Recovery of ferroelectric property after endurance testby positive reset voltage application for CeOx-capped ferroelectric HfO2 films
Kazuto Mizutani, T. Hoshii, H. Wakabayashi, K. Tsutsui, K. Kakushima

Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
E-mail: mizutani.k.ac@m.titech.ac.jp
Abstract
The influence of CeOx capping on HfO2 films on theferroelectric property is investigated. An improved
switching endurance was found with the capping, buta fatigue effect appeared. A better breakdown only inthe negative voltage application indicates themovement of oxygen ions in the capped layer to theunderlying HfO2 layer. A slight recovery with a better
endurance property was obtained by applying anegative voltage among the switching pulses.
1. Introduction
Ferroelectric HfO2 films have attracted considerable
attention owing to their scalability below 10 nm thickness[1]. Reliability issues, however, including switchingendurance, wake-up, or fatigue effects, remain. The
ferroelectric property in HfO2 films is influenced byoxidation time during atomic layer deposition (ALD)
processes; thus, the control of the oxygen vacancy (VO)concentration in HfO2 films is essential [2,3]. Recently,
an improvement in the endurance by CeOx capping on
Y-doped HfO2 (Y:HfO2) films was reported [4]. The
capping function is reported to supply or absorb O atoms
to the underlying HfO2 layer to control the VO
concentration in the HfO2 layer [5]. However, a detailed
analysis of the reliability is not clarified yet. In this study,
we show that oxygen ion (O2-) movement in the cappedlayer may be the origin for better endurance and theappearance of the fatigue effect.
2. Device structure
Metal-insulator-metal (MIM) capacitors were fabricatedon an n+Si substrate with bottom and top W electrodes.
7.5-nm-thick Y:HfO2 films were all deposited by ALD
using tetrakis-dimethylamino hafnium (TDMAH),
tris-isopropyl-cyclopentadienyl yttrium (iPrCp)3Y, andprecursors. The doping concentration in the Y:HfO2 layer
is designed to be 5 mol%. The capped CeOx layer with athickness of either 0.6, 1, or 2 nm was deposited bytris-ethyl-cyclopentadienyl cerium (EtCp)3Ce precursor.
The MIM capacitors were annealed at 500oC for 1 min or
100 min in a forming gas (3%H2+97%N2) atmosphere.
3. Ferroelectric property and endurance with
CeOx-capping
The MIM capacitor, annealed for 1 min, showed better
ferroelectric hysteresis loops when the capping thickness
is more than 0.6 nm (fig. 1). Annealing for a long period(100 min) further increases the ferroelectricity, but thecapping still has the advantage (fig. 2).
Switching cycle test at Vpp=4V revealed a wake-up
effect for non-capped capacitor and showed breakdown

before 108 cycles. The capped capacitor, on the otherhand, showed a reduced wake-up effect but with a fatigue
effect once the switching cycle exceeds 106 times. The
breakdown occurred before 1010 cycles, a significant
improvement to the non-capped capacitor (fig.3). The DC
current is measured regularly among the cycling test andis shown in fig. 4. Note that 0.5 V includes the switchingcurrent component and 1.5 V is the leakage component. Agradual current decrease at 0.5 V for capped capacitor
indicates the reduction of Pr along with the cycle test. We
can confirm that the suppressed leakage current
contributes to the prolonged endurance. The effect ismore pronounced for lower switching voltage(VPP=3.6V).
The breakdown measurements of the MIM capacitors
are shown in fig. 5. While no difference was found in thepositive voltage application, a larger breakdown voltage
was obtained in the negative direction with the capping.
As the oxygen ion conductivity of CeOx is known to belarge [6], one can suspect that O2- ions drift to theunderlying HfO2 layer to compensate for the created VO
filament in the layer (fig. 6 (a)).
Besides thecompensation of the VO in the HfO2 layer, the O2- ionsmay pin the ferroelectric domain wall and might be thereason for the fatigue effect (fig. 6 (b)).
4. Recovery of ferroelectricity with negative voltage
A reset voltage (Vreset) was applied to the cappedcapacitor after switching of 108 times. We observed arecovery in the ferroelectric property when a Vreset of 2 V
was used (fig. 7). By applying a Vreset for every 106 cycles,
the fatigue effect was found to be suppressed (fig. 8). The
impact of the positve Vreset is hypothesized to theextraction of O2- as well as electrons trapped at
CeOx/HfO2 interface.
5. Conclusion
The effect of CeOx capping on Y:HfO2 layer on theferroelectric properties is investigated. Improvement inferroelectricity with better endurance was obtained. Theeffect is presumably due to the oxygen ion movement inthe CeOx to compensate for the VO in the HfO2 layer. The
fatigue effect can be considered to be the O2- andelectrons to pin the domain. Applying a positive reset
voltage among the cycling test recovers theferroelectricity, unpin the domains.
Acknowledgment
The work was supported by JST COI Gran Number
JPMJCE1309. One of the authors, K. M., was supportedby The FUTABA Foundation.

