaSe interface appears to be very smooth.
Nanocomposite formation in the Fe3O4-Zn system by reaction milling
Authors: Takacs Laszlo , Pardavi-Horvath Martha
Published: July 2002

Magnetic nanocomposites of small iron particles embedded in nonmagnetic zinc oxide matrix have been prepared by ball milling, with an in situ displacement reaction between a metal oxide (Fe3O4) and a more reactive metal (Zn). The phase composition of the samples has been analyzed by x-ray diffraction. Metallic zinc disappears during the first 100 min of milling and the magnetization decreases to almost zero, indicating the formation of a nonmagnetic intermediate iron-zinc oxide phase. This intermediate phase decomposes into iron and ZnO upon further milling. The change in magnetic properties also reflects the decreasing size of the iron particles. The final particle size is about 9 nm, as estimated from x-ray diffraction linewidth measurements. The final product of the process is a semihard magnetic material with a room-temperature saturation magnetization of 40 emu/g and coercivity of 400 Oe. A significant fraction of the final Fe particles is superparamagnetic.
Anisotropy and magnetostriction measurement by interferometry
Authors: Harllee Peter S. , Bellesis George H. , Lambeth David N.
Published: July 2002

Extending the use of a laser-Doppler interferometer that measures magnetostriction by measuring the deflection of a thin-film cantilever sample to also measure the anisotropy is reported. When the sample is driven with a rotating magnetic field of constant magnitude, a number of harmonics of twice the rotating field frequency are evidence of the magnetization vector lagging and leading the drive field around the sample. Altering the magnitude of the drive field so that the H vector traces out an ellipse is found to null the harmonic signals. This ellipse has its major axis aligned with the hard axis. Using the Stoner–Wohlfarth single-domain-particle model, it is shown that a rotating field will cause the magnetization to rotate around the particle uniformly only if the difference in magnitude between the fields along the major and minor axes is equal to the anisotropy field. Measurements on isotropic and anisotropic samples confirm the theory.
A study of the accuracy of various Langmuir probe theories
Authors: Sudit Isaac D. , Woods R. Claude
Published: July 2002

Ion and electron densities have been measured in long, low pressure, cylindrical nitrogen and helium dc discharges using computer-controlled Langmuir probes. Cylindrical probe data have been analyzed with a variety of theories in order to determine the latter’s accuracy. Electron densities were obtained from the electron saturation currents using orbital motion limited (OML) theories, and from the electron retardation region of the probe trace by integration of the second derivative of the probe current. Ion densities were obtained from both OML and radial motion analysis of the ion saturation currents. Line integrated microwave interferometry and discharge current continuity considerations in the positive column have been used to obtain two independent electron density measurements. While both probe electron density methods agree very well with each other and reasonably well with the independent density measurements, the OML theory applied to the ions overestimates the plasma density by up to a factor of 10. The radial motion theory yields ion densities that show considerably better agreement with the electron densities than the OML theory. Ion and electron densities have also been measured with planar probes, but they were found to perturb the surrounding plasma more than the cylindrical ones.
Experimental evidence of resputtering of the yttria layer in a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film
Authors: Waytena G. L. , Hoff H. A. , Vold C. L. , Broussard P. R. , Claassen J. H. , Cestone V. C. , Sprague J. A.
Published: July 2002

Using transmission electron microscopy and energy dispersive x-ray spectroscopy analysis, the microstructure and composition of a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film deposited on MgO by off-axis sputtering at 670 °C and 100 mTorr was investigated. The YBa2Cu3O7−x layer in contact with the substrate was found to be mainly single phase. However, the top YBa2Cu3O7−x layer was embedded with 5–10 nm crystalline Y2O3 (yttria) particles, which disturbed the local microstructure. The top YBa2Cu3O7−x layer also had barium copper oxide particles covering its surface. The partial decomposition of the top YBa2Cu3O7−x layer may have been due to resputtering of the yttria layer which locally altered the deposition conditions. In particular, the oxygen pressure in the vicinity of the growing film may have increased due to sputtering of the yttria layer by plasma gas atoms and the sputtered target atoms. As a result, deposition occurred under conditions away from the thermodynamic stability line.
Optical absorption by colloidal precipitates in bismuth-implanted fused silica: Annealing behavior
Authors: Park Seung Y. , Weeks Robert A. , Zuhr Raymond A.
Published: July 2002

