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Gnaser H.,University of Kaiserslautern | Gnaser H.,Institute for Surface and Thin Film Analysis IFOS
Surface and Interface Analysis | Year: 2013

The energy spectra of Cs+ ions sputtered from silicon under 5.5-keV Cs+ bombardment were recorded for emission energies E ≤ 100 eV. The emitted ions were detected in a high-sensitivity double-focusing SIMS (Cameca IMS-4f). The influence of several instrumental parameters on the energy resolution and the peak position of the measured spectra were investigated to determine the instrument's resolution response. Specifically, entrance apertures with four different diameters (410, 280, 50, and 25 μm) were used in the energy analyzer, whereas the width of its exit slit was varied from 3300 to 15 μm. For the smallest of these values, the full width at half maximum (FWHM) of the Cs+ spectrum amounts to 2.2 eV. A box-type resolution function was found to describe the energy distributions over a substantial range. Using that function, the Cs+ distributions measured for various slit settings can be reproduced via the convolution of a single original function. The values of the spectral width obtained from this procedure are essentially identical with the FWHM values of the recorded spectra. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.

Gnaser H.,University of Kaiserslautern | Gnaser H.,Institute for Surface and Thin Film Analysis IFOS
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2011

The emission-energy spectra of atomic and molecular secondary ions sputtered from various metals and semiconductors (Al, Cu, In, Si, InP, and InSb) under 5.5-keV Cs+ irradiation were investigated. The emitted ions were detected in a high-sensitivity double-focusing secondary-ion mass spectrometer. Specifically, the energy distributions of Cs+, Cs2+, MCs+, and M+ ions (where M designates one of the target elements) were recorded for emission energies E ≤ 125 eV. All ion species exhibit a peak at low energy (E < 5 eV), but differ significantly in the respective fall-off to high emission energies. The influence of the oxygen partial pressure in the vicinity of the sputtered surface on the energy spectra was examined for Cs+ ions emitted from Si. With an increase of the ratio r of the O2 flux to the Cs+ flux, the spectra shift to higher emission-energy values, with the total shift amounting to ∼0.45 eV at a value of r ∼ 3.3. Concurrently, the intensity of Cs+ increases by ∼30%. The measured emission distributions of Cs+ ions from different samples were compared with the predictions of the electron-tunneling model of secondary-ion formation. It is found that the experimental spectra can be reproduced quite well when employing specific sets of parameters in that theoretical concept. The possible limitations of such a comparison are discussed. © 2010 Elsevier B.V. All rights reserved.

Wittmaack K.,Helmholtz Center Munich | Gnaser H.,University of Kaiserslautern | Gnaser H.,Institute for Surface and Thin Film Analysis IFOS
International Journal of Mass Spectrometry | Year: 2014

In the preceding paper an approach was described that allows measured secondary-ion energy spectra to be correlated with the original energy distribution. Here the model was applied to analyse energy spectra of Cs + ions sputtered from a Cs bombarded Si sample. The aim was to show that relevant properties of the employed IMS-4f sector field mass spectrometers as well as the mean surface binding energy of the Cs+ ions can be derived in one effort. The novel results may be summarised as follows. (i) The width of the energy slit can be determined in situ by comparing integral intensities for the chosen and the fully opened slit, the latter of known size. (ii) Pronounced yield losses occur if the width-to-thickness ratio of the slit is reduced below about 0.1. (iii) The broadening of energy spectra is predominantly defined by the slit width. Unexpectedly, the size of the circular contrast aperture plays only a minor role. The reason needs to be clarified. (iv) Measured spectra can be reproduced with a remarkable accuracy, better than ±8% throughout the whole spectrum. The origin of the energy scale can be determined with an uncertainty of ±0.05 eV. (v) Beyond a characteristic energy Ec the transmission decreases more rapidly with increasing emission energy E than the expected Ec/E fall-off. The difference is probably due to the detuning of the immersion lens, an action inevitably associated with increasing target-bias offset. (vi) The surface binding energy Es of Cs+ ions, derived from spectra measured with the largest contrast aperture, turned out to be surprisingly high, 3.9 ± 0.2 eV. Evidently, the Cs+ ions were emitted from sites featuring very strong ionic bonding. The derived value of Es presumably represents the centroid 〈Es⌠of a spectrum of binding energies which reflect different local configurations of damage around the Cs ions. Considering the much lower binding energy of adsorbed Cs atoms, we have to conclude that sputtered ions and neutrals do not exhibit the same energy spectrum. Hence the previous practise of using one global ionisation probability to correlate secondary ion emission with the sputtering of neutral atoms needs to be abandoned. © 2013 Elsevier B.V.

Gnaser H.,University of Kaiserslautern | Gnaser H.,Institute for Surface and Thin Film Analysis IFOS
Pure and Applied Chemistry | Year: 2011

The bombardment of the surface of a solid by energetic ions often results in pronounced surface modifications, leading to characteristic topographical features. In this report, the development of specific morphological nanostructures on surfaces under ion irradiation is discussed. The following aspects will be emphasized: (i) on an atomic scale, the generation of isolated defects such as adatoms and surface vacancies due to single-ion impacts, and their possible clustering; (ii) the transition from such individual defects toward extended morphological features on the surface and suitable scaling relations to describe them; (iii) the formation of highly periodic structures with nanoscale dimensions such as nanodots and "ripple"- like features, and the dependence of these nanostructcures on various ion-irradiation parameters and substrate materials; (iv) the theoretical concepts proposed to model the observed patterns which are thought to be related to (and caused by) the interplay between ion erosion and diffusion of adatoms (vacancies), thus inducing a surface reorganization. © 2011 IUPAC.

Gnaser H.,University of Kaiserslautern | Gnaser H.,Institute for Surface and Thin Film Analysis IFOS | Golser R.,University of Vienna
Surface and Interface Analysis | Year: 2011

The emission of negatively charged HfFn- (n ≤ 5) and WFn- (n ≤ 7) molecular ions sputtered from a mixed Hf-W-PbF2 fluoride sample by a 13 keV Cs+ ion beam was studied. The emitted ions were detected in a high-sensitivity double-focusing secondary ion mass spectrometer. In the HfFn- and WF n- series, HfF5- and WF 6- are the anions formed most abundantly, with their ratio amounting to HfF5-/WF6-∼6, whereas HfF5-/WF5-∼18. In particular, the rather high yield of the HfF5- anion could be essential for the sensitive detection of live 182Hf, a radionuclide (half-live ∼9 million years) that might originate from stellar events in the vicinity of the Earth. Generally, these high ion yields appear to correspond with a high electron affinity of the respective molecule. In addition, their formation may be affected by the atomic composition at the sputtering site. Because their constituents originate from (initially) separated sources (the individual Hf, W and PbF2 particles), the production of the HfFn- and WFn- molecular anions is envisaged to require an extensive mixing between these different reservoirs during sputtering. Furthermore, the energy distributions of these molecular anion species indicate that they are released from the surface via a collision cascade that also leads to the occurrence of fragmentation processes in the emission event. Copyright © 2010 John Wiley & Sons, Ltd.

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