RHEED at High Pressures - TorrRHEEDTM

Several techniques, such as laser deposition, CVD, MOMBE, etc., require a higher gas pressure in the process chamber during growth. The working gases are sometimes very reactive. Two factors limit the use of in- situ RHEED. First, a high gas pressure inside the RHEED gun may strongly reduce the filament lifetime. Second, at higher working pressures, the gas molecules inside the chamber may scatter the primary and the diffracted electron beams leading to a very diffuse RHEED diagram.

wpe5B.jpg (7033 Byte)

RHEED Systems from STAIB INSTRUMENTS provide effective solutions allowing in-situ RHEED observations under such difficult experimental conditions. With the new TorrRHEEDTM the system still features electronic beam rocking and azimuth control (patent pending).

For pressures up to a few 10 mTorr: the integrated differential pumping stage

For gas pressures inside the vacuum chamber up to a few 10 mTorr, a single differential pumping stage is sufficient to ensure long lifetime of the cathode. It is integrated with the gun and is located very close to the cathode. The efficiency of the differential pumping expressed as the ratio between the chamber and the gun pressure, is up to a factor 1000. The pressure inside the gun is kept at a few 10-5 Torr for a chamber pressure of a few 10 mTorr, allowing safe operation of the gun and a long lifetime of the cathode.

For pressures up to a few 100 mTorr: the double differential pumping stage

The scattering of electrons inside the chamber becomes stronger in this pressure range. The mean free path of electrons inside the chamber is reduced. To compensate, the RHEED screen is shifted closer to the sample. In addition, a second differential pumping stage is added between the gun and the chamber. A pumped tube protrudes inside the vacuum chamber and the differential pumping aperture at the end of the tube is located close to the sample. The electron mean free path through the high pressure region is thus limited to about 50 to 100mm, allowing safe RHEED operation up to a few 100 mTorr.

High beam voltage (35 to 50 kV) is also recommended for operation at such higher pressures, because the scattering effects decrease with energy.

hipres.jpg (19423 Byte)