bluuebunny/crossref_metadata_2025 huggingface dataset:
Scanning tunneling microscopy of x-ray optics
Authors: Green M. , Richter M. , Kortright J. , Barbee T. , Carr R. , Lindau I.
Published: July 2002

Results of scanning tunneling microscope studies of both single-layer and multilayer x-ray reflecting optics are presented. The surface roughness of thin Pt films, Pt with a thin overlayer of Rh, and W/C (C on top) multilayers, all sputter deposited on optical glass flats, was investigated. The use of glass substrates with overall height variation of one-tenth wave rather than one-fifth wave produces no improvement in small-scale surface smoothness (lateral dimensions <1000 Å). No evidence of crystallization was seen on any of the samples. Rhodium coated mirrors demonstrated smaller, more uniform features than the irregular hills of Pt. The amorphous C topped multilayers showed atomic scale features and low overall corrugation. rms surface roughness measurements are in excellent agreement with those derived by x-ray techniques. Surface feature size distribution for the Rh/Pt samples has been considered in relation to theories of optical scattering from rough surfaces.
Scanning tunneling spectroscopy study on graphite and 2H–NbSe2
Authors: Bando H. , Morita N. , Tokumoto H. , Mizutani W. , Watanabe K. , Homma A. , Wakiyama S. , Shigeno M. , Endo K. , Kajimura K.
Published: July 2002

Simultaneous measurements of scanning tunneling spectroscopy (STS) and scanning tunneling microscopy (STM) are performed on HOPG and 2H–NbSe2 in air. In order to see the distribution of local density of states related to different atomic sites and species STS images of normalized differential conductance are mapped. Correlation function between the STS images and corrugation in corresponding STM image is obtained as a function of the tip bias voltage.
Bounce-coated ablation layers on fusion targets
Authors: Gram R. Q. , Immesoete C. K. , Kim H. , Forsley L.
Published: July 2002

The current effort to achieve high compression of deuterium–tritium fuel by the Laboratory for Laser Energetics utilizes targets which are coated with uniform ablation layers of parylene. These layers are applied to the targets while they bounce on the vertically vibrating surface of a resonator. The low-power plasma required to prevent targets from sticking to each other and to the vibrating surface increases the deposition rate by a factor of 2 to 4. Careful control of plasma conditions is required to maintain bouncing while avoiding overly rapid polymerization, defect growth, embrittlement, and other adverse effects in the parylene layers. Ablation layer quality is studied as a function of parylene monomer pressure, argon pressure, and plasma power, and the optimum operating region is discussed. Automatic control of these parameters is described. A tensile strength of 36 MPa is achieved in the bounce-coated parylene layers, adding considerable strength to polymer shell targets. The permeability of these layers to Ar and He is measured.
Steps on Si(001)
Authors: Griffith J. E. , Kochanski G. P. , Kubby J. A. , Wierenga P. E.
Published: July 2002

Steps on vicinal Si(001) are either one or two atomic layers high. When the tilt of the surface about [11̄0] is 2°, alternating type-A and type-B single steps dominate. When the tilt angle is increased to 4°, the steps combine to form straight, evenly spaced type-B double steps. Type-A double steps do not occur. At kinks the double steps split into single steps. Tunneling images of the double steps reveal the rebonded atoms described by the Chadi model.
Interactions of high-energy ion beams with polyimide films
Authors: Matienzo L. J. , Emmi F. , Van Hart D. C. , Gall T. P.
Published: July 2002

Rutherford backscattering spectrometry (RBS), routinely used for the analysis of inorganic materials, is finding applications in the characterization of organic polymeric films, which may be sensitive to damage. To investigate ion beam effects on polyimide, pyromellitic dianhydride–oxydianiline films were exposed to He2+ ions in the energy range of 1.9 to 3.6 MeV, fluences of 0.25 to 3.0×1015 ions/cm2, and sample temperatures of 298 and 130 K. RBS, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, optical interferometry, and enhanced image analysis techniques were used to determine film changes during irradiation. RBS spectra collected in the range of specified fluences are essentially superimposable, except for differences in their signal-to-noise ratios. The damage is optically observed as a darkening of the film. At 130 K, there was no motion of the gold marker in the film, indicating no loss of mass. During irradiation, molecular rearrangements are detected and include cleavage of imide rings.
Nonlinear optical coefficients of Hg-based superlattices
Authors: Youngdale E. R. , Hoffman C. A. , Meyer J. R. , Bartoli F. J. , Chu X. , Faurie J. P. , Han J. W. , Cook J. W. , Schetzina J. F.
Published: July 2002

The first measurement of the third-order nonlinear optical susceptibility χ(3) in Hg-based narrow-gap semiconductor superlattices is reported. Four-wave mixing experiments were performed at CO2 laser wavelengths on a series of HgTe/CdTe and Hg1−xZnxTe/CdTe superlattices whose energy band gaps are approximately zero at T=0 K. The nonlinearity is believed to due to modulation of the free-carrier temperature and density by the optical beams. Theoretical estimates of the third-order nonlinear susceptibility due to this mechanism are in good agreement with the experimental results.
Neutral gas desorption and photoelectric emission from aluminum alloy vacuum chambers exposed to synchrotron radiation
Authors: Gröbner O. , Mathewson A. G. , Strubin P. , Alge E. , Souchet R.
Published: July 2002

