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August 1995

Researchers observe atomic-scale defects in semiconductors

WEST LAFAYETTE, Ind. -- In order to build a better semiconductor, a Purdue University engineer is looking for solutions by magnifying the problems.

James E. Angelo, assistant professor of materials engineering at Purdue, is part of a research team that has for the first time directly observed the atomic structure of defects in a semiconductor made of several different materials.

The team's findings may lead to a better understanding of the electrical properties of semiconductors and to the development of better-performing optical and electronic devices. Details of the research appear in the July 28 issue of Science magazine.

Semiconductors are tiny electrical switches used to control nearly all modern-day electrical and optical devices, such as CD players, remote control units and fiber optic communications networks.

"The goal of this research is to understand the structure of defects and how they affect the electrical properties of compound semiconductors, which has many important consequences," Angelo says. Another goal is to develop methods to eliminate the defects in semiconductors.

"As we try to make smaller and smaller electronic devices, the effects of defects in the semiconductors are going to be more important," he says.

Most semiconductors are silicon, but others, called compound semiconductors, are made from a combination of materials that must be carefully built, or grown, into crystals one thin layer at a time. Semiconducting materials are useful because the amount of electric current they conduct can be changed and controlled. Silicon semiconductors are used primarily in the transistors in electronic devices, while compound semiconductors like gallium arsenide are used for optical devices.

"One of the problems in fabricating these devices is that defects, or dislocations, in the structure are often electrically active, which can adversely affect the quality of the crystal and its performance," Angelo says. "You want the best possible crystal you can get, so it's important to understand the structure of these dislocations and their electrical properties."

Over the past year, Angelo and his colleagues observed the atomic structure of defects in thin semiconducting films made from the elements cadmium and tellurium, which they deposited atomic layer by atomic layer on top of a base of gallium arsenide. Cadmuim telluride semiconductors can be used in devices for fiber optic communications.

Using a powerful scanning transmission electron microscope at the Oak Ridge National Laboratory in Tennessee, the team took high-resolution images of the defects in the cadmium telluride semiconductor.

"Using conventional high-resolution methods, which are ways of looking at atoms, you can't resolve the different types of atoms in a structure," Angelo says. "Using the facility at Oak Ridge, we were able to identify the two types of atoms, cadmium and tellurium, and determine how they are arranged in the defects. This is the first time that anyone has been able to clearly identify the nature of these dislocations in a compound semiconductor."

In addition to Angelo, whose research specialty is electron microscopy, the team included Alastair McGibbon, currently at the University of Glasgow, Scotland, and Steve Pennycook of the Oak Ridge lab. The research was funded in part by the U.S. Department of Energy.

Angelo, who directs the microscopy facilities in Purdue's School of Materials Engineering, says the scanning transmission electron microscope at Oak Ridge is one of only a handful of instruments around the country that have such high-resolution capabilities.

"This is a relatively new instrument," he says. "It's been improved so that now it has a small enough probe to where you can actually image individual atoms one at a time. It's a very powerful tool for looking at defects in a host of materials."

Source: James E. Angelo, (765) 494-4107; Internet, jeangel@ecn.purdue.edu

Writer: Amanda Siegfried, (765) 494-4709; home, (765) 497-1245; Internet, amanda_siegfried@purdue.edu

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Abstract

Direct observation of dislocation core structures in CdTe/GaAs(001)

Alastair McGibbon, Department of Physics and Astronomy, University of Glasgow, Scotland, UK; Steve Pennycook, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN; James Angelo, School of Materials Engineering, Purdue University, West Lafayette, IN.

A strategy is presented for determining sublattice polarity at defects in compound semiconductors. Core structures of 60-degree and Lomar dislocations in the CdTe/GaAs(001) system have been obtained by the application of maximum-entropy analysis to Z-contrast images (Z is atomic number) obtained in a 300-kilovolt scanning transmission electron microscope. Sixty-degree dislocations were observed to be of the glide type, whereas in the case of Lomar dislocations, both a symmetric (Hornstra-like) core and an unexpected asymmetric structure made up of a fourfold ring were seen.


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