bluuebunny/crossref_metadata_2025 huggingface dataset:
Low temperature deposition of nanocrystalline silicon carbide films by plasma enhanced chemical vapor deposition and their structural and optical characterization
Authors: Rajagopalan T. , Wang X. , Lahlouh B. , Ramkumar C. , Dutta Partha , Gangopadhyay S.
Published: October 2003

Nanocrystalline silicon carbide (SiC) thin films were deposited by plasma enhanced chemical vapor deposition technique at different deposition temperatures (Td) ranging from 80 to 575 °C and different gas flow ratios (GFRs). While diethylsilane was used as the source for the preparation of SiC films, hydrogen, argon and helium were used as dilution gases in different concentrations. The effects of Td, GFR and dilution gases on the structural and optical properties of these films were investigated using high resolution transmission electron microscope (HRTEM), micro-Raman, Fourier transform infrared (FTIR) and ultraviolet-visible optical absorption techniques. Detailed analysis of the FTIR spectra indicates the onset of formation of SiC nanocrystals embedded in the amorphous matrix of the films deposited at a temperature of 300 °C. The degree of crystallization increases with increasing Td and the crystalline fraction (fc) is 65%±2.2% at 575 °C. The fc is the highest for the films deposited with hydrogen dilution in comparison with the films deposited with argon and helium at the same Td. The Raman spectra also confirm the occurrence of crystallization in these films. The HRTEM measurements confirm the existence of nanocrystallites in the amorphous matrix with a wide variation in the crystallite size from 2 to 10 nm. These results are in reasonable agreement with the FTIR and the micro-Raman analysis. The variation of refractive index (n) with Td is found to be quite consistent with the structural evolution of these films. The films deposited with high dilution of H2 have large band gap (Eg) and these values vary from 2.6 to 4.47 eV as Td is increased from 80 to 575 °C. The size dependent shift in the Eg value has also been investigated using effective mass approximation. Thus, the observed large band gap is attributed to the presence of nanocrystallites in the films.
Characterization of amorphous GexSi1−xOy for micromachined uncooled bolometer applications
Authors: Ahmed A. H. Z. , Tait R. N.
Published: October 2003

Thin films of GexSi1−xOy were prepared by reactive magnetron sputtering using simultaneous sputtering of silicon and germanium targets in an environment of oxygen and argon. Silicon and oxygen content were varied from 0 to 30 at. % separately and the effect of the addition of each element on electrical and optical properties of amorphous germanium was studied. The electrical and optical behavior of the compound with varying elemental composition is explained based on the oxidation behavior of the Si and Ge. Increasing the silicon content was found to inhibit the formation of germanium–oxygen bonds. Values of temperature coefficient of resistance as high as −5% K−1 were obtained at moderate resistivity values around 3.8×104 Ω cm. These characteristics could be used to enhance the performance of micromachined uncooled bolometers. The composition control enabled by cosputtering components allows resistivity and activation energy to be tailored to suit different design specifications.
Influence of rapid thermal annealing on a 30 stack InAs/GaAs quantum dot infrared photodetector
Authors: Stewart K. , Buda M. , Wong-Leung J. , Fu L. , Jagadish C. , Stiff-Roberts A. , Bhattacharya P.
Published: October 2003

In this article the effect of rapid thermal annealing (RTA) on a 30 stacked InAs/GaAs, molecular beam epitaxially grown quantum dot infrared photodetector (QDIP) device is studied. Temperatures in the range of 600–800 °C for 60 s, typical of atomic interdiffusion methods are used. After rapid thermal annealing the devices exhibited large dark currents and no photoresponse could be measured. Double crystal x-ray diffraction and cross sectional transmission electron microscopy studies indicate that this could be the result of strain relaxation. V-shaped dislocations which extended across many quantum dot (QD) layers formed in the RTA samples. Smaller defect centers were observed throughout the as-grown sample and are also likely a strain relaxation mechanism. This supports the idea that strained structures containing dislocations are more likely to relax via the formation of dislocations and/or the propagation of existing dislocations, instead of creating atomic interdiffusion during RTA. Photoluminescence (PL) studies also found that Si related complexes developed in the Si doped GaAs contact layers with RTA. The PL from these Si related complexes overlaps and dominates the PL from our QD ground state.
Time resolved dynamics of rapid melting and resolidification of Sb thin films under ns and ps laser pulse irradiation
Authors: Wiggins S. M. , Gawelda W. M. , Solis J. , Serna R. , Afonso C. N.
Published: October 2003

Real time reflectivity measurements with ps time resolution at the wavelength λ=514.5 nm have been used to analyze the rapid melting and solidification dynamics of Sb thin films on glass substrates irradiated with a visible laser pulse (λ=583 nm) of duration either 30 ps or 2 ns. Significant differences in the solidification dynamics have been obtained for the three different film thickness investigated (25, 50, and 200 nm) as well as between ps and ns laser pulse excitation. Bulk solidification is the dominant mechanism observed at high laser fluence in the thinner films, no matter the pulse duration used, while interfacial solidification is the only mechanism observed in the thicker film for both pulse durations. These results have been interpreted in terms of the differences of the effective thermal conductivity of the films. The comparison of the behavior of these films with the case of Sb-rich GeSb ones makes it clear that relatively minor changes in the film composition can lead to big changes in the solidification dynamics under similar heat flow conditions that are beneficial for the application of such films in phase change optical recording with ultrashort pulses. The optical properties of molten phase of Sb and of the solid material at the melting temperature have been also been determined at 514.5 nm.
Gd on GaN(0001) surface: Growth, interaction, and Fermi level movement
Authors: Xiao Wende , Guo Qinlin , Xue Qikun , Wang E. G.
Published: October 2003