The near-surface regions of high-purity fused silica disks (Corning 7940) were modified by implantation with Bi2+ ions at 160 keV and at room temperature. The glasses implanted with a nominal dose of 6×1016 Bi2+ ions/cm2 at ∼5 μA/cm2 were subsequently annealed in oxidizing and inert atmospheres. The annealing temperature ranged from 500 to 1100 °C. The doses retained before and after the anneals were determined from Rutherford backscattering. A broad optical-absorption band at ∼5 eV (248 nm) was resolved and attributed to surface plasmon resonance from colloidal bismuth metal particles precipitated after implantation and observed by transmission electron microscopy. After annealing in oxygen for 60 min at temperatures ≥1000 °C the bismuth in the sample decreased by ∼70%. Annealing in an oxidizing atmosphere oxidized the metallic precipitates, and oxidation changed the optical absorption of the samples. For samples annealed in argon, a relatively small decrease in retained dose by ∼12% was observed for temperatures ≥1000 °C. Based on the difference in the effects of annealing in oxygen and in argon, the bismuth atoms in the samples annealed in argon are not oxidized and remain in the particles. The small changes in the distribution of bismuth after annealing in argon are due to diffusion of bismuth atoms from colloidal particles to larger particles, i.e., Ostwald ripening, as well as evaporation from the surface of the fused silica.
Differential gain in coupled quantum well lasers
Authors: Akhtar A. I. , Xu J. M.
Published: July 2002

In this paper we examine the effect of coupling between the wells on the differential gain. We find, as for the gain peak, the differential gain at the gain peak is not a monotonic function of the barrier thickness. For relatively thick barriers inclusion of coupling reduces the differential gain but as the barrier gets thinner the differential gain can be significantly enhanced or suppressed. We also compare our calculations with those for single quantum well to show how the enhancement in differential gain in the multi quantum well laser, due to reduced state filling and higher modal gain is affected by well coupling. Without losing generality, the model assumes the quantum well region to be electric field free. Numerical examples are presented for a GaAs/Al0.2Ga0.8As double quantum well system.
Glancing-angle extended x-ray absorption fine structure study of strained InGaAs/GaAs heterostructures
Authors: Proietti M. G. , Turchini S. , García J. , Lamble G. , Martelli F. , Prosperi T.
Published: July 2002

The structural properties of strained InGaAs grown by molecular beam epitaxy on GaAs(100) substrates, have been studied by glancing-angle extended x-ray absorption fine structure (EXAFS). The very low incidence angle of the x-ray beam on the sample makes it possible to collect the signal coming from a thin quasi-surface layer allowing the study of a single strained sample built up by only 6 ML of InGaAs. The EXAFS results show that a slight deformation of the first shell Ga–As distance occurs and that the strain is accommodated also by bond-bending mechanism as deduced by the second and third coordination shells analysis. The lattice expands in the growth direction in agreement within the limits predicted by the elastic theory.
Broad photoluminescence band in undoped AlxGa1−xAs grown by organometallic vapor phase epitaxy
Authors: Kakinuma H. , Akiyama M.
Published: July 2002

We have studied the 77 K photoluminescence (PL) of undoped-AlxGa1−xAs (0.21⩽x⩽0.83) grown by organometallic vapor phase epitaxy. A deep broad (DB) PL band is found at 1.6–1.7 eV at a range of x from 0.21 to 0.63, with a maximum intensity at around x=0.5. Its large full width at half-maximum (∼200 meV) suggests that this emission originates from some impurity-defect complex. The Si- and C-doping dependencies of the PL reveal that the emission disappears in Si-doped n-type samples while it increases in intensity superlinearly with the hole concentration, thus, the emission center is C acceptor related. Furthermore, annealing Al0.52Ga0.48As samples in H2 flow eliminates the emission, while annealing in AsH3 flow increases the emission. Based on these results, we have considered the doping and V/III dependencies of various major point defects present in p-type GaAs and those of the combinations of the C acceptor and these defects. It is concluded that the DB band originates from the substitutional C–As antisite complex (CAs–AsGa). Mass-action rule analysis of the complex deduces a quadratic increase in the PL intensity with hole concentration, which generally explains the experimental results.
Diffraction of an acoustic plane wave by a rectangular plate
Authors: Hongo Kohei , Sugaya Hideyoshi
Published: July 2002

