Yan Rong Li

Laser deposition and dielectric properties of cubic pyrochlore bismuth zinc niobate thin films

The Bi1.5ZnNb1.5O7 (BZN) thin films with cubic pyrochlore structure were prepared on Pt∕SiO2∕Si substrates by pulsed laser deposition from a stoichiometric target. The crystallinity, surface morphology, and dielectric properties of BZN thin films at different substrate temperatures were investigated. It was found that the BZN films had a pure cubic pyrochlore structure when deposited at 550–650°C and the strong (222) texture at 650°C. The cubic BZN thin films deposited at 600°C had a dielectric loss lower than 0.004 and maximum voltage tunability of 6% with an applied bias field of 0.5MV∕cm under 100kHz.

Control of growth mode in SrTiO3 homoepitaxy under 500°C

Homoepitaxial SrTiO3 thin films were grown by laser molecular beam epitaxy. The growth mode was determined by in-situ reflective high energy electron diffraction, and the surface of the films was studied by ex-situ atomic force microscopy. At the deposition rate of 0.16A∕sec and the laser energy density of 6J∕cm2, layer-by-layer growth was observed above 460°C substrate temperature, while the Stranski-Krastanov growth mode, that is layer-by-layer growth plus island growth mode, prevailed between 460°C and 410°C. On further decreasing the substrate temperature, the island growth was determined under 410°C. With the optimization of deposition process in terms of laser energy density and deposition rate, the lowest crystallization temperatures of SrTiO3 films grown in layer-by-layer growth mode were obtained as low as 280°C. The effects of laser energy density on growth temperature were studied.

Growth dynamics and cell migration in ferroelectric thin films

Growth dynamics and unit cell migration of ferroelectric (Ba,Sr)TiO3 thin films were systematically studied with in situ reflective high-energy electron diffraction and atom force microscopy. By measuring the amplitudes of slow surface recovery oscillations of SrTiO3 film grown on (001) SrTiO3 surface, the activation energy of unit cell surface self-diffusion was surprisingly found to be only 0.29±0.01eV. A “unit-cell migration” model was developed to understand the epitaxial growth dynamics of ferroelectric thin films, which is a critical issue in oxide thin film growth and crucial in controlling the atomic structures.

Thickness uniformity of large-area double-sided thin films simultaneously deposited with biaxial substrate rotation

We have theoretically analyzed the thickness distribution of large-area double-sided thin films deposited with biaxial substrate rotation. The films were deposited from different physical vapor deposition sources (such as point, round-plate, cylinder, and conic sources) and have different thickness distribution characters. The calculated results show that the biaxial rotation is a suitable technique for simultaneous deposition on both sides from most sources if particles transfer diffusively.

Effects of oxygen pressure on the microstructure of LaNiO3 conductive thin film monitored by in situ reflection high energy diffraction

LaNiO3 (LNO) conductive thin films with different thicknesses were deposited on SrTiO3 (100) substrates in different oxygen pressures. Effects of oxygen pressure on microstructure of LNO conductive film have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray photoelectron spectroscopy (XPS). In the relatively low oxygen pressure, LNO film displays spotty RHEED pattern. When the thickness increases up to a critical value, about 30nm, the spotty RHEED pattern gradually changes to streaky pattern, and the RHEED oscillation curve appears. The streaky RHEED pattern of LNO film deposited in the relatively high oxygen pressure can be observed at the initial growth. With pumping the oxygen pressure to a relatively low value, the RHEED pattern gradually changes to spotty one. When the oxygen pressure is increased back to a high value, the RHEED pattern changes to streaky one again. This RHEED pattern transformation induced by the oxygen pressure is reversible. Ex situ XPS...

Some considerations on heater design for simultaneous deposition of large-area double-sided high-Tc superconducting thin films

Some important factors on heater design, which is used for the deposition of high-Tc superconducting thin films, have been discussed in this article. Compared to radiation heat transfer directly from the heater to the substrate, the conduction and convection due to the inlet gas (oxygen or/and argon) are negligible. A theoretical model based on radiation heat transfer has been proposed by which some heater configurations have been discussed in detail. The temperature distribution in a simultaneous deposition system with two sputtering sources is improved by either proper off-axis displacement accompanied with in-plane substrate rotation or increasing the distance of the two heating sheets. For a biaxial rotating substrate, a cylindrical heater is proposed. The temperature distribution in the heater can be modulated by varying the input power ratio of the two heating parts. And, the total radiation power imposed on the substrate in every out-of-plane revolution is excellently homogeneous.

Well-Ordered Self-Assembly Growth of Strain-Modulated SrTiO3 Thin Films: Templates for Complex Oxide Quantum Wires

Well-ordered self-assembled SrTiO3 thin film, as a template for complex oxide quantum wires, was fabricated on LaAlO3 (100) single crystal substrates with laser molecular beam epitaxy. The self-assembled growth was in-situ monitored by reflective high energy electron diffraction. The morphology evolutions of the films as a function of thickness were studied by ex-situ atomic force microscopy. As the thickness of the films increased from 3.875nm to 46.5nm gradually, the compressive stress-induced SrTiO3 films exhibited a periodic well-ordered ripple structure, which formed a unique nanoassembled template for the fabrication of quantum wires. Small angle X-ray scattering technique was employed to investigate the structure. Symmetric satellite peaks were discovered, indicating the well-ordered superstructure. In contrast, the similar superstructure was not observed during the growth of the tensile stress-induced LaAlO3 films on SrTiO3 substrates. The Compressive stress was considered as the main reason of the self-assembled growth, and systematical elucidation about strain mechanism was discussed. These results might provide an efficient method for the controllable formation of well-aligned template of quantum wire for complex oxide with desirable structure via proper modulation of strains.