Origin of azimuthal effect of RHEED intensity oscillations observed during MBE

On the basis of calculated RHEED intensities from stepped surfaces, the or igin of the azimuthal dependence of RHEED intensity oscillations observed during Si MBE growth on Si(001) is discussed. By combining the azimuthal dependence of RHEED intensity variations from stepped surface with a model growth mechanism, we obtain the result that in a certain incident beam azimuth one period of the oscillations corresponds to a biatomic height and to a monatomic height in other azimuths. The result is quite consistent with a recently reported experiment.     

Combined study of RHEED spot profiles and intensity oscillations during MBE growth of Ge on Ge(111)

The problem to define particular points in the growth sequence and to obtain quantitative data on the step-terrace structure of the growing surface is addressed in an analysis of RHEED spot profiles and intensity oscillations recorded simultaneously during MBE growth of Ge on Ge(111).     

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：     

Si衬底上分子束外延Ge,Si时的反射式高能电子衍射强度振荡观察

本文观察了在Si(100)和Si(111)衬底上分子束外延Si,Ge时的反射式高能电子衍射(RHEED)强度振荡现象。其振荡特性表明,外延一定厚度的缓冲层可以改善表面的平整性,较慢的生长速率或中断生长一段时间有利于外延膜晶体质量的提高。Si(100)上外延Si或Ge时,沿[100]和[110]方位观测到的振荡特性均为单原子模式,起因于表面存在双畴(2×1)再构;而Si(111)上外延Ge时,[112]方位观测到的振荡为双原子层模式,但在[110]方位观察到不均匀周期的强度振荡行为。两种衬底上保持RHEED     

Dynamics of film growth of GaAs by MBE from Rheed observations

Detailed observations have been made of the intensity oscillations in the specularly reflected and various diffracted beams in the RHEED pattern during MBE growth of GaAs, Ga _x Al_1–x As and Ge. The results indicate that growth occurs predominantly in a two-dimensional layer-by-layer mode, but there is some roughening, which is enhanced by deviations from stoichiometry and the presence of impurities. In the case of the GaAs (001) –2×4 reconstructed surface a combination of dynamic and static RHEED measurements has provided firm evidence for the presence of one-dimensional disorder features as well as surface steps.     

Sub-unit cell layer-by-layer growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>, MgO,and Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> thin films

The use of oxide materials in oxide electronics requires their controlled epitaxial growth. Recently, it was shown that Reflection High Energy Electron Diffraction (RHEED) allows the growth of oxide thin films to be monitored, even at high oxygen pressures. Here, we report the sub-unit cell molecular or block layer growth of the oxide materials Sr₂RuO₄, MgO, and magnetite using Pulsed Laser Deposition (PLD) from stoichiometric targets. Whereas a single RHEED intensity oscillation is found to correspond to the growth of a single unit cell for perovskites such as SrTiO₃ or doped LaMnO₃, in materials where the unit cell is composed of several molecular layers or blocks with identical stoichiometry, sub-unit cell molecular or block layer growth is established, resulting in several RHEED intensity oscillations during the growth of a single unit cell. PACS 61.14.Hg; 74.76.Db; 75.70.-i; 81.15.Fg     

Molecular beam epitaxy with synchronization of nucleation

A new MBE technique based on the possibility to maintain undamping RHEED oscillations by periodic increase of the surface supersaturation synchronized with the oscillations is presented. Surface supersaturation may be increased by any way that is suitable for a particular case. The technique has been referred to by the authors as MBE with nucleation synchronization and allows epistructures of any thickness to be grown with in situ control of their properties by RHEED or automatic ellipsometry. It can be applied both to elementary semiconductors and compounds like A_IIIB_V, A_IIB_VI, etc.     

Calculation of rheed from stepped Si(001) for interpretation of rheed oscillations during MBE

RHEED intensities from stepped Si(001) surfaces are calculated to show the azimuthai dependence of RHEED intensity oscillations during Si MBE on a single domain of Si(001). The results explain why different intensities are observed when the incident beam azimuth is along the [110] and the [1̄10] directions and they are consistent with recently reported experimental results. In addition the results of the calculations indicate that the preferred island growth direction is perpendicular to the bonding direction between the atoms in the growing layer and the underlying substrate layer.     

Oscillations in the surface structure of Sn-doped GaAs during growth by MBE

The presence of predeposited or surface-accumulated Sn during MBE growth modifies the reconstruction of the (001)GaAs surface, as observed by RHEED, at coverages as low as 0.025 monolayer. If growth is initiated on such a surface, oscillation in the intensity modulation of some of the RHEED streaks occurs, with a period equal to the monolayer deposition time of the GaAs. This oscillation decays away at a rate determined by the substrate temperature and Ga flux.     

Well defined superlattice structures made by phase-locked epitary using RHEED intensity oscillations

Long continuing intensity oscillations of the RHEED pattern in the [100] azimuth on a (001) oriented substrate were observed during MBE growth of GaAs and Al_xGa_1−xAs. Using these oscillations, growth rates of GaAs and Al_xGa_1−xAs, and the Al mole fraction x of the Al_xGa_1−xAs were accurately monitored during the growth. The phase of the RHEED oscillations was analyzed by computer and molecular beam shutters were operated at a particular phase of the oscillations. This computer controlled phase-locked epitaxy (PLE) was used to grow precisely defined (GaAs)₂(AlAs)₂ bi-layer superlattices. Raman scattering spectra of the bi-layer superlattice showed split lines characteristic of superlattices. From TEM observation of a GaAs-AlAs multi-layered structure, it was verified that one cycle of oscillations corresponds to one monolayer growth of GaAs and AlAs. This PLE has a great advantage over the conventional MBE growth method for the precise control of very thin films and superlattice structures because it is invulnerable to fluctuations of molecular beam flux intensity.     

