bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of charged defects in CdxHg1−xTe and CdTe crystals by electron beam induced current and scanning tunneling spectroscopy
Authors: Panin G. , Dı́az-Guerra C. , Piqueras J.
Published: July 2002

A correlative study of the electrically active defects of CdxHg1−xTe and CdTe crystals has been carried out using a scanning electron microscope/scanning tunneling microscope (SEM/STM) combined system. Charged structural and compositional defects were revealed by the remote electron beam induced current (REBIC) mode of the scanning electron microscope. The electronic inhomogeneities of the samples were analyzed with nm resolution by current imaging tunneling spectroscopy (CITS) measurements, which showed the existence of built-in electrostatic barriers as well as local variations of the surface band gap in the defect areas imaged by REBIC.
Theoretical analysis of the resistively coupled single-electron transistor
Authors: Korotkov Alexander N.
Published: July 2002

The operation of the resistively coupled single-electron transistor (R-SET) is studied quantitatively. Due to the Nyquist noise of the coupling resistance, degradation of the R-SET performance is considerable at temperatures T as small as 10−3e2/C (where C is the junction capacitance) while the voltage gain becomes impossible at T≳10−2e2/C.
Large low-field magnetoresistance over a wide temperature range induced by weak-link grain boundaries in La0.7Ca0.3MnO3
Authors: Wang X. L. , Dou S. X. , Liu H. K. , Ionescu M. , Zeimetz B.
Published: July 2002

Polycrystalline bulk porous samples with a large number of weak-link grain boundaries and high density polycrystalline bulk samples with strong-link boundaries were synthesized by conventional solid-state reaction and a partial melting technique. The weak-link samples showed large magnetoresistance (MR,Δρ/ρH=[ρ0−ρH]/ρ0), 20–30%, at a low magnetic field of 300 mT and over a wide temperature range from the magnetic transition at 235 to 77 K. The partially melted samples exhibited the same magnetoresistance behavior as that of single crystals with a maximum peak MR of 15% at a narrow temperature range around the ferromagnetic transition. It is suggested that grain boundaries are necessary but not sufficient to account for the MR at low field over a large temperature range. Weak-link grain boundaries rather than strong-link boundaries are responsible for the MR at low field over a large temperature range.
Franz–Keldysh oscillations from combined space-charge and grading fields as observed in graded emitter GaAlAs/GaAs heterojunction bipolar transistor structures
Authors: Huang Y. S. , Sun W. D. , Malikova L. , Pollak Fred H. , Low T. S. , Chang James S. C.
Published: July 2002

We have performed a systematic room-temperature photoreflectance study of Franz–Keldysh oscillations from the Ga0.77Al0.23As emitter and GaAs collector regions of a series of graded band-gap emitter GaAlAs/GaAs heterojunction bipolar transistor structures in which both the emitter grading and doping [space-charge field (SCF)] have been varied. It is found that the fields in the emitter depend on both the grading field and SCF, in agreement with the recent theory of Sun and Pollak [W. P. Sun and F. H. Pollak, J. Appl. Phys. 83, 4447 (1998)]. Good agreement for both the collector and emitter fields is found between experiment and simulation using a comprehensive, self-consistent model, including the photovoltaic effect.
Blue and red photoluminescence from Ge+ implanted SiO2 films and its multiple mechanism
Authors: Zhang Jia-Yu , Bao Xi-Mao , Ye Yong-Hong , Tan Xi-Lin
Published: July 2002

Under violet excitation, a strong blue band and a broad red band are emitted simultaneously from the Ge nanocrystal embedded SiO2 films fabricated by Ge+ implantation and annealing. The blue band exhibits a complex annealing behavior, and the photoluminescence excitation spectral analysis indicates that it results from a combination of several implantation-induced deficient centers. The peak position of the red band shifts from 600 to 640 nm when the mean size of Ge nanocrystals increases from 4.3 to 6.7 nm, suggesting that the red band comes from the radiative recombination of excitons confined in Ge nanocrystals.
Correlation between magnetic homogeneity, oxygen content, and electrical and magnetic properties of perovskite manganite thin films
Authors: Rajeswari M. , Shreekala R. , Goyal A. , Lofland S. E. , Bhagat S. M. , Ghosh K. , Sharma R. P. , Greene R. L. , Ramesh R. , Venkatesan T. , Boettcher T.
Published: July 2002

Perovskite manganese oxide materials known for the phenomenon of colossal magnetoresistance often exhibit anomalously large 1/f noise and large, temperature-dependent ferromagnetic resonance (FMR) linewidths. We show that in epitaxial films, these anomalies are very sensitive to oxygen partial pressure during film growth and to postdeposition thermal processing in oxygen, suggesting that oxygen stoichiometry plays a key role. We find that the temperature coefficient of resistance (TCR) at the metal–insulator transition increases and the FMR linewidth decreases as we increase the oxygen partial pressure during growth. Postdeposition heat treatment in oxygen leads to further increase in TCR and decrease in FMR linewidth, accompanied by a dramatic reduction in 1/f noise magnitudes.
Accurate growth rate determination on rotating substrates using electron diffraction dynamics
Authors: Braun W. , Möller H. , Zhang Y.-H.
Published: July 2002

