Search

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

Integrating the HfZrO2 (HZO) on silicon have attracted the attention of researches in the development of ferroelectric field effect transistor (FeFETs) for implementation in high-density memories and neuromorphic devices[1][2]. In this work, it has been investigated the thickness scalability of HZO thin films and the reduction of the equivalent-oxide-thickness (EOT) on Silicon using [(CH3)2N]4Hf and [(CH3)2N]4Zr as precursor and H2O and O3 as oxidants by means of atomic layer deposition. A TiN/HZO/Si structure was fabricated where the Si substrate has been prepared with a HF-last clean to remove the native oxide or SC1-last clean process to form a chemical oxide layer of 1.2nm. The EOT vs thickness analysis shows an interesting alteration in the dielectric constant which is understood as a phase formation in the HZO from the amorphous to the ferroelectric o-phase to be found in the 4 to 5nm region when annealed at 400 °C. Also, the interface thickness between silicon and HZO was reduced when the HZO is scaled down below the transition region and it is confirmed by transmission electron microscopy (TEM). This reduction in the interface is expressed as scavenging effects from the TiN top electrode when the samples are annealed at 400 °C. [1] J. Y. Kim, M.-J. Choi, and H. W. Jang, “Ferroelectric field effect transistors: Progress and perspective” APL Mater., 9, 021102 (2021) [2] E. T. Breyer, H. Mulaosmanovic, T. Mikolajick, and S. Slesazeck, “Perspective on ferroelectric, hafnium oxide based transistors for digital beyond von-Neumann computing,” Appl. Phys. Lett, 118, 050501 (2021)

Ferroelectrics have retrieved great interest since the discovery of a ferroelectric phase in HfO2.1 Especially, ferroelectric Hf0.5Zr0.5O2 (HZO) thin films have studied for application as a non-volatile memory and field-effect transistor, due to their high scalability and compatibility with CMOS process.2 In our previous work, we have achieved a low thermal budget process (400 °C) by controlling the tensile stress from the TiN electrode during the annealing process.3 Moreover, the TiN electrode can act as a barrier layer against hydrogen diffusion, therefore, the HZO films using the TiN electrodes can prevent degradation of the ferroelectric properties from hydrogen incorporation. However, during the deposition of HZO film using atomic layer deposition (ALD), the choice of oxygen source can affect the residue in the deposited film. When hydrogen-related substances are used as the oxygen source, the ferroelectric properties can be altered by the residues remaining in the HZO film. In this study, we adopted various oxygen sources such as H2O, deuterium oxide (D2O), and O3, for the investigation of hydrogen effect on the ferroelectric properties of the ALD-HZO films. The HZO films were deposited on the TiN bottom electrode by ALD using Hf[N(CH3)2]4 (TDMA-Hf) and Zr[N(CH3)2]4 (TDMA-Zr) as the precursors of Hf and Zr, while H2O, D2O, or O3 as the oxidant. After the TiN top electrode was deposited, rapid thermal annealing was done, and metal-insulator-metal capacitors were fabricated using a Pd/Au hard mask and wet etch process. The HZO films exhibited the formation of a non-centrosymmetric orthorhombic phase which is origin of the ferroelectricity, without consideration for the oxygen sources.4 However, it was confirmed that the ferroelectricity of the HZO film deposited with the oxygen source including hydrogen such as H2O and D2O, was degraded due to the more incorporation with hydrogen. As a result, the ferroelectric polarization of H2O-, and D2O-based HZO was lower than that of O3-based HZO. Simultaneously, the H2O- and D2O-based HZO exhibited higher leakage current than that of O3-based HZO. The results strongly suggest that the suppression of hydrogen incorporation is important to apply ALD-HZO films for next-generation memory application. This work is supported by the National Research Foundation of Korea (NRF) grant (No. NRF-2019R1F1A1059972), Brain Pool Program (No. 2019H1D3A2A01101691), and the Fostering Global Talents for Innovative Growth Program (No. P0008750). 1 T. S. Böscke, et al., Appl. Phys. Lett. 99, 102903 (2011). 2 S.J. Kim, et al., Appl. Phys. Lett. 113, 182903 (2018). 3 S.J. Kim, et al., Appl. Phys. Lett. 111, 242901 (2017). 4 S.J. Kim, et al., JOM 71, 246 (2019). Figure 1