The domain switching and structural characteristics of PLZT bulk ceramics and thin films chemically prepared from the same acetate precursor solutions

Lead lanthanum zirconate titanate (PLZT) ferroelectrics were produced in bulk ceramic and thin-film form from the same acetate precursor solutions in order to compare their electrical and physical properties. Bulk ceramics were hot pressed from chemically coprecipitated powders, and thin films were fabricated by spin coating on silver foil and platinum-coated silicon wafer substrates. A number of PLZT compositions were investigated, including ferroelectric memory materials near the morphotropic phase boundary with 2% La, memory and slim-loop ferroelectric x/65/35 (La/Zr/Ti) compositions with up to 12% La, as well as some antiferroelectric thin-film materials. Internal film stress from thermal expansion mismatch between films and substrates was found to contribute to differences in electrical properties and Curie temperatures between the thin film and bulk materials, as were interface layers between the films and substrates, mechanical clamping from the substrates and grain size.     

Electric characteristics of organic thin-film transistors and logic circuits with a ferroelectric gate insulator

Organic electronic devices using a pentacene have improved importantly in the last several years. We fabricated pentacene organic thin-film transistors (OTFTs) with dielectric SiO₂ and ferroelectric Pb(Zr_0.3,Ti_0.7)O₃ (PZT) gate insulators. The organic devices using SiO₂ and PZT films had the field-effect mobility of approximately 0.1 and 0.004 cm²/V s, respectively. The drain current in the transfer curve of pentacene/PZT transistors showed a hysteresis behavior originated in a ferroelectric polarization switching. In order to investigate the polarization effect of PZT gate dielectrics in a logic circuit, the simple voltage inverter using SiO₂ and PZT films was fabricated and measured by an output-input measurement. The gain of inverter at the poling-down state was approximately 7.2 and it was three times larger than the value measured at the poling-up state.     

Fabrication of one-transistor-capacitor structure of nonvolatile TFT ferroelectric RAM devices using Ba(Zr0.1Ti0.9)O3 gated oxide film.

In this study, the Ba(Zr0.1Ti0.9)O3 (BZ1T9) thin films have been well deposited on the Pt/Ti/SiO2/Si substrate. The optimum radio frequency (RF) deposition parameters are developed, and the BZ1T9 thin films deposition at the optimum parameters have the maximum capacitance and dielectric constant of 4.4 nF and 190. As the applied voltage is increased to 8 V, the remnant polarization and coercive field of BZ1T9 thin films are about 4.5 microC/cm2 and 80 kV/cm. The counterclockwise current hysteresis and memory window of n-channel thin-film transistor property are observed, and that can be used to indicate the switching of ferroelectric polarization of BZ1T9 thin films. One-transistor-capacitor (1TC) structure of BZ1T9 ferroelectric random access memory device using bottom-gate amorphous silicon thin-film transistor was desirable because of the smaller size and better sensitivity. The BZ1T9 ferroelectric RAM devices with channel width = 40 microm and channel length = 8 microm has been successfully fabricated and the ID-VG transfer characteristics also are investigated in this study.     

Hot-wire thin-film transistors on PET at 100 °C

Bottom-gate amorphous silicon thin-film transistors were fabricated using active layers deposited by r.f. and hot-wire (HW) chemical vapor deposition on polyethylene terephthalate (PET) and polyimide (PI) substrates. The maximum processing temperature was 100 °C for PET and 250 °C for PI. For transistors deposited at 100 °C by r.f. on PET and at 175 °C by HW on PI the transistor characteristics are comparable, although still inferior, to those of standard amorphous silicon transistors fabricated on glass substrates at 250 °C. HW transistors fabricated at 100 °C showed poor device characteristics. For devices fabricated at 100 °C, an extended anneal at this temperature was required to improve the transistor characteristics, independently of the film deposition technique used.     

