Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

Time evolution of interface roughness during thermal oxidation on Si(0 0 1)

The surface morphological change at an initial stage of thermal oxidation on Si(0 0 1) surface with O₂ was investigated as a function of oxide coverage by a real-time monitoring method of Auger electron spectroscopy (AES) combined with reflection high energy electron diffraction (RHEED). At 653 °C where oxide islands grow laterally, protrusions were observed to develop under the oxide islands as a consequence of concurrent etching of the surface. The rate of etching was measured from a periodic oscillation of RHEED half-order spot intensity I_(1/2,0) and I_(0,1/2). At 549 °C where Langmuir-type adsorption proceeds, it was observed that both I_(1/2,0) and I_(0,1/2) decrease more rapidly in comparison with an increase of oxide coverage and the intensity ratio between them decreases gradually with O₂ exposure time. These suggest that Langmuir-type adsorption occurs at sites where O₂ adsorbs randomly, leading to subdivision of the 2×1 and 1×2 domains by oxidized regions, and that Si atoms are ejected due to volume expansion in oxidation to change the ratio between 2×1 and 1×2 domains.     

Highly stable passivation of a Si(1 1 1) surface using bilayer-GaSe

As a stable and ‘epitaxial’ passivation of a Si surface, we propose the bilayer-GaSe termination of a Si(1 1 1) surface. This surface is fabricated by depositing one monolayer of Ga on a clean Si(1 1 1) surface and subsequent annealing in a Se flux at around 520 °C, which results in unreconstructed 1×1 termination of the Si(1 1 1) surface by bilayer-GaSe. We found by scanning tunneling microscopy observation that slow cooling of the clean Si(1 1 1) surface from 850 to 520 °C with simultaneous deposition of a Ga flux results in better termination of the Si(1 1 1) surface. It was also found that this surface is stable against heating around 400 °C in O₂ atmosphere of 3×10⁻³ Pa. By utilizing these properties of the bilayer-GaSe terminated surface, we have succeeded in fabricating ZnO quantum dots on this substrate.     

Low temperature fabrication of vanadium oxide films for uncooled bolometric detectors

Vanadium oxide films with temperature coefficient of resistant of −2.6% K⁻¹ have been fabricated on Si₃N₄-film-coated Si substrates by ion beam sputtering in a controlled Ar/O₂ atmosphere, at a relatively low growth temperature of 200 °C. The as-deposited films show no semiconductor-to-metal phase transitions even heated up to 150 °C. X-ray diffractometry shows that the main compound of the VO_x film is a metastable phase of vanadium dioxide (VO₂(B)) and the VO₂(B) film can be transformed into VO₂ film by post-growth annealing at 450 °C in flowing Ar atmosphere.     

Fabrication of p-Si/β-FeSi2 balls/n-si structures by MBE and their electrical and optical properties

We report on the formation technique of single-crystalline β-FeSi₂ balls (<100 nm) embedded in a Si p–n junction region by Si molecular beam epitaxy (MBE). β-FeSi₂ films grown on Si (0 0 1) by reactive deposition epitaxy (RDE) aggregated into islands after annealing at 850°C in ultrahigh vacuum. The islands of β-FeSi₂ aggregated further into a ball shape by following the Si MBE overgrowth at 750°C. It was found from X-ray diffraction (XRD) patterns that the epitaxial relationship between the two materials, and single-crystalline nature were preserved even after the annealing and the Si overgrowth. Capacitance–voltage (C–V) characteristics and transmission electron microscope (TEM) images revealed that a lot of defects were introduced around the embedded β-FeSi₂ balls with an increase of embedded β-FeSi₂ quantity.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Solid-phase epitaxy of CaSi2 on Si(1 1 1) and the Schottky-barrier height of CaSi2/Si(1 1 1)

Thin Ca films were evaporated on Si(1 1 1) under UHV conditions and subsequently annealed in the temperature range 200–650 °C. The interdiffusion of Ca and Si was examined by ex situ Auger depth profiling. In situ monitoring of the Si 2p core-level shift by X-ray photoemission spectroscopy (XPS) was employed to study the silicide formation process. The formation temperature of CaSi₂ films on Si(1 1 1) was found to be about 350 °C. Epitaxial growth takes place at T≥400 °C. The morphology of the films, measured by atomic force microscopy (AFM), was correlated with their crystallinity as analyzed by X-ray diffraction (XRD). According to measurements of temperature-dependent I–V characteristics and internal photoemission the Schottky-barrier height of CaSi₂ on Si(1 1 1) amounts to qΦ_Bn=0.25 eV on n-type and to qΦ_Bp=0.82 eV on p-type silicon.     

