Compositional congruency, correlation and high pressure polymorphism in electron-beam evaporated Pb1−xGexTe thin films

Abstract

Lead germanium telluride (Pb1−xGexTe), a pseudo-binary alloy of IV–VI narrow-gap semiconductors, PbTe and GeTe, is also recognized as a potential mechanically robust infrared high-index coating material. In order to clarify the difference of the stoichiometry between the layers and evaporants of Pb1−xGexTe, evaporation was carried out from the ingots of single crystals with different Ge concentration x using electron beam and resistance heating, respectively. The compositions of thin films were characterized using energy dispersive X-ray spectroscopy (EDX), the crystallographic structures were studied by X-ray-diffraction (XRD). It can be revealed that Ge concentration in thin films is approximately identical to that in the ingots when evaporation is carried out using electron-beam heating at an optimized substrate temperature. It is also demonstrated that, with an increasing of Ge concentration, Te concentration in films decreases gradually. As a consequence, the Te-rich characteristic presented in thin films will shift into the Te-deficient one due to the increasing of Ge concentration. The compositional interdependence can be attributed to the facts that Ge–Te bond energy is smaller than that of Pb–Te when Ge ions with a smaller ionic radius substitute for Pb ions with a larger one; as well as that the distance between a Ge ion and its adjacent Te ions is different due to the displacement of Ge ions from the cell center. It can be inferred that both the higher energy density delivered directly to evaporants from the electron beam and a rather lower value of thermal conductivity for PbTe-based semiconductor compounds are responsible for the congruent evaporation of Pb1−xGexTe. Furthermore, some high-pressure phases, such as rocksalt-type structure of GeTe and body-centered cubic structure Te-V, are presented in Pb1−xGexTe films which have a greater Ge concentration than that for ferroelectric phase transformation at room temperature. It can be assumed that existence of a rhombohedral ferroelectric phase structure in thin films leads to the occurrence of high-pressure polymorphism.