AFM tip-induced ripple pattern on AIII-BV semiconductor surfaces

Modification of c(8x2) InSb(0 0 1) surface induced by prolonged scanning with an atomic force microscope tip has been investigated. The experiment performed with loads of few tens of nanoNewtons resulted in creation of ripples perpendicular to the fast scan direction. It was found that terrace edges are acting as initial instabilities leading to development of the ripple pattern. As a result, information about initial surface topography is preserved in the ripple amplitude, even so the final height of the ripples and their periodicity are determined by the tip curvature.     

Thermal stability of SiGe films on an ultra thin Ge buffer layer on Si grown at low temperature

The thermal stability of SiGe films on an ultra thin Ge buffer layer on Si fabricated at low temperature has been studied. The microstructure and morphology of the samples were investigated by high-resolution X-ray diffraction, Raman spectra and atomic force microscopy, and using a diluted Secco etchant to reveal dislocation content. After thermal annealing processing, it is observed that undulated surface, threading dislocations (TDs) and stacking faults (SFs) appeared at the strained SiGe layer, which developed from the propagation of a misfit dislocation (MD) during thermal annealing, and no SFs but only TDs formed in strain-relaxed sample. And it is found that the SiGe films on the Ge layer grown at 300 °C has crosshatch-free surface and is more stable than others, with a root mean square surface roughness of less than 2 nm and the threading dislocation densities as low as ∼10⁵ cm⁻². The results show that the thermal stability of the SiGe films is associated with the Ge buffer layer, the relaxation extent and morphology of the SiGe layer.     

Calculated influence of work function on SE escape probability and Secondary Electron Emission yield

The influence of changes of the work function, ?, or electron affinity, χ, on the escape probability, A, of Secondary Electrons, SE, is derived from their angular and energy distributions, respectively ∂δ/∂α and ∂δ/∂E_k. Based on the evaluation of the spectral distribution of inner SEs, the present approach quantifies the dominant role of potential barrier on the SE emission and its change with surface treatments or thin film deposits. For instance it is shown that a 1 eV-increase of ? for Au leads to a decrease of A, and then of SE emission yield, δ, of about 50% while a 0.4 eV-increase of χ for potassium chloride induces a decrease of a factor 4 for A and then for the SEE yield δ. These results are summarized by empirical expressions of form A/A° = (?/?°)⁻³ for Au and A/A° =(χ/χ°)^−3/2 for KCl. Applied here to an insulating sample and to a metal, the present approach may be easily transposed to any kind of material of known Fermi energy and work function, metals, or known affinity, semiconducting and insulating samples. The large SEE yield values of inorganic insulators relative to that of metals are explained by larger values of their escape probability A - KCl: A° ∼ 25% for χ° = 0.6 eV; Au: A° ∼ 4% for ?° = 3.5 eV - combined to larger SE attenuation lengths and despite a less SE generation factor. This approach underlines the significant role of A on the large deviations between SEEY data as reported in literature and a strategy combining in situ δ and ? measurements is suggested to partly compensate the corresponding dispersion of experimental results. The present approach may be transposed to other energetic projectiles such as X-rays or ions and some practical consequences related to Scanning Electron Microscopy, mechanisms of contamination and crystalline contrasts, are pointed out.     

Fluorination of perovskite-related phases of composition La1−xSrxFe1−yCoyO3−δ

We report here on the fluorination of the perovskite-related phases La_1−xSr_xFe_1−yCo_yO_3−δ. The introduction of fluorine in place of oxygen is achieved through a low-temperature (400 °C) reaction with poly(vinylidene fluoride). X-ray powder diffraction data show that in all cases the fluorination leads to an expansion in the unit cell, which is consistent with partial replacement of oxygen by fluorine and consequent reduction in the oxidation state of iron and/or cobalt. This reduction in oxidation state is confirmed by X-ray absorption- and Mössbauer-spectroscopy. The Mössbauer spectra show complex magnetically split hyperfine patterns for the fluorinated samples, reflecting the interactions between Fe³⁺ ions, which are not possible in oxides containing Fe⁴⁺.     

