bluuebunny/crossref_metadata_2025 huggingface dataset:
AlGaAs/InGaAs/AlGaAs double pulse doped pseudomorphic high electron mobility transistor structures on InGaAs substrates
Authors: Hoke W. E. , Lyman P. S. , Mosca J. J. , McTaggart R. A. , Lemonias P. J. , Beaudoin R. M. , Torabi A. , Bonner W. A. , Lent B. , Chou L.-J. , Hsieh K. C.
Published: July 2002

Double pulse doped AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) structures have been grown on InxGa1−xAs (x=0.025–0.07) substrates using molecular beam epitaxy. A strain compensated, AlGaInAs/GaAs superlattice was used for improved resistivity and breakdown. Excellent electrical and optical properties were obtained for 110-Å-thick InGaAs channel layers with indium concentrations up to 31%. A room temperature mobility of 6860 cm2/V s with 77 K sheet density of 4.0×1012 cm−2 was achieved. The InGaAs channel photoluminescence intensity was equivalent to an analogous structure on a GaAs substrate. To reduce strain PHEMT structures with a composite InGaP/AlGaAs Schottky layer were also grown. The structures also exhibited excellent electrical and optical properties. Transmission electron micrographs showed planar channel interfaces for highly strained In0.30Ga0.70As channel layers.
Concentration dependent photoluminescence of Te-doped In0.5Ga0.5P layers grown by liquid phase epitaxy
Authors: Yoon I. T. , Ji T. S. , Oh S. J. , Choi J. C. , Park H. L.
Published: July 2002

Photoluminescence (PL) of Te-doped In0.5Ga0.5P epilayers grown by the liquid phase epitaxy technique has been investigated as a function of carrier concentration. The PL results are interpreted using a model taking into account nonparabolicity of the conduction band. Both the band filling as well as band tailing due to the Coulomb interaction of free carriers with ionized impurities and band shrinkage due to the exchange interaction between free carriers are considered in order to properly portray the observed features of the PL spectra. The theoretical calculations are in satisfactory agreement with the observed PL results. The PL line shape is well explained by a direct transition with a simple k-selection rule up to a carrier concentration of 2.0 ×1018 cm−3. Above the carrier concentration of 2.0 ×1018 cm−3, on the other hand, it is properly interpreted in terms of non-k-conserving transitions that arise from the indirect recombination of electrons in a highly filled conduction band. It was found that a concentration dependent gap shrinkage due to the exchange interaction in Te-doped In0.5Ga0.5P at 17 K is described by the relation Ece=2.34×10−8 n1/3 (eV). The concentration dependent effective mass has also been calculated using Kane’s three band model.
A micro-Raman investigation of the SCS-6 SiC fiber
Authors: Kim Jeongyong , Tlali Spirit , Jackson Howard E. , Webb James E. , Singh Raj N.
Published: July 2002

The microstructure of the SCS-6 SiC fiber, a fiber widely studied as a reinforcement in metal matrix and ceramic matrix composites, was characterized by spatially resolved Raman spectroscopy. Cross sections of as-received fibers and fibers annealed at 1640 °C were characterized in the backscattering geometry with the 514.5 nm line of an Ar+ laser. Characteristic graphitic peaks were observed in the several regions of the SCS-6 fiber, including the inner SiC region. The SiC transverse optical phonon mode was observed in the outer SiC region as a double peak, suggesting the presence of multiple polytypes in this region. Spectra of the outer SiC layer of this fiber in the zircon-matrix composite also showed a distinct Si peak. Silicon in this fiber is believed to be responsible for the observed anomalous thermal expansion behavior of the fiber between 1300 and 1400 °C.
Dependence of the third-order coefficients in Landau free energies for bcc→fcc structural transition on hydrogen concentration in zirconium hydrides
Authors: Ashida Yuh , Yamamoto Masahiro , Naito Shizuo , Mabuchi Mahito , Hashino Tomoyasu
Published: July 2002

Young’s modulus E and the modulus of rigidity G of zirconium hydrides ZrHx(0.9⩽x⩽1.65) at 941 and 1001 K have been obtained as a function of hydrogen concentration c by measuring resonance frequencies for bending and torsion vibrations of a polycrystalline wire. As c increases, observed E and G increase in the bcc β phase, slightly increase linearly in the β+δ phase, and then decrease in the fcc δ phase. On the basis of a phenomenological free energy in terms of strain components taking account of space group symmetry, two types of Landau expansion of the free energies for the β phase in terms of the strain components which play an important role in the structural phase transition between the β and the δ phases are examined. The observed E and G are assumed to be the same as the second-order coefficients of the free energy for the Bain distortions, which occur at the structural phase transition. The dependence of the third-order coefficients on c permits the expanded free energies to describe the fact that the β phase is more stable than the δ phase at low c.
K–Te photocathodes: A new electron source for photoinjectors
Authors: Bisero D. , van Oerle B. M. , Ernst G. J. , Verschuur J. W. J. , Witteman W. J.
Published: July 2002

K–Te photocathodes deposited on a Mo substrate have been successfully used as an electron source in the free electron laser of University of Twente. Long lifetimes have been measured: after more than 20 h of operation in the accelerator a K–Te cathode with 4.75% initial quantum efficiency still displays a 1.1% quantum efficiency at 259 nm. Moreover, the quantum efficiency of this cathode versus operation time can be fitted by an exponential decay curve, which saturates asymptotically to a 1.03% value, suggesting that a quantum efficiency close to 1% could be sustained for very long operation times. Films degraded by use can be recovered to a quantum efficiency which is close to the initial value, by heating the substrate at temperatures between 100 and 330 °C. A new procedure to obtain K–Te cathodes with high (up to 11%) quantum efficiencies is described.
Shift of surface Fermi level position toward the conduction band minimum by crystal defects near GaAs(001) surface
Authors: Hirota Y. , Maeda F. , Watanabe Y. , Ogino T.
Published: July 2002