Reflection high-energy electron diffraction oscillation frequencies are determined by measuring the width of the specular spot perpendicular to the surface during substrate rotation. Substrate rotation and data acquisition are phase locked to obtain exact rotation frequencies, allowing the inclusion of satellite peaks in the measurement. The method has a typical accuracy of well below 1% and provides a practical means to measure growth rates on rotating substrates.
Mechanism of shape formation of three-dimensional periodic nanostructures by bias sputtering
Authors: Kawakami Shojiro , Kawashima Takayuki , Sato Takashi
Published: July 2002

We previously demonstrated a process for fabricating three-dimensional (3D) periodic nanostructures composed of corrugated a-Si/SiO2 multilayers, which behave as 3D photonic crystals. In this process, bias sputtering is a key technique by which the pattern is self-forming. This letter clarifies the mechanism of the self-shaping effect of bias sputtering by comparing deposition simulation and experiments. The mechanism is decomposed into three main effects: diffuse incidence of neutral particles of film material, sputter etching by normally incident rare-gas ions, and subsequent redeposition of sputtered film material. Specifically, redeposition has a self-adjusting effect on the depth of holes or valleys, and is the key of formation of stable patterns.
Local characterization of electronic transport in microcrystalline silicon thin films with submicron resolution
Authors: Rezek B. , Stuchlı́k J. , Fejfar A. , Kočka J.
Published: July 2002

Two-dimensional maps of dark conductivity with submicron resolution have been obtained on in situ prepared hydrogenated microcrystalline silicon (μc-Si:H) layers used for solar cells by atomic force microscopy with conductive cantilever. Comparison of the morphology and current image allows clear identification of Si crystallites. Pronounced current decrease has been detected at the grain boundaries. The technique was used to study initial stages of μc-Si:H growth, and we show how the incubation layer, detrimental for solar cells efficiency, can be minimized by pulsed excimer laser crystallization of the initial amorphous layer.
Surface pattern and large low-field magnetoresistance in La0.5Ca0.5MnO3 films
Authors: Peng H. B. , Zhao B. R. , Xie Z. , Lin Y. , Zhu B. Y. , Hao Z. , Ni Y. M. , Tao H. J. , Dong X. L. , Xu B.
Published: July 2002

Formation of an ordered surface structure in La0.5Ca0.5MnO3 films due to the mismatch of the thermal expansion coefficient between the film and the substrate has been investigated. The surface pattern consists of grain chains located on regular orthogonal cracks. The cracks serve as weak-link grain boundaries, and unusually enhanced low-field magnetoresistance (−14.4% in 400 Oe at 90 K) has been observed, which may be explained by spin-polarized tunneling across the grain boundaries.
Transverse field-induced magnetization process in an ultrathin Au/Co/Au film
Authors: Stankiewicz A. , Maziewski A. , Ferré J. , Grolier V.
Published: July 2002

The in-plane (transverse) H∥ field-induced magnetization reversal process in a polycrystalline Au/Co/Au ultrathin magnetic film structure with strong perpendicular anisotropy has been studied. Faraday rotation microscopy shows that the domain structure, initially prepared under an H⊥ field, is not influenced under H∥. Only the value of the net perpendicular magnetization inside the domains is reduced. Our data are described in the frame of a simple model which assumes that the sample is composed of nanometer-sized uncoupled atomic terraces (patches) with different local anisotropies. The limits of our model are discussed.
CoSi
2
formation in the presence of interfacial silicon oxide
Authors: Detavernier C. , Van Meirhaeghe R. L. , Cardon F. , Donaton R. A. , Maex K.
Published: July 2002

Evidence is presented that a reactive capping layer may influence the interfacial reaction in a thin film diffusion system. As a prototype system, we describe the CoSi2 formation in the Ti/Co/SiOx/Si system. We observed that Ti from the capping layer transforms the SiOx diffusion barrier into a CoxTiyOz diffusion membrane, initiating silicide formation. The CoSi2 layer that is formed has a preferential epitaxial orientation. The epitaxial quality is dependent on annealing temperature, Co and Ti thickness. The epitaxial growth may be explained by a combination of Ti-interlayer mediated epitaxy and oxide mediated epitaxy.
Optical nonlinearity in low-temperature-grown GaAs: Microscopic limitations and optimization strategies
Authors: Haiml M. , Siegner U. , Morier-Genoud F. , Keller U. , Luysberg M. , Lutz R. C. , Specht P. , Weber E. R.
Published: July 2002

We have quantitatively measured the linear and the nonsaturable absorption as well as the absorption modulation and its recovery time in as-grown and annealed low-temperature (LT) GaAs. Correlation of the optical data with As antisite (AsGa) defect densities yields the absorption cross section and the saturation parameter of the dominant AsGa to the conduction-band defect transition. We show that this defect transition is mainly responsible for the large nonsaturable absorption in as-grown LT GaAs with fast recovery times. Reducing the AsGa density by annealing yields an optimized material with small nonsaturable absorption, high absorption modulation, and fast recovery times.
Pulse shaping using nonlinear dielectric SrTiO3
Authors: Findikoglu A. T. , Reagor D. W. , Rasmussen K. O/. , Bishop A. R. , Gro/nbech-Jensen N. , Jia Q. X. , Fan Y. , Kwon C.
Published: July 2002

We have studied electromagnetic pulse shaping in coplanar waveguide delay lines made from superconducting YBa2Cu3O7−δ electrodes on nonlinear dielectric SrTiO3 single-crystal substrates. The delay lines exhibited bias- and temperature-dependent nonlinearity and dispersion, with associated pulse-shaping effects. At low temperatures and under certain bias conditions, the interplay of dispersion and nonlinearity has led to stable and compact wave forms, reminiscent of microwave solitons.
Modulated rf discharges as an effective tool for selecting excited species
Authors: Cicala Grazia , Creatore Mariadriana , Favia Pietro , Lamendola Ritalba , d’Agostino Riccardo
Published: July 2002