Fabrication of One-Transistor-Capacitor Structure of Nonvolatile TFT Ferroelectric RAM Devices Using Ba(Zr 0.1Ti 0.9)O 3 Gated Oxide Film

In this study, the Ba(Zr_0.1Ti_0.9)O₃ (BZ1T9) thin films have been well deposited on the Pt/Ti/SiO₂/Si substrate. The optimum radio frequency (RF) deposition parameters are developed, and the BZ1T9 thin films deposition at the optimum parameters have the maximum capacitance and dielectric constant of 4.4 nF and 190. As the applied voltage is increased to 8 V, the remnant polarization and coercive field of BZ1T9 thin films are about 4.5 muC/cm² and 80 kV/cm. The counterclockwise current hysteresis and memory window of n-channel thin-film transistor property are observed, and that can be used to indicate the switching of ferroelectric polarization of BZ1T9 thin films. One-transistor-capacitor (1TC) structure of BZ1T9 ferroelectric random access memory device using bottom-gate amorphous silicon thin-film transistor was desirable because of the smaller size and better sensitivity. The BZ1T9 ferroelectric RAM devices with channel width = 40 mum and channel length = 8 mum has been successfully fabricated and the I_D-V_G transfer characteristics also are investigated in this study.     

Elastic modulus of a silicon thin film fabricated by nanotransfer printing

Transfer printing, a promising method for fabricating multi-scale structures on various substrates such as semiconductors and polymers, has been used to fabricate flexible devices with performance superior to that of conventional organic flexible devices. Although thin films might be expected to suffer damage during the transfer printing process, no reports of the degradation of mechanical properties during transfer printing have been published. The change in mechanical properties before and after transfer printing should be evaluated in terms of reliability and design for transfer printing to be successful. We propose a method of fabricating freestanding 200-nm-thick single-crystal silicon (SCS) thin-film specimens using transfer printing in order to investigate the mechanical properties of the transferred SCS thin-film specimens. The fabrication method combines several techniques such as semiconductor manufacturing, liftoff, and transfer printing processes. The core technology in this method is the fabrication of freestanding SCS thin-film structures suspended between two fixed ends. The mechanical properties of the freestanding SCS thin-film structures were measured using a microtensile machine capable of optical strain measurement. The test results provide insight into device design and reliability evaluation of flexible electronics fabricated by nanotransfer printing.     

Midinfrared modulation through the use of field-induced scattering in ferroelectric liquid crystals

The feasibility of the use of modulation devices based on field-induced transient scattering in ferroelectric liquid crystals (LC) to replace mechanical choppers used in uncooled infrared-imaging systems was investigated. Devices fabricated with ITO-coated ZnSe substrates and a ferroelectric LC path length of 25 μm were able to modulate optical radiation by transient forward scattering at rates approaching 20 kHz. Through the use of a commercial arbitrary waveform generator and associated PC-based software, drive waveforms were developed that produced a variable, square-wave optical-modulation pattern by the extension of the duration of the scattering state to periods ranging from hundreds of microseconds to milliseconds. The ability of these extended-scattering-mode (ESM) devices to modulate radiation in both the visible and midinfrared regions was verified in a simple experiment through the use of a Fourier-transform infrared spectrometer, in which an unoptimized ESM device displayed a 40% modulation dep th for IR radiation in the 8-12-μm region.     

Beyond Substrates: Strain Engineering of Ferroelectric Membranes

Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO₃, production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm⁻¹ and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.     

Resonant-cavity-enhanced thin-film AlGaAs/GaAs/AlGaAs LED's with metal mirrors

Resonant-cavity-enhanced light-emitting diodes (RCE LED's) are of increasing interest as a low-cost alternative to lasers for short-distance applications. We report on the characteristics of thin-film AlGaAs/GaAs/AlGaAs double-heterostructure RCE LED's with metal mirrors on both sides fabricated by means of epitaxial liftoff and bonded to silicon host substrates. The devices exhibit typical turn-on voltages of 1.3 V, operating resistances of 31 Ω, linewidths of 10.4 nm, efficiencies of 1.4%, dispersion half-angles of 23.7°, and stable output over more than 1700 h. These devices exhibit significant improvement over conventional LED's without additional complicated processing or growth steps, resulting in a manufacturable, low-cost device.     

