bluuebunny/crossref_metadata_2025 huggingface dataset:
High pressure ultrasonic and x-ray studies on monolithic SiC composite
Authors: Amulele George M. , Manghnani Murli H. , Li Baosheng , Errandonea Daniel J. H. , Somayazulu Maddury , Meng Yue
Published: January 2004

The equation of state (pressure-volume relationship) of a monolithic SiC composite has been determined separately by high pressure ultrasonic and x-ray techniques. The ultrasonic measurements were performed on a cylindrical specimen, 2.2 mm in diameter and 2.2 mm in length, in a large-volume uniaxial split cylinder press to 13.6 GPa. The synchrotron measurements were carried out on the polycrystalline sample loaded in a diamond anvil cell up to a pressure of 27 GPa. The room temperature bulk modulus obtained from the ultrasonic measurements is K0=216.5±1.1 GPa and the pressure derivative is K0′=4.19±0.09. The shear modulus obtained is G0=196.7±0.7 GPa with its pressure derivative G0′=0.95±0.03. Using K0′ obtained from the ultrasonic measurements in the Birch–Murnaghan equation to fit the synchrotron x-ray data, we obtain a bulk modulus of K0=218.4±4.9 GPa for the composite.
Optical propagation loss of ZnO films grown on sapphire
Authors: Ashida A. , Ohta H. , Nagata T. , Nakano Y. , Fujimura N. , Ito T.
Published: January 2004

Optical propagation losses from ZnO thin films grown on c-cut sapphire were measured for 632.8 nm light using prism coupling and a charge coupled device detection method. Propagation losses varied according to growth temperature and growth rate. No relationship between the distribution of crystallographic orientation and propagation losses was observed. However, propagation losses were found to be insensitive to changes in the distribution of grain orientations. Neither the depth of surface roughness nor the mean interval of the surface undulations was found to be a dominant factor contributing to the losses. A mean angle of surface roughness, α, is proposed as a parameter to comprehensively characterize both effects. Loss is found to depend on α for propagation of both TE0 and TE1 modes. This demonstrates that the parameter α effectively characterizes surface morphology for optical device applications.
Thermal contact conductance of adhered microcantilevers
Authors: Huxtable Scott T. , Cahill David G. , Phinney Leslie M.
Published: January 2004

The thermal contact conductance G for polycrystalline silicon cantilever beams that are adhered to an underlying substrate is examined using two different optical techniques. Using time-domain thermoreflectance, we measure G=9±2 MW m−2 K−1 at 25 °C and G=4±1 MW m−2 K−1 at 150 °C. The room temperature value is confirmed using a modified Ångström method, which establishes a lower limit of G>5 MW m−2 K−1. This contact conductance is a factor of 10–105 greater than values reported for metal–metal and ceramic–ceramic interfaces. The large interfacial conductance is consistent with the presence of a thin layer of water trapped between the cantilever and the substrate. The thermal conductivity Λ of the phosphorus doped polysilicon cantilever is nearly isotropic with Λcross plane=65 W m−1 K−1, and Λin plane=70 W m−1 K−1 at room temperature.
Surface states in template synthesized tin oxide nanoparticles
Authors: Cabot A. , Arbiol J. , Ferré R. , Morante J. R. , Chen Fanglin , Liu Meilin
Published: January 2004

Tin–oxide nanoparticles with controlled narrow size distributions are synthesized while physically encapsulated inside silica mesoporous templates. By means of ultraviolet-visible spectroscopy, a redshift of the optical absorbance edge is observed. Photoluminescence measurements corroborate the existence of an optical transition at 3.2 eV. The associated band of states in the semiconductor gap is present even on template-synthesized nanopowders calcined at 800 °C, which contrasts with the evolution of the gap states measured on materials obtained by other methods. The gap states are thus considered to be surface localized, disappearing with surface faceting or being hidden by the surface-to-bulk ratio decrease.
Energy distribution and spatial structure of electrosprays of ionic liquids in vacuo
Authors: Romero-Sanz I. , Fernandez de la Mora J.
Published: January 2004

The ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMI+BF4−) is electrosprayed in a vacuum from positively charged Taylor cones, and the full sprays are analyzed by time of flight mass spectrometry (TOFMS). They contain predominantly the ions (EMI-BF4)nEMI+ (n=0,1,2), at current levels of several μA. Current versus arrival time distributions I(t,L) are measured in a planar disk collector at varying sources to collector distances L, yielding ion-resolved angular current distribution in the polar angle range 21°<θ<37°. Comparison of experimental TOF curves to those expected for the measured angular distribution for a plane collector and a point source shows that the ions are essentially monoenergetic. Hence the present ion source is similar in quality to liquid metal ion sources, while offering a much richer range of chemical composition and mass, including large molecular positive and negative ions. Reinterpretation of the TOF curves as angular distributions shows (indirectly and provisionally) that most ions are at about 20°, with few passing below 10°.
Model of thermal erasure in neighboring tracks during thermomagnetic writing
Authors: Lyberatos A. , Hohlfeld J.
Published: January 2004

