bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress free and moisture insensitive silicon oxide dielectric films formed by molecular-beam deposition
Authors: Chand Naresh , Kola R. R. , Opila R. L. , Comizzoli R. B. , Krautter H. , Sergent A. M. , Tsang W. T. , Osenbach J. W. , Luftman H. S.
Published: July 2002

Silicon oxide, SiOx (1≤x≤2), formed by molecular-beam deposition (MBD) has many attractive optical, electrical, mechanical, and chemical properties which make it a suitable dielectric for many semiconductor device applications. It can be thermally evaporated at a much lower temperature than Si, SiO2, or Si3N4 and it condenses on cooler surfaces in uniform and adherent stoichiometric SiO (x=1) films when evaporated in high vacuum. At low deposition rates and at high pressures of oxygen, SiOx (1≤x≤2) films result. This allows variation of refractive index, stress, and other properties with x. MBD-SiOx films are insensitive to moisture absorption. A high quality thin film of SiO2 formed on the surface of SiOx, when exposed to an oxidizing ambient, protects the SiOx film underneath from the environment. Dielectric breakdown strength of the SiO films is comparable to that of other high quality deposited dielectrics. In general, the SiO (x=1) films are under tensile stress of <100 MPa, which is significantly lower than that observed in other dielectric films. Introduction of a small amount of oxygen during deposition reduces the tensile stress; at an O2 pressure of 5×10−7 Torr and above, the films are in compression. This allows the tunability of stress, and deposition of films essentially free from stress. Furthermore, both Si and SiO have similar values of the linear thermal expansion coefficients (average values between 23 and 350 °C: 3.37×10−6 and 2.7×10−6 °C−1, respectively). As a result, SiOx/Si films develop little stress during thermal cycling.
Theory of coherent phenomena in pump-probe excitation of semiconductor amplifiers
Authors: Girndt A. , Knorr A. , Hofmann M. , Koch S. W.
Published: July 2002

The ultrafast pump-probe signal of a two band semiconductor amplifier is theoretically analysed using Maxwell-Semiconductor-Bloch equations. It is shown that the coupling of the pump and probe pulse via the probe gain modification significantly contributes to the signal for short delay times between pump and probe. The probe signal exhibits dominant oscillatory interference-like structures which conceal intensity dependent ultrafast features. Despite the semiconductor is described with a two band model which does not include free carrier absorption and two photon absorption the results are qualitatively similar to those of recent experiments.
Electron capture and emission by the Ti acceptor level in GaP
Authors: Roura P. , Morante J. R. , Guillot G. , Bremond G. , Ulrici W.
Published: July 2002

Previously reported results on deep level optical spectroscopy, optical absorption, deep level transient spectroscopy, photoluminescence excitation, and time resolved photoluminescence are reviewed and discussed in order to know which are the mechanisms involved in electron capture and emission of the Ti acceptor level in GaP. First, the analysis indicates that the 3T1(F) crystal-field excited state is not in resonance with the conduction band states. Second, it is shown that both the 3T2 and 3T1(F) excited states do not play any significant role in the process of electron emission and capture.
Facing targets sputtered Fe-N gradient films
Authors: Jiang E. Y. , Sun D. C. , Lin C. , Tian M. B. , Bai H. L. , Ming S. Liu
Published: July 2002

Fe-N gradient films were prepared with a facing targets sputtering system. During deposition, the nitrogen pressure increased linearly up to a value, which is called the ‘‘ultimate pressure.’’ Composition profiles, microstructure, magnetic properties, and corrosion resistance of the films were investigated by various methods. The experimental results indicate that the Fe-N films possess some composition and structural gradients. The Fe concentration decreases from the substrate to the film surface from 100 to 66 at. %. The phases α″-Fe16N2, γ′-Fe4N, ε-FexN(2<x≤3) and ζ-Fe2N are present in the gradient films at different depths. Ms under the ultimate nitrogen pressure of 0.05 Pa has a value of 1803 emu/cc which is higher than that of bulk iron, this is attributed to the presence of Fe16N2. Increasing further the ultimate nitrogen pressure, Ms decreases monotonically. The corrosion resistance of the gradient film with higher nitrogen concentration near the surface is good enough for magnetic recording heads.
Temperature dependence of the Fowler–Nordheim current in metal-oxide-degenerate semiconductor structures
Authors: Pananakakis G. , Ghibaudo G. , Kies R. , Papadas C.
Published: July 2002

A comprehensive study of the temperature dependence of the Fowler–Nordheim (F–N) tunnel emission in a metal-oxide-semiconductor structure is conducted both theoretically and experimentally. The theoretical variations with temperature of the F–N emission is analyzed both for metallic and degenerate semiconductor cathode materials. The influence of the electron concentration of a degenerate semiconductor on the amplitude of the F–N current is demonstrated. A new analytical formula for the F–N current temperature dependence is derived from the exact expressions using the Sommerfeld expansion. This new analytical approximation proves to be much more efficient than the previous analytical formula developed by Good and Müller [Field Emission, Handbuch der Physik, Vol. 21 (Springer, Berlin, 1956)] and may be very useful for F–N current computer-aided-design-oriented numerical simulation. The experimental study of the F–N current in MOS capacitors clearly demonstrates the strong impact of temperature on the F–N emission above 250 °C. It is also shown that the pre-exponential and the exponential F–N coefficients can still be determined as a function of temperature. The relative variation with temperature of the experimental F–N current data can be well interpreted by the exact F–N emission formula provided that the temperature dependence of the semiconductor (metal) -oxide barrier height Φb is well accounted for by a quasilinear function of temperature. The absolute amplitude of the F–N current can also be satisfactorily predicted by the exact F–N theory while adjusting the semiconductor electron concentration.
Concentration-dependent near and above band edge absorption in doped InP and its effect on solar cell modeling
Authors: Augustine G. , Rohatgi A. , Jokerst N. M. , Dhere R.
Published: July 2002

