Enhancement of photoelectrochemical performance in ferroelectric films via the introduction of an Au buffer layer

The inefficient separation of photogenerated carriers has become a serious problem that limits the photoelectrochemical(PEC)performance of semiconductors.Herein,a sol-gel method was used to prepare BiFeO_(3) ferroelectric thin films with FTO and FTO/Au as substrates,respectively.The polarization electric field of the ferroelectric can more effectively separate the carriers generated in the photoelectrode.Meanwhile,the introduction of an Au buffer layer can reduce the resistance in the process of charge transfer,accelerate the carrier migration,and enhance the efficiency of the charge separation.Under light irradiation,Au/BiFeO_(3) photoelectrode exhibited an extraordinary improvement in PEC water splitting compared with BiFeO_(3).In addition,the ferroelectric polarization electric field causes band bending,which further accelerates the separation of electrons and holes and improves the PEC performance of the photoelectrode.This work promotes the effective application of ferroelectric films in PEC water splitting.

Review of My Research Work on Ferroelectric Films and Si-Based Device Applications: Opportunity and Challenge

Ferroelectric materials have many interesting physical properties such as ferroelectricity, pyroelectricity, piezoelectricity, and opto-electricity, and applying ferroelectric materials in the forms of thin and thick films and integrating them on the silicon substrate as electronic and MEMS devices is a very attractive research area and challenging. In this paper, we report our research works on ferroelectric MEMS and ferroelectric films for electronic device applications. Pyroelectric thin film infrared sensors have been made, characterized, and a 32×32 array with its size of 1cm×1cm has been obtained on Si membrane. Ferroelectric thin films in amorphous phase have been applied to make silicon based hydrogen gas sensors with the metal/amorphous ferroelectric film/metal device structure, and its turn-on voltage of about 4.5V at ～1000 ppm in air is about 7 times of the best value reported in the literature. For the application of electron emission flat panel display, ferroelectric BST thin films with excess Ti concentrations have been coated on Si tips, the threshold voltage of those ferroelectric film coated tips has been reduced about one order from ～70 V/μm to 4～10 V/μm for different Ti concentrations, and however, the electron emission current density has been increased at least 3～4 order for those coated tips compared to that of the bare Si tips. To fulfill in the thickness gap between thin film of typical ～1 μm made by PVD/CVD and polished ceramic wafer of ～50 μm from the bulk, piezoelectric films with thickness in a range of 1～30 μm have been successfully deposited on Si substrate at a low temperature of 650oC by a novel hybridized deposition technique, and piezoelectric MEMS ultrasonic arrays have been very recently obtained with the sound pressure level up to ～120 dB. More detailed results will be presented and mechanisms will be discussed.

Fabrication and optical properties of InGaN/GaN multiple quantum well light emitting diodes with amorphous BaTiO_3 ferroelectric film

BaTiO3 （BTO） ferroelectric thin films are prepared by the sol,el method. The fabrication and the optical properties of an InGaN/GaN multiple quantum well light emitting diode （LED） with amorphous BTO ferroelectric thin film are studied. The photolumineseence （PL） of the BTO ferroelectric film is attributed to the structure. The ferroeleetric film which annealed at 673 K for 8 h has the better PL property. The peak width is about 30 nm from 580 nm to 610 nm, towards the yellow region. The mixed electroluminescence （EL） spectrum of InGaN/GaN multiple quantum well LED with 150-nm thick amorphous BTO ferroelectric thin film displays the blue-white light. The Commission Internationale De L＇Eclairage （CIE） coordinate of EL is （0.2139, 0.1627）. EL wavelength and intensity depends on the composition, microstructure and thickness of the ferroelectric thin film. The transmittance of amorphous BTO thin film is about 93% at a wavelength of 450 nm-470 nm. This means the amorphous ferroelectrie thin films can output more blue-ray and emission lights. In addition, the amorphous ferroelectric thin films can be directly fabricated without a binder and used at higher temperatures （200 ℃-400 ℃）. It is very favourable to simplify the preparation process and reduce the heat dissipation requirements of an LED. This provides a new way to study LEDs.

Optimal parameter space for stabilizing the ferroelectric phase of Hf_(0.5)Zr_(0.5)O_(2) thin films under strain and electric fields

Hafnia-based ferroelectric materials, like Hf_(0.5)Zr_(0.5)O_(2)(HZO), have received tremendous attention owing to their potentials for building ultra-thin ferroelectric devices. The orthorhombic(O)-phase of HZO is ferroelectric but metastable in its bulk form under ambient conditions, which poses a considerable challenge to maintaining the operation performance of HZO-based ferroelectric devices. Here, we theoretically addressed this issue that provides parameter spaces for stabilizing the O-phase of HZO thin-films under various conditions. Three mechanisms were found to be capable of lowering the relative energy of the O-phase, namely, more significant surface-bulk portion of(111) surfaces, compressive c-axis strain,and positive electric fields. Considering these mechanisms, we plotted two ternary phase diagrams for HZO thin-films where the strain was applied along the in-plane uniaxial and biaxial, respectively. These diagrams indicate the O-phase could be stabilized by solely shrinking the film-thickness below 12.26 nm, ascribed to its lower surface energies. All these results shed considerable light on designing more robust and higher-performance ferroelectric devices.

