Mid-infrared near-field investigation of resonances in doped semiconductors
- Nahfeld-Untersuchungen von Resonanzen dotierter Halbleiter im mittleren infraroten Spektralbereich
Jung, Lena; Taubner, Thomas (Thesis advisor); Schäpers, Thomas (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2020
In recent years, the material class of semiconductors has gained considerable interest in the fields of optoelectronics as well as mid-infrared plasmonics. This is, since the incorporation of dopant atoms into semiconductors can distinctly change their electronic properties as free charge carrier density, electron mobility and electrical conductivity. The optical response of a semiconductor sample illuminated by mid-infrared light changes as well, since the electronic and optical properties are closely connected via the dielectric function. Consequently, a precise tuning of these properties allows for enormous flexibility in the design of optoelectronic, as well as plasmonic devices. In order to quantify device behavior, optical characterization techniques are a suitable tool for non-destructive characterization of doped semiconductors with few constraints regarding sample design. In scattering-type scanning near-field optical microscopy (s-SNOM), optical near-fields are excited by illumination of a scanning probe tip employing mid-infrared laser light. This characterization method combines the high lateral resolution of scanning probe techniques with the sensitivity of optical characterization methods to electronic properties. In the present thesis, s-SNOM is utilized to determine local charge carrier properties of doped semiconductor structures with nanoscale lateral resolution. Furthermore, their potential for applications in the field of plasmonics is explored. To this end, two distinct kinds of resonances relevant in doped semiconductors are discussed. On the one hand, the so called material resonance results from the excitation of collective electron oscillations. On the other hand, a coupling of these collective electron oscillations with light can lead to bound modes at the sample surface, which are therefore called surface plasmon polaritons. In the first part of this thesis, the characterization of doped semiconductor samples via their material near-field resonance is discussed. In this regard, theoretical model calculations are studied regarding their parameters impact on the near-field resonances. Furthermore, since the s-SNOM response is not purely surface-sensitive, it depends on the vertical composition of a sample given by surface layers (e.g. oxides or protection layers) as well as inhomogeneities (e.g. carrier accumulation or depletion at the surface). Thus, the expected near-field response of exemplary layer stacks of scientific or technological relevance is discussed. The knowledge gained is used to identify a charge accumulation layer at the surface of a doped indium arsenide sample by combining spectroscopic near-field measurements and theoretical modeling. In a next step, the high lateral resolution of s-SNOM is utilized to resolve charge carrier density gradients between doped and undoped parts in silicon nanowires. An evaluation scheme is introduced that allows for an assignment of charge carrier density values to positions along the nanowire despite a thin oxide layer with unknown optical properties covering the nanowire. In the last part, the capability of s-SNOM to image propagating surface plasmon polaritons is discussed. Wave patterns are imaged on a laterally homogeneous doped germanium sample, that are assigned to the excitation of surface plasmon polaritons at an edge of the sample.