Time-of-flight Medium Energy Ion Scattering Research Articles

A versatile time-of-flight medium-energy ion scattering setup using multiple delay-line detectors

We present the most recent upgrades of the time-of-flight medium energy ion scattering (TOF-MEIS) system in Uppsala. The experimental chamber features a 6-axis goniometer with a sample annealing stage and two delay line detectors for composition analysis with high depth resolution and depth-resolved crystallography. The first detector has a large solid angle and can be moved circularly around the target position which allows to detect backscattered or transmitted ions. The second detector features increased flight distance from sample to detector resulting in enhanced energy resolution. A reduction from 1.4 keV to 0.4 keV is achieved for 100 keV He scattered from an Au surface for 1 ns time resolution, equivalent to a depth resolution of 6 Å. This detector is equipped with an electrostatic electrode in order to deflect charged particles, which allows to study the charge state for scattered ions in the medium energy regime.

Determination of Dimension and Conformal Arsenic Doping Profile of a Fin Field Effect Transistors by Time-of-Flight Medium Energy Ion Scattering.

We have developed a methodology that analyzes the dimensions and conformal doping profiles in fin field effect transistors (FinFET) using time-of-flight medium energy ion scattering (TOF-MEIS). The structure of a 3D FinFET and As dopant profiles were determined by comprehensive simulations of TOF-MEIS measurements made in three different scattering geometries. The width and height of a FinFET and the As doping profiles in the top, side, and bottom of fin were analyzed simultaneously. The results showed the dimension and conformal doping profile of nanostructures with complex shape can be determined by TOF-MEIS nondestructively, quantitatively, and with subnm depth resolution without any sputtering and matrix effects.

Ion-beam based characterization of TiN back contact interlayers for CZTS(e) thin film solar cells

Time-of-Flight Elastic Recoil Detection Analysis (ToF-ERDA) and Time-of-Flight Medium-Energy Ion Scattering (ToF-MEIS) have been employed to investigate the potential of TiN thin films as intermediate layers on Mo back contact in CZTS(e) solar cells. TiN films of various thicknesses (20, 50 and 200 nm) were prepared with reactive DC magnetron sputtering and atomic layer deposition on Mo/SLG (soda-lime glass) substrates and annealed ex situ in either S or Se atmosphere. In situ annealing of the samples to different temperatures was also performed in the MEIS setup together with subsequent ToF-MEIS and ERDA analysis.The results of the sample and interlayer composition profiles, layer quality and thickness distributions are discussed in context with complementary experimental findings partially obtained previously by X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Scanning Electron Microscopy and Scanning Transmission Electron Microscopy – Electron Energy Loss Spectroscopy (STEM – EELS).

Quantitative Analysis of Calcium Phosphate Nanocluster Growth Using Time-of-Flight Medium-Energy-Ion-Scattering Spectroscopy.

One of the remaining challenges in material chemistry is to unveil the quantitative compositional/structural information and thermodynamic nature of inorganic materials especially in the initial nucleation and growth step. In this report, we adopted newly developed time-of-flight medium-energy-ion-scattering (TOF-MEIS) spectroscopy to address this challenge and explored heterogeneously grown nanometer-sized calcium phosphate as a model system. With TOF-MEIS, we discovered the existence of calcium-rich nanoclusters (Ca/P ∼ 3) in the presence of the non-collagenous-protein-mimicking passivating ligands. Over the reaction, these clusters progressively changed their compositional ratio toward that of a bulk phase (Ca/P ∼ 1.67) with a concurrent increase in their size to ∼2 nm. First-principles studies suggested that the calcium-rich nanoclusters can be stabilized through specific interactions between the ligands and clusters, emphasizing the important role of template on guiding the chemical and thermodynamic nature of inorganic materials at the nanoscale.

