Institut für Oberflächen- und Schichtanalytik GmbH

New applications in the realms of terahertz (THz) technology require versatile adaptive optics and powerful modulation techniques. Semiconductors have proven to provide fast all-optical terahertz wave modulation over a wide frequency band. We show that the attenuation and modulation depth in optically driven silicon modulators can be significantly enhanced by deposition of graphene on silicon (GOS). We observed a wide-band tunability of the THz transmission in a frequency range from 0.2 to 2 THz and a maximum modulation depth of 99%. The maximum difference between the transmission through silicon and GOS is Δt = 0.18 at a low photodoping power of 40 mW. At higher modulation power, the enhancement decreased due to charge carrier saturation. We developed a semianalytical band structure model of the graphene-silicon interface to describe the observed attenuation and modulation depth in GOS.

The influence of the plasma excitation frequency on the growth conditions and the material properties of microcrystalline silicon prepared by plasma enhanced chemical vapor deposition at low deposition temperature is investigated. It is found that an increase of the plasma excitation frequency leads to a simultaneous increase of the growth rate, the grain size, and the Hall mobility of microcrystalline silicon. This is attributed to an effective selective etching of disordered material creating more space to develop crystalline grains, while also more species for faster growth of the crystallites are available.

Joining different material types, like metal and thermoplastic fiber reinforced polymer composites (TP-FRPC), offers a large potential for innovative light weight applications. This kind of bonding depends on mechanical, physical, and chemical interactions and is, therefore influenced by joining partner surface treatments. This study describes adhesion models and the effect of surface treatments of AlMg3-CF/PA66-joints. Joining by means of induction heating is an appropriate joining technology for the bonding of metal/TP-FRPC as it is utilized by a rapid heat generation. The characterization of the bonding mechanisms and the influence of the surface treatments are presented by single-lap joints and by microscopic analyses.

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.

We present a study of the influence of the annealing temperature Ta on the magnetic properties of Co40Fe40B20 thin films. Using a vector network analyzer ferromagnetic resonance (VNA-FMR) setup and the magneto-optical Kerr effect, the dependence of the Gilbert damping parameter α, the exchange constant A, the saturation magnetization as well as the coercive field and the Kerr signal on Ta is reported. Additionally, the correlation with the crystalline properties of the films studied by X-ray diffractometry is discussed. We found that while the damping parameter α and the coercive field show sharp changes starting at a certain Ta value, the exchange constant A and the Kerr signal show a steady evolution. A differential modification of the film surface compared to the bulk is discussed as a possible reason. In any case, we found that the low damping values are preserved at the first onset of crystallization.

The emission of negative secondary ions (${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Si}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Ge}}^{\mathrm{\ensuremath{-}}}$, ${\mathrm{Au}}^{\mathrm{\ensuremath{-}}}$) from keV-${\mathrm{Cs}}^{+}$-irradiated elemental surfaces was monitored during the initial stages of Cs incorporation. Concurrently determined work-function variations \ensuremath{\Delta}\ensuremath{\Phi} were found to be 2.75 eV for graphite, 2.3 eV for Si, 0.84 eV for Ge, and 0.62 eV for Au. This lowering of \ensuremath{\Phi} results in an exponential increase of the sputtered ions' ionization probability ${\mathit{P}}^{\mathrm{\ensuremath{-}}}$. Values of ${\mathit{P}}^{\mathrm{\ensuremath{-}}}$ derived from the \ensuremath{\Delta}\ensuremath{\Phi} scaling are 0.19 for ${\mathrm{C}}^{\mathrm{\ensuremath{-}}}$, 0.093 for ${\mathrm{Si}}^{\mathrm{\ensuremath{-}}}$, 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for ${\mathrm{Ge}}^{\mathrm{\ensuremath{-}}}$, and 5.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for ${\mathrm{Au}}^{\mathrm{\ensuremath{-}}}$, and agree quantitatively with measured ion-yield data. \textcopyright{} 1996 The American Physical Society.

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, whereas roughly 30% are trapped within the interfacial layer (with a thickness of ∼ 0.8 nm); the remainder resides in the surrounding matrix. Cluster-size dependent P concentrations support the view of self-purification in the Si nanostructures.

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 beam implantation in InP. The results indicate that utilizing MCs+ molecular ions an accurate quantification of major components by SIMS is possible.