The effects of crystal defects near the surface on the position of surface Fermi level (EFS) are investigated using photoluminescence (PL) measurements and synchrotron radiation photoelectron spectroscopy (SRPES). For the lightly Si-doped GaAs(001) surface, PL measurements reveal that after heating to 500 °C a layer with lower PL peak intensities related to gallium vacancies than those of the bulk exists just under the thermal degraded layer. SRPES shows that EFS moves upward to 1.1–1.17 eV above the valence band maximum when this thermal degraded layer is removed by chemical etching and the excess arsenic on the surface, which is formed by rinsing the etched surface with deoxygenated and deionized water, is evaporated by heating in ultrahigh vacuum (UHV). After evaporation of excess arsenic on the surface by heating, the etching-depth dependence of EFS for a sample preheated in UHV correlated with the existence of this defect concentration layer. These results suggest that the position of EFS for the GaAs(001) surface is strongly influenced by crystal defects near the surface.
X-ray studies of defects and thermal vibrations in an organometallic vapor phase epitaxy grown GaN thin film
Authors: Xiong X. , Moss S. C.
Published: July 2002

A semiconducting GaN thin film with the 001 plane parallel to the surface grown by organometallic vapor phase epitaxy method on (110) sapphire was studied using x-ray diffraction. The line profiles of the GaN thin film along the [001] direction can be quantitatively reproduced assuming a strained lattice at the interface. The deformation and growth faults were determined to be equal and each is 0.2%. Least-squares refinement on 42 independent peaks, after correcting for the first-order thermal diffuse scattering, gives the values of the Debye–Waller factor for Ga (B11=0.28, B33=0.26) and N (B11=0.38, B33=0.26) atoms. The wurtzite positional parameter u for this GaN thin film was found to be 0.3730, 1% smaller than that in a strain-free single crystal (u=0.377), most probably resulting from the strain effects.
Al/Ti
x
W
1−x
metal/diffusion-barrier bilayers: Interfacial reaction pathways and kinetics during annealing
Authors: Bergstrom D. B. , Petrov I. , Greene J. E.
Published: July 2002

Polycrystalline bcc TixW1−x layers with mixed 011 and 002 texture were grown on oxidized Si(001) substrates at 600 °C by ultrahigh-vacuum (UHV) magnetron sputter deposition from W and Ti0.33W0.67 targets using both pure Ar and Xe discharges. Ti concentrations in the 100-nm-thick layers were 0, 6, and 33 at. % depending on target composition and sputtering gas. Al overlayers, 190 nm, thick with strong 111 preferred orientation, were then deposited in Ar at 100 °C with and without breaking vacuum. Changes in bilayer sheet resistance Rs were monitored as a function of time ta and temperature Ta during subsequent UHV annealing. Thermal ramping of Al/W and Al/Ti0.06W0.94 bilayers at 3 °C min−1 resulted in large (>fourfold) increases in Rs at Ta≃550 °C, whereas Rs in the Al/Ti0.33W0.67 bilayers did not exhibit a similar increase until ≃610 °C. Area-averaged and local interfacial reactions and microstructural changes were also followed as a function of annealing conditions. The combined results indicate that Al/W and Al/Ti0.06W0.94 bilayer reactions proceed along a very similar pathway in which monoclinic WAl4 forms first as a discontinuous interfacial phase followed by the nucleation of bcc WAl12 whose growth is limited by the rate of W diffusion, with an activation energy of 2.7 eV, into Al. In contrast, the W diffusion rate during the early stages of Al/Ti0.33W0.67 annealing is significantly higher allowing the formation of a continuous WAl4 interfacial blocking layer which increases the overall activation energy Ea, still limited by W diffusion, to 3.4 eV and strongly inhibits further reaction. We attribute observed increases in WAl4 nucleation and growth rates in interfacial Al/Ti0.33W0.67 to a “vacancy wind” effect associated with the very rapid (Ea=1.7 eV) diffusion of Ti into Al.
Use of metal-oxide-semiconductor capacitors in the analysis of low-temperature epitaxial Si films deposited by remote plasma-enhanced chemical vapor deposition
Authors: Sharma R. , Fretwell J. L. , Doris B. , Banerjee S.
Published: July 2002

This article discusses the electrical characterization of low-temperature intrinsic Si films deposited by remote plasma-enhanced chemical vapor deposition. Metal-oxide-semiconductor (MOS) capacitors were fabricated on films deposited over a range of temperatures. Conventional MOS measurements such as capacitance versus voltage, breakdown voltage, Zerbst plot, and charge-to-breakdown were used to analyze the capacitors. The results of these measurements not only yielded information about the electrical properties of the films, but also led to conclusions regarding structural quality and the presence of metal contamination. This, coupled with the fact that capacitor fabrication requires only a simple, moderate-thermal budget process, makes MOS capacitor measurements an attractive technique for the characterization of low temperature epitaxial Si films.
Thermoelectric power properties of graphitic nanotubule bundles
Authors: Tian Mingliang , Chen Lin , Li Fanqing , Wang Ruiping , Mao Zhiqiang , Zhang Yuheng , Sekine Hisashi
Published: July 2002

