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.     

Experimental and theoretical investigations into the origin of cross-contamination effects observed in a quadrupole-based SIMS instrument

The minimum-detection limits achievable in SIMS analyses are often determined by transport of material from surrounding surfaces to the bombarded sample. This cross-contamination (or memory) effect was studied in great detail, both experimentally and theoretically. The measurements were performed using a quadrupole-based ion microprobe operated at a secondary-ion extraction voltage of less than 200 V (primary ions mostly 8keV O ₂ ⁺ ). It was found that the flux of particles liberated from surrounding surfaces consists of neutrals as well as positive and negative ions. Contaminant species condensing on the bombarded sample could be discriminated from other backsputtered species through differences in their apparent energy spectra and by other means. The apparent concentration due to material deposited on the sample surface was directly proportional to the bombarded area. For an area of 1 mm² the maximum apparent concentration of Si in GaAs amounted to 5 × 10¹⁶atoms/cm³. The rate of contamination decreased strongly with increasing spacing between the bombarded sample and the collector. The intensities of backsputtered ions and neutrals increased strongly with increasing mass of the target atoms (factor of 10 to 50 due to a change from carbon to gold). The effect of the primary ion mass (O ₂ ⁺ , Ne⁺, and Xe⁺) and energy (5–10keV) was comparatively small. During prolonged bombardment of one particular target material, the rate of contamination due to species not contained in the sample decreased exponentially with increasing fluence. In order to explain the experimental results a model is presented in which the backsputtering effect is attributed to bombardment of surrounding walls by high-energy particles reflected or sputtered from the analysed sample. The level of sample contamination is described by a formula which contains only measurable quantities. Cross-contamination efficiencies are worked out in detail using calculated energy spectra of sputtered and reflected particles in combination with the energy dependence of the sputtering yield of the assumed wall material. The experimental findings are shown to be good agreement with the essential predictions of the model.     

Ion–ion interactions of LiPF6 and LiBF4 in propylene carbonate solutions

Self-diffusion coefficients of Li⁺ D_Li⁺, PF₆⁻ D_PF6⁻ and solvent propylene carbonate (PC) D_PC in LiPF₆−PC solutions were determined at 298 K by the pulse gradient spin echo (PGSE) NMR technique over the salt concentration range of 0.1–3.0 M (M = mol dm^– 3). The order of the diffusion coefficients was found to be D_Li⁺ < D_PF6⁻ < D_PC over the concentration range examined, and they were monotonically decreased with increasing the salt concentration. Haven ratio Λ/Λ_NMR, where Λ and Λ_NMR represent the ionic conductivity measured electrochemically and that estimated via the Nernst-Einstein equation using the diffusion coefficient, respectively, was evaluated as the measure of the ion–ion interaction in the LiPF₆–PC solutions. Though Λ/Λ_NMR values for LiPF₆-solutions decrease with increasing the salt concentration, they were greater than those for LiBF₄–PC solutions over the whole concentration range examined, which indicates that the ion pair formation ability of PF₆^– ion is weaker than that of the BF₄^– ion. The smaller value of the ionic conductivity for the highly concentrated LiPF₆–PC solution (above 2.0 M) than that of the LiBF₄-solutions can be attributed to the more rapidly increased viscosity relative to the LiBF₄-solution. Classic molecular dynamics (MD) simulations for the respective LiPF₆ and LiBF₄-solution of 0.5 and 1.0 M were also carried out based on the effective pair potentials. Diffusion coefficients, ionic conductivity and Haven ratio for these solutions were calculated from MD trajectories, and they qualitatively agree with those evaluated by experiments. Pair correlation functions g_LiO(r) (for Li⁺–O (PC) pair) and g_LiPF6(r) (for Li⁺–PF₆^– pair) or g_LiBF4(r) (for Li⁺–BF₄^– pair) revealed that the lithium ion weakly forms the contact ion pairs with PF₆^–, whilst strongly with BF₄^–, which supports the present experimental results. Moreover, the simulation results show that both anions in the contact ion pairs predominantly take the monodentate form, which is in contrast to the multidentate coordination predicted by ab initio calculation in gas phase.     

