bluuebunny/crossref_metadata_2025 huggingface dataset:
Unusual magnetic behavior in sputtered FeO and α-Fe2O3 thin films
Authors: Dimitrov D. V. , Hadjipanayis G. C. , Papaefthymiou V. , Simopoulos A.
Published: July 2002

A ferromagnetic-type behavior has been found in sputtered FeO and α-Fe2O3 thin films at low temperatures. This observation is in contrast to the bulk antiferromagnetic properties of FeO and α-Fe2O3. At room temperature the coercivity of the films is negligible, but below 150 K it increases rapidly and at 10 K it is larger than 3 kOe for both kinds of iron oxide films. The saturation magnetization of the films is large (with a maximum of 270 emu/cc in FeO and 140 emu/cc in α-Fe2O3 samples) and thickness dependent. Mössbauer spectra of α-Fe2O3 films could be analyzed by two magnetic components, one very close to bulk α-Fe2O3 and the other one identified as surface α-Fe2O3. Mössbauer spectra of FeO films showed that the samples were off-stoichiometric, with about 10% Fe deficiency. Clusters of tetrahedrally coordinated Fe3+ ions were observed in an environment similar to that in Fe3O4. The hysteresis behavior in FexO films can be explained by the defect clusters of Fe3+ ions, and by uncompensated spins in the surface component of α-Fe2O3 films. Shifted hysteresis loops were observed in field-cooled samples, indicating a strong exchange coupling between the regions with the net magnetic moment and the antiferromagnetic magnetic component.
Degradation of ZnS field-emission display phosphors during electron-beam bombardment
Authors: Sebastian J. S. , Swart H. C. , Trottier T. A. , Jones S. L. , Holloway P. H.
Published: July 2002

Phosphor thin films of ZnS:Mn on Si(100) have been subject to electron bombardment (0.6–4 keV) over a range of pressures from 1×10−6 to 5×10−8 Torr. Various gases including hydrogen, oxygen, and water vapor were introduced into the ambient during bombardment, to assess their effects on the phosphor surface. Auger electron spectroscopy data indicate that electron bombardment in the presence of O2 and H2O caused depletion of sulfur and accumulation of oxygen on the surface. Hydrogen also caused depletion of sulfur. Removal of sulfur was shown to be consistent with electron-beam dissociation of molecular species to atomic hydrogen and/or oxygen, followed by a surface reaction to form high vapor pressure sulfur compounds (e.g., SOx and H2S). In the case of oxygen and water vapor, ZnS was converted to ZnO or ZnSO4. These changes in surface chemistry reduced the intensity of cathodoluminescence from ZnS-based phosphors. The mechanisms leading to loss of CL intensity are discussed.
Investigations in the sheath region of a radio frequency biased inductively coupled discharge
Authors: Benck E. C. , Schwabedissen A. , Gates A. , Roberts J. R.
Published: July 2002

Temporally and spatially resolved optical emission, as well as Langmuir and electric probe measurements, were used to investigate the effects of radio frequency (rf) biasing near an electrode in an inductively coupled plasma cell. The plasma source is a modification of the Gaseous Electronics Conference rf Reference Cell. Emission from the atomic argon 750.387 nm transition was observed. With the lower electrode grounded, the optical emission did not exhibit any rf modulations. However, for a constant rf bias power of 9 W at frequencies from 1.695 to 33.9 MHz applied to the lower electrode, various waveforms were observed in the temporal evolution of the optical emission near the electrode as well as in the bulk plasma. Also, for pressures between 0.67 and 13.3 Pa of argon and a rf power of 9 W at a frequency of 2.712 MHz, the oscillations in the optical emission near the biased electrode showed the presheath/sheath region rapidly shrinking with increasing pressure. The dc sheath voltage drop, determined from Langmuir and electric probe measurements, did not exhibit a dependence on the applied rf frequency, but varied nearly linearly with the ratio of the bias power to the power dissipated in the plasma.
Effect of deposition temperature on the structural properties of n+ μc-Si:H films
Authors: Park B. , Murthy R. V. R. , Benaissa K. , Aflatooni K. , Nathan A. , Hornsey R. I. , Chamberlain S. G.
Published: July 2002

The role of the deposition temperature (Td) on the structural properties of doped hydrogenated microcrystalline silicon (n+ μc-Si:H) has been studied through grazing incidence x-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy measurements. The deposition temperature was varied from 200 to 400 °C to observe the crystalline and amorphous phase formations in these films. The results show that the films deposited at low temperatures are more crystalline in nature than their high temperature counterparts. The elimination of the hydrogen atoms from the growing surface of the film seems to be correlated with the transition from the amorphous to the crystalline phase. This may be attributed to the following. At low deposition temperatures, the impinging H atoms from H2 dilution interact with a silicon-hydrogen bond on the grown surface, forming a volatile H2 molecule, to leave a Si–Si network. The result of this interaction may be responsible for the increased crystallinity. At high deposition temperatures, a transition between the hydrogen out-diffusion from Si–H bonds and the hydrogen incorporation by H2 dilution may lead to formation of Si–H bonds, again on the surface at a particular chemical equilibrium. This causes the film to remain amorphous.
Electrical and optical properties of amorphous fluorocarbon films prepared by plasma polymerization of perfluoro-1,3-dimethylcyclohexane
Authors: Weber A. , Pöckelmann R. , Klages C.-P.
Published: July 2002

