Quantitative determination of element distributions in silicon based thin film solar cells using SNMS

The determination of elemental distributions in thin film solar cells based on amorphous silicon using electron beam SNMS is possible by quantifying the measured ion intensities. The relative sensitivity factors (RSFs) for all elements measured have to be known. The RSFs have been determined experimentally using implantation and bulk standards with known concentrations of the interesting elements. The measured RSFs have been compared with calculated RSFs. The model used for the calculation of the RSFs takes into account the probability for electron impact ionization and the dwell time of the neutrals inside the postionization region. The comparison between measured and calculated RSF shows, that this model is capable to explain the RSFs for most elements. Differences between calculated and measured values can be explained by the formation of hydride and fluoride molecules (in case of H and F) and influences of the angular distribution of the sputtered neutrals in case of Al. The experimentally determined RSFs have been used for a quantification of depth profiles of the i-, buffer-, p- and front contact layers of a-Si solar cells.     

Quantitative determination of element distributions in silicon based thin film solar cells using SNMS

The determination of elemental distributions in thin film solar cells based on amorphous silicon using electron beam SNMS is possible by quantifying the measured ion intensities. The relative sensitivity factors (RSFs) for all elements measured have to be known. The RSFs have been determined experimentally using implantation and bulk standards with known concentrations of the interesting elements. The measured RSFs have been compared with calculated RSFs. The model used for the calculation of the RSFs takes into account the probability for electron impact ionization and the dwell time of the neutrals inside the postionization region. The comparison between measured and calculated RSF shows, that this model is capable to explain the RSFs for most elements. Differences between calculated and measured values can be explained by the formation of hydride and fluoride molecules (in case of H and F) and influences of the angular distribution of the sputtered neutrals in case of Al. The experimentally determined RSFs have been used for a quantification of depth profiles of the i-, buffer-, p- and front contact layers of a-Si solar cells.     

New Approach to the Calculation of Relative Sensitivity Factors in Inductively Coupled Plasma Mass Spectrometry

The relative sensitivity factors (RSFs) of 68 elements in inductively coupled plasma mass spectrometry were determined. The ionization process in an inductively coupled plasma was found to be only approximately described by the Saha–Eggert equation. A relationship between the RSFs and the absolute electronegativities of atoms was found. This factor has the strongest effect on the accuracy of the calculations of RSFs for chemically active elements. The average relative systematic error of the calculations of RSFs taking into account absolute electronegativity was reduced to 0.30.     

Relative elemental sensitivity factors in non-resonant laser-SNMS

Using a reflectron time-of-flight mass spectrometer, the ionization process in non-resonant Laser postionization Secondary Neutral Mass Spectrometry (SNMS) has been investigated. In particular, the postionization efficiencies (PIE) achieved by multi photon and single photon absorption have been compared by ionizing ten elements sputtered from a NIST standard reference material by excimer laser radiation of 248 nm, 193 nm and 157 nm. Only in the case of single photon ionization (SPI) the measured laser intensity dependence of the PIE can be understood quantitatively in terms of corresponding theory. From the results, absolute values of the SPI cross sections have been evaluated for atoms of nine elements, which show a total variation over about two orders of magnitude. Furthermore, even in the regime of high laser intensity, where the ionization of all atoms is completely saturated, different elements have been detected with relative sensitivity factors which scatter over about one order of magnitude. This has been attributed to element dependent variations of the effective ionization volume which are caused by the different kinetic energy and angular distributions of different sputtered atoms.     

Relative elemental sensitivity factors in non-resonant laser-SNMS

Using a reflectron time-of-flight mass spectrometer, the ionization process in non-resonant Laser postionization Secondary Neutral Mass Spectrometry (SNMS) has been investigated. In particular, the postionization efficiencies (PIE) achieved by multi photon and single photon absorption have been compared by ionizing ten elements sputtered from a NIST standard reference material by excimer laser radiation of 248 nm, 193 nm and 157 nm. Only in the case of single photon ionization (SPI) the measured laser intensity dependence of the PIE can be understood quantitatively in terms of corresponding theory. From the results, absolute values of the SPI cross sections have been evaluated for atoms of nine elements, which show a total variation over about two orders of magnitude. Furthermore, even in the regime of high laser intensity, where the ionization of all atoms is completely saturated, different elements have been detected with relative sensitivity factors which scatter over about one order of magnitude. This has been attributed to element dependent variations of the effective ionization volume which are caused by the different kinetic energy and angular distributions of different sputtered atoms.     

