bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical studies of the passivation of GaAs surface recombination using sulfides and thiols
Authors: Lunt Sharon R. , Ryba Gail N. , Santangelo Patrick G. , Lewis Nathan S.
Published: July 2002

Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2−(aq) concentration, variation of the pH of aqueous HS−solutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH=8, 0.1-M HS−(aq) solutions displaying comparable luminescence intensity relative to those treated with pH=14, 1.0-M Na2S⋅9H2O(aq). Organic thiols (R-SH, where R=−CH2CH2SH or −C6H4Cl) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of (100)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S3 overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of As2S3 is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electron-donating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2S3 and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The time-resolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HS− and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate.
The dynamics and stability of thin liquid films during spin coating. I. Films with constant rates of evaporation or absorption
Authors: Reisfeld B. , Bankoff S. G. , Davis S. H.
Published: July 2002

A fixed volume of liquid is placed on a horizontal disk spinning at a constant angular speed. The liquid forms a film that thins continuously due to centrifugal drainage and evaporation or thins to a finite thickness when surface absorption counterbalances drainage. A nonlinear evolution equation describing the shape of the film interface as a function of space and time is derived, and its stability is examined using linear theory. When there is either no mass transfer or there is evaporation from the film surface, infinitesimal disturbances decay for small wave numbers and are transiently stable for larger wave numbers. When absorption is present at the free surface, the film exhibits three different domains of stability: disturbances of small wave numbers decay, disturbances of intermediate wave numbers grow transiently, and those of larger wave numbers grow exponentially.
The effect of Ar+ ion implantation on the electrical characteristics of Cr/P-Si Schottky diodes
Authors: Malherbe J. B. , de Witt B. , Berning G. L. P.
Published: July 2002

The effect of 150-keV Ar+ ion implantation of Cr/p-Si(100) Schottky contacts was studied by measuring the current voltage characteristics over the temperature range 293–373 K before and after different doses of ion bombardment. The characteristics obtained were used in a least-squares fitting procedure to determine the ideality constant n, the saturation current IS, the series resistance R, and the Schottky barrier height Φb. Ar+ ion bombardment resulted in higher values for n and R but a lower value for IS. The value of Φb increased by 10%. The apparent reason for the increase in Φb for p-type contacts as well as the decrease for n-type contacts is discussed in reference to the identical behavior earlier reported for low-energy inert gas ion beam etching and reactive ion etching of Si.
SiO2 thin films deposited by reactive ion-beam sputtering under ultraviolet light irradiation
Authors: Nomura Koji , Ogawa Hisahito
Published: July 2002

SiO2 thin films have been successfully prepared on glass substrates by reactive ion-beam sputtering with ultraviolet (uv) light irradiation from a low-pressure Hg lamp in a partial background pressure of oxygen. X-ray photoelectron spectroscopy, infrared transmission spectroscopy, refractive index measurements, and electron-spin-resonance measurements have been used to investigate the composition and oxygen defects in the films. The deposition process with uv light irradiation improves the film quality, that is, high densification and a high oxidation, because the chemically active O(1D) atoms produced by uv light irradiation are sure to enhance the chemical reaction for SiO2 growth. This process also makes the limitation of the suitable deposition rate increase about five times in comparison with the process without uv light. The preparation, composition, and electrical properties of the SiO2 thin films are described.
Graphite as a substrate for diamond growth
Authors: Dubray J. J. , Pantano C. G. , Yarbrough W. A.
Published: July 2002

The nucleation of diamond on highly oriented pyrolytic graphite (HOPG) substrates using hot-filament-assisted chemical-vapor deposition has been studied. Significant differences were observed between the basal {0001} and prism plane {hk*0} surfaces, and for various pretreatments of these surfaces. Observed nucleation densities were found correlate with the C KVV Auger transition signatures observed ex situ on these surfaces prior to diamond growth. An enhanced diamond nucleation density is observed on those carbon surfaces which give an Auger signature close to that observed for the prism or edge plane surface(s) of HOPG. The cleaved basal plane surface of HOPG appears to give by far the lowest nucleation density for diamond.
Theory and simulation of the formation and erasure of space-charge gratings in photoconductive polymers
Authors: Schildkraut J. S. , Buettner A. V.
Published: July 2002

Equations that describe the formation of space charge in a photoconducting polymer are presented. The equations are written in a nondimensional form that includes the electric-field dependence of charge generation efficiency, charge mobility, recombination rate, and trapping rate. The equations are solved numerically for the condition of illumination with an intensity distribution that varies sinusoidally with position. The effect of traps on the formation of a space-charge grating is modeled. Finally, the erasure of the space-charge grating by uniform illumination is considered.
Generation and transport of 140 kJ ribbon electron beam
Authors: Arzhannikov A. V. , Nikolaev V. S. , Sinitsky S. L. , Smirnov A. V. , Yushkov M. V. , Zotkin R. P.
Published: July 2002

