T. Neidhart

Surface analysis of PtxNi1−x single crystals

Abstract The surface composition of Pt 10 Ni 90 (100), Pt 25 Ni 75 (111) and Pt 50 Ni 50 (110) single crystal alloys has been determined by ion scattering spectroscopy (ISS) and ultraviolet photoelectron spectroscopy (UPS). Pt enrichment of the topmost layer due to preferential sputtering occurs independently of the surface orientation. Annealing the sputtered alloys results in face specific surface compositions: Pt enrichment for the (111) surface, Ni enrichment for the (110) surface and a composition close to the bulk value for the (100) surface. Exposure of the sputtered surfaces to oxygen induces segregation of Ni. Adsorbate induced segregation is correlated with segregation due to annealing. The determination of workfunctions by UPS results support the ISS results on surface composition. The experimental results on surface segregation are discussed with respect to thermodynamic calculations.

SPUTTER YIELDS OF INSULATORS BOMBARDED WITH HYPERTHERMAL MULTIPLY CHARGED IONS

The total sputter yield for Au, Si, GaAs, SiO2, MgO, LiF and NaCl bombarded with hyperthermal highly charged Arq+ ions (q = 1-9) has been measured. Only for alkali halides (LiF, NaCl) and to some extent for SiO2 potential sputtering (enhancement of the sputter yield with increasing charge state of the primary ion) has been observed. All other targets showed normal collision induced sputtering. From that result it is obvious that the mechanisms for sputtering can not be explained by the Coulomb explosion model, because in this model insulators like MgO and semiconductors like Si and GaAs should also show charge state dependence of the sputtering yield. Alkali halides and SiO2 are materials which are known for strong electron phonon coupling where electronic excitations in the valence band are localized by formation of self trapped excitons (STE) and/or self trapped holes (STH). During bombardment with highly charged ions the neutralization process in front of, at and below the surface causes the formation of STE and/or STH. Therefore the potential sputtering can be explained as a defect mediated sputtering process which is well known in electron stimulated desorption (ESD) where the decay of STH and/or STE into different colour centers leads at the end to the desorption of neutralized anions. The also created neutral cations are either evaporated (as it is the case for the alkali halides) or have to be removed by momentum transfer by the impinging projectiles. Therefore it is very likely that in the case of SiO2 for very low impact energy mainly only oxygen is enhanced sputtered, the surface is enriched in Si and the potential sputtering effect decreases with increasing ion dose.

Secondary ion emission from lithium fluoride under impact of slow multicharged ions

Abstract Secondary ion emission has been investigated for bombardment of polycrystalline lithium fluoride by slow multicharged Ar ions (charge state q ≤ 9, impact energy E k ≤ 500 eV). The F − ions originate from collisional energy transfer, almost independent of the primary ion charge, whereas the F + yield strongly increases with q . The F + ions are produced via interatomic Auger transitions from the F − 2p valence band into projectile states, and their desorption from LiF is controlled by Coulomb interaction of F + with Li + and F − surface ions, and LiF lattice relaxation. At high impact energy, emission of Li + is also mainly due to collisional energy transfer, but toward lower E k the primary ion charge plays an increasingly important role. The present measurements demonstrate that secondary ions account for less than 0.1% of our earlier measured total sputter yields from LiF.

Total sputter yield of LiF induced by hyperthermal ions measured by a quartz microbalance

Abstract The total sputter yield of LiF induced by singly charged hyperthermal He+, Ne+ and Ar+ ions with a kinetic energy of 5–500 eV is presented. The measurements have been performed with a highly sensitive quartz crystal microbalance. Results of the total sputter yield for Au show excellent agreement with the literature. The results on LiF show a large and slowly decreasing total sputter yield at low energies. This fact is interpreted as evidence for electronic processes in the sputtering of LiF.

Surface composition of PtxNi1−x single crystal alloys

Abstract We present results of the surface composition of the topmost layer of three low index planes of Pt x Ni 1−x single crystal alloys: Pt 10 Ni 90 (100), Pt 25 Ni 75 (111) and Pt 50 N 50 (110). The alloys were investigated by ion scattering spectroscopy (ISS). We observed Pt enrichment of the surface layers caused by preferential sputtering. After exposing the surfaces to oxygen, adsorbate induced segregation of Ni was measured. Annealing the sputtered alloys results in a site specific change in surface composition. We found Pt enrichment for the (111) surface and Ni enrichment for the (110) surface. The surface of Pt 10 Ni 90 (100) showed a Pt concentration close to the bulk composition. Work functions were determined by ultraviolet photoelectron spectroscopy (UPS). The UPS results support the ISS results of the surface composition. The results of thermodynamic calculations are discussed with respect to the experimental results on surface segregation.

Determination of electron-induced total sputter yield of LiF

Abstract We present measurements of the total sputter yield for LiF at 150°C induced by electrons with an impact energy between 10 and 500 eV. A nearly linear increase with kinetic energy has been measured. The decrease of the yield with increasing electron dose for energies below 100 eV is also shown. For very high electron dose the yield even becomes zero and simultaneously a red coloration of the surface is observed. Low energy ion scattering (LEIS) measurements at such a surface showed a Li enrichment to more than 90%. To reach such a composition with electrons between 40 and 80 eV kinetic energy sputtering of about 20 monolayers LiF is necessary.

Defect mediated sputtering of amorphous LiF induced by low-energy ion bombardment

Abstract Sputter yields of amorphous LiF caused by low-energy Ar+, Ar2+, and K+ impact have been computed using the binary collision simulation code Marlowe in conjunction with a simple, phenomenological model for defect production and diffusion. In the model used, the sputter yield is assumed to be made up by contributions due to (a) resonant or Auger neutralization (RN or AN), (b) vacancy formation due to momentum transfer, (c) inelastic energy loss channels other than RN or AN processes, and finally (d) direct sputtering of the constituents of the alkali halide solids. Our results are compared to recent investigations of the total sputter yield of Ar+ bombarded LiF films.

Desorption from LiF(100) by Singly- and Doubly-Charged Hyperthermal He Ions

Sputtering of a LiF(100) surface by singly- and doubly-charged He ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F− ions is only slightly higher for doubly-charged ions compared to singly-charged projectiles at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model [1,2] based on calculations by Walkup and Avouris [3].

Ionization of LiF by Hyperthermal Multiply Charged Ions

Sputtering of a LiF thin film (0, 25 μm) surface by singly- (He+, Ne+ and Ar+) and doubly-charged (Ne+ and Ar+) ions with impact energies between 10 and 500 eV has been performed. The yield of sputtered Li+ and F- ions is only slightly higher for doubly-charged ions compared to singly-charged projectiles at impact energies below 100 eV, whereas the F+ yield is substantially increased if doubly-charged projectiles are used. The experimental data are explained within the framework of a model [1,2,3] based on calculations by Walkup and Avouris [4]. A comparison is given with former measurements on a LiF single crystal [1,2,3].