Modulated NH3-fed rf glow discharges have been investigated; the emissions of the active species NH* and N2* have been collected during the modulation period (time on+time off). In this study, modulated discharges have been characterized by a constant time on of 7 ms and a tunable time off in the range of 0–1000 ms. It has been found that the power modulation represents an effective tool for selecting excited species: this procedure may be exploited in surface grafting treatments of polymers where high selectivity of chemical groups is requested, e.g., for preferentially grafting −NH2 groups onto polyethylene with respect to all other N-containing functionalities.
Enhanced group-V intermixing in InGaAs/InP quantum wells studied by cross-sectional scanning tunneling microscopy
Authors: Chen Huajie , Feenstra R. M. , Piva P. G. , Goldberg R. D. , Mitchell I. V. , Aers G. C. , Poole P. J. , Charbonneau S.
Published: July 2002

Cross-sectional scanning tunneling microscopy is used to study InGaAs/InP quantum-well intermixing produced by phosphorus implantation. When phosphorus ions are implanted in a cap layer in front of the quantum wells (in contrast to earlier work involving implantation through the wells), clear strain development is observed at the interfaces between quantum well and barrier layers after annealing. This is interpreted in terms of enhanced group-V compared to group-III interdiffusion.
The cause of the anomalously small electric field effect in thin films of Bi
Authors: Butenko A. V. , Shvarts Dm. , Sandomirsky V. , Schlesinger Y.
Published: July 2002

Measurements of the electric field effect (EFE) in thin films of Bi exhibit an unexpectedly small effect. Even though the additional charge density is comparable with the intrinsic carrier concentration, the measured EFE is minute. We show that this can be attributed to the following facts: (1) The additional charge due to EFE is divided between the electron and hole bands correspondingly to the density-of-states (DOS). The Bi film is in the quantum-size regime, thus the carriers DOS depend linearly on the corresponding effective masses of DOS. (2) The nature of the scattering mechanism is such that the ratio of the carrier mobilities is almost equal to the inverse ratio of the corresponding effective masses. This near equality is the cause of the anomalous EFE in Bi films.
The effects of x-ray induced structural changes on the microstructure of a-Si after thermal crystallization
Authors: Yu Kin Man , Walukiewicz Wladyslaw , Muto Shunsuke , Jin Hyun-Chul , Abelson John R.
Published: July 2002

We have investigated the effects of intense x-ray irradiation on the local relaxation in amorphous silicon films doped with Ge(a-Si:Ge) and subsequently the crystallization behavior of these films. The Ge–Si bond distance in the as-grown a-Si:Ge is slightly longer (∼0.004 Å) than for the crystalline case (2.378 Å). When the a-Si:Ge film was exposed to synchrotron x rays, the Ge–Si bond distance increased to a value closer to the sum of the covalent radii of Si and Ge (2.39 Å). This x-ray-induced bond length dilation is found to be x-ray dose dependent and is strongest in the sample irradiated with x rays for 20 h (corresponding to a dose of ∼1 photon/Si atom). The x-ray-induced bond dilation in the a-Si:Ge directly affects the crystallization of the films after irradiation. We found that the final grain size of the annealed Si crystal depends on the initial Ge–Si bond length in the amorphous film. The larger the RGe–Si (due to x-ray irradiation) the larger is the Si grain size after thermal annealing. The mechanism leading to the lattice relaxation in the amorphous phase and subsequently the crystal grain size after annealing due to x-ray irradiation will be discussed.
Domain structure of magnetic layers deposited on patterned silicon
Authors: Landis S. , Rodmacq B. , Dieny B. , Dal’Zotto B. , Tedesco S. , Heitzmann M.
Published: July 2002

Arrays of Si square dots down to 200 nm in size were patterned on silicon substrates, and thin films of different magnetic materials were sputter deposited on these patterned substrates. The magnetic film covers the top of the dots, the bottom of the grooves and to much less extent the sidewalls of the dots. Single domain magnetic dots were obtained for Co/NiO bilayers and Co/Pt multilayers, without significant direct coupling mediated by the magnetic deposit on the sidewalls of the dots. Our results indicate that, in these arrays, the magnetic pinning forces are stronger than the estimated value of the largest demagnetizing magnetostatic field on each individual dot. As a result, any magnetic configuration could in principle be stored in such arrays. This approach seems therefore very promising for the preparation of magnetic storage media with ultrahigh density.
Spatially resolved luminescence investigation of AlGaAs/GaAs single quantum wires modified by selective implantation and annealing
Authors: Liu Xingquan , Lu Wei , Li Zhi Feng , Chen Yi Dong , Shen S. C. , Fu Y. , Willander M. , Tan Hark Hoe , Yuan S. , Jagadish C. , Zou J. , Cockayne D. J. H.
Published: July 2002

