What is Molecular Beam Epitaxy (MBE) ?

MBE is a promising method for synthesizing thin epitaxial materials and heterostructure device structures where high purity and precise control of layer thickness and components are required. This results from the precise control of beam fluxes and growth conditions during MBE. The term “molecular beam” as used in MBE is somewhat of a misnomer since the species in the beams may be either atoms or molecules. During growth in an MBE system, thin films crystallize by means of reactions between thermal-energy molecular or atomic beams of the constituent elements and a substrate surface which is maintained at an elevated temperature in ultrahigh vacuum. The composition of the grown epilayer and its doping level are related to the relative arrival rates of the constituent elements and dopants, which in turn depend on the evaporation rates of the appropriate sources.

Because of MBE’s versatility, and high degree of control, MBE has become a valuable development and production tool.  An excellent introduction to MBE can be found at:

An Introduction to MBE
The MBE system is in an ultra-high vacuum environment to guarantee formation of a molecular ‘beam’. Without this level of vacuum, the atoms or molecules leaving the effusion cells would be scattered as residual gas molecules and never form a beam. This beam is directed at a heated substrate.
Similar to the spray of individual paint droplets from the nozzle of a spray-paint can, broad “beams" of individual atoms or molecules are generated by evaporation or sublimation of the pure source materials. However, unlike paint droplets, the atoms do not just stick where they land.  Instead, the heated substrate allows the recently added atoms to migrate a short distance until they find a favorable site to become part of the single crystal. 
An MBE growth is performed in a chamber consisting of at least a substrate manipulator, effusion cells, and monitoring equipment. 


The MBE system is in an ultra-high vacuum environment to guarantee formation of a molecular ‘beam’. Without this level of vacuum, the atoms or molecules leaving the effusion cells would be scattered as residual gas molecules and never form a beam. This beam is directed at a heated substrate.
Similar to the spray of individual paint droplets from the nozzle of a spray-paint can, broad “beams" of individual atoms or molecules are generated by evaporation or sublimation of the pure source materials. However, unlike paint droplets, the atoms do not just stick where they land.  Instead, the heated substrate allows the recently added atoms to migrate a short distance until they find a favorable site to become part of the single crystal. 
An MBE growth is performed in a chamber consisting of at least a substrate manipulator, effusion cells, and monitoring equipment. 

An MBE growth is performed in a chamber consisting of at least a substrate manipulator, effusion cells, and monitoring equipment. 
The typical construction of a MBE is shown in the following picture:




At the bottom there are the effusion cells to provide the molecular beam for either the bulk constituents or the dopants. These cells can be thermal evaporation cells (Knudsen cells), cells for gaseous media or plasma sources as well. In front of them is a shutter, this means a plate which could be brought into the beam for ‘switching’ the beam on and off. Opposite to the cells and the shutters is the substrate, mounted on a heatable and rotatable substrate holder. Often there are instruments for in-situ analysis like RHEED attached to the growth chamber. A cryopanel around the sample and the cells absorbs residual gases and provides a clean substrate environment.