Diagnostic study of laser ablated YBa2Cu3Oy plumes

Post-ablation ionisation in conjunction with a reflectron time-of-flight mass spectrometer has been used to investigate a number of species in the ablation plume from a YBa₂Cu₃O_y target. The experiments were carried out using a Q-switched Nd:YAG laser with typical intensities of ≈ 10⁸ W cm⁻² characteristic of the fluences (1 J cm⁻²) required for the pulsed laser deposition of thin superconducting films. By varying the delay between the ablation and the ionisation laser, the velocity distributions of several of the species from the target have been measured simultaneously. It has been observed that, although some of the atoms and molecules (i.e. Cu, Ba and BaO) have similar velocities, the atoms and oxides of Y (Y and YO) have very different velocities. The yttrium atoms and oxides were observed to be slower than the barium atoms and oxides at both ablation wavelengths examined (355 and 532 nm) and at two different distances from the target surface (2 and 3 mm). It is suggested that Ba, Cu and their oxides are ablated directly from the surface as neutrals, whereas Y and YO form clusters in the ablation plume. These clusters are then fragmented by the post-ionisation laser to produce Y and YO ions.     

Principle and analytical applications of resonance lonization mass spectrometry

Resonance ionization mass spectrometry (RIMS) is a very sensitive analytical technique for the detection of trace elements. This method is based on the excitation and ionization of atoms with resonant laser light followed by mass analysis. It allows element and, in some cases, isotope selective ionization and is applicable to most of the elements of the periodic table. A high selectivity can be achieved by applying three step photoionization of the elements under investigation and an additional mass separation for an unambiguous isotope assignment.An effective facility for resonance ionization mass spectrometry consists of three dye lasers which are pumped by two copper vapor lasers and of a linear time-of-flight spectrometer with a resolution better than 2500. Each copper vapor laser has a pulse repetition rate of 6.5 kHz and an average output power of 30 W.With such an apparatus measurements with lanthanide-, actinide-, and technetium-samples have been performed. By saturating the excitation steps and by using autoionizing states for the ionization step a detection efficiency of 4 × 10^–6 and 2.5 × 10^–6 has been reached for plutonium and technetium, respectively, leading to a detection limit of less than 10⁷ atoms in the sample. Measurements of isotope ratios of plutonium samples were in good agreement with mass-spectrometric data. The high elemental selectivity of the resonance ionization spectrometry could be demonstrated.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Spatially resolved ultra-trace analysis of elements combining resonance ionization with a MALDI-TOF spectrometer

A combined setup for spatially resolved mass analysis of trace amounts of elements and macromolecules is presented. Using a MALDI-TOF mass spectrometer, a laser spectroscopic setup for resonant ionization of neutral atoms has been implemented. This allows for an efficient and selective detection of trace elements by means of resonance ionization mass spectrometry (RIMS). The instrumental scheme is described, and methodological developments are presented. In a first application pure, laser desorption/ionization with TOF-MS was used to measure mass distributions of cosmic nanodiamonds. For further applications regarding the spatially resolved ultra-trace analysis of elements in solid samples, an implanted target was used to characterize both laser desorption/ionization and laser desorption/resonance ionization for the detection of trace elements within. A perspective of the setup is given and future investigations are outlined.     

Plasma chemistry in laser ablation processes

Based on the results of quantitative spectroscopic diagnostics (LIF in combination with time resolved emission spectroscopy) chemical dynamics in laser-produced plasmas of metallic (Ti, Al,), and graphite samples have been examined. The Nd-YAG (1064 nm, 10 ns, 100 mJ) and excimer XeCl (308 nm, 10 ns, 10 mJ) lasers were employed for ablation. The main attention was focused on the elucidation of a role of oxide and dimer formation in controlling spatio-temporal distributions of different species in the ablation plume. The results of the spatial and temporal analysis of a laser-produced plasma in air indicates the existence of diatomic oxides in the ablation plume both in the ground and excited states, which are formed from reactions between ablated metal atoms and oxygen. The efficiency of the oxidation reaction depends on the intensity and spot diameter of the ablation laser beam. The maximal concentration of TiO molecules are estimated to be of 1×10¹⁴ cm⁻³ at the time of 10 μs after the start of the ablation pulse. A comparison of spatial–temporal distributions of Ti atoms and excited TiO molecules allow us to find a correlation in their change, which proves that electronically excited Ti oxides are most probably formed from oxidation of atoms in the ground and low lying metastable states. The spectroscopic characterization of pulsed laser ablation carbon plasma has also been performed. The time–space distributions as well as the high vibrational temperature of C₂ molecules indicate that the dominant mechanism for production of C₂ is the atomic carbon recombination.     

