bluuebunny/crossref_metadata_2025 huggingface dataset:
Heteroepitaxial selective growth of InxGa1−xAs on SiO2-patterned GaAs(001) by molecular beam epitaxy
Authors: Lee S. C. , Dawson L. R. , Brueck S. R. J. , Stintz A.
Published: October 2004

Selective growth of InxGa1−xAs on a SiO2-mask patterned GaAs(001) substrate by molecular beam epitaxy (MBE) is reported. A simple model for selective growth is derived from the general rate equation of thin film growth. Based on this model, heteroepitaxial selective growth of InxGa1−xAs with x up to 0.07 is demonstrated at 595°C, which is within the range of typical MBE growth temperatures for GaAs but is above the desorption temperature of In atoms from a GaAs surface. An incorporation rate of In atoms of about 0.22% for a Ga flux of 2.0×1013atoms∕cm2s was measured at this temperature. The concentration of In atoms effectively incorporated into the InxGa1−xAs layer is linearly proportional to the supplied In flux in the range 0.7–9.2×1014atoms∕cm2s. The extremely low, but finite, In incorporation is utilized for the growth of 5nm thick InxGa1−xAs∕GaAs quantum wells and ∼300nm thick InxGa1−xAs layers. Together with selective growth, the surface morphology and optical properties of high-temperature grown InxGa1−xAs are characterized.
Demagnetizing factors for completely shielded rectangular prisms
Authors: Pardo Enric , Chen Du-Xing , Sanchez Alvaro
Published: October 2004

Fluxmetric and magnetometric demagnetizing factors Nf and Nm have been calculated using a finite-element method for rectangular prism 2a×2b×2c with susceptibility χ=−1, in the completely shielded state. Assuming the field is applied along the c dimension, the results are plotted and tabulated as functions of their aspect ratios c∕a and b∕max[c,a].
Bayesian analysis of the effective charge from spectroscopic bremsstrahlung measurement in fusion plasmas
Authors: Krychowiak M. , König R. , Klinger T. , Fischer R.
Published: October 2004

At the stellarator Wendelstein 7-AS (W7-AS) a spectrally resolving two channel system for the measurement of line-of-sight averaged Zeff values has been tested in preparation for its planned installation as a multichannel Zeff-profile measurement system on the stellarator Wendelstein 7-X (W7-X) which is presently under construction. The measurement is performed using the bremsstrahlung intensity in the wavelength region of ultraviolet to near infrared. The spectrally resolved measurement allows to eliminate signal contamination by line radiation. For statistical data analysis a procedure based on Bayesian probability theory has been developed. With this method it is possible to estimate the bremsstrahlung background in the measured signal and its error without the necessity to fit the spectral lines. For evaluation of the random error in Zeff the signal noise has been investigated. Furthermore, the linearity and behavior of the charge-coupled device detector at saturation has been analyzed.
Island growth in the atomic layer deposition of zirconium oxide and aluminum oxide on hydrogen-terminated silicon: Growth mode modeling and transmission electron microscopy
Authors: Puurunen Riikka L. , Vandervorst Wilfried , Besling Wim F. A. , Richard Olivier , Bender Hugo , Conard Thierry , Zhao Chao , Delabie Annelies , Caymax Matty , De Gendt Stefan , Heyns Marc , Viitanen Minna M. , de Ridder Marco , Brongersma Hidde H. , Tamminga Yde , Dao Thuy , de Win Toon , Verheijen Marcel , Kaiser Monja , Tuominen Marko
Published: October 2004

Atomic layer deposition (ALD) is used in applications where inorganic material layers with uniform thickness down to the nanometer range are required. For such thicknesses, the growth mode, defining how the material is arranged on the surface during the growth, is of critical importance. In this work, the growth mode of the zirconium tetrachloride∕water and the trimethyl aluminum∕water ALD process on hydrogen-terminated silicon was investigated by combining information on the total amount of material deposited with information on the surface fraction of the material. The total amount of material deposited was measured by Rutherford backscattering, x-ray fluorescence, and inductively coupled plasma–optical emission spectroscopy, and the surface fractions by low-energy ion scattering. Growth mode modeling was made assuming two-dimensional growth or random deposition (RD), with a “shower model” of RD recently developed for ALD. Experimental surface fractions of the ALD-grown zirconium oxide and aluminum oxide films were lower than the surface fractions calculated assuming RD, suggesting the occurrence of island growth. Island growth was confirmed with transmission electron microscopy (TEM) measurements, from which the island size and number of islands per unit surface area could also be estimated. The conclusion of island growth for the aluminum oxide deposition on hydrogen-terminated silicon contradicts earlier observations. In this work, physical aluminum oxide islands were observed in TEM after 15 ALD reaction cycles. Earlier, thicker aluminum oxide layers have been analyzed, where islands have not been observed because they have already coalesced to form a continuous film. The unreactivity of hydrogen-terminated silicon surface towards the ALD reactants, except for reactive defect areas, is proposed as the origin of island growth. Consequently, island growth can be regarded as “undesired surface-selective ALD.”
Analytic band Monte Carlo model for electron transport in Si including acoustic and optical phonon dispersion
Authors: Pop Eric , Dutton Robert W. , Goodson Kenneth E.
Published: October 2004

