Ternary Compound Synthesis of High Purity Copper, Indium, and Selenium

The picture on the left shows high purity (99.999%) pellets of copper (red), indium (silver), and selenium (black) in a quartz tube in the atomic proportions of 1:1:2 respectively. This is in preparation for the synthesis of these three elements in to the compound CuInSe2 by sealing the tube into a closed ampoule, heating and allowing chemical reaction to take place. With the compound at a temperature above its melting point, the liquid is allowed to freeze from one end by slowly lowering the sealed ampoule through a steep temperature gradient. As a result of this process, called the Bridgman method, a solid ingot of CuInSe2 is obtained, containing large single crystals, which can be studied physically on account of their interesting semiconducting properties.

The picture on the right shows triangular facets on a face of a single crystal of the semiconducting compound CuInSe2, as observed under an optical microscope after growth of the material from the liquid state (using a process called the Bridgman method). The facets are terraces of growth steps and their triangular shapes and 60o angles indicate that the surfaces are (112) crystallographic planes, which have a 3-fold symmetry in a chalcopyrite structure. Similar triangular pattersn, called etch pits, can be obtained on a (112) face of a crystal of CuInSe2 by etching chemically with an acid or thermally by heating above 600oC in a vacuum. Compounds basedon this material have potential in thin film photovoltaic solar cells, arising from their high absorption of light in a range of wavelengths where the sun has its greatest intensity.

Clifford Champness
McGill University
Honourable Mention, Category 2 (open)

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