Behaviour of amorphous GeTe and As2Te3 films under electron irradiation

Abstract

The effects of electron irradiation in producing structural and compositional changes in amorphous chalcogenide films and in amorphous chalcogenide films coated with metal, are the basis of several important applications. For example, crystallization of amorphous chalcogenides induced by the electron beam can be used as a memory transition [1] for use in highdensity computer memories. Also, under irradiation, silver is observed to diffuse away from irradiated areas in several types of chalcogenide/silver films. This is the basis for lithographic applications [2]. However, the basic structural and compositional changes occurring in chalcogenide and chalcogenide/silver films under electron irradiation are not well understood. In this investigation structural and compositional changes produced by electron irradiation in AszTe3 and GeTe films and films of these materials coated with silver have been studied.GeTe films deposited at room temperature on rocksalt substrates are amorphous. Under moderate electron irradiation, accomplished by focusing the electron beam in an EM6G transmission electron microscope or a Jeol 100C scanning transmission electron microscope, these films were observed to transform to polycrystalline GeTe (Fig. 1). The grain size could be controlled to some extent by varying the electron intensity. Similar effects have been observed by Chopra and Bahl [3] and by Aznfirez and M6ndez [4]. Films of GeTe coated with small amounts of silver, up to 15 wt %, were also amorphous. They were, however, stable in the electron beam. When larger amounts of silver were deposited with concentrations of 30 wt % or greater, a contact reaction was observed to take place producing polycrystalline films of the orthorhombic form of hessite (Ag2Te). These films with high silver concentrations were found to be stable under electron irradiation.As2Te3 films deposited on rocksalt at room temperature were also amorphous. On moderate electron irradiation, they were also transformed to the polycrystalline form, in an uncontrolled manner. The polycrystalline material was identified as As2Te3 from electron diffraction patterns. On deposition of small amounts of silver, up to 15 wt %, these films remained amorphous. However, crystallization in the electron beam became much more controllable on the addition of this small proportion of silver, and large grains were obtained (Fig. 2). By varying the irradiating intensity by a small amount, the growth of grains in As2Te3/Ag films could be speeded up, slowed down or Figure 1 A transmission electron micrograph of an area of GeTe crystallized in the electron beam.stopped. The growth and interaction of grains could be readily observed simply by adjusting the irradiating electron beam intensity. For larger amounts of silver, a contact reaction took place, producing films with either globules of material dispersed in a random fashion (Fig. 3) or randomly dispersed aggregates of much smaller particles. The globules have been identified as consisting largely of silver by X-ray Figure 2 A tr...