Stable ferroelectric properties of Hf
0.5Zr
0.5O
2thin films within a broad working temperature range
Dao Wang1,2, Jiali Wang1,2, Qiang Li1,2, Waner He1,2, Min Guo1,2, Aihua Zhang1,2, Zhen Fan1,2, Deyang Chen1,2, Minghui Qin1, Min Zeng1, Xingsen Gao1, Guofu Zhou2,3, Xubing Lu1,2*, and Junming Liu1,4
1Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, People’s Republic of China
2Guangdong Provincial Key Laboratory of Optical Information Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, 510006, People’s Republic of China
3National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou, 510006, People’s Republic of China
4Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, People’s Republic of China
*E-mail:luxubing@m.scnu.edu.cn
Received July 16, 2019; accepted August 1, 2019; published online August 21, 2019
The ferroelectric properties of 20 nm Hf0.5Zr0.5O2(HZO) thin films has been investigated in a wide temperature range from 100 K to 450 K. The remnant polarization of HZO thin films decreases slightly from 24.6μC cm−2(100 K) to 17.9μC cm−2(450 K), indicating a robust temperature stability. The capacitors also exhibit excellent endurance properties up to 109cycles at 100 K and 300 K, and their endurance cycles slightly degrades to 108at an elevated temperature of 450 K. The results show that HZO ferroelectric thin films have great potential for future emerging memory applications in various harsh temperature environments.©2019 The Japan Society of Applied Physics
T
he discovery of ferroelectricity in doped HfO2 thin films in recent years have revived great interest in high performance nonvolatile ferroelectric memory devices or negative capacitance field effect transistors (NCFETs).1–3)Compared with traditional ferroelectric mate- rials such as PZT, SBT and BTO etc,4,5)HfO2-based ferro- electricfilms offers several noticeable advantages such as its full compatibility with standard CMOS processes, high scaling potential, and high-k metal gate integrity.6) Ferroelectricity of HfO2 can be achieved by doping Si,1,7) Zr,8) Al,9) Y,10) La,11) Gd and Sr12) etc. combined with certain thermal treatment conditions. Among those reported doped ferroelectric HfO2thinfilms, Zr-doped HfO2thinfilms have been widely studied owing to the similar chemical properties between HfO2 and ZrO2, wide stoichiometric range and the low thermal budget (450 °C) to induce ferroelectricity.8,13) This low thermal budget process facil- itates the integration of ferroelectric circuits in the back-end of the line, allowing for a wider range of applications.13)On the other hand, Si7) and Al9) doped HfO2are less explored and one possible reason may be their high annealing temperature (>800 °C) to form the desirable phase for ferroelectricity.Extensive studies have demonstrated that various factors play an important role in the ferroelectric properties of doped-HfO2thin films such as dopant concentration, thick- ness, annealing temperature, surface energy, mechanical encapsulation, and seed layer etc.13,14) Temperature has been long considered an important factor that affects the ferroelectric properties of traditional ferroelectric materials.15–17)It is well known that the interface energy of the ferroelectric domain decreases with the increase of temperature, which makes the movement of the domain wall easier resulting in the decrease of the intrinsic Pr
value.18) Recently, attention has been also paid to the temperature dependence of the ferroelectric properties of the HfO2–ZrO2(HZO) thinfilms.8,19,20,21)
For example, Müeller et al. demonstrated that HZO thinfilms with a composition of Hf0.5Zr0.5O2have a stable ferroelectric phase in a temperature range from 100 K to 400 K.8) Park et al. reported the
ferroelectric hysteresis loops of HfxZr1-xO2 (x=0.2∼0.5) thin films as a function of temperatures from 298 K to 448 K.19) Park et al. also reported the electrocaloric effects of antiferroelectric HfxZr1-xO2 (x=0.1∼0.3) thin films at low temperatures from 25 K to 150 K.20)Smith et al. reported the polarization and pyroelectric response of 20 nm (Hf1-xZrx)O2thinfilms across a composition range of 0 ⩽x
⩽1 was assessed.21)
Summarizing the above mentioned results, there still lacks a systematic investigation on the impact of the measurement temperature on the typical ferroelectric and dielectric proper- ties of the HZO thin films. In this work, we carried out a systematic study on temperature dependent ferroelectric properties such as remanent polarization, coercive field, positive-up–negative-down (PUND) hysteresis loop, and endurance properties etc at various temperatures, which demonstrated that the electrical proeperties of Hf0.5Zr0.5O2 thin films show a robust temperature stability in a wide temperature range from 100 K to 400 K. Our present work will be very valuable in helping to further understand the electrical properties of the HZO thin films in various temperature environments and contribute to its future appli- cations in nonvolatile memory and negative capacitance transistor.