Morphology of sol-gel produced composite films for optical oxygen sensors

The results reported concern the characterization of thin layer SiO₂-based matrices with an oxygen sensing component Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline) immobilized, when a sol-gel process along with dip- and spin-coating deposition methods are used.SEM, TEM and AFM study, assisted by X-ray energy dispersive microanalysis reveals the influence of the precursors used, sol treatment and the coating conditions on the films morphology and Ru distribution in the matrices. Uniform and smooth surface is produced from tetraethoxysilane (TEOS). The presence of ormosils (methyltriethoxysilane, MtEOS and octyltriethoxysilane, OtEOS) significantly increases the surface roughness exhibited as dots on the SEM image. Their surface concentration and size depend on the number of immersions and withdrawal speed at the dip coating. Spin deposition leads to rather different morphology of the films, based on TEOS/OtEOS. Following commonly used sol preparation procedure (with 1.25-2.5 g Ru-complex/dm³ sol) microcrystallization of the complex occurs with formation of randomly distributed crystals 100-400 nm in size. The ultrasound treatment of the sol by means of ultrasound disintegrator leads to homogeneous distribution of the complex without observable crystallization and significant improvement of the film sensing properties (increase of Stern-Volmer constant and better linearity of the Stern-Volmer plots both in gaseous and aqueous media).     

Simultaneous imaging for surface and internal structure of polymer blend thin films

A novel experimental technique for three-dimensional (3D) visualization of phase-separated structure of polymer blend thin film was proposed. Polystyrene/poly(methyl methacrylate) (PS/PMMA) blend thin films with the thickness of approximately 100 nm were cut at extremely low angle by utilizing surface and interface cutting analysis system (SAICAS), and the cross-section was exposed as gradient surface with the width of approximately 2.5 μm. SFM investigation for the grazing cross-section imaged the detailed internal and surface phase separated structure of the (PS/PMMA) blend thin films on one image.     

Structural investigation of n-hexadecanoic acid multilayers on mica surface: Atomic force microscopy study

The structure of n-hexadecanoic acid (HA) multilayers formed by spreading an ethanol solution containing this molecule onto a freshly cleaved mica surface has been studied by atomic force microscopy (AFM). AFM images of multilayers obtained with different coating time showed that HA molecules first formed some sporadic domains on mica surface. With the proceeding of the coating process, these domains gradually enlarged and coalesced, until formed a continuous film finally. It was observed that HA molecules were always adsorbed on mica surface with tilted even-numbered layers structure. The height of the repeated tilted bilayer film was measured to be approximately 3.8 ± 0.2 nm, which implied a ∼60° tilt molecular conformation of the HA bilayers on mica surface. Phase image confirmed that the HA multilayers terminated with the hydrophilic carboxylic acid groups. The formation mechanism of the HA multilayers was discussed in detail. Thus, resulted hydrophilic surfaces are of special interest for further study in biological or man-made member systems.     

Impact of surface step heights of 6H-SiC (0 0 0 1) vicinal substrates in heteroepitaxial growth of 2H-AlN

Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H-SiC (0 0 0 1) vicinal substrates by high-temperature gas etching. 2H-AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed.     

Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification

A superhydrophobic ZnO nanorod array film on cotton substrate was fabricated via a wet chemical route and subsequent modification with a layer of n-dodecyltrimethoxysilane (DTMS). The as-obtained cotton sample was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning probe microscope (SPM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the cotton fabric sample was also studied by contact angle measurements. The modified cotton fabrics exhibited superhydrophobicity with a contact angle of 161° for 8 μL water droplet and a roll-off angle of 9° for 40 μL water droplet. It was shown that the proper surface roughness and the lower surface energy both played important roles in creating the superhydrophobic surface, in which the Cassie state dominated.     

Passivation of structured p-type silicon interfaces: Effect of surface morphology and wet-chemical pre-treatment

The effect of both surface morphology and wet-chemical pre-treatment on electronic surface and interface properties was investigated for mono- and polycrystalline silicon substrates with special surface structures. Surface charge, energetic distribution, and density of rechargeable states on these surfaces were determined by surface photovoltage (SPV) measurements. These results were correlated to previously reported findings on atomically flat Si(111) and Si(100) surfaces of monocrystalline wafers. In this paper, a specially optimised sequence of cleaning, wet-chemical oxidation, and oxide removal procedures is described in detail for the first time. This method was successfully applied in order to remove contaminations and damaged surface layers and to obtain atomically flat areas on substrates with evenly distributed atomic steps, polycrystalline and monocrystalline substrates with randomly distributed pyramids. A significant reduction in surface micro-roughness, interface state density, and recombination loss was achieved. Using passivation by wet-chemical oxidation or H-termination, respectively, the optimised surface state can be preserved by the time of following preparation steps and during subsequent a-Si:H plasma enhanced chemical vapour deposition (PECVD).