Yuhuan Xu

Ferroelectric thin films prepared by sol-gel processing

Abstract Ferroelectric thin films are playing a growing role as key elements in variety of devices. For various techniques of ferroelectric thin film preparation, the sol-gel processing is one of the most promising. In comparison with different deposition techniques, we survey the intrinsic advantages and the recent improvements of the sol-gel processing. In this review paper, several interesting topics, including epitaxial growth and grain-orientation, symmetric and asymmetric P-E hysteresis loops, heterojunction effect of the interface between film and substrate, to attest pyroelectricity of the films, electrooptic coefficients in poled and unpoled films, possibility of amorphous ferroelectrics, etc., are introduced.

The self‐biased heterojunction effect of ferroelectric thin film on silicon substrate

Several ferroelectric thin films with n‐type or p‐type conductivity, including undoped and doped lead zirconate titanate, barium titanate, strontium barium niobate, and potassium niobate, as well as lead barium niobate, were made on silicon single‐crystal substrates by the sol‐gel process. Self‐biased heterojunction effects were observed in both p‐ferroelectric thin film on n‐silicon and n‐ferroelectric thin film on p‐silicon by the measurement of current‐voltage characteristics. It has been observed that either p‐n junction or n‐p junction in the ferroelectric‐semiconductor systems behave like a rectifying diode. However, the junction effect is weak in the p‐ferroelectric thin film on p‐silicon system. A physical model based on the consideration of energy‐band theory has been constructed in explaining this effect. Possible applications of this effect are discussed.

Ferroelectric and pyroelectric properties of strontium barium niobate films prepared by the sol‐gel method

Thin films of Sr0.6Ba0.4Nb2O6 on silicon and fused silica substrates have been prepared by the sol‐gel method. The dense, transparent, and crack‐free films were fabricated by the dipping and then firing at a temperature of 700 °C. Ferroelectric properties were confirmed by P‐E hysteresis loops. The refractive index and optical transmission were measured. The temperature variation of the dielectric constant was obtained. The pyroelectric coefficient at room temperature was found to be 2×10−8 C/cm2 K.

Electrical and Optical Properties of MgO Thin Film Prepared by Sol-Gel Technique

MgO thin films with either (111) or (200) preferential orientation have been prepared on (100) Si substrates by sol-gel method after a heat-treatment at 800°C. The obtained (111) preferentially oriented MgO film has a dielectric constant of 7.0 with a loss factor of 5% and a dielectric strength higher than 8×105 V/cm. The optical refractive index, which depends on the film thickness, is 1.71 when the film thickness is 260 nm. The surface structure of the Si substrate is believed to affect the preferential orientation of the sol-gel derived MgO film. Specifically, the microstructures at the interface indicate an interdiffusion of Mg, O, and Si between the film and the substrate.

Ferroelectric Materials by the Sol-Gel Method

The sol-gel method is ideally suitable for the preparation of ferroelectric thin films such as LiNbO3, BaTiO3, KNbO3 and PZT. The preparation and properties of polycrystalline, amorphous and single crystal films of these ferroelectric oxides are summarized. The origin of “amorphous ferroelectricity” is discussed. Single crystal KNbO3 films have been successfully fabricated into planar waveguides and their ability to convert infrared laser into green light demonstrated.

Sol-gel derived ferroelectric PZT thin films on doped silicon substrates

Thin Pb(ZrxTi1-x)O3 films were fabricated by the sol-gel method. Films with thickness of 0.5 μm to 2 μm were coated on doped silicon substrates, using a multiple deposition process. The study on the ferroelectric hysteresis loop of the films using metal-ferroelectric-semiconductor configuration yielded remanent polarization of 2.2 × 10-2 C/m2 and coercive field of 7.5 kV/cm. The low remanent polarization and high coercive field of the PZT film compared to the bulk PZT is due to the small grain size. Asymmetric hysteresis loops were observed. The mechanism associated with the phenomenon is attributed to the space charge compensating the contact potential difference at the interface of ferroelectric-semiconductor.