Improving the quality of Al2O3/4H-SiC interface for device applications

The present paper focuses on the investigation of Al2O3/4H-SiC dielectric interface upon annealing, its consequent structural modifications, and the link to electrical properties. For this purpose, the test structures are prepared by depositing Al2O3, using atomic layer deposition (ALD), on low doped n-type 4H-SiC epitaxial layers. The structures are annealed from 300 °C to 1100 °C for different time duration (from 5 to 60 mins) and ambient such as, low vacuum (10−1 Torr), N2, and N2O. The structural studies on these samples are conducted using synchrotron-based high resolution x-ray photoelectron spectroscopy (HR-XPS), lab-based XPS, time of flight elastic recoil detection analysis (ToF-ERDA), and time of flight medium energy ion scattering (ToF-MEIS). The electrical response of capacitive structures is monitored through capacitance voltage (CV) measurements for as-deposited and annealed structures. It is found that the annealing at high temperatures, such as 1100 °C, and in N2 or N2O environment, improves the dielectric properties due to the introduction of a thin layer of about 1 nm stable SiO2 between the Al2O3 and 4H-SiC.

Ion-induced particle desorption in time-of-flight medium energy ion scattering

Abstract Secondary ions emitted from solids upon ion impact are studied in a time-of-flight medium energy ion scattering (ToF-MEIS) set-up. In order to investigate characteristics of the emission processes and to evaluate the potential for surface and thin film analysis, experiments employing TiN and Al samples were conducted. The ejected ions exhibit a low initial kinetic energy of a few eV, thus, requiring a sufficiently high acceleration voltage for detection. Molecular and atomic ions of different charge states originating both from surface contaminations and the sample material are found, and relative yields of several species were determined. Experimental evidence that points towards a predominantly electronic sputtering process is presented. For emitted Ti target atoms an additional nuclear sputtering component is suggested.

Interface between Al2O3 and 4H-SiC investigated by time-of-flight medium energy ion scattering

The formation of interfacial oxides during heat treatment of dielectric films on 4H-SiC has been studied. The 4H-SiC surface has been carefully prepared to create a clean and abrupt interface to Al2O3. An amorphous, 3 nm thick, Al2O3 film has been prepared on 4H-SiC by atomic layer deposition and rapid thermal annealing was then performed in N2O ambient at 700 °C and 1100 °C during 1 min. The samples were studied by time-of-flight medium energy ion scattering (ToF-MEIS), with sub-nanometer depth resolution and it is seen that, at both annealing temperatures, a thin SiOx (1 ⩽ x ⩽ 2) is formed at the interface. Our results further indicate that carbon remains in the silicon oxide in samples annealed at 700 °C. Additional electrical capacitance voltage measurements indicate that a large concentration of interface traps is formed at this temperature. After 1100 °C annealing, both MEIS and XRD measurements show that these features disappear, in accordance with electrical data.

Analysis of photon emission induced by light and heavy ions in time-of-flight medium energy ion scattering

Abstract We present a systematic analysis of the photon emission observed due to impact of pulsed keV ion beams in time-of-flight medium energy ion scattering (ToF-MEIS) experiments. Hereby, hydrogen, helium and neon ions served as projectiles and thin gold and titanium nitride films on different substrates were employed as target materials. The present experimental evidence indicates that a significant fraction of the photons has energies of around 10 eV, i.e. on the order of typical valence and conduction band transitions in solids. Furthermore, the scaling properties of the photon emission with respect to several experimental parameters were studied. A dependence of the photon yield on the projectile velocity was observed in all experiments. The photon yield exhibits a dependence on the film thickness and the scattering angle, which can be explained by photon production along the path of the incident ion through the material. Additionally, a strong dependence on the projectile type was found with the photon emission being higher for heavier projectiles. This difference is larger than the respective difference in electronic stopping cross section. The photon yield shows a strong material dependence, and according to a comparison of SiO2 and Si seems to be subject to matrix effects.