Absorption coefficients for n- and p-type InP are reported for the first time in the energy range of 1.3–3.0 eV in this paper. In order to obtain absorption coefficients in the higher energy range of 1.8–3.0 eV, extremely thin (∼0.2 μm) InP samples were fabricated and bonded to glass substrates. For measurements in the lower energy range of 1.3–1.6 eV, samples with thickness in the range of 0.5–0.9 μm were found to be optimum. For energies below 1.6 eV, the absorption coefficients are a strong function of the doping concentration. However, in the energy range of 1.8–3.0 eV there is little doping dependence and the measured absorption coefficients follow the theoretical calculations. In the case of n-type samples, absorption coefficients decrease with increasing doping concentration and the absorption edge moves to higher energy due to the Burstein–Moss effect. For p-type samples, the absorption edge shifts to lower energies due to transitions between band tails. Model calculations show that the use of accurate doping-dependent absorption coefficients reported in this paper, as opposed to the commonly used absorption coefficients of undoped InP, can result in significant improvement in the predicted internal quantum efficiency and device parameters of InP solar cells.
Phase transformations caused by spark planing of α-Fe surfaces
Authors: Cabanillas E. D. , Desimoni J. , Punte G. , Mercader R. C.
Published: July 2002

Spark planed surfaces of pure iron have been investigated using conversion electron Mössbauer spectroscopy (CEMS), scanning electron microscopy, and x-ray diffraction. Results reveal that as a consequence of spark erosion, phases not originally present, α′- and γ-iron, are formed at the surface of the samples. The limit of carbon content of γ-Fe is estimated from CEMS hyperfine parameters.
Absolute stress determination in orthotropic materials from angular dependences of ultrasonic velocities
Authors: Degtyar A. D. , Rokhlin S. I.
Published: July 2002

In this paper we describe a method to determine absolute plane stresses in orthotropic materials from the angular dependence of ultrasonic velocities. No prior information on material anisotropy (texture) is required since the stress dependent elastic constants and stresses are found simultaneously from velocity data using least-squares optimization. In the optimization algorithm no relation between the stress dependent elastic constants and stresses is assumed and therefore they are considered as uncoupled variables, but it is shown that the iteration process converges to the correct (coupled) values. The method is applicable for stress measurements in materials which have undergone a complicated (possibly plastic) history of loading and unloading; thus it can be used to determine both applied and residual stresses. To check the proposed technique we use synthetic sets of experimental data for different degrees of anisotropy and stress levels. Calculations using these synthetic data show that when the principal stress directions coincide with the symmetry axes the angular velocity data in the plane perpendicular to the stress plane may be used for reconstruction of both stress components. When the stress is off the symmetry axis, the shear and the difference of the normal stress components may be determined from the angular dependence of the velocities in the plane of stresses. In both cases the stress determination accuracy depends only on the accuracy of velocity measurements and is independent of stress level and material anisotropy.
Anisotropy of the critical current in silver sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes
Authors: Hu Q. Y. , Schalk R. M. , Weber H. W. , Liu H. K. , Wang R. K. , Czurda C. , Dou S. X.
Published: July 2002

The transport critical current of Bi:2223 tapes was measured in magnetic fields up to 15 T and at temperatures from 4.2 to 84 K. At high temperatures, the critical current density Jc is strongly anisotropic and the anisotropy increases rapidly with magnetic field, whereas at low temperatures the critical current is less anisotropic and the anisotropy is field independent above 1 T. In the former case we believe we are in a regime, where pinning limits Jc, at least within some parts of the tape, whereas in the latter case the limitation of Jc by Josephson weak links seems to be the dominant mechanism. In addition, a critical current hysteresis induced by flux trapping in a weak link network is observed, which is more pronounced at low temperatures. From transmission electron microscopy observations of the microstructure we find that the ‘‘brick’’ in the ‘‘brick wall’’ model turns out to be the colony instead of the grain inside the colony. Additionally it is found that colony boundaries parallel the ab plane and intersected boundaries occur much more frequently than boundaries parallel to the c axis, due to the misalignment of the colonies inside the tape. In a small region near the silver sheath colony misalignment is much smaller and boundaries parallel to the c axis may act as strong links at high temperatures as their interfaces are very clean and well matched.
Dislocations in lattice-mismatched wide-gap II-VI/GaAs heterostructures as laser light scatterers: Experiment and theory
Authors: Rouleau C. M. , Santana C. J. , Jones K. S. , Park R. M.
Published: July 2002

