Atomic-scale probing of metallic and semiconductor nanostructures

In scanning tunneling microscopy (STM) an atomically sharp metallic tip is brought in close proximity to a (semi) conducting sample to probe the electronic and topographic features of the surface. Three extensions of this technique, namely cross-sectional scanning tunneling microscopy (X-STM), scanning tunneling luminescence microscopy (STL), and spin-polarized scanning tunneling microscopy (SP-STM), are presented in this thesis. In the first technique, X-STM, a sample is cleaved along the (110) natural cleavage plane of a zinc-blende crystal to allow the observation of single dopants and embedded nanostructures such as quantum wells and quantum dots in a plane parallel to the growth direction. The second technique, STL, in which the STM-tip locally induces luminescence can be used to extend optical probing beyond the diffraction limit. In this respect, this technique has the potential to provide a wealth of information about light–matter interactions on the atomic-scale. Furthermore, the optical properties of a material system can be linked to its magnetic properties by studying the polarization of the STM-induced luminescence. In this respect, STL and the technique of SP-STM are complementary. In the latter technique a magnetic sensitive STM-tip is used to probe the electromagnetic properties of a surface on the atomic-scale, a highly sought after capability in modern day development of spintronics. Although the techniques of STL and SP-STM have great potential, the downside is that they are notoriously difficult to implement experimentally. This is reflected in the small number of groups that have succeeded in implementing one of the techniques, let alone both simultaneously. This is a pity since the complementary nature of these two techniques opens up a myriad of experiments with which the optical, electronic, and magnetic properties of materials can be simultaneously investigated with atomic-scale resolution. The ultimate goal of this thesis is to combine the two techniques of STL and SP-STM, with the technique of X-STM to study the properties of single dopants and embedded nanostructures. In this thesis the successful implementation of the before mentioned techniques in a single scanning tunneling microscopy is reported. It is shown that it is possible to relatively easily and cost-effectively implement luminescence detection into a commercially available Omicron low temperature STM. STM-induced luminescence could be collected efficiently from an Au(110)(1 × 3)-reconstructed surface. Furthermore, it was demonstrated that it is possible to simultaneously record the surface topography and the corresponding photon map, both with atomic resolution. The fact that a full luminescence spectrum is recorded at each grid point where the luminescence is collected allows for a spatially resolved spectral analysis. Besides successful STL on a metallic surface, the collection of luminescence from a highly Zn-doped GaAs semiconductor sample was demonstrated. Here, a threshold bias voltage for the onset of electroluminescence was found yet. No correlation between the shape, the position, and the intensity of the luminescence spectra with the positions of the dopants was found. The results show that the proposed collection system can be used to spectrally collect and analyze the STM-induced luminescence from both metallic and semiconductor material systems with atomic-scale resolution. Spin-polarized tunneling microscopy was demonstrated on a vicinal W(110)-surface covered with a thin iron film. This material system consists of alternating mono- and bilayer magnetic nanowires. Tips having an in-plane direction of magnetization and tips that proofed to have a slanted direction of magnetization were used. Respectively, three and four levels of magnetic contrast were observed with these tips, unambiguously demonstrating SP-STM. The next step along the lines of the current work is to extend SP-STM to dilute magnetic semiconductors, e.g. Mn in GaAs. Atomic-scale resolution with magnetic chromium tips has already been observed (not in the thesis) on this material system in X-STM measurements, an important step towards probing the magnetic properties of dilute magnetic semiconductors in the future. In the last decade the fabrication of quantum dots (QDs) has been intensively studied. The interesthas been, and still is, stimulated by applications of self-assembled QDs in optoelectronic devices. Nowadays, QDs are for instance applied or suggested in QD lasers, single electron transistors, and spin manipulation. It is well known that the optical and electronic properties of QDs are strongly affected by their size, shape, and chemical composition. In this thesis the size, shape and chemical composition of QDs and ways to control these properties have been intensively studied for various material systems and growth techniques by atom probe tomography (APT) and X-STM. The technique of APT is a conceptually completely different characterization technique as X-STM: atoms are evaporated from a high voltage biased specimen by a laser pulse and recorded at a detector. The technique allows the fully three-dimensional characterization of embedded nanostructures, carrying the geometrical and chemical analysis beyond the twodimensional cleavage plane to which the technique of X-STM is restricted. In this thesis, APT was bench marked against X-STM. It is shown that APT and X-STM complement each other very well. Where X-STM gives only two-dimensional cross-sections, APT provides a fully three-dimensional tomographic reconstruction, and where X-STM has a limited capability to distinguish chemical species, the mass-spectral analysis of APT offers the ability to distinguish different elements from each other. The two techniques were linked by means of computational methods that model surface relaxation. This analysis method emphasized structural features of the studied QDs that were not detected or neglected in previous measurements but are important in modeling the QDs. Control over the height of QDs allows the tuning of their emission wavelength and g-factor. Nowadays, several methods are available to achieve this in the Stranski-Krastanov growth mode, among which the use of surfactants, double-capping, indium flush, and strain engineering of the capping layer. In this thesis, the latter two techniques are investigated in detail by X-STM and Kinetic Monte-Carlo (KMC) simulations. X-STM studies have, and will continue, to provided a wealth of information giving a better understanding of the growth process, but the technique only provides a cross-sectional snapshot of the buried QDs after the completion of the growth. In this respect, techniques such as APT and KMC simulations can be of great complementary value and provide further insight into the details of the growth process. The work presented in this thesis is the first in which a realistic, fully three-dimensional KMC simulation is compared with experimental results. The agreement between the KMC simulations and the experimental results opens up the door for the use of KMC simulations in the future to predict the outcome of the growth process. In the last part of the thesis, the details and possibilities of droplet epitaxy as an alternative technique to grow self-assembled QDs were reported. Traditionally, QDs are grown in the strain driven Stranski-Krastanov mode. Defect free QDs can be grown with this technique, but the presence of strain in the material during the growth process is a major complicating factor. For one, strain can strongly modify the electronic structure and is the driving force behind QD decomposition and intermixing. The resulting structural imperfections can obscure the intrinsic properties of the QDs and hinder the linking of experiment, e.g. photoluminescence measurements, with a realistic QD model. In this thesis, it was shown that in this respect QDs grown by droplet epitaxy provide a much simpler approach. This technique involves the low temperature growth of unstrained liquid group III-elements droplets that are subsequently crystallized into QDs by the incorporation of group V-elements. It was shown that wetting layers (less than 1 bilayer) form on an (001)-oriented substrate and are absent on an (311)A-oriented substrate. As expected in lattice-matched material systems, the QDs were found to be unstrained. The results show that some degree of Al intermixing, attributed to local etching, with the GaAs QDs occurs during the growth in case of the (001)-oriented substrate. In case of the (311)A-oriented substrate, substantial interface fluctuations are present between the nanostructures and the buffer layer. These fluctuations are attributed to the destabilization of the interface by the liquid Ga droplet and subsequent reconfiguration of the surface in energetically more favorable facets. It is demonstrated in this thesis that quantum wires (QWRs) can be created by annealing uncapped QDs grown on the (311)A-oriented substrate. The QWRs show strongly polarized emission along the direction of the QWRs, a feature that is highly desirable in the fabrication of lasers that use cleaved crystalic surfaces as Fabry-Pérot mirrors