In-situ UHV Electron Microscopy of Surfaces

Surface micro-structure analysis is very important for surface study in material science. Observations of surface atomic steps and reconstructed structures have been made using several techniques: reflection high energy electron microscopy (RHEEM), low energy electron reflection microscopy (LEERM) and low energy electron diffraction microscopy (LEEDM).In the present experiment, observations of surface micro-structures have been made using a scanning type reflection high energy electron diffraction (RHEED) microscopy. This technique has certain advantages of easy combinations with multiple surface analyzing techniques such as scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS).A schematic diagram of the scanning RHEED microscope combined with the microprobe AES is shown in Fig. 1. RHEED patterns are observed on the fluorescent screen through a viewing port. To observe the micro-structure (scanning RHEED image or dark field image), a particular diffraction spot is selected by means of the other small fluorescent screen with an aperture.

The history and future of transmission electron microscopy (TEM) is discussed as it refers to the eventual development of instruments and techniques applicable to the real time in situ investigation of surface processes with high resolution. To reach this objective, it was necessary to transform conventional high resolution instruments so that an ultrahigh vacuum (UHV) environment at the sample site was created, that access to the sample by various in situ sample modification procedures was provided, and that in situ sample exchanges with other integrated surface analytical systems became possible. Furthermore, high resolution image acquisition systems had to be developed to take advantage of the high speed imaging capabilities of projection imaging microscopes. These changes to conventional electron microscopy and its uses were slowly realized in a few international laboratories over a period of almost 40 years by a relatively small number of researchers crucially interested in advancing the state of the art of electron microscopy and its applications to diverse areas of interest; often concentrating on the nucleation, growth, and properties of thin films on well defined material surfaces. A part of this review is dedicated to the recognition of the major contributions to surface and thin film science by these pioneers. Finally, some of the important current developments in aberration corrected electron optics and eventual adaptations to in situ UHV microscopy are discussed. As a result of all the path breaking developments that have led to today’s highly sophisticated UHV–TEM systems, integrated fundamental studies are now possible that combine many traditional surface science approaches. Combined investigations to date have involved in situ and ex situ surface microscopies such as scanning tunneling microscopy/atomic force microscopy, scanning Auger microscopy, and photoemission electron microscopy, and area-integrating techniques such as x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, temperature programmed desorption, high-resolution electron energy-loss and Fourier-transform infrared spectroscopies, and others. Material systems ranging from atomic layers of metals and semiconductors to biology related depositions are being investigated. In the case of biological materials, however, strict limitations to high-resolution applications are imposed by electron radiation damage considerations.

A phase diagram of the Sb/Si(001) interfacial system

In this paper we used in situ preparation and surface investigations of as grown HTSC films to avoid oxygen loss induced by cleaning procedure. A specially devised UHV evaporation chamber coupled with a surface analysis system LAS-620 is presented. By means of the pulsed laser ablation technique (Nd: YAG laser,1060nm,25ns) thin (0.2–0.6μm) HTSC films on metallic (Ag) and dielectric (SrTiO3, MgO) substrates are deposited. UV lamp ionisation source was used to introduce ionised oxygen to the substrate during film deposition with low system oxygen pressure. Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED) were applied to analyze the growth and surface properties of as grown films of p-type (Y1Ba2Cu3O7−X, Bi2Ca1Sr2Cu2O8−x) and n-type (Nd1.85Ce0.15CuO4−x) HTSC. The AES analysis shows no carbon contamination of the films.The XRD and LEED studies show that Y-films are epitaxial and Bi-,Nd- films are highly textured having the c axis perpendicular to the film. A (lxl) LEED structure of Y- films was observed.

Electron energy loss spectroscopy (EELS) is a versatile and sensitive probe of electronic and vibrational excitations at surfaces. The technique is extensively used as a research and analytical tool in a broad range of surface science applications including research on intrinsic physical and chemical properties of surfaces, chemisorption phenomena, and chemical processes at surfaces. The spectroscopy is based on measuring the energy loss and momentum transfer of electrons in a highly collimated, monochromatic (narrow energy spread) beam after the electrons scatter from a surface. Vibrational excitations of atoms and molecules adsorbed on surfaces, collective excitations of ordered chemisorbed structures, the intrinsic collective excitations of surface atoms (surface phonons), and collective electronic excitations (surface plasmons) can be studied using the technique. Under favorable conditions, vibrational losses from surface adsorbate concentrations below 10−3monolayer (1012atoms cm−2) can be detected. EELS is a surface science probe, typically requiring an ultrahigh vacuum (UHV) environment, and is generally employed in conjunction with complementary surface analytical probes such as Auger electron spectroscopy (AES), low‐energy electron diffraction (LEED) and thermal desorption mass spectroscopy (TDS). EELS is an ideal tool for determining surface chemical composition, and provides vibrational loss spectra similar to infrared reflection absorption spectroscopy (IRAS), Raman spectroscopy, and helium atom scattering spectroscopy (HAS).