Influence of the state of interfaces on the magnitude of the magnetoelectric effect in Co (Ni) films on PbZr<Subscript>0.45</Subscript>Ti<Subscript>0.55</Subscript>O<Subscript>3</Subscript> and GaAs substrates

Co (Ni) films on ceramic lead zirconate titanate (PbZr_0.45Ti_0.55O₃) ferroelectric substrates and single-crystal GaAs piezoelectric substrates are shown to exhibit a giant magnetoelectric response, which reaches 200 V/A at room temperature and mechanical Q of at least 1000. These findings are interpreted in terms of the formation of stable interfaces in thin-film heterostructures owing to good adhesion of dissimilar materials to substrate surfaces after ion beam sputtering/deposition processing. This will allow one to extend the applicability of the magnetoelectric effect at room temperature to commercially available microelectronic materials and integrate related hybrid structures into single-chip signal generation/processing devices.     

Metal organic chemical vapor deposition (MOCVD) of oxides and ferroelectric materials

The electrical properties of thin-film oxides span a tremendous range from insulating to superconducting. As a consequence, they are now finding an increasing number of applications in electronics and opto-electronics, both as stand-alone layers and for the provision of added functionality to electronic circuits, especially silicon. The modified precursor delivery technologies (e.g. aerosol or liquid delivery or injection techniques) coupled with improved precursors, and better engineering of the deposition kit, have transformed the prospects for the deposition of these materials by MOCVD. It is now possible to exploit the many traditional advantages of MOCVD, especially over other routes to thin-film oxide deposition. A wide range of oxides and particularly complex ferroelectric oxides are being produced on substrates of up to 200 mm in diameter. This paper highlights the progress and real advantages that MOCVD has for thin-film oxide deposition, including the design of improved precursors. Examples of the deposition of dielectric layers and lead-based ferroelectric layers for uncooled thermal imaging, and MEMS actuation applications are presented.     

Non-volatile domain nucleation and growth in multiferroic BiFeO3 films

We have presented a systematical study of the domain nucleation and growth behaviors in multiferroic BiFeO(3) (BFO) films. Both the ferroelectric and the ferroelastic switching dynamics were investigated. Several environmental parameters, including the polarization orientations, the monodomain-like matrix, and the ordered domain walls as local boundaries, were well controlled by thin-film strain engineering through changing the vicinal angles of the substrates. The tip-based domain dynamics was studied by subsequent piezoresponse force microscope (PFM) imaging of the domain evolution under external voltage pulses. For the nanodomains written in the monodomain-like environment, the domain wall performed the thermal activated motion. The as-grown 71° domain walls can act as pinning centers for the ferroelectric domain growth driven by low fields; moreover, ferroelastic nucleation near a 71° domain wall will cause the deformation of the domain wall. The ferroelastic domain growth possessed relatively small activation fields, and therefore usually performed non-activated motion. This study revealed the effects of local environments on the dynamics forming nanoscale domains, and opened a pathway for applications in novel non-volatile functional devices.     

Special features in the pyroelectric properties of PZT films containing excess lead oxide

Pyroelectric hysteresis in unipolar lead zirconate titanate (PZT) films was studied upon preliminary poling in a strong external electric field and immediately in the applied external field. The asymmetry of the observed hysteresis loops is considered within the framework of an electromechanical approach to the phenomenon of natural unipolarity (spontaneous polarization) in thin-film ferroelectric capacitors formed on silicon and glass-ceramic composite substrates.     

Relieving the Photosensitivity of Organic Field-Effect Transistors

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 10⁵ times and the threshold voltage shift (ΔV_T) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔV_T of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.     

