Infrared detectors are electronic devices designed to detect hot or cold spots by scanning the temperature and heat distribution of objects. All objects, whatever their temperature, give off varying degrees of thermal radiation. In its most familiar form, radiated heat can be sensed by our skin but not seen by our eyes. For example, a heated clothes iron feels hot to the touch but does not look different from a cold iron. Unseen heat is energy that can be identified by infrared devices, which convert an object�s thermal radiation into an electrical signal and then to a digital signal or picture of the object. Infrared devices turn invisible infrared heat into visible signals or images.

Incorporated in binoculars, telescopes, night-vision goggles, and cameras, infrared detectors have a wide range of uses. These include medical imaging for detecting infections and tumors, and aerial surveys for detecting crop infestations and blights. In the defense industry, infrared detectors help in conducting reconnaissance, surveillance, and flights in the dark. Commercial applications of infrared detectors include the detection of power hazards and failures in industrial equipment, and energy conservation measures such as scanning buildings for heat loss. Aerospace scientists are assisted by infrared detectors in the study of planets and stars. And infrared detectors aid law enforcement officials in apprehending poachers and illegal industrial polluters.

Although infrared devices can perform many useful functions, relatively few are manufactured because the best materials that detect infrared heat are extremely difficult to produce. NASA researchers are discovering that microgravity may provide the key to improving processing methods for these valuable materials, especially for mercury cadmium telluride (HgCdTe).

Creating Ideal Materials

Crystals of HgCdTe are used in infrared devices to detect, or sense, thermal radiation from objects. The atoms in these crystals line up in orderly rows and columns that create a three-dimensional structure, or lattice. The effectiveness of HgCdTe as an infrared detector depends on this orderly, crystalline structure. Irregularities, or defects, in the crystal�s structure adversely affect the ability of the material to convert heat signals and, consequently, of the device to form high-resolution pictures of an object.

Manufacturers try to avoid defects in HgCdTe crystals by solidifying the material under very controlled conditions. One method for controlling the formation, or growth, of the crystal is to solidify HgCdTe on top of another material that has a similarly ordered atomic structure. This base material is called a substrate. For HgCdTe, cadmium zinc telluride (CdZnTe) is the "substrate of choice" among scientists and commercial producers. CdZnTe serves as a stabilizing influence as the HgCdTe crystal layer forms because, during growth, the atoms between the active detector and substrate join together with a tight, interlocking match between the two lattices. This match between the two lattices reduces stress on the active detector as it grows. Like a Tinkertoy� set at the atomic level, CdZnTe joins together with HgCdTe to form, in effect, a continuous material.

This continuous material has several advantages when used in an infrared device. The two-layer material can be placed in the device so that only the substrate is exposed to the air, with the HgCdTe crystal sealed in an inert gas or a vacuum within the device. In this arrangement, CdZnTe protects the active detector material from the environment. Because CdZnTe is transparent to thermal radiation, it does not interfere with the transmission of heat signals to the active detector. The CdZnTe substrate allows maximum information to be received by the active detector, to be analyzed quickly and precisely.

Although CdZnTe is in many ways an ideal substrate, it is a relatively soft material in which structural irregularities, or defects, occur frequently. Just as a precise crystalline structure is necessary for optimum performance of HgCdTe, a precise crystalline arrangement is necessary for CdZnTe to perform well as a substrate. Defects that occur in CdZnTe crystals make the material costly to produce, and the expense would limit usage. The culprit behind such defects is gravity.

The Problem of Gravity

Gravity makes growing the CdZnTe substrate difficult on Earth. Defects, caused by gravity-driven effects such as hydrostatic pressure and buoyancy-induced convection frequently occur in the crystal, resulting in higher material costs and substrates of inferior quality. Hydrostatic pressure is the internal pressure variation within a body of liquid, caused by the weight of the liquid upon itself. This phenomenon is pervasive on Earth, and we take it for granted. One example is the increase in pressure experienced by a diver descending to the bottom of the ocean. Even in a body of liquid as small as water in a drinking glass, hydrostatic pressure is at work. The pressure at the bottom of the glass is greater than the pressure near the top, which forces the liquid to conform to the container wall.

