Formation of nanocrystalline TiO2 by 100 MeV Au

Nanocrystalline TiO₂ structures are formed by irradiation of 100 MeV Au⁸⁺ ion beam on amorphous thin films of TiO₂. Surface morphology of the nanocrystals is studied by Atomic Force Microscopy (AFM). Amorphous to nanocrystalline phase transformation is identified by Glancing Angle X-ray Diffraction (GAXRD) and Raman spectroscopic studies. Optical characterization is carried out by UV-VIS spectroscopy technique. Blue shift observed in absorption band edge indicates the formation of nanophase TiO₂ after irradiation. The impinging swift heavy ion (100 MeV Au⁸⁺) induces nucleation of nanoparticles along the ion trajectory through inelastic collisions of the projectile with electrons of the material. It is observed that the shape and size of nanoparticles formed is dependant on the irradiation fluence.     

Nano- and micro-scale patterning of Si (1 0 0) under keV ion irradiation

Evolution of Si (1 0 0) surface under 100 keV Ar⁺ ion irradiation at oblique incidence has been studied. The dynamics of surface erosion by ion beam is investigated using detailed analysis of atomic force microscopy (AFM) measurements. During an early stage of sputtering, formation of almost uniformly distributed nano-dots occurs on Si surface. However, the late stage morphology is characterized by self-organization of surface into a regular ripple pattern. Existing theories of ripple formation have been invoked to provide an insight into surface rippling.     

XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar⁺ ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10¹³ ions/cm² to 1 × 10¹⁸ ions/cm². Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N₂⁺ ions and 7.2 × 10¹⁷ ions/cm² are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar⁺ ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.     

Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

Depth control of a silicon structure fabricated by 100q keV Ar ion beam lithography

Ion beam lithography of a silicon surface using an Ar ion beam with an ion energy in the order of hundreds of keV is demonstrated in this study. A specially designed ion irradiation facility was employed that enabled generation and irradiation with a highly accelerated and highly charged Ar ion beam. An ion-beam-induced amorphous layer on a silicon substrate can be selectively etched in hydrofluoric acid, whereas, a non-irradiated area is scarcely etched and, consequently, a concave structure can be fabricated on the irradiated area. To control the depth of the structure, parameters for dependence of the depth on ion irradiation were investigated. As a result, the depth of irradiated area can be controlled by the ion energy that is adjusted by the acceleration voltage and the ion charge. In addition, the etch resistance of the irradiated area increases with an increase in ion energy due to the crystalline layer formed on the surface. Simulation results reveal that the depth is strongly related to the defect distribution induced by ion irradiation. These results indicate the potential use of this method for novel three-dimensional lithography.     

MeV-ion axial surface-channeling from an electron irradiated KCl(0 0 1) surface

Surface-channeling of protons incident with 5 mrad on an electron-irradiated surface is investigated using a 0.55 MeV beam of protons. The target surface is KCl(0 0 1), which is damaged by electron-stimulated desorption with 5 keV electron irradiation. The direction of the incident beam is adjusted along the 〈1 0 0〉 and 〈1 1 0〉 channeling conditions and the channeling-dips of the scattering yields are observed. The irradiation dose-dependence of the minimum yields and widths of the dips is measured. Two dips are compared, i.e., for the 〈1 0 0〉 and 〈1 1 0〉 channelings. By increasing the irradiation dose up to 2 × 10¹⁶ cm⁻², the dip around the 〈1 0 0〉 axis becomes opened, but that of the 〈1 1 0〉 axis becomes shallow. The irradiated surfaces are observed to have many overlapped terraces of sub-micron with monolayer steps. The surface morphology deformed by the irradiation effects to truncate trajectories of the 〈1 0 0〉 channeling protons. This roughness of the surface is more effective for the trajectories of the 〈1 1 0〉 channeling protons. The protons incident on the rough surface along the 〈1 1 0〉 axis are not reflected from the atomic row but reflected by a potential of the surface with steps. Results by a simple computer simulation of the trajectories of protons at stepped surfaces also indicate the scattering processes.     

HREELS study of the adsorption and evolution of diethylamine (DEA) on Si(1 0 0) surfaces

The adsorption of diethylamine (DEA) on Si(1 0 0) at 100 K was investigated using high-resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption (ESD). The thermal evolution of DEA on Si(1 0 0) was studied using temperature programmed desorption (TPD). Our results demonstrate DEA bonds datively to the Si(1 0 0) surface with no dissociation at 100 K. Thermal desorption of DEA takes place via a β-hydride elimination process leaving virtually no carbon behind. Electronic processing of DEA/Si(1 0 0) at 100 K results in desorption of ethyl groups; however, carbon and nitrogen are deposited on the surface as a result of electron irradiation. Thermal removal of carbon and nitrogen was not possible, indicating the formation of silicon carbide and silicon nitride.     

