Oliver Hartwig MSc Publications

Biography Oliver Hartwig started to study Physics in 2011 at the Karlsruhe Institute of Technology where he obtained his BSc degree in 2015. In the same year he moved to Munich to continue his studies in Applied and Engineering Physics at the Technical University of Munich. In 2018 he graduated with an MSc degree, focusing on “On-Chip Photon Generation and Detection Using Two-Dimensional Materials Coupled to Plasmonic Waveguides”. During his PhD at the University of the Bundeswehr Munich (UBWM) in the Group of Prof. Georg S. Duesberg, Oliver worked on the synthesis and characterization of 2D materials. While being in the last stages of completing his dissertation at UBWM, Oliver joined the Institute for Microelectronics, TU Wien in May 2023. In collaboration with the UBMW, Oliver currently investigates the growth of fluorides and 2D materials by molecular beam epitaxy (MBE).

In semiconductor technology, improvements in integration density and performance of field-effect transistors (FETs) require downscaling of device size and especially the thinning of gate dielectrics. However, this imposes challenges such as control over material quality and layer thickness, especially for amorphous materials such as SiO₂, Al₂O₃, and HfO₂. Recent reports demonstrated epitaxial growth of ultra-thin (2.2 nm), crystalline CaF₂ on Si(111), which promoted CaF₂ as a promising candidate to overcome existing challenges in FET device fabrication. CaF₂ is an ionic crystal with a lattice constant of 5.46 Å, matching that of silicon with 5.43 Å. It has a large band gap ( of SiO₂ is 8.9 eV), a good permittivity of ε = 6.8 (ε of SiO₂ is 3.9) and a high effective electron mass m_e = 1.0 m₀ (vs. 0.42 m₀ in SiO₂). For device fabrication, achieving controlled crystalline growth of thin CaF₂ layers on metals or graphitic materials is highly desirable, as these substrates can serve as contact electrodes. Additionally, the direct growth of CaF₂ on various semiconductors holds significant promise for advancing FETs based on materials beyond silicon. The growth of a crystalline CaF₂ layer depends on the meticulous and gradual evaporation of CaF₂ films in an ultrahigh vacuum environment (UHV). Molecular Beam Epitaxy (MBE) systems are typically the most effective in meeting the technical requirements for this process. Our research aims to establish controlled growth procedures, resulting in ultra-thin (< 3 nm), closed, and pinhole-free CaF₂ films on metallic substrates. The morphology of the CaF₂ films is studied by AFM scans, as shown in Fig. 1. A relatively thick (18 nm) CaF₂ layer was deposited on a gold surface by evaporation. A low RMS roughness of 0.7 nm could be determined for the CaF₂ film, which is only marginally larger than the underlying gold film's low RMS roughness of 0.35 nm. Conductive AFM scans confirmed that the film fully covers the gold surface. Deposition of gold electrodes on top of the CaF₂ is then carried out to enable capacitance measurements. We are currently investigating the deposition of much thinner (< 3 nm) CaF₂ films while maintaining full surface coverage.

Fig. 1: AFM scan of a CaF₂ film deposited on a gold substrate.