From Salt to Lasers and Beyond:

Molecular beam epitaxy takes crystal growing to a whole new level

BY ANITA MARTIN                                                                       

Crystal growing seems a favored pastime of science teachers.  Maybe you’ve tried it.  It takes little more than a mason jar, half a cup of salt (or alum powder if you’re feeling extravagant), some boiling water, a string, a pencil and a paper clip.  Tie the string to the pencil to the paper clip, drop it into salty water and a couple weeks later, you’ve got your very own string of salt crystals.

Not impressed?

Well, don’t give up on science yet.  If you can survive the crude beginnings of crystal construction, you might get to the good stuff.  Arthur Smith of Ohio University’s nanoscience research team can attest to it.

Smith devotes a lot of his time lately to crystals.  And his is no pencil and paper clip operation.  He uses a technique developed in the 1960s by physicists Leo Esaki and Raphael Tsu called molecular beam epitaxy (MBE).  It all takes place in a huge apparatus with many-sized metal tubes extending in all directions—somewhat like a demonic stainless steel Tinker toy set.  While it may be intimidating, the machine is a very sophisticated nanoscience research tool.  It consists of three chambers: one for a scanning tunneling microscope (STM) that examines crystalline nanosamples, another for quick changes of samples or STM tips and the third for MBE, where the samples are grown.

You may wonder why these tiny crystals require a machine that takes up half of a room.  Smith, his students and his colleagues have a lot more in mind than salt on a string.  They are producing what scientists call “new and exotic” hybrid materials.  That is, material compounds that combine electronic and magnetic properties in ways not commonly found in nature.  And with MBE, they can grow them with atomic precision, one lattice layer at a time.

To find out how, let’s zoom in on the MBE chamber, a steel sphere adorned with a few round windows and several long, metallic cylinders called ports.  Four of these, arranged in a cluster (called a flange), are filled with different metals.  These materials are placed in the ports as solids, but with the right amount of heat, they melt and evaporate.  Some even skip the liquid phase and begin evaporating directly from the solid state in a process called sublimation.  A fifth port, set apart from this sublime quartet, is filled with nitrogen.  Normally nitrogen comes in molecules of two atoms each (N₂), but for the purpose of MBE, these atomic duos are split in a hot nitrogen plasma into single N atoms.

Meanwhile, inside the chamber rests a tiny crystalline sheet less than a millimeter thick and about a centimeter in length and width, generally made out of crystalline minerals and gemstones such as sapphire or magnesium oxide.  The chamber itself contains an ultra high vacuum (UHV) with an air pressure of one ten-billionth of a Torr (air pressure at sea level is over 760 Torr). 

Next things start heating up.  The substrate is heated to help clean the surface of impurities such as oxygen and water.  Temperatures also rise for the flange foursome to the point of evaporation, and when everything is set, some of these evaporated elements are shot toward the substrate, along with single nitrogen atoms.  Once they reach the substrate, the atoms arrange themselves into a crystal lattice formation, mimicking the structure of their substrate.  They form this pattern out of the tendency of matter to seek the point of lowest energy, just as marbles cast over a Chinese checker board would come to rest in the holes.

So what’s the big deal about these little crystals?  For one, they are the raw material of a tool used for everything from curing blind spots to playing DVDs to monitoring movements within the earth’s tectonic plates.  Just take the grown material, do some processing and etching and add a few things like leads, contacts and switches -- and you can build lasers. 

What’s more, the research going on in Smith’s lab promotes better understanding of electronics, spintronics and magnetics and could eventually lead to nanotechnology that reaches even beyond the scope of our modern lasers.  So stick around—this crystal growing business only gets better.