InAs/GaInSb超晶格薄膜结构与电学性能

采用分子束外延(MBE)方法, 调节生长温度、Ⅴ/Ⅲ束流比等参数在(001)GaAs衬底上生长了InAs/GaInSb超晶格薄膜.结果表明:InAs/GaInSb超晶格薄膜的最佳生长温度在385~395 ℃, Ⅴ/Ⅲ束流比为5.7 :1~8.7 :1.高能电子衍射仪(RHEED)原位观测到清晰的GaAs层(4×2)、GaSb层(1×3)和InAs层(1×2)再构衍射条纹.获得的超晶格薄膜结构质量较好.随着温度的升高, 材料的载流子浓度和迁移率均上升.     

SrTiO3同质外延过程中的反射高能电子衍射图案分析

在激光分子束外延实验中,用RHEED原位监测了SrTiO3基片初始、退火以及同质外延过程中的表面形态.通过对RHEED图案分析,获取了表面面内的晶格常数振荡与衍射条纹的半高宽振荡现象,前者是由退火重构表面与薄膜之间的界面造成的,后者与二维岛边界的弛豫相关.另外还观察到了等离子体对入射电子束的影响而导致的RHEED强度振荡行为的相位移现象. 关键词： 反射高能电子衍射 SrTiO3 表面晶格常数及衍射强度振荡     

FeSe/SrTiO3高温超导体中的电子条纹相

单层FeSe/SrTiO₃中的界面超导增强是近年来高温超导领域的重要发现.该体系中SrTiO₃衬底对FeSe的超导增强机制已被广泛研究,其调控作用主要表现为两个方面:电荷掺杂和界面电声耦合.然而,关于FeSe薄膜本身的电子特性研究还不够充分.本文介绍该体系超导增强机制的新进展:FeSe薄膜中的电子条纹相及其与超导的关联.通过扫描隧道显微镜结合分子束外延生长技术,对不同厚度的FeSe薄膜进行了系统研究.我们发现FeSe薄膜中电子倾向于排成条纹状结构,并观测到该条纹相随层厚变化显现出从短程到长程的演化.条纹相是一种电子液晶态,它源于薄层FeSe中被增强的电子关联作用.表面电子掺杂一方面会减弱FeSe薄膜中的电子关联作用,逐渐抑制条纹相;另一方面会诱导超导相变,而剩余的条纹相涨落会对超导电性带来额外增强.我们的结果加深了对低维界面超导体系的认识,也揭示了FeSe薄膜本征的特异性,完善了对FeSe/SrTiO₃超导增强机制的理解.     

SrTiO_3(001)衬底上单层FeSe超导薄膜的分子束外延生长

在以前工作的基础上,进一步研究了SrTiO3(001)(STO)衬底上单层FeSe超导薄膜的分子束外延生长.首先,通过去离子水刻蚀、盐酸溶液腐蚀和纯氧气氛中退火等步骤,获得台阶有序、具有单一TiO2终止的原子级平整表面的STO衬底,这是前提条件.这个过程中酸的选择和退火过程中氧的流量是最为关键的因素.其次,在FeSe薄膜的分子束外延生长中,选择适当的Fe源和Se源束流以及衬底温度是关键因素.如选择适当,生长模式为step-?ow生长,这时得到的FeSe薄膜将是原子级平整的.最后一步为退火,这个过程会增强FeSe薄膜结晶性以及它与SrTiO3衬底间的结合强度.     

Si(111)分子束外延的生长动力学过程研究

本文用反射式高能电子衍射(RHEED)强度振荡研究了不同生长温度下Si(111)分子束外延的生长动力学过程,生长温度高于520℃(生长速率约0.15?/S)时,Si(111)外延为“台阶流”生长模式,生长温度低于475℃时,外延为“二维成核”双原子层生长模式,在较低温,甚至室温时,其外延仍为双原子层模式,但是镜向弹性散射束振荡和非弹性散射束振荡的叠加会造成RHEED强度在生长的最初阶段出现“类单原子层”模式的振荡特性。 关键词：     

