Ultrashallow junctions in silicon formed by molecular-beam epitaxy using boron delta doping

Low-temperature molecular-beam epitaxy was used to form highly conductive, ultrashallow layers in silicon using boron delta doping. Junction depths, determined with secondary ion mass spectrometry, ranged from 7 to 18 nm. A minimum resistivity of 3×10−4 Ω cm was obtained when the delta-doped layers were spaced 2.5 nm apart. The sheet resistances of the epitaxial layers, plotted as a function of junction depth, followed the theoretical curve for a box-doped layer having a boron doping concentration equal to the solid solubility limit, 6×1020/cm3. At a specific thickness, the minimum sheet resistance obtained by B delta doping was more than a factor of 5 less than that achieved by ion implantation.