Atomic Structure of Domain and Interphase Boundaries in Ferroelectric HfO₂

Abstract

Though the electrical responses of the various polymorphs found in ferroelectric polycrystalline thin film HfO 2 are now well characterized, little is currently understood of this novel material's grain sub-structure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and interconversion. Here, we apply scanning transmission electron microscopy to investigate the atomic structure of boundaries in these materials. In particular, we find orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic interphase boundaries formed throughout individual grains. The results inform how interphase boundaries can impose strain conditions that may be key to phase stabilization. Moreover, the atomic structure near interphase boundary walls suggests potential for their mobility under bias, which has been speculated to occur in perovskite morphotropic phase boundary systems by mechanisms similar to domain boundary motion.

Analysis of Performance Instabilities of Hafnia‐Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

“…This rotation in the structure results in a change in plane spacing which is visible in the lattice spacing measurements. The presence of coherent monoclinic/ orthorhombic interphase boundaries and 90° orthorhombic domains in Hf 0.5 Zr 0.5 O 2 is consistent with findings of microstructure in Gd-doped hafnia thin films, [22] which suggests they possess similar microstructure features. This monoclinic region has distinct symmetry and spacing compared to the other regions.…”

“…The presence of coherent monoclinic/ orthorhombic interphase boundaries and 90° orthorhombic domains in Hf 0.5 Zr 0.5 O 2 is consistent with findings of microstructure in Gd-doped hafnia thin films, [22] which suggests they possess similar microstructure features. [11,20,22] HAADF STEM additionally reveals that the ferroelectric/electrode interface is smoother for the bottom interface than for the top interface, which is consistent with findings for Gd-doped hafnia. [20] During the ALD deposition process, oxygen from the hafnia film likely reacts with the TiN electrode to form an interfacial TiO x N region.…”

“…Both orthorhombic portions also border a monoclinic region that is aligned to the[011] m zone axis. [11,20,22] HAADF STEM additionally reveals that the ferroelectric/electrode interface is smoother for the bottom interface than for the top interface, which is consistent with findings for Gd-doped hafnia. The {111}-type planes slightly misalign between the orthorhombic and monoclinic phase regions, as indicated by the lines in Figure 2a.…”

“…[20] The value close to 1 for the constant phase parameter n (see Table 1) indicates quite homogeneous properties of this layer. [27] However, our very thin hafnia thin films consist of mainly columnar grains [22] with a high fraction of oriented grain boundaries [17] strongly decreasing the grain boundary contribution. In literature, where other material systems and thicker layers are investigated, this is often interpreted as the difference between grains and grain boundaries.…”

On the Origin of the Large Remanent Polarization in La:HfO2

“…When the Hf and O atoms of the amorphous start moving toward their lattice positions in the crystalline phase, the presence of a tensile strain leads to increased in-plane interatomic distances. The prominent interphase boundaries found by Grimley et al [50] also point toward that conclusion. At this time of the process, the atoms might not sufficiently mobile to simply increase the number of atoms in the in-plane direction further, but sufficiently mobile to flip the forming unit cell around.…”