bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement: AES and XPS
Authors: Seah M. P.
Published: July 2002

An appraisal is made of the current status of measurement in Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). In many areas, the measurement accuracy is now exceptionally high and reproducible. For XPS, peak energy calibration values are presented with a traceable accuracy of 0.02 eV or better. Interlaboratory tests show that commerical instruments, calibrated using two peak energies, reproduce a third in the middle of the energy scale to ±0.035 eV. For selected area XPS instruments, relativistic corrections may be important and these nonlinear corrections are verified to ±0.05 eV. Instrument intensity–energy response functions are analyzed and interlaboratory tests show that relative functions between commercial instruments may be defined, by two methods which agree, to within about 4%. The area of analysis in XPS is defined, with reference methods, to evaluate even the most complex spectrometers. Traceability is then extended to sputter depth profiles. It is shown that a certified reference material of Ta2O5 on Ta may be used to establish depth scales and depth resolutions. Depth resolutions as high as 1.18 nm at 28.4 nm depth are observed. Rationalization of depth resolutions, using a compiled data base, show that the transferability of data allows one to question the validity or universality of conclusions concerning techniques for improving depth resolution.
Vacuum system design and tritium inventory for the charge exchange diagnostic on the Tokamak Fusion Test Reactor
Authors: Medley S. S.
Published: July 2002

The application of charge exchange analyzers for the measurement of ion temperature in fusion plasma experiments requires a direct connection between the diagnostic and plasma-discharge vacuum chambers. Differential pumping of the gas load from the diagnostic stripping cell operated at ≳ 10−3 Torr is required to maintain the analyzer chamber at a pressure of ≲ 10−6 Torr. The migration of gases between the diagnostic and plasma vacuum chambers must be minimized. In particular, introduction of the analyzer stripping cell gas into the plasma chamber having a base pressure of ≲ 10−8 Torr must be suppressed. The charge exchange diagnostic for the Tokamak Fusion Test Reactor (TFTR) is comprised of two analyzer systems designed to contain a total of 18 independent mass/energy analyzers and one diagnostic neutral beam rated at 80 keV, 15 A. The associated arrays of multiple, interconnected vacuum systems were analyzed using the Vacuum System Transient Simulator (vsts) computer program which models the transient transport of multigas species through complex networks of ducts, valves, traps, vacuum pumps, and other related vacuum system components. In addition to providing improved design performance at reduced costs, the analysis yields estimates for the exchange of tritium from the torus to the diagnostic components and of the diagnostic working gases to the torus.
Adsorbate orientation on platinum by polar-angle x-ray photoelectron diffraction
Authors: Wesner D. A. , Coenen F. P. , Bonzel H. P.
Published: July 2002

We used polar-angle x-ray photoelectron diffraction to study the orientation of C2H2, C2H4, and CO on Pt(111) and Pt(110) at 300 K. For C2H2 on Pt(111) and Pt(110) we find no C 1s intensity enhancement, consistent with a side-on bonded species. The same is true for C2H4 on Pt(110), but for C2H2 on Pt(111) we find an enhancement along the surface normal, supporting a vertical ethylidyne species. The C–C bond of C2H2 on Pt(110) remains intact. In the case of saturation coverages of CO, we find a C 1s enhancement along the normal for Pt(111) but not for Pt(110). This supports vertical CO (with the C down) on Pt(111) and tilted CO on Pt(110). Comparison to calculations indirectly supports a model for the tilted phase having p1g1 symmetry. At lower CO coverage on Pt(110) we find vertically adsorbed CO, in line with the idea that the tilting arises from molecular interactions in the adlayer.
Investigation of oxidation of CO on Pd with a mixed molecular beam
Authors: Xi Guangkang , Shao Jun , Shao Shumin , Li Shenglin , He Tianxi , Wang Junrong
Published: July 2002

The kinetics of CO oxidation on polycrystalline Pd at UHV are investigated with a mixed modulated beam. Analysis of the data in different temperature regions concludes a Langmuir–Hinshelwood mechanism for oxidation of CO on polycrystalline Pd. The characteristics of both mixed modulated and single modulated beams are compared and discussed. Best fits to the rate constant parameters characterizing the reaction are given. This reaction proceeds with an activation energy ELH =23.0±2 kcal/mol.
An investigation of small commercial quadrupole mass spectrometers
Authors: Austin W. E. , Mao Fu Ming , Yang Jin Man , Leck J. H.
Published: July 2002

This paper presents a detailed investigation of the characteristics of the small quadrupole mass spectrometers used extensively for partial pressure measurement. Interest has centered upon determining (a) the upper and lower limits to the useful pressure range, (b) the accuracy of measurement, and (c) the long-term stability of the characteristics. Measurements have been made to cover the whole range of the instruments. The general conclusions are that great care must be taken in operating and interpreting measurements if anything more than qualitative results are required. It is also noted that there are significant differences between the performance of instruments with similar sets of published specifications. The suggestion is made in the paper that an agreed practice be adopted for defining a precise format for presenting performance characteristics.
Effects of the plasma-wall sheath on transport
Authors: Mandrekas J. , Singer C. E. , Ruzic D. N.
Published: July 2002

