Computational Calculations of the Effects of Defects on HfO2 Properties

Emily Crabb, Massachusetts Institute of Technology

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Current Si-CMOS technologies are reaching their limit because transistors can only be made finitely small before quantum effects interfere with their behavior. As a result, it will not be possible to maintain Moore's law solely by relying on existing technology in the near future. One method to extend these technologies is to improve the gate materials used. Current Si-MOSFETs use HfO2 as a thin gate material with a high dielectric constant. However, one problem with HfO2 gates is leakage current that limits their efficiency. Impurities in the HfO2 gates increase their efficiency by decreasing this leakage current, but the exact mechanisms are not fully understood. We used density functional theory (DFT)-based calculations to examine the energetics of oxygen vacancies and nitrogen and fluorine defects in HfO2. DFT was used, as it is fast, relatively inexpensive and well-established as an electronic structure method for finding materials' properties. In order to account for electron correlations, we used the so-called DFT+U approach, in which an on-site parameter (U) is used for electronic correlations. For transition metal oxides like HfO2, the electron correlations are often too strong to use DFT to accurately determine the materials' properties and the U-parameter cannot accurately capture all effects stemming from correlations. As such, there is a growing area of research that uses Quantum Monte Carlo (QMC) simulations to better model these materials. QMC simulations are more expensive, as they require more computational power and time, but they also are capable of achieving greater accuracy. We plan to use QMC as a follow-on to the DFT calculations to elucidate the effect of correlations on the electronic structure.

Abstract Author(s): Emily Crabb, Heyondeok Shin, Anouar Benali, Olle Heinonen