Procedure of RHEED Experiment

1. Procedure of RHEED Experiment Three major steps:

2. 1. Pump down the chamber from the atmosphere pressure • Connect the flexible stainless steel hose associated with the turbo pump (Fig. 1) to the pump port of RHEED chamber (Fig. 2). • Turn on the mechanical pump. (Fig. 1). • (3) After about 20 min, when the pressure from the thermal couple pressure gauge of the mechanical pump is lower than 100 milli Torr, turn on the turbo pump. Turbo pump controller Mechanical pump Turbo pump Mechanical pump & Turbo pump Fig. 1

3. 1. Pump down the chamber from the atmosphere pressure Manipulator Right angle valve Pump port Nitrogen gas inlet Gate valve turning knob Fig. 2

4. Ion gauge controller Sublimation pump controller 1. Pump down the chamber from the atmosphere pressure (4) After about 3 - 4 min, when the pressure of the RHEED chamber is lower than 10-3 Torr, turn on the ion gauge associated with the chamber (Fig. 3). Fig. 4 Fig. 3

5. 1. Pump down the chamber from the atmosphere pressure (5) When the pressure is in the Lower range of 10-6 Torr, slowly open the gate valve. Turn on the ion pump(Fig. 5). If the pressure goes up, turn off the ion pump and wait the pressure goes down to previous lever and turn on the ion pump again. “Flash” the ion pump several times until the pressure shown by ion gauge does not go up. Then turn on the ion pump, close the “right angle valve”. Ion pump controller Sublimation pump could be used accompany with ion pump to achieve an optimal low pressure: Slowly turn up the current in the sublimation pump (Fig. 4) from 0 amp to 40 amp under “continuous”. Set current at “40 amp”, and then switch the knob to “cycle”. controller Fig. 5

6. 1. Pump down the chamber from the atmosphere pressure (6) Vent the turbo pump. Connect the nitrogen gas tube to the venting port of the turbo pump. Open the venting valve slowly and keep the pressure shown by couple pressure gauge below 100 mili torr. The rotating speed of turbo pump will gradually slow down. Then turn off the turbo pump. When the pressure shown by couple pressure gauge increases to atmosphere pressure, turn off the mechanical pump. Tips: During the pumping process, if the pressure indicated by ion gauge keeps at LOW×10-5 torr, it may be due to the leak of the gate valve. In this case, slowly open the gate valve, an increase of pressure will be observed. Then wait until the pressure goes to the Lower range of 10-6 Torr, and follow step (5).

7. 2. Using RHEED to obtain diffraction images Requirement of using RHEED: The pressure of the RHEED chamber is lower than high range of 10-8 Torr (low range of 10-7 Torr may be alright); turn off the ion gauge because the W filament generates light; turn off the sublimation pump because its Ti filament also generates light.

8. 2. Using RHEED to obtain diffraction images (1) Start camera (the black one near the computer monitor by pushing the button on the CCD camera controller. (Fig. 6) (2) Turn on the Computer: (program) CCD camera controller Fig. 6

9. 2. Using RHEED to obtain diffraction images (3) Turn on the RHEED gun switch on the back of the power supply. See Fig. 7. RHEED gun switch Grid voltage Fig. 7

10. Electron gun power supply 2. Using RHEED to obtain diffraction images (4) Turn on the electron gun supply on the rack. See Fig. 8. (5) Turn on the HV (on the panel of electron gun supply). See figure 8. Fig. 8

11. 2. Using RHEED to obtain diffraction images (6) Turn on the grid voltage. See Fig. 7. (7) Change the “HV” from 0 V to 9 kV (Slowly, the button shows “6/05”) (8) Turn on the filament (on the panel of electron gun supply) (9) Turn on the “V Limit” (the button shows “7/1”) (10) Increase the “I Limit” to “2 A” (the button shows “5/1”) Emission current is around 0.2 mA. (11) Close all the room light, open the shutter (locates inside the chamber and in front of the Phosphorous screen)

12. 2. Using RHEED to obtain diffraction images (12) Adjust the RHEED pattern so that it appears on the screen using the two-screw (x and y deflections) buttons on the right side of the panel. (13) Computer program (See appendix). (14) To shut down the system, do the reverse process from Step 10 to Step 3, then turn off the computer and close the CCD camera.

13. 3. Open the RHEED chamber or venting from high vacuum to atmosphere (1) Turn off the ion pump (gate valve is slightly leaking). (2) Make sure that the Ti sublimation pump is off. (3) Turn off the ion gauge and any instrument in the chamber that has hot filament. (4) Close the gate valve in the RHEED chamber. (5) Connect the Tygon tube for dry nitrogen gasto the nitrogen inlet port of the RHEED chamber. See Fig. 2. (6) Open the valve of nitrogen gas cylinder slowly, and use low pressure (close to atmosphere pressure). (7) Turn the “right angle valve” at counter-clock direction slowly, and stop when hearing a hissing sound from the chamber. (8) Open the chamber from the top of RHEED chamber where the manipulator is located on a 6 inch port.

14. 2. Using RHEED to obtain diffraction images • Appendix: Computer program for taking RHEED images • (1) Type “cd prog”, Type “cd Apps” , Type “./DataAcq” , Program Starts. • (2) On the toolbar of the program, go to “View”, open “Image sorter”. • (3) Go to “EFRHEED”, click “Buffer Scan”. • In the window of “EFRHEED Buffer Scan”, Gate Time is usually set as 7000(ms) or longer. Then click “OK”. • Double click the buffer in the “image sorter” to see the RHEED image. • To save the image, go to “Experiment”, go to “Save”, click “Buffer” to save one image or “Multiple buffers” to save several images.

15. 2. Using RHEED to obtain diffraction images (7) To analyze an image, go to “1d-2d”, go to “Radial profile”. Then in the RHEED image, select the region to measure. To check the radial profile data, go to “View”, go to “Line data sorter” and double click the corresponding buffer. (8) To save Radial profile data, go to “Save”, click “Line buffer” to save one set of data or “Multiple line buffers” to save several sets of data.