高氧压下氧化物薄膜同质和异质外延的RHEED实时监测

我们自行研制了具有三级差分气路可以在高气压下工作的RHEED系统(High-pressure RHEED),并利用本系统实时监测了(001)SrTiO3基片上SrTiO3:Nb、Ba0.5Sr0.5TiO3、YBa2Cu3O7单层薄膜,及Ba0.5Sr0.5TiO3/SrTiO3:Nb双层膜的生长过程.研究结果表明当镀膜室氧压高达21Pa时该系统仍然可以正常工作,并且能够获取较清晰的衍射图样.通过分析衍射图样我们发现,所有这些薄膜都是外延生长且晶体质量良好,但薄膜生长模式及表面平整度受沉积条件影响较大.在真空下薄膜基本上以层状模式生长,具备纳米级光滑的表面,且其表面平整度并不因膜厚的改变而变化;而在10Pa量级氧压下薄膜更倾向于以岛状模式生长,膜表面平整度较差,并且随膜厚的增加粗糙度上升.此外对多层薄膜而言,底层薄膜的表面和结构直接影响到顶层薄膜的质量和品质.     

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     

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.     

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     

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     

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.     

Design and development of an ultra-compact drum-shaped chamber for combinatorial pulsed laser deposition

We have designed a compact combinatorial pulsed laser deposition (PLD) chamber as a building block of a desktop laboratory for advanced materials research. Development of small-size systems for the growth and characterization of films would greatly help in interconnecting a variety of analytical tools for rapid screening of advanced materials. This PLD chamber has four special features: (1) a drum-shaped growth chamber, (2) a waterwheel-like combinatorial masking system, (3) a multi-target system having one feedthrough, and (4) a small reflection high-energy electron diffraction (RHEED) system. The performance of this system is demonstrated by the RHEED intensity oscillation during homoepitaxial growth of SrTiO₃ as well as by simultaneous fabrication of a ternary phase diagram of rare earth-doped Y₂O₃ phosphors.     

In situ RHEED analysis of epitaxial Gd2O3 thin films grown on Si (001)

Epitaxial Gd₂O₃ thin films were successfully grown on Si (001) substrates using a two-step approach by laser molecular-beam epitaxy. At the first step, a ～0.8 nm thin layer was deposited at the temperature of 200 ^°C as the buffer layer. Then the substrate temperature was increased to 650 ^°C and in situ annealing for 5 min, and a second Gd₂O₃ layer with a desired thickness was deposited. The whole growth process is monitored by in situ reflection high-energy electron diffraction (RHEED). In situ RHEED analysis of the growing film has revealed that the first Gd₂O₃ layer deposition and in situ annealing are the critical processes for the epitaxial growth of Gd₂O₃ film. The Gd₂O₃ film has a monoclinic phase characterized by X-ray diffraction. The high-resolution transmission electron microscopy image showed all the Gd₂O₃ layers have a little bending because of the stress. In addition, a 5–6 nm amorphous interfacial layer between the Gd₂O₃ film and Si substrate is due to the in situ high temperature annealing for a long time. The successful Gd₂O₃/Si epitaxial growth predicted a possibility to develop the new functional microelectronics devices.     

Growth and in situ high-pressure reflection high energy electron diffraction monitoring of oxide thin films

Interface and surface physics is an important sub-discipline within condensed matter physics in recent decades. Novel concepts like oxide-electronic device are prompted, and their performance and lifetime are highly dependent on the flatness and abruptness of the layer surfaces and interfaces. Reflection high-energy electron diffraction (RHEED), which is extremely sensitive to surface morphology, has proven to be a versatile technique for the growth study of oxide thin films. A differential pumping unit enables an implementation of RHEED to pulsed laser deposition (PLD) systems, ensuring an in situ monitoring of the film growth process in a conventional PLD working oxygen pressure up to 30 Pa. By optimizing the deposition conditions and analyzing the RHEED intensity oscillations, layer-by-layer growth mode can be attained. Thus atomic control of the film surface and unit-cell control of the film thickness become reality. This may lead to an advanced miniaturization in the oxide electronics, and more importantly the discovery of a range of emergent physical properties at the interfaces. Herein we will briefly introduce the principle of high-pressure RHEED and summarize our main results relevant to the effort toward this objective, including the growth and characterization of twinned La_2/3Ca_1/3MnO₃ thin films and ReTiO_3+δ/2 (Re = La, Nd; δ = 0 ～ 1) A _n B _n O_3n+2 structures, on YSZ-buffered ‘Silicon on Insulator’ and LaAlO₃ substrates, respectively, as well as the study of the initial structure and growth dynamics of YBa₂Cu₃O_7?δ thin films on SrTiO₃ substrate. Presently we have realized in situ monitoring and growth mode control during oxide thin film deposition process.     

