Tiny oxygen gaps may determine whether next-generation memory films store data well

Researchers have observed in real-time the critical moment when the performance of a next-generation memory material is determined. The secret lies in controlling oxygen vacancies, microscopic defects caused by missing oxygen atoms in the material. A team led by Professor Min Hyuk Park from Seoul National University collaborated with other scientists to track how a ferroelectric phase, responsible for storing information in the material, forms during heat treatment of hafnia-zirconia (HZO).

The study, published in Advanced Functional Materials, found that oxygen vacancies significantly impact both the crystallization temperature and the final crystalline phase of HZO, leading to varying memory storage performance. Ferroelectrics are materials that maintain their electrical state even without power, making them ideal for nonvolatile memory and low-power devices.

Hafnia-based ferroelectrics, in particular, are favored for their compatibility with silicon fabrication processes and potential for high-density memory.

However, HZO does not naturally possess ferroelectric properties. The ferroelectric phase requires specific atomic arrangements, which can be achieved under certain processing conditions. Instead of focusing solely on the final structure, the researchers examined the process of crystal formation from an amorphous thin film and the selection of crystalline phases during heat treatment.

The key variable in this study was oxygen vacancies. The researchers created HZO films with different oxygen-vacancy concentrations by varying the amount of ozone used during atomic layer deposition. Using synchrotron radiation, they performed real-time X-ray scattering to monitor the transformation of the amorphous films into crystals during heating and cooling.

The analysis revealed that higher oxygen-vacancy concentrations delayed the crystallization process. Films with more oxygen vacancies required higher temperatures to initiate crystallization and had slower lateral grain growth. In contrast, films with fewer oxygen vacancies had lower energy barriers for crystallization and allowed ferroelectric orthorhombic phase formation more readily.

This resulted in a 30°C lower crystallization temperature and higher memory storage signal (2Pr) at the same temperature (400°C) compared to films with higher oxygen vacancies.

The study emphasizes that oxygen vacancies are crucial process variables that control the onset of crystallization and the formation of the ferroelectric phase in HZO thin films. Controlling oxygen vacancies could lead to lower-temperature ferroelectric formation, which is beneficial for semiconductor processing to minimize thermal damage in back-end manufacturing. The findings contribute to advancements in next-generation memory technology.