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  • American Physical Society  (1,470)
  • American Institute of Physics (AIP)  (156)
  • Institute of Physics (IOP)  (58)
  • 1
    Electronic Resource
    Electronic Resource
    Fluorine atom abstraction by Si(100). I. Experimental (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3679-3695 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In the interaction of low energy F2 with Si(100) at 250 K, a dissociative chemisorption mechanism called atom abstraction is identified in which only one of the F atoms is adsorbed while the other F atom is scattered into the gas phase. The dynamics of atom abstraction are characterized via time-of-flight measurements of the scattered F atoms. The F atoms are translationally hyperthermal but only carry a small fraction (∼3%) of the tremendous exothermicity of the reaction. The angular distribution of F atoms is unusually broad for the product of an exothermic reaction. These results suggest an "attractive" interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. These results are in disagreement with the results of theoretical investigations implying that the available potential energy surfaces are inadequate to describe the dynamics of this gas–surface interaction. In addition to single atom abstraction, two atom adsorption, a mechanism analogous to classic dissociative chemisorption in which both F atoms are adsorbed onto the surface, is also observed. The absolute probability of the three scattering channels (single atom abstraction, two atom adsorption, and unreactive scattering) for an incident F2 are determined as a function of F2 exposure. The fluorine coverage is determined by integrating the reaction probabilities over F2 exposure, and the reaction probabilities are recast as a function of fluorine coverage. Two atom adsorption is the dominant channel [P2=0.83±0.03(95%, N=9)] in the limit of zero coverage and decays monotonically to zero. Single atom abstraction is the minor channel (P1=0.13±0.03) at low coverage but increases to a maximum (P1=0.35±0.08) at about 0.5 monolayer (ML) coverage before decaying to zero. The reaction ceases at 0.94±0.11(95%, N=9) ML. Thermal desorption and helium diffraction confirm that the dangling bonds are the abstraction and adsorption sites. No Si lattice bonds are broken, in contrast to speculation by other investigators that the reaction exothermicity causes lattice disorder. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479677
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  • 2
    Electronic Resource
    Electronic Resource
    Electrical conductivity in high-frequency plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 8993-8997 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The electrical conductivity is an important parameter in understanding the mechanism by which power is coupled to a radio-frequency (rf) discharge plasma, as well as in determining the external electrical characteristics of the discharge. We present the results of computations of the resistive and reactive components of the collisional impedance of an argon plasma at 13.56 MHz. The plasma conductivity is computed from the two-term solution to the Boltzmann equation, and includes the velocity dependence of the electron collision frequency, as well as non-Maxwellian electron energy distribution functions. We compare these results with those obtained from the widely used classical expression for plasma impedance, in which the electron collision frequency is computed either in the dc or high frequency limit. Our results show that neither of the classical limiting expressions are adequate for discharge pressures in the range of few mTorr to a few Torr, which includes the region of operation for many rf discharges used in many applications of plasma technology. Further, the classical formula assumes that in the high-frequency limit the plasma reactance is due entirely to electron inertia. We demonstrate that the plasma reactance may be strongly influenced, and in some cases dominated, by electron collisions. Results are presented in graphical form, which are useful in evaluating the importance of these effects on the interpretation of experimental results and the modeling of rf discharges. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362631
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  • 3
    Electronic Resource
    Electronic Resource
    A study of 2H trapping and release in 2H+-irradiated YBa2Cu3O7−δ /MgO〈100〉 (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 344-352 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A thin-film sample of YBa2Cu3O7−δ on MgO was irradiated at room temperature with 50-keV 2H+ (deuterium) to a dose of 1×1016 ions cm−2. The film was mainly c-axis textured film, ∼360–420 nm thick, deposited by sputtering on 〈100〉 MgO substrate. The as-implanted sample was divided into several pieces and annealed in a flowing oxygen ambient using (i) a rapid thermal annealing oven, at various temperatures between 450 and 940 °C, and (ii) a conventional annealing furnace, at various temperatures between 100 and 350 °C. Analysis by secondary-ion mass spectroscopy shows that the implanted 2H is a fast diffuser in the 123 phase. The apparent activation temperature (energy) for 2H release from the initial traps within the YBCO film during the anneal is estimated to be ∼175 °C (∼0.97 eV), which is obviously lower than the apparent activation temperature (energy) for 2H release from the initial traps within the MgO substrate (∼550 °C, i.e., ∼1.78 eV). At 200 °C the diffusivity of 2H in the YBCO film is estimated to be ∼1.4×10−13 cm2/s. In the irradiated MgO, during the anneal the migration and release of 2H is thought to be radiation enhanced around the higher damage region. No diffusional broadening or diffusion tail toward the deeply undamaged region was observed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360681
