An approach to the atomic structure of bulk metallic overlayers using tip-induced layer-by-layer dissolution

Well-grown 3-D copper clusters were obtained on a Pt(111) surface at intermediate overpotentials from a sulfuric acid solution. The hexagonal and triangular clusters grew layer-by-layer and were aligned to the orientation of the substrate, indicating epitaxial growth without rotation. By using the tip as an anodic electrode, the atomic structure of each layer of the cluster was monitored locally during dissolution. The results show that in the copper clusters the lattice constant relaxes directly to the bulk lattice constant in the second layer (at least after deposition of the third layer). Additional discussion on the dissolution kinetics is also included. The same method has been used to elucidate the structure of the Cu overlayer on Pt(100).     

Low-temperature growth of epitaxial (1 0 0) silicon based on silane and disilane in a 300 mm UHV/CVD cold-wall reactor

Epitaxial (1 0 0) silicon layers were grown at temperatures ranging from 500 to 800 °C in a commercial cold-wall type UHV/CVD reactor at pressures less than 7×10⁻⁵ Torr. The substrates were 300 mm SIMOX SOI wafers and spectroscopic ellipsometry was used to assess growth rates and deposition uniformities. High-resolution atomic force microscopy (AFM) was employed to verify the atomic terrace configuration that resulted from epitaxial step-flow growth. Deposition from disilane exhibited a nearly perfect reaction limit for low temperatures and high precursor flow rates (partial pressures) with measured activation energies of ≈2.0 eV, while a linear dependence of growth rate on precursor gas flow was found for the massflow-controlled regime. A similar behavior was observed in the case of silane with substantially reduced deposition rates in the massflow-limited regime and nearly a factor of 2 reduced growth rates deep in the reaction limited regime. High growth rates of up to 50 μm/h and non-uniformities as low as 1σ=1.45% were obtained in the massflow-limited deposition regime. Silicon layers as thin as 0.6 nm (4.5 atomic layers ) were deposited continuously as determined using a unique wet chemical etching technique as well as cross-sectional high-resolution transmission electron microscopy (HRTEM). In contrast, epitaxial silicon deposited in RPCVD at 10 Torr using disilane within the same temperature range showed imperfect reaction limitation. While activation energies similar to that of UHV/CVD were found, no partial pressure limitation could be observed. Furthermore, layers deposited using disilane in RPCVD exhibited a large number of defects that appeared to form randomly during growth. We attribute this effect to gas phase reactions that create precursor fragments and radicals—an effect that is negligible in UHV/CVD.     

An investigation of the internal temperature dependence of Pd-Pt cluster beam deposition: A molecular dynamics study

We investigated the internal temperature dependence of the Pd_1−aPt_a cluster beam deposition in the present study via the molecular dynamics simulations of soft-landing. By analysis of the velocity distribution and diffusion coefficient of the bimetallic cluster, Pd atoms with better mobility improved the diffusibility of Pt atoms. The radial composition distribution showed that a Pt-core/Pd-shell structure of the cluster formed at high internal temperatures through migrations of the Pd atoms from inner to surface shells. In the soft-landing process, the diffusing and epitaxial behaviors of the deposited clusters mainly depended on the internal temperature because the incident energy of the cluster was very small. By depositing clusters at high internal temperatures, we obtained a thin film of good epitaxial growth as the energetic cluster impact. Furthermore, nonepitaxial configurations such as scattered nonepitaxial atoms, misoriented particles, and grain boundaries of (1 1 1) planes were produced in the growth of the cluster-assembled film. As the size of the incident cluster increased, the internal temperature of the cluster needed for better interfacial diffusion and contact epitaxy on the substrate also rose.     

Nanoscale observations of the epitaxial growth of hashemite on barite (0 0 1)

The heteroepitaxial growth of hashemite BaCrO₄ on barite BaSO₄(0 0 1) from supersaturated aqueous solutions was observed in situ using an atomic force microscope (AFM). It was shown that the first hashemite layer grows via two-dimensional nucleation easily forming a complete epitaxial layer, which is likely to have a low level of intrinsic stress. Two-dimensional nucleation of the second and subsequent layers proceeds with significantly lower rates, and growth occurs with lower step velocities. These layers seem to have significant level of intrinsic stress and tend to reduce it via the formation of free surface normal to the growth layer (holes in the layer, dendrite-like shape of nuclei and steps, preferable formation of nuclei at the step edges). As a result, the initially flat surface becomes rough. The process described corresponds to the Stranski-Krastanov epitaxial growth mode, which is well known for growth of semiconductor and metal films but not previously recognised for crystals grown from aqueous solutions.     

