摘要:

Calf skin collagen molecules adsorbed on a graphite substrate were directly imaged by scanning tunneling microscopy in air. The right‐handed helical structures were observed and the average periodicity was quantitatively measured to be 31 Å, in good agreement with the value of about 28 Å estimated from biochemical studies. The intermolecular distance in a collagen bundle was 23 Å.

ISSN:0734-2101    

DOI:10.1116/1.577517

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have studied the change of Li+and F−secondary ion intensiters with the matrix transition from submonolayer LiF on gold to thick LiF by time‐of‐flight secondary ion mass spectrometry. Based on direct recoil particle measurements and Monte Carlo simulations, we estimated that the ratio of Li+/F−secondary ion yields did not change significantly with the matrix. From the results, it is suggested that the bond breaking model can be applied for submonolayer LiF on gold as well as for thick LiF.

ISSN:0734-2101    

DOI:10.1116/1.577519

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The SiO2/Si interface structure of thin oxide films thermally grown on Si(100), (111), and (110) surfaces under device processing conditions has been investigated using high‐resolution photoemission spectroscopy with synchrotron radiation. The intensity distribution of the so‐called suboxides, Si1+, Si2+, Si3+, displays a strong dependence on the crystallographic orientation of the substrate over the oxidation temperature range from 600 to 900 °C; Si1+is enhanced in intensity on Si(111) and (110), while the Si2+intensity is larger than the Si1+one on Si(100). This orientation dependence is explained in terms of the bond topology of the substrate surface. A Si(110) surface exhibits a rather large Si3+intensity as compared to Si(100), (111) surfaces, suggesting that Si–Si bonds on the outermost layer of a Si(110) surface are easily broken by oxygen atoms to generate the bridge bond Si–O–Si. The presence of an interfacial Si atom to which hydrogen is bonded is clearly observed. The total suboxide intensity, i.e., the sum of the suboxide intensities depends on both oxidation temperature and substrate orientation, which is interpreted by means of the interfacial roughness and the surface Si atom density of the substrate. It is shown that an ordered crystalline phase of SiO2is present at the interfacial region. The generation of this phase has a Si(111)‐preferred orientation.

ISSN:0734-2101    

DOI:10.1116/1.577520

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Experimental conditions for high fidelity electron spectroscopy are discussed when digitized sweep generator is used to drive spectrometer with lock‐in signal detection and computerized data acquisition/processing. Spectral quality were evaluated in terms of signal to noise ratio (S/N ratio) and full width at the half‐maximum (FWHM) inN(E) mode or energy separation of peak maxima in differential mode. Optimums of following terms were discussed from points of noise reduction ability and due time for measurement: (1) nature of noise signal; (2) sweep mode and repeat times of measurement; (3) parameters of the lock‐in amplifier and scanning speed; (4) numerical treatment on spectral data.

ISSN:0734-2101    

DOI:10.1116/1.577521

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A random walk adsorption–migration kinetics model has been developed to describe the time‐varying layer‐by‐layer adsorption buildup on a smooth surface under continuous molecular flux impingement. The basic approach of this stochastic model is to determine the transition probability for each adsorbed layer by detailed balancing of layer‐by‐layer adsorption, desorption, and migration rates at any given time, whereupon the net surface adsorption quantity as a function of time can be predicted. In essence, this theory constitutes a generalized Brunauer–Emmett–Teller model with adsorption kinetics and interlayer migration considerations. Based on this theory, the transient adsorption–migration problem can be described by a system of Kolmogorov quasilinear differential equations for a Markov process. A numerical scheme has been developed to solve this system of equations, and the solution has been tested with a heavy‐molecule DC‐704 lubricating oil outgassing‐deposition problem. Good theory/data agreement was obtained by using variable heats of desorption and migration, the latter depending on both the incident flux and the deposit quantity.

ISSN:0734-2101    

DOI:10.1116/1.577522

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Photon‐stimulated desorption (PSD) experiments on thermal oxide films on Si have been performed using synchrotron radiation with photon energies in the Si 2pcore‐level excitation region, to investigate the chemical nature of the film surface. Mass analysis of the PSD ions is performed using a time‐of‐flight technique. H+is found to be the dominant ion species that desorbs from the film surface. It is observed that the H+PSD ion yield decreases exponentially with increasing the irradiation time of incident photons. The H+PSD ion yield as a function of photon energy exhibits a close similarity in spectral shape to the photoabsorption spectrum of SiO2. The present experimental results confirm us that hydrogen atoms exists on the outermost layer of SiO2film, and provide direct evidence for Si–OH and Si–H termination of SiO2films.