(a) No Ce-cap

403020100-10-20-30-40(c) 1-nm Ce-cap(b) 0.6-nm Ce-cap

W
CeOx

W
Y:HfO2(7.5nm)(7.5nm)W(d) 2-nm Ce-cap

WCeOx

WCeOx

(2.0nm)(1.0nm)Y:HfO2

Y:HfO2(7.5nm)(7.5nm)WW-4 -3 -2 -1 0 1 2 3 4

Voltage (V)Polarization (µC/cm2)

Fig. 1 Hysteresis loops with different CeOx-capping layer
thicknesses.
50
40 without capping
30
Y:HfO220
100-10with capping-20
CeOx
Y:HfO2-30-40-50-5 -4 -3 -2 -1 0 1 2 3 4 5
Voltage (V)Fig. 2 Hysteresis loops with capacitors annealed at
500oC for 100 min.
35

without capping

PSW (µC/cm2)

30

Y:HfO2

25

Y:HfO2

10-1

withoutcapping

10-3
10-5

withcapping -V

10-7

CeOx
Y:HfO210-9-8 -7 -6 -5 -4 -3 -2 -1

10with capping

5CeOx
Y:HfO2

10-2
10-3

Vpp=4V
Vpp=3.6V

Y:HfO2(a)10-4with capping

10-6
10-7
10-8
105

CeOx
Y:HfO2

at 0.5V

106

107

108

109

1010

Switching cycles

34

567(b)

O2-CeOx

O2-compensate

pinningpolarization

conductive
filament

Y:HfO2
BEFig. 6 (a) O2- ions in the capping layer compensate theVO in the HfO2 layer to prevent from breakdown. (b)O2- ions can pin the ferroelectric domain to decrease
the ferroelectricity.
2520151050-5-10-15-20-25

before recovery

after recovery-3-2-101Voltage (V)232110111012

Fig. 4 Current (J) measured during switching cycles.
Note that J at 0.5 V includes the switching current
component and J at 1.5 V is the leakage current.

biased by +2V

20191817at 1.5V

10-52TE

PSW (µC/cm2)

Current density (A/cm2)

10-1

1Fig. 7 Recovery of the ferroelectricity with positive
Vreset application. (Vreset=2V for 1 s)

Fig. 3 Switching endurance under VPP=2 V at 500 kHz.
without capping

0Fig. 5 Breakdown measurement of the capacitors forboth negative and positive directions.

0 0 1 2 3 4 5 6 7 8 9 1010 10 10 10 10 10 10 10 10 10 10Switching cycles

100

without capping+Vwith capping CeOx

Voltage (V)2015+V
Y:HfO2

Y:HfO2(0.6nm)

W

Positive direction-V

101

Y:HfO2(7.5nm)-4 -3 -2 -1 0 1 2 3 4

Negative direction

103

Polarization (µC/cm2)

40
30
20
100-10-20-30-40

Current density (A/cm2)

Polarization (µC/cm2)

References[1] T. S. Böscke, et al, Appl. Phys. Lett., 99, 102903 (2011).
[2] M. Pešic, et al, Adv. Funct. Mater., 26, 4601 (2016).
[3] T. Mittmann, et al, IEDM, pp. 378 (2020).
[4] J. Molina, et al, IMW, 5.3 (2020).
[5] N. Umezawa, Appl. Phys. Lett., 96, 162906 (2010).
[6] M. Mamatrishat, et al, Vacuum, 86, 1513 (2012).

16106

without Vreset107
Switching cycles

108

Fig. 8 Relaxed fatigue effect with positive Vreset forevery 106 cycles.

8