The PHARAO Experiment:
The PHARAO Project of the Paul-Drude Institute (PDI) at BESSY II in Berlin is dedicated to the in situ real-time characterization of MBE growth using synchrotron x-ray diffraction. In collaboration with a diffractometer manufacturer and the scientific team at the PDI, VTS Createc designed and built the mission-critical MBE experiment which is now successfully operating at a dedicated beamline. The system has met or exceeded all specifications.
VTS Createc designed a set of independent growth chambers to extend the capabilities of the PHARAO system for additional specimen synthesis. The complete unit consists of three independent MBE Systems for:
A Versatile System:
Despite the restriction of being incorporated into a highly precise six-circle diffractometer, the MBE system needs to provide MBE growth conditions similar to stationary research systems. Large Be windows allow incidence and exit polar angles of 0 to 45°, and exit azimuthal angles of 0 to 110°.
The chamber features three liquid nitrogen cooled shrouds, ports for 7 effusion cells and a permanent video sample surveillance for alignment. The chamber is pumped by turbo, ion and Ti sublimation pumps and has a base pressure in the upper 10E-11 mbar range with arsenide growth.
The sample holders rigidly couple to the sample stage, which is directly connected to the innermost circle of the diffractometer. Sample movements are transferred into vacuum by torque-compensated bellows and a two-stage differentially pumped rotation feedthrough. Sample temperatures up to 1000°C are possible.
The high-precision RHEED system HP-4/Safire can be operated simultaneously with x-ray diffraction. During operation, the complete system inside the radiation safety hutch is remotely controlled by our EMERALT software. For sample introduction, a three chamber transfer system is integrated into the diffractometer platform.
Results:
The first measurement campaign has focused on surface x-ray diffraction studies of GaAs growth kinetics. With the x-ray intensities available at BESSY II, layer-by-layer growth can be studied with a time resolution down to 1 s.
On reconstructed surfaces, the terrace or island kinetics can be separated from the reconstruction kinetics by choosing appropriate diffraction conditions. The data indicates qualitatively different coarsening kinetics of islands and reconstruction domains at typical GaAs growth conditions.
This MBE System is manufactured by:
VTS-CreaTec
GmbH
Industriestr. 9
74391 Erligheim
Germany
mbe@vts-createc.com
www.vts-createc.com