Single Al0.5Ga0.5As/GaAs V-groove quantum wires (QWR) modified by selective implantation and rapid thermally annealing were investigated by spatially resolved microphotoluminescence (micro-PL). The PL from the necking region was clearly observed at room temperature. Optical properties of QWR and the adjacent quantum well structures were strongly degraded by the implantation. The recovery properties of the PL signals from all the structures were dependent on the implantation dose. A critical dose of 1×1013 cm−2 was found for the selective implantation, over which the PL from the necking region could not be recovered. Also the blueshifts of QWR and the necking-region PL peaks were observed for all the annealed samples. This blueshift is caused by the interface intermixing, which is very useful to increase the confinement of carriers in QWR region for optoelectronic device applications.
Detection of gate oxide charge trapping by second-harmonic generation
Authors: Fang J. , Li G. P.
Published: July 2002

The high-electric-field-induced trapped oxide charge and neutral oxide traps of a metal–oxide–semiconductor field-effect transistor gate oxide are investigated by surface second-harmonic light generation (SHG). The electric-field dependence of the SHG intensity is sensitive to the charge trapped at the interface between the oxide and the silicon substrate. The time dependence of the SHG intensity probes the characteristics of the neutral trap sites in the oxide.
Microstrip superconducting quantum interference device radio-frequency amplifier: Tuning and cascading
Authors: Mück Michael , André Marc-Olivier , Clarke John , Gail Jost , Heiden Christoph
Published: July 2002

The resonant frequency of an amplifier based on a low-temperature dc superconducting quantum interference device (SQUID) is tuned by varying the capacitance across the end of the input microstrip by means of a varactor diode. The resonant frequency can be reduced substantially, for example, from 190 to 110 MHz, while maintaining an on-resonance gain of 27–28 dB. Two microstrip SQUID amplifiers with the same resonant frequency are cascaded to achieve a gain of 33.5±1 dB at 386 MHz; with the SQUIDs tuned to different frequencies, the gain is 22 dB or higher from about 260 to 420 MHz.
Radiative recombination centers induced by stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures
Authors: Lüerßen D. , Bleher R. , Richter H. , Schimmel Th. , Kalt H. , Rosenauer A. , Litvinov D. , Kamilli A. , Gerthsen D. , Ohkawa K. , Jobst B. , Hommel D.
Published: July 2002

Stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures are found to induce enhanced radiative recombination visible as pairs of bright spots in microphotoluminescence intensity maps. Structural investigation by atomic-force microscopy and transmission electron microscopy (plan view as well as cross section) reveal that a widening and bending of quantum wells occurs when they are intersected by Frank-type stacking faults. The enlargement of the well width by up to 12 bilayers evokes an efficient localization of excitons. The localizing potential related to Shockley-type stacking-fault pairs is found to be much shallower.
Highly sensitive ultraviolet photodetectors based on Mg-doped hydrogenated GaN films grown at 380 °C
Authors: Yagi Shigeru
Published: July 2002

Excellent photoelectrical properties are reported for Mg-doped hydrogenated GaN (GaN:H) films grown at 380 °C. These films are fabricated using dual remote-plasma metalorganic chemical vapor deposition under hydrogen-rich conditions. Infrared spectra exhibit N–H and Ga–H vibration bands but not a Mg–H band. The spectral photoresponse of Al/Mg-doped GaN:H/Au sandwich-type cells reveals that the peak responsivity is 0.11 A/W at 360 nm with the dark current of 10−11 A at −1 V bias. The application in low-cost high-sensitivity visible blind ultraviolet sensors are exhibited for the films.
Absorbance spectroscopy and identification of valence subband transitions in type-II InAs/GaSb superlattices
Authors: Kaspi Ron , Moeller Charles , Ongstad Andrew , Tilton Michael L. , Gianardi Donald , Dente Gregory , Gopaladasu Prabhakara
Published: July 2002

We describe the molecular-beam epitaxy growth, as well as both the structural and optical characterization of a set of InAs/GaSb type-II strained-layer superlattice samples, in which the GaSb layer thickness is systematically increased. Absorbance spectroscopy measurements show well-defined features associated with transitions from the various valence subbands to the lowest conduction subband, and also a significant blueshift of the band edge when the GaSb layers thickness is increased. Empirical pseudopotential method calculations are shown to successfully predict the blueshift and help identify the higher-energy transitions.
Spin-tunnel-junction thermal stability and interface interdiffusion above 300 °C
Authors: Cardoso S. , Freitas P. P. , de Jesus C. , Wei P. , Soares J. C.
Published: July 2002

Spin tunnel junctions (CoFe/Al2O3/CoFe/MnIr) were fabricated with tunneling magnetoresistance (TMR) of 39%–41% after anneal at 300 °C, decreasing to 4%–6% after anneal at 410 °C. Junction resistance decreases from (0.8–1.6) to (0.5–0.8) M Ω μm2 during anneal. The pinned-layer moment decreases by 44% after anneal at 435 °C, but the free-layer moment does not change. The current–voltage characteristics change significantly and become asymmetric above 300 °C. Rutherford backscattering analysis (RBS) shows that above 300 °C, strong interdiffusion starts at the CoFe/MnIr interface with Mn moving into CoFe, causing the electrode moment to decrease. Mn eventually reaches the Al2O3/CoFe interface contributing to the TMR decrease. RBS analysis of a separate CoFe/Al2O3/CoFe structure shows only minor structural changes at the CoFe/Al2O3 interfaces after anneal at 435 °C, possibly leading to a second mechanism for the loss of interface polarization and TMR.
Low-voltage GaN:Er green electroluminescent devices
Authors: Heikenfeld J. , Lee D. S. , Garter M. , Birkhahn R. , Steckl A. J.
Published: July 2002

