Study on low-energy bombardment of Au (1 1 1) by noble metal atoms with molecular dynamics simulations

The low-energy bombardment of Au (1 1 1) surface by noble metal atoms is studied with molecular dynamics (MD) simulations. With the incident-energy dependence of adatom yields, sputtering yields, and vacancy yields for different projectiles, we find that the implantation of projectiles in shallow layers below surface can be distinguished by subplantation (in the first and second layers) and implantation (deeper than the third layer). The transition from subplantation to implantation occurs at the incident energy of about 45 eV for the low-energy bombardment of noble metal atoms on Au (1 1 1). The incident-energy dependence of defect yields is obviously different for the subplantation and implantation of projectiles. Based on our MD simulations, we discuss the influence of low-energy bombardment on film growth and the guide to the search for optimum deposition parameters.     

Atomic-level investigation of Al and Ni thin film growth on Ni(1 1 1) surface: Molecular dynamics simulation

Using molecular dynamics (MD) simulation, the structural characteristics of Al and Ni thin film growth on Ni(1 1 1) substrate according to the incident energy of adatoms were investigated. In case of Al on Ni(1 1 1), Al adatoms were grown basically through the layer-by-layer growth mode. On the other hand, Ni thin films on Ni(1 1 1) surface at low incident energy were shown to favor island growth. The steering effect due to atomic attraction, which results in rougher surface, was significantly observed at low incident energy. The growth mode of Ni film was, however, changed to follow layer-by-layer growth mode for the incident energy of 6 eV. The different aspects of surface morphology between Al and Ni deposition on Ni(1 1 1) surface could be successfully explained by the surface diffusion and impact cascade diffusion.     

Hydrocarbon film growth by energetic CH3 molecule impact on SiC (0 0 1) surface

Classic molecular dynamics (MD) calculations were performed to investigate the deposition of thin hydrocarbon film. SiC (1 0 0) surfaces were bombarded with energetic CH₃ molecules at impact energies ranging from 50 to 150 eV. The simulated results show that the deposition yield of H atoms decreases with increasing incident energy, which is in good agreement with experiments. During the initial stages, with breaking Si-C bonds in SiC by CH₃ impacting, H atoms preferentially reacts with resulting Si to form Si-H bond. The C/H ratio in the grown films increases with increasing incident energy. In the grown films, CH species are dominant. For 50 eV, H-Csp3 bond is dominant. With increasing energy to 200 eV, the atomic density of H-Csp2 bond increases.     

Bi induced step-flow growth in the homoepitaxial growth of Au(1 1 1)

Homoepitaxial growth of Au on Bi-covered Au(1 1 1) was studied at room temperature using reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). From observations of RHEED it is found that the Au(1 1 1) (23 × 1) reconstruction structure changes to a (1 × 1) by about 0.16-0.5 ML deposition of Bi and to a (2√3 × 2√3)R30° by about 1.0 ML deposition of Bi, respectively. The surface morphology evolution by Bi deposition leads to a change of Au homoepitaxial growth behavior from layer-by-layer to step flow. This indicates that the surface diffusion distance of Au atoms on the Bi-precovered (1 × 1) and (2√3 × 2√3)R30° surfaces is longer than that on the Au(1 1 1) (23 × 1) clean surfaces. A strong surface segregation of Bi was found at top of surface. It is concluded that Bi atoms acted as an effective surfactant in the Au homoepitaxial growth by promoting Au intralayer mass transport.     

Simplified method to prepare atomically-ordered TiO2(1 1 0)-(1 × 1) surfaces with steps and terraces

An effective way to prepare atomically-ordered rutile TiO₂(1 1 0) surfaces that have distinct step and terrace structures suitable for oxide thin film deposition is demonstrated. Only a two-step procedure, consisting of 20% HF etching and UHV-annealing at 1100 °C, was required to yield a clean (1 × 1) structure with step and terrace structures. Investigation of the surface using scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy reveals that carbon contamination is removed at around 800 °C, and straight steps with clear terraces appear at around 1000 °C.     

