Stability of GaAs/Si Superlattices During MBE Growth and Post-Growth Annealing
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J.K. WADE*, P.D. MORAN*, H.J. GILLESPIE*, G.E. CROOK** AND R.J. MATYI*** *Materials Science Program, **Dept. of Electrical and Computer Engineering, and ***Dept. of Materials Science and Engineering, University of Wisconsin, Madison, WI 53706
ABSTRACT The stability of GaAs/Si superlattices grown on GaAs substrates using molecular beam epitaxy is described. Typical superlattice structures consisted of ten periods of thin (less than 6.5A thick) layers of pseudomorphic silicon alternating with thick GaAs layers. We have examined the As2/Ga flux conditions required for the growth of high quality superlattices and have found that the structural perfection is extremely sensitive to the V/III flux ratio. The best superlattices were grown under condition that were just barely enough arsenic to produce a stable (2 x 4) surface reconstruction in the GaAs layers; increases in the arsenic overpressure resulted in a progressive trend towards 3-D growth of the GaAs on the pseudomorphic Si. In addition, we have examined the stability of GaAs/Si superlattices towards post-growth annealing. Double crystal x-ray diffraction scans showed little change in superlattice structure following rapid thermal anneals at 800°C; at 900'C, however, all but the first order satellite reflections disappeared. We attribute this behavior to the relaxation of pseudomorphic strin and the generation of misfit dislocations at the higher anneal temperature. INTRODUCTION The epitaxial growth of superlattice structures consisting of thin, pseudomorphic layers of silicon between thick layers of GaAs has attracted considerable attention recently for various reasons. For instance, the insertion of pseudomorphic layers of Si or Ge in AlAs/GaAs heterostructures results in an interfacial dipole that alters the valence and conduction band discontinuities [1]. Theoretical analyses of alloys of Group IV elements with GaAs have shown interesting changes in band structure as a function of alloy composition [2]. From a structural standpoint, the differences in lattice parameter, thermal expansion, and crystal symmetry between Si and GaAs makes the growth of GaAs/Si heterostructures an opportunity to explore the factors that limit the successful growth of materials that are highly mismatched. Recently a number of groups have reported the successful molecular beam epitaxy
(MBE) growth of superlattices consisting of thin (< 5A) layers of pure silicon alternating with thicker layers of GaAs [3-5]. X-ray rocking curves from these superlattices typically exhibit narrow satellite reflections; satellites out to the twenty-second order have been observed, indicating a high level of structural perfection [4]. Independent analysis by reflection high energy electron diffraction (RHEED) [3,4] and Raman spectroscopy [5] suggested that the silicon was present as continuous, pseudomorphic layers. Given that the silicon layers are extremely thin, it is natural to speculate on their stability during the epitaxial growth process or during post-growth processes such as rapid thermal ann
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