TiN/HZO/TiN capacitors were fabricated on p-type Si wafers with a 50 nm SiO2 layer. Firstly, the 150 nm TiN bottom electrodes were deposited on a SiO2/Si substrate by reactive sputtering at 270 °C. Next, 20 nm HZOfilms with a Hf:Zr ratio of 1:1 was deposited on a TiN/SiO2/Si substrate via atomic layer deposition (ALD) at a substrate temperature of 280 °C. Hf[N(C2H5)CH3]4, Zr[N(C2H5)CH3]4, and ozone were used as the Hf-precursor, Zr-precursor, and oxygen source, respectively. Then, 150 nm TiN thin film was deposited also by sputtering as the top electrode. The fabricated 20 nm HZO capacitors were annealed at 550 °C for 30 s in a N2atmosphere using a rapid thermal annealing system. Finally, the electrode patterns were formed by conventional photolithography and ion milling, and the top electrode area for the capacitors was 7.85×10−5cm2.
090910-1 https://doi.org/10.7567/1347-4065/ab3844
The crystal structure of the HZOfilms was analyzed using grazing-angle incidence X-ray diffraction (GIXRD) (PANalytical X’Pert Pro diffractometer). The microstructures of the HZO films was investigated by a high resolution transmission electron microscope (HRTEM, JEM2100F) using a cross-section specimen. The ferroelectric, dielectric, and leakage current properties were characterized by a Radiant Technologies Multiferroic II instrument, an Agilent B1500A semiconductor product analyzer, and an E4900A impedance analyzer, respectively. The temperature dependent ferroelectric, dielectric, and leakage properties were mea- sured in a Jannis variable temperature vacuum probe station.
Figure1(a) shows the GIXRD measurement results of the 20 nm HZO films. Diffraction peaks from the (111), (200), and (220) planes demonstrate typical ferroelectric orthor- hombic phases, which are consistent with the reported results.22) Ferroelectric behavior in HZO films originates from the existence of the orthorhombic phase with the Pca21
space group, which is metastable and
non-centrosymmetric.8,13) Figure 1(b) shows typical room temperature polarization–electricfield (P–E) hysteresis of the TiN/HZO/TiN capacitors with various scanning electric fields. The results reveal that the P–E loops change from a linear loop to a well saturated ferroelectric hysteresis when the sweeping electric field increases from 0.5 MV cm−1 to 1.5 MV cm−1. The remanent polarization value 2 Pr at 10 kHz and 1.5 MV cm−1scanningfield is ∼25.8μC cm−2, which proves that the HZO film has good ferroelectricity.
The cross-sectional HRTEM images of the TiN/HZO/TiN capacitors on a SiO2/Si substrate are shown in Fig. 1(c).
Clearly, the gate stack of TiN/HZO/TiN has sharp interfaces, and no noticeable inter-diffusion between the electrode and HZO film can be observed. The thicknesses of the TiN (bottom and top electrodes) and HZOfilms are about 150 nm and 20 nm, respectively. To gain a deep insight into the structural properties of HZO films, selected area electron diffraction patterns (SAED) were used to further investigate their phase structures. Figure 1(d) is a high resolution magnification image of the HZO film in Fig. 1(c), which reveals the polycrystalline nature of the films. Figure 1(e) shows the fast Fourier transformation (FFT) pattern of the selected area marked by a white frame in Fig. 1(d). The diffraction spots in the FFT pattern can be indexed to the (111) plane of orthorhombic phase. Figure 1(f) shows the filtered inverse FFT image, from which the distance of lattice fringes in HZO films was calculated to be approximately 2.92 Å, further proving the existence of a ferroelectric phase.
Figure2(a) shows the temperature dependentP–Ehyster- esis of HZO thin films, which reveals a gradual transition from a well saturated to a sub-loop behavior ferroelectric hysteresis in a wide temperature range from 100 K to 450 K.