Diffusion controlled solid-state reactions during the annealing of Al/Ni multilayers deposited by rf magnetron sputtering onto silicon substrates have been studied. The samples with an overall atomic concentration of ${\mathrm{Al}}_{0.5}{\mathrm{Ni}}_{0.5}$ consisted of four Ni and four Al sublayers with a double layer thickness of 50.3 nm. The temperature induced compositional and structural changes during the annealing periods of 45 min were determined by Auger electron spectroscopy (AES) sputter depth profiling and x-ray diffraction. Up to 120 \ifmmode^\circ\else\textdegree\fi{}C no intermetallic phase was detected, albeit the AES sputter depth profiles indicated a significant enrichment of Ni in the Al layers. At temperatures around 160 \ifmmode^\circ\else\textdegree\fi{}C these sublayers nucleated completely into the ${\mathrm{Al}}_{3}\mathrm{Ni}$ phase. With a further increase of the annealing temperature the reaction proceeded to the formation of ${\mathrm{Al}}_{3}{\mathrm{Ni}}_{2},$ and finally above 250 \ifmmode^\circ\else\textdegree\fi{}C to a homogeneous layer of bcc ordered AlNi.

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons. We clarify the influence of the electron-defect scattering lifetime on the spectral shape and the interface transmission on the THz amplitude, and how this is linked to structural defects of bilayer emitters. The results of our study define a roadmap of the properties of emitted as well as detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.

The unfolding of a sheared mechanically mixed third-body (TB) in tungsten/tungsten carbide sliding systems is studied using a combination of experiments and simulations. Experimentally, the topographical evolution and the friction response, for both dry and lubricated sliding, are investigated using an online tribometer. Ex situ X-ray photoelectron spectroscopy, transmission electron microscopy, and cross-sectional focused ion beam analysis of the structural and chemical changes near the surfaces show that dry sliding of tungsten against tungsten carbide results in plastic deformation of the tungsten surface, leading to grain refinement, and the formation of a mechanically mixed layer on the WC counterface. Sliding with hexadecane as a lubricant results in a less pronounced third-body formation due to much lower dissipated frictional power. Molecular dynamics simulations of the sliding couples predict chemical changes near the surface in agreement with the interfacial processes observed experimentally. Finally, online topography measurements demonstrate an excellent correlation between the evolution of the roughness and the frictional resistance during sliding.

The gradual incorporation of cesium into elemental samples of Al, Si, Nb, and Au bombarded with 5.5-keV ${\mathrm{Cs}}^{+}$ ions was investigated by monitoring the emission of sputtered positive ${\mathrm{Cs}}^{+}$ and molecular ions and the relative work-function changes (\ensuremath{\Delta}\ensuremath{\Phi}) induced from the shifts of secondary-ion energy distributions. With increasing Cs fluence and Cs surface concentration the work function is reduced, and reaches a stationary value at about 1\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Cs}}^{+}$/ ${\mathrm{cm}}^{2}$ for Si and Al, 5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{Cs}}^{+}$/${\mathrm{cm}}^{2}$ for Nb, and 1.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{Cs}}^{+}$/${\mathrm{cm}}^{2}$ for Au. The corresponding \ensuremath{\Delta}\ensuremath{\Phi} shifts then amount to -1.3\ifmmode\pm\else\textpm\fi{}0.1 eV for Al and Si, -0.9 eV for Nb, and -0.4 eV for Au. This lowering of the work function reduces the ionization probability of positive ${\mathrm{Cs}}^{+}$ ions by factors of about ten (Al), seven (Si), and three (Nb). In agreement with the electron-tunneling model of secondary-ion formation, this reduced ionization is observed only when the work function falls below a limiting value which is close to the ionization potential of Cs. Computer simulations of the Cs incorporation process result in stationary Cs surface concentrations of 12 at. % for Si, 10% for Al, 5.5% for Nb, and \ensuremath{\sim}2.5% for Au. These values scale inversely with these elements' sputtering yields. \textcopyright{} 1996 The American Physical Society.

