Low Energy Ion Beam Deposition Research Articles

Production of low-energy SiCH3+ and SiC2H7+ ion beams for 3C-SiC film formation by selecting fragment ions from dimethylsilane

Fragment ions produced from dimethylsilane (DMS) in a Bernas-type ion source were investigated. Dominant fragment ions were identified to be CH3+, Si+, SiH3+, SiCH3+, and SiC2H7+. We mass-selected SiCH3+ and SiC2H7+ ions and then produced low-energy SiCH3+ and SiC2H7+ ion beams. Subsequently, these ion beams were used to irradiate Si(111) substrates at 800 °C. The energies of SiCH3+ and SiC2H7+ ions were set at 100 eV. We found that the irradiation of these ions led to the deposition of silicon carbide (3C-SiC) films on the Si substrates. For comparison, SiC film formations on Si(111) substrates were also attempted by the chemical vapor deposition (CVD) method using DMS. The X-ray photoelectron spectroscopy measurements showed that atomic ratios of C to Si (C/Si ratio) of the films obtained by the irradiation of SiCH3+ and SiC2H7+ ion beams were 1.2 and 1.2, respectively. Conversely, the C/Si ratio of the film obtained by CVD was 1.9. Raman spectroscopy showed that the film obtained by CVD included diamond-like carbon. These results suggest that the low-energy mass-selected ion beam deposition technique using DMS-derived fragment ions is practicable for the 3C-SiC deposition on Si substrates.

Influence of Dispersion Interactions on the Thermal Desorption of Nonplanar Polycyclic Aromatic Hydrocarbons on HOPG

A combination of low energy ion beam deposition and mass resolved thermal desorption spectroscopy is applied to analyze the binding behavior of two nonplanar polycyclic aromatic hydrocarbons (PAHs) to highly oriented pyrolytic graphite (HOPG) surfaces—also concerning their lateral dispersion interactions. In particular, the fullerene precursor C60H30 (FPC) and rubrene C42H28 are studied. Due to their smaller contact areas, both molecules exhibit significantly weaker binding energies to the HOPG surface compared to planar PAHs of similar size: C60H30 is bound to the surface by 3.04 eV, which is 0.6 eV lower than for a fully planar homologue. For rubrene, an isolated molecule–substrate binding energy of 1.59 eV is found, which is about 1 eV less than that of the corresponding planar homologue hexabenzocoronene C42H18. In contrast to FPC, rubrene shows a significant (intermolecular) lateral dispersion contribution to the binding energy as the submonolayer coverage increases.

Desorption of Fullerene Dimers upon Heating Non-IPR Fullerene Films on HOPG

Low energy ion beam deposition of mass-selected fullerene ions was used to generate thin films of non-IPR fullerenes, C2n (2n = 48–58 and 52–68) on graphite under UHV. These were probed by mass-resolved thermal desorption spectroscopy. We observe the emission of dimers in addition to dominant monomer desorption for 2n = 58–68, while for 2n = 48–56 no dimer desorption was detected. Dimer signals were typically at <1% level compared to the monomers. The desorption temperature ranges of these dimer species were higher and much narrower than for the corresponding monomers. Building also on theoretical considerations, we came to the conclusion that coalescence reactions between the fullerenes must take place below 1000 K and that fullerenes with more than three pairs of adjacent pentagons cross-link too strongly to allow the desorption of dimers.

Formation of Sublimable Nanographene Oxides by Reacting Coronene Films with Atomic Oxygen

We have studied the oxidation of multilayer thin films of coronene (C24H12) on graphite under UHV conditions. Coronene films were grown by low-energy cluster ion beam deposition (LECBD, C24H12+, Ekin < 6 eV) and subsequently oxidized by exposing them to a beam of near-thermal atomic oxygen at room temperature. Mass spectra recorded during sublimation of the oxidized coronene films reveal a distribution of emitted species, C24H12On (n ≤ 7), which we assign as coronene epoxides. From the fragmentation pattern we infer that primarily rim epoxides are formed, which exhibit a higher stability than core epoxides according to DFT calculations. In addition to epoxides, XPS indicates that other oxidic functional groups, such as ethers, are also formed in smaller relative amounts as nonsublimable species. The overall yield for the thermal desorption of intact coronene oxides can be up to 7% of the deposited coronene, sufficiently high such that this procedure could be useful as a source of monodispersed nanoscale g...