We describe the implementation of a Monte Carlo model for electron transport in silicon. The model uses analytic, nonparabolic electron energy bands, which are computationally efficient and sufficiently accurate for future low-voltage (<1V) nanoscale device applications. The electron-lattice scattering is incorporated using an isotropic, analytic phonon-dispersion model, which distinguishes between the optical/acoustic and the longitudinal/transverse phonon branches. We show that this approach avoids introducing unphysical thresholds in the electron distribution function, and that it has further applications in computing detailed phonon generation spectra from Joule heating. A set of deformation potentials for electron-phonon scattering is introduced and shown to yield accurate transport simulations in bulk silicon across a wide range of electric fields and temperatures. The shear deformation potential is empirically determined at Ξu=6.8eV, and consequently, the isotropically averaged scattering potentials with longitudinal and transverse acoustic phonons are DLA=6.39eV and DTA=3.01eV, respectively, in reasonable agreement with previous studies. The room-temperature electron mobility in strained silicon is also computed and shown to be in better agreement with the most recent phonon-limited data available. As a result, we find that electron coupling with g-type phonons is about 40% lower, and the coupling with f-type phonons is almost twice as strong as previously reported.
Nanoscale GaN whiskers fabricated by photoelectrochemical etching
Authors: Grenko J. A. , Reynolds C. L. , Schlesser R. , Hren J. J. , Bachmann K. , Sitar Z. , Kotula P. G.
Published: October 2004

GaN whiskers with nanoscale dimensions have been fabricated by photoelectrochemical (PEC) etching in dilute H3PO4 electrolyte. Etching in lower concentration H3PO4 electrolyte for 1 h or for a short time of 5 min at a higher concentration results in individual whiskers with a density of ∼2×109cm−2 and diameters to 15nm. It is observed that ∼10% of them have formed nearly perfect hexagonal plates on the top of the whiskers, which appear to evolve into flowerlike features upon extended etching to 12 min. Such hexagonal plates have not been reported previously in the PEC etching of GaN. The presence of a dislocation along the central axis of the needles is clearly demonstrated, and the etch pattern is suggested to be related to the growth mechanism for GaN on sapphire. When etched for times >30min, these whiskers are typically arranged in clusters with a density of 2–5×107cm−2 and have ten or more whiskers contributing to the central top of the cluster.
Deposition of palladium nanoparticles on the pore walls of anodic alumina using sequential electroless deposition
Authors: Johansson A. , Lu J. , Carlsson J.-O. , Boman M.
Published: October 2004

Palladium nanoparticles were deposited using a sequential electroless deposition technique on the pore walls of nanoporous anodic alumina. For the particle deposition a Pd(NH3)42+ solution was soaked in the alumina membrane and a heated air flow was applied in order to reduce the palladium complex to palladium metal nanoparticles. By repeating the deposition process the size of the nanoparticles could be tailored in this investigation between 6 and 11nm. The size of the nanoparticles was also affected by the concentration of the Pd(NH3)42+ solution, i.e., higher concentration yielded larger particle mean diameters. The samples were investigated using high resolution scanning electron microscopy, x-ray diffraction (XRD), inductively coupled plasma with a mass spectrometer, high resolution transmission electron microscopy, and energy dispersive spectroscopy (EDS). Analysis revealed narrow size distributions of the particles as well as uniform particle coverage of the pore walls. No by-products were observed with EDS, and with the XRD analysis the metallic palladium crystallinity was confirmed.
Electronic transport in field-effect transistors of sexithiophene
Authors: Stallinga P. , Gomes H. L. , Biscarini F. , Murgia M. , de Leeuw D. M.
Published: October 2004

The electronic conduction of thin-film field-effect-transistors (FETs) of sexithiophene was studied. In most cases the transfer curves deviate from standard FET theory; they are not linear, but follow a power law instead. These results are compared to conduction models of “variable-range hopping” and “multi-trap-and-release”. The accompanying IV curves follow a Poole-Frenkel (exponential) dependence on the drain voltage. The results are explained assuming a huge density of traps. Below 200 K, the activation energy for conduction was found to be ca. 0.17 eV. The activation energies of the mobility follow the Meyer-Neldel rule. A sharp transition is seen in the behavior of the devices at around 200 K. The difference in behavior of a micro-FET and a submicron FET is shown.
Thermal transport cross section and phase function of longitudinal phonons for scattering by nanoparticles and microparticles
Authors: Prasher Ravi S.
Published: October 2004

This paper deals with the calculation of the thermal transport cross section and phase function of longitudinal phonons for scattering by nanoparticles and microparticles. Thermal transport cross section is different from the scattering cross section due to the anisotropic nature of scattering. Exact formulation of the phase function is given for the Rayleigh scattering. This paper also proposes an approximate method to calculate the thermal transport cross section for low contrast scatterers. The formulations developed in this paper will be useful for the predictive modeling of thermal conductivity of practical systems such as nanocomposites and nanomicro particle laden systems.
Dynamic optical probing of the magnetic anisotropy of nickel-ferrite nanoparticles
Authors: Raikher Yu. L. , Stepanov V. I. , Depeyrot J. , Sousa M. H. , Tourinho F. A. , Hasmonay E. , Perzynski R.
Published: October 2004