It is considered in this work that the dislocations that form upon attainment of the critical thickness in lattice-mismatched wide-gap II-VI/GaAs heterostructures can act as laser light scatterers. Experimentally, a near-normal-incidence HeNe laser probe has been employed during epitaxial growth, which generates both a specularly reflected laser light signal as well as a nonspecularly reflected, or scattered, light signal for epilayer thicknesses beyond the critical thickness. It has been determined that the scattered light originates within the bulk of the II-VI epilayer, as opposed to the free surface, based on observation of a π-phase shift between the specular and nonspecular reflections which were monitored simultaneously. A strong correlation has also been observed between the dislocation density as determined postgrowth by transmission electron microscopy analysis of ZnSe/GaAs heterostructures and the optical data (scattered light intensity) recorded in situ during the growth of such heterostructures as a function of epilayer thickness. Theoretically the refractive index perturbations necessary for such scattering have been considered to be the result of strong microelectric fields which surround the dislocations evolving during plastic deformation. Specifically, the field distribution around a dislocation is considered with regard to three different potentials, namely, the deformation potential, the charged dislocation potential, and the piezoelectric potential. The magnitudes of these fields are considered with reference to ZnSe and subsequently the electro-optic effect is evoked in order to argue that a refractive index perturbation (approximately 10−5–10−4) sufficient in magnitude to scatter light could result in the ZnSe/GaAs system as a consequence of such fields.
Threshold of stimulated emission in multivalley lead salts
Authors: Tomm J. W. , Mocker M. , Kelz T. , Elsaesser T. , Klann R. , Novikov B. V. , Talalaev V. G. , Tudorovskii V. E. , Böttner H.
Published: July 2002

A study of the threshold of stimulated emission for optically pumped PbSe and PbTe is presented. The influence of external parameters such as magnetic field and misfit strain on the threshold as well as the differential quantum efficiency of stimulated emission are analyzed. It is shown that effective energy gap shifts caused by the external parameters only weakly influence the threshold, whereas the main effect arises from the redistribution of the carriers in the multivalley band structure when the degeneracy of the carriers in the valleys at the L points of the Brillouin zone is lifted. A consistent qualitative theoretical explanation for the effects is given.
Time-delay measurements depending on lasing wavelengths of single small-size droplet dye laser
Authors: Taniguchi Hiroshi , Tomisawa Humihiro
Published: July 2002

Measurements of time delay of several nanoseconds depending on the lasing wavelengths from 200-μm-diam single-droplet dye laser are reported. The wavelength tuning is performed by injecting scattering particles into the droplet dye laser. It is suggested that the time delay depending on the lasing wavelengths by varying the quantity of the scatterers is related to the round-trip time required to support lasing in the droplet.
Characterization of electron cyclotron resonance plasmas by vacuum ultraviolet spectroscopy
Authors: Mehlman G. , Eddy C. R. , Douglass S. R.
Published: July 2002

We have recorded, in the energy range of ∼4–11 eV, radiation from electron cyclotron resonance microwave plasmas generated when coupling microwave energy in a number of gases and gas mixtures appropriate for plasma deposition or etching. Molecular, atomic, and ionic emissions are observed in this range attesting to the major collisional processes (dissociation, excitation, and ionization) with hot electrons. The neutral atom temperature has been measured by standard spectroscopic techniques and found to be in the range of 0.1–0.3 eV, much lower than the electron temperature of the same plasmas. The main discharge parameters (i.e., gas pressure, flow rate, microwave power) have been varied to interpret their influence on the plasma radiation in terms of plasma parameters.
Characterization of nanostructured carbon microwires fabricated by an aerosol process
Authors: Schleicher Bertram , Friedlander Sheldon K.
Published: July 2002

Nanometer aerosol particles produced by laser ablation from a carbon target were used to fabricate wires about 50 μm in diameter by agglomeration in a dc field. We report measurements of the electrical properties of these wires. As the voltage applied across a wire increases the resistance of the wire decreases. At certain voltage settings, a rapid change in the electric current occurred, accompanied by a change in shape of the wire as observed with an optical microscope. The rapid change may result from an increase in wire density when the voltage reaches certain critical values.
Raman scattering characterization of amorphous carbon from photothermal oxidation of fullerite
Authors: Manfredini M. , Bottani C. E. , Milani P.
Published: July 2002

Amorphous carbon has been produced by laser irradiation of single-crystal and polycrystalline fullerite, via cage opening reactions and coalescence of C60 units. A photothermal oxidation reaction is responsible for fullerene cage opening, and oxygen is also bounded to the final products of the coalescence process. Under inert atmosphere C60 clusters retain their structure even under very high laser fluence. The structure of the new carbon phase has been characterized by Raman scattering and energy-dispersive x-ray spectroscopy. A disordered amorphous carbon phase is observed. The evolution of Raman spectra gives information on the kinetics of coalescence and rearrangement of fullerene fragments.
Spectral gaps for elastic waves in continuous periodic composites
Authors: Mon K. K.
Published: July 2002