The 1 (1)/(2) -dimensional plasma transport code baldur, along with the neutral transport code degas are used to study the effects of secondary and photoelectron emission from the wall and limiters of a tokamak on the plasma transport. Since different electron emission mechanisms appear to be dominant in different parts of the scrapeoff, the two-region scrapeoff model included in baldur is particularly useful. The effect that secondary and photoelectron emission have on recycling through the reduction of the sheath potential is taken into account in degas by prescribing appropriate values of the sheath potential on each recycling surface, according to the dominant electron emission mechanism present. Results indicate that when photoelectron emission from the first wall increases, the ion and electron temperatures increase in the scrapeoff, while they remain unaffected near the center. When the secondary electron coefficient of the limiters is increased, the ion and electron temperatures drop in the scrapeoff while they increase near the center. In both cases the neutral density decreases slightly throughout the device.
Cryopumps for evacuating space simulation chambers
Authors: Schäfer G. , Häfner H.-U.
Published: July 2002

Cryopumps with pumping speeds of 5000 to more than 100 000 l/s for nitrogen are used for many applications and particularly for producing clean high vacuum in space simulation chambers. After some general notes on the different pumping mechanisms, some types of appendage and open-type refrigerator-cooled cryopumps are presented as well as a helium-gas-cooled cryopump with extremely low vibrations.
Oxidation kinetics for thin rare-earth metal films
Authors: Burnham Alan K. , Jameson Glen T.
Published: July 2002

We measured the oxidation rates of thin samarium, gadolinium, ytterbium, and yttrium films at room temperature in air by a simple optical transmission technique to help understand how to produce flat 100 nm or so thick films on 150-nm freestanding polymer substrates for use as x-ray lasers. The validity of the method is established with aluminum, whose oxide layer reaches a limiting thickness of about 5 nm within 1 h. The oxidation kinetics of different rare earths follow different rate laws. Ytterbium and gadolinium are best described by an inverse logarithmic rate law, yttrium by a direct logarithmic rate law, and samarium by a parabolic rate law. After 2 months, the oxide thicknesses of Y, Yb, Gd, and Sm are about 5, 9, 15, and 17 nm, respectively. Only Sm continues to oxidize rapidly, reaching 35 nm in 6.5 months. Spectroscopic results on Eu and Gd are consistent with previous results that the oxidized layer on the rare earths is usually a carbonated hydroxide. The results of our measurements are compared to those from initial oxidation studies by surface analysis techniques and high-temperature gravimetric studies.
Diagnosis and therapy for a recalcitrant parylene coater
Authors: Williams Joel M. , Rowen Jerome T.
Published: July 2002

An in-house built parylene coater produced coatings whose thicknesses varied 50% at the 1-μm level, whose surfaces were rough and flapjacklike, and whose cross sections looked like foam. This paper details the product response to various equipment and operational changes. The systematic analysis showed that a balance of vacuum pressure and pyrolysis temperature is needed to produce good-quality coatings.
Thermally stable tantalum nitride/silicon Schottky barriers
Authors: Ashok S. , Crookshank M. , Chu M. , Sellers G.
Published: July 2002

Tantalum nitride films deposited by reactive sputtering in an rf magnetron sputtering system have been found to form excellent Schottky barriers on n-type Si with an effective barrier height of 0.74 eV and an ideality factor of <1.1. Following a postdeposition 500 °C anneal, the Au/Ti–W/TaN/n-Si structures exhibit little degradation of electrical properties when sustained at that temperature. The current–voltage characteristics begin to degrade at 600 °C, and evaluation of the capacitance–voltage data of degraded devices shows a rapid rise in surface donor concentration at anneal temperatures above 600 °C, evidently caused by the penetration of Ta (a deep donor in Si) into Si.
Diagnostics by optical emission spectroscopy in the vicinity of the substrate during magnetron sputtering of Ti
Authors: Pech T. , Chabrerie J. P. , Ricard A.
Published: July 2002

The spatial distribution of excited states for neutral Ti and Ar, as well as for ionized Ti, have been investigated in Ar discharges at 0.27 Pa, in the neighborhood of a grounded substrate, at variable temperatures θs (up to ∼500 °C), for given values of the magnetron current (up to 4 A). Pronounced local variations of excited states, with a maximum of spectral line intensities Iλ at ∼ 1 mm from the substrate, characterizing a fairly thin anode sheath, have been detected. The secondary electron emission from the anode is only weakly influenced by θs. The Iλ intensities increase with the power P, faster for Ti and Ti+ than for Ar: ITi∝P1.6, ITi+∝P2.3, and IAr∝P0.8.
A new hybrid molecular pump with large throughput
Authors: Chu J. G.
Published: July 2002

A new design of a hybrid molecular pump with large throughput is described in this paper. It can be operated within the pressure range of 1 to 10−10 Torr for a clean vacuum with a maximum throughput of 10 Torr l/s. The maximum inlet pressure is above 1 Torr, which is almost 100 times higher than that of a conventional turbomolecular pump (TMP). The pump is constructed with ten disks of a TMP and nine disks of a Siegbahn molecular pump in series, with a clearance of 0.5 mm between rotating and stationary parts. The compression ratio for H2 is over 700, which is equivalent to that of a TMP with 31 disks.
A charged-particle analyzer for radio-frequency discharges
Authors: Smith A. M. , Beilby B. N. , Horwitz C. M.
Published: July 2002