The electrical and optical properties of amorphous fluorocarbon films (a-C:F) prepared by plasma polymerization have been investigated. Perfluoro-1,3-dimethylcyclohexane was introduced into a parallel plate plasma reactor operating at 13.56 MHz to deposit a-C:F films. Transparent a-C:F films with an F/C ratio of 2 were prepared at lower power densities of 10–300 mW/cm2 at deposition rates of 4–10 nm/min. The films have a low surface energy of 15 mN/m and are thermally stable up to 210 °C. The coatings exhibit low refractive index of 1.38, low dielectric constant of 2.3, and high transmittance in the visible range. The electret properties were evaluated by measuring the decay of a positive surface potential (SPD) on 25 μm thick a-C:F films. High rf power densities lead to an increased formation of conjugated C–C double bonds affecting the transmittance and SPD characteristic. However, the relative low thermal stability, insufficient planarization, and charge storage capabilities limit the applications of the a-C:F films for microelectronic and electrical applications.
Dually driven radio frequency plasma simulation with a three moment model
Authors: Kim Heon Chang , Manousiouthakis Vasilios I.
Published: July 2002

This article presents simulation results of a dually excited capacitive rf plasma reactor. A self-consistent three moment model is employed which is shown to accurately capture the ion flux and energy at the substrate. Self-dc biases at the powered electrodes are also self-consistently determined by relating surface charges through Gauss’ law. The simulation results of this rf triode system indicate that plasma density is predominantly determined by the primary electrode. Self-bias and ion bombardment energy at the secondary electrode both exhibit linear (logarithmic) dependency on the secondary rf power (frequency). This is qualitatively in good agreement with experimental results.
Synthesis and structure of Al clusters supported on TiO2(110): A scanning tunneling microscopy study
Authors: Lai X. , Xu C. , Goodman D. W.
Published: July 2002

Al clusters supported on TiO2(110) have been investigated using scanning tunneling microscopy. Al interacts strongly with the TiO2(110) surface at room temperature and becomes oxidized by abstracting oxygen from the interface region of the substrate. Highly oxidized Al clusters were found to disorder the TiO2 substrate in the low coverage regime (0.05 ML) illustrating that Al exhibits a strong chemical interaction with the oxide substrate. Distinct changes in the TiO2(110) substrate from a “row” structure to a “net” structure due to Al interacting with oxygen in the topmost layer of the substrate are also observed.
Desorption rate of surface hydrogen in SiGe gas-source molecular beam epitaxy using Si2H6 and GeH4
Authors: Hirose F. , Sakamoto H.
Published: July 2002

The desorption rate of surface hydrogen in the SiGe gas-source molecular beam epitaxy (GSMBE) using Si2H6 and GeH4 has been investigated by thermal desorption spectroscopy. We have experimentally obtained variations of kinetic energy and frequency factor in the surface-hydrogen desorption as a function of Ge content in the grown film. The desorption of surface hydrogen is enhanced by Ge addition to the growing surface, where both the decrease of activation energy and the increase of frequency factor in the hydrogen desorption take place. We present a rate equation of surface-hydrogen desorption as a function of Ge concentration for calculating growth rate in the SiGe GSMBE.
Angular-resolved valence-band spectroscopy of different reconstructed 3C–SiC (001) surfaces
Authors: Lübbe M. , Lindner K. , Sloboshanin S. , Tautz S. , Schäfer J. , Zahn D. R. T.
Published: July 2002

We present angular-resolved valence-band spectroscopy data for the (3×2) and c(2×2) reconstructed (001) surface of cubic silicon carbide. The two reconstructions were prepared by annealing the sample in a flux of silicon atoms. In this way single domain reconstructed surfaces were achieved which was confirmed by low energy electron diffraction. The orientation of the surface unit cell with respect to the substrate orientation excludes the alternate dimer-row model for the (3×2) reconstruction of the surface. Angular-resolved valence-band spectra were recorded along the [11̄0] direction of the sample. Both surfaces reveal the photoemission characteristics known from angular integrating experiments. By comparison of the reconstructions, surface derived photoemission features were identified. In addition to the known V1 and V2 states of the (3×2) reconstruction, we found a surface feature V3 in a limited range of the surface Brillouin zone which is to our knowledge reported for the first time. The results are compared to ab initio calculations of the surface band structure. The data for the c(2×2) reconstructed surface are in accordance to calculations for the bridging dimer model. By comparing the data of the (3×2) structure to calculations of the (2×1) surface band structure, we found indications for an assignment of the V1 and V2 states to dangling bonds and silicon dimers, respectively.
Design and experimental approach of optical reflection filters with graded refractive index profiles
Authors: Wang X. , Masumoto H. , Someno Y. , Hirai T.
Published: July 2002

A novel optical multilayer filter with graded refractive index profiles using a quarterwave stack was designed. This filter exhibits a high reflectance band, a sharp cutoff and a wide transmittance region as the result of sidelobe suppression. A 31 layer TiO2/SiO2 high reflectance filter with a graded refractive index profile was fabricated by helicon plasma sputtering on BK7 and Si (100) substrates. The deposition system was operated under ambient gas pressure of 1.8×10−1 Pa at room temperature. The measured transmittance of the spectrum was in good accord with calculated results. Transmission electron microscopic observations confirmed the expected microstructure of the filter.
Growth and structure of epitaxial CeO2 by oxygen-plasma-assisted molecular beam epitaxy
Authors: Kim Y. J. , Gao Y. , Herman G. S. , Thevuthasan S. , Jiang W. , McCready D. E. , Chambers S. A.
Published: July 2002

