326
V. Y. Zhuo et al.
materials is TiO 2 . Kwon et al. reported in situ studies on TiO 2 RRAM devices where
the CF was made of a conductive Ti-O phase, termed Magnéli phase, with a Ti n O 2n−1
stoichiometry [59]. Similar oxygen-deficient CF with Magnéli phase was reported
in WO 3 based RRAM systems by Tan et al. [75]. However, these observations rarely
yielded any valid switching dynamics in bipolar OxRRAM systems that do not have
stable intermediate phases such as HfO 2 and Ta 2 O 5 . This could be due to sample
surface contaminants and the reliance of TEM observations on just the ion mass
attribute. To circumvent this, a new technique was developed by Yang et al. in 2017
to characterize the ion transport and CF growth dynamics in bipolar TaO x and HfO x
based OxRRAM using electrostatic force microscopy (EFM) [76]. Specifically, EFM
is sensitive to the ion charge accumulation, has sub-10 nm spatial resolution with no
strict requirements on sample thickness nor vacuum level [77, 78]. By merging EFM
with systematic atomic force (AFM) and conductive atomic force microscopy (CAFM) characterizations [79], this approach is able to provide a clearer representation
of the dynamic oxygen ion transport during the resistive switching of OxRRAM. As
depicted in Fig. 5, this approach is able to detect the migration and accumulation of
oxygen ions to the interface and the following redox reactions as well as oxygen gas
formation, which led to oxygen-deficient CF and structural distortions in the memory
film. The formation of these CF in HfO 2 was directly identified using sphericalaberration (C s )-corrected TEM and reversible ion migration was proposed to be
responsible for the bipolar switching in HfO 2 [76].
Although bipolar switching in OxRRAM tends to be due to ionic motion and
electrochemical reactions caused by electric field effects, thermal effects still play a
crucial role during the RESET process [80, 81]. Panda et al. combined two models,
an analytical temperature model [82] and a filament dissolution model [83, 84] and
found that the temperature varies in a parabolic path within the CF, and is highest at
the middle of the CF and lowest at the electrodes [85], which is in agreement with
hypothesis that Joule heating assists the RESET mechanism in OxRRAM devices.
Material Selection Considerations
Since Joule heating is inevitable in the OxRRAM, it has a major influence on the
material selection. In order for the OxRRAM to have stable and reliable resistive
switching, both insulating and conductive phases are necessary. These two phases
should not chemically react with each other to form a new phase, even at elevated
temperatures caused by Joule heating. It should be a simple system with only two
thermodynamically stable solid-state phases, one insulating phase and one relatively
conductive phase for the conduction channel. Ideally, the conduction channel should
have a high oxygen solubility to serve as a reliable oxygen reservoir to conserve the
oxygen ions during repeated switching cycles [26]. TaO x and HfO x are the prime
candidates that fulfil these requirements, exhibiting switching endurance over 10
12
[86] and 10
10 cycles [87], respectively.
V. Y. Zhuo et al.
materials is TiO 2 . Kwon et al. reported in situ studies on TiO 2 RRAM devices where
the CF was made of a conductive Ti-O phase, termed Magnéli phase, with a Ti n O 2n−1
stoichiometry [59]. Similar oxygen-deficient CF with Magnéli phase was reported
in WO 3 based RRAM systems by Tan et al. [75]. However, these observations rarely
yielded any valid switching dynamics in bipolar OxRRAM systems that do not have
stable intermediate phases such as HfO 2 and Ta 2 O 5 . This could be due to sample
surface contaminants and the reliance of TEM observations on just the ion mass
attribute. To circumvent this, a new technique was developed by Yang et al. in 2017
to characterize the ion transport and CF growth dynamics in bipolar TaO x and HfO x
based OxRRAM using electrostatic force microscopy (EFM) [76]. Specifically, EFM
is sensitive to the ion charge accumulation, has sub-10 nm spatial resolution with no
strict requirements on sample thickness nor vacuum level [77, 78]. By merging EFM
with systematic atomic force (AFM) and conductive atomic force microscopy (CAFM) characterizations [79], this approach is able to provide a clearer representation
of the dynamic oxygen ion transport during the resistive switching of OxRRAM. As
depicted in Fig. 5, this approach is able to detect the migration and accumulation of
oxygen ions to the interface and the following redox reactions as well as oxygen gas
formation, which led to oxygen-deficient CF and structural distortions in the memory
film. The formation of these CF in HfO 2 was directly identified using sphericalaberration (C s )-corrected TEM and reversible ion migration was proposed to be
responsible for the bipolar switching in HfO 2 [76].
Although bipolar switching in OxRRAM tends to be due to ionic motion and
electrochemical reactions caused by electric field effects, thermal effects still play a
crucial role during the RESET process [80, 81]. Panda et al. combined two models,
an analytical temperature model [82] and a filament dissolution model [83, 84] and
found that the temperature varies in a parabolic path within the CF, and is highest at
the middle of the CF and lowest at the electrodes [85], which is in agreement with
hypothesis that Joule heating assists the RESET mechanism in OxRRAM devices.
Material Selection Considerations
Since Joule heating is inevitable in the OxRRAM, it has a major influence on the
material selection. In order for the OxRRAM to have stable and reliable resistive
switching, both insulating and conductive phases are necessary. These two phases
should not chemically react with each other to form a new phase, even at elevated
temperatures caused by Joule heating. It should be a simple system with only two
thermodynamically stable solid-state phases, one insulating phase and one relatively
conductive phase for the conduction channel. Ideally, the conduction channel should
have a high oxygen solubility to serve as a reliable oxygen reservoir to conserve the
oxygen ions during repeated switching cycles [26]. TaO x and HfO x are the prime
candidates that fulfil these requirements, exhibiting switching endurance over 10
12
[86] and 10
10 cycles [87], respectively.