In an aluminum alloy vacuum chamber exposed to synchrotron radiation, the photoelectron currents produced were measured with the photons incident at low angles on the side wall and compared with normal incidence. The calculated photocurrents for normal incidence, using published values of the photoyield for oxidized Al, agree to within 15% with the measured values. Differences in the photocurrent dependence on photon spectrum at normal and glancing incidence were attributed to low-energy photons being totally reflected and hence producing no photoelectrons. It was established that, at glancing angles of incidence down to 11 mrad, a substantial—more than 20%—fraction of the synchrotron radiation is scattered around the vacuum chamber from the initial point of impact. During exposure to synchrotron radiation, the gases desorbed were H2, CO, CO2, and CH4. The similar shapes of the dependence of the gas desorption and the photoelectron currents on the photon spectrum suggested that it is mainly the photoelectrons that are contributing to the desorption. It was estimated that electrons of 60 eV would produce the same gas desorption as synchrotron radiation with a critical energy of 3 keV.
Etch pit study of dislocation formation in Hg1−xCdxTe during array hybridization and its effect on device performance
Authors: Norton Peter W. , Erwin Anthony P.
Published: July 2002

The relationship between hybridization force, dislocation (etch pit) density, and device performance has been investigated for devices fabricated from Hg1−xCdxTe liquid phase epitaxy (LPE) material. Coldwelded indium column interconnects are the predominant method used to mechanically and electrically attach mosaic arrays to silicon multiplexers. The mismatch in thermal expansion between the Hg1−xCdxTe photodiode arrays and silicon multiplexers stresses the interconnects when the arrays are cooled to their operating temperature (typically 40–120 K) which can lead to failures. Increased hybridization force improves the mechanical stability of the interconnects but can introduce dislocations in the Hg1−xCdxTe. Dislocations have been reported to degrade the performance of some device structures. The dislocation density in the LPE film after growth was ≊8×105/cm2. We have found no significant change in dislocation density at hybridization forces up to ≊3.9×107 N/m2. At forces on the order of 5.9×107 N/m2 (≊10% of the Vickers hardness), the dislocation density in the films increased to 90×105/cm2 in the areas where the indium columns were. The diffusion limited performance of the test structures at 120 K was not significantly degraded by the increased dislocation density. At 80 K, however, where the devices were modeled to be generation–recombination limited, there was an increase in dark current with increased dislocation density. The structures used in the study were fabricated from LPE Hg1−xCdxTe grown on a CdTe substrate. Implantation of boron into the p-type material through a ZnS passivation layer was used to form junctions. A zero-bias junction resistance of 1000 to 7000 Ω cm2 was typical for the test structures which had a cutoff wavelength of 4.7 μm at 120 K.
Fabrication of hollow silica aerogel spheres by a droplet generation method and sol–gel processing
Authors: Kim N. K. , Kim K. , Payne D. A. , Upadhye R. S.
Published: July 2002

A system was designed to produce hollow silica aerogel spheres by combining a droplet generation method with sol–gel processing. Simultaneous passage of an alkoxide solution and a fill gas, through a dual-nozzle system, formed a continuous stream of hollow droplets in the fluid state. The spherical droplets were levitated in a basic atmosphere containing ammonia which converted the sol into a rigid gel state. Supercritical drying was used for the elimination of residual solvents without shrinkage and cracking.
The formation and control of direct current magnetron discharges for the high-rate reactive processing of thin films
Authors: Howson R. P. , Spencer A. G. , Oka K. , Lewin R. W.
Published: July 2002

The process of reactive dc magnetron sputtering is shown to provide a high-rate, large-area deposition system when stability is achieved using optical emission spectroscopy to control the reactive gas flow. The film properties are improved when the isolated substrate is allowed to acquire a bias by immersion in a plasma directed onto it from the source. These techniques give good results at high rates of production without the complications of rf or ion beam systems. Results for oxide systems of indium and titanium are presented. Indium oxide films gave a resistivity of 4×10−6 Ω m and titanium oxide a refractive value of 2.55 at 633 nm when deposited onto room-temperature substrates.
Thermal conductivity of thin films: Measurements and understanding
Authors: Cahill David G. , Fischer Henry E. , Klitsner Tom , Swartz E. T. , Pohl R. O.
Published: July 2002

Several techniques are reviewed with which thermal conductivity and phonon scattering can be measured in films of thicknesses ranging from angstroms to millimeters. Recent experimental results are compared critically with previous measurements. It is shown that phonons are very sensitive probes of the structural perfection of the films.
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o simulation of the interaction between ionic crystal surfaces and the atomic force microscope tip
Authors: Shluger A. , Pisani C. , Roetti C. , Orlando R.
Published: July 2002

The ab initio Hartree–Fock program crystal is used to simulate the interaction of three different kinds of tips of the atomic force microscope (AFM) with the MgO(100) surface, by employing the periodic slab model. The neutral silicon atom and the SiO molecule (Si or O oriented toward the surface) are used as tip models. The relevant AFM topographical resolution and its quality are analyzed. It is shown that, depending on the chemical and electronic structure of the tip, the interpretation of the images obtained could be even opposite. The best topographic image is expected for the weak constant force (≊2×10−8 N) repulsive regime between the tip and the ionic surface. Their mutual perturbation is studied in detail.
Local order, epitaxy, and electronic structure of the Bi/III–V semiconductor interfaces
Authors: Joyce J. J. , Nelson M. M. , Tang Ming , Meng Y. , Anderson J. , Lapeyre G. J.
Published: July 2002

