Periodicities in the X-Ray Diffraction of Low Order ALAS/GAAS Superlattices
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PERIODICITIES IN THE X-RAY DIFFRACTION OF LOW ORDER ALAS/GAAS SUPERLATTICES JOSEPH PELLEGRINO, S.QADRI*, W.TSENG, W.R.MILLER", AND J.COMAS, NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, MD., 'NAVAL RESEARCH LAB WASH. D.C.,-*ON SABBATICAL LEAVE FROM PENN. STATE UNIV., MIDDLETOWN PA. ABSTRACT In this work we examine the physical properties for the superlattice system (GaAs)nl (AlAs)n 2 /GaAs(100) for low values of n i and n2 , i.e., n1 = n2 = 3, 6, 12. Normal, interrupted growth, and migration enhanced epitaxy (MEE) growth techniques were used to grow the superlattice structures in a molecular beam epitaxy system. X-ray diffraction spectra were obtained, and the major and satellite peak positions were analyzed to obtain the superlattice periodicity. An analysis of the major diffraction peaks and their associated satellites produced superlattice periodicity in good agreement with theory. Diffraction peaks were also observed in regions adjacent to the primary diffraction peaks which did not occur in the expected satellite positions. An analysis of these peaks relative to the primary peak indicate periodicities corresponding to layer thickness greater than the intended period. One possible cause for these periodicities is growth conditions that exist during the growth of the superlattice which result in the deposition of fractional monolayers. In this study we present results which suggest that an arsenic-deficient growth condition may be a contributing factor in the deposition of fractional monolayers. INTRODUCTION The molecular beam epitaxy (MBE) growth technique permits the fabrication of superlattices with monolayer periodicities. Because these structures involve individual layers several tenths of a nanometer (angstroms) thick, X-ray diffraction can be effectively used for structural examination. There is much interest in the structure of superlattices and compositionally modulated materials formed by imposing an atomic-scale periodicity during the growth of a film. One advantage of the MBE deposition technique in this regard is that the growth rate can be controlled to a considerable degree by regulating the temperature of the effusion cells and the substrate. The ability to control the growth rate, as well as the availability of in-situ reflection-high-energy-electrondiffraction (RHEED) capabilities to measure the growth rates, makes MBE well suited for fabricating structures requiring precise layer thicknesses. One category of material which demands a high degree of layer thickness control at the atomic level is superlattices. High-quality reproducible superlattice structures are required for optical and quantum confinement devices such as high electron mobility transitors, surface lightemitting lasers, self-electro-optic effect devices and quantum well lasers. In this work we attempt to correlate certain growth parameters with the structural periodicity of the superlattice. EXPERIMENTAL The superlattice materials used in this study were fabricated using the MBE growth technique. Semi-insulating GaAs(100) was
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