高质量镍基超晶格薄膜的制备及表征

自发现镍基超导薄膜之后, 镍基材料体系已成为当前人们的研究热点. 而在相关报道中, 普遍认为高质量的镍基超导前驱体薄膜的制备对其拓扑还原后超导电性的实现具有重要影响. 前期研究表明高质量的前驱体薄膜只生长在SrTiO3 衬底附近, 这与我们的实验结果一致. 为了可控制备高质量的镍基超导前驱体薄膜, 本文利用脉冲激光沉积(PLD) 技术在SrTiO3 (001) 衬底上生长不同厚度的可层选择性还原的[SrTiO3 ]m/[ Nd0 .8Sr0 .2 NiO3 ]n[(STO)m/(NSNO)n ]超晶格薄膜. 采用反射高能电子衍射仪(RHEED) 及透射电子显微镜(STEM) 和 X 射线衍射(XRD) 技术对超晶格薄膜的结构进行原位检测及结构表征, 然后利用综合物性测量系统(PPMS) 测试薄膜的电磁输运性质. 结果表明, 超晶格薄膜的结构质量良好, 超晶格薄膜和高质量单层前驱体薄膜表现出类似的电阻和磁阻现象. 该研究为后续镍基超导薄膜的可重复制备提供了重要的参考.     

Mn掺杂SiC磁性薄膜的结构表征

利用反射式高能电子衍射(RHEED)、X射线衍射(XRD)和X射线吸收近边结构谱(XANES)等技术研究了在950 ℃条件下Si(111)衬底上共蒸发分子束外延方法制备的Mn掺杂SiC磁性薄膜的结构特征.RHEED结果表明,生长的Mn掺杂SiC薄膜为立方结构.XRD和XANES结果表明,在Mn掺杂量为0.5％和18%的样品中,Mn原子均是与SiC半导体介质中的Si原子反应生成镶嵌在SiC基体中的Mn₄Si₇化合物颗粒,并未观察到在SiC晶格中有替代式或间隙式的M     

GaSb/AlSb/GaAs应变层结构的分子束外延生长

本文以反射式高能电子衍射(RHEED)和其强度振荡为监测手段,在半绝缘GaAs衬底上成功地生长GaSb/AlSb/GaAs应变层结构,RHEED图样表明,GaSb正常生长时为Sb稳定的C(2×6)结构,AlSb为稳定的(1×3)结构,作者观察并记录GaSb,AlSb生长时的RHEED强度振荡,并利用它成功地生长10个周期的GaSb/AlSb超晶格,透射电子显微镜照片显示界面平整、清晰,采用较厚的AlSb过渡层及适当的生长条件,可在半绝缘GaAs衬底上生长出质量好的GaSb外延层,其X射线双晶衍射半峰宽小于 关键词：     

LaTiO3+x薄膜生长氧压对薄膜结构和输运性能的影响

利用脉冲激光沉积 (PLD)的方法,在750℃的生长温度,不同的氧气气氛下(6.6×10-4Pa～50Pa)制备了LaTiO3 x(LTO)薄膜.反射式高能电子衍射仪 (RHEED)实时监测表明我们所制备的薄膜表面平整光滑,面内取向较好.X射线衍射 (XRD)数据表明:LaTiO3 x薄膜特征峰的位置随生长氧压的变化而发生变化.当氧压增大时,衍射峰对应的2θ角减小,并且在中间氧压时,衍射峰宽化.我们认为氧压增大,使得薄膜中氧含量增加.随着x值的增加,薄膜的电输运性能发生显著改变.在高真空度条件下生长的薄膜表现出很强的金属性,当氧压增加时,材料的电阻率显著增大.实验中测量的所有样品的电阻率都保持正的温度系数,并且在一个相当大的温度区间内与温度的平方呈线性关系.     

高气压反射式高能电子衍射仪监控脉冲激光外延氧化物薄膜

在超高真空分子束外延（MBE）生长技术中，反射式高能电子衍射仪（RHEED）能实时显示半导体和金属外延生长过程，给出薄膜表面结构和平整度的信息，成为MBE必备的原位表面分 析仪.为了研究氧化物薄膜如高温超导（YBa₂Cu₃O₇） 、铁电薄膜（Sr_1-xBa_x TiO₃）及它们的同质和异质外延结构的生长机理，获得高质量的符合各种应用 需要的氧化 物多层薄膜结构，在常规的制备氧化 关键词： 高温超导薄膜 RHEED     