We study by means of numerical solution to the wave equation the possibility of spectral gaps in model elastic periodic composites with continuous variation of elastic properties. This is in contrast to recent studies of periodic composite with discrete interface. We observe that the presence of spectral gaps and their dependence on the periodic variation of the elastic properties are qualitatively similar to those reported for periodic composite with discrete interface.
Gas-source molecular beam epitaxy of ZnSe using elemental Zn and hydrogen selenide
Authors: Ohtsuka Takeo , Horie Kayoko , Akiyama Naoki , Yao Takafumi
Published: July 2002

We report the growth of unintentionally doped ZnSe by gas-source molecular beam epitaxy using elemental Zn and thermally cracked hydrogen selenide (H2Se). The dependence of the growth rate on H2Se flow rate indicates that the minority source flux limits the growth. The stoichiometric growth condition is established. It is found that the two-dimensional growth regime of ZnSe films extends down to 150 °C. The electrical and optical properties of ZnSe epilayers are characterized by Hall and photoluminescence measurements. Undoped ZnSe epilayers show n-type conduction with an electron concentration of 1.6×1016 cm−3 and Hall mobility of 490 cm2/V s at room temperature. Low-temperature photoluminescence spectra of undoped ZnSe epilayers are dominated by neutral donor-bound exciton emission located at 2.7968 eV. The residual donor impurities of In and Cl are detected in ZnSe epilayers by secondary-ion-mass spectroscopy measurements and are responsible for the n-type conduction. It is confirmed that the donor impurities originate from the H2Se gas source.
Epitaxial growth of AlAs/CoAl/AlAs(001) heterostructures by controlling the metal surface
Authors: Maeda Narihiko , Kawashima Minoru , Horikoshi Yoshiji
Published: July 2002

Epitaxial growth of CoAl films on AlAs(001) films was examined for films with Co contents of 47–58 at. %. The surface terminations of CoAl films were characterized, and the dependance of surface terminations and surface reconstructions on the Co/Al composition was examined. When AlAs/CoAl/AlAs heterostructures were fabricated by growing AlAs overlayers on CoAl films whose surface terminations were well defined, a mixture of AlAs(001) and AlAs(111) phases grew on the Al-terminated surface, whereas only the AlAs(001) phase grew on the Co-terminated surface. The Co-terminated CoAl surface thus is suitable for the overgrowth of AlAs. A high-quality epitaxial GaAs/AlAs/CoAl/AlAs/GaAs heterostructure was grown by combining the control of metal surface terminations with low-temperature migration-enhanced epitaxial growth of AlAs overlayers.
Some thermodynamic aspects of possible reverse phase transformations of carbon and BN during a shock wave experiment
Authors: Rakhimov Artour
Published: July 2002

A new model has been developed to describe the mechanism which determines the preserving or reversing of the phase transformation for diamond and hard BN (boron nitride) during a shock wave experiment. The model considers the principal distribution of parameters, such as stresses, temperature, mass velocity, and density both within these materials, and in their mixtures with other materials. Four main stages in the shock wave experiment are identified to reflect the qualitative changes in these parameters. The possibility of preserving synthesized hard phases of carbon and BN is shown to depend primarily on the characteristics of the release process after maximum pressures are achieved. If the release process is almost adiabatic, then the transformation of strain energy into heat and powerful tensile stresses causes graphitization. If the release process is almost isentropic, then conditions can be favorable for preservation of the hard phases.
Effects of azimuthal and radial spreads of canonical momentum on electron-beam focusing characteristics in the presence of space-charge forces
Authors: Cohen Moshe , Gover Avraham
Published: July 2002

A theoretical and numerical investigation of the effects of azimuthal and radial spreads of canonical momentum on an electron beam focused by a magnetic lens in the presence of space-charge forces is presented. The particles are inserted with an initial Gaussian distribution in the transverse space and in the momentum coordinates or with a uniform initial current distribution. The particle trajectory equation is derived for parameters of an arbitrary applied fields configuration with cylindrical symmetry, and a nonvanishing initial canonical momentum. In the absence of an initial momentum spread particles launched above a critical radial distance from the axis exhibit a phase-space tearing effect in the electron distribution. The inclusion of initial canonical momentum spread in the model allows for skewed trajectories with strong centrifugal force which prevents the appearance and overshadows the effect of strong space-charge forces near the axis, which are responsible for the phase-space tearing effect.
Structural analysis of imperfect GeSi superlattices grown on Ge(001) substrates
Authors: Tweet D. J. , Tatsumi T. , Miyanaga K. , Terashima K.
Published: July 2002

By combining the complementary techniques of x-ray diffraction and transmission electron microscopy we have been able to accurately determine the structure of imperfect GeSi superlattices (SL’s). The samples were epitaxially grown on Ge(001) substrates using Si2H6 and GeH4 source gases. In this report, details of the x-ray experiment and analysis are emphasized. In particular, a model is presented for calculating the diffracted intensity from a SL containing gradients in composition and thickness. Applying this model to the data it is found that between the first and last layers of each SL there exists a roughly 10% increase in the growth rate, without a corresponding change in the alloy composition. This is attributed to a slow increase in the substrate temperature, Tsub, of just a few degrees. A sample grown with a corresponding gradual decrease in the control temperature, TC, was found to be much more uniform.
Substrate temperature effect on the stability of hydrogenated amorphous silicon films deposited at high rates
Authors: Kleider J. P. , Longeaud C. , Barranco-Diaz M. , Morin P. , Roca i Cabarrocas P.
Published: July 2002