Field dependence of dynamic magneto-orientational birefringence in a ferrocolloid based on the nickel-ferrite nanoparticles is examined. The nanoparticles are electrostatically stabilized and suspended in glycerin at low-volume fractions Φ⩽0.75%. The colloids are tested under crossed magnetic fields: an alternating weak (probing) and a constant strong (bias) one. By comparison to a theoretical model of the birefringence relaxation, an evaluation of the mean nanoparticle magnetic anisotropy energy, Ea, is done. We get σ=Ea∕kT∼10, given that the particles are rather polydisperse: a log-normal distribution with exp[⟨lnd⟩]=6.5nm and the width s=0.55. With the parameter σ in such a range, the NiFe2O4 grains make a first example of a nanodisperse material (ferrofluid) whose anisotropy properties differ substantially from both magnetically soft (e.g., maghemite with σ<2) and magnetically hard (e.g., cobalt ferrite with σ>50) nanosubstances of the same grain size.
Thickness dependence of the magnetic properties of MnAs films on GaAs(001) and GaAs(113)A: Role of a natural array of ferromagnetic stripes
Authors: Däweritz L. , Wan L. , Jenichen B. , Herrmann C. , Mohanty J. , Trampert A. , Ploog K. H.
Published: October 2004

Systematic studies of as-grown MnAs films deposited by molecular-beam epitaxy on GaAs(001) and GaAs(113)A reveal that their magnetic properties and, in particular, their saturation magnetization are determined by the phase separation into stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs. Using a specific saturation magnetization MS*, which refers to the actual volume of α-MnAs, the thickness dependence of MS* can be described in a universal way. It is due to the variation of the stripe structure and the changing of the intra- and interstripe magnetic interaction. Values well above ∼1100emu∕cm3, obtained for the optimum film thickness at room temperature, are considered as an intrinsic property of a nearly defect-free MnAs in the fully magnetized state.
Layered compound Nb3SiC2 predicted from first-principles theory
Authors: Grechnev A. , Li S. , Ahuja R. , Eriksson O. , Jansson U. , Wilhelmsson O.
Published: October 2004

A previously unobserved ternary carbide, Nb3SiC2, belonging to the family of the so-called Mn+1AXn or MAX phases is predicted from first-principles calculations. It has a theoretical bulk modulus of 269Gpa, which is much higher than that of Ti3SiC2. The new phase is metastable with a formation energy of +0.02eV∕atom. We suggest that the phase may possibly be synthesized using thin film technology. The chemical binding of Nb3SiC2 is investigated using the balanced crystal orbital overlap population indicator and it is found to be dominated by the formation of Nb 4d–C2p covalent bonds.
Influence of composition on the piezoelectric effect and on the conduction band energy levels of InxGa1−xAs∕GaAs quantum dots
Authors: Migliorato M. A. , Powell D. , Liew S. L. , Cullis A. G. , Navaretti P. , Steer M. J. , Hopkinson M. , Fearn M. , Jefferson J. H.
Published: October 2004

We address fundamental issues relating to the symmetry of the shape and the nonuniform composition of InGaAs quantum dot islands. Using atomistic simulations in the framework of the Tersoff empirical potential, we study the effect of compositional gradients in the In distribution on the piezoelectric effect in quantum dots. We demonstrate that the internal piezoelectric fields contribute strongly to the experimentally observed optical anisotropies. This is confirmed by accurate high-resolution transmission electron microscopy analysis over hundreds of islands grown in different conditions that reveals the absence of structural anisotropy under our growth conditions.
Solid state microcavity dye lasers fabricated by nanoimprint lithography
Authors: Nilsson D. , Nielsen T. , Kristensen A.
Published: October 2004

We present a solid state polymer microcavity dye laser, fabricated by thermal nanoimprint lithography (NIL) in a dye-doped thermoplast. The thermoplast poly-methylmethacrylate (PMMA) is used due to its high transparency in the visible range and its robustness to laser radiation. The laser dye is Rhodamine 6G ClO4. This dye is shown to withstand temperatures up to 240 °C without bleaching, which makes it compatible with the thermal nanoimprint lithography process. The 1.55 μm thick dye-doped PMMA devices are fabricated on a SiO2 substrate, yielding planar waveguiding in the dye-doped PMMA with two propagating TE–TM modes. The laser cavity has the lateral shape of a trapezoid, supporting lasing modes by reflection on the vertical cavity walls. The solid polymer dye lasers emit laterally through one of the vertical cavity walls, when pumped optically through the top surface by means of a frequency doubled, pulsed Nd:YAG laser. Lasing in the wavelength region from 560 to 570 nm is observed from a laser with a side-length of 50 μm. In this proof of concept, the lasers are multimode with a mode wavelength separation of approximately 1.6 nm, as determined by the waveguide propagation constant(s) and cavity dimensions. The stamps used in this work were fabricated by UV-lithography, limiting the lateral dimensional control of the devices. The resolution of NIL is ultimately limited by the quality of the stamps. Using electron beam lithography for stamp fabrication, the NIL process presented here offers the possibility for adding mode-selecting elements, e.g., diffractive- or sub-wavelength optical elements.
Laplace-transform deep-level spectroscopy: The technique and its applications to the study of point defects in semiconductors
Authors: Dobaczewski L. , Peaker A. R. , Bonde Nielsen K.
Published: October 2004

