Positive Ion Yield Research Articles

Secondary Charged Particle Emission from Proton Conductive Oxides by Ion Impact

Secondary charged particle emission (SCPE) from a proton conductive perovskite oxide, namely, SrCe0.95Yb0.05O3-δ(SCO) film on Si by ion impact has been measured using a charge collector method. When the ion projected range is much longer than the film thickness, SCPE yields are found to be nearly independent of the ion beam current (IB) for IB < a few nA, giving SCPE yields under charge-accumulation free condition, to contrast with the results for the thick polycrystalline SCO which shows a strong dependence of SCPE on the ion beam current. Several corrections are required to evaluate the secondary positive ion yields and a method is described.

Femtosecond time-resolved photo-stimulated desorption from ionic crystals

We have used the pump–probe technique to measure the positive ion yield, from ionic crystals, as a function of time delay between two femtosecond laser pulses. The two-pulse technique allows direct observation of solid state and surface dynamics on a femtosecond timescale. We find the ion yield, from 265 nm irradiated MgO and KBr, depends critically on the time delay between pulses. For example, the Mg + desorption yield displays three distinct features; a coherence peak followed by a fast rise and decay features. In contrast, the yield of K + from KBr displays only the coherence peak and picosecond decay features. The observed ion detection thresholds suggest that, although the nanosecond laser ion desorption mechanism may be dominated by defect photoabsorption, significant electron–hole pair production may contribute to the femtosecond laser desorption mechanism. By determining the ultrafast time-dependence of positive ion emission, we hope to reveal the mechanism of laser ion desorption for both regimes.

Yield enhancement effect of low‐energy O2+ ion bombardment in Ga focused ion beam SIMS

AbstractThe effect of low‐energy oxygen ion irradiation during Ga focused ion beam (FIB) SIMS analyses has been investigated. The positive secondary ion yield in Ga FIB‐SIMS analysis was increased dramatically due to the bombardment of O2+ ions. The degree of yield enhancement increased with increased oxygen doses until it became constant above 1.5 × 1016 atoms cm−2, indicating saturation of the oxygen concentration at the analysis depth. Secondary ion images of a line‐and‐space pattern of boron in the silicon matrix have been obtained with 3 keV and 0.7 keV O2+ ions. It was revealed that the enhancement effect of irradiation with 0.7 keV O2+ ions is of the same degree as that with the 3 keV O2+ ions. From the perspective of sensitivity, however, a lower energy beam is more effective because the sputtering of the sample surface by the O2+ irradiation is reduced, resulting in a greater volume for analysis by the Ga+ beam. A secondary ion image of 0.6 at.% boron from the 0.5 µm line width with an analysis depth of 6 nm has been obtained readily by irradiation of the 0.7 keV O2+ beam. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Time-resolved femtosecond laser-induced desorption from magnesium oxide and lithium fluoride single crystals

We have used the pump–probe technique to measure the photostimulated positive-ion yield, as a function of time delay, between two ultraviolet laser pulses. We find that the ion yield from 265 nm femtosecond-irradiated MgO and LiF depends critically on the laser pulse delay in two-pulse correlation experiments. For example, single-pulse excitation of MgO produces a variety of ions including Mg +, MgO + and H +. If the femtosecond laser pulse is split into two sub-threshold beams and then recombined with a variable time delay, the Mg + desorption yield displays three distinct time regimes and persists for laser delays of over 100 ps. A pulse delay of only 500 fs nearly eliminates ion desorption except for Mg +. In contrast, the Li + desorption yield from LiF decays more rapidly, largely within the femtosecond pulse duration. A weak but measurable decay component of approximately 2 ps is indicated, however. We hypothesize that non-resonant excitation contributes to the ultrafast desorption mechanism, possibly through the creation of surface electron–hole pairs and allows direct observation of the surface dynamics.

