Conduction Mechanism and Improved Endurance in HfO2-Based RRAM with Nitridation Treatment
Yuan et al. Nanoscale Research Letters
Conduction Mechanism and Improved Endurance in HfO -Based RRAM with 2 Nitridation Treatment
Fang-Yuan Yuan 0
Ning Deng 0 3
Chih-Cheng Shih 1 2
Yi-Ting Tseng 1
Ting-Chang Chang 1 6
Kuan-Chang Chang 5
Ming-Hui Wang 2
Wen-Chung Chen 2
Hao-Xuan Zheng 1
Huaqiang Wu 0 3
He Qian 0 3
Simon M. Sze 4
0 Institute of Microelectronics, Tsinghua University , Beijing 100084 , China
1 Department of Physics, National Sun Yat-Sen University , Kaohsiung 80424 , Taiwan
2 Department of Materials and Optoelectronic Science, National Sun Yat-Sen University , Kaohsiung 80424 , Taiwan
3 Tsinghua National Laboratory for Information Science and Technology (TNList) , Beijing 100084 , China
4 Department of Electronics Engineering, National Chiao Tung University , Hsinchu 300 , Taiwan
5 School of Electronic and Computer Engineering, Peking University , Shenzhen 518055 , China
6 Advanced Optoelectronics Technology Center, National Cheng Kung University , Tainan 70101 , Taiwan
A nitridation treatment technology with a urea/ammonia complex nitrogen source improved resistive switching property in HfO2-based resistive random access memory (RRAM). The nitridation treatment produced a high performance and reliable device which results in superior endurance (more than 109 cycles) and a self-compliance effect. Thus, the current conduction mechanism changed due to defect passivation by nitrogen atoms in the HfO2 thin film. At a high resistance state (HRS), it transferred to Schottky emission from Poole-Frenkel in HfO2-based RRAM. At low resistance state (LRS), the current conduction mechanism was space charge limited current (SCLC) after the nitridation treatment, which suggests that the nitrogen atoms form Hf-N-Ox vacancy clusters (Vo+) which limit electron movement through the switching layer.
HfO2-based RRAM; Nitridation; Endurance; Space charge limit current
Recently, resistance random access memory (RRAM)
composed of an insulating layer sandwiched by two
electrodes has been widely studied as a promising candidate
for next-generation nonvolatile memory due to its
superior properties such as simple structure, low power
consumption, high-speed operation (< 300 ps), and
nondestructive readout [
]. Although most RRAM
devices have many properties superior to nonvolatile
memory, the high operation current of RRAM and
performance degradation are major issues in nonvolatile
memory in terms of the application of portable
The Pt/HfO2/TiN structure can supply a conduction
path which induces a resistive switching behavior [
]. However, the defects of amorphous HfO2 will
increase the number of leakage paths, leading to power
consumption and joule heating degradation. In this
work, the resistive switching layer of HfO2 was treated
by a solution with a urea/ammonia complex nitrogen
source as the nitridation treatment to enhance its
electrical switching properties.
The patterned TiN/Ti/SiO2/Si substrate was fabricated
with a standard deposition and etching process, after
which via holes can be formed (inset of Fig. 1a). Then, a
23-nm-thick HfO2 thin film was deposited into via holes
on the substrate by RF magnetron sputtering using a
pure HfO2 target. The sputtering power was fixed at RF
power of 150 W and was carried out in argon ambient
(Ar = 30 sccm) with a working pressure of 4 mtorr at
room temperature. The HfO2/TiN semi-finished device
was put into the reactive chamber and immersed into
the solution with a urea/ammonia complex nitrogen
source for nitridation treatment. During the nitridation
treatment, the solution was heated to 160 °C in the
system’s stainless steel chamber for 30 min. Then, the
110-nm-thick Pt top electrode was deposited by DC
magnetron sputtering on the HfO2 thin film to form
electrical devices with Pt/HfO2/TiN sandwich structures.
Finally, all of the electric characteristics were measured
by the Agilent B1500 semiconductor parameter analyzer.
The DC and pulse sweeping bias were applied to the
bottom electrode (TiN) while the top electrode (Pt) was
grounded during the electrical measurements. In
addition, Fourier-transform infrared spectroscopy (FTIR)
was measured by a Bruker VERTEX 70v spectrometer in
the middle infrared region.
Results and Discussion
An electroforming process is required to activate all of
the RRAM devices using a DC bias with a compliance
current of 10 μA, as shown in Fig. 1a. After the forming
process, the electrical current-voltage (I-V) properties of
the HfO2-based RRAM were compared at initial and
after the nitridation treatment. At LRS, the current was
obviously reduced compared to that of untreated HfO2
thin film, as shown in Fig. 1b. The current reduction can
be attributed to the defects passivated by the NH3
molecule in the treatment solution. We found that HRS
distribution is much more stable after the nitridation
treatment, as in the inset of Fig. 1b. The resistance states
are extracted with a reading voltage of 0.1 V during the
100 sweep cycles with DC operation (inset of Fig. 1b).
The resistance on/off ratio was slightly reduced after the
nitridation treatment. Interestingly, a self-compliance
resistive switching property was observed in these
HfO2based RRAM devices after the treatment, as shown in
Fig. 1c. After more than 103 sweep cycles, a repeatable
self-protective characteristic of the device without hard
breakdown was observed. The retention time was
evaluated at 85 °C and remained stable even after 104 s both
in HRS and LRS.
To further evaluate device performance, the endurance
tests of HfO2-based RRAM were performed for initial
and after the nitridation treatment, as shown in Fig. 2.