We present a comprehensive review of implementation and application of Laplace deep-leve1 transient spectroscopy (LDLTS). The various approaches that have been used previously for high-resolution DLTS are outlined and a detailed description is given of the preferred LDLTS method using Tikhonov regularization. The fundamental limitations are considered in relation to signal-to-noise ratios associated with the measurement and compared with what can be achieved in practice. The experimental requirements are discussed and state of the art performance quantified. The review then considers what has been achieved in terms of measurement and understanding of deep states in semiconductors through the use of LDLTS. Examples are given of the characterization of deep levels with very similar energies and emission rates and the extent to which LDLTS can be used to separate their properties. Within this context the factors causing inhomogeneous broadening of the carrier emission rate are considered. The higher resolution achievable with LDLTS enables the technique to be used in conjunction with uniaxial stress to lift the orientational degeneracy of deep states and so reveal the symmetry and in some cases the structural identification of defects. These issues are discussed at length and a range of defect states are considered as examples of what can be achieved in terms of the study of stress alignment and splitting. Finally the application of LDLTS to alloy systems is considered and ways shown in which the local environment of defects can be quantified.
Ion-beam-induced embedded nanostructures and nanoscale mixing
Authors: Satpati B. , Satyam P. V. , Som T. , Dev B. N.
Published: October 2004

Megaelectron volts ion-induced effects for discontinuous gold nanoislands and for continuous gold films on silicon substrate have been studied. Irradiation was carried out with 1.5MeV Au2+ ions at room temperature to various fluences. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry are used to study the ion-beam mixing in Au∕Si systems. At a fluence of 1×1014ionscm−2, a material push-in effect and a metastable Au-Si phase formation have been observed for Au nanoislands, while no push in or mixing has been observed for the case of continuous films. The mixed phase of Au-Si system is found to be crystalline in nature. The material push- in and ion-beam mixing effects that are observed in case of nanoislands appear to be due to combined effect of capillary driving force, ion-induced viscous flow, and ion-induced energy spike effects.
An investigation of eddy-current damping of multi-stage pendulum suspensions for use in interferometric gravitational wave detectors
Authors: Plissi M. V. , Torrie C. I. , Barton M. , Robertson N. A. , Grant A. , Cantley C. A. , Strain K. A. , Willems P. A. , Romie J. H. , Skeldon K. D. , Perreur-Lloyd M. M. , Jones R. A. , Hough J.
Published: October 2004

In this article we discuss theoretical and experimental investigations of the use of eddy-current damping for multi-stage pendulum suspensions such as those intended for use in Advanced LIGO, the proposed upgrade to LIGO (the US laser interferometric gravitational-wave observatory). The design of these suspensions is based on the triple pendulum suspension design developed for GEO 600, the German/UK interferometric gravitational wave detector, currently being commissioned. In that detector all the low frequency resonant modes of the triple pendulums are damped by control systems using collocated sensing and feedback at the highest mass of each pendulum, so that significant attenuation of noise associated with this so-called local control is achieved at the test masses. To achieve the more stringent noise levels planned for Advanced LIGO, the GEO 600 local control design needs some modification. Here we address one particular approach, namely that of using eddy-current damping as a replacement or supplement to active damping for some or all of the modes of the pendulums. We show that eddy-current damping is indeed a practical alternative to the development of very low noise sensors for active damping of triple pendulums, and may also have application to the heavier quadruple pendulums at a reduced level of damping.
Microcavity-like leaky mode emission from a planar optical waveguide made of luminescent silicon nanocrystals
Authors: Valenta J. , Ostatnický T. , Pelant I. , Elliman R. G. , Linnros J. , Hönerlage B.
Published: October 2004

The propagation of light emitted from silicon nanocrystals forming planar waveguides buried in SiO2 is studied both experimentally and theoretically. Experiments reveal that photoluminescence spectra detected from the sample facet mainly contains narrow (10–20nm full-with-at-half-maximum) polarization-resolved transverse electric and transverse magnetic modes instead of the usual broad nanocrystal emission band peaked at 700–800nm. A theoretical model developed in the framework of wave optics identifies these modes as substrate modes propagating along the waveguide boundary (not the usual modes guided inside the nanocrystal plane due to its graded index profile). This peculiar observation is the consequence of the high losses in the nanocrystalline waveguide and may occur in other dissipative waveguide structures under conditions that are discussed.
The morphotropic phase boundary and dielectric properties of the xPb(Zr1∕2Ti1∕2)O3-(1−x)Pb(Ni1∕3Nb2∕3)O3 perovskite solid solution
Authors: Vittayakorn Naratip , Rujijanagul Gobwute , Tan Xiaoli , Marquardt Meagen A. , Cann David P.
Published: October 2004

The solid solution between the normal ferroelectric Pb(Zr1∕2Ti1∕2)O3 (PZT) and relaxor ferroelectric Pb(Ni1∕3Nb2∕3)O3 (PNN) was synthesized by the columbite method. The phase structure and dielectric properties of xPZT-(1−x)PNN where x=0.4–0.9 and the Zr∕Ti composition was fixed close to the morphotropic phase boundary (MPB) were investigated. With these data, the ferroelectric phase diagram between PZT and PNN has been established. The relaxor ferroelectric nature of PNN gradually transformed towards a normal ferroelectric state towards the composition 0.7PZT-0.3PNN, in which the permittivity was characterized by a sharp peak and the disappearance of dispersive behavior. X-ray diffraction analysis demonstrated the coexistence of both the rhombohedral and tetragonal phases at the composition 0.8PZT-0.2PNN, a new morphotropic phase boundary within this system. Examination of the dielectric spectra indicates that PZT-PNN exhibits an extremely high relative permittivity near the MPB composition. The permittivity shows a shoulder at the rhombohedral to tetragonal phase transition temperature TRT=195°C, and then a maximum permittivity (36 000 at 10kHz) at the transition temperature Tmax=277°C at the MPB composition. The maximum transition temperature of this system was 326°C at the composition x=0.9 with the relative permittivity of 32 000 at 10kHz.
Electrical activation of high-concentration aluminum implanted in 4H-SiC
Authors: Negoro Y. , Kimoto T. , Matsunami H. , Schmid F. , Pensl G.
Published: October 2004