Ion and electron fluxes have been measured at the target electrode of an inverted radio-frequency (rf) diode discharge. The retarding-field analyzer uses a single suppressor electrode to select the charged particle polarity, and errors due to secondary particle generation have been minimized. The error sources in this apparatus are described, together with ways of avoiding them. This analyzer permits accurate determination of the directed particle flux with sampling aperture sizes ranging from 30 to 400 μm diameter. Examples of particle fluxes and energy distributions in both rf and direct current (dc) discharges are given, and it is shown that rf discharges have greater ion and electron fluxes at the target, and higher ion energies, than do comparable dc discharges.
A possible resolution of the valence-band offset controversy in HgTe/CdTe superlattices
Authors: Hui P. M. , Ehrenreich H. , Johnson N. F.
Published: July 2002

The valence-band offset controversy in HgTe/CdTe superlattices can be simply resolved by showing, as we do here, that a large offset value (Λ>300 meV) is indeed consistent with the magneto-optical data of Berroir et al. [Phys. Rev. B 34, 891 (1986)]. The superlattice (100 Å HgTe/36 Å CdTe) becomes semimetallic (SM) when Λ is increased from small values, but reverts to semiconducting (SC) behavior for Λ>300 meV. The band gap in this new regime occurs at the superlattice Brillouin zone face. The sensitivity to layer thicknesses of such SC→SM→SC transitions is discussed and experiments suggested.
Analysis of magnetic bit pattern by magnetic force microscopy
Authors: Wadas A. , Grütter P. , Güntherodt H.-J.
Published: July 2002

This paper describes the behavior of a tiny tip of the magnetic force microscope (MFM) over longitudinally directed magnetic bits, by a quantitative approach. Three possible magnetic structures are taken into consideration together with a realistic tip model. Magnetic and van der Waals interactions between the tip and the sample are derived analytically to simulate a force and a force gradient in the MFM. Contours of constant force gradient are presented and related to the known experimental data. The important role of van der Waals force gradient in constant force gradient measurements is shown. The force gradient from a magnetic interaction is partly negative and partly positive, it is balanced or increased by the always positive van der Waals force gradient. Asymmetry of the force gradient above the transition area is revealed when the MFM tip is tilted. By tilting the tip, the detection of two components of a stray field is performed. Lateral resolution of 90 nm could be achieved with our realistic tip model.
Laser-induced fluorescence characterization of ions in a magnetron plasma
Authors: Goeckner M. J. , Goree J. , Sheridan T. E.
Published: July 2002

We report for the first time laser-induced fluorescence measurements of a sputtering magnetron plasma. From these measurements we determined the density, average velocity, and temperature of the ions. The ion density profile is peaked at the same location as the electron density profile, which was measured with a Langmuir probe. The average ion velocity parallel to the cathode surface is less than our detection limit of 93 m/s. The ions at the edge of the discharge are room temperature, while in the center of the discharge they reach a temperature of 0.64 eV. These temperatures are attributed to acceleration of the ions by the electric field in the plasma, together with collisional scattering.
Observation of (5×1) surface reconstruction and surface smoothing of molecular-beam epitaxial ZnS:Ga by post-annealing
Authors: Ohta Shin-ichi , Kobayashi Satoshi , Kaneko Futao , Kashiro Ko-ichi
Published: July 2002

The (2×1) and (5×1) surface reconstructions accompanied with the surface smoothing have been observed at ZnS:Ga layers grown on (100) GaAs by molecular-beam epitaxy (MBE) and followed by postannealing at 600 °C for 20 min. The origin of these reconstructions are discussed and it is suggested that the reconstruction of (5×1) is due to the misfit strain relaxation in the ZnS:Ga epilayer or at the ZnS:Ga/GaAs interface. The simple lattice model of (2×1) and (5×1) surfaces are proposed on the basis of a dimer model. It is confirmed that the surface lattice models proposed give the diffraction patterns of (2×1) and (5×1) by a laser simulation.
Diagnostic techniques for plasma stability in electron cyclotron resonance plasma processing
Authors: Shufflebotham P. K. , Thomson D. J.
Published: July 2002

Three complementary, qualitative diagnostics concerned with the global properties of electron cyclotron resonance (ECR) microwave plasmas of the type used in materials processing applications are presented. Specifically, it is first shown that both ECR and sub-ECR plasmas can occur in distinct modes, between which discontinuous and sometimes bistable transitions take place. Each mode is characterized by a specific radial structure and dependence on system variables. We then describe several easily performed diagnostics which provide a means to classify the modes, delimit their range in the system’s parameter space and characterize their unstable transition regions. Some comments regarding the possible origins of the modes are also given.
Friction studies of ‘‘clean’’ and oxygen exposed Fe surfaces in ultrahigh vacuum
Authors: DeKoven Benjamin M. , Hagans Patrick L.
Published: July 2002

The coefficient of friction for both clean and oxygen exposed contacting polycrystalline Fe surfaces was measured in ultrahigh vacuum (UHV) using a pin on flat device. This system was chosen in order to examine the influence of very thin oxide films (≤20 Å) on friction. The surfaces were then examined in situ in the same UHV chamber using both x-ray photoelectron spectroscopy (XPS) and scanning Auger microprobe (SAM). The oxide thickness was determined using O (1s) XPS intensities. The loads used were 0.12–0.16 N. The Fe pin (radius=0.005 m) motion was linear over 0.004 m at a speed of 5×10−6 m/s. The friction coefficient measured for the ‘‘clean’’ (∼0.2–1.0 Å oxide) surfaces was 3.0–5.8. Large stick slips were seen during these measurements. The friction dropped monotonically for increasing oxide thickness (≤25 Å) to 1.7–2.0. Auger line scan studies of the O/Fe ratio in several scars showed a large depletion of O in the contacted area. Even though the Fe flat surface is deformed up to depths of 3 μm during sliding and loading, the presence of the very thin oxide layer (≤25 Å) moderates the friction.
Modeling of the energy deposition mechanisms in an argon magnetron planar discharge
Authors: Guimarães F. , Almeida J. , Bretagne J.
Published: July 2002