Self biased heterojunction effect of ferroelectric thin film on silicon

Abstract Several ferroelectric thin films with n-type or p-type conductivity, including undoped and doped PZT (Lead-Zirconate Titanate), BaTiO3 (Barium Titanate), SBN (Strontium-Barium Niobate), KNbO3 (Potassium Niobate) as well as PBN (Lead Barium Niobate) were made on the silicon substrates by the sol-gel process Self biased heterojunction effects were observed. It has been observed that either p-n junction or n-p junction in the ferroelectric-semiconductor systems behave like a rectifying diode. A physical explaination has been constructed in explaining this effect.

Preparation and Characterization of PbS Quantum Dots Doped Ormocers

A new method for the preparation of semiconductor PbS quantum dot-doped Ormocer (Organically Modified Ceramic) has been developed. The Ormocer matrix was prepared through the hydrolysis and condensation of alkoxysilane precursors (sol-gel process). Formation of PbS particles took place in the porous Ormocer through H2S gas reaction with a lead precursor incorporated at the solution stage. Control of the PbS dot size was achieved through the use of organically substituted trifunctional silanes. Particle formation and growth was studied under different experimental conditions (e.g. temperature and lead precursor concentrations) where nucleation and aggregation processes occurred. Determination of the average particle size was done by XRD. Transmission electron microscopy was also used to determine particle diameter as well as particle size distribution. Optical absorption spectra were measured at the UV-VIS wavelength range. Absorption edge blue shifts showed the quantum confinement effect in these materials. The non-hydrolyzed groups bonded to the silane prevented uncontrolled nucleation and aggregation during the particle formation and growth, i.e. at the solution stage by the homogeneous distribution of the Pb salts or at the xerogel state by the capping of the growing particles.

NANOCOMPOSITE OF SEMICONDUCTING FERROELECTRIC ANTIMONY SULPHOIODIDE DOTS-DOPED GLASSES

Abstract Antimony sulphoiodide (SbSI) is a semiconducting ferroelectric crystal. The SbSI possesses attractive properties, such as high pizoelectric constant, very high dielectric permittivity, and high electro-optic coefficient, but it is very difficult to grow a single crystal with bigger size. Currently, a method for synthesis of SbSI by liquid reaction has been developed in our laboratory. Nanocomposites of SbSI dots in sodium borosilicate glass as well as in organic modified silicate (ORMOSIL) matrix have been fabricated by the sol-gel technique. The dot size is controlled by processing conditions. The nanocomposites were characterized by x-ray diffraction and high resolution TEM. Quantum confinement effect was observed on optical spectra. Nonlinear optical property, the third order susceptibility χ(3). of the SbSI dots-doped glasses was measured by four wave mixing method at 532 nm. The value of χ(3) was found to be 6 χ 10−11 esu at this non-resonant wavelength.

Cadmium telluride quantum-dot-doped glass by the sol-gel technique

CdTe quantum dot (QD)-doped sodium borosilicate (NBS) glasses with particle sizes ranging from 2.4 nm to 8.5 nm were synthesized by the sol-gel technique. Two sol-gel methods were developed. Method (1) involves the immersion of CdO-doped gels in Na2Te/methanol (MeOH) solution for 1 to 4 days at 60 degrees Celsius, followed by heat-treatment at 540 degree Celsius or 570 degrees Celsius for 6 to 12 hours in an inert atmosphere. In method 2, CdTe QDs were directly formed at various temperatures (520 degrees Celsius to 570 degrees Celsius) under a reducing atmosphere. The synthesis of CdTe QDs in the NBS glass was accomplished using cadmium nitrate [Cd(NO3)2] and telluric acid (H6TeO6) as the starting materials. The number density and particle size distribution of the CdTe nanocrystallites depend on the heat- treatment conditions for the reduction of Te6+ to Te2- ions and their subsequent ions react with Cd2+ ions to form CdTe. The absorption edges of these glasses were blue-shifted due to quantum confinement depending on the concentration of the initial salt and the heat- treatment conditions.