The potential of ion beams for characterization of metal–organic frameworks

Ion scattering has been employed for depth-profiling of metal–organic frameworks (MOFs) to characterize the degree of post-synthetic uptake of [FeFe](mcbdt)(CO)6 (mcbdt=2,3-dithiolato-benzoic acid). The system investigated consisted of UiO-66 (UiO=University of Oslo) MOF thin films grown on p-type Si wavers in which a molecular proton reduction catalyst [FeFe](mcbdt)(CO)6 was introduced by postsynthetic exchange (PSE). We have characterized samples by Rutherford Backscattering spectrometry (RBS), Time-of-Flight Elastic Recoil Detection analysis (TOF-ERDA) and by Time-of-Flight Medium Energy Ion Scattering (TOF-MEIS). The beam induced sample modification during the analysis has been characterized by Scanning Electron Microscopy (SEM). No detectable sample modification was found for RBS and TOF-MEIS whereas TOF-ERDA had a clear impact in the present experiment. Composition profiles could be obtained and indicated enrichment of catalyst and/or catalyst residual near to and at the sample surface.

Energy loss of slow Ne ions in Pt and Ag from TOF-MEIS and Monte-Carlo simulations

Time-Of-Flight Medium Energy Ion Scattering (TOF-MEIS) experiments were performed to investigate the different energy loss mechanisms for slow Ne ions (a few keV/nucleon) in polycrystalline thin films of Ag and Pt deposited on Si. To disentangle electronic (Se) and nuclear (Sn) contributions to the stopping power, Monte-Carlo (MC) simulations using the TRBS (TRim for BackScattering) code were conducted. The plateau-width of the experimental signal recorded in backscattering geometry was analyzed in order to extract additional information on the effective nuclear stopping. Electronic stopping powers were found to show the expected velocity dependence while the contribution from nuclear stopping was found low. The implications for experiments performed to study energy loss or related parameters are discussed.

Soft landing of bare PtRu nanoparticles for electrochemical reduction of oxygen.

Magnetron sputtering of two independent Pt and Ru targets coupled with inert gas aggregation in a modified commercial source has been combined with soft landing of mass-selected ions to prepare bare 4.5 nm diameter PtRu nanoparticles on glassy carbon electrodes with controlled size and morphology for electrochemical reduction of oxygen in solution. Employing atomic force microscopy (AFM) it is shown that the nanoparticles bind randomly to the glassy carbon electrode at a relatively low coverage of 7 × 10(4) ions μm(-2) and that their average height is centered at 4.5 nm. Scanning transmission electron microscopy images obtained in the high-angle annular dark field mode (HAADF-STEM) further confirm that the soft-landed PtRu nanoparticles are uniform in size. Wide-area scans of the electrodes using X-ray photoelectron spectroscopy (XPS) reveal the presence of both Pt and Ru in atomic concentrations of ∼9% and ∼33%, respectively. Deconvolution of the high energy resolution XPS spectra in the Pt 4f and Ru 3d regions indicates the presence of both oxidized Pt and Ru. The substantially higher loading of Ru compared to Pt and enrichment of Pt at the surface of the nanoparticles is confirmed by wide-area analysis of the electrodes using time-of-flight medium energy ion scattering (TOF-MEIS) employing both 80 keV He(+) and O(+) ions. The activity of electrodes containing 7 × 10(4) ions μm(-2) of bare 4.5 nm PtRu nanoparticles toward the electrochemical reduction of oxygen was evaluated employing cyclic voltammetry (CV) in 0.1 M HClO4 and 0.5 M H2SO4 solutions. In both electrolytes a pronounced reduction peak was observed during O2 purging of the solution that was not evident during purging with Ar. Repeated electrochemical cycling of the electrodes revealed little evolution in the shape or position of the voltammograms indicating high stability of the nanoparticles supported on glassy carbon. The reproducibility of the nanoparticle synthesis and deposition was evaluated by employing the same experimental parameters to prepare nanoparticles on glassy carbon electrodes on three occasions separated by several days. Surfaces with almost identical electrochemical behavior were observed with CV, demonstrating the highly reproducible preparation of bare nanoparticles using physical synthesis in the gas-phase combined with soft landing of mass-selected ions.