The remnant polarization (±Pr) and coercive electric field (±Ec) were extracted from theP–Ehysteresis loops, as shown in Figs.2(b) and2(c). The ±Prvalues nearly remain constant when increasing temperature from 100 K to 400 K. Only at high temperatures from 400 K to 450 K, a noticeable decrease of ±Prvalues was observed. This proves that the remanent polarization of HZO thin films has good temperature stabi- lity. In addition, the wide temperature range from 100 K to 400 K fully covers the common temperature range for
Fig. 1. (Color online) (a) GIXRD pattern of a 20 nm HZOfilm annealed at 550 °C for 30 s in N2taken after chemical removal of the TiN top electrode. (b) Room temperatureP–Ehysteresis loops measured at various sweeping electricfields. (c) Cross-sectional HRTEM image of the TiN/HZO/TiN structure. (d) A high resolution magnification image of the HZOfilms. FFT pattern (e) andfiltered inverse FFT image (f) of the selected area marked by a white frame in Fig.1(d).
090910-2
industrial applications (from 233 K to 398 K).23) The +Ec
value nearly remains unchanged during the whole tempera- ture range, while the -Ec decreases with the increase of the measurement temperature. It was reported that the ferro- electric properties have a close relationship with their leakage current.7) To obtain a good understanding of the present temperature dependent ferroelectric properties of HZOfilms, we also carried out the current density–electricfield measure- ment at various temperatures, and the results were shown in Fig. 2(d). Below 250 K, the HZO thin films show a low leakage current density of ∼5×10−5A cm−2 at
±1.5 MV cm−1. Above 250 K, there still remains a low leakage current density of ∼5×10−5A cm−2 at
±1.0 MV cm−1. Then, the leakage current density starts to increase with the increase of measurement temperature when the electric field is larger than 1.0 MV cm−1. The results show that the HZOfilm has a good insulating quality, and the decrease of the Pr values at high temperature may be attributed to the increased leakage current. In order to exclude the impact of leakage currents on P–E curves, a PUND measurement24) was also done for HZO films at 100 K, 300 K, and 450 K, as shown in Fig.2(e). The intrinsic 2Pr
values are determined to be about 23μC cm−2, 20μC cm−2 and 12μC cm−2 at 100 K, 300 K and 450 K, respectively.
The decrease of the intrinsic polarization value has been proposed to be due to the following mechanism. The inter- face energy of ferroelectric domain decreases with the increase of temperature.18) Consequently, the movement of domain wall becomes easier, which results in the decrease of the polarization value.
To further investigate the ferroelectricity of HZOfilms, the temperature dependent capacitance–electricfield (C–E) char- acteristics were measured, and the results are shown in Fig. 3(a). The butterfly shaped hysteresis curves confirm again that HZO thin films can exhibit and retain obvious ferroelectric properties over a broad temperature range. The capacitance peaks observed in theC–Ecurves correspond to the coercive fields, which are 0.43 MV cm−1 and
−0.45 MV cm−1 at room temperature. Figure 3(b) shows the permittivity of HZO films at zero bias electricfield as a function of temperature. It shows an monotonic increase with the increase of temperature, and its value changes from 35 to 43 when the temperature increases from 100 K to 450 K. For materials in ferroelectric phase, the permittivity usually increases with temperature,23) which demonstrated that our results are consistent with the common reported experimental phenomena. The monotonic increase of permittivity with the temperature implies the Curie temperature of HZO films in our work should be above 450 K, which is favorable for its applications in a high temperature environment.
To reveal the impact of temperature on the reliability characteristics of HZO films, We also measured the fatigue and retention properties of HZO films at different tempera- tures. Figure4(a) shows the endurance properties of the HZO films measured at various temperatures of 100 K, 300 K, and 450 K. The remanent value remains very stable after 108 cycles at all of the temperatures. At low temperature of 100 K, the HZOfilm can withstand at least 109cycles and its Prvalue remains nearly constant. At elevated temperatures of 300 K and 450 K, thePrvalues of HZOfilms start to degrade after 108cycles. It should be mentioned that the Prvalue of 450 K shown in Fig.4(a) is slightly larger than that of 300 K.
This contradicts the results shown in Fig. 2(e). The reason can be analyzed as follows. In Fig. 4(a), the polarization value (Pr) was read without subtracting the leakage current part. It is well known that the measuredPrvalue was usually closely related to the leakage current,25,26) and a higher Pr
was often observed in a leaky film with larger leakage current. As shown in Fig. 2(d), the HZO thin film has a larger leakage current at 450 K than that measured at 300 K.
