The high-temperature behaviour of gallium and indium liquid metal ion sources

Energy distribution diagrams and derived data are presented for gallium and indium liquid metal ion sources operated at elevated temperatures. Results for the gallium source confirm that a secondary peak is formed on the energy distribution diagram at source temperatures above 250°C. Contrary to the findings of other research workers, data presented here show that the indium source displays similar characteristics to that of gallium. Off-axis data are also given, showing that secondary peak formation is not limited to the centre of the beam. Present hypotheses propose that secondary peak formation is the result of an increased contribution to emission by free-space field ionisation at elevated temperatures. Data presented here for the gallium and indium sources are discussed and the above hypotheses are examined. It is concluded that a field ionisation mechanism does not satisfactorily explain the form of the high temperature liquid metal ion source energy distributions.     

Direct recognition of non-radiative recombination centers in semi-insulating LEC InP:Fe using double excitation photoluminescence

In order to observe the effect of intra-band gap excitation on the photoluminescence (PL) properties of undoped InP and iron doped InP (InP:Fe), PL measurements were performed in InP crystals with thickness of 360 μm and area of about 4×3 mm², grown by the liquid encapsulated Czochralski (LEC) technique upon excitation with both Ar-ion laser and 980 nm light. The PL intensities for InP:Fe under 980 nm wavelength light illumination relative to no illumination increased by about 52%, 33%, and 12% for the 1.337, 1.380, and 1.416 eV peaks, respectively, at 10 K, whereas there was no illumination effect for undoped InP. This is a strong indication that Fe centers play a role as non-radiative recombination centers to decrease the PL intensity. PL experiments were performed in the spectral range of 1320-1440 meV for InP in the sample temperature range of 10-160 K. The electron and hole photoionization cross-sections at 980 nm wavelength light illumination were calculated as and , respectively.     

The measurement of energy spectra of neutral particles in low energy ion scattering

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×10¹⁰ ions/cm²) is much smaller than the dose needed for an ESA-spectrum (6×10¹³ ions/cm²). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV⁴⁰Ar⁺ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.     

On-axis energy analysis of69Ga+, Ga2+, and Ga 2 + ions emitted from a gallium liquid metal ion source

On-axis energy distribution measurements of 3 ionic species emitted from a gallium liquid metal ion source are reported. Voltage deficits, FWHM energy spreads, angular intensities and chromatic angular intensities are presented and compared with other published work. Ga⁺ deficit variations with beam current and temperature are discussed in detail. Ga ₂ ⁺ FWHM and deficit changes with source operating conditions are accounted for by a model of droplet fragmentation.     

Theoretical investigation of liquid metal ion sources: Field and temperature dependence of ion emission

We investigate the field and temperature dependence of the rate of ion formation by important mechanisms in Liquid Metal Ion Sources (LMIS). In addition to field evaporation and field ionization of thermally evaporated neutrals we identify a third mechanism, thermal-field evaporation, which is intermediate between the other two mechanisms. Field (or thermal-field) evaporation is found to be dominant in the normal operating regime of LMIS in agreement with the conclusions of Prewett et al. A jet-like protrusion model of LMIS shape, which is consistent with direct observations, allows high temperatures to be reached with a reasonable power input to the source. Thus a small, but sometimes important, contribution to the total ion current from field ionization of thermally evaporated atoms is expected.     

Optical absorption and photoluminescence studies of Eu-doped phosphate and fluorophosphate glasses