The growth morphology and electronic structure of the Bi/InP(110) interface is investigated and compared to the Bi/GaAs(110) system. High-resolution synchrotron radiation core level and valence band photoelectron spectroscopy is employed in the determination of the local order, epitaxy, and electronic structure development for these systems. Valence band spectra show the development of a density of states between the InP valence band maximum and the Fermi level in the coverage range 0–1 monolayer. These Bi-derived states show a 450 meV gap between the onset of emission and the Fermi level indicating the first monolayer is semiconducting. At all coverages beyond 1 monolayer, the Bi-derived states show emission at the Fermi level. Core-level intensity attenuation profiles indicate a Stranski–Krastanov growth mode. Computer fitting of the Bi 5d core level shows two bonding sites for Bi on the surface, with a third Bi 5d component at all coverages above 1 monolayer. A zigzag chain model used for the Bi/GaAs(110) system is proposed as an appropriate model for the Bi/InP(110) interface.
Time-resolved reaction studies on the atomic scale: NO and CO adsorption on stepped Rh surfaces
Authors: Kruse N.
Published: July 2002

The interaction of NO and CO with stepped surfaces of Rh field emitter tips has been studied by means of pulsed field desorption mass spectrometry (PFDMS) at temperature 300<T<600 K and steady gas pressures p<1.3×10−4 Pa. The repetition frequency of the pulses, i.e., the field free reaction time tR between the pulses has been varied in the range 100 μs≤tR≤1 s in order to obtain information about the kinetics of surface reactions. The pulsed field desorption (PFD) mass spectra obtained during NO adsorption always contain NO+, RhxNn+, and RhOn+ ions (x,n=1,2). This finding suggests that part of the adsorbed NO undergoes dissociation during tR. For short reaction times, i.e., low surface coverages, NO adsorption is found to be predominantly molecular. Using a probe hole ∼150 atomic sites close to the (100) pole of the tip surface have been analyzed in detail. The equilibration between adsorption and thermal desorption of NO has been monitored by tR variation. In this manner the mean lifetimes τ before first order thermal desorption can be determined. From the temperature dependence of the τ values the thermal desorption kinetics have been evaluated. An activation energy Ed=102 kJ/mol and a preexponential τ0=3×10−14 s are found. The PFD mass spectra obtained during CO adsorption on stepped Rh planes contain CO+ and Rh(CO)n+y ions (n=1,2; y=1–3). The tR dependence of these species has been measured and found to be consistent with a consecutive surface reaction between chemisorbed CO and Rh step atoms leading to adsorbed Rh subcarbonyls. Possibly Rh–Rh bonds are broken during Rh(CO)2 formation so that this species is liberated and able to diffuse into the terrace regions of the surface. Only a small amount of adsorbed CO undergoes dissociation at long reaction times.
Photoemission study of the negative electron affinity surfaces of O/Cs/Si(001)2×1 and O/K/Si(001)2×1
Authors: Abukawa T. , Enta Y. , Kashiwakura T. , Suzuki S. , Kono S. , Sakamoto T.
Published: July 2002

We have found that a negative electron affinity (NEA) surface can be formed by exposing a K-saturated Si(001)2×1 surface to oxygen gas just as in the formation of a NEA surface of O/Cs/Si(001)2×1. By x-ray photoelectron diffraction we have found that the arrangement of O and K atoms in the NEA O/K/Si(001)2×1 surface is essentially the same as that in the NEA O/Cs/Si(001)2×1 surface in which a double layer of alkali metal is preserved. Angle resolved ultraviolet photoelectron spectroscopy (ARUPS) has been applied to the two NEA surfaces. The resulting ARUPS spectra can give useful information on bonding at the NEA surfaces if they are combined with theoretical calculations.
Desorption studies of hydrogen and carbon monoxide from nickel surfaces using thermal desorption spectroscopy
Authors: Cabrera A. L.
Published: July 2002

The adsorption of hydrogen and carbon monoxide at room temperature on nickel samples was studied with thermal desorption spectroscopy. Desorption from nickel foil was studied using a mass spectrometric method in an ultrahigh vacuum system, while desorption from nickel powder was studied with a microreactor system using a thermal conductivity detector. Two hydrogen desorption peaks are observed at low heating rates (less than 3 K/s). These peaks correspond to the β1 and β2 states observed in single crystals studies. An activation energy of 7–10 kcal/mol is obtained for the low temperature peak (β1 state), assuming first order kinetics, while an energy of 28–30 kcal/mol is obtained for the higher temperature peak (β2 state) with second order kinetics. Desorption spectra using the two different experimental methods are very similar, and the kinetic data derived from the spectra agree well.

