Molecular Beam Epitaxy

Molecular Beam Epitaxy:
MBE is non-CVD epitaxial process using evaporation at very low pressures. Using it we can grow high-purity epitaxial layers of compound semiconductors. MBE operates at lower temp than CVD, it minimizes auto-doping, facilitates abrupt interfaces and good control of thickness, doping, and composition. It allows complicated doping profiles that have applications in microwave and photonic devices, e.g. linear doping profile for linear C-V of Varactor diodes, which are used as FM modulators.

In a highly evacuated chamber, heat a base material (substrate) such as silicon, germanium, or gallium arsenide to 500–600°C. Shoot precise beams of atoms or molecules in gas form (suitably heated) at the substrate from guns called effusion cells. A separate effusion cell is needed for each different beam, shooting a different kind of molecule at the substrate, depending on the nature of the crystal that is to be created. The molecules land on the surface of the substrate, condense, and build up very slowly and systematically in ultra-thin layers, and the complex, single crystal grows one atomic layer at a time. That's why MBE is called thin-film deposition system. It involves building up materials by manipulating atoms and molecules. It is a perfect example of self-assembly process in nanotechnology.

MBE is a very precise way of making a crystal (crystalline film) since it forms the film in highly controlled conditions: extreme cleanliness and an ultra-high vacuum (UHV), so no dirt particles or unwanted gas molecules can interfere with or contaminate the crystal growth. The ‘molecular beams’ are typically from thermally evaporated elemental sources, which must be extremely pure. Its chamber is cleaner than the conditions used in normal semiconductor manufacture and the pressure is on the lower limit of what's easily measurable. The typical growth rate is a few Å/sec and the beams can be shuttered in less than a second, to make abrupt transition between materials of the layer. It usually has three main vacuum chambers: a growth chamber, a buffer chamber, and a load lock.

An analogy is how an inkjet printer makes layers of colored print on a page by firing jets

of ink from hot guns. An inkjet printer has four separate guns firing inks of different colors

(cyan, magenta, yellow, and black). These slowly build up a complex colored image on

the paper. In MBE, separate beams of different molecules build up on the surface of the

substrate, though far more slowly than in inkjet printing—MBE can take hours!

maintain uniformity,

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