C58 on Au(111): A scanning tunneling microscopy study

C58 fullerenes were adsorbed onto room temperature Au(111) surface by low-energy (~6 eV) cluster ion beam deposition under ultrahigh vacuum conditions. The topographic and electronic properties of the deposits were monitored by means of scanning tunnelling microscopy (STM at 4.2 K). Topographic images reveal that at low coverages fullerene cages are pinned by point dislocation defects on the herringbone reconstructed gold terraces (as well as by step edges). At intermediate coverages, pinned monomers act as nucleation centres for the formation of oligomeric C58 chains and 2D islands. At the largest coverages studied, the surface becomes covered by 3D interlinked C58 cages. STM topographic images of pinned single adsorbates are essentially featureless. The corresponding local densities of states are consistent with strong cage-substrate interactions. Topographic images of [C58]n oligomers show a stripe-like intensity pattern oriented perpendicular to the axis connecting the cage centers. This striped pattern becomes even more pronounced in maps of the local density of states. As supported by density functional theory, DFT calculations, and also by analogous STM images previously obtained for C60 polymers [M. Nakaya, Y. Kuwahara, M. Aono, and T. Nakayama, J. Nanosci. Nanotechnol. 11, 2829 (2011)], we conclude that these striped orbital patterns are a fingerprint of covalent intercage bonds. For thick C58 films we have derived a bandgap of 1.2 eV from scanning tunnelling spectroscopy data confirming that the outermost C58 layer behaves as a wide band semiconductor.

The science and technology of low-energy ion-surface interactions

Low-energy ion-surface interactions have played a significant role in both the analysis and modification of the near-surface layers of solid materials. From ion scattering spectrometry to eV implantation, the technology of the ion beam or plasma system has played a key role in determining the effectiveness of the processes involved. The development and understanding of the technology required for low-energy ion implantation, ion-beam deposition, ion plating and reactive ion etching has relied heavily on the availability of analytical systems capable of characterising the nature of both the ion beams or plasmas and the surface region of the solid being treated. The present review follows the development of ion beam systems from the low-current analytical beams used for low and medium ion scattering and secondary ion and photon emission through to the high-current machines used for ion implantation and ion beam deposition. The transport concepts used in the latter machines formed the basis of high current, low-energy systems used commercially to produce the ultra-shallow junctions required for the current generation of MOS devices. These developments will be described in the context of the growth in understanding of the beam and plasma physics and the atomic collision processes involved. All the technological developments at Salford were carried out in the course of detailed studies of the atomic collision processes associated with ion trapping and release, ion scattering and sputtering. The results of some of these studies and the ways in which they have contributed to our current understanding of the physics of ion-surface interactions will also be reviewed.

Non-IPR C60 solids

Films comprising predominantly novel isomers of C(60) [=C(60)(nIPR)] have been generated by low energy ion beam deposition of vibronically excited C(60)(+) onto graphite followed by selective sublimation of C(60)(I(h)) from the deposited isomer mixture. The incident ions were generated by electron impact ionization/fragmentation of sublimed C(70). Images of the C(60)(nIPR) films obtained by applying atomic force microscopy show aggregates, which we attribute to covalently interlinked C(60)(nIPR) units. The covalent bonds are inferred from the significantly higher thermal stability of the C(60)(nIPR) films compared to the C(60)(I(h)) van der Waals solid-as measured by thermal desorption with mass spectrometric detection of the C(60) mass channel (the only desorbable species). In contrast to the characteristic doublet structure of the occupied valence band in the ultraviolet photoelectron spectrum of pure C(60)(I(h)), the valence band of C(60)(nIPR) films exhibits a triplet feature with the additional peak occurring at a binding energy of approximately 2.6 eV. This is an indicator of the electronic modifications induced by intermolecular bonding. C(60)(nIPR) films exhibit a narrower band gap than found for C(60)(I(h)). They also have significantly different chemical reactivity toward incorporation of thermal energy deuterium atoms. In order to model the experimental photoelectron spectra, various covalently linked oligomers of (#1809)C(60)(C(2v)), the second most stable conventional 60-atom fullerene cage, were calculated by means of the density functional theory. These spectral predictions together with analogous previous observations on related fullerene solids such as C(58) lead us to infer that C(60)(nIPR) films consist of fullerene cage isomers containing one or more adjacent pentagon pairs, which mediate covalent cage-cage interconnection.

