Leakage current suppression by layered insertion of Y2O3 for ferroelectric HfO2
Kazuto Mizutani1, Takuya Hoshii1, Hitoshi Wakabayashi1, Kazuo Tsutsui2, Kuniyuki Kakushima1○School of Engineering, Tokyo Institute of Technology, 4259 S2-20, Nagatsuta, Midori-ku, Japan
Institute of Innovative Research, Tokyo Institute of Technology, 4259 S2-20, Nagatsuta, Midori-ku, Japan
E-mail: mizutani.k.ac@m.titech.ac.jp
12Abstract

The effect of the localized distribution of Y atoms in 7nm-thick Y-doped HfO2 films on the ferroelectriccharacteristics is investigated. Although a slight decreasein the remnant polarization (Pr) is observed, ferroelectric
properties can be obtained with the localized distribution.
Suppressed leakage current with better switching cycle
endurance can be obtained with the localized distribution.
Reduced generation of oxygen vacancy (VO) might be thecharacteristic of the layered design.

1. Introduction

Ferroelectric HfO2 films have been studied extensively
for memory applications [1]. Among numbers of dopants
to obtain ferroelectric HfO2 films, the Y-atom-doping has
been widely used as the antiferroelectric tetragonal phasecan be suppressed [2]. The doping is performed byalternative injection of Hf and Y atom precursors inatomic-layer deposition followed by oxidation. Generally,
the vapor pressure of the precursors of Y atoms is quite
low; bubbling is commonly used to enhance gas delivery.
The deposition rate should be precisely tuned so as not tochange the composition of the Y-doped HfO2 film.
Meanwhile, ALD has the capability to deposit amonolayer (ML) or sub-ML by self-limit process [3].
Therefore, the controllability of the doping might be easy
with localized Y2O3 insertion in the HfO2. In this study,
the advantage of localized Y2O3 layers in HfO2 films isaddressed. The localized insertion of the Y2O3 layer might
be useful to prevent unintentional crystallization during
the deposition process, as is used for localized Al2O3 layer
for ZrO2 film in DRAM applications [4].

2. Sample fabrication

Metal-insulator-metal (MIM) capacitors were fabricatedon an n+Si substrate with bottom and top W electrodes.
About 7-nm-thick Y-doped HfO2 films were deposited byALD with 4 different Y2O3 and HfO2 thickness
combinations (Table 1). A Y-doped HfO2 film (sample 1),
with 13 sets of Y2O3/HfO2= 0.03/0.5nm can be considered
as a uniformly doped film (fig. 1(a)). On the other hand,sample 2 with Y2O3/HfO2=0.2/2.7nm for 2 sets is thelocally inserted film (fig. 1(b)). The localized presence ofY atoms can be confirmed by DF-TEM image. Note thatthe thickness of 0.2 nm is slightly less than 1 ML (0.23nm)of Y2O3. All the effective doping concentrations in the Ydoped HfO2 layers are designed to be 5 mol%. Annealing
was conducted at 600oC for 1 min in a forming gas(3%H2+97%N2) atmosphere. Capacitance-voltage (CV),polarization-voltage (PV) and leakage current (JV)
measurements were performed to elucidate the advantageof the localized insertion process.

3. Results and Discussions

PV hysteresis curves on different voltage sweeps and CV
characteristics of the samples are shown in fig. 2. Decent
ferroelectric hysteresis loops were observed for all thesamples when applied peak-to-peak voltage (Vpp) exceeds
4 V (about 2.7 MV/cm). The Pr obtained at 3 Vpp decreases
from 20 to 15 µC/cm2 by further localizing the Y2O3position. The CV curves revealed that high dielectric
constants of 34 and 32 could be obtained with uniform andlocalized doped films, respectively. A smaller dielectric
constant is useful for memory applications.
The switching cycle test with a pulse height of VPP of 4
V at 500 kHz shows a wake-up effect for all the samples(fig. 3) [5]. This fact indicates an incomplete transition ofthe ferroelectric phase during the annealing. A better
endurance before dielectric breakdown by a factor of 5 inthe switching cycles was obtained with localized Y2O3films. JV characteristics revealed better insulation with theY2O3 localized HfO2 films (fig. 4). The evolution ofleakage current measured at 1.5 V on the switching cycles
indicates a gradual increase before the breakdown event.
The behavior is a typical sign of defect-mediatedformation of a conductive filament composed of oxygen
vacancy (VO) in HfO2 films [6].

4. ConclusionsFerroelectric characteristics were obtained with uniform
and localized doped HfO2 films. A slight decrease in theinitial Pr from 20 to 15 µC/cm2 indicates an incomplete
transition to the ferroelectric phase in the HfO2. However,suppression in the leakage current, as well asimprovement in the switching cycle endurance, can beachieved with localized doped HfO2.

Acknowledgment

The work was supported by JST COI Gran Number
JPMJCE1309. One of the authors, K. M., was supportedby The FUTABA Foundation.

References[1] T. S. Boscke, Appl. Phys. Lett., 99, 102903 (2011).
[2] J. Niinisto, Electrochem. Solid-State Lett., 12, G1 (2009).
[3] J. Song, Jpn. J. Appl. Phys., 59, SMMB01 (2020).
[4] H. J. Cho, Solid-State Electronics 51, 1529 (2007).
[5] M. Pesic, Adv. Funct. Mater, 26, 4601 (2016).
[6] A. S. Sokolov, Appl. Surf. Sci., 434, 822 (2018).

Table1 Thicknesses design of the fabricatedsamples.

Capacitance (µF/cm2)

Polarization (µC/cm2)

Fig. 1 (a) Uniform and (b) localized Y2O3
distribution in HfO2 film. Localized Y atoms wasobserved even at 600oC annealing for 100 min.

403020100-10-20-30-40
5.0
4.5Sample 1

Sample 2

Sample 3

Sample 4

Sample 1

Sample 2

Sample 3

Sample 4

4.0
3.5
3.0-3 -2 -1 0

123-3 -2 -1 0

12-3 -2 -1 0
3Voltage (V)123-3 -2 -1 0

12

Figure 2 PV hysteresis loops and CV characteristics of the samples.

50 010 101 102 103 104 105 106 107
Switching cycles

Figure 3 Switching cycle
endurance of the samples. Wakeup effects were observed for allthe samples. A better endurancecan be obtained with Y2O3
localized HfO2 films.

10-3

Sample 1

10-4

Sample 4

Sample 3

10-5
10-6
10-7
10-8
10-9

00.5
1.0
Applied voltage (V)1.5

Figure 4 JV curves of before
switchingindicatebetterinsulating properties with Y2O3
localized HfO2 film.

10-1

Sample 3
Sample 4

10100

Sample 2

Sample 2

1510-2

Leakage current (A/cm2)

20

10-1

Current density (A/cm2)

PSW (µC/cm2)

25102 103 104 105
Switching cycles

106

Sample 130

Measured at 1.5 V

10-2

10-3100

101

107

Figure 5 Leakage current of thesamples at each switching cycles
measured at 1.5 V. An early
degradation is observed foruniformly doped HfO2 films.

3