异质外延MgO/SrTiO3薄膜中界面应力研究

本文利用激光分子束外延(LMBE)技术在SrTiO3(100)单晶基片上外延生长MgO薄膜，同时又在MgO(100)单晶基片上外延生长SiO3(STO)薄膜。通过反射高能电子衍射(RHEED)仪原位实时监测薄膜生长，研究薄膜的生长过程。并结合X射线衍射(XRD)仪来分析在不同的生长条件下，不同应力对薄膜外延生长的影响。在压应力情况下，MgO薄膜在STO基片上以单个晶胞叠层的方式生长，即以“Cubicon Cubic”方式进行外延；在张应力情况下，由于膜内位错较多，STO薄膜在MgO基片上以晶胞镶嵌的方式进行生长，即以“Mosaic”结构进行外延；提高生长温度，可以减少膜内位错，提高外延质量，使STO薄膜在MgO基片上以较好的层状方式外延生长。     

SrTiO3薄膜材料的高分辨率X射线衍射分析研究

本文应用高分辨X射线衍射(HRXRD+TAXRD)技术对外延生长的SrTiO3膜进行了分析,获得了有关该薄膜的晶体取向、衬底的结构特性以及弛豫态的点阵常数等信息,对今后改进SrTiO3系列样品生长工艺有重要的意义.     

Nb∶SrTiO3单晶基片上外延生长BaTiO3薄膜的漏电流机制分析

采用脉冲激光沉积法(PLD),在掺铌的钛酸锶单晶基片(Nb∶SrTiO3,NSTO)上外延生长了15nm厚度的BTO薄膜。并采用X射线衍射(XRD)、原子力显微镜(AFM)、高分辨透射电子显微镜(HRTEM)等技术,分别测试了BTO薄膜的晶相结构和微观形貌与结构,证实了薄膜的外延生长特征。以磁控溅射法制备金属铂(Pt)上电极,在室温下测试了BTO薄膜的电流-电压(I-V)曲线。通过拟合I-V曲线,结果表明:在正向电压下,薄膜的漏电流符合空间电荷受限电流机制(SCLC);在反向电压下,漏电流先为普尔-弗伦克尔发射机制(Poole-Frenkel emission),随着反向电压的升高,转变为福勒-诺德海姆隧穿机制(Fowler-Nordheim tunneling)。分析指出,在Pt/BTO界面存在一个可变的肖特基势垒,该势垒受到BTO薄膜内部迁移的氧空位影响,从而决定了上述漏电流机制。     

SrTiO3衬底上溅射法外延生长PLZT薄膜

用射频磁控溅射法在SrTiO3衬底上外延生长PLZT薄膜,研究了不同的溅射工艺对薄膜生长速率的影响,探讨了不同的后期热处理条件与薄膜取向度的关系.在SrTiO3衬底上成功制备出外延生长的、厚度达1.5μm的PLZT薄膜.     

激光分子束外延BaTiO3/SrTiO3超晶格的晶格应变研究

采用脉冲激光分子束外延(PLMBE)方法,通过优化的工艺参数,在SrTiO3(100)单晶基片上外延结构为(8/8)的BaTiO3/SrTiO3超晶格薄膜.综合利用反射式高能电子衍射系统(RHEED)、高分辨率X射线衍射(HRXRD)以及高分辨率透射电镜选区电子衍射(SAED)技术,研究超晶格薄膜的晶格应变现象和规律.研究结果表明,在制备的BaTiO3/SrTiO3超晶格薄膜中,BaTiO3晶胞面外晶格增大,面内晶格减小;而SrTiO3晶胞面内及面外方向晶格都被拉伸,但面外晶格拉伸程度较大,SrTiO3晶胞产生了与BaTiO3晶胞方向一致的四方相转变.     

超高真空中SrTiO3薄膜同质外延生长的过程研究

在超高真空下利用激光分子束外延(LMBE)方法基于SrTiO3(100)单晶基片同质外延SrTiO3薄膜.通过反射式高能电子衍射(RHEED)对生长过程进行原位监测,发现对基片的预热处理明显有利于改善其晶面结构,当在其上同质外延SrTiO3薄膜时,容易实现单晶层状生长模式,并得到原子级平整度的铁电薄膜.     

斜切基片上c-取向YBa2Cu3O7-δ薄膜的生长特性及开裂行为

用脉冲激光沉积方法在斜切SrTiO3(001)基片上制备了YBa2Cu3O7-δ薄膜,并用原子力显微镜、X射线衍射仪及透射电镜研究了薄膜表面形貌、取向特征、结晶性和显微结构.结果表明,薄膜表面呈"台阶-台面"结构,并伴有与台阶边缘垂直的裂纹产生,说明薄膜以"台阶流动"方式生长;而且薄膜具有c-轴取向,但其结晶性不好.在薄膜中没有观察到a-b孪晶的存在.薄膜的开裂和结晶性较差均是由于斜切基片台阶表面晶格四方畸变抑制了薄膜正交转变时a-b孪晶的产生,而使晶格错配应变不能释放造成的.     

