Strain Relief Mechanisms and Nature of Misfit Dislocations in GaAs/Si Heterostructures
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Strain relief mechanisms and nature of misfit dislocations in GaAs/Si heterostructures S.Sharan and J. Narayan Department of Materials Science and Engineering, North Carolina State University, Raleigh, N. C. 27695-7916 J. P. Salerno and J. C. C. Fan Kopin Corp. Taunton, Mass 02780 Abstract The nucleation and glide of misfit dislocations in GaAs/Si system is investigated using transmission electron microscopy. GaAs epilayers of different thicknesses were examined by electron microscopy (plan and cross-section) and the elastic strain remaining in the film has been related to the average spacing of the misfit dislocations at the interface. A model is developed based on minimum energy considerations to determine the strain-thickness relationship. The theoretical predictions of strain relaxation are compared with experimental observations using high resolution electron microscopy. Introduction Epitaxial growth of GaAs on Si has been the subject of increasing interest since such a system provides the potential for utilizing the benefits of GaAs (high mobility and direct band gap/optical phenomena) in conjunction with Si for advanced integrated device structures. Various methods have been utilized to grow epitaxial GaAs layers on silicon substrates. These include vapor phase epitaxy (VPE) [1] on Ge coated Si substrates as well as direct growth by molecular beam epitaxy (MBE) [2] and organometallic chemical vapor deposition (OMCVD) [3] on Si substrates. Despite several potential applications for GaAs on Si ranging from GaAs integrated circuits to monolithic integration of GaAs photonic and Si electronic devices, the quality of the epitaxial layer limits their use. The large misfit (4%) between GaAs and Si is responsible for a high density of misfit dislocations at the interface and a large number of emergent dislocations propagating through the epilayer. The misfit problem is further aggravated by the difference in thermal expansion coefficients of the two phases (aCGaAs - 6.8 x 10-6 0C-1 ; aSi = 2.56 x 10-6 0c-l) which leads to large tensile stresses in the epilayer. The lattice mismatch between a heteroepitaxial film and its substrate can be accommodated by elastic strain and/or misfit dislocations. Theoretical calculations based on energy balance between the growing coherent film and the film with misfit dislocations indicates that pseudomorphic growth takes place until a critical thickness is reached [4-9]. Misfit dislocations are formed to relax the lattice mismatch upon exceeding the critical thickness. For semiconductor films of diamond cubic and zinc-blende structures, experimental observations show a much smaller relaxation of the elastic strain as a function of epilayer thickness. The epilayer has residual elastic strain even at thicknesses much larger than the equilibrium critical thickness[1012]. It is realized that dislocation nucleation and glide can be impeded by various processes. There is a large Pierels-Nabarro frictional stress in semiconductors, which can impede the glide of dislocations toward the inte
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