Core-induced photofragmentation of acetonitrile adsorbed on Au(111) and Pt(111)

Photodissociation of acetonitrile (CH3CN) condensed, physisorbed on Au(111), and chemisorbed on Pt(111), has been studied at the C(1s) and N(1s) edges in H+ yield and/or in total positive ion yield. The H+ yield C(1s) absorption experiment on the condensed acetonitrile shows a selective fragmentation of the C–H bond at the C 1s→π*CH3 excitation, that is still observed when acetonitrile is physisorbed on Au(111) and chemisorbed on Pt(111). Transitions to the π*C≡N state give rise to unselective fragmentations, which are strongly quenched by charge transfer from the substrate when acetonitrile is chemisorbed on Pt(111). The π⊥*C≡N level that plunges into the Pt(111) is the most affected and vanishes in the total ion yield measurement. The quenching is weaker on the π∥*C≡N which is parallel to the surface, and thus is less hybridized with the metal than the π⊥*C≡N. For acetonitrile physisorbed on Au(111), the influence of the surface is limited to the quenching of the desorption of the heaviest outgoing fragments.

Femtosecond time-resolved laser-induced desorption of positive ions from MgO

We have used the pump-probe technique to measure the photostimulated positive ion yield as a function of time delay between two sub-threshold femtosecond laser pulses. We find that the ion yield from UV femtosecond irradiated MgO depends critically on the laser pulse delay, Δt, in two-pulse experiments. In single-pulse experiments, excitation of MgO produces a variety of ions including Mg+, MgO+, and a significant yield of H+. In contrast, if the femtosecond laser pulse is split into two sub-threshold beams and then recombined with a variable time delay, the ion yield may be drastically altered depending on the delay between pulses. The Mg+ desorption yield displays three distinct lifetimes and persists for laser delays of over 100 ps. A pulse delay of only Δt=500 fs nearly eliminates ion desorption except for Mg+. The use of a pair of delayed femtosecond laser pulses can thus control the species of the desorbed ion. The mechanism for femtosecond laser desorption is clearly different from nanosecond laser desorption. We hypothesize that the creation of electron-hole pairs by nonresonant two-photon excitation contributes to the ultrafast desorption mechanism.

Influence of the initial state of adatoms on the ratio of atoms and ions in a sputtered particle flow

The role of the initial state of adatoms in the formation of the charge composition of a sputtered particle flow is investigated. Calculations are carried out for alkali metal atoms adsorbed on a tungsten surface and subjected to mutual dipole-dipole repulsion. It is shown that the escape probability (yield) of positive ions diminishes as the surface density of adatoms increases, an effect that is enhanced when the initial state ofthe adatoms is taken into account.

Photon stimulated desorption of cations from yttria-stabilized cubic ZrO 2(100)

The positive ion yield resulting from the interaction of a pulsed 266-nm laser with yttria-stabilized cubic zirconia crystals is investigated. Although the photon energy (4.66 eV) is well below the nominal band-gap energy for ZrO 2 (5.0–5.5 eV), photon stimulated ion desorption (PSD) of Y +, Zr +, YO + and ZrO + begins at ∼2.5 MW/cm 2. We interpret this as the onset of laser ablation. Cation mass spectra collected using higher laser fluences resemble those obtained via secondary-ion mass spectrometry (SIMS). The similarity between the laser ablation and SIMS data demonstrates the importance of surface electronic structure effects in photon induced degradation of this material.

Secondary electron and positive ion emission from HTC superconductors by ion impact

We have measured the secondary electron emission (SEE) yield γ e and secondary positive ion yield γ + from high T c superconductors by ion impact at normal incidence, using a cylindrical Faraday cup, outside of which is surrounded by a shield, in vacuum of ∼10 −7 Torr. The bias V b was applied to the inner cylinder. For H + and Ar + impact on YBa 2Cu 3O 7 (YBCO), γ e and γ + reached constant values for V b > +25 and < − 90 V, respectively. The calculated reflection coefficient is much smaller than γ +, confirming that γ + is not due to the reflection of incident ions but secondary positive ions. It is found that γ e and γ + at saturation follow the electronic and nuclear stopping powers, respectively. It also appears that γ + is much larger than the calculated sputtering yield for H + impact.

Emission of secondary particles from metals and insulators at impact of slow highly charged ions

The emission of secondary electrons and ions from clean Au, C x H y Au and SiO 2 surfaces at impact of slow ( v ≈ 0.3 v Bohr) ions has been measured as a function of incident ion charge for 1 + ≤ q ≤ 75 +. Electron yields from thermal silicon dioxide films (150 nm on Si) are found to be lower than those from Au and C x H y Au for q > 3 +. Yields of negative secondary ions from SiO 2 and C x H y Au were recorded in parallel with electron emission data and exhibit a q n , n ≈ 4, dependency on incident ion charge. A direct comparison of collisional and electronic contributions to secondary ion production from SiO 2 films using a beam of charge state equilibrated Xe q= qeq (at 2.75 keV/u) shows positive and negative secondary ion yield increases with incident ion charge of > 400. Results are discussed in relation to key signatures of electronic sputtering by Coulomb explosions.