The model for the planar magnetron discharge is based on the resolution of the Boltzmann equation for the electron energy distribution function corresponding to the plasma region of the discharge. It assumes that electron transport is controlled by binary collisions; effects of Coulomb collisions are included. The influence of the main discharge parameters (gas density, discharge current and voltage) is studied. Comparison of the (Vd–Id) characteristics predicted by the model with experimental ones is made allowing some confidence in the model.
Microscopic view of scanning tunneling microscopy
Authors: Chen C. Julian
Published: July 2002

We present a theory of the atomic resolution in scanning tunneling microscopy (STM) in terms of localized surface states on the tip. The tunneling matrix elements arising from these tip states are evaluated with the derivative rule. For example, a pz surface state on the tip generates a tunneling matrix element proportional to [∂ψ/∂z] at the nucleus of the apex atom, and a d3z2−r 2 tip state generates a tunneling matrix element proportional to [3∂2ψ/∂z2−κ2ψ], (ψ is the sample wave function, κ is the decay constant of surface wave function, κ=(2meφ)1/2/ℏ ). To obtain analytic results of theoretical STM images, we further developed a simple independent-orbital model to describe the wave functions of the sample surface. With this model, we present qualitative and quantitative explanations of the observed atomic resolution on metals and semiconductors, the spontaneous switching of instrument resolution during imaging, and various tip-sharpening procedures.
Diagnostics of direct-current-magnetron discharges by the emission-selected computer-tomography technique
Authors: Miyake S. , Shimura N. , Makabe T. , Itoh A.
Published: July 2002

The dc magnetron discharges in Ar are commonly used for thin film deposition. It is essential to investigate the two-dimensional emission profile Φλ(z,r) in order to analyze the geometrical structure of the ring-shaped magnetron discharge. In the present work, an emission-selected computer tomography (ESCT) system was constructed to investigate the three-dimensional structure of the magnetron discharge in Ar. Photons from the discharge are detected by two identical scanning mirror systems, movable both parallel and perpendicular to the electrode, positioned outside the two small orthogonal windows on the chamber. Experiments have been performed in a planar magnetron with an aluminum target having a 100 mm diam and a 20 mm spacing in Ar at a pressure of 0.3 Pa, a current of 40 mA, and a voltage of 290 V. Three lines, Ari(419.8 nm), Arii(434.8 nm), and Ali(396.2 nm) are selected for the ESCT analysis and discussion of the structure of the discharge. Results from the tomographic imaging showed that the peak in photon production were found at the radius where the magnetic field became parallel to the electrode. The eroded ringlike pattern on the target surface coincided with the radial distribution of photon production in front of the target, and the radial position of the maximum depth of the erosion exactly corresponded to that of the peak emission. The overall features of dc magnetron sputtering were also shown in detail by the present ESCT technology.
Ionicity dependence of surface bond lengths on the (110) cleavage faces of isoelectronic zincblende structure compound semiconductors: GaP, ZnS, and CuCl
Authors: Lessor D. L. , Duke C. B. , Kahn A. , Ford W. K.
Published: July 2002

Previously measured low-energy electron diffraction (LEED) intensity data for the (110) surfaces of the isoelectronic series GaP, ZnS, and CuCl, are reanalyzed using dynamical LEED calculations, a new selection of independent structural variables, and an automated optimization procedure. The bond lengths of the atoms in the top layer and perpendicular displacements of the cation-anion sublattices in the top two layers are selected as the independent structural variables. All other bond lengths are taken to be equal to their bulk values. An automated gradient method search procedure is utilized to obtain these structural variables by optimizing the fit of the measured intensities by the calculated ones as determined by the x-ray R factor. This procedure permits a precise comparison of the bond lengths of species in the top layer from one material to another. Our major finding is that the bond lengths, d, between the anion and cation in the top layer depend on the spectroscopic ionicity, f, via [(d−dB)/dB]=0.025−0.14f, in which dB is the bulk value of the bond length. The bonds between surface and subsurface species do not exhibit a systematic dependence on f.
Vacancy migration, adatom motion, and atomic bistability on the GaAs(110) surface studied by scanning tunneling microscopy
Authors: Gwo S. , Smith A. R. , Shih C. K.
Published: July 2002

In this article scanning tunneling microscopy studies of atomic motion of defects on the GaAs(110) surface at room temperature are reported. The slow dynamic behavior of vacancies and As adatoms can be resolved within a time scale of about one minute. The vacancies and As adatoms are observed to move preferably along the [1̄10] direction, corresponding to a large anisotropy in the two-dimensional diffusion coefficients. We attribute such a large anisotropy to the energetics of atom migration in the presence of the GaAs(110) surface zigzag chain structure. Observations of As adatoms also indicate that they form linear chains along the zigzag chain. Finally, the fast dynamic behavior of atomic flip-flop action at a dual divacancy is discussed. The characteristic flip-flop time is estimated from the scanning speed and sampling frequency to be on the order of milliseconds.
Dynamics of the desorption of D2 and H2 from Cu(111)
Authors: Rettner C. T. , Michelsen H. A. , Auerbach D. J.
Published: July 2002

