List of thesis topics 2008-2009 Master of physics Solid-state physics on nanometer scale Theme 1: Magnetism on mesoscopic scale •

Magnetic coupling between an ultra thin Fe-layer and an Cr-single-crystal Supervision: Nikie Planckaert, Bart Laenens, Joost Demeter, Prof. André Vantomme, Prof. Kristiaan Temst In 2007, the Nobel prize in Physics was awarded to Albert Fert and Peter Grünberg for their discovery and understanding of the giant magnetoresistance (GMR) effect. Together with the many examples of practical applications in the magnetic recording industry, the award illustrates the importance of the study of magnetic coupling phenomena like exchange bias in ultra thin films. An exchange biased nanostructure exists of a ferromagnetic material (e.g. Fe) and an antiferromagnetically ordered material (e.g. Cr). In the proposed project, we will study the non-continuous coupling behavior at low temperature between an ultra thin Fe-layer and a Cr single crystal. To understand this behavior the total Fe/Cr-system as well as each layer individually needs to be characterized magnetically. Therefore, both conventional techniques as nuclear techniques will be used in the project. Even the possibility exists to take part in a neutron diffraction experiment at one of the neutron reactors in Europe.

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Magnetic anisotropy of epitaxial ferromagnetic films Supervision: Dr. Steven Brems, Prof. Chris Van Haesendonck In order to apply ferromagnetism for magnetic data storage, it is essential to be able to control the magnetization reversal process in the presence of an external magnetic field. Your research will focus on epitaxially grown iron films with pronounced magnetocrystalline anisotropy, which you will prepare by molecular beam epitaxy. You will determine the magnetization reversal process by combining anisotropic magnetoresistance measurements and magnetic force microscopy.

Theme 2: Vortex-materie en supergeleiding •

Nanostructured Superconductor/Ferromagnet hybrids Supervision: Dr. A.V. Silhanek, Dr. W. Gillijns, Dr. J. Van de Vondel, Prof. Victor Moshchalkov During this research project we will explore the interplay between two competing cooperative phenomena, namely Superconductivity and Ferromagnetism. We will focus our investigation on submicrometer scale permanent magnets in close proximity with a superconductor of similar shape and dimensions.

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Tunable pinning landscapes Supervision: Dr. J. Van de Vondel, Dr. W. Gillijns, Dr. A.V. Silhanek, Prof. Victor Moshchalkov

The aim of this project is to achieve a fully controllable pinning landscape for quantum flux lines in superconducting films. In order to achieve this goal we will profit from the magnetic stray field generated by micrometer scale coils made of either superconducting materials or normal metals.

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Vortex Imaging in Superconducting Nanostructures Supervision: Katrien De Keyser, Dr. Roman Kramer, Dr. Mariela Menghini, Prof. Victor Moshchalkov We will visualize superconducting vortex patterns in Superconductor/Ferromagnet hybrids. The combined use of the Bitter decoration technique and Scanning Hall Probe microscopy will provide a rich overview of the vortex distribution in a wide range of fields and temperatures. In specific conditions, the use of an applied transport current will allow the study of vortex dynamics in these systems.

Theme 3: Electron spectroscopy on semiconductor nanostructures •

Internal electron photoemission in nanolayers and nanostructures Supervision: Prof. V.V. Afanas’ev, Prof. A. Stesmans, S. Shamuillia The energy spectrum of electron states in nanometer-thin insulating layers and at their interfaces with, e.g., semiconductors (Si, Ge, etc.) can be characterized into great detail using observations of optically-induced electron transitions across the interface (the internal photo-emission effect, as compared to the better known classical solid/vacuum photoelectric effect ingeniously described way back by A. Einstein). This method allows the most straightforward and unique determination of the fundamental energy parameters of a two-solids nanostructure, like band gap, height of energy barriers and band offsets at the interfaces. The spectroscopy of internal photoemission will be applied to the study of insulating materials currently tested for use in new novel micro-and nanoelectronic devices, this within a framework of international collaboration.