The epitaxial growth of CeO2 films on SrTiO3(001) has been investigated over a wide range of growth parameters using oxygen-plasma-assisted molecular beam epitaxy. The lattice mismatch for CeO2 on SrTiO3(001) is 2.0% (compressive) if the film nucleates with a 45° rotation about [001] relative to the substrate (i.e., CeO2(001)‖SrTiO3(001) and CeO2[110]‖SrTiO3[100]). Pure-phase, single-crystalline epitaxial films of CeO2(001) with the above epitaxial relationship readily grew on SrTiO3(001) for substrate temperatures ranging from 550 to 700 °C. However, small amounts of (111) and (220) minority orientations also nucleated at the higher substrate temperatures. In addition, the film surface was observed to become progressively smoother with increasing substrate temperature due to more extensive island agglomeration. The highest-quality film surface grown at 700 °C is unreconstructed and oxygen terminated.
Polymerized C-Si films on metal substrates: Cu adhesion/diffusion barriers for ultralarge scale integration?
Authors: Chen Li , Kelber Jeffry A.
Published: July 2002

We report the synthesis of polymeric C-Si films by electron or ultraviolet photon bombardment of condensed vinyl silane derivatives. These films are resistant to thermal diffusion of Cu at temperatures below 800 K, even at thicknesses <100 Å. They also show potential for providing a covalently bonded interphase between Cu and, e.g., fluoropolymer dielectrics. Auger data show that these films are not SiC, and have chemical compositions very close to those of the vinyl silane precursors [e.g., Si:C=1:4 for vinyltrichlorosilane (VTMS)-derived films]. Temperature programmed desorption results show that the polymerization occurs via the vinyl group. Films derived from VTMS are adherent and stable on Ta substrates until 1000 K in ultrahigh vacuum (UHV). Diffusion of Cu deposited overlayers is not observed below 800 K, with dewetting occurring only at temperatures above 400 K in UHV. Perfluorobenzene moieties can also be incorporated into the growing film with good thermal stability, indicating that these films have the potential to adhesively couple Cu surfaces to vapor-deposited fluoropolymer films.
Method for measuring deuterium in erbium deuteride films
Authors: Brangan James R. , Thornberg Steven M. , Keenan M. R.
Published: July 2002

Determining the quantity of deuterium in an erbium deuteride film is essential for assessing the quality of the hydriding process but is a challenging measurement to make. First, the ideal gas law cannot be applied directly due to high temperature (950 °C) and low temperature (25 °C) regions in the same manifold. Additionally, the metal hydride does not release all of the deuterium rapidly upon heating and metal evaporation occurs during extended heating periods. Therefore, the method developed must provide a means to compensate for temperature inhomogeneities and the amount of deuterium retained in the metal film while heating for a minimal duration. This article presents two thermal desorption methods used to evaluate the kinetics and equilibria of the deuterium desorption process at high temperatures (950 °C). Of primary concern is the evaluation of the quantity of deuterium remaining in these films at the high temperature. A multiple volume expansion technique provided insight into the kinetics of the deuterium evolution and metal evaporation from the film. Finally, a repeated pump-down approach yielded data that indicated approximately 10% of the deuterium is retained in the metal film at 950 °C and approximately 1 Torr pressure. When the total moles of deuterium determined by this method were divided by the moles of erbium determined by inductively coupled argon plasma atomic emission spectroscopy, nearly stochiometric values of 2:1 were obtained for several erbium dideuteride films. Although this work presents data for erbium and deuterium, these methods are applicable to other metal hydrides as well.
Nanometer-scale design and fabrication of polymer interfaces using polydiacetylene monolayers
Authors: Mowery Mark D. , Cai Mei , Menzel Henning , Evans Christine E.
Published: July 2002

The incorporation of photopolymerizable diacetylene structures within spontaneously organized monolayers has facilitated the fabrication of robust interfacial assemblies with covalent molecular scaffolding. Through selective ultraviolet-induced polymerization, the resulting polydiacetylene monolayers permit precise structural control in the lateral domain through phototemplating. Results presented in this article demonstrate that these unique monolayer polymers also allow control of the vertical domain within the monolayer through variation of the conjugated polymer backbone position along the length of the alkyl chains. Small variations in the vertical position of the diacetylene groups of less than a nanometer are found to have a profound impact on both polymerization efficiency and the resulting conjugation length of the polymer interface. Furthermore, the relative position of this polymer backbone within the alkyl chains can be utilized to manipulate the overall properties of the assembly such as the long- and short-range order. In this article, spectroscopic and electrochemical techniques are utilized to demonstrate the range of interfacial environments obtainable with these unique polymer monolayers. These advances are currently being utilized to create robust assemblies with three-dimensional control of the physical and chemical properties on a micro- to nanometer scale.
Optical properties of carbon and carbon nitride films prepared by mass-separated energetic negative carbon and carbon nitrogen ions
Authors: Tsubouchi Nobuteru , Enders Burkhard , Chayahara Akiyoshi , Kinomura Atsushi , Heck Claire , Horino Yuji
Published: July 2002