A method is presented for the calculation of the thermal erasure of the recorded information in neighboring tracks and implemented for heat-assisted magnetic recording on FePt granular thin films with high perpendicular anisotropy. The thermal erasure in neighboring tracks decays exponentially with distance from the recording head. It is strongly dependent on the heating, grain size, and anisotropy but the write field has a relatively small effect. The track width is dependent primarily on the temperature profile of the laser beam. Secondary effects arising from the write field, anisotropy, intergranular magnetostatic, and exchange coupling are also considered. A more square shaped temperature profile of the laser beam results in substantial improvement in recording properties; enhancement of read signal and sharpness of the magnetic transition and reduction of transition jitter and lubricant loss.
Application of real-time, stroboscopic x-ray diffraction with dynamical mechanical analysis to characterize the motion of ferroelastic domain walls
Authors: Harrison Richard J. , Redfern Simon A. T. , Buckley Andrew , Salje Ekhard K. H.
Published: January 2004

The dynamic response of ferroelastic twins to an alternating stress has been studied in situ at high temperature using a stroboscopic x-ray diffractometer and combined dynamical mechanical analyzer (XRD-DMA). The XRD-DMA is designed to allow x-ray rocking curves to be collected while the sample is undergoing simultaneous dynamical mechanical analysis in three-point-bend geometry. The detection of diffracted x-rays is synchronized with the applied load, so that rocking curves corresponding to different parts of the dynamic load cycle can be obtained separately. The technique is applied to single-crystal LaAlO3, which undergoes a cubic to rhombohedral phase transition at 550 °C, leading to the generation of characteristic “chevron” twins. The rocking-curve topology is calculated as a function of crystal orientation for each chevron type. Systematic changes in the rocking curves during heating and cooling under dynamic load demonstrate a clear preference for chevrons containing {100}pc walls perpendicular to the sample surface and {110}pc walls oriented at 45° to the sample surface. These domain walls are oriented favorably with respect to the applied stress (i.e., they separate domains with contrasting components of spontaneous strain parallel to the sample length). Below 200 °C, the superelastic strain is accommodated by rapid advancement/retraction of vertical {100}pc needle domains, with little effect on the dynamic rocking curves. Above 200 °C, a dynamic shift in peak position between rocking curves measured at the maximum and minimum applied loads is detected. The onset of a dynamic response correlates with the loss of the {100}pc needle domains and the transformation of the microstructure to 45° {110}pc walls. Superelastic strain is then accommodated by domain wall displacement/rotation, causing the wall to sweep back and forth across the x-ray beam and diffraction to occur from alternate domains at the maximum and minimum points of the stress cycle. A second sample, oriented so that domain walls in all possible chevrons are unfavorably oriented with respect to the applied stress, shows very different behavior. The rocking curves consist of several well-separated peaks at the minimum load and a single broad diffraction signal at the maximum load. This is caused by the creation of a very high density of twin walls across the sample above a critical applied stress, leading to corrugation of the sample surface.
Nucleation kinetics of Si quantum dots on SiO2
Authors: Nicotra G. , Puglisi R. A. , Lombardo S. , Spinella C. , Vulpio M. , Ammendola G. , Bileci M. , Gerardi C.
Published: January 2004

The formation of Si quantum dots on SiO2 by chemical vapor deposition of SiH4 has been investigated in the range from the submonolayer to the complete coverage with Si. In order to investigate the very early stages of the nucleation process of Si on SiO2, the energy filtered transmission electron microscopy has been chosen as the main characterization technique, because of the high spatial resolution typical of the transmission electron microscopy analysis, coupled to the compositional information obtained by the electron energy loss spectroscopy. The plan view configuration has been used to measure the dot size distributions down to dimensions of about 1 nm, and in cross section to evaluate the dot wetting angle. For all the several experimental conditions, a wetting angle distribution has been obtained and has shown to be centered at about 90°. Data on the dot size distributions are shown and discussed in the framework of a continuous nucleation model, which has been implemented to take into account the dot coalescence process contribution. Through the comparison with experimental data the relevant thermodynamic parameters of the process have been evaluated: the critical radius, the free energy barrier for nucleation, and the concentration of nucleation sites.
X-ray photoelectron spectroscopy of nitromethane adsorption products on Si(100): A model for N 1s core-level shifts in silicon oxynitride films
Authors: Eng J. , Hubner I. A. , Barriocanal J. , Opila R. L. , Doren D. J.
Published: January 2004

N  1s core-level shifts from x-ray photoelectron spectra (XPS) are reported for the adsorption products of nitromethane (CH3NO2) on Si(100). Three spectral peaks are identified and these are associated with specific bonding environments for nitrogen by comparison to predicted core-level shifts from density functional calculations on a range of energetically feasible chemical structures. These species can be classified according to the number of N–O bonds (zero, one, or two) that they contain and, in this sense, they are comparable to the species believed to exist in oxynitride films on Si. Since the energetically feasible products of room-temperature CH3NO2 adsorption can be identified with more confidence than those resulting from ion bombardment and high-temperature processing in oxynitride films, nitromethane provides a model system that can aid in correlating spectral features with specific atomic-scale structures. This work supports an earlier proposal that the XPS peak of weakest binding energy is due to species with a dangling bond on nitrogen, while the most intense peak is due to the energetically preferred NSi3 species.
Laser micro-Raman spectroscopy of single-point diamond machined silicon substrates
Authors: Yan Jiwang
Published: January 2004