Direct evidence for the step density model in the initial stages of the layer-by-layer homoepitaxial growth of GaAs(111)A

Scanning tunnelling microscopy (STM) has been used to investigate the morphological basis of the specular beam intensity oscillations observed in reflection high-energy electron diffraction (RHEED) studies during the initial stages of GaAs(111)A homoepitaxy. Analysis of STM images after the deposition of controlled amounts of GaAs up to a coverage of 2 monolayers show a strong relationship between the step density and the RHEED specular beam intensity. It is shown that the RHEED oscillations observed during the initial stages of growth reflect the temporal variation in surface step density.     

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：     

The simulation of single-crystal growth by molecular beam epitaxy using a kinetic rate-equation model

A computer simulation of Molecular Beam Epitaxial (MBE) growth under typical growth conditions is presented. The method is based on a solution of kinetic rate equations that govern the time dependence of the concentration of islands of varying sizes and differing heights on the top of the surface during the MBE growth. The time dependence for the coverage of each monolayer () is calculated. The mode of the MBE growth is determined by a calculation of the RHEED (Reflection High-Energy Electron Diffraction) intensity. A calculation of the Exposed Coverage (EC) (which is defined as the number of surface atoms in each layer unscreened by other atoms directly above them) and the interface width (IW) (which determines the roughness of the growing surface) serves to confirm the growth mode of the MBE structure. The possible role of these type of calculations in determining the optimized growth conditions for the production of 2D growth in a general materials system is also described.On leave from Department of Microelectronics, Slovak Technical University, Bratislava, Slovakia     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

On the interpretation of the RHEED intensity oscillations during the growth of vicinal faces

A quantitative treatment of the crystallization kinetics in MBE growth of vicinal surfaces results the relation l ²=2D _sK between the surface diffusion coefficient D _s, the time for a monolayer deposition and the interstep distance l at which the RHEED intensity oscillations disappear. The correction factor K depends on the size and the energy of the two-dimensional critical nucleus and it is estimated to be smaller than 10^–2. The currently used interpretation of the RHEED intensity oscillations ignores the correction factor K and, therefore, the calculated values of D _s are several orders of magnitude smaller than its real values. The surface transport during the time of growth interruption is discussed in connection with the tendency to three dimensional growth at every second interface (where a deposition of the material with strong intermolecular bonds starts) of a small period superlattice.     

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     

Effects of diffraction conditions and processes on rheed intensity oscillations during the MBE growth of GaAs

The RHEED intensity oscillation technique has received wide-spread attention for the study of MBE growth dynamics, but insufficient consideration has been given to the diffraction conditions and processes involved. We report here a systematic investigation of the intensity oscillation behaviour as a function of diffraction parameters (azimuth, incidence angle, specular and non-specular beams), with constant growth conditions for GaAs films on GaAs (001) substrates.We show that many reported anomalies attributed to growth effects, such as phase differences and periodicity variations, can be accounted for entirely by diffraction events, provided it is realised that multiple scattering processes are the dominant cause of RHEED intensity variations during growth.The technique can provide valuable information on growth behaviour, but only if diffraction-dependent effects are first eliminated.     

SrRuO based heterostructures grown by pulsed laser deposition

In this report we demonstrate that high quality epitaxial heterostructures, based on metallic SrRuO₃ and insulating SrTiO₃ individual blocks a few unit cells thick, can be grown in a purely 2D, layer-by-layer mode, using pulsed laser deposition with in situ reflection high energy electron diffraction (RHEED) diagnostics. The thickness of each constituent block can be controlled at the level of a single unit cell. A detailed investigation carried out at the synchrotron facility, ESRF, by various X-ray techniques has demonstrated that each intensity oscillation of the RHEED specular spot corresponds strictly to the growth of a single perovskite unit cell, either SrRuO₃ or SrTiO₃. Furthermore, we show that, in these structures, the interfaces between the different constituent blocks are very sharp with a roughness of only one unit cell. Received 3 July 2002 / Received in final form 12 September 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: tebano@uniroma2.it     

In situ RHEED monitor of the growth of epitaxial anatase TiO2 thin films

Epitaxial anatase TiO₂ thin films were successfully grown on (0 0 1) SrTiO₃ substrates by the laser molecular-beam epitaxy (laser-MBE) method. The whole growth process is monitored by in situ reflection high-energy electron diffraction (RHEED). RHEED monitoring shows a transition from a streaky pattern to a spot pattern during deposition, indicating different growth modes of TiO₂ film. The RHEED patterns are in consistent with the RHEED intensity oscillation results. The atomic force microscopy (AFM) and X-ray diffraction (XRD) investigation show that the thin films have single crystalline orientation with roughness less than three unit cells.     

激光分子束外延BaTiO3薄膜最顶层表面原子平面与薄膜生长机理

用激光分子束外延技术在SrTiO₃(001)衬底上外延生长了高质量的BaTiO_{3< /sub>薄膜,薄膜的生长过程由反射式高能电子衍射仪(RHEED)原位实时监测,表明薄膜具有 二维层状生长模式.薄膜的晶体结构和表面形貌分别由X射线衍射和原子力显微镜表征,显示 该薄膜为完全c轴取向四方相晶体结构,其表面具有原子尺度光滑性.采用角分辨X射线光电 子谱技术(ARXPS),研究了BaTiO3薄膜表面最顶层原子种类和排列状况.结果表 明,BaTiO3 关键词： 激光分子束外延 3薄膜')" href="#">氧化物BaTiO3薄膜 最顶层表面 角分辨X射线光电子谱}