Behavior of Dopant-Related Defects in AlGaAs Superlattices
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BEHAVIOR OF DOPANT-RELATED DEFECTS IN AIGaAs SUPERLATTICES N.D. Theodore, P. Mei*, S.A. Schwarz**, C.B. Carter, C. Palmstrom**, J.P. Harbison" and L.T. Florez** Cornell University, Department of Materials Science and Engineering, Bard Hall, Ithaca, NY 14853; * Columbia University, Department of Electrical Engineering, 500 W. 120st St., New York, NY 10027; **Bell Communications Research Inc., 331 Newman Springs Rd., Redbank, NJ 07751.
ABSTRACT During the course of investigation of the mixing of highly silicon-doped GaAs/AlAs superlattices, defects such as dislocation loops and Si-rich precipitates were found to form in the specimens. These defects formed at particular doping levels upon annealing of the samples. The presence of the defects can be related to changes in mixing behavior. In the present study, transmission electron microscopy has been used to characterize the defects. Superlattices with varying silicon doping levels were annealed at different temperatures for varying time-periods, to observe the temperature-time behavior of the dislocation loops. The defects aggregate preferentially in the GaAs as opposed to the AlAs in the superlattice. A number of the dislocation-loops were investigated using high-resolution TEM. All the loops observed were interstitial in nature.
INTRODUCTION Superlattice mixing, induced by a variety of techniques in the AlGaAs system, has been studied extensively [1-151. The objective has been to obtain regions of different bandgap and mobility integrated on one substrate. Methods used to obtain mixing have included ion-implantation followed by annealing [1-51, diffusion from the surface [6-12], and doping during growth [13-15]. In the AlGaAs system, silicon-induced mixing has been of interest because of the fact that mixing can be obtained at relatively modest doping levels and can be confined to well-defined regions. In a previous study, AlAs/GaAs superlattices doped with Si during molecular beam epitaxy (MBE) growth were characterized after annealing; doping levels ranged from 10A8 to 1020 cm- 3 . The observation of defects related to doping levels in these superlattices has been reported. A correlation has also been reported between the presence of the defects (Si-rich precipitates or dislocation loops) and the inhibition of superlattice mixing [14,15]. In this study, the behavior of the dislocation loops related to doping levels is explored.
EXPERIMENTAL DETAILS Cross-section TEM specimens were prepared to evaluate the behavior of defects present in these superlattice structures. The structures consisted of 20 periods of alternating AlAs (100A) and GaAs (400A) layers grown by MBE on a GaAs substrate. The samples contained three regions doped during growth to 1020, 1019 and 1018 cm- 3 Si; each region consisted of four of the AlAs/GaAs periods. Specimens were annealed at temperatures ranging from 650-750 0 C and for periods of 0.5-3 hours in a H2 :Ar atmosphere; the surface of the sample was maintained in contact with an undoped GaAs wafer. Cross-section TEM specimens were pre
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