When the heating rate is high (larger than 12 K/s) only one hydrogen desorption peak is observed, indicating that experimental resolution decreases for very fast heating rates. Kinetic analysis of this spectra yields an activation energy similar to the β2 state. The two H2 desorption features (β1 and β2) obtained for Ni powder and foil agree well with previously reported studies on Ni single crystals, indicating that the presence of the two desorption states is insensitive to surface structure. The activation energy and order of the desorption for the β2 state obtained in this work is in general agreement with values reported for single crystals studies. The activation energy and order for the β1 state is in agreement with some studies performed with Ni single crystals, largely due to the assumption in prior work that desorption proceeds as a second order process for this state. Three carbon monoxide desorption peaks are observed for low heating rates (less than 3 K/s). Presumably, the two low temperature peaks which are associated with adsorbed molecular CO correspond to on-top and bridge site adsorption. The activation energy for the weakest bonding (on-top site/dominant peak) was obtained and corresponded to 6–10 kcal/mol. The activation energy for dissociated CO (high temperature peak) corresponded to 30–36 kcal/mol. Our ability to resolve the two types of adsorbed molecular CO is lost when very fast heating rates are used (12 K/s).
Kinetics and mechanics of Si2H6 surface decomposition on Si
Authors: Kulkarni S. K. , Gates S. M. , Greenlief C. M. , Sawin H. H.
Published: July 2002

A detailed study of the kinetics of Si film growth from disilane has been undertaken. In order to understand the growth kinetics at the atomic level, the mechanisms of disilane decomposition on Si(111) surfaces have been studied using modulated molecular beam scattering (MMBS) combined with surface science techniques: static secondary ion mass spectrometry (SIMS), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED).1 Modulated molecular beam spectrometry was performed over the temperature range of 50–850 °C and with beam fluxes from 1015 to 1016 cm−2 s−1.2 Rapid evolution of monosilane above 500 °C and unusually slow desorption of H2 (time constant≊1 s at 850 °C) is observed. We have modeled the MMBS data using reaction mechanisms derived experimentally. According to this model, adsorbed disilane decomposes producing SiH4 and chemisorbed SiH. Disilane also reacts with the chemisorbed SiH and emits SiH4 which is the main path for hydrogen removal from the surface. The SiH migrates by random ‘‘exploration’’ to ledge sites on the surface forming *SiH. The less significant path of surface hydrogen removal is the recombination of two *SiH desorbing H2.
Electron and ion transport in magnetron plasmas
Authors: Sheridan T. E. , Goeckner M. J. , Goree J.
Published: July 2002

We demonstrate experimentally that there is a strong link between electron transport and ion transport in sputtering magnetron plasmas. Electron densities and discharge currents for He, Ne, Ar, Kr, and Xe discharges are measured and used to infer the charged particle confinement time. We find that the confinement time increases with ion mass. Further we find fair agreement between the experimental results and a model which assumes that ion motion is collisionless and unmagnetized, and that the spatial dependence of the electric potential and ion source are similar in the different discharges.
Improved data acquisition and smoothing methods in high-resolution electron energy loss spectroscopy
Authors: Chen P. J. , Colaianni M. L. , Yates J. T.
Published: July 2002

A simple method for converting an analog high-resolution electron energy loss (HREEL) spectrometer to a digital instrument is described in this paper. Methods of Fourier transform smoothing of the digital data are described and a comparision to RC-smoothed analog data is made using synthetic analog data. The advantages of digital data acquisition in HREELS compared to analog methods are presented.
Scanning tunneling microscopy of silver containing salt of bis(ethylenedithio)tetrathiafulvalene
Authors: Bai C. , Dai C. , Zhu C. , Chen Z. , Huang G. , Wu X. , Zhu D. , Baldeschwieler John D.
Published: July 2002

The surface of silver containing salt of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) was studied with the computer-controlled scanning tunneling microscope developed in our laboratory. The crystal surface of the charge-transfer complex is well ordered and a regular array of corrugations is clearly visible. The prominent feature of the experimental scanning tunneling microscopy images is in agreement with the bulk crystal structure obtained by x-ray diffraction method.
Interfacial reactions between thin films of Ti–Ta and single crystalline Si
Authors: Ben-Tzur M. , Eizenberg M.
Published: July 2002

Interfacial reactions in a contact system comprising of thin films of two refractory metals, Ti and Ta, codeposited onto a single crystalline substrate of Si have been studied and compared with the behavior of each of these elements separately on Si. The work analyzed systematically the behavior of this alloy system, both in its ‘‘as deposited’’ state as well as in its interaction with Si, as a function of the alloy composition and the applied heat treatments. The analytical tools employed were Auger electron spectroscopy, transmission electron microscopy and x-ray diffraction. It has been found that only binary compounds, silicides of each of the metals, and their mutual solid solutions, and not any new ternary compounds, are formed. The reaction products at the early stages of the reaction are TaSi2 and the metastable (C49 structure) TiSi2. Extra diffraction lines were obtained, they possibly belong to Ta5Si3. The final stage of the reaction consists of a uniform intermixing of the disilicides, TiSi2 and TaSi2. Alloying causes a shift in the temperature of silicide formation to intermediate values between those for pure Ti and Ta (500 and 700 °C, respectively), and this value increases with Ta content in the alloy film.
Reactive sputter deposition of zirconium nitride/aluminum nitride multilayers: Chemical competition effects and structural characterizations
Authors: Meng W. J. , Sell J. A. , Waldo R. A.
Published: July 2002