Phosphate (P₂O₅+K₂O+BaO+Al₂O₃+Eu₂O₃) and fluorophosphate (P₂O₅+K₂O+BaO+BaF₂+Al₂O₃+Eu₂O₃) glasses with different Eu³⁺ ion concentrations have been prepared and characterized through optical absorption, photoluminescence and decay times. An intense red luminescence is observed from the ⁵D₀ emitting level of Eu³⁺ ions in these glasses. The relative luminescence intensity ratio of ⁵D₀→⁷F₂→⁵D₀→⁷F₁ transitions has been evaluated to estimate the local site symmetry around the Eu³⁺ ions. The emission spectra of these glasses show a complete removal of degeneracy for the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions. Second and fourth rank crystal-field (CF) parameters have been calculated together with the CF strength parameter by assuming the C_2v symmetry for the Eu³⁺ ions in both the phosphate and fluorophosphate glasses. Judd-Ofelt parameters have been evaluated from the luminescence intensity ratios of ⁵D₀→⁷F_J (J=2, 4 and 6) to ⁵D₀→⁷F₁ transitions. These parameters have been used to derive radiative properties such as transition probabilities, branching ratios, radiative lifetimes and peak stimulated emission cross-sections for the ⁵D₀→⁷F_J transitions. Decay curves of the ⁵D₀ level of Eu³⁺ ions in these two Eu³⁺:glass systems have been measured by monitoring the ⁵D₀→⁷F₂ transition (611 nm) at room temperature. The experimental lifetime of the ⁵D₀ level in the title glasses is found to be higher than Eu³⁺-doped niobium phosphate glasses. The analysis indicates that the lifetime of the ⁵D₀ level is found to be less sensitive to the Eu³⁺ ion concentration and addition of BaF₂ has no significant effect on the optical properties of Eu³⁺-doped phosphate glasses.     

Derivation of a modified paraxial formalism for two-dimensional trajectories in electron and ion sources of non-simple geometries

A modified paraxial formalism has been developed which describes two-dimensional charged particle beam dynamics in electron and ion sources with pointed or needle-type geometries. A Hamiltonian formalism, and a more exact treatment of energy conservation is used to derive the modified paraxial equation for two-dimensional trajectories in systems with axially symmetric prolate-spheroidal beams. Calculations have been done for a gallium field emission liquid metal ion source modeled by a hyperboloid of revolution and planar extractor. Two important conclusions emerge from these calculations: i) The dominant effect of space-charge, for source geometries with small radii of curvature, occurs in the region close to the apex (<0.05 n) and ii) beam divergence has a strong dependence on geometry. This latter effect is a consequence of large two-dimensional field gradients near the apex of sources with needle-type or pointed geometry.This work was supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-8108829     

Quantitative analysis of photoacoustic IR spectra

Photoacoustic spectroscopy is a method for directly measuring the absorption properties of gases and condensed matter. We have developed a photoacoustic gas cell for a rapid-scan Fourier spectrometer. With this cell we can extend the spectral range down to 180 cm^–1; more than one octave lower than previously reported useful broadband measurements. The photoacoustic spectrum of a microscope cover glass was measured from 180 ... 200 cm^–1 and normalized with respect to the spectrum of carbon black as a reference material. Starting from the one-dimensional equation of heat conduction we derived an expression for the surface temperature of single and double-layer samples. We calculated the surface temperature of the glass lamella and divided it by the corresponding values for carbon black using the thermal conductivity of carbon black as a fitting parameter. We show that the one-dimensional model calculation reproduces the experimental spectrum over the whole spectral range.     

The emission characteristics of an indium needle-type liquid metal ion source

Operational data for an indium needle-type liquid metal ion source are presented. Detailed comparisons are drawn between these characteristics and those of a similar gallium source. The behaviours of the two sources are found to be strikingly similar, indicating a common mechanism of ion emission.     

Shape of a liquid metal ion source

A model of the liquid-metal ion-source shape consisting of a jet-like protrusion on the end of a Taylor cone shape is shown to be consistent with a field evaporation mechanism of ion formation, fluid dynamic considerations, space charge effects and recent TEM observations. The diameter of the ion emitting area is found to be only a few tens of Å. Self-consistent numerical calculations of electric potential and particle trajectories predict emission characteristics which compare favorably with experimental results.On leave from Trinity Hall, Cambridge, UK     

On the mechanism of liquid metal ion sources

We have developed a theoretical model of liquid metal ion source operation which consistently explains the shape and size of the ion emitting region, the mechanism of ion formation and properties of the ion beam. We find that field evaporation is the main current generating mechanism and that field evaporation and subsequent postionization produce the doubly and higher charged ions. Field ionization of thermally evaporated neutrals may make a significant, but not dominant, contribution to the current of singly charged ions. Our model is consistent with experimental results on energy spread, energy deficit and charge state ratios and we are able to explain the stability of the emitted ion current.     

