The electron source in our RHEED guns are so-called Mixed-Metal-Type cathodes. They consist of a porous tungsten pellet mounted on top of a metal sleeve with an indirect heater inside. Distributed inside the pores of the pellet is a mixture of Ba-Ca-Al oxide.
The surface of the tungsten pellet is additionally coated with a layer of Osmium and Ruthenium. After the activation process of the cathode, a thin layer of barium on-top of the surface reduces the work function down to approx. 2 eV. This low work-function allows electrons to be emitted at a temperature of just below 1000°C.
There are several advantages to use a Mixed-Metal-Type cathode in a RHEED system, which overcome some of the obstacles of an electron source in form of a direct heated Tungsten wire.
Our Mixed-Metal-Type cathodes were developed originally for Travelling Wave Tubes, which are amplifier tubes to be used onboard of satellites. This application requires much higher currents compared to what is needed for a RHEED pattern. Especially servicing, like exchanging a filament is almost impossible in space. This is the reason why these cathodes are designed to have a lifetime of more than 15 years at 24/7 operation. Therefore a change of the electron source because of degradation of a direct heated wire is not necessary in our RHEED system. 

An important benefit of a Mixed-Metal-Type cathode is the sharp energy distribution of the resulting electron beam. The cathode is mounted inside a Wehnelt cylinder with a very small aperture of only 0.3 mm. When the cathode is operating, the strength of the electron beam is controlled through a negative voltage on the Wehnelt cylinder.
A space charge of electrons is formed above the surface of the cathode, just below the aperture of the Wehnelt. In this space-charge an equilibrium is set up between emitted and absorbed electrons right above the cathode surface.
As the cathode is designed to deliver a high current, only a small fraction of the emitted electrons is necessary to pass through the aperture towards the various electrodes which form the electron-optic lenses. The energy distribution of the electrons is therefore very sharp and is ideal for a brilliant, high contrast RHEED pattern. 

The electron gun structure with all its electrodes is made of a series of high precision parts. These parts are fixed together by insulating glass rods. In order to correct the electron beam for the smallest deviations and to optimize the beam shape we integrate individual correction magnets. One correction magnet is mounted at the entrance of the focus cylinder, the other one inside the big anode cylinder.

During manufacturing the electron gun is sealed inside a glass tube with a phosphor front-plate and is evacuated to UHV level.
The operator lights up the tiny “picture tube” and checks with a microscope the shape of the electron beam.
With a ring of correction coils outside the vacuum he can generate correction fields (dipole, quadrupole, hexapole ) and optimize the shape of the beam profile. Necessary corrections are “frozen” into the magnetic rings permanently. 

To direct the electron beam to the point of interest on the substrate two sets of deflection coils are mounted perpendicular to each other on the outside of the vacuum tube of the RHEED gun. The sensitivity of the deflection coils is enhanced by wrapping a special µ-metal shielding around. The possible deflection angle is practically only limited by the diameter and length of the entrance port to the chamber. On a DN40 CF flange and a typical length of the tube of 50 mm, the possible deflection angle is up to 11° in both directions.

All values of the deflection can be seen directly on the display, so it's easy to direct the beam on the substrate to fix positions repeatable. Like all other parameter once determined positions can be saved and reloaded. 

Asides the usual X-Y Deflection our RHEED guns are all equipped with a High Angle Beam Rocking function.
What is Beam Rocking? 

1) The unrocked electron beam passes through the magnetic field of a vertical deflection coil (right side) and hits the substrate under a certain angle (black line) given by the coil characteristics and the deflection current set.
  
2) Before the vertical deflection coil an additional set of coils is mounted, the Beam Rocking coils. When the electron beam is deflected already by the beam rocking coil, it enters the vertical deflection coil at a shifted position. If one wants to redirect the beam to the same position on the substrate with the vertical deflection coil, the resulting incident angle to the surface will be bigger (red line). 

The electron beam can be switched on and off by simply pressing a button on the Remote Control or can also be initiated via the USB or optional LAN TCP/IP interface. The control voltage of the electron beam at the Wehnelt cylinder will be set to block or open the aperture. With a small software program you can trigger this also according your requirements. There is no mechanics involved at all.

Electron beams are very sensitive against any external magnetic fields. Starting from the earth magnetic field to every pump, motor, magnetic material and especially all the unknown magnetic fields from the labs above and below your chamber create disturbances to the electron beam. The lower the energy of the electrons is, the higher the effect of magnetic disturbances will be.
In order to reduce the vulnerability of the electron beam to a minimum, we have shielded our RHEED guns with a high efficient, thick µ-metal shielding. To limit the free, unprotected path from the end of the RHEED Gun into the chamber, we offer additional, customized shielding. Developed once for our highly sensitive Cathode Ray tubes for professional applications, we have all the necessary Know-How to treat µ-metal in the proper way and to achieve a maximum shielding effect. 

The heart of our power supply is a state of the art Microprocessor based control board, which communicates via optical fibers with the various, galvanically isolated high voltage modules to supply all the necessary voltages for the RHEED gun.

The Power Supply is built into a single low profile 19” housing and is connected to the RHEED Gun via a 10m long cable as standard. 
An additionally Safety Switch can be used as an interlock. If the Safety Switch is open, all High Voltages are set to Zero and filament is off. 

Each parameter can be addressed either at the power supply directly or via a separate Remote Control. All individual settings are displayed on the display of the Power Supply and the Remote Control directly in front of the user. So its very easy to adjust the beam for best performance and locate it to the area of interest.

You can SAVE all parameter of a certain setting and LOAD them next morning again. The software takes care for a safe re-arrangement of all voltages, so a risk of sudden voltage changes and therefore possible arcs is minimized.
The fully digital setup allows it to control all parameter also either via an USB or an optional LAN TCP/IP interface through an external PC. The necessary control software is already integrated and can be easily implemented onto your system. For details see the manual in the download section. 

You can SAVE all parameter of a certain setting and LOAD them next morning again. The software takes care for a safe re-arrangement of all voltages, so a risk of sudden voltage changes and therefore possible arcs is minimized.