Hydrostatic pressure can lead to other forces, or stresses, that act on a solidifying material such as CdZnTe. These stresses can cause various kinds of defects. When a molten material comes into contact with the wall of the container (this contact is due to hydrostatic pressure), it penetrates small fissures and crevices. As the material solidifies, it may become "locked" to the container wall. Contact with the wall leads to several possibilities, for as the material solidifies, the crystal and the container wall contract at different rates. As a result, the crystal and container may separate from each other because the crystalline material typically contracts more than the outer container wall does. Another possibility is that the container wall can contract more than the crystal does and compress the crystal. Still another result might be that the crystal contracts and moves away from the wall, but because the two have become locked together, the solidifying material is put under tension. When this happens, the material might cause the outer wall to break, or the crystal might experience internal fractures or generate defects as a result of stress. These stresses are called "hoop" stresses, and the resulting defects make the crystal less usable.

Common defects that arise during the processing of CdZnTe include dislocations and twinning. Dislocation defects can develop in either of two ways: an edge of an extra plane of atoms can be introduced in one part of the crystal, or extra atoms can be added during the growth process to the "steps" of a spiral arrangement of atoms located around a center axis. Although the material is still usable in these instances, working around dislocation defects is costly. Twinning, on the other hand, is catastrophic. Twinning defects arise as the material solidifies and, rather than forming a continuous orderly arrangement of atoms, instead reverses the sequence of atoms periodically. The twinned material is electronically unacceptable, and the material must be thrown out, reducing yield and increasing cost.

The difficulties with processing CdZnTe in Earth�s gravity are a significant stumbling block in the production of infrared devices. But the collaboration between a researcher with an innovative idea and a government agency with a unique resource may lead to a way to overcome these obstacles.

The Microgravity Solution

"A good scientist is one who thoroughly evaluates unexpected results and recognizes when something good has happened," notes David Larson. Larson, a researcher and professor at the State University of New York at Stony Brook has had opportunities to evaluate unexpected results and has recognized good things that happened in experiments conducted early in NASA�s spaceflight research program. In the 1970s, Larson worked for Grumman Corporation, an aerospace and defense systems manufacturer, as one of the first microgravity materials science researchers. He led successful solidification investigations of magnetic materials on the first U.S. space station, Skylab, and during Apollo, and the Apollo-Soyuz Test Project. He applied the results of this early microgravity solidification research to semiconductor crystals grown in subsequent spaceflight investigations on the space shuttle. Those studies of magnetic materials focused on trying to improve the quality of crystals by improving the homogeneity (the uniform distribution of phases and chemical components) of the crystal material. But Larson saw a clue overlooked in the results of those studies � that improved crystalline quality and reduced lattice strain might be linked to detached solidification, a phenomenon that frequently occurred in microgravity conditions. Detached solidification happens when a crystal material grows, or solidifies, without touching the wall of its container.

When Grumman approached Larson about optimizing CdZnTe, a semiconductor material used by the company in the manufacturing of infrared devices for the military, he saw an opportunity to test his hypothesis concerning detached solidification. On the first United States Microgravity Laboratory (USML�1) mission, flown in 1992, Larson investigated the growth of CdZnTe in a microgravity environment. His goal was a solidified material with far fewer defects than materials grown on the ground.

In his preflight preparations for the experiment, Larson allowed for the absence of hydrostatic pressure in microgravity, but his plan still called for solidifying the material uniformly and in contact with the wall. If his experiment went as planned and resulted in a crystal with defects, the experiment would demonstrate that wall contact was indeed the cause of structural defects in the CdZnTe substrate material, not gravitation-dependent convection.

The dislocation defects that appear in CdZnTe crystals grown on the ground are almost eliminated in the crystal of CdZnTe grown in orbit during USML�1.

But the near absence of hydrostatic pressure in microgravity resulted in surprising events. What Larson found upon examining the USML�1 samples after their return to Earth was that a significant amount of material solidified without wall contact, and the flight samples showed dramatic reductions in dislocation defects and twinning in the portions that experienced detached growth. "What we found," states Larson, "was that the number of defects in the crystal that we grew was 50 times less than the best we had done on Earth.

The yield [of usable material], because of the reduced twinning, was better than 70 percent, instead of 5 percent on the ground � a big difference!" He adds, "The [greater] yield is what drops prices, and the quality is what tells you what can be done with it. And in fact, the sample also gave us good results as far as the 50-fold reduction in dislocations was concerned." Although the USML�1 experiment didn�t proceed exactly as planned, says Larson, "it made sense when we saw the results. We just didn�t quantitatively predict them."