In the untreated device after 106 sweeping cycles, the
HRS/LRS ratio significantly degrades from 100:1 to 5:1,
as shown in Fig. 2a. After the nitridation treatment,
however, even after more than 109 sweep cycles, the
device exhibited a stable HRS/LRS ratio, as in Fig. 2b.
These results indicate that the nitridation process
enhanced HfO2-based RRAM to perform with superior
switching features and reliability. To further investigate
these results, FTIR analysis was used to observe the
chemical alterations of the HfO2 thin film, as shown in
Fig. 3. A sharp peak at 1589 and 1311 cm−1 appeared
after the nitridation treatment, corresponding to the
symmetrical and asymmetrical stretching vibration peak
of an N–O bond [
]. Further, the peak intensity of N–
H bonds at 1471 cm−1 [
] increased due to the
nitridation process by urea/ammonia complex nitrogen source
(inset of Fig. 3). Therefore, we can infer the formation of
nitrogen-containing compounds after the nitridation
In order to clarify the resistive switching mechanism,
we analyzed the current conduction mechanism of the
HfO2 thin film with and without the nitridation
treatment, shown in Fig. 4a and d. For the untreated HfO2
thin film, the electrons were transferred through the
defects, such that the current conduction mechanism was
dominated by Poole-Frenkel conduction according to
the linear relationship between ln(I/V) and the square
root of the applied voltage (V1/2) on HRS, as shown in
Fig. 4b [
In contrast, HfO2-based RRAM exhibited the
Schottky emission mechanism according to the linear
relationship between ln(I/T2) and the square root of
the applied voltage (V1/2) of HRS, as shown in Fig. 4c
]. This is due to the decrease in defects and
dangling bonds, as bonds become passivated by
nitrogen atoms after the nitridation treatment. In addition,
we also analyzed the current conduction mechanism
with and without treatment at LRS in HfO2-based
RRAM. On LRS, the carrier transport mechanism of
the untreated HfO2-based RRAM was dominated by
ohmic conduction, where current decreases with
increasing temperature, as shown in Fig. 4e. After
nitridation treatment, the current conduction mechanism
transfers to space charge limited current (SCLC) with
a slope of 1.52. The I-V curve is not relative to
temperature, with a linear relationship between ln(I)
and the square of the applied voltage V2 on LRS, as
shown in Fig. 4f .
We proposed a model to explain the characteristics of
the current conduction mechanism, and it is shown as
Fig. 5. Thus, there are two offsetting dipoles associated
with N and O atoms and a Hf atom (i.e., the sequence
O–Hf–O is replaced by O–Hf–N–O) after doping N
atoms into HfO2 thin film. Because nitrogen electron
negativity is lower than oxygen, the dipole of Hf–N bond
is lower than the Hf–O bond, which creates a lower
dielectric constant region. When a positive bias is applied
during the SET process, a series of Hf–N–Ox vacancies
are formed due to their lower dielectric constant, then
forming vacancy clusters (Vo+). The conductive path
typically forms along with the Hf–N–Ox vacancy
clusters (Vo+) as nitrogen atoms capture oxygen ions around
the clusters, as shown in Fig. 5b. The presence of these
insulating Hf–N–Ox vacancy clusters (Vo+) results in
current reduction and the self-compliance effect found
in HfO2-based RRAM.
In summary, a self-compliance resistive switching
property was observed in a Pt/HfO2/TiN RRAM device after
the nitridation treatment. Endurance times reached 109
cycles and a retention time of more than 104 s was
achieved at 85 °C. Due to the smaller electron negativity
of the nitrogen atom when compared to the oxygen
atom, the dipole of the Hf–N bond is smaller than that
of the Hf–O bond, which creates a lower dielectric
constant region. During the SET process, the Hf–N–Ox
vacancy clusters (Vo+) form the conductive path. The
insulating Hf–N–Ox vacancy clusters (Vo+) protect the
device from hard breakdown and perform a
FTIR: Fourier-transform infrared spectroscopy; HRS: High resistance state;
LRS: Low resistance state; RRAM: Resistive random access memory;
SCLC: Space charge limited current
This work was performed at National Science Council Core Facilities
Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung
area and supported by the Ministry of Science and Technology of the
Republic of China under Contract No. MOST-106-2112-M-110-008-MY3 and
Availability of data and materials
All data are fully available without restriction.
FYY, YTT, and WCC carried out the sample preparation and the
measurements. CCS, MHW, and HXZ participated in the discussion. ND, TCC,
KCC, HW, HQ, and SMS supervised the project. All the authors have read and
approved the final manuscript.
Fang-Yuan Yuan is a doctor of the Institute of Microelectronics, Beijing,
Tsinghua University. Ning Deng and Huaqiang Wu are professors of the
Institute of Microelectronics, Beijing, Tsinghua University and Tsinghua
National Laboratory for Information Science and Technology (TNList). He
Qian is a professor of the Institute of Microelectronics, Beijing, Tsinghua
University and head of Tsinghua National Laboratory for Information Science
and Technology (TNList). Chih-Cheng Shih and Wen-Chung Chen are doctors
of the Department of Materials and Optoelectronic Science, Kaohsiung,
National Sun Yat-Sen University. Yi-Ting Tseng is a doctor of the Department
of Physics, Kaohsiung, National Sun Yat-Sen University. Ting-Chang Chang is
a professor of the Department of Physics, Kaohsiung, National Sun Yat-Sen
University and Advanced Optoelectronics Technology Center, Taiwan,
National Cheng Kung University. Kuan-Chang Chang is an assistant professor
of the School of Electronic and Computer Engineering, Shenzhen, Peking
University. Simon M. Sze is professor of the Department of Electronics
Engineering, Hsinchu, National Chiao Tung University.
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The authors declare that they have no competing interests.
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