Showing papers in "Journal of Applied Physics in 2001"

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

High-κ gate dielectrics: Current status and materials properties considerations

Abstract: Many materials systems are currently under consideration as potential replacements for SiO2 as the gate dielectric material for sub-0.1 μm complementary metal–oxide–semiconductor (CMOS) technology. A systematic consideration of the required properties of gate dielectrics indicates that the key guidelines for selecting an alternative gate dielectric are (a) permittivity, band gap, and band alignment to silicon, (b) thermodynamic stability, (c) film morphology, (d) interface quality, (e) compatibility with the current or expected materials to be used in processing for CMOS devices, (f) process compatibility, and (g) reliability. Many dielectrics appear favorable in some of these areas, but very few materials are promising with respect to all of these guidelines. A review of current work and literature in the area of alternate gate dielectrics is given. Based on reported results and fundamental considerations, the pseudobinary materials systems offer large flexibility and show the most promise toward success...

Nearly 100% internal phosphorescence efficiency in an organic light emitting device

Abstract: We demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host. Using bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)] doped into 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole, a maximum external quantum efficiency of (19.0±1.0)% and luminous power efficiency of (60±5) lm/W are achieved. The calculated internal quantum efficiency of (87±7)% is supported by the observed absence of thermally activated nonradiative loss in the photoluminescent efficiency of (ppy)2Ir(acac). Thus, very high external quantum efficiencies are due to the nearly 100% internal phosphorescence efficiency of (ppy)2Ir(acac) coupled with balanced hole and electron injection, and triplet exciton confinement within the light-emitting layer.

Growth process conditions of vertically aligned carbon nanotubes using plasma enhanced chemical vapor deposition

Abstract: The growth of vertically aligned carbon nanotubes using a direct current plasma enhanced chemical vapor deposition system is reported. The growth properties are studied as a function of the Ni catalyst layer thickness, bias voltage, deposition temperature, C2H2:NH3 ratio, and pressure. It was found that the diameter, growth rate, and areal density of the nanotubes are controlled by the initial thickness of the catalyst layer. The alignment of the nanotubes depends on the electric field. Our results indicate that the growth occurs by diffusion of carbon through the Ni catalyst particle, which rides on the top of the growing tube.

Bending instability in electrospinning of nanofibers

Abstract: A localized approximation was developed to calculate the bending electric force acting on an electrified polymer jet, which is a key element of the electrospinning process for manufacturing of nanofibers Using this force, a far reaching analogy between the electrically driven bending instability and the aerodynamically driven instability was established Continuous, quasi-one-dimensional, partial differential equations were derived and used to predict the growth rate of small electrically driven bending perturbations of a liquid column A discretized form of these equations, that accounts for solvent evaporation and polymer solidification, was used to calculate the jet paths during the course of nonlinear bending instability leading to formation of large loops and resulting in nanofibers The results of the calculations are compared to the experimental data acquired in the present work Agreement of theory and experiment is discussed

Taylor Cone and Jetting from Liquid Droplets in Electrospinning of Nanofibers

Abstract: Sessile and pendant droplets of polymer solutions acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and a flat plate, if the potential is not too large. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. It is widely assumed that when the critical potential φ0* has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone, having a half angle of 49.3°. In the present work we show that the Taylor cone corresponds essentially to a specific self-similar solution, whereas there exist nonself-similar solutions which do not tend toward a Taylor cone. Thus, the Taylor cone does not represent a unique critical shape: there exists another shape, which is not self-similar. The exp...

Bulk electronic structure of SrTiO3: Experiment and theory

Abstract: Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3 Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states The experimentally determined indirect band gap energy is 325 eV, while the direct band gap energy is 375 eV The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 325 eV up to 15 eV The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV Interband transitions from the O 2s band to the conduction bands appear at 26 eV A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons

Ultrathin (<4 nm) SiO2 and Si-O-N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

Abstract: The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Effective electron mobility in Si inversion layers in metal–oxide–semiconductor systems with a high-κ insulator: The role of remote phonon scattering

Abstract: The high dielectric constant of insulators currently investigated as alternatives to SiO2 in metal–oxide–semiconductor structures is due to their large ionic polarizability. This is usually accompanied by the presence of soft optical phonons. We show that the long-range dipole field associated with the interface excitations resulting from these modes and from their coupling with surface plasmons, while small in the case of SiO2, for most high-κ materials causes a reduction of the effective electron mobility in the inversion layer of the Si substrate. We study the dispersion of the interfacial coupled phonon-plasmon modes, their electron-scattering strength, and their effect on the electron mobility for Si-gate structures employing films of SiO2, Al2O3, AlN, ZrO2, HfO2, and ZrSiO4 for “SiO2-equivalent” thicknesses ranging from 5 to 0.5 nm.