Surface composition and structure of palladium ultra-thin films deposited on Ni(1 1 1)

Ultra-thin palladium films deposited on the Ni(1 1 1) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). For low coverage, LEED shows a (1 × 1) pattern similar to that of the substrate. For intermediate coverage, the LEED pattern displays extra spots around the main (1 × 1) spots, resembling a Moiré coincidence pattern, probably associated with the formation of Pd bi-dimensional islands oriented in different directions on the Ni(1 1 1) surface. The results obtained by XPS and XPD corroborate this finding. The LEED pattern displays this structure up to 500 °C. Annealing at 650 °C brings back the (1 × 1) pattern, which is associated with a Pd island coalescence and alloy formation by Pd diffusion in the first atomic layers of the Ni(1 1 1). In this paper we present a detailed study of this surface structure via a comparison between XPD experiment and theory.     

Blue luminescence from the InGaN multiple quantum wells

We have fabricated very high-quality In_0.13Ga_0.87N/GaN multiple quantum wells with thickness as small as on (0 0 0 1) sapphire substrate using metal organic chemical vapour deposition (MOCVD). We have investigated these ultra-thin multiple quantum wells by continuous wave (cw) and time resolved spectroscopy in the picosecond time scales in a wide range of temperatures from 10 K to 290 K. In the luminescence spectrum at 10 K we observed a broad peak at 3.134 eV which was attributed to the quantum wells emission of InGaN. The full-width at half-maximum of this peak was 129 meV at 10 K and the broadening at low temperatures which was mostly inhomogeneous was thought to be due to compositional fluctuations and interfacial disorder in the alloy. The ultra narrow width of the quantum well was found to have a very profound effect in increasing the emission linewidth. We also observed an intense and narrow peak at 3.471 eV due to the GaN barrier. The temperature dependence of the luminescence was studied. The peak positions and intensities of the different peaks were obtained after a careful Lorentzian analysis. The activation energy of the InGaN quantum well emission peak was estimated as 69 meV. The lifetime of the quantum well emission was found to be 720 ps at 10 K. The results were explained by considering the localization of the excitons due to potential fluctuations. At higher temperatures the non-radiative recombination was found to be very dominant.     

Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash

In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 °C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a complex nature of this superstructure with double-hexagonal symmetry and (2√3×2√3)-R30° periodicity. Among the possible candidate phases, including metastable FeSi₂ with a CaF₂ structure and FeSi_1+x with a CsCl structure, the best match of the interatomic distances to the measured 14.4 Å × 14.4 Å unit cell dimensions pointed to the hexagonal Fe₂Si (Fe₂Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I-V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to β-FeSi₂.     

Growth and thermal evolution of submonolayer Pt films on Ru(0 0 0 1) studied by STM

The growth of submonolayer Pt on Ru(0 0 0 1) has been studied with scanning tunneling microscopy. We focus on the island evolution depending on Pt coverage θ_Pt, growth temperature T_G and post-growth annealing temperature T_A. Dendritic trigonal Pt islands with atomically rough borders are observed at room temperature and moderate deposition rates of about 5 × 10⁻⁴ ML/s. Two types of orientation, rotated by 180° and strongly influenced by minute amounts of oxygen are observed which is ascribed to nucleation starting at either hcp or fcc hollow sites. The preference for fcc sites changes to hcp in the presence of about one percent of oxygen. At lower growth temperatures Pt islands show a more fractal shape. Generally, atomically rough island borders smooth down at elevated growth temperatures higher than 300 K, or equivalent annealing temperatures. Dendritic Pt islands, for example, transform into compact, almost hexagonal islands, indicating similar step energies of A- and B-type of steps. Depending on the Pt coverage the thermal evolution differs somewhat: While regular islands on Ru(0 0 0 1) are formed at low coverages, vacancy islands are observed close to completion of the Pt layer.     

Nano-faceting of fcc(1 1 0) surfaces controlled by adsorbates and atom deposition or removal

(1 1 0) surfaces of most fcc metals are only marginally stable against faceting into (1 1 1) orientations. Trace concentrations of adsorbates (surfactants) that prefer (1 1 1) over (1 1 0) facets can tip the balance and favor faceting of all or part of the (1 1 0) surface. The growth of such facets is impeded by island and vacancy-island nucleation barriers. However, during atom deposition or etching these barriers are reduced. Growth or removal conditions and facet stabilizing surfactant concentrations control the evolving faceted nano-patterns. Recent observations of hut-shaped nanocrystals formation on Al(1 1 0) are consistent with this model.     

Temperature-dependent facet development in the deposition of GaInP on non-planar surfaces

The low-pressure (20 mbar) organometallic vapour-phase epitaxy (LP-OMVPE) of GaInP on non-planar {001} GaAs substrates has been examined. The encountered and {1 0} faceting features develop along the bottom corner and the top edge configurations of the inverted and dovetail grooves, respectively. At higher temperatures (T ≥ 720°C) these features are no longer present. The results have been compared to computer simulations of surface concentration profiles, whereby the inversely proportional relation between temperature and supersaturation, along with varying growth rate on adjacent surfaces of different crystallographic orientations, is found to be the driving force behind the occurrence of these features. The stability of the observed facets is related to the decrease in dangling-bond densities upon surface reconstruction.