Development of semiconductor gas sensors capable of resistive switching induced by the target gas

Motivated by its potential in new generation of fast, densely integrated memories and in artificial neuron synapses, the resistive switching phenomenon is currently being intensively studied. In addition to this, there are many progressive possibilities of utilization of this phenomenon, which are yet to be developed. The phenomenon is typically observed in capacitor-like metal/metal oxide/metal cells with vertical electrode geometry. It was observed also in TiO₂ thin layers, which are used in semiconductor gas sensors. Although a planar electrode geometry is typically utilized in sensors, it has been shown recently that also sensors with the vertical electrode geometry may exhibit an outstanding sensing performance. The coexistence of both effects in the same structure thus makes novel functionalities viable. In particular, it allows for a sensor design, in which the sensor is capable of preserving information of a concentration change, which happened in the past. Such functionality is achieved via change of a resistance state when a selected threshold concentration is reached. The new resistance state, which carries the information about the previous concentration change, is maintained even when the original gas concentration is restored. The present thesis is devoted to the development of the suggested functionality in a vertical Pt/TiO₂/Pt structure with a nanocrystalline TiO₂ layer sensitive to the presence of hydrogen gas. Firstly, we proposed the theoretical principle of the sensor with a novel functionality and explained how the resistive switching induced by a target gas is allowed. Secondly, we prepared and characterized vertical Pt/TiO₂/Pt structures and subsequently examined the coexistence of both the bipolar resistance switching and hydrogen gas sensing in the same cell. Different forming process parameters and resistive switching modes were investigated with the aim to maintain the sensing performance. Special attention was devoted to the current compliance. In particular, we examined an impact of a current source utilization on the forming process and subsequent resistive switching. In the next part, we repeatedly demonstrated a resistive switch to the low resistance state induced by a hydrogen gas concentration decrease from 1 % to 0 %. The experimentally observed resistive switches performed in accordance with the theoretical description and demonstrated that the sensor design with suggested functionality is feasible. We observed them at 100 grad C as well as at room temperature, which may allow for reducing the power consumption of a final device. The functionality was confirmed also in the environment with an elevated relative humidity of 32 % and 52 %, which is close to real ambient air conditions. The sensor with suggested functionality can be utilized in long-term monitoring of oxidizing and reducing gases whose concentration should stay below or above a critical limit. Typical application could be in fabrication processes or environment monitoring. The memory functionality is achieved without any additional energy and space consuming control electronics. On the contrary, our device has a simple capacitor-like metal/metal oxide/metal structure scalable to the nanometer size. (Author)