This is a drawing of our dual growth chamber MBE system. MBE stands for Molecular Beam Epitaxy. MBE is a technique for growing thin, extremely well controlled, layers of crystalline material. It's basically like spray painting with atoms. Inside the large chambers at both ends, pieces of material (such as silicon and germanium) are heated to the point where they begin to evaporate a stream of atoms. These streams are directed at a mildly heated crystalline silicon wafer. When the evaporated atoms land on the wafer, they tend to stick and arrange themselves into new crystal. However, as in spray painting, we can block the flow of atoms by inserting something in their path (here, small metal shutters). So if we block the Ge stream, we end up growing only Si on the wafer. Or we can block the Si to grow Ge. Or we can block neither stream to grow an alloy of GeSi. The neat thing is that if we evaporate at slow enough rates, we can block and unblock the flow of atoms so quickly that we can control the composition of the growing crystal atomic layer by atomic layer!

 OK, why the big metal chambers? Because, unlike drops of paint, single atoms are so small that air molecules also get in their way (and the air molecules may react with the wafer). For the evaporating atoms to act like paint droplets, moving in nice straight lines, we've got to get rid of that air by pumping it away. The big metal chambers are vacuum enclosures. They are pumped by "cryopumps" that use helium compressors to cool an internal surface so cold (~15K) that almost all of the air simply freezes. That leaves a vacuum of approximately one tenth of a millionth of a millionth of atmospheric pressure (10E-10 torr) where everything works just fine. This vacuum is better than that the space shuttle experiences in orbit.

Link to Virtual Reality Tour of our MBE system