Growth, interfacial reaction, and Fermi level movement of Gd on n-type GaN(0001)-(1×1) surface are studied using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy-electron diffraction. Gd grows in a layer-by-layer-like mode and reacts with the substrate at the interface, leading to formation of metallic Ga at room temperature. A downward Fermi level movement is observed, and the resultant Schottky barrier height is 1.5 eV. Annealing promotes further diffusion and an interfacial Gd–Ga exchange reaction, reducing the Schottky barrier height.
Temperature and thickness dependence of molecular orientation of α-sexithienyl on Cu(111)
Authors: Kiguchi Manabu , Yoshikawa Genki , Saiki Koichiro
Published: October 2003

Atomic and electronic structures of α-sexithienyl(6 T) films grown on Cu(111) were studied by near edge x-ray absorption fine structure (NEXAFS) and S 1s x-ray photoemission spectroscopies. The polarization dependent NEXAFS indicated highly ordered arrays of 6 T molecules, with their molecular long axes parallel to the surface, at substrate temperatures lower than 300 K. On the other hand, their molecular long axes became normal to the surface at 360 K, while they initially adsorbed with their long axes parallel to the surface. The film thickness and substrate temperature dependence of the molecular orientation is discussed in terms of the van der Waals interaction between 6 T and Cu(111) and the kinetics of the growth process. It is demonstrated that the molecular orientation of the promising organic semiconductor 6 T can be controlled by substrate temperature.
Modeling of charge quantization and wave function penetration effects in a metal–oxide–semiconductor system with ultrathin gate oxide
Authors: Chim W. K. , Zheng J. X. , Koh B. H.
Published: October 2003

We investigated the charge quantization and wave function penetration effects in a metal–oxide–semiconductor structure using an in-house developed quantum mechanical (QM) capacitance–voltage (C–V) simulator (denoted as NUSQM), which solves the Schrödinger and Poisson equations self-consistently. The C–V results obtained from the NUSQM simulator were compared with those obtained from other QM simulators in the literature (i.e., U.C. Berkeley, U.T. Quant, NEMO, and North Carolina State University). The results in the present work are generally in good agreement with the other simulators in the inversion region while some discrepancies exist among the different simulators in the accumulation region. It was found that the wave function penetration has a non-negligible effect on the gate capacitance and resulting C–V curves when the oxide thickness decreases to 1 nm and below.
Optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells
Authors: Stevens Martin J. , Najmaie Ali , Bhat R. D. R. , Sipe J. E. , van Driel H. M. , Smirl Arthur L.
Published: October 2003

We report all-optical injection and coherent control of a ballistic charge current in GaAs/AlGaAs quantum wells. This current arises through quantum interference of one- and two-photon absorption of ∼100 fs pulses with parallel linear polarizations, and its magnitude can be controlled by adjusting the relative phase of the incident pulses. By monitoring differential transmission using a spatially resolved optical pump–probe technique, we observe evidence of carrier motion associated with this ballistic current. Results are consistent with a theoretical treatment specific to quantum wells, and are qualitatively similar to previous measurements in bulk GaAs.
Transient enhanced diffusion of arsenic in silicon
Authors: Solmi S. , Ferri M. , Bersani M. , Giubertoni D. , Soncini V.
Published: October 2003

The transient enhanced diffusion (TED) of As in silicon samples implanted at 35 keV with dose 5×1015 cm−2 has been investigated in the temperature range between 750 and 1030 °C by comparing experimental and simulated profiles. For temperatures higher than 900 °C the phenomenon is of modest entity and vanishes after a few seconds, whereas at lower temperatures diffusivity enhancements of some order of magnitude have been observed. The anomalous shift of the junction depth, evaluated at 2×1018 cm−3, is about 12 nm at 900 °C and increases up to 45 nm at 750 °C. It has been verified that the two are the contributions, that generate the interstitial excess responsible for the TED: (i) the implantation damage and (ii) the aggregation in clusters of the As atoms. From an experiment that allows us to separate the two contributions, we estimate that about one third of the TED observed in the first 20 min of annealing at 800 °C is due to the defects produced by clustering. The influence of clustering on the shape of the As profiles after diffusion at different temperatures is also discussed.
Elastic and mechanical properties of transparent nanocrystallized KNbGeO5 glass
Authors: Torres F. , Narita K. , Benino Y. , Fujiwara T. , Komatsu T.
Published: October 2003

KNbGeO 5 (i.e., 25K2O-25Nb2O5-50GeO2) glass with the glass transition 622 °C and crystallization onset 668 °C temperatures shows prominent nanocrystallization, giving K3.8Nb5Ge3O20.4 nanocrystals with the size of ∼30 nm. Dielectric, elastic and mechanical properties of transparent nanocrystallized KNbGeO5 glasses are examined using some techniques such as polarizability analysis, cube resonance spectra and Vickers nanoindentation. Electronic polarizability, shear modulus, bulk modulus, Young’s modulus (E), Debye temperature (θD), Vickers hardness, and deformation energies during nanoindentation increase largely due to the nanocrystallization, e.g., E=64.1 GPa and θD=500 K for the base glass and E=85.5 GPa and θD=575 K for a transparent nanocrystallized glass. The values of Poisson’s ratio and fractal bond connectivity of nanocrystallized glasses are the same as those of the base glass, suggesting that relative arrangements of constituent ions (K+, Nb5+, Ge4+) in nanocrystallized glasses are basically similar to those in the base glass. The present study suggests that nanocrystallization in glass results in not only more compact atomic arrangements but also in an increase in interatomic strength.
Self-organized growth and optical emission of silicon-based nanoscale β-SiC quantum dots
Authors: Wu X. L. , Gu Y. , Xiong S. J. , Zhu J. M. , Huang G. S. , Bao X. M. , Siu G. G.
Published: October 2003