Generation of a microsecond ribbon electron beam with a strongly elongated cross section 4 cm×140 cm in a magnetically insulated diode at 1 MV voltage and its transport at the distance 2 m in a slit vacuum channel with a guiding magnetic field about 1 T are described. High efficiency of the ribbon beam generation and transport at the total energy about 140 kJ is experimentally proven.
Thermal stability of Pt/Co multilayered films
Authors: Sumi S. , Kusumoto Y. , Teragaki Y. , Torazawa K. , Tsunashima S. , Uchiyama S.
Published: July 2002

The thermal stability of Pt/Co multilayered films with various underlayers has been studied by annealing up to 400 °C. In Pt/Co films sputtered on glass or amorphous AlN underlayers, their coercivity and the squareness of magnetic hysteresis loops are degraded by annealing above 300 °C. In contrast, in films sputtered on ZnO or Pt underlayers, the coercivity is remarkably increased by annealing at 300 °C. The film with the ZnO underlayer shows a hysteresis loop with perfect squareness even after annealing at 400 °C. The results of x-ray diffraction indicate that the ZnO underlayer not only enhances the fcc Pt/Co(111) orientation but also prevents the decay of the multilayered structure during annealing. The results of laser microannealing using a conventional disk drive system also indicate that the Pt/Co films with ZnO underlayers have superior resistance to laser damage.
Fluid simulations of glow discharges: Effect of metastable atoms in argon
Authors: Lymberopoulos Dimitris P. , Economou Demetre J.
Published: July 2002

A one-dimensional fluid simulation of a 13.56 MHz argon glow discharge including metastable species was performed as an example of a coupled glow-discharge/neutral-transport-reaction system. Due to the slow response time of metastables (∼10 ms) direct time integration of the coupled system requires ∼105 rf cycles to converge. This translates to prohibitively long computation time. An ‘‘acceleration’’ scheme was employed using the Newton–Raphson method to speed up convergence, thereby reducing the computation time by orders of magnitude. For a pressure of 1 Torr, metastables were found to play a major role in the discharge despite the fact that their mole fraction was less than 10−5. In particular, metastable (two-step) ionization was the main mechanism for electron production to sustain the discharge. Bulk electric field and electron energy were lower, and a smaller fraction of power was dissipated in the bulk plasma when compared to the case without metastables. These results suggest that neutral transport and reaction must be considered in a self-consistent manner in glow discharge simulations, even in noble gas discharges.
Characteristics and origin of the 1.681 eV luminescence center in chemical-vapor-deposited diamond films
Authors: Feng Tom , Schwartz Bradley D.
Published: July 2002

Employing photoluminescence (PL) spectroscopy, we have carried out a detailed study of the characteristics and origin of the 1.681 eV luminescence center in chemical-vapor-deposited (CVD) diamond films. In addition to the zero-phonon line (ZPL), the fine structure of the PL emission spectrum revealed vibronics corresponding to both phonon emission and phonon absorption. Most of the vibronics can be identified with known lattice phonons in a natural diamond, although a few of the peaks appear to be associated with a local vibrational mode. The emission intensity was found to be a strong function of the CH4/H2 ratio used to grow the diamond films. Decreasing the methane concentration from 2.0% to 0.25% resulted in an increase of about four orders of magnitude in the PL intensity. It has been found that residual stress on the order of 0.5 GPa in the diamond film quenches the PL emission intensity but has no measurable effects on the peak position and half-width of the 1.681 eV ZPL. Temperature has a marked influence on the PL emission characteristics. With increasing temperature, the emission intensity decreases, the peak position shifts to lower energy, and the half-width broadens. Some of these results provide further support to the conclusion arrived at recently by some researchers that the 1.681 eV center, characteristic of ‘‘high-quality,’’ CVD-grown diamond films, is not the GR1 center present in radiation-damaged bulk diamond. The plausibility of the Si-impurity model, proposed to explain the origin of this 1.681 eV center, was critically examined.
Plasma chemistry of He/O2/SiH4 and He/N2O/SiH4 mixtures for remote plasma-activated chemical-vapor deposition of silicon dioxide
Authors: Kushner Mark J.
Published: July 2002

Remote plasma-activated chemical-vapor deposition (RPACVD) provides a means to deposit thin dielectric films with low ion bombardment and while having high selectivity in generating precursors. In RPACVD of SiO2, gas mixtures of He/O2 or He/N2O are passed through a plasma, producing radicals and excited states that are mixed with silane downstream. Excited states produced in the plasma and precursor species produced by these reactions then flow to the substrate. Although high-quality SiO2 films can be produced by RPACVD, the gas-phase deposition precursors have not been identified. A two-dimensional plasma chemistry model is described, and results from that model are used in a discussion of possible gas-phase precursors for SiO2 deposition. In particular, the formation and transport of silanols (SiH2O and SiH3O) are examined as a function of gas mixture, power deposition, and geometry. It is found that the fluxes of SiH2O, SiH3O, and SiH3 are sufficient to account for the observed deposition rates; while systematic dependencies of the fluxes of HSiO and SiO discount them as being deposition precursors. He/N2O/SiH4 mixtures differ from He/O2/SiH4 mixtures by providing larger fluxes of SiH3 to the substrate, while the fluxes of SiH2O, SiH3O, and O2(1Δ) are significantly less.
Effective medium at finite frequency: Theory and experiment
Authors: Rousselle D. , Berthault A. , Acher O. , Bouchaud J. P. , Zérah P. G.
Published: July 2002