Using laser ablation/inductively coupled plasma mass spectrometry to bioimage multiple elements in mouse tumors after hyperthermia

In this study, we employed laser ablation/inductively coupled plasma mass spectrometry (LA-ICP-MS) to map the spatial distribution of Gd-doped iron oxide nanoparticles (IONPs) in one tumor slice that had been subjected to magnetic fluid hyperthermia (MFH). The mapping results revealed the high resolution of the elemental analysis, with the distribution of Gd atoms highly correlated with that of the Fe atoms. The spatial distributions of C, P, S, and Zn atoms revealed that the effect of MFH treatment was significantly dependent on the diffusion of the magnetic fluid in the tissue. An observed enrichment of Cu atoms after MFH treatment was probably due to inflammation in the tumor. The abnormal distribution of Ni atoms suggests a probable biochemical reaction in the tumor. Therefore, this LA-ICP-MS mapping technique can provide novel information regarding the spatial distribution of elements in tumors after cancer therapy.     

Production of cold bromine atoms at zero mean velocity by photodissociation

The production of a translationally cold (T < 1 K) sample of bromine atoms with estimated densities of up to 10(8) cm(-3) using photodissociation is presented. A molecular beam of Br(2) seeded in Kr is photodissociated into Br + Br* fragments, and the velocity distribution of the atomic fragments is determined using (2 + 1) REMPI and velocity map ion imaging. By recording images with varying delay times between the dissociation and probe lasers, we investigate the length of time after dissociation for which atoms remain in the laser focus, and determine the velocity spread of those atoms. By careful selection of the photolysis energy, it is found that a fraction of the atoms can be detected for delay times in excess of 100 μs. These are atoms for which the fragment recoil velocity vector is directly opposed and equal in magnitude to the parent beam velocity leading to a resultant lab frame velocity of approximately zero. The FWHM velocity spreads of detected atoms along the beam axis after 100 μs are less than 5 ms(-1), corresponding to temperatures in the milliKelvin range, opening the possibility that this technique could be utilized as a slow Br atom source.     

Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume

A two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 10(5) u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, tau(L) = 6 ns) and the Er:YAG laser (tau(L) approximately 120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process.     

Particle number density and velocity distributions in laser plumes

A time-of-flight mass spectrometer with electron impact ionization facility was used in investigations of the laser plume structure. Densities and velocity distributions of positively charged and neutral species were measured 12 cm downstream of the target. Velocities of particles in a plume were measured by the retarding potential method. The combination of a skimmer and declining electric field was used to suppress the influence of charged particles during the measurement of the neutral component parameters. In the case of YBaCuO ceramic laser ablation, a strong variation of the laser-induced plume composition was observed from its head to its tail. It seems to be accounted for by the difference of the starting (phase transition) temperatures of various layers of a plume. Ions detected mainly in the head of a plume were followed by atoms, molecules and clusters in inverse succession to their appearance in the plume under the light intensity increase. The characteristic of the number density dependence upon the laser spot diameter make it clear that most of the molecules BaO and YO are the direct product of ablation. In contrast, the detected clusters with masses up to 2000 amu are the product of condensation in the expanding plume under the conditions of the experiments.     

Laser Desorption Ionization of Melamine and Identification of Fragmentation Channels from Ion Velocity Distribution Measurements

Fragmentation frequently accompanies intact laser desorption ionization of a parent non-volatile compound, desorption and dissociation dynamics has been a subject of intense studies over the past decade. As a preliminary lest system for future laser desorption study of energetic compounds such as explosives and propellents, we studied UV laser desorption ionization of melamine at a laser power density of approximately 4.4 MW/cm². Several gas-phase dissociation channels of the parent and fragment ions formed in UV laser desorption ionization of melamine films can be identified from their velocity distributions. A phenomenological desorption temperature of the order of 20000 K is estimated from fitting the experimental velocity distributions to Maxwellian functions.     

Isotope determination of lead and bismuth by pulsed laser evaporation and resonance ionization time-of-flight mass spectrometry

Pulsed laser evaporation coupled with resonance ionization time-of-flight mass spectrometry has been used to measure the isotopic abundance of lead and bismuth. A pulsed Nd:YAG laser was used to evaporate the metal atoms, the evaporated atoms were then detected by one color two photon resonance ionization and time-of-flight mass spectrometry. The arrival time distributions of atoms evaporated by pulsed laser, and the isotopic abundances of Pb and Bi were measured. Our results show that this method is good enough for measuring the isotopic abundances of Pb and Bi with high sensitivity and selectivity.     

Laser-enhanced ionization spectroscopy of sodium atoms in an air-hydrogen flame with mass spectrometric detection

Sodium atoms in an air-hydrogen flame at atmospheric pressure have been selectively ionized by laser-enhanced ionization (LEI) spectroscopy, and the resulting ions have been drawn into a vacuum and detected by quadrupole mass spectrometry. A commercial inductively coupled plasma-mass spectrometer, modified for use with a flame rather than an ICP, was used to sample and detect the LEI ions. Following double-resonance LEI using pulsed dye lasers, the detected sodium ion signal was enhanced by a factor of 350 over that induced by thermal ionization alone. Using a 5 mm laser beam diameter, the LEI signal pulse was found to last for 0.54 ms (FWHM). Spatial studies in which the position of the laser beam relative to the mass spectrometer sampler cone was varied, demonstrated that the ions produced by LEI travel with the flame velocity into the mass spectrometer, with no significant losses due to recombination from as far as 13 mm from the interface.     

