Atomic layer etching of ferroelectric hafnium zirconium oxide thin films enables giant tunneling electroresistance

The ferroelectric properties of hafnium oxide and zirconium oxide based thin films are promising for applications in low power electronics, such as ultra-thin ferroelectric tunneling devices. However, the amount of ferroelectric phase in the film depends on their polycrystalline morphology, which changes with film thickness. Therefore, controlling the film thickness without changing the ferroelectric properties has remained challenging. Here, we propose the use of thermal atomic layer etching to decouple the ferroelectric phase stabilization from the film thickness. First, the ferroelectric phase fraction is maximized by crystallizing the film at an optimized film thickness. Subsequently, the ferroelectric film thickness is reduced to the desired range by atomic layer etching. We demonstrate the feasibility of this approach for a ferroelectric hafnium zirconium oxide film of 10 nm initial thickness, which we integrate into a double-layer ferroelectric tunnel junction. The atomic layer etch rate of ferroelectric hafnium zirconium oxide using HF and dimethylaluminum chloride is found to be ∼0.2 Å/cycle. Although the ferroelectric phase persists after atomic layer etching, the etching increases the surface roughness. For applications in ferroelectric tunnel junctions, we show that atomic layer etching of ferroelectric hafnium zirconium oxide can improve the read current by more than a factor of 200, while at the same time reducing the read voltage by 43%. The resulting tunneling electroresistance of about 2500 is the highest reported so far for polycrystalline hafnium zirconium oxide-based materials.