Besides their technological importance in electromagnetic applications, composites materials made of magnetic metal inclusions in an insulating matrix provide a stringent test of effective medium theories. While there exists numerous experimental and theoretical studies in the long wavelength approximation, that is, when induced currents can be neglected, much less information is available on the effects of, e.g., eddy currents on the effective permeability of composites, especially when it comes to a systematic comparison between experimental data and the various theoretical formulations. We intend to throw some light on this rather intricate problem by comparing experimental data with an extension of Bruggeman effective medium theory.
Structural and magnetic properties of ultrafine Fe-Pd particles
Authors: Yiping L. , Hadjipanayis G. C. , Sorensen C. M. , Klabunde K. J.
Published: July 2002

The structural and magnetic properties of evaporated ultrafine Fe-Pd particles have been studied near the equiatomic composition. Particles with an average size in the range of 65–360 Å were obtained with the argon pressure varied between 0.5–40 Torr. All of the as-made particles had a face centered cubic (fcc) structure and they were magnetically soft. A fcc to face centered tetragonal (fct) phase transformation was observed after annealing the powders at 500 °C resulting in room-temperature coercivities up to 3.6 kOe and a magnetization (at 55 kOe) around 95 emu/g. An enhanced magnetic moment of about 3.0μB per atom was found for Fe.
Design of a two-dimensional square mesh metal grating coupler for a miniband transport GaAs quantum-well infrared photodetector
Authors: Wang Yeong-Cheng , Li Sheng S.
Published: July 2002

Numerical analysis of a two-dimensional (2D) square mesh metal grating coupler formed on top of a bound-to-miniband (BTM) transition GaAs-based quantum-well infrared photodetector (QWIP) has been carried out. Two normalized parameters s and h (i.e., s=λ/g and h=a/g, where g is the grating period and a is the width of the aperture) are used in the simulations of the 2D square mesh metal grating structure formed on a GaAs BTM QWIP. Using two universal graphs displaying the total power and diffracted angle of the higher-order transmitted and reflected waves versus normalized wavelength, the optical absorption constant and coupling quantum efficiency of the QWIPs are calculated for different grating periods and aperture sizes in the wavelength range of 8–14 μm. A maximum coupling quantum efficiency of η=52% was obtained for the square mesh metal grating coupled BTM QWIP with g=3.3 μm and a=2 μm, which is higher than that of the 45° polished facet illumination (η=38%).
Bistable saturation due to single electron charging in rings of tunnel junctions
Authors: Lent Craig S. , Tougaw P. Douglas
Published: July 2002

The behavior of rings of four small-capacitance tunnel junctions that are charged with two extra electrons is examined. Single electron charging effects result in quantization of charge on the metal electrode islands. To minimize the total electrostatic energy, the electrons localize on opposite electrodes, leading to a charge alignment in one of two configurations. We consider such rings as cells that may be capacitively coupled to each other in a cellular automaton architecture. The interaction between cells results in strong bistable saturation in the cell’s charge alignment which may be used to encode binary information. Lines of such cells can be viewed as binary wires.
Surface chemistry of tin oxide based gas sensors
Authors: Gaggiotti G. , Galdikas A. , Kačiulis S. , Mattogno G. , S̆etkus A.
Published: July 2002

Four types of gas sensors based on SnOx thin film with and without additives (Pt and Sb) were investigated by means of x-ray photoelectron spectroscopy and scanning Auger microscopy. The sensors were deposited on Si substrates by using reactive dc magnetron sputtering. The temperature dependencies of the electrical resistivity response to CO gas exposure were measured in order to characterize all types of fabricated sensors. The surface chemical composition before and after various treatments (Ar+ ion sputtering, thermal annealings in ultrahigh vacuum, and oxygen up to 400 °C) was determined from core level and valence band spectra. The three main types of oxygen species were found on the sensors’ surface: oxygen in SnOx, adsorbed hydroxyl groups and adsorbed water. An ultrathin Pt overlayer, which enhances the gas sensitivity in a low operating temperature range, was found to be very porous. The addition of a Pt overlayer was promoting a formation of hydroxyl groups, while the surface oxygen species was independent of the Sb doping. The obtained results support the assumption that the changes of the sensors’ resistivity under reducing gas exposure are caused more by the change of the surface defects’ density than by variation of excess surface charge induced by adsorbed oxygen species.
Properties of Czochralski-grown RAIO3 (R: Dy, Ho, and Er) single crystals for magnetic refrigeration
Authors: Kimura H. , Numazawa T. , Sato M. , Ikeya T. , Fukuda T.
Published: July 2002

