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Study shows domain walls in ferroelectrics can be the most stable state, enabling high-density memory

A research team, led by Professor Junhee Lee from the Graduate School of Semiconductor Materials and Devices Engineering at UNIST, has demonstrated through quantum mechanical calculations that charged domain walls in ferroelectrics—once thought to be unstable—can, in fact, be more stable than the bulk regions.

This discovery opens new avenues for developing high-density semiconductor memory devices capable of storing information as binary states (0s and 1s) based on the presence or absence of domain walls.

This research was conducted in collaboration with researchers Pawan Kumar and Dipti Gupta, who served as the first author and co-author, respectively. The research is in the journal Â鶹ÒùÔºical Review Letters.

Ferroelectrics are next-generation semiconductor materials that can have their internal polarization direction altered by external electric fields. These materials form boundaries called domain walls where differing polarization orientations meet. Historically, the formation of domain walls was considered energetically costly and their transient nature made them unreliable for stable data storage.

Contrary to this conventional understanding, Professor Lee's team theoretically confirmed that in certain orientations of hafnium oxide (HfO₂)—a ferroelectric material—charged domain walls can possess lower total energy than the bulk material itself. This counterintuitive phenomenon challenges conventional solid-state physics understanding and is attributed to an unusual physical property known as "negative gradient energy."

Typically, domain walls involve a sharp change in polarization, resulting in positive gradient energy that opposes their formation. In the case of hafnium oxide, certain vibrational modes cause this gradient energy to become negative, thereby favoring the formation of charged domain walls.

This negative gradient energy partially offsets the electrostatic energy generated by bound charges at the domain walls. When combined with partial compensation through doping, the overall energy favors the stability of these domain walls over the bulk phase.

Professor Lee stated, "Our research theoretically establishes the conditions under which charged domain walls in ferroelectrics can be energetically stabilized." He further noted, "This insight could serve as a foundation for developing high-density memory devices that encode information based on the presence or absence of domain walls, corresponding to binary states 0 and 1."