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Polarization fluctuation behavior of lanthanum substituted Bi4Ti3O12 (Bi4-xLaxTi3O12, BLT) ferroelectric thin films has been examined. Remnant polarization exhibits an initial increase (Pup, 1-106 cycles) and a subsequent decrease (Pdown, 106-109 cycles) with switching cycles, whereas the dielectric constant exhibits a continuous decrease. By careful investigations on the effect of switching frequency and annealing atmosphere on the polarization fluctuation characteristics, we propose that this polarization fluctuation characteristic of BLT films is attributed to the competition between domain pinning and passive layer growing effect, due to the redistribution of oxygen vacancy related defect under external applied field. Pup behavior is dominated by the unpinning of pinned domain, while Pdown behavior is dominated by the reduction of applied field on BLT bulk layer, due to the growing of the passive layer between BLT and Pt electrode. By assuming the dielectric constant and initial thickness of passive layer, the passive layer was estimated to be about 2-5 times thicker than the initial state after 109 cycling. [ABSTRACT FROM AUTHOR]

Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials. Half metallicity, an intriguing physical property arising from the metallic nature of electrons with singular spin polarization and insulating for oppositely polarized electrons, holds a great potential for a 100% spin-polarized current for high-efficiency spintronics. Conventionally synthesized thin films hardly sustain half metallicity inherited from their 3D counterparts. A fundamental challenge, in systems of reduced dimensions, is the almost inevitable spinmixed edge or surface states in proximity to the Fermi level. Here, we predict electric field-induced half metallicity in bilayer A-type antiferromagnetic van der Waals crystals (i.e., intralayer ferromagnetism and interlayer antiferromagnetism), by employing density functional theory calculations on vanadium diselenide. Electric fields lift energy levels of the constituent layers in opposite directions, leading to the gradual closure of the gap of singular spin-polarized states and the opening of the gap of the others. We show that a vertical electrical field is a generic and effective way to achieve half metallicity in A-type antiferromagnetic bilayers and realize the spin field effect transistor. The electric field-induced half metallicity represents an appealing route to realize 2D half metals and opens opportunities for nanoscale highly efficient antiferromagnetic spintronics for information processing and storage. [ABSTRACT FROM AUTHOR]

Piezoresponse force microscopy (PFM) is a powerful technique to characterize ferroelectric thin films by measuring the dynamic electromechanical response. The ferroelectricity is commonly demonstrated by the PFM hysteresis loops and a 180° phase difference of PFM images before and after poling. Such ferroelectric-like behaviors, however, recently are also found in many non-ferroelectrics. Consequently, it is still a challenge to identify intrinsic ferroelectricity in various kinds of thin films. Here, using PFM, we systematically studied the electromechanical responses in ferroelectric thin films with fast (BaTiO3) and slow (PVDF) switch dynamics, and also in the non-ferroelectric (Al2O3) thin films. It is found that both of the ac voltage (Vac) and pulsed dc voltage (Vdc) play an important role in the PFM measurement. When the Vac amplitude is higher than a explicit threshold voltage (Vc), collapse of the PFM hysteresis loops is observed for the films with fast switch dynamics. By measuring PFM hysteresis loops at various Vdc frequencies, an explicit Vc could be found in ferroelectric rather than in non-ferroelectric. The existence of an explicit Vc as well as nonvolatile behavior is proposed as an important approach to unambiguously identify intrinsic ferroelectricity in materials regardless of switch dynamics. [ABSTRACT FROM AUTHOR]

The subband structure and occupation in the triangular quantum well at Al xGa1− xN/GaN heterointerfaces have been investigated by means of temperature dependent Shubnikov–de Haas (SdH) measurements at low temperatures and high magnetic fields under illumination. After the illumination of the heterostructures, the total two-dimensional electron gas concentration increases, and the SdH oscillation amplitudes are enhanced when there is no additional subband occupation. It is also found that the energy separation between the subbands decreases after the illumination. We suggest that the illumination decreases the electric field and thus weakens the quantum confinement of the triangular quantum well at Al xGa1− xN/GaN heterointerfaces. The GaN layer is thought to be the primary contributor of the excited electrons by the illumination. [ABSTRACT FROM AUTHOR]

Examines the fabrication of pyroelectric Ba[sub 0.8]Sr[sub 0.2]TiO[sub 3] thin films with a sol-gel process. Relationship between dielectric constants and temperature; Decrease of the coercive field; Use of thin films in uncooled infrared detectors and thermal imaging applications.