Thermoelectric power (TEP) properties of graphite nanotubule bundles were measured in the range 80–280 K. It was found that the TEP is positive and the magnitude at 280 K reaches about an order of +15 μV/K, far larger than that in highly oriented pyrolytic graphite. Moreover, in the studied range, the TEP can be approximately described by the formula S(μV)=0.167T−(70.2+0.085T)e−302.5/T derived based on a two-band model. The experimental results support such an idea that in the buckybundles both kinds of nanotubes, i.e., metallic tubes with a highly mobile velocity and semiconductive tubes with a narrow energy gap are included. The Fermi energy of the valence band for the metallic tubes is about −0.22 eV, and the average effective energy gap of the semiconductive tubes is estimated at about 52.2 meV. This conclusion is in good agreement with the theoretical predictions.
Transport properties and electroluminescence of siloxene
Authors: Rosenbauer M. , Stutzmann M.
Published: July 2002

The transport and electroluminescence properties of siloxene (Si6O3H6) in the as-prepared and annealed states are investigated. As-prepared siloxene is found to show a low dark conductivity (≈10−19 Ω cm−1) at room temperature which is thermally activated with an activation energy that is approximately half of the optical gap of siloxene (≈2.6 eV). The presence of ambient gases can increase the dark conductivity due to intercalation of gas molecules between the two-dimensional siloxene polymer sheets. The photoconductivity of siloxene is investigated with respect to its spectral and temperature dependences. Electroluminescence of siloxene is found for sufficiently high current levels.
Physical characteristics of band-gap engineered, photovoltaic detectors
Authors: Newman A. K. , Liu J. M.
Published: July 2002

A comprehensive understanding of the physics of a graded band-gap, photovoltaic detector has been achieved through a full scale finite difference simulation of the semiconductor equations and the Poisson equation in time and space. The results show that three characteristic times, Maxwell’s dielectric relaxation time, the electron transit time across a graded device period, and the minority electron lifetime, govern the transient response of the device completely. Varying device parameters to control these characteristic times will enable tailoring of device structures to optimize responsivities for applications requiring picosecond to nanosecond response speeds.
Highly irregular stacking structure in r-BN pressed up to 7.7 GPa at room temperature
Authors: He L. L. , Taniguchi T. , Sato T. , Horiuchi S.
Published: July 2002

The structural evolution in rhombohedral-type boron nitride (r-BN) pressed up to 7.7 GPa at room temperature is examined before and after pressing using high-resolution transmission electron microscopy. In the starting state, r-BN is in a platelike form with some folding. The plates consist of an almost perfect 3R structure. After pressing, a large number of stacking faults is formed and causes mixing of 3R and 2H in a few areas and a highly irregular stacking structure in most areas. This means that the coherent slip due to shearing between sp2 sheets is the dominant deformation mechanism in r-BN under high pressure at room temperature. All of these structures cause a broadening of the x-ray diffraction peaks. In some plates, the 2H structure is found in a relatively large area. It has a hexagonal lattice with the stacking sequence of …ABAB… and lattice constants of a=0.25 nm and c=0.67 nm. The structural evolution in r-BN revealed after pressing is in contrast to that in hexagonal-type BN, in which twinning is the dominant deformation mechanism that introduces the folding of plates under high pressure at high temperature.
Thresholdlike behavior of photoluminescence in laser heterostructure wafers
Authors: Grinberg A. A. , Sputz S. K.
Published: July 2002

Thresholdlike behavior of photoluminescence (PL) in heterostructure wafers is studied. It is shown that strictly speaking there is no PL threshold, and the thresholdlike dependence of PL on the pump power results from the combination of three factors: the PL spreading along the wafer surface, the change in the radiative fraction of electron-hole recombination, and the restricted aperture of the PL detector. The first two factors were found to be dominating in the wafers studied.
Effect of Cl incorporation on the performance of amorphous silicon thin film transistors
Authors: Choi Jong Hyun , Kim Chang Soo , Kim Sung Ki , Jang Jin
Published: July 2002

We have studied the effect of Cl incorporation on the performance of amorphous silicon thin film transistors (a-Si:H(:Cl) TFTs). The off-state leakage current of a-Si:H(:Cl) TFTs under light illumination is much lower than that of a-Si:H TFTs, because the photoconductivity of a-Si:H(:Cl) is much lower than that of conventional a-Si:H. The a-Si:H(:Cl) films deposited between [SiH2Cl2]/[SiH4]=0.04 and [SiH2Cl2]/[SiH4]=0.12 show p-type conduction, leading to the much lower photoconductivity. The TFT using a-Si:H(:Cl) deposited with [SiH2Cl2]/[SiH4]=0.04 exhibits a field effect mobility of 0.41 cm2/V s and a threshold voltage of 5.56 V; however the off-state leakage current under light illumination is two orders of magnitude smaller than that of a conventional a-Si:H TFT.
Ultraviolet laser light scattering diagnostics of the plume in pulsed-laser deposition process
Authors: Okada Tatsuo , Nakata Yoshiki , Maeda Mitsuo , Kumuduni W. K. A.
Published: July 2002