High-dose aluminum-ion (Al+) implantation into 4H-SiC (0001) and (112¯0) has been investigated. The dependences of the electrical properties on the implanted Al+ dose and on the annealing time were examined by Hall-effect measurements. A low sheet resistance of 2.3kΩ∕◻ (0.2μm deep) was obtained in a (0001) sample by implantation of Al+ with a dose of 3.0×1016cm−2 at 500°C and a subsequent high-temperature anneal at 1800°C for a short time of 1min. In the case of (112¯0) samples, even room-temperature implantation resulted in a low sheet resistance of 2.3kΩ∕◻ (0.2μm-deep) after anneal at 1800°C. The Hall data are compared with the calculated values determined by using the doping-concentration dependent ionization energy of Al acceptors. The experimentally obtained free-hole concentrations agree well with the theoretically expected values. Hole mobilities are not as high as the empirical mobilities obtained in Al-doped epitaxial layers. The differences in the electrical properties between the experimental data and expected values are discussed.
Emission properties of Ho3+∕Tb3+ Co-doped in Ge30Ga2As8S60 glass
Authors: Lee Tae Hoon , Heo Jong , Choi Yong Gyu , Park Bong Je , Chung Woon Jin
Published: October 2004

Emission properties of Ho3+ and Ho3+∕Tb3+ doped Ge30Ga2As8S60 glasses were investigated. Population densities of the I55 and I75 levels and the gain coefficients for the 1.6μm emission were calculated. Upon the addition of more than 0.05mol% Tb3+, population inversion between the I55 and I75 levels was achieved while it was not possible in the glass doped with Ho3+ only. The positive optical gain in the 1620–1750nm wavelength region was obtained with the addition of 0.3mol% Tb3+ ions.
Monitoring of cement hydration by broadband time-domain-reflectometry dielectric spectroscopy
Authors: Hager N. E. , Domszy R. C.
Published: October 2004

The broadband complex permittivity is monitored continuously in hydrating cement paste over the frequency range of 10kHz–8GHz and from initial mixing to several weeks of cure. Measurements are made by time domain reflectometry (TDR) dielectric spectroscopy, using an adjustable capacitance sensor, which can be embedded in the material in situ. The results are fit to a relaxation model, which includes terms representing (1) a Cole-Davidson relaxation near 1MHz, which grows initially and then decreases with an advancing cure; (2) a Debye relaxation near 100MHz, which grows initially and then decreases with an advancing cure; (3) a free-water relaxation near 10GHz, which decreases with an advancing cure; and (4) an ion conductivity and electrode polarization, which decreases with an advancing cure. The model is fit continuously as function of cure time extracting parameters for the relaxation amplitudes, relaxation frequencies, and distribution parameters as a function of cure time. The results are contrasted with measurements in tricalcium silicate and tricalcium silicate with varying ion content, revealing differences that may indicate the nature of the processes involved. Alternative methods for extracting reaction-rate information directly from the TDR transient are presented, providing a robust monitoring procedure usable in the field. Such methods are demonstrated using a variation in temperature and comparison with measurements of reaction heat obtained by calorimetry.
Excited state dynamics of Pr3+ in YVO4 crystals
Authors: Boutinaud Philippe , Mahiou Rachid , Cavalli Enrico , Bettinelli Marco
Published: October 2004

The quenching mechanism of the emission from the P03 level of Pr3+ doped into YVO4 has been investigated by means of optical spectroscopy techniques. Continuous wave and time-resolved emission and excitation measurements have been carried out as a function of the temperature and of the dopant concentration in order to obtain information on the energy-transfer processes involved in the host sensitization of the red luminescence and in the depopulation of the emitting states. The experiments have also been extended to LaVO4:Pr3+ and YPO4:Pr3+, i.e., two crystals for which the P03 emission is stronger than for YVO4:Pr3+, with the aim of studying the relationship between of the host properties and the optical behavior of the Pr3+ ion.
Polarization selection due to scattering losses in nonaxisymmetric vertical cavity surface emitting laser cavities
Authors: Riyopoulos S. , Nhan E.
Published: October 2004

A theoretical explanation is offered for the polarization pinning in vertical cavity surface emitting laser cavities with deliberately imposed lack of axisymmetry. The analytic computation of scattering losses shows that, for medium to small aperture sizes, differentiation in modal Q’s among the two polarizations is sufficient to overrule intrinsic near-degeneracies and maintain polarization throughout operation range. It is also shown that the prevailing polarization, the one with lowest round-trip losses, is aligned with the longest aperture dimension, in agreement with experiments.
Effect of Au cap layer on the magnetic properties and the microstructure for FePt thin films
Authors: Yuan F. T. , Chen S. K. , Chang W. C. , Horng Lance
Published: October 2004

