Metallic Superlattices

The earliest known research with metallic thin films was carried out in Germany by August Kundt in the late nineteenth century. Many years would pass, however, before thin films became a major focus in the field of physics. The introduction of computers and the search for practical methods of data storage are primarily responsible for the sudden increase in scientific interest in magnetic thin films that developed around the mid twentieth century. This interest has been steadily maintained since that time as thin film production methods have been improved, new technologies have evolved, and additional opportunities for thin film applications have emerged. In fact, tens of billions of dollars are spent each year on magnetic thin film research in the United States alone, and much more funding supports endeavors in the field worldwide.

The thin films utilized by Kundt in his pioneering work were produced via the lengthy and often difficult process of electroplating. Using this method, he was able to generate individual films of iron, nickel, and cobalt on plates of glass. His primary interest in the films was magneto-optical in nature and he successfully employed them to observe the Faraday effect. Kundt’s techniques and concentration dominated studies of thin films for several decades after his work with the intriguing films was reported. As technological capabilities advanced and demands changed, the field experienced a revolution.

Nickel-Copper on Mica

Early thin films all consisted of a single layer of a material on a substrate, but eventually the multilayered film, or superlattice, was invented. A superlattice consists of alternating layers of thin films deposited in an orderly manner. In metallic magnetic superlattices, at least one of the thin film materials is magnetic. IBM researchers Leo Esaki and Raphael Tsu are credited with first developing the concept of superlattices and with first successfully producing one of the multilayered structures. Working together at the IBM T. J. Watson Research Center, the pair made these substantial breakthroughs in the late 1960s.

To produce the world’s first manmade superlattice, Esaki and Tsu employed an advanced crystal growth technique known as molecular beam epitaxy (MBE). MBE must be carried out under ultra-high vacuum, clean environment conditions and involves targeting a stream of one or metals or other materials onto a surface a layer of atoms at a time. The deposition rate associated with the technique is typically a few nanometers per minute. MBE had been employed before Esaki and Tsu adopted the practice for their work, but the IBM scientists are thought to have been the first to employ it generate a new semiconductor material. Semiconductors are a class of solid crystalline substances with electrical conductivities that exceed those characteristic of insulators but are less than those of good conductors. Semiconductors are widely employed in computer chips and a variety of other electronics.

Iron/Nickel Oxide on Magnesium Oxide

In addition to MBE, a number of other methods of superlattice formation have been developed, including sputtering, lithography, and chemical vapor deposition. Particularly popular is the process of metal-organic chemical vapor deposition (MOCVD), which is sometimes alternatively known as metalorganic vapour phase epitaxy, organometallic vapour phase epitaxy, or organometallic chemical vapour deposition. This and other chemical methods of thin film growth entail the use of a precursor that experiences a chemical change along substrate surfaces. In MOCVD, the precursor is an organometallic gas. Due to the nature of gases, the substrate is completely surrounded by gas molecules so that upon chemical alteration all of the surfaces of the substrate are simultaneously and evenly covered with a layer of material.

No matter what deposition method is utilized to produce thin films or superlattices, to generate a high quality finished product, materials with good lattice matching must be employed. A perfect thin film should be extremely smooth, its surface appearing similar to a mirror when examined under an optical microscope. If the spacing between atoms of the substrate and the overlying layers of material are not properly aligned, a variety of physical deformities may develop. Flaws in thin films often include wrinkles, clumps, and striations. Such flaws generally prevent the practical employment of the thin films, though deformed films can provide useful information about what direction additional production attempts should take. Structurally metallic superlattices can be considered to fall into one of two basic groups: those with good epitaxial lattice matches so that a single crystal superlattice is formed and those with poor lattice matches so that each layer in the superlattice exhibits a different structure.

Gold-Nickel on Mica

A broad array of metals, other elements, and compounds have proven useful as components of superlattice structures. The properties of any particular superlattice depend both on the properties of the individual materials employed in its structure as well as with their interactions with one another. Because of the importance of the interplay of materials along thin film surface boundaries, research into this area of materials science is currently very intense. Of particular interest have been the interactions between layered systems including magnetic and nonmagnetic thin films.

Iron Oxide on Magnesium Oxide

Recent studies have found that superlattices containing magnetic and nonmagnetic components exhibit behavior that is not typical of either individual material type in bulk form. A number of couplings are known to occur between magnetic thin film layers separated by nonmagnetic spacer layers. Changes in coupling behavior can be altered by modifying the thickness of the layers or by employing different materials. By fine tuning coupling and thereby influencing superlattice behavior, engineers have helped improve the performance of various devices that employ multilayered thin films. Included among these devices are magnetic recording heads and sensors. Additional research, it is hoped, will result in a better understanding of thin film coupling and thereby lead to increased characteristic control capacity and even better superlattice-based technologies.

In order to study coupling, various methods must be available to characterize the magnetic properties of metallic superlattice structures. Several means of characterization are currently employed on a regular basis. Some of the most notable methods include surface magneto-optic Kerr effect microscopy (SMOKE), scanning electron microscopy with polarization analysis (SEMPA), Brillouin scattering of light, spin-polarized electron scattering, and neutron diffraction.

Iron/Nickel Oxide on Magnesium Oxide

Although superlattice research is ongoing, some devices have already been developed that make use of the unusual structures. One of the most intriguing applications of superlattices thus far is in laser technology. In the mid 1990s, a team of researchers working at Bell Labs, a branch of Lucent Technologies, invented a superlattice-based device called the quantum cascade laser. The unique semiconductor laser operates in a manner that can be considered akin to an electronic waterfall (hence the name). Electrons in the quantum cascade laser trickle down a kind of energy stairway in such a way that a single photon is emitted along each step. This cascading type of photon emission renders quantum cascade lasers more powerful than diode lasers and certain other laser varieties. Studies indicate that quantum cascade lasers are also remarkably reliable. The wavelength at which the laser operates depends chiefly upon the thickness of the layers that make up the superlattice it contains rather than the materials that comprise the structure. Thus a single material combination can be used and the laser will still be able to cover a large section of the electromagnetic spectrum.

In 1991, at the first International Workshop on the Science and Technology of Thin Films for the 21st century, it was suggested by a panel of participants focusing on magnetics that “the future lies in thin films.” Now that the 21st century has dawned, the prediction of the panel does not appear to be unfounded. Thin films have found their way into a number of devices and research in the field continues fast and furious. Many hopes for their widespread implementation in groundbreaking technologies remain to be realized, but the steady pace of work with thin films intimates that the future for them, especially advanced superlattices, is bright.