In order to investigate the gas phase condensation in the pulsed-laser deposition (PLD) process, an ultraviolet (uv) laser light scattering system has been developed and applied for the diagnostics of laser-ablated plumes during the pulsed-laser deposition process of high-Tc thin film fabrications. Time-of-flight measurements were carried out from 1 μs to 1 s after ablation. Rayleigh-scattered signals from the condensing particles have clearly shown that the condensation occurs in the gas phase under typical PLD conditions. The condensation was noticeable at a large delay time of 10 ms after ablation. At a shorter delay time of less than 500 μs, laser-induced fluorescence signals from molecules, which were accidentally excited by the uv laser, were also observed.
Persistent spectral hole burning of Eu3+ ions in sodium aluminosilicate glasses
Authors: Fujita Koji , Hirao Kazuyuki , Tanaka Katsuhisa , Soga Naohiro , Sasaki Hiroko
Published: July 2002

Hole burning spectroscopy has been carried out for glasses with compositions of (75−x)SiO2⋅xAlO3/2⋅25NaO1/2⋅1.0EuO3/2 with several aluminum oxide contents. Persistent hole was burned in the D05−F07 transition of Eu3+ at temperatures between 4.2 and 20 K in aluminosilicate glasses, while the hole burning was not observed above 4.2 K in silicate glass. In other words, an introduction of Al3+ ions in silicate glass promotes the formation of a stable hole. Hole spectra without any antiholes suggest that the holes are burned by an optically induced rearrangement of local structure around the Eu3+ ion. The homogeneous linewidth shows a linear dependence with T between 4.2 and 8 K and a nearly quadratic dependence for T>8 K.
Waterfall lasers
Authors: Casperson Lee W.
Published: July 2002

Laser concepts can be applied to a broad range of physical phenomena. One of the closest parallels occurs with the fluttering oscillations that are sometimes observed in the falling sheets of water associated with fountains, dams, and natural waterfalls. In many respects these fluid feedback oscillations are similar to the electromagnetic modes of typical lasers, and recognition of this similarity led to the interpretation of the waterfall behavior. Gain profiles for the waterfall oscillations are developed, and the relationship of experimental waterfall data to the laser-like models is considered in detail.
Depth distribution of silicon-ion induced defects in crystalline silicon
Authors: Chavan S. T. , Dhole S. D. , Bhoraskar V. N. , Kanjilal D. , Mehta G. K.
Published: July 2002

In crystalline silicon, irradiated with 60 and 80 MeV energy silicon ions, regions of high defect densities were located by repeatedly etching the sample and measuring the lifetime of minority carriers, τ. The values of τ  before etching were 10 and 8 μs in the 60 and 80 MeV ion irradiated sides, respectively, and 19 μs in the unirradiated side. On etching the samples, these values of the irradiated side initially increased slowly but later on, as the total thickness of the etched silicon approached the ion ranges, they increased steeply. However, the value of τ in the unirradiated side remained unchanged even after etching. These results show that the regions of high defect densities are situated below the surface at depths of 20 and 27 μm, respectively, in 60 and 80 MeV ion irradiated samples. These coincide, respectively, with the projected depth in silicon at which the 60 and 80 MeV silicon ions deposit a maximum amount of energy through nuclear collisions.
Analysis of abnormal x-ray diffraction peak broadening from InGaAs/GaAs multiple quantum wells
Authors: Kim In , Choe Byung-Doo , Park Sang Koo , Jeong Weon Guk
Published: July 2002

Strain relaxation behavior in In0.2Ga0.8As/GaAs multiple quantum well (MQW) structures with various barrier thicknesses but with fixed well thickness was characterized by analyzing the x-ray rocking curves and low temperature photoluminescence. For the thick-barrier samples which have good optical characteristics, it is observed that the superlattice x-ray peaks are broadened nonuniformly. This anomalous behavior can be successfully simulated by considering the paired dislocation character in the double-kink type strain relaxation. Meanwhile, the thin-barrier ones show deteriorated optical properties which can be attributed to dislocations located at the MQW-substrate interface. This observation shows that two different relaxation mechanisms are working for the thick- and thin-barrier cases, respectively.
Structural defects in Hg1−xCdxI2 layers grown on CdTe substrates by vapor phase epitaxy
Authors: Sochinskii N. V. , Muñoz V. , Espeso J. I. , Baruchel J. , Marı́n C. , Diéguez E.
Published: July 2002

Hg 1−x Cd x I 2 20–25-μm-thick layers with a uniform composition in the range of x=0.1–0.2 were grown on CdTe substrates by vapor phase epitaxy (VPE). The growth was carried out using an α-HgI2 polycrystalline source at 200 °C and in the time range of 30–100 h. The layers were studied by scanning electron microscopy (SEM) and high resolution synchrotron x-ray topography (SXRT). The SEM and SXRT images of Hg1−xCdxI2 VPE layers allow one to identify the defects affecting the layer structure. The two main types of structural defects in the layers are subgrain boundaries and densely spaced striations similar to those referred generally to as vapor grown HgI2 bulk crystals. The effect of the growth time on these defects has been analyzed and on the basis of this it has been possible to grow Hg1−xCdxI2 layers with low defect density.
A study of poly(p-phenylenevinylene) and its derivatives using x-ray photoelectron spectroscopy
Authors: Lee H. G. , Kim S. , Hwang C. , Choong V. , Park Y. , Gao Y. , Hsieh B. R.
Published: July 2002

Poly(p-phenylenevinylene) and its derivatives (DP-PPV and labeled R-7) were carefully studied using x-ray photoelectron spectroscopy. The C 1s peaks taken from spin-coated thin films of these materials were fitted with the consideration of the bonding state differences (–C=CH and C=C–, etc). The intensity of these C 1s peaks agrees well with the value estimated from the bonding state consideration.
Crystal-temperature dependence of picosecond two-beam coupling gains in semi-insulating semiconductors
Authors: Kusakabe Kazuhide , Tomita Yasuo
Published: July 2002

