Hubert Gnaser

SYNTHESIS AND ELECTRON FIELD EMISSION OF NANOCRYSTALLINE DIAMOND THIN FILMS GROWN FROM N2/CH4 MICROWAVE PLASMAS

Nanocrystalline diamond films have been synthesized by microwave plasma enhanced chemical vapor deposition using N2/CH4 as the reactant gas without additional H2. The nanocrystalline diamond phase has been identified by x-ray diffraction and transmission electron microscopy analyses. High resolution secondary ion mass spectroscopy has been employed to measure incorporated nitrogen concentrations up to 8×1020 atoms/cm3. Electron field emission measurements give an onset field as low as 3.2 V/μm. The effect of the incorporated nitrogen on the field emission characteristics of the nanocrystalline films is discussed.

ERBIUM LUMINESCENCE IN POROUS SILICON DOPED FROM SPIN-ON FILMS

Erbium photoluminescence at room temperature and at 77 K has been observed from porous silicon doped with erbium from a spin‐on silica gel film. Erbium incorporation into silicon dioxide at the surface of porous silicon and rapid thermal processing at temperatures higher than 1223 K were found to be a necessary prerequisite for erbium‐related luminescence in porous silicon. No erbium diffusion into monocrystalline silicon from the spin‐on films was observed. The depth‐dependent erbium concentration in the bulk of porous silicon was determined by secondary‐neutral‐ and secondary‐ion‐mass spectrometry depth profiling. The laterally resolved erbium distribution in the porous silicon was derived from energy‐dispersive x‐ray analysis. Possible mechanisms of erbium‐related luminescence in porous silicon are discussed.

SIMS depth profile analysis using MCs+ molecular ions

SummaryThe use of Cs+ primary ions in conjunction with the detection of MCs+ molecular ions (where M is the element to be monitored) in SIMS depth profiling is shown to be an efficient method of minimizing the variations of ion yields with sample composition, e.g., at the interface of multilayer structures. Depth profiles of several such samples demonstrate that MCs+ intensities follow closely the concentrations of the respective elements, providing the possibility of a (semi)quantitative analysis of major components by means of secondary ion mass spectrometry. As indicated by the similarity of their energy distribution data, the formation and emission process of MCs+ molecules seems coupled to that of Cs+ ions.

The emission of neutral clusters in sputtering

The mass, angle, and energy resolved emission of neutral clusters in sputtering was studied for a variety of metals and semiconductors. The main phenomena and results are the following: (i) Cluster emission from a series of transition metals reveals a prominent contribution of clusters to the total flux of ejected particles but there is no simple scaling of cluster intensities with the average sputtering yields. With increasing number of constituents, relative intensities of neutral clusters decrease much faster than those of secondary-ion clusters. (ii) The relative intensities of clusters emitted from amorphous and crystalline semiconductors are identical, but the energy spectra of Gen-clusters (n = 1–4) sputtered from Ge (111) peak at a slightly higher energy (1 eV) as compared to spectra taken from amorphous Ge. The intensities of all Gen-clusters exhibit the same dependence on emission angle; this holds for both the amorphous and crystalline Ge-sample. (iii) The flux of neutral monomers, dimers, and trimers sputtered from Cu(111), Ni(111), and Ag(111) crystals shows a pronouncedly anisotropic emission along the 〈110〉 lattice directions which is ascribed to a momentum alignment in the anisotropic part of the collision cascade. Energy spectra taken along 〈110〉 peak at higher energies than those obtained from a random emission angle.

Improved quantification in secondary‐ion mass spectrometry detecting MCs+ molecular ions

Molecular secondary ions composed of a sample atom M and a resputtered Cs+ projectile (MCs+) are successfully used for the quantification of secondary‐ion mass spectrometry (SIMS) data. For a variety of different specimens it is demonstrated that the yields of these rather ubiquitous species exhibit little or no dependence on sample composition (matrix effect) even in the presence of electronegative elements and are thus well suited for quantitative SIMS evaluation. Specifically, for series of binary and ternary systems (a‐Si:H, a‐SiGe:H, a‐SiC:H, and HCN) composition independent relative sensitivity factors could be established; thus, quantification by means of a single standard is feasible. Furthermore, from the correlation of the measured MCs+ secondary ion intensities relative sensitivity factors (and hence concentrations) can directly be derived, i.e., without any standard. The applicability of this quantification scheme is exemplified for a AlGaAs/GaAs multilayer structure and for the Ga focused ion...