On the energy spectra of secondary ions emitted from silicon and graphite single crystals

Secondary ion emission from silicon and graphite single crystals bombarded by argon ions with energies E ₀ varied from 1 to 10 keV at various angles of incidence α has been studied. The evolution of the energy spectra of C⁺ and Si⁺ secondary ions has been traced in which the positions of maxima (E _max) shift toward higher secondary-ion energies E ₁ with increasing polar emission angle θ (measured from the normal to the sample surface). The opposite trend has been observed for ions emitted from single crystals heated to several hundred degrees Centigrade; the E _max values initially remain unchanged and then shift toward lower energies E ₁ with increasing angle θ. It is established that the magnitude and position of a peak in the energy spectrum of secondary C⁺ ions is virtually independent of E ₀, angle α, and the surface relief of the sample (in the E ₀ and α intervals studied). Unusual oscillating energy distributions are discussed, which have been observed for secondary ions emitted from silicon (111) and layered graphite (0001) faces. Numerical simulations of secondary ion sputtering and charge exchange have been performed. A comparison of the measured and calculated data for graphite crystals has shown that C⁺ ions are formed as a result of charge exchange between secondary ions and bombarding Ar⁺ ions, which takes place both outside and inside the target. This substantially differs from the ion sputtering process in metals and must be taken into account when analyzing secondary ion emission mechanisms and in practical applications of secondary-ion mass spectrometry.     

Features of the energy distribution of secondary particles upon the ion bombardment of graphite

The secondary emission of carbon atoms from the (0001) plane of graphite nanocrystallites bombarded with argon ions with energies of 1 and 10 keV and the incidence angle α = 45° is investigated. The unusual oscillating energy distributions of secondary C⁺ ions with main maxima E _max in the range of 40–60 eV and peaks corresponding to the energies E ₁ ≈ 20, 30, 70, 80, and 100 eV have been revealed. The C⁺ ion yield decreases, the energy spectrum increases, and the maximum E _max shifts to larger energies E ₁ with increasing emission angle (with respect to the normal to the surface). The secondary-ion emission from the (0001) face of graphite is numerically simulated with allowance for the charge exchange of secondary ions to obtain a qualitative explanation of the observed results.     

Quantitative study of oxygen enhancement of sputtered ion yields. I. Argon ion bombardment of a silicon surface with O2 flood

A novel quantification approach is applied to determine in situ the amount of surface oxygen within the sputtered particle escape depth during steady-state sputter depth profiling of silicon under simultaneous oxygenation with an oxygen flood gas or with an oxygen primary ion beam. Quantification is achieved by comparing the secondary ion intensities of ¹⁶O that is adsorbed or implanted at the Si surface with the measured peak intensities of a calibrated ¹⁸O ion implant used as a reference standard. Sputtered ion yields can thereby be related to surface oxygen levels. In the present work the dependences of the partial silicon sputter yield Y and of the positive and negative secondary ion useful yields UY(X^±) (X = B, O, Al, Si, P) on the oxygen/silicon ratio, O/Si, in the sputtered flux are studied for ⁴⁰Ar⁺ bombardment of Si with simultaneous O₂ flooding. The silicon sputter yield is found to decrease with increasing flood pressure and O/Si ratio by up to a factor of 3. Both positive and negative secondary ion yields are enhanced by the presence of oxygen at the silicon surface. The useful ion yield of Si⁺ scales non-linearly with the atom fraction of surface oxygen; this behavior is shown to invalidate models that suggest that Si⁺ ion yield enhancement is dominated either by isolated oxygen atoms or by formation of SiO₂ precipitates. In contrast a microscopic statistical model that assumes that local Si⁺ ion formation depends only on the number of oxygen atoms coordinated to the Si atom to be ejected fits the ion yield data quantitatively.     

Trap centers in Au-implanted MOS structures

Trap centers in the Si-SiO₂ interface region of MOS structures doped by ion implantation of gold have been investigated using constant capacitance deep level transient spectroscopy (CC-DLTS). Gold doses of 10¹²–3 × 10¹³ cm^–2 were implanted into the back surface of the wafers and were then redistributed during a diffusion anneal for 30 min at 1100° or 900° C. Three Au-related trap levels have been observed in the interface region, which were attributed to the Au-donor (E _v +0.35 eV), the Au-acceptor (E _v +0.53 eV), and the Au-Fe complex (E _v +0.45 eV). The trap concentration profiles show that the Si-SiO₂ interface affects the Au concentration in a depth range of 1 m from the interface and that gettering of Au occurs at the interface. The interface state density is independent of the Au concentration at the interface even for concentrations of 10¹⁵ cm^–3.     