Laser micro-Raman spectroscopy was used to examine the silicon substrates machined by single-point diamond turning at machining scales ranging from 10 to 1000 nm under plane strain conditions. The results showed that the subsurface layer was partially transformed to amorphous, the extent of amorphization depending strongly on the undeformed chip thickness. The intensities of the crystalline phase and the amorphous phase show opposite tendencies with respect to the undeformed chip thickness. In brittle regime machining, Raman spectra differ depending on the test locations. The intensity of the amorphous phase reaches maximum near the ductile–brittle transition boundary. In ductile regime machining, the intensity of the amorphous phase decreased sharply as the undeformed chip thickness decreased. This work provides technological insights into the possibility of direct manufacturing of subsurface damage-free optical and optoelectronic products of silicon by ductile machining without the need for or with a decreased need for subsequent etching or chemomechanical polishing.
Thermal activation of Mg-doped GaN as monitored by electron paramagnetic resonance spectroscopy
Authors: Zvanut M. E. , Matlock D. M. , Henry R. L. , Koleske Daniel , Wickenden Alma
Published: January 2004

The microscopic process involved with thermal activation of the Mg acceptors in GaN epitaxial films is investigated using electron paramagnetic resonance (EPR) spectroscopy. Samples were heat treated in dry N2 for 30 min at temperatures between 200 and 1000 °C. Below 850 °C, the data are consistent with release of hydrogen from a Mg complex and corroborate the initial assignment of the EPR signal to the p-type acceptor. However, the Mg-related EPR signal is permanently quenched for anneals above 850 °C suggesting that additional point defects are created during high temperature treatment.
Electronic properties of the AlGaN/GaN heterostructure and two-dimensional electron gas observed by electroreflectance
Authors: Kurtz S. R. , Allerman A. A. , Koleske D. D. , Baca A. G. , Briggs R. D.
Published: January 2004

A contacted electroreflectance technique was used to investigate AlGaN/GaN heterostructures and their intrinsic electric field-induced properties. By studying variations in the electroreflectance with applied field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were identified. Barrier-layer composition and electric field were determined from the AlGaN Franz–Keldysh oscillations. For a high mobility heterostructure grown on SiC, measured AlGaN polarization electric field and two-dimensional electron gas density approached values predicted by a standard bandstructure model. The two-dimensional electron gas produced a broad, field-tunable first derivative electroreflectance feature. With a dielectric function calculation, we describe the line shape and relative amplitude of the two-dimensional electron gas electroreflectance feature for a wide range of electron density and applied field values.
Fundamental structure of steady plastic shock waves in metals
Authors: Molinari A. , Ravichandran G.
Published: January 2004

The propagation of steady plane shock waves in metallic materials is considered. Following the constitutive framework adopted by R. J. Clifton [Shock Waves and the Mechanical Properties of Solids, edited by J. J. Burke and V. Weiss (Syracuse University Press, Syracuse, N.Y., 1971), p. 73] for analyzing elastic–plastic transient waves, an analytical solution of the steady state propagation of plastic shocks is proposed. The problem is formulated in a Lagrangian setting appropriate for large deformations. The material response is characterized by a quasistatic tensile (compression) test (providing the isothermal strain hardening law). In addition the elastic response is determined up to second order elastic constants by ultrasonic measurements. Based on this simple information, it is shown that the shock kinetics can be quite well described for moderate shocks in aluminum with stress amplitude up to 10 GPa. Under the later assumption, the elastic response is assumed to be isentropic, and thermomechanical coupling is neglected. The model material considered here is aluminum, but the analysis is general and can be applied to any viscoplastic material subjected to moderate amplitude shocks. Comparisons with experimental data are made for the shock velocity, the particle velocity and the shock structure. The shock structure is obtained by quadrature of a first order differential equation, which provides analytical results under certain simplifying assumptions. The effects of material parameters and loading conditions on the shock kinetics and shock structure are discussed. The shock width is characterized by assuming an overstress formulation for the viscoplastic response. The effects on the shock structure of strain rate sensitivity are analyzed and the rationale for the J. W. Swegle and D. E. Grady [J. Appl. Phys. 58, 692 (1985)] universal scaling law for homogeneous materials is explored. Finally, the ability to deduce information on the viscoplastic response of materials subjected to very high strain rates from shock wave experiments is discussed.
Carrier transport in multilayer organic photodetectors: I. Effects of layer structure on dark current and photoresponse
Authors: Xue Jiangeng , Forrest Stephen R.
Published: January 2004

In this and the following article (Parts I and II, respectively), we discuss carrier transport in multilayer organic photodetectors. In Part I, we analyze carrier tunneling in multilayer organic photodetectors, both within the organic active region and at the anode/organic interface. The external quantum efficiency of an organic photodetector whose individual layers in the active region are ⩽30 Å thick shows an activation energy of ⩽0.1 eV, suggesting that the photogenerated carriers tunnel through the potential wells formed by the multilayer stack. In such photodetectors, electron tunneling injection from the anode into the organic active region dominates the dark current at T⩾200 K. Fitting of the dark current–voltage characteristics using a semiclassical tunneling model leads to barrier heights in agreement with experimental data. In Part II, the effects of anode preparation on performance of multilayer organic photodetectors are discussed.
Carrier transport in multilayer organic photodetectors: II. Effects of anode preparation
Authors: Xue Jiangeng , Forrest Stephen R.
Published: January 2004