A scalar field diffracted by a rigid rectangular plate can be computed with the formula that Otsuki derived by applying the properties of Weber–Schafheitlin’s discontinuous integral (also referred to as the Kobayashi potential). This method may be regarded as giving an eigenfunction expansion of the wave functions of the present configuration, although these functions are expressed by double infinite integrals. Expansion coefficients are determined from the matrix equations, and the matrix elements are given by double infinite integrals. An algorithm effective for computing these integrals is developed here by using the asymptotic expansions for the integrands. As a result, the original integral reduces to a finite double integral plus two finite single integrals. The numerical results for some diffraction patterns are presented. With relatively large plates, the results agree well with those obtained from the Kirchhoff diffraction integral.
Electron distribution and capacitance–voltage characteristics of n-doped quantum wells
Authors: Moon C. R. , Choe Byung-Doo , Kwon S. D. , Shin H. K. , Lim H.
Published: July 2002

The effect of multiple quantum well (MQW) parameters on the free carrier distribution (FCD) and the apparent carrier distribution (ACD) obtained from capacitance–voltage (C–V) profiling has been investigated using a self-consistent simulation technique and the C–V profiling technique. The FCD in MQW structures reveals large outer peaks and small inner peaks even when all the parameters of each quantum wells (QWs) are the same. Such a feature becomes more apparent as the barrier layer becomes thinner or the barrier doping level becomes lower. These characteristics are found to originate from the fact that the density of carriers confined in each well is mostly determined by the depletion region formed alongside the well via the charge neutrality condition. The ACD is found to vary drastically as the thickness or the doping level of barrier changes. When the Debye averaging process is prominent, the ACD peaks are broader and smaller than the FCD peaks and are displaced toward the bottom layer side. The ACD inner peaks even disappear completely when the Debye screening length is comparable to or larger than the QW period, while real free carriers are well confined in each well. The effect of temperature on the ACD through the Debye averaging process is also investigated. When the Debye length is much smaller than the QW period, the full width at half maximum of the ACD is determined by the change of the position expectation value of the two-dimensional differential carriers. This change of position expectation value is found to be much smaller than the well width for relatively narrow QWs. The accuracy of our simulation results is confirmed by the excellent agreements between the simulated ACDs and the measured ACDs of In0.2Ga0.8As/GaAs MQWs. As an example of extracting the qualitative informations from the measured C–V profile, the C–V profiles of partially strain relaxed InxGa1−xAs/GaAs MQWs with x=0.15 and x=0.25, grown by metal organic chemical vapor deposition, are discussed. These results show systematically how the QW parameters affect the FCD and the ACD.
Thermal diffusivity measurement of solid materials by the pulsed photothermal displacement technique
Authors: Bennis G. L. , Vyas R. , Gupta R. , Ang S. , Brown W. D.
Published: July 2002

A simple, noncontact technique for the measurement of thermal diffusivity of solids is experimentally demonstrated. The technique is based on the photothermal displacement effect. Excellent agreement between the quasistatic theory of photothermal displacement and the experiment has been obtained. The technique has been demonstrated by measuring the thermal diffusivities of GaAs and InGaAs/AlGaAs multiple quantum wells.
On laser induced single bubble near a solid boundary: Contribution to the understanding of erosion phenomena
Authors: Isselin Jean-Christophe , Alloncle Anne-Patricia , Autric Michel
Published: July 2002

Cavitation erosion is an especially destructive and complex phenomenon. In order to understand its basic mechanism, the dynamics of laser-induced vapor bubbles have been investigated. Special experimental devices have been used to record ultrafast visualizations and pressure measurements. From these measurements, the different sources of stresses, induced on the solid wall by the presence of the bubble (shock wave, microjet), have been characterized. The “water hammer” pressure associated with the microjet velocity varies up to 210 MPa. When the bubble collapses near a solid wall, the pressure emitted is less than in an infinite medium. Pressure values up to 2.5 MPa have been found. These values have been associated with the duration of the pressure applied to the solid wall, which is about 30 ns for the microjet and more than 300 ns for the shock wave. These results have been correlated with the analysis of damage created on the surface sample.
Temperature dependence of the thermal diffusivity of GaAs in the 100–305 K range measured by the pulsed photothermal displacement technique
Authors: Soltanolkotabi M. , Bennis G. L. , Gupta R.
Published: July 2002

We have measured the variation of the value of the thermal diffusivity of semi-insulating GaAs in the 100–305 K range. The method used is the pulsed photothermal displacement technique. This is a noncontact technique, and the value of the thermal diffusivity is derived from the temporal evolution of the signal rather than its amplitude. This makes the technique less susceptible to uncertainties. We find that the temperature dependence of the thermal conductivity of semi-insulating GaAs follows a power law as T−1.62, in disagreement with results obtained previously. Possible reasons for the deviation within this very important intermediate temperature range are discussed.
Measurement of x-ray photogeneration in amorphous selenium
Authors: Blevis I. M. , Hunt D. C. , Rowlands J. A.
Published: July 2002