Carbon and carbon nitride (CN) films were prepared under ultrahigh vacuum condition by ion beam deposition using isotopically mass-separated, energetic (50–400 eV) negative C2-12 and C12N-14 ions, respectively. The optical properties as well as structures and chemical composition of the films have been characterized and discussed as a function of the kinetic energy of C2− and CN− ions. The structures of carbon and CN films in this study were hydrogen-free amorphous carbon (a-C) like. The N/C composition ratio of the CN films was N/C∼0.4, although the arrival ratio of N/C was N/C=1. The CN film properties depended weakly on kinetic energy of CN− ions, while on a-C films there were kinetic energy dependence of C2− ions of optical constant observed.
Synthesis and current–voltage characterization of tin dioxide varistors
Authors: Kale A. S. , Seal S. , Date S. K. , Santhosh P. N. , Barve R. N.
Published: July 2002

An overview of the steady state electrical properties of tin dioxide varistors is presented. The varistors are manufactured in two ways: (1) a ceramic route by mixing and compaction of tin dioxide powder with various dopants and (2) a sol-gel method, where dopants are dispersed in their aqueous solutions. For a quantitative agreement with the experimental data, the electronic defect states in the bulk of the tin dioxide grains and at the grain boundary interfaces are studied. Scanning electron microscope and x-ray diffraction are employed to study the morphological and structural aspects of tin dioxide varistors. The effect of various sintering temperatures on their nonohmic or nonlinear characteristics was also investigated. The varistors prepared by the sol-gel method showed high nonlinearity, high density, and high threshold voltage properties than the ones produced by the ceramic route. This is probably due to a finer grain structure and a better densification of dopants along the grain boundaries.
Side-wall damage in a transmission electron microscopy specimen of crystalline Si prepared by focused ion beam etching
Authors: Kato N. I. , Kohno Y. , Saka H.
Published: July 2002

In focused ion beam (FIB) fabrication of cross-sectional transmission electron microscopy (X-TEM) specimens, highly accelerated ion beams sometimes cause serious damage. The damage can be induced in both the specimen surface and the side walls. We used the X-TEM technique to investigate the sidewall damage in crystalline Si. The damaged layer was found to be about 20 nm thick in the case of 30-keV-FIB etching. We evaluated several techniques for reducing the damage, such as gas-assisted etching (GAE) with iodine, low-energy FIB etching, and cleaning by broad argon ion milling or by wet etching. The damage depth was 19 nm for GAE and 10 nm for 10 keV FIB etching, and was reduced to 7 nm by 3 keV argon ion milling with a beam current of 20 μA and a tilt angle between the beam and the specimen of 4°. Wet etching using a mixture of nitric and hydrofluoric acid removed most of the damaged layer. The effect of the damaged layer on TEM observation was also investigated, and it was shown that removal of the damaged layer results in a high-resolution TEM image.
Unoccupied band structure of strained gadolinium
Authors: Komesu Takashi , Waldfried C. , Dowben P. A.
Published: July 2002

From spin-polarized inverse photoemission, the experimental spin-resolved unoccupied band structure of gadolinium on Mo(112) has been constructed. The spin-dependent unoccupied electronic structure near the Fermi level is dominated by shallow dispersion of a spin-minority band. A spin-majority and -minority bulk-band pair also exhibits some dispersion across the Brillouin zone.
Ion and neutral species in C2F6 and CHF3 dielectric etch discharges
Authors: Jayaraman RaviPrakash , McGrath Robert T. , Hebner G. A.
Published: July 2002

Relative concentrations of reactive ions, neutral radicals, resist and substrate etch products have been measured in dielectric etch chemistries using an uncollided beam mass spectrometer/ion extractor from Hiden Analytical. Analysis techniques employed include both electron impact ionization and dissociative ionization of neutral gas, and potential bias extraction of positive ions from the reactor discharge volume. Measurements were made in C2F6 and CHF3 discharges in an inductively coupled plasma (GEC) research reactor operating with power densities, pressures, gas compositions and wafer materials typical of those found in etch processing tools. Wafer substrates investigated included blanket silicon wafers and silicon wafers with varying amounts of photoresist coverage of the surface (20%, 80% and 100%). In C2F6 discharges CF3+ was consistently the dominant fluorocarbon ion present, in agreement with published cross sections for dissociative ionization. Smaller concentrations of CF+, CF2+, and C2F5+, were also observed, though the dissociative ionization production of C2F5+ was a factor of 5 smaller than would be expected from published cross section values. The presence of photoresist, even in small amounts, was found to produce marked changes in the discharge composition. For example, in C2F6 discharges, concentrations of SiFx etch products relative to concentrations of CxFy species were notably diminished and larger concentrations of water vapor were observed when resist was present. In CHF3 discharges, CF3+ and CHF2+ were found to be the main species present, along with smaller concentrations of CF2+, CF+, CHF+, CH+ and F+.
Reconstructed oxide structures stable in air: Silicate monolayers on hexagonal SiC surfaces
Authors: Starke U. , Schardt J. , Bernhardt J. , Heinz K.
Published: July 2002

