Journal of The Electrochemical Society, 155 共2兲 H92-H96 共2008兲
H92
0013-4651/2007/155共2兲/H92/5/$23.00 © The Electrochemical Society
Resistance Switching Behaviors of Hafnium Oxide Films
Grown by MOCVD for Nonvolatile Memory Applications
Seunghyup Lee, Wan-Gee Kim, Shi-Woo Rhee,* and Kijung Yong*,z
Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk
790-784, Korea
Resistive switching characteristics of hafnium oxide were studied for possible nonvolatile memory device applications. The HfO2
films were grown by metallorganic chemical vapor deposition 共MOCVD兲 at 400°C using tetrakis共diethylamido兲hafnium
关Hf共N共C2H5兲2兲4兴 as a precursor and oxygen gas as an oxidizing agent. The film was polycrystalline and had gradational compositions of Hf and O atoms. Current-voltage characteristics of the films were investigated with 1 mA compliance. A reproducible
resistance switching behavior was observed with high resistance ratio of about 104–109, which is higher than other comparable
materials, such as TiO2 and ZrO2. SET and RESET voltages were measured about 0.8 and 1.5 V, respectively, indicating that the
device can be operated below 2 V. The devices were operated in ohmic conduction mechanism. During the forming process,
characteristics of Schottky emission were observed, which indicates that conduction mechanisms between SET and forming
processes are different. The bipolar resistance switching behavior was also observed as well as unipolar resistance switching
behavior.
© 2007 The Electrochemical Society. 关DOI: 10.1149/1.2814153兴 All rights reserved.
Manuscript submitted September 7, 2007; revised manuscript received October 7, 2007.
Available electronically December 6, 2007.
Because the reversible resistance switching phenomena of chalcogenide thin films were reported,1 resistance switching behaviors
of various transition metal oxides such as Al2O3, TiO2, VO2, NiO,
Nb2O5, and Ta2O5 have been studied for the applications in nonvolatile memory devices.2-9 Because the resistance differences between low and high resistance states of these materials are large
enough to distinguish each state without total phase changes, these
materials attract great interest for possible applications in the resistive switching random access memory 共ReRAM兲. These ReRAM
devices have various advantages, such as simple device structure;
low-power, high-speed operation; high-density integration; and
compatibility of the current complementary metal-oxidesemiconductor 共CMOS兲 process.10 However, the mechanism of the
resistance switching behavior is not fully understood yet. Recently,
various conduction mechanisms are suggested, such as spacecharge-limited conduction, Schottky emission, ohmic conduction,
and Frenkel–Poole emission.11
Hafnium oxide has been studied as a promising candidate to
replace SiO2 in CMOS technology because of the high dielectric
constant 共k兲 and high resistance.12 Recently, because the oxides of
Ti and Zr, which are elements in the same group with Hf in the
periodic table, show good resistance switching behaviors, hafnium
oxide attracts great interest for a possible candidate as a nonvolatile
memory device material.13,14 Moreover, because the physical deposition methods have limitations in the fabrication of the nanoscale
memory devices, research on chemical deposition is necessary.
Thus, we have investigated the resistance switching behaviors of the
chemically deposited hafnium oxide films.
In this paper, HfO2 films were prepared by metallorganic chemical vapor deposition 共MOCVD兲 using tetrakis共diethylamido兲hafnium 关Hf共N共C2H5兲2兲4兴 as a precursor and oxygen gas as an
oxidant for possible applications in the nonvolatile memory devices.
To study the mechanism of the resistance switching behaviors, we
have analyzed the physical/chemical properties of HfO2 films and
also investigated current-voltage characteristics in various ways.
