Dopant incorporation kinetics and abrupt profiles during silicon molecular beam epitaxy

Abstract

Controlled dopant incorporation behavior during the growth of single crystal silicon films by molecular beam epitaxy (MBE) is crucial for most device applications. However, since all dopants except boron exhibit low incorporation probabilities and/or a high degree of surface segregation, achievement of sharp doping layers with well defined concentration levels is not straightforward. In order to overcome these problems, techniques involving either the use of accelerated low-energy ions (secondary and direct implantation) or solid-phase epitaxy regrowth have been developed. Recent results on dopant incorporation using low-energy secondary and direct implantation are presented in this paper. Using indium as a model dopant, it is shown that the incorporation probability during secondary implantation, as measured by secondary-ion mass spectroscopy, exhibits a complicated dependence on the film growth temperature and the indium flux. From a detailed investigation of the adsorption/desorption behavior of indium on Si(100) surfaces, it was found that the incorporation behavior was directly correlated to corresponding changes in the segregated indium surface overlayer. Physical models describing dopant incorporation during silicon MBE are discussed and particular emphasis is placed on a newly developed multi-site model. This model is based on an exchange process for dopant atoms moving between potential wells corresponding to different lattice sites in the near-surface region. Five different dopant sites, including surface, bulk and three intermediate sites were used and surface segregation, incorporation, and bulk diffusion were accounted for by solving simultaneous rate equations. The model is demonstrated in the case of both thermal and accelerated antimony doping, to fit experimental incorporation data both as a function of growth temperature and growth rate very well. Finally, results on δ-doped structures are also presented.