We have examined in detail the process of reactive sputtering of elemental Zr and Al targets simultaneously in an argon/nitrogen mixture. The total pressure and optical emission from both target plasma discharges have been monitored as a function of increasing nitrogen fraction. We compare this situation with the usual case of single target operation and demonstrate the existence of a chemical competition effect between the individual target plasma discharges, which manifests itself in preferential reaction of nitrogen with Zr and sequential nitriding of Zr and Al targets with increasing nitrogen fraction. We have further examined the structure and composition of single layer zirconium nitride and aluminum nitride films, deposited at different nitrogen fractions, by symmetrical and glancing angle x-ray diffraction, and by electron probe microanalysis, and our results corroborate the existence of chemical competition effects. We demonstrate the possibility of synthesizing zirconium nitride/aluminum nitride multilayered thin films with each individual component layer close to being stoichiometric, and we show the presence of sharp composition modulation and ordered interfaces in such multilayers.
Characterization of sharp phosphorus dopant features in silicon by secondary ion mass spectrometry
Authors: Vriezema C. J. , Zalm P. C. , Maes J. W. F. M. , Roksnoer P. J.
Published: July 2002

The in-depth distribution of a sharp P dopant feature in Si grown by vapor phase epitaxy has been determined by secondary ion mass spectrometry under various bombardment conditions. It is shown that with a low energy primary 0+2 beam supplemented by oxygen gas flooding of the sample surface both sub-ppm detection limits and good depth resolution can be obtained simultaneously. This combination is not possible by using either primary Cs+ bombardment, giving only good detection limits, or low energy oxygen bombardment, resulting in a good depth resolution but unfavorable detection limits.
Quantification of microstructural evolution in sputtered a-Si thin films by real time spectroscopic ellipsometry
Authors: An Ilsin , Nguyen H. V. , Nguyen N. V. , Collins R. W.
Published: July 2002

We have undertaken a real time spectroscopic ellipsometry (SE) investigation of amorphous silicon (a-Si) sputtered under conditions leading to the highest bulk bond-packing density. The spectral range for our observations is 1.5–4.5 eV, and the time resolution and repetition rate for complete spectra are ∼3 and ∼15 s, respectively. From the real time measurements, we have deduced the complete optical functions and band gap of the growing material. The real time data also provide quantitative information on the microstructural evolution during initial nucleation and growth. For a-Si on thermally oxidized c-Si, we find that the first monolayer covers ∼15% of the surface, and the first bulk density monolayer is formed after a cluster thickness of 21 Å. The advantage of SE over the single photon energy approach, is that void fractions and layer thicknesses can be deduced from each pseudodielectric function independently. This leads to unambiguous microstructural information at an unprecedented level of detail, in a compact, easily assimilated form for future comparisons with film growth simulations.
Free energy model for the analysis of bonding in a-SixNyHz alloys
Authors: Yin Z. , Smith F. W.
Published: July 2002

A free energy model (FEM) for the analysis of the bonding in amorphous covalent alloys is developed and applied to ternary a-SixNyHz alloys. The quasichemical approach to the thermodynamics of regular solutions is used to obtain the Gibbs free energy of mixing GM=HM−TSM for the alloys. The enthalpy of mixing HM is determined by the nearest-neighbor bond energies while the entropy of mixing SM is given by the number of possible bonding configurations in Si-centered tetrahedra. The concentrations of Si–N, Si–Si, Si–H, and N–H bonds have been determined as functions of temperature and composition by minimizing the free energy of the a-SixNyHz alloy. It is shown that chemical ordering (CO) in these alloys corresponds to a preference for Si–N and Si–H bonds at the expense of Si–Si and N–H bonds. The predictions of the FEM are shown to be in good agreement with experimentally determined bond concentrations in a-SixNyHz alloy films and it is demonstrated that, in general, neither the CO nor random bonding limits are valid.
Indium–tin oxide films radio frequency sputtered from specially formulated high density indium–tin oxide targets
Authors: Kulkarni Sudhakar , Bayard Michel
Published: July 2002

High density ITO (indium–tin oxide) targets doped with Al2O3 and SiO2 manufactured in the Tektronix Ceramics Division have been used to rf sputter ITO films of various thicknesses on borosilicate glass substrates. Sputtering in an oxygen–argon gas mixture and annealing in forming gas, resulted in ITO films exhibiting 90% transmission at 550 nm and a sheet resistance of 15 Ω/sq for a thickness of 1100 Å. Sputtering in an oxygen–argon gas mixture and annealing in air increased sheet resistance without a large effect on the transmission. Films sputtered in argon gas alone were transparent in the visible and the sheet resistance was found to be 100–180 Ω/sq for the same thickness, without annealing.
Optical anisotropy spectra of GaAs(001) surfaces
Authors: Chang Yia-Chung , Ren Shang-Fen , Aspnes D. E.
Published: July 2002

We present theoretical studies of the optical anisotropy spectra of GaAs(001) surfaces with different reconstructions at growth temperatures. The Ga-rich (4×2) missing-dimer surface, three different phases (α, β, and γ) of the As-rich (2×4) surface, and an As-rich c(4×4) surface are considered. Total energy calculations within a nearest-neighbor tight-binding model are performed to determine the surface geometry for each reconstruction considered. The difference between reflectance spectra with [110] and [1̄10] polarization for these surfaces is analyzed and compared to available data.
1/f noise in Bi2Sr2Ca2Cu3–O and its impact on bolometer performance
Authors: Zindler R. W. , Parsons R. R.
Published: July 2002

