Aidong Li

Ferroelectric-field-effect-enhanced electroresistance in metal/ferroelectric/semiconductor tunnel junctions

We propose a tunneling heterostructure by replacing one of the metal electrodes in a metal/ferroelectric/metal ferroelectric tunnel junction with a heavily doped semiconductor. In this metal/ferroelectric/semiconductor tunnel diode, both the height and the width of the tunneling barrier can be electrically modulated due to the ferroelectric field effect, leading to a colossal tunneling electroresistance. This idea is implemented in Pt/BaTiO3/Nb:SrTiO3 heterostructures, in which an ON/OFF conductance ratio above 10

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors

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Ferroelectric properties of Bi3.25La0.75Ti3O12 thin films prepared by chemical solution deposition

can be readily achieved at room temperature. The colossal tunneling electroresistance, reliable switching reproducibility and long data retention observed in these ferroelectric tunnel diodes suggest their great potential in non-destructive readout nonvolatile memories.Ferroelectric tunnel junctions (FTJs), composed of two metal electrodes separated by an ultrathin ferroelectric barrier, have attracted much attention as promising candidates for non-volatile resistive memories. Theoretical and experimental works have revealed that the tunnelling resistance switching in FTJs originates mainly from a ferroelectric modulation on the barrier height. However, in these devices, modulation on the barrier width is very limited, although the tunnelling transmittance depends on it exponentially as well. Here we propose a novel tunnelling heterostructure by replacing one of the metal electrodes in a normal FTJ with a heavily doped semiconductor. In these metal/ferroelectric/semiconductor FTJs, not only the height but also the width of the barrier can be electrically modulated as a result of a ferroelectric field effect, leading to a greatly enhanced tunnelling electroresistance. This idea is implemented in Pt/BaTiO3/Nb:SrTiO3 heterostructures, in which an ON/OFF conductance ratio above 10(4), about one to two orders greater than those reported in normal FTJs, can be achieved at room temperature. The giant tunnelling electroresistance, reliable switching reproducibility and long data retention observed in these metal/ferroelectric/semiconductor FTJs suggest their great potential in non-destructive readout non-volatile memories.

Processing- and composition-dependent characteristics of chemical solution deposited Bi 4−x La x Ti 3 O 12 thin films

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

Characteristics of LaAlO3 gate dielectrics on Si grown by metalorganic chemical vapor deposition

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were prepared on platinum coated silicon substrate by chemical solution deposition. The layered-perovskite phase was obtained by rapid thermal annealing the spin-on films at 650 or 700 °C for 180 s. Scanning electron micrographs showed uniform surfaces composed of spherical grains. The grain size increased with increasing annealing temperature. The remanent polarization and coercive field of 650 °C annealed film were 12.3 μC/cm2 and 48.9 kV/cm, respectively, and those of 700 °C annealed films were 18.2 μC/cm2 and 51.1 kV/cm. BLT thin films showed little polarization fatigue under 250 kV/cm bipolar cycling at 50 kHz, while fatigue properties deteriorated with decreasing cycling field and frequency. At various frequencies from 1 Hz to 50 kHz, nonvolatile polarization Pnv showed nearly no degradation over an initial period of cycling, then decayed logarithmically with switching cycles. The onset of logarithmic decay of Pnv was found to increase linearly with ...

Preparation of perovskite conductive LaNiO3 films by metalorganic decomposition

Thin films of lanthanum substituted bismuth titanate, Bi 4−x La x Ti 3 O 12 (BLTx), were prepared by chemical solution deposition. Crystallized BLTx films were obtained by rapid thermal annealing at a temperature as low as 650 °C. Structural and electrical characteristics of crystalline BLTx films were studied as functions of La composition. Structure characterization was conducted by x-ray diffraction and Raman spectroscopy. The lowest lattice vibration mode around 116 cm −1 showed softening with increasing lanthanum composition. Surface morphology of BLTx films were recorded by scanning electron microscopy. BLTx films have saturated hysteresis loops with remnant polarization of 9.7, 12.3, and 4.5 μC/cm 2 , respectively, for x = 0.50, 0.75, and 1.00 films. BLT0.75 films showed fatigue-free behavior over 250 kV/cm, 50 kHz cycling, which could be compared with that of SrBi 2 Ta 2 O 9 thin films. Fatigue resistance decrease at lower cycling field. The field dependence of fatigue property was discussed briefly in terms of competition between domain pinning and field-assisted unpinning.