Si-based β-SiC quantum dots (QDs) were fabricated for exploring efficient blue emission from β-SiC nanostructures. Microstructural observations and x-ray photoemission spectroscopy reveal that the β-SiC QDs with sizes of 5–7 nm are embedded in the SiO2 and graphite matrices, displaying a locally tetragonal symmetry. Photoluminescence spectral examinations show two narrow blue-emitting bands at 417 and 436 nm, which are determined by both quantum confinement and surface structure of the β-SiC QDs. Electron spin resonance investigation demonstrates that the photoexcited carriers partially come from the β-SiC QD core with a widened band gap, whereas the radiative recombination occurs in Si excess defect centers at the β-SiC QD surface. A theoretical calculation about electronic states caused by the vacancy defects in the gap of balls formed with excess Si atoms at the surfaces of the β-SiC QDs supports our assignment to the two blue-emitting origin.
Capillary condensation monitored in birefringent porous silicon layers
Authors: Künzner N. , Gross E. , Diener J. , Kovalev D. , Timoshenko V. Yu. , Wallacher D.
Published: October 2003

We performed studies on the capillary condensation of various substances in optically anisotropic porous silicon layers. Their strong in-plane form birefringence has been utilized to analyze the polarization state of the transmitted light when molecules penetrate into the pores. The polarization state of the transmitted light is correlated with the filling fraction of the pores by an effective-medium model for anisotropic porous materials. The experimentally obtained adsorption/desorption isotherms show hysteresis typical for capillary condensation in porous materials. We discuss the shape of the hysteresis loops in the framework of the morphology of the layers. Pore-size distributions derived from adsorption/desorption isotherms are presented.
Improvement of the thermal stability of silver metallization
Authors: Kim H. C. , Alford T. L.
Published: October 2003

The thermal stability of Ag(Al) alloy thin films on SiO2 has been investigated and compared to pure Ag thin films on SiO2 substrates. The prevention of Ag agglomeration on the SiO2 layer is a critical issue to improve the thermal stability of Ag metallization. We deposited Ag(Al) alloy thin films on SiO2 and compared them to pure Ag thin films by performing various analyses: Rutherford backscattering spectrometry, x-ray diffractometry, optical microscope, and four-point probe. Ag(Al) thin films showed good thermal stability on the SiO2 layer without any diffusion barrier and the Ag alloy thin films were stable up to 600 °C for 1 h in vacuum. No agglomeration was found during annealing for 1 h at 600 °C in vacuum. The electrical resistivity of the as-deposited Ag (3.1 at. % Al) thin film is slightly higher than that of pure Ag thin film. However, the resistivity of Ag(Al) samples annealed at high temperatures (up to 600 °C for 1 h in vacuum) remained constant due to no occurrence of agglomeration. It is confirmed in this study that the thermal stability of Ag metallization on SiO2 substrates can be enhanced by using Ag(Al) alloy thin films.
Perpendicular magnetic anisotropy of La0.67Sr0.33MnO3 thin films grown on CaMnO3 buffered SrTiO3
Authors: Wang Zhi-Hong , Cristiani G. , Habermeier H.-U. , Zhang Zhen-Rong , Han Bao-Shan
Published: October 2003

La 0.67 Sr 0.33 MnO 3 ( LSMO ) thin films were grown onto CaMnO3(CMO) buffered SrTiO3(100) by pulsed laser deposition. Because of the in-plane compressive strain induced by the lattice mismatch between CMO and LSMO, a perpendicular magnetic anisotropy (PMA) was obtained in the overlayer LSMO. Using the magnetic force microscopy, stripe magnetic domains in association with the PMA were observed at room temperature. Furthermore, the magnetoresistance with in-plane magnetic field parallel and vertical to the measuring current was studied at 5 and 300 K, and its correlation with the magnetic anisotropy has been discussed.
Electron transport through coupled quantum dots
Authors: Li Shu-Shen , Abliz Ahmad , Yang Fu-Hua , Niu Zhi-Chuan , Feng Song-Lin , Xia Jian-Bai , Hirose Kenji
Published: October 2003

The transmission through coupled quantum dots (CQDs) is calculated using the coupled-channel recursion method. Our results reveal that the conductance peaks move to high energy as the CQDs radius decreases or the period increases. If we increase the transverse momentum the conductance peaks move to high energy. Applying this characteristic, we can design a switch device using CQDs by applying a static electric field perpendicular to transmission direction. The theoretical results qualitatively agree with the available experimental data. Our calculated results may be useful for the application of CQDs to photoelectric devices.
Influence of Ta/Si atomic ratio on the interdiffusion between Ta–Si–N and Cu at elevated temperature
Authors: Lai L. W. , Chen J. S. , Hsu Wu-Shiung
Published: October 2003