Magnetization of RAlO3 (R: Dy, Ho, and Er) single crystals along the c axis have been measured by the superconducting quantum interference device magnetometer. The single crystals are nominally pure and large sized, grown by the Czochralski method. Paramagnetic properties depend on the rare-earth elements. According to the magnetic entropy change, ErAlO3 single crystals are promising materials for the magnetic refrigerants using the Carnot cycle in the temperature range below 20 K.
Modeling the patterned two-dimensional electron gas: Electrostatics
Authors: Davies John H. , Larkin Ivan A. , Sukhorukov E. V.
Published: July 2002

We present analytical calculations of the potential in a two-dimensional electron gas (2DEG) generated by patterned polygon gates on the surface of a heterostructure. They give the bare and screened potentials and reveal the effect of different boundary conditions on the surface. The formulas for the bare electrostatic potential from patterned gates are simple enough to be plotted in spreadsheets; they give threshold voltages, estimates of the region occupied by the 2DEG, and the energies of some collective infra-red excitations. We also consider the screened potentials in linear response, where no part of the 2DEG is fully depleted, which can again be found within an electrostatic approximation. The behavior of the exposed surface between the gates affects the potential strongly. Surface states provide perfect pinning of the Fermi energy in the ‘‘equipotential’’ model, the usual assumption, but this requires charge to move to the surface from the 2DEG. The charge on the surface is held fixed in response to a gate voltage in the ‘‘frozen’’ model, which typically gives a lower cutoff voltage but stronger confinement of electrons in a split-gate wire. The difference between the two models is large, emphasizing that accurate modeling needs a thorough understanding of the surface states.
Mechanism for stress-induced leakage currents in thin silicon dioxide films
Authors: DiMaria D. J. , Cartier E.
Published: July 2002

Leakage currents introduced in the low-field, direct-tunneling regime of thin oxides during high-field stress are related to defects produced by hot-electron transport in the oxide layer. From these studies, it is concluded that the ‘‘generation’’ of neutral electron traps in thin oxides is the dominant cause of this phenomenon. Other mechanisms due to anode hole injection or oxide nonuniformities are shown to be unrealistic for producing these currents. Exposure of thin oxides to atomic hydrogen from a remote plasma is shown to cause leakage currents similar to those observed after high-field stress, supporting the conclusion that these currents are related to hydrogen-induced defects.
Efficient blue polymer light-emitting diodes from a series of soluble poly(paraphenylene)s
Authors: Yang Y. , Pei Q. , Heeger A. J.
Published: July 2002

Three derivatives of poly(paraphenylene) (PPP) have been synthesized, all with excellent solubility in common organic solvents. Efficient blue polymer light-emitting diodes (LEDs) are demonstrated using these PPPs as the semiconducting and luminescent polymers. Double-layer polymer LEDs (consisting of a hole transport layer in addition to the electroluminescent layer) emit blue light with external quantum efficiencies between 1% and 3% photons per electron, when using indium tin oxide as the anode and calcium as the cathode. Using internal field emission (Fowler–Nordheim tunneling) of single carrier devices for both electrons and holes, the energies of the top of the π band and the bottom of the π* band have been determined as, respectively, 5.7 and 2.3 eV below the vacuum. The operating voltages of these LEDs have been lowered by using a porous polyaniline anode, or by blending PPP with a hole transport material. LEDs using air stable cathodes, silver, indium, aluminum, and copper, were also demonstrated. Device quantum efficiencies, between 0.3% and 0.8%, were obtained.
A simulation of electromigration-induced transgranular slits
Authors: Wang Weiqing , Suo Z. , Hao T.-H.
Published: July 2002

An on-chip aluminum interconnect carries an intense electric current at an elevated temperature, motivating atoms to diffuse in the solid state, and inducing voids that may cause an open failure. Recent observations have shown that a void sometimes collapses to a slit running nearly perpendicular to the electric current direction. Such a slit often lies inside a grain rather than along a grain boundary. An earlier calculation showed that diffusion on the void surface, driven by the electric current, can cause a circular void to translate in an infinite, isotropic interconnect. It was suggested recently that this solution may be unstable, and that two forces compete in determining the void stability: surface tension favors a rounded void, and the electric current favors a slit. A linear perturbation analysis, surprisingly, revealed that the translating circular void is stable against infinitesimal shape perturbation. Consequently, the slit instability must have resulted from finite imperfections. This article reviews the experimental and theoretical findings, and describes a numerical simulation of finite void shape change. We determine the electric field by a conformal mapping of complex variables, and update the void shape for a time step by a variational method. The simulation shows that a finite void shape imperfection or surface tension anisotropy can cause a void to collapse to a slit.
Intrinsic and extrinsic capacitances of in-plane-gated transistors
Authors: de Vries D. K. , Stelmaszyk P. , Wieck A. D.
Published: July 2002

The intrinsic and extrinsic capacitances of in-plane-gated (IPG) transistors are calculated by conformal mapping. The capacitance per unit length between coplanar areas is only weakly dependent on the separation. It is essentially determined by the dielectric constant only and has a value of ∼18 pF/m in air. In both ion-implanted and trench-etched IPG transistors the geometry is of minor importance for the low- and high-frequency behavior.
Photoreduction and oxidation of as-deposited microcrystalline indium oxide
Authors: Xirouchaki C. , Kiriakidis G. , Pedersen T. F. , Fritzsche H.
Published: July 2002

