Effect of electromechanical boundary conditions on the properties of epitaxial 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 Ginzburg-Landau equation (DGLE). 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 epitaxial 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.     

旋涂法制备氧化锆介质层及其在薄膜晶体管中的应用

采用旋涂法制备了氧化锆介质层薄膜,重点讨论了退火温度以及旋涂转速对薄膜性能的影响及作用机制。研究发现高温后退火一方面使得氧化锆水合物脱水形成氧化锆,另一方面促使氧化锆薄膜结晶。此外,转速较高时,其变化对薄膜厚度及粗糙度无显著影响。当转速为5 000 r/min、退火温度为300℃时,制备的绝缘层厚度具有良好的厚度均匀性,粗糙度为0.7 nm,漏电流为3.13×10^-5 A/cm²（电场强度1 MV/cm）。最终,利用ZrO₂薄膜作为栅极绝缘层,在玻璃基板上制备了铟镓锌氧化物-薄膜晶体管（IGZO-TFT）,其迁移率为6.5 cm²/（V·s）,开关比为2×10⁴。     

First nucleation steps during deposition of SiO2 thin films by plasma enhanced chemical vapour deposition

The initial nucleation stages during deposition of SiO₂ by remote plasma enhanced chemical vapour deposition (PECVD) have been monitored by XPS inelastic peak shape analysis. Experiments have been carried out on two substrates, a flat ZrO₂ thin film and a silicon wafer with a native silicon oxide layer on its surface. For the two substrates it is found that PECVD SiO₂ grows in the form of islands. When the SiO₂ particles reach heights close to 10 nm they coalesce and cover completely the substrate surface. The particle formation mechanism has been confirmed by TEM observation of the particles grown on silicon substrates. The kinetic Monte Carlo simulation of the nucleation and growth of the SiO₂ particles has shown that formation of islands is favoured under PECVD conditions because the plasma species may reach the substrate surface according to off-perpendicular directions. The average energy of these species is the main parameter used to describe their angular distribution function, while the reactivity of the surface is another key parameter used in the simulations.     

Kinetic stabilization of Fe film on GaAs(1 0 0): An in situ X-ray reflectivity study

We study the growth of Fe films on GaAs(1 0 0) at a low temperature, 140 K, by in situ X-ray reflectivity (XRR) using synchrotron radiation. The XRR curves are well modeled by a single Fe layer on GaAs both at the growth temperature and after annealed at the room temperature. We found that the surface became progressively rougher during the growth with the growth exponent, β_S = 0.43 ± 0.14. The observed β_S is attributed to the restricted interlayer diffusion at the low growth temperature. The change of the interface width during growth was minimal. When the Fe film was annealed to room temperature, the surface smoothed, keeping the interface width almost unchanged. The confinement of the interface derives from that the diffusion of Ga and As proceeds via the inefficient bulk diffusion, and the overlying Fe film is kinetically stabilized.     

Thin film stress measurement by instrumented optical fibre displacement sensor

Residual stress can adversely affect the mechanical, electronic, optical and magnetic properties of thin films. This work describes a simple stress measurement instrument based on the bending beam method together with a sensitive non-contact fibre optical displacement sensor. The fibre optical displacement sensor is interfaced to a computer and a Labview programme enables film stress to be determined from changes in the radius of curvature of the film-substrate system. The stress measurement instrument was tested for two different kinds of thin film, hard amorphous carbon nitride (CN) and soft copper (Cu) films on silicon substrates deposited by RF magnetron sputtering. Residual stress developed in 500 nm thick CN thin films deposited at substrate temperatures in the range 50-550 °C was examined and it was found that stress in CN films decreased from 0.83 to 0.44 GPa compressive with increase of substrate temperature. Residual stress was found to be tensile (121 MPa) for Cu films of thickness 1500 nm deposited at room temperature.     

The role of oxygen and hydrogen partial pressures on structural and optical properties of ITO films deposited by reactive rf-magnetron sputtering

