Dietmar Hirsch

Phenomenology of iron-assisted ion beam pattern formation on Si(001)

Pattern formation on Si(001) through 2u2009keV Kr+ ion beam erosion of Si(001) at an incident angle of =30° and in the presence of sputter co-deposition or co-evaporation of Fe is investigated by using in situ scanning tunneling microscopy, ex situ atomic force microscopy and electron microscopy. The phenomenology of pattern formation is presented, and experiments are conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Special attention is given to the determination of morphological phase boundaries and their origin. Height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering are found to be of relevance for pattern formation.

Iron-assisted ion beam patterning of Si(001) in the crystalline regime

We present ion beam erosion experiments on Si(001) with simultaneous sputter co-deposition of steel at 660?K. At this temperature, the sample remains within the crystalline regime during ion exposure and pattern formation takes place by phase separation of Si and iron-silicide. After an ion fluence of F???5.9???1021?ions?m?2, investigations by atomic force microscopy and scanning electron microscopy identify sponge, segmented wall and pillar patterns with high aspect ratios and heights of up to 200?nm. Grazing incidence x-ray diffraction and transmission electron microscopy reveal the structures to be composed of polycrystalline iron-silicide. The observed pattern formation is compared to that in the range of 140?440?K under otherwise identical conditions, where a thin amorphous layer forms due to ion bombardment.

Ge2Sb2Te5 phase-change films on polyimide substrates by pulsed laser deposition

Growth of Ge2Sb2Te5 phase-change films on flexible polyimide substrates by pulsed laser deposition (PLD) is demonstrated. The effect of annealing temperature on the crystalline nature of the films was studied. A decrease of (200) lattice plane distance with the increase of annealing temperature was revealed for the films grown on both polyimide and Si substrates, which was related to tensile stress in the crystallized films. Optical reflectivity measurements showed a high reflectivity contrast between full crystalline and amorphous films. The results indicate an excellent potential for applications of these PLD-deposited Ge2Sb2Te5 films on flexible polyimide substrates in optical data storage.

AFM tip calibration using nanometer-sized structures induced by ion beam sputtering

Atomic force microscopy (AFM) is usually the instrument of choice for the investigation of the surface roughness of thin films. Often a detailed image and roughness analysis is hindered by tip artifacts. Many of these artifacts arise from a spatial convolution or dilation of the actual tip and the shape of the surface features imaged. Therefore a careful tip evaluation and calibration is important for a reliable roughness evaluation. In this study about a process for the fabrication of self- assembled nanometer-sized surface structures using low- energy ion sputtering of semiconductor surfaces is reported. The dimension of these structures (typically between 10 and 100 nm), the distance between them and their shape can be tuned by the parameters of the sputter process. With the help of surfaces prepared by this way the influence of the actual AFM tip quality on the measured surface topography was evaluated. Furthermore, it is shown that the tip quality has a strong influence on the parameters extracted from first- and second-order statistics of the surface roughness. This applies particularly with regard to surfaces characterized by a low surface roughness (approximately 1 nm) as generally obtained by means of thin film technologies.

Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO3–BaTiO3 Superlattices

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO3-BaTiO3 superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO3-BaTiO3. Surprisingly, the highest ME voltage coefficient of 55 V cm-1 Oe-1 at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO3 layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO3 and BiFeO3. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices.

Properties of ns-laser processed polydimethylsiloxane (PDMS)

The medical-grade polydimethylsiloxane (PDMS) elastomer is a widely used biomaterial in medicine and for preparation of high-tech devices because of its remarkable properties. In this work, we present the experimental results on drilling holes on the PDMS surface by using ultraviolet, visible or near-infrared ns-laser pulses and on studying the changes of the chemical composition and structure inside the processed areas. The material in the zone of the holes is studied by XRD, ?-Raman analyses and 3D laser microscopy in order to obtain information on the influence of different processing laser parameters, as wavelength, fluence and number of consecutive pulses on the material transformation and its modification.

Terrace morphology on fused silica surfaces by Ar+ ion bombardment with Mo co-deposition

The morphology evolution of self-organized nanopatterns induced during Ar+ ion bombardment (IB) with Mo co-deposition on fused silica (SiO2) surfaces at different incidence angles and fluences was investigated by using atomic force microscopy and transmission electron microscopy. For pure IB at incidence angles from 30° to 70°, SiO2 surfaces evolve from being flat, via ripples, to direction-transversed ripples. In contrast, at the same ion fluence and incidence angles, the simultaneous Mo co-deposition leads to significant terraced structures with significantly enhanced roughness and wavelength. Our observations show that the concurrent Mo co-deposition during IB can reduce the critical incidence angle and the fluence level of terrace formation. Owing to the guidance of the IB-induced morphology, at incidence angles where a well-ordered ripple-mode can be generated, well-ordered terrace morphology is more likely to be formed. Terraced structures are initiated and further grow until the appearance of the nonlinear phase, i.e., where the ripple amplitude is sufficiently high. The enhanced terrace morphology on smooth SiO2 results from the interplay between pure IB and Mo co-deposition. The phase separation is attributed to the formation of crystalline MoOx on the side facing the impurity.