Effect of 160 MeV Ni ion irradiation on PbS quantum dots

PbS quantum dots of average size 10 nm are encapsulated in a matrix (polyvinyl alcohol (PVA)) following chemical route. They are irradiated with 160 MeV Ni¹²⁺ ion beam with fluences 10¹²-10¹³ ions/cm². Red shift in the absorption response in the optical absorption spectra reveal size enhancement of the quantum dots after irradiation and was confirmed by transmission electron microscopy (TEM). Photoluminescence (PL) study was carried out with excitation wavelength 325 nm on both unirradiated and irradiated samples at different fluences and fluence-dependent surface states and excitonic emission is observed in the PL study. The Huang-Rhys coupling constant decreases significantly after swift heavy ion (SHI) irradiation and shows a decreasing trend with increase in ion fluence.     

Interface study of ion irradiated Cu/Ni/Cu(0 0 2)/Si magnetic thin film by X-ray reflectivity

The irradiation effect of 1 MeV C⁺ on the interface and magnetic anisotropy of epitaxial Cu/Ni system with a perpendicular magnetic anisotropy was investigated by using magneto-optical Kerr effects, grazing incident diffraction and X-ray reflectivity. The magnetic easy-axis was altered from the direction along the surface normal to in-plane and the strain in the Ni layer was relaxed after ion irradiation. Though the interface between the top Cu layer and the Ni layer becomes rough, the contrast of electron densities of Cu and Ni layer increases and the grain-growth occurs during ion irradiation. These phenomena arise from thermo-chemical driving force, i.e. heat of formation, which may be a crucial factor in determining the interface shape in the case of indirect energy transfer mechanism. Therefore, the change of the magnetic anisotropy of the Ni/Cu system after ion irradiation is not due to the formation of the intermixed layer at the interface. The ion irradiation effects on the grain-growth and enhancement of the electronic contrast between Ni and Cu are explained by the interfacial atomic movement caused by thermo-chemical driving force.     

Protons’ generation by laser irradiation at 5 × 10 W/cm from silicon dielectric targets containing an excess of hydrogen

A study of silicon plasma generated in vacuum by 532 nm Nd:YAG laser at intensities of about 5 × 10⁹ W/cm² from dielectric targets containing a relatively huge quantity of hydrogen was presented.Time-of-flight technique was employed to measure the particles’ energy and the relative yield with respect to other ion species. Plasma-accelerated ions show Coulomb-Boltzmann-shifted distributions depending on their charge state.Mass quadrupole spectrometry allowed the estimation of the relative hydrogen amount inside the different samples considered: silicon (Si), silicon nitride (Si₃N₄) and hydrogenated annealed silicon (Si(H)) as a function of the ablation depth and irradiation time.Depth profiles of the laser craters permit to calculate the ablation yield at the used laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the protons’ generation process occurring inside the plasma, due to the possible influence of the hydrogen excess on the treated samples in comparison to the not-hydrogenated silicon ones.     

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.     

GaN nucleation on (0 0 0 1)-sapphire via ion-induced nitridation of gallium

The growth of epitaxial GaN films on (0 0 0 1)-sapphire has been investigated using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). In order to investigate the mechanism of the growth in detail, we have focused on the nitridation of pre-deposited Ga layers (droplets) using ion beam-assisted molecular beam epitaxy (IBA-MBE). Comparative analysis of XPS core-level spectra and LEED patterns reveals, that nitride films nucleate as epitaxial GaN islands. The wetting of the surface by GaN proceeds via reactive spreading of metallic Ga, supplied from the droplets. The discussed growth model confirms, that excess of metallic Ga is beneficial for GaN nucleation.     

Focused ion beam induced swelling in MgO(0 0 1)

The interplay between swelling and milling phenomena in determining the morphology of Focused Ion Beam (FIB) -processed MgO(0 0 1) was investigated by atomic force microscopy. At the early stages of ion irradiation, before milling erosion is observed, MgO shows a relevant swelling behaviour with protrusion of the bombarded areas up to 6 nm for a dose of 5 × 10¹⁶ ions cm⁻². The effect is mainly ascribed to subsurface defect accumulation, while the low Ga ions concentration, as measured by in-depth Auger analysis, seems to exclude a contribution from ion implantation. In order to explain and control the morphology of Fe/NiO FIB patterned sub-micron structures on MgO substrates, we have also investigated FIB effects on Fe(0 0 1) and NiO(0 0 1) single crystals. Absent or negligible swelling has been observed on these materials.     

