Using doping superlattices to study transient-enhanced diffusion of boron in regrown silicon

Abstract

A boron-doped silicon superlattice consisting of three boron spikes separated by 1700 Å of undoped silicon has been grown by molecular beam epitaxy and used to study the evolution of point defects following an amorphizing implant of Si+. After MBE growth, the wafer was implanted at 77 K with either 146 or 292 keV Si+ at a dose of 5×1015/cm2. These implants produced amorphous layer depths that coincided with the depths of either the middle B peak or just below the deepest B peak. The samples were then annealed at 800 °C in an Ar ambient. Secondary-ion-mass spectrometry and transmission electron microscopy were used to monitor the diffusion of the boron spikes upon annealing and the evolution of the extended defects upon annealing, respectively. For the lower-energy sample, an enhancement in the B diffusivity of over 500× was observed for both the surface B spike and the deepest B spike. The higher-energy implant shows conclusively that the back flow of interstitials into the regrown region is coming from the end-of-range damage just below the amorphous/crystalline interface. These results show that for these implant conditions the end-of-range damage does not act as a barrier to flow of interstitials to the surface. In addition it is noted that boron in the regrown silicon does not cluster whereas the boron below the amorphous crystalline interface does. Both of these features must be accounted for when modeling boron diffusion in regrown silicon.