ISSN:0734-2101    

DOI:10.1116/1.577523

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The sticking coefficient γ of ground‐state Cl (Cl 2P3/2) and spin–orbit excited Cl* (Cl 2P1/2) on lightly dopedn‐Si(100) was measured using resonance‐enhanced multiphoton ionization of Cl and Cl* at mTorr pressure in a Knudsen cell. For Cl only an upper limit of γ≤5×10−5could be obtained and for Cl* γ=4.6×10−4was measured. These γ values are temperature independent in the range of 300–600 K and the sum corresponds to a spontaneous etch rate of 9.4 Å/min for Si(100). SiCl2was the principal etch product under these spontaneous etch conditions, and no SiCl4was found over the given temperature range.

ISSN:0734-2101    

DOI:10.1116/1.577524

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The structure and composition of air‐exposed Si(100) surfaces irradiated by a KrF (248 nm, 5.0 eV, 22 ns pulses) excimer laser in ultrahigh vacuum (UHV) have been studied using Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and reflection high‐energy electron diffraction (RHEED). The near‐surface region was compositionally profiled using secondary‐ion mass spectrometry (SIMS). Pulsed laser irradiation at energies below the melting threshold (≊500 mJ cm−2) led to significant reductions in surface C and O concentrations but detectable contamination remained even after 200 pulses. However, surfaces exposed to several laser pulses at energies just above the melting threshold were atomically clean (less than 0.01 monolayer of impurities) as judged by AES and EELS and exhibited a 2×1 RHEED pattern. The peak in the EELS spectrum associated with surface dangling bond states was slightly lower in intensity and higher in energy than results obtained from thermally cleaned samples, while other surface states associated with the 2×1 reconstruction and noninteger RHEED streak intensities were also slightly less intense. SIMS profiles of the irradiated wafers revealed C and O impurities distributed throughout the melt volume. Thus, excimer laser irradiation produces clean Si(100) surfaces by a combination of desorption (accounting for approximately 1/3 to 1/2 of the decrease in impurity concentration) and dissolution leaving a disordered (2×1) surface. Annealing irradiated wafers at 600 °C for 1 min in UHV produced sharp 2×1 RHEED patterns and EELS spectra identical to those obtained from thermally cleaned (≥900 °C) surfaces.

ISSN:0734-2101    

DOI:10.1116/1.577525

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have observed an etching reaction on the Al/Si(111) surface (θAl≥1) induced by the chemisorption of atomic hydrogen at low temperature (T=250–400 K). The major silicon containing product detected by temperature programmed desorption (TPD) is silane (SiH4). An approximate sixfold increase in silane production is observed in the presence of aluminum, with a decrease in the TPD peak temperature from 645 K on Si(111) to 335 K on Al/Si(111). A kinetic model is presented to explain this catalyzed etching reaction.

ISSN:0734-2101    

DOI:10.1116/1.577526

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The longitudinal nuclear relaxation times of tritons in equilibrium deuterium–tritium (eD–T, actually D2–DT–T2) adsorbed onto amorphous silica aerogel has been measured from 4.2 to 23 K from 0.5 to 6.2 monolayers of coverage. Below 7 K, the relaxation time dramatically increases with decreasing temperature, especially for low coverages. A value of 16 s for 0.5 monolayer at 4.2 K may be compared with the usual electric quadrupole–quadrupole (EQQ) determined relaxation time of 0.1 s. It is shown that absorption of 10% of the tritium beta particle energy occurs in aerogel fully loaded with D–T. This decreases the concentration ofJ=1 T2and increases the nuclear relaxation time. Similar energy absorption calculations in the thin layers shows that long relaxation times are expected, and that the measured values must be caused by spin–lattice relaxation with the aerogel surface. Both (temperature)−7and exponential mechanisms are considered with no final decision being possible. Electric field gradients from the aerogel walls are considered the likely relaxation mechanism. These findings suggest that a long triton relaxation time may be achievable in ultrapure DT in the silica aerogel. Such a result would be of considerable importance to nuclear polarized inertial confinement fusion targets.

ISSN:0734-2101    

DOI:10.1116/1.577527

出版商:American Vacuum Society    

年代:1991

数据来源: AIP