Indium oxide (InOx) films with a thickness of 100–500 nm were prepared by dc magnetron sputtering onto Corning 7059 glass substrates. The as-deposited films were microcrystalline as revealed by x-ray diffraction analysis. The conductivity of the as-deposited films was of the order of 10−3–10−2 Ω−1 cm−1 and increased up to the order of 101–102 Ω−1 cm−1 by exposure to ultraviolet light hν≥3.5 eV in vacuum. By subsequently exposing the same films to an oxidizing atmosphere they reverted to the insulating state. Photoreduction and oxidation have been shown to change the conductivity properties of one and the same film in a fully reversible manner. In this article, we demonstrate that the large reversible conductivity changes produced by UV photoreduction and oxidation are not limited to amorphous InOx but are equally observed in microcrystalline InOx.
Substrate selection for high-temperature superconducting thin films
Authors: Phillips Julia M.
Published: July 2002

Substrate selection presents particular challenges for the production of high-quality high-temperature superconducting (HTS) thin films suitable for applications. Because the substrate is generally a passive component, it is often ignored and assumed to have a negligible effect on the structure residing on top of it. There is also a technological motivation to use substrates that conventional wisdom would argue are unlikely to support high-quality HTS films. These facts have led to rediscovery of many of the fundamental issues governing the role of the substrate in determining the properties of the thin film(s) it supports. For this reason, the study of issues in substrate selection for HTS materials presents a microcosm for substrate selection more generally. We consider the major issues governing the role of the substrate in HTS thin-film technology and discuss many of the material classes and specific materials that have been studied for their suitability as substrates for HTS films.
Third-order nonlinear optical properties of sol–gel derived α-Fe2O3, γ-Fe2O3, and Fe3O4 thin films
Authors: Hashimoto Tadanori , Yamada Tetsuya , Yoko Toshinobu
Published: July 2002

The third-order nonlinear optical properties of sol–gel derived α-Fe2O3, γ-Fe2O3, and Fe3O4 thin films have been investigated by the third-harmonic generation method. Especially, the effects of the valence and coordination number of Fe ions on the third-order nonlinear optical susceptibility, χ(3), have been examined. The χ(3) values of α-Fe2O3, γ-Fe2O3, and Fe3O4 thin films were 5.8×10−11, 2.1×10−11, and 4.0×10−10 esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that χ(3) of α-Fe2O3 and γ-Fe2O3 was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe3+ ions as well as by virtual transition from O 2p orbitals to Fe 3d orbitals while χ(3) of Fe3O4 was enhanced by both one- and three-photon resonances. The higher second hyperpolarizability, γ(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens.
Mechanical strength of GeSi alloy
Authors: Yonenaga Ichiro , Sumino Koji
Published: July 2002

The characteristic mechanical behavior of bulk Ge1−xSix alloys with compositions x=0–0.4 grown by the Czochralski technique is investigated as a function of the composition and temperature by means of compressive deformation and is compared with that of elemental Ge and Si semiconductors. Both the yield stress and the flow stress at any strain increase with increasing Si content in the composition range investigated. The yield stress of the GeSi alloy is temperature insensitive at high temperatures, showing that the flow stress of an alloy semiconductor has an athermal component that is absent in elemental or compound semiconductors. The hardening mechanism in alloy semiconductors is discussed.
Role of charge compensation mechanism in La-modified Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics: Enhanced ordering and pyrochlore formation
Authors: Gupta Surya M. , Viehland Dwight
Published: July 2002

Selected area electron-diffraction (SAED) investigations were performed on La-modified (1−x)Pb(Mg1/3Nb2/3)O3–x(PbTiO3) ceramics for compositions near the morphotropic phase boundary for various nonstoichiometric ordering conditions of Mg and Nb on the B-site cation positions. Studies were performed on specimens with La contents between 0 and 10 at. % and for x=0.35. Specimens were fabricated either by: (i) keeping the average Mg/Nb/Ti ratio unchanged and while creating B-site vacancies (La–PMN–PT), or (ii) changing the average Mg/Nb/Ti ratio (PLMN–PT). For La–PMN–PT, SAED studies revealed the presence of F spots for high La contents. The superlattice reflection intensity was found to increase with increasing La content. However, for PLMN–PT, distinct diffuse superlattice reflections were observed at low La contents, which then decreased significantly in intensity with increasing La substitution. Quantitative analysis using standardless energy-dispersive x-ray spectroscopy was also performed. These investigations revealed that enhanced pyrochlore formation accompanies enhanced B-site cation ordering with increasing La content for La–PMN–PT.
Temperature dependence of ZnGeP2 birefringence using polarized light interference
Authors: Fischer D. W. , Ohmer M. C.
Published: July 2002