Ta–Si–N thin films with comparable N content but different Ta/Si ratios (Ta57Si5N38, Ta53Si11N36, and Ta29Si32N39) were prepared by cosputtering from Ta and Si targets, in an Ar+N2 atmosphere. All as-deposited Ta–Si–N films exhibit the amorphouslike (nanocrystalline) structure. The Ta–Si–N films were then applied as diffusion barriers in a Cu–SiO2 metallization system. After annealing at 800 °C for 30 min, the Ta57Si5N38 barrier crystallizes into Ta2N phase but the other two systems exhibit minuscule diffraction peaks. However, the Cu/Ta53Si11N36 couple shows almost no interdiffusion while the interdiffusion is apparent for Cu/Ta57Si5N38 and Cu/Ta29Si32N39 systems. Cu/Ta–Si–N interdiffusion is evidently not related to the crystallization of Ta–Si–N. The Cu/Ta–Si–N interdiffusion behavior is discussed on the basis of the Ta atom mobility and the atomic density of Ta–Si–N of various Ta/Si ratios.
Improved metal–oxide–nitride–oxide–silicon-type flash device with high-k dielectrics for blocking layer
Authors: Choi Sangmoo , Cho Myungjun , Hwang Hyunsang , Woo Kim Jung
Published: October 2003

We have investigated a sputter-deposited Al2O3/HfO2 stack as a blocking layer of metal–oxide–nitride–oxide–silicon capacitors. Compared with a SiO2 blocking layer, the sample with Al2O3/HfO2 stack shows lower write/erase (W/E) voltage, higher speed, and longer data retention time. The improvement of electrical characteristics can be explained by lower leakage current density and less voltage drop of high-k blocking layer which in turn increase the effective thickness of tunneling oxide. After stressing the W/E voltage of ±10 V voltage for 1 ms, the flat-band voltage shift of 4 V and 10 years of retention time was obtained.
Temperature dependent contactless electroreflectance study of intersubband transitions in a self-assembled InAs/InP (001) quantum dot structure
Authors: Malikova L. , Pollak Fred H. , Masut R. A. , Desjardins P. , Mourokh Lev G.
Published: October 2003

Contactless electroreflectance, in the range of 18<T<300 K, has been used to investigate a self-assembled InAs/InP (001) quantum dot (QD) structure fabricated by metalorganic vapor phase epitaxy. Signals have been observed from all the relevant portions of the sample including the QDs, wetting layer, InP cap layer, and/or substrate. The energies of the QD transitions are in good agreement with an envelope function calculation based on the physical parameters of the QDs as observed from transmission electron microscopy and other relevant parameters. The temperature dependence of the energies of all transitions has been fit using both the semiempirical Varshni and Bose–Einstein-like expressions. It was found that extracted Varshni and Bose–Einstein fitting parameters are in the range of those for most III–V materials, both bulk and quantum well. However, the temperature variation for the energy transitions in the QDs is somewhat larger that reported for bulk InAs.
Magnetic storage device using induced magnetic reversal of a cobalt element array
Authors: Chen Hanning , Whittenburg Scott L.
Published: October 2003

The effects of the applied field, cell size, and cutting area on the “seed” induced magnetic reversal of a cobalt element array have been studied by a stochastic dynamic micromagnetics code using the Laudau–Lifshitz–Gilbert equation. Three magnetic reversal mechanisms under different magnitudes of the applied field have been investigated by examining the energy profiles. To minimize the effect of the thermal fluctuations on the switching time, an applied field with magnitude around 0.7 or 0.8 T and an element array with cutting area less than 10 nm×10 nm are required. By using the smaller cellsize, the switching time and the storage density of the element array can be improved. A sinusoidal applied field with a period of 0.1 ns was used to generate a single switching event.
Structural and dielectric properties of Ga-modified BiFeO3–PbTiO3 crystalline solutions
Authors: Cheng Jin-Rong , Li Nan , Cross L. Eric
Published: October 2003

The dielectric and structural properties of Bi(GaxFe1−x)O3–PbTiO3 (BGF–PT) crystalline solutions have been investigated. Studies have focused on phase-pure perovskite materials. With increasing Ga content, we have found: (i) a coexistence of tetragonal and rhombohedral ferroelectric phases; (ii) a multicell perovskite structure for BGF–0.3PT; and (iii) a maximum tetragonal c/a ratio for the composition BGF–0.6PT with x=0.25. Also, Ga modification increases the electrical resistivity to ⩾1012 Ω cm, reduces the dielectric loss or tan δ over a wide temperature range, and increases the dielectric constant K. We have developed BGF–PT materials with values of K>400 and of tan δ<0.03.
Nonmagnetic semiconductor spin transistor
Authors: Hall K. C. , Lau Wayne H. , Gündoğdu K. , Flatté Michael E. , Boggess Thomas F.
Published: October 2003

We propose a spin transistor using only nonmagnetic materials that exploits the characteristics of bulk inversion asymmetry (BIA) in (110) symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in (110) InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in (110) quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field (Rashba effect). This device may also be used as a spin valve, or a magnetic field sensor.
Temperature dependence of the radiative recombination coefficient of intrinsic crystalline silicon
Authors: Trupke T. , Green M. A. , Würfel P. , Altermatt P. P. , Wang A. , Zhao J. , Corkish R.
Published: October 2003

