Measurement of phosphorus segregation in silicon at the atomic scale using scanning tunneling microscopy

In order to fabricate precise atomic-scale devices in silicon using a combination of scanning tunneling microscopy (STM) to position dopant atoms and molecular beam epitaxy to encapsulate the dopants it is necessary to minimize the segregation∕diffusion of dopant atoms during silicon encapsulation. We characterize the surface segregation∕diffusion of phosphorus atoms from a δ-doped layer in silicon after encapsulation at 250°C and room temperature using secondary ion mass spectrometry (SIMS) and STM. We show that the surface phosphorus density can be reduced to a few percent of the initial δ-doped density if the phosphorus atoms are encapsulated with 5 monolayers of epitaxial silicon at room temperature. We highlight the limitations of SIMS to determine phosphorus segregation at the atomic scale and the advantage of using STM directly.