Joachim Ahner

Development and performance of the nanoworkbench: A four tip STM for conductivity measurements down to submicrometer scales

A multiple-tip ultrahigh vacuum (UHV) scanning tunneling microscope (MTSTM) with a scanning electron microscope (SEM) for imaging and molecular-beam epitaxy growth capabilities has been developed. This instrument (nanoworkbench) is used to perform four-point probe conductivity measurements at μm spatial dimension. The system is composed of four chambers, the multiple-tip STM∕SEM chamber, a surface analysis and preparation chamber, a molecular-beam epitaxy chamber, and a load–lock chamber for fast transfer of samples and probes. The four chambers are interconnected by a unique transfer system based on a sample box with integrated heating and temperature-measuring capabilities. We demonstrate the operation and the performance of the nanoworkbench with STM imaging on graphite and with four-point-probe conductivity measurements on a silicon-on-insulator (SOI) crystal. The creation of a local FET, whose dimension and localization are, respectively, determined by the spacing between the probes and their positio...

Adsorbate–adsorbate repulsions—the coverage dependence of the adsorption structure of CO on Cu(110) as studied by electron‐stimulated desorption ion angular distribution

The coverage dependent orientation of CO adsorbed on a Cu(110) surface was studied by the electron‐stimulated desorption ion angular distribution (ESDIAD) technique. A neutral excited (CO*) species is imaged and in addition positive ions are measured. The adsorption temperature was varied between 32 K and 150 K. By applying the ESDIAD technique at a temperature below 80 K it was possible to decrease the beamwidths drastically, to determine the angular distributions better than ±0.5°, and to study the adsorption of CO chemisorbed and physisorbed on the surface. With increasing CO coverage we observe three distinct ESDIAD patterns. Starting from a normal beam pattern with an elliptical cross section with the major axis oriented in the 〈110〉 direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into a pattern of two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the 〈110〉 directions by ∼9°. With further increasing CO c...

Adsorption and electron-stimulated dissociation of ammonia on Cu(110): an ESDIAD study

Abstract The adsorption of NH 3 by Cu(110) has been studied at 80 K. At low coverages, NH 3 adsorbs on atop Cu sites with the C 3v axis normal to the surface, and the NH bonds occupy all azimuthal directions as observed by ESDIAD, which detects a symmetrical ring pattern. As the coverage rises to ∼0.1 ML, tilting of the NH 3 molecules in the 〈001〉 directions occurs, yielding off-normal H + ESDIAD beams. Electron beam damage (200 eV) results in the production of NH 2 (a) and H(a) species, both of which are observed by ESDIAD to produce characteristic H + beam directions. The cross-section for electron-induced dissociation of NH 3 is 1.9 ± 0.2 × 10 −16 cm −2 at 200 eV.

Patterning of sub-10-nm Ge islands on Si(100) by directed self-assembly

A process is reported for creating arbitrary patterns of sub-10-nm Ge islands on a Si(100) substrate by directed self-assembly. Carbon-based templates are created on Si substrates by electron-beam-induced deposition using high-resolution electron beam lithography. Ozone etching, followed by annealing in ultra-high vacuum, yields small (<4nm) SiC nucleation sites for subsequently deposited Ge. Quantitative analysis of atomic force microscope images reveals templated Ge islands with mean diameter d∼8nm, averaging 2000±500 atoms per island, with controlled spacings as small as 35 nm, and 2 nm absolute positional accuracy. The Ge∕Si nanostructures reported here may find use in end-of-scaling classical computing and single-electron devices and spin-based quantum computing architectures.

The adsorption conformation of chemisorbed pyridine on the Cu(110) surface

The adsorption conformation of pyridine on the Cu(110) surface has been studied using temperature programmed desorption, low energy electron diffraction, and electron stimulated desorption ion angular distribution techniques. Pyridine adsorbs on Cu(110) via the nitrogen lone pair orbital. The molecular symmetry axis is perpendicular to the surface. At low coverage, the aromatic ring plane of adsorbed pyridine is oriented parallel to the 〈001〉 plane, which is perpendicular to the close-packed copper atom rows on Cu(110). At high coverage, the aromatic ring plane of pyridine is azimuthally rotated by 25 (±5)° away from the 〈001〉 direction as a result of an attractive contribution from nonparallel aromatic ring–aromatic ring interactions superimposed on the overall repulsive adsorbate–adsorbate interactions. The zero coverage desorption activation energy of pyridine on Cu(110) is 0.97 eV and the repulsive adsorbate–adsorbate interaction energy is 0.16 eV/monolayer. At near saturation coverage, (5×3) and (4×3...

