Page 94 - Chalcogenide Glasses for Infrared Optics

72 Cha pte r T h ree

FIGURE 3.1 The Asarco copper refi ning electroplating tank house at Amarillo.

production of the primary metals. When primary metal production is
down and demand for the by-products exceeds the available supply,
the price may rise dramatically, as for any other commodity. Another
factor is purity. To be useful in electronic materials production, the
purity standards are five 9s at a minimum sometimes reaching purity
levels of seven 9s. There has been a tremendous improvement in
purification and verification of purity techniques since the start of the
semiconductor industry. The production of high-purity chalcogenide
glasses has benefited as well from this development. For example,
from one supplier the listed impurities in a high-purity six 9s arsenic
in parts per million (ppm) are C, 0.09; Na, 0.03; and Cl, 0.08.
For five 9s sulfur from a supplier, the impurities listed in parts per
million were Ca < 0.2, Cu < 0.5, Fe < 0.5, Mg < 0.2, Si < 1.0, and Ag < 0.2
with the values for the common impurities C and H O not even men-
2
tioned. Residual impurities vary among the sources of the ore used in
the production of the primary metal and the production means used.
Major impurities affecting optical performance of chalcogenide
glasses are gas molecules of water, hydrogen sulfide, hydrogen sele-
nide, and hydrogen telluride. Carbon, silica, and other oxides pro-
duce unwanted absorption in wavelength regions of interest. Metals
such as iron produce absorption at the short wavelengths near the
beginning of transmission. Sulfur glasses and selenium glasses are
considered insulators or semi-insulators in their electronic conduc-
tion. The impurity iron can dope the glasses, just as if they were
crystalline, forming a deep energy level in their forbidden gaps
which results in increased absorption in visible or near-infrared light.
Unfortunately, the analytical results furnished with the high-purity
element may have been developed for other larger, more important
applications than infrared glass. The impurities of consequence may