The use of low-mobility photoconductors in x-ray imaging, especially amorphous selenium (a-Se), is currently of great interest. Detailed measurements of the photoconversion gain factor W± and gain fluctuations σ have been obtained to facilitate applications using a-Se and to provide a better understanding of the charge formation process. The measurements were made with an x-ray spectrometer constructed using a-Se as the x-ray absorber. Pulse height spectra of known monoenergetic x-ray sources were measured to determine the response of the a-Se. Both the gain and fluctuations calculated from the data were found to depend on the x-ray energy ε, and the applied field E, indicating the recombination of a larger amount of charge that was created initially.
Initiation of high power microwave dielectric interface breakdown
Authors: Neuber A. , Hemmert D. , Krompholz H. , Hatfield L. , Kristiansen M.
Published: July 2002

A simple model of vacuum/dielectric/vacuum interface breakdown initiation caused by high power microwave has been developed. In contrast to already existing models, a spatially varying electron density normal to the interface surface has been introduced. Geometry and parameter ranges have been chosen close to the conditions of previously carried out experiments. Hence, physical mechanisms have become identifiable through a comparison with the already known experimental results. It is revealed that the magnetic field component of the microwave plays an important role. The directional dependence introduced by the magnetic field leads to a 25% higher positive surface charge buildup for breakdown at the interface downstream side as compared to the upstream side. This and the fact that electrons are, in the underlying geometry, generally pulled downstream favors the development of a saturated secondary electron avalanche or a saturated multipactor at the upstream side of the dielectric interface. The previously observed emission of low energy x-ray radiation from the interface is explained by bremsstrahlung generated by impacting electrons having initially a higher energy than the average emission energy. Final breakdown is believed to be triggered by electron induced outgassing or evaporation, generating a considerable gas density above the dielectric surface and eventually leading to a gaseous breakdown.
Longitudinal and lateral stress measurements in shock-loaded gabbro and granite
Authors: Millett J. C. F. , Tsembelis K. , Bourne N. K.
Published: July 2002

Plate impact experiments have been performed on two igneous rocks of different grain size. Shock stresses have been measured using embedded manganin stress gauges, up to ∼12 GPa. In the coarser grained material data was obtained by impacting rock flyer plates onto previously characterized targets. Results are compared and contrasted to the existing data for other geological materials. In the finer-grained material lateral stress was also measured. In combination with the longitudinal stresses, these results have been used to obtain the materials shear strength under shock loading conditions. Results suggest that the material is deforming in an inelastic manner.
Kerr microscopy observations of magnetization process in microfabricated ferromagnetic wires
Authors: Yokoyama Y. , Suzuki Y. , Yuasa S. , Ando K. , Shigeto K. , Shinjo T. , Gogol P. , Miltat J. , Thiaville A. , Ono T. , Kawagoe T.
Published: July 2002

The magnetization process in microfabricated NiFe wires was observed using a Kerr microscope. Magnetic wires were made from a 20-nm-thick NiFe film by using lift-off techniques. Their width W and length L were designed as W=0.5, 1.0 and 2.0 μm and L=50 μm, respectively. One end of the wire was connected to a square shaped head with a side of 2W, which is designed to act as a domain wall source. In each wire, necks with different width of 0.2W, 0.6W, and 0.8W were introduced as artificial pinning sites of a domain wall. By using an oil-immersion lens (NA=1.3) and a Hg lamp, magnetization reversals in very narrow wires, as narrow as 0.5 μm, were clearly observed. It is confirmed that domain wall penetration, pinning, depinning, and also the direction of wall motion are controllable using square shaped head and necks with optimized width.
Comparison of linewidth enhancement factors in midinfrared active region materials
Authors: Olesberg J. T. , Flatté Michael E. , Boggess Thomas F.
Published: July 2002

We report calculations of the linewidth enhancement factor for five midinfrared active region materials. The linewidth enhancement factors for two type-I quantum wells based on InAsSb are 2.5 and 5.4, which represent a reduction of up to a factor of 2.6 with respect to bulk InAs0.91Sb0.09. However, active region materials based on the type-II, InAs/GaInSb system have linewidth enhancement factors near 1.0, which is a factor of 2–5 reduction compared to the type-I quantum wells. The reduction of the linewidth enhancement factor is associated with both a reduction of the mismatch between the conduction and valence band densities of states and the presence of conduction band dispersion. We describe an additional optimization that is possible in the type-II materials: Carefully placed intersubband absorption features can be used to further reduce the linewidth enhancement factor. We show that linewidth enhancement values as low as 0.3 can be obtained in the type-II superlattices when fabricated into a distributed feedback structure.
Neutron reflectometry as a tool to study magnetism (invited)
Authors: Felcher G. P.
Published: July 2002

