Effect of capping layer on the ferroelectricity of hafnium oxide

In this work, we investigate the effect of a capping layer of aluminum oxide (AlO_x) on dopant-free hafnium-oxide (HfO₂) ferroelectric layer. According to our experimental results, the AlO_x-capped HfO₂ ferroelectric capacitor exhibits the significant improvement on ferroelectric polarization strength and also presents a comparable electrical stress resistance in comparison with control HfO₂ ferroelectric capacitor with a very weak ferroelectric hysteresis. An additional nitrogen treatment on AlO_x capping layer effectively alleviate the stress-induced generation of defect traps near the interface of tantalum nitride (TaN) electrode under negative-bias voltage stress. Therefore, the performance improvement can be ascribed to the introduction of AlO_x capping layer and well-controlled nitrogen plasma treatment to improve interface traps and stabilize the ferroelectric phase transformation during high-temperature annealing process.