Characterization of laser ablation and ionization in helium and argon: A comparative study by time-of-flight mass spectrometry

Laser ablation and ionization in ambient helium and argon gases were studied by multiple-stage time-of-flight mass spectrometry. Measurements made at different gas pressures indicated that there exists an optimal pressure for adequately cooling energetic ions and reducing multiply charged ions that are higher for He than for Ar. The temporal distributions of ions were compared at various laser fluences and gas pressures, and the broad distributions for He could be ascribed to elastic scattering and thermodynamic processes. The diffusion of ions in He resulted in a longer delay before the instrument registered its maximal signal. Ions with different masses were observed to have the same kinetic energies in He, which was confirmed using the SIMION software, while ion movement was hydrodynamically controlled in Ar. The velocities of singly and doubly charged ions were also studied, and doubly charged ions showed much higher kinetic energy because of their frontal location in the plasma expansion.     

Spatial distributions of the number densities of neutral atoms and ions for the different elements in a laser induced plasma generated with a Ni-Fe-Al alloy

Spatially-resolved emission spectroscopy, including spatial devonvolution of the spectra, has been used to determine the three-dimensional distributions of the relative number densities of neutral atoms and ions of the elements present in a laser-induced plasma generated with a Ni-Fe-Al alloy. The method is based on the precise measurement of the local electronic temperature from Saha–Boltzmann plots constructed with Fe I and Fe II lines. The plasma was generated in air at atmospheric pressure using a 1064-nm Nd:YAG laser, and the emission was detected in the time window 3.0–3.5 μs. The ionization fraction was very high (above 0.9) for the three elements in the sample, only decreasing behind the expanding plasma front. The relative number densities were obtained from the emissivities of selected elemental lines as well as the temperature. The error in this procedure was estimated, and it was found that it is largely due to the uncertainties in the transition probability values used. The spatial distributions of the total relative number densities of the three elements were shown to coincide within the error, a result which is relevant to the development of models of plasma emission used in analytical applications. The ratios of the total number densities of the elements in the plasma were compared to their concentration ratios in the sample; however, the relatively high errors in the relative number densities did not permit any definitive conclusions to be drawn about the stoichiometry of the laser ablation process.     

Theoretical considerations of laser induced fluorescence and ionization spectrometry: how close to single atom detection

The potential capability of detecting single atoms and/or of approaching the intrinsic detection limit, i.e. the limit where all external causes of noise in the system are eliminated, is discussed with respect to laser induced fluorescence and ionization spectrometry. These approaches have been chosen because of their widespread use in laboratories involved in analytical laser spectroscopy. Among the lasers, tunable dye lasers pumped by N₂, Nd-YAG, Excimer, Cu vapor lasers and flashlamps are considered, and flames, graphite furnaces, plasmas and low pressure glow discharges are considered as atom reservoirs. From practical considerations, it is shown that none of the conventional analytical approaches used can satisfy the requirement of a true single atom detection technique, the only exception being provided by the ionization method coupled with atomization under vacuum.     

Influence of wavelength,irradiance, and the buffer gas pressure on high irradiance laser ablation and ionization source coupled with an orthogonal Time of Flight Mass Spectrometer

Influence of laser wavelength, laser irradiance and the buffer gas pressure were studied in high irradiance laser ablation and ionization source coupled with an orthogonal time-of-flight mass spectrometer. Collisional cooling effects of energetic plasma ions were proved to vary significantly with the elemental mass number. Effective dissociation of interferential polyatomic ions in the ion source, resulting from collision and from high laser irradiance, was verified. Investigation of relative sensitivity coefficients (RSC) of different elements performed on a steel standard GBW01396, which was ablated at 1064 nm, 532 nm, 355 nm, and 266 nm, has demonstrated that the thermal ablation mechanism could play a critical role with the first three wavelengths, while 266 nm induces non-thermal ablation principally. Experimental results also indicated that there is no evident discrepancy for most metal elements on RSCs and LODs among four wavelengths at high irradiance, except that high boiling point elements like Nb, Mo, and W have higher RSCs at higher irradiance regions of 1064 nm, 532 nm, and 355 nm due to thermal ablation. A geological standard and a garnet stone were also used in the experiment subsequently, and their RSCs and LODs for metal elements show nonsignificant dependence on wavelength at designated irradiances. All results reveal that relatively uniform sensitivity can be achieved at any wavelength for metal elements in the solids used in our experiments at an appropriate irradiance for the low pressure high irradiance laser ablation and ionization source.