Molecular dynamics investigation of deposition and annealing behaviors of Cu atoms onto Cu(0 0 1) substrate

The deposition growth and annealing behaviors of Cu atoms onto Cu(0 0 1) are investigated in atomic scale by molecular dynamics (MD) simulation. The results indicate that the film grows approximately in a layer-island mode as the incident energy is from 1 to 5 eV, while surface intermixing can be significantly observed at 10 eV. The surface roughness of the film decreases with increasing the incident energy, and the film after annealing becomes smoother and more ordered. These phenomena may be attributed to the enhanced atomic mobility for higher incident energy and thermal annealing. It also indicates that atomic mixing is more significant with increasing both the incident energy and substrate temperature. In addition, the peak-to-peak distances of radial distribution function (RDF) clearly indicate that the films before and after annealing are still fcc structure except for that at the melting temperature of 1375.6 K. After annealing, the film at the melting temperature returns to fcc structure instead of amorphous. Moreover, the residual stress and Poisson ratio of the film are remarkably affected by the thermal annealing. Furthermore, the density of thin film is obviously affected by the substrate temperature and annealing process. Therefore, one can conclude that high incident energy, substrate temperature and thermal annealing could help to enhance the surface morphology and promote the microstructure of the film.     

Femtosecond pulsed laser deposition of Ge quantum dot growth on Si(1 0 0)-(2 × 1)

Ge quantum dots were grown on Si(1 0 0)-(2 × 1) by femtosecond pulsed laser deposition at various substrate temperatures using a femtosecond Ti:sapphire laser. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to analyze the film structure and morphology. The morphology of germanium islands on silicon was studied at different coverages. The results show that femtosecond pulsed laser deposition reduces the minimum temperature for epitaxial growth of Ge quantum dots to ∼280 °C, which is 120 °C lower than previously observed in nanosecond pulsed laser deposition and more than 200 °C lower than that reported for molecular beam epitaxy and chemical vapor deposition.     

Theoretical and experimental studies of the structure and dynamics of the CaF2(1 1 1) surface

The structure and dynamics of the CaF₂(1 1 1) surface were investigated by means of low-energy electron diffraction (LEED) and molecular dynamics (MD) simulations at 300 K. LEED beam intensities were recorded as a function of electron energy and were analyzed with the tensor LEED approach. Positions as well as mean square amplitudes of the ions in the first layers were fitted to the experimental I(E) curves. According to both LEED and MD, the CaF₂(1 1 1) surface structure is similar to the bulk-terminated structure with only small relaxation of the outermost ions. Moreover, both methods show an enhancement of vibrational amplitudes in the outermost F-Ca-F triple layer.     

Surface structures of K on Mo(1 1 0) surface investigated by RHEED

Phase transitions of K on Mo(1 1 0) have been studied by RHEED technique. As Ba and Cs structures on the bcc(1 1 0) surface, surface structures of K were hexagonal from RT to 250 °C for θ > 0.9 ML. The hexagonal structure successively expanded from α to γ structure with Nishiyama-Wassermamm (N-W) orientation relationship. The nearest neighbor spacing in the α structure at RT was 4.50 Å, which is very closed to the atomic distance of K in metal, and stretched to 5.14 Å in the γ structure at T = 200 °C. At temperatures greater than T = 250 °C, the γ structure oriented in N-W and Kurdjumov-Sachs (K-S) relationships at the same time and stayed up to the temperature of 450 °C. These two orientations of γ structure also appeared in all temperature range for 0.4 < θ < 0.9 ML.     

Characterization of bicrystalline epitaxial LaNiO3 films fabricated on MgO (1 0 0) substrates by pulsed laser deposition

A series of metallic LaNiO₃ (LNO) thin films were deposited on MgO (1 0 0) substrates by pulsed laser deposition (PLD) under the oxygen pressure of 20 Pa at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD) was used to characterize the crystal structure of LNO films. θ-2θ scans of XRD indicate that LNO film deposited at a substrate temperature of 700 °C has a high orientation of (l l 0). At other substrate temperatures, the LNO films have mixed phases of (l l 0) and (l 0 0). Furthermore, pole figure measurements show that LNO thin films, with the bicrystalline structure, were epitaxially deposited on MgO (1 0 0) substrates in the mode of LNO (1 1 0)//MgO (1 0 0) at 700 °C. Reflection high-energy electric diffraction (RHEED) and atomic force microscopy (AFM) were also performed to investigate the microstructure of LNO films with the high (l l 0) orientation. RHEED patterns clearly confirm this epitaxial relationship. An atomically smooth surface of LNO films at 700 °C was obtained. In addition, bicrystalline epitaxial LNO films, fabricated at 700 °C, present a excellent conductivity with a lower electrical resistivity of 300 μ Ω cm. Thus, the obtained results indicate that bicystalline epitaxial LNO films could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Effect of surface steps on sputtering and surface defect formation: molecular-dynamics study of 5 keV Xe bombardment of Pt(1 1 1) at glancing incidence angles