Ultrathin oxide layers on hexagonal SiC surfaces were studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). SiC(0001) and SiC(0001̄) samples were ex situ prepared using thermal hydrogen etching or a microwave powered hydrogen plasma treatment. A well ordered (3×3)R30° reconstructed surface is observed by LEED immediately upon introduction into vacuum. The samples contain oxygen of approximately one layer equivalent bonded to Si atoms as indicated by AES. From a full dynamical LEED structure analysis carried out for the SiC(0001̄) surface the crystallographic structure is determined: The silicon oxide is arranged as a silicate (Si2O3) layer on top of the SiC substrate, forming rings of (3×3)R30° periodicity with twofold coordinated oxygen atoms in the topmost position. The oxygen incorporation into the surface presumably proceeds via rapid oxidation in air of the well ordered topmost substrate bilayer. The extreme stability of the resulting surface reconstruction is caused by the absence of dangling bonds in the surface terminating silicate layer. On the SiC(0001) surface a similar silicate type layer terminated by its oxygen atoms is found by the LEED analysis. However, the adlayer and substrate are bonded via a linear Si–O–Si bond.
Two point calibration scheme for the linearization of the spinning rotor gauge at transition regime pressures
Authors: Šetina J.
Published: July 2002

Commercial spinning rotor gauges (SRGs) use a special linearization procedure to compensate for the pressure dependence of the rotor deceleration rate in the transition regime from 0.1 to 100 Pa. These procedures have been found to have large errors above 10 Pa [J. Šetina and J. P. Looney, Vacuum 44, 577 (1993)], but can be significantly improved. An extensive set of experimental data of rotor deceleration rate versus gas pressure up to 130 Pa was acquired. A group of six SRGs and four gases (N2, Ar, He and H2) were used in the study. Temperature measurements were also included to account for heating effects. The data show that the differences between rotor/thimble combinations are large enough to cause differences of several percent if one uses the linearization functions currently in use without adjustable parameters. To get the best accuracy, one parameter is left in our linearization procedure to be determined by calibration. We call it the Knudsen length of the rotor/thimble assembly. This is the second calibration constant of a SRG that needs to be determined for accurate pressure measurements above 1 Pa. A method to determine the second calibration constant at 100 Pa is proposed. The first calibration constant is the already well-known rotor accommodation coefficient and is determined at pressures less than 0.1 Pa. The error in reading of the new linearization procedure with the two calibration parameters is less than 2% over the entire range from molecular regime up to 130 Pa.
Low energy cathodoluminescence spectroscopy of etched 6H-SiC surfaces
Authors: Young A. P. , Jones J. , Brillson L. J.
Published: July 2002

We have performed low energy cathodoluminescence spectroscopy (CLS) measurements of localized states near 6H-SiC (0001) and (0001̄) surfaces under ultrahigh vacuum (UHV) conditions and with varying depth sensitivities. CLS reveals several electronic states deep within the SiC band gap at least two of which exhibit strong surface-dependent properties. HF etched C and Si surfaces yielded pronounced emission from near band edge transitions and also from transitions involving a set of deep levels with substantial emission intensity only a few tens of nanometers from the free surface. Lower incident beam voltages result in excitation only a few tens of nanometers from the surface and a dramatic appearance of new peaks in CLS spectra reveal a dramatic dependence of emission intensities on the excitation energy, indicating bulk-related features at 0.95 eV versus surface emissions at 1.10, 1.20, 1.35, and 1.65 eV. These measurements reveal the sensitivity of UHV-CLS techniques to localized states at SiC surfaces and their dependence on the chemical preparation used.
Parametrization of Laframboise’s results for spherical and cylindrical Langmuir probes
Authors: Karamcheti A. , Steinbrüchel Ch.
Published: July 2002

Three new aspects regarding the analysis of Langmuir probe data are presented. First, we demonstrate that the numerical results of Laframboise for spherical probes can be parametrized easily for arbitrary ratios of the probe radius rp to the Debye length λD. The ion current can be expressed in the form a(−X)b, where a and b are parameters depending on rp/λD, and X is the dimensionless probe voltage. This functional form is the same as the one for cylindrical probes reported previously, but the values of a and b are different. Second, we use numerical simulations to show that unless the plasma potential Vs is known, it is in general difficult to determine accurately the form of the ion current characteristic Ii(Vp), and thus the ion density Ni, from typical probe data. This is because Ii(Vp), Ni, and rp/λD are interdependent. Third, the simulations indicate that the apparent electron energy distribution is very sensitive to the exact form of Ii(Vp) and to the method by which Ii(Vp) is subtracted from the total probe current to obtain the electron current. A linear extrapolation of Ii(Vp) is often adequate for determining the electron temperature, but assuming a constant ion current leads to electron energy distributions that appear to have two components with different electron temperatures. Additional issues discussed include the consequences of a slightly collisional probe sheath and the importance of end-effect corrections with a cylindrical probe.
Silicon nitride islands as oxidation masks for the formation of silicon nanopillars
Authors: Ha Jeong Sook , Park Kang-Ho , Yun Wan Soo , Lee El-Hang
Published: July 2002

We have used silicon nitride islands as oxidation masks for the formation of nanometer-scale silicon pillars. For the growth of silicon nitride islands on the Si(111)-7×7 surface, two different methods were used: (1) 100 eV N2+ ion exposure at room temperature followed by subsequent post annealing at 980 °C and (2) N2 exposure at 800 °C. Scanning tunneling microscope images for the two differently prepared surfaces showed a submonolayer coverage of nanometer-size silicon nitride islands. On these surfaces, oxygen was exposed at high temperatures where silicon etching was dominant over oxide formation. It was found that those silicon nitride islands successfully worked as oxidation masks to form silicon nanopillars as high as several nanometers via a selective oxygen etching of silicon. Silicon nanopillars of uniform sizes but with nonuniform heights were obtained in the case of N2+ ions, while those with a distribution of nonuniform sizes but homogeneous heights were formed in the case of N2 gas. Such different results are explained in terms of differences in the nitridation reaction between the two preparation methods.
Moisture stability and structure relaxation processes in plasma-deposited SiOF films
Authors: Pankov V. , Alonso J. C. , Ortiz A.
Published: July 2002