Temperature dependences of time-resolved picosecond beam coupling gains and losses in semi-insulating undoped GaAs and CdTe:Ga are investigated experimentally at a wavelength of 1.064 μm. In the temperature range of 283–363 K a weak temperature dependence of the free-carrier energy transfer gain is observed at high fluence excitation, while photorefractive beam coupling gains and nonlinear absorption losses are found to exhibit no clear temperature dependence. A numerical simulation of the dynamics of photoexcited carriers and space-charge fields is also performed in order to examine the experimental results.
Strains in HgTe/Hg0.1Cd0.9Te superlattices grown on (211)B Cd0.96Zn0.04Te substrates
Authors: Li M. , Gall R. , Becker C. R. , Gerhard T. , Faschinger W. , Landwehr G.
Published: July 2002

Strains in HgTe/Hg0.1Cd0.9Te superlattices grown on (211)B Cd0.96Zn0.04Te substrates have been investigated by high-resolution x-ray diffraction. The lattice mismatch, the tensile as well as the shear strain have been obtained by measuring symmetric and asymmetric diffraction profiles in different azimuths. These measured strain parameters are then used to extract from the diffraction profiles the chemical composition and thickness of individual layers constituting the superlattice period. The analysis is based on the theory of elasticity, in which the strain tensor components in partially relaxed epitaxial layers are calculated by minimizing the strain energy density. The method presented in this article is valid for the strain analysis of partially relaxed epitaxial layers grown on arbitrarily oriented surfaces.
Response to “Comment on ‘Quantum well infrared detectors’ ” [J. Appl. Phys. 81, 7076 (1997)]
Authors: Levine B. F.
Published: July 2002

It is shown that a previous comment on quantum well infrared photodetectors is not correct.
Evidence of a temperature induced enhancement of giant magnetoresistance in Fe1−xCox/Cr superlattices
Authors: Yu C. T. , Westerholt K. , Theis-Bröhl K. , Zabel H.
Published: July 2002

We have studied the giant magnetoresistance (GMR) of antiferromagnetically coupled Fe1−xCox/Cr superlattices in the temperature range between 77 K and 296 K. We find that the field induced net change of the magnetoresistance (ΔR) increases with increasing temperature. This unusual temperature dependence of the GMR effect can be modeled by a positive linear temperature dependent term together with a negative quadratic temperature dependent term. The temperature induced GMR enhancement is attributed to inelastic phonon scattering, which becomes spin-dependent in a system with strong spin asymmetry of the electronic band structure.
Coupled electron and nonequilibrium optical phonon transport in a GaAs quantum well
Authors: Paulavičius G. , Mitin V. V. , Bannov N. A.
Published: July 2002

The self-consistent Monte Carlo technique has been used to solve coupled nonlinear kinetic equations for electrons and optical phonons confined in a GaAs quantum well. We have studied the influence of nonequilibrium phonons on quasi-two-dimensional electron transport for a lattice temperature of 30 K and for a wide range of applied electric fields. A substantial difference in generation and decay times as well as the confinement inside the GaAs/AlAs heterostructure-bounded active region lead to a significant growth of nonequilibrium optical-phonon population generated by a heated electron gas. We have found that when the phonon generation (as well as phonon reabsorption by the quasi-two-dimensional carriers) becomes significant, there are substantial effects on transport in the quantum well. We show that for low electron concentrations, the hot optical-phonon distribution reflects the main features of the carrier distribution; indeed, it preserves an average quasi-momentum in the forward (opposite to electric field) direction. However, hot-phonon feedback to the electron system is found to be not essential in this case. For high electron concentrations, enhanced nonequilibrium optical-phonon reabsorption results in phonon distribution which spreads significantly in the quasi-momentum space and essentially loses the characteristic of the forward-peaked anisotropy. The interactions with the confined electron subsystem typically result in an isotropic phonon distribution. In this case, nonequilibrium optical phonons lead to an increase in the mean electron energy and a reduction in the carrier drift velocity.
Fracture toughness estimation of thin chemical vapor deposition diamond films based on the spontaneous fracture behavior on quartz glass substrates
Authors: Kamiya Shoji , Sato Masaki , Saka Masumi , Abé Hiroyuki
Published: July 2002

This article is on the experimental estimation of the fracture toughness of thin diamond film deposited by the microwave plasma chemical vapor deposition method on a quartz glass substrate. Because of their differences in the coefficient of thermal expansion, diamond films on a quartz glass substrate suffer tensile stress at room temperature and show various kinds of spontaneous fracture behavior, reflecting the mechanical properties of the films. On the basis of detailed observation of cracking patterns and also measuring the residual stress with the aid of Raman spectroscopy, the fracture toughness of the film having thickness of around 1 μm has been estimated here satisfactorily without the help of any difficult microscopic experiment. The fracture toughness of the film of thickness 0.35 μm is found to be around half of that obtained with much thicker films.
Transport properties of Ni–WS2 photoconductive thin films
Authors: Lignier O. , Couturier G. , Salardenne J.
Published: July 2002