Green light emission has been measured from Er-doped GaN electroluminescent devices (ELDs) at an applied bias as low as 5 V. The GaN–Er ELDs were grown by solid source molecular beam epitaxy on Si (111) substrates. We have achieved this low-voltage operation (ten-fold reduction in optical turn-on voltage) by using heavily doped (∼0.01 Ω cm) Si substrates and by decreasing the GaN–Er layer thickness to several hundred nanometers. A simple device model is presented for the indium tin oxide/GaN–Er/Si/Al ELD. This work demonstrates the voltage excitation efficiency of Er3+ luminescent centers and the compatibility of GaN rare earth-doped ELDs with low-voltage drive circuitry.
Wave function-dependent mobility and suppression of interface roughness scattering in a strained SiGe p-channel field-effect structure
Authors: Sadeghzadeh M. A. , Horrell A. I. , Mironov O. A. , Parker E. H. C. , Whall T. E. , Kearney M. J.
Published: July 2002

The 4 K Hall mobility has been measured in a top-gated, inverted, modulation-doped Si/Si0.8Ge0.2 structure having a Si:B doping layer beneath the alloy. From comparisons with theoretical calculations, we argue that, unlike an ordinary enhancement-mode SiGe p-channel metal–oxide–semiconductor structure, this configuration leads to a decrease of interface roughness scattering with increasing sheet carrier density. We also speculate on the nature of the interface charge observed in these structures at low temperature.
Theory of electrostatic probe microscopy: A simple perturbative approach
Authors: Gómez-Moñivas S. , Sáenz J. J. , Carminati R. , Greffet J. J.
Published: July 2002

A theoretical approach to electrostatic scanning probe microscopy is presented. We show that a simple perturbation formula, originally derived in the context of scattering theory of electromagnetic waves, can be used to obtain the capacitance and the electrostatic force between a metallic tip and an inhomogeneous dielectric sample. For inhomogeneous thin dielectric films, the scanning probe signal is shown to be proportional to the convolution between an effective surface profile and a response function of the microscope. This provides a rigorous framework to address the resolution issue and the inverse problem.
A reaction-path Hamiltonian described with quasirectilinear vibrational coordinates constructed from a nonlinear combination of curvilinear internal coordinates: Formulation
Authors: Okuno Yoshishige
Published: July 2002

We present for a polyatomic reaction system a reaction-path Hamiltonian described with a reaction coordinate and quasirectilinear vibrational coordinates that are constructed from a nonlinear combination of curvilinear internal coordinates. To determine the vibrational coordinates we use a quasipotential-energy expression in which, in a Taylor-series expansion of the potential energy around the reaction path, the usual derivatives with respect to the internal coordinates are replaced by the corresponding covariant derivatives. The vibrational coordinates are determined so that (1) when the angular momentum is assumed to be zero, the respective expressions for the quasipotential energy and the kinetic energy have diagonal forms in the second-order terms and the first-order terms with respect to the vibrational coordinates in the internal configuration subspace perpendicular to the reaction path, and (2) the covariant second derivatives of the potential energy with respect to the vibrational coordinates coincide with the usual second derivatives. The reaction and vibrational coordinates determined presently not only are separated from rotations but are also invariant under the transformation of the original internal coordinates. The present reaction-path Hamiltonian is expected to be of advantage for obtaining an appropriate picture of the reaction-path dynamics under the zero-angular momentum assumption.
Diffusing species in the proton generation process in Si/SiO2/Si structures
Authors: Girault V.
Published: July 2002

In this article, a study of the proton generation in Si/separation by implantation of oxygen (SIMOX) SiO2/Si structures is reported. The interest was focused on the ability to create mobile charges in SIMOX oxides following a forming gas anneal composed of 92% of N2 and 8% of H2 at 700 °C. The kinetics of the charge generation at 700 °C are presented for two SIMOX oxide thicknesses. A study of the influence of the forming gas pressure is also reported. From both types of experiments, it was concluded that the charge generation mechanism is an interface controlled reaction and that molecular hydrogen H2 or atomic hydrogen H are not the limiting diffusing species in the charge generation. By varying the pressure of the forming gas atmosphere, it was shown that the charge generation is dependent on the square root of the forming gas pressure. From these results and other authors’ work, a model is proposed in which the proton generation is a two-step process.
Rotator-II to rotator-I phase transition in alkanes
Authors: Mukherjee Prabir K.
Published: July 2002

The rotator-II to rotator-I phase transition in alkanes is studied within Landau theory. The effect of pressure on the transition is also examined. The transition is found to be first order, even at elevated pressure, with the transition temperature increasing with pressure. The various thermodynamic quantities are calculated near the transition. The Landau coefficients are calculated for various chain lengths from experimental data. The values of the coefficients are decreasing with increasing chain length.
Stability of large-area electron-beam diodes
Authors: Friedman M. , Swanekamp S. , Obenschain S. , Chan Y. , Ludeking L. , Smithe D.
Published: July 2002

In this letter, we report on experimental measurements of an instability in the large-area electron-beam diodes used to pump krypton–fluoride (KrF) lasers. The instability is identified as the transit-time instability and it is shown that it modulates the electron beam (spatially and temporally), producing a wide spread in the energy and momentum distributions of electrons emerging from the diode. These effects can enhance the energy deposited in the foils and adversely affect the energy-transfer efficiency to the KrF gas. Analysis and simulations of the instability suggest that resistively loaded slots in the cathode should eliminate the instability.
The structure of deuterated methane–hydrate
Authors: Gutt C. , Asmussen B. , Press W. , Johnson M. R. , Handa Y. P. , Tse J. S.
Published: July 2002