The radiative recombination coefficient B(T) of intrinsic crystalline silicon is determined as a function of temperature over the temperature range 77–300 K. We observe that B(T) decreases as a function of temperature and we compare our results to previously published contradictory data from the literature. The radiative recombination coefficient is calculated from the absorption coefficient for band-to-band transitions, which we determine at different temperatures from photoluminescence spectra measured on planar high resistivity float zone silicon wafers. Photoluminescence spectra could be detected over a large range of more than five orders of magnitude, which allowed us to determine extremely low values of the absorption coefficient in the spectral range where absorption processes are accompanied by the simultaneous absorption of up to four phonons.
Configurations, energies, and thermodynamics of the neutral MgH complex in GaN
Authors: Wright A. F. , Myers S. M.
Published: October 2003

Atomic configurations corresponding to local-energy minima for the neutral MgH complex in wurtzite GaN are identified using density-functional theory and the generalized-gradient approximation for exchange and correlation. MgH binding energies, H local-mode vibration frequencies, and configurational degeneracies for the six lowest-energy configurations are used, along with corresponding results for isolated H+, to compute equilibrium H state populations in Mg-doped GaN as a function of temperature. For a Mg concentration of 1×1019/cm3 and a H/Mg concentration ratio of 0.99, MgH is found to be the majority H species at room temperature with isolated H+ becoming the majority species at T≈550 °C. Among the MgH states, one is found to dominate at all temperatures. The dominant configuration consists of H at an antibonding site of a N neighbor of the substitutional Mg, with the Mg–N and N–H bonds nearly aligned and the N–H bond oriented at an angle of ∼109° with the c axis. The H stretch-mode frequency of the dominant state is consistent with the peak observed in Fourier-transform infrared reflection spectra from Mg-doped GaN samples.
Fokker–Planck approach to extending the one-flux method of carrier transport in semiconductors to variable energies
Authors: McKinnon W. R.
Published: October 2003

The one-flux method of McKelvey, Longini, and Brody [Phys. Rev. 123, 51 (1961)] is extended to arbitrary distributions of the fluxes in energy, based on a Fokker–Planck approach to the Boltzmann equation. The method is used to discuss the backscattering of carriers from a high-field region. It is shown that a key result of the one-flux method, that the backscattering vanishes in the limit of high fields, holds more generally than under the assumptions of the one-flux method.
Beam fanning reversal in the ferroelectic relaxor Sr0.61Ba0.39Nb2O6 at high external electric fields
Authors: Goulkov M. , Imlau M. , Granzow T. , Woike Th.
Published: October 2003

Beam fanning has been studied in the cerium-doped ferroelectric relaxor strontium–barium–niobate (Sr0.61Ba0.39Nb2O6:Ce, SBN:Ce) under the application of high external electric fields. Spatial reversal in the distribution of the scattered light is achieved both below and above the phase transition. It is shown that the photorefractive response in SBN strongly depends on the state of the polar structure of the crystal, which can be controlled by external fields. Coherent illumination of the crystal greatly facilitates the repoling process, causes a considerable refinement of the domain order in the ferroelectric phase and assists the stabilization of the polar structure in the relaxor regime. The model of scattering centers associated with refractive index anomalies located on domain walls is applied to receive detailed information about the distribution of initial seed scattering in the crystal bulk and the efficiency of nonlinear amplification of the scattering.
Modeling plate impact response of particle–polymer composite
Authors: Zhang Duan Z.
Published: October 2003

A recently obtained constitutive relation [Zhang et al., Phys. Rev. E 66, 051806 (2002)] is used for polymers to model plate impact experiments on a composite material consisting of elastic particles and a polymer binder. The stress relaxation in the polymer is described by a kernel in a history integral. The kernel implies a 1/t stress relaxation for a short time t, and an exponential stress relaxation for a long time. It is found that the binder behavior, especially the stress relaxation at a short-time scale, plays an essential role in the wave dispersion observed during the experiments. Although it is difficult to implement the history dependent constitutive relation directly into a numerical calculation, a mathematical approximation of the kernel is found in the form resembling a prony series. The approximation enables easy implementation of the constitutive relation into numerical calculations. While the conventional use of a prony series requires a series of experiments to determine the parameters, in the present article, the parameters in the series expansion are uniquely determined by the kernel. Furthermore, not all the terms in the series are needed for a specific problem of interest. Only those terms with the relaxation times in the range of the time scales of the physical process are important.
Anisotropy of optical properties of conjugated polymer thin films by spectroscopic ellipsometry
Authors: Losurdo Maria , Bruno Giovanni , Irene Eugene A.
Published: October 2003

Optical properties of thin films of conjugated poly(arylenephenylene) polymers have been determined using variable angle spectroscopic ellipsometry. Dispersion of the complex refractive index parallel, N∥=(n∥+ik∥), and perpendicular, N⊥=(n⊥+ik⊥), to the film plane are reported for the photon energy range 1.5–5.0 eV. The energy dispersion in the optical properties has been analyzed using a three-phase model and Lorentzian dispersion functions for the polymers. The resonant frequencies of the Lorentzian oscillators describe the fundamental π-π* optical transition polarized parallel to the polymer chain and indicate additional optical transitions polarized perpendicular to the polymer chain that are introduced by interchain interactions. Dependence of the optical anisotropy and fundamental π-π* optical transition on the polymer backbone is discussed.
Geologic lithofacies identification using the multiscale character of seismic reflections
Authors: Strauss Moshe , Sapir Micha , Glinsky Michael E. , Melick Jesse J.
Published: October 2003

