High-Speed Scanning Tunneling Microscopy on Thin Oxide Film Systems

Dünne Silizium- und Germaniumdioxidfilme auf Ru(0001)-Kristallen werden hinsichtlich dynamischer Prozesse untersucht. Zwischen Oxidfilm und Substrat befinden sich Sauerstoffatome, die eine ent-scheidende Rolle in diesen Systemen spielen. Zunächst werden diese Sauerstofflagen auf Ru(0001) mittels Hochgeschwindigkeits-Rastertunnelmikroskopie (STM) analysiert. Daraufhin wird die GeO2-Monolage auf Ru(0001) bei hohen Bildraten mit einer selbstentwickelten halbautomatischen Netz-werkdetektion untersucht. Schließlich wird die SiO2-Bilage auf Ru(0001) mit konventionellen sowie mit schnellen STM-Messungen bei Raumtemperatur und bei 600 K abgebildet. Um schnelle Messungen bei hohen Temperaturen zu realisieren, wird ein Hochgeschwindigkeits-STM konstruiert, welches bei unterschiedlichen Temperaturen betrieben werden kann. Unkon-ventionelle Spiralgeometrien ermöglichen verzerrungsfreie Bilder in weniger als 10 ms aufzunehmen. Die adsorbierten Sauerstofflagen werden erstmals bei hohen Bildraten untersucht. Die experimen-tellen Ergebnisse werden durch extern durchgeführte Dichtefunktionaltheorie-Berechnungen ergänzt. In den auf Ru(0001) bei Raumtemperatur stabilen Sauerstofflagen O(2×2), O(2×1) und 3O(2×2) werden dynamische Prozesse beobachtet. Die Besetzung des Zwischenzustandes entlang des Diffusionspfades und schnelle "Umklapp"-Prozesse eindimensionaler Linien werden auf atomarer Ebene aufgelöst. Komplexe Domänengrenzen in der GeO2-Monolage auf Ru(0001) werden mit Hochgeschwindigkeits-STM abgebildet. Die Messungen an der SiO2-Bilage auf Ru(0001) zeigen dynamische Änderungen des Abbildungskontrasts, die mit den mobilen Sauertsoffatomen an der Grenzfläche zusammenhängen können. Messungen bei hohen Temperaturen zeigen dynamische Kontraständerungen von mesoskopischen Strukturen. Diese Messungen stellen die ersten schnellen Hochtemperatur-STM-Aufnahmen des Siliziumdioxidfilms dar und bilden die Grundlage für künftige Studien zu dynamischen Veränderungen in dünnen Oxidschichtsystemen.Dynamics related to thin silicon- and germanium dioxide films that are grown on Ru(0001) crystals are investigated. Between the film and the metal support oxygen species are present that play a crucial role for these film systems. First, these oxygen adlayers on Ru(0001) are analyzed by high-speed scan-ning tunneling microscopy (STM) with the focus on dynamic processes. In a next step, the monolayer of germanium dioxide (germania) supported on Ru(0001) is studied at elevated frame rates and with a self-designed semi-automated network detection. Finally, the bilayer of silicon dioxide (silica) on Ru(0001) is studied by conventional and by high-speed STM both at room temperature and at 600 K. To realize fast STM measurements at elevated temperatures, a high-speed STM is designed that can operate at variable temperatures. Images are acquired in less than 10 ms with unconventional spiral scan patterns. The dynamics in oxygen adlayers are investigated for the first time at elevated frame rates. Experimental results are supported by density functional theory (DFT) calculations performed externally. Dynamic events are observed in the oxygen adlayers that are stable on Ru(0001) at room temperature, namely O(2×2), O(2×1), and 3O(2×2). The occupation of an intermediate state along the oxygen diffusion pathway and fast "flipping" events of atomic one-dimensional stripe patterns are observed. On the germania monolayer on Ru(0001), complex domain boundary structures are resolved with high-speed STM. In high-speed scans on the silica bilayer on Ru(0001), dynamic changes of the imaging contrast are observed that may relate to the mobile species in the oxygen interfacial layer. Measurements at elevated temperature reveal dynamic contrast changes of mesoscopic features. These measurements constitute the first high-speed STM scans on the silica film at elevated temperatures and form the basis for future studies with the focus on dynamic processes in thin oxide film systems

