Molecular secondary ion emission from adenine overlayers in dependence on the primary ion species and substrate material

The influence of the primary ion species (He⁺, Ne⁺, Ar⁺, Kr⁺, Xe⁺ and SF₅⁺) and substrate material (graphite, Al, Cu, Ag and Pb) on the secondary ion emission from molecular overlayers of the purine base adenine was investigated in dependence on the layer thickness. The measurements showed an increasing yield with increasing mass of the primary ions and its number of constituents. The yield enhancement, defined as the ratio between the maximum yield obtained from approximately a monolayer coverage of adenine to the yield obtained from a multilayer coverage, was shown to depend on the substrate material. However, a clear dependence on the primary ion species was not found.     

Influence of primary ion species on the secondary cluster ion emission process from SAMs of hexadecanethiol on gold

In order to investigate the secondary cluster ion emission process of organo-metallic compounds under keV ion bombardment, self-assembled monolayers (SAMs) of alkanethiols on gold are ideal model systems. In this experimental study, we focussed on the influence of the primary ion species on the emission processes of gold-alkanethiolate cluster ions from a hexadecanethiol SAM on gold. For this purpose, we carried out time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements using the following primary ion species and acceleration voltages: Ar⁺, Xe⁺, SF₅⁺ (10 kV), Bi⁺, Bi₃⁺(25 kV), Bi₃²⁺, Bi₅²⁺, Bi₇²⁺ (25 kV).It is well known that molecular ions M⁻ and gold-alkanethiolate cluster ions Au_xM_y⁻ with M = S-(CH₂)₁₅-CH₃, x − 3 ≤ y ≤ x + 1, x, y > 0, show intense peaks in negative mass spectra. We derived yields Y_SI exemplarily for the molecular ions M⁻ and the gold-hexadecanethiolate cluster ions Au_y+1M_y⁻ up to y = 8 and found an exponentially decreasing behaviour for increasing y-values for the cluster ions.In contrast to the well-known increase in secondary ion yield for molecular secondary ions when moving from lighter to heavier (e.g. Ar⁺ to Xe⁺) or from monoatomic to polyatomic (e.g. Xe⁺ to SF₅⁺) primary ions, we find a distinctly different behaviour for the secondary cluster ions. For polyatomic primary ions, there is a decrease in secondary ion yield for the gold-hexadecanethiolate clusters whereas the relative decrease of the secondary ion yield ξ_Y with increasing y remains almost constant for all investigated primary ions.     

Enhanced photoluminescence of Ar implanted sapphire before and after annealing

In this paper, we studied the changes in the photoluminescence spectra of the Ar⁺ ion implanted mono-crystalline sapphire annealed at different atmospheres and different temperatures. Single crystals of sapphire (Al₂O₃) with the (1 0 1¯ 0) (m-samples) orientation were implanted at 623 K with 110 keV Ar⁺ ions to a fluence of 9.5×10¹⁶ ions/cm². Photoluminescence measurement of the as-implanted sample shows a new emission band at 506 nm, which is attributed to the production of interstitial Al atoms. The intensity of emission band at 506 nm first increased then decreased with increase in annealing temperature. For the same annealing temperature, the intensity of PL peak at 506 nm of the sample annealed in air was higher than the sample annealed in vacuum. The experimental results show that the intensity of the PL peak at 506 nm of Ar-implanted sapphire can be enhanced by subsequent annealing with an enhancement of nearly 20 times. The influence of thermal annealing of the Ar-implanted samples on the new 506 nm emission band was discussed.     

Study of the formation and unimolecular fragmentation of Si<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>O<Stack><Subscript><Emphasis Type="Italic">m</Emphasis></Subscript><Superscript>+</Superscript></Stack> clusters under ion bombardment

The dependences of the emission and fragmentation of clusters sputtered by Xe⁺ ions from the surface of Si _n O _m ⁺ on the oxygen pressure near the bombarded surface are studied using secondary ion mass spectrometry. It is shown that the process of Si _n O _m ⁺ cluster formation under ion bombardment can be described within the framework of the mechanism of combinatorial synthesis by taking into account the mutual reversibility of the reactions of formation and unimolecular decay.     

