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LEED / AES Spectroscopy

Electrons emitted by the cathode are focused at normal incidence onto the surface of the sample by a series of electrostatic lenses. The first grid of the optics and the last electrode of the electron gun are at the potential of the crystal, so that a field-free space is created. The majority of the electrons back-scattered from the crystal have lost energy in the scattering process (inelastic scattering). These electrons contain no diffraction information and have to be separated from the elastically scattered electrons which are back - diffracted from the solid surface. After allowing the scattered electrons to drift radially away from the crystal surface, the separation of the elastic and inelastic components is made by using a retarding potential that repels all electrons that have lost energy in the scattering process. The elastically scattered electrons pass through the retarding field and impinge on the spherical fluorescent screen to create the diffraction pattern. The pattern can be observed visually, reproduced photographically, or recorded by a video system.

Surface Science Applications Overview

One of the major growth areas in modern research is the study of surfaces. Surface science is a very vast field consisting of numerous sciences; however, one of the most significant is Surface Crystallography. It studies the atomic arrangement and symmetry of a crystal surface. Each crystal surface consists of different patterns on the surface. The study of Surface Crystallography starts with the basics of the arrangement of atoms at a surface, the techniques used to study the surface, and the applications used in the real world. Atoms at a surface are arranged into patterns depending on the crystal. This kind of geometrical information about a specific surface can be obtained by using the best known technique called Low-energy electron diffraction or LEED. In addition to LEED development, there was a significant event in the introduction of Auger electron spectroscopy or AES as a means of analyzing the chemical composition of surfaces. This information obtained from these techniques is crucial for the development of new microprocessors, new solar cells, flat panel displays, hard disks, and thin film coatings.

Surface Crystallography may not be one of the most simplest sciences; however, it serves a very good purpose. The basis of understanding surface crystallography must start with the basics of the arrangement of atoms and their patterns. The concept of surface crystallography can be better understood by these macroscopic analogies: a brick driveway consists of different coloured bricks creating various patterns on the surface; and the tiles in a bathroom consist of assorted patterns which are repeated on the surface of the floor. These patterns are as a result of atoms being repeated on the surface. In a crystalline solid, which contains either atoms, molecules, or ions which occupy a specific position called a lattice point, the basic repeating unit of the arrangement of atoms or molecules is a unit cell. The arrangement of these unit cells in a crystalline solid vary depending on the type of solid. Diagram 1 shows the fourteen different types of unit cells of crystal structures. The two most simplest but important unit cells are the body-centred cubic (BBC) and the face-centred cubic (FCC). A body-centred cubic crystal unit cell (diagram 2) consists of two atoms in total on the inside: an eighth of an atom on each of the eight corners and a whole atom in the middle. Within the bulk (the inside atoms), each atom has eight nearest neighbours. A face-centred cubic crystal unit cell (diagram 2) consists of four atoms in total on the inside: an eighth of an atom on each of the eight corners and a half an atom on each face. Within the bulk, each atom has six next-neighbours. These unit cells can be cut at a certain angle to produce a new surface with respect to the unit cells. The most common surfaces of a cubic crystal are (001 or 100), (110), and (111) which are shown in diagram 3 (Clarke, 1985, p. 34). These three common cubic surfaces of the body-centred unit cell and the face-centred unit cell are shown in diagram 4. Through observing the body-centred unit cell with the different surfaces, the most densely packed surface is the (110). In the face-centred cubic unit cell, the most densely packed surface is the (111). Diagram 5 shows two different crystals and there surfaces. This is the basis for understanding the principle of surface crystallography. The best known technique to study the surfaces of crystals is called Low energy electron diffraction.

The oldest surface science technique to study the structure of crystalline surfaces is called Low energy electron diffraction or LEED. Although numerous other techniques have been recently proposed which also provide information concerning the geometrical arrangement of atoms at a crystal surface, none have been studied as widely as LEED. This technique gives direct information about the symmetry, spacing of atoms, and dimensions of the unit cells in a surface. Diagram 6 illustrates the basic principle of LEED.