Characterization of Ion Beam Deposited 107Ag Thin Films on Si(111) Surface by means of Rutherford Backscattering Spectroscopy and Reflection High Energy Electron Diffraction

The crystalline quality of the 107Ag films on Si(111) surface grown by mass-separated low energy ion beam deposition (IBD) has been studied by means of combined techniques of reflection high energy electron diffraction (RHEED), low energy electron diffraction, and Rutherford backscattering spectroscopy (RBS). Although crystalline quality of the films hardly depends on Ag ion energy in the range of 30-100 eV for RHEED patterns, the RBS-channeling spectra clearly show the difference in the minimum channeling yield Xmin and the increasing rate of the channeling yield dx(z)/dz. The Ag ion energy to obtain a well-crystallized Ag film is found to be 30-50 eV. In addition, it is shown that excellent quality of the Ag films grown at elevated temperature can be obtained by means of IBD.

Characterization of Ion Beam Deposited 107Ag Thin Films on Si(111) Surface by means of Rutherford Backscattering Spectroscopy and Reflection High Energy Electron Diffraction

The crystalline quality of the 107Ag films on Si(111) surface grown by mass-separated low energy ion beam deposition (IBD) has been studied by means of combined techniques of reflection high energy electron diffraction (RHEED), low energy electron diffraction, and Rutherford backscattering spectroscopy (RBS). Although crystalline quality of the films hardly depends on Ag ion energy in the range of 30-100 eV for RHEED patterns, the RBS-channeling spectra clearly show the difference in the minimum channeling yield Xmin and the increasing rate of the channeling yield dx(z)/dz. The Ag ion energy to obtain a well-crystallized Ag film is found to be 30-50 eV. In addition, it is shown that excellent quality of the Ag films grown at elevated temperature can be obtained by means of IBD.

有機金属イオンビーム法によるヘテロエピタキシャルＳｉＣナノドットの結晶成長

Silicon carbide (SiC) is a remarkable material for electronic and optoelectronic device applications. Using the organometallic ion beam deposition, the heteroepitaxial SiC nano-dots were fabricated on Si(100) substrate at the low-temperatures (500-800°C). The formation of the SiC nano-dots was performed by the low-energy ion beam deposition system. The single precursor of methylsilicenium ion (SiCH3+) was generated from dimethylsilane (SiH2(CH3)2) plasma. The ion beam was extracted at 25 keV, and mass-selected by a sector magnet. The SiCH3+ ions were decelerated to 100 eV in front of Si(100) substrate. The SiC nano-dots formed on Si(100) were heteroepitaxial 3C-SiC(100). The size of the SiC nano-dots was the length of 100-200 nm and the height of 10-30 nm. The SiC nano-dots formed by the organometallic ion beam deposition showed the nano-tile structure.

Ga 1−xMn xSb grown on GaSb with mass-analyzed low-energy dual ion beam deposition

The Ga 1− x Mn x Sb samples were fabricated by the implantation of Mn ions into GaSb (1 0 0) substrate with mass-analyzed low-energy dual ion beam deposition system, and post-annealing. Auger electron spectroscopy depth profile of the Ga 1− x Mn x Sb samples showed that the Mn ions were successfully implanted into GaSb substrate. Clear double-crystal X-ray diffraction patterns of the Ga 1− x Mn x Sb samples indicate that the Ga 1− x Mn x Sb epilayers have the zinc-blende structure without detectable second phase. Magnetic hysteresis-loop of the Ga 1− x Mn x Sb epilayers were obtained at room temperature (293 K) with alternating gradient magnetometry.

