Reflection High-Energy Electron Diffraction (RHEED) has proven to be a very versatile technique for growth and surface studies of thin films. The combination of the forward scattering geometry of RHEED with the arrangement of pulsed laser deposition, in which the plasma plume is incident normally on the substrate, enables diffraction features to be monitored during growth. In RHEED, high-energy electrons with energies of 10-35 kV are incident on the surface of a crystal at a grazing angle of a few degrees (1-5°). The high-energy electrons only interact strongly with the first few layers of a material, because of the grazing angle. This makes RHEED extremely sensitive to surface morphology, with which it is possible to investigate the surface structure and morphology in-situ and real time during growth.
Monolayer growth control
RHEED can be used for the structural analysis of crystal surfaces and the investigation of the growth kinetics of thin films by monitoring the variations in the intensity of various features in the RHEED pattern. It has been shown that during growth, the intensity of RHEED patterns (during experiments typically the intensity of the main specular spot is tracked) show an oscillatory behaviour, which directly relates to the growth rate. The oscillations are associated with periodic changes in the roughness of the growing surface, as atoms forming a new monolayer gradually fill vacant surface lattice sites.
- The system includes a 30keV RHEED-gun to which an differentially pumped extension tube is connected with a pinhole close to the substrate. This design, developed at the University of Twente, makes it possible to carry out RHEED measurements at higher pressures, typically necessary for high crystalline growth of oxide materials. The entire RHEED-gun and extension can be mounted on an XY-stage to allow for electron beam adjustment with respect to the substrate. A valve can be included to isolate the RHEED gun from the chamber to allow atmospheric pressure annealing.
- Opposite to the electron gun a phosphor screen is mounted on a flange that extends into the chamber to position the screen as close as possible to the substrate. A metal shield is attached to the phosphor screen to avoid contamination. Data acquisition for the software is realised with a camera capturing the image on the screen. An optional shutter can be mounted to protect the phosphor screen during depositions without RHEED.