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Probing the structural quality of semiconductor/heterostructures using electron spin resonance Supervision: Prof. A. Stesmans, Prof. V.V. Afanas’ev, M. Jivanescu, K. Keunen When dimensionally downward scaling solid structures from the macro over the microscopic down to the nano (quantum) size, the relative importance of surfaces and interfaces gradually increases, even towards taking an all dominant role: One atomic-size imperfection my imply life or dead for the envisioned functional (physical) property. The technique of magnetic resonance, applied to electrons (ESR), is the unique tool enabling, besides characterization, identification on true atomic scale of imperfections in fully non-destructive way through sensing the magnetic moment of unpaired electrons. Actual challenges imply the interfaces in Ge/GeO2/SiO2 and GaAs/oxide entities, and a natural defect in SiO2.

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First-principles modeling of high-mobility semiconductors/insulator interfaces Supervision: Prof. M. Houssa, Prof. V.V. Afanas’ev, M. Scarozza, Prof. A. Stesmans A next step in boosting progress and performance of semiconductor devices implies, in replacement of the current Si, incorporation of semiconductors of higher intrinsic carrier mobility. When introduced at the nano scale, this will require new analysis and physics insight. The objective of this work is to compute and model the structural and electronic properties of interfaces of high-mobility semiconductors (e.g., Ge) with high-quality metal oxide insulators. As a propelling tool, the results

will be correlated, when possible, to experimental results obtained through structural and electrical observations.

Theme 4: Ionimplantation – structure, doping, properties •

Lattice site location of phosphorus in germanium Supervision: Stefan Decoster, Prof. André Vantomme For many applications in micro-electronics, the behaviour of impurity atoms (such as electrical, optical, ... dopants) in semiconductors such as Si, Ge, SiGe, GaAs, GaN,... plays a very important role. A very crucial factor is their microscopic lattice location in the semiconductor lattice, which is known to highly influence its electronic or optical properties. In this study, the lattice location of several impurity atoms in Ge will be investigated. The past few years, these experiments have been performed with rare earth elements (Er) and transition metals (Fe, Ag, Cu...). The future work will be mainly focused on phosphorus (P), one of the most important and promising dopant elements in Ge. We will study the influence of the annealing temperature, the implanted fluence and the effect of codoping on the lattice location of P. The goal is to determine the stability of the lattice location, the possible diffusion of P in Ge, and the microscopic configuration of P-related defects. The technique we will use is based on emission channeling (EC), after implantation of radioactive impurity atoms. De EC-experemints are performed at the ISOLDE-facility in CERN, Geneva.

Theme 5: Self-organization and growth mechanisms of thin layers •

Development of artificial neural networks for the analysis of real time RBS data Supervision: Dr. Dries Smeets, Jelle Demeulemeester, Prof. André Vantomme Rutherford Backscattering spectrometry (RBS) is one of the most powerful quantitative ion beam techniques to determine the composition of a thin film. Especially the depth sensitivity makes it extremely valuable for the study of thin film solid phase reactions. Even more valuable data concerning diffusion rates and the role of diffusing elements can be gathered when RBS is performed during annealing, i.e. real time RBS. However, the time consuming analyses of hundreds of spectra typically taken during an experiment prohibits the real break-through of real time RBS. Recently, tests of a novel analysis technique called Artificial Neural Networks (ANN) have been undertaken with great success and the RBS community is therefore looking forward to further development of ANN’s dedicated to real time RBS. In this project a certain amount of time is reserved to acquire experimental expertise in RBS, the main goal is to further develop and integrate ANN for the analysis of real time RBS data.

