Conduction Mechanism of Resistive Switching Layer

Fig. 3.4.1, Fig. 3.4.2 and Fig. 3.4.3 show the double-logarithmic plots of current-voltage curve for negative bias in set process for 9-cycle, 19-cycle and 38-cycle with the fitting results. Obviously they show I∝V at first in low voltage region and then show I∝V² characteristics with the increase of voltage bias. This is the typical behavior of trap-controlled space-charge-limited current (SCLC) behavior. Because it happens before set process gets finished, this SCLC behavior stands for HRS carrier conduction mechanism[11][14].

And Fig. 3.4.4, Fig. 3.4.5, Fig. 3.4.6 show the double-logarithmic plots of current-voltage curve for positive bias in reset process for 9-cycle, 19-cycle and 38-cycle with the fitting results and I∝V relation can be observed. So the Ohmic conduction mechanism could stands for the carrier conduction mechanism of LRS in the resistive memory. Also the HRS fit well as SCLC here, this echo the statement in the above paragraph.

Fig. 3.4.1 SCLC conduction fitting of 9-cycle in HRS by set process

Fig. 3.4.2 SCLC conduction fitting of 19-cycle in HRS by set process

Fig. 3.4.3 SCLC conduction fitting of 38-cycle in HRS by set process

Fig. 3.4.4 Ohmic conduction fitting of 9-cycle in LRS by reset process and SCLC fitting in HRS

Fig. 3.4.5 Ohmic conduction fitting of 19-cycle in LRS by reset process and SCLC fitting in HRS

Fig. 3.4.6 Ohmic conduction fitting of 38-cycle in LRS by reset process and SCLC fitting in HRS

3.5 Resistive Switching Mechanisms for Multi-layer Resistive Switching Film

The evolution of the resistive switching hysteresis is demonstrated in previous section. Even though the basic physical resistive switching mechanisms are not clear in details, we still work on that. Overall, the memory devices fabricated by multi-layer stacking as resistive switching films could be treated as the transition-metal oxides such as HfO2 mixed with Al2O3 in different level. In other words, the Al2O3 has different distribution in resistive switching layers. And 38-cycle device has the widest distribution of Al2O3. According to the research by X. F. Wang et al [29], Al atoms in HfO2 thin films would introduce a significant amount of oxygen vacancies. And it was reported that the single Al2O3 films usually show random resistive switching behaviors during repeated current–voltage measurements [30].

S. Yu et al [31] confirmed that the Al diffusion into HfO2 may be mainly responsible for the improved switching uniformity. Therefore, the oxygen vacancies in transition-metal oxides plays an important role, and it is widely believed that resistive switching is triggered by an electrical migration of anions, such as oxygen anions, which are typically described by the motion of corresponding vacancies [9]. The resistive switching from HRS to LRS is associated with the enrichment of oxygen vacancies arranged in chains under an electric field. Thus, stabilizing the generation of oxygen vacancies is the key issue for stabilizing the switching behaviors. The ab initio calculations

reveal that the oxygen vacancy formation energy in a HfO2 supercell is effectively reduced from 6.53 to 4.09 eV in the vicinity of Al atoms [32], while it is almost unaffected at locations several atoms away from the impurities. Therefore, the chains of oxygen vacancy are easier to generate and be localized along the Al atoms, and more stable conducting filaments are expected to be formed during different switching processes, which could account for the experimentally observed the difference among these devices of switching parameters as shown in previous section.

For our devices, we have known that 38-cycle meets Al2O3 every 6 monolayer and 9-cycle meets Al2O3 every 24 monolayer as we mentioned in chapter 2. Because the chains of oxygen vacancy are easier to generate and be localized along the Al atoms, the 38-cycle device could have so many oxygen vacancies that when electron current flows in the oxide after the forming process. These oxygen vacancies generate unstable conducting filaments that electron current will spread out very easy, because there are oxygen vacancies almost everywhere. For the reason that Al2O3 spread out more in resistive switching layers of the 38-cycle device, it is more difficult to form stable conducting filaments. Therefore, the performance of 38-cycle could be attributed to the random formation of the conducting filaments in the resistive switching layer during each switching process. On the contrary, The-9-cycle device does not happen this way. The Al toms in 9-cycle device distribute tighter and it helps us to control the oxygen vacancies generation, so the conducting filaments are more stable after the forming process, as illustrated in Fig. 3.5.1. Accordingly, 9-cycle has better performance than 38-cycle and 19-cycle is between two parties.

Fig. 3.5.1 Abridged general view of proposed conducting filaments distribution in 9-cycle and 38-cycle

Chapter 4 Conclusion

We first demonstrate the resistive switching characteristics of Transparent Resistive Random Access Memory Devices (TRRAM) with HfO2/Al2O3 multi-layer stacking via different ALD deposition cycles as resistive switching films for 9-cycle, 19-cycle and 38-cycle devices respectively. The endurance times, average set operation voltage, standard deviation of set voltage distribution; average reset operation voltage, standard deviation of reset voltage distribution for the three devices are: 1250 times, -1.258V, 0.162V; 1.275V, 0.193V (9-cycle), 950 times, -1.927V, 0.92V;

1.867V, 0.375V (19-cycle) and 300 times, -1.706V, 0.932V; 1.412V, 1.131V (38-cycle). The-9-cycle device has the best performance than the other two (19-cycle and 38-cycle) from the electrical characteristics shown above. For the reason that Al2O3 spread out more in resistive switching layers of the 38-cycle device, it is more difficult to form stable conducting filaments.

In addition, our 9-cycle device with endurance times up to 1250 times, -1.258V, 1.275V set/reset operation voltage and 0.162V, 0.193V standard deviation of reset voltage distribution is relatively excellent compare to recent published transparency memory devices [10]. It was claimed that oxygen vacancy forming energy could be lower down by Al atoms which leads conducting filaments formed more stable in previous non-transparent RRAM [11]. Even though, the performances of TRRAM in this thesis are still not comparable to non-transparent RRAM. To the best of our knowledge, we first

apply this mechanism to control the conducting filament formation in TRRAM. Not only echo the previous study of the mechanism but also demonstrate TRRAM with the ever highest performance. The multi-layer stacking as the resistive switching films in TRRAM shows great potential in the future modern electronics.

Chapter 5 Future Work

Even though the performance of TRRAM in this thesis is excellent, it still does not match the criterion of NVM application today. Due to the proposed resistive switching mechanism in this thesis, changing the deposition cycles could improve the performance and changing the Al2O3/HfO2 ratio may give a big help too. Furthermore, this thesis use tungsten probe as the top electrode might not be the best choice, deposition a metal between the resistive switching layer and ITO should make the performance better. Hopefully the future research could focus on these views to further make the modern TRRAM in commercial applications come true.

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