We present the results of a high-resolution neutron diffraction experiment with a fully deuterated methane hydrate type I at temperatures of 2, 100, and 150 K. Precise crystallographic parameters of the ice-like D2O lattice and the thermal parameters of the encaged methane molecules have been obtained. The parameters of the host lattice differ only slightly from values found for hydrates with asymmetric guests included, which leads to the conclusion that the host lattice of structure I is only a little adaptive. At low temperatures (2 K) the methane molecules in both types of cages present in structure I occupy positions in the center of the cages. At higher temperatures the thermal parameters in both types of cages reflect the surrounding cage geometries or more precisely the translational potentials of the cages. The orientational scattering length density of the CD4 molecules has been analyzed in terms of a multipole expansion with symmetry adapted functions [Press and Hüller, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. A29, 252 (1972); Press, ibid. A29, 257 (1972)]. In both types of cages we found only small modulations of a spherically symmetric scattering density accounting for almost free rotations of the methane molecules. The large and asymmetric cage leads to a somewhat more pronounced modulation of the orientational density than in the small dodecahedral cage. The orientational probability distribution function (PDF) remains nearly unchanged from 2 to 150 K. At 200 K we observed the time-resolved decomposition of the hydrate structure I into hexagonal ice Ih.
Preparation and photoluminescence of nanocrystalline MSO4:xTb3+ (M=Ca, Sr, and Ba; x=0.001–0.005)
Authors: Gong Xiong , Liu Lei , Chen Wenju
Published: July 2002

A series of nanocrystalline MSO4:xTb3+ (M=Ca, Sr, and Ba; x=0.001–0.005) samples have been prepared by the co-precipitation method following low-temperature treatment. The x-ray diffraction technique yielded an average grain size of approximately 24 nm. Based upon the experimental data and theoretical calculations, it is concluded that the photoluminescence quenching in nanocrystalline MSO4:xTb3+ results from the dipole–quadrupole interaction.
Nanoquasicrystallization of binary Zr–Pd metallic glasses
Authors: Murty B. S. , Ping D. H. , Hono K.
Published: July 2002

The formation of an icosahedral phase during crystallization of Zr–Pd binary amorphous alloys is reported. The icosahedral phase forms in the nanocrystalline state with a grain size of about 10 nm as a metastable transient phase during the transformation from an amorphous to a crystalline state in Zr70Pd30 and Zr65Pd35 alloys. The amorphous phase coexists with the nanoquasicrystalline phase before complete crystallization to Zr2Pd occurs.
Principles of strain relaxation in heteroepitaxial films growing on compliant substrates
Authors: Kästner G. , Gösele U.
Published: July 2002

In spite of various reports that the density of misfit dislocations threading a growing heteroepitaxial thin film can be considerably reduced by means of using a “compliant substrate,” the underlying physical mechanisms are not well understood. The common theoretical models suppose that the growing film can preferably relax in an elastic way by slipping on this kind of substrate. This idea, however, requires us to suppose macroscopic slip displacements. Such displacements are disregarded in the common theories and not reported to occur experimentally. This very doubtful free-slipping hypothesis has been used to establish a force balance (“strain partitioning”) between the growing film and the template layer below it and, consequently, to theoretically derive an enhanced critical thickness for the onset of slip of misfit dislocations. In the present article, more realistic mechanisms are discussed, including early plastic relaxation at a low film thickness where multiplicative interaction of dislocations hardly occurs. Possibilities for an enhanced elastic relaxation of the film in the case of Stranski–Krastanow island growth are discussed.
Fundamental relationship of excitonic photoluminescence intensity with excitation density in semiconductor quantum well structures
Authors: Jin S. R. , Ramsteiner M. , Grahn H. T. , Ploog K. H. , Li Z. H. , Shen D. X. , Zhu Z. Q.
Published: July 2002

The fundamental relationship between excitonic photoluminescence (PL) intensity and excitation intensity in semiconductor quantum well structures is developed. This relationship is further simplified in the regime of low excitation, and used for a fit function of the Arrhenius plot of time-integrated PL intensity. The proposed four fit parameters are definitely correlated to the distinct characteristic quantities of the sample material, which are the binding energy of excitons, the activation energy, the scattering time, and the background concentration in the well. The validity of the model has been confirmed using our experiments.
The phonon wake behind a charge moving relative to a two-dimensional plasma crystal
Authors: Dubin Daniel H. E.
Published: July 2002

In a recent experiment a wake was created in a two-dimensional lattice of charged dust grains by a charge moving parallel to the lattice plane. Multiple “Mach cones” were observed in the wake. This paper describes a linear theory of the phonon wake caused by a charge moving relative to a crystalline lattice. The theory predicts multiple structures in the wake that are qualitatively similar to those observed in the experiments. These structures are caused by constructive interference of compressional phonons excited by the moving charge, combined with the strongly dispersive nature of these phonons.
Experimental proof-of-principle investigation of enhanced Z3DT in (001) oriented Si/Ge superlattices
Authors: Koga T. , Cronin S. B. , Dresselhaus M. S. , Liu J. L. , Wang K. L.
Published: July 2002

