Xi Zou

Non-volatile memory based on the ferroelectric photovoltaic effect

The quest for a solid state universal memory with high-storage density, high read/write speed, random access and non-volatility has triggered intense research into new materials and novel device architectures. Though the non-volatile memory market is dominated by flash memory now, it has very low operation speed with ~10 μs programming and ~10 ms erasing time. Furthermore, it can only withstand ~105 rewriting cycles, which prevents it from becoming the universal memory. Here we demonstrate that the significant photovoltaic effect of a ferroelectric material, such as BiFeO3 with a band gap in the visible range, can be used to sense the polarization direction non-destructively in a ferroelectric memory. A prototype 16-cell memory based on the cross-bar architecture has been prepared and tested, demonstrating the feasibility of this technique.

Chemoselective Photodeoxidization of Graphene Oxide Using Sterically Hindered Amines as Catalyst: Synthesis and Applications

We report a green and efficient method for chemoselective deoxidization of graphene oxide via the ultraviolet irradiation catalyzed with 2,2,6,6-tetramethyl-4-piperidinol. While the sp(2)-hybridized oxygen functional groups are removed after the reduction, the epoxy and hydroxyl groups are retained in the chemoselectively reduced graphene oxide (CrGO). The obtained CrGO nanosheets exhibit the high solubility and excellent electronic stability, which allows for the fabrication of thin film devices through a solution processing. As a proof of concept, a CrGO-based write-once-read-many-times memory device with the desirable stability and long-time operation is fabricated.

Characterization and manipulation of mixed phase nanodomains in highly strained BiFeO3 thin films.

The novel strain-driven morphotropic phase boundary (MPB) in highly strained BiFeO(3) thin films is characterized by well-ordered mixed phase nanodomains (MPNs). Through scanning probe microscopy and synchrotron X-ray diffraction, eight structural variants of the MPNs are identified. Detailed polarization configurations within the MPNs are resolved using angular-dependent piezoelectric force microscopy. Guided by the obtained results, deterministic manipulation of the MPNs has been demonstrated by controlling the motion of the local probe. These findings are important for an in-depth understanding of the ultrahigh electromechanical response arising from phase transformation between competing phases, enabling future explorations on the electronic structure, magnetoelectricity, and other functionalities in this new MPB system.

Mechanism of polarization fatigue in BiFeO3.

Fatigue in ferroelectric oxides has been a long lasting research topic since the development of ferroelectric memory in the late 1980s. Over the years, different models have been proposed to explain the fatigue phenomena. However, there is still debate on the roles of oxygen vacancies and injected charges. The main difficulty in the study of fatigue in ferroelectric films is that the conventional vertical sandwich structure prevents direct observation of the microscopic evolution through the film thickness during the electric field cycling. To circumvent this problem, we take advantage of the large in-plane polarization of BiFeO(3) and conduct direct domain and local electrical characterizations using a planar device structure. The combination of piezoresponse force microscopy and scanning kelvin probe microscopy allows us to study the local polarization and space charges simultaneously. It is observed that charged domain walls are formed during the electrical cycling, but they do not cause polarization fatigue. After prolonged cycling, injected charges appear at the electrode/film interfaces, where domains are pinned. When the pinned domains grow across the channel, macroscopic fatigue appears. The role of injected charges in polarization fatigue of BiFeO(3) is clearly demonstrated.

Study of strain effect on in-plane polarization in epitaxial BiFeO3 thin films using planar electrodes

Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films using traditional sandwich capacitor devices, because of high leakage current. We employed a planar electrode device with different crystallographical orientations between electrodes along different electric field orientation to directly measure the in-plane polarization-electric field (P-E) hysteresis loops in fully strained thin films. At high misfit strains such as -4.4%, the pure Tetrogonal-like phase is obtained and its polarization vector is constrained to lie in the (010) plane with a significantly large in-plane component, ~44 {\mu}C/cm2. First-principle calculations are carried out in parallel, and provide a good agreement with the experimental results. Our results pave the way to design in-plane devices based on T-like BFO and the strategy proposed here can be expanded to study all other similar strained multiferroic ultrathin films.

Charge trapping-detrapping induced resistive switching in Ba0.7Sr0.3TiO3

Intensive research has been devoted to the resistive switching phenomena observed in many transitional metal oxides because of its potential for non-volatile memory application. To clarify the underlying mechanism of resistive switching, a planar device can provide information that is not accessible in conventional vertical sandwich structures. Here we report the observation of resistive switching behavior in a Pt/Ba0.7Sr0.3TiO3/Pt planar device. Using in-situ scanning Kelvin probe microscopy, we demonstrate that charge trapping/detrapping around the Pt/Ba0.7Sr0.3TiO3 interface modulates the Schottky barrier, resulting in the observed resistive switching. The findings are valuable for the understanding of resistive switching in oxide materials.

Domain structure and in-plane switching in a highly strained Bi0.9Sm0.1FeO3 film

We report the domain structure and ferroelectric properties of a 32 nm-thick Bi0.9Sm0.1FeO3 film epitaxially grown on a LaAlO3 (LAO) substrate. This film exhibits a monoclinic Mc phase, with its monoclinic distortion and anisotropy of in-plane (IP) lattice parameters being both smaller than those of pure BiFeO3 (BFO) grown on LaAlO3. Polarization hysteresis loops measured using a quasi-planar capacitor show an in-plane polarization up to 30 μC/cm2. Piezoresponse force microcopy demonstrates that a 180° in-plane polarization switching accompanied by a 90° domain wall rotation takes place after electric poling. First-principles calculations suggest the differences between highly strained Sm-substituted and pure BiFeO3.

Mechanism of polarization fatigue in BiFeO3: The role of Schottky barrier

By using piezoelectric force microscopy and scanning Kelvin probe microscopy, we have investigated the domain evolution and space charge distribution in planar BiFeO3 capacitors with different electrodes. It is observed that charge injection at the film/electrode interface leads to domain pinning and polarization fatigue in BiFeO3. Furthermore, the Schottky barrier at the interface is crucial for the charge injection/accumulation process. Lowering the Schottky barrier by using low work function metals as the electrodes can also improve the fatigue property of the device, similar to what oxide electrodes can achieve.

Domain tuning in mixed-phase BiFeO3 thin films using vicinal substrates

The structural and ferroelectric domain variants of highly strained BiFeO3 films grown on vicinal LaSrAlO4 substrates were studied by piezoelectric force microscopy and high-resolution x-ray reciprocal space mapping. Through symmetry breaking of the substrate surface, ferroelastic domain variants in the highly strained MC phase BiFeO3 can be greatly reduced in thinner, purely tetragonal-like films. More interestingly, in thicker, mixed phase films, the structural variants can also be tailored by substrate vicinality. These findings lead to better understanding of the phase evolution and polarization rotation process in the strain-driven polymorphic phase system.

Temperature controlled c axis elongated low symmetry phase BiFeO3 thin film on STO substrate

BiFeO3 thin films with a mixture of tunable R-like and c axis elongated low symmetry phase (T-like phase) are fabricated on STO (001) substrate through controlling of the substrate temperature. Almost pure T-like phase can be grown on STO substrate at 600°C. Comparing with the situations on LAO (001), it is found that, strains from the LAO substrate may be the only reason that induces the T-like phase at higher temperatures. At lower temperatures, the island growth induced strains alone can also generate T-like phase on STO substrate.