Thermal stability of ultra-shallow junctions in silicon formed by molecular-beam epitaxy using boron delta doping

Ultra-shallow junction layers are required for deep submicron CMOS and quantum devices. Low-temperature (320 °C) molecular-beam epitaxy was used to form highly conductive, ultra-shallow layers in silicon using boron delta doping. The as-grown 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 cm−3. Minimal change was detected in either the atomic profiles or the resistivity after a 10 s rapid thermal anneal (RTA) or a 10 min furnace anneal (FA) up to 700 °C. The sheet resistance of the as-grown shallow junction are substantially less than that obtained by ion implantation. Only after the 800 °C FA did the MBE-grown layers degrade to have as large a sheet resistance as the best ion implanted layers.