Sputtering the surface of a TiAlN(C,O) specimen with a $14.5\ensuremath{-}{\mathrm{keV}\mathrm{}\mathrm{Cs}}^{+}$ ion beam produces negative molecular ions of ${\mathrm{N}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{CO}}^{\mathrm{\ensuremath{-}}}.$ They are detected in a double-focusing mass spectrometer operated at a mass resolution of $\ensuremath{\sim}13000;$ this provides a separation of interfering species in this mass region (e.g., ${\mathrm{Si}}^{\mathrm{\ensuremath{-}}},$ ${\mathrm{AlH}}^{\mathrm{\ensuremath{-}}}$) and a mass assignment at a precision of $\ensuremath{\sim}{10}^{\ensuremath{-}4}\mathrm{amu}.$ The flux of ${\mathrm{N}}_{2}^{\mathrm{\ensuremath{-}}}$ relative to the stable ${\mathrm{N}}_{3}^{\mathrm{\ensuremath{-}}}$ amounts to about 0.3. On average, one ${\mathrm{N}}_{2}^{\mathrm{\ensuremath{-}}}$ is formed for ${10}^{7}$ N atoms sputtered and a similar rate is observed for ${\mathrm{CO}}^{\mathrm{\ensuremath{-}}}.$ The flight time through the mass spectrometer ($\ensuremath{\sim}10\ensuremath{\mu}\mathrm{s}$) sets a lower limit for the stability of these molecular anions. The abundant production of electronically excited molecules in sputtering is proposed to constitute the prerequisite for the formation of ${\mathrm{N}}_{2}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{CO}}^{\mathrm{\ensuremath{-}}}:$ these excited molecules may capture a surface electron forming thus a temporary negative-ion state (Feshbach resonance). By contrast, the upper limit for the emission of ${\mathrm{N}}^{\mathrm{\ensuremath{-}}}$ is found to be less than one ${\mathrm{N}}^{\mathrm{\ensuremath{-}}}$ per ${10}^{10}$ nitrogen atoms ejected, in agreement with the very short lifetimes of the excited ${\mathrm{N}}^{\mathrm{\ensuremath{-}}}$ anions.

Titania films, doped with erbium, were fabricated on porous anodic alumina from a precursor. The samples, subjected to thermal processing up to 1270 K, exhibited strong luminescence at 1.53 μm associated with transitions of ions in the xerogel. The intensity of photoluminescence increased with the number of xerogel film depositions onto alumina. Secondary ion mass spectrometry analyses show that the pores of anodic alumina are filled by the xerogel after sequential spinning of ten layers. Cooling of the samples to 4.2 K gives enhancement of the intensity of the Er-related band at 1.53 μm and narrowing of the full width at half maximum from 20 to 10 nm. © 2001 The Electrochemical Society. All rights reserved.

We study the electronic properties of phosphorus doped Si nanocrystal/SiO2 superlattices and determine the carrier concentration by transient current analysis. This is achieved by encapsulating the multilayers between two electrical insulation layers and controlling the carrier mobility by a defined layer to layer separation. A saturation of the voltage dependent ionized carrier density is observed which indicates complete substitutional dopant ionization and allows to calculate the dopant induced charge carrier density. It is found that the doping efficiency of the superlattice is only 0.12% considering the full ionization regime which explains the unusual small dopant effect on transport characteristics.

Small doubly charged negative cluster ions ${\mathrm{SiC}}_{n}^{2\ensuremath{-}}$ (with $n=6,$ 8, and 10) are produced by sputtering the surface of a SiC specimen with a 14.5-keV ${\mathrm{Cs}}^{+}$-ion beam. They are detected in a double-focusing mass spectrometer that covers a dynamic abundance range of about ${10}^{9}.$ The emission yields of these dianionic clusters amount to roughly ${10}^{\ensuremath{-}4}$ of the corresponding singly charged cluster ions. For both types of ion species, the abundance distributions decrease monotonically with an increasing number of C atoms in the cluster. This observation is ascribed to fragmentation processes that are due to the high amount of internal energy relayed to the cluster species in the sputtering event. Apart from this decomposition caused by excitation, the flight time through the mass spectrometer of \ensuremath{\sim}15 \ensuremath{\mu}s establishes a lower limit with respect to the intrinsic lifetimes of both the singly and doubly charged ions.

