Nanometer -scale studies of interdiffusion and segregation in semiconductor structures.

Abstract

Interdiffusion and segregation are fundamental processes that lead to changes in structural and compositional uniformity of heterostructures, which in turn affect the performance of electronic and photonic devices. As devices are continually reduced in size, the distribution of the chemical species near heterointerfaces, often determined by the interdiffusion and segregation processes, becomes of crucial importance. In this dissertation, a combination of cross-sectional scanning tunneling microscopy and spectroscopy, cross-sectional transmission electron microscopy, and high-resolution x-ray diffraction has been employed to investigate the interdiffusion and segregation processes in molecular-beam-epitaxially-grown heterostructures. This work provides insight into these processes, including direct measurements of the interdiffusion and segregation lengths, on an atomic scale. The effects of residual strain on these fundamental parameters are also discussed. In the non-stoichiometric AlAs/GaAs superlattices, results suggest that Al-Ga interdiffusion induces an apparent superlattice disordering. In addition, misfit strain presumably affects the preferential As precipitation along the GaAs side of each AlAs/GaAs interface. The preferential As precipitation, less apparent after <italic>ex situ</italic> annealing, may be diminished by Al-Ga interdiffusion at the AlAs/GaAs interfaces. In the ZnSnP₂/GaAs superlattices, an asymmetry in interface abruptness is observed, with the GaAs on ZnSnP₂ interfaces apparently much smoother than the ZnSnP₂ on GaAs interfaces. This asymmetry is likely due to strain-enhanced surface segregation of Sn occurring during the growth of GaAs on ZnSnP₂. Furthermore, the ZnSnP₂ layers appear inhomogeneous, due to compositional fluctuations caused by the presence of ZnSnP₂ and ZnSnAs₂-rich regions. Investigations of <italic>in situ</italic> and <italic>ex situ</italic> post-growth thermal annealing of InAs/GaAs quantum dot superlattices suggest that the organization of these superlattices is affected by annealing-induced dot dissolution. Annealing-induced variations in the positions of the In atoms between the dot arrays enable direct measurements of In-Ga interdiffusion and In segregation lengths. Finally, the effects of residual strain on the dissolution of dots in dot arrays are discussed.