An experimental proof-of-principle of an enhanced Z3DT (thermoelectric figure of merit) is demonstrated using (001) oriented Si/Ge superlattices. The highest value of the experimental Z3DT at 300 K for a (001) oriented Si(20 Å)/Ge(20 Å) superlattice is 0.1 using κ=5 Wm−1 K−1, for the in-plane thermal conductivity, which is a factor of seven enhancement relative to the estimated value of Z3DT=0.014 for bulk Si. The good agreement between experiment and theory validates our modeling approach (denoted as “carrier pocket engineering”) to design superlattices with enhanced values of Z3DT. Proposals are made to enhance the experimental values of Z3DT for Si/Ge superlattices even further.
A theoretical approach to the O(1D)+H2O(X  1A1) reaction: Ab initio potential energy surface and quasiclassical trajectory dynamics study
Authors: Sayós R. , Oliva Carolina , González Miguel
Published: July 2002

An ab initio study of the ground potential energy surface (PES) of the O(1D)+H2O system has been performed, employing Møller–Plesset methods. From the stationary and additional points calculated, the ground PES has been modeled as a triatomic system, with an OH group of the H2O molecule treated as a single atom of 17.0 amu. The rate constant of reaction (1), O(1D)+H2O→2OH (main reaction channel), estimated from the quasiclassical trajectory (QCT) calculations is reasonably close to the recommended experimental value. For the relative translational energies explored (ET=0.234, 0.303, and 0.443 eV) and H2O at T=300 K, the QCT OH vibrational populations are in good agreement with the experimental values reported for the new OH fragment, but the QCT OH average rotational energies are in general quite larger than the experimental ones. Regarding the stereodynamics, for ET=0.234 eV there is not a clear tendency to a particular rotational alignment of the OH product with respect to the initial relative velocity vector, in agreement with experiments. The QCT results also show that nearly all reactive trajectories leading to reaction (1) take place through an insertion microscopic mechanism, which, even at the highest ET value considered (0.443 eV), is mainly (70%) a nondirect one. The collision complex has an average lifetime of about three rotational periods and a geometry around that of the HO(OH) hydrogen peroxide molecule. The QCT results concerning the microscopic mechanism of reaction (1) are in agreement with the suggested ones by the experimentalists to interpret their results. The present study should be considered as a starting point in the study of reaction (1) from which different aspects on the dynamics may be learned.
The Ã 2Σ+ state of Ar⋅NO
Authors: Lozeille Jérôme , Gamblin Stuart D. , Daire Sophia E. , Wright Timothy G. , Smith David M.
Published: July 2002

The Ã 2Σ+ state of Ar⋅NO is studied using (1+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Higher quality spectra than obtained in other studies allow the identification of a number of previously unreported features. The spectrum is analyzed using two models: a rigid van der Waals complex in which NO is weakly bonded to Ar; and a complex in which the free internal rotation of NO is hindered by the anisotropy caused by the presence of the Ar atom. It is concluded that as the intermolecular stretch is excited, then the anisotropy decreases, and the angular motion of the complex becomes more and more like that of a free rotor. Near the origin, the complex has an average geometry approaching linear, whereas when the intermolecular stretch is excited, an average geometry closer to T-shaped occurs; however, when the anisotropy is small, the concept of geometry becomes ill-defined.
Effects of mechanical stress on the volume phase transition of poly(N-isopropylacrylamide) based polymer gels
Authors: Takigawa T. , Araki H. , Takahashi K. , Masuda T.
Published: July 2002

The effects of mechanical stress on the volume phase transition of a poly(N-isopropylacrylamide) (PNIPA) gel as well as a copolymer gel composed of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) were investigated in the relatively low stress region. The PNIPA gel without elongational stress showed the behavior close to the second order phase transition. The character of the first order transition became clear under tension, and the transition temperature increased with increasing applied stress. Similar behavior was observed for the NIPA-SA copolymer gel, but the copolymer gel showed the first order transition in the whole stress range investigated. The thermodynamical linear region, where the transition temperature varies linearly with applied stress, was narrower than the mechanical linear region determined by the stress–strain relation of the gels. The change in the transition behavior by the application of the mechanical stress originated chiefly from the volume change in the gels by the applied mechanical stress. It was found that the curve of the transition temperature against applied stress corresponds to the phase boundary between the swollen and collapsed phases for the gels. On the basis of the experimental data, a phenomenological model describing the volume phase transition of the polymer gels is proposed in the frame of the Landau-type free energy expression.
High-scan-range cryogenic scanning probe microscope
Authors: Urazhdin S. , Maasilta I. J. , Chakraborty S. , Moraru I. , Tessmer S. H.
Published: July 2002

We have designed and constructed a scanning probe microscope operable at temperatures down to 260 mK within a top-loading helium-3 cryostat. It achieves a large scan range with the sample situated near the bottom of the scanning head—maximizing the cooling efficiency of the liquid helium. The scan head is completely thermally compensated, thus eliminating thermal expansion and contraction on cooling and warm-up, as well as thermal drift during operation. We demonstrate the performance using two distinct scanning probe methods: scanning tunneling microscopy and charge accumulation imaging.
Effect of foreign particles on the growth of faceted crystal faces
Authors: Liu X. Y.
Published: July 2002