SrTiO3斜切基片上外延生长YBa2Cu3O7-δ薄膜的扫描探针显微镜研究

采用激光脉冲沉积法在钛酸锶SrTiO3 (0 0 1)斜切基片上外延生长YBa2 Cu3 O7 δ薄膜 ,在大气环境下采用扫描探针显微镜对YBa2 Cu3 O7 δ薄膜的表面纳米形貌进行直接观察。发现YBa2 Cu3 O7 δ薄膜具有相对光滑的表面形貌 ,薄膜表面由沿SrTiO3台阶趋向外延生长的纳米台阶组成 ,薄膜生长模式主要以台阶媒体生长为主。     

MOCVD同质外延生长的单晶β-Ga2O3薄膜研究

采用金属有机化学气相沉积(MOCVD)技术,在(010)Fe掺Ga₂O₃半绝缘衬底上进行同质外延生长Ga₂O₃薄膜,系统性地研究了生长温度(880℃/830℃/780℃/730℃)和生长压强(80/60/40/20 Torr)对外延薄膜表面形貌、晶体质量以及电学特性等的影响。结果表明随着生长温度和压强的增加：薄膜生长速率分别呈现出略微增加和大幅下降的趋势;薄膜表面阶梯束(step bunching)的生长方式逐渐增强,并且呈现出沿着[001]晶向生长的柱状晶粒;高分辨X射线衍射(XRD)扫描显示薄膜均只在(020)面存在衍射峰,表明生长的薄膜为纯β相单晶薄膜,且半高宽可达到45.7 arcsec;霍尔测试表明780℃和60 Torr的生长条件下薄膜的室温电子迁移率最高。本文为基于MOCVD的Ga₂O₃同质外延生长提供了系统的参数指导,为高质量Ga₂O₃薄膜的制备奠定了基础。     

取向对PLD法制备BiFeO3薄膜性能的影响

以快速等离子烧结法(SPS)制备的BiFeO3块体为靶材,用激光脉冲沉积(PLD)法在不同衬底上制备了BiFeO3(100)/LaNiO3(100)/Si(100)、BiFeO3(111)/LaNiO3(111)/SrTiO3(111)、BiFeO3(110)/Pt/TiO2/SiO2/Si、BiFeO3(110)LaNiO3(110)/Pt/TiO2/SiO2/Si不同择优取向的薄膜,并对薄膜进行了XRD和SEM分析。X射线衍射结果表明,BiFeO3薄膜外延沉积在导电层衬底上,并且它们具有相同的高度取向。SEM分析表明,薄膜上的晶粒是柱状形态,表面光滑致密且颗粒分布非常均匀,晶粒的边界和尺寸也能被清晰地观察到。通过铁电铁磁性能研究,BiFeO3(111)择优取向性能最佳。SrTiO3衬底上(111)取向的BiFeO3薄膜铁电剩余极化值达到了30.3μC/cm2,漏电流为1.0×10-3 A/cm2,饱和磁化强度为20.0kA/m。     

Formation of SrTiO3 from Sr-oxalate and TiO2

SrTiO₃ powder has been prepared from Sr-oxalate and TiO₂ precursors, instead of using titanyl-oxalate. Sr-oxalate was precipitated from nitrate solution onto the surface of suspended TiO₂ powders. Crystallization of SrTiO₃ from the precursor was investigated by TGA, DTA and XRD analysis. It is evident that precursor, upon heating, dehydrates in two stages, may be due to the presence of two different types of Sr-oxalate hydrates. Dehydrated precursor then decomposes into SrCO₃ and TiO₂ mixture. Decomposition of SrCO₃ and simultaneous SrTiO₃ formation occur at much lower temperature, from 800 °C onwards, due to the fine particle size of the SrCO₃ and presence of acidic TiO₂ in the mixture. The precursor completely transforms into SrTiO₃ at 1100 °C. About 90 nm size SrTiO₃ crystallites are produced at 1100 °C/1 h, due to the lower calcination temperature and better homogeneity of the precursor.     

Interstitial and substitutional zirconium in SrTiO3

We investigate Zr in SrTiO₃ (STO) as a model for nuclear waste forms in which the fission product ⁹⁰Sr eventually decays to stable Zr through beta emission. The transformation of a divalent into a tetravalent constituent is expected to affect the long-term structural and chemical stability of this solid. Computational methods of electronic structure theory, specifically the density functional theory (DFT) within the supercell model, are used to predict the thermodynamic stability and electronic states of interstitial and Sr- or Ti-substituted Zr atoms in the STO lattice. Native defects such as vacancies and antisites are also considered. When Zr replaces Sr, its most stable configuration is to simply occupy the Sr site. For Zr added to the lattice, its most stable configuration is to replace a Ti, making a Zr_Ti impurity plus a Ti interstitial. Zr_Sr is predicted to be a double electron donor, Zr_Ti is electrically inactive and interstitial Zr and Ti are predicted to be quadruple donors, with all donor levels in the conduction band. The interstitials are all predicted to increase the crystal volume, and lead to a tetragonal distortion of the lattice. Experiments with injection of Zr and O atoms into STO qualitatively confirm these predictions of crystal structural changes.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Spin-dependent tunnelling in La0.7Sr0.3MnO3/SrTiO3 superlattices

La_0.7Sr_0.3MnO₃ is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La_0.7Sr_0.3MnO₃/SrTiO₃ epitaxial superlattices were grown on LaAlO₃(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La_0.7Sr_0.3MnO₃ and SrTiO₃. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I–V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I–V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.     

