| As MOSFETs scale to the deep-submicron regime, the need for ultra-shallow junctions and modulation-doping structures has brought an increasing demand for silicon epitaxial layers with abrupt doping profiles. For these devices, arsenic is an attractive N-type dopant because of its high solubility and low diffusion rate, but suffers from severe surface segregation during epitaxy, making high-concentration incorporation with abrupt transitions difficult.; This dissertation describes arsenic surface segregation and incorporation during Si molecular beam epitaxy (MBE) using a unique combination of solid and gas sources. Using disilane gas for silicon and dimer molecules for arsenic sources, it is shown that relatively high substrate temperatures are needed to activate surface reactions during growth. Surface segregation of arsenic under these conditions is investigated and a new segregation energy model is proposed based on surface 2-D islanding of arsenic. Arsenic incorporation in SiGe at these high temperatures is much improved compared to that in silicon, which is attributed to competitive surface segregation. Replacing disilane with an elemental silicon source, on the other hand, eliminates surface reaction steps and enables deposition at lower temperatures, where surface segregation becomes kinetically suppressed. Under these conditions extremely high arsenic concentrations can be achieved. In this work, we demonstrated Si (100) epilayers with As concentrations up to 4 × 1021 cm−3 and doping transitions better than 3 nm/decade. Other mechanisms that can limit arsenic incorporation in Si and SiGe in this regime are discussed. Electrical properties of heavily doped as-grown and annealed materials are investigated and correlated to atomic-scale defects. While electrical properties in thicker epilayers are limited by bulk values, confining dopants to a thin sheet a few nanometers thick leads to significant improvements in both dopant activation and carrier mobility. The former is correlated to geometric suppression of arsenic clustering and the latter to quantum confinement. Effects of layer thickness and spacing are also discussed. |