We have determined rotational and vibrational distributions for D2 and H2 desorbed from Cu(111) and have determined velocity distributions of the desorbed molecules for a wide range of vibrational and rotational states. The quantum state populations display strong deviations from a Boltzmann distribution, and the velocity distributions are highly supersonic with properties that depend strongly on quantum state. Molecules desorbed at 925 K were detected in a state-specific manner using laser ionization. Velocity distributions were obtained by measuring the flight times of D+2 ions in a field-free region. Vibrational populations of D2(v=1), D2(v=2), and H2(v=1) were found to be a factor of 20, 79, and 18 above that expected for equilibration at 925 K, respectively, relative to v=0 populations. Boltzmann plots of the rotational state distributions display distinct curvature, particularly for molecules in the ground vibrational state. H2 and D2 molecules in the ground vibrational state have mean kinetic energies of ∼0.6 eV for molecules in low rotational states. This energy was found to increase slightly with increasing J at low J, pass through a maximum at a rotational energy of ∼0.15 eV, and then fall as rotational energy is further increased. H2(v=1), D2(v=1), and D2(v=2) molecules have mean energies of ∼0.30, 0.45, and 0.25 eV, respectively. In each of these cases, the mean energy also varies significantly with rotational state. The velocity distributions of molecules in v=0 have a speed ratio of ∼0.35, which decreases with both increasing rotational and vibrational energy. Results are discussed in terms of a simple dynamical picture of desorption via an anisotropic potential energy barrier.
Design of disk molecular pumps for hybrid molecular pumps*
Authors: Shi Liqun , Wang Xiaozhen , Zhu Yue , Pang Shijin
Published: July 2002

In recent years, the various types of hybrid molecular pumps combining a molecular drag pump (MDP) with a turbomolecular pump (TMP) have been developed in order to raise their pumping throughput and operating pressure. Some of them have been put on the market. In this article the pumping performances of the disk MDP, whose pumping channels are cut on a rotating disk, were calculated using a new model based on the Monte Carlo method. The relationship between the pump performance to the structural parameters was studied. The above pumping performance is also compared with that of the pumps having channels on stationary disks. When a pump is constructed in a series of two stages, the pumping speed and the compression ratio of the present pump for nitrogen gas increases by 35% and 64%, respectively, as compared with the previous type. It is believed that the pump having channels on the rotating disk has both kinds of pumping actions. Hence, it possesses the advantages of both MDP and TMP.
Synchrotron radiation (5–50 eV) induced degradation of fluorinated polymers
Authors: Simons J. K. , Frigo S. P. , Taylor J. W. , Rosenberg R. A.
Published: July 2002

The photochemistry of solid, free-standing films of poly(tetrafluoroethylene) (PTFE) and poly(hexafluoropropylene–tetrafluoroethylene) (FEP) was investigated by monitoring the neutral photodesorption products produced upon irradiation with synchrotron radiation (5–50 eV). CF3, C2F5, C3F5, C3F7, and C4F7 were the predominant species observed in the mass spectra of the irradiated polymers. Oligomers at least as large as 293 amu (C7F12) were also detected. Distinct differences are observed in the photodesorption yields and time responses of the yields based on the energy of the initial excitation. An edge jump at 22 eV in the photon-stimulated desorption spectra of both PTFE and FEP films is assigned to the ionization of the C(2s) states of the polymers. The energy dependence of the photodesorption yields is accounted for by considering two separate reaction pathways.
Magnesium cluster-beam deposition on glass and Si(111)
Authors: Hagena O. F. , Knop G. , Fromknecht R. , Linker G.
Published: July 2002

Magnesium cluster beams have been obtained from supersonic nozzle expansions. The low boiling point of Mg (1363 K) allows conditions for clustering to be reached with and without argon as a carrier gas. This article reports on cluster size and intensity as a function of expansion conditions, and discusses the characteristics of magnesium films obtained by cluster-beam deposition. The source parameters were temperature T0≤1600 K, total pressure p0≤5000 hPa, magnesium partial pressure pMg≤2600 hPa, supersonic conical nozzle: diameter d=0.25 mm, cone angle 2α=10°, and cone length l=27 mm. For the Ar/Mg mixture the cluster-beam intensity corresponded to deposition rates of up to 83 nm/s at a 0.3 m distance from the nozzle. This exceeds the ‘‘ideal’’ intensity from a sonic nozzle operated with the same magnesium mass flow by about a factor of 3.5. For the neat Mg vapor the deposition rates extended up to 190 nm/s, but at a higher Mg mass flow compared to the Ar/Mg mixture. The cluster beams were deposited on room-temperature glass and Si(111) substrates. Films about 1000 nm thick were examined by x-ray diffraction. Compared to films produced by atomic beam deposition with a typical polycrystalline structure the cluster films are distinguished by a preferential orientation of (002) planes parallel to the substrate. This feature was observed for both types of cluster beams, with and without argon as a carrier gas, and for both glass and Si substrates. The highly specular film surfaces turned usually into a golden-yellow color after exposure to air, with the notable exception of the film formed with the highest intensity (510 nm/s). The surface structure studied with an atomic force microscope showed an increase in grain size with increasing deposition rate.
Interaction of hydrogen with chemical vapor deposition diamond surfaces: A thermal desorption study
Authors: Chua Lye Hing , Jackman Richard B. , Foord John S. , Chalker Paul R. , Johnston Colin , Romani Simon
Published: July 2002