Polarization analysis of the light emitted by an optically pumped3He gas

Optical measurements of the nuclear polarizationM of an optically pumped pure³He gas have been performed. They consist in monitoring the circular polarization of various helium lines emitted by the discharge. We have demonstrated the possibility to use the orange line (587.5 nm) to measureM and we have calibrated this technique over the pressure range 0.4–6.5 Torr. The circular polarization of the light has been analyzed using a new device which includes a static polarimeter and photodiodes as light sensors. The intensity of the discharge was modulated and the detection was made at the frequency of modulation.Laboratoire associé au Centre National de la Recherche Scientifique et a l'Université P. M. Curie     

Infrared studies on ion irradiated quartz

Damage produced inα- and fused quartz bombarded with low energy (∼ 100 keV) D⁺, He⁺ and Ar⁺ ions, has been studied by observing the changes in their spectrum. Besides bulk reflectivity, the attenuated total reflection spectrum has also been studied, the latter with a view to obtaining the surface polariton frequencies. It is observed that for the same fluence, the changes following D⁺ irradiation are much higher compared to that for Ar⁺ irradiation. The variation of the surface polariton frequency inα-quartz with the damage energy deposited has the same trend as observed earlier for refractive index. Some annealing studies have also been performed in argon-irradiated samples. These studies indicate that whereas in fused quartz the damaged layer recovers completely, inα-quartz there is a residual amorphization even after annealing. A two-layer model is proposed which gives a reasonable simulation of the observedir properties.     

Measurement of energy spread and angular intensity of a cesium liquid metal ion source

This paper determines the optimum range of total ion current for a cesium liquid metal ion source for use as a focused ion beam (FIB). This range is determined from a figure of merit calculated from measurements of the angular intensity and energy spread of emitted ions. Judging from both the figure of merit and the tail of the energy distribution curves, the total emission current should be set near 1 A for a cesium FIB with a high current density. Assuming that both Cs- and Ga-FIBs have the same diameter, the relative current density of a Cs-FIB is expected to be approximately 80% that of a Ga-FIB.     

Spectroscopic studies of copper and carbon ion irradiated polypropylene

The samples of polypropylene (PP) have been irradiated with 120 MeV ⁶⁴Cu⁹⁺ and 70 MeV ¹²C⁵⁺ ion beams, with the fluence ranging from 1 × 10¹³ to 1 × 10¹¹ ions/-cm⁻². UV-VIS and FTIR techniques have been used to study the chemical and optical properties of these irradiated polymers. UV spectra revealed that the optical-gap energy decreases by 54 % with copper ion irradiation at the fluence of 1 × 10¹³ ions/cm², whereas at the same fluence, carbon beam decreases the optical-gap energy by 20%. FTIR analysis of ion irradiated samples revealed the presence of -OH, C = O and C = C bonds. Alkyne formation has been observed only in the case of copper ion irradiation.      

Thermoluminescence and photoluminescence of LiNaSO4:Eu irradiated with 24 and 48 MeV Li ion beam

Thermoluminescence (TL) of LiNaSO₄:Eu phosphor, irradiated with 24 and 48 MeV ⁷Li ions at different fluences in the range 5×10⁹-1×10¹² ion/cm², has been studied. The samples from the same batch were also exposed to γ-rays from a Cs¹³⁷ source for comparative studies. The TL glow curves of the materials, irradiated with ⁷Li ions, have similar structures to that of γ-irradiated sample. They have a simple structure with a prominent peak at 412 K along with small one at around 481 K. The intensity ratios of 412-481 K peaks have been observed to increase with fluence increasing, while that of γ-irradiated sample shows a reverse trend. This could be attributed to the changes in the recombination center populations due to ⁷Li ions, that have been implanted inside the matrix of LiNaSO₄:Eu, during irradiation and might also act as a source for new trapping and luminescent centers. The implantation has been confirmed by TRIM calculations. The penetration depths (where the ion comes to rest) are found to be 145 and 463 μm corresponding to 24 and 48 MeV ion beam energies, respectively, which are less than the thickness of the sample chips (∼800 μm). The efficiencies of LiNaSO₄:Eu to 24 and 48 MeV ⁷Li ions measured relative to γ-rays of Cs¹³⁷ are found to be 0.007 and 0.024, respectively. Theoretical analysis of the glow curves of the samples irradiated by ⁷Li ions and γ-rays were done by glow curve deconvolution method to determine trapping parameters of various peaks. The experimentally observed linearity/sublinearity has been discussed in the frame of track interaction model. Photoluminescence studies in the ⁷Li ions irradiated and un-irradiated samples show that europium ions have incorporated in the host in their divalent (emission at 440 nm) as well as trivalent (emissions at 594, 615 and 700 nm) forms. The intensities of the emission bands of these ions have been observed to increase with fluence increasing.     

