Improvement of Ferroelectricity and Fatigue Property of Thicker HfxZr1−XO2/ZrO2 Bi-layer
Authors: Onaya Takashi , Nabatame Toshihide , Inoue Mari , Jung Yong Chan , Hernandez-Arriaga Heber , Mohan Jaidah , Kim Harrison Sejoon , Sawamoto Naomi , Nagata Takahiro , Kim Jiyoung , Ogura Atsushi 
Published: December 2020

HfxZr1−xO2 (HZO) has been an attractive material for future ferroelectric memory devices because of their high scalability ~10 nm, stable ferroelectricity over a wide Hf:Zr composition range, and compatibility with CMOS manufacturing process. [1] Considering the ferroelectric device manufacturing, the stable ferroelectricity in the thicker film region is required. However, it has been reported that the ferroelectricity of HZO films is generally degraded in the thick ferroelectric film region > 10 nm. [2] To obtain the superior ferroelectricity in the thicker film region, we pay attention to the ferroelectricity of the HZO based thick ferroelectric structures (> 20 nm) with paraelectric HfO2 and anti-ferroelectric ZrO2 layers. In this work, we studied the ferroelectricity and fatigue properties of the thick HZO/ZrO2 and HZO/HfO2 stack structures fabricated using atomic layer deposition (ALD).

The TiN/HZO/ZrO2/TiN capacitors were fabricated as follows: An HZO layer was deposited on TiN bottom electrode by ALD at 300 °C using (Hf/Zr)[N(C2H5)CH3]4 (Hf:Zr = 1:1) cocktail precursor and H2O gas. The thickness of the HZO layer were varied from 0 to 10 nm. Next, a 10-nm-thick ZrO2 layer was deposited at 300 °C using (C5H5)Zr[N(CH3)2]3 and H2O gas. After fabrication of HZO/ZrO2 bi-layer, post deposition annealing (PDA) was performed at 600 °C for 1 min in a N2 atmosphere. Finally, TiN top electrode was fabricated by DC sputtering. The capacitors with HZO single layer and HZO/HfO2 bi-layer were also prepared as references.

Figure 1 shows the remanent polarization (2P
r) of the HZO single layer, HZO/HfO2, and HZO/ZrO2 bi-layers from polarization-electric field (P-E) hysteresis curves as a function of the HZO thickness while the HfO2 and ZrO2 thicknesses were kept at 10 nm. The maximum 2P
r (12 µC/cm2) of the HZO single layer was obtained when the thickness of the HZO layer was 10 nm. However, the 2P
r of the HZO single layer gradually decreased with the HZO thickness due to the formation of the stable monoclinic phase which shows paraelectricity. [2] The ferroelectric properties of ZrO2 and HfO2 single layers were hardly observed because both layers exhibited anti-ferroelectricity and paraelectricity, respectively. The HZO/HfO2 bi-layer maintained extremely small 2P
r regardless of the HZO thickness. On the other hand, the switching properties of HZO/ZrO2 bi-layer were divided into three regions such as anti-ferroelectricity in less than 12.5 nm, mixing phases of anti-ferroelectricity and ferroelectricity in 15 nm, and ferroelectricity in over 17.5 nm. The maximum 2P
r of 11 µC/cm2 could be obtained in the 20-nm-thick of HZO/ZrO2 bi-layer, and was approximately 2 times larger than that (6 µC/cm2) of the 20-nm-thick HZO single layer. This superior ferroelectricity of the HZO/ZrO2 bi-layer may be due to the improvement of quality of the HZO layer and the transformation from anti-ferroelectric to ferroelectric phase in the ZrO2 layer. [3, 4]

Figure 2 shows endurance properties of HZO (10 nm) single layer, HZO/ZrO2 (20 nm), and HZO/HfO2 (20 nm) bi-layers using positive-up negative-down (PUND) method. For the HZO single layer and the HZO/ZrO2 bi-layer cases, the wake-up effect was observed until 104 cycles, while the HZO/HfO2 bi-layer had small 2P
r in the entire switching cycle region. The large difference of 2P
r properties between the HZO layer and the HZO/ZrO2 bi-layer was observed in over 104 cycles. Noted that the HZO/ZrO2 bi-layer was larger 2P
r than that in pristine condition after 106 cycles, while the 2P
r of the HZO single layer was dropped by around 30%. Therefore, it is thought that ZrO2 layer play an important role in preventing the HZO/ZrO2 bi-layer from the degradation of ferroelectricity.

In conclusion, the thick HZO/ZrO2 bi-layer exhibited superior ferroelectricity and endurance properties compared to the HZO/HfO2 bi-layer and thin HZO single layer. Based on these results, the HZO/ZrO2 bi-layer using the combination of HZO and ZrO2 layers is one of the promising structures for future ferroelectric devices application.

This study was partially supported by JSPS KAKENHI (JP18J22998 and K4H002). The authors thank all staff members of the Nanofabrication Group of NIMS for their support in fabricating the MFM capacitors.

[1] J. Müller et al., Nano Lett. 12, 4318 (2012).

[2] M. H. Park et al., Appl. Phys. Lett. 102, 242905 (2013).

[3] T-.Y. Hsu et al., Smart Mater. Struct. 28, 084005 (2019).

[4] S. Shibayama et al., Jpn. J. Appl. Phys. 59, SMMA04 (2020).

Figure 1