Ultra-thin film and interface analysis of high-k dielectric materials employing Time-Of-Flight Medium Energy Ion Scattering (TOF-MEIS)

We explore the potential of Time-Of-Flight Medium Energy Ion Scattering (TOF-MEIS) for thin film analysis and analyze possible difficulties in evaluation of experimental spectra. As a model system high-k material stacks made from ultra-thin films of HfO2 grown on a p-type Si (100) substrate with a 0.5nm SiO2 interface layer have been investigated. By comparison of experimental spectra and computer simulations TOF-MEIS was employed to establish a depth profile of the films and thus obtaining information on stoichiometry and film quality. Nominal film thicknesses were in the range from 1.8 to 12.2nm. A comparison of the results with those from other MEIS approaches is made. Issues regarding different combinations of composition and stopping power as well as the influence of channeling are discussed. As a supporting method Rutherford-Backscattering spectrometry (RBS) was employed to obtain the areal density of Hf atoms in the films.

Electronic stopping power of hydrogen in HfO2 at the stopping maximum and below

Electronic energy loss of hydrogen ions in HfO2 was investigated in a wide energy range in the medium and low energy ion scattering regime. Experiments by Time-Of-Flight Medium-Energy Ion Scattering (TOF-MEIS) with proton and deuteron projectiles were performed in backscattering geometry for nm-films of HfO2 on Si with an ultrathin SiO2 interface layer prepared by ALD. At energies around the stopping maximum excellent agreement is found with earlier results from Behar et al. (2009) [45] and theoretical predictions. Towards lower energies discrepancies between experiment and calculations increase slightly. The low energy data exhibits excellent velocity proportionality and indicates the absence of clear effects due to distinct electronic states. Thus, no apparent velocity threshold can be extrapolated from the experiments within the uncertainty of present data. The magnitude of the energy loss is discussed in terms of a free-electron model and compared with the expected electron densities from plasmon frequencies.

Electronic stopping power of slow 20Ne ions in Au obtained from TOF-MEIS and Monte-Carlo computer simulations

The electronic stopping power of slow Ne ions (v>0.6a.u. per nucleon, i.e. 180keV) in polycrystalline Au was investigated in a backscattering approach. Spectra were recorded in a Time-Of-Flight Medium-Energy Ion Scattering (TOF-MEIS) experiment using a gold thin film of several nm thickness deposited on Si. The stopping power was extracted from fitting energy converted TOF-spectra by Monte-Carlo simulations for the ion trajectories. This permits an accurate disentanglement of elastic losses and inelastic losses which are due to nuclear collisions and interactions of the ion with the electronic system of the solid, respectively. The resulting electronic stopping cross section is found to exhibit excellent velocity proportionality in the whole investigated energy range. Data is compared to other experimental datasets for similar ion energies.

Time-of-flight medium energy ion scattering study of epitaxial [formula omitted] superlattice structures

The ability to determine structural and compositional information from the sub-surface region of a semiconductor material has been demonstrated using a new time-of-flight medium energy ion scattering (ToF-MEIS) system. A series of silicon-silicon/germanium ( Si Si 1 − x Ge x ) heterostructure and multilayer samples, grown using both solid source molecular beam epitaxy (MBE) and gas source chemical vapour deposition (CVD) on Si(100) substrates, have been investigated. These data indicate that each individual layer of Si 1 − x Ge x ( x ∼ 0.22) in both 2 and 3 period samples, can be uniquely identified with a resolution of approximately 3 nm. A comparison of MBE and CVD grown samples has also been made using layers with similar structures and composition. ToF-MEIS showed that the MBE grown samples exhibit a much higher Ge concentration per layer than CVD grown samples. Surface segregation of Ge was observed for the MBE grown samples, but appears to be suppressed in the case of CVD grown samples. These data indicate the ability of ToF-MEIS to probe the differences in the kinetics associated with Ge incorporation and segregation between the two techniques. The total Ge content of each sample was confirmed independently using conventional Rutherford backscattering spectrometry (RBS).