The magnitude 1/f noise in the high Tc superconducting transition region of Bi2Sr2Ca2Cu3–O materials was investigated. The results are similar to those previously reported for Y1Ba2Cu3–O. The results indicate that thermal fluctuations are the source of excess 1/f noise detected in the lower temperature region of the transition. In the higher temperature region, however, thermal fluctuation 1/f noise decreases, and is overpowered by an undetermined source of 1/f noise. The implications on bolometer performance are discussed.
Quantitative correlation of infrared absorption with nuclear magnetic resonance measurements of hydrogen content in diamond films
Authors: McNamara K. M. , Gleason K. K. , Robinson C. J.
Published: July 2002

Hydrogen concentrations in polycrystalline diamond films were found to correlate well with the integrated intensity of the CH-stretch region in the infrared (IR) spectrum for films with a total hydrogen content <0.10 at. %. Comparison of IR to nuclear magnetic resonance (NMR) measurements yielded an effective absorption coefficient for the CH-stretch region of 4.3±0.8×103 ℓ mol−1 cm−2, similar to literature values for condensed-phase hydrocarbons. In several films, the dominant stretch modes occur at ∼2850 and 2920 cm−1, typical of CH2 groups. The presence of these modes is accompanied by an increase in the Gaussian linewidth in the NMR spectrum, indicating a decrease in the interproton spacings within the film. In films with higher total hydrogen contents, a simple linear relationship between hydrogen content and intensity in the CH-stretch region is no longer applicable.
Comparison of Cu(111) in aqueous electrolytes and in ultrahigh vacuum: An optical second harmonic generation study
Authors: Wong E. K. L. , Friedrich K. A. , Robinson J. M. , Bradley R. A. , Richmond G. L.
Published: July 2002

The rotational anisotropy of the second harmonic intensity has been used to provide information about the structural and electronic properties of a Cu(111) electrode surface in 0.01 M NaClO4. The experiments have involved measurement of the changes in the second harmonic (SH) intensity and relative phase of the SH response as the surface is biased at various potentials. Complementary measurements of the SH response from Cu(111) examined in ultrahigh vacuum (UHV) have also been performed and indicate that when this surface is in solution, it has very different optical (electronic) properties from the native surface at all applied potentials studied. Dosing of the sample in UHV with oxygen provides evidence for the existence of oxygen containing species on the surface in the electrochemical environment despite rigorous exclusion of molecular oxygen during both the surface preparation and the optical measurements. These studies demonstrate the utility of making comparative measurements of the surface in different environments, for which the SH technique is ideally suited.
Computer-aided procedure for optimization of layer thickness uniformity in thermal evaporation physical vapor deposition chambers for lens coating
Authors: Bosch Salvador
Published: July 2002

A computer-aided method to improve the thickness uniformity attainable when coating multiple substrates inside a thermal evaporation physical vapor deposition unit is presented. The study is developed for the classical spherical (dome-shaped) calotte and also for a plane sector reversible holder setup. This second arrangement is very useful for coating both sides of the substrate, such as antireflection multilayers on lenses. The design of static correcting shutters for both kinds of configurations is also discussed. Some results of using the method are presented as an illustration.
Deposition of deuterium and metals on divertor tiles in the DIII–D tokamak
Authors: Walsh D. S. , Doyle B. L. , Jackson G. L.
Published: July 2002

Hydrogen recycling and impurity influx are important issues in obtaining high confinement discharges in the DIII–D tokamak. To reduce metallic impurities in DIII–D, 40% of the wall area, including the highest heat flux zones, have been covered with graphite tiles. However, erosion, redeposition, and hydrogen retention in the tiles, as well as metal transport from the remaining Inconel walls, can lead to enhanced recycling and impurity influx. Hydrogen and metal retention in divertor floor tiles have been measured using external ion beam analysis techniques following four campaigns where tiles were exposed to several thousand tokamak discharges. The areal density of deuterium retained following exposure to tokamak plasmas was measured with external nuclear reaction analysis. External proton-induced x-ray emission analysis was used to measure the areal densities of metallic impurities deposited upon the divertor tiles either by sputtering of metallic components during discharges or as contamination during tile fabrication. Measurements for both deuterium and metallic impurities were taken on both the tile surfaces which face the operating plasma and the surfaces on the sides of the tiles which form the small gaps separating each of the tiles in the divertor. The highest areal densities of both deuterium (from 2 to 8 × 1018 atoms/cm2) and metals (from 0.2 to 1 × 1018 atoms/cm2) were found on the plasma-facing surface near the inner strike point region of each set of divertor tiles. Significant deposits, extending as far as 1 cm from the plasma-facing surface and containing up to 40% of the total divertor deposition, were also observed on the gap-forming surfaces of the tiles.
Analysis of GaAs(100) surfaces prepared with various wet and in situ sample treatments
Authors: Wang X.-S. , Self K. W. , Maboudian R. , Huang C. , Bressler-Hill V. , Weinberg W. H.
Published: July 2002