Investigation of Mn-implanted n-Si by low-energy ion beam deposition

Mn ions were implanted to n-type Si(0 0 1) single crystal by low-energy ion beam deposition technique with an energy of 1000 eV and a dose of 7.5×10 17 cm −2. The samples were held at room temperature and at 300 °C during implantation. Auger electron spectroscopy depth profiles of samples indicate that the Mn ions reach deeper in the sample implanted at 300 °C than in the sample implanted at room temperature. X-ray diffraction measurements show that the structure of the sample implanted at room temperature is amorphous while that of the sample implanted at 300 °C is crystallized. There are no new phases found except silicon both in the two samples. Atomic force microscopy images of samples indicate that the sample implanted at 300 °C has island-like humps that cover the sample surface while there is no such kind of characteristic in the sample implanted at room temperature. The magnetic properties of samples were investigated by alternating gradient magnetometer (AGM). The sample implanted at 300 °C shows ferromagnetic behavior at room temperature.

Improvement in ion beam quality for thin-film formation

Low-energy ion beam technique is useful for the study on ion–surface interaction and deposition processes. In the low-energy ion beam system, ions are accelerated and decelerated to a desired energy just before deposition. The high-energy neutrals contained in the low-energy ion beam cannot be decelerated and make damage in the deposition film. By rejecting high-energy neutrals from a low-energy ion beam, improvement of the quality of the beam might be achieved. Thus, we modified the low-energy ion beam deposition system in order to reduce them. With this modification, the amount of high-energy Ar neutrals in a 100-eV Ar + ion beam was reduced to 0.5% in this work.

Growth temperature effect on a self-assembled SiC nanostructure

Self-assembled nanostructure fabrication technology holds much potential to further the interests of nanoscience and nanotechnology. Using a low-energy mass-selected ion beam deposition system, we fabricated self-assembled silicon carbide (SiC) nanotiles. Methylsilicenium ions (SiCH 3 +) used as single precursors were generated from dimethylsilane (SiH 2(CH 3) 2). Low-energy SiCH 3 + ions (100 eV) were deposited on a Si(100) substrate at 500–600 °C. The characteristics of the self-assembled SiC nanotiles were analyzed by reflection high-energy electron diffraction (RHEED), a scanning electron microscope (SEM) and an atomic force microscope (AFM). The self-assembled SiC nanostructure displayed the properties of a zinc-blende structure (3C–SiC) and heteroepitaxial growth. The shape of the self-assembled SiC nanotiles was rectangular with an edge length of 150–200 nm and a height of 10–30 nm. Optimum temperature was found to be 600 °C due to the sensitivity of crystal quality and shape of the self-assembled SiC nanotiles to growth temperature.

Momentum effects of the molecular ion beam in SiC crystal growth

Silicon carbide (SiC) films were grown on Si(111) substrates by low-energy ion beam deposition for the purpose of investigating momentum effects on crystal growth. Dimethylsilane (SiH 2(CH 3) 2) was ionized to obtain methylsilicenium ions (SiCH 3 +) with Si–C bonding for use as precursors. The ion beam was extracted at 25 m keV for efficient transport from the ion source to the deposition chamber, and SiCH 3 + ions were mass-selected by a sector magnet. Ions transported at high energy were decelerated in front of the substrate to desired energies of 20–100 eV. Ion beam current at the substrate was 0.1–0.2 μA. Temperature at the Si(111) substrate was held at 750 °C during ion beam deposition and pressure in the deposition chamber atmosphere was kept below 3.9×10 −7 Pa. Incident SiCH 3 + ions measured with an energy analyzer were found to have monochromatic energy distributions, e.g. the FWHM was only 1 eV in the case of 100 eV. Crystal structures of the SiC thin films were analyzed in situ by reflection high-energy electron diffraction (RHEED). Under conditions of incident SiCH 3 + ions energies of 60–100 eV, 3C–SiC(111) was grown on Si(111) substrate. On the other hand, under conditions of incident ion energies of 20–50 eV, 2H–SiC was grown on Si(111) substrate. In conclusion, the momentum of low-energy ions was found to exert an effect on the determination of SiC crystal structures.