The interaction of hydrogen with a chemical vapor deposition (CVD) deposited polycrystalline film on Si(100), cleaned by heating to 1500 K, has been examined. No adsorption is observed unless prior activation with a hot filament is carried out. However atomic H thereby created is found to adsorb with a high sticking probability, subsequently desorbing in the temperature range 1100–1225 K. Etching of the diamond surface, producing gaseous methane and ethane is also seen. The reactivity patterns are rather similar to those observed on C(100) and C(111) surfaces, suggesting these are suitable model substrates for the study of reaction mechanisms associated with CVD diamond growth. However, a major difference is also seen in that a mixture of hydrogen, silane, and disilane, with other minority species, is seen to evolve from the surface of the CVD diamond film at 1275 K when exposed to the reacting gas and is not detected from single-crystal diamond surfaces. The results indicate that this arises from the selective etching of Si present at grain boundaries in the deposited films.
Investigation of synchrotron radiation-induced photodesorption in cryosorbing quasiclosed geometry
Authors: Anashin V. V. , Malyshev O. B. , Osipov V. N. , Maslennikov I. L. , Turner W. C.
Published: July 2002

The results of 4.2-K photodesorption experiments in two quasiclosed geometries are reported, (1) a simple tube and (2) a tube with a coaxial perforated liner. These experiments were designed to measure separately the desorption coefficients of tightly bound and physisorbed molecules. The results are important for the beam tube vacuum of the next generation of superconducting proton colliders that have been contemplated—the 20-TeV Superconducting Super Collider in the United States and the 7.3-TeV Large Hadron Collider at CERN.
Spatially resolved optical emission for characterization of a planar radio frequency inductively coupled discharge
Authors: Beale D. F. , Wendt A. E. , Mahoney L. J.
Published: July 2002

Planar radio frequency (rf) inductively coupled plasmas (ICP) are currently being investigated as sources for materials processing. We have characterized an Ar plasma ICP discharge using spatially resolved optical emission spectroscopy. Abel inversion of line-integrated intensities of Ar* emission yields two-dimensional (r,z) profiles of emission in a cylindrical volume, driven at one end by a spiral antenna with rf power at 13.6 MHz. Measurements were made over a pressure range of 10–100 mTorr, and power levels of 100 and 200 W. In all cases emission intensity was found to peak in a ring-shaped region at one end of the cylinder adjacent to the location of the antenna, where the rf field is strongest. A local maximum is also observed in most cases near the center of the volume where the electron density peaks [L. J. Mahoney, A. E. Wendt, E. Barrios, and C. J. Richards, J. Appl. Phys. 76, 2041 (1994)].
Fabrication of relaxed Si1−xGex layers on Si substrates by rapid thermal chemical vapor deposition
Authors: Dutartre D. , Warren P. , Provenier F. , Chollet F. , Pério A.
Published: July 2002

Chemical vapor deposition has been applied to the fabrication of a relaxed SiGe buffer on (100) Si substrates. Our structures consist of a 0%–x% graded layer and a uniform Si1−xGex capping layer, with x between 32% and 52%. First, the variation of the threading dislocation density with grading rate has been determined. Our results clearly show an abrupt transition between two domains: steep gradings correspond to a high dislocation density, and smooth gradings to low dislocation densities. Second, the surface morphology has been studied at different steps of the buffer fabrication. In addition to the classical 〈110〉 crosshatch morphology, we report for the first time 〈100〉-oriented undulations. This new morphology, only found on steep gradings, is attributed to a Stranski–Krastanov growth mode caused by large surface strains. Finally, we propose a new picture of the role of the grading: a steep grading, with a large surface strain, induces very undulated growths; in this case, small dislocation loops and a high density of threading dislocation are generated. Our calculations allow us to derive a simple criterion: grading rates lower than 137% Ge/μm will guarantee a relaxation of the low mismatched type. Following this, very low threading dislocation densities (103 cm−2) were indeed achieved.
GaAs formation by reduction of As2O3 and Ga2O3 at SiO2/GaAs oxides/GaAs interfaces
Authors: Jimeńez I. , Moreno M. , Martín-Gago J. A. , Asensio M. C. , Sacedón J. L.
Published: July 2002

SiO2/GaAs interfaces have been obtained by deposition of Si on oxidized GaAs(110) surfaces, with the reduction of As and Ga oxides to form Si oxide. High resolution photoemission studies have been performed to elucidate the chemical state of the atoms liberated from the oxide. The reduction of As oxides, faster than that of Ga oxides, produces an As enrichment at the interface, that creates deviations from GaAs stoichiometry. This As excess appears in the high resolution spectra as a component with an energy shift between 0.45 and 0.65 eV from the GaAs substrate signal. Subsequent reduction of Ga oxides produces the decrease of the component related to As excess. In any stage of the process no segregation of Ga appears from the spectra. Finally the As-excess component vanishes, showing that it reacts with the liberated Ga atoms to form again GaAs. A quantitative analysis of the photoemission intensities has allowed the establishment of a structural model of the atomic arrangement during the chemical changes.
Molecular beam epitaxy of GaN(0001) utilizing NH3 and/or NH+x ions: Growth kinetics and defect structure
Authors: Lee N.-E. , Powell R. C. , Kim Y.-W. , Greene J. E.
Published: July 2002