Influence of interface structures on Sn thin film growth on Si(1 1 1) surface

Using coaxial impact collision ion scattering spectroscopy, we have investigated Sn thin film growth on Si(1 1 1)√3×√3-Sn and hydrogen-terminated Si(1 1 1) surfaces at room temperature. Sn formed crystalline film with β-Sn structure on Si(1 1 1)√3×√3-Sn surface, but on the hydrogen-terminated Si(1 1 1) surface, the epitaxial growth of Sn thin film was disrupted, and Sn grew as a polycrystalline film. The growth orientational relationship of the Sn film grown on Si(1 1 1)√3×√3-Sn surface was found to be

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. In the works, we found that interface structure plays a decisive role for the growth mode, crystallinity, and growth orientation of the growth of thin film.     

Elemental steps in the growth of thin β-Ga2O3 films on CoGa(1 0 0)

The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga₂O₃ films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm⁻¹. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga₂O₃ are found also on the terraces. After an exposure of 200 L O₂ at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga₂O₃.     

Effects of carbonization and substrate temperature on the growth of 3C-SiC on Si(1 1 1) by SSMBE

The growth of 3C-SiC on Si(1 1 1) substrate was performed at different carbonization temperatures and substrate temperatures by solid-source molecular beam epitaxy (SSMBE). The properties of SiC film were analyzed with in situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The best carbonization temperature of 810 °C was found to be optimal for the surface carbonization. The quality of SiC film grown on Si at substrate temperature of 1000 °C is best. The worse crystalline quality for the sample grown at higher temperature was attributed to the large mismatch of thermal expansion coefficient between SiC and Si which caused more dislocation when sample was cooled down to room temperature from higher substrate temperature. Furthermore, the larger size of single pit and the total area of the pits make the quality of SiC films grown at higher temperature worse. More Si atoms for the sample grown at lower temperature were responsible for the degradation of crystalline quality for the sample grown at lower temperature.     

A PdO(1 0 1) thin film grown on Pd(1 1 1) in ultrahigh vacuum

We present experimental results demonstrating that a high quality PdO(1 0 1) thin film can be grown on Pd(1 1 1) in ultrahigh vacuum by oxidizing the metal at 500 K using an oxygen atom beam, followed by annealing to 675 K. Low energy electron diffraction (LEED) images show that the [0 1 0] direction of the PdO(1 0 1) thin film aligns with the [−1 1 0] direction of the Pd(1 1 1) substrate, and that the PdO film grows in three degenerate domains, rotated 120° relative to one another. Based on excellent agreement between the experimental and simulated LEED patterns, we conclude that the surface structure of the PdO thin film deviates minimally from bulk-terminated PdO(1 0 1). Recent temperature programmed desorption (TPD) experiments also provide evidence that the PdO(1 0 1) thin film on Pd(1 1 1) is terminated by the stoichiometric surface in which half of the Pd atoms are coordinatively unsaturated (cus), corresponding to a cus-Pd atom density equal to about 35% of the surface density of Pd(1 1 1). The ability to generate a well-defined PdO(1 0 1) surface in ultrahigh vacuum should provide new opportunities for conducting model surface science studies of PdO, particularly studies aimed at elucidating the reactivity of PdO(1 0 1) toward species important in commercial applications of Pd catalysis.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Thermal stability of the structural features in the superhydrophobic boehmite films on austenitic stainless steels

Boehmite thin film with 50–100 nm surface flake structure has been synthesized on AISI 316 type austenitic stainless steel by immersing boehmite gel film into boiling water. When further coated with hydrolyzed (heptadecafluoro-1,1,2,2-tetrahydrodecyl) trimethoxysilane (FAS), the boehmite film becomes superhydrophobic with a contact angle for water of 152°. The superhydrophobic property results from both the nanoscale surface flake structure and the low surface energy of the FAS top layer. The topography of such film was revealed by atomic force microscope (AFM) and a set of roughness parameters of such film was discussed. The degradation of superhydrophobicity of the surface was studied as a function of the heat-treatment temperatures. Below 600 °C, the surface remained to be superhydrophobic with the FAS top layer. Above 700 °C, the surface was not superhydrophobic anymore due to a gradual loss in surface roughness which was revealed by field emission scanning electron microscope (FESEM). A phase change from boehmite to γ-Al₂O₃ occurred during the heat-treatments from 700 to 900 °C which was studied by the selected area electron diffraction (SAED) patterns from the transmission electron microscope (TEM) measurement.     

Si epitaxial growth on LaAlO3(0 0 1)

We report on the growth of Si on c(2 × 2) reconstructed LaAlO₃(0 0 1) surfaces at high substrate temperature (700 °C) by molecular beam epitaxy. An initial Volmer-Weber mode is evidenced using reflection high energy electron diffraction (RHEED), X-ray photoelectron diffraction (XPD) and atomic force microscopy. After the deposition of a few monolayers, the islands coalesce. Using X-ray photoelectron spectroscopy, we demonstrate that Si islands exhibit an abrupt interface with the LaAlO₃ substrate without formation of silicate or silica. Finally, combined RHEED and XPD measurements show the epitaxial growth of Si with a unique Si(0 0 1)//LaAlO₃(0 0 1) and Si<1 0 0>//LAO<1 1 0> relationship.     

