Molecular Beam Epitaxy & Vacuum Technology

1. Ultra High Vacuum and Much Ado About Almost Nothing Donna Barton, Cedar Hills Elementary School, Jacksonville, FL Steve Crandall, Inverness Middle School, Inverness, FL Dr. Maitri Warusawithana, Department of Physics, Florida State University Abstract Vacuum Experiment Demonstrator Design 5 E Learning Cycle Model The original design of the tube was similar to a demonstrator tube constructed by John Quinn. First, this teacher vacuum demonstrator would have a circumference large enough to be able to insert ones hand in the tube to place various items in for testing and for cleaning purposes. Second, it would be long enough to demonstrate a coin and feather drop effectively. Originally, the plan called for putting similar metal flanges on each end which would be epoxied in place and then have another metal plate and O-ringsto seal the tube. After some discussion at the Physics Department Machine Shop, Ian Winger suggested large rubber plugs would work in place of metal flanges and would also be easier and more cost effective to reproduce. Polycarbonate would be the preferred tube material in order to handle the outside atmospheric pressure when under vacuum. The next session students would then explore their ideas in a paper inquiry lab where teams are given two identical pieces of paper and asked to take measurements and make observations about the paper. The students will then be asked to make a prediction about what will happen if both papers are dropped parallel to the floor, from the same height, at the same time. Students will then drop the papers several times and record what happens. After this students will crumple one paper into a ball and again take measurements. Again students will make predictions and then, as before, drop both pieces from the same height and record the results. The Explain and Evaluate session involve students creating an interactive poster that discusses the goal of their investigation, their explanation and their evidence and reasoning to support their explanation. Students will use their notes as well as text and internet resources to research laws and theories that might support their reasoning. In the final session students would share their work using a round robin format which would be followed up by a discussion to synthesize all the perspectives into one explanation that the students agree is the most valid. The Elaborate sessions allow teachers from elementary to high school level to adapt this to many other areas. Similar to the Paper Drop, students might design and test Paper Whirligig Wings, using standard size/mass pieces of paper. Students would apply what they learned by changing the design to alter the flight of the whirligig by changing the edge of the wings straight, saw-toothed, or scalloped. Molecular Beam Epitaxy is a method of growing crystalline films where molecular beams of atoms physically arrange themselves onto a crystalline surface under ultra high vacuum conditions. These unique films cannot be found in nature and may be used for devises like molecular semiconductors, lasers, resistors, and transistors. The regulatory devices used to prepare, create, and test these crystals can tightly control layer composition and thickness at an atomic scale. Solid metals are heated in effusion cells to obtain molecular beams through sublimation or evaporation. The ultra high vacuum environment gives source molecules a large mean free path, forming a straight beam which is then deposited upon a heated substrate. The National Science Foundation granted the Florida State University in Tallahassee funds for the Research Experience for Teachers program that invites classroom teachers to work with science professionals doing such research. Two teachers were assigned positions with the MBE team at the National High Magnetic Field Laboratory at FSU. These teachers worked to create fairly low tech and easy to duplicate demonstrators that can show some of the interactions of matter and energy under vacuum conditions. Figure 5: Paper Drop Experiment - same shape. Figure 3: Demonstrator Design able to achieve medium vacuums < 30 inHg. Figure 1:Molecular Crystal Films will grow Under Ultra High Vacuum in the Molecular Beam Epitaxy Chambers. Figure 6: Paper Drop Experiment – different shape. Inquiry Lesson Design Molecular Beam Epitaxy & Vacuum Technology The vacuum unit may be used to teach students at various levels about how things may or may not be affected by a vacuum environment. Our first inquiry lesson will focus on force and motion. This unit also utilizes the 5 E model. The Engage and Explore sessions of the unit involves accessing prior knowledge, and finding out what types of misconceptions students may have. Students will write and discuss their ideas of what it means to have a “vacuum”. The teacher would then ask students to make a hypothesis about what would happen if a coin and a feather were dropped into the tube at the same time. Students would watch this repeated several times and record what they see happening. After some discussion, the teacher would then introduce the vacuum pump and discuss what happens when the pump is turned on and how this is measured. Once the tube is evacuated to 30 inHg the students would again write about what they think will happen and why. After some discussion the teacher would then demonstrate several times by turning the tube to show how the feather and coin now hit almost at the same time. The characteristic feature of all MBE techniques is the beam nature of the mass flow toward the substrate. Vacuum conditions are indispensible because admissible values of the total pressure of the residual gas in the vacuum reactor have to be ensured in order to preserve the beam nature of mass transport in the reactor. This MBE chamber is capable of achieving ultrahigh vacuum down to billionths of a millibar.The demonstrators constructed are capable of a medium vacuum down to tenths of a millibar. Two parameters, related to pressure, are the mean free path of the gas molecules penetrating the vacuum and the concentration of the gas molecules or number of molecules per unit volume. Mean free path is the average distance traversed by the molecules between successive collisions. In standard MBE reaction chambers the molecular beam is generated in effusion cells and controlled with shutters. There is about 0.2 meters between the outlet orifices of the beam sources and the substrate crystal surface. On the way to the substrate the beam molecules may encounter molecules of unwanted residual gas atoms if the pressure in the chamber is not low enough. Collision scattering processes degrade the beam nature of the mass flow, considering the very slow growth rates and requirement of negligible amounts of impurities. The mean free path of the particles in the medium vacuum demonstrator chamber is on the order of 1 mm, which is considerably less than the mean free path of ~100 km in the Ultra-High Vacuum of the MBE. Acknowledgements Dr. Maitri Warusawithana, Jonathan Ludwig, Priyashree Roy, Minseong Lee, Julien Frougier, Ashley Huff, Kedrick Vaughans, John Quinn, Ian Winger, Jose Sanchez References Chambers, A., Fitch, R. K., & Halliday, B. S. (1998). Basic Vacuum Technology. Philadelphia PA: Institute of Physics Publishing. Dasgupta, N., Indian Institute of Technology [Lecture 10]. Molecular Beam Epitaxy. Retrieved from http://freevideolectures.com/Course/2328/VLSI-Technology/10 Herman, M.A. & Sitter, H. (1989). Molecular Beam Epitaxy: Fundamentals and Current Status. New York, NY: Springer-Verlag Berlin Heidelberg. Wagendristel, A., & Wang, Y. (1994), An Introduction to Physics and Technologies of Thin Films. River Edge, New Jersey: World Scientific Publishing Co. Pte. Ltd. Warusawithana, M. (2009). “Nanostructured thin film synthesis: A new frontier at the Mag Lab”. Florida State University National High Magnetic Field Laboratory Mag Lab Reports, Volume 16 No.4, 5 – 6. Figure 4: Feather and Coin Drop Experiment where pink feather and copper coin are falling to the bottom of the tube at the same rate under vacuum.. Figure 2: MBE Cross Section View.