We address the effects of anode preparation on the performance of multilayer organic photodetectors introduced in the accompanying article (Part I) [J. Xue and S. R. Forrest, J. Appl. Phys. 95, 1859 (2004)]. Using atomic force microscopy and ultraviolet photoemission spectroscopy, we find that the surface morphology and electronic properties of indium–tin–oxide (ITO) anodes depend strongly on the anode preparation. ITO films deposited by rf magnetron sputtering at room temperature have smoother surfaces and 0.1–0.3 eV lower work functions than commercially obtained ITO films. Oxygen plasma and ultraviolet (UV)-ozone treatment can both increase the ITO work function, although the mechanisms leading to the increase have different origins. The dark current in a multilayer organic photodetector is reduced when the anode has a high work function, which agrees with our conclusion in Part I that electron tunneling injection from the anode into the organic active region dominates the dark current. The quantum efficiency of multilayer organic photodetectors is reduced by ∼10% for ITO anodes exposed to oxygen plasma or UV ozone, possibly due to an increased density of defect states at the anode/organic interface.
Coherent and stochastic charge tunneling in ferromagnetic single electron transistors
Authors: Jalil M. B. A. , Wang X.
Published: January 2004

We present a model of tunneling magnetotransport across a ferromagnetic single electron transistor (FM-SET). The model applies the “orthodox” theory and the Master equation method to describe the stochastic nature of separate tunneling events. The coherent transport of an individual charge through a tunnel barrier is modeled by a free-electron Hamiltonian through a trapezoidal barrier, and a two-band model within the FM electrodes. Single electron charging effect is incorporated into both stochastic and coherent parts of the model. The calculated tunneling resistance Rt is dependent on the thickness and voltage-dependent profile of the tunnel barriers. A bias modulation of the tunneling magnetoresistance (TMR) is obtained with the same period as the Coulomb staircase pattern. This modulation is present even for “homogeneous” FM-SETs, where each junction is of the same material composition. By contrast, previous studies based on the Julliere model predict total suppression of TMR modulation for such FM-SETs. The TMR modulation is found to be highly dependent on barrier thickness and the material composition of the junctions, implying further adjustable parameters for optimal TMR ratio. Finally, the bias modulation of TMR persists at high temperatures, even though the Coulomb staircase steps have been virtually smeared out.
Modeling of organic light-emitting diodes with graded concentration in the emissive multilayer
Authors: Gusso A. , Ma Dongge , Hümmelgen I. A. , da Luz M. G. E.
Published: January 2004

We model the electrical behavior of organic light-emitting diodes whose emissive multilayer is formed by blends of an electron transporting material, tris-(8-hydroxyquinoline) aluminum (Alq3) and a hole transporting material, N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4-diamine. The multilayer is composed of layers of different concentration. The Alq3 concentration gradually decreases from the cathode to the anode. We demonstrate that these graded devices have higher efficiency and operate at lower applied voltages than devices whose emissive layer is made of nominally homogeneous blends. Our results show an important advantage of graded devices, namely, the low values of the recombination rate distribution near the cathode and the anode, so that electrode quenching is expected to be significantly suppressed in these devices.
Shock-wave compression of a porous material
Authors: Resnyansky A. D. , Bourne N. K.
Published: January 2004

Porous materials may exhibit highly nonstationary behavior under shock-wave loading. The majority of existing experiments have measured the dependence between shock-wave velocity and particle velocity to define the Hugoniot for subsequent derivation of an equation of state. Such equations of state are nonconvex, which leads to significant thermodynamic and numerical problems. The present article suggests an experimental configuration and mathematical model, to overcome these difficulties. The experiment is based on a setup resulting in a continuous record of the stress profile with time using embedded manganin gauges. The model employs a homogenization approach enabling us to obtain a hyperbolic system of equations, which is completed with a convex equation of state so as to be suitable for implementation in commercial hydrocodes. Using available data for porous aluminum, an approach is elaborated for construction of constitutive equations. The model is tested with the present stress profiles in sand and demonstrates good agreement.
Effect of intrinsic stress on preferred orientation in AlN thin films
Authors: Gan B. K. , Bilek M. M. M. , McKenzie D. R. , Taylor M. B. , McCulloch D. G.
Published: January 2004

We examine the effect of ion impact energy on the intrinsic stress and microstructure of aluminum nitride thin films deposited using a filtered cathodic arc. The dependence of intrinsic stress on ion impact energy is studied over the range from 0 to 350 V using dc bias and up to several kV for a fraction of the ions using pulse bias. For dc bias, the stress reaches a maximum at 200 V and decreases with further increase in ion bias. The preferred orientation of the crystallites was studied by cross-section transmission electron microscopy and diffraction. We found that there is a preference for the c crystallographic axis to lie in the plane of the film under high intrinsic stress conditions (4 GPa), whereas a c-axis orientation perpendicular to the plane of the film was observed for low intrinsic stress (0.25 GPa). We carried out calculations of the expected distribution of intensity in cross-sectional electron diffraction patterns to predict the effect of rotation freedom of crystallites with the c axis pinned. The calculated patterns agreed well with experiment.
Lateral short range ordering of step bunches in InGaAs/GaAs superlattices
Authors: Hanke M. , Schmidbauer M. , Köhler R. , Kirmse H. , Pristovsek M.
Published: January 2004