Using low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy, we have analyzed GaAs(100) surfaces prepared with different ex situ and in situ sample treatments. After acid etching, the surfaces are extremely susceptible to carbon contamination, which is found to induce faceting. Sulfur passivation using (NH4)2S can reduce the amount of carbon contamination while preserving the surface order. On GaAs(100) surfaces, the sulfur overlayer forms a (2×1) superstructure after annealing to remove the residual oxide. P2S5 is found to be ineffective in chemically passivating the GaAs surface. An oxide layer on the surface renders it relatively inert to chlorine etching. On the oxide-free surface, however, chlorine can easily remove both sulfur and GaAs layers, leaving the surface relatively As rich. We propose a new preparation procedure for clean, stoichiometric, and ordered GaAs(100) surfaces that consists of acid etch, sulfur passivation and in situ Cl2 exposure.
Double Langmuir probe diagnostic of a resonant cavity microwave discharge
Authors: McColl W. , Brooks C. , Brake M. L.
Published: July 2002

An Asmussen resonant cavity operating at 2.45 GHz provides the basis for the application of probe theory to a microwave discharge. A double Langmuir probe is inserted into a discharge produced by a microwave resonant cavity. Typical operating pressures range from 0.5 to 50 Torr in helium, with continuous wave microwave powers ranging from 60 to 120 W at 2.45 GHz. Typical probe data indicates electron densities in the range of 1011–1012 cm−3 with electron temperatures of 5 to 20 eV. The probe data is compared with the results of a model based upon the electromagnetics of the discharge.
Challenges and progress in the scale up of CuInSe2 thin film photovoltaic technology
Authors: Ermer J. , Gay R. , Pier D. , Tarrant D.
Published: July 2002

The development of high-efficiency, low-cost thin film CuInSe2 (CIS) photovoltaic technology is progressing in laboratory research and engineering development efforts. In research laboratories, higher deposition temperatures, improved junction and window layers, and I–III–VI2 multinary absorbers have resulted in cell efficiencies approaching 15%. In engineering development efforts, low-cost processing techniques and durable commercial module packages have been demonstrated and large area (0.4 m2) prototype modules have been fabricated with power outputs that exceed 40 W and aperture area efficiencies that exceed 10%. The challenge facing engineering development now is to improve the processing yields on large area modules to a level which justifies a transition to pilot manufacturing. Recent work has focused on characterizing and improving the uniformity of CIS junctions and the reliability of monolithic circuit interconnects. Diagnostic techniques used to characterize the adhesion and uniformity of the individual cell layers and to spatially resolve the quality of devices and interconnects are discussed. Optical beam induced current measurements and scanning electron microscopy have been used to identify four unique defect types which stem from either defects in glass substrates or particulate contamination during fabrication.
Electronic and optical properties of room temperature sputter deposited indium tin oxide
Authors: Lee S. B. , Pincenti J. C. , Cocco A. , Naylor D. L.
Published: July 2002

The purpose of this article is to demonstrate a simple technique for magnetron sputter deposition of thin ITO films at room temperature without postdeposition annealing. Using a gas mixture of 0.7% O2 balanced with Ar and 50 W rf power, thin films with resistivity as low as 8.2×10−4 Ω cm have been fabricated. The sputter deposited films have optical transmittance of over 80% in the visible range. The absorption edge located in the mid-infrared (4000–400 cm−1) has been observed to shift toward increasing wave number (decreasing wavelength) with increasing resistivity of the thin ITO films. At 0.7% O2 concentration it has been found that the resistivity is insensitive to the total pressure of the sputtering chamber over the range of 1.4–8.0 mTorr.
Metastable states’ population densities in Gd atomic beam at high-rate evaporation by electron beam heating
Authors: Nishimura Akihiko , Arisawa Takashi , Ohba Hironori , Shibata Takemasa
Published: July 2002

Metastable states’ population densities in gadolinium atomic beams, which were produced by an axial electron beam gun, were measured by laser absorption spectroscopy at the atomic densities on the order of 1012 atoms/cm3. The following results were obtained. (1) The atomic excitation temperature derived from the metastable states distributions is considerably lower than the evaporation surface temperature. With increasing evaporation rates, the atomic excitation temperature decreased to about 500 K. (2) The densities of both ground and first metastable states increased linearly with the deposition rates as measured by a quartz crystal sensor on the beam axis, which indicates that the atomic beam is almost free from the cluster formation caused by rapid expansion into a high vacuum.
Effect of boron doping on the surfaces of diamond thin films
Authors: Shinar R. , Leksono M. , Shanks H. R.
Published: July 2002

Auger and electron energy loss spectroscopies were used to study chemical and structural properties of as-grown surfaces of boron-doped diamond thin films prepared by microwave plasma-assisted chemical vapor deposition. Results are compared to surface properties of as-deposited undoped diamond thin films. As the boron level in the film increased energy losses associated with π electron bonds (sp2 sites) were enhanced at the surface. Similarly, the surface oxygen level of the films increased with increasing boron content. On the other hand the characteristic diamond Raman peak at 1332 cm−1 was narrower than in the undoped films and the broad band at about 1500 cm−1 associated with nondiamond carbon decreased. Surface morphology micrographs obtained from scanning electron microscopy (SEM) measurements indicated that the introduction of boron enhances the appearance of the (111) planes. Surface characterization together with the Raman and SEM results suggest that boron doping enhances surface reactivity resulting in increased adsorption of oxygen and carbonaceous species following the growth process. This behavior has implications on surface treatments necessary for device applications.
Minority carrier lifetimes in molecular beam epitaxy grown AlxGa1−xAs/GaAs double heterostructures doped with aluminum
Authors: Sheldon P. , Keyes B. M. , Ahrenkiel R. K. , Asher S. E.
Published: July 2002