Effect of electromechanical the properties of epitaxial boundary conditions on ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzbur~Landau equation （DCLE）. The numerical solution for BaTiO3 film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxia] ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.

MINIMUM SIZE OF 180 DEGREE DOMAINS IN FERROELECTRIC THIN FILMS COVERED BY ELECTRODES

Ferroelectric domain switching under low voltage or short pulses is of interest for the development of high-density random access memory （FRAM） devices. Being necessarily very small in size, instability and back switching often occur when the external voltage is removed, which creates serious problems. In this investigation, a general approach to determine the minimum size of ferroelectric domain to avoid back switching was developed, and as an example, a 180° domain in a ferroelectric thin film covered by the upper and lower electrodes was considered in detail. We note that our approach is generally applicable to many other fields, including phase transformation, nucleation and expansion of dislocation loops in thin films, etc.

Orientation of Bi_(3.2)La_(0.8)Ti_3O_(12) Ferroelectric Thin Films with Different Annealing Schedules

Fatigue-free Bi3.2La0.8Ti3O12 ferroelectric thin films were successfully prepared on p-Si （100） substrates using metalorganic solution deposition process. The orientation and formation of 5-layers thin films were studied under different processing conditions using XRD. Experimental results indicate that increase in annealing time at 700 ℃ after preannealing for 10 min at 400 ℃ can remarkably increase （200）-orientation of the films derived from the precursor solutions with two contents of citric acid. Meanwhile, high content of citric acid increases the film thickness and is conducive to the α-orientation of the films with the preannealing, and low concentration of the solution is conducive to the c-orientation of the films without the preannealing.

Photovoltaic and photoelectric response of Sn_(2)P_(2)S_(6) ferroelectric films

In the present work,the comparative study of the electric response of Sn_(2)P_(2)S_(6) films to the visible light action was carried out without and under external electric field.The light flux modulated by a chopper induces the integral response comprising time-dependent photoelectric and photovoltaic components.It was shown that spontaneous polarization influences the electric response characteristics.

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg Landau-Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interracial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectrie materials.

Effect of Annealing on Ferroelectric Properties of Bi_ (3.25)La_(0.75)Ti_3O_ (12) Thin Films Prepared by the Sol-gel Method

Bi3.25La0.75Ti3O12（BLT）thin films were prepared on Pt/Ti/SiO2/Si substrate by the sol-gel method.The effect of annealing on their structures and ferroelectric properties was investigated.The XRD patterns indicate that the BLT films annealed at different temperatures are randomly orientated and the single perovskite phase is obtained at 550℃.The remmant polarization increnses and the coercive field decreases with the annealing temperature increasing.The leakage current density of the BLT films annealed at 700℃ is about 5.8×10^-8A/cm^2 at the electrie field of 250kv/cm.

ANALYTICAL SOLUTION FOR RESPONSE OF FERROELECTRIC THIN FILM TO INFRARED RADIATION

Micro-bolometer pixel is an essential element in the infrared focal plane array （IRFPA） of infrared detectors. Its response to infrared radiation is analyzed in this paper. The pixel structure is modeled as a composite laminate thin plate whose sides are measured with the thickness from 0.1-1 μm. Its middle ply is a ferroelectric thin film. Its top surface is covered with a gold or platinum infrared absorber, while the bottom surface is deposited with platinum or lanthanum-nickel. Meanwhile both surfaces are a pair of electrodes. The top surface receives infrared radiation pulses successively. For the very tiny micro bolometer pixel, it is assumed that the infrared radiation is uniformly distributed on the plate. Furthermore, as the ratio of the side length to the thickness of the plate is dramatically large, it is assumed that heat transfer only takes place across the thickness of the plate. The thermal-electric-mechanical coupling governing equations are solved in a form of Fourier series. Results of the displacement, temperature variation and electric output signals of the micro bolometer pixel structure under infrared radiation are obtained, analyzed and compared with experimental data.

The hysteresis loops of a ferroelectric bilayer film with surface transition layers

Based on the transverse Ising model in the framework of the mean field approximation, this paper discusses a ferroelectric bilayer film with the surface transition layers within each constituent slab and an antiferroelectric interracial coupling between two slabs. The hysteresis loop of a bilayer film is investigated. The results show that the surface transition layer in a ferroelectric bilayer film plays a significant role in realizing the multiple-state memory.