In 1927, working in the United States, Davisson and Germer discovered that by firing electrons with energies lying between 15 and 200 electron-volts (eV) at crystal of nickel, angular variations in the reflected intensity were produced consistent with electron diffraction (Clarke, p.1). Their discovery gave experimental evidence of the wave nature of electrons. Independently in Britain, G. P. Thomson and Reid used electrons with energies between 3900 and 16500eV to produce diffraction patterns from transmission through a thin sample of celluloid (Clarke, p.1). A few months later, Thomson reported the observation of transmission diffraction patterns from a thin film of platinum using electrons with energies between 30000 and 60000eV (Clarke, p.1). For the next 30 years, the technique of low energy electron diffraction was inactive except for the work done by Farnsworth (Lu, 1995, p.1).

The low energy electron diffraction technique operates by sending a beam of electrons from an electron gun to the surface of the sample being tested. Diagram 7 illustrates the schematic diagram of LEED. An electron gun consists of a heated cathode and a set of focusing lenses which sends the electrons between 20-300keV (Clarke, p. 82). As the electrons collide with the surface of the sample, they diffract in numerous directions depending on the surface crystallography. Once the electrons diffract, they head back towards three girds followed by a phosphor covered screen. The first grid is grounded and basically serves as a shield which protects the second grid as a result of its negative potential. The second grid acts as filter by allowing only the electrons with higher energies to pass through. The lower energy electrons are blocked out due to the fact that they disorder the image creating a clouded image. Once the electrons pass through the second grid, they come to third and final grid. This grid separates the pervious negative grid from the phosphor screen which carries a positive charge. As the electrons land on the phosphor screen they create a phosphor glow. The intensity of the glow depends on the intensity of the electron. The pattern of these glows is the pattern of the atoms on the surface of the crystal structure. These are the images produced by LEED and diagram 8 shows a few examples of images of silicon surfaces.

The general method for analyzing these diffraction patterns was to manually take several dozen pictures. However when computers became available, these photographs were scanned and digitized and the computer ran a program to do the analysis thus saving some time on the experimentalist's part. Later a design was constructed such that the electrons were diffracted directly into a special camera and computer with an imaging software which immediately digitized and analyzed the pattern. Diagram 9 shows the actual LEED.

The principle technique used to the determine the concentration of elements on the surface is called Auger Electron Spectroscopy (AES). Diagram 10 shows the basic principle of Auger Electron Spectroscopy. When a beam of electrons is fired at the surface of the material these beams simulate several interactions. One of those interactions is Auger Electron Spectroscopy. The principle of Auger operates by allowing a high-energy electron from the beam to eject an electron from its orbit creating an empty hole in the orbit. As this occurs, another electron from a higher orbit moves to fill the empty space. As the electron changes from a higher to a lower orbit, it releases energy. This energy might eject a third electron from another orbit. By measuring the energy of the emitter electron called Auger electron, the atom can be identified. Different atoms have different atomic orbits therefore different Auger energies. One of the techniques for measuring the energy of the Auger electrons is the same in the LEED setup. It consists of the three grids and instead of the screen present in LEED, an electron collector is located in the Auger. The electron gun in the Auger technique operates at higher energy than in the LEED.

LEED-AES apparatus must operate in ultra high vacuum environment to allow travelling of the electron and to ensure extremely high purity the analyzed surface. The high vacuum environment is created by specially designed vacuum vessels (chambers) made out of stainless steel. The chamber is evacuated via combination at vacuum pumps. The vacuum level in the operated chambers with LEED-AES is similar to the vacuum level in space.

New micro electronic material consists of very thin multi-layered structures. The properties of these "sandwiches" depends on the type of material and structure. The information about the composition and structure is very important for the construction of these "sandwiches".

Recently LEED-ACES technique was very useful in the development of the new solar cell material (Meter, p.1900). The researchers found the new surface treatment process which generated special surface structure on the silicon surface. This new structure has the properties required by the new generator of solar cells. In this development the LEED-ACES apparatus works together with coating and surface processing equipment. Directly after coating or surface processing the investigated surfaces are analyzed by LEED-ACES.

As the search for the electronic materials continues, the significance of the basic surface characterization is crucial in this process. The quantities measured by LEED-ACES play an important role in finding the right properties of the material surface at the atomic scale.


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