Optical anisotropy induced by ion bombardment of Ag(001)

Grazing incidence ion bombardment results in the formation of nanoripples that induce an anisotropic optical reflection The evolution of the reflectance anisotropy has been monitored in situ with reflectance anisotropy spectroscopy. The Rayleigh-Rice theory (RRT) has been used to analyze the optical spectra quantitatively and provides the evolution of the average ripple period and root-mean-squared surface roughness. After an incipient phase, both the increase in the periodicity and the roughness vary roughly with the square root of the sputter time. Additional high-resolution low-energy electron diffraction (HR-LEED) measurements have been performed to characterize details of the average structure created by ion bombardment

Kinetics of the adsorption of atomic oxygen (N2O) on the Si(001)2x1 surface as revealed by the change in the surface conductance

The adsorption behaviour of N2O on the Si(001)2 × 1 surface at 300 K substrate temperature has been investigated by measuring in situ the surface conductance during the reaction process. For comparison we monitored in the same way the adsorption of O2 on the same surface which ultimately leads to the flat band situation. The adsorption of atomic oxygen as released by decomposition of the N2O molecule, in contrast with molecular oxygen, was found to result in an increase of the band bending. The difference in behaviour of the change of the surface conductance between the two solid-gas reactions can be explained by considering that the adsorption of O2 will also remove deep-lying backbond states in addition to the dangling bond (DB) and dimer bond (DM) related surface states. It is well known that only the DB and DM surface states are affected by N2O. The surface conductance measurements (SCM) presented in this paper complement our previous spectroscopic differential reflectivity measurements and Auger electron spectroscopic results for the system Si(001)2 × 1 + N2O; we have found evidence that the second step of the proposed three-stage adsorption process of atomic oxygen can be divided into two substages. From our SCM data we could derive that the distance between the valence band edge and the Fermi energy of the clean Si(001)2 × 1 surface is 0.32 ± 0.02 eV, which is in agreement with previous photoemission results

The influence of inter-atomic transitions in Auger valence band spectroscopy: oxygen on Si(001)2x1

In this paper we will show that the description of an Auger process in terms of a process confined to one atom is in general not adequate and the Mulliken population is only in very specific cases a good alternative in evaluating the strength of inter-atomic transitions. The ionicity of the chemical bond cannot be used as a direct measure of the contribution of inter-atomic Auger transitions, as will be demonstrated in the case of the oxygen KVV Auger transitions in gaseous molecular oxygen and oxygen chemisorbed on the Si(001) surface. A full evaluation of inter-atomic transition rates shows that their strength depends on the inter-atomic distance as well as on the screening of the initial core hole

Flux heterogeniety through incidence angle and particle energy in steering-enhanced growth

During growth Van der Waals forces between the incident atoms and the substrate lead to steering with sometimes important implications for the morphology of the molecular-beam-epitaxy- or sputter grown films. Deterministic classical trajectory calculations, modelling the atom-substrate interaction with a Lennard Jones potential, have been run to provide insight in the influence of the polar angle of incidence and the energy of the atoms on steering for 1 and 3 monolayer high islands. For low energies (10¿100 meV) substantial flux redistribution has been found. A major part of the flux directed to the ascending steps is transferred on top of the protrusion, while even reversal of the lateral velocity at the descending steps occurs. At grazing incidence strong deviations from geometric shadowing behavior is observed even for energies as high as 1 to 10 eV

Plasmon resonance shift during grazing incidence ion sputtering on Ag(001)

Grazing incidence ion sputtering was used to create shallow ripple patterns on a Ag(001) surface. The anisotropic plasmon resonance associated with this ripple pattern can be sensitively measured with Reflection Anisotropy Spectroscopy. A slight red shift of the resonance energy is observed with increasing ion fluence. The observed resonance feature is described well with a skewed Lorentzian line shape. This line shape is the small roughness length scale limit of the Rayleigh Rice perturbation approach. The width of this line shape is directly related to imaginary part of the dielectric function, which shows a roughness induced reduction of the electron mean free path. The observed change in resonance energy and strength with ion fluence is discussed

Optical characterization of the evolution of ion-induced anisotropic nanopatterns on Ag(001)

Reflection anisotropy spectroscopy is used as an in situ probe for the emergence and evolution of surface patterns on Ag(001) during oblique incidence ion bombardment. The information is extracted from plasmon resonances induced by the nanoscale patterns, utilizing the fact that smooth Ag(001) is optically isotropic. The Rayleigh-Rice perturbation approach delivers the temporal development of the average periodicity and amplitude of the surface patterns. For ion bombardment at a polar angle of incidence of 70∘ along a ⟨110⟩ azimuth, strongly anisotropic surface features develop, giving rise to a single plasmon resonance, which is described well with a one-dimensional power spectral density function. For a smaller polar angle of incidence of 61.5∘ multiple plasmon resonances are observed, which demand a two-dimensional power spectral density function for a perfect description. These result compare well with high-resolution low-energy electron diffraction data, taken after ion bombardment at both angles of incidence. The optical data, obtained at 61.5∘, show coarsening and seem to suggest scaling of the periodicity and roughness, with critical exponents 0.27 and 0.40, respectively

Alloying of Pd thin films with Nb(001)

Annealing at elevated temperatures (1000–1600 K) of at least 10 ML thick Pd films deposited on Nb(0 0 1) has been found to result in a substrate capped by a pseudomorphic monolayer of Pd. This 1 ML thick Pd cap layer was characterised with a combination of UPS and DFT-calculations. UPS, RHEED and AES show that this cap layer protects the Nb(0 0 1) surface against (oxygen) contamination, which is a well known problem of Nb substrates. AES sputter profiling indicates that a major part of the Pd material in excess of the pseudomorphic monolayer is dissolved in the Nb lattice just below the surface. XPD shows that these dissolved Pd atoms occupy substitutional sites in the substrate. The analysis of the XPS-anisotropy also provides some information about the concentration and positions of the Pd and Nb atoms in the alloyed samples

Alloying, de-alloying and reentrant alloying in (sub-)monolayer growth of Ag on Pt(111)

An in-situ nanoscopic investigation of the prototypical surface alloying system Ag/Pt(111) is reported. The morphology and the structure of the ultrathin Ag-Pt film is studied using Low Energy Electron Microscopy during growth at about 800 K. An amazingly rich dynamic behaviour is uncovered in which stress relieve plays a governing role. Initial growth leads to surface alloying with prolonged and retarded nucleation of ad-islands. Beyond 50% coverage de-alloying proceeds, joined by partial segregation of Pt towards the centre of large islands in violent processes. Upon coalescence the irregularly shaped vacancy clusters are filled by segregating Pt, which then take a compact shape (black spots). As a result at around 85% coverage the strain of the initially pseudo-morphological film is almost completely relieved and Pt-segregation is at its maximum. Further deposition of Ag leads to transient re-entrant alloying and recovery of the pseudo-morphological layer. The black spots persist even in/on several layers thick films. Ex-situ atomic force microscopy data confirm that these are constituted by probably amorphous Pt(-rich) structures. The (sub-)monolayer films are very much heterogeneous