Energy loss and charge state distribution of reflected ions in N 6,7+ ion-Al(110) surface collisions

We have measured scattering angle dependent energy spectra of reflected projectiles following collisions of N 6,7+ on Al(110) along a random direction, at grazing incidence and keV energies. In a single measurement, this technique allows simultaneous determination of the positive ion yields and their respective energy loss. The most remarkable outcome of these measurements is the appearance of an initial charge state effect on the energy loss. Furthermore an extreme sensitivity of these multiply charged ions to the first layer is observed.

W Values of Protons Slowed Down in Molecular Hydrogen

For protons completely slowed down in molecular hydrogen, the mean energy W required for forming an ion pair has been studied in the energy range between 1 keV and 10 MeV, partly by measurements and partly by calculations. The experimental W values were determined for collimated beams of monoenergetic protons with energies between 1 keV and 100 keV by measuring the positive ion yield they produce in a plane-parallel ionisation chamber filled with hydrogen gas. The calculated W values were determined (i) using the Monte Carlo method for energies between 1 keV and 100 keV and (ii) within the framework of an anlytical model based on the continuous slowing down approximation in the energy range between 1 keV and 10 MeV, taking into account the proton stopping power, the effect of charge exchange, the different ionisation cross sections for protons and neutral hydrogen particles, and the contributions of secondary electrons to the total ionisation yield. The complete data set for W as a function of energy shows the typical decrease with increasing energy in the region of up to about 5 keV, followed by a smoothly oscillating shape for higher energies. It can be assumed that the oscillating shape of W as a function of energy is mainly caused by charge-exchange processes and by the ionisation yield contribution of secondary electrons.

Inverse velocities of sputtered monatomic ions: Systematics and quantification

The emission of secondary ions of about fifteen different elements, sputtered from Ti-base metal specimens, has been studied by SIMS. Both positive and negative ion yields have been measured at different exit energies up to ca 350 eV. It is found that when the logarithm of ionizability is plotted versus the inverse of the exit velocity, each element suggests a straight line behavior at energies above ca 20 eV. The gradient of the straight line is related to the respective 1st ionization potential (for positive ions) or electron affinity (negative ions). This behavior gives considerable support to the premises of modern theory of ionization in sputtering. Furthermore, the straight line plots for different elements are seen to converge as exit velocity increases; the intercepts at zero inverse velocity are found to be proportional to the respective element concentrations. This in principle offers a means of quantification in elemental analysis by SIMS, a method that does not require any external standards. The usefulness of the new method is demonstrated for ten elements sputtered from two specified titanium-base alloy standards from NIST.

Relative sensitivity factors of B related to SiGe alloy composition on secondary ion mass spectrometry with an oxygen primary ion beam

This paper describes the useful yields of positive 11B, 28Si, and 70Ge ions produced from the surface of B-doped Si1−xGex alloys when the surface is irradiated by an 8 keV positive O2 ion beam. Boron doping into alloys was done by implanting 11B ions with an energy of 30 keV at a dose of 1×1015 cm−2. The reactive sensitivity factors (RSFs) of 11B to the matrix isotopes of 28Si or 70Ge are determined using the useful yields, and the relationships between the RSFs and the alloy compositions are shown. Also described are the RSFs of 11B estimated from the local thermal equilibrium model with two internal standard elements of Si and Ge. The theoretical RSFs were a factor of 3–4 greater than the experimental RSFs.

The freon flooding technique in SIMS analysis

The efficiency of the positive secondary ion emission when exposing Cu and stainless steel samples to a freon 12 (CF 2CI 2) gas under Cs + ion bombardment has been investigated. The results show a significant enhancement of secondary positive ion yield in both cases. For the Cu sample a much greater enhancement was found under the freon gas flooding in comparison with that under oxygen.

Electronic effects in ion-stimulated desorption of positive halogen ions from semiconductor surfaces

Energy distributions of ions emitted during either 6–10 keV O 2 + or low-energy (<1 keV) electron bombardment have been studied for different ion species implanted into gallium arsenide. Some positive ions, such as F + and Cl +, have been shown to exhibit close similarity between ion-stimulated and electron-stimulated energy distributions indicating the dominance of electronic effects in ion-induced ejection of these ions. A tentative explanation based on an interatomic Auger process is given for the ion-stimulated ejection and results are correlated with measurements of the positive secondary ion yield dependence on ionization potential.