Fabrication and electrical properties of sol-gel derived BaTiO3 films with metallic LaNiO3 electrode

Amorphous LaAlO3 (LAO) gate dielectric thin films have been deposited on Si substrates using La(dpm)3 and Al(acac)3 sources by low-pressure metalorganic chemical vapor deposition. The growth mechanism, interfacial structure, and electrical properties have been investigated by various techniques. The ultrathin films show smaller roughness of ∼0.3 nm, larger band gap of 6.47 eV, and good thermal stability. The growth follows a chemical dynamic control mechanism. High-resolution transmission electron microscopy confirms there exists no interfacial layer, or only thinner ones, between LAO and Si. X-ray photoelectron spectroscopy analyses reveal that the thinner interfacial layer is compositionally graded La–Al–Si–O silicate and Al element is deficient in the interfacial layer. The reliable value of equivalent oxide thickness around 1.2 nm of LAO/Si has been achieved.

Co-doped titanate nanotubes

Perovskite conductive LaNiO3 films, 250 nm thick, were prepared by metalorganic decomposition. Rutherford backscattering spectrometry was used to determine the film thickness and composition. The x‐ray diffraction patterns of LaNiO3 films indicated that the lowest temperature for crystallization is about 530 °C. The measurement of resistivity as a function of annealing temperatures showed that the good metallic conductive LaNiO3 films could be obtained at 550 °C. The films with the lowest resistivity (4.0×10−4 Ω cm) were obtained on quartz by annealing in oxygen at 700 °C.Perovskite conductive LaNiO3 films, 250 nm thick, were prepared by metalorganic decomposition. Rutherford backscattering spectrometry was used to determine the film thickness and composition. The x‐ray diffraction patterns of LaNiO3 films indicated that the lowest temperature for crystallization is about 530 °C. The measurement of resistivity as a function of annealing temperatures showed that the good metallic conductive LaNiO3 films could be obtained at 550 °C. The films with the lowest resistivity (4.0×10−4 Ω cm) were obtained on quartz by annealing in oxygen at 700 °C.

Structure and electrical properties of Bi3.15Nd0.85Ti3O12 ferroelectric thin films

Metallic LaNiO3 (LNO) films were prepared on LaAlO3 (LAO) by metalorganic decomposition (MOD) and their application as the bottom electrode in sol-gel derived BaTiO3 (BTO) thin film was studied by means of x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. BTO film on LNO-coated LAO exhibited preferred (100) orientation and smooth surface with fine grains (∼50 nm). Electrical measurements on BTO film capacitor showed good ferroelectric hysteresis, lower loss tangent and good insulating properties. These results indicated the BTO/LNO heterostructure fabricated by sol-gel and MOD technique to be a promising combination for microelectronic device applications.Metallic LaNiO3 (LNO) films were prepared on LaAlO3 (LAO) by metalorganic decomposition (MOD) and their application as the bottom electrode in sol-gel derived BaTiO3 (BTO) thin film was studied by means of x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. BTO film on LNO-coated LAO exhibited preferred (100) orientation and smooth surface with fine grains (∼50 nm). Electrical measurements on BTO film capacitor showed good ferroelectric hysteresis, lower loss tangent and good insulating properties. These results indicated the BTO/LNO heterostructure fabricated by sol-gel and MOD technique to be a promising combination for microelectronic device applications.

Preparation, characterization and photocatalytic properties of ZnTiO3 powders

Co-doped titanate nanotubes were synthesized through 130 °C treatment of Co-doped TiO2 powders in high concentration NaOH aqueous solutions in a hydrothermal autoclave. The obtained nanotubes were identified to be of a monoclinic layered trititanate structure, the same as H2Ti3O7, their un-doped counterpart. The incorporation of Co ions was identified by energy dispersive spectroscopy. The Ti:Co atomic ratio in the obtained nanotubes was found significantly less than the value in the powder precursor. This indicates that the precursor powders were deformed in NaOH solution before the formation of nanotubes. Magnetic measurement showed a hysteresis loop at room temperature.