We have measured the birefringence (Δn) of ZnGeP2 throughout the 14–500 K temperature range using polarized light interference spectra obtained in transmittance from 0.7 to 12 μm. It was found that Δn continuously increases with temperature for all wavelengths but that the amount of increase is not constant with temperature as often assumed. Δn increases by approximately 0.006 when the temperature changes from 14 to 500 K. The temperature derivative of the birefringence, d(Δn)/dT, was determined at 50 K increments and its value was found to be quite dependent on the exact temperature range over which it was measured. It increases linearly with temperature, having a coefficient of 3.3×10−8/K. Actual d(Δn)/dT values range from 5×10−6/K at the lowest temperature to 19×10−6/K at the highest. We conclude that the anomalously low phase matching angles observed for second-harmonic generation in ZnGeP2 are not associated with errors in birefringence values as previously speculated but instead are most likely due to nonlinear absorption phenomena.
A semianalytic radio frequency sheath model integrated into a two-dimensional hybrid model for plasma processing reactors
Authors: Grapperhaus Michael J. , Kushner Mark J.
Published: July 2002

In high plasma density ([e]>1011–1012 cm−3) reactors for materials processing, the sheath thickness is often <100 s μm while the reactor dimensions are 10 s cm. Resolving the sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency (rf) biased sheaths which has been integrated into a two-dimensional model for plasma etching reactors. The basis of the sheath model is to track the charging and discharging of the sheath in time, and use a one-dimensional analytical model to obtain the instantaneous sheath voltage drop based on the sheath charge and the plasma conditions at the sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200–800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl2 will be discussed. We found that the sheath voltage wave form remains nearly sinusoidal, and that the plasma density, and consequently the ion flux to the surface, scale primarily with inductively coupled power.
Use of time-resolved Raman scattering to determine temperatures in shocked carbon tetrachloride
Authors: Pangilinan G. I. , Gupta Y. M.
Published: July 2002

We report on the use of time-resolved Raman scattering data to determine temperatures in liquid carbon tetrachloride shocked by step-wise loading to a peak pressure of 12 GPa. Changes in our previously reported experimental configuration have resulted in a significant improvement in the signal-to-noise ratio in the data and permitted us to analyze the stokes and antistokes intensities for more than one vibrational mode (314 and 460 cm−1 modes). Hence, better precision can be obtained for shock temperatures on the nanosecond time scales. Temperatures are reported in the 500–1000 K range with an experimental precision of approximately 8%–10%. Our data show that the equation of state for carbon tetrachloride reported in the literature provides temperature values that are significantly lower than the measured values. Various aspects of using Raman scattering data for determination of shock temperatures are discussed.
A comparative study of Si doping in GaAs layers grown by molecular beam epitaxy on GaAs(110) and GaAs(001) surfaces
Authors: Zhou T. C. , Zhou X. C. , Kirk W. P.
Published: July 2002

A comparison of Si doping behavior in GaAs layers on (110) and (001) surfaces under the same growth conditions shows that the autocompensation ratio in (110) layers is usually higher than in (001) layers. However, under certain conditions (low substrate temperature, high As4 pressure, and low Si flux), the free electron concentration in the (110) layers can actually be higher. We attribute this behavior to Ga vacancy traps in the (001) surface layers; whereas layers on the (110) surface remain almost defect free. Our results help to clarify the mechanism of defect generation in the (001) layers, which leads to reduced carrier concentration at low growth temperatures.
Persistent photoconductivity in a two-dimensional electron gas system formed by an AlGaN/GaN heterostructure
Authors: Li J. Z. , Lin J. Y. , Jiang H. X. , Asif Khan M. , Chen Q.
Published: July 2002

Persistent photoconductivity (PPC) effect associated with a two-dimensional electron gas (2DEG) in an AlGaN/GaN heterojunction device has been observed. As a consequence, the device was observed to be sensitive to light and the sensitivity was associated with a permanent photoinduced increase in the 2DEG carrier mobility and density. By formulating the PPC buildup and decay kinetics, we attributed the observed increase in the 2DEG carrier density and mobility to the photoionization of deep level impurities (DX like centers) in the AlGaN barrier material. In the PPC state, we were able to continuously vary the 2DEG carrier density in a single sample through photoexcitation and it has been found that the 2DEG carrier mobility increases almost linearly with the carrier density in the 2DEG channel. At 10 K, an electron mobility of 5800 cm2/V⋅s has been obtained in a PPC state. Implications of these observations on the device applications based on AlGaN/GaN heterojunctions have also been discussed.
Erbium implanted thin film photonic materials
Authors: Polman A.
Published: July 2002

Erbium doped materials are of great interest in thin film integrated optoelectronic technology, due to their Er3+intra-4f emission at 1.54 μm, a standard telecommunication wavelength. Er-doped dielectric thin films can be used to fabricate planar optical amplifiers or lasers that can be integrated with other devices on the same chip. Semiconductors, such as silicon, can also be doped with erbium. In this case the Er may be excited through optically or electrically generated charge carriers. Er-doped Si light-emitting diodes may find applications in Si-based optoelectronic circuits. In this article, the synthesis, characterization, and application of several different Er-doped thin film photonic materials is described. It focuses on oxide glasses (pure SiO2, phosphosilicate, borosilicate, and soda-lime glasses), ceramic thin films (Al2O3,Y2O3, LiNbO3), and amorphous and crystalline silicon, all doped with Er by ion implantation. MeV ion implantation is a technique that is ideally suited to dope these materials with Er as the ion range corresponds to the typical micron dimensions of these optical materials. The role of implantation defects, the effect of annealing, concentration dependent effects, and optical activation are discussed and compared for the various materials.
Characterization of pulse-modulated inductively coupled plasmas in argon and chlorine
Authors: Hebner G. A. , Fleddermann C. B.
Published: July 2002