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  • 4
    Electronic Resource
    Electronic Resource
    Raman studies of laser-written patterns in YBa2Cu3Ox films (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 2929-2934 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Patterns on YBa2Cu3O6.87 films, laser written in O2 or N2 atmospheres, and with various writing power densities and scanning speeds, have been studied by Raman scattering. For laser writing in O2 at atmospheric pressure, with low power density and fast scanning speed, it is found systematically that the oxygen content of YBa2Cu3Ox can be reduced. Only when the writing power density is high and the scanning speed is low (e.g., 2.3 mW/μm2 and 1 μm/s), do the laser-irradiated lines in an O2 atmosphere approach full oxygenation (x→7). For laser writing in N2 at atmospheric pressure, the oxygen stoichiometry always decreases, rendering the lines nonsuperconducting. The structural phase transition from orthorhombic to tetragonal can be identified with laser writing in a N2 atmosphere with a writing power density of 1.9 mW/μm2 and a scanning speed of 20 μm/s. The oxygen O(4) line (∼500 cm−1) and the Ba line (115 cm−1) disappear, and the intensity of the Cu(2) line (144 cm−1) rises sharply when the phase transition occurs. This study shows how Raman scattering can be used as a routine technique, which is fast, sensitive, and nondestructive, and is able to measure oxygen content on a fine scale (a few μm) during high-Tc superconductor device processing. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363148
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal stability of W ohmic contacts to n-type GaN (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 278-281 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: W was found to produce low specific contact resistance (ρc∼8.0×10−5 Ω cm2) ohmic contacts to n+-GaN (n=1.5×1019 cm−3) with limited reaction between the metal and semiconductor up to 1000 °C. The formation of the β–W2N and W–N interfacial phases were deemed responsible for the electrical integrity observed at these annealing temperatures. No Ga out-diffusion was observed on the surface of thin (500 A(ring)) W contacts even after 1000 °C, 1 min anneals. Thus, W appears to be a stable contact to n+-GaN for high temperature applications. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362816
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  • 6
    Electronic Resource
    Electronic Resource
    Fluorine atom abstraction by Si(100) II. Model (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5190-5204 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model is developed to describe the kinetics of the three scattering channels—unreactive scattering and dissociative chemisorption via single atom abstraction and two atom adsorption—that are present in the interaction of F2 with Si(100). The model provides a good description of the non-Langmuirian coverage dependence of the probabilities of single atom abstraction and two atom adsorption, yielding insight into the dynamics of the gas–surface interaction. The statistical model is based on the premise that the two dissociative chemisorption channels share a common initial step, F atom abstraction. The subsequent interaction, if any, of the complementary F atom with the surface determines if the overall result is single atom abstraction or two atom adsorption. The results are consistent with the orientation of the incident F2 molecular axis with respect to the surface affecting the probability of single atom abstraction relative to two atom adsorption. A perpendicular approach favors single atom abstraction because the complementary F atom cannot interact with the surface, whereas a parallel approach allows the F atom to interact with the surface and adsorb. The fate of the complementary F atom is dependent on the occupancy of the site with which it interacts. The model distinguishes between four types of dangling bond sites on the Si(100)(2×1) surface, based on the occupancy of the site itself and that of the complementary Si atom in the Si surface dimer. The results show that the unoccupied dangling bond sites on half-filled dimers are about twice as reactive as those on empty dimers, which is consistent with an enhanced reactivity due to a loss of a stabilizing π interaction between the two unoccupied dangling bonds on a dimer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481092
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  • 7
    Electronic Resource
    Electronic Resource
