Using infrared nano-spectroscopy to analyze the influence of defects on the local electronic properties in resistively switching oxides and chalcogenides

• Analyse des Einflusses von Defekten auf die lokalen elektronischen Eigenschaften von resistiv schaltenden Oxiden und Chalcogeniden mittels infraroter nano-Spektroskopie

Lewin, Martin; Taubner, Thomas (Thesis advisor); Wuttig, Matthias (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

In this thesis the local charge carrier properties of new storage materials for information technology are studied on the nanometer scale. This work is embedded in the collaborative research center 917 "Nanoswitches" and focusses on the materials Sb2Te3 and SrTiO3, which are exemplary for the classes of resistive-switching oxides and phase change materials respectively. In both material classes the electrical resistance can be switched reversibly by applying voltage pulses. However, the exact switching process is not yet fully understood. In order to enable a targeted optimization and scaling of the components, it is particularly important to better understand the influence of defects on the local charge carrier properties. Established analysis methods, however, are either hindered by the often existing nanometer-thick surface layers (electrodes, protective layers, oxides) or do not guarantee a non-destructive examination. Both limitations are circumvented in this thesis by investigating the local charge carrier properties in Sb2Te3 and SrTiO3 with the help of a scattering-type scanning near-field optical microscope (s-SNOM, penetration depth up to 100 nm) in combination with infrared laser beam sources (non-destructive). Hauer et al. observed 2015 in Sb2Te3 nanoplatelets using s-SNOM the formation of triangular domains with a strong contrast in the infrared spectral range.[1] It was argued that the infrared contrast might result from different carrier densities as a consequence of the special growth process of the platelets. In this thesis a detailed s-SNOM growth study of Sb2Te3 is performed. Using s-SNOM chemically synthesized nanoparticles of different morphologies and thin films grown by molecular beam epitaxy are investigated in the infrared spectral range. Such, it is demonstrated that the domain formation is an intrinsic property of Sb2Te3, which can occur independently of the growth method also in technologically relevant systems. The evaluation of normalized near-field spectra shows that the infrared spectrum of the individual domains can be described very well by the Drude model of free charge carriers, where the plasma frequency is spectrally shifted between the domains. As the most likely explanation, different carrier densities as a result of different densities of substitution atoms formed during growth are identified as the cause. The observed intrinsic charge carrier domains are thus relevant for any electronic application of Sb2Te3 and could be found in many other semiconductors with Van-der-Vaals like bonding. Since the plasma frequency of the charge carriers in SrTiO3 is typically in the terahertz range, the local electronic properties in SrTiO3 cannot be directly investigated by s-SNOM using commercial continuous-wave laser sources. In this thesis near-field spectra of the phonon response of SrTiO3 are analyzed. It is shown that the electronic properties can be indirectly extracted from the near-field phonon resonance in SrTiO3 due to plasmon-phonon coupling. Based on this method in combination with electron microscopic methods the accumulation of electrons at grain boundaries in donor-doped SrTiO3 ceramics is observed. As an application example for future investigations of the electronic switching process, the infrared spectra of locally switched spots in SrTiO3 thin films are analyzed. To demonstrate the potential of infrared nanoscopy for in-operando measurements on buried structures, a vertical resistive switching SrTiO3 cell with a graphene cover electrode is imaged using s-SNOM. Strong inhomogeneities are identified in the local infrared scattering signal of the cell, which result presumably from unknown extended defects in the SrTiO3 substrate. In the main part of this thesis, concepts from nano-optics (plasmon-phonon coupling) are successfully applied to characterize the properties of resistively switching oxides. Conversely, the results of this work suggest to apply the functional materials investigated as an active medium for the control of surface phonon polaritons in photonics. As an outlook, a concept is presented for the use of resistively-switching oxides as basic building blocks for rewritable optical meta-surfaces consisting of only one active layer. [[1] B. Hauer et al. In: Nano Letters, vol. 15, no. 5 (2015), pp. 2787-2793.]