The metastability of the electronic properties of hydrogenated amorphous silicon (a-Si:H) films deposited at high rates by the rf glow discharge decomposition of mixtures containing 40% silane in helium is compared with that of device quality a-Si:H material deposited at 250 °C under standard low deposition rate conditions. The density of states above the Fermi level of the films obtained under helium dilution decreases for deposition temperatures increasing from 250 to 350 °C, both after annealing and after light soaking. At 350 °C, the density of states becomes comparable in both states to that of device quality low deposition rate a-Si:H. We observe a correlation between these results and the degradation of the photoconductivity and the below-Fermi-level defect density measured in situ during light soaking.
Grain-boundary diffusion modeling and efficiency evaluation of thin-film diffusion barriers considering microstructure effects
Authors: Gui Xiang , Friedrich Loran J. , Dew Steven K. , Brett Michael J. , Smy Tom
Published: July 2002

We have developed a novel grain-boundary diffusion model using the transmission-line matrix method. In conjunction with a two-dimensional Monte Carlo thin-film growth simulator, this model can be employed for the analysis of impurity diffusion in thin-film diffusion barriers with realistic microstructures. In the model, the impurity at the upper surface of the barrier layer may diffuse through rapid and irregularly shaped grain-boundary diffusion paths to reach the bottom surface. Calculations of the impurity concentration and out-diffusion flux as a function of elapsed diffusion time and position enable the evaluation of the effectiveness of the barrier layer at a microstructure level. Consequently, the diffusion process is depicted with less assumptions and more precisely than previously available approaches. This paper details the grain-boundary diffusion modeling method outlined above, with the emphasis on the treatment of various boundary conditions. A representative application to titanium nitride (TiN) thin-film diffusion barriers is also demonstrated, as befits the high level of interest in this particular material for very large scale integrated devices.
Studies of thin strained InAs, AlAs, and AlSb layers by spectroscopic ellipsometry
Authors: Herzinger C. M. , Snyder P. G. , Celii F. G. , Kao Y.-C. , Chow D. , Johs B. , Woollam J. A.
Published: July 2002

The optical constants for thin layers of strained InAs, AlAs, and AlSb have been investigated by spectroscopic ellipsometry and multi-sample analyses. These materials are important for high-speed resonant tunneling diodes in the AlAs/InAs/In0.53Ga0.47As and AlSb/InAs material systems. Understanding the optical properties for these thin layers is important for developing in situ growth control using spectroscopic ellipsometry. Ex situ room-temperature measurements were made on multiple samples. The resulting fitted optical constants are interpreted as apparent values because they are dependent on the fit model and sample structure. These apparent optical constants for very thin layers can be dependent on thickness and surrounding material, and are generally applicable only for layers found in a similar structural context. The critical point features of optical constants for the strained layers and for the thin unstrained cap layers were found to differ from bulk values, and three principle effects (strain, quantum confinement, and thin-barrier critical-point broadening) have been identified as responsible. Of these three, the broadening of the E1 and E1+Δ1 critical points for thin barrier material is the newest and most pronounced. This thin barrier effect is shown to be a separate effect from strain, and is also observable for the AlAs/GaAs system.
Fully dense Sm2Fe17Nx permanent magnets prepared by shock compression
Authors: Mashimo Tsutomu , Tashiro Suguru , Hirosawa Satoshi , Makita Ken
Published: July 2002

Fully dense Sm2Fe17Nx bulk bodies with porosities less than 5% were prepared by shock compression in a certain low-pressure region, using the magnetically aligned powder pellets with an initial porosity of 46%–52% and without binder. Below a driving shock pressure (single shock wave using a copper standard capsule) of about 10 GPa, the recovered specimens did not consolidate well, and above about 20 GPa they began to decompose to α-Fe and Sm nitride. The consolidated state and porosity depended on the driving shock pressure, the thickness and porosity of the starting powder pellet, and the thickness of the impact plate. It was found that the shock consolidated bulk bodies had a Curie temperature of approximately 475 °C, and that the magnetic properties were sensitive to shock pressure, porosity, alignment, etc. The largest values of coercivity and maximum energy product of the recovered bulk specimens prepared in this study were 7.1 kOe and 16.8 MGOe, respectively.
Nature of compensating luminescence centers in Te-diffused and -doped GaSb
Authors: Dutta P. S. , Méndez B. , Piqueras J. , Dieguez E. , Bhat H. L.
Published: July 2002

Diffusion of tellurium in undoped p-GaSb has been carried out. Using the cathodoluminescence and photoluminescence techniques, the luminescence centers in Te-diffused samples have been identified and compared with the Te-doped bulk GaSb. Fundamental differences in the radiative levels are observed between the diffused and the as-grown doped samples. Evidence for self-compensating acceptor complexes are seen in diffused samples. With short and moderate diffusion times, a compensating acceptor complex VGaGaSbTeSb is observed. For long diffusion times, the dominant acceptor center has been attributed to the antisite defect GaSb or related complex. The reasons for the formation of various acceptor centers have been discussed.
A model of the Temkin isotherm behavior for hydrogen adsorption at Pd–SiO2 interfaces
Authors: Eriksson M. , Lundström I. , Ekedahl L.-G.
Published: July 2002