Mn implanted GaAs by low energy ion beam deposition

High dose Mn was implanted into semi-insulating GaAs substrate to fabricate embedded ferromagnetic Mn–Ga binary particles by mass-analyzed dual ion beam deposit system at room temperature. The properties of as-implanted and annealed samples were measured with X-ray diffraction, high-resolution X-ray diffraction to characterize the structural changes. New phase formed after high temperature annealing. Sample surface image was observed with atomic force microscopy. All the samples showed ferromagnetic behavior at room temperature. There were some differences between the hysteresis loops of as-implanted and annealed samples as well as the cluster size of the latter was much larger than that of the former through the surface morphology.

FexSi grown with mass-analyzed low-energy dual ion beam deposition

Heavily iron-implanted silicon was prepared by mass-analyzed low-energy dual ion beam deposition technique. Auger electron spectroscopy depth profiles indicate that iron ions are shallowly implanted into the single-crystal silicon substrate and formed 35nm thick FexSi films. X-ray diffraction measurements show that as-implanted sample is amorphous and the structure of crystal is partially restored after as-implanted sample was annealed at 400°C. There are no new phases formed. Carrier concentration depth profile of annealed sample was measured by Electrochemical C–V method and indicated that FexSi film shows n-type conductivity while silicon substrate is p-type. The p–n junction is formed between FexSi film and silicon substrate showing rectifying effect.

(Ga,Mn,N) compounds growth with mass-analyzed low energy dual ion beam deposition

The (Ga,Mn,N) samples were grown by the implantation of low-energy Mn ions into GaN/Al 2O 3 substrate at different elevated substrate temperatures with mass-analyzed low-energy dual ion beam deposition system. Auger electron spectroscopy depth profile of samples grown at different substrate temperatures indicates that the Mn ions reach deeper in samples with higher substrate temperatures. Clear X-ray diffraction peak from (Ga,Mn)N is observed in samples grown at the higher substrate temperature. It indicates that under optimized substrate temperature and annealing conditions the solid solution (Ga,Mn)N phase in samples was formed with the same lattice structure as GaN and different lattice constant.

Structure of carbon nitride films prepared by mass-separated low-energy ion beam deposition

Carbon nitride films were prepared on Si(1 0 0) wafers by alternating irradiation with mass-separated ion beams of C + and N + at the ultra-high vacuum pressure 10 −7 Pa. The ion energy of both ion species was 50 eV. The Si substrate was kept at room temperature (rt), 460, 800 or 1130 K during deposition, and the effects of the substrate temperature on the film structure and chemical bonds formed were investigated. The films were characterized by transmission electron microscopy, X-ray diffraction, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy. The CN film prepared at rt was amorphous. The nitrogen content decreased from 27 at.% in the film prepared at rt, to 20 at.% in the film prepared at 1130 K. The amorphous CN film prepared at rt consisted of sp 2 CN, sp 3 CN, sp 2 CC, and sp 3 CC bonds, whose bond angles were distorted. This distortion of the chemical bonds decreased, and the structure of the CN film changed to a graphite-like one on increasing the growth temperature. The graphite-like film has a highly preferential crystal orientation, and the c-axis of the graphite basal plane is parallel to the film growth surface. The basal plane is also curved and interlinked because of the presence of in-plane sp 2 CN and 3D sp 3 CN bonds.

Epitaxial growth of β-SiC on Si (100) by low energy ion beam deposition

Epitaxial β-silicon carbide (SiC) films on mirror-polished (100) Si substrates were deposited using low energy (150 eV) ion beam modification at different ion doses (1×10 17/cm 2, 1×10 18/cm 2, 1×10 19/cm 2). The Si substrates were exposed to a broad ion beam of hydrocarbon, argon, and hydrogen ions, generated in a Kaufman ion source. These ions reacted with the Si substrates and formed the epitaxial β-SiC films. Amorphous carbon films were generated on top of β-SiC films. Some voids were found with transmission electron microscopy (TEM) at the interface between the Si substrate and the β-SiC film. By studying the ion dose dependence of the quality and thickness of the SiC and amorphous carbon films, we suggest that the formation of the epitaxial β-SiC was driven by thermal diffusion of Si atoms in SiC and preferential etching of non-epitaxial SiC crystals by H ions.