Chemical-etch-assisted growth of epitaxial zinc oxide

We use real-time spectroscopic polarimetric observations of growth and a chemical model derived therefrom, to develop a method of growing dense, two-dimensional zinc oxide epitaxially on sapphire by metalorganic chemical vapor deposition. Particulate zinc oxide formed in the gas phase is used to advantage as the deposition source. Our real-time data provide unequivocal evidence that: a seed layer is required; unwanted fractions of ZnO are deposited; but these fractions can be removed by cycling between brief periods of net deposition and etching. The transition between deposition and etching occurs with zinc precursor concentrations that only differ by 13%. These processes are understood by considering the chemistry involved.Comment: 9 pages, 5 figure

Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping

It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds.Comment: 9 pages, 3 figure

Electrodynamics of surface-enhanced Raman scattering

We examine SERS from two perspectives: as a phenomenon described by the Laplace Equation (the electrostatic or Rayleigh limit) and by the Helmholtz Equation (electrodynamic or Mie limit). We formulate the problem in terms of the scalar potential, which simplifies calculations without introducing approximations. Because scattering is not usually calculated this way, we provide the necessary theoretical justification showing that the scalar-potential description is complete. Additional simplifications result from treating the scatterer as a point charge q instead of a dipole. This allows us to determine the consequences of including the longitudinal (Coulomb) interaction between q and a passive resonator. This interaction suppresses the mathematical singularities that lead to the unphysical resonant infinities in first and second enhancements. It also modifies the effective restoring-force constant of a resonant denominator, which permits us to explore the possibility of dual resonance through a molecular pathway. We apply the formalism to spherical inclusions of radius a for q located at polar and equatorial positions. For small a, of the order of 1 nm or less, the low-l multipole terms are important. For the more relevant case of radii of the order of 10 nm and larger, the q-sphere interaction can be approximated by a model where q interacts with its image charge for a dielectric plane, and the singularity shifts in a discrete manner from \epsilon(\omega)=-2 to \epsilon(\omega)=-1. These results are supported by more accurate calculations taking retardation into account, although the use of only one spherical-harmonic term does not fully represent the difference between forward- and backscattering.Comment: 25 page

Real-time observation of bond-by-bond interface formation during the oxidation of (111) Si

Atomic-level structure of solids is typically determined by techniques such as X-ray and electron diffraction,1, 2, 3, 4 which are sensitive to atomic positions. It is hardly necessary to mention the impact that these techniques have had on almost every field of science. However, the bonds between atoms are critical for determining the overall structure. The dynamics of these bonds have been difficult to quantify. Here, we combine second-harmonic generation and the bond-charge model of nonlinear optics5, 6 to probe, in real time, the dynamics of bond-by-bond chemical changes during the oxidation of H-terminated (111)Si, a surface that has been well characterized by static methods. We thus demonstrate that our approach provides new information about this exhaustively studied system. For example, oxidation is activated by a surprisingly small applied macroscopic strain, and exhibits anisotropic kinetics with one of the three equivalent back-bonds of on-axis samples reacting differently from the other two. Anisotropic oxidation kinetics also leads to observed transient changes in bond directions. By comparing results for surfaces strained in different directions, we find that in-plane control of surface chemistry is possible. The use of nonlinear optics as a bond-specific characterization tool is readily adaptable for studying structural and chemical dynamics in many other condensed-matter systems.Comment: 15 pages, 4 figure

Quantitative assessment of linear noise-reduction filters for spectroscopy

Linear noise-reduction filters used in spectroscopy must strike a balance between reducing noise and preserving lineshapes, the two conflicting requirements of interest

Ordering and Absolute Energies of the L6c and X6c Conduction Band Minima in GaAs

Resolved critical point structures in Schottky-barrier electroreflectance spectra of Ga3dV-sp3 conduction band transitions in the 20-22-eV range provide a direct proof that the L6C equivalent minima lie approximately 170±30 meVbelow the X6C minima in GaAs. This ordering, opposite to that assumed and apparently supported by previous experiments, is in fact consistent with these experiments and provides natural explanations for many formerly puzzling features of GaAs