Simulation of photoacoustic IR spectra of multilayer structures

In photoacoustic spectroscopy a sample in a closed gas cell is heated by periodically modulated light. When thermal waves generated by the warm spots inside the sample reach the surface they heat up the adjacent gas. This causes periodic pressure variations which are detected by a microphone. We present a matrix method which enables us to calculate the surface temperature of a multilayer structure with any number of homogeneous lamellae of any optical and thermal properties. This method is based on the multiple reflections and interferences of thermal waves inside the system of lamellae. Photoacoustic spectra simulated by this method reproduce the measured spectra. This is demonstrated for a mylar (polyethyleneterephthalate, PETP) foil coated with a thin antimony layer.     

Frequency- and wavelength-modulation spectroscopies: Comparison of experimental methods using an AlGaAs diode laser

Low-wavelength modulation (1 kHz), high-wavelength modulation (100 MHz) and two-tone frequency modulation (390±5 MHz) spectroscopies are systematically compared by measuring the minimum detectable absorption achieved using an AlGaAs diode laser tuned on a third-overtone methane transition at 886 nm. From the S/N behavior has been extrapolated a minimum relative absorption (1 Hz of bandwidth) of 4.5(1)×10^–7 for the LMW, 9.7(3)×10^–8 for the HWM and 6.4(2)×10^–8 for the TTFM. In the LWM case the detection-limit value is represented by the laser amplitude 1/f excess noise, while for the high-frequency detection techniques this contribution is negligible with respect to other noise sources. These detection limits well agree with the calculated quantum limited values based on measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus.     

Swift heavy ion induced photoluminescence studies in Aluminum oxide

Aluminum oxide, a promising material for high temperature applications, is synthesized by combustion route and characterized by X-ray diffraction technique. Photoluminescence of aluminum oxide bombarded with 120 MeV swift Au⁹⁺ ions have been studied at room temperature. The observed PL emission with peak at ～420 nm is attributed to F-center while the two more weak emissions with peaks at ～482 and ～525 nm are attributed to aggregates of F-centers. It is found that PL intensity increases with increase in Au⁹⁺ ion fluence up to ～1×10¹³ ions cm^?2 and thereafter it reaches saturation. The Fourier transform Infrared spectroscopy results show the destruction of Al?O?H bonds whereas the XRD results indicate the surface amorphization of Al₂O₃.     

Investigation of partial sputtering of lithium from a binary Al/Li alloy with laser induced fluorescence

Sputtering investigations of an Al/Li alloy containing 9.1 at-% of lithium have been performed for 6 keV helium ion bombardment. Absolute particle densities and velocity distributions of the sputtered neutral lithium atoms were measured with laserinduced fluorescence. The amount of sputtered lithium was found to be constant for target temperatures ranging from room temperature up to 500° C. The mean transport velocity and the sputtering yield of the Li component have been calculated from the measurements. Thermal evaporation of neutral Li atoms could be measured independently of the presence of the helium beam for target temperatures above 300° C. The experimental results indicate that the surface is covered by lithium with at least several atomic layers even under highcurrent ion irradiation.Preliminary results have been presented at the SYMPOSIUM ON SPUTTERING, Spitz/Austria (1986)