A Paradigm Material

Explaining the unexpectedly good results from the USML�1 experiment, Larson knew, would require conducting a follow-up experiment that would show definitively that the reduction in defects was due to the lack of contact between the growing crystal and the container wall. After all, Larson reasoned, some other effect of the microgravity environment might account for the reduction in defects. For his next spaceflight experiment, flown on USML�2 in 1995, Larson needed to artificially create hydrostatic pressure, otherwise absent in space, so that the liquefied CdZnTe would be forced into contact with the wall during solidification. Forcing wall contact required redesigning the container so that a piston would push against the liquefied material, simulating hydrostatic pressure. Creating this effect would ensure total contact with the container wall. If the crystal grown in this way had a large number of defects, Larson�s hypothesis would be confirmed.

With the second spaceflight experiment, Larson not only wanted to prove that contact with the container wall caused defects, he also wanted to take the next step, which was to design a unique detached solidification experiment -� growing the crystal without touching the container wall. He also wanted to increase the amount of detached material grown. Designing this second half of the experiment involved a novel attempt to purposely grow detached crystal material in space. Preparations required rigorous months of complex theoretical work to try to anticipate and predict all possibilities involved in growing an increased amount of nearly defect-free paradigm material. Larson explains, "Nothing like this could be done on the ground because of hydrostatic pressure. It involved risk. It was not like we were going to quantitatively judge a difference between flight and ground experiments. We were trying something unique."

When the space shuttle carrying the USML�2 mission touched down, Larson retrieved two experiment samples. One contained defects; the other was nearly flawless. The sample with defects, grown with wall contact, showed highly strained material in which twins were common at the surface. This sample "verified our understanding that it was contact with the container wall and resultant hoop stresses that contributed to defects," says Larson. But the nearly flawless sample, grown largely without wall contact, held a surprise: it was of unprecedented size, and the incidence of twin formation was virtually zero. Larson recalls feeling stunned when he first looked at the X-ray picture of the experiment. "I�m looking at it and I�m saying, �No one predicted this.�" He adds, "This result would only have happened in the microgravity environment. It could not have happened on the ground."

A Unique Laboratory

Using the unique laboratory of a microgravity environment advances both science and technology, as Larson demonstrated in his investigations into the crystal growth process of CdZnTe. In the absence of gravity-related complications, Larson not only definitively showed that defects arise during crystal growth processing as a result of contact with the container wall, but he also showed that growing near-perfect, paradigm material is possible. Such insights gained from microgravity experimentation can lead to improved crystal growth processing on the ground. With a paradigm space-grown crystal in hand, for instance, researchers can begin working on how to reproduce that improvement on Earth. And eventually, Larson predicts, "we can begin growing high-quality material on demand, rather than waiting for one chance in twenty and lucking into a good sample."

Growing high-quality material by eliminating twinning and dislocation defects in ground processing would have enormous commercial potential. Millions of dollars are lost each year in the infrared detector industry due to defective material. Improving ground techniques for growing semiconductor crystals could reduce costs of manufacturing important infrared devices used by doctors, pilots, law enforcement officials, industrial workers, and environmental and aerospace scientists. In addition, higher-quality crystals with enhanced performance could also be produced, thus enabling infrared devices to create higher-resolution images.

Learning how to improve processing for CdZnTe can also be applied to making improvements in other materials. For instance, three groups are currently funded by NASA for next-generation experiments investigating detached solidification. Each group will develop techniques allowing crystals of electronic or semiconductor materials to grow without wall contact while in spaceflight. The materials the groups choose will be selected with the goal of creating paradigm samples that could serve as the benchmark for optimizing the material�s performance.

The microgravity environment can provide invaluable insights into continued scientific experimentation. Says Larson, "We are trying to apply that environment to scientific problems by identifying and optimizing materials for improved performance." Getting rid of gravity-related complications seems to be the key to enabling scientists to study the origins of defects in materials produced on Earth. For example, "Until recently, there has been no theory that addressed the origin of twinning defects and why they�re there. We just knew they were there and we couldn�t get rid of them," says Larson. Now, through continued scientific efforts combining the use of the microgravity environment with innovative approaches to solving the mysteries of crystal defects, scientists are gathering knowledge and developing new strategies for crystal processing that may greatly advance various technologies, including infrared devices.

MicroGravity News - SpaceDaily Special Report

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