Phosphorus doping of Si nanocrystals embedded in silicon oxynitride determined by atom probe tomography

Silicon nanocrystals (SiNCs) embedded in a silicon oxide matrix were studied by 3D atom probe tomography (APT). The distribution of the SiNC diameter was found to have a mean value of 3.7 ± 0.8 nm. The elemental composition of these particles was determined by employing two different approaches: (i) The proximity histogram method and (ii) a cluster identification algorithm based on maximum-atom separations. Both approaches give very similar values in terms of the amount of P, O, and Si within the SiNCs: the mean atomic concentrations are cP = 0.77% ± 0.4%, cO = 12.3% ± 2.1%, and cSi = 85.3% ± 2.1%. A detailed cluster analysis implies that, on average, a 4.5-nm SiNC would contain around 30 P atoms, whereas a 2.0-nm SiNC would contain only around 3 P atoms. Radial concentration profiles obtained for these SiNCs indicate that the P content is inhomogeneous and possibly enhanced at the boundary as compared to the interior of the NCs. About 20% of the P atoms are found to be incorporated into the SiNCs, wherea...

Preferential emission of lighter isotopes in the initial stage of sputtering

Abstract Initially, the flux of sputtered ions is strongly enriched in the lighter isotopes. During sample erosion, this enrichment gradually decreases and finally a steady-state value is reached. Isotopic variations were monitored for primary beam (14.5 keV O−) fluence increments of ∼1 × 1015 ions/cm2 by means of secondary ion mass spectrometry. Extrapolation of the data to the zero-fluence limit shows that the ratio 46Ti+/50Ti+ (sputtered from TiO2) is initially higher by a factor of ∼1.063 relative to the steady-state flux. The same value for 6Li+/7Li+ (sputtered from LiAlSi2O6) is ∼1.054. Enrichments of comparable magnitude were observed also for Ga and Mo isotopes sputtered from GaAs and Mo, respectively. Applying a theoretically predicted enrichment factor at low fluence of (Mj/Mi)2m, with Mj and Mj being the mass of the light and heavy isotope, respectively, values of m ranging from 0.16 to 0.45 are derived from the experiments. A momentum asymmetry in the collision cascade, as found in recent computer simulations, might cause these strong effects.

Analysis of solids by secondary ion and sputtered neutral mass spectrometry

A mass spectrometer is described, which allows the analysis of sputtered neutral and charged particles as well as of residual gas composition. This combined SIMS, SNMS, and RGA instrument consists of a scanning primary ion beam column, an electron impact ionizer, an electrostatic energy filter and an rf quadrupole mass analyzer.Various examples of surface and bulk analysis are presented which demonstrate the beneficial complementary features of these techniques. These are, in particular: a substantial reduction of the matrix effect and fewer complications with samples of low electrical conductivity in SNMS, and the possibility of measuring the depth distribution of gases included in small cavities in the solid in the SNMS/RGA mode. SIMS, on the other hand, allows in many cases higher detection sensitivities.

Location and Electronic Nature of Phosphorus in the Si Nanocrystal--SiO2 System.

Up to now, no consensus exists about the electronic nature of phosphorus (P) as donor for SiO2-embedded silicon nanocrystals (SiNCs). Here, we report on hybrid density functional theory (h-DFT) calculations of P in the SiNC/SiO2 system matching our experimental findings. Relevant P configurations within SiNCs, at SiNC surfaces, within the sub-oxide interface shell and in the SiO2 matrix were evaluated. Atom probe tomography (APT) and its statistical evaluation provide detailed spatial P distributions. For the first time, we obtain ionisation states of P atoms in the SiNC/SiO2 system at room temperature using X-ray absorption near edge structure (XANES) spectroscopy, eliminating structural artefacts due to sputtering as occurring in XPS. K energies of P in SiO2 and SiNC/SiO2 superlattices (SLs) were calibrated with non-degenerate P-doped Si wafers. results confirm measured core level energies, connecting and explaining XANES spectra with h-DFT electronic structures. While P can diffuse into SiNCs and predominantly resides on interstitial sites, its ionization probability is extremely low, rendering P unsuitable for introducing electrons into SiNCs embedded in SiO2. Increased sample conductivity and photoluminescence (PL) quenching previously assigned to ionized P donors originate from deep defect levels due to P.

Energy and Angular Distributions of Sputtered Species

Energy and angular distributions of sputtered species from a wide variety of target materials (metals, semiconductors, alkali halides, frozen gases, and organic solids) are discussed. Bombardment energies in the range from a few 10 eV to roughly 100 keV are considered, covering irradiation conditions for which nuclear (elastic) collisions dominate the energy dissipation processes. In the linear cascade regime, energy spectra of neutral, excited, and ionized atoms and molecules are presented. The emission characteristics of clusters and large organic molecules are described in considerable detail. In this context, computer simulations can clearly elucidate pertinent ejection mechanisms. Angular distributions from amorphous, polycrystalline, and single-crystal materials illustrate the distinct influence of the sample structure on the spectra. Emission distributions at low impact energies indicate the occurrence of anisotropic cascades and of collision sequences involving only a small number of recoil generations. High-density cascades are examined for cluster-ion bombardment. As compared to atomic irradiation, for cluster impact an additional low-energy component is found in energy spectra. For large cluster projectiles (composed of hundreds of atoms) the angular distributions of sputtered material exhibit a pronounced emission at very oblique polar angles.