Effect of size on phase transformation temperatures in Ge/Bi/Ge films

Abstract

Despite numerous studies the nature of phase transformations of nanomaterials in confinement remains among grand scientific challenges of nanoscience and nanotechnology. We address this issue in a comprehensive experimental study of melting and crystallization of 3–100 nm thick bismuth films confined between germanium layers. Layered Ge/Bi/Ge films modeled a “nanosized object – matrix” system and were formed by successive deposition of the components in a high vacuum. Two experimental approaches were used to trace the phase state of the system. The first one is based on studying crystalline structure during in situ heating and cooling of the samples in a transmission electron microscope. The second one is a differential method based on the registration of an abrupt change in the morphology of the films during their melting. It has been shown that the eutectic temperature is not a monotonic function of size. It decreases gradually as the size of the Ge/Bi/Ge system reduces and reaches its lowest value ≈ 190 °C for a ≈5 nm thick Bi film. For a thinner Bi film, the liquid phase formation temperature quickly rises in the system. The data on the metal-induced crystallization of amorphous germanium films in contact with bismuth, as well as the data on the crystallization of a liquid phase confined between Ge layers are presented. The processes of interphase interaction in the studied system are discussed.