The complete Raman spectrum of nanometric SnO2 particles

Abstract: 14 and space group P4 2 /mnm. The unit cell consists of two metal atoms and four oxygen atoms. Each metal atom is situated amidst six oxygen atoms which approximately form the corners of a regular octahedron. Oxygen atoms are surrounded by three tin atoms which approximate the corners of an equilateral triangle. The lattice parameters are a5b 54.737 A, and c53.186 A. The ionic radii for O 22 and Sn 41 are 1.40 and 0.71 A, respectively. 1 The 6 unit cell atoms give a total of 18 branches for the vibrational modes in the first Brillouin zone. The mechanical representation of the normal vibration modes at the center of the Brillouin zone is given by 2,3 G5G 1 ~ A1g!1G 2 ~ A2g!1G 3 ~ B1g!1G 4 ~ B2g! 1G 5 ~ Eg!12G 1 ~ A2u!12G 4 ~ B1u!14G 5 ~ Eu!, ~1! using the Koster notation with the commonly used symmetry designations listed in parenthesis. The latter will be used throughout this article. Of these 18 modes, 2 are active in infrared ~the single A2u and the triply degenerate Eu), 4 are Raman active ~three nondegenerated modes, A1g , B1g , B2g , and a doubly degenerate Eg), and two are silent ( A2g , and B1u). One A2u and two Eu modes are acoustic. In the Raman active modes oxygen atoms vibrate while Sn atoms are at rest ~see Fig. 1 in Ref. 4!. The nondegenerate mode, A1g , B1g , and B2g , vibrate in the plane perpendicular to the c axis while the doubly degenerated E g mode vibrates in the direction of the c axis. The B 1g mode consists of rotation of the oxygen atoms around the c axis, with all six oxygen atoms of the octahedra participating in the vibration. In the A2g infrared active mode, Sn and oxygen atoms vibrate in the c axis direction, and in the Eu mode both Sn and O atoms vibrate in the plane perpendicular to the c axis. The silent modes correspond to vibrations of the Sn and O atoms in the direction of the c axis (B1u) or in the plane perpendicular to this direction ( A2g). According to the literature, the corresponding calculated or observed frequencies of the optical modes are presented in Table I. When the size of the SnO2 crystal is reduced, the infrared spectrum is modified because the interaction between electromagnetic radiation and the particles depends on the crystal’s size, shape, and state of aggregation. 8‐1 0 Experiments using Raman spectroscopy have also reported spectrum modification, at least partially. Low frequency bands have been observed previously in SnO2, 11 and several authors have reported the existence of bands not observed in single-crystal or polycrystalline SnO 2 which have been found to be closely related to grain size. 12‐15 However, some of these reports do not adequately explain the origin of the abnormal spectrum. The aim of this article is to present a complete Raman spectrum of SnO2 nanoparticles. The analysis comprises ~i! modification of the normal vibration modes active in Raman when the spectra are obtained from nanocrystals of SnO2 ~‘‘classical modes’’ !, ~ii! the disorder activated surface modes in the region around 475‐775 cm 21 , and ~iii! the appearance of the acoustic modes in the low-frequency region of the spectra.

Phonon heat conduction in a semiconductor nanowire

Abstract: A model for phonon heat conduction in a semiconductor nanowire with dimensions comparable to the phonon mean free path is developed. It is based on the solution of Boltzmann’s equation, which takes into account (i) modification of the acoustic phonon dispersion due to spatial confinement, and (ii) change in the nonequilibrium phonon distribution due to partially diffuse boundary scattering. Numerical simulation is performed for a silicon nanowire with boundaries characterized by different interface roughness. Phonon confinement and boundary scattering lead to a significant decrease of the lattice thermal conductivity. The value of this decrease and its interface roughness and temperature dependence are different from the predictions of the early models. The observed change in thermal resistance has to be taken into account in simulation of deep-submicron and nanometer-scale devices.

Polarization fatigue in ferroelectric films: Basic experimental findings, phenomenological scenarios, and microscopic features

Abstract: The reduction in switchable polarization of ferroelectric thin films due to electrical stress (polarization fatigue) is a major problem in ferroelectric nonvolatile memories. There is a large body of available experimental data and a number of existing models which address this issue, however the origin of this phenomena is still not properly understood. This work synthesizes the current experimental data, models, and approaches in order to draw conclusions on the relative importance of different macro- and microscopic scenarios of fatigue. Special attention is paid to the role of oxygen vacancy migration and electron injection into the film and it is concluded that the latter plays the predominant role. Experiments and problems for theoretical investigations, which can contribute to the further elucidation of polarization fatigue mechanisms in ferroelectric thin films, are suggested.

Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films

Abstract: In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were small in fine grain PZT films (especially those under 1.5 μm in thickness). However, as the films became thicker (>5μm), nonlinear behavior between the converse piezoelectric coefficient and the electric driving field was observed. This indicated that there was significant ferroelectric non-180° domain wall motion under high external excitation in thicker films. The activity of the non-180° domain walls was studied through non-180° domain switching. For fine grain films with film thicknesses less than 2 μm, non-180° switching was negligible. Transmission electron microscopy plan-view micrographs evidenced non-180° domain fringes in these films, where the vast majority of grains were 50–100 nm in diameter and showed a single set of domain fringes. Taken together, these measurements suggest that the pinning of non-180° domain walls is very strong in films with thickness less than 2 μm. In thicker films, non-180° domain switching was evidenced when the poling field exceeded a threshold field. The threshold field decreased with an increase in film thickness, suggesting more non-180° domain wall mobility in thicker films. Non-180° domain switching in large grained PZT films was found to be much easier and more significant than in the fine grained PZT films.

Materials characterization of ZrO2–SiO2 and HfO2–SiO2 binary oxides deposited by chemical solution deposition

Abstract: The thermal stability, microstructure, and electrical properties of xZrO2⋅(100−x)SiO2 (ZSO) and xHfO2⋅(100−x)SiO2 (HSO) (x=15%, 25%, 50%, and 75%) binary oxides were evaluated to help assess their suitability as a replacement for silicon dioxide gate dielectrics in complementary metal–oxide–semiconductor transistors. The films were prepared by chemical solution deposition using a solution prepared from a mixture of zirconium, hafnium, and silicon butoxyethoxides dissolved in butoxyethanol. The films were spun onto SiOxNy coated Si wafers and furnace annealed at temperatures from 500 to 1200 °C in oxygen for 30–60 min. The microstructure and electrical properties of ZSO and HSO films were examined as a function of the Zr/Si and Hf/Si ratio and annealing temperature. The films were characterized by x-ray diffraction, mid- and far-Fourier transform infrared (FTIR), Rutherford backscattering spectroscopy, and Auger electron spectroscopy. At ZrO2 or HfO2 concentrations ⩾50%, phase separation and crystallizatio...

Electron irradiation effects in single wall carbon nanotubes

Abstract: We determine, with excellent agreement between theory and experiment, the behavior of single wall carbon nanotubes during uniform electron irradiation. Calculations utilizing known ejection threshold energies predict that an isolated nanotube will damage preferentially on surfaces that lie normal to the electron beam. A minimum incident electron energy of 86 keV is required to remove a carbon atom by a knock-on collision for this geometry. Higher electron energies are required for any other geometry, and at energies exceeding 139 keV every atom on a nanotube is susceptible to ballistic ejection. Transmission electron microscopy observations of nanotubes using 80–400 keV electrons corroborate these conclusions. Based upon empirical observations, we also explain damage processes in nonisolated nanotubes.

Dielectrophoretic liquid actuation and nanodroplet formation

Abstract: Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis(DEP) makes possible a variety of microelectromechanicalliquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including “wall-less” flowstructures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ∼7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ∼60 kHz. A simple RC circuit model explains this frequency-dependent behavior. DEP actuation of small water volumes is very fast. We observe droplet formation in less than 0.1 s and transient, voltage-driven movement of water fingers at speeds exceeding 5 cm/s. Such speed suggests that actuation can be accomplished using preprogrammed, short applications of the rf voltage to minimize Joule heating.

Elastic, piezoelectric, and dielectric properties of multidomain 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals

Abstract: The elastic, piezoelectric, and dielectric constants of 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 domain engineered single crystal were determined experimentally by using ultrasonic and resonance methods. It was confirmed that the single crystal system has large electromechanical coupling coefficient k33 (∼94%) and piezoelectric constant d33 (∼2800 pC/N) if the poling is done along the [001] of pseudocubic directions. A soft shear mode with a velocity of 880 m/s was observed in the [110] direction with displacement in [110]. Using the measured data, the orientation dependence of phase velocities and electromechanical coupling coefficients were calculated. The origin of experimental errors and their influence on measured results are also examined.

Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition

Abstract: Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.

