Growing oxide thin films in a low-energy electron microscope

  By combining low-energy electron microscopy (LEEM) with pulsed laser deposition (PLD), we have created a unique set-up to study the first stages of growth of complex metal oxides. We demonstrate this by investigating the growth of SrTiO3 (STO) and LaAlO3 (LAO) on STO in real-time. We follow growth by monitoring the intensity and the full-width-half-maximum (FWHM) of the specular diffracted beam at various energies. For layer-by-layer growth, we find the anticipated intensity peaks at the completion of each layer, and an oscillatory FWHM with the maximum at half-layer coverage. In the LAO on STO case, for optimal growth conditions and a LAO thickness above the critical thickness of 4 unit cells the interface between the band insulators shows conductivity. We obtain an electronic fingerprint of the growing material, by measuring the intensity of the specular beam as a function of energy at regular intervals during growth. Extending this fingerprint with the intensity dependence on the momentum parallel to the surface allows us to extract the band dispersion of unoccupied electron states of the sample surface. Significant differences in the unoccupied band structure develop between samples which are conducting and non-conducting.  The work described in this thesis is supported by the Netherlands Organisation for Scientific Research (NWO) by means of a ”NWO Groot” grant and by the Leiden-Delft Consortium NanoFront. The work is part of the research programmes NWOnano and DESCO, administered by the Foundation for Fundamental Research on Matter (FOM), which is part of NWO.Quantum Matter and Optic

Op-power in diabetic retinopathy

The present study is mainly concerned with the role of the OPs in diagnosing early diabetic retinopathy. In 1966 Simonsen had already found a clear relation between background diabetic retinopathy and the disappearance of OPs at the ascending limb of the b-wave of the ERG. Many investigators have studied this relationship ever since, but because of a deficiency in a quantification measure of the OPs, the clinical application has been unsatisfactory. Recent developments involving digitalising of the ERG signal and the accompanying related increase in signal processing techniques, nowadays offer us the opportunity of developing a reliable system of OP quantification. Diabetic retinopathy is characterized by multiple vascular lesions of the eye fundus. The clinical course is quite variable, but one or another feature may predominate the fundus picture at a given time. The knowledge of the pathogenesis in diabetic retinopathy remains incomplete as yet; as does the precise cell localisation of OPs and their relation to diabetic retinopathy. As long as this knowledge is incomplete a classification and measurement system of the retinal function in this disease will be defective

Formation of a mixed ordered termination on the surface of LaAlO3(001)

Quantum Matter and Optic

Quantitative analysis of spectroscopic Low Energy Electron Microscopy data: High-dynamic range imaging, drift correction and cluster analysis

For many complex materials systems, low-energy electron microscopy (LEEM) offers detailed insights into morphology and crystallography by naturally combining real-space and reciprocal-space information. Its unique strength, however, is that all measurements can easily be performed energy-dependently. Consequently, one should treat LEEM measurements as multi-dimensional, spectroscopic datasets rather than as images to fully harvest this potential. Here we describe a measurement and data analysis approach to obtain such quantitative spectroscopic LEEM datasets with high lateral resolution. The employed detector correction and adjustment techniques enable measurement of true reflectivity values over four orders of magnitudes of intensity. Moreover, we show a drift correction algorithm, tailored for LEEM datasets with inverting contrast, that yields sub-pixel accuracy without special computational demands. Finally, we apply dimension reduction techniques to summarize the key spectroscopic features of datasets with hundreds of images into two single images that can easily be presented and interpreted intuitively. We use cluster analysis to automatically identify different materials within the field of view and to calculate average spectra per material. We demonstrate these methods by analyzing bright-field and dark-field datasets of few-layer graphene grown on silicon carbide and provide a high-performance Python implementation