In the present paper we report on structural investigations of fivefold In0.2Ga0.8As/GaAs superlattices which have been grown by means of metal organic chemical vapor deposition on vicinal GaAs(001) substrates. Cross-sectional transmission electron micrographs exhibit an initially flat and nonfaceted grooved surface, while step bunching occurs during subsequent growth stages with an inclined vertical inheritance approximately 45° off the (001) direction. A reconstructed sample cross section on the base of high resolution x-ray diffraction data qualitatively confirms the local morphology proved by transmission electron microscopy. Moreover, a line shape analysis of diffusely scattered intensity using Gauss profiles indicates a lateral short range ordering of step bunches.
Network of flatband solar cells as a model for solid-state nanostructured solar cells
Authors: Burgelman Marc , Grasso Catelijne
Published: January 2004

Nanostructured solar cells are too complex for standard modeling. Here, we decouple the effects at a microscopic (nm) scale from those at a macroscopic (μm) scale. The three-dimensional nanoporous geometry is simplified to a quasiperiodic nm-scale ordering of “unit cells,” each consisting of a TiO2 sphere and its p-semiconductor or dye/p-conductor shell. It is shown that, due to the periodic boundary conditions, such a unit cell is essentially field free; it is called here a “flatband cell.” The band diagram, including possible discontinuities, the Fermi levels, carrier density and recombination (interface and bulk) of these flatband cells is given accurately by a back-of-the-envelope calculation, as confirmed by numerical simulation. The unit cells are connected in a one-dimensional electrical network, which also accounts for the transport effects in the porous TiO2 network and for the contact effects at the electrodes. Results for solid-state nanostructured cells are: the interconnection geometry is disadvantageous for the open circuit voltage Voc; it is tolerant to moderate resistance in the n-TiO2 network, but not to resistance in the p-conductor.
Levitation of an iron ball in midair without active control
Authors: Sasaki S. , Yagi I. , Murakami M.
Published: January 2004

An iron ball floats in midair in a plastic box when several iron balls were attracted by a permanent magnet. A complex interaction between magnetized sphere materials and a lifting magnet enabled the suspension of an iron ball. The balls in the first row are simply attracted by the lifting magnet. The ball in the second row is also attracted by the lifting magnet, however, due to the repulsive forces exerted from the balls sitting above, it can float in midair. We also found that there are two stable positions for the ball to float. The floating ball could be transported from one equilibrium position to another by simply rotating the lifting magnet. This will make it possible to construct a noncontact load transport device.
Excimer emission from cathode boundary layer discharges
Authors: Moselhy Mohamed , Schoenbach Karl H.
Published: January 2004

The excimer emission from direct current glow discharges between a planar cathode and a ring-shaped anode of 0.75 and 1.5 mm diameter, respectively, separated by a gap of 250 μm, was studied in xenon and argon in a pressure range from 75 to 760 Torr. The thickness of the “cathode boundary layer” plasma, in the 100 μm range, and a discharge sustaining voltage of approximately 200 V, indicates that the discharge is restricted to the cathode fall and the negative glow. The radiant excimer emittance at 172 nm increases with pressure and reaches a value of 4 W/cm2 for atmospheric pressure operation in xenon. The maximum internal efficiency, however, decreases with pressure having highest values of 5% for 75 Torr operation. When the discharge current is reduced below a critical value, the discharge in xenon changes from an abnormal glow into a mode showing self-organization of the plasma. Also, the excimer spectrum changes from one with about equal contributions from the first and second continuum to one that is dominated by the second continuum emission. The xenon excimer emission intensity peaks at this discharge mode transition. In the case of argon, self-organization of the plasma was not seen, but the emission of the excimer radiation (128 nm) again shows a maximum at the transition from abnormal to normal glow. As was observed with xenon, the radiant emittance of argon increases with pressure, and the efficiency decreases. The maximum radiant emittance is 1.6 W/cm2 for argon at 600 Torr. The maximum internal efficiency is 2.5% at 200 Torr. The positive slope of the current–voltage characteristics at maximum excimer emission in both cases indicates the possibility of generating intense, large area, flat excimer lamps.
Suppression of the burn-in effect in InGaP/GaAs heterojunction bipolar transistors by constant period of voltage stress
Authors: Chong Kwok-Keung , Chen Hsun-Chin , Houng Mau-Phon , Wang Yeong-Her , Lin Shr-Ting
Published: January 2004