Anomalous barium filling fraction and n-type thermoelectric performance of BayCo4Sb12

Abstract: Barium-filled skutterudites BayCo4Sb12 with an anomalously large filling fraction of up to y=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show that Ba0.44Co4Sb12 is paramagnetic, which implies that some of the Co atoms in BayCo4Sb12 have acquired a magnetic moment. The presence of the two different valence states of Co (Co3+ and Co2+) leads to the anomalously large barium filling fraction even without extra charge compensation. All samples show n-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity of BayCo4Sb12 is significantly depressed as compared to unfilled Co4Sb12. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 for Ba0.24Co4Sb12 at 850 K.

p-type conductivity in CuCr1−xMgxO2 films and powders

Abstract: CuCr1−xMgxO2, a wide band gap semiconductor with the delafossite structure, has been synthesized in bulk and thin-film form. Bulk undoped CuCrO2 is almost black and has moderate conductivity with p-type carriers. Upon doping with 5% Mg, the conductivity increases by a factor of 1000. In films, the best p-type conductivity is 220 S cm−1 in CuCr0.95Mg0.05O2, a factor of 7 higher than previously reported for Cu-based p-type delafossites. Undoped films have a conductivity of order 1 S cm−1. Films are usually polycrystalline on amorphous substrates, but undoped films can be c-axis oriented if deposited at or above 650 °C. Optical and ultraviolet transmission data indicate a direct band gap of 3.1 eV.

Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells

Abstract: The influence of the organic layer thickness on short-circuit photocurrent spectra and efficiency is investigated in heterojunction photovoltaic cells with the electron donor materials poly(p-phenylenevinylene) (PPV) and Cu-phthalocyanine (CuPc), respectively, together with C60 as electron acceptor material. The main process of photocurrent generation after light absorption, exciton generation, and exciton diffusion in the bulk of the absorbing material is given by the exciton dissociation at the donor–acceptor interface. We determined a strong dependence of the optimum layer thickness of the absorbing material on the exciton diffusion length by systematically varying the layer thickness of the electron donor material. Additionally, a significant photocurrent contribution occurred due to light absorption and exciton generation in the C60 layer with a subsequent hole transfer to PPV, respectively, CuPc at the dissociation interface. Using a simple rate equation for the exciton density we estimated the exci...

Energetics of native defects in ZnO

Abstract: We have investigated the formation energies and electronic structure of native defects in ZnO by a first-principles plane-wave pseudopotential method. When p-type conditions are assumed, the formation energies of donor-type defects can be quite low. The effect of self-compensation by the donor-type defects should be significant in p-type doping. Under n-type conditions, the oxygen vacancy exhibits the lowest formation energy among the donor-type defects. The electronic structure, however, implies that only the zinc interstitial or the zinc antisite can explain the n-type conductivity of undoped ZnO.

Lithium doping of semiconducting organic charge transport materials

Abstract: We study the effects of lithium (Li) incorporation in the cathodes of organic light-emitting devices. A thermally evaporated surface layer of metallic Li is found to diffuse through, and subsequently dope, the electron transporting organic semiconducting thin films immediately below the cathode, forming an Ohmic contact. A diffusion length of ∼700 A is inferred from analyses of the current–voltage and secondary ion mass spectrometry data. The conductivity of the Li-doped organic films is ∼3×10−5 S/cm. Photoemission spectroscopy suggests that Li lowers the barrier to injection at the organic/cathode interface, introduces gap states in the bulk of the organic semiconductor, and dopes the bulk to facilitate efficient charge transport.

Interfacial chemistry of Alq3 and LiF with reactive metals

Abstract: The electronic structure and chemistry of interfaces between tris-(8-hydroxyquinoline) aluminum (Alq3) and representative group IA and IIA metals, Al, and Al/LiF have been studied by x-ray and ultraviolet photoelectron spectroscopies. Quantum-chemical calculations at the density functional theory level predict that the Alq3 radical anion is formed upon reaction with the alkali metals. In this case, up to three metal atoms can react with a given Alq3 molecule to form the trivalent anion. The anion formation results in a splitting of the N 1 s core level and formation of a new feature in the previously forbidden energy gap. Virtually identical spectra are observed in the Al/LiF/Alq3 system, leading to the conclusion that the radical anion is also formed when all three of these constituents are present. This is support by a simple thermodynamic model based on bulk heats of formation. In the absence of LiF or similar material, the reaction of Al with Alq3 appears to be destructive, with the deposited Al reacting directly with the quinolate oxygen. We proposed that in those circumstances where the radical anion is formed, it and not the cathode metal are responsible for the electron injection properties. This is borne out by producing excellent injecting contacts when Ag and Au are used as the metallic component of the cathode structure. © 2001 American Institute of Physics.

