Nano-optical mapping of permittivity contrasts and electronic properties at the surface and beneath
Aachen / Publikationsserver der RWTH Aachen University (2015) [Dissertation / PhD Thesis]
Page(s): VIII, 134 S. : Ill., graph. Darst.
Scattering-type scanning near-field optical microscopy (s-SNOM) enables optical measurements with a resolution far below the limit of diffraction via the optical near-field interaction between a sharp illuminated tip and the sample. Since near-fields are not restricted to the immediate surface of the sample, the method enables a quantitative investigation of buried structures. Furthermore, in combination with tunable monochromatic light sources, s-SNOM allows for local optical spectroscopy on a lateral scale below 50 nm. Within the scope of this work the mid-infrared spectral range between 5 µm and 11 µm is applied to investigate samples with respect to their free charge carriers and optical phonons.The capabilities of s-SNOM are regarded from two perspectives. In the first part of this work, general questions concerning the imaging properties and the underlying physics are addressed. In this context, a new model for the theoretical description of the near-field interaction with stratified samples is presented, which enables a quantitative analysis of measured contrasts. Additionally, the resolution and sensitivity to objects covered by a dielectric layer is experimentally investigated.A fundamental difference between diffraction-limited spectroscopy and near-field spectroscopy is that in s-SNOM the evanescent fields at the tip provide high in-plane components of the k-vector. The significance of these components for the excitation of surface waves (surface plasmon polaritons and surface phonon polaritons) is discussed. Also the case of strongly dispersive surface waves in emerging two-dimensional systems like graphene is considered.In the second part, the applicability of s-SNOM for the investigation of nanostructures is demonstrated on samples beyond specifically designed model systems. On three different materials it is shown that s-SNOM provides insight into phenomena that are hard to address with other methods:The charge carrier density of doped indium arsenide nanowires can be quantified with an accuracy of 10% without the need for contacting the sample electrically. This is by far sufficient for the detection of slight unintended variations in the charge carrier concentration. Therefore, s-SNOM can be thought of as a routine tool for the measurement of the doping efficiency in nanostructures.On chemically synthesized antimony telluride platelets, a previously unknown highly symmetric pattern of domains has been detected. Using spectroscopy and model calculations the origin of these domains can be explained with different charge carrier densities. A comparison to differently prepared samples of the same material shows that this finding is related to the specific crystallization process of platelets synthesized from solution.Finally, the effect of extended crystallographic defects in silicon carbide epitaxial layers on the local phononic properties is investigated. It is found that common types of extended defects in epitaxial layers strongly influence the near-field scattered by the tip. Implications on potential silicon carbide-based devices relying on surface phonon polaritons are discussed.Together, the applications on nanowires, platelets, and epitaxial films demonstrate the suitability of s-SNOM for the investigation of systems and materials that are of high relevance in current research. The advances in the mathematical description of near-field contrasts enables an analysis of measurements on vertically inhomogeneous samples. This has a high significance for the establishment of the method as a noninvasive metrology for the characterization of thin films. Altogether, this work is a demonstration of the potential of mid-infrared near-field microscopy as a versatile approach to address fundamental material-specific questions in modern solid state physics.