Abstract

Lead telluride is one of the important infrared high refractive index coating materials beyond 3.8 microns. Because of the great difference in vapor pressuse between lead and tellurium, in a deposited PbTe layer more Te will be lost. Conventionally, evaporable PbTe is prepared from stoichiometric proportions of pure constituents. From the phase diagram of Te and Pb it is obvious that the resulting material is always a p-type semiconductor. In using the conventionally prepared material to deposit coatings even at a rather low substrate temperature of, say, 100°C, a strong n-type PbTe layer is formed, which will lead to a strong free carrier absorption. As the substrate temperature is increased even more free carriers will be introduced. In order to obtain a PbTe layer of good quality a compensative process is required. This has been realized by O₂ bombardment during deposition or by baking in the air after deposition.⁽¹⁾⁽²⁾ In addition, a n-type PbTe layer is less stable. In certain conditions it can be compensated either by O₂ or by S and Se. The latter is just the case of preparing multilayer coatings, in which case the refractive index and absorption of a fresh PbTe layer will be shifted and hence the accuracy of optical thickness mornitoring will be reduced.⁽³⁾⁽⁴⁾

© 1984 Optical Society of America