The characteristics of pulse-modulated inductively coupled plasmas in argon and chlorine have been experimentally investigated. Measurements were performed for peak rf powers between 150 and 400 W at 13.56 MHz, duty cycles between 10 and 70%, and pulse repetition frequencies between 3 and 20 kHz. Over this parameter space, measurements were performed of the time dependent forward and reflected rf powers into the matching network, coil voltage, rf variation of the plasma potential, electron density, and Cl− density. These measurements indicated that for the first 5–30 rf cycles of each pulse, the discharges probably were operating in a capacitively coupled discharge mode with rf variations in the plasma potential of several hundreds of volts and relatively low electron density. Measurements of the electron density in pulse-modulated chlorine discharges indicated that the plateau electron density was a function of the duty cycle; the plateau electron density was lower for higher duty cycles. This may indicate that the ratio of Cl to Cl2was changing with duty cycle. In addition, a microwave radiometer was used to provide an indication of the time-dependent electron temperature. Large spikes in the microwave radiation temperature were noted at the turn-on of the rf power pulses and, in some cases, at the transition from a capacitively coupled to an inductively coupled plasma.
Diagnostics of the early phase of an ultraviolet laser induced plasma by spectral line analysis considering self-absorption
Authors: Hermann J. , Boulmer-Leborgne C. , Hong D.
Published: July 2002

We have performed time- and space- resolved plasma diagnostics during ultraviolet excimer laser ablation of a Ti target in a low pressure N2 atmosphere. Spectral lines emitted from metal vapor ions in the early plasma phase (t⩽200  ns) have been analyzed and compared to line profiles computed for a plasma in a local thermodynamic equilibrium.
Comprehensive characterization of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN
Authors: Carrano J. C. , Li T. , Grudowski P. A. , Eiting C. J. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report on the material, electrical, and optical properties of metal–semiconductor–metal ultraviolet photodetectors fabricated on single-crystal GaN, with active layers of 1.5 and 4.0 μm thickness. We have modeled current transport in the 1.5 μm devices using thermionic field emission theory, and in the 4.0 μm devices using thermionic emission theory. We have obtained a good fit to the experimental data. Upon repeated field stressing of the 1.5 μm devices, there is a degradation in the current–voltage (I–V) characteristics that is trap related. We hypothesize that traps in the GaN are related to a combination of surface defects (possibly threading dislocations), and deep-level bulk states that are within a tunneling distance of the interface. A simple qualitative model is presented based on experimental results. For devices fabricated on wafers with very low background free electron concentrations, there is a characteristic “punch-through” voltage, which we attribute to the interaction of the depletion region with the underlying low-temperature buffer layer. We also report GaN metal–semiconductor–metal photodetectors with high quantum efficiencies (∼50%) in the absence of internal gain. These photodetectors have a flat responsivity above the band gap (measured at ∼0.15 A/W) with a sharp, visible-blind cutoff at the band edge. There is no discernible responsivity for photons below the band-gap energy. We also obtained record low dark current of ∼800 fA at −10 V reverse bias. The dark current and ultraviolet photoresponse I–V curves are very flat out to VR>−25 V, and do not show evidence of trap-related degradation, or punch-through effects.
Pressure sensing with a flexural plate wave resonator
Authors: Butler M. A. , Hill M. K. , Spates J. J. , Martin S. J.
Published: July 2002

A silicon nitride membrane supported on a silicon substrate and having a sealed air cavity underneath has been used as a pressure sensor. Membrane resonances are excited and detected using conductor lines patterned on the membrane in combination with an applied magnetic field. The resonant frequency depends strongly upon membrane tension and thus the differential pressure across the membrane.
Scaling of pinning forces in NdBa2Cu3O7−δ superconductors
Authors: Koblischka M. R. , Higuchi T. , Yoo S. I. , Murakami M.
Published: July 2002

The scaling of the volume pinning forces, Fp=jc×Ba, is performed versus the reduced fields bf=Ba/Bfp (where Ba is the applied field, and Bfp denotes the peak position of Fp,max) and h=Ha/Hirr (where Hirr denotes the irreversibility field) for two different NdBa2Cu3O7−δ superconductors, a single crystal and a melt-processed sample. In both cases, an excellent natural scaling is found to hold. Based on these good scaling results, we extend the scaling analysis towards lower temperatures. There, the scaling parameters are no longer directly accessible within our experimental field range, but can be deduced from the Fp(Ba) curves normalized using fitting values of Bfp and Fp,max so that these overlap with the high temperature curves. It is shown that such a procedure allows to obtain reasonably good estimates of Fp,max and Hirr, which leads in turn to an estimation of the irreversibility line well down to 0 temperature. At low temperatures, deviations from the analytical dependencies appear which are due to interference with the central and intermediate peak in the magnetization loops. The shape of the irreversibility line is discussed.
Surfactant effects of thallium in the epitaxial growth of indium arsenide on gallium arsenide(001)
Authors: Storm D. F. , Lange M. D. , Cole T. L.
Published: July 2002

