Program Speaker: Dr. Ralph Dawson, Crystal Grower.
By Drs. Scott and Susan Wilson
Dr. Ralph Dawson, who recently retired from Sandia National Laboratories as a crystal grower, spoke to the Guild about basic crystal classes and their unique crystal lattice arrangements.
For thirty years, Dr. Dawson grew crystals using a technique known as molecular beam epitaxy (MBE). Molecular beam epitaxy allows the crystal grower to precisely grow very thin layers of atoms (known as mono-layers) with controlled thicknesses. This technique permits highly advanced semiconductor structures to be grown, such as Vertical Cavity Emitting Lasers (VCELs).
The materials that Dr. Dawson works with are mainly III-V compounds. These are binary (2 component) chemical compounds formed from one element taken from the 3rd column of the periodic table, along with one element taken from the 5th column of the periodic table. Hence, the name “three-five compounds”.
Examples of these types of compounds are Gallium-Arsenide (GaAs) and
Indium-Phosphide (InP). These compounds are of great interest in the manufacturing
of semiconductor lasers (your CD player has one).
In his introduction, Dr. Dawson described the three degrees of crystallization that a solid material may take: amorphous, polycrystalline, and a single crystal. The differences between these three types are based upon the size of an ordered region within the material.
An ordered region is a volume within where the atoms (or molecules) exhibit regular geometric or periodic arrangements (think of the regular spacing of boxes on graph paper). Amorphous material, such as glass, has order only on a length scale of a few atoms (very, very small). Polycrystalline material, such as agate, has order over many atoms (very small).
In both cases above, the ordered regions vary in size and orientation with respect to each other (rotated or displaced). Single crystal material, mainly what we faceters work with, has a high degree of order over a long range (several millimeters).
A single crystal region is called a grain. Adjacent crystal grains are separated by grain boundaries. These grain boundaries affect how well a material conducts electricity, and they may also influence the strength of the material.
The periodic arrangement of the atoms in the single crystal is called the “lattice” (remember the analogy of the stacked boxes). The 3D (three dimensional) lattice is a periodic repetition of a group of atoms. Since the lattice structure has repetition within, there must be some fundamental unit being repeated across the whole lattice. This fundamental unit is called the unit cell. By stacking unit cells above, below, and next to each other, we can build the full lattice structure to fill any given volume in the crystal.
There are seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, cubic, hexagonal, and trigonal. Fourteen possible unit cells exist and are known collectively as the Bravais lattices. Two things need to be kept in mind: which crystal system (i.e. cubic or tetragonal, etc.) and which unit cell structure (i.e. body-centered, face-centered, primitive, or base-centered).
Dr. Dawson explained the symmetry found in a crystal. Since the crystal is formed with repeating unit cells, it logically follows that there will be some symmetry in the arrangement of the crystal lattice.
The crystal symmetry can be seen by rotating models of the different crystal lattice structures. For example, if the crystal structure is cubic, then the lattice will look like a box with an atom at each corner of the box. If we hold the box to look only at the front of the box, then we see only four atoms (one at each of the four corners). If we rotate the box to look at one of the other sides, it will appear exactly the same to us. There is no visible difference in the four sides of the box. This is an example of four-fold symmetry.
To satisfy the interests of the group, Dr. Dawson spoke about cleavage
planes in material. Crystals will cleave (break apart along crystal planes)
where the atomic bonds are weakest. Bond strength is a function of the
distance between adjacent atoms. The closer the atoms are to each other,
the stronger the bond.
Dr. Dawson mentioned that one must take into account the density of bonds on adjacent layers. For example, on a given crystal plane, the bond strength between the atoms on either side of the plane may be weak. However, many atoms may be connected together across the plane and prevent the crystal from cleaving along that plane. Those bonds may be weak, but there are a lot of them.
Conversely, on a different crystal plane we may have strong bonds, but very, very few atoms bonding across the plane. It is possible that the crystal will cleave along this plane relatively easy.
There is one crystal lattice arrangement that Dr. Dawson identified as THE most technologically important for mankind: the diamond structure. Clearly, the diamond structure is that exhibited by diamonds, with the lattice points being carbon atoms. Other materials may crystallize in the diamond structure, and among them is the element silicon. Silicon (pronounced sil-i-kun, NOT sil-i-cone) is used extensively in the semiconductor industry to make all the integrated circuits and transistors that run our computers, cars, phones, and our lives.
Diamond has carbon atoms that make up the lattice, while silicon has silicon atoms that make up the lattice. The crystal lattice arrangement is identical, but not so the atoms!
Instead of using only one type of atom in the diamond crystal lattice arrangement, we use two types of atoms, like gallium atoms and arsenic atoms, to make it interesting. We then obtain the zinc-blende crystal lattice. The overall structure is still that of diamond. However, by using two types of atoms, we give it a different name. Many of the crystals Dr. Dawson grows in his laboratory exhibit the zinc-blende structure.
Dr. Dawson merely began to scratch the surface of crystal structure and crystal growth. At the urging of many Guild members, Dr. Dawson graciously accepted to speak in more detail at a later date on work performed when he grew crystals using molecular beam epitaxy. Thank you, Dr. Dawson.
From the Editor: Dr. Ralph Dawson addressed the Guild on how growing crystals for the semi-conductor industry can provide insight into problems found in faceting. Dr. Dawson’s well-explained concepts brought clarity into my mind and inspired Eureka! moments for me. Dr. Dawson’s explanation of how crystal growth can be faster on some planes than on others helped me to understand how crystals often have a tubular shape, down the C axis.
One Eureka! moment arrived when Dr. Dawson explained two, three, and four-fold symmetry using visual aids to show the three types of symmetry three-dimensionally. These well-constructed visual aids represented various forms of crystals, both simple and complex.
Another Eureka! moment occurred when Dr. Dawson clarified the reasons
for cleavage found in crystals. Using his crystal props again, Dr. Dawson
showed how the directions for cleavage formed along the flat surfaces,
where the individual atoms were at their greatest distance from one another.
This leaves the atomic structure weak in that particular direction, and
cleavage then occurs along the longest bond length. Thanks to Dr. Dawson,
several concepts became crystal clear.