Two significantly abrupt increases of dc current gain (β) at the opposite extremes of base-emitter voltages (Vbe) linked by a relatively slight increase of β in the range of 1.25⩽Vbe⩽1.75 V are found in InGaP/GaAs heterojunction bipolar transistors (HBT’s). It has been proved that the burn-in effect directly causes the second abrupt increase of β occurring at Vbe>1.75 V instead of elsewhere. In addition, choosing Vbe>1.75 V, a higher base-emitter voltage results in a faster increase rate with improvement in current gain transient. Based on these observations, a constant period of voltage stress (CPVS) with Vbe=2.0 V and Vce=3.0 V for 5 min with a specific choice of Vbe>1.75 V (the voltage range corresponding to the second abrupt increase of β) is thus applied to promote the electrical performance of HBT’s. After applying a CPVS, the burn-in effect is substantially suppressed without showing any second abrupt increase of current gain. Although the current measured under both reverse and forward biases is greatly reduced for the base-emitter junction, a CPVS slightly degrades the electrical properties of a base-collector junction. Also, it has been proved that bulk recombination, rather than the surface recombination current, the base contact recombination, or the space-charge recombination, is the dominant base current component causing the burn-in effect. The electrical improvement of the base-emitter junction is due to the annihilation of H-related traps in the base region after a CPVS. Once H− ions form, we propose that these ions are too fast to drift toward the base-collector junction under the reverse bias of the base-collector voltage. After migration through the extrinsic base region, H− ions are supposed to be trapped near the base-collector space region, which results in the degradation of the base-collector junction after a CPVS.
Effect of incident current and energy on surface charging and electron emission of hydrogenated diamond films induced by low energy electron irradiation
Authors: Hoffman A. , Akhvlediani R. , Laikhtman A. , Lafosse A. , Guillotin J.-P. , Azria R.
Published: January 2004

The influence of incident electron current on surface charging and electron emission of hydrogenated diamond films subjected to low energy electron irradiation was investigated. For incident electron energies Ei in the 5–20 eV range, it was found that for sufficiently low incident currents (<∼8 nA) the secondary electron emission (SEE) decays in intensity under continuous irradiation, albeit a constant onset of SEE is maintained. For higher incident currents the surface charges negatively. The SEE decay rate and surface charging were found to depend on incident electron energies, obtaining maximum values for incident electron energies of ∼9 eV. Increase of the substrate temperature resulted in an enhanced electron emission and less efficient surface charging. These effects are explained on the basis of electron trapping in the near-surface region, which results in the formation of a depletion layer and upward surface band bending. It is suggested that the main mechanism of electron trapping is by resonant electron attachment of incident electrons onto C-H bonds present within the hydrogenated diamond film surfaces.
Influence of growth rate on charge transport in GaSb homojunctions prepared by metalorganic vapor phase epitaxy
Authors: Kindl Dobroslav , Toušková Jana , Hulicius Eduard , Pangrác Jiřı́ , Šimeček Tomislav , Jurka Vlastimil , Hubı́k Pavel , Mareš Jiřı́ J. , Krištofik Jozef
Published: January 2004

Dark current–voltage (I–V) characteristic measurement in the temperature range from −190 °C to 65 °C was carried out on GaSb p-n homojunctions prepared by low-pressure metalorganic vapor phase epitaxy. It was shown that the charge transport mechanism in these homojunctions is strongly affected by the growth rate of GaSb epitaxial layers. Samples prepared at higher growth rate (40 nm/min.) exhibit an anomalous low-temperature peak of tunneling current which can be explained by the presence of a narrow band of energies due to high concentration of native defects, probably GaSb antisites. The same defect levels are responsible for the generation–recombination current which dominates in these samples at higher temperatures. On the other hand, quite different behavior was found in the case of slowly grown (20 nm/min) samples. At sufficiently low temperatures, a current maximum near 50 mV of forward voltage points out a band-to-band tunneling as a prevailing transport mechanism. With increasing temperature, however, this maximum disappears as at least one side of the junction becomes nondegenerate. Combination of the tunneling via energy states in the band gap and the thermal current governs the forward I–V characteristics at higher temperatures, whereas the direct tunneling remains dominant in the reverse direction.
Temperature dependence of excitonic recombination in lateral epitaxially overgrown InGaN/GaN quantum wells studied with cathodoluminescence
Authors: Khatsevich S. , Rich D. H. , Zhang X. , Zhou W. , Dapkus P. D.
Published: January 2004

We have examined in detail the optical properties of InGaN quantum wells (QWs) grown on pyramidal GaN mesas prepared by lateral epitaxial overgrowth (LEO) in a metalorganic chemical vapor deposition system that resulted in QWs on {1-101} facets. The effects of In migration during growth on the resulting QW thickness and composition were examined with transmission electron microscopy (TEM) and various cathodoluminescence (CL) imaging techniques, including CL wavelength imaging and activation energy imaging. Spatial variations in the luminescence efficiency, QW interband transition energy, thermal activation energy, and exciton binding energy were probed at various temperatures. Cross-sectional TEM was used to examine thickness variations of the InGaN/GaN QW grown on a pyramidal mesa. CL imaging revealed a marked improvement in the homogeneity of CL emission of the LEO sample relative to a reference sample for a conventionally grown In0.15Ga0.85N/GaN QW. The characteristic phase separation that resulted in a spotty CL image profile and attendant carrier localization in the reference sample is significantly reduced in the LEO QW sample. Spatial variations in the QW transition energy, piezoelectric field, and thermal activation energy were modeled using excitonic binding and transition energy calculations based on a single-band, effective-mass theory using Airy function solutions. Band-edge and effective-mass parameters were first obtained from a strain- and In-composition-dependent k⋅p calculation for wurtzite InxGa1−xN, using a 6×6 k⋅p Hamiltonian in the {1-101} representations. The calculations and experiments confirm a facet-induced migration of In during growth, which results in a smooth compositional variation from x≈0.10 at the bottom of the pyramid to x≈0.19 at the top. We demonstrate the existence of a strong correlation between the observed thermal activation behavior of QW luminescence intensity and the associated exciton binding energy for various positions along the pyramidal InGaN/GaN QWs, suggesting exciton dissociation is responsible for the observed temperature dependence of the QW luminescence in the ∼150 to 300 K range.
Spin-dependent Hall effect in semiconductor quantum wells
Authors: Huang H. C. , Voskoboynikov O. , Lee C. P.
Published: January 2004