The effect of increasing the excitation level of the sodium atom on the structure of the NaArn polyatomic exciplexes

In this work, the geometrical structures, the formation energies, and electronic states of the Na(ms)Ar_n polyatomic exciplexes with m = 3-6 and n = 2-5 are studied by using a quantum-classical method. The interaction potential between an electronically excited sodium atom and argon atoms are calculated by using a one-electron model involving electron-Ar and electron-Na⁺ pseudopotentials, in which the Hamiltonian is diagonalized at every optimization step in the Basin Hopping algorithm. The relationship between the position of the electronically excited levels and the cluster geometry is investigated as a function of the excitation level and of the spatial extension of the excited electron orbital. We show that the equilibrium structures of the ground state Na(3s)Ar_n and those of the electronically excited states Na(4s)Ar_n, Na(5s)Ar_n, and Na(6s)Ar_n are significantly different. As a result of the detailed examination of the relationships between the geometrical structure and density distribution of the Na valence electron of the Na^∗Ar_n with n = 2-5 polyatomic exciplexes, we can see that for the Na(4s)Ar_n polyatomic exciplexes, the two extreme geometries, neutral Na(3s)Ar_n and ionic Na⁺Ar_n compete. It appears that none of them is the actual one. For Na(5s)Ar_n and Na(6s)Ar_n the valence electron is very weakly bound to the ionic core and described by a more diffused orbital so that the geometry and the formation energies of this excited state called Rydberg states converge towards those of the ionic cores.     

Silicon clusters produced by femtosecond laser ablation: non-thermal emission and gas-phase condensation

Neutral silicon clusters Si_n (up to n=7) and their cations Si_n⁺ (up to n=10) have been produced by femtosecond laser ablation of bulk silicon in vacuum and investigated using time-of-flight mass spectrometry. Two populations of the Si_n⁺ clusters with different velocity and abundance distributions in the ablation plume have been clearly distinguished. Possible mechanisms of cluster formation (Coulomb explosion, gas-phase condensation, phase explosion) are discussed. PACS 52.38.Mf; 61.46.+w; 79.20.Ds     

Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

Hydrogenated carbon clusters produced by highly charged ion impact on solid

The emission of small (hydrogenated) carbon cluster ions C_nHm ⁺ (n =2-22) upon highly charged Xe^q+ (q =20-44) impact on C₈₄ surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation of a certain carbon cluster ion Cn ⁺ is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted as an entity in agreement with “equilibrium” and “shock wave” models. Received 4 February 2000     

Metastable Decomposition of Ar3+ Cluster Ions Into Ar2+ and Ar+

In this note we present the first account of a study of metastable (unimolecular decay) and collision-induced dissociation of Ar₃+ cluster ions using an experimental setup consisting of a molecular beam-electron impact ion source and a double focussing (reversed Nier Johnson geometry) mass spectrometer. The existence of the following metastable decay processes (accessible by our sampling time window) could be demonstrated: Ar₃⁺* → Ar₂+ and Ar₃⁺* → Ar⁺. The processes were studied as a function of electron impact energy. The present results are of importance in order to provide some guidance for the development of appropriate theoretical models for the dissociation of cluster ions.     

Characterization of ion species of silicon oxide films using positive and negative secondary ion mass spectra

Secondary ion species of silicon oxide films have been investigated using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Characterization of thermally grown SiO₂ films on silicon has been performed. A diagram showing secondary ion spectra of SiO₂ films in both positive and negative polarities indicates the pattern of change in polarities and intensities of ion species from SiO⁺ to Si₅O₁₁⁻. The ions mostly change from positive to negative polarity between Si_nO_2n−1 and Si_nO_2n. Ion peaks with the strongest intensities in the respective cluster ions correspond to the Si_nO_2n+1 negative ion. Intensities of ion species of Si_nO_2n+2 appear negligibly small. Ion species of Si₃O⁺, Si₃O₂⁺ and Si₃O₃⁺ have been found at the interface between silicon and SiO₂ films. The intensity patterns of these ion species compared to those of SiO₂ films indicate that most of these species are not emitted from the SiO₂ films, but likely from the SiO structures.     