A forward acoustic model shows that geologic lithofacies groups can be identified by the character of the wavelet transform of their seismic reflection response even for incident signals with a wavelength much larger than the dominant bed thickness. The same model shows that multiple interbed reflections can be neglected. This allows the use of an analytical relation of the linear reflection response expressed as a convolution between the incident signal and the scaled derivative of the acoustic impedance. The relation is applied to solve the inverse problem for the acoustic impedance, using orthogonal discrete wavelet transform (DWT) and Fourier transform methods; good agreement is obtained between the well log wavelet spectrum and both the forward modeled seismic data and the real seismic data. It is found that the DWT approach is superior, having a better signal-to-noise ratio and more localized deconvolution artifacts. A population of well logs containing a wide range of lithologies and bed thicknesses, which are categorized into lithofacies groups, is used to define the conditional probability of a wavelet transform response given a lithofacies group. These conditional probabilities are used to estimate the lithofacies probability given a seismic wavelet response via a Bayesian inversion.
Nonlinear electric field dependence of piezoresponse in epitaxial ferroelectric lead zirconate titanate thin films
Authors: Chen Lang , Nagarajan V. , Ramesh R. , Roytburd A. L.
Published: October 2003

Landau–Devonshire-type phenomenological thermodynamic theory is employed to explain the electric field dependence of piezoelectric properties of tetragonal single domain PbZrxTi1−xO3 (PZT). The strong nonlinearity of the converse piezoelectric coefficient under a large external electric field is proved to be intrinsic both in bulk crystal and epitaxial tetragonal PZT thin films. The tunability of piezoelectric responses by an external electric field and its dependence on the film/substrate misfit and elastic compliance of thin films are characterized quantitatively. The theoretical predictions are in good agreement with the experimental results of piezoresponse scanning microscopy. Due to the large tunability of piezoresponse in it, PZT 50/50 (x=0.5) epitaxial film is a promising candidate for use in future tunable devices.
Low-temperature GaAs films grown on Ge and Ge/SiGe/Si substrates
Authors: Andre C. L. , Boeckl J. J. , Leitz C. W. , Currie M. T. , Langdo T. A. , Fitzgerald E. A. , Ringel S. A.
Published: October 2003

The growth and structural properties of low-temperature GaAs (LT-GaAs) films grown on Ge/SiGe/Si substrates using solid-source molecular-beam epitaxy were investigated. Identical structures were also grown on both Ge and GaAs substrates in order to ascertain the effects of heterovalent interfaces, lattice mismatch, and surface morphology on the structural properties and excess As incorporation of LT-GaAs. Triple-axis x-ray diffraction measurements revealed nearly identical lattice expansion due to excess As incorporation for LT-GaAs layers on all substrates, with the excess As concentration estimated to be 0.34%. Subsequent in situ annealing resulted in complete layer relaxation coupled with the formation of randomly distributed As precipitates of similar sizes throughout the LT-GaAs layers on each substrate as determined by transmission electron microscopy. Secondary ion mass spectroscopy measurements confirmed the incorporation of excess As to be identical for growth on each substrate type, indicating that LT-GaAs with ideal structural properties can be grown on compositionally graded SiGe/Si substrates with no apparent impact from crosshatched surface morphology or heterovalent interfaces.
High conductivity modulation doped AlGaN/GaN multiple channel heterostructures
Authors: Heikman Sten , Keller Stacia , Green Daniel S. , DenBaars Steven P. , Mishra Umesh K.
Published: October 2003

A methodology for the design of modulation doped AlGaN/GaN multiple channel heterostructures is presented. Doping is utilized in conjunction with polarization effects to achieve high carrier mobility and high sheet carrier density in each channel, while maintaining a low energy barrier for majority carrier transfer between channels. Several eight-period Si-doped Al0.22Ga0.78N/GaN heterostructures were grown by metalorganic chemical vapor deposition, according to the methodology. Sheet electron densities around 7.7×1013 cm−2, and room temperature electron mobilities as high as 1200 cm2/V s were measured. Applications for the structure include lateral current spreading layers in III-nitride visible light and UV emitters and detectors, and high conductance source and drain access regions in AlGaN/GaN high electron mobility transistors.
Ultralow dissipation Josephson transistor
Authors: Giazotto Francesco , Taddei Fabio , Heikkilä Tero T. , Fazio Rosario , Beltram Fabio
Published: October 2003

A superconductor–normal metal–superconductor (SNS) transistor based on superconducting microcoolers is presented. The proposed four-terminal device consists of a long SNS Josephson junction whose N region is, in addition, symmetrically connected to superconducting reservoirs through tunnel barriers (I). Biasing the SINIS line allows modification of the quasiparticle temperature in the weak link, thus controlling the Josephson current. We show that, in suitable voltage and temperature regimes, large supercurrent enhancements can be achieved with respect to equilibrium, due to electron “cooling” generated by the control voltage. The extremely low-power dissipation intrinsic to the structure makes this device relevant for a number of electronic applications.
Frequency and temperature dependent dielectric and conductivity behavior of KNbO3 ceramics
Authors: Sundarakannan B. , Kakimoto K. , Ohsato H.
Published: October 2003