It is shown that the annealing under Ar of sputtered WSx amorphous films deposited on Ni coated substrates gives bidimensional polycrystalline 2H–WS2 films. Ni enhances the formation of large crystallites. The temperature dependence of the mobility and its dependence versus the Ni content clearly show that transport properties are governed by grain boundaries. A basic grain boundary model like the model of Seto is well suited to explain the electrical properties. The photoconductivity enhancement resulting from nickel is assigned to a decrease of the number of the electrical barriers. However, noise measurements are not consistent with Hall measurements; a model is thus proposed to explain the 1/f noise excess in relation to the traces of Ni–W revealed by x-ray diffraction.
Magnetization of an epitaxial YBa2Cu3O7−δ film at the critical field sweep rate
Authors: Ji Z. M. , Wu R. X. , Zhou G. D. , Zuo J. P. , Zhou J. , Wang H. B. , Xu W. W. , Cheng Q. H. , Yang S. Z. , Wu P. H. , Geng J. F. , Jin X.
Published: July 2002

We have measured the magnetic hysteresis loops of an epitaxial YBa2Cu3O7−δ thin film using our recently developed device which can provide the field magnitude in the range of 0–1000 Oe and the field sweep rate up to 107 Oe/s. The shape of the hysteresis loop measured changes with the field-sweep rate up to the critical sweep rate; and over the critical sweep rate the ac magnetization reaches its real critical state where magnetization does not change even with a further increase in the field-sweep rate. The critical sweep rate is about 106 Oe/s at 77 K. With the hysteresis loops and Bean model, we have calculated the magnetization critical current density (Jcac) which is consistent with that obtained by I–V measurements. We have also studied flux motion and activation energy under the high sweep rate magnetic field. At temperature 77 K, the velocity of the flux motion is of the order 10 m/s and the pinning energy U0/k is about 339 K which is much smaller than the magnetization decay measurement.
The response of high-Tc SQUID magnetometers to small changes in temperature
Authors: Milliken F. P. , Koch R. H. , Brown S. L. , Altman R. A. , Gallagher W. J. , Haupt S. G. , Lathrop D. K.
Published: July 2002

We have investigated the response of the flux-locked output of several high-Tc SQUID magnetometers to small changes in temperature and for magnetic fields 0–30 μT. The temperature response DT≡dΦS/dT is observed to be linear in the applied magnetic field Ba and can be as large as 500 mΦ0/K where ΦS is the flux through the SQUID loop and Φ0 is the flux quantum. Our measurements can be explained using a simple model that takes into account the geometry of a given device and is based on the idea that DT is due to the temperature dependence of the superconducting penetration depth. Our results can be used to optimize device performance in applications where the noise of a device is dominated by ambient temperature fluctuations.
Water degradation of a- and c-axis oriented HgBa2CaCu2Ox superconducting thin films
Authors: Yun S. H. , Karlsson U. O.
Published: July 2002

Degradation of HgBa2CaCu2Ox superconducting thin films with a- and c-axis oriented normal to the substrates in water has been studied with electrical measurement, x-ray diffractometry, scanning electron microscopy, and energy dispersive x-ray spectrometry. We observed the different degradation behavior of a- and c-axis oriented HgBa2CaCu2Ox thin films normal to substrates in water immersion experiments. The a-axis oriented films are much more stable than the c-axis oriented films in water. After a 60 min immersion, the zero-resistance temperature of the a-axis oriented film is still above 110 K, but the zero resistance is not observed in the c-axis oriented film in water even for 5 min. The intensity of (00l) peaks is suppressed at one order of magnitude faster than that of (l00) peaks as the total immersion time increase.
Synthesis of oriented thin films of ferroelectric SbNbO4 on Si by pulsed laser ablation
Authors: Chattopadhyay Soma , Ayyub Pushan , Multani Manu , Pinto R.
Published: July 2002

The stibiotantalite family includes a number of ferroelectrics and antiferroelectrics with excellent potential for applications. We report the deposition of phase-pure, oriented thin films of SbNbO4 on Si(100) substrates using pulsed laser ablation. The deposition conditions were optimized with respect to substrate temperature, laser parameters, and the ambient oxygen pressure. The films were characterized by x-ray diffraction and electron microprobe analysis, while their microstructure was studied by atomic force microscopy and scanning electron microscopy. Dielectric studies indicated that these films are ferroelectric with a TC≅500 °C and εmax≅200.
Ultraclean etching of GaAs by HCl gas and in situ overgrowth of (Al)GaAs by molecular beam epitaxy
Authors: Kadoya Y. , Yoshida T. , Noge H. , Sakaki H.
Published: July 2002

Etching of GaAs by HCl gas has been systematically studied with emphasis on the quality of the heterointerfaces formed by the direct overgrowth of AlGaAs on the etched GaAs surfaces in an ultrahigh vacuum processing system. The interfaces were characterized by the transport properties of the two-dimensional electron gas (2DEG) and the photoluminescence from the quantum wells, both of which were formed at the interfaces. We show that the interfaces prepared by the etching and overgrowth at the molecular beam epitaxy-grown clean GaAs surfaces can be quite clean and almost damage-free so that high mobility 2DEG can be formed. We also report the properties of the interfaces prepared by the etching and overgrowth at the air-exposed GaAs surfaces, to show that the impurities adsorbed during the air exposure are efficiently removed and 2DEG can be formed at the interfaces.
The determination of phases formed in AlSiCu/TiN/Ti contact metallization structure of integrated circuits by x-ray diffraction
Authors: Fortin V. , Gagnon G. , Caron M. , Gujrathi S. C. , Currie J. F. , Ouellet L. , Tremblay Y. , Biberger M.
Published: July 2002

