Synthesis and resistive switching mechanisms of mott insulators based on undoped and Cr-doped vanadium oxide thin films : as function of nanostructure and material properties
- Synthese und resistive Schaltmechanismen von Mott-Isolatoren basierend auf undotieren und Cr-dotierten Vanadiumoxid Dünnschichten als Funktion der Nanostruktur und der Materialeigenschaften
Rupp, Jonathan Amadeus; Waser, Rainer (Thesis advisor); Lemme, Max Christian (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020
The rapid evolution of electronics and their performance progress in the past decades call for extremely fast, scalable and power efficient memory technologies at the lowest cost. The dominating contemporary memory types of information technology (dynamic random access memory "DRAM" and Flash) shortly approach their physical and technological limits beyond which no further scaling is neither possible nor economically feasible. Hence, there is an urgent need for research of alternative memory and logic concepts. One novel memory class consists of a very simple two terminal device structure of an electrically active thin film sandwiched between two electrodes. After its working principle, it is called resistive switching random access memory ("ReRAM" or "RRAM"). Stored information is represented by the resistance of the electrically active thin film which can be switched between at least two distinguishable states. Macroscopically, resistive switching is controlled by applying an appropriate electrical potential to the device. Depending on the nanoscopic switching mechanism, the device responds with a volatile or a non-volatile change in resistance. In the past few years, ReRAM technology increased in popularity due to its promising device properties with excelling speed, scalability, energy efficiency and endurance. Nowadays, it is seen as one hot candidate to be able to compete both with DRAM as well as Flash and could even open new fields of computation towards neuromorphic circuits. In this thesis, the potential of (and control over) resistive switching mechanisms in undoped and chromium doped vanadium oxide thin films is explored. The material class of vanadium oxides is well known for its abundance of extraordinary electric and magnetic properties such as the presence of electron correlations and the formation of Mott-insulating states in VO2 and Cr-doped V2O3. Therefore, three different synthesis processes are established to determine the (crucial) influence of defect density on electrical switching properties. Low oxygen content thin films are reactively sputtered at room temperature (I) which result in amorphous undoped and Cr-doped VOx=1.5-2, at elevated temperatures (II, > 673 K) for crystalline Cr-doped V2±ΔyO3 and at room temperature with a post-reduction step (III), resulting in Cr-doped V2O3 with excellent stoichiometry. The three established synthesis processes generate largely different morphological and electrical properties in the same type of material. Moreover, resistive switching mechanisms and kinetics of ReRAM devices are investigated in a large temperature range between 80 K and 370 K. At least two volatile and at least four non-volatile types of switching mechanisms have been identified and have been classified with respect to crystallinity, defect density, Cr-doping, stack symmetry, device size and current compliance. Two volatile switching types could be tracked back to mechanisms such as crystallographic phase change in (Cr:)VO2 and a thermal feedback event in Cr:V2O3. Four non-volatile mechanisms may result as consequence of ionic drift, local valence change (e.g. by oxygen vacancies), thermochemical redox reactions and electron-electron correlations. Lastly, the resistive switching performance of ultra-thin (10 nm) Cr-doped V2O3 films is probed by local conducting atomic force microscopy in ultra-high vacuum. A mix of volatile and non-volatile characteristics can provide a multitude of operation principles in the same device. Finally, strong scaling potential below dimensions of less than 250 nm³ makes the material class attractive for selector as well as memory applications.
- Chair of Materials of Electrical Engineering II and Institute of Materials of Electrical Engineering 
- Chair of Electronic Devices