Polarized-neutron specular reflectometry (PNR) was developed in the 1980’s as a means of measuring magnetic depth profiles in flat films. Starting from simple profiles, and gradually solving structures of greater complexity, PNR has been used to observe or clarify a variety of magnetic phenomena. It has been used to measure the absolute magnetization of films of thickness not exceeding a few atomic planes, the penetration of magnetic fields in micron-thick superconductors, and the detailed magnetic coupling across nonmagnetic spacers in multilayers and superlattices. The development of new scattering techniques promises to enable the characterization of lateral magnetic structures. Retaining the depth sensitivity of specular reflectivity, off-specular reflectivity may be brought to resolve in-plane structures over nanometer to micron length scales.
Magnetic properties of single crystal Co2MnGe Heusler alloy films
Authors: Ambrose T. , Krebs J. J. , Prinz G. A.
Published: July 2002

Single crystal Co2MnGe Heusler alloy films were epitaxially grown on GaAs substrates by molecular beam epitaxy. The films have the bulk L21 crystal structure as determined by reflection high energy electron diffraction patterns and x-ray diffraction. A high saturation moment of 1006 emu/cm3 at room temperature has been measured. Ferromagnetic resonance of these ordered alloy films shows narrow linewidths and a small anisotropy. A room temperature resistivity of 115 μΩ cm and a 0.11% anisotropic magnetoresistance has also been determined.
Photoluminescence of InAs1−xSbx/AlSb single quantum wells: Transition from type-II to type-I band alignment
Authors: Yang M. J. , Bennett B. R. , Fatemi M. , Lin-Chung P. J. , Moore W. J. , Yang C. H.
Published: July 2002

Infrared photoluminescence has been used to study the band-gap energy of InAs1−xSbx digital superlattices and band alignment of InAs1−xSbx/AlSb quantum wells at 5 K. It is found that the InAs1−xSbx digital alloys have a smaller effective band gap than InAs1−xSbx random alloys. In addition, the valence band offset between type-II InAs/AlSb is determined to be 130 meV. This number reduces as the Sb mole fraction in InAs1−xSbx is increased, and the alignment between InAs1−xSbx/AlSb becomes type I when x>0.15.
Initial stages of metal/organic semiconductor interface formation
Authors: Hill I. G. , Mäkinen A. J. , Kafafi Z. H.
Published: July 2002

We have used photoelectron spectroscopies to study the metal/organic semiconductor interfaces formed by depositing three different metal–quinolate derivatives on Ag, with the goal of better understanding the initial stages of interface formation. We find very consistent results at all three interfaces, which exhibit strong energy level shifts within the first molecular layer, followed by a nearly “flat-band” condition. These results were analyzed in the context of the interface dipole and “band-bending” models. We conclude that the interface dipole model, extended to account for the differences in polarization screening in the first molecular layer, most accurately describes our findings. In this article we present the most thorough description of the early stages of metal/molecular organic semiconductor interfaces to date.
The strong metal–support interaction (SMSI) in Pt–TiO2 model catalysts. A new CO adsorption state on Pt–Ti atoms
Authors: Ocal Carmen , Ferrer Salvador
Published: July 2002

In order to investigate the microscopical aspects of the SMSI state in catalysts we performed photoemission experiments on samples consisting of submonolayer Pt deposits on a Ti(0001) surface and on a TiO2 thin film. Annealing Pt/TiO2 samples at ∼500 °C caused a change in the electronic structure of Pt atoms which was reflected in a narrowing of the Pt 4f7/2 XPS line. The width of this line became smaller than the corresponding one for a bulk Pt sample. This was interpreted as the result of an electron transfer between Pt and Ti atoms that we designed for brevity as alloying. CO adsorption on alloyed Pt–Ti exhibited in Pt/TiO2 samples an unusual C1s binding energy of 291.3 eV. The uptake of CO per Pt atom in Pt/TiO2 samples was reduced, by a factor of 2 when CO was adsorbed on surfaces previously annealed at 500 °C which is the temperature for the SMSI state in Pt/TiO2 catalysts.