Size Effects of the Critical Temperature in Ferroelectric Thin Films

The size effects of the critical behaviors for the systems of interacting spins are discussed extensively inliterature.In this paper,the finite-size dependence of the critical temperature and susceptibility of the ferroelectric thinfilm are investigated numerically based on the four-state Potts model with the nearest-neighbor interactions between thedipole moments.The four orientations of the domains exist in the ferroelectric film and the movement of the domainwalls determines the polarization switching process besides the boundary conditions of the film.The critical exponentsare obtained and our investigations show that the boundary conditions play the important roles for the ferroelectricproperties of the thin films and the critical behavior of the thin films strongly depends on the feature of the surface.

Influence of lateral size on dielectric properties of ferroelectric thin films with structure transition zones

By taking into account structural transition zones near the lateral and thickness direction edges,this paper uses a modified transverse Ising model to study dielectric properties of a finite size ferroelectric thin film in the framework of the mean-field approximation.The results indicate that the influence of the lateral size on the dielectric susceptibility cannot be neglected and lateral structural transition zones could be a crucial factor that improves the mean susceptibility of the fixed size film.

Strain tunability of dielectric and ferroelectric properties in epitaxial lead titanate thin films

Many distinguished properties of epitaxial ferroelectric thin films can be tunable through the misfit strain.The strain tunability of ferroelectric and dielectric properties in epitaxial lead titanate ultrathin films is numerically investigated by using a phase field model,in which the surface effect of polarization is taken into account.The response of polarization to the applied electric field in the thickness direction is examined with different misfit strains at room temperature.It is found that a compressive misfit strain increases the coercive field and the remanent polarization while a tensile misfit strain decreases both of them.The nonlinear dielectric constants of the thin films with tensile misfit strains are much larger than those of the thin films without misfit strains,which are attributed to the existence of the a/c/a/c multiple domains in the thin films under tensile misfit strains.

Epitaxy surface effect on the first-order phase transition properties in a ferroelectric thin film

By modifying the interchange interactions and the transverse fields on the epitaxy surface layer, this paper studies the phase transition properties of an n-layer ferroelectric thin film by the Fermi-type Green＇s function technique based on the transverse Ising model with a four-spin interaction. The special attention is given to the effect of the epitaxy surface layer on the first-order phase transition properties in the parameter space constructed by the ratios of the bulk transverse field and the bulk four-spin interaction to the bulk two-spin interaction with the framework of the higher-order decoupling approximation to the Fermi-type Green＇s function. The results show that the first-order phase transition properties will be changed significantly due to the modification of interchange interaction and transverse field parameters on the epitaxy surface layer. The dependence of the first-order phase transition properties on the thickness of ferroelectric thin films is also discussed.

Enhanced Ferroelectric Polarization in Laser-ablated Bi4Ti3O12 Thin Films by Controlling Preferred Orientation

Polycrystalline Bi_4Ti_3O_（12） thin films with various fractions of a-axis, c-axis and random orientations have been grown on Pt（111）/Ti/Si O_2/Si substrates by laser-ablation under different kinetic growth conditions. The relationship between the structure and ferroelectric property of the films was investigated, so as to explore the possibility of enhancing ferroelectric polarization by controlling the preferred orientation. The structural characterization indicated that the large growth rate and high oxygen background pressure were both favorable for the growth of non-c-axis oriented grains in the Bi_4Ti_3O_（12） thin films. The films with high fractions of a-axis and random orientations, i e, f（a-sxis） = 28.3% and f（random） = 69.6%, could be obtained at the deposition temperature of 973 K, oxygen partial pressure of 15 Pa and laser fluence of 4.6 J/cm^2, respectively. It was also noted that the variation of ferroelectric polarization was in accordance with the evolution non-c-axis orientation. A large value of remanent polarization（2 Pr = 35.5 μC/cm^2） was obtained for the Bi_4Ti_3O_（12） thin films with significant non-c-axis orientation, even higher than that of rare-earth-doped Bi_4Ti_3O_（12） films.

High Ferroelectricities and High Curie Temperature of BiInO3PbTiO3Thin Films Deposited by RF Magnetron Sputtering Method

Properties of ferroelectric xBiInO3-（1-x）PbTiO3（xBI-（1-x）PT） thin films deposited on（101） SrRuO3/（200）Pt/（200） MgO substrates by rf magnetron sputtering method and effects of deposition conditions are investigated.The structures of the xBI-（1-x）PT films are characterized by x-ray diffraction and scanning electron microscopy.The results indicate that the thin films are grown with mainly（001） orientation. The chemical compositions of the films are analyzed by scanning electron probe and the results indicate that the loss phenomena of Pb and Bi elements depend on the pressure and temperature during the sputtering process.The sputtering parameters including target composition, substrate temperature, and gas pressure are adjusted to obtain optimum sputtering conditions. To decrease leakage currents,2 mol% La2 O3 is doped in the targets. The P-E hysteresis loops show that the optimized xBI-（1-x）PT（x = 0.24） film has high ferroelectricities with remnant polarization2 Pr = 80μC/cm2 and coercive electric field 2 EC = 300 kV/cm. The Curie temperature is about 640℃. The results show that the films have optimum performance and will have wide applications.