A model, so-called heterogeneous two-dimensional (2D) nucleation model, is presented to describe the effect of foreign particles on the growth and surface instability of growing crystals. The occurrence of foreign bodies on growing surfaces of crystals will serve as precursors for heterogeneous 2D nucleation, so that it will promote rapid growth at relatively low supersaturations at the regions where the foreign particles are in contact with the crystal surface. At high supersaturations, due to the limitation of effective transport, heterogeneous 2D nucleation will become kinetically unfavourable. Then, foreign particles have less or no effect on crystal growth. To check the model, the theoretical results are confronted with experimental ones. A novel phenomenon of a needlelike pattern of surface instability caused by solid particle induced growth was observed for the first time for naphthalene crystals grown from the melt. This particulate-induced surface instability occurs in a completely different regime of G/V compared to the dendriticlike surface instability, which turns out to be in excellent agreement with our prediction.
On the origin of stress in magnetron sputtered TiN layers
Authors: Kamminga J.-D. , de Keijser Th. H. , Delhez R. , Mittemeijer E. J.
Published: July 2002

A recently proposed model has been used to describe the state of stress in magnetron sputtered TiN layers in which the stresses are believed to be caused by atomic peening. The state of stress in the layer is described by a combination of: (i) a hydrostatic state of stress, caused by the introduction of the misfitting atoms, and (ii) a biaxial state of stress induced by the equalization of the lateral dimensions of the substrate and the layer, dilated due to the misfitting atoms and the thermal misfit due to the cooling down of the layer/substrate assembly to room temperature. The implications of the thus obtained total state of stress on x-ray diffraction measurements have been clarified and a quantitative elaboration of the growth stress as a function of the amount and type of misfitting particles has been given. It has been deduced that the growth stresses are caused by about 1 wt % Ti atoms on nitrogen sites in the TiN lattice. By comparing x-ray diffraction results of layers of different thickness, deposited simultaneously on two different substrates, it has been concluded that the growth stress in the layers depends on the layer thickness, whereas the thermal stress is equal for all layers on a given substrate. The observed layer thickness dependence of the growth stress has been associated with a (macro)strain depth profile in the layers. The distinct diffraction line broadening observed for all layers cannot be due to smallness of crystallite size and the macrostrain-depth profiles, it is ascribed to (localized) lattice defects as dislocations and low angle grain boundaries.
Near-infrared Yablonovite-like photonic crystals by focused-ion-beam etching of macroporous silicon
Authors: Chelnokov A. , Wang K. , Rowson S. , Garoche P. , Lourtioz J.-M.
Published: July 2002

We report on the fabrication of three-dimensional (3D) Yablonovite-like photonic crystals by focused-ion-beam (FIB) etching of macroporous silicon. Crystals containing up to 25×25×5 lattice cells are fabricated with a submicronic period of ∼0.75 μm. Photonic band gaps at wavelengths close to 3 μm are demonstrated from reflection measurements and confirmed by numerical calculations. The combination of plasma or chemical etching with FIB micromachining appears to be promising for the fabrication of a large variety of multiple-period 3D photonic crystals at optical wavelengths.
First-principles molecular-dynamics study of surface disordering of the (0001) face of hexagonal ice
Authors: Mantz Yves A. , Geiger Franz M. , Molina Luisa T. , Molina Mario J. , Trout Bernhardt L.
Published: July 2002

In order to study surface disordering of ice at temperatures below the bulk melting point as a function of depth into the bulk, Car–Parrinello molecular-dynamics simulations of a periodic model of the hexagonal ice (0001) surface were carried out. Partial disorder in the uppermost bilayer was observed at a simulation temperature of 190 K, which is ∼30 K below the estimated bulk melting point, qualitatively validating earlier classical molecular-dynamics studies of this phenomenon. Over 0.5 ps, the time scale of a simulation, there were three particularly useful (and complementary) measures of disorder: The pair distribution function g(r), the distance of the oxygen atoms from the bottommost bilayer, and the distribution of angles φ and θ formed by the molecular dipole vector and the Cartesian axes. Our results set the stage for future studies addressing the effect of the disordered ice surface on heterogeneous atmospheric chemistry.
Hydrogenic impurity in multilayered quantum wires
Authors: Hsieh Cheng-Ying , Chuu Der-San
Published: July 2002

The binding energy of the ground state of a hydrogenic impurity located at the center of a multilayered quantum wire (MLQW) is studied within the framework of the effective-mass approximation. The MLQW consists of a core wire (GaAs) coated by a cylindrical shell (Ga1−xAlxAs) and then embedded in the bulk (Ga1−yAlyAs). The calculation was performed by using a trial wave function. To make a comparison, the ground and excited states (1s, 2p, and 3d states) binding energies of a hydrogenic impurity located at the center of a single-layered quantum wire (QW) are also calculated. It is found for small wire radius, the ground-state binding energy of the hydrogenic impurity located at the center of a MLQW behaves very differently from that of a single-layered QW.
Area-controlled growth of InAs quantum dots and improvement of density and size distribution
Authors: Tatebayashi Jun , Nishioka Masao , Someya Takao , Arakawa Yasuhiko
Published: July 2002

We propose and demonstrate a scheme (area-controlled growth) for controlling where self-assembled InAs quantum dots form, using a SiO2 mask and selective area metalorganic chemical vapor deposition growth. Using this technique, quantum dots can be formed in only selected areas of a growth plane. However, in the regions where dots are formed there is variation of dot density and size along the mask stripe direction because of the diffusion of species in the vapor phase. We achieve more uniform distributions of dot density and size by changing the mask pattern. Using this growth technique, it is possible to fabricate integrated optical devices containing an external reflector together with quantum dots serving as the active layer of a semiconductor laser.