Time-of-flight investigation of the intensity dependence of laser-desorbed positive ions from SrF2

The intensity dependence of the total and specific yields of positive ions desorbed from SrF2 under 193 nm and 308 nm excimer-laser irradiation has been investigated by the time-of-flight method. The following positive ion species have been detected: F+, Sr+, Sr++, SrF++ and SrF2+. The Sr+ and SrF+ emission yields are found to increase as En, where E represents the laser energy per pulse. The exponent n is related to defect-initiated neutral particle emission and gas-phase ionization. The influence of surface damage on this power dependence is investigated. The F+ emission yield showed a quite different behaviour compared to that of the Sr+ and SrF+ emission. At both wavelengths the total positive ion emission yields saturate at a certain laser energy. In the saturation regime the SrF+ emission vanishes and alternative emission of F+ and Sr+ was observed at both wavelengths, but the total emission yield in the saturation regime (F+ + Sr+) remained constant. A Scanning Electron Microscope (SEM) was used to investigate the damage spots after laser irradiation for thermal effects.

Evidence for strongly enhanced yields of negative molecular secondary ions due to bombardment with SF n cluster ions

We have performed the first comparative study of the effect of molecular ion bombardment on the yield of positive and negative molecular ions sputtered from adsorbed layers. Mass spectra of differently treated silicon samples were measured by TOF-SIMS using 27–33 keV Xe + and SF − 6 primary ions at impact angles between 56° and 66°. The spectra of positively charged secondary ions are dominated by characteristic peaks due to C n H + m , ( m, n = 0, l, 2, …) ions and, prior to etching, by the well-known plasticizer peak at mass number 149 u. The secondary ion yields Y + are higher for SF − 6 than for Xe + impact, the yield ratio R + = Y + (SF − 6)/ Y + (Xe +) amounting to a factor of about 2 at low masses ( M < 35 u) and a factor of 4 to 6 at higher masses. This moderate yield enhancement may tentatively be attributed to a corresponding difference in the cross sections for ion induced desorption. The spectra of negatively charged secondary ions, observed under SF − 6 bombardment, exhibit high yields of H −, O −, OH −, C n H − m ( n = 0, 1, 2, …; m = 0 and 1), (SiO 2) − n and (SiO 2) n X −, where X can be H, O and OH. For M < 35 u the yield ratio R − (≈ 3) is similar to R +, but with increasing molecular ion mass R − increases rapidly to become as large as ≈ 100 for M > 80. Comparative studies using primary ion beams of Xe +, SF + 5 and SF − 5 showed that the strong yield enhancement observed in the negative SIMS mode is due to the molecular character of the projectile and only marginally (if any) related to the projectile charge. In an application of this cluster bombardment technique we show that changes in composition of the topmost layers, brought about by etching of the sample, can be studied in detail by comparing the yields of bulk-type high-mass molecular ions like (SiO 2) n X −( X = H or OH).

A coincidence counting study of spontaneous desorption

The “spontaneous” emission of ions and photons from surfaces was studied with coincidence counting time-of-flight mass spectrometry. Photons were produced by the spontaneous desorption process with an experimental yield of about 0.01%. Both positive and negative secondary ion yields were measured under various conditions. Results imply classes of desorption events. This may indicate that several types of primary ions are being produced during a single experiment. There was no detectable change in the types of primary ions involved in the spontaneous desorption process as a function of sample potential within the range examined. Positive and negative valine molecular ion yields were found to be similar.

Interaction of atomic and molecular MeV ions with surface

Secondary ion emission was studied using atomic and molecular ions bombarding organic and metallic targets. The energy of the projectile ions varied from 200 keV to 3 MeV. Relative yields of positive and negative secondary ions desorbed from phenylalanine and aluminum targets were identified by time-of-flight mass spectrometry. C q+ , O q+ , N q+ , CO + and CO 2 + (0 ≤ q ≤ 7) were the beams used in this study. The yield obtained for [M−H] −, [M + H] + and H − and H + secondary ions indicate the existence of two mechanisms for the desorption process: the kinetic and the surface mechanism.