Barriers to Strain Relaxation in Epitaxial Fluorides on Si(111)
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BARRIERS TO STRAIN RELAXATION IN EPITAXIAL FLUORIDES ON Si(lll)
Weidan Li*, Steve Hymes**, Shyam P. Murarka**, Leo J. Schowalter* * Center for Integrated Electronics and Physics Department, Rensselaer Polytechnic Institute, Troy, New York 12180 "**Centerfor Integrated Electronics and Material Engineering Department, Rensselaer Polytechnic Institute, Troy, New York 12180 Abstract
The mechanical stress of epitaxial SrF 2 and CaF2 films on Si(1 11) substrates has been measured as a function of temperature by the substrate curvature technique. The temperature dependence of the stress in the SrF 2 film is interpreted in terms of an energy barrier to dislocation motion. When the strain energy is smaller than the value needed to overcome the barrier, the change in stress is due mainly to elastic deformation. As the temperature change increases, the strain energy becomes large enough to overcome the barrier, at which point plastic deformation initiates. Unlike SrF 2 , the stress behavior of the CaF 2 film for increasing temperature is quite different from its behavior for decreasing temperature. This unusual behavior is not understood at this time. I. Introduction Epitaxial growth of group HA fluorides on semiconductors is attractive for both fundamental research and potential applications in the optoelectronic and micro-electronic industry(l). For example, the SrF 2/Si system has both a large lattice mismatch (6.8%) and a large thermal expansion mismatch (SrF 2 has a thermal coefficient of expansion, TCE, 7.4 times larger than Si). This allows detailed studies of growth and strain relaxation mechanisms in a both lattice and thermal mismatched system to be carried out in a well controllable epitaxial system. For use as a buffer layer between compound semiconductor and Si, CaF 2 has been found to provide significant ability for strain relief. Consequently, the growth of strain free GaAs on Si demonstrates promise for device applications( 2). In recent years, the residual strains in both CaF 2 and SrF 2 on Si(1 11) have been studied intensively( 3- 5). The conclusions may be summarized briefly as follows: the residual strain in the fluoride films at room temperature, when cooled down from the deposition temperature, is tensile in spite of the fact that the fluorides have a larger lattice constant than Si. Furthermore, the tensile strain decreases with the increase of the film thickness for films grown thicker than a thermal critical
thickness. These phenomena can be explained by the competition between the lattice mismatch and the thermal expansion mismatch( 4 ,5). The residual strain is completely relieved in the CaF 2 epi-layers thicker than 300 nm, while it remains in the SrF 2 films even when the film thickness exceeds 600 nm. The film thickness dependence of the residual strains in SrF 2 epi-layers on Si(1 11) has been modeled phenomenologically by applying the force balance theory. In this model, the equilibrium condition was assumed and a frictional force distinct from the conventional Peierls force was introduced
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