Experimental
Hafnium oxide thin films were directly deposited on a
Pt/Ti/SiO2 /Si substrate by MOCVD using TDEAH 兵tetrakis共diethylamido兲hafnium; Hf关N共C2H5兲2兴4其 and O2 as precursor and oxidant,
respectively, at a substrate temperature of 400°C. The thicknesses of
Pt, Ti, and SiO2 layers on a Si wafer were 150, 20, and 150 nm,
* Electrochemical Society Active Member.
z
E-mail: kyong@postech.ac.kr
respectively, as shown in Fig. 1a. Immediately prior to deposition,
wafers were precleaned with acetone and ethanol for 20 min, respectively, in an ultrasonic cleaner. The temperature 共vapor pressure兲 of the Hf共N共C2H5兲2兲4 bubbler was fixed at 80°C 共1 Torr兲.15
Argon 共99.9995%兲 was used as a buffer flow to keep flow uniform
and a carrier gas to transport source vapor into the reactor. The O2
flow rate and the flow rate of Ar carrier gas for Hf共N共C2H5兲2兲4 were
fixed at 10 and 15 sccm, respectively. The buffer flow rate was
200 sccm. The thicknesses of the HfO2 films were controlled by
deposition time. After HfO2 film deposition, Au top electrodes were
fabricated by evaporation using metal shadow mask. A schematic
diagram of the metal-insulator-metal device 共MIM兲 is shown in Fig.
1b.
The thicknesses of HfO2 films were measured using ellipsometry
共MX75/100T兲. The cross-sectional atomic structure of the films was
investigated using high-resolution transmission electron microscopy
共HR-TEM兲. The depth profile of films was analyzed by X-ray photoelectron spectroscopy 共XPS兲. Grazing incidence XRD with Cu K␣
radiation was employed to investigate the crystalline properties of
the films. The current-voltage 共I-V兲 characteristics of the devices
were studied using an E5270A.
Results and Discussion
Film properties.— Figure 2 shows the high-resolution transmission electron microscopy 共TEM兲 image of the HfO2 film deposited
on the Pt/Ti/SiO2 /Si substrate at 400°C, which indicates the film
has a polycrystalline structure with microcrystallites. The inset is an
XRD pattern of the HfO2 thin film deposited on a Si共100兲 substrate
at 400°C. We used Si substrate for XRD analysis because XRD
peaks may be hidden in the case of the Pt/Ti/SiO2 /Si substrate.16
The XRD peaks were analyzed using the joint committee for power
diffraction studies 共JCPDS兲 database, and the film was identified as
a polycrystalline HfO2. In polycrystalline films, because conduction
paths 共i.e., current flow channels兲 are formed easier rather than
single crystalline films, the resistive switching can be observed at
much lower voltages.17 Thus, the fabrication of the MIM device was
processed without postannealing of the polycrystalline HfO2.
In Fig. 3, the XPS Hf 4f and O 1s spectra at difference depth
from the results of depth profile of the HfO2 film are presented to
understand the chemical binding states in the film. Figures 3a-c
represent the XPS results of Hf 4f in surface, intermediate and interface region, respectively. In Fig. 3a, the binding energies of Hf 4f
were 18.4 and 20.1 eV Hf4+. Also a feature of lower binding energy
appeared with a weak intensity. This indicates that surface layer is
mainly composed of stoichiometric HfO2. Because the sampling
depth of XPS is deep, thus, a low binding energy feature seems to
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Journal of The Electrochemical Society, 155 共2兲 H92-H96 共2008兲
H93
Figure 2. High-resolution TEM image of HfO2 film in Au/HfO2 /Pt MIM
structure. The inset shows the XRD pattern of the HfO2 film grown on the Si
substrate at 400°C by MOCVD.
the surface layer has a nonconductive insulation region. However, as
the depth deepened, the oxygen peak intensity decreased rapidly and
the film got nonstoichiometric with oxygen deficiencies. Finally, in
the interface layer, the oxygen peak disappeared and the film was
reduced to Hf0, which can be attributed to Hf-metal layer. This
implies that at the interface region conductive Hf-metal layer is
formed and in the transition region the HfOx and Hf-metal layer
coexist with oxygen deficiencies.
Figure 1. 共a兲 Cross-sectional TEM image of the HfO2 film deposited on
Pt/Ti/SiO2 /Si substrate. 共b兲 Schematic diagram of the HfO2 resistance
switching memory device with Au and Pt electrodes.
originate from sublayers. At the interface region the binding energies were shifted to 14.6 and 16.2 eV 共Hf0兲 as shown in Fig. 3c.
Figure 3b shows that Hf 4f peaks are clearly deconvoluted into two
binding energies, indicating an intermediate state between surface
and interface region. This region can be defined as a transition layer.
