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Computational Modelling of Oxygen Defects and Interfaces in Monoclinic HfO2

Bradley, SR; (2016) Computational Modelling of Oxygen Defects and Interfaces in Monoclinic HfO2. Doctoral thesis , UCL (University College London). Green open access

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Hafnia (HfO2) is of particular interest for microelectronics applications. It is currently used as a high-k insulating dielectric in the latest generations of transistors and as the insulator in new metal-insulator-metal resistive memory devices (ReRAM). Oxygen defects within HfO2 are thought to have a signi cant impact on the performance of transistors and are seen to be a leading factor in dielectric breakdown. This same breakdown phenomenon is exploited in HfO2 ReRAM devices to form reversibly switchable conducting laments. In this work computational modelling is used to look at the properties of oxygen defects in monoclinic HfO2, to explain their mechanisms of formation and di usion, both in the bulk material and at interfaces. The properties of oxygen vacancy aggregates ranging from 2 to 4 vacancies are analysed. It is shown that the most stable aggregates form the largest void within the bulk lattice and the larger aggregates have a higher binding energy per vacancy. Negatively charged aggregates are found to be stable whereas positively charged aggregates are not. The properties of Frenkel pairs is also calculated leading to the development of a model for the formation of conductive laments in HfO2; by generation of oxygen vacancy aggregates, via Frenkel pair formation during electron ooding conditions, and oxygen interstitial out-di usion. A Pt/HfO2 interface model is created and its structural and electronic properties are analysed. The stability of oxygen defects at the interface is calculated. It is shown that at low oxygen concentrations, vacancies may be more stable at the interface however at high oxygen concentrations, oxygen interstitial ions are unlikely to penetrate the Pt electrode and so, remain within HfO2. The stability of oxygen defects across a Si/SiO2/HfO2/TiN interface are calculated and a model of oxygen scavenging process, from SiO2 and into defective TiN, is suggested.

Type: Thesis (Doctoral)
Title: Computational Modelling of Oxygen Defects and Interfaces in Monoclinic HfO2
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Keywords: HfO2, Defects, Interfaces, oxygen vacancy, oxygen interstitial, DFT, multiple vacancy, VASP, Frenkel Defect
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1473850
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