On the investigation of surface structure by reflection high energy electron diffraction (RHEED)

RHEED (Reflection High Energy Electron Diffraction) has been used to investigate surface structures. A formalism which allows fast, precise determination of the sizes, shapes and relative orientation of the lattices of well-ordered surface structures from the distances between the diffraction lines has been developed. This formalism is illustrated by the analysis of copper, gold and silicon surfaces and various surface structures formed on these. The relative precision of the measurements can in special cases be of the order of 10^?4 and will in general be better than 10^?3.     

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.     

关于RHEED振荡技术优化GaAs(110)量子阱生长的研究

在GaAs(110)衬底上生长的半导体材料有诸多优良性能，使得在非极性GaAs(110)衬底上获得高质量各类异质结材料，成为近年来分子束外延生长关注的课题.考虑GaAs(110)表面是Ga和As共面，最佳生长温度窗口很小；反射式高能电子衍射的(1×1)再构图案对生长温度和V/Ⅲ束流比不敏感，难于通过观察再构图案的变化，准确地找到最佳生长条件.作者在制备GaAs(110)量子阱过程中，观察到反射式高能电子衍射强度振荡呈现出的单双周期变化.这意味着不同工艺条件下，在 GaAs(110)衬底上量子阱有单层和双层两种生长模式.透射电子显微镜和室温光致荧光光谱测量结果表明：在双层生长模式下量子阱样品光学性能较差，而在单层生长模式下量子阱光学性能较好，但是界面会变粗糙.利用这一特点，我们采用反射式高能电子衍射强度振荡技术，找到了一种在GaAs(110)衬底上生长高质量量子阱的可行方法. 关键词： 反射高能电子衍射 量子阱 分子束外延     

Structure determination of the (100) surface of rhodium by low energy electron diffraction

Elastic low-energy electron diffraction intensity data have been measured as a function of energy for two directions of incidence for the (100) surface of rhodium. The dynamical perturbation programs of Van Hove and Tong have been used for analysing these new experimental data, and it is concluded that the normal face-centered cubic registry is maintained to the surface layer. A preliminary comparison between measured and calculated I(E) curves indicates the topmost interlayer spacing to be 1.96 ± 0.10 Å, and therefore possibly slightly expanded from the bulk interlayer spacing of 1.90 Å.     

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     

Comparison of cross sections in high resolution electron energy loss spectroscopy and infrared reflection spectroscopy

Electron energy losses and absorption of infrared radiation are both caused by the dipole moment of the surface vibration. A comparison of absolute intensities between both techniques should therefore be possible. In this paper the appropriate formulas are derived. For the adsorption system CO on Pt(111) which has been investigated by both techniques a perfect agreement is found. For a number of adsorbate systems the effective ionic charge is calculated from previously published electron energy loss data.     

Energy loss spectra,auger spectra and high energy electron diffraction of A-15 superconductors

We have measured the electron energy loss spectra of V₃Si, Nb₃Al, Nb₃Sn, Nb₃Ge (annealed) and Nb₃Ge (sputtered) from 3 to 70 eV. Numerous new structures were discovered in this study which could be related to interband transitions, plasma excitations, and core transitions. We have also measured the high energy electron diffraction patterns and Auger spectra which characterized the lattice structures and compositions of these surfaces.     

Geometrical resonance of the Rayleigh sound wave absorption as a method for direct measurements of the electron specular reflection coefficient from the metal surface

A theory of resonant absorption of the Rayleigh sound waves in metals has been developed for the case of weak magnetic fields (i.e. the Pippard type geometrical oscillations). The amplitude and shape of resonance lines have been studied in relation to the nature of electron reflection from the surface. It has been shown that observations of the geometrical resonance oscillations may serve for investigating directly surface reflectivity of the metal with respect to conduction electrons.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Variation of relative intensities between surface and bulk plasmon losses due to crystal orientations for aluminium in low energy electron reflection loss spectroscopy

The contrast change of secondary electron images due to the crystal orientations is observed by the ultra high vacuum scanning electron microscope (UHV-SEM) for crystal grains of clean surface of polycrystalline Al in the primary energy E_p of 200 eV to 5 keV. The low energy electron loss spectra are measured by the cylindrical mirror analyzer. The relative intensity ratio between surface and bulk plasmon loss spectra was dependent on the crystal orientations. The SEM images taken by the surface and bulk plasmon signals at E_p = 230 eV show the inverse contrast depending on the grains. The inversion of the relative intensities between the surface and bulk plasmon losses is explained qualitatively by taking into account of variation of the penetration depth of the incident beam caused by the electron channeling.     

