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  • American Institute of Physics (AIP)  (9)
  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 7338-7343 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Si(100) and (111) surfaces have generally been classified as relatively inert toward molecular hydrogen adsorption. In the present work, we show using surface second harmonic generation that the sticking probability increases dramatically for slightly misoriented substrates, which are of interest in many integrated circuit applications. At temperatures lower than 30–50 K below where desorption becomes significant, defect sites serve as adsorption conduits from which hydrogen atoms diffuse onto the rest of the surface. The adsorption rate therefore has the activation energy of surface diffusion, found to be 0.78 eV on Si(100) and 1.50 eV on Si(111). The small energy barrier and pre-exponential factor (∼10−11 cm2/s) for diffusion on (100) seem to result from the need to break Si–Si dimer-pair bonds prior to hopping of an H atom. At higher temperatures, the adsorption mechanism changes, and H2 sticks to both surfaces with a fairly large probability near 0.1. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 4963-4972 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photoreflectance (PR) has until now been a technique used primarily for characterization of bulk electronic band structure in semiconductors. However, since reflectance is an interfacial phenomenon, PR should in principle be sensitive to surface treatment as well. The present work appears to be the first quantitative use of PR to measure the effects of surface treatment. In particular, PR spectra of air-oxidized and of annealed GaAs(100) are shown to be substantially different from those of the clean surface. Spectra at the E0 (1.4 eV) and E1 (3.0 eV) transitions respond quite differently to these treatments, and the differences are explained in terms of an optical interference phenomenon due to excitons that occurs only at E0. Effects of slow surface states have been observed with PR for the first time. The effective barrier between these states and the bulk decreases from 21±2 to 16± 2 meV upon air oxidation of the clean surface.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 218-219 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A sample mount is described that is particularly useful for cooling and uniformly heating thin, fragile semiconductor samples in ultrahigh vacuum while maintaining optical flatness. The design is based on spring-loaded clips and permits rapid sample exchange. Difficulties with certain conventional methods of temperature measurement are avoided.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 6597-6604 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Surface diffusion of hydrogen, deuterium, and CO on Rh(111) has been investigated by laser-induced thermal desorption (LITD) and compared with previous results for these species on Pt(111) and on other metals. As the coverage θ of deuterium increases from 0.02 to 0.33, the preexponential factor D0 remains constant at 8×10−2 cm2/s, but the diffusion activation energy Ediff rises from 3.7 to 4.3 kcal/mol. Ediff for hydrogen is 0.6 kcal/mol lower than for deuterium, consistent with the difference in zero-point energy. For CO, Ediff =7 kcal/mol at all coverages, but D0 rises from 10−3 to 10−2 cm2/s between θ=0.01 and 0.40. Values of Ediff for these adsorbates vary by several orders of magnitude for surfaces on which heats of adsorption are essentially identical. These differences appear to correlate with differences in heats of adsorption in different binding states which form saddle point configurations in surface diffusion. Ediff is found to be nearly identical to the reaction activation energies for the CO and hydrogen oxidation reactions on Rh and on several other transition metal surfaces. This suggests that surface diffusion of the reducing agent may be the rate-limiting step in their bimolecular surface reactions.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 2557-2565 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: For diffusion-controlled reactions in three dimensions, continuum mechanics provides a quantitative relation between the steady-state reaction rate constant k and the diffusion coefficient D. However, this approach fails in two dimensions, where no steady-state solution exists on an infinite domain. Using both Monte Carlo methods and analytical techniques, we show that previous attempts to circumvent this problem fail under real laboratory conditions, where fractional coverages often exceed 10−3. Instead, we have developed a rigorous and general relation between k and D for all coverages on a square lattice for the reaction A+A→A2. For short times or high coverages, the relation k=πD/γ holds exactly, where γ denotes the two-dimensional packing fraction. For lower coverages, however, k