Session R12: Dynamics at silicon surfaces

Electron Phonon Interaction at the $\mbox{Si(111)-7}\times \mbox{7}$ Surface

In recent years, it has become possible to study the electron-phonon interaction of metals by high resolution photoelectron spectroscopy. However, virtually no such experimental information is available for the case of semiconductors. It is shown that electron-phonon interaction at the $\mbox{Si(111)-7}\times \mbox{7}$ surface provides a natural explanation for the unusual band dispersion of the metallic surface states. While local density theory predicts a hole-like dispersion for the adatom bands that cross the Fermi level $E_F $, photoemission finds an electron-like band with a break in the slope at 0.1~eV below $E_F $. Such behavior follows naturally from electron phonon interaction with the surface phonon mode at $\hbar \omega =70\mbox{ meV}$ that dominates electron energy loss spectra. It has been assigned to adatom vibrations by molecular dynamics calculations. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and the temperature dependent phonon broadening yield very similar coupling parameters around $\lambda =1.1$.   

Ultrafast Electron Dynamics of Silicon Surface States: Second-Harmonic Hole Burning Spectroscopy on Si(111)7x7

We used second harmonic generation (SHG) as an all-optical, surface-specific probe of spectral hole-burning to measure the ultrafast dynamics of electronic excitations into the adatomU1 band of the Si(111)7x7 surface. The time-delayed SHG obtained with tunable $\sim $100 fs probe pulses was measured in response to the excitation of a $\sim $100 fs pump pulse at 1.54 eV. The transient holes induced by the pump corresponded to a homogeneous dephasing time as short as 15 fs. Carrier-carrier scattering was responsible for the linear dependence of the dephasing rate on pump fluence. The hole-burning spectra also reveal a strong coupling between the localized dangling-bond states and the surface phonon mode at 570 cm$^{-1}$ associated with the adatoms. The technique can be extended to measurement of electron dynamics at any interface between two centrosymmetric media accessible to light.   

Silicon surface evolution at high temperature in UHV.

The step distribution, associated with the miscut of a Si wafer can be modified by controlled step flow. There are many studies of step-flow dynamics during sublimation or deposition at high temperature. In our group, atomically flat, step-free surfaces were previously obtained over areas of up to 50 by 50 microns by annealing Si samples in UHV. The surface of a normal crystal wafer does not usually show a step-terrace structure. Instead, a Czochralski Si wafer, mirror polished by chemical-mechanical polishing, has an rms roughness of 1-2 nm. A sizable thermal budget has to be used to remove this initial roughness or that induced by reactive ion etching. We report here results of the evolution of step distributions over a range of temperatures. By inducing a temperature gradient on our Si samples we were able to study the stages that occur as the surface transforms towards an atomically flat one. The samples were later reannealed to see the morphological changes that occurred on previously examined regions. We have also investigated the processes of trench formation near the walls of etched craters and of ridge development around the edges of mesas structures.   

Interacting dimer rows on Si(001): Reconstructed Si(001)

A continuum model involving elastic interactions of dimer rows on a two-dimensional surface is presented. It produces a logarithmic stress-domain interaction energy from the obtained partition function. Using dimer rows as the building blocks of a reconstructed surface thus generalizes Alerhand and Marchenko models of the stress- domain interaction on terraces, which have been previously used to predict step- height transition on reconstructed Si(001). The double-layer step phase is determined to be more stable that the single-layer step phase for typical temperatures and miscut angles. A mixed phase region in the temperature-versus-miscut-angle phase diagram is found.   

Gold on silicon: Can theory and experiments for Si(111)-(5x2)-Au be reconciled?

Since its discovery 30 years ago, the 5x2 reconstruction of Si(111) induced by 2/5 monolayer of Au has been widely studied for its unusual one-dimensional electronic properties. Despite quite detailed and consistent data from a variety of surface science techniques, there is still no structural model consistent with the constraints inferred from those data. Here we combine density-functional theory (DFT) and scanning tunneling microscopy (STM) data to systematically develop and investigate new candidate structural models. The underlying 5x2 parent reconstruction is decorated by Si adatoms, whose density is at most one adatom per two 5x2 unit cells, and at equilibrium is half of this limit. We consider first the adatom-free parent reconstruction. Starting from simple principles of dangling bonds and surface stress relief, we construct a well-defined set of chemically plausible models that can be exhaustively generated by combinatorics. Several recently proposed models for Si(111)-(5x2)-Au are examples from this set. We then consider the role of the Si adatoms. We find that their role is two-fold: to passivate dangling bonds and to dope the parent band structure. Using the complete model, detailed predictions for STM imagery and surface band structure compare favorably with experiment.   