Sn doped In₂O₃ films are deposited by rf-magnetron sputtering at 300 °C under Ar, Ar + O₂ and Ar + H₂ gas ambients. For the film prepared under argon ambient, electrical resistivity 6.5 × 10⁻⁴ Ω cm and 95% optical transmission in the visible region have been achieved optimizing the power and chamber pressure during the film deposition. X-ray diffraction spectra of the ITO film reveal (2 2 2) and (4 0 0) crystallographic planes of In₂O₃. With the introduction of 1.33% oxygen in argon, (2 2 2) peak of In₂O₃ decreases and resistivity increases for the deposited film. With further increase of oxygen in the sputtering gas mixture crystallinity in the film deteriorates and both the peaks disappeared. On the other hand, when 1.33% hydrogen is mixed with argon, the resistivity of the deposited film decreases to 5.5 × 10⁻⁴ Ω cm and the crystallinity remains almost unchanged. In case of reactive sputtering, the deposition rate is lower compared to that in case of non-reactive sputtering. HRTEM and first Fourier patterns show the highly crystalline structure of the samples deposited under Ar and Ar + H₂ ambients. Crystallinity of the film becomes lower with the introduction of oxygen in argon but refractive index increases from 1.86 to 1.9. The surface morphology of the ITO films have been studied by high resolution scanning electron microscopy.     

Stress dependence of growth mode change of epitaxial layered cobaltite γ-Na0.7CoO2

We report stress dependence of growth characteristics of epitaxial γ-Na_0.7CoO₂ films on various substrates deposited by pulsed laser deposition method. On the sapphire substrate, the γ-Na_0.7CoO₂ thin film exhibits spiral surface growth with multi-terraces and highly crystallized texture. For the γ-Na_0.7CoO₂ thin film grown on the (1 1 1) SrTiO₃ substrate, the nano-islands of ∼30 nm diameter on the hexagonal grains are observed. These islands indicate that the growth mode changes from step-flow growth mode to Stranski-Krastanow (SK) growth mode. On the (1 1 1) MgO substrate, the large grains formed by excess adatoms covering an aperture between hexagonal grains are observed. These experimental demonstrations and controllability could provide opportunities of strain effects of Na_xCoO₂, physical properties of thin films, and growth dynamics of heterogeneous epitaxial thin films.     

(111)取向生长Pb(Zr0:95Ti0:05)O3纳米晶薄膜的铁电与热释电特性

" 在Pt/Ti/SiO2/Si基片上用溶胶-凝胶法与快速退火工艺制备了300 nm厚的锆钛酸铅Pb(Zr0:95Ti0:05)O3 (PZT95/5)反铁电薄膜．结果显示600~700 ℃晶化处理的钙钛矿PZT95/5薄膜具有高度(111)取向生长特性．薄膜的电性能测量采用金属-铁电-金属电容器结构．在20 V电压作用下,600~700 ℃晶化处理的PZT95/5薄膜显示出饱和电滞回线．在1 kHz下,600、650和700 ℃晶化的薄膜介电常数与损耗分别为519与0.028、677与0.029、987     

A sagittally focusing double‐multilayer monochromator for ultrafast X‐ray imaging applications

The development of a sagittally focusing double‐multilayer monochromator is reported, which produces a spatially extended wide‐bandpass X‐ray beam from an intense synchrotron bending‐magnet source at the Advanced Photon Source, for ultrafast X‐ray radiography and tomography applications. This monochromator consists of two W/B₄C multilayers with a 25 Å period coated on Si single‐crystal substrates. The second multilayer is mounted on a sagittally focusing bender, which can dynamically change the bending radius of the multilayer in order to condense and focus the beam to various points along the beamline. With this new apparatus, it becomes possible to adjust the X‐ray beam size to best match the area detector size and the object size to facilitate more efficient data collection using ultrafast X‐ray radiography and tomography.     

Zn、Cu共掺杂TiO_2∶SiO_2薄膜材料的光学性能研究

用溶胶-凝胶法制得Zn、Cu共掺杂的TiO_2∶SiO_2凝胶,旋转法于玻璃基底涂膜,制得Zn、Cu共掺杂的TiO_2∶SiO_2薄膜,探讨了煅烧温度、煅烧时间及掺杂比例对其结构、形貌和性能的影响。采用XRD、FESEM、FTIR等测试技术对薄膜进行表征,并考察了其对甲基橙的光催化降解性能。XRD测试结果显示:薄膜样品的晶型为锐钛矿型,结晶良好。SEM谱图显示:薄膜微粒粒径小,分布均匀,表面平整、致密且无明显裂痕;紫外-可见光谱(UV-Vis)表明:Zn、Cu共掺杂的TiO_2∶SiO_2薄膜在紫外区和可见光区的吸光度明显增加,提高了对光的利用率;光催化性能测试表明:与纯相TiO_2对比,Zn、Cu共掺杂的TiO_2∶SiO_2薄膜对甲基橙的光催化降解率有较大提高,在600℃下焙烧2h的掺杂的量比为n(Ti)∶n(Si)∶n(Zn)∶n(Cu)=3∶2∶1.5∶4的薄膜样品光催化降解率最高。