Line shape and symmetry analysis of core-level electroreflectance spectra of GaP

No dependence of electroreflectance line shapes upon polarization direction or crystal orientation is found for any core-level electroreflectance structure from the Ga 3dv core levels to the conduction bands in GaP. Matrix-element effects that are responsible for anisotropy in sp3 valence-conduction-band electroreflectance spectra appear to be too weak to be detected in core-level spectra. The result may be general. The field-induced modulation line shape, Δε1, for the Ga 3dv32,52−Xc6 critical points is obtained from the dependence of the spectra and the generalized Seraphin coefficients upon angle of incidence. The line shape is further analyzed to obtain the Δε1 spectrum for Ga 3dv52−Xc6 alone. This procedure yields a spin-orbit splitting Δ3d=0.43±0.02 eV. A weighting of 0.65 ± 0.05 is also obtained for the j=32band relative to the j=52 band. This is in good agreement with the 4:6 ratio expected on d-band occupancy, showing that the matrix elements are also independent of j. The line shape of Δε1 is in good agreement with that predicted by the lifetime-broadened, Coulomb-enhanced Franz-Keldysh theory given by Blossey. The line shape shows a broadening of 160 meV for this transition, and a momentum matrix element about 1/3 as large as that characteristic of sp3 valence-conduction-band transitions

Electroreflectance of GaAs and GaP to 27 eV using synchrotron radiation

Electroreflectrance (ER) spectra of GaAs and GaP, taken with the Schottky-barrier method, exhibit to 27 eV the strong structural enchancement and high resolution characteristic of similar measurements below 6 eV. Above 20 eV, a new set of critical points is observed between the flat valence bands derived from the Ga 3d core levels and the local extrema of the sp3 conduction bands. The attained resolution, of the order of 100 meV, enables us to resolve clearly the spin-orbit splitting of 0.45 eV of the 3d-derived valence bands. The following critical-point energies have been determined in GaAs and GaP, respectively. sp3 valence conduction: E1′, 6.63 ± 0.05 eV, and 6.80 ± 0.05 eV; E1′+Δ1′, 6.97 ± 0.05 eV (GaAs only); E0\u27\u27(Γv15→Γc12), 10.53 eV, and 9.38 ± 0.1 eV; E0\u27\u27\u27(Γv15→Γc1), 8.33 ± 0.1 eV, and 10.27 ± 0.1 eV, E1\u27\u27, 9.5 ± 0.2 eV, and 10.7 ± 0.2 eV. E5, E6, and E7 structures are observed at 15.1, 16.7, and 17.9 eV in GaAs, and at 14.7, 16.1, and 18.6 eV in GaP. Relative values of 3d core to sp3 conduction-band matrix elements are estimated for several states and show that the lowest 3d core-level ER structures arise from transitions terminating at the Xc1conduction-band minimum. We calculate an exciton or core-hole interaction shift of 150 meV for GaP and 200 meV for GaAs, which indicates that core-hole effects are probably small for these materials. Spectral features with initial structure less than 100 meV in width are observed above 20 eV, showing that broadening effects are much smaller in this energy range than previously believed

Temperature Coefficients of Energy Separations between Ga 3d Core Levels and sp3 Valence-Conduction Bands in GaP

The measured temperature coefficients of the energy separations between the Ga 3d core levels and the top (Γ8V) and bottom (X6C) of the sp3 valence and conduction bands in GaP between 110 K and 295 K are (+1.0±0.5)×10−4 eV K−1and (-2.4±0.5)×10−4 eV K−1, respectively. They are described within experimental accuracy by the Debye-Waller, hydrostatic, self-energy, and spatially averaged screened-ion core potential interactions of the sp3 bands alone. No significant core-level contribution is observed

Modulation spectroscopy at non‐normal incidence with emphasis on the vacuum‐uv spectral region

Expressions are given to analyze modulation spectra taken at non‐normal incidence. These expressions are used to determine the optimum angle of incidence to maximize the signal‐to‐noise ratio. Significant improvements are shown to be obtained in the vacuum‐uv spectral region by making measurements at relatively large angles of incidence. We apply these expressions to evaluate the field‐induced change in the dielectric function for the 20.5–21.0‐eV core‐level doublet in GaP from Schottky‐barrier electroreflectance data. The line shape obtained is consistent with that of a field‐modulated M 0critical point modified by a Coulomb attraction between the core hole and the excited electron