Circularly symmetric light scattering from nanoplasmonic spirals

In this paper, we combine experimental dark-field imaging, scattering, and fluorescence spectroscopy with rigorous electrodynamics calculations in order to investigate light scattering from planar arrays of Au nanoparticles arranged in aperiodic spirals with diffuse, circularly symmetric Fourier space. In particular, by studying the three main types of Vogel's spirals fabricated by electron-beam lithography on quartz substrates, we demonstrate polarization-insensitive planar light diffraction in the visible spectral range. Moreover, by combining dark-field imaging with analytical multiparticle calculations in the framework of the generalized Mie theory, we show that plasmonic spirals support distinctive structural resonances with circular symmetry carrying orbital angular momentum. The engineering of light scattering phenomena in deterministic structures with circular Fourier space provides a novel strategy for the realization of optical devices that fully leverage on enhanced, polarization-insensitive light-matter coupling over planar surfaces, such as thin-film plasmonic solar cells, plasmonic polarization devices, and optical biosensors.     

Strain engineering of piezoelectric properties of strontium titanate thin films

The in-plane and out-of-plane piezoelectric properties of (001) strontium titanate (SrTiO₃, STO) epitaxial thin films on pseudo-cubic (001) substrates are computed as a function of in-plane misfit strain. A nonlinear thermodynamic model is employed, which takes into account the appropriate mechanical boundary conditions, the electromechanical coupling between the polarization and the in-plane lattice mismatch, and the self-strains of the ferroelastic and ferroelectric phase transformations. The piezoelectric behavior of epitaxial STO films is described in various strain-induced ferroelectric phase fields in a temperature range from ?50 to 50 °C. The calculations show that by carefully tailoring in-plane misfit strains in both tensile and compressive ranges, piezoelectric coefficients that are of the order of prototypical lead zirconate titanate and other lead-based piezoceramics can be realized. These results indicate that strain engineered STO films may be employed in a variety of sensor and actuator applications as well as surface acoustic wave devices and thin-film bulk acoustic resonators.     

Multiferroics: progress and prospects in thin films

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties - and in turn promise new device applications - as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.     

Fabrication of fully transparent nanowire transistors for transparent and flexible electronics

The development of optically transparent and mechanically flexible electronic circuitry is an essential step in the effort to develop next-generation display technologies, including 'see-through' and conformable products. Nanowire transistors (NWTs) are of particular interest for future display devices because of their high carrier mobilities compared with bulk or thin-film transistors made from the same materials, the prospect of processing at low temperatures compatible with plastic substrates, as well as their optical transparency and inherent mechanical flexibility. Here we report fully transparent In(2)O(3) and ZnO NWTs fabricated on both glass and flexible plastic substrates, exhibiting high-performance n-type transistor characteristics with approximately 82% optical transparency. These NWTs should be attractive as pixel-switching and driving transistors in active-matrix organic light-emitting diode (AMOLED) displays. The transparency of the entire pixel area should significantly enhance aperture ratio efficiency in active-matrix arrays and thus substantially decrease power consumption.     

Structural defects and local chemistry across ferroelectric–electrode interfaces in epitaxial heterostructures

We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr_0.2Ti_0.8O₃, PbZr_0.52Ti_0.48O₃, and BaTiO₃ deposited on SrRuO₃/SrTiO₃, were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric–electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr_0.2Ti_0.8O₃/SrRuO₃/SrTiO₃ thin films. Conversely, Pb diffusion was found to be prevalent in PbZr_0.52Ti_0.48O₃/SrRuO₃/SrTiO₃ thin films. The BaTiO₃/SrRuO₃/SrTiO₃ heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.     

Monolithic formation of thin films on the vertical surface of a substrate by a dual-ion-beam sputtering technique

We have developed a novel method for the selective deposition of thin films on the vertical surface of a planar substrate with a vertical step. This was done with a dual-ion-beam sputtering apparatus that is equipped with two ion-beam sources. Using this technique a multilayer filter was monolithically formed on the vertical surface of a Si substrate on which a photodetector had been fabricated, and clear filtering-photodetecting characteristics were observed. This technique can be applied to the monolithic integration of thin-film devices and waveguide-type optical devices with a vertical end facet.