● Molecular Beam Epitaxy (MBE):

The preparation of high quality epitaxial thin films needs a carefully controlled layer by layer growth process. Hence, low deposition rates are highly preferred. In order to avoid the disturbing influence of residual gas impurities ultrahigh vacuum (UHV) conditions are necessary allowing at the same time the installation of other sensitive in situ characterization equipments such as RHEED, and LEED either in the deposition chamber or in a connected preparation chamber without vacuum breaking. The construction of the evaporation sources must consider all the special conditions of a precise low rate deposition with controlled evaporation rates. In this sense MBE is a refined vaporation method for epitaxy of pure metals, defect-free semiconductors and superlattices with perfect interface morphology and even for the deposition of high quality epitaxial oxides and magnetic perovskites.

Figure 38.3: Schematic drawing of a Molecular Beam Epitaxy set up with growth (left) and analysis chamber (right).

Figure 38.3 shows the typical arrangment of MBE set up [3] with a growing chamber and analysis chamber. Knudsen cells are used as evaporation sources, allowing the stabilization of a defined vapor pressure inside the cell by a precise temperature control of the heater. In the case of chemical compounds, the Knudsen cell is replaced by a cracker section for the decomposition of the vaporized material. Further, instead of thermal cracking, the modern effusion cells use plasma sources, either with RF-discharges or as ECR microwave discharges, which are useful also for the activation of reactive gases introduced into the UHV system for the epitaxial growth of oxides.