Resistive switching in ZrO$_{2}$ based metal-oxide-metal structures

There is an increasing interest in resistive switching (RS) random-access memories (RRAM) for future electronics. A popular class of materials nowadays investigated for redox based resistive switches is transition metal oxides. A great challenge for the RRAM production is to develop a well-performing RS device compatible with a highly scalable CMOS technology. Among the group IV metal oxides, ZrO2 is of interest for RRAM because of its chemical similarity to HfO2 (exploited for high-k gate and RRAM applications) and because it is a fast ion conducting material. The goal of this work is a deeper understanding of the influence of the (i) metal-oxide-metal (MOM) layer stacks configuration, (ii) the oxide films microstructure, (iii) and their defect structure on the appearance of different switching modes, i.e. unipolar (UP) and bipolar (BP). The thesis is based on the work which was supported by a European Marie Curie action and performed at the Research Center Jülich in collaboration with the Finnish ALD Center of Excellence in Helsinki. The first part deals with the fabrication of ZrO2 thin films by an industrial compatible atomic layer deposition (ALD) process, the chemical, structural and morphological characterization of the films, the growth of ZrO2/TiO2 bilayers, the integration of the layers into metal-oxide-metal (MOM) devices and the electrical characterization with focus on the RS behavior. In the second part the effect of the device structure, in particular the thickness of the electrochemical active electrode (EAE) and the ZrO2 film morphology, on the RS switching polarity of Pt/ZrO2/(EAE) cells is discussed. ZrO2 films and ZrO2/TiO2 bilayers were grown by ALD and were carefully structurally and electrically characterized. The variation of the ALD oxygen sources, O3 or H2O, influenced the morphology, structure, and at some extent the RS properties of the ZrO2 films. The ZrO2 films grown from Zr[N(CH3)C2H5]4 (TEMA-Zr) at 240 °C were polycrystalline with a mixture of cubic/tetragonal phases. ALD/H2O-ZrO2 films exhibited a random oriented polycrystalline structure, whereas the ALD/O3-ZrO2 films consisted of preferrably oriented cubic shaped grains. The ozone based ALD ZrO2 process resulted in devices with a higher yield and reproducibility in contrast to water based one. Pt/ZrO2/Ti/Pt structures with a Ti top electrode (TE) thickness of 5 to 20 nm showed unipolar type RS behavior, while by increasing the Ti TE thickness a gradual change of switching polarity from unipolar to bipolar with a completely bipolar type RS behavior for a Ti TE thickness of 40 nm is found. The RS behavior of the ZrO2/TiO2 bilayer structures revealed a dependence on the stacking sequence of the layers. The switching in Pt/ZrO2/TiO2/Ti/Pt devices was unipolar, comparable to Pt/ZrO2/Ti/Pt cells. In contrast, bilayers with the reverse structure, Pt/TiO2/ZrO2/Ti/Pt, showed non-switching behavior. The effect of the cells stack structure on the polarity of the RS behavior was studied in detail for 20 nm thick ZrO2 films grown by an ozone based ALD process and integrated into Pt/ZrO2/Ti/Pt cells while the thickness of the electrochemically active electrode (Ti) was varied from 0 nm to 40 nm. Cells with a thin EAE (< 20 nm Ti) exhibited a UP-type RS behavior, while cells with thick EAE (> 20 nm Ti) showed BP switching characteristics at a lower current compliance as the UP switching cells. A detailed structural analysis of the ozone grown ZrO2 films revealed a polycrystalline structure of columnar shaped grains with a meta-stable cubic-tetragonal ZrO2 phase. The phase stabilization was attributed to an incorporation of a certain amount (percentage regime) of process related impurities. In the second part of the thesis an empirical model for the polarity dependence of the RS in the ALD ZrO2 based devices as a function of the EAE thickness was suggested. The model assumed a columnar shaped microstructure and certain impurity content for the ZrO2 films. In addition, the results of current-voltage behavior, temperature dependency of the resistance states and impedance spectroscopy (IS) measurements of different devices in different RS states were considered. Impedance spectroscopy measurements of UP and BP type switching devices with the same ZrO2 films but with different EAE thicknesses were carried out for the states prior to electroforming (pristine), after RS into the ON-state and after RS into the OFF-state. The different devices in their pristine states exhibited nearly identical IS characteristics while the ON and OFF states of the UP and BP devices revealed pronounced differences. This result indicated that the effect of the EAE thickness on the switching polarity was primarily generated during the electroforming process. In the model, the UP switching in ZrO2 based devices with thin EAE was described as a result of a noncomplete local reduction of the grain boundary cores, which might gave rise to a hard thermal breakdown and the formation of metallic like conduction paths. On the contrary, the suggested idea for BP switching of ZrO2 based devices with thick EAE based on the formation of local semiconducting oxygen depleted ZrO2-x filament-like regions along certain positions, probably preferably at grain boundary cores. These filaments might be formed and ruptured by a thermal assisted oxygen vacancy drift. The combination of the structural analysis with measurements of the temperature dependences of the resistance states and the impedance spectroscopic characterization led to the idea of a micro-structural grain boundary model which could describe the difference in unipolar and bipolar type RS in Pt/ZrO2-x/(EAE) devices as a function of the thickness of the electrochemically active electrode. Furthermore, the proposed ‘grain boundary model’ was compared to the ‘virtual electrode model’. The careful comparison showed that both scenarios might explain the impedance spectroscopy results for the BP RS ZrO2-based devices, while a combination of the two might be the most reasonable scenario in the description of BP switching in the studied ZrO2 based devices.