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Growth of Ni silicide thin films with a lowered contact resistance by the addition of rare earth metals Supervision: Dr. Dries Smeets, Jelle Demeulemeester, Prof. André Vantomme, Prof. Kristiaan Temst Metal silicides (e.g. CoSi2, TiSi2, NiSi,…) represent a class of intensively studied materials. Interest in the formation, growth and properties of these silicide thin films is motivated by their low resistivity, which makes them applicable for dedicated micro-electronic devices. At the moment NiSi is used as contact layer material because of its low resistivity. Because of the on going down-scaling of these devices, not only the resistivity of the silicides, but also their thin film microstructure and schottky barrier raised by the silicide-silicon contact, gain importance as determining properties for the

transistor performances. The challenge for the full integration of NiSi lies within its rather high schottky barrier with respect to n-Si. The aim of this project is therefore to lower this barrier by adding rare earth elements which exhibit a much lower barrier. In this project Ni/rare earth silicide thin films will be grown by reactive deposition in a molecular beam epitaxy system. Structural and electrical properties will be studied by ion beam techniques, x-ray diffraction and barrier height measurements.

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Surface passivation of semiconductors (Ge en GaAs) Supervision: Dr. Dries Smeets, Claudia Fleischmann, Kristof Paredis, Prof. André Vantomme, Prof. Kristiaan Temst In the search for faster electronic devices novel materials such as Ge or GaAs are currently under great investigation in the semiconductor industry in terms of their capability to replace the conventional Si. However, growing thin films (dielectric material) on top of these substrates causes defects at the interface, which will inhibit the undisturbed function of the final product. To produce a defect-free interface a passivation layer (S or Se) is deposited onto the surface of the Ge or GaAs substrate. In this way the subsequent growth of the dielectric material can be optimized, i.e. defects at the interface are effectively reduced. The goal of this thesis project is to investigate the chemical and structural properties of the surface passivation layer. Furthermore, the influence of gasses and heat treatments on the passivation will be explored. This is achieved by using analytical techniques such as scanning tunneling microscopy, electron diffraction techniques, Auger electron spectroscopy etc. available in the IMBL lab or at Imec.

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Growth of nanostructures by self assembly via low angle deposition Supervision: Prof. Kristiaan Temst, Prof. Margriet Van Bael Self-assembly has become a valuable technique for the production of nanostructures: nanometer sized islands are produced by the controlled deposition of an atomic beam on a substrate in ultra high vacuum (UHV). In this research project, we would like to explore a recent development in selfassembly, namely the growth of nanostructures that form when the atomic beam is deposited at a very small incident angle with respect to the substrate (‘glancing angle deposition’). The strong selfshadowing effects result in a specific growth mode in which the material preferentially grows on top of the existing precipitates. This way, instead of a continuous two-dimensional layer, structures of separate pillars (10 to several 100 nm wide) are formed, also called ‘sculptured thin films’. You will learn to operate the UHV deposition setup and you will characterize the structure of the resulting samples by atomic force microscopy (AFM) and electron microscopy (SEM). We will also study the magnetic properties and specifically the strong anisotropy of Co-nanowires formed in this way.

Theme 6: Physical properties of carbon nanotubes •

The electromechanical properties of two-dimensional graphene Supervision: Tom Moorkens, Dr. Alexander Volodin, Prof. Chris Van Haesendonck Recently, it became possible to isolate individual atomic layers of carbon from a graphite crystal. These so-called graphene layers have unique two-dimensional electrical transport properties that you will investigate in the framework of your master thesis. In particular, you will study how the electrical conduction process can be modulated by locally deforming a graphene layer with the tip of an atomic force microscope.

Theme 7: Nanoparticles •

Optical properties of nanoclusters Supervision: Dr. Marcel Di Vece, Prof. Peter Lievens In this master thesis project the physics of nanoscale objects will be studied by the interaction of light with nanoclusters. The nanoclusters will be produced by a gas phase cluster source. The light with which the nanoclusters will be illuminated stems from a Nd:YAG laser. The resulting photoluminescence from the nanoclusters will be measured by a photoluminescence setup as a function of the light wavelength and time. Through this experiment the exotic properties of nanoclusters will be investigated. Also, the nanoclusters can be used as optical switching device by electrically charging, after which the optical response changes. A third important line of research is the optical field enhancement effect. In the vicinity of the nanoclusters the electric field will be enhanced to several orders of magnitude, influencing the optical properties of neighbouring optical active materials. A careful balance between experiment and theory can be found together with the student.