A simple electrostatic model of the adsorbate–adsorbate interaction of hydrogen atoms at a Pd–SiO2 interface is presented. The model predicts a hydrogen adsorption isotherm of the Temkin type. It is found that, in practice, an upper limit for the hydrogen response of a Pd-metal-oxide-semiconductor device exists. The value (in V) is equal to the difference of the initial heats of adsorption (in eV) of the interface and the Pd bulk, respectively. Furthermore, a corresponding maximum hydrogen concentration, at the interface, of 1×1018 m−2 is predicted. The predictions are in good agreement with previously observed experimental data.
Bistable behavior of a medium-deep center related to EL5 and EL6 in n-type bulk GaAs
Authors: Shiraki Hiroyuki , Tokuda Yutaka , Sassa Koichi
Published: July 2002

We report characteristic behaviors of medium-deep electron traps (EL5, EL6, etc.) in n-type bulk GaAs crystals observed by isothermal constant-capacitance–voltage transient spectroscopy (CCVTS). Each CCVTS spectrum of EL5 and EL6 was broader than a theoretical one expected for a single level, and was found to consist of more than two trap components. Anomalous filling time dependence of CCVTS peak heights for these traps was observed when the filling pulse width was varied in a wide range, i.e., one component of EL6 decreased to about 50% of its maximum, while one component of EL5 increased and saturated. The decrement of the constituent of EL6 was nearly equal to the increment of that of EL5. These variations could be reversed by controlling electron occupation fractions of these traps by means of changing the interval time between two adjacent filling pulses in application of the isothermal “double-pulse CCVTS” method. Such interaction was commonly observed in n-type bulk GaAs irrespective of carrier concentration and growth method. These behaviors of medium-deep electron traps are interpreted well by a model of an electronically controlled bistable reaction.
Formation of cationic silicon clusters in a remote silane plasma and their contribution to hydrogenated amorphous silicon film growth
Authors: Kessels W. M. M. , Leewis C. M. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

The formation of cationic silicon clusters SinHm+ by means of ion–molecule reactions in a remote Ar–H2–SiH4 plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is based on SiH4 dissociation in a downstream region by a thermal plasma source created Ar–H2 plasma. The electron temperature, ion fluence, and most abundant ion emanating from this plasma source are studied as a function of H2 admixture in the source. The electron temperature obtained is in the range of 0.1–0.3 eV and is too low for electron induced ionization. The formation of silicon containing ions is therefore determined by charge transfer reactions between ions emanating from the plasma source and SiH4. While the ion fluence from the source decreases by about a factor of 40 when a considerable flow of H2 is admixed in the source, the flux of cationic silicon clusters towards the substrate depends only slightly on this H2 flow. This implies a strong dissociative recombination of silicon containing ions with electrons in the downstream region for low H2 flows and it causes the distribution of the cationic silicon clusters with respect to the silicon atoms present in the clusters to be rather independent of H2 admixture. The average cluster size increases, however, strongly with the SiH4 flow for constant plasma source properties. Moreover, it leads to a decrease of the ion beam radius and due to this, to an increase of the ion flux towards the substrate, which is positioned in the center of the beam. Assuming unity sticking probability the contribution of the cationic clusters to the total growth flux of the material is about 6% for the condition in which solar grade a-Si:H is deposited. Although the energy flux towards the film by ion bombardment is limited due to the low electron temperature, the clusters have a very compact structure and very low hydrogen content and can consequently have a considerable impact on film quality. The latter is discussed as well as possible implications for other (remote) SiH4 plasmas.
Percolation models for gate oxide breakdown
Authors: Stathis J. H.
Published: July 2002

Computer calculations of the formation of a percolation path across a finite lattice are used to model dielectric breakdown. The classical scaling relations for percolation are expected to be valid only for large (finite) systems near pc. We investigate the opposite limit of very small samples, comparable to the lattice spacing. It is shown that relatively simple numerical calculations can quantitatively describe the statistics and thickness dependence of oxide breakdown in thin samples. The critical defect density for breakdown shows a strong decrease with thickness below about 5 nm, then becomes constant below 3 nm. Both of these features can be quantitatively explained by percolation on a finite lattice. The effective defect “size” of about 3 nm is obtained from the thickness dependence of the breakdown distributions. The model predicts a singular behavior when the oxide thickness becomes less than the defect size, because in this limit a single defect near the center of the oxide is sufficient to create a continuous path across the sample. It is found that a given percolation path has a probability of about 10−3 for initiating destructive breakdown. We investigate both homogeneous percolation and percolation in a nonuniform density of sites.
The He2 potential at small distances
Authors: Ceperley D. M. , Partridge H.
Published: July 2002

Quantum Monte Carlo methods have been used to determine the exact Born–Oppenheimer interaction energy of two helium atoms with internuclear separations between 0.5 and 1.8 Å. There is reasonable agreement with potentials derived from scattering data, however the semiempirical Aziz potential is too repulsive for separation less than 1.8 Å. We propose a new potential for this region.
The description of N2 and F2 potential energy surfaces using multireference coupled cluster theory
Authors: Laidig William D. , Saxe Paul , Bartlett Rodney J.
Published: July 2002