Satellite radar observation feasibility for large infrastructure public works

A very large infrastructural work is being undertaken in Delft. A tunnel is going to be constructed to replace the current rail viaduct. As in any large infrastructural work, the monitoring of the land deformations during the period of the tunnel’s construction is highly essential. In this project, one study was performed to analyse the feasibility of PSInSAR as an independent technique for monitoring of land subsidence. The driving mechanisms for deformation were studied to find out the relations among them. Soil geophysics, hydrology, infrastructures and thermal expansion were studied in relation with deformation. Besides these, the traditional deformation monitoring methods were also studied, since they are the competitors of this technique. LiDAR, photogrammetry, tachymetry, levelling and GPS were considered in this study for the comparison with radar. The major attention has been given in this study to assessment of the geolocalisation quality of PSInSAR observations. We have found that the accuracy and point density of PSInSAR is sufficient for deformation monitoring. While its repeat interval might not be able to detect quick failure mechanisms, other techniques have proven to be excellent complements for this deficiency. Radar measurements are also good for validation in other fields, showing that they correlate well with thermal expansion and soil mechanics theory. Further work should be directed to improving geolocalisation and deformation models

Formation of a conducting LaAlO3/SrTiO3 interface studied by low-energy electron reflection during growth

The two-dimensional electron gas occurring between the band insulators SrTiO3 and LaAlO3 continues to attract considerable interest, due to the possibility of dynamic control over the carrier density and due to ensuing phenomena such as magnetism and superconductivity. The formation of this conducting interface is sensitive to the growth conditions, but despite numerous investigations there are still questions about the details of the physics involved. In particular, not much is known about the electronic structure of the growing LaAlO3 layer at the growth temperature (around 800∘C ) in oxygen (pressure around 5×10−5 mbar), since analysis techniques at these conditions are not readily available. We developed a pulsed laser deposition system inside a low-energy electron microscope in order to study this issue. The setup allows for layer-by-layer growth control and in situ measurements of the angle-dependent electron reflection intensity, which can be used as a fingerprint of the electronic structure of the surface layers during growth. By using different substrate terminations and growth conditions we observe two families of reflectivity maps, which we can connect either to samples with an AlO2 -rich surface and a conducting interface or to samples with a LaO-rich surface and an insulating interface. Our observations emphasize that substrate termination and stoichiometry determine the electronic structure of the growing layer, and thereby the conductance of the interface.Quantum Matter and Optic

Growing a LaAlO3/SrTiO3 heterostructure on Ca2Nb3O10 nanosheets

The two-dimensional electron liquid which forms between the band insulators LaAlO3 (LAO) and SrTiO3 (STO) is a promising component for oxide electronics, but the requirement of using single crystal SrTiO3 substrates for the growth limits its applications in terms of device fabrication. It is therefore important to find ways to deposit these materials on other substrates, preferably Si, or Si-based, in order to facilitate integration with existing technology. Interesting candidates are micron-sized nanosheets of Ca2Nb3O10 which can be used as seed layers for perovskite materials on any substrate. We have used low-energy electron microscopy (LEEM) with in-situ pulsed laser deposition to study the subsequent growth of STO and LAO on such flakes which were deposited on Si. We can follow the morphology and crystallinity of the layers during growth, as well as fingerprint their electronic properties with angle resolved reflected electron spectroscopy. We find that STO layers, deposited on the nanosheets, can be made crystalline and flat; that LAO can be grown in a layer-by-layer fashion; and that the full heterostructure shows the signature of the formation of a conducting interface.Comment: 11 pages, 7 figure

Formation mechanism of Ruddlesden-Popper-type antiphase boundaries during the kinetically limited growth of Sr rich SrTiO3_{3} thin films

We elucidated the formation process for Ruddlesden-Popper-type defects during pulsed laser deposition of Sr rich SrTiO3 thin films by a combined analysis of in-situ atomic force microscopy, low energy electron diffraction and high resolution scanning transmission electron microscopy. At the early growth stage of 1.5 unit cells, the excess Sr results in the formation of SrO on the surface, resulting in a local termination change from TiO2 to SrO, thereby forming a Sr rich (2 × 2) surface reconstruction. With progressive SrTiO3 growth, islands with thermodynamically stable SrO rock-salt structure are formed, coexisting with TiO2 terminated islands. During the overgrowth of these thermodynamically stable islands, both lateral as well as vertical Ruddlesden-Popper-type anti-phase boundaries are formed, accommodating the Sr excess of the SrTiO3 film. We suggest the formation of thermodynamically stable SrO rock-salt structures as origin for the formation of Ruddlesden-Popper-type antiphase boundaries, which are as a result of kinetic limitations confined to certain regions on the surface