FORMATION OF CRACKS IN EPITAXIAL OVERLAYERS

The one-dimensional model of epitaxial interfaces developed by Frank a nd van der Merwe and refined to account for the anharmonic and non-con vex character of the interatomic bonding is used to study the formatio n of cracks in epitaxial films. In our model, the force which keeps th e atoms together displays a maximum (the theoretical tensile strength of the material), and the chemical bonds become distorted, i.e. long ( weak) and short (strong) bonds alternate in expanded overlayers. In co mpressed overlayers, the bonds in the cores of the misfit dislocations are expanded and can break along the dislocation lines at small misfi ts. In expanded overlayers, the bonds in the core of the misfit disloc ations are compressed and cannot break. However, due to the anharmonic ity of the potential and the distortion of the chemical bonds, coupled kink-antikink configurations could replace single kinks beyond the me tastability limit of the latter. The bonds in the cores of the antikin ks are strongly elongated. They can easily break as the force which ke eps the atoms together is several orders of magnitude weaker than the theoretical tensile strength of the material and the overlayer can cra ck along the dislocation lines at large absolute values of the misfit. Therefore compressed overlayers can crack at small positive misfits, whereas expanded overlayers are more prone to crack at large absolute values of the negative misfit. Finite 2D islands crack easier than inf inite overlayers when compressed and more difficult when under tensile stress.