Fluorinated silicon dioxide (SiOF) films have been prepared by remote plasma enhanced chemical vapor deposition using SiF4, O2, H2, and He gases. Fourier transform infrared spectroscopy has been used to study structural changes in SiOF films caused by interaction with atmospheric moisture. It is shown that the increased incorporation of fluorine into the SiOF network during film deposition gradually increases the value of the average Si–O–Si angle in the SiOF network, 〈θ〉, from 138° to 153°. It is concluded that the value of about 144° corresponds to nearly relaxed SiOF network and therefore can be considered as an equilibrium angle for SiOF films. Subsequent increase in the fluorine concentration in the film results in 〈θ ) larger than the equilibrium 〈θ〉 (“overequilibrium”) and causes structural relaxation of the SiOF network towards the equilibrium 〈θ〉 during the post-deposition period. Both “under-” and “overequilibrium” values of 〈θ〉 seem to be the sources of SiOF film structural instability and increased reactivity with moisture. However, film hydrolysis and structural relaxation processes in the SiOF films characterized by “underequilibrium” values of 〈θ〉 are effectively suppressed by F presence in moderate concentrations. On the contrary, SiOF films characterized by “overequilibrium” 〈θ〉 have low density network and heavily absorb atmospheric water, which strongly promotes both structural relaxation and hydrolysis during the post-deposition period.
Chemical vapor deposition of pyrolytic boron nitride from borazine
Authors: Demin V. N. , Asanov I. P. , Akkerman Z. L.
Published: July 2002

The chemical vapor deposition of pyrolytic boron nitride from borazine (the B3N3H6–N2 system) in the temperature range of 1300–1800 °C and chamber pressures of 1–10 Torr has been studied using a cold-wall reactor. The density, phase composition and B/N ratio of the samples (0.2–0.8 mm thick) have been measured. The deposition process from borazine is controlled by diffusion and produces stoichiometric boron nitride with a high fraction of the hexagonal phase of boron nitride in the samples. Employing borazine as a precursor reduces the temperature of formation of h-BN in comparison with the well-known BCl3–NH3–N2 system.
Kinetic study of 3C-SiC growth on Si by pyrolyzing tetramethysilane in low pressure radio frequency-induction heated chemical vapor deposition reactor
Authors: Kim Kwang Chul , Nahm Kee Suk , Hahn Yoon Bong , Lee Yeon Sik , Byun Hun-Soo
Published: July 2002

Single crystal 3C-SiC films were grown on Si(111) substrate using tetramethylsilane [TMS,(CH3)4Si] in a low pressure radio frequency-induction heated chemical vapor deposition reactor. The growth rate of 3C-SiC film increased with increasing TMS flow rate and growth temperature. The growth kinetics of 3C-SiC film was determined by analyzing the experimental data on the growth rate. The growth reaction of 3C-SiC film was the first order of TMS partial pressure in the reactor. The reaction rate controlling step changed from the surface reaction in the temperature range of 1100–1250 °C to the diffusion of reactive gases at higher temperatures above 1250 °C. The activation energies were 72.2 kcal/min for the surface reaction-controlling step and 38.2 kcal/min for the diffusion-controlling step. At the growth temperatures, TMS was dissociated into hydrogen, Si atoms, and hydrocarbon gases such as CH3⋅, CH4, C2H2, and C2H4. The effective reactive species for the growth of 3C-SiC turned out to be gaseous Si atoms and methyl radicals in this growth system. The mechanisms of TMS decomposition and SiC growth were discussed in detail based on quadrapole mass spectrometer analysis.
Coverage dependence of activation barriers: Nitrogen on Ru(0001)
Authors: Diekhöner L. , Mortensen H. , Baurichter A. , Luntz A. C.
Published: July 2002

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses several previously unknown higher coverage states are sequentially filled up to a N coverage of almost 1 ML, N/Ru. These states show well-defined temperature programmed desorption (TPD) peaks. Recent density functional calculations demonstrate that the N–Ru bond strength decreases significantly with N coverage, and in fact that high coverage N adsorbate states are not thermodynamically stable relative to associative desorption. The observed high coverage states must, therefore, be only metastable, with lifetimes determined by the height of the barrier between gas phase N2 and the adsorbed atomic state. Analysis of the TPD in combination with the theoretical adsorption energies allows us to estimate these coverage dependent barriers. Independent measurements of the coverage dependent barrier heights were also obtained via the technique of laser assisted associative desorption i.e., by measuring the translational energy distribution of desorbing N2 via time of flight techniques induced by a short laser induced temperature jump. A barrier increases of ⩾1 eV with N atom coverage was observed by both methods.
Tribochemistry of monodispersed ZDOL with hydrogenated carbon overcoats
Authors: Chen Chao-Yuan , Bogy David B. , Bhatia C. Singh
Published: July 2002

