Title: DFT models of ferroelectric-paraelectric and ferroelectric-antiferroelectric boundaries in hafnium-zirconium oxides
Presenters: Kisung Chae 1 ,2, Kyeongjae Cho, 2 and Andrew Kummel, 1
1 Department of Chemistry and Biochemistry, University of California San Diego
9500 Gilman Dr, La Jolla, CA 92093, USA
2 Department of Materials Science and Engineering, The University of Texas at Dallas
800 W Campbell Rd, Richardson, TX 75080, USA
Ferroelectric (FE) hafnium-zirconium oxide (HZO) thin films have many advantages in next-generation device applications such as negative capacitance field effect transistor (NCFET), ferroelectric random access memory (FERAM) or multi-state memory. Due to extensive previous studies, it has been identified that the orthorhombic (o) phase with a space group of Pca21 (No. 21) is responsible for the observed ferroelectricity and various factors such as doping, strains and electric field have been proposed to stabilize the o-phase over the monoclinic (m) phase which is known to be the ground state in pure bulk stress-free films. Here, density functional theory (DFT) is employed to show that o-phase is stabilized due to the significant in-plane tension formed during the experimental annealing process in which the as-deposited amorphous phase is crystallized into the more dense crystalline phases. HZO can form atomically sharp interfaces including phase and/or domain boundaries as shown by a recent transmission electron microscope study; this was also modeled with DFT. Stack models were made for paraelectric (PE) and FE as well as AFE and FE. In both cases, polarization in the FE layer is perpendicular to the interface, which is most relevant to NCFET and multistate memory applications, For the PE/FE stack, the perpendicular polarization was rotated to be parallel to the interface in less than one unit cell. Because of the sharp rotation at the interface, the electric field building up in the FE layer is perfectly transmitted to the PE layer, indicating minimal depolarization at the interface. In the same way, potential transmission was also seen for AFE/FE stack. In this case, formation of interface generates tetragonal phase formation which is higher in energy. This work was supported by DARPA SRC JUMP ASCENT theme 1 task 59.
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