Gas-source molecular beam epitaxy (GS-MBE), utilizing Ga and NH3, and reactive-ion MBE (RIMBE), incorporating both thermal NH3 and low-energy NH+x ions, were used to grow single crystal GaN(0001) layers on Al2O3(0001) at temperatures Ts between 700 and 850 °C with deposition rates of 0.2–0.5 μm h−1. The RIMBE experiments were carried out with incident NH+x/Ga flux ratios JNH+x/JGa=1.9–3.2 and NH+x acceleration energies ENH+x=45–90 eV. Plan-view and cross-sectional transmission electron microscopy analyses showed that the primary defects in the GS-MBE films were threading dislocations having either pure edge or mixed edge/screw characteristics with Burgers vectors b̄=1/3〈21̄1̄0〉, basal-plane stacking faults with displacement vectors R̄=1/6〈022̄3〉, and prismatic stacking faults with R̄=1/2〈1̄101〉. In the case of RIMBE films, no stacking faults or residual ion-induced defects were observed with ENH+x=45 eV and Ts≥800 °C. However, increasing ENH+x to ≥60 eV at Ts=800 °C gave rise to the formation of residual ion-induced point-defect clusters observable by transmission electron microscopy (TEM). Increasing Ts to 850 °C with ENH+x≥60 eV resulted in the ion-induced defects aggregating to form interstitial basal and prismatic dislocation loops, whose number densities depended upon the ion flux, with Burgers vectors 1/2〈0001〉 and 1/3〈21̄1̄0〉, respectively. Unlike previously published results for RIMBE growth with N+2, GaN growth kinetics with NH+x were not found to be a strong function of either ion-to-thermal flux ratios or ion acceleration energies.
Lateral and vertical dopant profiling in semiconductors by atomic force microscopy using conducting tips
Authors: De Wolf P. , Snauwaert J. , Hellemans L. , Clarysse T. , Vandervorst W. , D’Olieslaeger M. , Quaeyhaegens D.
Published: July 2002

The determination of the spatial distribution of charge carriers in semiconducting structures with an atomic force microscope (AFM) is presented. This new technique is based on the measurement of the spreading resistance of a conducting AFM tip on a sample as the tip is scanned over the surface. The high spatial resolution of this method allows for its application on the cross section of a device providing one-dimensional (vertical) and two-dimensional (lateral) profile information. First, a detailed study is presented regarding the properties of the tip (tip material and preparation, lifetime) and the evolution of I–V characteristics with pressure on uniformly doped silicon. From this work a model for the electric properties of the microcontact is deduced. It includes an ohmic contribution to the overall resistance that is related to the plastically deformed area, and contributions from a barrier as well as from tunneling. Through the determination of a calibration curve, whereby the resistance was measured on homogeneously doped substrates with varying resistivities, the quantification properties of the method are determined. Finally, this method has been applied for junction delineation on the cross section of a device. The results show a 50 nm resolution and a sufficient sensitivity to encompass a dynamic range of concentrations between 1014 and 1019 cm−3. These measurements were found to be in good agreement with those performed by the classical spreading resistance profiling technique and by secondary ion mass spectroscopy.
Zincblende–CdSe on GaSb(110): Characterization of epitaxial growth and electronic structure
Authors: Neuhold G. , Horn K. , Magnusson K. O. , Evans D. A.
Published: July 2002

Substrate-stabilized pseudomorphic growth offers the chance to study the electronic structure of a particular semiconductor in different crystal structures, and to investigate the influence of structural differences on bulk and surface states. We have grown layers of CdSe in the zincblende modification on cleaved GaSb(110) surfaces by molecular beam epitaxy. The growth mode and structure of the overlayer were studied by means of low energy electron diffraction and photoemission using synchrotron radiation. The attenuation of substrate core level intensities with CdSe deposition indicates layerwise growth. Interface reaction leads to the liberation of Sb, which floats on the growth front, and the formation of a Ga–Se compound, as signaled by changes in substrate and overlayer core level line shape. The valence band offset for this lattice-matched interface system is 1.09 eV, such that the heterojunction is of the staggered type, in agreement with predictions based on the dielectric midgap energy model.
Role of temporal delay in dual-laser ablated plumes
Authors: Witanachchi S. , Mukherjee P.
Published: July 2002

A novel laser ablation process using two lasers of different wavelengths has demonstrated the capacity to grow essentially particulate-free thin films without compromising the ionic excitation and kinetic energy of the plume species. We demonstrate in this article, the critical role of the interpulse delay when a pulsed CO2 laser spatially overlaps an excimer (KrF) laser on the ablation target during this dual-laser ablation process. At the optimum delay, the excitation and the ionization of the plume were significantly enhanced. The time-of-flight ion probe studies indicate more than a fourfold increase in the ion kinetic energies for this optimized process in comparison to the single-laser ablated plumes. The choice of a suitable delay also facilitates an optimization of elimination of particulate ejection that is typical of single-laser ablation. The morphology of the dual-laser ablated Er-doped Y2O3 films deposited on Si substrates as a function of interpulse delay is also discussed.
Irradiation-induced decomposition of Al2O3 during Auger electron spectroscopy analysis
Authors: Park Jae-Won , Pedraza Anthony J. , Allen William R.
Published: July 2002