The ground state potential energy surfaces (PES’s) for diatomic nitrogen and fluorine are examined using a version of our recently published linearized multireference coupled cluster method (MR-LCCM). Comparison calculations employing a variety of standard ab initio techniques such as single reference configuration interaction singles and doubles (CISD), many-body perturbation theory (MBPT), coupled cluster single and doubles (CCSD), and multireference (MR)-CISD were also performed. In addition, the PES’s were also investigated using the newly developed CCSDT-1 method, which includes the dominant effect of T̂3. These single reference procedures fail in various ways (with the possible exception of the CCSDT-1 method), while the MR-LCCM method is shown to compare favorably to the more traditional MR-CI techniques. Like the MR-CIs, the MR-LCCM energy curves dissociate correctly and the two are nearly parallel.
Direct inelastic scattering of N2 from Ag(111). III. Normal incident N2
Authors: Kummel Andrew C. , Sitz Greg O. , Zare Richard N. , Tully John C.
Published: July 2002

We have probed the angular momentum orientation of N2 scattered from cold Ag(111) when the N2 approaches the surface along the surface normal. Using resonance enhanced multiphoton ionization (REMPI) and pulsed molecular beam techniques, we are able to probe the flux backscattered along the surface normal. In accordance with the restrictions on cylindrically symmetric systems, the molecules backscattered along the surface normal have no angular momentum orientation nor does the entire scattered flux integrated over all exit angles. However, for detection away from the surface normal, we observe substantial angular momentum orientation; the degree and direction of orientation depends upon the rotational state being probed. Molecular dynamics calculations reproduce the experimental results semiquantitatively. The calculations show that for N2 incident along the surface normal, the exit angle is largely determined by the two-dimensional impact parameter of the molecule within the crystal unit mesh. However, the final rotational state, orientation, and alignment are determined largely by the molecular orientation geometry of the N2 during the collision. In essence, we have found a dynamical process which can partially differentiate between the two hidden initial conditions in a gas–surface collision: the two-dimensaional impact parameter and the molecular orientation geometry.
The lag between the Hamiltonian and the system configuration in free energy perturbation calculations
Authors: Pearlman David A. , Kollman Peter A.
Published: July 2002

The energetic response of two test systems to changes in the governing Hamiltonians (empirical potential energy functions) are examined. For these two systems—neon in water using periodic boundary conditions and partially solvated trypsin—a systematic lag is observed between changes in the Hamiltonian and changes in a component of the energetic trajectory which is relevant to free energy perturbation (FEP) calculations. This observation appears inconsistent with one of the underlying assumptions of a commonly used implementation of FEP simulations: ‘‘slow growth’’ or ‘‘thermodynamic integration.’’ The calculations presented here suggest that the relevant component of the system energetics lags the Hamiltonian by a minimum of about 0.6 ps. Implications of this lag for designing reliable methodologies to use with various implementations of FEP calculations are discussed.
A 193 nm laser photofragmentation time-of-flight mass spectrometric study of CH3SSCH3, SSCH3, and SCH3
Authors: Nourbakhsh S. , Liao C.-L. , Ng C. Y.
Published: July 2002

We have measured the time-of-flight (TOF) spectra for SCH3, CH3, and SSCH3 formed in the photodissociation processes, CH3SSCH3+hν(193 nm)→2SCH3 and CH3+SSCH3. The dissociation energies for the CH3S–SCH3 and CH3SS–CH3 bonds determined at 0 K by the TOF measurements are 72.4±1.5 and 55.0±1.5 kcal/mol, in agreement with the literature values. The threshold value for the formation of S2 measured by the TOF spectrum for S2 is in accord with the thermochemical threshold for the process, SSCH3+hν(193 nm) →S2+CH3. The threshold energy determined from the TOF spectrum for S is found to be consistent with the thermochemical threshold for the photodissociation process, SCH3+hν(193 nm) →S(1D)+CH3, an observation supporting that S atoms are not produced in the ground S(3P) state in the 193 nm photodissociation of SCH3. This observation is rationalized by symmetry correlation arguments applied between the S+CH3 product and SCH3 states.
A comparison of Pt overlayers on α-Al2O3(0001), ZnO(0001)Zn, and ZnO(0001̄)O
Authors: Roberts S. , Gorte R. J.
Published: July 2002