Observation of clusters in a sputtering ion source

This paper reports previously unpublished results which were obtained in 1966. We systematically investigated the dependence of cluster ion intensities on the bombarding gases He⁺, Ar⁺ and Xe⁺ (energies: 4 to 12 keV, current densities: 100 mA/cm²). Frequently, the observed structures in the relative cluster intensities were quite puzzling, e.g. for A1 and Si. Attempts to correlate these structures to crystal configurations failed, nor did any pattern develop from simple valency considerations alone. Initial ion energy distribution measurements from 0 to 1200 eV showed significant differences for atomic ions and cluster ions. This effect is used to reduce interference problems caused by cluster ion peaks in SIMS applied to trace analysis of solids. The results are discussed and compared with those of other investigators, also including cluster formation by vaporization and sparking. Extending known theoretical considerations may possibly afford a general understanding of the intensity structure. The formation and ejection mechanisms of clusters, however, remain unknown. Thermal effects to explain the latter are definitely discounted by the magnitude of the observed initial cluster energy (> 10 eV). Also discussed are two phenomena which demonstrate the presence of the bombarding gas in the surface.     

Molecule-doped rare gas clusters: structure and stability of ArnNO(X 2Π1/2/3/2), n ≤ 25, from new ab initio potential energy surfaces of ArNO

High level ab initio calculations carried out for the ²A′ and ²A″ states of ArNO(X ²Π) predict a crossing near the T-shape configuration, with the ²A′ minimum being slightly deeper. Spin-orbit coupling is included through a model treatment and results in two potential energy surfaces with similar topologies, nearly parallel to each other and close to the averaged non-relativistic surface. These results are used to construct a DIM-like model for Ar _n NO clusters. The lowest energy cluster structures are found to resemble those for Ar _n+1 with NO lying in the surface. The set of major magic numbers (structures of pronounced stability) is also the same as for the Ar _n+1 clusters, and is emphasized further by the detachment of NO, which requires a larger energy than for detachment of a single Ar atom. The relations of the difference between the two dissociation energies and of the Ar _n NO(1/2 → 3/2) excitation energy to the magic numbers are discussed.     

Messungen des γi-Koeffizienten an reinen Mo- und Fe-Oberflächen in Edelgasen

Es werden Messungen des γ_i-Koeffizienten an reinen Katodenoberflächen für die Kombinationen Mo—Ne⁺, Ar⁺, Kr⁺, Xe⁺ und Fe—Ne⁺ mitgeteilt, die mit einer von Varney angegebenen dynamischen Methode im Bereich 30 ? X/p₀ ? 400 V Torr^?1 cm^?1 durchgeführt wurden. Es wird gezeigt, daß die mit dieser Methode im Gas gemessenen γ_i-Werte sich aus den Vakuumwerten von Hagstrum ableiten lassen, wenn

• 1 . die Rückstreuung der emittierten Elektronen und
• 1 . die Molekülionenbildung

berücksichtigt werden. Die Nachlieferung von Elektronen durch Photonen (δ/α) und Metastabile (ω/α) wird unter Verwendung von ω/α-Werten aus Zündspannungsmessungen abgeschätzt. Measurements of the γ_i-coefficient on pure cathode surfaces are described for the combinations Mo—Ne⁺, Ar⁺, Kr⁺, Xe⁺ und Fe—Ne⁺ with a dynamic method according to Varney. These measurements were carried out in the range of 30 ? X/p₀ ? 400 V Torr^?1 cm^?1. It is shown that the measured values correspond to these of Hagstrum obtained in vacuum, if

• 1 . backscattering of emitted electrons and
• 1 . formation of molecules

are taken into account. An estimation is made for electron emission coefficients of photens (δ/α) and metastables (?/α) by calculated ω/α-coefficients from measurements of ignition voltage.     

Quantum chemical study of the properties of an SiO<Subscript>2</Subscript>/Si(100) interface implanted with boron ions

Quantum-chemical calculations of the properties of a B⁺ ion-implanted SiO₂/Si(100) interface are presented. Dependencies of the total energy of a B⁺ ion cluster system on the location of B⁺ ions in oxygen and silicon vacancies are calculated, along with the geometric and electronic characteristics of the equilibrium cluster states with implanted boron ions.     