We have investigated the effect of interfacial diffusion on the magnetic properties of Au∕Fe51Pt49 bilayer thin films. The samples were prepared in two stages. First, an ordered Fe51Pt49 continuous thin film was sputtered on a quartz substrate. Then an aurum cap layer was deposited onto the Fe51Pt49 film at room temperature, followed by a postannealing at 300–800°C to promote the interfacial diffusion. A high coercivity of 23.5kOe was achieved in the isotropic polycrystalline FePt-based film. Transmission electron micrographs indicate that the bilayer sample exhibits Ll0 ordered FePt grains with a diameter of about 20nm, which were partially isolated by Au phase. We consider that the isolation of FePtLl0 grains and the grain refining effect can be two contributory factors for the enhanced coercivity.
Formation of one-dimensional surface grooves from pit instabilities in annealed SiGe∕Si(100) epitaxial films
Authors: Gray J. L. , Hull R. , Floro J. A.
Published: October 2004

Growth of Si0.7Ge0.3 on Si under kinetically limited conditions results in the formation of shallow strain-relieving pits that only partially penetrate the wetting layer. Upon annealing at the growth temperature of 550°C, these pits elongate in one of the ⟨100⟩ directions and obtain near-{105} facets. The length-to-width aspect ratio of the resulting grooves can be as large as 20. Material ejected from the pits accumulates along the sides of the elongated pit forming shallow islands with a shape that exhibits a monotonic dependence on island size, and eventually evolves to {105} facets. We discuss the origins of this roughening behavior, which may provide a route for self-assembly of highly anisotropic quantum nanostructures.
High-dielectric-tunability of ferroelectric (Pb,Sr)TiO3 thin films on (001) LaAlO3
Authors: Liu S. W. , Lin Y. , Weaver J. , Donner W. , Chen X. , Chen C. L. , Jiang J. C. , Meletis E. I. , Bhalla A.
Published: October 2004

Ferroelectric (Pb,Sr)TiO3 (PSTO) thin films were epitaxially grown on (001) LaAlO3 (LAO) by using pulsed laser deposition. Microstructural characterizations with x-ray diffraction and transmission electron microscopy indicate that the as-grown films have excellent single crystalline quality and a (001)PSTO∕∕(001)LAO and [100] PSTO∕∕ [100] LAO interface relationship. Dielectric property measurements reveal that the as-grow films have a very high dielectric constant value of 3100 and very large dielectric tunability of 48% at 40V∕cm at room temperature. These excellent results suggest that the highly epitaxial ferroelectric (Pb,Sr)TiO3 thin films can be developed for room-temperature tunable microwave elements in wireless communication applications.
Bottom-up soft-lithographic fabrication of three-dimensional multilayer polymer integrated optical microdevices
Authors: Huang Yanyi , Paloczi George T. , Poon Joyce K. S. , Yariv Amnon
Published: October 2004

We develop a method to efficiently fabricate three-dimensional multilayer polymer microchips for integrated optical applications. This method uses soft lithography to mold the core structures on top of the cladding layers. By repeating the process, a three-dimensional multilayer integrated optical microdevice, which consists of several layers of individual planar optical devices, is fabricated. We demonstrate a dual-layer microring resonator optical filter device where the devices in the different layers show essentially identical transmission responses.
Ferroelectric properties of 0.87Na0.5Bi0.5TiO3–0.13PbTiO3 thin film prepared by metalorganic solution deposition
Authors: Wang Z. , Yang C. H. , Tao Y. , Wang C. L. , Zhao M. L.
Published: October 2004

0.87 Na 0.5 Bi 0.5 Ti O 3 – 0.13 Pb Ti O 3 thin film has been prepared by metalorganic solution deposition on Pt∕TiO2∕SiO2∕Si and SiO2∕Si substrates. The film shows good switching endurance under bipolar stressing up to 3×1010 switching cycles with a remanent polarization 2Pr of about 3μC∕cm2 and a coercive field Ec of approximate 56kV∕cm. The high frequency C–V curve is indicative of good film/substrate interface characteristics. The film could be used in storage capacitors, CMOS integrated devices, and insulation gate field-effect transistors.
Quantum capacitance in nanoscale device modeling
Authors: John D. L. , Castro L. C. , Pulfrey D. L.
Published: October 2004

Expressions for the “quantum capacitance” are derived, and regimes are discussed in which this concept may be useful in modeling electronic devices. The degree of quantization is discussed for one- and two-dimensional systems, and it is found that two-dimensional (2D) metals and one-dimensional (1D) metallic carbon nanotubes have a truly quantized capacitance over a restricted bias range. For both 1D and 2D semiconductors, a continuous description of the capacitance is necessary. The particular case of carbon nanotube field-effect transistors (CNFETs) is discussed in the context of one-dimensional systems. The bias regime in which the quantum capacitance may be neglected when computing the energy band diagram, in order to assist in the development of compact CNFET models, is found to correspond only to the trivial case where there is essentially no charge, and a solution to Laplace’s equation is sufficient for determining a CNFET’s energy band diagram. For fully turned-on devices, then, models must include this capacitance in order to properly capture the device behavior. Finally, the relationship between the transconductance of a CNFET and this capacitance is revealed.
Raman study of As outgassing and damage induced by ion implantation in Zn-doped GaAs
Authors: Barba D. , Aimez V. , Beauvais J. , Beerens J. , Drouin D. , Chicoine M. , Schiettekatte F.
Published: October 2004