Tribochemical studies of the lubricant molecular weight effect on the tribology of the head/disk interface were conducted using hydrogenated (CHx) carbon disks coated with ZDOL lubricant. The studies involved drag tests with uncoated and carbon-coated Al2O3–TiC sliders and thermal desorption experiments in an ultrahigh vacuum tribochamber. The studies showed that the lubricant interaction with the carbon overcoat varies as a function of lubricant molecular weight. The friction coefficient increases as the molecular weight increases. The higher friction is due to the higher viscosity. The friction and catalytic decomposition mechanisms of ZDOL are described. In general, the perfluoropolyethers polymers are decomposed by chain scission involving the breakage of the backbone bonds to yield free-radical segments. Chain scission can occur by three mechanisms: (1) random degradation, (2) depolymerization, and (3) weak-link degradation. Our studies further support previous observations that catalytic reactions occurred at the endgroups. The lower number of endgroups for ZDOL with higher molecular weight reduces the possibility of the occurrence of catalytic reactions. Moreover, the ZDOL desorbed peak temperatures shifted to higher temperatures with increasing molecular weight in thermal desorption tests. The spreading diffusion coefficient of ZDOL decreases with increasing molecular weight. As the mobility of the lubricant chain decreases, the desorption energy needed to break the lubricants increases, resulting in higher desorption peak temperatures.
The ZnSe(110) puzzle: Comparison with GaAs(110)
Authors: Duke C. B. , Paton A. , Kahn A. , Tu D-W
Published: July 2002

New low-temperature (T∼200 K) elastic low-energy electron diffraction (ELEED) intensity data obtained for ZnSe films grown epitaxially on GaAs(110) are analyzed using an R-factor methodology. The differences between the measured ELEED intensities for ZnSe(110) and those for GaAs(110) reveal the possibility that these two surfaces may not exhibit comparable atomic geometries. A bond-length-conserving top-layer rotation of the Se species outward and the Zn inward corresponding to ω1=4° provides a description of the measured intensities (Rx =0.22, RI =0.21) comparable to that afforded by the ZnSe(110) analog of the GaAs(110) structure (a relaxed version of an ω1=29° structure, Rx =0.24, RI =0.16). Since GaAs and ZnSe exhibit essentially identical bulk lattice constants, the possibility that their (110) surfaces exhibit different atomic geometries poses a puzzle within the framework of current understanding of this topic.
Power loss mechanisms in radio frequency dry etching systems
Authors: van den Hoek W. G. M. , de Vries C. A. M. , Heijman M. G. J.
Published: July 2002

The relation between the total radio frequency (rf) generator output power and the power that is actually dissipated in the plasma in rf dry etching systems was investigated. Power loss processes are described, their relation with the construction of the etch system is demonstrated, and their influence on the discharge is discussed. A method to estimate the power losses occurring in the etch system is described. For three different dry etching systems it is demonstrated that the stray capacitance of the system plays an important role in the power loss processes. In asymmetrical systems the dc self-bias voltage is shown to be a reliable parameter in determining the actual power dissipated in the plasma.
Scanning force microscopy with micromachined silicon sensors
Authors: Nonnenmacher M. , Greschner J. , Wolter O. , Kassing R.
Published: July 2002

Scanning force microscopy (SFM), operated in the attractive imaging mode, enables the precise measurement of the force between tip and sample over a tip–sample distance ranging from contact to tens of nanometers. The basic long range interactions (>1 nm: i.e., hydrodynamic, electrostatic, van der Waals, and capillary forces) between tip and sample have been measured and will be discussed. Each force leads to a different mode of operation in profiling samples. The most critical part of the SFM is the force sensor. Exact knowledge of the sensor properties is required for the interpretation of SFM measurements. We have used micromachined silicon sensors consisting of a monolithic silicon cantilever with integrated silicon tip and have performed a detailed characterization of the tip geometry and resonance properties. Examples of surface images on different samples (conductors, insulators and biological materials) and structures, ranging from atomic steps up to several microns high features, have been investigated to demonstrate capabilities and problems in SFM imaging.
Observation of phosphorylase kinase and phosphorylase b at solid–liquid interfaces by scanning tunneling microscopy
Authors: Lee Gil , Evans D. Fennell , Elings Virgil , Edstrom Ronald D.
Published: July 2002

Enzymes can be immobilized at a solid–liquid interface and observed with a scanning tunneling microscope (STM) using the inherent charge of proteins and an external potential applied to the STM substrate. Phosphorylase kinase, and dimers and oligomers of phosphorylase b have been observed at the interface of aqueous solutions and highly oriented pyrolytic graphite (HOPG). The lateral dimensions of phosphorylase kinase determined by STM at the solid–liquid interface are from 74%–78% of the dimensions determined by STM at the solid–air interface [Biochem. 28, 4939 (1989); Biophys. J. (in press)]. The phosphorylase b lateral dimensions of both enzymes are between 1.9 and 2.5 nm greater than the dimensions determined by x-ray crystallography. The vertical dimensions determined by STM at the solid–liquid and solid–air interfaces are in reasonable agreement with each other. Mixtures of the two enzymes show aggregates in which the complexes of the two enzymes are identifiable. This technique will make it possible to use STM to study the structure of protein systems at nearly physiological conditions and the nature of the interaction of proteins with surfaces.
IgG antibody and antibody–antigen complex imaging by scanning tunneling microscopy
Authors: Olk Charles H. , Heremans Joseph , Lee Peter S. , Dziedzic Daniel , Sargent Nicholas E.
Published: July 2002