Figure 3d represents XPS spectra of O 1s at different depth. It
showed that O 1s peak intensity decreased with increasing depth.
Moreover, the peak intensity fast decreased especially at the interface region. These results suggest that the interface layer has oxygen
deficient Hf-metallic states. From our XPS analysis, we believe that
our grown HfO2 film has a three-layered structure: surface, transition, and interface layer. Because the film is stoichiometric HfO2,
Unipolar resistive switching.— Figure 4a shows typical currentvoltage 共I-V兲 characteristics of a 12 nm HfO2 MIM device with Au
top electrode and Pt bottom electrode. If the current through a film is
too high, then hard-breakdown occurs in the film. Therefore, a
proper current compliance is necessary. Because the SET and RESET voltages increased with higher current compliance due to the
increase of the conduction paths,18,5 we set 1 mA of current compliance for low-voltage operations. Initially, the resistance switching
characteristics were not observed with high resistance 共⬃1011 ⍀ at
1 V兲 in the voltage region below 2 V; however, the leakage current
was suddenly increased and the film became the low resistance state
共LRS兲 with the bias of ⬃5 V 共forming process兲. The inset of Fig. 4
shows the forming process of 12 nm HfO2 MIM device with 1 mA
of current compliance. After the forming process, typical bistable
resistance switching behavior was observed in the low-voltage region below 2 V. A bias sweeping was applied to the LRS film from
0 to 1 V, and the resistance was measured ⬃102 ⍀ at 0.5 V. At a
certain voltage of ⬃0.8 V, which is referred to as the RESET voltage, the leakage current had fallen drastically and the film was in the
high-resistance state 共HRS兲. With another bias sweeping to the HRS
film from 0 to 2.5 V, the resistance was measured ⬃107 ⍀ at 0.5 V.
During the sweeping, leakage current suddenly increased and the
HRS film reverted to an LRS film with a bias around 1.8–2.2 V,
which refers to SET voltage. The resistance ratio between HRS and
LRS of 12 nm HfO2 film with Au and Pt electrode at 0.5 V was
about 104–109, and this value is relatively high compared to other
materials, such as ZrO2 and TiO2.10,3 This high-resistance ratio is
due to the high resistance of the HRS HfO2 film.
In order to understand the mechanism of the resistance switching
behaviors, log I–log V curve is presented in Fig. 4b. As reported by
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Journal of The Electrochemical Society, 155 共2兲 H92-H96 共2008兲
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Figure 3. XPS spectra of Hf 4f in 共a兲 surface region, 共b兲 intermediate region, and 共c兲 interface region in HfO2 film deposited on the Pt/Ti/SiO2 /Si substrate.
共d兲 XPS spectra of O 1s peak at different depth of the HfO2 film.
Lee et al.,13 the curve is ohmic at LRS, showing a linearity, which
indicates that the current flows through conducting paths. When the
film became HRS, the curve was still linear but showed some fluctuations in the high-voltage region. It is believed that some of conducting paths, which are formed at LRS, remain after the paths are
ruptured during the RESET process. Therefore, the HRS curve
shows ohmic conduction due to the left conduction paths even
though some fluctuations are observed in the high-voltage region.
Moreover, the forming process showed a different mechanism
from the SET process. Figures 4c and d show the log I-V1/2 curves
of the forming process and the typical SET process, respectively. In
case of the forming process, in the high-voltage region, the curve
was linear, which indicated the current flows in Schottky emission
mechanism expressed by the following equation,19 described as
冉
I = AT2 exp
−q共B − 冑qE/4i兲
kT
冊
关1兴
where A is the Richardson constant; T, the absolute temperature; q,
the electronic charge; B, the potential barrier at the metal/dielectric
interface; E, the electric field in the dielectric; i, the dielectric constant; and k, the Boltzmann constant. However, in the case of the
SET process, Fig. 4 showed a nonlinear curve. Thus, it is thought
that the first soft breakdown 共forming process兲 occurs in Schottky
emission mechanism, and once the film is formed, the other soft
breakdowns 共SET process兲 occur in different mechanisms.
