Amazing how the price of memory and especially SD cards has dropped. 8GB stores a ton of stuff. I think I’ll pick up one or two for my PDA so I can store more video and music.
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The cold weather is taking a bite out of my game and I have not had the ability to practice or play with the advent of the year end holidays. I went out yesterday, after spending some time in the gym, and had some trouble getting the ball to go straight. I was saved from further mishap by a
phone call so I had to leave the range.
I have made some updates to the blog on the back end so I hope to provide more frequent updates in the future. Until then …
Molecular Beam Epitaxy (MBE), is one of a number of methods of thin-film deposition. In solid-source MBE, ultra-pure elements such as gallium and arsenic are heated in separate quasi-knudsen effusion cells until they each slowly begin to evaporate. The evaporated elements then condense on the wafer, where they react with each other, forming, in this case, single-crystal gallium arsenide. The term “beam†simply means that evaporated atoms do not interact with each other or any other vacuum chamber gases until they reach the wafer, due to the large mean free path lengths of the beams.
A computer controls shutters in front of each furnace, allowing precise control of the thickness of each layer, down to a single layer of atoms. Intricate structures of layers of different materials may be fabricated this way. Such control has allowed the development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots. Such layers are now a critical part of many modern semiconductor devices, including semiconductor lasers and light emitting diodes.
During operation, RHEED (Reflection High Energy Electron Diffraction) is often used for monitoring the growth of the crystal layers.
The ultra-high vacuum environment within the growth chamber is maintained by a system of cryopumps, and cryopanels, chilled using liquid nitrogen to a temperature of 77 kelvins (−196 degrees Celsius). The wafers on which the crystals are grown are mounted on a rotating platter which can be heated to several hundred degrees Celsius during operation.
Molecular beam epitaxy is also used for the deposition of some types of organic semiconductors. In this case, molecules, rather than atoms, are evaporated and deposited onto the wafer. Other variations include gas-source MBE, which resembles chemical vapor deposition but in vacuum.
Molecular beam epitaxy was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and A. Y. Cho.