We have examined two preparations of biomolecules labeled with colloidal gold markers using scanning tunneling microscopy. In the first preparation, we studied gold labeled IgG antibodies. In the second preparation, we studied unlabeled IgG antibodies which were complexed with gold labeled antigen (bovine serum albumin). Images of these noncrystalline biological specimens, obtained under atmospheric conditions, revealed features on a nanometer scale. The images show details of molecular organization in addition to the classical antibody configuration.
Structural study of a wholly aromatic polyamide with a rigid pendent group by scanning tunneling microscopy
Authors: Hawley M. E. , Benicewicz B. C.
Published: July 2002

The scanning tunneling microscope was used to study nonconducting wholly aromatic polyamide chains deposited onto highly oriented pyrolytic graphite (HOPG) substrates. Images, taken in air, were analyzed to address questions concerning the attachment sites of rigid pendent groups to the polymeric molecular backbone as well as to answer general, larger morphological questions relating to such issues as possible helical coiling of chains, monomeric or dimeric, and multichain aggregation. Films were prepared from solutions of the polyamide dissolved in dimethylacetamide and differing in salt content to change the degree of association between chains and, therefore, the tendency of chains to form large bundles. Evidence was found for coiling of multimolecular chains and a tendency to form large ordered multimacromolecular assemblies.
Electropolymerization of glutaraldehyde observed by scanning tunneling microscopy and its implications for scanning tunneling microscopy imaging of organic material
Authors: Heckl Wolfgang M. , Smith D. P. E.
Published: July 2002

Here we show an example of scanning tunneling microscopy triggering and subsequently investigating a chemical reaction on a nanometer scale. It is observed that applying a high electric field to a graphite substrate homogeneously coated with a thin film of glutaraldehyde causes the glutaraldehyde molecules to form clusters ranging from 5 to 15 molecules in size. Studies on the lateral distribution of the clusters suggest that a lateral flux of charge between the negatively biased tip and the grounded substrate causes a type of electropolymerization of these molecules. We believe that a significant current flows due to charged species which have been created by the primary tunneling electrons and which subsequently diffuse over a large distance. We therefore propose that, in general, lateral charge transfer may play an essential role in imaging low-conductivity molecules by distributing the current between tip and substrate over a large sample area, thereby considerably reducing the current density.
Molecular resolution of polysaccharides by scanning tunneling microscopy
Authors: Miles M. J. , Lee I. , Atkins E. D. T.
Published: July 2002

Scanning tunneling microscopy has been used to obtain images in constant current mode in air at molecular resolution of the polysaccharides: cellulose azure, hydroxypropylcellulose (hpc), and xanthan. The molecules were deposited from solution onto a graphite substrate. The hpc samples were observed to form readily ordered molecular arrays on the substrate. The molecular dimensions of width and repeat distance along the molecule were found to be close to values previously determined by x-ray diffraction. The observed axial periodicity of 2 nm for xanthan is close to that expected for a double-helix molecule.
Atomic force microscopy of charge density waves and atoms on 1T–TaSe2, 1T–TaS2, 1T–TiSe2, and 2H–NbSe2
Authors: Garnaes J. , Gould S. A. C. , Hansma P. K. , Coleman R. V.
Published: July 2002

In this paper, Atomic force microscopy (AFM) measurements on TaSe2 and 1T−TaS2 will be presented clearly resolving both the atoms with a z-deflection of ≊0.5 Å, and the charge-density waves (CDWs) also with a z deflection of ≊0.5 Å. All of the measurements have been carried out at room temperature using freshly cleaved crystals in air. It has been possible to distinguish very clearly between the commensurate CDW structure on 1T−TaSe2 and the incommensurate CDW structure on 1T−TaS2. For the commensurate CDW structure on 1T−TaSe2, the rotation angle between the atomic lattice and the CDW superlattice is measured as 14.2°±0.5. For the incommensurate CDW structure on 1T−TaS2, the rotation angle between the atomic lattice and the CDW superlattice is measured as 12.1°±0.3. A detailed analysis of the Fourier transformation suggest no appreciable regions of commensurate CDWs at an angle of 13.9°. Instead, the analysis favors a continuous modulation of the CDW amplitude, in agreement with recent STM data. The mechanism involved in the AFM response to the CDW structure and the magnitude of the z deflection will be discussed. In addition, high resolution AFM scans of atoms on 1T−TiSe2 and 2H−NbSe2 are presented that have no CDWs at room temperature.
Scanning tunneling microscopy of 1T–TaSe2 at room temperature
Authors: Raina G. , Sattler K. , Müller U. , Venkateswaran N. , Xhie J.
Published: July 2002

We studied charge density waves on 1T–TaSe2 using scanning tunneling microscopy at room temperature in air. The images reveal three distinct types of the charge density wave vertices, showing triangular, rhombic and hexagonal atomic modulations. The various vertex structures cannot be explained by only one type of periodic lattice distortion namely the ‘‘Star-of-David’’ cluster unit. Two other cluster units besides the ‘‘Star-of-David’’ are proposed, based on the assumption that the maxima of the charge density waves can have different positions on the triangular atomic lattice. We also observed arrangements of the charge density wave lattice which we explain by simultaneous multi tip imaging and switching between different tunneling tips.