Dislocations in Germanium: Mechanical Properties

Germanium, having a face-centered cubic (fcc) diamond lattice, is brittle at room temperature like other Group IV semiconductors. This means that when applying a load on the material, it breaks before it deforms plastically. Gallagher [1], soon followed by some other Groups [2,3], was the first to report that when a stress is applied at high temperatures (in his work above 500° C) Ge becomes ductile. In other words, it deforms permanently by glide of dislocations (plastic flow). It was soon discovered that the glide planes were (111), that is, the planes of highest packing density in the structure [1–4], while the slip direction turned out to be <110> [5], the direction of the Burgers vector with modulus b.

As will be seen later, the plastic deformation of Ge is determined by the dynamic properties of the dislocations. The fact that Group IV elements are characterized by covalent bonding of four nearest neighbors gives rise to some unique mechanical behavior [6]. It is expected that the intrinsic dislocation or Peierls energy, taking into account the periodic structure of the lattice, is rather high in fcc diamond. This is related to the bonding rearrangement required in the core structure when moving a dislocation from one equilibrium site to another [6–8]. This means that at room temperature a high surface hardness is observed upon indentation, while at the same time the material breaks before it deforms. Another typical behavior is the yielding found in the stress-strain curve under dynamic load, using a constant strain rate e [6]. However, before dealing with the plastic flow of germanium, first, the elastic properties will be summarized and, more specifically, the first- and higherorder elastic constants discussed. They are keys to the understanding of the linear expansion coefficient (±_e) with temperature, which in turn is important for describing thermal stresses that may occur when a temperature gradient is present in the material or when two films with different ±_e are in contact with each other (epitaxial growth).