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Controlled growth of nanocrystals for the growth of carbon nanotubes Supervision: Dr. Dries Smeets, Dr. Kristof Paredis, André Vantomme, Kristiaan Temst The scientific interest in the formation and properties of nanocrystals has recently gained considerable interest, largely driven by the enormous number of possible application in different fields. The position and diameter of the particles and the particle density are determining factors in the properties and applicability of the nanoparticles. In this project we will investigate the influence of several parameters on fundamental processes such as surface diffusion of atoms, nucleation of nanocrystals etc., that determine the final position and properties of the nanoparticles. These nanoparticles will subsequently be used for the growth of carbon nanotubes, in close collaboration with IMEC.

Theme 8 Clusters and laser spectroscopy •

Laser spectroscopic and mass spectrometric research of doped clusters Supervision: Dr. Nele Veldeman, Jorg De Haeck, Pieterjan Claes, Prof. Peter Lievens How does the composition of doped silicon clusters influence their geometry and electronic structure? That is the central research question of this master thesis subject. We produce beams of mixed clusters of a few up to several hundreds of atoms with a laser vaporization source, and study their properties with laser spectroscopy and mass spectrometry. With deep infrared spectroscopy we can identify vibrational transitions that are characteristic for the cluster geometry, while visible light (or near infrared) spectroscopy identifies the electronic transitions. The employed technique is so-called action spectroscopy where absorption of laser light results in dissociation, fragmentation of ionization. For this research we have several laser systems available providing nanosecond pulsed light with wavelengths tunable between 195 and 2000 nm. For the absorption of infrared light, through the vibrational degrees of freedom in a frequency range from 100 cm-1 up to 500 cm-1, we move to the free electron laser FELIX (Free Electron Laser for Infrared eXperiments) of the FOM Institute for Plasma Physics Rijnhuizen (Nieuwegein, Nederland).

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Zero-energy SIMS Supervision: Nico Vanhove, Prof. Peter Lievens, Prof. Wilfried Vandervorst Due to the ongoing scaling of semiconductor structures (Moore’s law), the position of dopant atoms which determine the electrical properties of a device becomes more important. Until now, secondary

ion mass spectrometry (SIMS) is used to determine these dopant profiles due to its high sensitivity. However, quantitative depth profiles with a high depth resolution (sub-nm) are becoming more and more challenging due to the ion-substrate interaction. Zero-energy SIMS aims at eliminating these drawbacks by replacing the ions by a combination of electrons and a reactive gas mixture, and by ionizing the emitted species by laser post-ionization. First results are very promising. In this thesis subject the technique will be optimized further, with emphasis to the fundamental etching and laser post-ionization mechanisms.

Theme 9: Nanostructures under extreme conditions •

Molecular magnets Supervision: Wim Decelle, Dr. Johan Vanacken, Prof. Victor Moshchalkov Probing the nanoscale world is often performed through mesoscopic systems that provide a bridge to our macroscopic world. In the field of magnetism, one can find such mesoscopic systems in the form of molecular magnets. In general these nanoscale clusters provide a high spin ground state, f.e. S=10 for the prototype molecular magnet Mangenese-12 Acetate (Mn12Ac), in combination with superparamagnetic behaviour when grown into single crystals. The result is a set of mesoscopic systems that show intrinsically quantum mechanical processes, such as Quantum Tunneling of the Magnetization (QTM), Quantum Magnetic Deflagration and many more, in studies of the magnetization of millimetre-size single crystals. Our lab focuses on the behaviour of such molecular magnets when exposed to highly non-adiabatic conditions in pulsed magnetic fields.

