Amorphous versus crystalline GeTe films. I. Growth and structural behavior

Abstract

As a part of our structural, optical, and electrical studies of amorphous versus crystalline GeTe films, this paper (Part I) reports on the growth and structural properties of the thin-film polymorphs (amorphous, rhombohedral, and NaCl) of GeTe. When deposited above ∼130°C, the GeTe films are polycrystalline. Epitaxial or partially oriented growth of the rhombohedral phase is obtained on NaCl and mica at deposition temperatures ∼250°C. An electric field ∼4000 V/cm applied laterally on the substrate during deposition lowers the epitaxial temperature. At temperatures above 250°C, the high-temperature NaCl structure grows epitaxially. Below a deposition temperature of ∼130°C, the films are amorphous. The amorphous phase transforms very rapidly via random nucleation centers to the crystalline phase at a well-defined temperature of 145±3°C accompanied by the release of heat ∼1±0.2 kcal/mole (∼kT_m, where k is Boltzmann's constant and T_m is the melting point of GeTe). The transformation temperature increases linearly with decreasing thickness below 350 Å. The amorphous→crystalline transformation induced thermally as well as by electron beam irradiation has been studied inside the electron microscope. The crystalline phase is found to grow in the form of spherulites and dendrites. If induced by electron beam irradiation, the spherulites are polycrystalline rhombohedral in structure while the dendrites exhibit oriented NaCl structure.