We suggest that the mechanism, regarding how the conduction
paths are formed and ruptured in HfO2 film of MIM device and why
the forming process and SET process present different behaviors, is
related to the structure of the films. As the XPS results showed in
Fig. 3, the film is composed of three layers. Once the high voltage is
applied to a fresh film, the oxygen atoms in Hf-O transition layer
can be diffused out. Because of the oxygen deficiencies in the transition layer, conduction paths are easily formed from the defects and
then conductivity of the layer becomes increased. With higher applied voltage, the electrons passing through the insulating layer are
increased by Schottky effect. Eventually, the insulating layer breaks
down and the whole film becomes LRS 共forming process兲. With
other voltage sweeps without current compliance, a high current
passes the conduction path and at a certain voltage, and due to the
heat generated by the high current, the conduction paths in transition
layer are ruptured and the film becomes HRS while some of conduction paths remain and insulating layer stays broken down 共reset
process兲.3,13 When another voltage is applied with current compliance, conduction paths in the transition layer form again and the film
becomes LRS by oxygen diffusion and remained conduction paths
共set process兲. Because the insulating layer is already broken down,
conduction is not affected by Schottky emission during the set process.
Bipolar resistive switching.— Figure 5 shows a typical currentvoltage curve of the bipolar resistance switching characteristics of
the HfO2 MIM device after forming process. HfO2 film showed
bipolar resistance switching characteristics as well as unipolar characteristics, such as TiO2 and ZrO2 film.20,21 However, it showed
rather unstable behaviors compared to other films. Moreover, the
resistance ratio between HRS and LRS was relatively low. We believe that this is due to the high resistance of HfO2. Even the thickness of the film is much thinner 共12 nm兲, the order of resistance at
HRS is much higher 共⬃108 ⍀ at 0.5 V兲 compared to the films of
other ReRAM research. This is believed to be due to higher bandgap
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Journal of The Electrochemical Society, 155 共2兲 H92-H96 共2008兲
H95
Figure 4. Typical I-V characteristics of Au/HfO2 /Pt device at room temperature on 共a兲 log-linear and 共b兲 log-log scale. The inset of 共a兲 shows the I-V
characteristics of the forming process. Log I-V1/2 plot of 共c兲 forming process, and 共d兲 SET process.
of HfO2 共5.6 eV兲 than TiO2 共3.2 eV兲 and ZrO2 共4.8 eV兲.22-24 In
Fig. 5, bias was applied to an electroformed high-resistance state
HfO2 film from 2.8 to −2.1 V. When the voltage was ⬃1.3 V, the
film turned to LRS with current jump from 10−6 to 10−5 A, while
the film turned back to HRS with current drop when the voltage was
around −1.3 V. When another bias was applied to the device from
−2.1 to 2.8 V again, the film stayed at HRS. This result shows that
the bipolar and unipolar resistance switching characteristics coexist
in HfO2 film with Au and Pt electrode.
Conclusion
We investigated the resistance switching behaviors of HfO2 film
with Au and Pt electrode for possible applications in nonvolatile
memory devices. The films showed good resistance switching characteristics with high-resistance ratios about 104–109 under 2 V. We
believe the conduction mechanisms in our MIM device are dominantly ohmic conduction with conduction paths. Especially, in the
high voltage region at high-resistance state 共HRS兲 film, the characteristics were quite unstable, showing some fluctuations. Moreover,
the forming process showed Schottky emission, which was different
conduction mechanism from the SET process. The bipolar resistance
switching characteristics were observed as well as unipolar resistance switching behavior.
Acknowledgments
Figure 5. Bipolar resistance switching characteristics of the 12 nm HfO2
film in Au/HfO2 /Pt device at room temperature.
This work was supported by grant no. KRF-2006-521-D00126
from the Korea Research Foundation and the Korean Research
Foundation Grant funded by the Korean Government 共MOEHRD兲
共KRF-2005-005-J13101兲, and grant no. RTI04-01-04 from the Re-
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H96
Journal of The Electrochemical Society, 155 共2兲 H92-H96 共2008兲
gional Technology Innovation Program of the Ministry of Commerce, Industry and Energy 共MOCIE兲.
Pohang University of Science and Technology assisted in meeting the
publication costs of this article.
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