The growth of Pt films on ZnO(0001)Zn and ZnO(0001̄)O and their adsorption properties for CO have been studied and compared to results obtained for Pt on α-Al2O3(0001). Samples were prepared by vapor deposition of Pt onto flat substrates in ultra high vacuum, and metal coverages were measured using a quartz-crystal, film thickness monitor. On both ZnO surfaces, Auger electron spectroscopy (AES) and transmission electron microscopy (TEM) indicate that Pt grows in as a 2D film at 300 K, in contrast to results for Pt on α-Al2O3(0001) where 3D particles are formed under these conditions. Furthermore, transmission electron diffraction (TED) showed that the Pt grew as an ordered overlayer on the ZnO crystals. Heating the Pt/ZnO surfaces resulted in the formation of Pt particles starting at ∼650 K, but the Pt particles were still oriented with the ZnO. On both ZnO surfaces, temperature programmed desorption (TPD) curves for CO from high Pt coverages (>one monolayer) and from Pt particles formed by heating thinner films were identical to TPD curves for CO from Pt on α-Al2O3(0001). The main desorption features were observed at 400 and 510 K and are interpreted as being due to different site geometries on the Pt by comparison to results from Pt single crystals. However, for Pt at submonolayer coverages prior to particle formation, the TPD curves for CO from Pt on ZnO(0001)Zn were shifted down in temperature by ∼60 K compared to results for Pt on ZnO(0001̄)O. These results are interpreted as indicating a chemical interaction between Pt and the Zn+2 ions which are in direct contact.
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o and experimental quartic centrifugal distortion constants of acetone, pyrazole, and γ-pyrone
Authors: Wlodarczak G. , Demaison J. , van Eijck B. P. , Zhao Ming , Boggs James E.
Published: July 2002

The complete harmonic force field of acetone has been computed at the Hartree–Fock level with the 4–21 basis set of Gaussian orbitals. Seven scale factors optimized for related molecules were used to scale the computed force field. The mean deviation from the experimental vibrational frequencies is only 9.7 cm−1, except for the two methyl torsions. In order to obtain the best possible force field, a new set of seven scale factors was derived by fitting the computed spectrum of acetone to the observed spectrum. The only major change was for the C–C–C bending scaling factor. The final mean deviation was only 5.2 cm−1. The ground state rotational spectra of pyrazole and γ-pyrone have been reinvestigated in the millimeter-wave range. Thanks to millimeter-wave spectroscopy, accurate rotational and centrifugal distortion constants have been determined for acetone, pyrazole, and γ-pyrone. The experimental quartic centrifugal distortion constants are compared with the values calculated from ab initio force fields. A good agreement is found, even for the two-top molecule acetone for which the differences between the experimental and calculated T constants are smaller than 10%.
A refined droplet approach to the problem of homogeneous nucleation from the vapor phase
Authors: Dillmann A. , Meier G. E. A.
Published: July 2002

A theory of homogeneous nucleation from the vapor phase is presented, which is based on an extension of Fisher’s semiphenomenological droplet model for the Gibbs free energy of formation of a cluster. This droplet model allows translational, rotational, vibrational, and configurational degrees of freedom of the cluster as well as the variation of surface tension with cluster size. By suitable choice of the three free model parameters, known nucleation theories such as the classical Becker–Döring–Zeldovich theory and others are obtained as special cases. Since, however, an ansatz for the Gibbs free energy of formation allows the construction of an equation of state for real gases below the critical point, a new method of determining the model parameters is suggested, which is based on the idea of forcing the identity of the constructed equation of state with an experimentally verified one. Thus, all free model parameters can be determined purely from well-known handbook properties. It is shown that the new theory gives a better prediction of observed nucleation rates than the classical one.
Molecular dynamics of SF6 in porous silica
Authors: Brodka A. , Zerda T. W.
Published: July 2002

Molecular dynamics of SF6 in small cylindrical pores in amorphous silica is studied by computer simulations. The solid is represented by an atomic model that takes into account microscopic structure of the surface. Adsorbent is modeled as an assembly of six Lennard-Jones potentials. Rotational and translational correlation functions are calculated and discussed in terms of temperature, density, pore diameter, and surface potential. Changes in the motion along the pore axis and pore radius are analyzed. It is shown that surface interactions determine translational and rotational diffusion of molecules in the monolayer, but at distances greater than two molecular diameters, the dynamics can be compared to that in the bulk liquid.
Line-by-line measurements of interference parameters for the 0–1 and 0–2 bands of CO in He, and comparison with coupled-states calculations
Authors: Thibault F. , Boissoles J. , Le Doucen R. , Farrenq R. , Morillon-Chapey M. , Boulet C.
Published: July 2002

Here we present the first line-by-line measurements of interference parameters Y0k describing line-mixing effects in the weak overlapping regime for He-broadened CO lines in the 0–1 and 0–2 bands at 296 K. A detailed analysis of the line shape at intermediate perturber pressures (up to about 10 atm) has been performed, starting from previous theoretical calculations, which has demonstrated the possibility of an individual measurement of Yk parameters. The method is based on the existence of a component of the line shift, quadratic with the perturber pressure (density) and proportional to Y0k. Comparison of our measurements with results predicted from coupled-states calculations shows good overall agreement. Linear pressure shifts have also been simultaneously measured. The uncertainty on the data is important since linear shifts are very small; however, some features can be considered as significant although we have no clear understanding of them. Possible explanations are discussed.
A theoretical study of hyperfine coupling constants
Authors: Perera S. Ajith , Watts John D. , Bartlett Rodney J.
Published: July 2002

Isotropic hyperfine coupling constants of first-row atoms from B–F and the BH2 radical are calculated analytically from the coupled-cluster (CC) relaxed density with a variety of extended basis sets. We employ both restricted and unrestricted Hartree–Fock reference functions, with the CC singles and doubles (CCSD), CCSD with noniterative triples [CCSD+T(CCSD) and CCSD(T)] methods. The latter provide excellent agreement with experiment. We also consider the role of orbital relaxation and atomic basis functions in accurate predictions.