Combined optical imaging and electric transport studies on resistive switching and light-induced phase transitions in strongly correlated insulators

Abstract

The emerging field of neuromorphic computing strongly demands for memristive devices in order to emulate basic functionalities of the human brain. In recent years, there is growing interest in devices based on strongly correlated insulator thin films, which undergo thermally driven metal-to-insulator transitions (MIT) and insulator-to-metal transitions (IMT). These materials show memristive behavior due to resistive switching, i.e. the change of electical resistance induced by an external stimulus. In the three publications of this cumulative PhD thesis, we adress distinct research questions in the scope of the fundamental mechanism behind resisitve switching, spatial switching dynamics, and thermodynamical aspects of laser light-induced phase transitions. In publication 1, we investigate the characteristic length scales of conductive filaments in planar two-terminal thin film devices of the perovskite nickelates NdNiO3 and SmNiO3. For potential applications, it is crucial to have knowledge about the spatial filament dynamics, e.g. for miniaturization, but this fundamental question has been mostly unnoticed so far. In a study with complementary imaging techniques, namely optical widefield and scattering-type scanning near-field optical microscopy and with theoretical support by numerical simulations, we find key parameters, that determine the size of the filament. We identify the resistivity contrast during MIT/IMT to be a key parameter for the filament’s size. Larger resistive drops and sharp switching leads to narrower filaments with higher current density. Besides of that, bias current, temperature and thermal conductivity of the substrate play important roles, too. In publication 2, we present a study on resistive switching in the prototypical Mott insulator V2O3, touching the fundamental question of the origin of resistive switching. By use of optical widefield microscopy, we visualize the thermally driven strain-induced phase separation during MIT/IMT as well as volatile resistive switching in a planar two-terminal thin film device. The current-voltage-characteristics and the shape of the filaments are well reproduced by a numerical model, unveiling clear signs of an electrothermal breakdown, and correlating local heterogeneities like strain directly to the switching dynamics. In publication 3, we study laser irradiation of a V2O3 thin film and gain thermodynamical insights into the first-order nature of the Mott transition. We irradiate the sample by scanning a focused laser beam across the thin film surface and acquire optical widefield micrographs in situ. Laser irradiation modifies the phase configuration depending on the thermal history: during IMT, the laser induces predominantly metallic domains, whereas during MIT, it predominantly induces insulating domains. Most likely, the laser beam drives metastable states into stable ones in a non-thermal way. A numerical model supports this hypothesis.