Arsenic segregation at Si(lO0) surfaces during annealing (890–970°C) has been studied by medium energy ion scattering (MEIS), Rutherford backscattering spectrometry (RBS), ion scattering spectrometry (ISS), and Auger electron spectroscopy (AES). The unique depth resolution of MEIS revealed that arsenic segregated in a two-dimensional layer at the clean Si surface during annealing. For a surface with a native oxide the arsenic piled up at the Si/oxide interface. This segregation peak was no longer present on the Si surface after conventional contact opening. By metallizing arsenic junctions with Al:Si, 1% it was found that a segregation annealing step inhibited Al/Si interdiffusion. Diodes as shallow as 180 nm could be metallized without spiking.