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Plasmon enhanced photoluminescence in quantum-dot coatings Supervision: Dr. Damien Saurel, Dr. Johan Vanacken, Prof. Victor Moshchalkov The shrinking of component size in nanoelectronics (Moore’s law) has encountered the physical limits of electronic transport, affecting potential future technological progresses. The most promising solution here is to go for optical intra- and inter-chip interconnects, which are much faster than conventional metallic interconnects. The issue is then to be able to manipulate photons in the sub wavelength scale, i.e. the nanometric scale. Remarkably, in that limit nanomodulated metallic films can combine the better of the two worlds: surface plasmons in these films can very efficiently enable the light propagation, together with the still efficient electrical conductivity, allowing to reach superlensing, cloaking, unusual-non-linear effects or luminescence amplification nano-emitters. The study of these phenomena in special designed nanostructures (together with IMEC) is done via UV to NIR optical spectroscopy / photoluminescence in pulsed magnetic fields.

Theme 10: Quantum design of complex materials •

Probing the magneto-electrical properties with scanning probes Supervision: Dr. Steven Brems, Dr. Alexander Volodin, Prof. Chris Van Haesendonck For your master thesis you will investigate thin films of complex oxides that show at the same time ferroelectric and ferromagnetic order. The coupling between both order parameters in these socalled magneto-electrical materials allows to control the ferromagnetic domain structure by applying an electrical field. You will determine this coupling using scanning probes that are able to detect with nanometer resolution both the electrical and the magnetic response.

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Tailored oxide-semiconductor interfaces Supervision: Prof. Jean-Pierre Locquet Oxide-semiconductor interfaces are at the core of many semiconductor devices. For your master thesis, you will explore a novel approach to create high quality interfaces between oxides and a high mobility semiconductor such as Ge or GaAs. You will determine the properties of the interfaces using structural and electrical measurements.

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Magnetoelectricity in oxide heterostructures Supervision: Prof. Jean-Pierre Locquet Magnetoelectric materials can combine spin and charge – the two major ‘information carriers’ of condensed matter – for use in novel devices. For your master thesis you will grow different magnetoelectric thin films and determine their structural, magnetic and electric properties.

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Analysis on the basis of numerical field simulationof a table-top synchrotron Supervision: Prof. Herbert De Gersem, Dr. Johan Vanacken, Prof. Jean-Pierre Locquet Electrical fields are used for accelerating charged particles, whereas magnetic fields are applied to bend particle beams. Synchrotron radiation originates due to the bending of high-energy beams. Currently, a table-top synchrotron based on this principle is evaluated. The goal of this master thesis is the simulation of the electromagnetic fields and the tracking of the particles with numerical field simulation and trajectory tracking software tools. This should allow to gain understanding of the working principle and the performance of the device. Parameter studies and optimisation steps will possibly lead to improvements of the device.

Theme 11: Nanobiofysica •

Nanoclusters for biosensor applications Supervision: Prof. Peter Lievens, Prof. Margriet Van Bael Recent research trends combine physics at the nanometre scale with biological systems such as DNA or proteins. Relevant examples are protein microarrays for the simultaneous detection of many different proteins for early diagnosis of specific diseases. To optimise the density of proteins, a new generation of protein ‘nanoarrays’ is being developed. A possible route towards dense protein nanoarrays is to use atomic clusters on a surface to immobilise individual proteins. The purpose of this thesis research is to create and optimise patterns of atomic clusters on biocompatible surfaces, which will in a later stage be used to bind individual proteins. You will use a laser vaporization cluster source to produce gold clusters of only a few nanometer in size and deposit them on different surfaces. You will investigate the resulting cluster patterns by scanning tunnelling and atomic force microscopy.

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Electromagnetic field simulation of transcranial magnetic stimulation (TMS) Supervision: Prof. Herbert De Gersem Transcranial magnetic stimulation consists of exciting a particular area of a (human) brain by a current in an externally applied coil. This technique is not only used as a therapy for e.g. epilepsia and tinnitus, but also as a method in experimental psychology. The focussing of the magnetic field into a small area of the brain is troublesome. Moreover, the fat layer between skull and skin is responsible for a large impedance experienced by the electromagnetic field. In this master thesis, images obtained by magnetic resonance imaging (MRI) and computer tomography (CT) are processed and used for the

calculation of the electromagnetic fields on the basis of a finite-difference method. The dependence of the magnitude and distribution of the electromagnetic field on the size and the shape of the therapy coils are studied.