We report on the role of thallium as a surfactant in the growth of InAs on GaAs(001) by molecular beam epitaxy. Thallium suppresses the transition from two-dimensional to three-dimensional growth modes for temperatures below 440 °C without incorporating significantly into the bulk InAs lattice. It extends the range of temperatures at which InAs may be grown, and appears to improve the crystalline quality of growth at 520 °C. Preliminary results indicate that at low temperature (280 °C) thallium does not prolong pseudomorphic growth beyond the normal critical layer thickness, but rather may form interfacial InTlAs with a greater concentration of thallium than has been achieved to date.
Quantitative study of nitridated sapphire surfaces by x-ray photoelectron spectroscopy
Authors: Hashimoto Tadao , Terakoshi Yoshitami , Yuri Masaaki , Ishida Masahiro , Imafuji Osamu , Sugino Takashi , Itoh Kunio
Published: July 2002

The characteristics of an amorphous oxynitride layer formed by nitridation are studied with x-ray photoelectron spectroscopy in order to reveal the role of nitridation before GaN growth in metalorganic chemical vapor deposition. The intensity variation of the N 1s peak with nitridation time is analyzed quantitatively using a diffusion-based model. The effective diffusion coefficient of nitrogen-related species in the amorphous layer is estimated to be 4×10−18 cm2/s at 1000 °C. The thickness of the amorphous layer is on the order of monolayers because of this small diffusion coefficient. The nitrogen concentration in the layer, however, is as high as the order of the site density of O atoms on the sapphire surface. These characteristics imply that the role of the amorphous layer formed by nitridation is to modify surface energy with high-density N atoms rather than to form a buffer-like layer.
Generalized theory of the photoacoustic effect in a multilayer material
Authors: Hu Hanping , Wang Xinwei , Xu Xianfan
Published: July 2002

In this work, a generalized expression of the photoacoustic (PA) effect in a multilayer material is derived. This expression takes thermal and optical properties and geometry of a multilayer structure, as well as the thermal contact resistances between layers into consideration. In addition, a composite piston model consisting of a thermal piston and a mechanical piston of the PA effect is developed and interpreted from the viewpoint of thermodynamics. Mistakes occurring in the thermal piston model and in the composite piston model developed before are pointed out and corrected. It is also shown that the PA effect has an isochoric character for the thermal piston and a polytropic character with a polytropic factor of (2−1/γ) for the mechanical piston. The theory developed in this work is in good agreement with the experimental results, and is applicable to a wide range of photoacoustic problems.
Sulfur doped graphite prepared via arc discharge of carbon rods in the presence of thiophenes
Authors: Glenis S. , Nelson A. J. , Labes M. M.
Published: July 2002

Sulfur doped graphite is obtained by arc vaporization of graphite rods in the presence of thiophenes. The surfaces of the carbon rods become coated with a layer of a sulfur-doped carbon. X-ray diffraction indicates a small d spacing expansion, and there is a slight decrease in the electrical conductivity of the doped layer. X-ray photoelectron spectroscopy confirms that sulfur incorporation has occurred at a level of 0.6% and that the sulfur atoms are in a neutral state. X-ray photoemission results indicate that S incorporation induces a shift of the valence band toward the Fermi level.
Thermal equilibrium concentrations of the amphoteric dopant Si and the associated carrier concentrations in GaAs
Authors: Chen Chang-Ho , Gösele Ulrich M. , Tan Teh Y.
Published: July 2002

Expressions of the thermal equilibrium concentrations of Si in GaAs have been obtained in terms of fundamental constants of the involved materials. Silicon is an amphoteric dopant in GaAs, with four species: a neutral and an ionized shallow donor species occupying Ga sublattice sites, and a neutral and an ionized shallow acceptor species occupying As sublattice sites. The concentration of an ionized Si species is expressed by the concentration of the appropriate neutral species and the GaAs crystal Fermi level or the carrier concentration and the band gap energy level positions. The thermal equilibrium concentrations of the two neutral species are expressed by the relevant Gibbs free energies of formation and the As4 vapor phase pressure in the ambient. Using these equations, the long observed relations between the carrier and Si concentrations in different experiments involving both n- and p-type Si doping produced GaAs are quantitatively explained. A difference of ∼1.55 eV in the effective formation enthalpy between the neutral Si atoms occupying the As and Ga sublattice sites has been identified. Moreover, at high temperatures, the GaAs intrinsic Fermi level energy Ei appears to be higher than the midgap energy Eg/2 by ∼20–80 meV.