Therefore, the polarization value at 450 K is larger than that at 300 K. In Fig.2(e), the polarization value obtained by the PUND method is the intrinsic value, which obeys the general rule that the polarization decreases with the increase of the temperature. Figure 4(b) shows the room temperature P–E loops measured in the pristine state and after 103, 106and 109
Fig. 2. (Color online) Temperature dependent ferroelectric and leakage current properties of TiN/HZO/TiN capacitor. (a)P–Ehysteresis loops, (b) remanent polarization, (c) coercivefield, (d) leakage current density. (e) PUND hysteresis loops.
090910-3
times’cycles, which shows a clear evolution ofP–Ecurves with various endurance cycles. Well-saturated P–E curves remains after at least 106 cycles. Although a pinched hysteresis loop was observed up after 109 cycles, it still keeps a well-behaved ferroelectric hysteresis loop.
Figure 4(c) shows the PUND measurement results of the HZO film in the pristine state and after 103, 106 and 109 times’cycles. For the intrinsic ferroelectric properties without leakage current, theP–Ehysteresis loops are very similar in the pristine state and after various endurance cycles, and its remanent polarization value and coercivefield remain nearly unchanged. Only a slightly deformed P–E curve was observed after 109 cycles. Figure 4(d) shows the retention properties of ferroelectric polarization for the HZO film at room temperature. The 2 Pr value has decreased by about 51.8% of its original values after 105s’ retention. The retention of the 2 Pr value is not so good for the present HZO thinfilms. One of the possibility is that there may exist a high depolarizationfield in the HZO thinfilms during the retention measurement. Lomenzo et al.27) reported that the
bottom TaN electrode will be partially oxidized due to the usage of ozone during the ALD process of Si:HfO2thinfilms.
Since the HZO thin films in our work were also fabricated using ozone to provide an oxygen source, oxidation of the bottom TiN electrode may also occur. Consequently, one thin oxide dielectric layer may exist between the HZO thinfilms and the TiN metal electrode, which will induce a high depolarization field deteriorating the long term retention property of Pr. Another possible reason may be due to the charge traps in the interface between the TiN electrode and HZO thin film, which may also degrade the long term retention property.
In summary, we fabricated the ferroelectric HZOfilms by atomic layer deposition, and the impact of the measurement temperature on their ferroelectric properties has been inves- tigated. GIXRD measurements demonstrated that the HZO films exhibit a polycrystalline orthorhombic structure. The HZOfilms have excellent room temperature ferroelectric and leakage properties including a large 2Prof 25.8μC cm−2and a low current density of ∼5×10−5A cm−2 at
Fig. 3. (Color online) (a)C–Ecurves observed at various temperatures. (b) Permittivity at zero bias electricfield as a function of temperature.
Fig. 4. (Color online) (a) Endurance characteristic of the HZOfilm measured at 100 K, 300 K and 450 K, respectively. Polarization switching electricfield and reading electricfield are 1.0 MV cm−1and 1.5 MV cm−1, respectively.P–E(b) and PUND (c) hysteresis loops of HZOfilm in the pristine state and after various endurance cycles. The measurements were performed at room temperature and 10 kHz. (d) Retention properties of the HZOfilm measured at room temperature. Polarization switching electricfield and reading electricfield are 1.5 MV cm−1.
090910-4
−1.0 MV cm−1. The HZOfilms also show excellent opera- tion reliability with an endurance cycles up to 109. Most importantly, the ferroelectric properties of HZO films show an excellent temperature stability in a broad temperature range from 100 K to 400 K. The Curie temperature of HZO films is above 450 K. These findings imply that HZO ferroelectric films have great potential for future device applications in a broad temperature range of at least 100 K to 400 K.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51431006, 51872099). X. B. L. acknowledges the support of the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016). This work was also supported by the Guangdong Innovative Research Team Program (No. 2013C102), the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No.
2017B030301007), and the 111 Project. This work was also supported by the Innovation Project of Graduate School of South China Normal University (No.
2018LKXM014) and Challenge Cup Gold Seed Cultivation Project of South China Normal University (No. 19HDKB03).
1) T. S. Böscke, J. Müller, D. Bräuhaus, U. Schröder, and U. Böttger,Appl.
Phys. Lett.99, 102903 (2011).