Dielectric and conductivity measurements were carried out on the potassium niobate ceramics both as a function of temperature (50 to 550 °C) and frequency (102 to 106 Hz). A low-frequency dielectric relaxation in the temperature range of 100 to 200 °C is observed and analyzed with the Cole–Cole function. The activation energy of dielectric relaxation is estimated to be 0.84 eV. Frequency dependent conductivity data are analyzed with an augmented Jonscher relation. Potassium niobate exhibits universal conductivity behavior. Activation energies obtained for the dc conductivity and the hopping frequency are 1.01 and 0.94 eV, respectively. A possible mechanism for both the low-frequency dielectric relaxation and the frequency dependent conductivity is proposed based on activation energies and off stoichiometry of KNbO3, which is resulted due to potassium oxide evaporation during preparation processes.
Ab initio study of 3C inclusions and stacking fault–stacking fault interactions in 6H-SiC
Authors: Iwata H. P. , Lindefelt U. , Öberg S. , Briddon P. R.
Published: October 2003

Ab initio supercell calculations of cubic inclusions in 6H-SiC are performed. The cubic inclusions can be created in perfect 6H-SiC by the propagation of successive partial dislocations having the same Burgers vector in neighboring basal planes, i.e., multiple stacking faults. We have studied the electronic structures and the total energies of 6H-SiC single crystals that contain one, two, three, and four stacking faults, based on density functional theory in local density approximation. Our total energy calculations have revealed that the second stacking fault energy in 6H-SiC is about six to seven times larger than that of an isolated stacking fault, which is actually opposite the trend recently obtained for 4H-SiC [J. Appl. Phys. 93, 1577 (2003)]. The effects of spontaneous polarization on the electronic properties of stacking disorders are examined in detail. The calculated positions of the quantum-well-like localized bands and stacking fault energies of 3C inclusions in 6H-SiC are compared with those previously determined in 4H-SiC, and the possibility of local hexagonal to cubic polytypic transformations is discussed in light of the formation energy and quantum-well action.
Origin and implications of the observed rhombohedral phase in nominally tetragonal Pb(Zr0.35Ti0.65)O3 thin films
Authors: Kelman Maxim B. , McIntyre Paul C. , Gruverman Alexei , Hendrix Bryan C. , Bilodeau Steven M. , Roeder Jeffrey F.
Published: October 2003

The structural and electrical properties of Pb(Zr0.35Ti0.65)O3 (PZT) thin films ranging in thickness from 700 to 4000 Å have been investigated. These (001)/(100)-textured films were grown by metalorganic chemical vapor deposition on (111)-textured Ir bottom electrodes. It was observed that, in the as-deposited state, the thinnest PZT films are rhombohedral even though bulk PZT of this composition should be tetragonal. Thicker films have a layered structure with tetragonal PZT at the surface and rhombohedral PZT at the bottom electrode interface. In this article we investigate the origin of this structure and its effect of the ferroelectric and dielectric properties of PZT capacitors. It has been suggested that thin films stresses can affect the phase stability regions of single domain PZT. This possibility has been investigated by piezoresponse microscopy and thin film stress measurements. In the as-deposited state the majority of PZT grains contain a single ferroelastic domain, whereas after a high temperature anneal, a large fraction of the grains contain several ferroelastic domains. Wafer curvature measurements in combination with x-ray diffraction stress measurements in the Ir bottom electrode showed that the as-deposited PZT films are, within experimental error, stress free at room temperature. Landau–Ginbzurg–Devonshire formalism was used to explain the origin of the rhombohedral phase as a result of substrate constraint on single domain PZT grains. Annealing was found to affect the relative volume fractions of the rhombohedral and tetragonal phases and the electrical properties of PZT films. Intermediate temperature anneals increased the volume fraction of the rhombohedral phase and the coercive field extracted from the polarization-electric field hysteresis loops. After a high temperature anneal (650 °C) the majority of the grains transformed into a polydomain state, decreasing the volume fraction of the rhombohedral phase and the coercive field. If the high temperature anneal was performed after deposition of the top electrode, the coercive field became independent of the PZT thickness.
Viscosity and elastic constants of thin films of amorphous Te alloys used for optical data storage
Authors: Kalb J. , Spaepen F. , Leervad Pedersen T. P. , Wuttig M.
Published: October 2003

The biaxial modulus and the linear coefficient of thermal expansion of sputtered amorphous Ge4Sb1Te5, Ge2Sb2Te5, and Ag0.055In0.065Sb0.59Te0.29 thin films were determined from stress versus temperature measurements on two different substrates. Viscous flow was measured by stress relaxation at constant temperature using wafer curvature measurements. The shear viscosity increased linearly with time, which can be attributed to bimolecular structural relaxation kinetics. The isoconfigurational activation energy was 1.94±0.09 eV for Ge4Sb1Te5, 1.76±0.05 eV for Ge2Sb2Te5, and 1.33±0.09 eV for AgInSbTe. These values scale with the absolute melting temperatures of the material.
Crystallographic and optical properties of epitaxial Pb(Zr0.6,Ti0.4)O3 thin films grown on LaAlO3 substrates
Authors: Vilquin B. , Bouregba R. , Poullain G. , Murray H. , Dogheche E. , Remiens D.
Published: October 2003

Pb ( Zr 0.6 , Ti 0.4 ) O 3 (PZT) thin films are grown in situ on LaAlO3 substrates by rf magnetron sputtering. The relationship between structural and optical properties is investigated as a function of growth temperature. The ferroelectric films exhibit satisfying crystallization with epitaxial growth from 475 °C. The optical refractive index value is 2.558, in agreement with the bulk value. The films show homogeneous structure and the squarelike shape of the index profile along with the PZT thickness suggests a good interface quality with the substrate. The crystallographic and optical properties measured on our films tend to demonstrate the suitability of in situ grown PZT films for optical applications.
Curvature estimation for multilayer hinged structures with initial strains
Authors: Nikishkov G. P.
Published: October 2003