X-ray diffraction (XRD) analysis was performed in order to determine the crystallographic phases formed in AlSiCu/TiN/Ti contact metallization multilayers of very large scale integration/ultralarge scale integration devices deposited over SiO2 and Si substrates as a function of the annealing temperatures, the oxidation treatment of the TiN diffusion barrier, the presence of a TiN antireflective coating (ARC) and the barrier thickness. The most striking results of this study are the formation of the Ti7Al5Si12 phase and the recrystallization of Al after a 550 °C annealing for a nonoxidized 50 nm TiN barrier deposited on Si substrate. This Ti7Al5Si12 phase formation and the Al recrystallization tend to be blocked when an oxidized TiN barrier is used. It is also suggested that an air break process tends to lower the junction spiking problems since Ti7Al5Si12 is believed to be detrimental to the contact metallization layers. These effects were very weak for the 95-nm-thick TiN barrier. In addition, a TiN-ARC layer reduces the Al recrystallization on SiO2 substrate. Even after an important Ti7Al5Si12 formation, the transmission electron microscopy results showed that a TiN layer is still present but probably ineffective from an electrical point of view. Although the 550 °C anneal produced important changes, no significant difference was observed between the XRD spectra after a 450 and a 500 °C anneal. The identified phases by the XRD analysis were in agreement with a calculated Al-Ti-O-N quaternary isotherm diagram.
Self-similarity and coarsening of three dimensional particles on a one or two dimensional matrix
Authors: Viñals Jorge , Mullins W. W.
Published: July 2002

We examine the validity of the hypothesis of self-similarity in systems coarsening under the driving force of interface energy reduction in which three dimensional particles are intersected by a one or two dimensional diffusion matrix. In both cases, solute fluxes onto the surface of the particles, assumed spherical, depend on both particle radius and interparticle distance. We argue that overall mass conservation requires independent scalings for particle sizes and interparticle distances under magnification of the structure, and predict power law growth for the average particle size in the case of a one dimensional matrix (3D/1D), and a weak breakdown of self-similarity in the two dimensional case (3D/2D). Numerical calculations confirm our predictions regarding self-similarity and power law growth of average particle size with an exponent 1/7 for the 3D/1D case, and provide evidence for the existence of logarithmic factors in the laws of boundary motion for the 3D/2D case. The latter indicate a weak breakdown of self-similarity.
Interlayer exchange coupling: Effect of magnetic and outer nonmagnetic films
Authors: Zhang Xiangdong , Li Lie-Ming , Zhan Yiehong , Li Bo-Zang , Pu Fu-Cho
Published: July 2002

Based on the one-band tight-binding model, we systematically investigate the interlayer exchange coupling and the angular dependence of the coupling energy in a magnetic sandwich covered on both sides by nonmagnetic films. Our results show that (i) the thickness of magnetic and outer nonmagnetic films influence significantly the oscillatory behavior of exchange coupling, (ii) the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers, (iii) the nonoscillatory component of the coupling varies generally with the thickness of magnetic or outer nonmagnetic films, and (iv) the results in the case where the thickness of both magnetic or both outer nonmagnetic films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic or outer nonmagnetic films is fixed while the other is varied. These results are qualitatively in agreement with the experimental measurements.
Open-tube solid-state diffusion of Zn into n-type GaAs0.35P0.65 from ZnO oxide films and electroluminescence
Authors: Park Choon Bae , Kim Keunjoo
Published: July 2002

A thermal annealing method was utilized in the diffusion process of Zn in the formation of the p-type layer on GaAs0.35P0.65 films. The thermal diffusion of Zn from the ZnO source film on the n-type substrate provides for the formation of a p-type layer and shows a stable signal of electroluminescence. The Zn diffusion mechanism was investigated by introducing two distinct processes of diffusion and reaction into the kick-out model involving Ga Frenkel defects at the diffusion front. The interstitial diffusion process requires an activation energy of 5.3 eV which is much larger than the substitutional reaction energy of 3.5 eV. The Zn diffusion process through the interstitial–substitutional exchange of Zn ions is governed by the diffusion-limited process. Furthermore, the Ga out-diffusion is related to the Frank–Turnbull model involving Schottky defects at the surface. The dissociation of the compositions of the GaAs0.35P0.65 layer into the GaAs-like phase on the film surface was observed by room-temperature photoluminescence.
Impurity-limited mobility of semiconducting thin wires in n-type gallium arsenide
Authors: Wu Chhi-Chong , Lin Chau-Jy
Published: July 2002

The impurity-limited mobility of semiconducting thin wires for the nonparabolic band structure of electrons in n-type gallium arsenide has been investigated by scattering from ionized impurities or from the uniform distribution of remote impurities. Results are shown that the impurity-limited mobility due to scattering from background impurities increases monotonically and slowly with increasing temperature, while the impurity-limited mobility due to scattering from remote impurities increases rapidly with temperature. It is also shown that the mobility for both types of impurities decreases with increasing wire radius. However, for scattering from remote impurities, the mobility appears slowly decreasing with the wire radius at higher temperatures.
Evidence of an atomistic universal structural entity for high-temperature superconductivity
Authors: Blackstead Howard A. , Dow John D.
Published: July 2002