A unique configuration of PbZr0.4Ti0.6O3 multilayer stack was designed and grown on F-doped tin oxide thin film by spin casting and annealing process. The multilayer system exhibits a broad reflection band with peak reflectivity over 95% and band width no < 40 nm, a dielectric constant of 520 and dielectric tunability of ~49% at 1 MHz, a remanent polarization of 46.8 μC/cm2, and a polarization loss of < 5% after 109 polarization switching cycles, rendering excellent performance as 1D photonic crystals and as ferroelectric and dielectric media. This material structure may find application in photonic band-gap engineering, microwave tunable devices, and integrated optoelectronics. [ABSTRACT FROM AUTHOR]

Examines the effects of individual layer thickness on the microstructure and electrical properties of sol-gel derived barium strontium titanate thin films. Measurement of thickness layer growth resulting in columnar grains; Influence of the nature of substrates on microstructure and ferroelectricity; Characteristics of barium strontium titanate.

Reports that tetragonal Ba[sub 0.8]Sr[sub 0.2]TiO[sub 3] thin films with large columnar grains have been prepared on Pt/Ti/SiO[sub 2] substrates using sol-gel processing. Broadening of the ferroelectric phase transition in the prepared thin films; Potential for uncooled infrared detector and thermal imaging applications.

Memristors have been intensively studied in recent years as promising building blocks for next-generation nonvolatile memory, artificial neural networks and brain-inspired computing systems. However, most memristors cannot simultaneously function in extremely low and high temperatures, limiting their use for many harsh environment applications. Here, we demonstrate that the memristors based on high-Curie temperature ferroelectrics can resolve these issues. Excellent synaptic learning and memory functions can be achieved in BiFeO3 (BFO)-based ferroelectric memristors in an ultra-wide temperature range. Correlation between electronic transport and ferroelectric properties is established by the coincidence of resistance and ferroelectricity switch and the direct visualization of local current and domain distributions. The interfacial barrier modification by the reversal of ferroelectric polarization leads to a robust resistance switching behavior. Various synaptic functions including long-term potentiation/depression, consecutive potentiation/depression and spike-timing dependent plasticity have been realized in the BFO ferroelectric memristors over an extremely wide temperature range of −170 °C ∼ 300 °C, which even can be extended to 500 °C due to the robust ferroelectricity of BFO at high temperatures. Our findings illustrate that the BFO ferroelectric memristors are promising candidates for ultra-wide temperature electronic synapse in extreme or harsh environments. [ABSTRACT FROM AUTHOR]

We design and fabricate a good performance silicon photoconductive terahertz detector on sapphire substrates at room temperature. The best voltage responsivity of the detector is 6679V/W at frequency 300 GHz as well as low voltage noise of 3.8 nV/Hz1/2 for noise equivalent power 0.57 pW/Hz1/2. The measured response time of the device is about , demonstrating that the detector has a speed of . The achieved good performance, together with large detector size (acceptance area is 3 ), simple structure, easy manufacturing method, compatibility with mature silicon technology, and suitability for large-scale fabrication of imaging arrays provide a promising approach to the development of sensitive terahertz room-temperature detectors. [ABSTRACT FROM AUTHOR]

