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AA5.03 - Design and Optimization of Transition Metal Oxide-Based Resistive Switching Devices for Data Storage and Computing Systems 
April 8, 2015   8:30am - 9:00am

There is presently an extensive effort to develop non-volatile resistive random access memories (RRAM) based on transition metal oxides [1-4]. These generally work by the formation of a conductive filament of oxygen vacancies between the two electrodes. In one model, the ‘hour glass model’ [2], the oxygen vacancies migrate between two ‘reservoirs’, allowing the filament neck to increase or decrease in size during the SET and RESET process. HfO2, TiO2, Ta2O5, and Al2O3 are the typically used oxides, together with a scavenging metal that allows the vacancy creation process during the initial filament forming process. However, there has been no clear materials selection criteria given in terms of how to maximize memory endurance or retention lifetime. Here, we determine the standard operating conditions in terms of O chemical potential and Fermi energy. It is shown how the choice of scavenger metal can be used to fix the O chemical potential. This then fixes the O vacancy formation energy, because that formation energy varies between ~6.1 eV at pO2 = 0 eV, to ~ 0.2 eV at for example the Hf/HfO2 equilibrium potential pO2 = -5.8 eV. Setting this formation energy then ensures that the total number of O vacancies is conserved during memory cycling, maximizing endurance, and stops formation of any O interstitials. Choice of the O chemical potential/scavenging metal allows the endurance and retention time to be optimized. The various migration energies and charge state energies are calculated for the oxides. It is argued that Ta2O5 has the preferred properties, having lower migration energies, charge state energies at the preferred Fermi energies, and having a wider stability zone of its amorphous phase than HfO2. On the other hand, TiO2 has sub-oxide phases which complicate electrode processes, while Al2O3 has too high defect formation and migration energies. It can only be used as an oxide modifier. 1. R Waser, et al, Adv Mats 21 2632 (2009) 2. R DeGraeve, et al, Tech Digest VLSI (2013)p8.1; Tech Digest VLSI (2012); 3. S Clima et al, APL 100 133102 (2012) 4. J J Yang et al, Nature Nanotechnol 3 429 (2008) 5. L Goux et al, ECS Solid State Lets 3 Q79 (2014)

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