Both Al-treated pBN crucibles and Al-doped Ga melts have been used to reduce Ga-cell related oval defect densities in molecular beam epitaxy (MBE) grown epilayers. This practice, although effective in reducing the oval defect density, results in small amounts of Al contamination in the grown films. In this work, we have grown diagnostic Al0.3Ga0.7As/GaAs double heterostructure (DH) devices doped with Al in order to determine the effect of Al contamination on the minority-carrier lifetime. The active region of the DH structures was doped with Al at 5×1018 atoms/cm3 to emulate the levels observed when Al-treated pBN crucibles are used. Al impurity levels and distribution profiles were characterized by secondary ion mass spectrometry, and minority-carrier lifetimes of DH devices were characterized using time-resolved photoluminescence (PL) measurements. Device structures grown with various GaAs active layer thicknesses were used to calculate the interface recombination velocity (S) and the bulk lifetime (τB). We found that Al doping of the GaAs active layer adversely affects the measured PL decay time. GaAs active layers of DH devices doped with Al exhibited poor bulk lifetimes (τB≤120 ns) limited by Shockley–Read–Hall recombination. In contrast, DH devices grown with Al-free active layers resulted in significantly improved PL decay times with bulk lifetimes as high as τB=700 ns.
Deposition of aluminum oxide films with high refractive index
Authors: Shih K. K. , Dove D. B.
Published: July 2002

Aluminum oxide (Al2O3) films are used in a variety of applications including optics and microelectronics. We have deposited aluminum oxide films on 5 in.2 polished fused quartz substrates by the rf sputtering method. It was found that properties of these films such as refractive index and density varied significantly while the deposition conditions were held constant. Films with a high refractive index (n≳1.7) were denser and were not attacked by the chemical solution, while the films with a low refractive index (n<1.6) were less dense and readily attacked. Films with a high refractive index can be reproducibly obtained when a surface layer of the fused quartz substrates was removed by sputter etch before the film deposition. The amount of sputter etch required depended upon the deposition rate to be employed subsequently. The results are consistent with a model in which a hydrated surface layer on the fused quartz substrates provided a source of water molecules that may migrate into the growing Al2O3 films.
End-point detection by sputtered neutral mass spectrometry in ion milling of prepatterned semiconductor and high-T
c superconductor films
Authors: Jaekel Christian , Barth Roland , Roskos Hartmut G. , Kurz Heinrich
Published: July 2002

We explore the use of sputtered neutral mass spectrometry (SNMS) for semiconductor and superconductor device processing. SNMS allows us to precisely determine the end point at interfaces during sputter etching of prepatterned semiconductor and superconductor thin films. By postionizing the sputtered neutrals the detection sensitivity is similar for all elements. The dynamic range of up to three orders of magnitude makes precise end-point detection possible even if the mask allows sputter etching of only a small fraction of the surface of the specimen.
Influence of the composition of the altered layer on the ripple formation
Authors: Elst K. , Vandervorst W.
Published: July 2002

The ripple formation on Si under oxygen bombardment is studied in detail. The influences of temperature, oxygen flooding, primary ion, and the flux of the primary beam are explored and the results are compared with the existing models. As none of the models is capable of explaining all features consistently, a new mechanism is proposed that uses the heterogeneity of the altered layer. The model takes the history of the primary ion incorporation into account and it is implemented into a simulation program. Comparisons between experimental results and simulations using the new model are presented and good overall agreement is found.
Dependence of material properties of radio-frequency magnetron-sputtered, Cu-doped, ZnTe thin films on deposition conditions
Authors: Gessert T. A. , Li X. , Coutts T. J. , Mason A. R. , Matson R. J.
Published: July 2002

Because of its ability to be doped highly p type, and because its valence-band discontinuity with CdTe is small, ZnTe is believed to have an important use as an interface layer for electrical contacts to crystalline and thin-film CdTe devices. Although this and other potential uses have been identified, relatively little is known about the fundamental properties of ZnTe in its thin-film form, and even less is known about possible effects specific to sputter deposition. In this study, we investigate the effects of deposition conditions on the compositional, electrical, optical, and structural properties of thin-film ZnTe produced by rf-magnetron sputtering. Results indicate that, as is often observed when sputtering high-vapor-pressure compounds, rf-sputtered ZnTe films are slightly Zn deficient. Although this may be undesirable when producing high-resistivity undoped films, it may be advantageous when producing low-resistivity p-type films because it may encourage incorporation of substitutional Group I dopants, such as Cu. We have determined that the choice of deposition parameters can indeed affect the stoichiometry of the sputtered ZnTe film, and that Cu can be used effectively as a p-type dopant. Additionally, we have been able to produce Cu-doped ZnTe with carrier concentrations in excess of 1020 cm−3.