Narrow-gap Hg1−xCdxTe material with a composition x of about 0.3 plays an extremely important role in mid-infrared detection applications. In this work, the optical properties of doped HgCdTe with x ≈ 0.3 are reviewed, including the defects and defect levels of intrinsic VHg and the extrinsic amphoteric arsenic (As) dopants, which can act as shallow/deep donors and acceptors. The influence of the defects on the determination of band-edge electronic structure is discussed when absorption or photoluminescence spectra are considered. The inconsistency between these two optical techniques is demonstrated and analyzed by taking into account the Fermi level position as a function of composition, doping level, conductivity type, and temperature. The defect level and its evolution, especially in As-doped HgCdTe, are presented. Our results provide a systematic understanding of the mechanisms and help for optimizing annealing conditions towards p-type As-activation, and eventually for fabricating high performance mid-infrared detectors. [ABSTRACT FROM AUTHOR]

Using the density-functional first-principles calculations, we investigate the electronic structures of poly(vinylidene fluoride) PVDF/graphene composite systems. The n- and p-doping of graphene can be flexibly switched by reversing the ferroelectric polarization of PVDF, without scarifying the intrinsic π-electron band dispersions of graphene that are usually undermined by chemical doping. The doping degree is also dependent on the thickness of PVDF layers, which will get saturated when PVDF is thick enough. In PVDF/bilayer graphene (BLG) heterostructure, the doping degree directly determines the local energy gap of the charged BLG. The sandwich structure of PVDF/BLG/PVDF can further enhance the local energy gap as well as keep the electric neutrality of BLG, which will be of great application potentials in graphene-based nanoelectronics. [ABSTRACT FROM AUTHOR]

The magnetic properties of Ni-doped monolayer MoS2 are investigated using the density function theory. The results show that two Ni-doped systems of the nearest-neighbor configuration are ferromagnetic. The p–d hybridization between the Ni dopant and its neighboring S atoms results in the splitting of energy levels near the Fermi energy. These results suggest the p–d hybridization mechanism for the magnetism of the Ni-doped MoS2 monolayer. The magnetic moment disappears with increasing Ni–Ni distance. Our studies predict the nearest two-Ni-doped MoS2 monolayers to be candidates for thin dilute magnetic semiconductors. Moreover, the formation energy calculations indicate that it would be easier to incorporate Ni atoms into a S-rich MoS2 monolayer in the experiment. [ABSTRACT FROM AUTHOR]

The pyroelectric coefficient and pyroelectric infrared response in the sol-gel derived Ba[sub 0.8]Sr[sub 0.2]TiO[sub 3] thin films have been studied for possible infrared detector applications. The measured pyroelectric coefficient is larger than 3.1x10[sup -4] C/m[sup 2] K at temperatures ranging from 10 to 26 °C and reaches the maximum value of 4.1x10[sup -4] C/m[sup 2] K at 16 °C. The infrared detectivity of 4.6x10[sup 6] cm Hz[sup 1/2] W[sup -1] has been obtained at 19 °C and 10 Hz in the Ba[sub 0.8]Sr[sub 0.2]TiO[sub 3] films deposited on thick (500 μm) platinum coated silicon substrates. The better infrared response can be expected by the improvement in the thermal isolation of pyroelectric element and the electrode materials. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new type of incidence-angle-variable infrared spectroscopic ellipsometer working in the 2100-10 000 wave number range has been designed and constructed. For the system, the analyzer and polarizer were driven by two microstepping motors having hollow shafts and rotating synchronously with a speed ratio of 2:1, i.e., A=2P. The incidence angle can be varied from 30° to 90° with an accuracy of 0.01°. The doubled Fourier transforms as both functions of the wave number and the azimuthal angles of the polarizer and analyzer were carried out and integrated in the system. Two sets of ellipsometric parameters measured in the experiment have been used to test the data self-consistency of the system. In data reduction, the error arising from the slight anisotropy of the stray light was corrected. In application, the complex dielectric function of the Au film was measured with a data accuracy better that 1% in the entire spectral range. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]