Au-Induced Reconstructions on the Si(111) Surface

By depositing Au onto a Si(111)-(7$\times $7) surface at elevated temperatures, the existence of (5$\times $2), ($\sqrt 3 \times \sqrt 3 )$, and (6$\times $6) reconstructions have long been studied as a function of Au coverage. The corresponding structures however are yet to be satisfactorily determined. By using scanning tunneling microscopy (STM), we have investigated these Au-induced surface structures at LN2 temperature. With STM image quality exceeding currently published results, we are able to identify fine details in those reconstructions, which may shed new light on constructing structural models.   

Ab Initio Studies of Indium Diffusion on the Si(111)-7x7/Ge(111)-5x5 Surface

The mechanisms for the self-assembly of ordered arrays of indium magic clusters on the Si(111)-7x7/Ge(111)-5x5 surface have been investigated theoretically using ab initio total energy simulations. Plane wave density functional methods have enabled us to obtain the potential energy surface for one indium atom adsorbed on the Ge(111)-5x5 surface. Using these ab initio-derived energies as input, a classical molecular dynamics simulation has yielded values for the temperature-dependent, trajectory-dependent, microscopic self-diffusion coefficients of an indium atom on the Ge(111)-5x5 surface. For comparison, we also have calculated the diffusion coefficient obtained through fully ab initio molecular dynamics where we have used the non-self-consistent Harris Functional due to computational cost. Also, as a third approach, the harmonic approximation of the transition state theory has been applied and is assessed in light of the two molecular dynamics simulations. The single-atom results show promise for providing useful information on how the magic clusters on semiconductor surfaces may be formed.   

Evolution of Si Surface Morphology under Oxygen Etching

We have studied the surface morphologies produced after oxygen etching of the following Si surfaces: (001), (111), (113), (5 5 12), and (112). Atomic force microscopy data show the evolution of the surface morphology as a function of dosage (50 to 400 Langmuirs) for sample temperatures from 700 to 900 \r{ }C and a pressure of 3x10$^{-7}$ Torr. We have found that certain orientations are relatively stable against extended etching, whereas others are unstable and produce faceted morphologies. The (001), (111), and (113) surface orientations are stable and produce morphologies composed of terraces with islands caused by etching around oxide-induced pinning sites. As expected, the island density decreases as temperature increases, yielding an effective activation energy of 2 to 4 eV. High-index surfaces such as (5~5~12) and (112), however, are unstable against extended etching and produce faceted sawtooth morphologies. These sawtooths are aligned along the [\underline {1}10] direction and are primarily composed of the more stable (111) and (113) planes. Further studies are in progress to determine if steady-state morphologies exist at dosages above 1000 Langmuirs.   

Dissociation pathways and multiple adsorption configurations of ammonia on Si(001)

The adsorption of ammonia (NH$_{3})$ molecules on the Si(001) surface has been investigated by combining the scanning tunneling microscopy (STM) and the \textit{ab initio} pseudopotential calculations. An NH$_{3}$ molecule had been known to spontaneously dissociate into NH$_{2}$ and H and form the so-called \textit{on-dimer }configuration (OD). In OD, NH$_{2}$ and H bond with two Si atoms of one Si dimer. However, our study shows that there exist \textit{two }distinct configurations of NH$_{3}$/Si(001). One is the existing model of OD, which is found to be the lowest energy state. The other is identified to be an \textit{inter-dimer }configuration (ID), where NH$_{2}$ and H adsorb at two Si atoms at the same side of two adjacent dimers along the dimer row. The simulated STM images of OD and ID are in good agreement with experimental observations, proving the existence of two distinct adsorption configurations of NH$_{3}$/Si(001).   