表面吸附K原子的多层FeSe/SrTiO3(001)薄膜的抗磁响应的原位测量

SrTiO₃（001）单晶表面上生长的单层FeSe薄膜显示出了超乎寻常的高温超导电性,其超导增强机制的一个重要因素是电子由衬底转移到了单层FeSe薄膜当中.基于此认识,研究者们在吸附了钾（K）原子的多层FeSe薄膜表面上观察到了类似超导能隙的隧穿能谱和光电子能谱.但这种自上而下的电子掺入方式在多层FeSe薄膜表面上可能引起的高温超导电性,还缺乏零电阻或迈斯纳效应等物性测量实验的直接证实.本研究利用自行研制的一台特殊的多功能扫描隧道显微镜,在生长于SrTiO₃（001）衬底上的多层FeSe薄膜表面上,不但观察到了超导能隙随K吸附量的变化,而且利用原位双线圈互感测量技术,成功地的观察到了该薄膜的抗磁响应,并由此确定了该薄膜样品呈现迈斯纳效应的超导转变温度为23.9 K.其穿透深度随温度的变化呈二次幂指数关系,表明该体系的超导序参量很可能具有S^±配对对称性.     

高质量FeSe单晶薄膜的制备及相关性能表征

在铁基超导体中,FeSe具有最简单的晶体结构和化学组成,而且其超导转变温度具有较大的调控空间,因此适合作为超导机理研究和应用的载体.高质量样品的研制是物性研究和器件应用的前提,本文系统地研究了利用激光脉冲沉积技术制备FeSe薄膜的工艺条件,在多种衬底上成功地制备出高质量的β-FeSe薄膜,并首次实现了超导临界转变温度从小于2 K到14 K的连续调控,这为FeSe超导机理研究提供了样品支持.为探究FeSe薄膜超导电性变化的起因,从β-FeSe超导电性与晶格常数c正相关出发,基于简单的费米面填充假设,第一性原理计算可以很好地解释晶格常数c的变化规律,但该假设并不能完全符合角分辨光电子能谱实验给出的电子结构演变过程.因此β-FeSe薄膜的超导电性、晶格结构和电子结构三者之间的关系还有待澄清,该问题的解决将为FeSe超导机理研究提供重要的线索,而上述系列高质量的β-FeSe薄膜样品恰好能为该问题的研究提供理想的载体.本文根据实验和已有的相关研究结果,详细介绍了FeSe薄膜的脉冲激光沉积制备及其优化,以期为后续的薄膜研究应用提供参考.     

低维超导的实验进展

超导自发现以来, 已成为凝聚态物理领域最重要的方向之一. 近年来, 低维材料制备技术的进步使得一维或二维的超导特性实验研究成为可能. 本文在简要介绍超导现象的基础上, 重点回顾了近些年二维超导薄膜和一维超导纳米线的制备和电输运研究, 以及在低维超导体中发现的相移、近邻效应、铁磁超导相互作用和高温超导等新奇的现象, 并对该领域的进一步发展做出了展望.     

Observation of triangular terraces and triangular craters of CaF2 film on Si(1 1 1) substrate

We have investigated the growth mode and surface morphology of CaF₂ film on Si(1 1 1)7×7 substrate by reflection high-energy electron diffraction (RHEED) using very weak electron beam and atomic force microscopy (AFM). It was found by RHEED intensity oscillation measurements and AFM observations that three-dimensional (3D) islands grow at RT; however, rather flat surface appears with two-dimensional (2D) islands around 300 °C. Especially, at high temperature of 700 °C, characteristic equilateral triangular terraces (or islands) with flat and wide shape grow with the tops directed toward [1 1 −2] of substrate Si(1 1 1). On the other hand, the desorption process of the CaF₂ film due to electron stimulated desorption (ESD) was also examined. It was found that the ESD process at 300 °C forms characteristic equilateral triangular craters on the film surface with the tops (or corners) directed toward [−1 −1 2] of substrate Si(1 1 1), provided that the film was grown at 700 °C.     