    Multidimensional dynamics in the electron stimulated desorption of ammonia from Pt(111) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6318-6329 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more "spinning'' rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the ν2 symmetric deformation or "umbrella'' mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a1 electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two-dimensional potential energy surfaces (intramolecular bond angle and surface bond length) reveal that near-inverted molecules deexcite to a repulsive hard wall of the adsorbate–substrate interaction and desorb. Spinning excitation derives from the rotational barrier of the inverted molecule. Both isotope effects are direct consequences of desorption via inversion. In general, multidimensional dynamics must be considered in the study of stimulated surface processes. Our calculations also indicate that excited-state forces at equilibrium molecule–surface distances are an order of magnitude less than those derived from a currently accepted image-potential model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468385
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  • 8
    Electronic Resource
    Electronic Resource
    Photochemistry of adsorbed molecules. XV. Localized atomic scattering in the photolysis of HI/LiF(001) and HI/NaF(001) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5005-5019 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have compared the "elastic'' H atom angular-scattering, Pel(θ′), of HI(ad) photolyzed at 248 nm for LiF and NaF substrates, both experimentally and theoretically. The observed Pel(θ′) for elastic scattering were similar for LiF and NaF, with a peak in the range 43°–48°. The Pel(θ′) for H from HI/NaF was broader than that for H from HI/LiF. Energy distributions, P(ET′), for H recoiling from HI/LiF and HI/NaF are also reported. A semiempirical potential-energy surface (pes) is given for HI adsorbed on NaF, based on an earlier one for LiF [J. Chem. Phys. 94, 978 (1991)]. Ab initio "density function theory'' (DFT) calculations were performed to verify the suitability of this pes; the DFT points were in satisfactory agreement with the semiempirical pes. For HI/LiF the DFT calculation gave a heat of adsorption of about 9 kcal/mol, the semiempirical model gave 5 kcal/mol, and experiment gave 7 kcal/mol. We have computed H atom scattering by the classical trajectory method for two different models one involving dense and the other perforated arrangements of the HI adsorbate molecules. Comparison with the experimental results showed that the second model gave a better description of both Pel(θ′) and P(ET′) for HI/LiF and HI/NaF. The calculations showed that the angular and energy distributions of the scattered H were sensitive to the size of the vacancies between adjacent adsorbate molecules. The study also indicates the importance of unit-cell size in relation to the size of the adsorbate molecule in determining adsorbate geometry and hence the energy and angular distributions of the scattered photofragments. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472339
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  • 9
    Electronic Resource
    Electronic Resource
    Magnetic studies of R-Ba-Cu-O superconductors made by rapid solidification (abstract) (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 5913-5913 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have examined the magnetic hysteresis behavior and critical current density of R-Ba-Cu-O superconductors (R=Y, Dy, Er) made by melt spinning. The samples had a spherical shape with a diameter in the range 0.2–1.5 mm. The as-spun samples became superconducting after a heat treatment at around 900 °C. The critical temperature was measured with a SQUID magnetometer and found to be in the range 60–80 K. The critical temperature showed a strong dependence on sample size and was maximum for samples with a diameter approximately 0.6 mm. The critical current density was estimated to be 2×103 A/cm2, which is comparable to the values reported for bulk samples. Scanning electron microscope data showed a very fine grain structure with an average grain size of 2 μm. Detailed magnetic, structure, and chemical composition analyses will be reported.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.342200
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  • 10
    Electronic Resource
    Electronic Resource
    InAs self-assembled quantum dots grown on an InP (311)B substrate by molecular beam epitaxy (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 4186-4188 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Self-assembled InAs quantum dots (QDs) have been grown by solid-source molecular beam epitaxy on a (311)B InP substrate. Transmission electron microscopy clearly shows that a high density of smaller InAs islands can be obtained by using such a high index substrate. After introducing a lattice-matched underlying In0.52Al0.24Ga0.24As layer, the InAs QDs are much more uniform in size and form two-dimensional well ordered arrays. The photoluminescence (PL) spectra also confirm that the InAs QDs grown on underlying In0.52Al0.24Ga0.24As have a better quality than those grown in the In0.52Al0.48As matrix. A simple calculation indicates that the redshift of the PL peak energy mainly results from InAs QDs on underlying In0.52Al0.24Ga0.24As of large size. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1350616
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