Ferromagnetism in Electrospun Co-doped SrTiO3 Nanofibers

By employing electrospinning technique, subsequent calcination in air and annealing process in hydrogen, uniform Co-doped SrTiO₃ nanofibers with concentrations of Co between 0 and 0.20 were successfully produced. Their morphologies and detailed structures were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray powder diffraction. And, the chemical states of Co were determined by X-ray photoelectron spectroscopy. It was shown that, after calcination, Co²⁺ was well incorporated into the perovskite structure of SrTiO₃, and the nanofibers possessed smooth surface with diameters of 50–100 nm. Magnetic properties of the hydrogen-annealed and -unannealed nanofibers were both measured by physical property measurement system from 50 to 300 K. It was explored that the Co addition and the hydrogen annealing process were both very important to the generation of the observed ferromagnetism in SrTi_1−x Co _x O₃:H₂ nanofibers. In hydrogen-annealed SrTi_0.80Co_0.20O₃:H₂ nanofibers, a saturation magnetization of 0.74 emu/g and an average moment of 0.122 μ_B/Co were obtained. The origins of the enhanced ferromagnetism in SrTi_1−x Co _x O₃:H₂ nanofibers were analyzed according to the chemical state of Co and the mediation of oxygen vacancies.     

Cr掺杂SrTiO3光催化材料中Cr的稳定性

采用固相法制备了不同掺杂量、不同煅烧温度和不同取代位置的Cr掺杂SrTiO3样品,研究了其在不同pH值的溶液中Cr的稳定性。结果表明:即使掺杂量为1%,溶液中还是会有Cr相关的离子存在;溶液中Cr相关离子的浓度,随着掺杂量的增加而增大,随着煅烧温度的升高而减少。Cr在取代Sr位掺杂的样品中比取代Ti位掺杂的样品更稳定。Cr在中性溶液中最稳定,在酸性溶液中是以Cr3+离子存在,在中性和碱性溶液中以Cr2O27-与CrO24-离子存在。     

Porous SrTiO3 spheres with enhanced photocatalytic performance

Porous SrTiO₃ spheres were successfully synthesized by a convenient hydrothermal method, employing SrCl₂ as Sr source and titanate nanotube as Ti precursor. In this reaction, when short titanate nanotube was used as Ti precursor, porous SrTiO₃ spheres were generated for the aggregation of the nanotube@SrTiO₃ heteronanostructure. Whereas long titanate nanowire was used as the Ti precursor, solid SrTiO₃ spheres were obtained due to the SrTiO₃ which grows up gradually on the titanate nanowire. The morphology and the pore size of the SrTiO₃ sphere structures can be easily controlled by simply adjusting the reaction time, reaction temperature and the Ti precursor. The porous SrTiO₃ spheres exhibited enhanced photocatalytic activity which could achieve 100% degradation of Rhodamine B with a UV irradiation for 20 min.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Non-volatile memory properties of metal/SrTiO3/SiO2/Si structures

Non-volatile MIOS-type semiconductor memory elements were fabricated on silicon using electron-beam-evaporated SrTiO₃ as the second insulator. The charge storage properties were characterized for Au/SrTiO₃/SiO₂/Si structures. Our results show that a short-time post-deposition oxygen annealing is essential to anneal out the radiation damage resulting from electron beam deposition. The devices on n-type silicon substrates show fast switching for a positive applied pulse and a much lower switching speed (longer than 20 ms) for a negative pulse, which is believed to be caused by the minority carrier restriction. The devices show a logarithmic decay of the flat-band voltage as a function of time, with a rate of 0.4 V decade^-1 for stored electrons and 0.5 V decade^-1 for stored holes. The devices can survive 10⁴ write-erase cycles of endurance testing. An inversion of the surface silicon layer is found for devices on p-type substrates subjected to high temperature oxygen annealing.     

Band-edge luminescence from SrTiO3: No polaron effect

We studied the band-edge photoluminescence (PL) of electron-doped SrTiO₃ and strongly photoexcited SrTiO₃ crystals. Two band-edge PL peaks are clearly observed at low temperatures. The PL peak energies coincide with the high- and low-temperature onsets of the optical absorption, corresponding to the phonon-assisted band-to-band optical transition. This result is the direct evidence that the free electrons and holes exist in the conduction and valence bands, and radiative recombination of free electrons and free holes causes the band-edge PL.