Extrinsic Dopant Effects on Oxygen Vacancy Formation Energies in ZrO2 with Implication for Memristive Device Performance

• Published in: ACS applied electronic materials Vol. 1; no. 4; pp. 467 - 477
• Main Authors: Yildirim, Handan, Pachter, Ruth
• Format: Journal Article
• Language: English
• Published: American Chemical Society 23.04.2019
• Subjects: resistive random access memory (ReRAM); bulk ZrO2; density functional theory (DFT); ion doping; oxygen vacancy formation energy; resistive switching (RS) uniformity
• ISSN: 2637-6113; 2637-6113
• DOI: 10.1021/acsaelm.8b00090

Despite much progress in development of oxide-based resistive random-access memory (ReRAM) devices that have an inherent memory and advantages in increased speed of operation, higher density, nonvolatility, ease of integration, and low power, many challenges remain, particularly relating to conductive filament (CF) formation and rupture and associated device uniformity. Oxygen vacancies (Vos) play an important role for both the valence change memory (VCM), based on formation and rupture of Vo or oxygen ion-mediated CFs, and the electrochemical metallization mechanism (ECM), in which a metallic CF is formed by the cations of an active electrode. In this work, to provide guidelines for ReRAM device improvements, we investigated the role of dopants within the ZrO2 resistive switching layer. Density functional theory calculations for 20 dopants, which span a range of electron configurations, valence, and atomic radii, identified a preference toward either substitutional or interstitial doping, useful for considering VCM or ECM cells, respectively. The propensity toward reduction of Vo formation energies (OVFEs) to improve device performance was elucidated for all dopants and found to be in good agreement with available experimental data, validating our predictions for dopant selection. Moreover, significantly reduced dopant formation energies were calculated in the presence of Vos for interstitially doped Ni, Cu, and Ag atoms, suggesting their facile incorporation into ZrO2 from the electrode, which can enhance the formation of metallic CFs in ECM cells. The concept of devices based on such a hybrid mechanism can improve performance of ECM cells. We also found that the dopants affect OVFEs locally, enhance clustering of Vos near dopants, and enable spatial control of the conducting pathways. Finally, our electronic structure analyses, which provide information about the generation of mid-gap states, can motivate experimental analysis of the transport mechanism in the device.