The characteristic spectral lines of target atoms in the impact of<Superscript>40</Superscript>Ar<Superscript><Emphasis Type="Italic">q</Emphasis>+</Superscript> ions on metal surfaces

We report the measured results of the 200 nm—1000 nm characteristic spectral lines of target atoms when highly charged ions⁴⁰Ar ^q+(6≤q≤14) with the same kinetic energy and⁴⁰Ar⁶⁺ with different kinetic energies are incident upon Al, Ti, Ni, Ta and Au surfaces, respectively. The results for¹²⁹Xe⁶⁺,¹²⁹Xe¹⁰⁺ and¹²⁹Xe¹⁵⁺ with the same kinetic energy (150 keV) incident upon a Ta surface are also reported. These results show that when the projectile and target are properly selected (⁴⁰Ar¹²⁺ impinges on Al,¹²⁹Xe⁶⁺ impinges on Ta), the spectral intensity of characteristic spectral lines of the target atom is effectively enhanced, and is not strongly dependent on the kinetic energy of the incident ions.     

Lattice location and dopant behavior of group II and VI elements implanted in silicon

Outdiffusion, lattice location and electrical behavior of Zn, Cd, Hg and Se, Te implanted into silicon at 50 keV were investigated by backscattering and channeling effect of 1 MeV He⁺ ions and by Hall effect and sheet resistivity measurements. All the species exhibited outdiffusion with thermal processing. A significant fraction of Zn, Cd and Hg, when implanted into a substrate of 350°C, occupied regular interstitial lattice sites, while 50–60 per cent of the Se and Te atoms were on substitutional lattice sites. Selenium implanted at room temperature and mercury implanted into a substrate of 350°C exhibited depth dependent lattice location. The implanted layers showed n-type behavior: the maximum value of number of carriers/cm² was less than the number of implanted ions/cm² in all cases. The highest electrical activity was observed for Se corresponding to 25 per cent of the substitutional component.     

Modification of Zinc-Implanted Silicon by Swift Xenon Ion Irradiation

50 keV ⁶⁴Zn⁺ ions to a dose of 5 × 10¹⁶ cm^–2 are implanted into substrates of single-crystal n-type silicon. Then the samples are irradiated at room temperature with 167 MeV ¹³²Xe²⁶⁺ ions with a fluence ranging from 1 ×10¹² up to 5 × 10¹⁴ cm^–2. Changes in the structure and properties on the sample surface and in its body are studied by scanning electron microscopy, energy dispersive microanalysis, atomic force microscopy, time-of-flight secondary ion mass spectrometry, and photoluminescence.     

Self-organized dot patterns on Si surfaces during noble gas ion beam erosion

The erosion of target materials with energetic ions can lead to the formation of patterns on the surface. During low-energy (?2000 eV) noble gas (Ne⁺, Ar⁺, Kr⁺, Xe⁺) ion beam erosion of silicon surfaces dot patterns evolve on the surface. Dot structures form at oblique ion incidence of 75° with respect to surface normal, with simultaneous sample rotation, at room temperature. The lateral ordering of dots increases while the dot size remains constant with ion fluence, leading to very well ordered dot patterns for prolonged sputtering. Depending on ion beam parameters, dot nanostructures have a mean size from 25 nm up to 50 nm, and a mean height up to 15 nm. The formation of dot patterns depends on the ion/target mass ratio and on the ion energy. The temporal evolution and the lateral ordering of these nanostructures is studied using scanning force microscopy (AFM).     

Messungen des γi-Koeffizienten,der Zündspannung und des normalen Katodenfalls der Glimmentladung an gasbedeckten Mo- und Fe-Oberflächen in Edelgasen

Measurements of the γ_i-coefficient, the ignition voltage and the normal cathode fall of the glow discharge on gas-covered cathode surfaces for the combinations Mo—Ne⁺, Ar⁺, Kr⁺, Xe⁺ and Fe—Ne⁺ are described. H₂, N₂ and O₂ are used for covering the cathodes with a monomolecular adsorption layer. Measurements are carried out with a dynamic method according to VARNEY in the range 30 ? X/p₀ ? 400 V cm^?1 Torr^?1.