Room temperature micro-Raman investigations of LO phonon and LO phonon-plasmon coupling is used to study the As outgassing mechanism and the disordering effects induced by ion implantation in Zn-doped GaAs with nominal doping level p=7×1018cm−3. The relative intensity of these two peaks is measured right after rapid vacuum thermal annealings (RVTA) between 200 and 450°C, or after ion implantations carried out at energies of 40keV with P+, and at 90 and 170keV with As+. These intensities provide information regarding the Schottky barrier formation near the sample surface. Namely, the Raman signature of the depletion layer formation resulting from As desorption is clearly observed in samples submitted to RVTA above 300°C, and the depletion layer depths measured in ion implanted GaAs:Zn are consistent with the damage profiles obtained through Monte Carlo simulations. Ion channeling effects, maximized for a tilt angle set to 45° during implantation, are also investigated. These results show that the Raman spectroscopy is a versatile tool to study the defects induced by postgrowth processes in multilayered heterostructures, with probing range of about 100nm in GaAs-based materials.
Analytic, graphical, and geometric solutions for the band edges of one-dimensional photonic band gap materials
Authors: Szmulowicz Frank
Published: October 2004

The conditions for the band gap edges of one-dimensional photonic band gap (PBG) materials are presented in forms that lead to alternate analytic, graphical, and geometric solutions and interpretations. Heretofore unavailable analytical conditions for the band edges of quarter wave∕half wave and eighth wave∕half wave PBG structures are derived. The graphical solution has the form of an intersection of an invariant (material dependent) figure with a straight (structure dependent) line, a convenient separation not possible with the Kronig-Penney (KP) equation. The geometric solution is represented by the sides and angles of simple triangles. The present formalism should help in visualizing the PBG band gap formation beyond the view offered by the KP equation.
Generation and behavior of pure-edge threading misfit dislocations in InxGa1−xN∕GaN multiple quantum wells
Authors: Lü W. , Li D. B. , Li C. R. , Zhang Z.
Published: October 2004

The relaxation of the misfit strain by the formation of misfit dislocations in InxGa1−xN∕GaN multiple quantum wells grown by metal-organic chemical-vapor deposition was investigated by the cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. It is found that the misfit dislocations generated from strain relaxation are all pure-edge threading dislocations with burgers vectors of b=1∕3⟨112¯0⟩. The misfit dislocations arise from the strain relaxation due to the thickness of strained layer greater than the critical thickness. The relaxation of strained layer was mainly achieved by the formation of dislocations and localization of In, while the dislocations changed their slip planes from {0001}to{101¯0}. With the increasing temperature, the efficiency of photoluminescence decrease sharply. It indicates that the relaxation of the misfit strain has a strong effect on optical efficiency of film.
Diffusion of boron in 6H and 4H SiC coimplanted with boron and nitrogen ions
Authors: Usov I. O. , Suvorova A. A. , Kudriavtsev Y. A. , Suvorov A. V.
Published: October 2004

The diffusion behavior of boron (B) and nitrogen (N) implanted in 6H and 4H silicon carbide (SiC) samples was investigated using secondary ion mass spectroscopy. The samples were either coimplanted with B and N ions or implanted with each element alone. The annealing was performed at 1700°C for times ranging from 10to1800s in argon ambient or in the vapors of silicon and carbon. Transmission electron microscopy has been used to determine the structural properties of implanted layers after the annealing. The N concentration profiles remained unchanged after the annealing. B atoms showed transient enhanced out- and in-diffusion. The coimplantation reduced the fraction of mobile B atoms participating in out- and in-diffusion processes and resulted in an increase in the density and decrease in size of dislocation loops formed in the implanted layer. The B diffusion coefficients in both SiC polytypes have been determined and a diffusion mechanism has been discussed.
Effects of high-dose 40MeV proton irradiation on the electroluminescent and electrical performance of InGaN light-emitting diodes
Authors: Khanna Rohit , Allums K. K. , Abernathy C. R. , Pearton S. J. , Kim Jihyun , Ren F. , Dwivedi R. , Fogarty T. N. , Wilkins R.
Published: October 2004

InGaN multi-quantum-well light-emitting diodes (LEDs) in the form of unpackaged die with emission wavelengths from 410 to 525nm were irradiated with 40MeV protons to doses of 5×109–5×1010cm−2. The highest dose is equivalent to more than 100 years in low-earth orbit. The projected range of these protons is >50μm in GaN and thus they traverse the entire active region. The electroluminescent intensity from the LEDs decreased by only 15%–25% even for the highest doses and the reverse breakdown voltage increased by 1–2V from their control values of ∼21–29V. The percentage change in breakdown voltage and electroluminescence intensity was independent of the initial emission wavelength over the range investigated, within experimental error. The GaN LEDs exhibit extremely good stability to these high-energy proton irradiations with no measurable change in contact resistance or contact morphology.
p
-type conduction in N–Al co-doped ZnO thin films
Authors: Lu J. G. , Ye Z. Z. , Zhuge F. , Zeng Y. J. , Zhao B. H. , Zhu L. P.
Published: October 2004

p -type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3Ωcm with a Hall mobility of 0.43cm2∕Vs and carrier concentration of 2.25×1017cm−3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region.
Vertical composition gradient in InGaAs∕GaAs alloy quantum dots as revealed by high-resolution x-ray diffraction
Authors: Hanke M. , Grigoriev D. , Schmidbauer M. , Schäfer P. , Köhler R. , Sellin R. L. , Pohl U. W. , Bimberg D.
Published: October 2004