Ti-catalyzed Si nanowires by chemical vapor deposition: Microscopy and growth mechanisms

Abstract: Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi2 is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth.

Minority carrier lifetime degradation in boron-doped Czochralski silicon

Abstract: The minority carrier lifetime in boron-doped oxygen-contaminated Czochralski (Cz) silicon is strongly reduced under illumination or carrier injection. This process can be fully reversed by a 200 °C anneal step. In several recent studies it was shown that boron and oxygen are the major components of the underlying metastable Cz-specific defect. The energy level of the defect in its active state A was determined to be around midgap [Schmidt et al., J. Appl. Phys. 86, 3175 (1999)] while the energy level of the defect in its passive state P is very shallow. The Cz-specific defect in its passive state can be identified with the shallow thermal donor. The kinetics of the excess carrier-induced transformation from state P to state A can be described using recombination-enhanced defect reaction theory. On the basis of these experimental facts different solutions for the reduction or elimination of the metastable defect are suggested. Two promising solutions are discussed in more detail: the use of gallium-doped C...

The dynamic competition between stress generation and relaxation mechanisms during coalescence of Volmer–Weber thin films

Abstract: Real-time measurements of stress evolution during the deposition of Volmer–Weber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystalline Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior occurs even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the film/substrate interfacial strength. For Ag films, interfacial shear dominated the early relaxation behavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the Al/SiO2 interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures...

Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system

Abstract: In this study, the phase transition behavior of the BaTiO3–BaZrO3 system was studied using micro-Raman scattering and dielectric measurement techniques. BaZrxTi1−xO3 ceramics were prepared for x=0.00, 0.05, 0.08, 0.15, 0.20, and 1.00 compositions using a solid-state reaction technique. A single-phase perovskite structure of the ceramics was identified by the x-ray diffraction technique. The basic phase transition temperatures in these compositions were studied in the temperature range of 70–575 K. The tetragonal to cubic transition temperature was found to decrease with increasing Zr content. The orthorhombic to tetragonal transition temperature that increases with an initial increase in Zr content merges with the tetragonal–cubic transition for x⩾0.15 compositions. Raman spectra of rhombohedral and orthorhombic phases could not be distinguished. Excellent agreement between the crystallographic transition temperatures obtained by both techniques suggested that Zr substituted octahedra were uniformly distr...

AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy

Abstract: We report on an extensive study of the two-dimensional electron gas (2DEG) structures containing AlN layers. It is shown that the presence of large polarization fields in the AlN barrier layer in AlN/GaN heterostructures results in high values of the 2DEG sheet density of up to 3.6×1013 cm−2. Room-temperature sheet resistance of 180 Ω/□ is demonstrated in the AlN/GaN structure with a 35 A AlN barrier. As a result of reduced alloy disorder scattering, low-temperature electron mobility is significantly enhanced in AlN/GaN heterostructures in comparison to AlGaN/GaN structures with similar values of the 2DEG sheet density. The growth of GaN cap layers on top of AlN/GaN structures with relatively thick (∼35 A) AlN barriers is found to lead to a significant decrease in the 2DEG sheet density. However, inserting a thin (∼10 A) AlN layer between AlxGa1−xN and GaN in the AlxGa1−xN/GaN (x∼0.2–0.45) 2DEG structures does not affect the 2DEG sheet density and results in an increase of the low-temperature electron mob...

Discharge phenomena of an atmospheric pressure radio-frequency capacitive plasma source

Abstract: Discharge phenomena of a nonthermal atmospheric pressure plasma source have been studied. An atmospheric pressure plasma jet (APPJ) operates using rf power and produces a stable homogeneous discharge at atmospheric pressure. After breakdown, the APPJ operation is divided into two regimes, a “normal” operating mode when the discharge is stable and homogeneous, and a “failure” mode when the discharge converts into a filamentary arc. Current and voltage (I–V) characteristics and spatially resolved emission intensity profiles have been measured during the normal operating mode. These measurements show that the APPJ produces an alpha (α) mode rf capacitive discharge. Based upon a dimensional analysis using the observed I–V characteristics, a rough estimate is made for plasma density of 3×1011 cm−3 and an electron temperature of 2 eV. In addition, the gas temperature of 120 °C has been spectroscopically measured inside the discharge. These plasma parameters indicate that the APPJ shows promise for various mater...