Structure of the Si(011)-(16 × 2) surface and hydrogen desorption kinetics investigated using temperature-programmed desorption

D₂ temperature-programmed desorption (TPD) was used to probe the structure of the Si(011)-(16 × 2) surface. Deuterium was adsorbed at 200°C to coverages θ_D ranging up to complete saturation (approximately 1.1 ML) and the sample heated at 5°C s⁻¹. TPD spectra exhibited three second-order desorption peaks labelled β₂, β*₁ and β₁ centered at 430, 520 and 550°C. Of the proposed models for the Si(011)-(16 × 2) reconstruction, the present TPD results as a function of θ_D provide support for the adatom/dimer model with the β₂ peak assigned to D₂ desorption from the dihydride phase, while the β*₁ and β₁ peaks arise from adatom and surface-atom monohydride phases.     

STM and LEED observations of erbium silicide nanostructures grown on Si(1 0 0) surface: atomic-scale understandings

In this work, erbium silicide is grown on the Si(1 0 0) surface by depositing Er onto the substrate and annealing at 600–700 °C. Many nanowires of Er silicide are formed with lengths in the range 10–100 nm. The formation and evolution of this nanostructure are investigated at atomic scale directly with scanning tunneling microscopy and low-energy electron diffraction. The direction of these nanowires is found perpendicular to that of Si dimer rows. It is shown that Er coverage and annealing temperature have an effect on the formation of nanowires. On the surface between nanowires, new (5×2) and c(5×4) reconstructions are observed, giving an implication to understand the growth behaviors of Er silicide on Si(1 0 0) surface.     

Preparation of tantalum oxide thin films by photo-assisted atomic layer deposition

Tantalum oxide thin films were prepared by photo-assisted atomic layer deposition (Photo-ALD) in the substrate temperature range of 170–400 °C using Ta(OC₂H₅)₅ and H₂O as precursors. The constant growth rates of 0.42 and 0.47 Å per cycle were achieved for the films grown by normal ALD and Photo-ALD, respectively. The increased growth rate in Photo-ALD is probably due to the reactive surface by photon energy and faster surface reaction. In Photo-ALD, however, the constant growth rate started at lower temperature of 30 °C and one cycle time shortened up to 5.7 s than that of normal ALD. The films grown by normal ALD and Photo-ALD were amorphous and very smooth (0.21–0.35 nm) as examined by X-ray diffractometer and atomic force microscopy, respectively. Also, the refractive index was found to be 2.12–2.16 at the substrate temperature of 190–300 °C, similar to that of the film grown by normal ALD. However, the remarkably low leakage current density of 0.6×10⁻⁶ A/cm² to 1×10⁻⁶ A/cm² at applied field of 1 MV/cm is several order of magnitude smaller than that of normal ALD, probably due to the presence of reactive atom species.     

Effect of different Ge predeposition amounts on SiC grown on Si (1 1 1)

The SiC films were grown by solid source molecular beam epitaxy (SSMBE) on Si (1 1 1) with different amounts of Ge predeposited on Si prior to the epitaxial growth of SiC. The samples were investigated with reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), and X-ray diffraction (XRD). The results indicate that there is an optimized Ge predeposition amount of 0.2 nm. The optimized Ge predeposition suppress the Si outdiffusion and reduce the formation of voids. For the sample without Ge predeposition, the Si outdiffusion can be observed in RHEED and the results of XRD show the worse quality of SiC film. For the sample with excess amount of Ge predeposition, the excess Ge can increase the roughness of the surface which induces the poor quality of the SiC film.     

Optical second-harmonic investigations of the isothermal desorption of SiO from the Si(100) and Si(111) surfaces

The isothermal desorption of SiO from the Si(100) and Si(111) surfaces was investigated by means of optical second-harmonic generation (SHG). Due to the high adsorbate sensitivity of this method, desorption rates could be measured over a wide range from 10⁻¹ to 10⁻⁶ ML s⁻¹. From their temperature dependence between 780 and 1000 K, activation energies of E_A=3.4±0.2 eV and E_A=4.0±0.3 eV and pre-exponential factors of ν₀=10^16±1 s⁻¹ and ν₀=10^20±1 s⁻¹ for SiO desorption were obtained for Si(100) and Si(111), respectively. In the case of the Si(100) surface, a pronounced decrease of the first-order rate constants was observed upon increasing the initial coverage from 0.02 to 0.6 ML. The results are interpreted in terms of coverage-dependent oxygen-binding configurations, which influence the stability of the oxide layer.