Lead germanium telluride: a mechanically robust infrared high-index layer

A mechanically robust infrared high-index coating material is essential to the infrared interference coatings. Lead germanium telluride (Pb1−x Ge x Te) is a pseudo-binary alloy of IV–VI narrow gap semiconductors of PbTe and GeTe. In our investigation, the hardness and Young’s modulus of thin films of Pb1−x Ge x Te, which were deposited on silicon substrates using electron beam evaporation, were identified by means of nanoindentation measurement. It is demonstrated that layers of Pb1−x Ge x Te have greater hardness and Young’s modulus compared with those of PbTe. These mechanical behaviors of layers can be linked to a ferroelectric phase transition from a cubic paraelectric phase to a rhombohedral, ferroelectric phase. Moreover, the strength loss in the layers of Pb1−x Ge x Te can be also explained in light of strong localized elastic-strain fields in concentrated solid solutions. In addition, it is observed that layers of Pb1−x Ge x Te are highly transparent and refractive in the mid- and long-wave infrared spectral range (~3–40 μm). A conclusion can be drawn that a mechanically robust infrared high-index layer can be obtained using Pb1−x Ge x Te as starting materials.