Interpretation of friction and wear of ceramics in terms of surface analysis

Epitaxial growth of CoO films on semiconductor and metal substrates by constructing a complex heterostructure

Cd 2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES)

Electrochemical reactivity of 2,2',5,5'-tetrahydroxybiphenyl and related compounds adsorbed at pt (111) surfaces: studies by eels, leed, auger spectroscopy and cyclic voltammetry

Reconstruction of the 6H–SiC(0001) √3×√3-R30° surfaces by adsorption of hydrogen in ultra high vacuum

Electron Beam Induced Deposition (EBID) allows deposition of three-dimensional micro- and nano-structures of conductive and insulating materials on a wide range of substrates. The process is based on the decomposition of molecules of a pre-selected precursor by a focused electron beam. In recent decades EBID of several metals, namely Au, W, Cu and Pt, from different families of precursors, has been achieved and the technique has found some application for small-scale production of laboratory devices and for repair of masks and micro-optoelectronic devices. The weak point of the technique is at present the low purity of the deposited material, caused by metal-organic precursors and by the lack of selectivity of the electron-induced decomposition process. This work is dedicated to EBID of Rh nanostructures from the precursor [RhCl(PF3)2]2. High metal content deposits are expected because the precursor does not contain C atoms and because Rh is one of the less reactive metals. [RhCl(PF3)2]2 as EBID precursor has been characterized by vapor pressure, mass spectrometry and surface residence time measurements. The vapor pressure of 7.5 Pa at room temperature reveals that the precursor is sufficiently volatile for room temperature EBID. The knowledge of the vapor pressure allows also to estimate the number of precursor molecules delivered to the reaction area per unit time. Mass spectrometry measurements allow to know the decomposition path of the precursor under electron impact in the gas phase. The measured spectrum indicates that the molecule decomposes by successive loss of PF3 groups, as confirmed by density functional theory calculations. This is compatible with a high metal content deposit. Residence time measurements show that [RhCl(PF3)2]2 does not decompose on stainless steel surfaces. The measured residence time of 2 ms allows to estimate that the activation energy for desorption of [RhCl(PF3)2]2 on stainless steel is about 0.6 eV and that precursor molecules can travel distances in the micrometer range before being desorbed. EBID structures obtained from [RhCl(PF3)2]2 have been characterized with a wide range of techniques for a better knowledge of the material properties and the deposition process. The deposit morphology has been studied by Transmisssion Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) to characterize the different steps of the deposition process. Crystallographic analysis is carried out by TEM in diffraction mode. Chemical analysis is carried out by Auger Electron Spectroscopy (AES) and Electron Energy Loss Spectroscopy (EELS). Morphological analysis of deposits carried out at different exposure times reveals that the first phase of the growth process, in close proximity to the substrate, is characterized by an increase of the deposit height and the deposit diameter. On the other hand the second phase of the growth process is characterized by increasing height and constant diameter. TEM contrast profiles of dots and Atomic Force Microscopy (AFM) sections of lines have clearly shown that the EBID rate is highest in the center of the beam and decreases in the peripheral regions. Deposition at variable distances from the precursor source allowed to obtain hollow structures, whose morphology reveals that the precursor reaches the reaction area mainly by direct gas phase transport. Structural analysis and TEM revealed that, independently of the deposition conditions, the deposited material is made up of Rh nanocrystals immersed in a lighter amorphous matrix. Chemical analysis by Auger Electron Spectroscopy revealed that, after removal of the C rich contamination layer by Ar ion sputtering, the average composition of the deposits is about: 60 at.% Rh, 20 at.% P, 5 at.% Cl, 7 at.% N, 8 at.% O. The absence of C and the presence of N and O in the deposit bulk have been confirmed by Electron Energy Loss Spectroscopy. This technique allowed also to prove that Rh is dominant also in deposits of sub-micrometer size (not analyzable with AES) and to determine the elemental distribution in the deposit with nanometer resolution. Comparison of the deposit composition and the positive ions detected by mass spectrometry revealed that EBID, compared to low pressure gas phase ionization, involves a higher number of events, i.e. multi-electron decomposition and rearrangements of partially decomposed species.

Formation of a passive layer in surface oxidation of Gd: A leed and AES study

Growth and surface structure of vanadium oxide on anatase (0 0 1)

CuCl growth on the reconstructed surface of (0001) haematite

CHF 3 adsorption and decomposition on clean and oxygen covered Al(111)