We present a theoretical study of the spin-dependent scattering of electrons from screened attractive and repulsive impurities in III-V semiconductor quantum wells. The effective one-band Hamiltonian and the Rashba spin–orbit interaction are used. We demonstrated that the asymmetry of the spin-dependent skew-scattering and side-jump effect can lead to a quite large spin-dependent (anomalous) Hall effect at zero magnetic field in all-semiconductor quantum well structures. Our theory predicts a measurable spin-dependent Hall angle that reaches about 2.5×10−3 rad for a CdTe/InSb/CdTe quantum well with impurities doped in the center of the well.
Formation of a quasi-two-dimensional electron gas in GaN/AlxGa1−xN heterostructures with diffuse interfaces
Authors: Mkhoyan K. A. , Silcox J. , Yu Z. , Schaff W. J. , Eastman L. F.
Published: January 2004

Calculations of the electronic energy levels and the distribution of the quasi-two-dimensional electron gas (Q2DEG) at the GaN/AlxGa1−xN interface that take into account the graded nature of the interface are presented in this article. Mapping of the interface using scanning transmission electron microscopy annular dark-field imaging, the changes in the N K-edge and the integrated intensity of the Al L2,3-edge revealed that the interface can be up to 20 Å wide. Self-consistent calculations in the local density approximation estimate the sensitivity of the Q2DEG formed at the interface to various parameters, including the width of the interface, the concentration of bound charge, ambient temperature, and the geometrical sizes of the structure.
Observation of vacancy ordering structure in GaP nanobelts
Authors: Tsai Jin-Sheng , Chen Fu-Rong , Kai Ji-Jung , Chen Chia-Chun , Huang Rong-Tan , Wang Ming-San , Huang Gi-Chia , Guo Gia-Gia , Yu Min-Uan
Published: January 2004

III–V semiconductor GaP nanobelts were synthesized with a Fe2O3 catalyst on Si substrates by thermal evaporation at high temperatures. These nanobelts, typically, have a width from 50 to 600 nm and a length of up to several hundred micrometers. Their thickness varies from 10 to 40 nm. A vacancy ordering structure was observed near the edge of the GaP nanobelts. The vacancy ordering structure was analyzed using high-resolution transmission electron microscopy, electron diffraction patterns, and computer simulation. The unit cell of the vacancy ordering structure in the GaP belt is orthorhombic with lattice parameters a=3.767 Å, b=6.525 Å, c=18.456 Å, and α=β=γ=90°. In the [111] projection, the structure is a R3x3 120° superstructure, while in the [211] projection, it exhibits a superstructure in both the (1̄1̄3) and (1̄31̄) planes. This defective structure can also be visualized as a long period structure with a superstructure in the (111) stacking plane.
Approximate analytical solution to the space charge problem in nanosized Schottky diodes
Authors: Donolato C.
Published: January 2004

An analysis is given of the space charge region that is induced in a semiconductor by a circular Schottky contact. Using the depletion approximation, the resulting free-boundary problem for Poisson’s equation is formulated and approximately solved in oblate spheroidal coordinates. Expressions are derived for the boundary of the space charge region and the related depletion potential. Calculations of the thickness of the Schottky barrier as a function of the diode size, down to the nanometer range, show good agreement with published results obtained numerically.
AlGaN/GaN current aperture vertical electron transistors with regrown channels
Authors: Ben-Yaacov Ilan , Seck Yee-Kwang , Mishra Umesh K. , DenBaars Steven P.
Published: January 2004

AlGaN/GaN current aperture vertical electron transistors with regrown aperture and source regions have been fabricated and tested. A 2 μm thick GaN:Si drain region followed by a 0.4 μm GaN:Mg current-blocking layer were grown by metalorganic chemical vapor deposition on a c-plane sapphire substrate. Channel apertures were etched, and a maskless regrowth was performed to grow unintentionally doped GaN inside the aperture as well as above the insulating layer, and to add an AlGaN cap layer. Cl2 reactive ion etching was used to pattern the device mesa, and source, drain, and gate metals were then deposited. Devices were achieved with a maximum source-drain current of 750 mA/mm, an extrinsic transconductance of 120 mS/mm, and a 2-terminal gate breakdown of 65 V while exhibiting almost no DC-RF dispersion for 80 μs pulsed I–V curves. The suppression of DC-RF dispersion was shown to result from the absence of the large electric fields at the surface on the drain-side edge of the gate that are present in high electron mobility transistors. Parasitic leakage currents, which were present in all devices, have been studied in detail. Three leakage paths have been identified, and methods to eliminate them are discussed.
Effect of polishing stress on Raman spectra of the Mn–Zn ferrite
Authors: Yamashita Osamu , Ikeda Teruki
Published: January 2004