Using molecular-dynamics simulation, we study sputtering and defect formation induced by 5 keV Xe⁺ ion impact on a Pt(1 1 1) surface at oblique and glancing incidence angles. Impact on a terrace produces yield maxima at ?=60-65° incidence angle towards the surface normal. Beyond 75-80°, no damage is produced due to projectile ion reflection. Impact on a dense-packed step, however, produces defects in sizeable numbers up to glancing incidence, ?=85°. The dependence of the yields on the incidence angle and distance of the impact point of the projectile to the step are discussed.     

Observation of a (√3 × √3)-R30° reconstruction on GaN(0 0 0 1) by RHEED and LEED

A new reconstruction of √3 × √3-R30° has been observed on a GaN film grown on a 6H-SiC (0 0 0 1)-√3 × √3 surface using RHEED and LEED experimental techniques. The experimental LEED PF shows that the GaN film is Ga-terminated hexagonal. The surface is a mixture of two structures with a single bilayer height difference between them. One is a √3 × √3-R30° reconstruction with Ga-adatoms occupying the T4 sites. Another is a Ga-terminated 1 × 1 with no extra Ga on top. The area ratio of the √3 × √3 part to the 1 × 1 part is slightly larger than 1. The first principle total energy calculations and Tensor-LEED I-V curves simulations further confirm this structure model.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Angle-resolved study of hydrogen abstraction on Si(1 0 0) and Si(1 1 1): Evidence for non-activated pathways

The abstraction of chemisorbed hydrogen on Si(1 0 0) and Si(1 1 1) induced by atomic hydrogen has been investigated by studying with a rotatable mass spectrometer the angle-resolved molecular hydrogen desorption from a Si surface exposed to a chopped beam of atomic hydrogen. The angular distributions of desorbing molecules can be fitted independent of the surface temperature and the surface reconstruction by a cosⁿθ function with n < 1 for Si(1 0 0) and Si(1 1 1). These results are interpreted by non-activated pathways involving site-specific hot-atom abstraction on two adjacent silicon atoms with one having a dangling bond. Possible mechanisms according to the surface reconstructions are discussed.     

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880-920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Electronic structure study of reconstructed Au-SiC(0 0 0 1) surfaces

A study of surface and interface properties of reconstructed Au-SiC(0 0 0 1) surfaces is reported. Two reconstructions were prepared on SiC(0 0 0 1), a √3 × √3R30° and a Si-rich 3 × 3, before Au deposition and subsequent annealing at different temperatures. For the Si-rich 3 × 3 surface the existence of three stable reconstructions 2√3 × 2√3R30°, 3 × 3 and 5 × 5 are revealed after deposition of Au layers, 4-8 Å thick, and annealing at progressively higher temperatures between 500 and 950 °C. For the 2√3 surface two surface shifted Si 2p components are revealed and the Au 4f spectra clearly indicate silicide formation. The variation in relative intensity for the different core level components with photon energy suggests formation of an ordered silicide layer with some excess Si on top. Similar core level spectra and variations in relative intensity with photon energy are obtained for the 3 × 3 and 5 × 5 phases but the amount of excess Si on top is observed to be smaller and an additional weak Si 2p component becomes discernable.For the √3 surface the evolution of the core level spectra after Au deposition and annealing is shown to be distinctly different than for the Si-rich 3 × 3 surface and only one stable reconstruction, a 3 × 3 phase, is observed at similar annealing temperatures.     

Methylthiolate-induced reconstruction of Ag(1 1 1): A medium energy ion scattering study

Medium energy ion scattering (MEIS), using 100 keV H⁺ incident ions, has been used to investigate the structure of the Ag(1 1 1)(√7 × √7)R19° -CH₃S surface phase. The results provide the first direct evidence that this structure does involve substantial reconstruction of the Ag surface layer. The measured absolute scattered ion yields and blocking curves are in generally good agreement with a specific structural model of the surface based on a reconstructed layer containing 3/7 ML Ag atoms, previously suggested on the basis of scanning tunnelling microscopy (STM) and normal incidence X-ray standing wave (NIXSW) studies. However, the MEIS data indicate that any rumpling of the thiolate layer, is small, and probably ?0.2 Å. This value is smaller than the amplitude suggested in the STM and NIXSW studies, but could be entirely consistent with the earlier experimental data.