Formation and thermal stability of sub-10-nm carbon templates on Si(100)

We report a lithographic process for creating high-resolution (<10nm) carbon templates on Si(100). A scanning electron microscope, operating under low vacuum (10−6mbar), produces a carbon-containing deposit (“contamination resist”) on the silicon surface via electron-stimulated dissociation of ambient hydrocarbons, water, and other adsorbed molecules. Subsequent annealing at temperatures up to 1320K in ultrahigh vacuum removes SiO2 and other contaminants, with no observable change in dot shape. The annealed structures are compatible with subsequent growth of semiconductors and complex oxides. Carbon dots with diameter as low as 3.5nm are obtained with a 200μs electron-beam exposure time.

Fabrication of FePt nanoparticles for self-organized magnetic array

A self-organized array of magnetic nanoparticles can be potentially used to increase the storage density in magnetic recording. One challenge in this approach is to obtain long-range order assemblies of the nanoparticles. One method to solve this problem is to pattern a substrate having circumferential patterns, whose dimensions are within the coherent length of the self-organized array. By patterning the disk into topographically confined circumferential patterns with such dimensions, thermally stable magnetic nanoparticles may be used to fabricate magnetic recording disks. The circumferential patterns in this case are formed on the disk substrate with dimensions of 100–500 nm and depths of 5–20 nm, prepared by electron beam lithography and reactive ion etching techniques. The monodispersed FePt nanoparticles were synthesized by thermal decomposition of iron pentacarbonyl and reduction of platinum salt simultaneously in the presence of surfactant molecules, achieving a size distribution of 3.15±0.20 nm. ...

Probing the role of carbon microstructure on the thermal stability and performance of ultrathin (<2 nm) overcoats on L10 FePt media for heat-assisted magnetic recording.

An understanding of the factors influencing the thermal stability of ultrathin carbon overcoats (COCs) is crucial for their application in heat-assisted magnetic recording (HAMR) at densities ≥ 1 Tb/in(2). Two types of non-hydrogenated ultrathin (∼1.5 nm) COCs were investigated after being subjected to laser-induced localized heating (at temperatures > 700 K) as envisaged in HAMR. Filtered cathodic vacuum arc (FCVA)-processed carbon with tuned C(+) ion energies of 350 eV followed by 90 eV provides significantly higher sp(3) C-C hybridization than magnetron sputter deposition even at very low thicknesses of ∼1.5 nm. As a result, the FCVA-deposited ultrathin carbon overcoats displayed excellent thermal stability along with improved wear and corrosion resistance. On the other hand, the sputtered carbon exhibited carbon loss and topographical and structural changes after laser irradiation owing to lower sp(3) hybridization. Therefore, this study highlights the pivotal role of carbon microstructure, primarily sp(3) hybridization, in non-hydrogenated carbon overcoats to maintain excellent thermal stability during the recurring high-temperature cycles in a HAMR process.

Charge-based scanning probe readback of nanometer-scale ferroelectric domain patterns at megahertz rates

We present a method for data storage in continuous ferroelectric (FE) media, applicable to storage systems based on one or more scanning probes. Written FE domains are read back in a destructive fashion by applying a constant voltage of magnitude greater than the coercive voltage, as is done in FE random access memory (FeRAM). The resulting flow of screening charges through the readback amplifier provides sufficient signal to allow readback of domains of minimum dimension of the order of 10 nm at MHz rates, orders of magnitude faster than previously demonstrated techniques for readback of domains in continuous FE media.

The frustrated translational mode of CO on Cu(110): Azimuthal anisotropy studied by helium atom scattering—A comparison with time-of-flight electron stimulated desorption of ion angular distribution measurements

The frustrated translation of isolated CO molecules parallel to the surface, or T mode, has been studied on a Cu(110) substrate using helium atom scattering. A small but significant anisotropy in T-mode vibrational frequencies was found for the on-top site species with oscillator frequencies of 3.6 and 3.2 meV along the [110] and the [001] directions, respectively. These results are compared with recent results from time-of-flight electron stimulated desorption of ion angular distribution measurements [Ahner et al., Phys. Rev. Lett. 79, 1889 (1997)], where also a significant anisotropy has been observed.