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Methods in Surface Physics Experimentation in Ultra-High Vacuum Environments. Hasan Khan ( U niversity of Rochester ), Dr. Meng-Fan Luo ( N ational Central University ). Introduction. Objective.
in Ultra-High Vacuum Environments
Hasan Khan (University of Rochester), Dr. Meng-Fan Luo (National Central University)
Surface physics is an emerging field of science involving the study of the surface properties of solids in a vacuum environment. Solid substrates are cleaned and put into a ultra-high vacuum (UHV) environment, where they come into contact with other substances and may display unique properties. Surface physics has applications in a number of fields, including semiconductor science.
We describe the general procedure and methods involved in surface physics experimentation, including establishing a vacuum using pumps, cleaning the sample’s surface, and measuring parameters using various forms of spectroscopy.
Theory and Methods
The first step in preparing for a surface physics experiment is establishing an ultra-high vacuum for the sample to be studied in. The sample must be studied in a vacuum to prevent contaminants from altering the initial surface properties of the solid. The experimental setup used in surface physics experiments is composed primarily of a vacuum chamber connected to various pumps, filaments, and gas lines. The sample is held on a sample holder inside the chamber that is free to move so that it may interact with different components of the system during the experiment.
A vacuum is created inside the chamber using three separate pumps. A rotary pump brings the pressure in the chamber down from 760 torr (atmospheric) to ~10-3 torr. After this, a turbomolecular pump is needed to bring the pressure down further, else the pressure will begin to increase. The pump brings the pressure in the chamber down to ~10-7 torr. At this point, the chamber is baked to for 1-2 days to remove water molecules that may have been adsorbed to the inner walls of the chamber. Finally, an ion pump is used to further bring the pressure in the chamber down to ~10-9 torr. A degassing process is also performed in which several components connected to the chamber are also heated to remove captured gas molecules. After this procedure, an ultra-high vacuum has been established.
Before testing, the sample must be cleaned of all possible contaminants on the surface. An ion sputter gun is used to shoot argon ions at the surface of the sample to remove the first few monolayers. After this, annealing is performed on the sample where it is heated to remove any argon ions that may have been adsorbed onto the surface. This process is repeated several times until the surface has been adequately cleaned. During this process, the contamination level of the surface is measured through a method called Auger Electron Spectroscopy (AES), which measures the surface composition of the sample as a function of intensity. Once intensity of foreign elements and compounds is low enough, the surface is clean and ready for testing.
Figure 1. This graph represents various pumps used in UHV experiments as a function of the pressure ranges they are usable in. Normally, only the rotary, turbomolecular, and ion pumps are used.
Figure 2. This picture depicts the cleaning of a solid surface by use of an ion sputter gun, which fires ions that chip away layers of the surface. The sample is then annealed to remove any adsorbed ions.
Figure 3. Shown above is the typical experimental setup used in UHV experiments, consisting of the vacuum chamber, spectroscopy equipment, and pumps (not shown).
Testing of the sample involves the adsorption of gases or the deposition of metals onto the solid surface. In the case of a NiAl substrate, typically an oxide thin film of Al2O3 is formed by exposing the chamber to oxygen gas. This new Al2O3/NiAl surface is then used as the testing environment.
The growth of nanoclusters of various substances is typically the goal of such experiments. For example, if Au is being tested, Au is typically deposited on the surface in vapor form and the sample is then heated to show signs of growth. A similar procedure is performed with gases.
Various forms of spectroscopy, such as Low-Energy Electron Diffraction (LEED) and Reflection High-Energy Electron Diffraction (RHEED) are used to observe the growth of nanocluster patterns. A scanning tunneling microscope (STM) may be used to image the nanoclusters directly.
Lüth, Hans. Surfaces and Interfaces of Solids. 2nd Ed. Springer-Verlag Berlin Heidelberg, 1993. Print.
Image Credit (Figure 2): http://pprco.tripod.com/SIMS/Theory.htm
Surface physics is a new and exciting field of study, however the methods used to conduct surface physics experiments are typically long and complex. It is important to be well-informed of the proper procedure used in carrying out such experiments in a vacuum environment to prevent possible contamination that may alter results.
In the future it is desired that the procedure for surface physics experiments become simpler and more controlled.