depends on time in both constant-coverage (adsorption allowed) and transient-coverage (adsorption forbidden) regimes. In both cases, k decreases in response to the evolution of nonrandom adsorbate configurations on the surface. These results indicate that diffusion-limited surface reactions may be identified unambiguously in the laboratory and also provide a quantitative link between diffusion parameters and experimentally determined recombination rate parameters. Practical experimental methods highlighting such effects are outlined. © 1996 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 7190-7197 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mechanism of H2O adsorption on GaAs(100) has been elucidated by an adaptation of the photoreflectance (PR) technique for surface kinetic measurements. Being an optical method, PR is especially well-suited for probing weakly bonded adsorption systems where the pressures required for significant interaction ((approximately-greater-than)10−5 Torr) preclude the use of traditional electron or ion spectroscopies. H2O adsorbs through a physisorbed state. This species can desorb or react to a chemisorbed form, which in turn can desorb. Both the physisorbed and chemisorbed species are undissociated. We interpret exceptionally low values for the prefactors associated with the chemisorbed state in terms of an adsorbate-induced surface reconstruction.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 943-948 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Various methods have been used up to now in order to simulate the effects of inhomogeneous electric fields on the modulation spectra of semiconductors. Some methods involve partitioning a layer with a continuously varying field into a series of discrete steps. We show that in the case of a wide space-charge region, solving the resulting set of reflectance equations directly can give misleading results under some conditions because of subtle numerical problems. However, a WKB approach exists that avoids these problems and can be used for GaAs under essentially all conditions. Our simulations lend support to an excitonic mechanism for spectral "rotation'' in photoreflectance of GaAs even at room temperature.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 6958-6967 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Surface diffusion of Sb on Ge(111) has been measured with the newly developed technique of optical second harmonic microscopy. In this method, concentration profiles at submonolayer coverage are imaged directly by surface second harmonic generation with 5 μ spatial resolution. A Boltzmann–Matano analysis yields the coverage dependence of the diffusivity D without parametrization. Experiments were performed at roughly 70% of the bulk melting temperature Tm. In the coverage range 0≤θ≤0.6, the activation energy Ediff remains constant at 47.5±1.5 kcal/mol, but the pre-exponential factor D0 decreases from 8.7×103±0.4 to 1.6×102±0.4 cm2/s. Both Ediff and D0 are quite large, which is consistent with high-temperature measurements in other systems. The inadequacies of current theories for high-temperature surface diffusion are outlined, and a new vacancy model is proposed for low-coverage diffusion. The model accounts semiquantitatively for the large values of Ediff and D0, and suggests that these quantities may be manipulated using doping levels and photon illumination. An islanding mechanism is proposed to explain the decrease in D0 with θ.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 6772-6777 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Surface diffusion of In on Ge(111) has been measured by optical second harmonic microscopy. This technique employs surface second harmonic generation to directly image submonolayer surface concentration profiles. The coverage dependence of the diffusivity D can then be obtained from a Boltzmann–Matano analysis. In the coverage range 0.1〈θ〈0.48, the activation energy Ediff decreased with increasing coverage, ranging from 31 kcal/mol at θ=0.1 to 23 kcal/mol at θ=0.48. Over the same coverage range, the pre-exponential factor D0 decreased from 5×102 to 1×10−1 cm2/s. This gradual change reflects a change in diffusion mechanism arising from the disordered nature of the Ge(111) surface. At low coverages, In adatoms sink into the top layer of Ge, and diffusion is dominated by thermal formation of adatom-vacancy pairs. At high coverages, diffusion occurs by normal site-to-site hopping. The gradual change in diffusion parameters with coverage was interrupted by an apparent phase transition at θ=0.16. At this point, both Ediff and D0 peaked sharply at 41 kcal/mol and 6×105 cm2/s, respectively. The desorption energy Edes was measured by temperature programmed desorption. Edes decreased from 60 kcal/mol at submonolayer coverages to 55 kcal/mol at multilayer coverages.
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