Comparative studies of As and B diffusion in polysilicon-on-single-crystal Si systems have been performed by Cross-sectional Transmission Electron Microscopy (XTEM) and Secondary Ion Mass Spectrometry (SIMS). Arsenic and BF2 implanted in 300 nm Low Pressure Chemical Vapor Deposition (LPCVD) polysilicon are diffused into the underlying Si substrate by Rapid Thermal Annealing (RTA) or furnace annealing. Diffusion profiles of Asappear to be continuous across the poly/single crystal Si interface except for a peak at the interface and show a gradual increase towards the interface within the polysilicon layer due to an inhomogeneous distribution of grain sizes in Asimplanted polysilicon layers. On the other hand, indiffusion of B gives a discontinuous doping profile at the interface due to accumulation of B-defect complexes created by dissolution of defectclusters in polysilicon. At low anneal temperatures, the B profiles in single crystal Siare shallower than the As profiles. This is because most of the B in polysilicon films is immobile during annealing near the peak region where the solid solubility limit is exceeded, and because of low B segregation at grain boundaries, thereby reducing the impactof grain boundaries in terms of high diffusivity. At high anneal temperatures, the native oxide at the interface breaks up and causes the polysilicon layer to align epitaxiallywith respect to the underlying substrate. Time to breakup of interfacial oxide depends on dopant species and anneal temperatures. Oxide breakup takes longer for As-doped samples than for B-doped samples. For high thermal budget anneals, B diffusion into the substrate is greater than As diffusion because of higher B diffusivities in single crystal Si.