We examine the influence of crystal growth on the spin-pumping induced inverse spin Hall effect in Fe/Pt bilayers. The morphology of the Fe/Pt interface influences the effective spin mixing conductance. The increase of growth temperature leads to smoother and larger grains at the interface that enhance the effective spin mixing conductance. The spin current injection efficiency into Pt, measured by the inverse spin Hall effect, is maximized by optimizing the epitaxy of Pt on Fe. In magnetic field dependent measurements, the presence of a strong magnetic anisotropy gives rise to two distinct inverse spin Hall effect voltage peaks.

The GaAs(110) surface was exposed to normal-incidence ${\mathrm{Ar}}^{+}$ ion bombardment at energies ranging from 200 eV to 3 keV. The structural, electronic, and compositional modifications induced were monitored by low-energy electron diffraction, electron energy-loss spectroscopy (EELS), and by Auger-electron spectroscopy for different flux densities, accumulated fluences, and specimen temperatures. The fluences necessary for the amorphization of the near-surface region at room temperature amount to some 1\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{Ar}}^{+}$/${\mathrm{cm}}^{2}$ and increase both with decreasing flux density and with decreasing impact energy, the latter effect being very pronounced below 1 keV. The alterations of the electronic states as inferred from EELS exhibit a similar fluence dependence but are less sensitive to the bombardment energy. Prolonged ion irradiation causes an As depletion of the surface which saturates at fluences \ensuremath{\ge}1\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Ar}}^{+}$/${\mathrm{cm}}^{2}$, again dependent on the impact energy and flux density. The steady-state Ga/As surface concentration ratio (${\mathit{c}}_{\mathrm{Ga}}$/${\mathit{c}}_{\mathrm{As}}$${)}_{\mathrm{\ensuremath{\infty}}}$ is 1.25 (relative to the bulk composition) at 200 eV and increases to \ensuremath{\sim}1.45 at 3 keV. These surface composition changes are reduced for ion bombardment at elevated specimen temperatures and almost no variations are observed with the sample held at 730 K.

Micro machining processes are characterized by the influence of size effects like the influence of the workpiece micro structure on the process and ultimately on the process results. So far, the influence of grain orientation on the process results is state of the art and mostly investigated on single-crystal materials, but the influence of the grain orientation on the process and process results in micro machining of polycrystalline materials with a hexagonal close packed (hcp) crystal structure like commercially pure (cp)-titanium is, to the best knowledge of the authors, not completely understood. Within this research, the correlation between the grain orientation and the surface topography, process forces, and chip formation is researched by orthogonal cutting. This process offers the possibility of clearly linking the cutting behavior with the resulting surface and the influences of the size effect. The results show a clear correlation between the grain orientation and the topography height, the ripping out of material, and the resulting surface. A correlation between the grain orientation and the forces in the X-, Y-, and Z-direction could also be shown. The chip formation and thus the resulting chip thickness differs depending on the grain orientation. In addition, micro milling tests were conducted and influences on burr formation and process forces are shown. At low spindle speeds the burr width or the burr shape are a function of the grain orientation. At high spindle speeds this effect can no longer be observed. With the results of this research, it is possible to compensate the influence of the grain orientation on the process results and to enhance the efficiency and the resulting surface quality of micro machining processes.

Abstract In this paper the low temperature deposition of nanocrystalline and ultrananocrystalline diamond (UNCD) films is compared and discussed. NCD films were prepared by hot filament chemical vapor deposition from a 1% CH 4 /H 2 mixture, while microwave plasma chemical vapour deposition was used to deposit UNCD films from a mixture of 17% CH 4 /N 2 . The resulting films have been thoroughly characterized concerning their morphology and structure by scanning electron microscopy and concerning their crystalline properties by X‐ray diffraction. The composition was analyzed by X‐ray photoelectron spectroscopy (XPS), whereas XPS and Raman spectroscopy were applied to get information on the bonding structure of the films. The most important result of this study is that the composition, structure, morphology, and bonding environment of UNCD hardly change if the deposition temperature is lowered from 770 to 530 °C or even 450 °C. In contrast, there are drastic changes of the nature of NCD films if the temperature is reduced to 700 °C or even lower. Interestingly, the sp 2 /sp 3 ratio of the NCD films remains low and constant in the temperature range investigated. Rather, the nature of the sp 2 grain boundary material undergoes drastic changes if the temperature is lowered below 700 °C. In addition, the films become inhomogeneous on a micrometer (not nanometer) scale. Possible reasons for these observations will be discussed throughout the paper.