2) J. Müller, T. S. Böscke, D. Bräuhaus, U. Schröder, J. Sundqvist, P. Kücher, T. Mikolajick, and L. Frey,Appl. Phys. Lett.99, 112901 (2011).
3) M. H. Park, Y. H. Lee, T. Mikolajick, U. Schröder, and C. S. Hwang,MRS.
Commu8, 795 (2018).
4) I. Khan, K. Chatterjee, B. Wang, S. Drapcho, L. You, C. Serrao, S.
R. Bakaul, R. Ramesh, and S. Salahuddin,Nat. Mater.14, 182 (2015).
5) F. P. G. Fengler et al.,Adv. Electron. Mater.3, 1600505 (2017).
6) J. Müller et al., IEDM Tech. Dig. Dec. 2013 10.8.1-10.8.4.
7) S. Mueller, J. Müller, U. Schroeder, and T. Mikolajick,IEEE. T. Device.
Mat. Re.13, 93 (2013).
8) J. Müller, T. S. Böscke, U. Schröder, S. Mueller, D. Bräuhaus, U. Böttger, L. Frey, and T. Mikolajick,Nano Lett.12, 4318 (2012).
9) S. Mueller, J. Müller, A. Singh, S. Riedel, J. Sundqvist, U. Schroeder, and T. Mikolajick,Adv. Funct. Mater.22, 2412 (2012).
10) J. Müller et al.,J. Appl. Phys.110, 114113 (2011).
11) M. G. Kozodaev, A. G. Chernikova, E. V. Korostylev, M. H. Park, U. Schroeder, C. S. Hwang, and A. M. Markeev,Appl. Phys. Lett.111, 132903 (2017).
12) M. Hoffmann, T. Schenk, I. Kulemanov, C. Adelmann, M. Popovici, U. Schroeder, and T. Mikolajick,Ferroelectrics480, 16 (2015).
13) M. H. Park et al.,Adv. Mater.27, 1811 (2015).
14) A. Chernikova et al.,Appl. Mater. Inter8, 7232 (2016).
15) M. S. Senousy, R. K. N. D. Rajapakse, and M. S. Gadala,Acta Mater.57, 6135 (2009).
16) J. Bechhoefer, Y. Deng, J. Zylberberg, C. Lei, and Z. G. Ye, Am. J. Phys.
77, 783 (2007).
17) A. Muliana,Int. J. Solids Struct.48, 2718 (2011).
18) M. Vopsaroiu, J. Blackburn, M. G. Cain, and P. M. Weaver,Phys. Rev. B 82, 024109 (2010).
19) M. H. Park, H. J. Kim, Y. J. Kim, K. D. Kim, Y. H. Lee, S. D. Hyun, and C.
S. Hwang,Adv. Mater.28, 7956 (2016).
20) M. H. Park, H. J. Kim, Y. J. Kim, T. Moon, K. D. Kim, and C. S. Hwang, Nano Energy12, 131 (2015).
21) S. W. Smith, A. R. Kitahara, M. A. Rodriguez, M. D. Henry, M.
T. Brumbach, and J. F. Ihlefeld,Appl. Phys. Lett.110, 072901 (2017).
22) H. J. Kim, M. H. Park, Y. J. Kim, Y. H. Lee, W. Jeon, T. Gwon, T. Moon, K. D. Kim, and C. S. Hwang,Appl. Phys. Lett.105, 192903 (2014).
23) Y. Zhou, J. Müller, S. Knebel, S. Knebel, D. Bräuhaus, and U. Schröder, Appl. Phys. Lett.100, 082905 (2012).
24) Y. Zhou, Y. Guan, M. M. Vopson, J. Xu, H. L. Liang, F. Cao, X. L. Dong, J. Mueller, T. Schenk, and U. Schroeder,Acta Mater.99, 240 (2015).
25) S. K. Singh, K. Maruyama, and H. Ishiwara,Integr. Ferroelectr.98, 83 (2008).
26) H. Yan, F. Inam, G. Viola, H. Ning, H. Zhang, Q. H. Jiang, T. Zeng, Z.
P. Gao, and M. J. Reece,J. Adv. Dielectr.01, 107 (2011).
27) P. D. Lomenzo, Q. Takmeel, C. Zhou, C. M. Fancher, E. R. Lambers, N.
G. Rudawski, J. L. Jones, S. Moghaddam, and T. Nishida,J. Appl. Phys.
117, 134105 (2015).
090910-5