The Fermi energy in acceptor doped SrTiO3 and BaTiO3

In order to evaluate the presence of space charge layers and the magnitude of band bending at electrode interfaces of mixed ionic-electronic conductors we have evaluated the Fermi energies in the bulk and at interfaces of acceptor-doped SrTiO3, BaTiO3 and (Ba,Sr)TiO3. While the interface Fermi energy can be directly obtained using photoelectron spectroscopy (XPS) if conducting electrode materials are deposited, the determination of the bulk Fermi energy is more challenging due to the high resistivity of the samples. One approach is to use XPS on thin films deposited on conducting samples. In general, we observed a good agreement between upper and lower limits of Fermi energies at thin films surfaces and at interfaces. Surprisingly, the Fermi energy is hardly observed below EF-EVB≈2eV (see Fig. 1), although defect chemistry calculations predict values as low as EF-EVB≈2eV for acceptor doped samples, such as Fe-doped SrTiO3 or Mn-doped BaTiO3.c,d Even at anode interfaces of ionically polarized Fe-doped SrTiO3 single crystals,e at which the oxygen vacancy concentration should be very low, we have not observed lower Fermi energies. Please click Additional Files below to see the full abstract

Deep level analysis of homoepitaxial ZnO doped with N

High intrinsic carrier concentration (n-type) • Efforts to reduce this effect: • Homoepitaxy1 • Non-polar orientations • Similar samples exhibit residual doping as low as ~1014 cm-3 (2) The path to p-type doping • Many dopants proposed • N is a promising candidate • Simple NO is a deep level • Complex levels have shallower energies • N-related levels observed near the VB by many groups • Energies between 130 meV and 160 meV from VB

The energy level of the Fe2+/3+-transition in BaTiO3 and SrTiO3 single crystals

An approach to determine the defect energy levels of the Fe impurities in BaTiO3 and SrTiO3 single crystals using electrical conductance measurements is presented. The defect levels are obtained from the dependence of the activation energy of electrical transport on the oxygen vacancy concentration, which is varied by stepwise re-oxidation of a reduced sample. An energy level at 0.7–0.8 eV below the conduction band minimum ECB is identified for BaTiO3, which can be assigned to the Fe2+/3+-transition in good agreement with literature. In contrast, the conductivity of Fe-doped SrTiO3 does not show a defect energy level in the upper half of the band gap, indicating that the Fe2+/3+-transition in SrTiO3 is near the conduction band minimum. The often reported alignment of defect energy levels, which is fulfilled for the Fe3+/4+-transition in BaTiO3 and SrTiO3, does not hold for the Fe2+/3+-transition in these compounds. This limits the applicability of Fe-doped SrTiO3 as a model system for studying resistance degradation in acceptor-doped high-permittivity dielectrics

Public-Key Cryptographic Processor for RSA and ECC

We describe a general-purpose processor architecture for accelerating public-key computations on server systems that demand high performance and flexibility to accommodate large numbers of secure connections with heterogeneous clients that are likely to be limited in the set of cryptographic algorithms supported. Flexibility is achieved in that the processor supports multiple public-key cryptosystems, namely RSA, DSA, DH, and ECC, arbitrary key sizes and, in the case of ECC, arbitrary curves over fields GF (p) and GF (2 m). At the core of the processor is a novel dual-field multiplier based on a modified carrysave adder (CSA) tree that supports both GF (p) and GF (2 m). In the case of a 64-bit integer multiplier, the necessary modifications increase its size by a mere 5%. To efficiently schedule the multiplier, we implemented a multiply-accumulate instruction that combines several steps of a multiple-precision multiplication in a single operation: multiplication, carry propagation, and partial product accumulation. We have developed a hardware prototype of the cryptographic processor in FPGA technology. If implemented in current 1.5 GHz processor technology, the processor executes 5,265 RSA-1024 op/s and 25,756 ECC-163 op/s- the given key sizes offer comparable security strength. Looking at future security levels, performance is 786 op/s for RSA-2048 and 9,576 op/s for ECC-233.