Evidence is presented that the spatial regions responsible for superconductivity in NdBa2Cu3O7, Nd2−zCezCuO4, and Nd2−zCezSr2Cu2NbO10 are different in the three materials, based on the locations of the superconducting condensates as extracted from: pair-breaking data, the locations of the charge reservoirs, the effects of Ce doping, crystal-field splitting, and the different charges on the Nd2−zCezCuO4 in “free” bulk Nd2−zCezCuO4 and in “superlattice” Nd2−zCezCuO4/SrO/NbO2/SrO/CuO2/. Analyses show that the universal entity responsible for superconductivity is not a spatially extended or planar structure, but is atomistic charge-reservoir oxygen. This suggests that Ba1−aKaPb1−bBibCu3 should be reclassified as a high-temperature oxide superconductor, with Tc≈32 K for b→1. Predictions of the critical temperatures of Nd2−zCezSr2Cu2NbO10 and Nd2−zCezSr2Cu2TaO10, based on this picture, are successful.
Optical characterization of the in-plane switching effect utilizing multidomain structures
Authors: Klausmann Hagen H. H. , Aratani Sukekazu , Kondo Katsumi
Published: July 2002

Color shift and gray scale reversal for very large viewing angles are analyzed theoretically for in-plane switching. It follows that the color shift depends on the change of the effective birefringence dΔn, whereas gray scale reversal depends on the effective angle between the polarizers and the liquid-crystal director angle. Experimental results confirmed the theory that small gray levels result in a larger color shift. Likewise, the viewing angles in which a gray scale reversal occurs, correlate with the theory. To counter these drawbacks, two multidomain structures are proposed, neither of which requires additional orientation processes. With these multidomain structures, color shift was suppressed and no gray scale reversal was observed. However, the multidomain structures reduce maximum transmission, the extent of which depends on the type and design of the structure.
Are storage phosphors a useful tool for soft x-ray imaging diagnostics? Spectral sensitivity and spatial resolution in the 0.07 to 14 nm range
Authors: Nensel B. , Thielemann P. , Decker G.
Published: July 2002

Two types of storage phosphors—KCl:Eu and BaFBr:Eu—were investigated with respect to the application as an imaging diagnostic in the soft x-ray region. Therefore the dependencies of storage phosphor structure on the sensitivity and spatial resolution were investigated at 0.15 nm. The results reveal that it is possible to improve spatial resolution in comparison to commercial storage phosphors used for medical diagnostics. Furthermore, the sensitivity of storage phosphors was measured in the wavelength range of 0.07–14 nm with three different x-ray facilities.
Temperature and emission angle dependence of photoluminescence from an InGaAs/GaAs quantum well in a microcavity structure
Authors: Lim Dae Ho , Yang Gye Mo , Lim Kee Young
Published: July 2002

The spontaneous emission from an InGaAs/GaAs quantum well embedded in a semiconductor vertical-cavity structure is studied. The spontaneous emission enhancement at the cavity mode is clearly identified from the photoluminescence measurement, depending on the wavelength separation between the excitonic emission and the cavity resonance mode which is tuned by changing the measurement temperature and the emission angle. The emission intensity of the cavity resonance mode is increased as the excitonic emission approaches the cavity mode, thereby obtaining evidence of coupling between the exciton and the cavity mode. The emission wavelength of the cavity mode shifts to shorter wavelengths as the emission angle increases, which is clarified with theoretical calculations performed with the transfer-matrix method.
Surface reconstruction of InP(001) upon adsorption of H2S studied by low-energy electron diffraction, scanning tunneling microscopy, high-resolution electron energy loss, and x-ray photoelectron spectroscopies
Authors: Shimomura M. , Sanada N. , Ichikawa S. , Fukuda Y. , Nagoshi M. , Mo/ller P. J.
Published: July 2002

Reconstruction of an InP(001) surface structure upon H2S adsorption has been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), high-resolution electron energy loss (HREELS), and x-ray photoelectron spectroscopies (XPS). The HREELS result indicates that H2S is dissociated on the surface even at RT, leading to evolution of hydrogen from the surface. LEED patterns show (2×4) and (2×1) structures for the surface with sulfur coverages, 0 and 0.5–1 ML, respectively. A complex pattern appears at 0.25 ML. We find in STM images that the (2×1) structure starts to form even at 0.1 ML sulfur coverage at 350 °C. The (2×1) structure is almost established at 0.5 ML although the protrusions of about two atoms in size exist on the surface. The new (2×1) structure begins to grow from protrusions above 0.5 ML on the (2×1) surface found at 0.5 ML and it is established at about 1 ML where the c(2×2) structure with small domain is found. XPS result shows one chemical state of sulfur at 0.55 ML with the binding energy (BE) at 161.8 eV. Above 0.55 ML, S 2p spectra contain two components with the BE at 161.8 and 162.2 eV. Based on the angle dependence of the spectrum and the result of the STM images, these can be ascribed to sulfur on and under the surface, respectively. Sulfur is found not to be bonded to phosphorus atoms at 350 °C.
Structural properties and photoluminescence study of CdSe/Si epilayers deposited by laser ablation
Authors: Perna G. , Capozzi V. , Ambrico M.
Published: July 2002

Structural and optical characterization of CdSe thin films deposited by laser ablation technique on silicon (100)- and (111)-oriented substrates are reported. The effect of the substrate orientation on the growth and luminescence features of the two types of epilayers are investigated. Photoluminescence spectra of CdSe films measured from 10 up to 300 K and as a function of the laser excitation intensity give detailed information on the extrinsic levels localized in the forbidden gap. Temperature dependence of the energy of the n=1 exciton line has been fitted by the Varshni’s equation and by an expression containing the Bose–Einstein occupation factor for phonons. Parameters related to the electron-phonon interaction have been obtained. Temperature dependence of the broadening of exciton linewidth has been studied in terms of an expression containing both exciton-optical phonon and exciton-acoustic phonons coupling constants. Evaluated fitting parameters have shown the dominant contribution of optical phonons in mechanism of the exciton line broadening.