RHEED study of titanium dioxide with pulsed laser deposition

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the growth of thin films of titanium dioxide (TiO₂) on (1 0 0) magnesium oxide (MgO) substrates by pulsed laser deposition (PLD). The deposition is performed with a synthetic rutile TiO₂ target at low fluence. The topography and structure of the deposited layers are characterized using in situ high pressure RHEED and atomic force microscope (AFM). Based on these observations the growth mode of the films is discussed. The results will be compared to earlier results obtained for the growth of TiN films on (1 0 0) MgO.     

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.     

Growth of Cu Films on Si(111)-7×7 Surfaces at Low Temperature: A Scanning Tunnelling Microscopy Study

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at lowtemperature are investigated by scanning tunnelling microscopy (STM) andreflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rateproduces films with different morphologies, which is the result of Ostwald ripening.     

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：     

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.     

Adjusting chemical bonding of hard amorphous CSixNy thin films by N*-plasma-assisted pulsed laser deposition

Hard amorphous carbon silicon nitride thin films have been grown by pulsed laser deposition (PLD) of various carbon silicon nitride targets by using an additional nitrogen RF plasma source on [100] oriented silicon substrates at room temperature. The influence of the number of laser shots per target site on the growth rate and film surface morphology was studied. Up to about 30 at. % nitrogen and up to 20 at. % silicon were found in the films by Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). The XPS of the films showed a clear correlation of binding energy to the variation of PLD parameters. The films show a universal hardness value up to 23 GPa (reference value for silicon substrate 14 GPa) in dependence on target composition and PLD parameters. The results emphasise the possibility of variation of chemical bonding and corresponding properties, such as nanohardness, of amorphous CSi_xN_y thin films by the plasma-assisted PLD process.     

Layer-by-layer growth of MgO thin films controlled by inserting a TiN seed layer using an in-situ pulsed laser deposition

Completely (200)-oriented MgO films were grown on Si(100) with insertion of a TiN seed layer by pulsed laser deposition (PLD). Compared with the conventional direct ablation of a metal Mg or ceramic MgO target, we successfully demonstrated an effective way to improve the crystallinity of MgO thin films. By using TiN as a seed layer, high-quality MgO films with an atomic-scale smooth surface of about 0.55 nm (Ra) were obtained. Moreover, it is found that the quality of MgO films was independent of the thickness of the TiN seed layer. The improved crystalline quality of the MgO films was attributed to the layer-by-layer growth mode during the deposition of MgO films, which was monitored in-situ by reflection high energy electron diffraction (RHEED). PACS 68.55.Jk; 81.15.Fg     

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.     

Layered MoS2 grown on c ‐sapphire by pulsed laser deposition

Layered growth of molybdenum disulphide (MoS₂) was successfully achieved by pulsed laser deposition (PLD) method on c ‐plane sapphire substrate. Growth of monolayer to a few monolayer MoS₂, dependent on the pulsed number of excimer laser in PLD is demonstrated, indicating the promising controllability of layer growth. Among the samples with various pulse number deposition, the frequency difference (A_1g–E¹_2g) in Raman analysis of the 70 pulse sample is estimated as 20.11 cm^–1, suggesting a monolayer MoS₂ was obtained. Two‐dimensional (2D) layer growth of MoS₂ is confirmed by the streaky reflection high energy electron diffraction (RHEED) patterns during growth and the cross‐sectional view of transmission electron microscopy (TEM). The in‐plane relationship, (0006) sapphire//(0002)MoS₂and sapphire//MoS₂ is determined. The results imply that PLD is suitable for layered MoS₂ growth. Additionally, the oxide states of Mo 3d core level spectra of PLD grown MoS₂, analysed by X‐ray photoelectron spectroscopy (XPS), can be effectively reduced by adopting a post sulfurization process. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Observation of a registered phase in Cs/W(110)

RHEED and AES studies at room temperature have revealed a new metastable state of Cs ad-atoms on W(110) which are in registry at least along [1&#x0304;10] W. The ad-atom density in this state is 0.81 ± 0.03 ML, where 1 ML corresponds to the final density of Cs atoms in the saturated close packed hexagonal structure. This new structure has been observed as the final structure in dynamic deposition conditions, i.e. under the constant pressure of the Cs beam. The previously observed 1 ML structure arises when deposition and AES observations are alternated. The possibility that impurities provide the means of overcoming a nucleation barrier to this final structure is discussed.