Growth and photoluminescence properties of inclined ZnO and ZnCoO thin films on SrTiO₃（110） substrates

ZnO thin film growth prefers different orientations on the etched and unetched SrTiO 3(STO)(110) substrates.Inclined ZnO and cobalt-doped ZnO(ZnCoO) thin films are grown on unetched STO(110) substrates using oxygen plasma assisted molecular beam epitaxy,with the c-axis 42 inclined from the normal STO(110) surface.The growth geometries are ZnCoO[100]//STO[110] and ZnCoO[111]//STO[001].The low temperature photoluminescence spectra of the inclined ZnO and ZnCoO films are dominated by D 0 X emissions associated with A 0 X emissions,and the characteristic emissions for the 2 E(2G)→ 4A2(4F) transition of Co 2+ dopants and the relevant phonon-participated emissions are observed in the ZnCoO film,indicating the incorporation of Co 2+ ions at the lattice positions of the Zn 2+ ions.The c-axis inclined ZnCoO film shows ferromagnetic properties at room temperature     

Growth and morphology of thin films of aromatic molecules on metals: the case of perylene

The morphology and growth of perylene films on copper and gold surfaces have been characterized by XPS, AFM, SEM and polarization microscopy. Deposition at cryogenic temperatures leads to amorphous but homogeneous films whereas growth a room temperature results in a formation of disjointed crystalline islands. A similar morphology was observed after thawing the amorphous films which were grown before at low temperature and hence demonstrates a pronounced dewetting. Furthermore, it was found that the geometry of the resulting islands depends on the actual substrate surface which is attributed to the formation of seed layers and their influence on the subsequent film growth. The presently described dewetting and island formation appears to be a quite general phenomenon of organic film growth which needs to be considered in the interpretation of spectroscopic data and STM measurements for organic thin films. PACS 68.37.-d; 68.37.Ps; 68.55.Jk     

LaTiO3(110)薄膜分子束外延生长的精确控制和表面截止层的研究

LaTiO₃ 是一种典型的强关联电子材料, 其(110) 薄膜为通过晶格对称性、应变等的设计调控外延结构的物理性质提供了新的机会. 本文研究了SrTiO₃(110) 衬底表面金属La 和Ti 沉积所引起的微观结构变化, 进而利用电子衍射信号对分子束外延薄膜生长表面阳离子浓度的灵敏响应, 发展了原位、实时、精确控制金属蒸发源沉积速率的方法, 实现了高质量LaTiO₃(110) 薄膜的生长和对阳离子化学配比的精确控制. 由于LaTiO₃中Ti³⁺ 3d 电子的库仑排斥作用, 氧原子层截止的(110) 表面更容易实现极性补偿, 因此生长得到的薄膜表面暴露出单一类型的氧截止面.     

Thickness dependence of microstructures in La0.9Sr0.1MnO3 thin films grown on exact-cut and miscut SrTiO3 substrates

The thickness dependence of microstructures of La_0.9Sr_0.1MnO₃ (LSMO) thin films grown on exact-cut and miscut SrTiO₃ (STO) substrates, respectively, was investigated by high-angle X-ray diffraction (HXRD), X-ray small-angle reflection (XSAR), X-ray reciprocal space mapping and atomic force microscopy (AFM). Results show that the LSMO films are in pseudocubic structure and are highly epitaxial [0 0 1]-oriented growth on the (0 0 1) STO substrates. The crystalline quality of the LSMO film is improved with thickness. The epitaxial relationship between the LSMO films and the STO substrates is [0 0 1]_LSMO[0 0 1]_EXACT-STO, and the LSMO films have a slight mosaic structure along the q_x direction for the samples grown on the exact-cut STO substrates. However, an oriented angle of about 0.24° exists between [0 0 1]_LSMO and [0 0 1]_MISCUT-STO, and the LSMO films have a mosaic structure along the q_z direction for that grown on the miscut STO substrates. The mosaic structure of both groups of the samples tends to reduce with thickness. The diffraction intensity of the (0 0 4) peaks increases with thickness of the LSMO film. The XSAR and AFM observations show that for both groups, the interface is sharp and the surface is rather smooth. The mechanism was discussed briefly.     

Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF_2 substrate by pulsed laser deposition(PLD) method. Measurements on FeSe/CaF_2 samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140-160 K, much higher than ~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF_2 films by PLD method with the FeSe/SrTiO_3 films by molecular beam epitaxy(MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF_2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF_2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO_3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.