Both require latching upon large, nearly flawless crystals. In the case of room-temperature radiation detectors, gamma rays can be detected by crystals of a novel compound blended from cadmium, zinc, and telluride (CZT) - which in fact has a diamond-like structure. Larger crystals are more likely to catch stray radioactive particles that create an electronic signal upon impact, but they may contain more defects that degrade the material and reduce the quality of the signal.
So, Jason Heffelfinger (8715), a materials scientist with microscopy expertise, has been examining fine details of CZT crystal defects in electron micrographs. His work, being presented at the December Materials Research Society meeting in Boston, should help improve processing and fabrication, important to both commercial crystal growers and the Sandia research team that is developing CZT detectors to identify or monitor nuclear material.
"This is really one of the first microscopic pictures of what's going on within CZT crystals and detectors," says Ralph James, a distinguished member of technical staff in Materials Processing Dept. 8230. Ralph is co-chairing a symposium at the conference at which about 200 researchers are expected to hear some 140 presentations on CZT and other semiconductor sensors and imaging devices over the course of five days.
"The idea is to improve the electrical properties of the material," Jason says. "Even if you capture a very small signal, you can still have to extract that signal through a very large piece of defective material."
Noise may be introduced by the presence of cracks or bubbles or along the boundary of two different grain orientations. These regions tend to be rich in tellurium precipitates, which separate out during cooling and act like a short-circuit path that degrades the material and disrupts the electrical signal.
The compound is made under high heat and pressure. It may take two months to grow an ingot of 10 kilograms, Jason says, and only about one-fourth or less of the ingot will be of good enough quality to detect radiation.
CZT has only been developed for roughly five years as a radiation-sensor material and has not yet been characterized in very fine detail, Jason says. With high-resolution microscopy, details can be discerned as small as about 1.4 to 1.7 angstroms. "We hope we can detail atomic structure," Jason says.
Challenges include isolating a defect in the first place and creating a thin sample, since CZT is much more brittle than silicon. In fact, it can be damaged with an electron beam.
Transmission electron microscopy reveals structural information from thin-foil specimens. This approach (as well as scanning electron microscopy) can also reveal surface structure and chemical information by exciting X-rays characteristic of the probed material. Gaining information about composition by these means is known as X-ray energy-dispersive spectrometry (XEDS).
Jason also plans to probe the material by analyzing the residual gas in pipe-shaped voids. He wants to see if the voids contain argon or other gases present when the ingot is grown. This inquiry requires breaking the material in place under a vacuum.
Although defects may inhibit formation of large single crystals, a slightly tellurium-rich compound may also have better gamma-ray detection properties, he says.
Jason has conducted this basic materials research since coming to Sandia in January, with Doug Medlin (8715) and Bruce Brunett (8230).
Other Sandia CZT research is being presented by Ralph, Richard Anderson (8716), Dean Buchanauer (8716), Haim Hermon (8230), Nathan Hilton (8230), Jim Lund (8230), Michael Schieber (8230), and John Van Scyoc (8230).
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