The role of carbon impurities on the Si(0 0 1)-c(4 × 4) surface reconstruction: Theoretical calculations

In this work we employ the state-of-the-art pseudopotential method, within a generalized gradient approximation to the density functional theory, combined with a recently developed method for the calculation of HREELS spectra to study a series of different proposed models for carbon incorporation on the silicon (0 0 1) surface. A fully discussion on the geometry, energetics and specially the comparison between experimental and theoretical STM images and electron energy loss spectra indicate that the Si(1 0 0)-c(4 × 4) is probably induced by Si-C surface dimers, in agreement with recent experimental findings.     

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.     

The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE

Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si¹⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si¹⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si⁴⁺ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si₃N₄ by molecular beam epitaxy (MBE).     

Adsorption and decomposition of triethylsilane on Si(1 0 0)

The adsorption and decomposition of triethylsilane (TES) on Si(1 0 0) were studied using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), electron stimulated desorption (ESD), and X-ray photoelectron spectroscopy (XPS). TPD and HREELS data indicate that carbon is thermally removed from the TES-dosed Si(1 0 0) surface via a β-hydride elimination process. At high exposures, TPD data shows the presence of physisorbed TES on the surface. These species are characterized by desorption of TES fragments at 160 K. Non-thermal decomposition of TES was studied at 100 K by irradiating the surface with 600 eV electrons. ESD of mass 27 strongly suggests that a β-hydride elimination process is a channel for non-thermal desorption of ethylene. TPD data indicated that electron irradiation of physisorbed TES species resulted in decomposition of the parent molecule and deposition of methyl groups on the surface that desorbed thermally at about 900 K. Without electron irradiation, mass 15 was not detected in the TPD spectra, indicating that the production of methyl groups in the TPD spectra was a direct result of electron irradiation. XPS data also showed that following electron irradiation of TES adsorbed on Si(1 0 0), carbon was deposited on the surface and could not be removed thermally.     

Photoluminescent and thermal behavior of 120 MeV silicon and 84 MeV oxygen ion irradiated PVDC

Photoluminescent, structural and thermal changes induced in polyvinylidenechloride (PVDC) films by irradiation with energetic silicon and oxygen ions have been determined using different techniques viz PL, FTIR, UV-vis, XRD, TGA and AFM. Noticeable photoluminescence was observed in PVDC after irradiation with 84 MeV oxygen ions at fluence 3.2×10¹¹ ions cm⁻², which is attributed to the small carbon cluster formed in the polymer due to irradiation. Quenching and shifting in the position of the PL band towards the longer wavelength side of the spectrum was observed with the increase in ion fluence. UV-vis and FTIR analysis have strongly corroborated the results of PL spectroscopy. X-ray diffractograms and TGA curves show decrease in crystallinity and weight loss, respectively, of PVDC films after irradiation. The strong correlation between the behaviors of different properties were analyzed and discussed in the present paper.     

Resistless patterning of a chlorine monolayer on a Si(0 0 1) surface with an electron beam

We achieved electron beam (e-beam) patterning without a photoresist on a Cl-terminated Si(0 0 1) surface. Synchrotron radiation photoemission spectroscopy and scanning photoelectron microscopy were employed to investigate the surface chemical state and pattern formation. The Cl-Si bonds were easily broken by the irradiation with an e-beam of 1 keV, leading to a pattern formation through the adsorption of residual molecules of water and hydrocarbon at the exposed Si dangling bond sites. In addition, we demonstrated the selective adsorption of desired molecules on the surface by e-beam irradiation in environments consisting of different gases, such as oxygen, ammonia, and 1-butanethiol.     

Monolayer passivation of silicon(0 0 1) surface by selenium

Monolayer passivation of the silicon(0 0 1) surface by selenium is investigated in an ultrahigh vacuum environment with a solid selenium source by reflection high-energy electron diffraction and residual gas analysis. It is found that precisely one monolayer of selenium is deposited on silicon(0 0 1) when the silicon substrate temperature is set slightly above the selenium source temperature and the passivation time ensures a little overdose of selenium above one monolayer. The temperature settings prevent selenium condensation on silicon(0 0 1), which makes selenium deposition on silicon(0 0 1) a thermodynamically self-limited process to exactly one monolayer.