Using the RHEED System
The reflection high energy electron diffraction
RHEED gun
(RHEED) system consists of an electron gun with
focusing optics and a phosphor screen protected by a
shutter. In its standard mode of operation a well
collimated beam of electrons hits the sample surface at
grazing incidence and the diffraction pattern is
manipulator
projected on the screen opposite the gun. For very thin
films the system can also be used as a transmission
diffraction apparatus. The purpose of this document is
to give you a basic introduction to the safe and proper
operation of the equipment for the experiments that you
will perform as part of Physics 206: Modern Physics
chamber
and Physics 308: Experimental Physics Laboratory.
A schematic of the vacuum chamber with the
RHEED system is shown in Figure 1. The electron gun
phosphor
is the thing covered by the shiny metal cylinder at the
Figure 1
screen
back of the chamber, and the phosphor screen is the
greenish looking window opposite the gun. A safety
interlock has been integrated into the cylindrical cover on the gun so that the high voltage cannot
be turned on if it has been removed. Another safety feature is the leaded window glass that
blocks x-rays created by the high energy electrons as they decelerate on impact with the
phosphor screen.
Inside the vacuum electrons are extracted from a filament that is heated by passing a
current through it. Electric and magnetic optics that are part of the gun accelerate and focus the
electrons into a narrow beam that can be deflected in two orthogonal directions. You have
control over six parameters: 1) filament current, 2) beam energy, 3) grid, 4) focus, 5) X
deflection, and 6) Y deflection.
1) The filament current controls the temperature of the filament and therefore the
maximum electron emission current. Care must be taken not to use such a high value
that the filament is burned out – the maximum filament current you should use is 1.50
A.
2) The beam energy ranges from 1.0 keV to 20.0 keV and determines the kinetic energy,
momentum, and wavelength of the electrons in the beam.
3) The grid setting is used to limit the beam diameter and therefore the beam current. A
more tightly collimated beam which ultimately produces more sharply defined
diffraction patterns comes at the cost of reduced intensity.
4) The focus value determines the position of the focus and the beam divergence. To
obtain the best operating conditions requires adjusting both the grid and focus
settings.
5) The RHEED gun has been installed so that the X deflection control moves the beam
in a horizontal plane.
6) The RHEED gun has been installed so that the Y deflection control moves the beam
in a vertical plane.
Use the following procedure to begin operating the RHEED system to produce either
transmission or surface diffraction patterns.
1) Open the shutter that covers the phosphor screen.
2) Check to make sure the pressure in the vacuum chamber is below 1 × 10-5 Torr. If it
is not do not operate the electron gun.
3) Make sure the beam energy and filament current controls are both set to zero, then
turn on the power supply. It is easier to see the position of the beam relative to the
sample if the beam is unfocused to begin with, so turn the grid and focus settings on
the remote control down to 1.0 or less. The X and Y deflection settings are not that
important at this stage, but you can set them to 5.0 if you want the beam to be
approximately centered.
4) Turn the beam energy up to 5.0 keV over a span of about 10 seconds.
5) Turn the filament current up to 1.0 A over a span of about 15 seconds. Then increase
the filament current in 0.1 A increments over 10 seconds each until you see a beam
spot on the phosphor. Under no circumstances should you exceed 1.5 A filament
current.
6) At this point you can use the unfocused beam to cast a shadow image of the sample
and sample holder on the screen in order to more accurately position each. For
example, the sample may be too far to one side or another so that only a small
fraction of the screen is being used. Move the sample with the manipulator to correct
for this and reposition the electron beam with the remote control X and Y deflections.
7) Once the sample is appropriately oriented you can turn up the beam energy (again,
take about 10 seconds for each 5.0 keV increase) and adjust the grid and focus
conditions to achieve the best image possible. The optimum grid and focus settings
depend on beam energy – use the following table as a guide to approximate settings:
Energy
Focus
Grid
(keV)
(setting)
(setting)
10.0
2.02
1.52
15.0
2.90
2.00
20.0
3.66
2.32
8) Do not allow a well focused beam to hit the screen directly for long times because
localized heating can damage the phosphor layer.
9) To turn off the system when you are finished making your measurements, turn the
filament current to zero, turn the beam energy to zero, turn the power supply off, and
close the shutter so it covers the screen.