Lecture10清洁表面的制备及分子束外延.ppt

1,清洁表面的获得 1.1 超高真空的意义 1.2 清洁表面的获得手段 2. 分子束外延简介 3. 外延生长 3.1 外延生长的定义 3.2 外延生长的动力学过程 4. 反射式高能电子衍射 5. 分子束外延应用实例,实验4：清洁表面的制备及分子束外延,2,超高真空(Ultrahigh Vacuum)技术 获得清洁表面的前提,为什么需要超高真空技术？,由气体动力学方程，单位体积的气体分子与单位金属表面的碰撞频率,n单位体积内气体分子个数 ca平均速度,而均方根速率为,又有,3,得到,当P的单位取torr，T的单位取K，m由分子量M取代，有,(cm-2 s-1),N2分子量为28，室温293 K，1 torr压力下， = 3.881020 cm-2 s-1,典型固体表面原子密度约为1015 cm-2 ，假设碰撞到表面上的分子完全被吸附，则形成一个单层在10-6 torr的压力下仅需3s,在气压为10-10 Torr 或 10-11 Torr 时，吸附单分子层的时间将达几小时到几十小时。,除了极少数特例外(Au,石墨)，所有的清洁表面都必须在超高真空中获得和保持。,4,超高真空 为各种表面科学研究、材料生长和器件应用的基础,表面结构、能态 表面化学反应 表面吸附和生长动力学 表面纳米结构的制备和物性 磁性薄膜 分子自组装薄膜,半导体薄膜精确掺杂 分子束外延生长 大规模集成电路 溅射镀膜,5,1958年，第一界国际技术会议建议采用“托”(Torr)作为测量真空度的单位。国际单位制(SI)中规定压力的单位为帕(Pa)。,1标准大气压(1atm)1.013105Pa(帕) 1Torr1/760atm1mmHg 1Torr133Pa,在表面科学中，用L(langmuir)表示气体在样品表面的暴露量，定义：,1L = 10-6 torrs,例如：当系统压力为10-6 torr ，通入某气体的时间为1秒，则暴露量为1L,*,常用物理量,6,德国Omicron超高真空表面分析设备,7,德国Createc超高真空低温STM/AFM,8,The outer space is in UHV !,9,UHV,获得清洁表面的一般手段,1. 解理(Cleavage): 在真空中用机械方法分解块材样品，暴露出清洁表面,10,获得清洁表面的一般手段,1. 解理(Cleavage): 在真空中用机械方法分解块材样品，暴露出清洁表面,得到的表面具有和体相相同的化学成份比（对于化合物很重要）。,只适用于脆性的，有解理面的晶体 卤盐(NaCl，KCl)，氧化物(ZnO,TiO2,SnO2)，半导体(Ge, Si, GaAs） 对金属材料不适用 只能得到特定的解理面 例如： 碱卤盐: 100； ZnO：1010 III-V化合物半导体：110 元素半导体Si, Ge: 111,11,* 易解理面：满足电中性条件（non-polar surface）： 立方结构的碱卤盐: 100； ZnO：1010 III-V化合物半导体：110 元素半导体Si, Ge: 111,Cleavage is only possible along certain crystallographic directions which are determined by the geometry and the nature of the chemical bonds. The number of bonds being cut, or the compensation of electric fields within the cleavage plane in the case of ionic crystals, are determining factors.,12,Cubic ionic crystal structure (NaCl, KCl, MgO): 100,Wurtzite crystal structure (ZnO, ZnS): 1120,Zinc blende structure (GaAs, InAs, GaN): 110,Diamond structure (Si, Ge): 111,13,例：两种材料解理面的STM图,InP(110),Bi2Sr2CaCu2O8+d (BiO plane),14,2. 表面原位清洁处理 离子束溅射（Ion beam sputtering） 高温退火处理,Bombarding a surface with energetic ions leads to the removal of surface material. This process is called ion erosion or sputtering and is a common tool for surface modification. It is widely spread in industry as a simple technique to roughen, smoothen, or clean technical surfaces.,15,不限制晶面和材料种类，适用面广： 例如：溅射退火处理获得清洁金属表面(Au,Ag,Cu,Pt,W,Mo); 溅射退火获得清洁半导体表面（Ge, SrTiO3） 高温退火获得清洁Si表面,Ion gun,Ar+,Au(111),Si(111),容易改变表面的化学配比（择优溅射）,16,俄歇深度分析,17,磁控溅射镀膜,18,In 1962, Navez et al. showed that low energy ion erosion of glass surfaces can lead to the formation of self-organized periodic patterns. Since then, patterns have been found on a whole variety of materials, such as metals, semiconductors, and insulators, which shows the universality of the formation process.,溅射诱导表面自组织结构,19,3. 原位薄膜生长手段（CVD,IBD,MBE）,除可获得清洁表面外，更重要的是生长薄膜材料。 特别是分子束外延(MBE)，能以原子层的精度控制材料的生长，获得薄膜、半导体超晶格、量子点、自组装分子薄膜等人工晶体材料,20,分子束外延（Molecular Beam Epitaxy）,Invented in late 1960s at Bell Laboratories by J. R. Arthur and A. Y. Cho.,Al,As,Ga,Effusion cells,RHEED screen,Electron gun,Sample stage,It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho,21,分子束外延（Molecular Beam Epitaxy）,Invented in late 1960s at Bell Laboratories by J. R. Arthur and A. Y. Cho.,atmosphere (102 torr),low vacuum (10-6 torr),Ultrahigh vacuum (10-10 torr),22,分子束外延（Molecular Beam Epitaxy）,Invented in late 1960s at Bell Laboratories by J. R. Arthur and A. Y. Cho.,23,Early