Closed-form estimate of curvature for hinged multilayer structures with initial strains is developed. The finite element method is used for modeling of self-positioning microstructures. The geometrically nonlinear problem with large rotations and large displacements is solved using step procedure with node coordinate update. Finite element results for curvature of the hinged micromirror with variable width is compared to closed-form estimates.
Optical, photoelectric, and photorefractive properties of Ti-doped CdTe crystals
Authors: Gnatenko Yu. P. , Borshch A. O. , Kukhtarev N. , Kukhtareva T. , Faryna I. O. , Volkov V. I. , Bukivskij P. M. , Gamernyk R. V. , Rudenko V. I. , Paranchych S. Yu. , Paranchych L. D.
Published: October 2003

The photorefractive characteristics of Ti-doped CdTe semi-insulating crystals were measured. Additional optical and photoelectric measurement confirmed that titanium has advantages over other dopants and that this material has better characteristics for potential applications. The high optical holographic gain coefficient, Γ≈0.60 cm−1, low background absorption, kb≈0.2 cm−1, high optical quality and homogeneity, and almost monopolar (electronic) photoconductivity show that these materials can be effectively used for both optical and photoelectric applications in the near infrared region. Studies of the optical absorption and photodiffusion current made it possible to determine the nature and energy structure of impurity and intrinsic defects as well as to establish their role in the photorefractive effect. It is shown that the excited impurity T14(F) state is in resonance with the conduction band. As a result, autoionization of electrons to the conduction band under laser excitation takes place. An energy-level diagram, both of impurity and intrinsic defects, in the CdTe:Ti crystals was constructed.
Effects of La substituent on ferroelectric rhombohedral/tetragonal morphotropic phase boundary in (1−x)(Bi,La)(Ga0.05Fe0.95)O3–xPbTiO3 piezoelectric ceramics
Authors: Cheng Jin-Rong , Cross L. Eric
Published: October 2003

Crystalline solutions of (1−x)(Bi,La)(Fe0.95Ga0.05)O3–xPbTiO3 (BLGF–PT) have been fabricated with La concentrations of 0, 10, and 20 at. %. The BLGF–PT system has been found to have excellent insulation resistivity ⩽1013 Ω cm. In addition, La substituent was found to decrease the coercive field, resulting in much improved dielectric and piezoelectric properties. A shift in the morphotropic phase boundary of BLGF–PT with increasing La content was identified for x=0.3, 0.4, and 0.43. We have achieved optimized dielectric constant, loss factor, Curie temperature, remnant polarization, and piezoelectric d33 properties of 881, 0.037, 386 °C, 30 μC/cm2 and 163 pC/N, respectively, for 0.6BLGF–0.4PT with 10 at. % La. These results clearly demonstrate that BLGF–PT is a competitive alternative piezoelectric material to Pb(Zr,Ti)O3 with reduced lead content.
Simple one-dimensional model for electronic structure calculation of unbiased and biased silicon quantum dots in Coulomb blockade applications
Authors: Sée Johann , Dollfus Philippe , Galdin Sylvie , Hesto Patrice
Published: October 2003

This article presents a simple one-dimensional (1D) model of electronic structure calculation able to treat quantum dots (QDs) under bias voltage. With a view to investigating complex Coulomb blockade devices with multiple QDs, this model aims at providing accurate information on the QD eigenstates within reasonable and optimized computation time. First, the electronic structure of an unbiased QD is obtained from a self-consistent solution of the coupled Schrödinger/Poisson equations as a function of the dot size and the charging state. By comparison with three-dimensional (3D) calculations of total energy at given QD volume, we found that the 1D spherical approximation appears to be very good for a wide range of dot shapes. We develop two techniques to include the effect of external 3D bias potential that breaks the symmetry: (i) a perturbation method and (ii) an expansion of the wave function on the eigenstates of the unbiased dot. If the validity of the first technique is limited to small dots and/or low bias voltage, the latter gives excellent results over a wide range of dot sizes and bias voltages. The results obtained for a single dot device using this 1D model are carefully and successfully compared with a full 3D calculation.
Intersubband absorption in V-groove quantum wires
Authors: Gvozdić Dejan M. , Schlachetzki Andreas
Published: October 2003

This article describes intersubband optical transitions in V-groove quantum wires and the resulting absorption. It is demonstrated that the optical absorption spectrum can be modified and controlled by varying the quantum wire dimensions. In addition, the importance of the conduction band nonparabolicity in calculating the absorption spectrum is pointed out. Finally, a possible application of this structure as a converter to the far-infrared wavelength range is discussed.
Effect of humidity on the ac conductivity of nanoporous TiO2
Authors: Garcia-Belmonte G. , Kytin V. , Dittrich T. , Bisquert J.
Published: October 2003

Impedance spectroscopy measurements are performed on a sintered network of TiO2 nanoparticles at different humidity. A transition from nearly normal to dispersive transport is observed at a certain frequency depending on the humidity. The low-frequency conductivity depends on the humidity by power law with an exponent of 1.73, while the low-frequency dielectric constant remains constant. The results can be explained by hopping proton conduction over the percolation threshold. We propose that chemisorbed water molecules mainly supply the protons and the hopping sites are provided by physisorbed water molecules.