Thinner Is Better for Ferroelectric HfO₂ Films

Researchers found that ultrathin films of doped HfO₂ showed enhanced ferroelectricity, in sharp contrast to other ferroelectrics such as perovskites. Piezoresponse force microscopy (PFM) and scanning capacitance microscopy (SCM) verified that ferroelectric polarization switching occurred in films as thin as 1 nm.

Ferroelectric films less than 2 nanometers thick are needed to realize next-generation logic and memory devices. Hafnium oxide (HfO₂) based films are especially attractive because they are compatible with modern semiconductor processes. Unfortunately, the ferroelectric response of many materials is suppressed at such small thicknesses.

A U.S. team led by researchers at the University of California–Berkeley has demonstrated stable ferroelectricity in ultrathin films of Hf_0.8Zr_0.2O₂ (HZO) grown on silicon by low-temperature atomic layer deposition (ALD). Along with other measurements, nanoscale characterization with PFM and SCM provided conclusive evidence of spontaneous, switchable polarization. Data showed that the enhancement persisted in films as thin as two unit cells, or 1 nm, of the fluorite-structure HZO.

The results suggest a way to enhance electric polarization at the nanoscale by exploiting confinement strain in ultrathin films. With this approach, monolithic integration of doped HfO₂ on Si/SiO₂ could enable polarization-based memories and ferroelectric transistors capable of replacing current technologies such as flash memory.

Instruments used

MFP-3D, Cypher

Techniques used

Piezoresponse force microscopy (PFM) was conducted on an Asylum Research MFP-3D AFM to image written domain structures and measure switching spectroscopy piezoelectric hysteresis loops. The experiments used Asylum’s exclusive dual amplitude resonance tracking (DART) mode, which provides increased sensitivity to weak signals and minimizes crosstalk artifacts. Additional PFM experiments were performed on a Cypher AFM with the Interferometric Displacement Sensor (IDS) option. By directly measuring cantilever deflection, the IDS eliminates artifacts due to electrostatic coupling and improves reproducibility. The Cypher was also used for scanning capacitance microscopy (SCM) measurements of differential capacitance dC/dV at microwave frequencies (1.8 GHz). Asylum’s SCM option offers significantly higher performance than other available modules, including faster scanning and high sensitivity and resolution.

Citation: S. Cheema, D. Kwon, N. Shanker et al., Enhanced ferroelectricity in ultrathin films grown directly on silicon. Nature 580, 478 (2020). https://doi.org/10.1038/s41586-020-2208-x

Note: The data shown here are reused under fair use from the original article, which can be accessed through the article link above.