Desorption of Hydrogen from Si(111) by Resonant Excitation of the Si-H Vibrational Stretch Mode

We report first measurements of non-thermal, non-electronic resonant photodesorption of hydrogen from a Si(111) surface using tunable infrared radiation. The wavelength dependence of the desorption yield Shows a peak at 0.26 eV, the energy of the Si-H vibrational stretch mode. The corresponding excitation cross section is determined to be $2.3\times10^{-18}$ cm$^2$. The desorption yield is quadratic in the infrared intensity and quartic in the electric field along the Si-H bond direction. We postulate that two neighboring hydrogen adatoms, once simultaneously excited in their vibrational stretch mode, can desorb associatively at room temperature.   

Empirical modeling of STM imaging and tunneling spectroscopy of adsorbates on the Si(111)7x7 surface.

To accurately simulate experimentally measured STM results have developed an atomic orbital model for the top layer atoms of the Si(111)7x7 surface. Adatom, restatom and corner hole dangling bonds are represented by sp$^{3}$ hybrids of hydrogen-like orbitals adjusted to have the correct work function and including tip-induced electric field effects. Peak energies, widths and other DOS information, were extracted from known experimental data and theoretical calculations.$^{1}$ Restatom and adatom charge transfer was included. Back-bonding orbitals for adatoms, restatoms and dimers were also used. Bulk DOS data was used for deeper bonding orbitals. The Tersoff-Hamann approach is used calculate the tunneling current.$^{2}$ We will present numerically calculated topographic images as well as spectra and compare to experiments. [1]Brommer \textit{etal}, \textit{Surf. Sci.} \textbf{314}, 57 (1994). [2]Tersoff and Hamann, \textit{Phys. Rev. B,} \textbf{31}, 805 (1985).   

Chemical fingerprinting at the atomic level with scanning tunneling spectroscopy

Atomic-scale chemical analysis has been a long-standing goal of materials characterization. It requires the ability to interrogate individual atoms and to recognize their distinguishing signatures. Scanning tunneling spectroscopy based on scanning tunneling microscopy makes it possible to map the local electronic density of states for clean surfaces and for those with adsorbates at 80 K. We have developed a protocol that allows us to obtain the spectral fingerprints of halogen atoms on Si(001), and we use those fingerprints to distinguish between adatom species for surfaces with Cl and Br mixed adsorbates. The key to the process is the energy distribution of the antibonding states that depend on the halogen species. The protocol developed here, with its emphasis on the distribution and distinction of energy states, should be applicable to other systems, and should yield new insights regarding chemical identification.   

Spin echo resolved grazing incidence neutron scattering

Conventionally the spin-echo technique is used to encode energy transfer in neutron inelastic scattering experiments. However, spin-echo may encode the neutron momentum transfer, allowing the probing of large length scale structures without the need of a tight neutron beam collimation and resulting intensity loss. This concept has recently been successfully utilized for small angle scattering experiments in transmission geometry as well as scattering in the grazing incidence geometry at a reflectometer (EVA, Institut Laue-Langevin, Grenoble) equipped with neutron resonance spin echo circuits as well as appropriate neutron spin rotators. The tests included the measurement in transmission geometry of suspensions of polystyrene balls (1500 {\AA} radius) as well as anodized aluminum oxide 2D gratings, as well as the measurement in reflection geometry of the characteristic lengths of assemblies of polystyrene droplets dewetted from a silicon surface. The results are guiding the design of an instrument for the Spallation Neutron Source capable of studying the structure and working of biological membranes.   

Epitaxial growth and structure of the high-k dielectric Pr$_{2}$O$_{3}$ on Si(001) and Si(111)

Keeping the capacitance of CMOS devices constant while shrinking further the gate dimensions requires intolerably thin SiO$_{2}$ layers for next generation devices. Thus, replacing SiO$_{2 }$with a material with larger static permittivity k and matching properties is at present a very urgent task. Among others, the sesquioxide Pr$_{2}$O$_{3}$ is discussed as a possible candidate material. We deposited ultra thin films of Pr$_{2}$O$_{3 }$by e-beam evaporation$_{ }$on atomically clean (001) and (111) surfaces. We studied the growth from the very early stages using low energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, grazing incidence X-ray diffraction, X-ray reflectivity and X-ray photoelectron spectroscopy. On Si(111), Pr$_{2}$O$_{3 }$grows pseudomorphically in the hexagonal phase. On Si(001) surface, a 0.6 nm thick layer of cubic Pr$_{2}$O$_{3 }$ grows followed by the formation of a silicate layer on top. Growth under 10$^{-8}$ mbar O$_{2}$ atmosphere is needed in order to prevent Pr-silicide formation.