Self-assembling of C and Sn in Ge

Self-assembling of isoelectronic C and Sn impurities in Ge is predicted. The formation of the 1C4Sn tetrahedral cells is thermodynamically profitable in Ge-rich C_xSn_yGe_1−x−y (4x<y) alloys in the ultra dilute C impurity limit with 1×10^-8x1×10^-3. The concentrations of Sn atoms when all C atoms are surrounded only by Sn atoms are estimated for the lower molecular beam epitaxy, intermediate annealing and higher bulk crystallization temperatures. The origin of this phenomenon is a considerable decrease of the strain energy after self-assembling. The same self-assembling in Si is thermodynamically non-profitable due to the large cohesive energy of Si–C chemical bonds.     

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10¹⁸ cm⁻³). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10⁻⁴ (Al), 8.0×10⁻⁵ (Al/Au) and 1.2×10⁻⁵ Ω cm² (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.     

Secondary-ion emission from clean and oxygen-covered beryllium surfaces: II. Energy dependence

The secondary-ion energy distribution obtained by sputtering clean and oxygen-covered Be has been analyzed in terms of competing processes in secondary ion emission. The ion energy distribution N⁺(E) has been separated into an ionization coefficient R⁺(E) and a total energy distribution, N(E), i.e. N⁺(E) = R⁺(E) N(E). Experimentally, the dependence of R⁺(E) on both energy and oxygen coverage indicated a linear superposition of adiabatic tunneling and resonanance ionization processes from clean and oxygen-covered portions of the surface with no contributions to the secondary-ion yield from regions of intermediate coverage. Total energy distributions of sputtered Be atoms have been deduced and the principal features agree with the predictions of the collision cascade sputtering model. Variations of the energy distributions with oxygen coverage are in accord with small changes expected in the surface binding energy as a result of surface oxidation.     

The influence of superradiant relaxation on associative ionisation yield of laser excited potassium atoms

The influence of superradiant relaxation of excited 7P _1/2 potassium atoms on their associative ionisation yield has been investigated. The excitation was carried out by a laser pulse in a tube containing potassium vapour with concentration 10¹⁴ cm^–3. A seven-fold drop of ionisation yield was found and spontaneous luminescence pulse distortions by superradiance (SR) were measured in the wide interval of excited atoms densities N _e ⁰ 10⁸–10¹³ cm^–3. The values of associative ionisation rate constant and SR threshold atomic densities are obtained, and the shapes and quantum yields of SR pulses are evaluated from the experimental results.     

Electronic structure of high-temperature superconductors II. La-Ba-Cu-O systems

Electronic structure of La_2–x Ba _x CuO₄ superconductors has been studied. Model systems La₁₀CuO ₂₂ ^–12 , La₁₀cuO ₁₄ ⁺⁴ and La_gBaCuO ₁₄ ⁺³ with various Ba positions have been investigated using the quasirelativistic INDO molecular orbital method. The oxidation state of copper atoms corresponds to Cu(II). The influence of Ba/La substitution on copper atoms is much lower than on their oxygen ligands sphere.     

Radiation damage and annealing studies of ion bombarded cobalt

The effects of implantation-induced radiation damage on the thermal oxidation of cobalt have been studied. Bombardment by both Co⁺ self-ions and by Xe⁺ has been studied as a function of ion dose, energy and annealing temperature. A major increase in oxidation was observed for doses of >10¹⁶ Co⁺ cm^–2 in agreement with previous studies on Al. The oxidation behaviour as a function of annealing temperature was markedly different for Co⁺ and Xe⁺ bombarded samples. For Co⁺ bombarded samples, damage anneals rapidly in the temperature range 20–300°C due to thermally assisted repair of point defects and vacancy clusters. However, for Xe⁺ bombardment, it is proposed that the higher annealing temperatures required for damage repair arise due to the stabilisation of three-dimensional vacancy clusters by the oversized Xe atoms. The increase in oxidation after annealing in the temperature range 300–500°C is thought to be due to vacancy release mechanisms which may affect oxide nucleation.     

High-Resolution Laser Spectroscopy of YbCl: The AΠ–XΣ Transition

The A²Π–X²Σ⁺ transition of ¹⁷⁴Yb³⁵Cl and ¹⁷²Yb³⁵Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm⁻¹. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A²Π state are A_e = 1491.494(2) cm⁻¹ and R_e = 2.4433(1) Å, and for the X²Σ⁺ state, R_e = 2.4883(2) Å and γ_e = 4.59(2) × 10⁻³ cm⁻¹. The interaction between the X²Σ⁺ and A²Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.     