Raman scattering measurements were made on polycrystalline Mn0.62Zn0.30Fe2.08O4 ferrites with various degrees of stress generated during the polishing process, in the temperature range from 175 to 603 K. Raman spectra were found at energies of 337, 464, and 620 cm−1 in substrates with low stress. The integral intensities at 337 and 620 cm−1 increased significantly with decreasing residual stress stored in the surface. The former integral intensity remained constant with increasing temperature and decreased in the vicinity of the Néel point, while the latter one decreased linearly with increasing temperature through the Néel point. However, their linewidths changed little with stress and temperature, unlike the case of GaAs. These two spectra of the Mn–Zn ferrite, therefore, are considered to arise predominantly from the spin-dependent phonon scattering, in addition to the usual optical phonon scattering. It was thus found that the Raman intensity associated closely with the magnetic ordering is highly sensitive to the residual stress in the crystal, as in the case of the optical phonon, and Raman spectroscopy is also applicable to the evaluation of the stress in soft magnetic ferrites.
Anomalous positive magnetoresistance in Cox–C1−x granular films on Si substrates
Authors: Zhu D. D. , Zhang X. , Xue Q. Z.
Published: January 2004

We report the anomalous positive magnetoresistance (PMR) in Cox–C1−x granular films prepared on Si(100) substrates by pulsed laser deposition method. A much larger positive MR, up to 22% at magnetic field B=1 T and x=2 at. %, is observed at room temperature than at low temperature. Interestingly, the room-temperature response has a B2/3 dependence when B<1 T and a B1/2 dependence when B>1 T. Different from the low-temperature PMR, the room-temperature PMR are related with the Si(100) substrates. Furthermore, we show that the intrinsic positive MR at low temperature arise from shrinkage of the wave functions of localized electronic states, while the extrinsic MR effects of the Cox–C1−x/Si structure are obviously determined by another mechanism, which may exist in other similar structures. This in turn has important implications for the optimization of such materials.
Efforts to improve carrier mobility in radio frequency sputtered aluminum doped zinc oxide films
Authors: Agashe C. , Kluth O. , Hüpkes J. , Zastrow U. , Rech B. , Wuttig M.
Published: January 2004

This study addresses the electrical and optical properties of radio frequency magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films. The main focus was on the improvement in carrier mobility μ to achieve simultaneously high transparency for visible and particularly near-infrared light and low resistivity. The influence of Al concentration in the target, film thickness, sputter power, deposition pressure, and substrate temperature on material properties was investigated. The structural, compositional, electrical and optical properties were studied using x-ray diffraction, secondary ion mass spectrometry (SIMS), room temperature Hall effect measurements and spectral photometry, respectively. All ZnO:Al films were polycrystalline and preferentially oriented along [002]. The grain size along the direction of growth increased with higher Al doping and with increasing film thickness. The SIMS measurements revealed that the Al concentration in the film was nearly the same as in the target. Carrier concentration N and mobility μ are determined by the target Al concentration. In addition μ is influenced by the film thickness and the sputter pressure. For each Al concentration, the highest μ was generally observed at low deposition pressures. By using a target with low Al2O3 concentration of 0.5 wt %, μ could be improved up to 44.2 cm2/V s while maintaining the electrical resistivity ρ as low as 3.8×10−4 Ω cm. For these films the transparency in the near-infrared wavelength range strongly improved which makes them particularly interesting for the application in optoelectronic devices like thin-film solar cells. The μ-N dependence for films deposited under diverse conditions was studied to identify a practical limit for μ.
Site-selective study of Nd3+ optical centers in Ca3Sc2Ge3O12 laser garnet crystals
Authors: Romero J. J. , Jaque D. , Bausá L. E. , Cavalli E.
Published: January 2004

The Nd3+ multicenter distribution in a Ca3Sc2Ge3O12 garnet laser crystal has been analyzed by low temperature site-selective laser spectroscopy. Two nonequivalent Nd3+ centers have been identified and characterized in this host lattice. The effect of codoping the crystal with MgO on the Nd3+ center distribution has also been investigated, since this codoping enables considerable enhancement of the doping levels of these crystals. Three different centers were observed in the codoped crystals, together with a reduction of inhomogeneous emission broadening. The nature of the optical centers present in both cases is discussed by considering the charge compensation mechanisms involved in accommodation of Nd3+ ions.
Characteristics of a terahertz photomixer based on a high-electron mobility transistor structure with optical input through the ungated regions
Authors: Satou A. , Ryzhii V. , Khmyrova I. , Ryzhii M. , Shur M. S.
Published: January 2004

We develop a device model for a terahertz photomixer that utilizes the excitation of plasma oscillations in the channel of a device similar to a high-electron mobility transistor (HEMT). The device design assumes vertical optical input through the ungated source–gate and gate–drain regions. Using this model, we calculate the characteristics of the HEMT photomixer: the responsivity as a function of the signal frequency for devices with different geometrical and physical parameters, and the dependence of resonant frequency on the length of the gated and ungated portions of the channel and the gate voltage. We compare also the performance of the HEMT photomixer with that of a similar device but one in which the optical input is through the substrate.