stages: Three-temperature method (1958) Surface kinetic of the interaction (1960s) Surface chemical processes extensively studied (1970) 1980s: Introduction of gas-source Pulsed beam growth The observation of RHEED oscillations Coupling of MBE-related operations with UHV processing steps 1990s: Multidimensional quantum well (MD-QW) Lattice-mismatched pseudomorphic epitaxial growth,分子束外延（Molecular Beam Epitaxy）,Invented in late 1960s at Bell Laboratories by J. R. Arthur and A. Y. Cho.,24,概念： 分子束外延（MBE）是在超高真空条件下使用一束或者多束热蒸发原子束和在晶体表面进行外延生长的技术. Technique to grow crystalline thin films in ultrahigh vacuum (UHV) with precise control of thickness, composition and morphology 功能： 精确控制化学组分和掺杂浓度 以单原子层的精度控制材料的生长过程 各种低维结构材料：金属/半导体/功能分子薄膜、异质结、超晶格、量子点 特点： 超高真空(10-10 torr) 生长速率低 （1单原子层/秒） 生长温度低（550C for GaAs） 高纯度原子/分子束源 实时监测生长过程,分子束外延（Molecular Beam Epitaxy）,25,控制参数： 衬底/外延材料的选择 蒸发速率 衬底温度 生长的切换,理论挑战： 非平衡的动力学过程 微观动力学过程介(宏)观结构的关系 自组织生长(如量子点阵列)的机制,26,Molecular beam epitaxy (MBE) is used for the growth of semiconducting materials like: i) Group IV elemental semiconductors like Si, Ge, and C ii) III-V-semiconductors: arsenides (GaAs, AlAs, InAs), antimonides like GaSb and phosphides like InP iii) II-VI- semiconductors: ZnSe, CdS, and HgTe,Electrons move in GaAs five times faster than in silicon.,27,The Gas-Source MBE (GS-MBE) III-V semiconductors, group-V materials are hydrides such as arsine (AsH3) or phosphine (PH3) Metalorganic MBE (MO-MBE) group-III materials are metalorganic compounds. e.g., TEGa（三乙基镓） and TMIn（三甲基铟） Solid-Source MBE (SS-MBE) group-III and -V molecular beams.,Types of MBE,28,Effusion cells,Knudson cell,e-beam evaporator,29,RF plasma source,30,Introduction to Epitaxy,31,什么是“外延(Epitaxy)”,Royers definition of epitaxy (1928): “Epitaxy occurs only when the crystallization process involves the parallelism of the two lattice planes that have networks of identical or quasi-identical form and of closely similar spacings”,外延生长是指在一个晶体表面上生长晶体薄膜，并且得到的薄膜和衬底具有（1）相同的晶体结构、（2）相同的取向。 Homoepitaxy (同质外延)：在衬底上生长同种材料（可以和衬底有不同的掺杂浓度）。E.g. Si-Si, SiC-SiC Why homoepitaxy? - less defects, precise doping Heteroepitaxy (异质外延)：在衬底上生长异种材料（成份、晶格常数）。E.g. Ge-Si, AlAs-GaAs,32,33,(001)Ni / (001)Cu: 100Ni / 100Cu,Si: a0=5.431 CoSi2: a0=5.365 NiSi2: a0=5.406 GaAs: a0=5.654 Fe: a0=2.866 (110)Fe / (110)GaAs: 200Fe / 100GaAs SrRuO3: a0=5.567 ; b0=5.530 YBa2Cu3O7: a0=3.82 b0=3.88 (001)YBCO / (001)SRO: 100YBCO / 110SRO,a,b,c,thin film,substrate,外延关系标记方法,34,休息15分钟,35,分子束外延生长的原子过程,非平衡动力学过程,36,分子束外延生长的原子过程,分子束外延生长是在加热的衬底上进行，在生长过程中发生了下列表面动力学过程： 构成薄膜的原子或者分子被沉积并吸附在衬底表面 吸附分子在表面迁移、分解 原子被融合到衬底或者外延层的晶格中 没有融入晶格的原子或者其它基团重新热脱附离开表面,非平衡动力学过程,37,38,39,40,waiting time,diffusion rate,Attempt frequency , e.g.,Energy barrier , e.g. for hopping diffusion,Arrhenius law:,41,Transition states and energy barriers affect ONLY the non-equilibrium dynamics of the system,Et,t,detailed balance condition stationary P(s) exp- Es / (kBT) for states of type a,b,. in absence of deposition and desorption: system approaches thermal equilibrium,42,an example: Ehrlich-Schwoebel instability,additional Ehrlich-Schwoebel barrier hinders inter-layer diffusion,non-equilibrium, kinetic effect: additional barrier ES is irrelevant for equilibrium properties of the system,Monatomic Fe chains on Cu(111) vicinal surface, by Jiandong Guo (cf. Phys. Rev. B, 73, 193405, 2006).,43,44,45,46,47,反射式高能电子衍射 （Reflection High Energy Electron Diffraction）,48,Reciprocal space,ki,kf,k = G,Real Space,49,(00),(01),(02),(03),(04),(03),(02),(01),(05),(04),(05),Side View: 2D Reciprocal Space,LEED,50,Side View,RHEED,2D Reciprocal Space,Top View,51,电子束反射示意图,RHEED束斑强度,覆盖度,层状生长,RHEED Oscillation,52,Different stages of layer-by-layer growth by nucleation of 2D islands and the corresponding oscillation of the intensity of the diffracted RHEED beam.,RHEED Oscillation,53,Wolmer- Weber,Stranski- Krastanov,Frank-van der Merwe,表面上薄膜生长的三种典型模式,层状生长,岛状生长,SK 生长, = s/o + o - s Volmer-Weber : 0 Frank-van der Merwe: 0 S-K mode: 0 for d 0 for d dc,s/o,o,s,54,表面上薄膜生长的三种典型模式,55,分子束外延生长 应用实例,56,1. 量子阱 （Quantum Wells）,57,58,59,60,61,Conventional semiconductor lasers,Once an electron has emitted a laser photon by jumping from the upper to the lower energy level, it remains in the valence band.,Conventional interband semiconductor lasers that emit electromagnetic radiation through the recombination of electronhole pairs across the material band gap,The band gap decides the wavelength of the laser. so to get the laser with different wavelength we have to choose a different material.,2. Quantum Cascade Laser （量子级联激光）,62,QC lasers rely only on electrons, so they are also called the unipolar lasers. Intraband transitions between quantized conduction band states in coupled quantum wells.,In quantum cascade structures, electrons undergo intersubband transitions and photons are emitted. The electrons tunnel to the next period of the structure and the process repeats.,Since the position of the energy levels in the system is primarily determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of QCLs over a wide range in the same material system.,In a unipolar QCL, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted. This gives rise to the name cascade and makes a quantum efficiency of greater than unity.,Quantum Cascade Laser （量子级联激光）,63,64,65,Mismatched heteroepitaxial systems for quantum dots: III-V compounds arsenides (InGaAs/AlGaAs ,InAs/InGaAs, InAlGaAs /AlGaAs ) phosphides (InAs/InP, InP/InGaP ), antimonides and nitrides (GaN/AlN), IV-IV compounds Ge/Si and SiGe/Si II-VI compounds CdSe/ZnSe and Mixed-group compounds InAs/Si.,3. 量子点 （Quantum Dots）,66,AFM image of InAs/GaAs Quantumdots.,67,68,69,E = Eg + Ee + Eh E - emission energy Eg - quantumdot bandgap energy Ee - electron confinement energy Eh - hole confinement energy Nt: Exciton binding energy is neglected.,70,4. 巨磁阻超薄膜,71,72,Magnetoresistance,73,74,Giant MagnetoResistance,产生巨磁电阻最简单的体系：由两层磁性金属夹一层非磁金属组成的体系。在磁性材料内部，特别是磁与非磁材料之间的界面处，不同自旋的电子有不同的散射几率. 如果电子的自旋反平行于一般的磁化方向，其散射就较强。这些电子的电阻将比平行自旋的电子的电阻来得大。其次，当电子进入非磁材料时，它们全部有相同的散射，与它们的自旋方向无关. 在第二界面处和在最后一层磁性材料中，反平行自旋的电子再次有比平行自旋电子更大的散射.,75,从相互作用性质看多层膜GMR,磁性金属非磁磁性金属多层结构中，非磁金属层为纳米标度，该层太厚得不到GMR。 原因：相邻磁层必须有耦合或自旋相互作用（核自旋）与（d电子自旋）之间的相互作用。这种相互作用是通过非磁金属中的自由电子间接实现耦合的（即RKKY机制）。 RKKY耦合会引起相邻磁层的自发反铁磁磁化。,76,“spin valve“ effect:,GMR read heads allows the reading of extremely small magnetic bits at an areal density of 2.69 gigabits per square inch.,77,IBM工程师斯图尔特帕金(Stuart Parkin)在IBM Almaden研究中心Racetrack实验室。,78,小结 清洁表面的获得 1.1 超高真空的意义 1.2 清洁表面的获得手段 2. 分子束外延简介 3. 外延生长 3.1 外延生长的定义 3.2 外延生长的动力学过程 4. 反射式高能电子衍射 5. 分子束外延应用实例,79,下一次课： 12月27日：超高真空技术简介,