Structural and magnetic properties of rapidly solidified (Ce0.2Fe0.8)1-x Al x ribbons determined by X-ray diffraction and Mössbauer spectrometry

(Ce_0.2Fe_0.8)_1-x Al _x (0 x 0.9) ribbons have been prepared by planar flow casting under an He atmosphere with a linear velocity of 29 m s^-1. Analyses of the ribbons by X-ray diffraction and⁵⁷Fe Mössbauer spectrometry in the temperature range 4–300 K show that all the ribbons are crystalline. With increasingx, the observed phases are Ce(Fe, Al)₂, Ce₂(Fe, Al)₁₇, CeFe_4+y,Al_8-y and the single fcc aluminium phase. For the different phases, the line intensities of the Mössbauer spectra agree with previous results on the preferential substitution sites for aluminium. A coherent hyperfine parameters set was deduced from fitting spectra in the temperature range 4–300 K.     

Mixed hyperfine interactions in Fe-Mg-O: magnetic behaviour

The complex Mössbauer spectra exhibited by Fe_xO (x0.91) and (Fe_1–y Mg _y )_xO (y=0.15–0.85) powder samples at liquid helium temperature have been analysed by a Hamiltonian treatment to allow for the significant electric field gradients present at the Fe²⁺ defect sites. The magnetic behaviour of the defect clusters are considered in terms of antiferromagnetic couplings, consistent with the spin glass-like behaviour reported recently for magnesiowüstite.     

Resonant Raman scattering by N-related local modes in AlGaAs/InGaAsN multiquantum wells

We report resonant Raman scattering and secondary ion mass spectrometry measurements on InGaAsN/AlGaAs multiquantum wells grown by plasma-assisted molecular beam epitaxy. The appearance of a strong TO band at resonance with nitrogen (N)-related electronic levels has been observed. The N-induced vibration mode at 470 cm⁻¹ changes in intensity and shape with increasing N and In content. A new vibration mode has been observed at 320 cm⁻¹, whose intensity scales with the N concentration. This mode is not present in InGaAsN films, so it is linked to the presence of Al. Its frequency is close to the B₁ silent mode of wurtzite GaN. It is attributed to the formation of GaN pairs, near the MQW interfaces as a consequence of the preferential Al–N bonding.     

Luminescence of orthovanadates M 3 + M3+(VO4)2 and M 2 + M4+ (VO4)2

The absorption, excitation, and luminescence spectra of vanadates of type M ₃ ⁺ M³⁺(VO₄)₂ and M ₂ ⁺ M⁴⁺(VO₄)₂ are studied, where M⁺ is Na, K, Rb, Cs; M³⁺ is Al, Sc; M⁴⁺ is Zr, Sn, Ti. The luminescence spectra maxima are located at 490–510 nm, while those of the excitation spectra are at 360–375 nm. Temperature characteristics of luminescence and thermostimulated luminescence are studied. The question of activation of complex vanadates by rare-earth ions is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 24–28, March, 1976.     

Al and Cu NMR study in UCu5Al

We have studied the microscopic properties of the tetragonal UCu₅Al Kondo compound by ²⁷Al and ^63,65Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed χ>χ. Plots of K(T) against χ(T) at temperatures T≥100 K yield the transferred hyperfine fields of +5.9 kOe/μ_B for ²⁷Al nuclei, and +5.3 and −7.0 kOe/μ_B for ⁶⁵Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T<100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.     

Monoatomic and cluster beam effect on ToF-SIMS spectra of self-assembled monolayers on gold

Self-assembled monolayers represent well-defined systems that is a good model surface to study the effect of primary ion beams used in secondary ion mass spectrometry. The effect of polyatomic primary beams on both aliphatic and aromatic self-assembled monolayers has been studied. In particular, we analysed the variation of the relative secondary ion yield of both substrate metal-cluster (Au_n⁻) in comparison with the molecular ions (M⁻) and clusters (M_xAu_y⁻) by using Bi⁺, Bi₃⁺, Bi₅⁺ beams. Moreover, the differences in the secondary ion generation efficiency are discussed. The main effect of the cluster beams is related to an increased formation of low-mass fragments and to the enhancement of the substrate related gold-clusters. The results show that, at variance of many other cases, the static SIMS of self-assembled monolayers does not benefit of the use of polyatomic primary ions.