DOPANT INCORPORATION PROCESSES IN SILICON GROWN BY MOLECULAR BEAM EPITAXY.

Abstract

We have investigated the doping processes in Si MBE when two commonly used dopants, Ga and Sb, are evaporated onto the growing Si, using isothermal desorption combined with Auger electron spectroscopy and mass spectroscopy as well as an analysis of the doping profiles of the grown films. Doping proceeds from a surface phase of the dopant adsorbed on the surface rather than directly from the flux. Dopant atoms, first adsorbed into this phase have a certain mean residence time thereafter which they are desorbed. Within this time, they may be incorporated into the growing film. Changes in the flux do not cause instantaneous changes in the concentration of the surface phase but take a time of the order of the residence time to reach steady state. This results in a delay for the bulk doping level to reach the intended value and a consequent smearing of the profile. The kinetics of this incorporation process have been studied and the delays are found to be significant at some of the growth temperatures. The possible mechanisms of dopant incorporation and schemes to achieve abrupt profiles are described.