The effect of electron fluence on the decomposition of sapphire (Al2O3) was studied in situ by Auger electron spectroscopy (AES). The decomposition was primarily detected by monitoring the evolution of the low kinetic energy Auger transitions of aluminum in Al2O3 (54 eV) and in metallic aluminum (68 eV). The decomposition of sputter-cleaned sapphire started at a fluence of ∼4.9×1019 electrons/cm2 (7.8 C/cm2). This fluence was independent of the electron fluxes used in this work, except the lowest, which indicates that heating due to electron bombardment does not significantly affect the decomposition behavior. Electron-induced decomposition takes place in a minimum of the first five atomic layers of the substrate, as revealed by the evolution during irradiation of the high energy Al peaks associated with Al2O3 (1388 eV) and metallic aluminum (1396 eV). Comparison of the evolution of low and high kinetic energy Auger transitions demonstrates that the decomposition kinetics are much faster for the first monolayer than for the subjacent atomic layers. The surface condition strongly influences the decomposition kinetics. Thus, a carbon layer adsorbed at the alumina surface significantly increases the threshold dose for decomposition. The carbon layer most probably acts as a diffusion barrier for the oxygen produced during decomposition. An equation for the decomposition rate of the first monolayer of alumina is established. The integral of this equation gives a good fitting to the experimental data. It is found that the Auger signal of aluminum from sapphire does not disappear even if the entire region has been decomposed. This effect is due to backscattered electrons that promote Auger electron excitations outside the irradiated region.
Formation of polytetrafluoroethylene thin films by using CO2 laser evaporation and XeCl laser ablation
Authors: Inayoshi Muneto , Hori Masaru , Goto Toshio , Hiramatsu Mineo , Nawata Masahito , Hattori Shuzo
Published: July 2002

Laser evaporation and laser ablation methods were applied to the preparation of polytetrafluoroethylene (PTFE) thin films. In the case of the laser evaporation method, PTFE targets were evaporated by a continuous wave (cw) CO2 laser (10.6 μm), and fluorocarbon thin films were formed at a deposition rate of as high as 2 μm/min for a laser power of 10 W. The chemical composition and structure of the deposited film corresponded to those of a PTFE target, which was confirmed by x-ray photoelectron spectroscopy and Fourier transform infrared absorption spectroscopy analyses. In the laser ablation method, PTFE targets were ablated by a XeCl excimer laser (308 nm). It is found that the deposited films contained a small amount of fluorine atoms on the surface. From these experiments, the successful formation of PTFE thin films was demonstrated for the first time using cw CO2 laser evaporation method.
Stochastic charge fluctuations in dusty plasmas
Authors: Matsoukas Themis , Russell Marc , Smith Matthew
Published: July 2002

We consider the stochastic properties of charge fluctuations of noninteracting particles surrounded by a stationary, undisturbed plasma of electrons and singly charged positive ions. For unscreened Coulomb interaction in the orbit–motion limit the mean particle charge and the variance of the fluctuations are proportional to the particle size. The result applies to both Maxwellian and non-Maxwellian electrons as well as to charging by ion winds. When the polarizability of the particle is included in the electrostatic interaction, both the ion and electron currents to the particle increase and the mean particle charge becomes more negative. Except for particles in the low nanometer range, the effect of the polarizability can be neglected at the electron temperatures encountered in typical plasmas.
TiN barrier layer formation by the two-step rapid thermal conversion process
Authors: Kim Youn Tae , Jun Chi-Hoon , Lee Jin Ho , Baek Jong Tae , Yoo Hyung Joun
Published: July 2002

We formed TiN barrier layers on single-crystalline silicon substrates by thermal conversion of Ti films at various temperatures in an ammonia ambient using a rapid thermal process with a sequential two-step temperature cycle. The first-step temperatures were held in the low-temperature range of 400–450 °C for 60–300 s to minimize Ti/Si interaction while keeping reasonable interaction of Ti/NH3 and nitrogen diffusion through the Ti layer to maximize the thickness of the TiN layer. Then, the second-step was carried out at relatively high temperatures, 700–1000 °C, for 5–90 s to reduce Ti/Si interaction during the silicidation process. By the first steps of the low temperature process, sheet resistances increased with annealing time up to 60 s due to the deep penetration and high concentration of nitrogen in the Ti film, followed by saturation at 60–120 s; they steadily decreased beyond 120 s. Sheet resistance increases were dominated by the nitrogen-rich Ti layer formed during the first steps of long-time nitrogen diffusion. With the second steps of the high temperature process, nitrogen enriched Ti layers were converted to Ti-rich TiN layers, resulting in abrupt decreases in the sheet resistance due to silicidation, densification of TiN, and conversion of the remaining Ti to TiN layers. By means of a two-step rapid thermal conversion process of the 1000 Å Ti layer under long-time nitridation cycle conditions with optimal thermal conversion conditions (first step: 400 °C/90 s; second step: 700 °C/60 s), we obtained TiN/TiSi2 bilayers of 700/1500 Å thicknesses with the TiN thickness ratio relative to the totally converted layer in excess of 30%. These results indicate that the thickness ratio of the TiN layer prepared by a two-step process relative to the totally converted layer is double that obtained by a one-step process, while it also provides reduced total thickness of the thermally converted layer.