Interaction of water molecules with bare and deuterated polycrystalline diamond surface studied by high resolution electron energy loss and X-ray photoelectron spectroscopies

In this work we study the interaction of water molecules with deuterated and bare polycrystalline diamond surfaces upon exposure to water vapor by X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HR-EELS). To distinguish the molecular origin of hydrogen bonds (i.e. C–H, O–H, C–O–H, etc.) formed on the diamond surface upon interaction with the water molecules, deuterated and hydrogenated gases were used in our experiments. Diamond films were deposited from a deuterated gas mixture to induce C(di)-D surface terminations. Water adsorption on bare diamond surface gives rise to the appearance of well defined and pronounced C–H and C–OH vibrational HR-EELS peaks and an intense O (1s) XPS peak. These chemically adsorbed water fragments survive 300 °C anneal temperature under ultra-high vacuum conditions. Annealing at 600 °C of the water exposed bare diamond surface results in disappearance of the C–OH vibrational modes alongside with a pronounced reduction of the C–H vibrational modes, whilst only upon annealing to ~ 800 °C the O (1s) XPS peak decreased substantially in intensity. We associate these effects with dissociative adsorption of the water molecules on the bare diamond surfaces.Water exposure onto a deuterated surface, on the other hand, does not result in the appearance of the C–OH vibrational peaks but only to an increase of the C–H vibrational HR-EELS mode along side with the appearance of a weaker XPS O (1) peak, as compared to the same experiment, performed on the bare surface. 300 °C anneal significantly diminishes surface oxygen concentration, as monitored by XPS. We associate these results with H₂O decomposition reactions and also with molecular adsorption on deuterated diamond surfaces. Annealing of the water exposed deuterated diamond surface, results in a pronounced decrease and disappearance of the O (1s) XPS peak at a temperature of ~ 800 °C.     

Identification of surface vibrations on clean and oxygen covered Pt(111) surfaces with high resolution electron energy loss spectroscopy (EELS)

Clean and oxygen covered {111} recrystallized Pt surfaces were studied by EELS after surface preparation at 150≤T≤165OK. The clean surface shows Stokes as well as anti-Stokes lines of surface phonons at ±195^?1. Adsorption of small amounts of (<10^?2 monolayers) of O₂ or H₂ leads to substrate-derived phonon losses at ±380cm^?1. Oxygen exposure at different pressures, times and temperatures leads to atomic and/or molecular adsorption as well as oxide-related features which have been identified by EELS.     

Application of spectroscopic photoemission and low energy electron microscope to high-k gate dielectrics: Relationship between surface morphology and electronic states during Hf-silicide formation

We have applied the spectroscopic photoemission and low energy electron microscope to study high-k gate dielectrics and have performed the following in situ operations during ultrahigh vacuum annealing: real-time observation of surface morphology and microregion photoelectron spectroscopy measurements. Changes in surface morphology and electronic states were consistent with the models previously reported in the case of HfO₂/Si. No clear differences between void regions and nonvoid regions have been observed in microregion photoelectron spectra for poly-Si/HfO₂/Si, regardless of phase separation in real space. These results have suggested that the initial void formation occurs in about 100-nm wide regions for both HfO₂/Si and poly-Si/HfO₂/Si.     

Application of the thermal spike model for explanation of variations of surface structure of highly oriented pyrolytic graphite under bombardment by 86Kr and 209Bi fast ions with high ionization energy loss

Temperature effects in the highly oriented pyrolytic graphite (HOPG) under bombardment by ⁸⁶Kr (253 MeV) and ²⁰⁹Bi (710 MeV) heavy ions are studied in the framework of a three-dimensional thermal spike model. It is shown that the surface temperature of an HOPG target under bombardment by bismuth ions can exceed the sublimation temperature at particular values of the electron-phonon interaction coefficient. At the same time, the temperature at the target surface during bombardment of HOPG by krypton ions does not exceed the sublimation temperature over a wide range of variations in the electron-phonon interaction belongs. The calculations allow the explanation of the observed changes in the surface structure of HOPG single crystal under bombardment by ²⁰⁹Bi and ⁸⁶Kr ions.