Shape and composition profiles of self-organized In0.6Ga0.4As∕GaAs quantum dots (QDs) were investigated using diffuse x-ray scattering of a fivefold QD stack. To reveal the QD morphology, numerical scattering simulations of QDs with different morphologies were performed based on three-dimensional strain fields calculated by the finite element methods. Comparing our simulations to the data, we proved that the In concentration increases from the wetting layer to the top of the quantum dots. Moreover, we conclude that the In concentration of the wetting layers is significantly lower than the average value in the QDs.
Interband cascade lasers grown on GaAs substrates lasing at 4 microns
Authors: Hill Cory J. , Yang Rui Q.
Published: October 2004

Antimonide-based type II interband cascade lasers grown on GaAs substrates have been demonstrated at emission wavelengths longer than 4μm at temperatures up to 270K in pulsed-mode operation. Investigations by transmission electron microscopy, atomic force microscopy and x-ray diffraction are presented and discussed to gain insights into material issues such as defects and dislocations associated with device performance.
Experimental observation of delay-induced radio frequency chaos in a transmission line oscillator
Authors: Blakely Jonathan N. , Corron Ned J.
Published: October 2004

We report an experimental study of fast chaotic dynamics in a delay dynamical system. The system is an electronic device consisting of a length of coaxial cable terminated on one end with a diode and on the other with a negative resistor. When the negative resistance is large, the system evolves to a steady state. As the negative resistance is decreased, a Hopf bifurcation occurs. By varying the length of the transmission line we observe Hopf frequencies from 7–53 MHz. With the transmission line length fixed, we observe a period doubling route to chaos as the negative resistance is further reduced providing the first experimental confirmation of an existing theoretical model for nonlinear dynamics in transmission line oscillators [Corti et al., IEEE Trans. Circ. Syst., I: Fundam. Theory Appl. 41, 730 (1994)]. However, other experimental results indicate limitations to this model including an inability to predict the Hopf frequency or to produce realistic continuous wave forms. We extend the model to include finite bandwidth effects present in a real negative resistor. The resulting model is a neutral delay differential equation that provides better agreement with experimental results.
Polarization-sensitive complex Fourier domain optical coherence tomography for Jones matrix imaging of biological samples
Authors: Yasuno Yoshiaki , Makita Shuichi , Endo Takashi , Itoh Masahide , Yatagai Toyohiko , Takahashi Mari , Katada Chikatoshi , Mutoh Manabu
Published: October 2004

A polarization-sensitive Fourier domain optical coherence tomography (PS-FD-OCT) system is demonstrated. This OCT system is based on a spectral interferometer, does not require mechanical axial scanning, and enables phase information to be used in an OCT image. Owing to this phase information, the system requires only two measurements for determining the Jones matrix images and Müller images of biological samples. This system reveals the birefringence properties of the inner surface of a porcine esophagus.
Ultraviolet light responses in photovoltaic properties of TiO2∕conducting polymer heterostructure devices
Authors: Umeda Tokiyoshi , Hashimoto Yuuki , Mizukami Hiroyoshi , Shirakawa Tomoki , Fujii Akihiko , Yoshino Katsumi
Published: October 2004

Photovoltaic properties of heterostructure devices of titanium oxide (TiO2) and conducting polymer thin films have been studied. Normal photovoltaic properties were stably observed upon the visible light irradiation of the wavelength range corresponding to the absorption of a conducting polymer, and unique photovoltaic properties were also observed upon the ultraviolet (UV) light irradiation of the wavelength range corresponding to the absorption of TiO2. UV light irradiation caused marked increases in the short-circuit current at the visible-wavelength range and in the open-circuit voltage. These properties have been discussed by considering the enhancement of the built-in field which originates from the hole accumulation caused by the trap levels in the TiO2 layer or TiO2∕conducting polymer interface.
Ultrahigh room-temperature hole mobility in a SiGe quantum well
Authors: Myronov M. , Parry C. P. , Mironov O. A. , Parker E. H. C.
Published: October 2004

We report an ultrahigh room-temperature hole drift mobility obtained in a Si0.2Ge0.8 quantum well with a parabolic-like Ge profile, which has at its core a p-type modulation-doped (MOD) Si∕Si0.2Ge0.8∕Si0.65Ge0.35∕Si(001) heterostructure. High-conductivity holes at 293 K with a drift mobility of 3600cm2V−1s−1 at a sheet carrier density of 4.94×1012cm−2 were obtained in the Si0.2Ge0.8 quantum well after optimum annealing at 750 °C for 30 min. Hall mobility and sheet carrier density of this heterostructure are 1776cm2V−1s−1 and 2.37×1013cm−2, respectively. Structural characterization of the as-grown and the annealed samples revealed that the annealing had caused Si0.2Ge0.8 channel broadening, smearing of interfaces, and formation of a parabolic-like Ge profile that significantly improved room-temperature hole transport properties. The reported values of hole mobility are much higher than in the bulk Ge.
Probing electron charging in nanocrystalline Si dots using Kelvin probe force microscopy
Authors: Salem M. A. , Mizuta H. , Oda S.
Published: October 2004

By using Kelvin probe force microscopy, we investigate the contact potential difference (CPD) of nanocrystalline silicon (nc-Si) dots with various sizes before and after the dots charging. Few electrons are injected into the nc-Si dots using the atomic force microscope tip. A remarkable change in the dot potential is observed under the normal ambient conditions. Since the change in the dot potential represents the charging energy of the nc-Si dots, the number of electrons stored in the individual dots can be estimated by comparing the calculated charging energy and the measured CPD change caused by charging. We demonstrate that charge quanta are indeed injected and directly detected by this method by analyzing the dot diameter dependence of the CPD change for each dot.